OCR Text |
Show 12 Due to the fluctuations in the mass flow measurements, the limit for the minimum excess air ratio at which the CO-concentrations can be tolerated varied slightly from day to day. For this reason control of the excess air ratio via an oxygen sensor was necessary. In this way a residual 02-content of 0.5-1.0% could be established at all operational conditions thus keeping the CO-values near the detection limit. This results in an increase of the boiler efficiency compared with conventional firing systems of this size. IV.3. Influence of exhaust gas recirculation Due to an increase in flame temperature, reduction of the exhaust gas recirculation rate in the secondary air results in the production of thermal NOx. Fig. 10 shows the dependence of the CO- and NOx-emissions on the recirculation rate for oil no.2 as well as for heavy fuel oil. Within the investigated range of 5-45% exhaust gas recirculation, the influence on the CO-emissions was negligible. 70.0 _----------------------....., 0, _-+-- CO (oil no.6) ............. N 60.0 '0_ o -x-NOx-NOxo (oil no.6) - - 0 - - _ ....... _ ~ 50.0 ~ ---=:!--~w--.,.; .;;.-:¢.:::---..:;:-~~~-~ NOX~ no.: 0 E Q. 40 .0 ~ ~ ~ 30 . 0.~ ~ en CI) E w 20 .0 .~ 10.0 ~ ~ --.- NOx-NOxo (oil no.2) - - 0 - NOx (measured for oi 1 . 2~-~-----+------+-----+-----+-----+ opt. primary air excess air ratio , load 85% ...... .... L..-_______ --.-I ,, ' , res. 02=1% ~ .... --... - -- 0 .0 ~----+__--r_.-~~~~~~~.~~~~~~~,~~---~ 0 .0 Fig. 10: 5 .0 10.0 15.0 20 .0 25 .0 30 .0 35 .0 40 .0 Recirculation rate [%] Influence of the recirculation rate on the thermal NOx - production (comparison heavy fuel oil- oil no.2) |