Abstract
To solve the problem of the high NOx emissions of W-shaped boiler during the combustion of low-quality coal, a technique combining an additional separated over fire air (SOFA) ports above the furnace arch and fuel-lean coal/air ports retrofit was carried out in a 600 MW subcritical W-shaped boiler. The results indicated that the combustion characteristics after the low-NOx combustion system retrofit could be classified into two stages. An optimal air-fuel distribution could be obtained between lower and upper furnace when the excess air coefficient equaled 0.89. At the moment, both the low NOx emissions and the optimal combustion economy could be achieved. On the other hand, the fuel-lean coal/air ports retrofit shifted down to the lower furnace effectively improved the combustion stability, especially for using low-quality lean coal and anthracites. Furthermore, the new A-layer secondary-air in furnace arch (replacing the original fuel-lean coal/air ports) with higher temperature and faster velocity was favorable to ignite, strengthen the downward coal/air mixture and lengthen the fuel-rich coal/air downward distance.
