Biomass energy comes from organic materials produced by plants and animals. Biomass can include wood, agricultural waste, food waste, vegetable oils, and biofuel crops like corn and soy. As with fossil fuels, burning biomass for electricity generation produces greenhouse gas emissions, such as carbon dioxide. On a large scale, use of biofuels leads to deforestation and the destruction of biodiverse old-growth forests, replacing natural polycultures and with monoculture tree plantations. The clearing of forests to make way for biofuel crops like corn and soy leads to further deforestation, eliminating carbon sinks and increasing pollution.

Biomass power plants and machines that run on biofuel use a wide array of solid and liquid fuels to produce electricity.

1. Wood: Bioenergy and biofuel products can derive from wood chips, wood pellets, sawdust, tree trunks, and black liquor—a waste product from paper mills. This category also includes firewood, perhaps the oldest form of fuel used by humans. Wood fuel is the largest biomass energy resource today.

2. Agricultural products: Energy crops like corn and soy create biomass via the process of photosynthesis. Corn can convert into ethanol, soy can become biodiesel, and agricultural waste such as sugar cane bagasse can convert into cellulosic ethanol via anaerobic decomposition.

3. Sewage: Both human sewage and animal manure contain renewable organic material that can function as fuel when processed.

4. Solid waste: The municipal solid waste systems of many cities contain biological material that engineers can tap as a source of energy. In addition to municipal waste, valuable solid waste may also come from the commercial sector.

To convert biomass for the production of energy, engineers have four principle options.

• Direct combustion: Direct combustion of biomass means burning it. The resulting heat can produce steam, which powers turbines.
• Conversion to liquid fuel: Biomass can be converted to liquid fuel via biological, chemical, and thermochemical processes.
• Conversion to gaseous fuel: Biomass can be turned into gas fuel via thermochemical conversion or biological conversion.
• Conversion to solid fuel: Thermochemical conversion can turn biomass into solid fuel, although this fuel has fewer practical applications than gaseous or liquid fuel.

Around the world, biomass has supplanted some forms of fossil fuel use. The paper industry uses a sizable chunk of the world’s biomass energy, particularly liquid biofuel. Wood pellets, wood chips, and firewood are used in home heating applications, and some natural gas alternatives come from sewage waste, animal fats, and vegetable oil. The transportation fuel sector also uses biomass energy; in the United States, most gas stations sell a mixture of 90 percent gasoline and 10 percent ethanol, and some trucks and large vehicles run on a blend of petroleum diesel and biodiesel.