Depending on context, "biomass" can indicate the total mass of biological organisms in a particular ecosystem, or dead plants and animals able to be used as fuel. The latter meaning, however, does not include fossil fuels (coal, oil, and natural gas), even though they came from plants and animals originally. Biomass instead refers to fuel derived from plants and animals that have died usually within the past several months and at most within the past several decades.

Biomass is further divided into the following two categories:

Two difficulties encountered in implementing a biomass solution are the need to harvest a large volume of material spread over a wide area and the need to preprocess materials before use. In Japan, city garbage, agricultural waste, and in some places even chicken droppings are used as fuel to produce electricity; owing to the above difficulties, however, of total biomass equivalent to 26 million kL of crude oil, only the equivalent of 4.62 million kL, or 18%, was utilized in 1999.

Worldwide, Brazil and the United States use agricultural waste and grain to produce ethanol as a gasoline replacement. Europe and the United States use biomass from trees to produce energy, and many other projects are being implemented.

Energy from biomass as a raw material

Burning fossil fuels produces carbon dioxide emissions, increasing the atmospheric concentration of the gas. Burning biomass also produces carbon dioxide emissions, yet without a net addition of carbon to the atmosphere. The reason is that plant and animal matter utilized as biomass is destined to decompose and release its carbon content in any case. In essence, biomass energy is solar energy retrieved by returning biological carbon to the atmosphere. In this cycle, energy circulates, making it both clean and carbon neutral.

Used in an increasing variety of ways, biomass may be burned as is or chemically transformed into methane or methanol for heat and power. A metric ton of sugar cane can produce 0.2 metric tons of ethanol, and the daily waste of three cows can be used to provide one day's worth of electricity for a household.

One typical use of biomass is electricity generation. In Japan, the most common method is the incineration of city garbage, which produces steam to rotate a turbine and generate electricity. Globally, the use of bagasse (crushed sugar cane residue), chaff, and other agricultural wastes is common and can produce the following amounts of electricity:

1 metric ton of bagasse 2 metric tons of steam 500kWh
1 metric ton of chaff 4 metric tons of steam 1,000 kWh

As another example, biomass could run a 50,000 kW steam turbine for a year (365 days):

The area of this forest would be about twice as large as the 23 wards of Tokyo, or somewhat larger than Lake Biwa.