Selective Catalytic Reduction Oi NO_x And Enhancement Mechanism By NH₃ At Low Temperature On Activated Carbon Catalysts

Nitrogen oxides(NOx)is one of main air pollutants in China.Selective catalytic reduction(SCR)denitrification technology has been widely used in coal-fired power plants.Corresponding commercial catalysts such as V2O5/TiO2 or V2O5-WO3/TiO2 exhibit excellent SCR activity in the range of 300-400℃.Flue gas temperatures of coal-fired industrial boilers and industrial furnaces(such as iron and steel production process furnaces)are usually lower than 200℃.Thus,the development of low-temperature denitration technology suitable for low-temperature flue gas(<200℃)is of great industrial application significance.Selective catalytic reduction of NOx by NH3(NH3-SCR)on activated carbon catalysts is an environment-friendly low-temperature denitrification technology.The coupling of key step of intrinsic reaction at molecular level and enhancement mechanism at material level is the key scientific issue for further development and utilization of activated carbon as SCR catalysts.In this study,intrinsic reaction paths of NH3-SCR on activated carbon catalyst were calculated based on density functional theory(DFT),revealling key steps of the reaction at molecular level.According to the key steps,strengthening paths were proposed,and experimental study was carried out at material scale.The above research provides some reference for the development and industrial application of selective catalytic reduction technology of NOx by NH3 on activated carbon catalyst at low temperature.Intrinsic reaction paths of NH3-SCR on the activated carbon catalyst were calculated based on density functional theory.Two kinds of active sites,including unsaturated C atoms and high spin population C atoms,are determined according to whether the catalytic reaction could be completed.Intrinsic reaction paths and corresponding key steps for standard SCR and fast SCR are determined,respectively.Fast SCR intrinsic reaction consists of a series of sub-reactions,including the reaction between NO2 and NH3,NH2NO decomposition reaction,reaction between HNO3 and NO,and HONO decomposition reaction.Reaction between HNO3 and NO is the key step of fast SCR intrinsic reaction.Standard SCR intrinsic reaction consists of NO oxidation to NO2 and fast SCR reaction.Corresponding key step is continuous NO oxidation to NO2 at high spin population C atoms.The above DFT calculation results provide two enhancement methods for NH3-SCR reaction at material level.On the one hand,NO oxidation to NO2 can be enhanced by regulating physical and chemical properties of activated carbon catalyst.On the other hand,part of NO in flue gas can be oxidized to NO2 by ozone oxidation,low-temperature plasma and other technologies.The improvement of NO2/NOx molar ratio is beneficial to the occurrence of fast SCR reaction.Based on theoretical calculation results that NO oxidation to NO2 on activated carbon catalyst is the key step of standard SCR intrinsic reaction,the influence of O-containing functional groups and pore structure on NH3-SCR reaction was studied.A series of activated carbons with similar pore structure and varied O contents were prepared.Experimental results showed that the amount of carbonyl and lactone groups had little influence on NOx conversion of NH3-SCR.Thus,the influence of O-containing functional groups on NH3-SCR reaction was eliminated as far as possible.A series of activated carbons catalysts with typical pore structure parameters were prepared through the combination of carbonization methods and activation methods.Microporous specific surface area and the pore hierarchy(the proportion of meso-/macroporous volume to total pore volume)are both key pore structure parameters affecting denitrification performance of NH3-SCR.As the microporous specific surface area increased,the amounts of active sites increased,thus enhancing NO oxidation to NO2.Construction of hierarchical pore structure is beneficial to the diffusion of reactants and products,thus enhancing the NO oxidation to NO2·HAC2 with highest microporous specific surface area and hierarchical pore structure exhibited most excellent SCR denitrification efficiency.NOx conversion of NH3-SCR reached 33.81%at 200℃.Regulating the pore structure of activated carbon catalyst,that is,improving microporous specific surface area and constructing hierarchical pore structure,is one of methods to enhance denitrification performance of NH3-SCR.The influence of NO2/NOx molar ratio on NH3-SCR reaction on activated carbon catalyst was studied,and the reaction mechanism of NH3-SCR involving NO2 was clarified.Experimental results showed that increasing NO2/NOx molar ratio significantly enhanced the denitrification performance of NH3-SCR reaction on activated carbon catalyst.The optimal NO2/NOx molar ratio is 1.0,and NOx conversion reached 79.5%at 210℃.The results of N2 adsorption-desorption and FT-IR indicated that NH4NO3 was an important intermediate for the NH3-SCR reaction involving NO2.Reaction mechanism can be summarized as the generation and consumption of NH4NO3.The reaction between NH3 and NO2 results in the formation of NH4NO3.The reaction between NH4NO3 and NO leads to the consumption of NH4NO3.At low temperature,the reaction rate of reaction between NH4NO3 and NO is low.Unreacted NH4NO3 is deposited on the activated carbon catalyst,resulting in the coverage of the active sites.Thus,NOx conversion decreases gradually.With the increase in temperature,the reaction between NH4NO3 and NO accelerates.When all NH4NO3 is consumed,NOx conversion tends to be stable.NO2 participation transforms the key step of NH3-SCR reaction from NO oxidation to the reaction between NH4NO3 and NO(NH4NO3 can be decomposed into HNO3 and NH3)with a lower energy barrier.Improving NO2/NOx molar ratio is one of methods to enhance denitrification performance of NH3-SCR on activated carbon catalyst.The influence of pore structure on the selective catalytic reduction of NO2 by NH3 was studied,and the effect of typical flue gas conditions was also investigated.The increase in microporous specific surface area significantly enhanced the denitrification performance of selective catalytic reduction of NO2 by NH3.FGAC900 with highest microporous specific surface area exhibited the most excellent low-temperature denitrification performance,and NOx conversion reached 97.80%at 150℃.Meanwhile,the increase in microporous specific surface area reduced the optimal denitrification temperature(temperature corresponding to the highest NOx conversion)from 210℃ of FGAC to 150℃ of FGAC900.O2 concentration had little effect on NOx conversion.The gas hourly space velocity had a great influence on NOx conversion.Reducing gas hourly space velocity is beneficial to a higher NOx conversion.NH3/NO2 molar ratio had a great influence on NOx conversion.The NH3/NO2 molar ratio of 1.1 could ensure a higher NOx conversion and complete consumption of NH3.Water vapor and SO2 both inhibited the denitrification performance of FGAC900.When 8 vol.%H2O was added into dry simulated flue gas,NOx conversion decreased from 97.80%to 90.71%at 150℃.The inhibition of H2O is reversible,which is caused by the competitive adsorption between NO2 and H2O molecules.The inhibition of SO2 is irreversibe.