At present, primary iron metal is commonly produced through the CO₂ intensive carbothermic reduction of iron oxides in blast furnaces at a temperature of around 1600°C. Since carbon is used as both reducing agent and fuel for the process, blast furnace pig-iron cannot eliminate its CO₂ emissions. Therefore, in recent years, carbon-free electrochemical processes have been widely investigated as potential green alternative routes for the production of iron and iron-base alloys (1–4) (assuming renewable electricity).

A large project that has been focused on alternative ways of producing iron was Ultra Low CO₂ in Steelmaking (ULCOS) in which new smelting reduction concepts were studied. The ULCOWIN electrolytic production of iron from suspensions of iron oxide particles in a highly concentrated sodium hydroxide solution at 110°C was demonstrated at laboratory scale. It has been shown that the iron particles are reduced in the solid state, which differs from the conventional electrowinning processes where the metal is deposited through the reduction of dissolved metal cations. Previous works with this process achieved high Faradaic yield (80–95%) (1–4).

Based on the above-mentioned studies, the technology for alkaline pulp iron electrowinning is being studied for the first time from a secondary mineral source, namely bauxite residue from the alumina refining industry. This technique is referred to in the SIDERWIN project (5), which aims among others to produce iron from alternative low-grade iron sources, currently incompatible with the conventional steel making processes.

Bauxite ore is treated within the Bayer process to produce metallurgical grade alumina which is the raw material for aluminium production. Bauxite ore depending on its origin contains 40–60% alumina and the rest is a mixture of iron (20–30%), silicon and titanium oxides. When bauxite ore is treated with caustic soda, the aluminium hydroxides or oxides contained within are solubilised, with approximately 50% of the bauxite mass being transferred to the liquid phase, while the remaining solid fraction constitutes the bauxite residue, often termed as ‘red mud’ due to its colour. Depending on the grade of the bauxite ore used, bauxite residue, on a dry basis, is produced from 0.9 to 1.5 mass ratio to the alumina product (6). The high volume and alkalinity of this byproduct make its valorisation a major challenge worldwide (7).

Bauxite residue is an untapped secondary raw material source considering the presence of valuable substances such as iron (30–45 wt%), aluminium (15–25 wt%), silicon, calcium, titanium and sodium oxides as well as smaller concentrations of critical or industrially important elements such as rare earth elements (REEs) (mainly cerium, lanthanum, scandium, yttrium and neodymium), vanadium, chromium and others (6). The recovery of the major metals from bauxite residue has never yet been implemented while a lot of processes have been proposed. Despite the laboratory-scale success of much of the work so far, currently the industrial utilisation of bauxite residue is estimated at just 2–4 million tonnes, accounting for less than 3% of the annual bauxite residue production (8).

The main constituent of bauxite residue is iron oxide and it can make up to 45% of the mass of the bauxite residue. In fact, the red colour of bauxite residue is caused by iron(III) oxides (mostly haematite, Fe₂O₃) (9). In general, due to its high alkaline (Na₂O ≈ 10%) and its titanium (TiO₂ ≈ 4–15%) content, bauxite residue is not suitable for use as an iron ore substitute in blast furnaces.

Bauxite residue reductive smelting processes can be applied by several technologies (Corex, Finex^®, HIsmelt, Romelt, AusIron and electric arc furnace (EAF)) for the production of pig iron (10, 11). So far, two methods have been applied at pilot scale for bauxite residue reductive smelting: the Romelt method (12) and the EAF (13–16). The Moscow Institute of Steel and Alloys (MISA) (Russia), with National Aluminium Company Ltd (NALCO) and the joint venture company Romelt-Steel Authority of India Ltd (RSIL) (India), studied the pyrometallurgical process of bauxite residues using the Romelt method (12). The advantage of this method is that materials can be used with moisture levels of up to 10 wt%. The main disadvantage is the high energy consumption and the poor quality of pig iron with a high concentration of sulfur and phosphorus (10). In EAF reductive smelting, a mixture of bauxite residue, carbon and fluxes is treated at 1500–1700°C to form pig iron with higher than 95% iron recovery (6, 13, 17). Recovery of residual iron can be further improved by later magnetic separation in slag dust (18). Post-melting slag can be used to produce rockwool or building materials (6, 19) and for the recovery of non-ferrous metals and REE (9, 18, 20, 21). However, such methods have not been industrialised as they are not competitive to established iron and steel making processes.

The present paper describes a highly promising electrochemical method for sustainably extracting the iron from bauxite residue, in an alkaline environment, which is in-tune with the alkaline environment of the Bayer process.

The process is conducted in an electrolysis cell consisting of a borosilicate glass beaker (250 ml) closed with a specially configured cylindrical silicon bung (45 cm diameter). The experimental apparatus is shown in Figure 1.

Fig. 1.

A three-electrode configuration was used; the cathode was a rectangular shaped stainless steel V2A plate (110 mm height, 10 mm length, 1 mm width), the anodes were two rectangular shaped nickel plates (100 mm height, 10 mm length, 2 mm width). All electrodes were centred according to the silicon bung’s cylindrical axis in specially configured holes so that the electrodes were in certain positions and at fixed distance to each other. The working electrode’s surface area that was immersed in the solution was defined to be 8 cm². The reference electrode that was used was a commercial Hg | HgO | NaOH (1 M) electrode (RE-61AP, ALS Co Ltd, Japan) which was immersed in a distinct glass.

Bauxite residue was supplied by MYTILINEOS-Aluminium of Greece. Samples were solubilised via fusion method according to which a quantity of bauxite residue remained at 1000°C for 1 h with a mixture of Li₂B₄O₇/KNO₃ followed by direct dissolution in 6.5% nitric acid solution. Chemical analysis was performed by atomic absorption spectroscopy (AAS) with the use of a PerkinElmer 2100 atomic absorption spectrometer. The chemical analysis of the samples used in this study is shown in Table I. The chemical analysis showed that Fe₂O₃ is the main content of bauxite residue.

Mineralogical analysis of bauxite residue (Figure 2) showed that the main iron oxide phase is haematite while a small amount of goethite also exists. The haematite to goethite mass ratio in bauxite residue is 4.2 (22).

Fig. 2.

Prior to each experiment, the stainless-steel cathode and the nickel anodes were polished with sandpaper and were rinsed with demineralised water. Furthermore, the cathode was weighed. The electrolyte was a 50 wt% NaOH aqueous solution corresponding to molarity of 25 mol kg^–1, to which was added 10 wt% bauxite residue solid particles. Typically, a mixture of 152.4 g solid NaOH with purity higher than 99% (CHEM-LAB NV, Belgium) and 152.4 g demineralised water were mixed under stirring for about 30 min. After the electrolyte’s homogenisation, 33.9 g of bauxite residue were slowly added to the solution within 5 min.

The slurry temperature was measured via a probe that was wrapped in a polytetrafluoroethylene (PTFE) shrink tubing to avoid current leakage. The slurry was stirred using a 1 cm ringed cylindrical magnetic bar and magnetic hot plate (IKA^® RCT basic, IKA Works Inc, USA) at a rotational speed of 500 rpm to keep bauxite residue particles suspended. The small size of the magnetic bar was chosen to minimise the attraction of magnetite particles to the stirrer. The cathode, the anodes and the reference electrode were connected via their respective terminal to a potentiostat (SourceMeter^® 2461 Series, Keithley, Tektronix Inc, USA).

Cyclic voltammetry and electrolysis tests, under chronopotentiometry mode, were performed in a three electrode cell connected to a potentiostat (2461 Series, Keithley) and the obtained experimental data were analysed. Regarding cyclic voltammetry, it should be noted that both working and counter electrode were platinum wires while the reference electrode was the above mentioned Hg/HgO commercial electrode.

Electrolysis tests were performed under galvanostatic mode. The duration of the experiment was 2 h. After the end of each experiment, the cathode was thoroughly rinsed with distilled water in order to remove the remaining electrolyte, and was dried at 100°C for 24 h, before being weighed. The difference between the mass of the cathode prior to and after the end of electrolysis was considered as the mass of the deposit. This value was used to deduce the current efficiency according to Faradaic law.

2.1 Determination of Metallic Iron

3.1 Cyclic Voltammetry

3.2 Effect of Current density

3.3 Effect of Temperature

The present work has demonstrated the possibility to electrochemically reduce iron from bauxite residue in alkaline pulps. The process temperature proved to be the most crucial parameter that substantially affects the Faradaic process efficiency. At the current level of process development, a Faradaic efficiency of 71.58% was achieved at pulp density of 10 wt% bauxite residue in a 50 wt% NaOH solution at 130°C. The applied current has to create a cathodic potential higher than –1.4 V vs. Hg/HgO (in 1 M NaOH) in order to avoid the hydrogen evolution which takes place at cathodic potentials lower than –1.4 V.

The cathode in the case of bauxite residue pulps electrolysis is not uniformly covered by electroactive iron oxide particles (mainly haematite) and therefore there is not a uniform cathodic potential on the whole cathode surface. This affects the current efficiency of the process which is always substantially lower than that observed in pure haematite ore electrolysis.

Bauxite residue is produced as a byproduct of the Bayer process, an alkaline leaching process taking place at temperatures 120–250°C. Therefore, the present process has great potential for integration in the established Bayer process as a symbiotic step to valorise the (currently wasted) iron portion of the bauxite ore.

The adsorption of air and water contaminants on an activated carbon surface is frequently used in air and water treatment systems. Conventional biological treatment processes are efficient in removing biodegradable organics. These processes, however, have limited ability to remove lignins, humic substances, pesticides and residual colour and odour-producing organic compounds. The performance of activated carbon in removing such organic compounds has been proven. Activated carbon filters used for water treatment in homes are usually made of either powdered activated carbon (PAC) or granular activated carbon (GAC). PAC is made of powdered carbon with a particle size of <0.18 mm and diameter in the range of 0.25–1.15 mm. It is generally used in batch type reactors and is subsequently filtered off. It is used in the treatment of liquids and cleaning of flue gases. GAC is produced from 0.2–5 mm particle size irregular carbon. For liquid and gas phase substances, it is applied on adsorption columns (1–3).

The increase in production and accompanying energy demand which has emerged in the last century with the increasing population is becoming important. The environmental impacts caused by the increase in production draw attention and higher consumption creates problems for our limited natural resources. The necessity of treatment for the prevention of soil, air and water pollution is among the topics that are especially emphasised at international conferences. It is necessary for humanity to take joint decisions to increase the sensitivity of countries on environmental issues and to implement the necessary legal measures (4).

Biochar is a byproduct with high carbon content produced through the conversion of biomass into syngas by advanced thermal gasification using partial oxidation. Intensified gasification of biomass through partial oxidation is a recently developed, environmentally clean and renewable energy technology (5). Biochar obtained as a byproduct from gasification of woody biomass mainly contains carbon (85 wt%), nitrogen, hydrogen, phosphorus, calcium, magnesium and iron. Production of activated carbon from biochar is possible. To convert biochar into activated carbon, it is necessary to remove the specific contaminants to complete the activation.

Biomass is a renewable energy source but can cause environmental issues when it is not properly utilised. Therefore, it needs to be dealt using appropriate technology as compared to other renewable energy sources. Since thermal utilisation of biomass is carbon neutral, it does not cause global warming, and due to the quantities available it has high energy potential. Thermochemical methods are generally used to obtain energy from biomass. Known thermochemical methods are combustion, pyrolysis and gasification. The gasification process has several advantages over other thermal processes: the ability to dissolve material at lower reactor volumes, the formation of low amounts of contaminants and more efficient utilisation of the produced syngas. Compared to pyrolysis, it has the advantage of working autothermally without the need for external energy. Compared to other processes, gasification of woody biomass is stated to be one of the most suitable options for optimising the conversion efficiency of the fuel’s chemical energy (6–7). The stoichiometric reaction of woody biomass with partial oxygen results in flammable product gases and thus gasification. The basic equation of the gasification of biomass is as in Equation (i):

(i)

Various types of reactors are available for gasification. To not digress from the subject, the kinetics of gasification reaction was not elaborated. Fixed-bed biomass gasification reactors are used for syngas generation and biochar production. Fixed bed gasification reactors efficiently convert biomass to syngas and are the type of reactor suitable for the production of biochar as a byproduct with high carbon content (85–90 wt% carbon) (8–9).

The biomass fed into the gasification system passes through four sections in the reactor and is converted into syngas and biochar as a byproduct. The biomass passes through the drying, pyrolysis, reduction and oxidation zones in the reactor and is converted into biochar at elevated temperatures (800–1100ºC). The amount of biochar produced depends on the biomass type and the operational conditions. 10–20% of the biomass fed to the reactor is produced as biochar (10–12).

Recently, researchers have begun to be interested in biochar produced as a byproduct from biomass by thermal conversion pathways (>700ºC), such as pyrolysis, thermal carbonisation, flash carbonisation and gasification (13). Many studies have suggested that biochar can be used in CO₂ adsorption, soil remediation and air pollution removal (14–16). Additionally, the use of the syngas produced by thermochemical conversion of biomass as a renewable energy source provides an advantage while the efficiency potential of biochar produced as a byproduct will make these systems very attractive (17).

Physical activation improves the surface pores of biochar and affects the chemical properties of the surface (such as surface functional groups, hydrophobicity and polarity). The physical activation methods used for biochar are generally steam and gas activation.

The steam activation of the biochar is usually carried out after the thermal carbonisation of the biomass. The surface porosity of biochar increases after pyrolysis. Also, with steam activation, the activated biochar will gain higher porosity. The chemical formulae are shown in Equations (ii) and (iii) (18):

(ii)

(iii)

With the H₂O (steam) and carbon reaction during the activation process, three outcomes emerge: (a) removal of the volatiles and tar from the surface; (b) formation of new micropores; and (c) expansion of existing pores (19–20).

Studies on steam activation have shown that it increases the biochar’s surface area and micropores. For example, the surface area of biochar (136–793 m² g⁻¹) obtained by rapid pyrolysis (800ºC, 45 min) was increased as a result of steam activation. All these measurements were performed with BET isotherm (21). In another study, biochar (700ºC) from tea residue biomass was activated by steam and its surface area was obtained as 576.1 m² g⁻¹, pore volume as 0.109 cm³ g⁻¹ and pore diameter as 1.998 nm (before activation, the surface area was 342.2 m² g⁻¹, pore volume 0.022 cm³ g⁻¹ and pore diameter 1.756 nm) (22). The steam activated biochar formed at 700ºC indicated the highest absorption capacity (37.7 mg g⁻¹) at pH 3, with a 55% growth in absorption ability compared to non-activated biochar produced under the same conditions. Consequently, activation with steam has potential to enhance the adsorption capability of biochar. At 700ºC, produced biochar derived from plant-based biomass detected relatively low surface areas (9.27 m² g⁻¹). Due to the formation of tar during thermal decomposition, the pores in the biochar are blocked (23). Steam activated biochar derived from bamboo waste had a larger surface area compared to non-activated bamboo biochar. Optimum conditions for activation were 850ºC and 120 min activation time. Under these conditions, the BET surface area of activated biochar was 1210 m² g⁻¹ and total pore volume was 0.542 cm³ g⁻¹. This study showed that bamboo waste could be used to arrange new micropores in activated biochar through steam activation (24). In further research, activation of rice husk biochar was carried out at 800ºC using steam. Micro- and mesoporous structured biochar were produced and 1365 m² g⁻¹ surface area was obtained at the end of 15 min (25).

Another paper presents a study into the effect of different activation conditions and adsorption characteristics of biochar evaluated from tyre rubber waste. Steam was used as an oxidising agent and total micropore volumes and BET surface areas increased to 0.554 cm³ g⁻¹ and 1070 m² g⁻¹, respectively. Consequently, steam was observed to generate a narrower extensive microporosity (26). An experimental test represents the production of activated biochar from barley straw using steam activation. Activation was conducted to maximise the micropore volumes and BET surface area of the biochar. Optimal conditions for steam activation were a hold time of 1 h at 700ºC. The micropore volume and surface area of the activated biochar were 0.2304 cm³ g⁻¹ and 552 m² g⁻¹, respectively (27). A further study investigated activated biochar produced from date stone wastes by steam activation. The effect of activation hold time on surface textural structure properties of raw date stone and biochar were studied. The results indicated the presence of cellulose and hemicellulose in the raw material, and the predominance of carbon content. The highest microporous volume was 0.716 cm³ g⁻¹ and specific surface area was 635 m² g⁻¹, obtained through biochar activated under steam at 700°C for 6 h (28). Another study was conducted to investigate the effect of sulfur in activated biochar prepared from apricot stones by steam activation. The activation temperature and time tested were in the ranges of 650–850ºC for 1–4 h. The experimental results revealed that the surface area of the biochar was 1092 m² g⁻¹ at activation conditions of 800°C for 4 h. The experimental results indicated that commercial production of porous activated biochar from apricot stones is reasonable (29). The significance of the nature and composition of biomass was demonstrated by the steam activation of three different biomass sources including wheat straw, coconut shell and willow (30). For these three biomass substances, activated carbons with specific surface areas of respectively 246 m² g⁻¹, 626 m² g⁻¹ and 840 m² g⁻¹ were produced.

