| OCR Text |
Show - 200 C\I 0 ~0 -<0 E> 150- a. a. ~ CIJ c: 0 ~ '00 100- .~ E Q) 0 C\I z 50- 1:3 c: -e- NOx as x ~ N20 0 Z 0 I I I I I 0.0 0.5 1.0 1.5 2.0 2.5 3.0 air staging height, m Fig.6 Effect of air staging height on NOx and N20 emissions. The effects of secondary air ratio (i. e. secondary air/total air ratio) and air staging height (above the primary air inlet) on NOx and N20 emissions are shown in Fig. 5 and 6, respectively. It can be noted that the NOx emission decreased with increasing secondary air ratio and air staging height, whilst the N20 emission was relatively insensitive to the secondary air ratio and it decreased only slightly with the air staging height. Detailed analyses of the effects of air staging on NOx and N20 emissions are described elsewhere CNang et ai, 1994). In view of above experimental results, the CFB combustion is advantageous in controlling the NOx emission whilst it is a source of the N20 emission. It appears, from Fig. 4, that both NOx and N20 emissions could be controlled by reducing the overall excess air factor. Since a lower excess air factor could result in a lower combustion efficiency (Gulyurtlu and Cabrita, 1985), the strategy of applying a very low excess air factor may only be applicable when techniques are available which could enhance the combustion performance. The results shown in Figs. 2 and 3 and in Wang et aI (1994) suggest, on the other hand, that the NOx and N20 emissions could be controlled in stages. For example, the NOx |
