Process aspects in combustion and gasification Waste-to-Energy (WtE) units

• Devices for combustion, gasification, and pyrolysis of municipal solid waste are described.
• Combinations of pyrolyser/gasifiers and combustors are sometimes used for ash melting.
• Sorting at the source and at the plant affects the composition of the waste to be converted.
• To avoid corrosion, steam data are low and so are the efficiencies for electricity production.
• Combination of gasification with gas cleaning and combustion raises the efficiency.

The utilisation of energy in waste, Waste to Energy (WtE), has become increasingly important. Waste is a wide concept, and to focus, the feedstock dealt with here is mostly municipal solid waste. It is found that combustion in grate-fired furnaces is by far the most common mode of fuel conversion compared to fluidized beds and rotary furnaces. Combinations of pyrolysis in rotary furnace or gasification in fluidized or fixed bed with high-temperature combustion are applied particularly in Japan in systems whose purpose is to melt ashes and destroy dioxins. Recently, also in Japan more emphasis is put on WtE. In countries with high heat demand, WtE in the form of heat and power can be quite efficient even in simple grate-fired systems, whereas in warm regions only electricity is generated, and for this product the efficiency of boilers (the steam data) is limited by corrosion from the flue gas. However, combination of cleaned gas from gasification with combustion provides a means to enhance the efficiency of electricity production considerably. Finally, the impact of sorting on the properties of the waste to be fed to boilers or gasifiers is discussed. The description intends to be general, but examples are mostly taken from Europe.