What Is Biomass?

Biomass is organic material derived from living or recently living organisms that can be used as a source of energy or as a raw material for chemical production. The category includes woody material and agricultural residues, dedicated energy crops such as switchgrass and miscanthus, organic municipal solid waste, animal manure, and aquatic feedstocks such as algae. Biomass is considered a renewable energy resource because the carbon it releases during combustion or conversion was recently captured from the atmosphere by photosynthesis, in contrast to fossil fuels, which release carbon that has been sequestered for millions of years. The U.S. Department of Energy's bioenergy program frames biomass as a versatile feedstock capable of supplying heat, electricity, liquid transportation fuels, and bio-based chemicals.

Composition and Sources

The chemical composition of biomass feedstocks varies considerably by type. Lignocellulosic biomass, which constitutes the majority of plant material available for energy use, consists of three major structural polymers: cellulose, which is a linear polymer of glucose units; hemicellulose, a branched polymer of five- and six-carbon sugars; and lignin, an aromatic polymer that provides rigidity and resistance to biological degradation. The proportion of these components differs significantly between, for example, agricultural straws, hardwood, softwood, and grasses, which affects both the ease of processing and the energy content per unit mass. Starch-based feedstocks such as corn grain and sugarcane are more easily converted to ethanol than lignocellulosic materials because the glucose units are more accessible to enzymatic hydrolysis. Algae represent a distinct feedstock category with high lipid content suitable for biodiesel production and the potential for cultivation on non-arable land using industrial CO2 or wastewater as nutrients.

Conversion Pathways

Biomass is converted to useful energy forms through thermochemical, biochemical, or physical-chemical pathways. In direct combustion, the oldest and simplest thermochemical route, biomass is burned to produce heat, which can drive steam turbines for electricity generation. Gasification converts biomass at high temperatures in a low-oxygen environment into synthesis gas, a mixture of carbon monoxide and hydrogen that can be used in combustion turbines or further processed into liquid fuels via Fischer-Tropsch synthesis. Pyrolysis, conducted in the absence of oxygen at temperatures between 400 and 600 degrees Celsius, produces bio-oil, biochar, and syngas in proportions that depend on process conditions. The U.S. Energy Information Administration's biomass explainer documents these conversion routes and their current contribution to the U.S. energy supply. Biochemical routes rely on microorganisms or enzymes: fermentation by yeasts or bacteria converts sugars to ethanol, while anaerobic digestion by microbial communities breaks down organic matter to produce biogas, a mixture of methane and carbon dioxide.

Environmental Considerations

The carbon balance of biomass energy depends on land use, feedstock type, and conversion efficiency, and cannot be assumed favorable in all cases. When forests are cleared to grow energy crops, the stored carbon in vegetation and soil can take decades to recapture through crop growth, resulting in a temporary increase in atmospheric CO2. Land-use change for biomass production can also displace food crops, affecting food security and commodity prices. These concerns have motivated life-cycle assessment frameworks that quantify the full greenhouse gas balance of biomass energy systems from feedstock cultivation through end use. Research published through the IEA on bioenergy addresses the policy frameworks and sustainability criteria used internationally to govern biomass procurement for energy applications.

Applications

Biomass has applications in a range of fields, including:

• Electricity and heat generation at industrial and district scales
• Transportation fuel production as ethanol and biodiesel
• Biorefinery operations producing bio-based chemicals and plastics
• Waste management through anaerobic digestion of organic municipal waste
• Agricultural soil amendment using biochar from pyrolysis processes