OCR Text |
Show 9/15 there. The decreasing rate of NO concentration is also faster. This result means the char-N contributes to the N20 formation. N20 concentration at the furnace exit attains 450 ppm, the value of which is about 4.5 times as much as that of TH coal. The above-mentioned result suggests that N20 is easily formed by way of the following reactions related to HCN rather than those to NH3 as HCN + ° ~ NCO + H NCO + NO ~ N20 + CO NH3 + OH ~ NH2 + H20 NH2 + OH ~ NH + H20 NH + NO ~ N20 + H Thi s tendency we 11 agrees wi th that obta i ned by bubb 1 i ng f1 ui di zed bed coa 1 combustion presented elsewhere. 14) 2.2 N20 Formation Mechanisms after End of-Volatile Matter Combustion Not only the gas spec ies concentration but also the residual fraction of C, H and N elements in the burning particles are analyzed. Therefore the nitrogen mass ba 1 ance can be obta i ned from both gas and so 1 i d phase i nformat i on relating to such nitrogen compounds as HCN, NH 3, NO, N20 and char-No Figures 4 and 5 show the nitrogen molar fraction contai-ned in gas and solid phase along the furnace axis, corresponding to Figs. 2 and 3, respectively. In both figures C.R i (i: gas species) indicates the conversion ratio from fue1-N to species "i" based on nitrogen atoms, and C.RN(}+N20 shows the conversion ratio from NO to N20. As seen from Fig.4, in case of TH coal, C.Ri except C.RNO and C.RN20 becomes almost constant in the downstream region over 500 mm. This result suggests that this increase of N20 is caused by the change of NO (C.RNO+N20 ). In Fig.5, on the other hand, the increase of N20 is attained by not only the change of NO but also the oxidation reaction of char-No Figure 6 shows the total sum of nitrogen element in NO and N20 along the furnace axis, based on Figs. 4 and 5. In this f i gure the value of N20 is corrected to be double of the N20 concentration i n order to identify both values as the nitrogen element basis. This figure indicates the value of ( NO + 2 N20 ) |