Influence Of Oil Secondary Air On Single Phase Flow And Combustion In Down-Fired Boiler

Since the introduction of the domestic down-fired boiler in the 1980s,it has a unique structure and has good adaptability to low-volatility coals.At present,it has become the main boiler for burning low-volatility coals.However,because of its own characteristics of coal,its operating efficiency is low,and it is difficult to control pollutant emissions.Nowadays,the society’s awareness of environmental protection is increasing.Relevant policies require that new energy power generation be given priority to the grid,and thermal power units often face pressure load demand.The combustion stability of the down-fired boiler itself is affected by factors such as coal type,and it is difficult to work stably under ultra-low load.Therefore,the optimization of down-fired boilers and the development of new combustion technologies are urgent.This paper mainly analyzes the influence of the oil secondary air on the flow and combustion of a 300 MW Foster Wheeler-type down-fired boiler through single-phase cold test and numerical simulation research,and provides a factual and theoretical basis for subsequent research.Firstly,the single-phase cold mode test bed is built,and the flow characteristics in the furnace under different oil secondary air angles and different oil secondary air rates are analyzed through experiments.It is found that,under the same arch structure,the increase of oil secondary air rate can significantly increase the airflow downthrust depth.With the decrease of the Angle between the oil secondary air and the vertical direction,the area of the airflow back flow area also increases obviously under the ejection-carrying action of the oil secondary air.The parallel arrangement of the oil secondary air and the pulverized coal air is beneficial to improve the utilization rate of the furnace.At the same time,the influence of the oil secondary air on the combustion characteristics of the boiler is analyzed by numerical calculation for the original structure of the boiler and the parallel arrangement of the oil secondary air and the concentrated pulverized coal air.It is found that,in the original structure,the increase of oil secondary air rate is beneficial to move down the high temperature area and prolong the residence time in the pulverized coal furnace.However,when the oil secondary air rate is increased to 25%,the oil secondary air join to the pulverized coal air before the ignition and mixed into the airflow to compress the combustion space of pulverized coal.In the structure where the oil secondary air is arranged in parallel with the pulverized coal air,the increase of the oil secondary air rate is also conducive to the downward movement of the high temperature zone and the prolongation of the reaction time.Meanwhile,in the new structure,the oil secondary air at different air rates is added into the combustion reaction after ignition,without squeezing the combustion space of the pulverized coal.Under the two structures,the increase of oil secondary air rate makes the temperature distribution in the lower furnace more uniform,and at the same time,the area with high oxygen and low temperature near the furnace wall and the cold ash bucket is beneficial to prevent slagging and corrosion.Under different working conditions,the nitrogen oxide emission concentration of the boiler does not change more than 13%.Combined with the analysis in this paper,the oil secondary air can significantly improve the downdraft depth and affect the temperature distribution in the furnace.The Angle of oil secondary air can affect the ignition time of pulverized coal and the utilization rate of furnace space,so the appropriate oil secondary air rate and Angle can significantly improve the combustion performance of equipment.According to the work results in this paper,the boiler can open the oil secondary air to increase the depth of pulverized coal down during normal operation,but the oil secondary air rate should not exceed 15% under the original structure of the boiler.It can be tried to arrange the oil secondary air parallel to the pulverized coal air to increase the feasible range of the oil secondary air rate and improve the utilization rate of the furnace space.