W火焰锅炉SNCR脱硝及其对SCR入口流场的影响

由于W火焰锅炉NO_x排放浓度高，一些电厂采用低氮燃烧+SNCR脱硝+SCR脱硝的耦合脱硝方式，但运行中出现了氨逃逸浓度严重超标的问题。以某600 MW超临界W火焰锅炉为对象，采用CFD数值模拟辅以实测验证方法研究了锅炉负荷、尿素喷射层流场、氨氮摩尔比等因素对SNCR脱硝及SCR入口流场分布的影响。研究发现：SOFA风沿前后墙非等间距布置是造成尿素喷射层速度场及温度场不均的主要原因；随锅炉负荷降低，喷枪层截面平均温度趋向于SNCR最佳脱硝温度，脱硝效率逐渐增加；SCR入口截面温度、流速及NO_x浓度分布皆不均匀，随SNCR脱硝效率提高，SCR入口截面NO_x浓度分布偏差增大，不同负荷时SCR入口截面NO_x相对偏差达35%～53%。SNCR脱硝严重影响SCR脱硝反应器入口NO_x浓度均匀性，最终导致SCR氨逃逸浓度严重超标，建议通过优化SOFA风布置及在SCR入口段加装烟气混合器加以解决。

Study on SNCR Denitration of W-flame Boiler and Its Effect on the Flow Field of SCR Inlet Section

Because of the high NO_x concentration of W-flame boiler, the coupling denitrification technology consisting of low nitrogen combustion, SNCR and SCR has been applied by several power plants. However, severe violations of the ammonia escape concentration of SCR have taken place in the actual operation since the coupling denitrification technology was carried out. In this paper the effects of boiler load, flow field of urea spray section and ammonia-nitrogen molar ratio on the SNCR denitrification and inlet flow field distribution of SCR were studied for a 600 MW W-flame boiler by numerical simulation and experimental verification. It is found that the non-uniform arrangement of SOFA along the front and rear walls of the boiler is the major cause for the unevenly distributed velocity and temperature fields of urea spray layer. With the decrease of boiler load, the average temperature of reducing agent spray section tends to be close to the optimum denitrification temperature, and the NO_x removal rate increases gradually. The temperature, velocity and NO_x concentration at the inlet section of SCR is not uniformly distributed. Specifically, with the increase of SNCR denitrification rate, the unevenness of NO_x concentration distribution at SCR inlet section intensifies such that the relative standard deviation of NO_x may reach 35%~53% at different boiler loads. Moreover, the usage of SNCR has significant impacts on the NO_x distribution across the SCR inlet section, which may eventually lead to the serious violation of the concentration of ammonia escaping from SCR. To solve this problem, it is suggested to optimize the SOFA arrangement and install new type of gas mixer at the entrance section of SCR.