OCR Text |
Show is a refined and proven technology and is used in modern power stations, in the UK and worldwide. Coals of any rank and high ash content can be fired successfully, but it is essential that the coal quality is very consistent, otherwise flame instability occurs. Pf firing is also used for cement kilns and other large-scale industrial furnaces. In recent years, there have been developments to use pf as a means of converting existing oil-fired boilers to coal. Hamworthy have developed a system in which pf is fired with oil to provide flame stabilisation. It remains to be seen whether or not pf can be marketed at an economic price to make conversions worthwhile. Other options under development are to provide the pf in the form of an oil or water slurry, with stabilising agents to prevent settling out, so that it can be stored in existing oil tanks. An important consideration with pf is the need to achieve a high collection efficiency of the fine fly ash, and the cost of this may prove excessive in the context of a small boiler conversion. All of the conventional firing systems must be optimised to suit the particular coal which is being used and it is necessary to make adjustments when the coal is changed if boiler efficiency is to be maintained. A second disadvantage is that, in most conventional systems, the combustion temperature is above the ash fusion point, so that molten ash droplets may be carried into the boiler tube bank and cause fouling. This is particularly the case for pf firing and spreader stokers. A second cause of boiler fouling is that alkali metal salts are distilled from the ash at above 1000 C and then recondense on to boiler surfaces. The extent of this depends on the coal's sodium content, which is related to the more readily measured chlorine content. As deposits accumulate in a boiler, heat transfer is reduced and the efficiency falls. It is usually economic to shut down a package boiler every six to twelve weeks for cleaning. The operating period can be only two weeks for a high-chlorine coal. In larger boilers it is usually economic to use compressed air or steam-jet 'soot blowers' to extend the period between shut-downs. 20-7 |