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Show For outputs in the range 2 MW to 5 MW (7 t/h steam) the most cost-effective approach is to burn singles coal on a static grate with an above-bed spreading system (Figure 1). This concept was pioneered by GWB Parkinson Cowan (now Thorn EMI), and their Vekos boiler was the best-selling coal packaged boiler through the 1970's. Most boiler makers now offer similar units and Saacke market a front-fired system for conversion of existing boilers. This firing method is restricted to free burning singles and has the disadvantage that the ash must be manually removed from the boiler, three times daily at full output. The ash is pulled from the boiler into a clinker crusher, which delivers it to an automatic conveying system. Several companies are developing automatic de-ashing units and these are expected to be commercially available soon. Coals which swell and stick together when heated are usually reserved for the production of metallurgical coke, but they can be fired successfully in a reciprocating-grate coking stoker, which provides agitation of the fuel bed. For outputs above 5 MW it is usually economic to burn smalls rather than singles. In package boilers, smalls are fired using a chain grate stoker. These units are well developed and provide fully automatic load following and ash extraction. A recent development, initiated by the NCB, is the provision of electrical ignition, initiated by time clock. This eliminates the waste of coal in the traditional overnight 'banking'. Chain grates are also used for outputs below 5 MW when automatic de-ashing is required and they can burn singles if necessary. For water tube boilers up to about 100 MW (130 t/h), smalls coal is fired using either a travelling grate, which is similar to a chain grate, or a spreader stoker above a moving grate, to provide de-ashing. Both types of stoker are well developed and fully automatic. Above 100 MW the size of grate becomes excessive and it is then economic to pulverise the coal and fire it in suspension, forming a flame. The pulverised fuel (pf) size is typically 90% below 70 microns and the mills are expensive, power consuming, and require considerable maintenance. However, pf firing 20-5 |
