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The biomass-to-liquid (BTL) process concept is a process for converting various types of biomass to liquid fuels and uses a variety of processes (Table B-23).

Table B-23 Conversion of biomass to fuels.

Starch/sugar crops
Hydrolysis
Sugar
Fermentation
Refining
Ethanol
Lignocellulose biomass
Pyrolysis/liquefaction	Gasification	Anaerobic digestion
Bio-oil	Synthesis Gas	Bio-oil
Hydrotreating	Fischer-Tropsch	Biogas
Hydrocarbon fuels	Hydrocarbon fuels	Gas treating
		Methane
Oil-producing plants
Pretreatment
Vegetable oil
Esterification
Biodiesel

There are two types of biomass feedstock that can be employed to produce BTL fuels – woody and herbaceous. Woody feedstock comprises wood chips, wood powder and sawdust, obtained from ordinary forestry (wood logs), short-rotation forestry, various wood residues, and wood waste. Herbaceous feedstock includes chaffed dedicated energy crops and straw.

Owing to biomass composition, woody feedstock is better suited for energy applications than herbaceous feedstock. Woody biomass also has a larger production potential for energy (including BTL) application; however, the production potential of herbaceous biomass is currently under-explored as the path to useful products is open, for example (Table B-24):

Table B-24 Routes to liquid fuels from biomass.

Biomass
	Pretreatment
		Gasification
			Synthesis gas
			Fischer-Tropsch
				Upgrading
					Gasoline
					Diesel fuel
			Methanol synthesis
				Methanol-to-gasoline
						LPG
						Gasoline

Biomass pyrolysis is a process by which a biomass feedstock is thermally degraded in the absence of air/oxygen. It is used for the production of solid (charcoal), liquid (tar and other organics), and gaseous products. These products are of interest as they are possible renewable sources of energy. The study of pyrolysis is gaining increasing importance, as it is not only an independent process but it is also a first step in the gasification or combustion process, and has many advantages over other renewable and conventional energy sources. The actual reaction scheme of pyrolysis of biomass is extremely complex because of the formation of over a hundred intermediate products.

Plasma pyrolysis provides high temperature and high energy for reaction as the reaction sample is rapidly heated up to a high temperature. This review also covers the experimental and modeling study status of plasma-assisted pyrolysis.

Biofuels produced via gasification routes include direct gasoline and diesel substitutes made from gas-to-liquid processes (i.e., the Fischer-Tropsch process), methanol, ethanol, mixed alcohols, and hydrogen. Gas-to-liquids technologies are utilized commercially using natural gas or stranded natural gas as feedstock. Coal was used extensively by Germany in WWII and is still used in the Sasol (South Africa) facilities for gasoline and diesel fuel synthesis along with a wide variety of other products.

In a biomass-to-liquids process, the feedstock undergoes a pretreatment or selection (sizing, drying, and sorting) and is then gasified in a reactor. The gas product (carbon monoxide, hydrogen, low-boiling hydrocarbon derivatives, tars, and particulate material) undergoes extensive cleanup to remove catalyst poisons and other undesirable components. This is followed by gas processing/reforming where the hydrogen/carbon monoxide ratio is adjusted before entering the (Fischer-Tropsch) synthesis reactor. The liquid synthesis reactor contains catalyst material and operates at elevated pressure and temperature forming hydrocarbon compounds or alcohols from the synthesis gas. The liquids can be further refined to the desired end product.

See also: Biomass – Gasification, Biomass to Syngas, Synthesis Gas.