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Show 12/15 2.3 Influence of Coal Properties on N20 Formation Figure 7 shows the relation between the N20 conversion and the combustion efficiency at the furnace exit obtained by burning nine different types of coal. The N20 conversion is evaluated as the total fuel nitrogen basis. This figure finds that the N20 conversion decreases with the increase of combustion efficiency. Figure 8 rearranges the results of Fig.7 as a function of fuel ratio. In this figure the N20 conversion increases with increasing the fuel ratio. As the general tendency, the higher combustion efficiency can be attained by the coa 1 of the lower fue 1 rati o. These two fi gures· show thi s behavi or as a function of N20 concentration. When such coals with high combustibility as TH coal are burned, the combustion atmosphere s urroundi ng the coa 1 part i c 1 es becomes hi gh-temperature reduci ng condition in which N20 is easily decomposed. In case of coals with low combustibility like MT coal, however, the atmosphere surrounding the coal particles becomes opposite to the former case since the evolution of volatile matter is limited. Therefore N20 formation is promoted so much. 3. Reaction Paths of N20 Formation/Destruction Figure 9 summarizes the main N20 formation/destruction schemes in coal combustion under the low temperature condition. The sources of N20 are divided into volatile-N and char-No HCN and/or NH3 are the main constituents evolved from the nitrogen compounds in volatile matter. N20 is formed by the oxidation reaction of HCN and NH3 by way of NCO and NH radicals, respectively. After the volatile matter almost burns out, N20 is homogeneously produced by way of the reaction of NO with char-C. Further the char-N also contributes to forming N20 by the heterogeneous direct reaction with NO. With respect to the N20 destruction, on the other hand, H radical plays the important role in homogeneous decomposition of N20. Additionally such particles as char and CaO also decompose N20 catalytically. as shown in elsewhere. 16) |