Although physical activation using steam significantly increases surface area and porosity, there is a disadvantage of using steam activation. This is the loss of aromaticity and polarity in steam activated carbons compared to genuine biochar (31). However, using a combination of CO₂ and steam activation has been reported to produce activated carbons with better surface area and pore structure compared to using only CO₂ or steam for activation. In one study, activated carbon was produced from biochar obtained from olive kernel biomass using CO₂, steam and a combination of CO₂ and steam. A combined CO₂ and steam activation under similar experimental conditions was reported to produce a higher surface area (1187 m² g⁻¹) compared to the specific surface area obtained by using only CO₂ (572 m² g⁻¹) or steam (1074 m² g⁻¹) (32). From the aforementioned studies, it can be concluded that the reaction of steam with carbon occurs in a shorter period compared to the reaction of CO₂ with carbon. While generally micropore activated carbons are produced by using CO₂ in physical activation, meso- and macropores are formed in the structure during activation with steam (33). The reason for the difference in pores is attributed to the conversion of developed micropores into wide meso- or macropores and the faster reaction of the fixed carbon in the biomass structure of the steam. Also, it is possible to produce more pores using steam as it can penetrate the inner surface of the fixed carbon. On the other hand, CO₂ stagnates in its reaction with the fixed carbon and therefore more homogeneous micropores are added to the structure due to activation using CO₂. Determining the specified exposure time of the biomass at high temperatures with the activation agent is a critical decision to achieve high surface properties. The significance of biochar steam activation time was emphasised in some studies. It has been determined that there is a decrease in the surface pores of biochar exposed to long activation time (34).

Chemical and physical processes are needed to increase the utilisation value of biochar. Several processes are required such as separation of biochar into suitable granule size, obtaining carbon black, activation of pores by chemical processes and the sizing of activated carbon. The failure to use biochar produced as a byproduct of biomass gasification constitutes a disadvantage in all respects considering the spread of such technologies. Biochar produced as a byproduct by gasification of woody biomass is only used as a soil conditioner in small amounts. It is generally burned inefficiently in combustion boilers. To this end, this study investigates the conversion of biochar into activated carbon, which is a widely used valuable product. Several studies have emphasised that surface area and pore volume increase with the conversion of biomass at high temperatures by thermal methods (35). Thus, it is envisaged in the present study that the process of converting biochar obtained by gasification at high temperatures (800–1000ºC) into activated carbon by specific activation methods may have many advantages. The need for extra energy from external sources such as utilisation of fossil fuels to reach these temperatures in conventional carbon conversion systems causes high operational costs. Considering all this, it is expected that high quality activated carbons obtained from biochar will be available in the market at low costs.

In this experimental study, oak woodchip was gasified in an updraft gasifier. Byproduct biochar was activated in the activation unit integrated into the gasification system.

The proximate and ultimate analyses of the oak woodchips were conducted, and the results are presented in the Results and Discussion section. Proximate and elemental analyses were performed to determine the usability of biochar produced after the gasification of biomass for the production of activated biochar. Proximate analyses of moisture (ASTM D7582-12), ash (ASTM E1755-01(2020)), fixed carbon (ASTM D3172-13) and volatile matter (ASTM D7582-12) were made according to standard methods. The lower heating value was determined using ASTM D5865-13 standard method. The amounts of carbon, hydrogen, nitrogen and sulfur were determined in the element analyser and the amount of oxygen was determined using the standard ASTM D3176-09.

Using a thermal analyser, the thermogravimetric carbonisation analysis of oak woodchips was performed. Approximately 10 mg samples with an average particle size of 0.25 mm were heated at 10ºC min⁻¹ from room temperature to 800ºC under nitrogen flow. Throughout the measurements, the nitrogen flow was kept constant at 10 cm³ min⁻¹.

Activation of the biochar produced in the gasification system in the Gebze Technical University (GTU) Gasification Laboratory was performed. For the preparation of the biochar and activated biochar, oak woodchips were first carbonised by gasification at 800–1000ºC for 1–2 h. Then, the resultant biochar produced as a byproduct from the gasifier was activated by steam at three different temperatures (700ºC, 750ºC and 850ºC) utilising thermal heat generated from a syngas burner at different activation times (30 min, 60 min and 90 min). 5.5 kg byproduct biochar was used in each run. During the final process, the reactor was cooled under inert injection of nitrogen gas and then the activated carbon was removed from the reactor and weighed in order to determine the burn-off undergone in the reaction. For steam activation, a stainless-steel vertical reactor illustrated in Figure 1 was used and integrated to the system to heat each 5.5 kg biochar sample. Throughout the experiment, an exact heating rate and steam flow rate (~20ºC min⁻¹, 1.3 kg min⁻¹, respectively) were applied. Referring to Equation (ii), the optimal stoichiometric steam amount was determined and calculated for the system.

Fig. 1

The nitrogen adsorption or desorption isotherms were determined at 77 K by means of an automatic adsorption instrument to identify the textural properties of the produced biochar and activated biochar. Before the gas adsorption measurements, the samples were degassed at 300ºC under vacuum for 5 h. The N₂ adsorption isotherm was achieved over a relative pressure, P:P₀, ranging from roughly 10⁻⁶ to 1. The BET and t-plot methods were employed to determine the surface area, micropore surface area and pore volume of the biochar and activated biochar, respectively. Relative pressures in the 0.01–0.15 range were applied to evaluate the BET surface areas. The total pore volumes (V_t, cm³ g⁻¹) were considered to be the liquid volumes of N₂ at high relative pressure near unity (~0.99) (36–37).

SEM analysis was performed to investigate the surface, textural, porosity and structural properties of activated biochar produced under different conditions. SEM analyses were taken by enlarging ×500 to observe changes in surface morphological structure before and after activation.

To examine the crystal structure, XRD profiles of each sample were obtained at room temperature, using a copper Kα X-ray source, under 40 kV and 30 mA analysis conditions. Diffraction data were taken on a scale ranging from 2θ = 0–90º.

In order to qualitatively determine surface functional groups, FTIR spectra were obtained at room temperature, with the support of diamond orbital attenuated total reflection (ATR) accessory, by scanning 128 times in the range of 500–4000 cm⁻¹ band at 4 cm⁻¹ separation sensitivity. Samples were placed directly on the diamond crystal and were analysed by applying pressure to allow it to fully interact with the diamond crystal.

CHP production technologies using various wood residues and agricultural biomasses are commercially available at different sizes (1, 2). The state-of-the-art CHP technologies have been under severe financial stress in changing European markets characterised by a rapid addition of variable renewable energy (VRE) capacity and stagnating electricity demand (3). Consequently, many new thermal generators are currently designed to produce only hot water for heating purposes instead of CHP (4, 5). As a result, there is a clear need for new flexible district heating and CHP production solutions in Europe that can maintain economic feasibility under increasing VRE penetration.

On the other hand, advanced transportation biofuels have been the focus of intensive development since early 2000, but industrial deployment has been postponed (6–8). One fundamental reason for this is the attempt to reach satisfactory economics by exploiting economies of scale; which leads to extremely large-scale plant concepts (>300 MW) that are eventually deemed too risky by the investors. Large-scale plants also suffer from incomplete utilisation of byproduct heat, as it is difficult to find such large heat consumers who could effectively exploit the heat supply. Thus, the biomass utilisation efficiency of stand-alone plants rarely exceeds 55% lower heating value (LHV) even with the best available technologies (8).

In response to the growing share of solar and wind power in the energy systems and consequent need for converting surplus electricity into storable form, power-to-gas (P2G) and power-to-liquids (P2L) concepts have recently been suggested for managing the temporal mismatch between solar energy supply and heat and power demand (10). However, the simple P2G and P2L concepts producing, for example, synthetic natural gas (SNG) or methanol from excess electricity and CO₂ suffer from poor round-trip efficiency (typically <40%) when the final product after storage is once again converted to electricity (11). In addition, annual operation times for these plants, including fuel synthesis, are low especially in the northern European countries, typically only ca. 2000 h year⁻¹. As a solution to this problem hybrid systems have been suggested, where electrolysis technology is used to boost the biomass gasification and chemical synthesis plant (12, 13).

This paper deals with the experimental development activities related to one promising hybrid production concept, FlexCHX, illustrated in Figure 1 (14). This process produces heat, power and an intermediate energy carrier, FT wax, which can be refined to transportation fuels using existing oil refining equipment. In the summer, renewable fuels are produced from biomass and hydrogen; the hydrogen is produced from water via electrolysis that is driven by low-cost excess electricity from the grid. During the dark, winter season, the plant is operated with just biomass in order to maximise the production of much-needed heat, electricity and FT wax.

Fig. 1

In principal, the FlexCHX process can be realised at large scale using pressurised fluidised-bed gasification developed in Finland (14–16) or at smaller size range using the new staged fixed bed (SBX) gasifier developed in an ongoing European Union (EU) Horizon 2020 project (17, 18). In both gasification systems, the feedstock is gasified at moderate temperatures to generate a tar-containing raw gas, which is filtered at high temperature, and led to the catalytic reformer, where tars and hydrocarbon gases are reformed and the yields of hydrogen and carbon monoxide are increased. After final cleaning, the gas can be led into chemical synthesis units producing intermediate products, which can be refined to transportation fuels or chemicals using large-scale industrial units.

One of the most significant issues in syngas purification is the fate of light hydrocarbons (mainly methane, C2 hydrocarbons and benzene) and ‘tars’ produced during gasification. These hydrocarbons can constitute a significant proportion of the overall carbon content of the gas, and their conversion to syngas is therefore essential to improve syngas yield, as well as prevent poisoning of FT catalyst and plant fouling downstream of the gasifier. Technologies already exist for trapping tars at low temperatures such as water scrubbing and solvent wash, although these techniques will not address lighter hydrocarbons, and some tars with low dew points. A more elegant solution, utilised in the FlexCHX process, is the high temperature reforming of hydrocarbons to syngas immediately downstream from the gasifier.

In the FlexCHX project, previous knowhow of VTT, Finland, on catalytic reforming technology is combined with the catalyst knowhow of Johnson Matthey, UK. VTT’s technology with combinations of zirconia, noble metal and nickel catalysts has already been demonstrated in fluidised-bed gasification applications aiming to synfuel applications (15, 21). In FlexCHX project, this existing expertise is used to design an optimised reformer reactor for the raw gas of the pressurised SXB gasifier. The primary reduction of tar content already in the gasifier and on the filter cake together with new impurity tolerant catalysts developed by Johnson Matthey in the project will offer new, more efficient and robust design alternatives for the process. The platinum group metal (pgm) catalysts provide a potential for improved conversion efficiency to be achieved already at lower temperatures.

In addition to the reforming of tars and hydrocarbon gases, the catalytic reformer has another key role in the FlexCHX process in bringing the main gas components (carbon monoxide, water, CO₂ and hydrogen) towards equilibrium. The H₂:CO molar ratio should be close to two in the FT synthesis, while it is considerably lower in the raw feed or gas entering the reformer. During the heating season, the gasifier is operated with a mixture of steam and oxygen as gasification agents and the raw gas contains still typically 40% steam. This steam is then further consumed by reforming reactions and the main gas components approach the equilibrium of water gas shift reaction (Equation (i)):

(i)

The target molar ratio of H₂:CO can be achieved by controlling the steam feed of the reformer. During summer season, less steam is used while CO₂ is separated from the syngas and recycled back to the gasification process. In this case, the CO₂ is consumed by reforming reactions and the equilibrium of the water gas shift reaction is pushed towards high carbon monoxide contents. This makes room for the use of additional electrolysis hydrogen. This concept could not be realised without the catalytic reformer that on one hand consumes steam and CO₂ and on the other hand catalyses water gas shift reaction.

This paper is focused on the pilot scale development of catalytic reformer as part of the FlexCHX process. Results from four pilot test campaigns are presented and discussed.

2.1 Gasification Pilot Plant

2.2 Reformer Catalysts

2.3 Gasifier Feedstocks

Four test campaigns summarised in Table III were realised at the SXB pilot plant by the end of March 2020. The pilot plant was operated continuously without any interruptions in these test runs. Each test was started by preheating the plant at first in hot air and then by combusting wood in the gasifier reactor. This took 24–30 h, before the gasifier was switched from combustion to gasification mode. The flue gases from preheating periods were also led through the filter and reformer, which were gradually heated up as well. When the gasifier was turned from combustion to gasification, feeding of oxygen and nitrogen mixture to both beds was started and the catalyst beds were gradually heated to the target operation temperatures. Measurements were carried out in 3–20 h long periods, during which the mass flow rates of input streams were kept as constant as possible. Elemental mass balances and performance indicators of the reformer were calculated for the set point periods based on average measuring results.

One typical challenge of tar reforming has been soot formation (23), which over time decrease the catalyst activity and increase the pressure drop so that finally the reformer must be oxidised to remove the soot. In these pilot tests, the pressure drop of the reformer stayed constant at all set points reflecting just the changes in gas flow rate and operation temperature. As an example of the variation, Figure 6 shows the measured pressure drop in test run 20/11. Evidently, the applied pre-reforming taking place in the second stage of the gasifier together with the applied method of oxygen feeding into the reformer prevented soot formation reactions from taking place.

Fig. 6

The main measured results and calculated performance figures for selected four set points are presented in Table IV. Set points SXB 19/34B and 19/34E are tests where the reformer was loaded with nickel catalysts and set points SXB 20/11A and 20/11D are for the second reformer loading including the development catalysts B and C. Two of the set points (19/34B, 20/11D) represent operation with clean biomass with very low sulfur content and two set points (19/34E, 20/11A) are for bark gasification. The hydrogen sulfide contents of gas were typically ca. 20–30 ppm and 100 ppm respectively.

The tar concentrations measured after the filter unit were in the range 6.0–6.1 g m⁻³ with clean wood and 7.1–8.2 g m⁻³ for bark gasification. The benzene content varied in the range 9.7–13.1 g m⁻³ at these set points. Volumetric concentrations and volume flow rates in this article are presented in the conditions of standard temperature and pressure (STP) defined as 273.15 K and 101.325 kPa. Finally, methane and C2–C5 hydrocarbon concentrations in the reformer inlet were in the range 6.3–7.1 vol% and 1.2–1.5 vol% respectively. The C2–C5 hydrocarbons consisted mainly ethylene and ethane, which represented over 95% of these light hydrocarbon gases. These inlet concentrations of hydrocarbon gases and tars are of the same order of magnitude as determined for pressurised steam-oxygen blown fluidised-bed gasification of same feedstocks (15, 27). Consequently, similar hot gas filtration and catalytic reforming solutions can be applied for both gasifier types and the reforming results obtained in this study are applicable for fluidised-bed gasifiers as well. At these set points, the raw gas was cooled before filtration to 500–600°C in order to remove alkali-metal vapours during filtration as described in (28, 29). With wood residues, this gasification process could also be realised without cooling the gas before filtration, as is assumed in the process evaluation studies presented in (17).

In the first reformer stage, the gas temperature was raised by partial combustion reactions from 512–536°C to the maximum-targeted temperature, usually in the range 930–1000°C. The oxygen feed rate was controlled to ensure a reasonable temperature variation range at each target set point. Once endothermic steam and CO₂ reforming reactions occurred, the temperature decreased. The oxygen feed used in the first reformer stage was of the same order of magnitude in 2019 tests with nickel catalysts and in the 2020 test with the development catalysts.

The inlet temperature of the second reformer stage varied at these set points in a narrow range of 748–779°C. The second stage was also operated by controlling the oxygen feed so that the maximum temperature occurring close to the top of the catalyst bed was kept in the target range. As before, the temperature decreased as a result of reforming of remaining tars and part of methane occurred. The higher efficiency of the first reformer stage in 2020 tests can be seen indirectly from higher maximum temperatures and from the fact that clearly lower oxygen feed rates were needed in the second stage than were needed with the nickel beds. The higher efficiency of the second catalyst stage is clearly linked to the lower outlet methane contents as well as in 120–150°C lower gas outlet temperature.

The space velocities for both reformer stages are calculated for the whole catalyst bed volumes including both the nickel and precious metal sections in the 2020 tests. In the 2020 test, the share of nickel and precious metal catalyst in the first stage were 10% and 90% and in the second stage 40% and 60%. The gas hourly space velocity (GHSV) is given in Table IV both in actual average temperature and pressure and in standard STP (273.15 K, 101.325 kPa). The molar volume of ideal gas (22.41 dm³ mol⁻¹) is used in converting molar flows to volume flows. In the first stage, the gas volume flow is calculated as a sum of inlet raw gas flow and flow rate of added oxygen and nitrogen. The final gas flow after the reformer is used in calculating the GHSV for the second catalyst bed.

