Biomass conversion technologies can be broadly divided into thermochemical, chemical and biological processes. Thermochemical processes include gasification and pyrolysis, chemical conversion includes processes such as transesterification, while biological processes include anaerobic digestion and alcoholic fermentation.

Rapeseed oil can be converted into biodiesel by transesterification.

There are many technologies for processing and using biomass in the energy and transport sectors. In addition to direct combustion, biomass can be converted by thermochemical, chemical or biological processes into various liquid or gaseous fuels.

Combustion

The primary direct use of biomass is combustion in boilers for heat and power production. The released thermal energy can be used to generate electricity in a Rankine cycle or distributed as heat to consumers. Biomass does not necessarily require complex processing before combustion. Wood is commonly used in the form of chips, straw can be burned in bales, and biomass can also be compressed into pellets. As biomass may contain significant moisture, it is often dried before combustion. Maintaining a steady supply of large quantities of biomass can, however, be logistically demanding.

Gasification

Gasification is a thermochemical process in which biomass is converted at high temperatures, typically above about 700 °C, with a controlled amount of air, oxygen and/or steam. The main product is a combustible gas containing mainly carbon monoxide, hydrogen and other gaseous compounds. Biomass gasifiers may use fixed-bed or fluidised-bed designs.

Fixed-bed gasifiers are relatively simple and are often used for smaller installations, while fluidised-bed gasifiers provide more uniform mixing and temperature distribution and are better suited to larger throughputs. Fluidised-bed gasifiers typically operate at temperatures of about 800—900 °C. Biomass gasification generally proceeds through four main stages: drying, pyrolysis, oxidation or partial combustion, and reduction. The composition and heating value of the product gas depend strongly on the feedstock, gasifying agent and reactor design. Air-blown gasifiers typically produce a low-calorific fuel gas with a heating value of about 4—6 MJ/m³, which can be burned in boilers, engines or gas turbines after suitable gas cleaning.

At the 1900 Exposition Universelle in Paris, Rudolf Diesel demonstrated an engine running on peanut oil. Plant oils were of particular interest to France because peanuts were produced in its overseas colonies.

Pyrolysis

In anaerobic digestion, manure provides organic material for methane-producing microorganisms.

Pyrolysis is a thermochemical process in which biomass is decomposed by heating in the absence of oxygen. Depending on the temperature and heating rate, it produces varying proportions of solid char, liquid bio-oil and combustible gases. At higher temperatures, a larger proportion of gaseous products such as hydrogen, methane, carbon monoxide and carbon dioxide is formed. The products of pyrolysis can be used as fuels or as feedstocks for further chemical processing. Bio-oil can be burned directly after suitable treatment or further upgraded, while char can be used as a solid fuel or processed into biochar.

In fast pyrolysis, biomass is heated rapidly in the absence of oxygen and the resulting vapours are quickly cooled and condensed, maximising the yield of liquid bio-oil.

Transesterification

Transesterification is a chemical process used to produce biodiesel from vegetable oils, animal fats or used cooking oils. The triglycerides in the oil or fat react with an alcohol, usually methanol, in the presence of a catalyst such as sodium hydroxide or potassium hydroxide. The main products are fatty acid methyl esters, which form biodiesel, and glycerol as a by-product. Biodiesel is produced in dedicated processing plants or biorefineries equipped for oil extraction, purification and transesterification.

Anaerobic Digestion

Biogas produced in biogas plants can be used for heat and electricity generation.

Anaerobic digestion is a biological process in which microorganisms break down organic matter in an oxygen-free environment. Under suitable conditions, this process produces biogas, whose main combustible component is methane. Raw biogas typically has a heating value of around 20—23 MJ/m³ and can be used for heat and electricity generation.

Biogas production is a multistage biological process involving hydrolysis, acidogenesis, acetogenesis and methanogenesis. During hydrolysis, complex carbohydrates, proteins and fats are broken down into simpler soluble compounds. These are converted into organic acids, hydrogen and carbon dioxide during the following stages. Finally, methanogenic microorganisms convert mainly acetate, hydrogen and carbon dioxide into methane-rich biogas.

The first known anaerobic digestion plant was built in 1859 at a leper colony in Mumbai, India.

Alcoholic Fermentation

A silo for storing purpose-grown crops before processing.

Alcoholic fermentation is a biological process in which microorganisms convert sugars into ethanol and carbon dioxide under anaerobic conditions. The process is exothermic and releases heat.

On an industrial scale, bioethanol is produced from crops rich in sugars or starch, such as maize, sugar beet, sugar cane, potatoes and cereals. Bioethanol is commonly used as a transport fuel or blended with petrol.

Dough rises because yeast converts sugars into carbon dioxide and ethanol during alcoholic fermentation. The carbon dioxide forms bubbles in the dough, while most of the ethanol evaporates during baking.

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