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

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

Fig. 1

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

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

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

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

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

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

(i)

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

Fig. 2

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

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

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

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

Fig. 3

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

Fig. 4

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

Fig. 5

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Fig. 3

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

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

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

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

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

Fig. 4

3.2.1 Direct Combustion

3.2.2 Fuel Cell Systems

3.2.3 Catalytic Processes Involved in Ammonia to Power

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Fig. 3

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

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

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

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

Fig. 4

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

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

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

Fig. 5

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

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

Fig. 6

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

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

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

Fig. 7

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

Fig. 8

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

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

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

Fig. 9

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

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

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

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

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

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

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

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

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

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

Fig. 1

2.1 Biomass Conversion to Synthetic Fuels

2.2 Waste Conversion to Synthetic Fuels

2.3 Hydrogen and Carbon Dioxide Conversion to Synthetic Fuels

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

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

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

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

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

3.1 Representation of Synthetic Fuel Routes

3.2 Economic Viability of Synthetic Fuel Routes in the Long Term

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

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

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

4.1 Improvements to Model Design

4.2 Challenges and Opportunities for the Future of Energy Systems Modelling

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

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

In addition to the CO₂ conversion technologies described in Part I (1), there are several other approaches that are being investigated, although in general these are currently at a lower technology readiness level (TRL). With the sun being a desirable source of renewable power, the direct use of solar energy to produce chemicals from CO₂ akin to artificial photosynthesis is instinctively appealing. This is exploited in photochemistry using a photocatalytic (PC) reactor, which uses photons from the visible and ultraviolet spectrum to generate electrons from irradiating photosensitive semiconductors and transfers the electrons from a valence band to a conducting band in connection with a photocatalyst to promote the reduction of CO₂. Thus, an immediate advantage stems from not converting the solar energy to electricity first to supply the electrons, which brings in the associated upstream inefficiencies, as in the power-to-X type of conversion routes or electrochemical reduction. Commonly observed products from the photochemical reduction of CO₂ include CO, methanol, formic acid and methane (Figure 1). However, the process is complex to understand involving many mechanisms, especially for multiple proton and electron transfers for products such as methanol and methane with ‘CO₂ activation’ being a rate-determining step for many products. Much research effort is being concentrated on developing photocatalysts from earth-abundant, low-cost and less-toxic catalysts such as those based on iron, nickel, manganese and copper that are more scalable than those metals on the second and third row of the Periodic Table, such as rhenium, ruthenium and iridium (3). There has also been recent work looking at using metal-free photocatalysts such as graphene, nitrides and carbides (4). Using solar energy for CO₂ conversion does impart a degree of inflexibility regarding the energy source compared with for example an electrochemical route, with the energy conversion efficiency being key for cost and scalability. Although much promising research is being conducted in this area, the CO₂ conversion rates often observed in such systems under development appear impractically low for industrial implementation with some of the technical research challenges resting with developing photocatalysts that promote a high CO₂ reduction efficiency and product selectivity (5, 6).

Fig. 1

The attributes of photochemistry and electrochemistry can be combined in the approach known as photoelectrochemistry, conducted in a photoelectrochemical (PEC) cell, and has been investigated for CO₂ reduction to fuels and chemicals. A major benefit over a pure electrochemical approach is that a significant part of the energy necessary for the conversion is supplied by light, omitting or at least lowering the transmission and rectification losses associated with using solely an electrical supply (7). Fundamentally, PEC cells comprise an anode and cathode immersed in an electrolyte, with at least one of the electrodes being a photosensitive semiconductor that absorbs light and through photoexcitation provides some of the required electron flow (the current). The technology still faces challenges from complex reaction pathways, mass transfer conditions and large photovoltage requirements to generate sufficient product selectivities and solar-to-chemical efficiencies (8).

A process that would utilise renewable electricity for CO₂ conversion is non-thermal plasma (NTP) technology using a configured plasma reactor. Similar to a low-temperature electrochemical reactor, a plasma reactor has the ability to be switched on or off in response to the intermittency characteristic of the energy supply and requirements for grid-balancing. It also exhibits high reaction rates, attains steady-state quickly and can produce a range of hydrocarbon and oxygenated species, operating at near atmospheric temperature and pressure. The ability to enable thermodynamically uphill reactions to occur at ambient conditions arises from the generation of a low temperature ionised gas with electrons energised by applying an electric field to typically between 1 eV and 10 eV. This range is ideal for exciting molecular and atomic species and breaking chemical bonds, with the OC=O in CO₂ requiring 5.5 eV (9). Different types of plasma are being investigated for CO₂ conversion, involving various reactor configurations and how the electricity is supplied and its power rating, with the main three being dielectric barrier discharge (DBD), microwave (MW) plasmas and gliding arc (GA) plasmas, generating different performance for CO₂ conversion (10). The DBD configuration is probably the most widely investigated in this area. Some advantages and disadvantages of the main plasma technologies for CO₂ reduction are presented in Figure 2.

