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Show -10- 3.4.2 Natural Gas Gas firing has been one of the traditional methods of reducing NOx in boilers, since it contains no bound nitrogen. However, the CEMFLAM results suggest that fuel NOx is not significant and- that virtually all the NOx generated in a high temperature kiln is thermal NOx. Since gas has a lower emissivity than coal, it must generate higher flame temperatures to maintain heat transfer rates, and hence production. It is possible therefore, that gas firing may generate more NOx than a coal flame, all other factors being equal. It may be thought that the ignition of a gas flame may occur earlier than a coal flame, but this is unlikely because the ignition temperature of natural gas is in the order of 650°C (1200 0 F), whereas it is only in the order of lSO-300 oC for coal (300-600 0 F). Therefore, the ignition of the light volatiles in coal can easily be initiated, given a suitable burner design. 3.4.2.1 HOx Emissions from Conventional Gas Burners The CEMFLAM trials did not examine gas firing and relatively little gas is burnt in high temperature rotary kilns, gas firing being restricted to kilns where ash contamination of the product is unacceptable or where gas is very cheap. There is therefore relatively little published data on NOx emissions from gas fired kilns, the only data known to FCT is given in table 2. All these burners have a relatively high jet momentum and none, except the precalciner kiln, are producing low levels of NOx. Precalciner kilns in any case produces lower NOx, because up to 60% of the fuel is burnt at a lower temperature. In the case of site B, approximately 40% of the fuel is burnt in the precalciner. If this factor is allowed for, the corrected emissions from the kiln would be in the order of 1400 ppm. These results would suggest that for a given burner design, the NOx emissions on gas would not be significantly lower than for medium volatile coal and that the lowest effective NOx levels would be achieved with high volatile coal. 3.4.2.2 HOx Emissions from Precessing Jet Gas Burners A new low NOx gas burner for rotary kilns, known as the Gyro-Therm, has been developed by the University of Adelaide in Australia, together with Adelaide Brighton cement. This burner has been shown to achieve substantial reductions in NOx on small gas fired cement |