Pyrolysis

Pyrolysis is a thermochemical decomposition of materials at elevated temperature in the absence or near absence of oxygen. In pyrolysis, complex organic molecules break into smaller components, yielding solid char, condensable liquids (often called pyrolysis oil or tar), and non-condensable gases such as synthesis gas. The product distribution depends on feedstock, temperature, heating rate, and residence time.

Feedstocks commonly include biomass (wood, agricultural residues), coal, plastics, and tires. The process is used for energy recovery, material recycling, and chemical feedstock production. In biomass processing, three main modes are recognized: slow pyrolysis, which favors char formation; fast or flash pyrolysis, which maximizes liquid yields; and high heating rates with short vapour residence times that promote liquids.

Product streams comprise char, liquid oil, and gas. The liquids can be upgraded to fuels or chemical feedstocks, while char (such as biochar) can be used for soil amendment or carbon sequestration. Gas produced can be used for process heat or further upgrading.

Equipment ranges from fixed-bed kilns and retorts to rotary and fluidized-bed reactors. Gasification is related but distinct, operating with limited oxygen to produce synthesis gas (CO and H2) rather than a single pyrolysis oil.

Environmental and practical considerations include waste volume reduction and potential energy generation, balanced against energy input requirements and the need for gas cleaning and tar management. The term derives from Greek, meaning fire and separation, reflecting the heat-driven decomposition without combustion.