Pyrolysis is the thermal decomposition of the volatile components of an organic substance in the temperature range of 400-1,400°F (200-760°C) and in the absence of air or oxygen, forming syngas and/or liquids. An indirect source of heat is used. A mixture of un-reacted carbon char (the non-volatile components) and ash remains as a residual. Burned toast is an example of pyrolysis.
Gasification takes this to the next step. It occurs in a higher temperature range of 900-3,000°F (480-1,650°C) with very little air or oxygen. In addition to the thermal decomposition of the volatile components of the substance, the non-volatile carbon char that would remain from pyrolysis is converted to additional syngas. Steam may also be added to the gasifier to convert the carbon to syngas. Gasification uses only a fraction of the oxygen that would be needed to burn the material. Heat is supplied directly by partial oxidation of the carbon in the feedstock. Ash remains as a residual.