together with other measures such as reforestation. Basic concepts: heat into mechanical energy In a fossil fuel power plant the chemical energy stored in fossil fuels such as coal, fuel oil, natural gas or oil shale and oxygen of the air is converted successively into thermal energy, mechanical energy and, finally, electrical energy. Each fossil fuel power plant is a complex, custom-designed system. Multiple generating units may be built at a single site for more efficient use of land, natural resources and labor. Most thermal power stations in the world use fossil fuel, outnumbering nuclear, geothermal, biomass, or concentrated solar power plants. The second law of thermodynamics states that any closed-loop cycle can only convert a fraction of the heat produced during combustion into mechanical work. The rest of the heat, called waste heat, must be released into a cooler environment during the return portion of the cycle. The fraction of heat released into a cooler medium must be equal or larger than the ratio of absolute temperatures of the cooling system (environment) and the heat source (combustion furnace). Raising the furnace temperature improves the efficiency but complicates the design, primarily by the selection of alloys used for construction, making the furnace more expensive. The waste heat cannot be converted into mechanical energy without an even cooler cooling system. However, it may be used in cogeneration plants to heat buildings, produce hot water, or to heat materials on an industrial scale, such as in some oil refineries, plants, and chemical synthesis plants. Typical thermal efficiency for utility-scale electrical generators is around 37% for coal and oil-fired plants, and 56 – 60% (LEV) for combined-cycle gas-fired plants. Plants designed to achieve peak efficiency while operating at capacity will be less efficient when operating off-design (i.e. temperatures too low.) Practical fossil fuels stations operating as heat engines cannot exceed the Carnot cycle limit for conversion of heat energy into useful work. Fuel cells do not have the same thermodynamic limits as they are not heat engines. The efficiency of a fossil fuel plant may be expressed as its heat rate, expressed in BTU/kilowatthour or megajoules/kilowatthour. Plant types Steam In a steam turbine power plant, fuel is burned in a furnace and the hot gasses flow through a boiler. Water is converted to steam in the boiler; additional heating stages may be included to superheat the steam. The hot steam is sent through