Draft PPS18: Renewable Energy
Annex 1 Energy from Waste (Thermal Processes) Technology: Gasification
D13. Gasification is a process of partial combustion, which enables operators to effectively control the temperature of the process, with consequent mitigation of pollutants. A gas is formed when the fuel reacts with sufficient oxygen to maintain a high reaction temperature but with insufficient oxygen to complete combustion. This gas can then be used in engines, boilers or turbines to generate power.
D14. For all these processes the useful energy in the waste is generally released by combustion, although increasingly syngas from pyrolysis and gasification is being used as a source of hydrogen for fuel cells. In the context of fuel cells, pyrolysis and gasification as processes have the advantage of producing a homogeneous gas from which hydrogen can be extracted.
D15. Pyrolysis and gasification are still developing but experience thus far has demonstrated that the superior control of the combustion offered by these processes can create much lower levels of contaminants in the exhaust gas when compared with typical grate combustion. Pyrolysis and gasification systems can generally be implemented at smaller scales than conventional incineration, making them more flexible in meeting the needs of smaller communities, and reducing local air quality impacts through minimisation of waste transportation. The ash resulting from pyrolysis and gasification followed by combustion has also been found to be more stable and less polluting than that from conventional incineration.
D16. Waste can be pre-treated in a variety of ways to improve its combustion efficiency and extract recyclable materials such as metal and glass. Treatments include shredding, sorting and separation, and drying. The equipment used for sorting waste will typically include rotating and vibrating screens, magnetic separators, air separators and manual picking belts. Some more innovative systems use high temperature washing. The pressure from recycling targets will mean that all MSW plant will have extensive materials recovery facilities.