| OCR Text |
Show The initial interest was in coal oil mixtures, COM, containing up to 60% of coal by weight, representing about 40% of the heat content. By using a specially prepared low-ash coal, the problems of conversion are minimised, but savings arise only from replacing 40% of the oil. It is now generally considered that COM will not be economic at the price differential between oil and coal in Europe, but it may be used elsewhere if low cost coal is available. Development work on COM has been carried out in the UK by BP and Shell. More recently, attention has turned to using a coal water slurry, CWS, as a means of effecting 100% oil replacement. However the problems of conversion are increased and, in addition, there is a loss in boiler efficiency of about 4% because of the fuel's high water content (40-50%). As with COM, it is preferable to use a coal with minimum ash content, and this involves a secondary ash separation stage after the coal is pulverised, e.g. by froth flotation. Thus the coal in a CWS will be substantially more expensive than lump coal as fired in a stoker or fbc, and somewhat more expensive than pulverised coal. Even if the preparation and firing of CWS becomes commercially possible, it may prove preferable to replace oil boilers before the end of their useful life rather than fire CWS in them, particularly in the case of low-cost shell boilers. Ash Handling The ash content of UK washed coals is in the range 5 to 10%: the part-washed coals supplied to CEGB and other very large users contain about 18% ash. The nature of the ash after combustion, and the method for collecting and handling it, is dependent on the firing method. However, in most cases, coal ash will be dusty and potentially abrasive. With stoker firing most of the ash remains on the grate: a small proportion is entrained in the combustion gas but dust emission standards can be met with a simple cyclone. With a moving grate, ash will fall off the end into the ash handling system but, with a static grate or conventional underfeed stoker, it must be transferred manually from the boiler into the ash system. Pneumatic conveying is usually the preferred method of conveying ash from the boiler to the storage hopper. Dense 20-27 |
