Wood, wood pulp, and wood products have been burned for fuel since prehistory. Now, however, wood waste is burned for clean energy. Alternatively, wood waste is converted chemically into a fuel gas. In either case, the energy can be used for space heating or to drive a steam turbine for electricity.
Quite a bit of the waste of civilization is classed as wood products. Examples include tree trimmings, wood chips, lumber from demolition, and stained waste paper. (Clean waste paper and cardboard are typically recycled.) Some industries generate large amounts of such waste, in particular the paper industry. Municipal incinerators and the like will commonly make arrangements to accept the waste from such industrial producers.
Waste wood is used in two different ways, either burned directly or processed chemically. Material for burning will be cut into regular small pieces. A high moisture content may warrant preheating, for water in the fuel cuts down noticeably on the energy output. Wood that has varnish, paint, or various heavy metals may emit toxic vapors when burned. This requires the incinerator to be enclosed and to scrub its exhaust.
Alternative to burning, wood products may be processed chemically. The simplest common treatment is heating in the absence of oxygen. This will produce cabon monoxide (CO), hydrogen, and perhaps other gases such as methane (CH4). As it happens, this is not a new concept. Coal gasification (right) also produces this mixture, which is called syngas. Syngas, or synthetic gas, lighted street lamps in the 19th century before electrical lighting.