| OCR Text |
Show If, for any reason, one or both of these criteria are not fulfilled, combustible products, in the form of gaseous molecules or charred solid particles, will remain in the exhaust gas stream or in the fuel bed, and pass from the furnace into the flue gas system or ash removal system. Often these unburnt combustibles burn with entrained air in the flue gas system causing overheating and possible subsequent damage. The solid particles can also lead to visible atmosphere pollution. In order to burn efficiently it is necessary to allow some air in excess of the theoretical requirement in order to give adequate oxygen supply. When combustion takes place in air the active combustion agent, oxygen, is present at a level of only 21% by volume. The remaining 79% (mainly nitrogen) is inactive and serves only as a source of sensible heat loss because it enters the furnace at combustion air temperature yet exits at a much higher temperature in the flue gas, carrying with it heat supplied from the combustion of the fuel and often overloading the fume cleaning system (precipitators, etc.). This cold ballast of nitrogen not only represents a loss of useful heat but it also drastically lowers flame and combustion zone temperature. As a rule of thumb every 1% enrichment of primary air in an air fuel burner has the effect of increasing the apparent volatile content of the fuel by 3%. This is typical of say 15-20% of combustion air being supplied as a primary air. Where low volatile content is coincident with high inerts the obvious result of this effect is that lower C.V. fuels can be combusted in a stable manner. In general, therefore, an increase in the percentage oxygen in combustion air to levels above the normal 21% reduces heat load to the stack gas, increases the efficiency of combustion and improves the ability to combust difficult (i.e. low cv) materials. The use of an oxygen-assisted combustion technique is now a widely practised and accepted method of achieving increased process efficiency and throughput in various industries. For example, oxygen enrichment techniques are used in blast furnaces to increase the effective blast rate, and steel reheat furnaces to increase production throughput. Oxygen-fuel burners are now in use in the majority of metallic and non-metallic industries to boost furnace output, the most common examples being assisted melting on electric arc furnaces, glass tanks, copper smelting, and within the frit industry. Since the advent of the cryogenic air separation plant for oxygen product, industrial oxygen has become readily available either in gaseous or liquid form. With the continuing improvements in air separation technology, oxygen is increasingly becoming more of a cost-efficient and energy-efficient commodity, and as a consequence Air Products has, Page 2 |
