Pyrolysis is a form of incineration where there is no oxygen for combustion.  It is used for organic waste, which decomposes rather than burns.  Due to the lack of oxygen the waste doesn’t convert to carbon dioxide and water but breaks down (cracks) chemically to produce a gas that contains a mixture of combustible constituents including carbon monoxide, hydrogen, methane and a broad range of other volatile organic compounds.  This gas, called “syngas”, can then be burnt to produce energy.  The resulting solid contains the non-combustible materials and is high in carbon.  The solid is often called “char”.