Nmmerical Simulation Of Combustion Characteristics And NO_x Emissions For A Down-fired Utility Boiler With Arch-arranged OFA

The Down-fired boiler is an ideal boiler for hard-to-burn coal combustionbecause of the high temperature and long travel distance in the furna ce. In china,anthracite and lean coal account for more than40%of the power coal. Thus, thedown-fired boiler has been widely applied and developed. However, practicaldown-fired boiler operations still suffer from the problem of high NO_xemissions.The arch-arranged OFA proposed in this paper can decrease NO_xemissions andkeep high coal burnout rate.Numerical simulation of combustion characteristics and NO_xemissions for a660MWe FW down-fired boiler with the new OFA arrangement was conducted onthe FLUENT software. By comparing and analyzing the results, the optimal OFAratio and angle was determined.With the fixed air amount, as the OFA ratio increases from0%to25%, thepenetration depth of the arch-injected fuel-rich flow changes little while that of theOFA flow increases, which is favorable for the mix of the OFA and the upflowinggas. Besides, the high temperature zone in the furnace center moves upward,deteriorating the combustion of coal particles in the lower furnace. The carboncontent in fly ash increases from4.23%to6.79%. However, the NO_xemissions inthe furnace exit decrease from1559mg/m~3to1051mg/m~3. Therefore,comprehensively considering the effect of the OFA ratio on the flow andcombustion characteristics as well as the NO_xemissions and carbon content in flyash of the furnace exit, an OFA ratio of20%is recommended.With the fixed OFA ratio, OFA angle mainly affect the flow and combustioncharacteristics in the zones nearby the furnace arch and the upper furnace. The mixpoint of the OFA flow and the upflowing gas descends with the increasing of theOFA angle, prolonging the distance for burnout of the coal particals in the mixedflow. However, the horizontal momentum of the OFA flow decrease intensely and itmixs badly with the upflowing gas at large OFA angle. Concurrently, part of theOFA is involved into the recirculation zones under the furnace arch at wide OFAangle, weakening the reducing atmosphere of the lower furnace. As the OFA angleincreases from15°to30°, the average gas temperature in the upper furnace firstlyincreases and then decreases, reaching the highest point at20°. In contrast, theaverage O2content firstly declines and then ascends, reaching the bottom at20°.The carbon content in fly ash and the NO_xemissions of the furnace exit show thesame trend as the average O2content. The former arrives at the lowest point at20°and the later at25°. When the OFA angle increases from20°to25°, the NO_xemissions decreases by29mg/m~3but the carbon content in fly ash rises by0.22%, Based on the above analysis, the optimal OFA angle is20°.