The achieved conversion efficiencies were calculated based on average measuring results and mass balances for each set point. The results are shown in Figures 7 and 8. Methane conversion with the nickel catalyst in 2019 tests was close to 50% with both clean wood and bark. In the 2020 tests with the development catalysts, clearly higher methane conversions were achieved: 75.6% with clean wood and 66.1% with bark. The conversion of C2–C5 hydrocarbon gases was complete in the 2020 tests, while with the nickel catalyst used in 2019 tests the reformed gas contained 0.05–0.1% of C2 hydrocarbon gases despite higher reformer outlet temperature. Tar conversions with the nickel catalyst were 99.3% with clean wood and 97.4% with bark. In the 2020 tests, tar conversions were complete and only some trace concentrations of 2.7 mg m⁻³ could be found from the reformed gas. No significant differences could be found in benzene conversions determined with the nickel catalyst and with the development pgm catalysts. This may be due to the higher operating temperature of the second reformer stage in the 2019 experiments. It can be expected that benzene conversions can be increased by increasing the operation temperature of the second reformer stage.

Fig. 7

Fig. 8

One of the challenges of gas cooling, compression and final cleaning is the deposition problems caused by heavy tars. The conversion of heavy tars was almost complete at all set points but again the results obtained with the development catalyst in 2020 were even better as can be seen in Figure 9, which shows the measured inlet and outlet concentrations of tar components which have higher molecular weight than naphthalene.

Fig. 9

The increase in sulfur content of the raw gas, when changing from clean wood to bark, had already clear effects on methane and benzene conversions with both tested catalyst set ups. According to our previous experiences with nickel catalysts (15, 30) this deactivation is reversible at least in the concentration range of 10–150 ppm typical to gasification of clean wood, bark and forest residues. All conversion efficiencies were further reduced significantly, when the feedstock was changed from bark to sunflower husk, which contained roughly five times more sulfur than bark.

In general, the developed catalytic reformer technology is able to convert tars and hydrocarbon gases into syngas and to bring the gas composition towards equilibrium of the water gas shift reaction. Thus, the reformer seems to fulfil both key roles it has in the hybrid biofuel production concept FlexCHX. The residual tar contents are almost negligible, which makes it possible to remove them by activated carbon simultaneously with bulk sulfur removal in the first guard bed. High methane conversion and low outlet temperature achieved with the development pgm catalyst is beneficial for the overall process efficiency and makes it possible to design concepts where the tail gases of FT synthesis are recycled back to the gasification process.

Stable reformer operation with no signs of soot formation could be achieved with both reformer loadings tested in the 2019 and 2020 test runs. Evidently, the overall method of tar control applied in the SXB gasification process has been successful. The very high primary tar content typical to updraft gasifiers is reduced in the secondary gasification zone to similar levels as achieved in fluidised-bed gasifiers. Dust particles are efficiently removed by metal filters, which makes it possible to apply fixed-bed reformer designs. The resulting raw gas could then be efficiently reformed in the two-stage fixed bed reformer.

When the reformer was operated in a way that the maximum temperature in both beds was kept stable by controlling the oxygen feed rate, the difference in the activity of reformer catalysts could be seen in the gas outlet temperatures. With the more active pgm catalysts used in 2020 tests, the outlet temperature from the reformer was 120–150°C lower than in the 2019 test runs. Higher tar conversions could be achieved by the development catalysts already with lower outlet temperature than were achieved with the commercial nickel catalysts.

Methane conversions with the nickel catalysts used in 2019 tests were of the order of 40%, while with the development catalysts used in 2020 methane conversions were in the range 71–87% with clean wood and 48–68% with bark. The major part of C2–C5 hydrocarbon gases were reformed in all test runs. The reformer performance with woody biomass was very good, while in the test runs with high-sulfur sunflower husk; the conversions were lower than targeted. The solution could be higher operation temperatures or lower space velocities for feedstocks having high sulfur contents.

Ammonia decomposition was rather modest in these test runs. Evidently, the operation temperatures were too low, and the catalyst selection was not efficient for ammonia decomposition. In future tests, the effect of increased temperature will be studied. Alternatively, the reformer can be designed to be realised with three beds, where the final bed would be dedicated for ammonia decomposition. Alternatively, ammonia can be scrubbed from the syngas by simple acid scrubbing but if the concentration is high, the acid consumption becomes large.

The next steps in the project are to utilise these experimental results to help make conceptual design for an industrial scale FlexCHX plant. The design will incorporate a full technoeconomic assessment for this hybrid production concept of biofuels and heat. Further pilot gasification tests will also be carried out in order to optimise the catalyst loadings and the performance of the catalysts under different conditions is studied with simulated gases in the laboratory.

Methane market opportunities will keep emerging; most energy forecasts currently predict that natural gas will play an important future role in the global energy sector. Projected long-term growth rates for gas are around 2% per year and analysts are expecting natural gas to overtake coal in the global energy arena in the next two decades (1, 2). Security of supply is improving to meet this demand with the USA becoming a large liquid natural gas (LNG) export player in recent years as the price of natural gas continues to drop while liquid petroleum gas (LPG) and carbon exploitation associated costs increase. All these scenarios could also allow for economically viable opportunities for stranded gas and biomethane utilisation.

Sustainable exploitation is a key driver of natural gas consumption’s future growth. The role of natural gas as a crucial stage strategy vector to reduce emissions could be supported by carbon pricing policies with the development of low carbon technologies where natural gas is implemented. Sustainable gas sources combined with renewable energy applications and process efficiency innovations are all required to tackle global emissions. Some of these first and second generation lower carbon or fuel switch technologies are expected to merge and support future methane applications and opportunities (3–5).

An extensive technoeconomic and viability review paper was presented in parallel by the partners in C123, the reader is directed there for further details (6). In the present paper, technical progress and remaining challenges are reviewed.

Disruptive technologies need to evolve to support new challenges including biomethane composition (higher amounts of CO₂ in the gas), alternatives to natural gas flaring, transformations to easy-to-transport chemicals, hydrogen production and carbon capture, utilisation and storage (CCUS). Current stranded natural gas reserves opportunities include compressed natural gas (CNG), gas to liquids (GTL), gas to solids (GTS) (also known as solidified natural gas (SNG)) and gas to wire (GTW). GTL increases the ease of transport through a physical change of the natural gas into a liquid (7). Two additional state-of-the-art utilisation options include gas to polymers (GTP) and gas to olefins (GTO). For both GTO and GTP, natural gas is converted into syngas, which is used to produce methanol as the feedstock source for the methanol to olefins (MTO) process. The olefins, i.e. ethylene and propylene, are then converted into polymers, polyethylene and polypropylene.

C123 “Methane oxidative conversion and hydroformylation to propylene” is a €6.4 million European Union (EU) H2020 project running from 2019 to 2023. The consortium consists of 11 partners from seven different countries (Norway, Belgium, France, UK, Germany, Azerbaijan and The Netherlands) with six industrial partners, two research and technology organisations, two universities and one association, all of whom have extensive previous experience in national and international research and innovation projects. C123 will evaluate how to best valorise unexploited methane resources by an efficient and selective transformation into easy-to-transport liquids such as propanol and propanal. In C123 the selective transformation of methane to C3 products will be realised via a combination of OCoM and hydroformylation. The C123 process aims to validate the implementation in two energetically and economically relevant, complementary and sustainable routes depending on the natural gas source exploited:

C123 will explore and evaluate the viability of this technology for biogas, associated gas and marginal gas fields, all with different challenges. Biogas is produced from organic matter such as sewage sludge, cow manure, agricultural waste and the organic fraction of municipal solid waste. Large concentrations of CO₂, about 36%, and impurities such as hydrogen sulfide (100–10,000 parts per million (ppm)) are usually found in biogas. Associated petroleum gas (APG), is a form of natural gas found with deposits of petroleum, either dissolved in the oil or above the reservoir. Associated gas is often wasted by flaring, under-utilised in low value applications such as onsite electricity generation or reinjection for enhanced oil recovery, or sold. Marginal fields are abandoned or non-developed fields that can have limited economic viability, unfavourable crude oil characteristics or high gas and low oil reserves. Marginal gas reserves account for approximately 15% of the world’s proven gas reserves.

All C123 technologies exist at TRL3, and the objectives of C123 are their further development to TRL5 by an optimised integration of catalyst and process. The C3 commodity chemicals propanal and propanol can be transformed further into propylene and fed into the US$6 billion polypropylene market or transformed into other valuable chemical products. The breakthrough innovation is the replacement of propylene production via the very energy intensive steam cracking process with production by less energy demanding and more selective build-up from smaller molecules.

The C123 transformation of methane into C3 chemical building blocks will be developed through in the technical work packages (WPs) WP2, WP3 and WP4 described in Figure 1. The goal will be optimisation of the overall carbon and energy efficiencies and cost effectiveness of the integrated OCoM and hydroformylation processes through the application of a set of collaborative technologies involving both catalyst development and formulation and reactor design.

Fig. 1

The expected major advancements in WP2 OCoM with respect to the state-of-the-art are the achievement of a higher methane conversion per pass, simpler heat management, higher energy efficiency, better carbon utilisation and an optimum product stoichiometry for an efficient hydroformylation process. The latter will be realised by tuning the output of the OCoM process to provide the optimal C₂H₄:CO ratio. Some hydrogen, required for hydroformylation and also processes downstream of the hydroformylation step, will be formed during OCoM, but optimising the amount of hydrogen is not part of the overall targets of OCoM. Any extra hydrogen for the overall process will be supplied externally. The hydroformylation step will need to tolerate CO₂ in the reactor feed, as that is provided by both the feed gas and the byproduct recycle. C123 will thus improve the atom economy and will circumvent the bottlenecks of the state-of-the-art oxidative coupling of methane (OCM) process which optimises only the ethylene production.

The goal of WP3 is production of the liquid C3 intermediates propanol and propanal through the development of a heterogeneous hydroformylation catalyst and process optimised for conversion of the OCoM effluent. This is advantageous for transportation and can be used for on-demand production of propylene by dehydration of propanol, or for further conversion of propanal to valuable downstream chemicals propionic acid. While industrial hydroformylation is a homogeneous process, a heterogeneous catalyst for the C123 hydroformylation is envisaged, allowing simplified separation and global process integration with OCoM.

WP4 will comprehensively develop and optimise process concepts from the simultaneous tuning of the catalyst properties, the operating conditions and the reactor configuration. It includes an innovative integrated reactor design that optimises heat management and transfer, mass transfer and recycling, thus improving energy and carbon efficiency, as well as achieving high product yields. This requires an optimal combination of the two processes, including possible equalisation of the operating pressure (around 10 bar) of both OCoM and hydroformylation in order to minimise the costs of pressurisation and improve reactor integration efficiency. Further, the additional reaction steps, the hydrogenation of propanal and dehydration to propylene, as well as appropriate purification and separation steps need to be efficiently included. The C123 process innovation is expected to result in an increase in carbon efficiency of at least 25%. Overall, at least 30% of fossil fuel consumption can be saved, and this can potentially increase up to 100% when any external hydrogen required by the overall process is electrolytically generated with electricity from renewable energy sources.

For about four decades, i.e., since the pioneering work of Keller and Bhasin, (8) OCM into ethylene has been a goal of the petrochemical industry. It has made scientists and industrials dream of converting a low-value feedstock fuel, such as natural gas or methane into ethylene, i.e., the base chemical with the highest global production volume. The promising perspectives offered by this reaction came with severe challenges. Indeed, methane oxidative coupling products such as ethane and ethylene are more easily activated than the reactant methane by the typically employed metal oxide catalysts, such as Li/MgO, Sr/La₂O₃ and NaWMn/SiO₂ (9). Basicity was recognised as an interesting catalyst property to reduce the interaction between ethylene and the catalyst. Nevertheless, C2+ yields seldomly exceeded 20% not to mention 30% which, at times, was considered as a minimum threshold value for commercial viability, see Figure 2. The oxidative character of the reaction comes with a pronounced exothermicity, rendering temperature control difficult and triggering parasitic phenomena at the high reaction temperatures, i.e., 800°C or higher, such as wall effects in the case of improper reactor material selection.

Fig. 2

Almost the entire periodic table has been probed for finding the best suited elements to be included in OCM catalysts (11). Li/MgO and its tin promoted version belonged to the first generation of investigated catalysts and provided good perspectives, despite shortcomings with respect to stability. La₂O₃ and SrO₂ were other catalysts that resulted in high activities at the expense of the C2 selectivity. More recently, NaWMn on SiO₂ has been identified as a more moderately active but more selective catalyst. These catalysts have been studied in a series of European integrated projects, including, among others, ‘Towards Optimised Chemical Processes and New Materials Discovery by Combinatorial Science’ (TOPCOMBI) and ‘Oxidative Coupling of Methane followed by Oligomerization to Liquids’ (OCMOL), and within C123 they constitute the benchmark materials (12, 13). In addition to a better understanding of these catalysts in these projects, the perception arose that no adequate combination of catalyst and operating conditions was available to allow an economically viable, single-pass conversion of methane into ethylene via oxidative coupling with a sufficiently high selectivity. It became clear that, rather, an entire process concept would be required to meet this purpose, an aspect which was also recognised by Siluria Technologies, USA, who were the first to implement OCM technology at the pilot scale.

The necessity of a proper process concept was already recognised. The OCMOL project proposed the integration of OCM with (dry) methane reforming, mainly to recuperate the heat provided by OCM, see Figure 3.

Fig. 3

The resulting syngas was subsequently valorised by methanol synthesis and MTO conversion. Whereas each of the individual process steps could be designed in a competitive manner, the needs imposed on the separation proved to go significantly beyond the state of the art. Moreover, only about 10% of the carbon in the end products was the result of oxidative coupling, while 90% was incorporated by the conventional syngas route. Siluria Technologies took advantage of the presence of non-negligible amounts of ethane in shale gas to accommodate a post-bed ethane cracking zone in their process concept, which of course imposes constraints on the feedstocks that should be processed.

Considering the lessons learned from the work on OCM, the following C123 hypotheses and constraints were put forward:

As an answer to the above, the concept of the OCoM was conceived, still critically relying on OCM, yet potentially embodying a variety of alternative methane conversion routes not just to maximise the C2 yield, but to produce an effluent suitable for hydroformylation. Apart from an OCM reactor, the OCM process will also take advantage of methane conversion via reforming and partial oxidation and by incorporating a water-gas shift reactor. The heat produced by OCM can still drive the reforming reaction as was the case in the OCMOL project; however, the goal in C123 is to mix the effluents into an adequate proportion for hydroformylation rather than perform difficult separations. Achieving full oxygen conversion in the OCM reactor will be key to achieve this goal. The CO₂ produced can be recycled to the OCM reactor for CO₂ induced OCM or to the reforming reactor for dry reforming. Ethane formed can be dehydrogenated oxidatively or by interaction with CO₂. Subsequent hydroformylation to propanal and propanol efficiently combines products formed in relatively high amounts, i.e., carbon monoxide and ethylene, which would, otherwise, require difficult separation steps.

In order to establish a proper C123 process implementation, research advances are required along various directions. A suitable combination of catalytic material and operating conditions is required. However, this time the goal is not to maximise the per pass ethylene yield but to produce the most promising product spectrum: hydrogen, carbon monoxide and ethylene for subsequent hydroformylation. As a result, an innovative process concept is required for the efficient conversion of methane into a hydroformylation feedstock (14). The common denominator, serving both challenges, is the fundamental modelling of the reaction and transport phenomena involved, both at the catalyst pellet and the reactor scale, see Figure 4. Such a fundamental model is the mathematical translation of the experimental insight into the investigated system. The goal is not to prove the model, but rather to indicate when the model (hypothesis) is not adequate and, hence, it is an extremely useful tool to assess the potential validity of model assumptions.

Fig. 4

Within C123, the activities on OCoM are, hence, focused along three lines: (a) further catalyst development and operating conditions screening; (b) microkinetic modelling of the OCoM reactions; and (c) process concept development. As evident from the above, the further catalyst development and operating conditions screening mainly serves the need of providing the relevant information for evaluating various alternative process concepts and, of course, as a basis for the training of the OCoM microkinetic model. Three benchmark catalysts, i.e. two Sr/La₂O₃ catalysts and one NaWMn/SiO₂ catalyst, have been shared among the project partners by Johnson Matthey, UK. A crucial aspect for a proper performance evaluation is the pretreatment of the catalyst samples. Bosch et al. (13, 15) specifically focused on the crystal phases obtained in nanoparticle catalysts and identified some interesting differences as a function of the calcination atmosphere, i.e. whether or not it contained oxygen, see Figure 5.

Fig. 5

The microkinetic model, see Figure 4, accounts for intraparticle gradients for reactants, products, radicals and surface species. Particularly for the most reactive radicals and surface species, significant gradients were found to develop, even if reactant and product concentrations had a negligible gradient. According to the model, methane is mainly activated by oxidised sites on the catalyst surface. Practically no methane activation occurs in the interstitial phase, i.e. outside of the catalyst particles. The coupling steps, on the other hand, do proceed homogeneously, both in the catalyst pores (intraparticle phase) and between the catalyst pellets (interstitial phase). Highly porous catalyst materials appear to hold a lot of promise, on the conditions that sufficient surface sites remain for the methane activation. More particularly within C123, the impact of CO₂ in the feed on the catalyst performance is assessed. Limited experimental information is available and, at present, both positive and negative impacts are reported. Hence, additional experimentation at intrinsic kinetics conditions will be performed to elucidate the true behaviour. In the meantime, preliminary simulations have already been performed to probe the capability of the available microkinetic model to account for the effects induced by CO₂. Indeed, CO₂ formation is included already in this model, see Table I for the considered elementary steps. However, now that CO₂ is assuming the role of the oxidant, there may be a need for tailoring the rate coefficients of the already included steps involved in the production and consumption of CO₂ and incorporation of additional reaction steps. For the time being, a moderating effect on the methane conversion has mainly been observed after including CO₂ in the considered feedstock.