Fig. 2

One of the main CO₂ conversion reactions studied is CO₂ splitting and dissociation to CO and molecular oxygen, with CO being potentially used as a product itself or as a feedstock for fuels and chemicals. Other reactions involve a hydrogen source co-reactant such as methane (dry reforming of methane mainly to syngas but also forming hydrocarbons and oxygenates), hydrogen or water (hydrogenation to added-value fuels and chemicals). Akin to many processes the energy efficiency, reactant conversion and product selectivity are important parameters to evaluate from experiment and target achieving sufficient values (for example energy efficiency of >60% (10)) to merit further scale-up towards industrial commercialisation, with the specific energy input (SEI) being considered an important factor for influencing such performance for plasma technologies. NTP naturally involves radical-based chemistries, so improving selectivity of the desired product and lessening the burden on downstream separation costs is undoubtedly a challenge. The reported product yields and energy efficiencies for producing added-value hydrocarbons and oxygenates are generally lower than for CO and syngas type products, with the presence of molecular oxygen (from CO₂ dissociation) leading to the undesired oxidation of formed hydrocarbons (12). The use of heterogeneous catalysts such as metal oxides or zeolites to improve performance including selectivity is one such option, notably used in a packed-bed DBD configuration (13, 14). Any observation of high energy efficiencies appears to come at the expense of low conversions with augmenting both representing a long-term challenge involving further understanding of the complex plasma chemistry (11, 15). As with many novel CO₂ conversion reactors, the scale-up of plasma technology would lend itself to being modular, addressing the needs of small-scale, distributed production. When considering scale-up there will however be the need for hazard management regarding the in situ generation of molecular oxygen from the CO₂ together with flammable products from the CO₂ conversion, with potential ignition sources from the plasma generation equipment requiring the necessary safeguarding.

In general, the technologies described in this section are at lower TRLs than those mentioned in Part I of this review (1), especially when compared with the thermochemical routes. They each share some of the overall performance challenges of improving the energy efficiency and product selectivity to make them viable as contenders for future industrial scale-up. The use of realistic CO₂ feeds should be evaluated at the laboratory scale to ascertain the performance impact of inert or reactive gaseous components and seek to define tolerable catalyst poison levels, depending on the CO₂ source. Supporting studies such as technoeconomic assessment (TEA) and life cycle assessment (LCA) at the early stage are also recommended for these technologies to assist in understanding their viability for further development in the CO₂ utilisation space.

Microbial electrosynthesis (MES) is a combined biological and electrocatalytic process, where reducing energy cells in the form of electricity is provided to microbial for the synthesis of organic materials from CO₂. In MES, bioreactors fitted with electrodes transfer electrons to host organisms via hydrogen, a reduced mediator, or by direct transfer (Figure 3). This is thought to be an efficient means of providing reducing energy compared to the classic fermentative routes (16).

Fig. 3

MES was first demonstrated in 2010 with the production of acetate from CO₂ by a biofilm of the acetogenic Sporomusa ovata on a graphite cathode, which generated hydrogen (17). This was at a rate of 1.6 g acetate m^–2 day^–1 (relative to the cathode surface area). Since then, several examples using different wild type or engineered microbes converting CO₂ to chemical products have been published. With improvement in electrode design, microbial engineering and strain selection, improved yields of acetate from CO₂ have been achieved. Using a mixed culture biofilm and three-dimensional macroporous cathodes, acetate production at 685 g m^–2 day^–1 was demonstrated (18).

Apart from acetogens, the Knallgas bacterium Cupriavidus necator, which can use hydrogen or formate from the cathode as a source of reducing energy, has also been used in MES. Through genetic engineering and MES, C. necator has been used for the production of branched chain alcohols, alkanes and terpenes.

The use of MES for CO₂ conversion is still far from industrial use due to limitations in productivities and expense. Electron uptake by microbes is low, needing a wider cathode surface area and to be made with more biocompatible and cheaper materials. Limitations of the biological pathways themselves (slow carboxylation kinetics, high acetate products when using acetogens) also lead to low titres. How successfully these bottlenecks are addressed will determine whether MES will be more widely adopted as a means of CO₂ fixation in the future (16).

For the majority of C1 molecules, well developed routes exist for their manufacture from syngas which can be adapted to use CO₂ and renewable hydrogen to give a sustainable process. It is therefore hard for new processes to displace these existing technologies. Even in the case of a product such as formaldehyde, which is made commercially through highly efficient methanol oxidation processes (19), it is difficult to imagine a new process displacing the two well-established methods.

There are a couple of exceptions to this thinking. The first is the production of CO or syngas, which are very important intermediates in the production of a range of fuels and chemicals. The hydrogenation of CO₂ to CO is not a well-established technology: it runs at high temperatures and suffers from equilibrium limitations, problems with carbon laydown and formation of unwanted hydrocarbons. Hence there is an opportunity to use new processes such as electrochemical CO₂ reduction to drive these reactions.

