Syngas To Jet A1 - The Production Of Renewable Aviation Fuel
Abstract
Growing concerns over the environmental impact of the aviation industry has acceleratedthe search for a renewable, ’drop-in’ fuel that can reduce the industry’s current dependence onkerosene based Jet-A1. Much of this effort has focussed on producing a fuel from a sustainablesyngas source, either by Fischer-Tropsch synthesis or alcohol synthesis. Many aspects of thesetechnologies are well understood, as they have been used before in other petrochemical applications. Producing the syngas from a renewable, biomass source could lead to considerable reductions in the overall greenhouse gas emissions by jet fuel. However, the overall feedstockto fuel pathways for a biomass jet fuel are still in development and the commercial viability ofsuch a fuel will be heavily dependent on a number of external market factors. Therefore, while these fuels may be used in the future, they have little commercial prospects in the short term.
Introduction
Recent scientific studies have suggested that an 80% reduction below 1990 levels of global green-house gas emissions (GHG) will be required by 2050 to stabilise the climate and avoid the mostdestructive impacts of climate change. GHG emissions from aviation represent 3. 5% of the total anthropogenic radiative force, with a predicted to rise to 5% by2050. Although this is a small proportion of overall emissions, the industry’sreliance on a single non-renewable fuel has prompted mounting pressure for the developmentof renewable jet fuels (RJF).
Jet A is the principal grade of aviation fuel burnt in commercial aircraft in the United States, while Jet-A1 is an almost identical grade used in the rest of the world. Jet-A1 is made fromsweet, non-waxy crudes by atmospheric distillation. As demand for commercialaviation grows, research is focussing on the development of a sustainable, ”drop in” fuel, whichwill be able to replace traditional Jet-A1 fuel with little to no changes in existing infrastructureor engine technology required. This literature review focusses on previous and ongoing work to replace petroleum-based Jet-A1with a synthetic aviation fuel produced from syngas made from a biomass source. It aims tooutline the manufacturing routes that such a process might employ, understand the technical and commercial risks involved, and finally to analyse the environmental impacts of producing a ”green” aviation fuel.
Manufacturing routes
Fischer-Tropsch synthesis
The Fischer-Tropsch (FT) process involves the synthesis of liquid fuels from syngas, which isa mixture of carbon monoxide and hydrogen. The syngas can1be produced from carbonaceous feedstocks, including coal, natural gas and biomass. Once purified, a catalyst such as iron or cobalt is used to convert the syngas to long-chain paraffins. Hydrocracking is used to convert these to the desired chain length required forjet fuel. This fuel is known as synthetic paraffinic kerosene (SPK). Currently, SPKs are frequently use by commercial airlines in a 1:1 blend with petroleum-derivedkerosene. Sasol has supplied such a fuel, with the synthetic component made from coal via F-Tsynthesis to the Johannesburg Airport since 1999. However, these SPKs are produced from syngas made from fossil fuel based sources. Thereis no commercial scale production of F-T fuels using biomass as the feedstock, although ademonstration plant in the Netherlands has produced a low sulfur fuel oil from a biomass-derived syngas. This also produced synthetic natural gas and electricity as co-products of thefuel oil, which could help to increase the profitability of such a manufacturing route. Several promising pilot plants have been built in the past, only for plans to beabandoned. However, these pilot projects have allowed the fuel to be certified for commercialuse. Moreover, several airlines, including have established collaborations with the remainingdemonstration projects - a positive sign for the future of this manufacturing route.
The F-T process for producing fuel is well established, and a new plant could call on pre-vious research and industry experiences, helping to mitigate some technical and commercial risks. However, there are a few large companies that hold many of the patents associated with process using F-T synthesis to convert syngas to liquid fuels. This can be a costly barrier tosmaller companies wishing to enter the market.
Alcohol synthesis
et-A1 can be produced from a biomass source by first converting syngas to alcohols, and then producing jet fuel from this alcohol. This pathway is often referred to as alcohol to jet (ATJ). The alcohol can be produced in one of two ways - by catalytic synthesis or by fermentation. One of the advantages of the ATJ pathways is their ability to produce alcohols produced fromdiverse feedstocks. This could result in a process less vulnerableto feedstock shortages, than, say, a process based on FT synthesis.