Awaiting more detailed results from catalyst development, operating conditions screening and microkinetic modelling, the process concept development has already been started by making a stoichiometric analysis. The minimum amount of methane for producing a maximum amount of hydroformylation feedstock is determined from stoichiometric considerations and idealistic conversion scenarios. Such a scenario will serve as a benchmark for comparing actual implementations in a later stage of the project, initially based on literature reported kinetics, later based on microkinetics developed as part of C123.

Hydroformylation is the catalytic synthesis of an aldehyde from an alkene and a synthesis gas mixture. Aldehydes are convenient building blocks for a large range of organic compounds, including alcohols, carboxylic acids and amines, making hydroformylation a commercially attractive synthesis process (16). The reaction mechanism proceeds through a series of fundamental organometallic reactions, including ligand exchange, alkene insertion, oxidative addition and reductive elimination (17). Rhodium complexes are the most active catalysts, and although more expensive, they have generally replaced less active and selective cobalt catalysts. Both linear and branched aldehydes are produced from all C3+ alkenes, and the linear:branched ratio can be controlled via the reaction conditions, particularly the phosphine ligands bound to rhodium. Linear aldehydes are generally the preferred products.

The reaction parameters have been well established for nearly all alkenes, in particular propylene, since the hydroformylation product from propylene, n-butyraldehyde, is so industrially important. However, the literature is rather scarce on the conditions for the hydroformylation of ethylene, a simpler molecule with no stereoselectivity issues (18–21).

As discussed above, coupling a robust heterogeneous ethylene hydroformylation process with OCoM could disrupt the current technology. Benefits include a more circular economic process, responsible use of stranded, flared or biogas and improved transport in the value chain. The OCoM step is a high temperature (650–900°C), atmospheric pressure reaction. When tuned properly, this OCoM process will provide an optimised feedstock of ethylene and carbon monoxide for the hydroformylation process, which operates around 100°C and 20–40 bar pressure in the current industrial, homogeneous processes. A suitable heterogeneous catalyst will keep both processes in the gas phase, reduce precious metal losses during operation and address corrosion issues associated with solvent use. In addition to a tuned feedstock for hydroformylation, another operational goal is the reduction of the pressure difference between the two parts of the process (i.e. OCoM and hydroformylation). A detailed understanding of the reaction variables for homogeneous ethylene hydroformylation will guide catalyst and process development of a heterogeneous version of the reaction and the overall C123 process scheme.

The integrated process will attempt to avoid interstage purifications and pressure switches between each step. That means that the OCoM will have to operate under pressure, but also that the hydroformylation might have to operate at lower pressure than usual, and in a stream that contains CO₂, water and other impurities from previous stage such as residual methane and ethane. The hydroformylation C123 WP goals are development of stable heterogeneous hydroformylation catalysts, development of an integrated engineering concept for hydroformylation and demonstration of the process in an industrial environment at TRL5.

A screening study, involving both batch scale and high-throughput experiments, was carried out to determine the optimum catalyst and reaction parameters for the homogeneous ethylene hydroformylation. 11 different rhodium catalysts and 13 different phosphines with varying electronic and steric profiles (see Table II) were screened under two different CO:C₂H₄:H₂ feed gas compositions, several different pressures, different feed gas:catalyst ratios, a range of excess phosphine molar ratios and with argon or CO₂ as diluent gas. In all cases, only propanal was detected as product. No propanol was detected, even at higher pressures and with an excess of hydrogen in the feed gas.

As shown in Figure 6, the best catalysts are the known hydroformylation catalysts Rh(CO)H(PPh₃)₃, 1, and Rh(CO)(acac)PPh₃. A 10–20 fold excess of phosphine proved optimal, at least when PPh₃ was used. The high throughput screening studies showed that a 10-fold excess of the π-accepting phosphite ligands P(Otol)₃, P(O-tBu₂Ph)₃ and P(2-fur)₃ gave activities on par with the benchmark ligand PPh₃, but a definitive activity ranking of phosphines was difficult because of the high overall catalyst activity.

Fig. 6

To verify the high throughput results and investigate both the effect of CO₂ and the catalyst loading on activity, a series of batch studies were performed, and the results are given in Table III. As can be seen from the first six entries in Table III, there is very little difference in the turnover number (TON) for propanal formation, regardless of ligand, feed gas pressure or diluent gas. At least for the modest pressures and low C₂H₄:Rh ratios, CO₂ does not have an adverse effect on propanal TON. The enhanced effect of the π-acid ligands is most pronounced with the highest C₂H₄:Rh ratios. While P(Otol)₃ gives slightly higher TON than PPh₃ with the highest ratios (compare entries 8 and 9 with entries 11 and 12), the effect of the ligand P(2‐fur)₃ is dramatic, with this ligand giving TONs 2.5 times greater than those with PPh₃ (compare entries 8 and 9 with entries 17 and 18).

The high selectivity of the hydroformylation reaction to propanal is an important factor for the process design and impact of C123. Propanol is the preferred product over propanal since it is easier to transport and requires only a dehydration step to the valuable C3 product propylene. The economic and sustainability impact of an extra hydrogenation process step for the conversion of propanal to propylene will need to be evaluated. On the other hand, the selectivity for propanal even with excess hydrogen will lessen the need for hydrogen from the OCoM step, which may favour the OCoM process development.

Importantly, the production of only propanal in the homogeneous reaction does not guarantee that the gas phase, heterogeneous hydroformylation reaction will show the same selectivity. These results will regardless be very valuable in the design of the heterogeneous catalyst that will be developed in the next phase of the project, in addition to supporting the modelling work at Ghent University (UGent), Belgium, and in building a set of experimental data for the hydroformylation process optimisation and the WP4 process integration toolbox.

As indicated above, a gas phase hydroformylation reaction is preferred for the C123 process. While there are some examples of heterogeneous hydroformylation catalysts (19–22) the development of such a catalyst with the same activity and selectivity as the widely-used homogeneous ones remains a challenge. The C123 approach for development of a heterogeneous hydroformylation catalyst will therefore involve a more traditional funnelling strategy. First generation catalysts will be synthesised by tethering appropriate organometallic rhodium complexes in porous supports. Testing and iterative synthesis will provide a set of innovative, heterogeneous hydroformylation catalysts that will achieve the key performance indicators (KPIs) for TRL4. Second generation hydroformylation catalysts will be developed from the first generation catalysts by using relatively well-established shaping residence time distribution (RTD) protocols to select one or two catalysts that meet the KPIs for TRL5. For the hydroformylation catalysts, a part of the risk management strategy will be the use of homogeneous catalysts as backup, while still pursuing project work.

Johnson Matthey will functionalise high surface area silica surfaces with phenyl phosphine groups, and ultimately with rhodium complex catalysts, to form heterogeneous hydroformylation catalysts. Suitable organosilanes can interact with silica displacing surface silanol groups, creating a covalent bond. Two approaches have been selected for this functionalisation using ethoxy and methoxy silane organic precursors to anchor organic groups over the original silanol. Fumed silica, silicagel and templated mesoporous MCM-41 will be used to compare the effect of pore and surface area in the silica functionalisation.

The first approach will use an amine as an anchor group to then react the basic ligand with a rhodium salt, as shown in Figure 7. Aminopropyl trimethoxy silane grafted silicas have been widely reported in the literature as they can selectively remove acid molecules such as CO₂ and hydrogen sulfide (23). Direct impregnation techniques can attach propyl amines to silica, but there is evidence of more ordered surface coverages and optimisation through a toluene excess silane reflux approach over dehydrated silica. Surface secondary amines can then react with phenyl phosphine groups, attaching rhodium organometallic complexes such as Wilkinson’s catalyst or 1 to the silica surface (24–26).

Fig. 7

The second, more ambitious approach involves incorporation of a larger monophosphine organosilane precursor, 2-(diphenylphosphino)ethyl-triethoxysilane on silica, as shown in Figure 8. The surface anchored phosphine can then be coupled to rhodium salts such as [(COD)Rh(μ-Cl)]₂ (COD = 1,5-cyclooctadiene), [(NBD)Rh(μ-Cl)]₂ (NBD = norbornadiene) or [(COT)₂Rh(μ-Cl)]₂ (COT = cyclooctene) that can selectively incorporate other phosphines by sequential reaction with, for example, bis(diphenylphosphino)ethane dppe.

Fig. 8

The goal of the SINTEF, Norway, approach is to synthesise a metal organic framework (MOF) based material that has a large number of phosphines decorating the pores of the MOF. The idea is that, after introduction of rhodium to the material, a traditional organometallic reaction mechanism can be accessed, in that the rhodium has access to an abundance of phosphine moieties to both steer the fundamental reaction steps and prevent instability and leaching. This effect of this concept was illustrated by the incorporation of 1 into a (PTA)-MIL-1010(Cr) MOF (PTA = phosphotungstic acid). The PTA immobilised the rhodium complex within the MOF pores, yet provided homogenous catalyst-like selectivities in the hydroformylation of 1-octene in toluene (27).

Rather than incorporating the necessary phosphine moieties during the demanding MOF synthesis, we will investigate a post-synthetic modification approach called solvent-assisted ligand incorporation (SALI) (28). In particular, it has been shown that a range of ligands with pendant carboxylic acid and phosphoric acid groups can react with the μ₃-OH functionalities of MOFs built up with Zr₆(μ₃-O)₄(μ₃-OH)₄(H₂O)₄(OH)₄ nodes. There are a wide range of MOFs with varying pore sizes and shapes that are built up from this inorganic building block, such as NU-1000, the UiO series and MOF-808 (29). Our hypothesis is that full incorporation of phosphine ligands with an appropriate tether within a zirconium MOF, followed by addition of an appropriate fraction of a rhodium complex such as 1, will provide a heterogeneous version of a homogeneous hydroformylation catalyst (see Figure 9).

Fig. 9

Our initial attempt to make a suitably tethered PPh₃ variant, specifically (PPh₃)₂P(p-C₆H₄CH=CHCOOH) (see Equation (i)), provided instead the phosphine oxide 2. Reaction of 1 with the MOF NU-1000 provided evidence for incorporation of the tethered ligand into the MOF. Appropriately tethered ligands based on phosphites should be less prone to oxidation and have been synthesised.

(i)

The concept of porous macroligands, i.e. a porous solid acting as the organic ligand of a molecular complex, has been introduced and used recently by Canivet et al., at the National Centre for Scientific Research (CNRS) in Lyon, France, for the heterogenisation of active molecular catalysts to combine the advantages of high activity and versatility of molecular catalysts and sustainability of easy to separate and easy to recycle heterogeneous catalysts (30). Following this strategy Canivet’s team at CNRS will develop novel porous organic polymers which will embed efficient organometallic hydroformylation catalysts (31–33). Porous organic polymers formed from functionalised PPh₃ and biphephos have already been used as supports for the rhodium-catalysed hydroformylation of 1-octene in toluene and ethylene, propylene and 1-butylene in fixed bed reactor. Here, easily accessible phosphine or bipyridine based vinyl monomers will be used in controlled radical polymerisation leading to solid porous organic matrix with high surface area and pore accessibility (Figure 10). Further functionalisation with cobalt or rhodium precursors will give access to heterogenised and site-isolated hydroformylation catalyst within stable microporous structures.

Fig. 10

The high versatility of porous organic polymers will be used to advantageously tune the hydrophilic/hydrophobic balance of the hosting pore as catalytic nanoreactor, and the confinement within the micropore will play a crucial role by influencing transport, reaction rate and product selectivity. Moreover, the intrinsic swelling behaviour of porous organic polymers will ensure that the solubilised gaseous species will reach the active site while larger aldehydes or alcohols produced will freely transfer to the reaction medium.

Two microkinetic models will be developed for the hydroformylation of ethylene, i.e. one dedicated to the homogeneous and one to the heterogeneous process. The kinetic parameters will be determined via regression of the models to a comprehensive set of intrinsic kinetic data, i.e. data acquired in the absence of mass and heat transfer limitations. By performing such a detailed model construction, i.e. no rate determining steps are assumed, for homogeneous hydroformylation the critical steps in the selective conversion of ethylene to propanal can be unraveled. A distinction will be made between catalyst descriptors (for example, adsorption parameters) and kinetic descriptors (such as activation energy). Preliminary simulations using the model developed for homogeneous hydroformylation have shown that the increase of the propanal yield with the total pressure is nicely captured by the model.

Following the same principles a microkinetic model will be developed for heterogeneously catalysed hydroformylation, and similar relationships in terms of catalyst and kinetic descriptors will be established. This will allow the unravelling of the determining factors to optimise the heterogeneous catalyst activity and selectivity, and the models will be further used in the reactor design.

WP2 and WP3 will work in close collaboration with continuous exchange between both WPs as well as with WP4 in order to ensure overall integration. WP3 focus will be on the optimisation of the reaction and process conditions for maximised product yield and ethylene conversion, as well as achieving more active and more stable catalysts that can be operated at higher temperatures without deactivation and at lower pressures (close to those for OCoM) without decreasing conversion. This will be achieved through the combined development of heterogeneous catalysts optimised for operation in fixed reactor beds and maximum selectivity toward propanol with competitive performance with homogeneous catalysts and reactor design.

The results from OCoM (WP2) and hydroformylation (WP3) catalyst development and reactor design will be transferred into WP4 for the integrated process design and validation in relevant environment (TRL5) for both the modular and the add-on routes, to allow a conceptual design of the fully integrated units with all process steps, and the development of a possible scheme for an integrated commercial process.

The process design of the C123 technology is characterised by the combination of several reactions which are performed under different conditions and the integration of required purification and separation steps. In addition to the OCoM and hydroformylation reactions described before as novel core elements, a hydrogenation and a dehydration reaction also have to be included in the process. Furthermore, purifications and separations need to be implemented. Although these are in principle based on state-of-the-art technologies, at this point improved or innovative concepts and solutions have to be taken into account. Only by consideration of the interaction and optimal integration of all steps and process operations can an efficient and sustainable approach matching the efficiency and sustainability targets of the C123 project be achieved. Accordingly, this chapter is focusing on the base considerations and key aspects of process design for an implementation on technical scale and under industrially relevant conditions.

Methane shows a lower reactivity towards oxygen than higher hydrocarbons and olefins are more reactive than paraffins. Therefore, it is no surprise that OCoM reactions suffer from low selectivity at high conversion. Only at low conversion are highly selective reactions, for example to ethane and ethylene, feasible, although the difficult separations and high recycle ratios have an overwhelming impact on process economics.

In conventional approaches of OCoM (9, 10), carbon monoxide is considered an undesired byproduct, requiring additional effort in process design for separation, further conversion and recycling. Siluria Technologies, for example, based its technology on methanation and recycling of carbon oxides in order to increase the overall process efficiency (34). In contrast, C123 is taking advantage from the carbon monoxide production; by allowing the reaction to produce equimolar amounts of ethylene and carbon monoxide, propanal can be further produced by hydroformylation. Subsequent hydrogenation to propanol and dehydration yields propylene as final product. Propylene is very important chemical key intermediate, and propanal and propanol could be potential value products.

The challenge of a process design is the number of subsequent reaction steps which have completely different demands on conditions, which makes the process complex and requires compromises and optimisation.

The OCoM reactor is under strong kinetic control (8, 35). Thermal runaways leading to total oxidation are always possible and need to be avoided by a careful reactor design. A low pressure helps to push back the typically unselective gas phase reactions. Nevertheless, the reaction heat release is tremendous and requires a very effective cooling. Also, temperatures and heat recovery are challenging. While it looks attractive to use endothermic ethane dehydrogenation to recover high temperature heat after the OCoM reactor, providing the respective ethane stream requires cryogenic distillation. The process limitations affect the reactor design and vice versa.

Reaction thermodynamics already indicate the required reaction conditions for the subsequent steps. In addition, certain vapour-liquid-equilibria (VLE) separations require a high pressure. As the OCoM pressure is low, the pressure staging of the overall process is accordingly also subject to an optimisation.

The hydroformylation reaction (14, 36) has an equilibrium limitation at high temperature and low pressure, (Figure 11) and is therefore usually performed at 15 bar or more, i.e. significantly above the OCoM pressure. While in conventional hydroformylation the reaction partners are the main components in the gas phase, for the C123 process the overall equilibrium conversion is further reduced by inert gases, as the partial pressure of the educts is decreased by inert dilution (including especially methane, ethane, CO₂). Hence the process design has an impact on the maximum achievable hydroformylation conversion. The hydroformylation as targeted in the C123 project is designed as a heterogeneous system. In contrast to a homogeneous system, the catalyst is immobile and remains in the reactor avoiding dedicated efforts for catalyst separation and recycling.

