almost all coal, petroleum, nuclear, geothermal, solar thermal electric, and waste incineration plants, as well as many natural gas power stations are thermal. natural gas is frequently burned in gas turbines as well as boilers. the waste heat from a gas turbine, in the form of hot exhaust gas, can be used to raise steam by passing this gas through a heat recovery steam generator (hrsg). the steam is then used to drive a steam turbine in a combined cycle plant that improves overall efficiency. power stations burning coal, fuel oil, or natural gas are often called "fossil fuel power stations". some biomass-fueled thermal power stations have appeared also. non-nuclear thermal power stations, particularly fossil-fueled plants, which do not use cogeneration are sometimes referred to as "conventional power stations".