Catalytic synthesis to produce ethanol
This method involves converting syngas to methanol and ethanol, which are converted to paraf-fins and aromatics via a dimethyl ether intermediate. The dimethyl ether is reacted over acatalyst to form jet fuel range hydrocarbons and aromatics. The synthesis ofthe alcohol is conducted under high pressure (>40 bar) and at a temperature of 250-320 ◦C inthe presence of a catalyst.
Fermentation to produce ethanol
A LanzaTech led team has demonstrated that jet fuel can be produced by biomass syngas fer-mentation followed by catalytic conversion. This process is less capital and energy intensivethat the FT process. This process has a number of chemical co-products, such as butadieneand gasoline. This has the potential to reduce the cost of production for RJFs. Following gasification and clean up steps, the syngas is introduced into a gas fermentationbioreactor containing microbial biomass suspended in a nutrient broth. The bacteria contin-uously secrete ethanol and the broth is continuously distilled for ethanol recovery. An anaerobic bacterium known as Clostridium ljungdahlii is used. One advantage of this process is that a specific CO/H2 ratio is not required for optimalfermentation rate of reaction.
Catalytic upgrading (using standard refinery unit operations) removes oxygen from the alcoholand extends the carbon chain length, creating the desired blend of hydrocarbons for jet fuels. The ethanol is first dehydrated to produce ethylene. This is oligomerized to produce longer-chain olefins, which are then hydrogenated and fractioned into the desired jet fuel. The individual technologies required to convert syngas to alcohol are considered tobe mature, as they are widely used in other petrochemical applications. However, the completefeedstock-to-fuel process chains are still in development. This process has not been previously employed on a commercial scale. However, both LanzaT-ech and INEOS Bio have operational demonstration plants which employ fermentation processesto prodce alcohols from waste and other organic materials. This ethanol is then transported toanother site where it is converted into jet fuel.
Technical risks
Fischer-Tropsch synthesis
Choosing an appropriate catalyst and operating conditions for this process is challenging. TheF-T process can be operated at high temperatures (between 300 ◦C and 350 ◦C), using an ironcatalyst, or at lower temperatures (between 200 and 240 ◦C) at which iron or cobalt catalystscan be used. The process is highly exothermic, which influences the efficiency of the process.
Cobalt catalysts are preferred for the synthesis of paraffins, as they are more stable towards de-activation by water and syngas is not consumed in the water-gas shift reaction, as is the case foran iron catalyst. They are often preferred for a biomass based syngas source. However, iron catalysts are cheaper and have lower methane selectivity and lower sensitivity topoisons. Jet-A1 is required to meet certain specifications - for example, it must have a freezing pointwithin a certain range. To do this, the aromatics and naphthenes should be increased in theresulting jet fuel. This requires multiple processing steps, which can increase the cost of theproduct and decrease the efficiency of the process. This inefficiency can be reduced by us-ing a multifunctional K-Fe-Co-Mo-γ-Alumina catalyst but these technologies are still underdevelopment.
Alcohol synthesis
The syngas to methanol reaction is highly exothermic, and special reactors must be designed to efficiently remove heat from the system. The conversion of syngas to alcohols is usually carried out in the presence of a copper catalyst. However, these catalysts can last as little as two years, as they undergo slow deactivation by sintering and poisoning by sulfur. This increases the operating expenditure of the process - either the catalyst must befrequently replaced or the syngas must be purified to a very low sulfur concentration. Very large amounts of fresh water are required for the operation of a bio-refinery which employsthis process. Water management would be a considerable challenge for this sort of plant, withconsiderable environmental impacts.
Although this manufacturing route outperforms the Fischer Tropsch route in terms of car-bon utilization and thermal efficiency, the expected jet fuel price is higher, due to the highercapital expenditure required. Further developments in the selectivity of catalysts against lighthydrocarbons will be required to allow for a decrease in the size of equipment.
Commercial risks
EIA projects that jet fuel consumption by commercial carriers will continue to grow over thenext years, the rate of which will depend on economic growth and oil prices. However, forbiomass based jet fuel to be successful, it must be cost competitive with petroleum-based fuels. The capital investment required for a commercial-scale renewable jet fuel plant could be inthe region of US $ 100 million. A study by the European Commission showed that in 2013, renewable jet fuels were about two to three times more expensive thanpetroleum based fuels.
