DFT Study On The Effect Of Sodium In High Alkali Coal On The Heterogeneous Reduction Of NO By Char

In China,coal resources in the energy production and consumption system will still occupy a dominant position for a long time.However,NO_x emitted by coal-fired power plants is one of the important pollutants of atmospheric pollution in China.How to realize the clean and efficient utilization of coal is an important issue that China urgently needs to solve.Xinjiang Zhundong Coalfield is the largest self-contained coalfield in China,but it belongs to high-sodium coal,which results in the characteristics of NO_x emission during combustion and utilization remain unclear.Therefore,with the large consumption of coal resources,the realization of clean and efficient utilization of Zhundong high-sodium coal has very important practical and strategic significance for ensuring national energy security and environmental protection needs.The excess air coefficient of the reburning zone of coal-fired boilers is less than 1,and the reducibility in this zone is strong,which is conducive to the NO reduction reaction.It mainly includes the heterogeneous reduction reaction of NO at the active site of coal char surface and the homogeneous reaction process of NO in gas-phase flue gas.During the combustion and utilization of high-sodium coal,the existence and migration of sodium element catalyze the heterogeneous reduction reaction of NO in the reburning zone of the boiler.Based on the density functional theory,the mechanism of this catalytic effect is analyzed.The main findings are as follows:(1)The doping of sodium atoms on the surface of char structure can promote the adsorption of NO molecules by char,but different surface structures are affected differently.A variety of reasonably simplified char structure models were selected as the main research objects,and the changes in the adsorption energy of char to heterogeneously adsorb NO molecules before and after doping sodium atoms on the char structure were calculated.The adsorption of NO molecules by unsaturated char structures with active sites is a strong chemical adsorption.The stable adsorption of NO on the surfaces of armchair and zigzag char are-211.7 k J/mol and-564.6 k J/mol,respectively.After doping with sodium atom,the adsorption energy increased to-303.6 k J/mol and-570.3 k J/mol,respectively.The adsorption of NO molecules on the surface of the saturated char structure with no active site is a weak physical adsorption,and the adsorption energy is only-15.6 k J/mol,but after doping with sodium atom,the adsorption energy is significantly increased to-126.7 k J/mol.The adsorption behavior was changed from physical adsorption to chemisorption.(2)The sodium atom catalyzes the heterogeneous reduction reaction of NO on the surface of a char model composed of pure carbonaceous structure.The surface structures of typical armchair and zigzag char are selected as the research objects,and the energy changes of the heterogeneous reduction reaction paths of the two structures before and after sodium doping are compared and analyzed.The highest energy barrier for heterogeneous reduction of NO reaction in Armchair-type char structure is reduced from 164.54 k J/mol when undoped to 117.20 kJ/mol after doping with sodium atom.After doping with sodium atom,the total energy barrier for the NO_x reduction of the zigzag char structure in the heterogeneous phase decreased from 129.90 k J/mol when it is undoped to 99.51 k J/mol when it is doped.(3)The alkali metal sodium atom can also play a catalytic role in the reaction for reducing heterogeneous phase of zigzag containing pyridine nitrogen and armchair nitrogen containing char structure.The specific performance is that the highest energy barrier of the reaction of the armchair-type nitrogen-containing char structure after the heterogeneous reduction of NO reaction and the introduction of sodium atoms is reduced from 55.38 k J/mol before doping to 40.10 k J/mol after doping.The highest energy barrier of nitrogen char structure in the heterogeneous reduction of NO reaction after the introduction of sodium atom decreased from 216.49 k J/mol before doping to 184.28 k J/mol after doping.(4)There are differences in the characteristics of the pyrene nitrogen-containing char structure of armchair and zigzag type and the char structure of pure carbonaceous structure for the heterogeneous reduction reaction of NO.Compared to the armchair-type char structure,the heterogeneous reduction reaction of NO molecules on the surface of the zigzag-type char structure is more likely to occur;compared to the zigzag-type nitrogen-containing char structure,the heterogeneous reduction reaction of NO molecules on the surface of the armchair-type char structure more likely to occur.