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Show 2/15 INTRODUCTION Recently N20 has been noticed as one of the gases effective to global greenhouse effect and one of the tri gger speci es on the chemi ca 1 destruction 0 f stratospheric ozone layer. 1,2) Most of N20 is considered to be formed from the agricultural activities and natural ecological cycles. The exhaust gas re- 1 eased from coa 1 combus tors can be cons i dered as one of the a 1 ternat i ve sources of N20 production. In general the characteristics of N20 formation depend on the types of combustor and theirs operating conditions, especially the combustion temperature. The study on N20 formation/destruction has been already reported by using pulverized and bubbling/circulating fluidized bed coal combustors. 3-12) In summarizing the previous results, only a little concentration of N20 is released from the high temperature pulverized coal combustors. In the bubbling fluidized bed coal combustion, on the other hand, relatively higher N20 concentration is detected so far. Our previous report13) studied the characteristics of N20 formation/destruction experimentally and theoretically under high temperature pulverized coal combustion condition. The experimental results show that N20 is mainly formed during the initial stage of volatile matter combustion, but that it reaches a low value at the exit of furnace. The maximum N20 concentration in different types of coal depends on the axial temperature gradient, the content of volatile matter and the flame structure formed around coal particles. N20 concentration at the exit increases with increasing combustion air ratio. It is elucidated by the chemical kinetic analysis during combustion that N20 is mainly produced by way of the oxidation reactions of both HCN and NH 3, but that it is decomposed by H radicals which are produced by CO oxidation. In another previous study14) at lower combustion temperature, the N20 formation/destruction characteristics have been discussed by using a lab-scale bubbling fluidized bed coal combustor and by simulating numerically the volatile matter combustion. This study finds that N20 concentration increases with decreasing the bed temperature and with increasing excess air ratio. N20 seems to be mainly . formed during the volatile matter combustion. HCN and NH 3• evolved as volatile- N species, strongly contribute to N20 f0rmation. Especially HeN contributes more to N20 formation than does NH 3. |