Study On NO_x Reduction By Injection Of Different Reducing Agents Into High Temperature Reducing Atmosphere

As increasingly stringent regulations on NO_x emission,i.e.lower than50 mg/Nm³,have been implemented,it is of great significance to achieve effective NO_x removal.Injecting reducing agent into high temperature reducing zone can avoid the oxidation of reducing agent.Therefore,temperature window for reaction can be extended and further NO_x reduction can be achieved.Based on the 21.4k W drop tube furnace,the high temperature reducing atmosphere was simulated by gas feeding unit.NH₃ and CH₄were injected into high temperature reducing zone respectively and the influence of reaction temperature,oxygen concentration,residence time and mole ratio of reducing agent to NO_x on NO_x reduction efficiency was investigated.Furthermore,chemical kinetic modeling was conducted to investigate the NO_x reduction mechanism by injection of reducing agent into high temperature reducing zone.Experimental results of NO_x reduction by ammonia injection into high temperature reducing zone indicate that maximum NO_x reduction potential is obtained for no oxygen and NO_x reduction will be inhibited with the increase of oxygen concentration when temperature exceeds1573K.When temperature is lower than 1473K,presence of oxygen will promote NO_x reduction efficiency but excess oxygen will lead to oxidation of ammonia and thus stimulate NO_x formation.When initial NO_x concentration is～600ppm and residence time is 1.0s,the optimal temperature and optimal oxygen concentration are 1573K and 0.0%respectively,under which condition a maximum NO_x reduction efficiency of 98.99%is clearly observed.GADM98 mechanism is adopted to analyze the NO_x reduction mechanism and results show that chain reactions can take place under the premise of generation of NH₂ radicals.NH₂ radicals initially result from NH₃/O₂ reactions in the presence of oxygen while from thermal decomposition of NH₃ in the absence of oxygen,which has a strong temperature dependence.Methane has also been employed as reducing agent to explore NO_x reduction by injecting reducing agent into high temperature reducing zone and experimental results reveal that the maximum NO_x removal efficiency is obtained for no oxygen or trace oxygen when temperature is above1473K and excess oxygen will impair NO_x reduction efficiency.However,the optimal oxygen concentration is 0.3%at 1373K and NO_x reduction is inhibited for trace oxygen or excess oxygen.When residence time is 1.0s,optimal temperature and optimal oxygen concentration are 1573K and0.1%and under such conditions a NO_x reduction efficiency of 69.26%is obtained.For kinetic modeling,predictions of GRI 3.0 give a reasonably good match of experimental data for low O₂/CH₄.NO can be reduced into N₂ by methane injection under the premise of the formation of CH_nradicals,which initially results from CH₄/NO reactions in the presence of oxygen and from thermal pyrolysis of CH₄ for no oxygen.Similarly,thermal pyrolysis of CH₄ is also strongly temperature dependent.