Incineration is the most commonly used waste-to-energy process used in Denmark. Steam, heat or power are produced in any combination, and plants can be up to 98% efficient. However, there are other technologies that produce gas, liquid and solid combustible fuels. Other thermal technologies include gasification, depolymerisation and pyrolysis. Non-thermal technologies include anaerobic digestion, fermentation and mechanical-biological treatment.

The following diagram illustrates incineration with wet flue gas treatment —
see it larger and read its key. Diagram courtesy Rambøll.

In Copenhagen's CHP plants, municipal waste is delivered by road and tipped into huge concrete bunkers. It's mixed and extracted by crane grabs and fed into shafts leading to the furnaces. Here the waste sits on moving grates that take it through the flames. Hot exhaust gases leave the furnaces and heat water in a series of heat exchangers. The hot water, or steam, can be used directly for district heating, or the steam used to drive turbines to produce electricity.

Denmark's waste-to-energy plants represent a different approach to waste disposal than that found outside Scandanavia on a large scale. In 2008, Copenhagen, helped by legislation, sent only 2.2% of its 904,216 tonnes of waste to landfill — 20 times less than in 1988. The Danish average is 6-10%. Copenhagen is aiming for a figure of zero, as outlined in its Waste Management Plan 2012 (adopted 27th November 2008). A further 25-40% goes to waste-to-energy plants and the rest is recycled.

Among the factors that have made waste recovery for heat and power effective in Denmark is the consistent way that the combination of waste incineration and district heating have been promoted over a long period. Both political will and taxes and subsidies have been aimed at their joint development. Denmark's climate makes heating essential for most of the year, and landfill sites had already started to fill up by the early 20th century.

Incineration continues to cause controversy outside Scandanavia, though it is used widely in Japan and Taiwan. The by-products and residues can be hazardous. The Danes count the benefits as including the reduction of waste volumes, the production of energy, the conservation of fossil fuel resources and the diminution of CO2 emissions. Others have argued against the process on account of the cost of meeting air pollution standards and reaching the high technical standards needed for efficiency. To make the energy recovery aspect work, a reasonably sized district heating scheme is required.