The Path to Sustainable Aviation

2 April 2026

By Prof. Don MacElroy (Professor Emeritus in Chemical Engineering, University College Dublin, and Chairman of the NEG8 Carbon Advisory Board)

1. Introduction

In the field of carbon emissions mitigation, it is now widely recognised that aviation is one of the key hard-to-abate sectors. Aircraft require fuels with high energy density, which makes direct electrification to decarbonise impractical particularly for long-haul flights. With this in mind aviation is gradually assuming a leading role, and will continue to do so in the decades ahead, in global efforts to decarbonise human activities. The last ten to twenty years have seen a surge in research and development of technologies towards the production of man-made high energy density jet fuels. These products, more generally known as Sustainable Aviation Fuels (SAFs), are hydrocarbons which fall in the kerosene carbon range C₈-C₁₆.

Currently the technological development of SAFs has proceeded along two primary lines of action: bio-SAF and electro or e-SAF. The former relates to utilisation of bio-resources including cooking and plant oils (leading to hydro-processed esters and fatty acids (HEFA)), agricultural waste, municipal solid waste, woody biomass and dedicated energy crops. A fundamental requirement in the use of these resources is that they do not impede or influence primary agricultural needs (notably food). In a recent IATA report (1) it has been acknowledged that, at most, approximately 60% of SAF needs will be accounted for by the bio-route by mid-century and that the remaining 40% (eSAF sourced through Power-to-Liquid or PtL) will need to come from synthetic processes involving green hydrogen (produced primarily by electrolysis of water) and CO₂ derived from carbon capture.

While the carbon required for SAF production is plentiful, a hard-ceiling faced by biogenic CO₂ arises due to:

• Limited feedstock availability, competing land use and seasonal variability;
• The carbon required is not necessarily in the same location where SAF is to be produced and,
• As already alluded to, an increase in biogenic carbon cannot arise without placing pressure on agrifood production.

The focus of this chapter will be on eSAF and hence the utilisation of non-biogenic carbon whose origins primarily include (i) point source emissions from power generation and industry and (ii) distributed domestic, commercial and transport emissions. Both of these sources have their disadvantages and advantages:

Point source CO₂

• Limitations on purity gas purity
• Require transport to the location of eSAF production
• With the drive to net-zero, it is expected that point source emissions will fall significantly in the coming decades
• The primary advantage of point source CO₂ capture at this time is its relative ease and hence moderate cost

Distributed current and legacy atmospheric CO₂ emissions

• Because of its dilution (430 ppm), CO₂ can be costly to remove from the atmosphere via Direct Air Capture (DAC)
• A major advantage, however, is that DAC can be located directly at the site of e-SAF production
• The captured atmospheric CO₂ is very pure

In the following we will focus only on those eSAF technologies which will require development in the coming decades to utilise purified CO₂ (sourced primarily from atmospheric emissions) and H₂ derived via electrolysis of water (e-H₂ or green Hydrogen). These technologies have been summarized by Neto et al (2) and include:

(1) Indirect CO mediated hydrocarbon production via the Reverse Water Gas Shift (RWGS) reaction followed by Fischer-Tropsch Synthesis (FTS)

CO₂+H₂ = CO + H₂O(g)    (º= +41.2 kJ/mol CO₂ (RWGS))   [1]

nCO + 2nH₂  =  C_nH_2n(g) + nH₂O(g)        (FTS)   [2]

nCO + (2n+1)H₂  =  C_nH_2n+2(g) + nH₂O(g)       (FTS)  [3]

The FTS standard heats of reaction in the range 8 < n <16 are highly exothermic and approximately -163 kJ/mol CO₂

(2) Indirect Methanol mediated route: CO₂ reduced to Methanol followed by olefin synthesis, oligomerization and finally hydrogenation to hydrocarbons as expressed in compact form by reaction [5] below:

CO₂+3H₂ = CH₃OH(g) + H₂O(g)    (º= -49kJ/mol CO₂)   [4]

nCH₃OH(g) +H₂ = C_nH_2n+2(g) + nH₂O(g)   [5]

The standard heat of reaction in the latter case is approximately -73 kJ/mol CO₂.

(3) Direct route: CO₂ converted directly with H₂ to hydrocarbons with either CO or methanol formed directly as intermediates on the surface of a single catalyst:

nCO₂ + (3n+1)H₂  =  C_nH_2n+2 (g) + 2nH₂O(g)   [6]

with a standard heat of reaction ~ -122 kJ/mol CO₂.

