OCR Text |
Show It will usually be preferable to dry and crush a coal with more than about 40% ash. Because of the relatively higher mass flow of air per unit area of grate for fbc in comparison with conventional coal burning equipment, higher specific heat release rates are possible. Still higher heat release rates can be achieved by increasing the pressure in the combustion chamber, providing that the pressure energy can be recovered in a gas turbine, (see Section 2.2.4). Another advantage of fbc is the high rate of heat transfer to cooling surfaces immersed in the fluidised bed. Because up to 50% of the heat released during combustion can be removed by in~bed cooling, fluidised bed boilers can be smaller and hence less expensive than conventional systems. However, this saving is partly offset by the more expensive (4) start-up systemv . The excellent mixing, with resultant uniform distribution of fuel, gives a near-uniform temperature within the bed, which is controlled automatically within the range 800 C to 950 C. These comparatively low combustion temperatures, much lower than for conventional firing systems are below ash fusion temperature eliminating the fouling of boiler surfaces by molten ash and alkali salts common with conventional firing. Additionally absorption of sulphur dioxide can be achieved within the bed by continuous addition of crushed limestone with the coal This captures the sulphur dioxide as calcium sulphate, which is collected with the coal ash. The theoretical addition of limestone is 3.15% for every 1% of sulphur in the coal. The stone cannot be fully utilised and the theoretical addition usually reduces sulphur emission by 50%. A retention efficiency of about 80% can be achieved by increasing the rate to twice theoretical, depending on the stone reactivity and operating conditions. The capital investment to retain sulphur in this way is considerably less than for the wet scrubbing systems needed with conventional firing. Also there is not such a large loss in boiler efficiency arising from sulphur retention. For these reasons, fbc is expected to entirely supersede conventional firing in countries with high 20-9 |