Engineering Application Of The Air-staging De-NO_x Combustion In A Coal-fired Power Plant

Reducing the NO_x emission from the boilers is the most important work for acoal-fired power plant in these days. Recently, the air-staging de-NO_xretrofit wasconducted in the Unit#2of Shanghai Waigaoqiao Power Company, colaborating withthe SCR equipments installed in the same time. This paper conducted a series theoreticalanalyses and application research on the engineering of this project.Based on the literature study and theoretical analyses, it was found the nitrogencontent and volatile in the coal can increase the NO_xformation; while the former one ismore significant more the later one. On the other hand, the increase of nitrogen contentdecreases the conversion of nitrogen into NO_x, and thus the increase rate of NO_xformation is smaller than that of the nitrogen content.The results of the operation in the latest a few months showed that the air-stagingeffectively reduced the NO_xemission from the boiler and the reduction rate was40%~70%. And with the SCR technology, NO_xemission could be controlled under thelatest national emission standard in the cost-effective way, while the boiler efficiencywas kept nearly unchanged. The application also showed that the air-staging could not besimply treated as sending the air into the boiler separately. The distance between theover-fire-air (SOFA) port and the highest primary air port should be kept sufficientlylong, such that there is sufficient space and time for the NO_xreduction. As long as thefurnace is high enough, the retrofit could be mainly applied to the SOFA setting whilelittle on the burners; otherwise, it should be on both SOFA and burners. The effect ofNO_xreduction of the air-staging was influenced by the boiler load, the mill operation, thefuel type and the SOFA flow rate etc. NO_xreduction was effective in a high boiler load,but insignificant at a lower boiler load due to the high excess air ratio. It was also foundthe steam temperature in the reheater was reduced, especially in the low boiler load whenair-staging was applied, opposite to the findings of some literatures. This could be causedby the changes of the high temperature region and increase of the highe temperature areain the furnace, which resulting in higher heat absorption of the water membrance.Compared with the SCR technology, the air-staging technology had a higheffectness/cost ratio. It was simpler, easier and less expensive in engineering and barelywith any operational cost. The air-staging effectively reduced the pressure of the post-combustion flue gas treatment. Together with SCR, the NO_xemission control in acoal-fired power plant can be done with much less constrction and operation cost.