2. Indirect RWGS-FTS

The Fischer-Tropsch Synthesis (FTS) process was developed approximately a century ago to convert solid fuels (primarily coal) into liquid fuels. The FTS process itself, now at a TRL (Technology Readiness Level) of 9 came into being as a means to provide much needed hydrocarbon feedstocks during depletion of national resources and/or difficult trading periods. FTS requires synthetic gas (syngas composed of H₂ and CO) to achieve this transformation and in recent years steam reforming or gasification at high temperatures (> 1000K) of solid carbonaceous materials is being partially replaced by utilising non-biogenic CO₂ with e-H₂ in the syngas production step (operating at high temperatures ~ 800-1200K and moderate pressures ~ 1-25 bar). While the latter approach is still at a relatively low TRL (~5-6) it is a key component in the drive towards net-zero with abundant atmospheric CO₂ serving as a valuable feedstock in the production of sustainable fuels.

With the requisite CO₂ and H₂ feeds the combined RWGS-FTS process in its most rudimentary form is as depicted in Figure 1.

Figure 1. The elements of the RWGS-FTS process. A. The RWGS reactor. B The FTS reactor. C. Gas-liquid separator. D Liquid separation unit.

The conversion into liquid hydrocarbons occurs using a catalyst (Fe or Co) and the FTS reactor (typically a fixed or fluidized bed of porous particles) is operated at 200-300 ^oC and 10-50 bar depending on product requirements. As indicated by the nature of the highly exothermic reactions taking place in the reactor (reactions [2] and [3] above) significant levels of heat will be generated and heat management is an important design consideration in the process (e.g. the endothermic RWGS reaction is supported by the heat evolved in FTS). Lowering the temperature favours heavier hydrocarbons while higher temperatures leads to lighter hydrocarbons. Downstream from the reactor the products are cooled, separated and ultimately refined. To satisfy the need for high yield production of C₈-C₁₆ (e-kerosene) the FTS reactor operating conditions are typically ~ 250 ^oC and ~25 Bar using an iron based catalyst (see for example Dell’Aversano et al (3), Li et al (4)).

A key aspect of the hydrocarbon synthesis in FTS is the product distribution and particularly the chain growth probability, , which is subject to the Anderson-Schulz-Flory (ASF) chain length distribution.

where W_n is the mass fraction of hydrocarbons with chain length n. Theoretically a low value of ~ 0.5 corresponds to a high weight fraction of C₂-C₄ hydrocarbons (undesirable for e-SAF) while a value greater than 0.8 will provide a high weight fraction of the desirable C₈-C₁₆ hydrocarbons (5,6). The growth probability is increased towards the e-kerosene range by decreasing temperature, using a lower H₂/CO ratio in the feed to the FTS reactor, increasing the pressure and appropriate catalyst selection with promoters. A clear disadvantage of the FTS process is the resultant wide distribution of hydrocarbon products requiring significant upgrading to e-kerosene. While the lower molecular weight gases and liquids and the high molecular oils and waxes have value, in a production process targeted for jet fuel production they impose a significant burden on the provision of expensive e-H₂ (particularly with regard to energy). For example, Hirunsit et al (7) report that the energy intensity for the provision of H₂ alone to a simulated RWGS-FTS process can reach as high as 80.77 MWh/(tonne C₈-C₁₆) which should be compared with 3.39 MWh/(tonne C₈-C₁₆) required to operate the individual RWGS and FTS processes themselves. This outcome is largely a result of the excess production of C₅-C₇ and C₁₇₊ hydrocarbons during FTS leading to an e-kerosene production rate of just 0.08 kg C₈-C₁₆/kg CO₂ feed. Future developments in catalyst design (see for example (8) and Section 3 below) will certainly improve this situation and significantly lower the cost of e-SAF production which, for RWGS-FTS, is currently priced at approximately €8/kg (depending on renewable energy/e-H₂ costs). Today Jet A1 costs approximately €1.1/kg.

3. Direct CO₂-FTS

Two primary drawbacks of RWGS-FTS technology are (i) the requirement of two separate reaction systems to produce hydrocarbons from CO₂ and H₂ and (ii) the moderate selectivity of the FTS process itself toward the production of hydrocarbons in the eSAF range. Recent work on reactor system design has led to the development of FTS systems which are significantly less complex than RWGS-FTS and which can produce eSAF range hydrocarbons more effectively. The key to this development has been research on tandem catalysts which can perform both the RWGS reaction and the hydrocarbon production steps simultaneously i.e. reaction [6] above. The work conducted to date has been primarily at laboratory scale (so the TRL level is still ~ 3-4) however a wide variety of catalyst materials have been fabricated which can target specific ranges of hydrocarbons in the FTS process as the need arises (9-18).