Fig. 11

The propanal hydrogenation is again an equilibrium limited reaction performed at elevated but still limited temperature. In order to maximise the conversion, sufficient pressure is required (Figure 12). To provide the overall hydrogen demand for both steps (hydroformylation and hydrogenation) an external hydrogen source needs to be considered which is preferably based on renewable resources or energy.

Fig. 12

The dehydration is preferably performed at lower pressure and elevated temperature (Figure 13). Next to a beneficial increase of reaction rates and equilibrium conversion, typical catalysts for this reaction require a minimum temperature of >250°C. The endothermic dehydration from propanol to propylene can be performed in a heated reactor (for example, tube bundle fixed bed reactor) or in an adiabatic multi-stage reactor with intermediate cooling.

Fig. 13

Although the intention of the C123 project is to minimise the intermediate separation and purification effort, a minimum set of such process operations will be required (such as separation, drying). Due to different temperatures of the specific reactions, heat exchangers need especially to be implemented accompanied by the respective pressure drop.

The light unconverted gases hydrogen, carbon monoxide and ethylene, but also byproducts such as ethane, need to be separated from the hydroformylation product. Cryogenic separations themselves already require significant technical effort and equipment and are therefore expensive. If cryogenic separation cannot be avoided the effective removal of water and CO₂ using, for example, adsorptive removal and drying over molecular sieves are mandatory and further increase the technical and financial effort. Accordingly low temperatures are preferably avoided to maintain the process efficiency. Further, to minimise the cryogenic effort a minimum pressure is also required for these steps. The column design requires the detailed knowledge of the column feed stream and therefore of the performance of all reactors. Again, the separation efficiency also defines the reactor feed streams. The C2/C3 separation gives a C3 product stream with low gas impurities. The overhead fraction contains different light molecules including some uncondensed C3. At this point of the process an extractive distillation can for example be used to separate a gas recycle from the C3 product, avoiding a cryogenic process step. A further separation of this gas recycle stream is not possible without a cryogenic process. Again, the OCoM selectivities highly impact the process design: if this stream cannot be sent to combustion or if some species shall be recovered due to other reasons, this stream could be either recycled to an existing petrochemical plant or the cryogenic separation needs to be implemented as its own process unit.

Due to numerous feedbacks and interferences between reactor performance, separation efficiency and the overall process due to operating conditions, heat integration and the influence of trace components, none of these designs can be done separately. Specialised solutions need to be identified and combined in an optimal manner to ensure high efficiency and sustainability of the overall process. Accordingly, this task requires special expertise not only in the basic principles of all individual process units – including especially reaction and separation steps but also in the technical implementation on industrial scale. Only a few companies specialised in engineering, procurement and construction (EPC) of world scale petrochemical plants such as Linde Engineering, Germany, have the appropriate knowledge and experience to develop and provide a reliable and sustainable technical solution.

The C123 project aims to develop new technology for the upgrading of stranded natural gas or biogas to the easy-to-transport C3 commodities propanal and propanol, which can thereafter be transformed to propylene for the growing polypropylene market. The technology development aims to improve upon OCM technology by encouraging the production of carbon monoxide in addition to ethylene in the OCoM process and to develop a heterogeneous hydroformylation catalyst that provides comparable activity and selectivity to the well-known homogenous process.

There are many challenges ahead for this idea as the OCoM and hydroformylation processes are very different in nature and need to be united in as efficient an overall process as possible. Specific issues that impact the direction of OCoM and heterogeneous hydroformylation catalyst development and the overall process include the following:

Fortunately, the C123 project has an experienced consortium team that is tackling these issues in a unified catalyst and process development strategy, to bring this potentially disruptive technology to TRL5. C123 will also evaluate the process market viability and the end user requirements for the different final product opportunities that can be derived from C3 commodities. The methane sources and the challenges presented by stranded gas such as infrastructure and transport will also be reviewed.

“Accelerating the Transition to a 100% Renewable Energy Era” is part of the series Lecture Notes in Energy that contains 24 papers from multiple authors. The notes provide a topical and comprehensive source of information on achieving the transition to a low-carbon energy system, which is essential in the fight against climate change as we transition from our use of fossil fuels to clean energy.

The book provides in-depth analysis of the various solutions that will contribute to this change, such as hydrogen fuel, low carbon buildings and cities, security of supply, energy grids and energy storage. The collection of papers provides the necessary data, case studies and analysis to frame the topic and explore the challenges and potential solutions.

There has been a fundamental change in our awareness of our connection with the natural environment. In January 2020, Australia experienced devastating wildfires, and there was significant flooding in the UK. At the same time, a new zoonotic virus, COVID-19, was beginning to spread around the world. It was more evidence, if any were needed, that there is a clear link between our actions and the natural world. The subsequent COVID-19 lockdowns, where people had to stay at home in their local areas, also enabled a deeper connection with nature as we spent more time at home or outdoors and were able to see the change in seasons and hear the birdsong instead of traffic. Air quality in many cities also improved.

Reducing our impact on the environment is critical if we are to avoid the acceleration and escalation of climate change and reduce the likelihood of future pandemics. Quite simply, we are running out of time and yet many of the technologies and solutions are already available. In the UK, the Government is responding to the upsurge in public sentiment around these issues by recently announcing its Ten Point Plan (1) and is looking to put land stewardship at the centre of farming. An ambitious greenhouse gas reduction target of 68% by 2030 has been set and the sale of new petrol and diesel vehicles will cease in 2030. Later in 2021, Glasgow is hosting the crucial 26th United Nations Climate Change Conference of the Parties (COP26) meeting (2). There is cause for optimism although challenges still remain. Understanding the deployment of new technology will be central to achieving our climate goals.

This book represents a comprehensive source of information on renewable energy in terms of the various challenges, solutions and technologies. Each chapter is presented in the format of a scientific paper, which is understandable given the academic and expert authors that have contributed. The papers are detailed and are mostly accessible although I would say that some chapters are for deep subject matter experts and require the full attention of the reader. The content can quickly become dense or assume expert knowledge. The book is designed for both postgraduates and non-specialist researchers.

From my personal perspective as a sustainability professional, I was able to benefit from reading the book. It has helped me to understand the renewable energy landscape and many of the specific topics within this. For example, in Johnson Matthey we are seeking to incorporate renewable energy at our manufacturing sites and are also developing our businesses in manufacturing hydrogen fuel cells and battery cathode materials for electric vehicles. Reading the book has deepened my understanding of the different technologies and the choices that are necessary.

The first chapters provide a helpful introduction before moving into more specific topic areas. The initial chapter shows the global energy consumption by sector for buildings (36%), industry (31%), transport (28%) and others (5%). The subsequent chapter states that pathways limiting global warming to 1.5ºC require rapid and extensive transitions in all sectors and also makes the point that there are many other advantages to a low-carbon future, such as improved air quality and social prosperity. Progress has been slow but there is evidence for optimism with an increase in renewable electricity generation of around 3.1% per year between 2010 and 2016, which could approach 100% by 2050. Worryingly, the paper also states that current fossil fuel production will peak between 2030 and 2035 but would have needed to peak in 2020 to align with the Paris Agreement goals (3).

The subsequent chapters build upon the introduction by presenting deep-dives into the opportunities and challenges that have been outlined. I particularly enjoyed reading the papers associated with the role of hydrogen, as this is an important growth area for Johnson Matthey and it is a fantastic resource for all those that wish to deepen their understanding. Other papers investigate the challenges and solutions associated with energy storage and supply, either for hydrogen or the complexities of moving away from centralised electricity storage to microgrids, and address the challenges of intermittent supply from renewable sources (the phrase ‘dark calm’ is a term that I was not aware of in this context). These technologies and challenges are likely, however, to touch our lives over the coming years as we transition to a low-carbon economy and should therefore be of interest to all, not just those working in this topic area.

This book presents a comprehensive and detailed analysis on how to transition to a renewable energy future. It will be of value to anyone with an interest in the topic and will be particularly valuable to those that are working in this field, such as researchers or those in the corporate environment. The papers (lecture notes) are scientific and detailed and require the reader’s full attention and in some instances I would argue they require a certain amount of pre-existing subject matter expertise.

In conclusion, the book is a curation of lecture notes that has been successful in providing a holistic and topical analysis of the challenges and solutions related to the urgent need for decarbonising our energy systems. As such, it is an asset to those wishing to improve their knowledge in this arena.

The capital investment needed to achieve the IMO target of reducing carbon emissions from shipping by at least 50% by 2050 would be approximately US$1–1.4 trillion from 2030 to 2050 if green ammonia is adopted as primary zero-carbon fuel, according to the analytical work conducted by University Maritime Advisory Services (UMAS) and Energy Transitions Commission (ETC) and published as a brief by the Global Maritime Forum, an industry group backed by shipping and port operators in January 2020 (2). If shipping were to fully decarbonise by 2050, this would require extra investments of approximately US$400 billion over 20 years, making the total investments needed between US$1.4–1.9 trillion. The report claims “Under different assumptions, hydrogen, synthetic methanol, or other fuels may displace ammonia’s projected dominance, but the magnitude of investments needed will not significantly change for these other fuels.”

While making the calculations, the authors broke down the investment into two main areas: (a) ship related investments, which include engines, onboard storage and ship-based energy efficiency technologies; and (b) land-based investments, which comprise capital costs for hydrogen production, ammonia synthesis and the land based storage and bunkering infrastructure. As shown in Figure 1, the biggest share of investment is needed in the land-based infrastructure and production facilities for low carbon fuels, which make up more than 85% of the total investment. Hydrogen production via water electrolysis takes up around half of the total land-based investments needed, while ammonia synthesis, storage and bunkering infrastructure fulfil the other half. Only 13% of the investments needed are related to the ships themselves, which include the machinery and onboard storage required for a ship to run on ammonia both in new build ships and, in some cases, for retrofits.

Fig. 1

In addition to capital costs, the operational costs should also be considered while assessing the long-term economic feasibility and identifying the levelised cost of (green) ammonia (LCOA). It is outlined by Cédric Philibert at the IEA that “ammonia production in large-scale plants based on electrolysis of water can compete with ammonia production based on natural gas, in areas with world-best combined solar and wind resources.” Lately, this statement has been confirmed by Nayak-Luke et al. who found that with the current technology, islanded green ammonia can only be produced at US$473 per tonne at the most favourable geographic locations, but by 2030 this will decrease to a highly competitive US$310 per tonne (4). They have identified five key variables that have a significant impact on the estimated LCOA for islanded production which are levelised cost of electricity, electrolyser capital expenditure, minimum Haber-Bosch process load, maximum rate of Haber-Bosch process load ramping and renewable energy supply mix (5, 6). In practice, a combination of improvements on these key variables in a convenient geographical location (i.e., with favourable supply profiles) has the potential to make this carbon-free process economically viable for the first time and replace conventional ammonia production. Nevertheless, these calculated values are even now cheaper than the current anhydrous ammonia price, which is in the range of US$500–600 per tonne in the US (7) but still more expensive than LNG and MGO (8). Therefore, a key component of the commercial adoption of green ammonia as an energy vector in the future will probably be the level of incentives provided or regulation enforcing its use. The most likely incentive could come in the form of CO₂ taxation and credits. Based on the calculations of Argus Media, UK, the CO₂ pricing in Europe needs to be at least doubled to level the playing field for green vs. brown ammonia (9). Furthermore, the results reveal that significant utility grid backup is required for an all-electric ammonia plant built with present-day technologies. The total levelised cost of ammonia is driven in large part by the cost of producing hydrogen via intermittent renewable sources and operation of Haber-Bosch process. In order to reduce the costs, research is required to develop new, cost-effective yet highly efficient catalysts for electrolysers and ammonia production by either thermal or electrochemical methods.

Safety and environmental hazards for selected marine fuels are presented in Table I. As seen from the table, all fuels pose hazards in some way. Compared to the alternatives, ammonia is less flammable, thus presents a lower fire risk. The risks from cryogenic burns are also lower than for liquid hydrogen or LNG as ammonia can be liquefied easily by increasing pressure to ~10 bar at room temperature or by cooling to –33°C at atmospheric pressure, due to strong hydrogen bonding between molecules.

The main risks of ammonia arise from its toxic and corrosive nature. Ammonia is a gas at atmospheric pressure and room temperature, which is lighter than air. It has a strong odour, which can be detected at concentrations as low as 5 ppm; therefore, its smell provides an adequate early warning for a leakage. The US National Institute of Occupational Safety and Health (NIOSH) recommendations state that the maximum permissible time-weighted average (TWA) exposure of anhydrous ammonia for an 8 h workday of 40 h week is 25 ppm. The short-term exposure limit (STEL) or the concentration at which exposure of longer than 15 min is potentially dangerous is 35 ppm. The concentration at which the gas is immediately harmful to life or health (IDLH) is 500 ppm (16).

In addition, when anhydrous ammonia, either in gas or liquid phase, comes in contact with the human body, three types of injuries may result (17):

Therefore, the existing safety principles and systems used throughout the global ammonia industry would need to be deployed on ships and the crew onboard need to be equipped with suitable chemically resistant protective clothing and breathing apparatus.

Ammonia is also labelled as very toxic to aquatic life with long lasting effect. When liquid ammonia is spilled directly into water, most of it will dissolve into the water forming a balance of mostly ammonium hydroxide and a little ammonia depending on the pH and temperature of the water (Equation (i)) (18):

(i)

The remaining ammonia will evaporate resulting in a gas cloud with unpleasant smell. The dissolved ammonia is a serious threat to aquatic organisms killing most in close proximity as lethal concentrations can easily be exceeded. Long lasting effects of ammonia spillage are related to the time that the aquatic life requires to restore its original state through the nitrogen cycle (Figure 2). In this cycle, dissolved ammonia species are converted to nitrite (NO₂⁻) and nitrate (NO₃⁻) by Nitrosomonas and Nitrobacter bacteria, respectively, which is then used by plants. As this process consumes part of the available oxygen in water, the oxygen for other organisms, especially for the ones that are higher up the food chain such as fish, becomes limited, thus threatening their lives.

Fig. 2

When ammonia is combusted, it releases NOx species. NOx in the atmosphere contribute to photochemical smog, the formation of acid rain precursors, the destruction of ozone in the stratosphere and to global warming (19, 20). Despite the detrimental effect of NOx, control methods for reducing NOx emissions are already widely in place in land-based industrial installations and in the transport sector. One of the most common techniques is selective catalytic reduction (SCR) or deNOx technology. In this process, a reductant gas (ammonia or hydrogen) is added to the NOx-containing exhaust gas which is then passed over a catalyst that converts the NOx (NO and NO₂) to naturally occurring nitrogen and water (21, 22). The maritime sector has also had more than two decades of experience with SCR. More than a thousand SCR systems have been installed on marine vessels in the past decade (23). Despite the fact that SCR is a well-known process and the safe transportation and use of ammonia is well-established, it is clear that new applications will require careful risk assessment and additional control measures. If ammonia is going to be used as a new marine fuel, then the existing safety principles and systems used throughout the global ammonia industry would need to be adapted and deployed on ships to ensure that the risks of ammonia leakage and NOx formation are negligible. It has been reported that an average car needs only approximately 30 ml of ammonia per 100 km to neutralise any NOx emissions using SCR technology (24). If the vehicles run with ammonia as a fuel, this amount is unimportant with respect to the fuel tank volume. Similar calculations should also be performed for maritime sector in order to decide on the most feasible deNOx technology. The preliminary risk assessment forms using ammonia and hydrogen as marine fuels onboard and hazard mitigation strategies were reported by de Vries (25) which need to be improved and tested before implementation. It is also essential that the global use of ammonia at large-scale is well-thought out from a wider perspective in the roadmap. The effect of anthropogenic activities on the overall nitrogen cycle is generally overlooked in the literature. It has only been recently that MacFarlane and coworkers (26) provided a detailed discussion on cycling of nitrogen compounds and their environmental effects. As they stated, our understanding of the mechanisms of the global nitrogen cycle is not yet complete. Hence, further investment to basic scientific research is required to comprehend the environmental impacts of increased quantities of fixed nitrogen before implementing ammonia technology for transport. Finally, besides toxicity, the corrosive nature of ammonia also needs to be taken into account while selecting materials for storage and operation. Ammonia forms complexes with copper, brass and zinc alloys (27). Ammonia corrosion on these metals is even more drastic when there is some moisture. As previously discussed, ammonia is an alkaline reducing agent and it reacts with acids, halogens and oxidising agents.

The roadmap for the adoption of green ammonia as a marine fuel involves alterations of two systems in parallel, which are the ammonia manufacturing process and shipping propulsion structure. This ammonia-based economy will emerge through multiple generations of technology development and scale-up in the next 30 years.

