Numerical Simulation Of The Combustion Retrofit And Optimization For A 600MW FW Type Down-Fired Boiler

Increasingly serious air pollution on the health of our people live has caused a serious threat. Thermal power station has been the target of public criticism. So energy conservation is the trend for the thermal power units with efficient combustion and low nitrogen transformation in recent years. In order to save cost and time of trials, the use of CFD software for firing simulation and optimization is an important method for research today. This paper simulated for a down-fired boiler power plant of 600MW class on a trial basis, analyzing characteristics of furnace internal combustion and pollutants generation, which has important engineering significance.Down-fired boiler is used for low reactivity coal, mainly anthracite and lean coal. Because volatile content of the low reactivity coal is less than 10%, whose combustion characteristics are difficult to catch fire, bad combustion stability, long burning time, high NOx generation capacity. Thus seeking measures to ensure stable pulverized coal combustion and reducing NOx emissions are very necessary. In this paper, the CFD software has been used for W flame boiler combustion numerical simulation. The temperature field, the field of oxygen and NOx formation in different conditions were analyzed and compared, various programs were designed to compare the pros and cons in order to find the measures to reduce NOx emissions and achieve stable and efficient combustion of pulverized coal.First, the paper discussed the influence of overfire air height on furnace combustion. Overfire air vents are used in Tangentially boilers and cyclone coal-fired boiler widely, but less applying on down-fired boiler. In the simulation, overfire air vents are arranged in the front and rear walls of the furnace, and the front and rear wall vents are arranged in a staggered form, in order to increase the intensity of the disturbance. OFA height simulation is set four conditions, the height of each OFA working conditions is raised one meter. The simulation results indicate that with the height of overfire air increasing, the average oxygen and the average temperature level increase along the horizontal cross-section going up in the furnace, NOx emissions reducing. In this section of the simulation, it was found that locating overfire air vents near the throat portion is advantage for the recirculation zone under the arch.Secondly, the paper discussed the impact of the OFA rate on the furnace combustion. The main idea of air classification is that the pulverized coal in the primary combustion zone is in an atmosphere of excess air ratio less than 1. The amount of NOx generated would be less. The greater rate of OFA, the less NOx generated, but mechanical incomplete combustion losses would be greater. It is possible to make the big screen superheater temperature exceed, so it is necessary to find the best ratio of OFA. The simulation results indicated that the bigger OFA rate, the higher the average temperature on the horizontal cross section of the furnace, the less NOx emissions. When OFA rate reaches 20%, it can reach the central chamber of high temperature flue gas, which helps unburned carbon combustion.This article also simulated a comprehensive improvement of working conditions, which is closing A, D wind, E wind being halved, these air being flowed into the furnace by overfire air vents, in order to achieve the objective of both optimizing ignition and combustion of pulverized coal and reducing NOx emissions. After closing the A, D wind, the simulation results indicate that the recirculation would not mixed with the cold secondary air, the temperature level has been improved, the flue gas temperature of the recirculation zone around the pulverized coal has improved significantly, volatile combustion has also been advancd, which helps to heat the concentrated pulverized coal. Moreover, the OFA is helpful for reducing the NOx emissions.Considering the above simulation results, the improved working condition of closing A, D wind and set OFA wind has a certain effect in reducing NOx emissions and optimizating coal ignition and other aspects.