But some paper and pulp factories, which make wood products, obtain the energy for their manufacturing processes by burning bark, sawdust, and other wood waste. Wood is also used to make charcoal. Biomass materials other than wood are also used as fuel. For example, heat produced by burning nutshells, rice and oat hulls, and other by-products of food processing is often used to operate plant equipment. Liquid fuels Liquid fuels are made mainly from petroleum, but some synthetic liquid fuels are also produced.
Liquid fuels are easy to store and transport.
They are the major source of energy for automobiles, airplanes, and other vehicles. In addition, liquid fuels are used to heat buildings. Petroleum, also called crude oil, ranges from clear yellow-brown oils to thick, black tars. Some crude oil is burned as fuel in stoves and boilers without processing. However, most petroleum is refined to produce such fuels as gasoline, diesel oil, and kerosene.