Figure 2. Hematite nanoparticles with catalytic promoters K and Mn are activated to magnetite which forms the catalyst required to reduce CO₂ to CO in the RWGS reaction locally on the substrate surface. Magnetite itself is carburized forming the Hägg carbide providing the FTS catalytic pathway to the desired hydrocarbons (12).

One proposed tandem scheme is illustrated above in Figure 2 (12). It is postulated that the Fe-K-Mn catalytic system depicted here (simplified for illustrative purposes) promotes the electronic transfers and molecular adsorption/desorption processes required to complete the direct CO₂-FTS process steps to hydrocarbons. The K and Mn promoters are specific to this system and may be replaced by other elements (e.g. Na, Cs or Cu, Zn) however the pairing K, Mn exhibited the highest selectivity (47.8%) for e-kerosene range hydrocarbons in the studies reported in (12). While tandem schemes of the kind illustrated in Figure 2 underpin the basis for achieving the hydrocarbon chain propagation required to enhance the yield of hydrocarbons in the desired e-SAF range (for example, C₅₊ selectivities approaching 60% have been observed in recent work (19)), the resulting distributions are still relatively broad and characterised by modified chain probability distributions similar in form to the ASF distribution (see for example (9, 19)). Molecular modelling (see for example (20) is providing insights into the underlying mechanistic details of the catalytic processes taking place however Krausser et al (21) sound a word of caution from a process perspective with a more complete understanding of the impact of catalyst structure, activity and selectivity needed for optimal process design.

With the relative simplicity of the tandem catalytic concept it is clear that significant savings should be observed in CAPEX (elimination of the separate RWGS reactor as employed in the indirect RWGS-FTS approach) and in OPEX (the endothermic RWGS reaction taking place side-by-side with the exothermic FTS reaction on a molecular scale allows for nanoscale heat management). While the above approach is still at an early stage of development, it is attracting considerable interest. In view of its relative immaturity commercial development of the direct CO₂-FTS process is limited. Only one company (utilising the research reported in (12)), a spin-out from the University of Oxford (OXCCU), reports a roadmap towards commercialisation of modular systems delivering approximately 10 ktpa by 2028 and projects and licensing by 2030 with a projected 50% lower CAPEX than RWGS-FTS and reduced OPEX through efficient heat management (22).

4. Indirect CO₂-Methanol Mediated Route

In this approach (which is frequently referred to as methanol-to-jet fuel (MtJ)) the feedstocks CO₂ and H₂ are initially converted to methanol. Methanol is, itself, a very important industrial chemical feedstock for a wide variety of applications (e.g. plastics, adhesives, synthetic fabrics and pharmaceuticals), not the least of which is as an eFuel in its own right (notably for use in the maritime sector). However for the purpose of eSAF production the methanol enters a process chain as depicted in Figure 3 below. Summaries of the process steps are provided in (2, 3) with extended details provided in (23) and involve, in series: (i) olefin synthesis; (ii) oligomerization and finally (iii) hydrogenation to predominantly C₈-C₁₆ hydrocarbons using specialised catalysts.

Figure 3. A schematic of the Methanol-to-Jet fuel (MtJ) process

 

The methanol synthesis reaction

CO₂+3H₂ = CH₃OH(g) + H₂O(g)   (º = -49kJ/mol CO₂)                                [7]

is conducted in the range 220-280 ^oC and 30-80 bar using Cu/Zn/Al catalysts.

Olefin synthesis

2CH₃OH(g) = (CH₂)₂ + 2H₂O(g)   (º = -15 kJ/mol CO₂)                                [8]

occurs at a significantly higher temperature but lower pressure using a microporous zeolite catalyst (e.g. H-ZSM-5).

Oligomerization

(1/2) n (CH₂)₂ = C_nH_2n(g)   (º ~ -47.5 kJ/mol CO₂)                                                   [9]

takes place with a return to lower temperature and higher pressure reaction conditions in the presence of, for example, a catalyst of the Nickel based silica-alumina or H-ZSM-5 class of materials.

Hydrogenation

C_nH_2n(g) + H₂ = C_nH_2n+2(g)   (º = -10.4 kJ/mol CO₂)                                     [10]

also proceeds at conditions similar to oligomerization but using a reduced metal catalyst (e.g. Ni, Pd or Pt supported on alumina).

Each of the reactions listed above are exothermic to a greater or lesser extent and hence favour lower temperatures and, with the exception of the olefin synthesis reaction with increasing moles in the products, higher pressures. The integrated process indicated in Figure 3 does not currently exist although each of the individual stages form part of the operations at a variety of sites globally. At this time a number of companies (e.g. ExxonMobil (24), Topsoe (25)) are investigating this technology with the possibility of commercial production within the next five years.

