First generation biofuels‒including ethanol and biodiesel‒are oxygenated, meaning oxygen is present in the fuel molecules. This reduces their performance characteristics and compatibility with existing distribution and engine infrastructure.
Unlike biodiesel or ethanol, advanced biofuel processing technologies produce fuels which are fungible with conventional petroleum-based fuels. These fuels generally meet or exceed the performance criteria of the petroleum fuels they replace. Advanced biofuels are generally produced through one of the processing pathways below.
Hydroprocessing
The hydroprocessing of 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 refining facilities and operated at similar costs to petroleum refining. It can also take place in stand-alone refineries.
Biomass-to-Liquid: 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.
Biomass-to-Liquid: Gasification
Biomass gasification involves combusting biomass at high temperatures with controlled amounts of oxygen and steam, creating a synthetic gas (or syngas) consisting of carbon monoxide and hydrogen. Syngas is then converted into a synthetic paraffinic wax via the Fischer-Tröpsch process. As in hydroprocessing, this paraffin wax is then selectively cracked and isomerized to produce liquid fuels.
Fermentation-Based Biomass-to-Liquid Synthesis
In this approach, the cellulosic biomass is converted into water-soluble C5 and C6 sugars through either a chemical or enzymatic process. These sugars are fermented with microorganisms that directly metabolize them into hydrocarbon fuels. A number of companies are pursuing different microorganisms for this function, including heterotrophic algae, yeast, and bacteria.