Classified according to the fuel or energy used, solid fuel boilers, burning solid fuels such as coal. Liquid fuel boilers, which use liquid fuels such as heavy oil. Gas fuel boilers use gaseous fuels such as natural gas. Waste heat boilers use waste heat from industries such as metallurgy and petrochemical industry as heat sources. An atomic energy boiler that uses a thermal generator that releases heat energy from a nuclear reactor as a heat source. Waste boiler, a boiler that uses waste, bark, waste liquid, etc. as waste. Other energy boilers? Steam generators or water heaters that use geothermal, solar energy and other energy sources.