Pyrolysis is the thermal decomposition of organic material, such as biomass, in the absence of oxygen. Biomass, which consists of plant or animal material used as fuel to produce electricity or heat, is primarily composed of carbon, hydrogen, and oxygen (approximated by the formula CH₁.₄₄O₀.₆₆). In the absence of oxygen, the material does not combust but instead undergoes chemical decomposition. The major constituents of biomass—cellulose, hemicellulose, and lignin—thermally decompose into a mixture of combustible gases and solid residues known as charcoal.

The gaseous products of pyrolysis include permanent gases like carbon dioxide (CO₂), carbon monoxide (CO), hydrogen (H₂), and light hydrocarbons such as methane (CH₄). Additionally, many of these gases can be condensed into a liquid known as pyrolysis oil or bio-oil, which is a valuable renewable fuel. Bio-oil can be further processed to produce various chemicals and fuels, offering a versatile alternative to conventional fossil fuels.

Pyrolysis also produces biochar, a form of charcoal that can be used to improve soil fertility, sequester carbon, and reduce greenhouse gas emissions. The process conditions, such as temperature and heating rate, significantly influence the yield and composition of the pyrolysis products, making it possible to optimize the process for specific applications like energy production, chemical synthesis, or environmental management.