Reaction Mechanism Of Heterogeneous Reduction Of NOx On Carbonaceous Surface

NO and NO₂produced by pulverized coal fired boiler combustion in power plant are important causes of acid rain,photochemical smog,stratospheric ozone destruction,global warming and other disasters.Effective control of NOx has always been paid attention to by researchers.Staged combustion in furnace to reduce NOx through heterogeneous carbon based surface is a commonly used low nitrogen control method,but there are still a lot of theoretical problems to be solved.Based on the density functional theory（DFT）calculation method of quantum chemistry theory,this study studies the chemical reaction mechanism of heterogeneous reduction of NOx on carbonaceous surface in the furnace,reveals the heterogeneous reduction reaction path process of NOx in the process of coal combustion from the molecular level,and provides more theoretical basis for staged combustion control of NOx in coal-fired power plants.The main contents are as follows:Firstly,the reaction mechanism of homogeneous reduction of NO and NO₂with CO was studied by using the density functional theory of quantum chemistry,and the geometry and energy of each stagnation point in the reaction path of homogeneous NO and NO₂reduction were optimized.The results show that:1)the decisive step of homogeneous NO reduction is the formation of NO dimer,and the activation energy is 254.6 k J/mol;the homogeneous NO₂reduction only experiences one transition state,The activation energy is 117.0 k J/mol.2)The activation energy of homogeneous NO₂reduction is smaller,and the reaction rate constant at the same temperature is greater than that of homogeneous NO reduction,which indicates that NO₂is easier to react with CO and then be reduced than NO.Secondly,the heterogeneous reduction mechanism of NO and NO₂on carbonaceous surface was calculated by density functional theory using zigzag type and armchair type carbonaceous surface models.The results show that:1)the decisive step of heterogeneous reduction of NO is the formation of N₂;the path reaction of NO₂reduction can be divided into two stages:NO₂reduction stage and carbon oxide release stage,and the the rate determining step of NO₂reduction stage is NO release.2)The activation energies of heterogeneous reduction of NO and NO2 on zigzag and armchair model are 86.94 k J/mol,104.01 k J/mol and 52.16 k J/mol and 125.00 k J/mol,respectively.The activation energies of heterogeneous reaction are less than those of homogeneous reaction,indicate that the carbonaceous surface can promote the reduction of no and NO₂.3)CO molecule can reduce the energy barrier and accelerate the reaction rate of each stage in the path of NO₂reduction,which can promote the heterogeneous reduction of NO₂on carbonaceous surface.Finally,based on the calculation method of density functional theory,K was selected as the research object to explore the influence of metal loading on the heterogeneous reduction reaction of NO and NO₂from the molecular level,and the reaction paths before and after K loading were both calculated,and the different paths were compared and analyzed based on energy change.The results show that the decisive step energy barriers of heterogeneous reduction reaction of NO and NO₂on the surface of clean carbonaceous are 74.4 k J/mol and 166.9 k J/mol,respectively,and those energy barriers above on the carbon surface loaded with K are 47.2 k J/mol and 55.5 k J/mol,respectively.The K atom on the carbonaceous surface can reduce the energy barrier and promote the heterogeneous reduction of NOx.