Haldor Topsøe, a Danish catalysis company, presented a roadmap to all-electric ammonia plants (28) at the 2018 AIChE Annual meeting. According to its vision and strategy, ammonia production will be decarbonised in the 2030s by electrifying the production of hydrogen and nitrogen feedstock. The company is currently working on development of solid oxide electrochemical cell (SOEC) powered by renewable sources to produce nitrogen and hydrogen syngas using water and air which will then be used as a feedstock for Haber-Bosch process. In 2025, its aim is to demonstrate the production of ammonia via SOEC and Haber-Bosch processes at a scale of 500–1000 kg ammonia per day. After that, it intends to commercialise the technology starting from 2030. Until SOEC technology is mature enough to substitute the current brown ammonia production method, the company is suggesting to use a hybrid system (conventional and electrified Haber-Bosch) to decrease the amount of CO₂ emission whilst supplying the demand.

A more comprehensive roadmap to the ammonia economy has lately been published by Doug MacFarlane and coworkers (26). In this roadmap, the authors envisage renewable ammonia being produced in the future at a scale that is significant in terms of global fossil fuel use. The paper diagrams an evolution of ammonia synthesis through three overlapping generations of technology development and scale-up (Figure 3). Generation 1 (Gen1) involves the integration of sequestration or offsets to current-day Haber-Bosch ammonia production in order to bring the net carbon impact of the ammonia production to zero (blue ammonia). Generation 2 (Gen2) remains the Haber-Bosch process with existing and new plants, but hydrogen is derived from renewable sources (green ammonia). As the Haber-Bosch process is a well-established technology, the authors anticipate that ammonia production will remain dominated by it over the next two decades. Generation 3 (Gen3) rules out the need for the Haber-Bosch process by direct electrochemical conversion of nitrogen in water to ammonia. This renewable-powered entirely electrochemical ammonia production technology is expected to enter the market at scale as soon as it achieves commercial readiness index (CRI) 1 and start significantly contributing to global ammonia production thereafter, as plant size and capacity increases. The timeline of Gen3 to enter and dominate the market is highly dependent on progress in catalyst development. While several thousand catalysts were screened in the development of thermal ammonia synthesis, relatively few catalysts have been tested systematically for electrochemical activity. At present, the electrochemical ammonia production rates remain over an order of magnitude away from US DoE targets as mentioned in Section 2.2, Part I (1). Therefore, continuous development of routes to new materials, more control experiments and extended stability studies are necessary before the implementation of Gen3.

Fig. 3

A case specific policy analysis reported by the Organisation for Economic Co-operation and Development (OECD) (29) anticipates that it would be feasible to scale-up low-carbon ammonia production and deploy ammonia fuel technology swiftly enough to reduce the carbon emission from maritime shipping by up to 80% by 2035. In the OECD’s 80% carbon factor reduction scenario (Figure 4(a)), hydrogen and ammonia will fuel around 70% of the mix of ships. This, along with the increase in the uptake of biofuels (22%) and LNG (5%), could diminish the use of oil-based fossil fuels significantly to around 3% by 2035. Another scenario analysis performed by UMAS (3) suggests that ammonia is likely to represent the least-cost pathway for international shipping and play a leading role in replacement of fossil fuels with a rapid growth after 2040 and between 75–99% market share by 2050 (Figure 4(b)).

Fig. 4

The roadmap for the adoption of ammonia as a marine fuel was limited to the fuel mixing trajectories to reduce carbon emissions without specifying a specific timeline for the development of propulsion engine systems that are adapted to run with ammonia until the report of Environmental Defence Fund, USA, published in 2019 (30). The report focuses on ammonia in combustion engines and fuel cells. A possible roadmap for development and adoption of these technologies is depicted in Figure 5. The authors anticipate that the use of green ammonia in ship propulsion systems will most likely begin in the 2020s with modified ICE given that the shipping industry is dominated by the use of these engine types. MAN ES (31) and Alfa Laval, Sweden, (32) have already started developing a dual-fuel combustion engine to run with liquefied petroleum gas (LPG) and ammonia. Starting from 2020, further development is required in the use of green ammonia in fuel cells to pave their way for deployment in the 2030s. With the current state of technology readiness, the initial fuel cells are expected to be the PEM type that might give way to SOFCs over time.

Fig. 5

Following the direction of IMO towards the reduction of harmful gas emissions by 2050, the maritime sector is getting ready for an energy switch. Many reports (33–36) can be found in the literature that discuss the alternative fuels in a comparative manner to reduce GHG emissions from shipping. Of these alternative fuels, ammonia is prominent due to its carbon neutral chemical formula, high energy density, established production, transportation and storage infrastructure and competitive cost as discussed through this review. However, to satisfy the energy demand of the maritime industry, the production capacity of ammonia needs to be expanded substantially (i.e. 2.5 times larger production (~500 million tonnes per year)) to decarbonise the international fleet (30)) and production routes have to be green in order to reduce emissions of CO₂. This means that together with other sectors, shipping will add additional stress to renewable electricity production, around the order of magnitude of the current power sector, which itself is yet to fully decarbonise. One possibility for maritime is to build their own infrastructure for the production of green ammonia on existing ports and on offshore marine farms. Lately, the main ports of Morocco have been identified as potential locations to produce and store green ammonia (30). For instance, Jorf Lasfar Port has an existing ammonia storage infrastructure with a total capacity of 100,000 tonnes due to the ammonium phosphate fertiliser production complex of the state-owned OCP group. Upon integration of 300 MW solar panels near the port, it is envisaged that 700 tonnes of ammonia per day, which is equivalent to the daily fuel consumption of about four post-Panamax size vessels, can be produced and stored. In the case study, the daily amount of renewable electricity to produce green ammonia for fuelling all container and dry bulk vessels passing through Morocco’s ports is calculated as 280 GWh which is only 0.6% of the renewable (wind and solar) energy capacity of Morocco. Taking this case study as a basis, more research on technoeconomic analysis for green ammonia production needs to be performed in different ports, considering not only onshore but also offshore options and the use of a combination of two or more intermittent renewable energy sources (solar, wind, wave or tidal) to provide a virtually continuous supply and thereby improve the efficiency and cost-effectiveness of the whole process.

Today, the price of green ammonia is significantly higher than for brown ammonia and for conventional marine fuels such as HFO, MGO and LNG. However, looking towards the future where fossil fuels must be substituted, the price of ammonia is expected to be in the same range in comparison with other renewable alternatives such as biofuels and hydrogen. The high cost of green ammonia derives from the capital cost of electrolysers, which take up almost half of the total land-based investments. To bring these expenses down, the usage of expensive noble metal based catalysts should be reduced or ideally be replaced with earth-abundant alternatives.

The adoption of ammonia as a marine fuel in the short term is envisaged to be driven by ICE under current market and regulatory conditions. The preliminary small-scale test results reported by MAN ES and Wärtsilä demonstrate that the technology is ready to start working on a full-scale pilot with relatively few additional design modifications. Although ammonia combustion in ICEs does not contribute to carbon emission, thus can be regarded as a clean solution compared to fossil fuels, it is not 100% harmful emission free and requires NOx elimination. Therefore, ICEs should predominantly be seen as an important intermediate step to introduce ammonia as a new fuel in the maritime industry before pursuing towards truly 100% zero emission shipping by using fuel cells. In the medium to long term, ICEs are expected to leave their places to SOFCs as technology develops and price levels drop.

It should also be noted that there is no single solution and transition to zero emission will be through a combination of several technologies including new fuel sources and vessel efficiency improvements such as renewable assisted propulsion, hydrodynamics, paints and hull coatings, velocity optimisation, engine and ship design. Balcombe et al. (37) assessed GHG emission reductions via the use of alternative fuels (LNG, methanol, biofuels, hydrogen, nuclear and electricity) by incorporating various energy efficiency measures and they concluded that the decarbonisation requirements of the maritime industry could be met via a combination of several technological and operational pathways. Such a combined assessment is currently missing for green ammonia. It is recommended that future research activities focus on collective impact of changing fuels and implementing efficiency measures. In addition, an integrated system engineering is required to assess several factors such as space for onboard energy storage, energy requirement for a round trip, geography, infrastructure, costs and safety to decide on the ultimate energy transition pathways based on individual shipping operation conditions.

Overall, our analyses indicate that an effective fuel switching in maritime industry can only be achieved through engagement and synchronisation of three sectors, which are science and technology, industry and business, governance and policy. For a constructive transition, we need a round table that can link the key players from these industries and enable them work in a collaborative manner by involving in consortium projects. The global maritime energy shift council members may consist of, but not limited to, representatives from shipping companies, port managers, (renewable) energy firms and associations, politicians, policy makers, financial sectors and investors. Last but not least, the involvement of scientists should also not be forgotten. The efficiency and cost-competitiveness of the whole power-to-ammonia-to-power cycle explicitly depend on the development of new state-of-the-art materials and establishing an integrated system engineering. Scientists need support for carrying on fundamental research but also for increasing the commercial readiness of the discoveries. Various solid-state materials and techniques, that offer cost, efficiency and performance benefits, have already been reported in the literature and more will continue to come in the near future. However, there is a gap between transfer of knowledge to application. To increase the TRL value of these technologies within a compressed time frame and for large-scale implementation of carbon-free energy, scientific entrepreneurship should be encouraged and supported more.

Lastly, among the transportation sector, the shipping industry has long been criticised for being too conservative and too passive to change. In an interview with ShippingWatch, Henrik O. Madsen, the former CEO of major classification company DNV GL, stated “The attitude in the industry is mainly that any new regulation introduced is basically negative. I could hope that, going forward, they will change from seeing every new regulation as a risk to instead also thinking of a regulation as an opportunity” (38). To make this come true, local and international authorities need to join their forces and lead the round table meetings to bring innovative ideas collectively that can disrupt the conservativeness and fragmented nature of the maritime sector and help them change for a better future and business opportunities.

Following the directions, policies and roadmaps of IMO and national regulatory authorities, a number of ventures are already underway to test viability of ammonia in the shipping sector. The engine manufacturers, MAN Energy Solutions (MAN ES, Germany) and Wärtsilä, Finland, are currently developing two-stroke and four-stroke engines, respectively, designed to operate on ammonia and anticipate that the first ammonia engine could be in operation in 2024 (30, 31). Both companies reported that they had successfully conducted a preliminary study into ammonia combustibility, which revealed that slow flame velocity, slower heat release and combustion characteristics of ammonia were no obstacle to combustion in these engines (32). Based on their research on combustion in smaller engines and turbines, the challenges related to ammonia combustion are determined to be the high nitrogen oxides (NOx) generation, low flammability and low radiation intensity. Further full-scale engine tests will continue to overcome these challenges in 2021. These tests will serve as the platform for the ammonia engine development at Copenhagen Research Centre of MAN ES and the Sustainable Energy Catapult Centre’s testing facilities of Wärtsilä at Stord, Norway. Following that, Lloyd’s Register (LR, UK) has granted Approval in Principle to Dalian Shipbuilding Industry Company (DSIC, China) and MAN ES for an ammonia-fuelled 23,000 twenty-foot equivalent unit (TEU) ultra-large container ship (ULCS) concept design, the first ammonia as fuel design of its kind in China (33). MS Color Fantasy, the world’s largest roll on/roll off (RORO) cruise liner, has also plans to pilot ammonia as a marine fuel (34). In addition, like Enviu’s THRUST programme from The Netherlands, another non-profit organisation, the Mærsk Mc‐Kinney Møller Center for Zero Carbon Shipping, was launched in Denmark on 25th June 2020 (35). The organisation aims to bring the best minds from science, engineering and business in order to implement new energy systems and technologies for shipping. Although it is not clear yet how the decarbonisation of shipping will be achieved, given the tremendous drive around ammonia as a potential zero-carbon emission fuel, more ammonia-related shipping projects are expected to be announced in the near future.

Besides the efforts of individual companies on developing and expanding their ammonia powered technologies, recently there has been a tremendous increase in the announcement of consortium projects aiming to demonstrate ammonia-fuelled vessels operating at sea. The ShipFC consortium could secure €10 million fund from the EU’s research and innovation programme Horizon 2020 under its Fuel Cells and Hydrogen Joint Undertaking (FCH JU) to deliver the world’s first high-power fuel cell to be powered by green ammonia (36). The ShipFC project is being run by a consortium of 14 European companies and institutions, coordinated by the Norwegian cluster organisation NCE Maritime CleanTech. The project aims to demonstrate an offshore vessel, Viking Energy, which is owned and operated by Eidesvik AS, Norway, and on contract to energy major Equinor, Norway, powered only with a large 2 MW ammonia fuel cell to sail up to 3000 h annually. One of the main objectives is to ensure that a large fuel cell can deliver total electric power to shipboard systems safely and effectively. This is the first time an ammonia-powered fuel cell, scaled up from 100 kW to 2 MW, will be installed on a vessel. The design, development and construction of ammonia-fuelled solid oxide fuel cell (SOFC) will be undertaken by Prototech, Norway. Testing will be executed at the Sustainable Energy Norwegian Catapult Centre and the ship-side ammonia system will be supplied by Wärtsilä. It is envisaged that the ammonia fuel cell system will be installed in Viking Energy, UK, in late 2023. The ultimate goal is to demonstrate that long-distance, emission-free voyages on big ships are possible.

Another European based consortium in the Nordic region was announced in May 2020 (37). The Global Maritime Forum has launched The Nordic Green Ammonia Powered Ships (NoGAPS), a major consortium that aims to prove the feasibility of a large ammonia-powered deep-sea vessel by 2025. Funded by Nordic Innovation, partners of the project include Danish Ship Finance, shipowner J. Lauritzen, engine maker MAN ES, Ørsted energy group and consultancy group Fürstenberg Maritime Advisory, all from Denmark, along with Oslo-based bank DNB, the class society DNV GL, chemical group Yara International and the Helsinki-listed Wärtsilä.

In Japan, an industry consortium is collaborating in a project to develop ships designed to use ammonia as fuel and go beyond onboard ship technology to include “owning and operating the ships, supplying ammonia fuel and developing ammonia supply facilities.” The participants of the consortium are Nippon Kaiji Kyokai (ClassNK), Imabari Shipbuilding, Mitsui E&S Machinery, MAN ES, Itochu Corporation and Itochu Enex (38). In addition, on 6th August 2020, NYK Line, Japan Marine United Corporation and ClassNK signed a joint research and development (R&D) agreement for the commercialisation of an ammonia-fuelled ammonia gas carrier (AFAGC) that would use ammonia as the main fuel, in addition to an ammonia floating storage and regasification barge (A-FSRB) for offshore bunkering and stable supply of ammonia fuel (39).

It is likely that more ammonia propelled shipping demonstration projects will be announced in the following years. The winners of the contest will dominate their positions in the value chains to deploy zero-carbon vessels and bunkering infrastructure across the sector.

Recently ammonia has taken considerable attention and pointed as one of the most promising alternative chemical energy and hydrogen-carriers in many technical reports (19, 40), white papers (23, 41) and research articles (18, 22), due to the following reasons:

To meet IMO’s targets and ultimately decarbonise the maritime sector, vessels powered by zero GHG emitting fuels need to be implemented to the international shipping fleet in the early 2020s. Ammonia offers several potential advantages over hydrogen and the conventional marine fuels such as HFO, MGO and LNG. However, several factors such as sustainable production routes, power generation, cost of transition and safety and environmental aspects still need to be considered thoroughly before the implementation and deployment of an ammonia-powered fleet. The following sections of the paper will cover these aspects. It is also noted that there are many valuable studies that have assessed the potential of ammonia as an alternative fuel for transport (17–23). This paper adds to this body of literature by providing collective, up-to-date knowledge, introducing state-of-the-art and emerging technologies as well as identifying the critical research gaps necessary for practical application of these technologies. The paper follows an approach to show the picture from a wide-ranging perspective that is of interest particularly for industry without overwhelming with technical details. Instead, the key and recent studies have been identified, summarised and cited in the paper for interested readers to explore further.

As described above, green ammonia production incorporates two catalytic processes: (a) hydrogen production from water electrolysis; and (b) ammonia synthesis from hydrogen and nitrogen via Haber-Bosch reaction. The high cost of commercial electrolysers arises from the usage of expensive noble metals such as platinum and palladium on a carbon support as catalysts in the electrochemical cells. The catalyst itself has taken up a considerable portion of the total system and capital cost, especially if there is degradation or corrosion on the carbon support. Hence, one crucial aspect of the development in hydrogen evolution reaction (HER) technology is to replace the catalysts with earth-abundant alternatives to produce hydrogen in a more economical way. Mo et al. (54) has recently reported that inexpensive silver catalysts, particularly the cubic form of silver nanoparticles, can clearly exhibit superior HER activity over platinum at the same metal content by altering the rate-determining step in a proton exchange membrane (PEM) electrolyser when practically more negative potential is applied. High activity was attributed to the weaker Ag–H bond at the surface than Pt–H which is more favourable for H recombination to form H₂. This study is significant to rectify the misconception that platinum is always at the ‘optimal volcano’ position among all monometals in HER, which has led to an inaccurate description of the surface electrocatalysis under real PEM conditions at high workload. Beside this scientific achievement with a monometallic catalyst, start-up company Hymeth, Denmark, announced in 2019 that it would commence the production of Hyaeon^TM which is a low temperature and high pressure electrolyser, at a commercial scale after completing tests. The company uses an inexpensive trimetallic nickel-copper-iron core-shell electrocatalyst, possessing high electrochemical activity for both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) (55). Another method of hydrogen evolution is photocatalytic water splitting. This process benefits from direct usage of solar renewable energy without the requirement for the installation of an extra electricity generator such as photovoltaic panels or wind turbines to supply power to electrolysers. Although various studies have been reported in the past decade (56), no practical application has been implemented yet mainly due to low catalytic activities, a narrow range of light absorption and poor quantum efficiencies (QE) (the measure of the effectiveness of a light absorbing material to convert incident photons into electrons) as a result of fast recombination of charge carriers. In 2019, Tsang and coworkers (57) reported a nitrogen-doped titania nanocatalyst on MgO(111) photocatalyst that has a hydrogen evolution rate of over 11,000 μmol g⁻¹ h⁻¹ in the absence of any sacrificial reagents at 270°C. An exceptional range of QE from 81.8% at 437 nm to 3.2% at 1000 nm was also stated. High activity was attributed to formation of oxygen vacancies upon introducing nitrogen into the titania structure and prolongation of exciton lifetime over the polar MgO(111) surface. The technology readiness level (TRL) of this invention is currently at TRL3–4 but it has a strong potential in the future to harness solar energy (light and heat) for hydrogen production in large scale.

