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Show Property Stoichiometric Air Requirement (lb air/lb fuel) C/H ratio Fuel Heating Value (Btu/lb) Ignition Temperature (OF) Volume of Flue Gas (ftl/million Btu) gross nett Table 3 Comparison of Fuel Properties Fuel Oil 13.68 7.8:1 18,100 17,065 430 9040 Natural Gas 16.75 3.5:1 22,100 20,075 1005 10,400 As shown 10,400 ft l of flue products per million Btu (nett) of energy released are produced for gas compared to 9040 ftl/million Btu for oil. This has important implications for the kiln induced draught fan capacity. The situation is exacerbated by the higher flue gas temperatures associated with gas firing which will create a larger volumetric flow of exhaust products. The obvious solution of increasing the induced draft fan capacity was not possible owing to electrical power limitations. Fuel oil contains high order hydrocarbons which readily crack to form soot giving a highly luminous flame. By way of contrast, the relatively low carbon content of natural gas means that gas flames have a low emissivity even when the burner is designed to produce a luminous flame. Figure 10 shows the typical heat flux profiles produced by similar burners operating on fuel oil and natural gas. As can be seen the oil flame releases a major part of its heat close to the burner nozzle giving a burning zone close to the kiln nose ring. The gas on the other hand releases its heat further down the kiln at the tail of the flame. This results in shift in the burning zone further up the kiln which is typical for gas firing in kilns. The region of negative heat flux close to the burner nozzle is caused by heat being initially absorbed by the burner jet be fore it is hot enough to radia te to its surroundings. The reduced radiative heat trans.fer from a gas flame results in less of the fuel's energy from being transferred to the kiln charge. This in turn leads to higher gas temperatures at the exhaust end of the kiln which not only increases the volume of flue gases but also increases the kiln heat losses. This occurs not only as a resul t of increased sensible losses from the flue gas itself but also from higher cold end shell losses. A change from fuel oil to natural gas firing will result in a reduced kiln thermal efficiency leading to a higher specific fuel consumption. The increase in fuel consumption can vary between 5 to 15% depending upon the process involved, kiln system and burner design. This increase in fuel consumption, however, is often more than offset by the lower cost of the natural gas, making it a more economical fuel to burn in terms of operating costs. |