Investigation On Removal Characteristics Of NO_x By CO Based On Adsorption-Reduction Decoupling Method

Coal is a major component of energy consumption of China,and most of the coal is used through direct combustion.However,this utilization will produce a large amount of NOx,which will harm human health and the ecological environment,seriously.Currently,NH3-SCR technology is an effective controlling measure for the fixed-source NOx emission,but the NH3 leak in this technology causes problems such as blockage of downstream equipment and formation of haze in the atmosphere.CO is the main component of pyrolysis gas in the coal semi-coke process of our research group,which has the characteristics of cost-effectiveness,easy to obtain and easy to handle by-products.In recent years,more attention has been paid to the efficient removal of NO.in the three-way catalytic reaction.However,the higher concentration of O2 in the flue gas strongly inhibits the reduction of NOx by CO,resulting in difficulties to directly apply like NH3 in the reduction of NOx.x.Therefore,our group proposed a novel process of NOx adsorption-reduction decoupling to avoid the O2 inhibition.The process of NOx reduction by CO is artificially divided into two processes:adsorption process and reduction process.The catalyst rotates to the reduction zone to realize regeneration after NOx adsorption in the flue gas zone,while the surface adsorbed NOx is reduced and desorbed into the reducing gas.The catalyst continues to rotate and alternately circulates in the adsorption zone and the reduction zone for the purpose of removing NOx from the flue gas.In order to obtain the promotion and application of the technology,in this paper,the characteristics of NOx adsorption-reduction decoupling were investigated for NOx removal by CO based on adsorption-reduction decoupling NOx removal reaction experimental system.The feasibility of the novel NOx removal process was investigated.The controlling factors affecting the dynamic removal of NOx were investigated.The influence of the physicochemical properties of the catalyst on NOx adsorption capacity and reduction activity was analyzed.The coupling effect of catalyst NOx adsorption performance and reduction performance was discussed for NOx removal performance.Through these studies,theoretical support and reference are provided for the design and application of adsorption-reduction decoupling NOx removal process.The process of adsorption-reduction decoupling NOx removal was established.The effects of operating parameters such as temperature,CO concentration and reduction zone time for NOx removal of dynamic decoupling were investigated over Fe/ZSM-5 catalyst.Studies suggest that NOx in flue gas can be efficiently removed,and significantly improve the NOx removal efficiency when CO as a reducing agent in an oxygen-containing atmosphere,but most of the NOx in the oxygen-containing flue gas through adsorption-desorption process proceeds with anaerobic reductive gas,the NOx reduction is very easy in an oxygen-free reducing gas.Increasing the temperature significantly reduces the NOx adsorption capacity of the catalyst,but at the same time increases the NOx reduction activity,which together determines the dynamic NOx removal performance.Besides,the NOx adsorption capacity of catalyst dominates at high temperatures in adsorption-reduction decoupling NOx removal process.Increasing the O2 concentration could significantly promote NOx adsorption and NOx removal performance.Changing the reduction zones time and CO concentration essentially changes the amount of CO reduction,and increasing the CO amount could also improve the NOx removal performance.In the dynamic NO,removal process,NOx in the flue gas zone is first adsorbed on the catalyst surface in the form of free nitrate ions,bridge nitrates,bidentate nitrates,etc.,and in the reduction zone,CO will occupy the Fe active sites,and the nitrate ions and NO+ are gradually driven away from the catalyst surface and promote NOx desorption and NOx removal during the adsorption phase in the next cycle.Optimized zeolite-based catalysts of dynamic NOx removal process.Fe is the main active component of Fe/ZSM-5 catalyst and appropriate increase of Fe content could increase NOx adsorption capacity,but too high Fe loading to reduce the dispersion of surfactant surface active component,Ba species store NOx in the form of Ba(NO3)2.The catalytic activity of Fe-Ba/ZSM-5 catalyst is directly related to the concentration of a-Fe2O3 species.Free nitrate ions adsorbed on the catalyst surface are the main species that increase the NOx adsorption capacity,while the bridge nitrate does not increase the NOx adsorption capacity.Increasing the catalytic activity of the catalyst promotes the decomposition and desorption of NOx on the catalyst surface and the regeneration of the catalyst at 250?.Meanwhile,increase the low-temperature catalytic activity of the decouple NOx removal catalyst can significantly improve the NOx removal efficiency.Further improving NOx removal performance in the dynamic decoupling process,the metal oxide catalysts containing Cu,Fe,and Ce were developed.It is noted that Cu2+ions are the active sites for the oxidation conversion of nitric acid.The interaction between CuO and supports advantageously promotes the reduction of Cu oxides and the diffusion of surface oxygen species at low temperatures,forming a low-temperature catalytic active center.Excessive Cu loading lead to increasing Cu accumulation on the catalyst surface,inhibiting the contact of NO and CO with the activated Cu species,and reducing the catalyst reactivity.In the support.Fe is doped to the Ce lattice to increase the CeO2 and Fe2O3 grain size.The low concentration of Fe3+ ions can easily replace the Ce4+ ions in the CeO2 lattice.Increasing the Fe3+ ion concentration causes the gap of the CeO2 structure to be occupied by Fe3+,and lower Fe/Ce molar ratio increases the oxygen vacancy concentration and the synergistic effect between the support and the copper species,thereby increasing the reactivity of the catalyst.The characteristic analysis demonstrated that NOx species tend to be stored in the Fe active site,the addition of Fe species to the catalyst can help the catalyst to adsorb more nitrite and nitrate;CO tends to adsorb on CeO2 and is stored as carbonate on the surface of the catalyst.Supported copper can convert the inactive nitrite to the active intermediate nitrate and ONNO in the CO +NO reaction,thereby promoting the catalytic reduction of NO by CO.According to the evolution of species on the surface of the catalyst,it can be seen that after NO dissociates into N and O,N combines with NO to form intermediate product N2O and finally decomposes into N2,and at the higher the temperature,the faster N2O is reduced to N2.However,the strong adsorption capacity of Ce in the ruthenium-rich catalyst leads to the combination of dissociation N and CO to form NCO species,which disappears with the increasing in Fe content and temperature in the support,and the reaction pathway to N2O as an intermediate product,which dominate the reaction that reduction of NO by CO.The coupling mechanism of NOx adsorption and reduction based on metal oxide catalysts was studied to reveal the dynamic decoupling characteristics.It was found that the metal oxide catalyst had poor activity for oxidation of NO to NO2,which resulted in a small NOx adsorption capacity of the metal oxide catalyst.The adsorption of NOx on the catalyst surface was mainly nitrite species.The competitive adsorption between H2O and NOx reduces the ability of the catalyst to adsorb NOx,however,the rich surface 0 of the Ce-based catalyst is beneficial for the conversion of NO to NO2 and weakens the inhibition of H2O.The 4Cu/CF catalyst surface has weakly adsorbed NO+ and ONNO species,which promotes its high NOx removal efficiency at 100-200?.The OH" of H2O dissociation could help to oxidize weakly adsorbed NO to stable NO2,enhancing the stability of NOx adsorbed species,and the dissociated product H+ accelerates the migration and desoiption rate of NOx on the catalyst surface,which lead to the decrease the inhibition of NOx adsorption and reduction by H2O in dynamic decoupling,so that H2O only slightly reducing the NOx removal efficiency in the dynamic decoupling process and enhances the CO oxidation efficiency.