Another energy intensive and costly process in ammonia production is the Haber-Bosch process where hydrogen and nitrogen react at 15–25 MPa and 400–450°C using an iron-based catalyst (either magnetite or wurtzite). Low equilibrium single-pass conversion (~15%) necessitates the recycle of unreacted gases, leading to higher energy consumption (58). Compared with commercial iron catalysts, ruthenium-based catalysts offer advantages in Haber-Bosch reaction because they are relatively active at low pressure. Ruthenium with a higher electron density in d-orbitals, in assistance with strong electron donor dopants such as alkali metals, can donate electrons into the anti-bonding orbital of adsorbed nitrogen, facilitating its dissociation, and thus, can work under lower pressure. However, ruthenium-based catalysts have found limited uses in conventional Haber-Bosch processes because they are relatively more expensive and are easily poisoned by carbon deposition from methane in syngas (59). The electrified Haber-Bosch system, where hydrogen is derived from water, does not contain methane, so the carbon poisoning effect can be well avoided. However it is also known that another surface poisoning of ruthenium sites by competitive strong hydrogen dissociative adsorption limits the overall reaction rate. Lately some workers have demonstrated that changing the surface polarity by either decorating terrace sites of ruthenium nanoparticles with Li⁺ (60) or using an electrostatically polar MgO(111) in place of nonpolar MgO as the support (61), can significantly alleviate the hydrogen poisoning and facilitate an unprecedented ammonia production rate. Another outstanding study reported by Hattori et al. (62) has demonstrated the ability of ruthenium catalysts to produce ammonia from nitrogen and hydrogen at a temperature as low as 50°C. The researchers used a stable electron-donating heterogeneous catalyst, cubic CaFH, a solid solution of calcium fluoride and calcium hydride formed at low temperatures to achieve high performance with an extremely small activation energy of 20 kJ mol⁻¹ at 50°C, which is less than half that for conventional catalysts.

If the future green ammonia production via Haber-Bosch process is carried out in decentralised, islanded locations in small scale, then hydrogen manufactured from an electrolyser at lower pressure and temperature would require coupling with an efficient catalyst to achieve high ammonia production rate. In this manner, ruthenium stands out from the other alternatives and high cost may actually not be a disadvantage. In fact, developing countries, particularly ones located in Africa may use this opportunity to attract investment as they have high renewable solar energy capacity and resources for platinum group metals.

Regarding the electrochemical approach to synthesise ammonia, there are a number of potential candidates, which have recently been demonstrated to be active for this reaction (63–65). The goal of electrochemical ammonia synthesis, in contrast to electrified Haber-Bosch process, is to catalyse the direct reaction of nitrogen with water to form ammonia at ambient pressure. The potential elimination of the separation and purification steps for hydrogen when water is used as the reductant for nitrogen, along with the input of electrochemical energy at milder conditions, is very attractive. However, the nitrogen molecule is highly inert towards reduction, much more so than the most common electrochemical solvent, water. In principle the reaction can proceed under ambient conditions, as seen in biology, however translating this chemistry into an industrial process while retaining practical rates and efficiencies has shown to be challenging. The vast majority of reports (Figure 3) fall below the targets set by the US Department of Energy (DoE) in the Advanced Research Projects Agency-Energy (ARPA-E) Renewable Energy to Fuels Through Utilization of Energy-Dense Liquids (REFUEL) programme for feasible industrial installations (current density >300 mA cm⁻² and current efficiency >90%, which is equivalent to an effective rate of 9.3 × 10⁻⁷ mol cm⁻² s⁻¹). Although the present rates remain over an order of magnitude away from DoE targets, continuous progress is being made both in mechanistic understanding of the reaction and in the development of routes to new materials. Finding the ideal combination of mediator, catalyst and electrolyte components to optimise selectivity and yield rate, while decreasing energy costs, is thought to be the key goal of research in this field (66) for commercial feasibility.

Fig. 3

Given the fact that green ammonia production from water electrolysis followed by Haber-Bosch process would be the most convenient route with current technology, several green ammonia demonstration or production plants with a wide range of capacities have been announced in the past few years. Table II summarises these projects including the key players and their targets.

The construction of the first three pilot plants given in Table II has been completed. They are currently up and running to carry out R&D toward ammonia synthesis and power generation from ammonia in a cost-effective way by utilising renewable energy. The initial test results were reported to be very promising (74–77), paving the way to larger scale, mega projects as announced by several companies from Australia, New Zealand, The Netherlands, Spain and Saudi Arabia.

Today, commercial manufacturing of green ammonia is not available anywhere. But, with renewed interest and global drive, it is highly likely that by 2030, there will be a body of demonstration plants that can show the viability of producing ammonia from renewable energy at scale.

With an energy density of 12.7 GJ m⁻³, ammonia would require a larger volume of space onboard in order to deliver the same power as conventional marine fuels. For instance, if a HFO fuel tank has a volume of 1000 m³, an ammonia fuel tank would require 2.75 times more space than that of HFO to provide the same power (30). This might make ammonia appear unfeasible; however, the space requirement for ammonia remains significantly smaller compared to other carbon-free options as the tank volume would be 4117 m³ for liquid hydrogen at –253°C; 14,000 m³ for a Tesla Model 3 battery (Tesla, USA) and 120,896 m³ for the battery pack of Corvus Energy, Norway, the marine battery market leader (30). Even carbon-based methanol does not offer significant advantage, needing a tank volume of 2333 m³. Therefore, the space requirement for ammonia-propelled shipping is not found to be unrealistic or inapplicable (24).

Two kinds of propulsion systems (direct combustion and fuel cells) that could use ammonia as a marine fuel stand out regarding the current and emerging technologies. Figure 4 illustrates the simplified configuration of these propulsion systems.

Fig. 4

3.2.1 Direct Combustion

3.2.2 Fuel Cell Systems

3.2.3 Catalytic Processes Involved in Ammonia to Power

So far, none of these propulsion technologies for ammonia has yet been commercialised and deployed for shipping but a design study for such a vessel was recently published by de Vries (43). The author reviewed all options covering ICE, PEMFC, AFC and SOFC for marine applications. It has been concluded that the SOFC scores best in efficiency but lacks power density, load response capability and is still too expensive. The ICE is second in efficiency and thus more efficient than the PEMFC and the AFC (in case these are operated close to maximum power). Additionally, the ICE is less expensive, more robust with acceptable power density and load response. Based on these comparisons, the ICE has been identified as the best option for maritime applications at the current technology status but SOFCs are considered to have a lot of potential in the future.

As mentioned in Section 1.1, MAN ES and Wärtsilä are working on the development of the ammonia-fuelled engine for shipping. The overall message from MAN ES is that the liquid gas injection (LGI) engine family that works with dual-fuel is a good candidate for the conversion to ammonia and the ships running with LNG can be retrofitted for ammonia operation as the tanks used for storage of LNG with the same requirements can also be used for ammonia (30, 91). However, when designing the storage and propulsion systems, the chemical properties of ammonia should be taken into consideration. Due the corrosive nature of ammonia, copper, brass and zinc alloys need to be avoided as discussed in Part II (92).

Today’s fuels and chemicals industries have their roots in 20th century investments in fossil fuel extraction and conversion. Highly integrated petrochemical complexes and refineries have evolved to deliver products that have transformed how we feed, clothe, shelter, make healthier and move people across the globe. The environmental impacts of these industries are under ever tighter scrutiny, particularly as we accelerate towards a future with net zero and circularity at the forefront of our thinking.

This Johnson Matthey Technology Review issue explores technological advances that may evolve to form the backbone of the future fuels and chemicals industries: technologies which will deliver even greater benefits for human life, while treading lightly on our planet.

There is no ‘silver bullet’ as we transition our industries to their future state. Many advocate that they have ‘the answer’, but the reality is that we must deploy every tool at our disposal today, while continuing to innovate the step-out solutions of tomorrow. The good news is that the fundamentals are in place. Renewable energy costs are falling, we understand the chemical conversions that need to be implemented, and we have the technologies to abate unavoidable greenhouse gas emissions.

The chemical industry has always adapted to different feedstocks, driven by price or geopolitical pressures. In recent years shale gas has revolutionised the US chemical industry, whilst China retains a strategic interest in coal. Methane offers significant carbon reduction potential and while it can be readily processed by syngas routes, direct upgrading options continue to be explored. These technologies, applied to renewably derived methane, maintain their relevance in a net zero world.

Bio-derived fuels are already established in transport applications where legislation drives today’s investments (1). While some commercial routes to bio-derived chemicals exist, the same driving force for change is not there. However, brand owners of consumer facing products are making strong commitments to sustainability and decarbonisation (2) which may create the market pull for biobased plastics, surfactants and formulating agents. Contaminated plastics, which cannot be easily recycled back to monomers or polymers, as well as municipal solid wastes, are emerging as feedstocks that can be reconstituted into chemical products via pyrolysis or gasification.

Carbon dioxide from industrial processes or direct air capture will become an important carbon source in the decades ahead. The direct or indirect addition of renewable energy is needed to move back up the thermodynamic hill. Hydrogen generated by splitting water will be a key enabler, as subsequent conversion via syngas into alcohols and hydrocarbons leads into downstream value chains. As well as water electrolysis, electrochemical technologies that reduce carbon dioxide directly are being developed and scaled. These may enable future chemical production at more localised scale.

Regardless of feedstock, designing energy and atom efficient processes remains key. 90% of chemical processes today are already catalysed (3), and we can expect this to grow. When developing new processes the mantra of ‘right catalyst, right reactor, right process’ remains truer than ever. Recent progress by Johnson Matthey and its partners in Low Carbon Hydrogen (4) and FT-CANS^TM (5) technologies serve to illustrate this point.

Catalytic processes will continue to evolve to deliver more atom and energy efficient flowsheets. Seamless integration of renewable electricity, decarbonised hydrogen and carbon capture will become the norm for chemical plants of the future. Advances in reactor design and control may revolutionise process operation, and the use of lower density feedstocks may drive some applications towards distributed, intensified modules. More hybrid bio-thermochemical processes are likely to emerge as advances in biotechnology are leveraged. The aim here must be to preserve some of the precious functionality within the biomass feed.

It is interesting to reflect on whether the portfolio of chemicals that make up the industry toolkit today will change significantly in the years ahead. For example, the challenges of recycling a more divergent range of plastics is likely to incentivise the industry to work mostly with decarbonised variants of what is known, rather than seeking radically new polymers.

Fuel slates will shift as transport becomes increasingly electrified, and hydrogen will take up a role as an energy vector alongside electricity. It will be valued for its ability to store and distribute low carbon energy, as well as being the solution to decarbonisation challenges in heavy transport and industry. Kerosene type fuels derived from biomass, waste or e-fuels will continue to dominate long-haul aviation (6). There is much debate around shipping, with hydrogen, methanol and ammonia vying to be selected as the future fuel of choice (7). While production of these molecules is well established, the scale of investments needed to move these commodity chemicals into the fuels arena will surely drive innovation in these mature processes.

Markets that are likely to see most innovation in product design are those in which chemicals are unavoidably released into the environment, such as pesticides, fertilisers, detergents and water-soluble polymers. Biodegradability in real conditions and the minimisation of eco-toxicity will drive the search for materials with improved environmental functionality.

The transition of the fuels and chemicals industries towards a circular, net zero future will require disruptive transformation in how we innovate and deploy technologies. New ecosystems will evolve as the power generation, agriculture and waste management sectors integrate with the chemicals and fuels sectors to meet the challenge. The transition will not be achieved by technology solutions alone. The right legislative frameworks, incentives and business models must be established to create a level playing field in which all embedded environmental impacts are accounted for. Deployment at scale is also essential to move quickly down the cost curves.

We are at the start of a revolution in our industries, but one at which efficient and effective catalyst technology sits at the heart. Industry’s current skills in deploying thermocatalysis will be complemented by advances in bio, electro and possibly photocatalysis. I for one am excited to see what we can collectively achieve over the coming decades as we step up to deliver the solutions needed for a cleaner, healthier world.

Propanal, also known as propanaldehyde or propionaldehyde, is a liquid, with an ethereal pungent odour (6). Propanal is mainly used as a chemical intermediate in the production of n-propanol and propionic acid, for example. The market for propanal itself is not large and the annual quantity imported or manufactured in Europe is in the range of 100–1000 tonnes according to the European Chemicals Agency (ECHA) (7). Main producers consume it internally. Hydroformylation of ethylene with synthesis gas (syngas) is the usual process to produce propanal. An alternative route, through isomerisation of allylic alcohol, would nevertheless also be possible (i.e. enol reaction).

According to the trade data, there was a significant volume of exports of propanal in 2019 (8, 9), amounting to 286,000 tonnes worth US$725 million.

Trade data are accessible through the harmonised system (HS) code (numbers used to classify traded products), for example 291219 which includes “acyclic aldehydes, without other oxygen function (excluding methanal, ethanal, butanal, benzaldehyde)”. Therefore, it is not possible to isolate propanal in the trade data, but because it is the main product share under this code, general trends are relevant.

The data in Figure 3 are computed from the export volumes and take into account the average distance travelled by the product weighted by the trade value. The concentration parameter is an indication of the diversity of customers (importing countries); a concentration value of 1 means export to a single country. The trade balances (exports-imports) are used as the indicator, but the distance is based on exports only. Chinese and US products travel longer distances than products made in Germany. The data illustrate that for this product (propanal), there would certainly be a demand for small or local production, with small units, avoiding long distance transports and representing a potential market of US$100 million. Stranded gas, complemented with biogas, make methane an ubiquitous resource for small plants. Taking into account the higher cost for importing small volumes of products and other logistics costs, a local small production cost compares more favourably than with world market prices. In addition, production on-site and on-demand contributes to the reduction of other associated costs such as inventory and safety and require specific local investigation.

Fig. 3

1-Propanol is a colourless liquid whose odour and flavour are alcoholic and earthy (10). Due to its excellent solvent properties, 1-propanol is used in various applications, including lubricants, coating products, dispersing agents, pesticides, surface agents, cleaning products and adhesives. This material is also used for packaging and food-contact applications and was recently also used in some sanitiser gels in combination with isopropanol.

Due to the wide range of potential applications, the market is more developed and the annual quantity imported or manufactured in Europe is in the range of 10,000–100,000 tonnes according to the REACH registrations (11). 1-propanol has 85% of the propanol market, with 15% for isopropanol (2-propanol). 1-propanol can be produced through hydrogenation of propanal, or directly from syngas through the Fischer-Tropsch process developed by Sasol, in which it is separated from the mix of products. Interestingly, there does not seem to be a commercial fermentation route to produce biobased 1-propanol.

The propanol HS trade code is 290512, including both 1-propanol and 2-propanol. Because 1-propanol represents 85% of the market, the trade data mostly represent the targeted product. Asian countries produce mostly 2-propanol (for example by Tokuyama, ISU Chemical, LCY Chemical Corp, Zhejiang Xinhua Chemical Company, LG Chem). For the readers who would be interested in 2-propanol trade and productions, we suggest to look at the phenol trade data. Acetone is coproduced with phenol and 2-propanol can be produced either by hydrogenation of acetone, or direct hydration of propylene, or fermentation (just starting). So if acetone is the main source for 2-propanol, it would be linked with phenol production. But this is not the scope of the present paper.

The net difference between exports and imports has been computed from the annual data available from Trade Map between 2010 and 2018 and is shown in Figure 4. This was done to identify where the main producers are located and to eliminate the countries only involved in trading. Imports and exports are based on the HS trade codes for both propanol isomers.

