Modeling of SO2 removal in fabric filter

Fabric filters are involved in most semi-dry flue gas desulfurization process and represent ability of SO₂ removal. SO₂ removal efficiency in fabric filter after a semi-dry scrubber is investigated. Experimental results showed that SO₂ inlet concentration has little effect on SO₂ removal efficiency, SO₂ removal efficiency increases as flue gas inlet temperature increases and relative humidity affects SO₂ removal efficiency significantly. The kinetic model based on shrinking core theory has been presented. It is found that, in the beginning, when calcium hydroxide conversion ratio is less than 0.3, SO₂ removal process is mainly controlled by chemical reaction (Model-2); and when calcium hydroxide conversion ratio is greater than 0.3, SO₂ diffusion through product layer is rate limiting (Model-3). The experimental results in fabric filter are successfully correlated by Model-3.     

Kinetic model of CaO/fly ash sorbent for flue gas desulphurization at moderate temperatures

Characteristics of the sulphation reaction between SO₂ and CaO/fly ash sorbent were analyzed based on TGA results to develop a kinetic model for a dry moderate temperature (400–800 °C) FGD process. It was found that SO₂ diffusion within sorbent particles involved three sub-processes: inter-particle diffusion, inter-grain diffusion and diffusion through product layers and the diffusion dominated the whole sulphation reaction process. The activation energy for product layer diffusion E_diff of 49.3 kJ mol⁻¹ being greater than the chemical reaction activation energy E_a of 13.9 kJ mol⁻¹ verified the importance of the diffusion. Predictions using the kinetic model in which k₀ varies with temperature agree well with the experimental data.     

NOx removal by selective noncatalytic reduction with urea solution in a fluidized bed reactor

A fluidized bed reactor has been developed to overcome the plugging problem of urea injection by employing a sparger rather than nozzles in the SNCR process for simultaneous removal of SO₂ and NO_x. In a developed fluidized bed reactor, the optimum temperature to remove NO_x is shifted to lower values, the reaction temperature window is widened with the presence of CO in flue gas, and NO conversion is higher than that in a flow reactor. The optimum amount of urea injection in the reactor is found to be above 1.2 based on the normalized stoichiometric molar ratio (NSR) with respect to NO conversion. In the simultaneous removal of SO₂/NO, conversions of SO₂ and NO reach 80–90%, nearly the same values for the individual removal of SO₂ and NO above 850 ‡C.     

氧燃烧方式下石灰石直接固硫反应模拟

氧燃烧方式下高浓度CO2气氛使得石灰石与SO2的气固反应存在直接固硫和间接固硫两种方式。在热重分析仪上进行了石灰石直接硫化的实验,考察了温度、SO2浓度对直接固硫反应的影响。基于球形颗粒气体扩散理论,在未反应收缩核模型的基础上推导出一种从实验数据计算化学反应速率常数和SO2反应级数的新方法。同时在已有研究的基础上改进了产物层扩散系数的计算方法,并采用未反应收缩核模型对不同温度、SO2浓度条件下石灰石直接固硫反应进行模拟,模拟结果与实验结果较为吻合。在所建立模型的基础上定性讨论了温度、孔隙率、平均孔径对产物层有效扩散系数的影响,发现温度对有效扩散系数影响很显著,而孔隙率、平均孔径的影响较小。     

Study of the sulfation kinetics between SO2 and CaO catalyzed by TiO2 nano-particles

The catalytic activity of TiO₂ nano-particles, prepared by a sol-gel method, was studied when added in the reaction between SO₂ and CaO. The reaction products were analyzed by infrared spectrophotometry (IR) and specific surface area analysis and the kinetics and mechanisms of the sulfation catalyzed by the addition of TiO₂ are discussed. The results indicate that nano-TiO₂, which serves as an active catalytic center, enhances O₂ transfer and is helpful in the diffusion of SO₂ from the product layer to the inner unreacted CaO. As a result, the desulfurization efficiency increased. The results also suggest that the SO₂ and NO must both be removed simultaneously in order to keep the sulfation rate. The desulfurization reactions are first order for SO₂ concentration and zero order for O₂ concentration and include two zones, the surface reaction zone and the product layer diffusion zone, with later being the rate limiting step. The apparent activation energy of the desulfurization reaction decreased with the addition nano-TiO₂ as compared to that without. The unreacted shrinking reaction core model was used to investigate the reaction kinetics and was shown to describe the course of desulfurization. Lastly, the results obtained through calculation agree with the empirical data.     

The gas—solid trickle-flow reactor for the catalytic oxidation of hydrogen sulphide: A trickle-phase model

The oxidation of H₂S by O₂ producing elemental sulphur has been studied at temperatures of 100–300°C and at atmospheric pressure in a laboratory-scale gas-solid trickle-flow reactor. In this reactor one of the reaction products, i.e. sulphur, is removed continuously by flowing solids. A porous, free-flowing catalyst carrier has been used which contains a NaX zeolite acting as a catalyst as well as a sulphur adsorbent. In order to describe mass transfer in the trickle-flow reactor, a reactor model has been developed in which a particle-free, upflowing gas phase and a dense, downflowing gas-solids suspension, the so-called trickle phase, are distinguished. Within the trickle phase, diffusion of the reactants parallel to reaction in the catalyst particles takes place. The mass transfer rate from the gas phase to the trickle phase has been evaluated by the reaction of H₂S with SO₂, which is a much faster reaction than the reaction with O₂. From the experiments and from the reactor model calculations it appears that for the H₂S-O₂ reaction no mass transfer limitations occur at temperatures up to about 200°C, whereas at 300°C gas-phase mass transfer and diffusion within the dense solids suspension offer resistance to reaction.     

