Pyrolysis is the anoxic (oxygen-free) decomposition of organic matter by heat. The process begins with torrefaction as temperature exceeds 200C - organic materials become completely dehydrated and volatile components escape. Some degradation of the dry solid material occurs with browning (Malliard reaction), and gradual further degradation. At temperatures above 400C, biopolymers such as cellulose and lignin – major components of plant fiber – will break apart at the molecular level. The majority of carbon, along with trace metals, will remain in elemental form as solid residue, and other elements (primarily hydrogen and oxygen), and some carbon will reform into smaller molecules which are volatile at such temperature. Much of the reformed substances can be condensed as “pyrolysis oil” or distillate and the volatile gases are generally combustible. Burning them is sometimes employed to heat the pyrolysis process.

Pyrolysis is not limited to plant fiber feedstock. Plastics, tires, and composite materials with organic components can be treated. The rate of heating in pyrolysis affects the distribution of pyrolysis products, with rapid heating favoring the production of pyrolysis oil and slow heating favoring the production of elemental carbon product. Pyrolysis is most often conducted at temperatures of 500C – 1000C. Primitive processing can be conducted batchwise, but most industrial-scale pyrolysis is rotary-kiln or fluidized-bed. Sometimes auger-driven or multiple-hearth operations are employed. Heating can be achieved by contact with fluidizing or combustion gases, microwaves, or contact with molten salt.

The most basic practice of pyrolysis is generating charcoal – a worker cuts and stacks wood in a dense pile and covers it with soil. Ignition points and one or more chimneys are excavated, so the wood can be started burning. Then the soil cover is restored and the accumulated heat in the partially-burned wood pile completes the pyrolysis.

In more advanced practices of pyrolysis where product output goals are to be optimized, it is necessary to shred, chop or grind the feedstock to uniform reduced size. More uniform and rapid heating can be achieved, and the process completes more quickly.

The carbon content of wood varies with type, but on a dry-weight basis it is commonly held to be approximately 50% - setting a theoretical upper limit for the yield of biochar from pyrolysis.