Model And Application Research On Transport And Adsorption/Reaction Of Multi-Pollutants And Carbon Dioxide

Porous media are widely used in the fields of energy,environment,materials,chemical industry,etc.,and play a crucial role in many technologies for improving the efficiency of energy conversion and utilisation,and reducing the emission of multi-pollutant and greenhouse gas.In-depth study on the transport properties of multi-pollutant and greenhouse gas in porous media and the coupling effect between mass transfer and adsorption/reaction is of great significance to enhance the material performance,improve the efficiency of multi-pollutant removal and greenhouse gas capture,and realise the synergistic effect of pollution and carbon reduction in the energy use process.However,the study of mass transfer and adsorption/reaction in porous media still faces many problems and challenges.For example,the pore structure of porous media is usually complex,with a large number of pore channels,irregular shape,large size span,and randomness and non-uniformity in pore distribution;the mechanism of multiple physicochemical effects in the mass transfer process is not clear,including the interaction between the mass transfer components,the interaction between the mass transfer components and the pore channels,and the capillary condensation effect in the pore channels,etc.;The pore structure of porous media under actual working conditions has a evolution process caused by deposition,condensation and other phenomena will dynamically affect the mass transfer process,which in turn affects the adsorption and reaction in the porous media.To address the above issues,this paper focuses on the mechanism of mass transfer and adsorption/reaction process in porous media,taking the selective catalytic reduction removal of nitrogen oxides and carbon dioxide adsorption and capture as examples,combining high-precision structural characterisation of porous media and computational 3D reconstruction methods,to establish a model of mass transfer and adsorption/reaction for carbon dioxide and pollutants,and to investigate the influences of pore structures,surface sites,physicochemical properties,competitive adsorption/transport and other factors on the mass transfer and adsorption/reaction process.By revealing the dynamic evolution of the structure of the porous media under the influence of the physicochemical effects and the coupling mechanism of mass transfer and adsorption/reaction,guidance will be provided for the optimal design of the structure of porous functional materials.The main results obtained in the paper are as follows:（1）The dual control-volume model of NO,NH₃,SO₂ molecules in Ti O₂ nanopores was constructed by molecular dynamics method,and the diffusivities of each gas component in non-equilibrium state were solved.The effects of temperature,surface sites and pore size on the diffusivities were investigated,and the results showed that molecules with smaller molecular weights and larger dipole moments are more easily affected by hydroxyl sites in the pore during diffusion,while molecules with larger molecular weight and smaller dipole moment are less affected by the hydroxyl sites.The influence of surface sites on the diffusivities of gas molecules decreases with the increase of the pore size,and when the distance between gas molecules and the wall is larger than the cut-off distance of short-range interactions,the surface sites can only influence them through long-range interactions and momentum transfer.In the diffusion of mixed molecules,the effect of temperature and surface sites on diffusivities can be transferred by intermolecular collisions,therefore the effect of hydroxyl sites on the diffusivity of NH₃ are significantly reduced due to the competing transport of SO₂ molecules,which have larger molecular weights.（2）A dual control-volume model was established for the mass transfer and adsorption process of CO₂ molecules in graphene layered pores.The effects of temperature,pore size,amino sites,and H₂O doping on the diffusion and adsorption of CO₂ molecules were studied,and the CO₂ diffusivities under different conditions were solved.The results indicate that the adsorbed CO₂ molecules form double layers in the pore,with the adsorption capacity of the first layer mainly determined by the adsorption area in the pore,while the density of amino sites mainly affects the adsorption capacity of the second layer.When the size of the pore decreases,the CO₂ molecules adsorbed in the pore are gradually subjected to van der Waals forces and electrostatic forces from the opposite surface and sites,causing the adsorption position to shift towards the opposite side,resulting in a decrease in the number first layer CO₂and an increase in the number of second layer CO₂.When the pore size reaches 17.7?,the force on CO₂ molecules increases significantly and accumulation and blockage occur in the pore.The doping of H₂O can lead to the formation of H₂O clusters in pore at low temperatures.In pore containing amino sites,H₂O clusters are stretched by van der Waals forces and electrostatic forces from the sites,making it easier to contact the pore wall and cause capillary condensation to block the pores,hindering the diffusion of CO₂.In addition,although the adsorption selectivity of H₂O on graphene is weaker than CO₂ at high temperatures,H₂O clusters will diffuse into the pores and occupy the pore surface at low temperatures,reducing the adsorption area of CO₂.