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).