Another molecule which could be of interest to develop new routes to is formic acid. Currently, formic acid is largely produced by hydrolysis of methyl formate, which is in turn made by carbonylation of methanol (20). Direct routes from CO₂ have been developed based on ruthenium homogeneous catalysts and on a range of heterogeneous catalysts (21) as well as electrochemical methods (22, 23). One challenge here is in the low current demand for formic acid. Its current uses are mainly as a preservative for animal feeds and silage, as well as in the leather and textile industries (20). Proposed future uses include as a fuel, a hydrogen storage and transport medium and also as a feedstock for biological processes to make higher carbon chain length products. Given the toxic and corrosive nature of formic acid (20), it remains to be seen to what extent these applications are realised.

The next classification of molecules to consider is those with a few carbon atoms. The key challenge here is typically selectivity to the chosen product. Unlike for C1 atoms, there are very few well-established catalytic processes for selectively converting C1 feeds such as syngas into C2–C4 products. The exception to this is dimethylether, which is produced in two steps from CO₂ by dehydration of methanol using acidic catalysts such as zeolites or alumina (24). Dimethylether is unusual as a C2 molecule which does not contain a C–C bond. It is proposed as a fuel of the future due to having similar combustion properties to diesel, although as a gas at room temperature it would require handling in a different way to current fuels.

Ethylene is an important intermediate in the production of chemicals, notably polyethylene but also a wide range of other molecules, and so a route from CO₂ to ethylene would be very useful to decarbonise ethylene production. There is no established direct heterogeneous or homogeneous catalytic conversion route from CO₂ to ethylene. By far the most progress has been made with electrochemical conversion, where in contrast to other molecules such as methanol, ethylene has been made with 70% Faradaic efficiency (FE) over copper catalysts (25, 26) at practical current densities of 500–1100 mA cm^–2. With further commercialisation, this could provide a viable pathway to a range of fuels and chemicals using ethylene as an intermediate. The main competing technology is a two-step method using the methanol-to-olefins process.

A range of other C2–C4 molecules have been prepared using electrochemistry, but only at laboratory scale and generally as part of a mixture of products. For example, Opus 12 reports having made eleven different compounds (27) in this range. Of other molecules formed electrochemically, ethanol is the best studied but is still only reported in modest selectivity. Ethanol is also reported as being produced by heterogeneous catalysis and biological approaches (see box).

Beyond C2, propanol has been reported as being made electrochemically from CO₂ on a molybdenum sulfide electrode (28) but only at <5% FE. At C4, biological systems can be very efficient in producing a number of molecules, particularly oxygenates such as butanediols or butyrates. Some systems are able to produce C3 molecules such as propionates, but generally even number chains are preferred due to the C2 coupling mechanism employed.

For larger molecules, direct synthesis from CO₂ becomes increasingly challenging. Biological systems offer perhaps the best route to making C6 molecules, but only a small number of examples have been published. However, the biological approach seems to offer a more selective direct conversion than is possible by other routes such as catalysis or electrochemistry. Interestingly, a number of the examples of C6 and even C8 production have been given through MES (29, 30).

As the product’s carbon chain becomes longer, using coupled processes becomes more relevant (Figure 5). It is important to note that not all molecules need to be accessed in a single step from CO₂, and in many cases it will be preferable to use two efficient steps rather than one less efficient one. Some examples of this are already well-known, for example reduction of CO₂ to CO which can then be converted to fuel range hydrocarbons by the Fischer-Tropsch (FT) process.

Fig. 5.

Efficient processes to make larger molecules tend to be based on a biological coupling of C1 or C2 molecules which can be prepared in different ways. Catalytic coupling processes such as Guerbet coupling of ethanol (31) or FT coupling of syngas tend to give a range of products, which is acceptable for end uses in fuels and in some cases solvents, but not for the majority of applications. Two-step processes with a second biological step can be based on a wide range of feedstocks, including formic acid (32), methanol (33) or syngas (34). An advantage of this approach is that all these molecules can be made readily by catalytic or electrochemical reduction of CO₂, enabling efficient processes. One possible concern is that the feedstock can be consumed by the microorganism and be converted back into CO₂. Approaches which have the potential to overcome this include MES, where feeding (renewable) electrons into the reaction supplies the microbes with energy without feedstock being consumed.

There are many viable routes to convert CO₂ using sustainable technologies, and many products are accessible, from C1 through to C10 and beyond. Different technological approaches suit different molecules (Figure 4) and have their own advantages and drawbacks. The use of two-step processes widens the range of molecules even further, and in some cases allows more energy-efficient syntheses to be developed. Key to making sustainable processes based on CO₂ is the supply of renewable energy as light, electricity or another source. In the case of electricity, intermittency is an important issue when power from solar panels or wind turbines is used. Chemical processes in general do not like to be switched on and off or turned up and down, although low temperature electrochemical and biological processes are more tolerant to this than most. However, low utilisation rates will stop processes being viable from a capital cost perspective.

Comparing Methods for Conversion of Carbon Dioxide into Ethanol