贯流锅炉表面燃烧改造后热力性能的数值模拟研究

Numerical Simulation Study on Thermal Performance of Tubular Boiler after Surface Combustion Modification

采用表面燃烧器是目前贯流锅炉低氮改造的主流技术。基于ANSYS软件,以实际运行的锅炉为研究对象,对贯流锅炉进行表面燃烧改造后的燃料和空气混合物的流动特性,温度特性,传热特性及排放特性等热力性能进行数值模拟研究,得出冷热态燃料和空气混合物及烟气的流动速度特性,热态烟气流场分布,炉膛内的温度场及CH_(4)分布,CO_(2)和NO_(x)的浓度分布,水管壁面温度分布等。通过模拟发现,烟气经过第一回程出口时其流动方向发生改变,并且由于烟气流通截面的缩小,随着不断靠近第一回程出口处,烟气速度不断增大,在内侧管和外侧管所形成的通道中存在最大速度。通过对受热面辐射热负荷计算发现,表面燃烧改造后,辐射热负荷远较锅炉原设计时采用扩散式燃烧器时显著减小。烟气第一回程处外侧水管受到未经充分换热的高温烟气高速的垂直冲刷,造成该处部分外侧水管温度较高,过热和热疲劳应力的反复作用使该处水管易产生密集型裂纹。从传热学的角度分析得出贯流锅炉表面燃烧改造后第一回程出口处的水管易产生裂纹是由于过热疲劳所致。

The use of surface burners is currently the mainstream technology in the low nitrogen transformation of tubular boilers.Based on the ANSYS software and taking the actual running boiler as the research object,conducts numerical simulation research on the flow characteristics,temperature characteristics,heat transfer characteristics and emission characteristics of the fuel and air mixture after the surface combustion transformation of the cross-flow boiler,and obtains the flow velocity characteristics of the cold and hot fuel and air mixture and flue gas,the hot flue gas flow field distribution,the temperature field and CH_(4)distribution in the furnace,and the concentration distribution of CO_(2)and NO_(x),The water pipe wall temperature distribution,etc.Through simulation,it is found that the flow direction of the flue gas changes when it passes through the first return exit,and due to the reduction of the flow section of the flue gas,the flue gas velocity increases as it gets closer to the first return exit,and the maximum velocity exists in the channel formed by the inner tube and the outer tube.The calculation of radiant heat load on the heating surface shows that the radiant heat load is significantly reduced after the surface combustion transformation compared with the original design of the boiler with the diffused burner.The outer water pipe at the first return of the flue gas is vertically washed by the high temperature flue gas without sufficient heat exchange at high speed,resulting in high temperature of part of the outer water pipe,and the repeated action of overheating and thermal fatigue stress makes the water pipe easy to produce dense cracks.From the point of view of heat transfer,it is concluded that the water pipe at the outlet of the first return passage afer surface combustion transformation of cross-flow boiler is prone to crack due to overheat fatigue.