Renewable jet fuels are nearly chemically identical to conventional fossil jet fuels, only they are produced from renewable resources.

Unlike biodiesel or ethanol, advanced biofuel processing technologies produce fuels which are completely fungible with conventional petroleum-based fuels. These fuels generally meet or exceed the performance criteria of the petroleum fuels they replace. Renewable jet fuels under development, the conversion technologies that produce them, and the resources from which they are made, are described below:

HEFA (or HRJ), Produced from Hydroprocessing Natural Oils
HEFA (hydroprocessed esters and fatty acids), sometimes also called HRJ (hydroprocessed renewable jet) are produced by “refining” natural oils much like petroleum is refined today. Hydroprocessing natural oils (plant oils or animal fats) involves converting these oils from lipids to hydrocarbons through the addition of hydrogen. The first step converts the lipids to fully saturated hydrocarbons, or synthetic paraffins, by saturating oxygen bonds and double-carbon bonds with hydrogen. These hydrocarbons are then selectively cracked and isomerized to produce primarily diesel, jet fuel, and propane. This process can be integrated into existing fossil fuel refining facilities and operated at similar costs to petroleum refining. They can also be added on to first generation biodiesel production facilities, or built from scratch in stand-alone refineries.
**Note that HEFA fuels are ASTM certified for commercial use in up to a 50/50 blend with conventional jet fuel.

FT-SPK, Produced from Biomass Gasification and Fischer-Tröpsch Synthesis
FT-SPK (Fischer-Tröpsch Synthetic Paraffinic Kerosene) is made by gasifying (*you can think of gasifying as vaporizing) biomass using heat and controlled amounts of oxygen and steam and then using the F-T process to turn the vapors (called synthetic gas or ‘syngas’ consisting of carbon monoxide and hydrogen) into liquid fuels. The syngas is converted into a synthetic paraffinic wax via the Fischer-Tröpsch catalytic process. As in hydroprocessing, this paraffinic wax is then selectively cracked and isomerized to produce liquid fuels that are compatible with existing infrastructure. Note that this process can also be used to turn coal and natural gas into non-renewable fuels generally referred to as CTL (coal to liquid) and GTL (gas to liquid) fuels.
**Note that FT-SPK fuels are ASTM certified for commercial use in up to a 50/50 blend with conventional jet fuel.

ATJ (Alcohol to Jet), Produced from Alcohol Oligomerization
ATJ (Alcohol to Jet) is made via alcohol oligomerization which involves linking short-chain alcohol molecules (e.g. ethanol or butanol) together to form jet-fuel range hydrocarbons. There are several chemistries that can be employed to oligomerize alcohols. In each of these processes, water and/or oxygen are removed from the alcohol molecules, and hydrogen is added. Because the starting alcohol volume is reduced in order to produce a marginally more valuable hydrocarbon jet fuel (at current market prices), the economic rationale of these conversions must be critically examined on a company-by-company basis.

PTJ (Pyrolysis to Jet), Produced from Biomass Pyrolysis
Pyrolysis refers to the thermal decomposition of biomass into low-quality pyrolysis oil. Biomass is heated under extreme temperatures in the absence of any reactive gaseous compounds. The biomass is converted into low-quality oil which must then be upgraded for processing into fuels. Hydroprocessing, described above, is the principal method by which pyrolysis oil is converted to fuels. Gasoline, aromatics, and chemicals are the principal products of this production method, although jet fuel can be co-produced.

FRJ (Fermented Renewable Jet), or Fermentation-Based Biomass-to-Liquid Synthesis
In this approach, sugars are fermented by microorganisms that directly metabolize them into hydrocarbon fuels that can be used as jet fuel with little or no additional chemical processing. A number of companies are pursuing different microorganisms for this function, including heterotrophic algae, yeast, and bacteria.

The renewable jet fuels described above are generally produced from lingo-cellulosic biomass (e.g. forestry and agricultural residues), natural oils (e.g. plant oils and animal fats), and sugars (e.g. from sugar cane, sugar beet, or sweet sorghum). These broad feedstock categories are described below.
In addition to renewable fuels produced from converted biomass, natural oils, or sugar feedstocks, there are early-stage technologies under development that seek to directly catalyze renewable fuels from CO or C02 via chemical, metal, and biological catalysts.

Biomass Feedstock
Biomass includes any plant biomass containing sugar, starch, cellulose or other complex carbohydrates as target compounds for conversion into fuels. These feedstocks include municipal, agricultural, and forestry wastes. Dedicated biomass energy crops are also used, including sugarcane, switchgrass, sorghum, miscanthus, willow, poplar, or macroalgae.

Natural Oils Feedstock
Natural Oils feedstocks are triglycerides or fatty acids and can be extracted (or secreted from) plants and microorganisms. Oilseed plants, including Camelina, Jatropha, Castor, Oil Palm, and other crops can produce oil-bearing seeds. Aquatic microorganisms, such as algae and cyanobacteria, can accumulate and sometimes secrete natural oils. Yields vary dramatically as do the amount and types of land used for these crops. The scale potential and environmental impacts of natural oils feedstocks will accordingly be highly variable and site-specific.

Sugar
Some plant sugars are readily extractable/digestible, such as parts of cane sugars. Sugars can also be derived from biomass via either a chemical or enzymatic hydrolysis process, unlocking water-soluble C5 and C6 sugars. The scale potential for biofuels derived from cellulosic sugars is massive, but these technologies are not yet fully commercialized – though several companies may be close. There are also early stage companies developing photosynthetic microbes that can fix C02, synthesize, and then secrete sugars for collection, thus decoupling sugar production from agricultural land use.

CO, CO2
Several biofuel conversion processes utilize waste CO or CO2 from other industrial processes as their primary input. These gases can be converted directly to fuels via fermentation, photosynthesis, or chemical catalysis.