| China is the world's largest coal producer and consumer.Coal has always dominated China's energy structure for a long time.The nitrogen oxides(NOx)produced by coal combustion are one of the main pollutants that damage the ecological balance and endanger human health.Besides,the CO2 generated in the combustion process is the main cause of the greenhouse effect.Oxy-fuel combustion technology(O2/CO2 flue gas recirculation combustion technology)can achieve comprehensive control of CO2,SO2,and NOx,and is a clean coal combustion technology with great development potential and advantages.Therefore,it is of great significance to study the NOx emission characteristics and its influencing factors in oxy-fuel combustion of pulverized coal.In this paper,the NOx emission characteristics of methane and pulverized coal under oxy-fuel combustion are simulated using a detailed chemical reaction mechanism.First,using Chemkin software,the nitrogen conversion characteristics of CH4/NH3,CH4/NO,and CH4/NH3/NO flames were simulated,and the effect of high concentration of CO2 on nitrogen conversion during combustion was investigated.The calculation results show that for CH4/NH3 flames,in the O2/CO2 atmosphere,CO2 generates a large amount of CO through the reaction of CO2+H=CO+OH,which changes the composition of the O/H/OH elementary pool and increases the O,H,OH concentration,thereby promoting the conversion of NH3 to NO.Under rich-fuel combustion conditions,CO2 concentration has a greater effect on the nitrogen conversion characteristics and exists three zones,while it has less effect on the lean fuel combustion conditions.For CH4/NO and CH4/NH3/NO flames,the presence of high CO2 concentration is not conducive to the reduction of NO.The reactions of CHi-NO and NHi-NO are important reactions in the NO reduction process,which are beneficial to reduce NO emissions.Secondly,based on the CRN network method,a one-dimensional model of pulverized coal Oxy-fuel combustion is established and simulated according to previous experiments.The simulation results show that the conversion rate of fuel nitrogen under Oxy-fuel combustion is lower than that of air,and Oxy-fuel combustion has better NOx emission reduction potential.The concentration of HCN and NH3 under oxy-combustion is higher than that of air,which is beneficial to the reburning reactions of NO.CO2 atmosphere has little effect on the release of volatile nitrogen,and HCN is the main precipitation path of char nitrogen.No thermal NOx will be generated during Oxy-fuel combustion,and the Zelvioch mechanism will proceed in the reverse direction with a certain reduction effect on NOx.The influence of heterogeneous reaction in Oxy-fuel combustion is greater than that in air atmosphere,where char-no reaction is beneficial to reduce fuel nitrogen conversion rate,while gasification reaction will increase nitrogen conversion rate.Increasing the length of the primary combustion zone can reduce nitrogen conversion.The conversion rate of fuel nitrogen decreases first and then increases with the depth of classification,and there is an optimal classification depth.Flue gas recirculation is beneficial to the occurrence of NOx reburning reactions.When there is no initial NO in the reaction gas,water vapor will weaken the generation of NO,and when there is initial NO,it will suppress the reburning reactions of NO and increase the emission of NO.With the increase of particle size,the NOx emission decreases first and then increases.There is an optimal particle size so that the NOx emission reaches a minimum value.The volatile content,nitrogen content,specific surface area and reactivity of the coal all have an impact on NOx emissions.NOx emission characteristics of different coal types vary widely... |
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