Experimental Study On Flameless Combustion And NO_x Formation Characteristics Of Low Volatile Carbon-Based Fuel

Flameless combustion technology has been promoted from gas fuel combustion to solid fuel combustion in recent thirty years due to its good combustion stability and low NOx emissions.This dissertation uses the method of fuel preheating and air stage to achieve flameless combustion for low-volatile carbon-based fuels to obtain stable combustion and ultra-low NOx emission control strategy,and provide technical for engineering utilization.In this dissertation,on a test rig for carbon-based fuel flameless combustion,operating characteristics of a preheating burner,combustion characteristics of high temperature preheated fuel,and characteristics of NOx formation are studied.The influence factors,such as the fuel itself(volatile content and fuel particle size),nozzle structure and air distribution method,are systematically analyzed.After fuel is preheated,high-temperature coal gas and coal char are generated,and are jetted into a down-fired combustor to be completely burned.Most of the volatiles in fuel are released in the preheating burner,and the nitrogen in the volatiles is then released and converted into N2,and only a small amount is converted into NOx precursors.The properties of preheated fuel are not only related to the properties of the fuel itself,but also closely related to the temperature and air equivalence ratio of the preheating burner.Experimental results show that:the higher the volatile content and specific surface area,the higher the content of combustible components in the coal gas;as the combustion temperature of the preheating burner rises(caused by the change of fuel particle size or the ratio of fuel to air volume),the H2 content in the coal gas increases,while the CO content decreases;compared with the original fuel,the particle size of coal char is smaller(within 100?m),the specific surface area is significantly increased,but the average pore diameter is reduced to varying degrees.Based on the central symmetrical arrangement of the secondary air nozzles,the relevant experimental researches on the basic characteristics of flameless combustion and NOx generation of high-temperature preheated fuel are carried out,and the influence of different operating parameters is discussed.The results show that:the NOx emissions can be effectively reduced by properly increasing the air equivalence ratio of the preheating burner and increasing the release rate of fuel nitrogen during the preheating process;reducing the distance and angle of the secondary air jet and the preheated fuel jet can also reduce NOx emissions to a certain extent,but the effect is not significant;the secondary and the tertiary air-staging has more obvious effect on the preheated fuel combustion and NOx emissions.When the secondary air equivalence ratio is about 0.4,the NOx emission is the lowest;inducing the tertiary air at a suitable position in the down-fired combustor and enhancing the disturbance in the reduction zone,NOx generation can be effectively suppressed.By the means,NOx original emissions can be controlled about 50 mg/Nm~3(@6%O2).In the multi-channel coaxial jet nozzle,the combustion and NOx generation characteristics of preheated fuel are also discussed.The study shows that:when the ratio of the internal and the external secondary air is close to 1,the combustion is the most uniform and the NOx emissions are the lowest;by adding CO2 to the internal secondary air channel to simulate flue gas recirculation,the overall temperature of the combustor is reduced and NOx emissions are reduced.The research on different nozzle structures shows that in this experimental system,the direct jet of the internal secondary air is more advantageous than the swirl jet,and the NOx emissions are significantly lower.Through the above studies,it is found that the high-temperature preheated fuel for low-volatile solid carbon-based fuels(Shenmu semi-coke and coal gasification fly ash)can easily achieve stable flameless combustion.Under flameless mode,the temperature of the down-fired combustor is more uniform,the whole combustion zone is dark red and transparent with no visible flame fronts.Compared with the flame combustion of preheated fuel,the NOx emissions can be reduced by a half or more.Experiments show that the air equivalence ratio of the preheating burner and the velocity of the secondary air jet are not the main factors affecting the flameless combustion achievement of the preheated fuel.The preheated fuel can achieve flameless combustion within a wide range of parameters.However,the unreasonable arrangement of the tertiary air nozzles and the swirling of the internal secondary air will concentrate the combustion process in a small area,resulting in the flame combustion of the preheated fuel.