Definitions of the Terms Gasification and Pyrolysis

Established Thermal Treatment (TT)

Until recently the only established Thermal Treatment technology was Incineration.

Incineration usually involves the combustion of unprepared (raw or residual) MSW. To allow the combustion to take place a sufficient quantity of oxygen is required to fully oxidise the fuel. Typically, incineration plant combustion (flame) temperatures are in excess of 850°C and the waste is converted into carbon dioxide and water. Any non-combustible materials (e.g. metals, glass) remain as a solid, known as Bottom Ash, that contains a small amount of residual carbon.

Advanced Thermal Treatment (ATT)

Pyrolysis

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 300°C to 800°C 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 non-combustible 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 of the syngas can be collected by cooling the syngas, potentially for use as a liquid fuel.

Gasification

Gasification can be seen as a process which lies 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 750°C.

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.)