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100 Evaluation of Opportunities for Converting Indigenous UK Wastes to Wastes and Energy AEA/ED45551/Issue 1 Utilising SRF in isolation for energy generation is not common in the UK. There are currently two sites that use this material. Slough Heat and Power in Berkshire is a CHP plant with a generating capacity of 101MW. This site takes up to 100,000 tonnes of material each year – a combination of clean wood waste and SRF as one of its boiler systems is WID compliant. The second site at Allington, in Kent which takes 500,000 tonnes per annum of minimally treated SRF to produce 43MW electricity from two large bubbling fluidised bed boilers. 178 There are several industries where making use of SRF is already established, usually for the generation of heat. Cement kilns are one such industry, the paper industry is the other. 166 The British Cement Association reports that all but one of the 15 cement kilns in the UK use SRF. 7.1.2 Mechanical / Heat Treatment of MSW to solid recovered fuel C & I waste MSW Mechanical Heat Treatment (MHT) Process Description Mechanical Heat Treatment (MHT) is a term that is used to describe configurations of mechanical and thermal, including steam, based technologies. The purpose of these processes is to separate a mixed waste stream into several component parts, to give further options for recycling, recovery and in some instances biological treatment. The processes also sanitises the waste by destroying bacteria. The basic flowchart for a Mechanical Heat Treatment process 179 is shown below in Figure 16. SRF Figure 16 Flowchart representing Mechanical Heat Treatment 178 http://www.kentenviropower.co.uk/default.asp 179 A description of MHT processes can be found in “Mechanical Heat Treatment of Municipal Solid Waste”, published by Defra in 2007.
Evaluation of Opportunities for Converting Indigenous UK Wastes to Wastes and Energy AEA/ED45551/Issue 1 The most common system being promoted for the treatment of MSW using MHT is based around a thermal autoclave. The development for use with MSW started in the early 1990’s, and a number of patents have been issued. A second type of MHT system is a non-pressurised heat treatment process, where waste is mixed to a slurry with water and then heated in a rotating kiln prior to mechanical separation. This process requires a pre-treatment stage, which includes the use of a shredder. Both the autoclave process and the non-pressurised treatment processes have the following effect on the waste: • Biodegradable materials, including paper and card, are broken down into a high biomass fibre for which the main use is as a fuel product; • Glass bottles and tins have their labels removed as the glue disintegrates under the action of the heat; • Plastics are softened, and labels are removed. Certain types of plastics are deformed by the heat, but remain in a recognisable state, whereas other plastics soften completely forming hard balls of dense plastic. Both processes produce product streams (fibre product, plastic, etc) using well-established mechanical processing techniques (screening, density separation, magnetic separation, eddy current separation). The resulting outputs are relatively clean ‘hard’ recyclables (tins, glass and plastics with no labels and most of the food waste removed) a fibrous material from the breakdown of paper, card and green/kitchen waste constituents, and a reject fraction. Both systems heat the waste to temperatures in the range of 120-170°C, which is sufficient to destroy bacteria present in the waste. This has benefits in terms of storage, transport and handling of the outputs as they are sanitised, and are free from the biological activity that may give rise to odour problems. There is also a significant volume reduction in the waste. Table 52 shows the typical specification of a mechanical heat treatment plant. About 15-20% of the input waste is landfilled. Table 52: Typical specification of a mechanical heat treatment process Product stream Wt % of input MSW Fibre product 60-65 Metal products 4-5 Mixed plastic product 7-12 Aggregate substitute product 4-8 Landfilled 180 15-20 Total 100 Other products from the process Suppliers of MHT plants claim that these plants can recycle up to 20% of the input waste. However, whilst established markets for the metal products are available, the total level of recycling achieved will depend on the availability of markets for both the mixed plastic product and the aggregate substitute product. If these markets are not available then these products will be landfilled. Commercial status Some demonstration projects are operating under the Defra Waste Innovation Programme, such as the Huyton MHT site in Merseyside. 180 The landfilled reject stream is mainly other combustible materials. 101
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South East 327 - - 55% of MSW by 20
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Appendix 3 MBT Methods Specific in
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Figure 27 Orchid Environmental proc
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