Influence Of The Ratio Of Inner Secondary Air And Outer Secondary Air On Combustion Characteristics Of A Swirl Burner Under Sub-stoichiometric Ratio

At present, coal still dominates in the structure of China’s energy productionand consumption, and the main utilization forms of coal is direct to be burnt toprovide heat and produce electricity. The gradual increase of the NO_xemissionwhich has enormous harm gives the rise to the vigorous development of the NO_xcontrol technology which is efficient in economy, stable in operating and apt to beused massively. The technology of low NO_xburner and air-staging combustiontechnology are used widely due to their economy in the part of installing andoperating. In order to get a better NO_xcontrol performance, low-NO_xburnertechnology and air-staged combustion technology are usually used in combination.The low NO_xburner is under sub-stoichiometric ratio condition when these twokinds of technology are combined together, while the research on the combustionperformance of the burner, the formation characteristics of NO and the effect of therelated operating factor is few in literatures.This paper investigates the principle of the influence of diffident stoichiometricratio and diffident ratio of inner secondary air to external secondary(R_SA)undersub-stoichiometric ratio on pulverized coal combustion process and NO formationand reduction process of the primary combustion zone on a pulverized coalcombustion test system with a centrally fuel rich swirl coal combustion burner （CFRburner） through the measurement and analysis of local gas components （O₂, CO,NO）, gas temperature, char burnout rate and elements（C、N） release rate. Still, theinfluence of sub-SR on the performance of pulverized coal combustion, NOformation and reduction characteristic, flame center position and slagging tendencyon the wall is studied. To describe the combustion process of char and the formation and reduction process of NO precisely, a method to determine the boundary of thefuel-rich zone within the swirl pulverized coal flame is proposed and a parameterCR^reNOto scale the conversion degree of the released fuel-N to NO is defined.Char burnout rate varies greatly in the fuel-rich zone which is the main placefor devolatilization and the heterogeneous reaction of pulverized coal. The overallNO emission concentration decreases as SR declines, and the main reasons are thatthe release rate of N elements decreases with SR decreasing, while the CR^reNOdecreases rapidly as SR declines. With the decreasing of SR, the ignitionperformance of the centrally fuel rich burner is not significantly influenced, theposition of the flame center moves downstream significantly, the slagging tendencyis aggravated. The optimal SR is0.862by analyzing the test results comprehensively,considering the economic, environmental and safe performance.R_SAhas significant impact on the combustion characteristic and the feature ofthe formation and reduction of NO in the primary combustion zone. As the R_SAincreases, the mixing point of secondary air and primary air shifts to an earliermoment, and the intensity of mixing is greater, and the moment of the supplement ofO₂for the fuel-rich zone is earlier and the amount is larger which leads to the earlierand greater increase of the local stoichiometric ratio, accelerating the process ofburnout of char and release of elements at the early stage, while leading to thesignificant increase in the amount of NO formation.As the R_SAdecreases from5:5to3:7, the emission concentration of NO reduces48%, and the combustion efficiency decreases slightly, and a relative low R_SAisbenefit for the reduction of slagging tendency of water wall, so a relative low R_SAisappropriate in the real application.