Alternative jet fuels can be produced from a variety of feedstocks including renewable biomass, waste or fossil feedstocks, such as coal and natural gas. The focus here is on sustainable alternative fuels that have the potential to reduce life cycle CO2 emissions.
The figure below presents a simplified view of pathways for the production of sustainable alternative fuels. Only the routes that have already been approved or that are currently being submitted for approval to ASTM are represented.
Please click on image to enlarge.
There are mainly three families of bio-feedstock that can be used to produce alternative fuel jet fuels: the family of oils and fats, or triglicerides, the family of sugars, and the family of lignocellulosic feedstock.
Fuels made from oils and fats
Triglicerides currently come largely from oil crops, animals fats and used cooking oil. Production from micro-algae is an additional promising pathway that is currently in the research and development stage. Triglicerides contain oxygen that needs to be removed in order to produce jet fuel components, as those are pure hydrocarbons. Different processes are proposed for this, but the currently approved method is the
Hydroprocessed Esters and Fatty Acids (HEFA) process.
Fuels made from sugars and starch
Sugars come from sugar crops and cereals starch. They are mainly associated with fermentation routes that generally produce alcohols, which are further upgraded into hydrocarbons. This is the “alcohol-to-jet” route. Advanced fermentation has also been developed that directly produces hydrocarbons which can be upgraded into jet fuel components. It should be noted that fermentation has also been developed from industrial waste gas in the form of carbon monoxide. Cultivation of algae is also a way to use waste gas to produce feedstocks: CO2 is indeed needed to grow algae. Currently approved methods that follow these processes are
Synthetic Iso-paraffin (SIP) (formerly referred to as Direct-sugar-to-Hydrocarbon (DSHC) and
Alcohol to Jet Synthetic Paraffinic Kerosene (ATJ-SPK).
Fuels made from lignocellulose
Lignocellulose is found in the wall of plants’ cells and in wood, and come from various energy crops, as well as from agriculture or forest residues and from macro-algae. Lignocellulose can be directly converted into hydrocarbons using thermochemical processes such as
Fischer-Trospch (FT), pyrolysis or catalytic cracking. The Fischer-Tropsch processes (Synthetic Paraffinic Kerosene (FT-SPK) and
Synthetic Kerosene with Aromatics (FT-SKA)) can also be used to convert municipal solid wastes, coal or natural gas.
Lignocellulose can also be transformed into sugar and can thus be used for the aforementioned fermentation routes. In a similar way, sugars can be transformed into oil by yeast or micro-algae and thus further processed into jet fuel through deoxygenation.
Additional advanced routes
Additional routes are also being studied to produce alternative fuels directly from CO2, including CO2 captured from the atmosphere, without using biomass. Conversion then uses renewable energy to break down CO2 into CO and O2, and water into H2 and O2, and then recombines CO and H2 in liquid hydrocarbon using the Fischer-Tropsch synthesis. These processes (e.g. solar fuels) are currently in the research stage.
Thus, there are a large number of processes under development that allow for processing of almost all kinds of feedstock into aviation fuel components, which offers flexibility for regional adaptation and optimization.
Most of the various pathways do not directly produce a drop-in jet fuel. They produce components that need to be blended with Jet A-1 to obtain the final drop-in fuel. It should also be noted that although they are not the processes currently deployed for road-transportation, these processes co-produce fuels that can be used for road transportation.
