Biomass (energy)

Biomass refers to organic matter from recently living organisms that is used for bioenergy production. This includes a wide variety of materials like wood, agricultural residues, and organic waste from households. Most biomass used for energy comes from plants, though some definitions include algae or animals. Bioenergy is a type of renewable energy that can help mitigate climate change. While people often use the terms biomass and biofuel interchangeably, biofuel is actually a specific subset. Biofuel refers to liquid or gaseous fuels used for transportation, such as those made from corn or soy.

There are two main categories of biomass: primary and secondary. Primary biomass is harvested directly for energy purposes. This includes wood, perennial energy crops, and certain food crops. Secondary biomass consists of residues and waste. These are by-products from materials originally harvested for other reasons. For example, wood residues come from forestry operations or the wood processing industry. Agricultural residues, such as straw, are also a major source of secondary biomass.

To make raw biomass more practical, it must undergo conversion. This process upgrades the material into higher-grade fuels. One method is thermal conversion, which uses heat as the primary driver. Torrefaction is a mild form of thermal conversion. It involves heating organic material to 400–600 °F in a low-oxygen environment. This process removes low-energy parts and leaves behind a high-energy solid. This product is water-resistant and can even compete with coal for electricity generation.

Other thermal methods include pyrolysis and gasification. Pyrolysis heats material to 800–900 °F in the near complete absence of oxygen. This produces bio-oil, charcoal, methane, and hydrogen. Gasification uses much higher temperatures, between 1,400 and 1,700 °F. It injects controlled amounts of oxygen or steam into the vessel. This creates synthesis gas, or syngas, which can power diesel engines or generate electricity. Chemical conversion is another path, such as transesterification. This process converts vegetable oils and animal fats into biodiesel.

Biochemical conversion uses the natural processes of microorganisms to break down molecules. Common methods include fermentation, anaerobic digestion, and composting. Fermentation is used to turn biomass into bioethanol. In the United States, corn is a major source for bioethanol production. The European Union is the largest producer of biodiesel, often using oil-producing crops like rapeseed. These "first-generation" biofuels are made from food crops. Currently, global bioethanol and biodiesel production provide 2.2 and 1.5 EJ of energy per year, respectively.

Forests play a massive role in the global biomass supply. Forests provide 85% of all biomass used for energy worldwide. Of the 4 billion hectares of global forest area, one third is used for commercial purposes. In the European Union, forests provide 60% of all biomass energy. Wood is the largest biomass source used today. However, traditional wood use for cooking in developing countries can be highly polluting. The World Health Organization estimates this pollution causes 3.8 million deaths annually. The United Nations aims to phase out this traditional use by 2030.

Using biomass has significant implications for the environment and land use. Burning wood releases carbon dioxide, but this can be offset by planting new trees. New trees remove carbon dioxide from the air as they grow. However, farming biomass can also reduce biodiversity and degrade soils. It can consume large amounts of water for irrigation and require fertilizers. There is a balance to maintain between energy needs and food production. Experts believe global land and water resources are adequate to support both food and carbon-neutral energy needs.