（3）High precision characterization,computer 3D reconstruction,and visualization of the nanoscale pore structure of commercial SCR catalysts were performed using a FIB-SEM.To solve the problems of weak conductivity and poor imaging performance of commercial SCR catalysts,pre-treatment methods such as sample gold spraying and brushing conductive carbon paste were adopted.The high brightness of sample pore edges was avoided from affecting image segmentation by using backscatter.The median filtering and Otsu method were determined as the optimal filtering and segmentation methods for catalyst sample images.The reconstructed catalyst sample has a three-dimensional open pore structure with dual connectivity,meaning the skeleton and the majority of pores can be connected separately.There is a complex connected network with a large number of pores and throats inside the sample.The topological structure information of the sample was extracted using the maximum ball method,and a pore network model was established.The pore size and volume distribution,pore throat size and length distribution,coordination number,and connectivity and blind pores in the model were analyzed.The results showed that the pore size distribution obtained from reconstruction was consistent with the BJH pore size distribution measured by N₂ adsorption experiments,proving the effectiveness of such extraction and reconstruction.Futhermore,a calculation method for the effective diffusion coefficient and gas concentration distribution of pore networks based on pore throat diffusivities was proposed.The calculation of 3D diffusion is simplified by solving a series of linear equations based on one-dimensional pore throat diffusion,which saves computational resources and time while considering the characteristics of nanopore structure.（4）On the basis of 3D pore network model of commercial SCR catalysts,a mass transfer and reaction model was established with taking ammonium bisulfate deposition into consideration.The dynamic evolution of pore structure and gas concentration during ammonium bisulfate deposition process were studied,and the results showed that the deposition of ammonium bisulfate in pores has strong non-uniformity.When the model is completely blocked due to the deposition of ammonium bisulfate,the proportion of pore volume occupied by ammonium bisulfate in the total pore volume of the model is usually low because of the blockage of the critical path connecting the inlet and outlet.We studied the effects of temperature,inlet gas concentration,water content,ammonium bisulfate decomposition efficiency,capillary condensation on the deposition process of ammonium bisulfate.It was found that under the influence of capillary condensation,the blockage process can be roughly divided into two stages.The first stage is blockage in small pore pores with low gas concentration in the middle and near the outlet of the sample,and the second stage is blockage in large pore pores near the inlet of the sample.（5）A mass transfer and reaction coupling model for honeycomb SCR catalyst was constructed,and the spatial distribution of various gas components involved in the SCR and SO₂ oxidation process in the catalyst pores and walls was studied.The results showed that the position of chemical reactions occurring in the catalyst walls would change due to different reaction rates and diffusion rates of reactants in the catalyst.SCR reaction mainly occurred in thin layers（about 0.2 mm）near the surface of catalyst walls,while SO₂ oxidation occurred throughout the entire catalyst walls.Based on the research,a low sulfur conversion SCR catalyst structure with layered walls was proposed.175 cases were simulated under actual flue gas conditions in coal-fired power plants,with an active layer thickness ranging from 0.05 mm to 0.6 mm,a temperature ranging from 320℃to 400℃,and a GHSV ranging from 2000 h^-1to 10000 h^-1.The results showed that for SO₃,increasing the active layer thickness would significantly increase its concentration.And the effect of layer thickness on NO distribution would significantly decrease when the active layer thickness near the catalyst inlet exceeded0.15 mm and the active layer thickness near the outlet exceeded 0.1 mm.In cases with different thicknesses of active layers,an increase in GHSV will reduce the residence time of reactant gases in the catalyst,resulting in the reduction of both NO and SO₂ conversion.However,in cases with thicker active layers and slower GHSV,an increase in temperature sometimes reduces the conversion of NO.This is because the SCR reaction rate is relatively fast,and the increase in reaction rate caused by higher temperature does not significantly improve the overall conversion.Instead,the increase in reactant diffusion rate reduces the residence time.Based on this,an optimized thickness of the catalyst active layer and a related operating condition controlling strategy were proposed to reduce sulfur oxidation while ensuring denitrification efficiency and meeting ultra-low emission requirements.