OCR Text |
Show or dilute phase can be used, although there is a preference for suction dilute phase because it ensures that there is no dust emission through holes eroded in the conveying pipe. Mechanical conveying systems generally suffer from excessive wear due to ash ingress into moving components, although conveyor belts submerged in water are sometimes appropriate. Water sluice systems are under development. For stokers in which clinkers may form, such as static grates, it is necessary to install a clinker crusher before the conveying system. In the case of fbc and pf firing, most of the ash is entrained in the combustion gases, although it is still usually necessary to make provision to remove ash from the combustion chamber. The ash entrained from fbc is somewhat less fine than that from pf and is thus collected more readily. High efficiency cyclones are sufficient to meet the emission standards for smaller boilers, but are not adequate for the stricter standards applying to larger boilers. Bag filters, or a lower cost combination of cyclones and bag filters, are used in larger industrial boilers and are extremely effective, albeit somewhat expensive. Electrostatic precipitators are used in power station boilers. I 5. Conclusions The realisation that the availability of oil and gas will become increasingly limited, leading to higher prices, has resulted in a resurgence of interest in coal, particularly as a fuel for industrial applications. This interest is stimulating improvements in traditional coal combustion technology as well as the development and commercialisation of new technologies such as fluidised bed combustion. The amenity of coal is being improved by increasing use of pneumatic conveying, with consequent reduction in capital cost and maintenance requirements. Increases in reliability and automation, including automatic ignition, have been achieved in traditional firing equipment. Fluidised bed systems are providing a high level of amenity and reliability, while maintaining high efficiency for extended periods by virtually eliminating boiler fouling. Modern coal firing readily meets the requirements of UK Clean Air Acts and techniques exist to further reduce emissions, albeit with an increase in the capital and operating costs. 20-28 |