During combustion, the chemical energy originally derived from sunlight is released as heat.

Biomass stores solar energy captured by plants through photosynthesis in organic compounds. During combustion, this chemical energy is released as heat. Oxygen combines with carbon and hydrogen in the fuel, producing mainly carbon dioxide and water. Unlike fossil fuels, the carbon released from biomass was recently absorbed from the atmosphere during plant growth and can be reabsorbed as new biomass grows, provided that the resource is sustainably replenished. Replacing fossil fuels with sustainably sourced biomass can reduce lifecycle greenhouse-gas emissions. Biomass, particularly clean woody biomass, generally contains less sulphur than many fossil fuels, although the exact composition varies with the fuel. Biomass combustion can produce nitrogen oxides, particulate matter, carbon monoxide and other pollutants, so modern plants use combustion control and flue-gas cleaning systems to limit emissions. Nitrogen-oxide formation depends on combustion temperature, fuel nitrogen content and operating conditions. Biomass combustion also produces fly ash and fine particulate matter, with the amount and composition depending strongly on the type of biomass and combustion technology. Modern plants therefore use particulate-control systems such as filters or electrostatic precipitators to reduce these emissions.

The carbon released during biomass combustion was previously absorbed from the atmosphere during plant growth.

The ash content and composition of biomass vary considerably with the type of fuel. Clean woody biomass generally produces relatively little ash, while bark, straw and other agricultural residues may contain substantially more. Biomass ash often contains nutrients such as potassium, calcium and phosphorus, but it may also contain trace metals and other compounds that affect its suitability for recycling or disposal. High concentrations of alkali metals can also contribute to slagging, fouling and corrosion in combustion equipment.

Growing plant matter — a potential source of biomass for energy production.