| Gas-fired boilers have aroused much concern because of their advantages,like low emission of sulfate,weak corrosion of flue gas to boilers and long service life.With the development of numerical simulation technology and computational fluid dynamics,gas-fired boilers can be numerical simulated by CFD software that can help to guide the boiler design and actual operation according to the simulation results,which has become an important method to do research on gas-fired boiler.In this paper,FLUENT software is used to numerically stimulate a certain type of 58 MW gas boiler based on its own features on the following aspects: the excess air coefficient,inlet radius,load rate,vortex intensity,the ratio of secondary air inlets,combustion and pollutant emissions.The optimum working conditions of this type of boilers under the above parameters are put forward.Firstly,the temperature field and concentration field of the boiler furnace are simulated numerically.The temperature distribution and the law governing NOx generation are researched by changing the inlet radius（0.5m,0.75 m,1.0m）,the excess air coefficient（1.0,1.2,1.4）and load rate（60%～120%）,and then the emission of pollutant gases,NOx and CO,in the outlet is finally obtained.The results show that when the excess air coefficient increases,the high temperature area in the furnace decreases,the overall temperature decreases,NOx emission increases first and then decreases.What’s more,the change of CO emission is the same as NOx.With the increase of inlet radius,pollutant emission decreases with high excess air coefficient,and decreases with low excess air coefficient.As the load rate increases,the air outlet temperature increases and pollutant emission decreases.In addition,this paper explores the influence of swirling flow at the inlet on the temperature of the boiler outlet and NOx emission.The results show that with the increase of the inlet angular velocity,the temperature in the furnace first presents a downward trend,and the high temperature region also gradually decreases.When the inlet angular velocity is higher,the range of the temperature change in the furnace becomes larger,and the area with high temperature increases greatly.NOx emissions first decreased and then increased,which is with the lowest value at 6rad/s.The CO emission increases slightly with the increase of angular velocity,and the increase becomes larger when the angular velocity of the inlet is higher.According to the above results of numerically stimulating 58 MW gas-fired boiler,the boiler with high pollutant discharge is selected for the secondary air treatment,and the focus is on the influence of the ratio of primary and secondary air on the internal temperature of the furnace and pollutant discharge of the outlet.The results show that with the increase of secondary air ratio,the overall temperature in the furnace decreases and the high temperature area also decreases.NOx emissions first decreases and then increases,and when the secondary air ratio is 50%,NOx emissions are lower than those without secondary air.CO emissions shows an upward trend,and when the secondary air ratio is less than 25%,the emissions are lower than those without secondary air.In order to further reduce the pollutant emission of boiler,OFA was added to the boiler,and the variation trend of pollutants in the furnace and outlet was studied.The results show that the flue gas recirculation system and the secondary air exhaust air system can reduce the NOx and CO emissions of the boiler outlet,and the reduction of NOx in the OFA is larger.In addition,with the increase of exhaust air proportion,NOx emissions in the outlet first increased and then decreased,and reached the lowest value when the exhaust air proportion was 25%.The CO emission of the outlet decreases with the increase of the proportion of exhaust air,and decreases when the proportion of exhaust air is higher than 25%.After conducting comprehensive analysis of the numerical stimulation results and combining with the actual applicability of the 58 MW boiler,the temperature inside the furnace and the pollutant emissions can be reduced to the greatest extent when the following parameters are met: the excess air coefficient is between 1.2 and1.4,the inlet radius between 0.75 and 1.0m,the load rate between 100% and 120%,the inlet swirl angular velocity between 4rad/s and 6rad/s,the proportion of secondary air between 37.5% and 50%,and the proportion of OFA is near 25% by flue gas recirculation.It provides some reference for the actual operation of gas boiler. |
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