OCR Text |
Show and carbon particles in the freeboard. The data seems to support such a claim. Figure 6 indicates that combinations of high-temperature levels, moderate to high excess air rate, and low feed rates can tend to produce higher NO levels. The combinations of these parameters result in more x r combustion occurring in the bed, starving the freeboard of carbon particles. The net result is higher NO levels. Usually in an FBC system NO formation is basically from the nitrogen in the coal with some additional NO from the thermal reaction with nitrogen in the combustion air x Reference 4 indicates that NO concentrations may be further reduced by x lowering the combustion zone into the bed, thus establishing the potential of NO reductions by occidental char to occur in the bed as well as x in the freeboard. Conclusions Based on the results of the study, the char exhibited excellent combustion and emission characteristics over a wide range of operating conditions. The small particle size of the char created several operational problems, but overall combustion of char seems applicable to atmospheric fluidized-bed combustion. Data indicated that emissions were easily controlled within Federal guidelines while maintaining combustion efficiencies of greater than 90 percent. The only definable trend in the data, within the accuracy of the experiments, revealed that NO emissions increased with increas-r x ing excess air rates. None of the independent variables dominated as a combustion controlling mechanism, but each in part contributed to maintain the high levels of combustion efficiency obtained in the tests. It appears that the optimum operating conditions occurred at bed temperatures near 1550°F and excess air rates of approximately 5 percent. 22-17 |