Incineration Waste is burned in an airstream to ensure complete combustion.
Hot gas from the furnace is used to raise steam and this is used in a steam cycle to generate electricity. Relatively low steam temperatures limit electrical efficiency to about 27%. This compares to coal power stations, which operate at 35% efficiency. Using the heat in a district heating system can bring the overall efficiency up to 70%.
Gasifi cation A gasifier heats waste to a temperature in excess of 1,000 0C in an atmosphere that doesn't contain enough oxygen to allow full combustion.
This yields a gas called syngas which can be used as a fuel. The efficiency of generating electricity is limited at 30%, largely because the uncooled gas is too hot to compress to go into a gas turbine.
Pyrolysis Waste is heated in complete absence of oxygen to a temperature up to 850 0C. This yields a gas, which is not as clean as that from a gasifier, but can be burned in engines.
Electricity generation efficiency is about 30%. Some of the energy in the original waste is still present as carbon in the 'ash' from the process, but it can be recovered by either combustion or gasifi cation.
Anaerobic Digestion A biological process where biodegradable waste is subjected to the action of bacteria. The product is a gas containing methane and carbon dioxide. Growth of the bacteria requires energy and not all the organic material in the waste is converted to gas so there are some losses. The process produces a digestate that can be used as a fertiliser. The overall conversion efficiency is 19.3%.
Mechanical/Biological Treatment (MBT) Several processes are used to separate waste into its constituent parts to maximise recycling. These produce a dried, organic material that could be burned. Some energy is used to grow bacteria. Heat given out by the bacteria is used to dry the waste.
Air is passed through the waste to carry the water vapour away with a further loss of energy from this process. Air is passed through a flame to destroy odours and biological contamination. Burning the derived fuel to produce electricity in a power plant is 30% efficient.