De Jong et al. (2015). This price differential may be about to decrease, as governments impose policies that make fossil fuels more expensive and the economics of theoil market causes the price of kerosene based fuels to rise. In 2012, airlines flying to Euro-pean airports began having to add the cost of carbon dioxide emission allowances to the costof buying jet kerosene. Higher costs for extraction of oil, due to new fields being located indeep or complex locations, are exerting upward pressures on oil prices. Environment penalties in other transport sectors, such as shipping, could lead to these sectorsturning to kerosene as an alternative fuel source, which could increase prices for this crude cut. However, even by 2020, rising costs of carbon credits and fossil fuels may notbe sufficient on their own to make renewable jet fuels economic.
The commercial viability of the plant will be heavily dependent on feedstock costs, whichcontribute heavily to the cost of production Rising food prices today indicate increasing com-petition for arable land, while other forms of transport and power generation are resulting inincreased demand for energy crops such as wood pellets. This is likely to cause a rise in thecost of feedstocks required to produce renewable jet fuels.
Environmental impact
Life cycle analysis
Fossil fuels are made from geologically sequestered carbon sources, which is released as car-bon dioxide when they are burnt. Biomass feedstocks absorb carbon dioxide when they grow and release an equal amount during combustion. This means that renewable jet fuels have a”biomass credit” that offsets the combustion of carbon dioxide in the life cycle analysis. However, there are many factors other than combustion which affect the footprint ofa fuel. For example, Wong has demonstrated that land use change, necessary for increased biomass production, has the potential to release significant greenhouse gas emissions.
Strattan reports than biomass to liquids (BTL) plants without carbon capture have life cycleGHG emissions that are less than 20 % of conventional jet fuel. However, suchcalculations are complicated, and cannot be considered exact or objective.
Particulate matter emissions
Studies have shown reductions in particulate matter emissions of 39 % and 62% respectively for50 % and 100 % F-T based fuels respectively when compared with a conventionally produced JetA1. Biofuel refineries are much cleaner than those of crude oil, which release4millions of pounds of cancer-causing chemicals such as benzene, butadiene and formaldehydeand other pollutants which cause heart disease and asthma into the environment during theconversion into usable products.
Conclusions
A significant amount of time and energy has been invested into finding a sustainable alternativeto the fossil fuel based Jet-A1, particularly on the production of jet fuel from a biomass-basedsyngas. Two viable manufacturing routes have emerged from this work - Fischer-Tropsch syn-thesis and alcohol synthesis. Alcohol synthesis involves converting syngas to jet fuel via ethanol and can be performed by acatalyst or by fermentation, while the Fischer-Tropsch process converts the syngas directly fromgas to liquid fuels. Although the Fischer-Tropsch process has the advantage of being widelyused in existing fossil fuel based jet fuel production, with the resulting fuel already certified foruse in commercial aviation, market entry may be difficult as a small number of large companieshold the patents to many of the technologies involved. In contrast, most alcohol synthesis plantsare still in their pilot phases and more research and testing will be required before the fuels canbe used by commercial airlines. However, this does open up the opportunity for a new playerto establish their dominance in the market.
There would be significant environmental benefits associated with switching from conventionaljet fuel to a biomass-based renewable jet fuel. Not only would this ensure a sustainable source, it would result in reduce greenhouse gas emissions over the life cycle of the fuel, as well asreduced particulate matter emissions.
There are a number of technical risks associated with both processes. The Fischer-Tropsch process is energy intensive, especially if the resulting fuel is to have appropriate characteristicssuch that it can be blended with conventional jet fuel. In contrast, the alcohol synthesis routeis water intensive and water management would be a significant challenge if this route were tobe used. Both processes suffer from expensive operating costs due to catalyst expenses.
The commercial viability of such a plant is not certain. Although the jet fuel consumption by commercial carriers is projected to grow into the future, profitability would be heavily de-pendent on the ready availability of cheap feedstock and high crude and natural gas prices. Given the volatility of these markets, it might be difficult to justify the large capital investment required for a commercial-scale renewable jet fuel plant. As other transport sectors turn to more sustainable fuel sources, the aviation industry willno doubt face pressure to conform. Producing jet fuel from biomass syngas would yield nu-merous advantages. However, a number of technical and commercial challenges will have tobe overcome before such a fuel can be used on a large scale in commercial aviation and thelength of time that this takes will depend on ongoing scientific research and external market conditions.
