Advanced Thermal Treatment

Advanced thermal treatment is the overall name given to processes that employ pyrolysis and/or gasification in dealing with municipal solid waste (MSW). These are different to the process of incinerating MSW: a process that uses sufficient quantities of oxygen to ensure full combustion of mixed MSW. The gasification and pyrolysis of solid materials is not a new concept. They have been used extensively to produce fuels such as charcoal, coke and town or producer gas. It is only in recent years, however, that they have been commercially applied to the treatment of MSW. In the UK the development of pyrolysis and gasification technologies is in its infancy although large scale plants have been built and are in operation in Europe, North America and Japan. (Source: DEFRA web site)

Pyrolysis

In contrast to combustion, pyrolysis is the thermal degradation of a substance in the absence of oxygen. This process requires an external heat source to maintain the temperature required. Typically, relatively low temperatures of between 300ºC to 800ºC are used during pyrolysis of materials such as MSW. The products produced from pyrolysing materials are a solid residue and a synthetic gas (syngas). The solid residue (sometimes described as a char) is a combination of noncombustible materials and carbon. The syngas is a mixture of gases (combustible constituents include carbon monoxide, hydrogen, methane and a broad range of other volatile organic compounds). A proportion of these can be condensed to produce oils, waxes and tars. The syngas typically has a net calorific value (NCV) of between 10 and 20 MJ/Nm3. If required, the condensable fraction can be collected by cooling the syngas, potentially for use as a liquid fuel.

Gasification

Gasification can be seen as a process between pyrolysis and combustion in that it involves the partial oxidation of a substance. This means that oxygen is added but the amounts are not sufficient to allow the fuel to be completely oxidised and full combustion to occur. The temperatures employed are typically above 750ºC. The main product is a syngas, which contains carbon monoxide, hydrogen and methane. Typically, the gas generated from gasification will have a net calorific value (NCV) of 4 - 10 MJ/Nm3. The other main product produced by gasification is a solid residue of non-combustible materials (ash) which contains a relatively low level of carbon. [For reference, the calorific value of syngas from pyrolysis and gasification is far lower than natural gas, which has a NCV of around 38 MJ/Nm3.]