While the MtJ process is relatively complex involving multiple reaction systems and concomitant intermediate separation processes it does possess a number of advantages as an approach toward the production of eSAF. Most notably MtJ is a targeted process (hence it is not subject to the broad ASF product distribution of FTS) producing eSAF at high yields with only modest by-product formation outside the C₈-C₁₆ range. A case in point is the simulation work of Hirunsit et al (7) which demonstrates that the energy intensity of the MtJ process is predicted to be 3.5 times lower than the RWGS-FTS process due to its efficient utilisation of H₂ feedstock. Under optimum process conditions this route towards eSAF can also limit production of undesirable compounds which appear in fossil based fuels and hence lead to a ‘cleaner’ aviation fuel.

5. Direct CO₂(Methanol Mediated) Route

In contrast to the direct CO₂-FTS route, an approach in which all of the four steps outlined in Figure 3 are combined in some form using tandem catalysts (i.e. a CO₂-MtJ approach) has not yet been developed and is unlikely to be until the mid-2030s or early 2040s. This arises largely due of the complexity of the reaction route and kinetics, the sensitivity of the mechanism of the transformation of CO₂/H₂ to the desired eSAF product range to the catalyst(s) properties including composition, active site distribution and support morphology and finally the management of intermediates and by-products (e.g. CO and water) as they arise during the sequence of the reactions. A number of recent research studies in this area are summarised by Neto et al (2) and Elwalily et al (23) discuss possible opportunities for integrating pairs of the reaction steps [7] – [10] to simplify the process layout including:

1. Combining reactions [7] and [8] to achieve the direct conversion of CO₂ and H₂ to olefins can simplify the overall process and reduce energy demand at a temperature and pressure intermediate to those for the individual steps (see for example (26) in which a mixed metal oxide/zeolite catalyst was employed). However, research is required to develop efficient multifunctional catalysts to limit the negative effects of by-product water on catalyst activity and to enhance carbon-carbon bond formation.
2. Integrating the oligomerization and hydrogenation steps into one process utilising zeolite supported hydrogenation catalysts (e.g. Ni or Pd) is also another possibility. The indicated operating conditions for the individual steps [9] and [10] imply this is achievable and can enhance energy efficiency. However it does eliminate recycling of low molecular weight olefins and can lead to by-product spread in the chain length distribution of final products.

Finally, in view of the value of methanol in its own right as a chemical feedstock for a variety of processes and as an eFuel (.e.g. for shipping), it may also be worthwhile focussing on development of an integrated process involving just the three reactions [8]-[10] rather than the more complicated CO₂-MtJ approach.

Conclusion

Of the four approaches outlined here for the production of eSAF from non-biogenic CO₂ and green hydrogen the indirect methanol-mediated route (MtJ) is the most mature although the direct CO₂-FTS approach is developing rapidly. At this time a key challenge in either case is the availability and cost of e-H₂. The route to green H₂ is predominantly through electrolysis of water with alkaline electrolyzers (AE), proton exchange membrane electrolyzers (PEM), solid oxide electrolyzers (SOEC) or anion exchange membrane electrolyzers (AEM). The IATA (1) projected requirements for e-SAF production worldwide by 2050 are 176 Mtpa. The e-H₂ requirement for this estimate to be met is in the range 25-50 Mtpa under ideal conditions (efficient H₂ feedstock utilization) which would in turn require between 1500 and 3000 TWh of electrical energy (based on the above electrolyzer technologies).

From the perspective of Ireland’s needs, if we can assume that only long-haul flights to and from Ireland by mid-century operate using IATA’s proposed mix of bio-SAF and eSAF then to satisfy Ireland’s annual eSAF needs, the energy required to produce the e-H₂ required (based on today’s jet fuel demands) would be ~ 1.9-3.1 TWh (~0.22-0.35GW). Employing the DAC technology developed by NEG8 Carbon to source pure CO₂ and with the availability of waste heat, then the CO₂ feedstock required for eSAF (0.78 Mtpa) on long-haul flights would require 0.35 TWh electrical energy i.e. the feed to an eSAF facility would require between 2.25 and 3.45 TWh annually (5-10% of total electrical energy consumption) to maintain this component of the production process.

This is not an inconsiderable demand in energy and it is clear that significant efficiency improvements are required in the process technologies associated with eSAF itself and the availability of abundant renewable energy (particularly low cost off-peak and/or dispatch-down power) if these requirements are to be met.

For more:

• What is eFuel?
• What is eMethanol?
• Sustainable Aviation Fuel (SAF)
• Methanol-to-Jet Fuel for Sustainable Aviation
• Lift Off for NEG8 Carbon in Sustainable Aviation Fuel
• CORSIA vs EU ETS

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