| OCR Text |
Show The effect of excess air factor on NOx and N20 emissions is shown in Fig. 4. Here, the operating conditions (apart from temperature) were the same as those for Fig. 3. It shows that both NOx and N20 emissions in the CFB increased with increasing excess air factor. Taking as an example when the excess air factor in the riser was increased from 5% to 65%, the N20 emission increased from 100 to 210 ppmv (3% 02), and the NOx emission increased from 95 to 265 ppmv. This result agrees with that of Hiltunen et al (1991) but differs from what was reported by Moritomi et al (1990), in which the emissions of NO and N20 were almost invariable with increasing excess air factor. The increase in both NOx and N20 emissions with increasing excess air factor may be explained in light of the combustion characteristics of coal under similar conditions. During experiments we observed an increase in CO concentration at the base of the riser from 0.1% to 0.140/0, and then to 0.3% when the excess air factor was increased from 5% to 200/0, and then to 40%, respectively. This indicates that coal devolatilization could be enhanced by an increase in the excess air factor. The subsequent combustion of volatiles therefore produced more NH3 and HCN, which caused both NOx and N20 emissions to increase. - 200 (\I 0 ~ 0 -<0 E> 150 0. 0. (J) c: 0 (J) 100 .!:!l E (J) 0 (\I z 50 "'0 c: -e- NOx ~ X --E9- N20 0 Z 0 0.0 0.1 0.2 0.3 0.4 0.5 secondary air ratio Fig.S Effect of secondary air ratio on NOx and N20 emissions. |
