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Show The sulphur content of UK coal is relatively low, typically 1.5%, and UK oil also has a low sulphur content. A policy of flue gas dispersion, introduced in the mid 1950's, has led to ground level concentrations of SO2 which comply with European Standards. The Act specifies the stack height which must be used, to ensure dispersion, in relation to the boiler house capacity and the fuel sulphur content. However, local planning requirements sometimes place restrictions on stack heights, which effectively puts a limit on the fuel sulphur content. An alternative would be to incorporate sulphur retention, but this is not economic at present because sufficient low sulphur fuels are available. There is no specified standard for the emission of N0X from boilers. However, local inspectors have authority to impose 'best practicable means' to minimise emissions from chemical, processing and other plants, and this enables them to place restrictions on SO2 and N0X emissions from boilers at such sites. Combustion of Coal (2) There is a very wide variety of coal firing equipment available and it is necessary to select the most appropriate type for each application, taking into account the coal size-grade and its rank, i.e. volatile content and its tendency, when heated, to swell and stick together forming coke. 2.1 Conventional firing methods For small heating boilers, up to 2 MW, which will use free-burning singles, the traditional firing method is an underfeed stoker. Conventional designs are well proven and reliable, but require manual lighting and ash removal (twice daily, at full output) and are somewhat inefficient. Following NCB development initiatives, manufacturers now offer more efficient units incorporating electrical ignition controlled. by time clock and automatic de-ashing. In recent years, two reciprocating grate stokers have been introduced, providing fully automatic operation for outputs up to 2 MW. These are the Escom Stoker and the Proctor Mini-Coking Stoker. Several units of both designs are now in commercial operation. 20-4 |