Fig. 4

According to the trade data, there was a significant volume of 1-propanol and 2-propanol exports in 2019 (9, 12):

These data illustrate the market activity. The same country can be an exporter and an importer of the same chemical compound. It is obvious for large countries like China, Russia or the USA, where the east and west coasts can more easily import from other countries than to transfer product from the other side of the country. Therefore export volumes and value from major producers are more relevant to assess potential for production units in other countries. In addition the volume produced is not necessarily in line with the exported volume, since a lot of captive use can exist. However, the share of exports to the production capacity is an important indicator to identify the potential risks linked with a specific producer or producing country.

Exporting countries sell the product in many countries (low concentration factor) Figure 5. Moreover, the average distance is similar to that of propanal. Germany and The Netherlands export to Europe, while South Africa, South Korea and the USA export at longer distances. The largest shares of export values are in the USA, where products travel on average 6000 km. This supports the notion that smaller, modular C123 plants for local production should have a commercial interest, with a cumulated value of US$350 million (this value is calculated based on the export values from South Africa, USA and South Korea from which the product travel more than 6000 km). When products travel on long distance, they not only consume a lot of energy for transport, but also contribute that way to global warming but are also more sensitive to energy price variations and as seen recently to unpredictable events like COVID-19 and country resilience.

Fig. 5

Propionic acid is a colourless pungent odorous liquid (15). Propionic acid is manufactured to be used as preservative and anti-mould agent in animal feed and grain (16). It is also used as a chemical building block for the production of herbicides, pharmaceuticals, dyes, textile and rubber products, plastics, cosmetics and perfumes. In addition, propionic acid is a preservative and flavouring agent in packaged foods.

The annual quantities imported or manufactured in Europe is in the range of 100,000–1,000,000 tonnes, according to ECHA (17). The calculated average of annual differences between exported and imported quantities are shown in Figure 6. Propionic acid is produced either by oxidation of propionaldehyde or by a Reppe process, i.e. the hydroxy-carboxylation of ethylene (18). The HS trade code 291550 includes propionic acid, its salts and esters. Because propionic acid is needed for the creation of its salts, the traded volumes are relevant for discussion.

Fig. 6

According to the trade data, propionic acid export volumes in 2019 were as follows (9, 19):

The capacity expansions which have been made recently by several producers are a good sign that the market demand is growing, whether the growth is for captive use or to satisfy customer demands.

In contrast with the exporting countries of the two previous products, the export concentration (Figure 7) is higher for propionic acid and is above 0.8 in case of China. This means that exporters have a limited number of customers in targeted regions. Nevertheless, the average distance from a given exporting country to their markets is similar. Sweden appears as an exporting country to neighbouring northern Europe, while Germany exports worldwide. Again, the USA has the largest share in export value, with products traveling more than 5000 km. Once again this supports the notions that it would make sense to have more localised production of this C3 chemical.

Fig. 7

The variation of the trading value (import and export prices) for 1-propanol and 2-propanol from the USA (9), together with the price of crude oil (Brent is selected as a better world price reference than West Texas Intermediate) (24), US ethylene (25) and propylene (9), the traditional feedstocks for the C3 products, is shown in Figure 8. It illustrates that the value of propanol replicates the price of crude oil, ethylene and propylene and the dependence between the prices of the raw material and products of interest. When propanol is at the same price per tonne as propylene, there is more value in propanol, since the dehydration to propylene also corresponds to a weight loss in material.

Fig. 8

Importantly, the price of the product depends on the production process as well. In South Africa propanol is produced by Sasol via the Fischer-Tropsch process, which resulted in an average export price of US$765 tonne^–1 (9), compared to the USA where propanol is produced through the hydrogenation of propionaldehyde, resulting in an export unit value (calculated as a ratio between the exported value and the volume exported) at around US$1008 tonne^–1 (9). For more details on the way the values are calculated, the reader can refer to the website in reference.

In a correlation matrix, values can be between –1 and +1. A positive value means that two parameters vary in the same direction. A value close to 0 means that the parameters are relatively independent, while a value close to 1 means that the parameters are highly dependent on each other.

The influence of raw material costs on final product prices in a process can be analysed with the help of a correlation matrix. The correlation matrix in Figure 9 is made for US exports, because the country is producing the three targeted products in large quantities. Thus, we can isolate the price fluctuations of the geo-economic environment to reflect the connections to raw materials.

Fig. 9

The value of exported ethylene correlates less with the value of crude oil (value of 0.76) than with the value of propylene (value of 1). This is most probably due to the fact that recently, ethane crackers have increased ethylene production in the USA. Propanal, which is produced through ethylene hydroformylation, correlates poorly with ethylene (value of 0.52), because most of the production is consumed internally and only part of it is sold on the market. Propanol correlates with the price of crude oil (0.67) and ethylene (0.53), as this is the major market; while propionic acid, used in feed additives, has a rather small market and therefore poorly correlates with feedstocks (values of 0.07–0.17). Products correlate poorly between themselves (for example, propanol and propionic acid has a value of 0.04). This shows that there are opportunities for each product and that a balanced product portfolio is probably a wise strategy. In addition, the targeted products have a high growth potential (pre-COVID-19 estimates), of around 6–8% compounded annual growth rate (CAGR). Therefore, propylene, propanal, n-propanol and propionic acid are deemed good C3 candidates for the C123 technology.

In 2015, the global community committed to limiting warming to “well below 2ºC” and to pursuing efforts to limit warming to “1.5ºC above pre-industrial levels” (1). Global CO₂ emissions must halve by 2030 if we are to have a chance of reaching the 1.5ºC target (2). This will require dramatic transformations in all aspects of energy systems around the world. The UK Government, for example, has responded by enacting a new target of net zero greenhouse gas (GHG) emissions by 2050 (3).

Much more stringent climate targets set worldwide has brought renewed attention on the environmental burdens of aviation. The emissions impact of international flying can be 30 times greater than the low-carbon alternative of international rail per passenger kilometre (4). The global aviation industry is responsible for 2% of all anthropogenic CO₂ emissions, but sectoral emissions are set to grow at an annual rate of 4% along increase in international and domestic air travel demands (5), notwithstanding the impacts of the COVID-19 pandemic. In addition, the warming effect of aviation is doubled due to nitrous oxide and water vapour emissions at high altitudes.

While previous decarbonisation efforts for aviation were provided by other sectors via carbon offsetting schemes (5), this is more difficult to justify under net-zero futures. Decarbonising aeroplanes via new technologies is challenging as high energy density fuel is required for long-distance air travel. Electrification of aircraft has shown some progress as of late, but only for small-scale aircrafts (~20 seats), and it is set to remain focused on short-distance air travel for the foreseeable future (6). Without reliance on early-stage technologies and heavy infrastructure investments, low-carbon replacement (i.e., ‘drop-in’) fuels that meet jet fuel specifications are desirable. Bio-based and synthetic jet fuels have strong potential for decarbonisation. These options would have low or zero CO₂ emissions over their lifecycle. Recognising this, ASTM International, USA, has internationally certified several sustainable aviation fuels (SAF) for commercial use.

Table I lists the five alternative jet fuel (AJF) pathways approved so far for commercial airlines. In all cases, neat AJF must be blended with conventional jet fuel (i.e. fossil-based) before it can meet specific properties and molecular components that of standard jet fuel. At this time, the highest possible blend percentage of AJF is 50% by volume. This limit is expected to be increased over time (8), for example through efforts to modify certified routes and pursue enhanced conversion strategies to include additional hydrocarbon products, such as aromatic content, to allow for greater blending volumes of the AJF product (9). Possible feedstocks are of biomass origin, with varying pre-treatment and conversion processes. FT synthetic paraffinic kerosene (FT-SPK) and FT synthetic paraffinic kerosene with aromatics (FT-SPK/A) pathways are the most favourable options in terms of technology maturity and versatility of feedstock, and can take in virtually any carbon based raw material (10).

Many of the AJF pathways involve synthetic products. The term ‘synthetic fuels’ is widely used as an umbrella definition describing fuels produced from coal, natural gas or biomass through chemical conversion into synthetic crude or synthetic liquid products. Recently, the term is increasingly associated with relatively clean fuels produced from low-carbon feedstocks. These types of fuels have various potential applications across the energy system in line with net zero ambitions (11), with some organisations envisaging up to 15% of final energy consumption from synthetic fuels in their modelling scenarios (12).

There are many processing options in the production of synthetic fuels. Typically, a carbon source is converted into synthesis gas (syngas, a mixture of carbon monoxide and hydrogen) through gasification. This in turn is synthesised into useful hydrocarbons, which are refined or upgraded for end use. Synthesis is usually via either FT synthesis or methanol synthesis. These syngas platforms produce premium alternative fuels that are compatible with existing infrastructure. Much of the current industry and academic focus is on FT synthetic fuels from sustainable feedstocks: biomass (13–15), waste (16–18) and captured CO₂ (19, 20). Therefore, this paper centres on FT jet fuel applications in energy system models.

Energy system models are often used to inform low-carbon energy policies (21). They model the entire energy system from domestic production of fuel resources, commodity processing, to secondary energy carriers and end-use energy service demands across the economy (22). Energy system models balance various interactions, delivering energy services at minimum global cost while meeting GHG targets. Such modelling methods allow for systematic experimentation of multidimensional variables corresponding to climate, technology, economy and policy (23).

In this paper, we examine whether there is a need to improve the representation of the role of synthetic fuels in energy system models, using three models as case studies: UK TIMES, JRC-EU-TIMES and TIAM-UCL. The paper is organised as follows. Section 2 reviews synthetic jet fuel manufacture through the sequence of gasification and FT processes, including all three sustainable feedstocks that can be processed through this route, i.e. biomass, waste and hydrogen and captured CO₂. Section 3 presents current available technologies in the three models and compares model outputs of jet fuel production in decarbonisation scenarios that are consistent with the Paris Agreement. It outlines the role of synthetic jet fuel in aviation according to the models through to 2050. Section 4 discusses the scenario outputs and recommends improvements in model design and evaluates challenges and opportunities for synthetic fuels in the future of energy systems modelling. We draw conclusions in Section 5.

Synthetic jet fuel is produced from biogenic sources (for example, biomass and waste) in three stages. First, syngas is produced through gasification of the feedstock and is cleaned and conditioned. Alternatively, syngas components could be collected from elsewhere, for example by mixing hydrogen from electrolysis with CO₂ from industrial flue gases. Second, middle distillates are produced from the syngas through FT synthesis. Third, the FT liquids (or ‘syncrude’) are refined and upgraded to high-quality jet fuel. Syngas from non-biogenic sources are conditioned to suit FT synthesis and subsequently processed through the same steps as the above. Figure 1 shows the schematic line-up of possible FT routes to synthetic jet fuel.

Fig. 1

2.1 Biomass Conversion to Synthetic Fuels

2.2 Waste Conversion to Synthetic Fuels

2.3 Hydrogen and Carbon Dioxide Conversion to Synthetic Fuels

Although technoeconomic assessments of sustainable synthetic fuels have been performed (9, 35, 36), their role in low-carbon energy systems is not well understood. A recent study (37) touches on the importance of biomass-derived synthetic transport fuels in a global energy system, but is focused on the role of bioenergy and CO₂ removal technologies. A study (38) examined the potential role of FT synthetic fuels in the global energy system as a major alternative energy carrier, but their assumptions on relatively new technologies are outdated or they are not represented at all. European Union (EU) (39) and global (40) energy system modelling studies that focus on the competitiveness of PtL production pathways have model limitations. The global study does not differentiate between synthetic manufacturing processes (such as FT and methanol synthesis), but instead includes a proxy for all synthetic fuels that does not reflect their varying feedstocks and costs. This model also assumes gasoline, diesel and jet fuels are indistinguishable, yet fuel standards for each of these are quite different in reality. The EU study uses the JRC-EU-TIMES model, whose assumptions are examined in Section 3.1. A national-scale study (41) explores the opportunities for power-to-gas and PtL using an energy system simulation model (EnergyPLAN), but here we examine cost-optimisation models as we aim to understand whether synthetic fuels are likely to be economically-viable in the future.

We examine the role of synthetic fuels in three energy system models operating at different spatial scales: TIAM-UCL (global), JRC-EU-TIMES (EU) and UK TIMES (national). The TIMES Integrated Assessment Model was developed by the International Energy Agency (IEA, France) Energy Technology Systems Analysis Program (IEA-ETSAP). This ETSAP-TIAM model has 15 regions representing global decarbonisation to the year 2100 (42). A version was subsequently developed at University College London (UCL), UK, that included a UK region (TIAM-UCL), as well as a number of improvements particularly around resource availability (43). The JRC-EU-TIMES model represents the 27 EU countries and close neighbours (for example Norway, Switzerland, UK) as separate regions (44). The single-region UK TIMES energy system model is jointly developed by UCL and the UK Government, and has informed a number of UK decarbonisation policies including the Clean Growth Strategy (45).

All three models use TIMES model generator, which is developed by IEA-ETSAP (46). TIMES is a bottom-up (i.e. technology-rich) technoeconomic least-cost optimisation model. It is used to identify decarbonisation pathways for energy systems, over long time horizons that meet all projected energy service demands across the economy.

TIMES includes detailed representations of both current and potential future energy technologies. Technologies are characterised by their efficiency (input and output), cost (capital expenditure and operating expenditure) and lifetime. Energy commodities are produced and consumed by technologies, and can be traded between regions. Commodity shadow prices are endogenously calculated through supply and demand curves (47).

In this section, we first examine which synthetic fuel production technologies are included in each model and how they are parametrised, in order to identify whether the approaches and data assumptions are consistent. We then examine a comparable decarbonisation scenario in each model to investigate which of these technologies are deployed, in order to understand whether these technologies might have a substantive role in future energy systems. Through these two analyses, we can ascertain whether there is a need to improve the representation of these technologies in these models.

3.1 Representation of Synthetic Fuel Routes

3.2 Economic Viability of Synthetic Fuel Routes in the Long Term

Synthetic fuels have received little attention in energy system models in the past because of their perceived high costs compared to other decarbonisation approaches, and because there are large uncertainties in the plant cost and performance data. The only exception has been for technologies using fossil fuels as feedstocks, where there are historical precedents based on energy security needs. As climate science has evolved, decarbonisation targets have become more stringent. While synthetic fuels were expected to have at most a minor role in future energy systems with emissions at 60% or 80% below 1990 levels, the JRC-EU-TIMES scenario in Section 3.2 show that they could make an important contribution to net zero systems.

Yet the choice and level of deployment of these technologies varies substantially between the three models. One reason is that there is uncertainty within the modelling community about which of these technologies is likely to be technically feasible, and about the cost and performance of the technologies. UK TIMES represents only a single inflexible FT reactor, with only biomass as a feedstock, and with no CCS option. The value of the plant is further reduced by assuming that only 50% of the output can be biokerosene, with lower-value biodiesel comprising the remainder. This technology has no role in 2050 as biomass is more economically used in negative emission plants. In contrast, JRC-EU-TIMES has a range of plants using both biomass and captured CO₂, and with CCS options. In the JRC-EU-TIMES net zero scenario, the CCS versions of both sets of technologies make important contributions. Given the stringent climate targets, in TIAM-UCL synthetic jet fuels are produced exclusively by FT processes with CCS using biomass feedstock (energy crops, and agricultural and forestry residues). Fossil FT processes are not used but kerosene from crude oil is still produced, with the associated emissions offset by ‘negative emissions’ from bioenergy with CCS (BECCS) electricity generation plants. The availability of other GGR technologies (such as DAC and afforestation) does not reduce pressure on biomass sources but it does change the role of BECCS for climate mitigation (37).

These scenarios suggest a close relationship between negative emissions, CCS and synthetic fuels. This is illustrated by comparing the net zero scenarios in the European and UK models. The JRC-EU-TIMES model assumes that CO₂ sequestration cannot exceed 1000 million tonnes CO₂ per year in 2050, and this limit is reached. For this reason, virtually all sequestration capacity is used for negative emissions and synthetic fuels have an important role. Since the UK has a large CO₂ sequestration capacity compared to the European average, the sequestration limit is not reached in UK TIMES. Natural gas is a substantial source of CO₂, and there is less need for synthetic fuels.

4.1 Improvements to Model Design

4.2 Challenges and Opportunities for the Future of Energy Systems Modelling

Aviation is carbon-intensive and must reduce its emissions to meet current climate goals. Decarbonisation of the sector is challenging, but there are opportunities through switching to low-carbon synthetic jet fuels. Energy system models are valuable for understanding the role of synthetic fuels in climate mitigation. Our evaluation of three models in this paper has identified gaps in technology and input feedstock options for synthetic fuel production. We have identified a variety of potential model improvements to better represent synthetic fuels in the future. This would ideally be coupled with a better understanding of the fuel quality from each production process, and the implications for jet fuel blending.

Model scenario outputs show synthetic jet fuels could make an important contribution to net zero systems. This will mainly rely on improving cost competitiveness compared with conventional jet fuel coupled with negative emissions. Importantly, the scenarios show that there is a close relationship between negative emissions, CCS and synthetic fuels. This means that the share of synthetic fuels in net zero scenarios will also depend on the assumptions made on CCS and negative emissions. Given the stringent climate targets and the long lifetimes of synthetic fuel production plants, further research on synthetic fuels is needed in the context of energy system modelling to fully determine its capabilities in emissions reduction.