As pressures have increased to make the most efficient use of energy, combined heat and power has become an increasingly important sector. Engines are usually run on some form of gas, which in many cases would otherwise enter the atmosphere without yielding any energy, for example methane emitted from landfill sites or disused coal mines. In these examples, not only does CHP enable the energy in the methane to be put to good use, it also helps to reduce greenhouse gas emissions (methane is a far more potent greenhouse gas than carbon dioxide).
Because of its usefulness in reducing greenhouse gas emissions, CHP often attracts grants and subsidies. However, these are usually conditional on the toxic exhaust emissions being maintained at very low levels. Since the engines employed rarely come with any exhaust gas aftertreatment technology as standard, it is normal to retro-fit them with it. The stable operating conditions and round-the clock use of most CHP engines means that significant quantities of toxic emissions can be rendered harmless.
CHP plants with a thermal output in the range 1-50 mW will have their emissions regulated by the recently-introduced Medium Combustion Plant Directive (MCPD).