Effect Of Flue Gas Recirculation On NO_x Emissions Characteristics Of Gas Non-premixed Combustion

Nitrogen oxides(NO_x)are one of the major atmospheric pollutants and have a serious impact on the environment,climate and human health.With the increase of fossil energy consumption,the environmental pollution caused by NO_x emissions is also becoming more and more serious.In the face of ever-increasing nitrogen oxide emission reduction and environmental protection pressure,Chinese scholars are actively developing low-nitrogen combustion technology.Current low-nitrogen combustion technologies commonly used in industrial combustion equipment include:low excess air ratio,lean-burn combustion technology,swirl combustion technology,oxygen-enriched combustion technology,and flue gas recirculation technology etc.The flue gas recirculation technology has the advantages of less investment,simple system,and high denitration efficiency.Beijing has stipulated that the NO_x concentration in the new gas boiler shall not exceed 30 mg/m~3from April 1,2017.It can be predicted that the national NO_x emission standards for gas boilers will gradually increase.Reducing nitrogen oxide emissions from gas combustion is an urgent and important challenge.At present,the flue gas recirculation technology is mainly used in coal-fired boilers and is less used in industrial gas equipment.Therefore,it is importtant to study the effect of flue gas recirculation technology on the NO_x emission characteristics in the combustion process of gas.In this paper,on a small combustion experimental device with liquefied petroleum-gas as fuel,the effects of different excess air ratios,gas flow rate,and flue gas recirculation rate on NO_x emissions from liquefied petroleum gas combustion were studied,and the three-dimensionalnumerical simulations of the test conditions were performed.In addition,the numerical simulation method was used to study the optimization of water spray on the flue gas recirculation.The effect of the dimethyl ether/liquefied petroleum gas co-firing technology on reducing NO_x emissions was also obtained.Through the comparison of experiments and simulation results,it is found that the high temperature zone in the combustion chamber is mainly distributed in the middle of the furnace.With the increase of the excess air coefficient,the combustion chamber temperature and theNO_x emission concentration showed a trend of increasing first and then decreasing.Whenα=1.1,the NOx concentration peaked and then gradually decreased.Whenα=1.2-1.3,it has both high combustion efficiency and low NO_x emission.When the gas flow rate increases,the combustion chamber temperature monotonously increases,and the NO_x concentration also increases.When the flue gas recirculation rate is less than 20%,the NO_x emission concentration decreases with the recirculation rate increasing.Whenα=1.1 and 1.2,the flue gas recirculation rate is 20%,the NO_x concentration is around 60-76 mg/m~3.When the flue gas recirculation rate exceeds 20%,the CO concentration sharply increases and the combustion becomes unstable.Through numerical simulation,it is found that the water spray rate in the air has a significant effect on the combustion process,with the increase of the water spray rate,the NO_x emission concentration would decrease,the CO content would increase.When the water spray rate is controlled at about 0.2,the use of smaller flue gas return flow rate can achieve good NO_x emission reduction effects.When the flue gas return rate is9%,the NO_x concentration is 17.6 mg/m~3.When dimethyl ether is blended with liquefied petroleum gas,on the one hand,the flash evaporation mixing effect of dimethyl ether,on the other hand,the rapid NO_x generation rate in the fuel-rich area is reduced,and both of them can effectively reduce NO_x emissions.Compared with pure flue gas recirculation,when dimethyl ether volume ratio is 0.3,flue gas recirculation can achieve better NO_x emission reduction effect.Finally,the flue gas recirculation technology was applied to a WNS1-1.0-Q horizontal internal combustion natural gas steamboiler.When the flue gas recirculation rate is 20%,the NO_x concentrationis 39 mg/m~3.