Sulfonation of solid polystyrene using gaseous sulfur trioxide

Kinetics of the heterogeneous sulfonation of polystyrene (PS) beads using gaseous SO₃ was investigated. Scanning electron microscopy (SEM) and electron diffraction scattering spectroscopy (EDS) was employed to study the kinetics of diffusion of SO₃ into the PS particles. The diffusion of SO₃ through the barrier of sulfonated polystyrene (SPS) on the beads surface was the primary parameter determining the rate and the yield of the sulfonation reaction. The measurement of the time dependence of the thickness of sulfonated layer formed on the solid PS surface provided for the hypothesis that the sulfonation in heterogeneous phase was diffusion controlled. Diffusion coefficients of SO₃ in PS at ?5°C, 22°C, and at 50°C and activation energy of SO₃ diffusion to the solid PS were determined from these experimental data assuming in the first approximation a simple diffusion unaffected by the ongoing sulfonation reaction. The experimental data were fitted using Johanson‐Mehl‐Avrami‐Jerofyeev‐Kolgomorov's equation to obtain an overall rate constant of heterogeneous sulfonation on solid PS surface. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

Dynamic analysis of changes in diffusion and sorption parameters in the reaction of SO2 with activated soda ash

Variations in pore structure cause significant changes in diffusion resistance and adsorption characteristics during the reaction of SO₂ with activated soda. Breakthrough and moment analysis of the SO₂-activated soda ash reaction showed that the effective diffusion coefficient of SO₂ in the solid product (mostly Na₂SO₃) was about three times smaller than its value in the original activated soda. About 30% of the diffusion flux of SO₂ in the solid product was found to be due to surface diffusion. The adsorption equilibrium constant of SO₂ on the solid product was found to be about half the value of the adsorption equilibrium constant on activated soda. It was also determined that SO₂ was irreversibly adsorbed on the solid product Na₂SO₃ at 200°C and 3.7 atm. The gaseous product CO₂ was not adsorbed on the solid product while it was slightly adsorbed on activated soda.     

Microwave catalytic NOx and SO2 removal using FeCu/zeolite as catalyst

Non-thermal plasma technology is a promising process for flue gas treatment. Microwave catalytic NO_x and SO₂ removal simultaneously has been investigated using FeCu/zeolite as catalyst. The experimental results showed that a microwave reactor with FeCu/zeolite only could be used to microwave catalytic oxidative 91.7% NO_x to nitrates and 79.6% SO₂ to sulfate; the reaction efficiencies of microwave catalytic reduction of NO_x and SO₂ in a microwave reactor with FeCu/zeolite and ammonium bicarbonate (NH₄HCO₃) as a reducing agent could be up to 95.8% and 93.4% respectively. Microwave irradiation accentuates catalytic reduction of SO₂ and NO_x treatment, and microwave addition can increases SO₂ removal efficiency from 14.5% to 18.7%, and NO_x removal efficiency from 13.4% to 18.7%, separately. FeCu/zeolite catalyst was characterized by X-ray diffraction (XRD), X-ray photoelectron spectrum analysis (XPS), scanning electron microscopy (SEM) and the Brunauer Emmett Teller (BET) method. Microwave catalytic NO_x and SO₂ removal follows Langmuir-Hinshelwood (L-H) kinetics.     

Effect of attrition on particle size distribution and SO2 capture in fluidized bed combustion under high CO2 partial pressure conditions

In both pressurized and oxygen-enriched fluidized bed combustion the partial pressure of CO₂ in the reactor becomes high, which affects SO₂ capture by limestone. Both of these technologies are also applicable to decreasing greenhouse gas emissions; the first one by increasing the efficiency of electric energy production and the latter by enabling capture of carbon dioxide for storage.Attrition increases the reaction rate by removing the sulphated layer on the particle, thus reducing the diffusion resistance. In the well-known solution for the shrinking core model the reaction time can be presented as the sum of the contributions of the kinetics and diffusion. It is shown that the effect of attrition can be expressed as an auxiliary term in this expression. A method to extract the diffusivity of the product layer from the SO₂ response in a bench-scale fluidized bed test using a limestone sample with a wide particle size distribution is presented. Based on a population balance model, a method to estimate the particle-size-dependent attrition rate from measured particle size distributions of the feed and bed material is illustrated for a 71-MW_e pressurized power plant. In addition attrition and its effect on the optimization of the limestone particle size for sulphur capture in oxygen-enriched combustion are discussed.     

Enhancement mechanism of SO2 removal with calcium hydroxide in the presence of NO2

The enhancement mechanism of SO₂ removal by the presence of NO₂ under low temperature and humid conditions was studied in a fixed bed reactor system. The presence of NO₂ in the flue gas can enhance SO₂ removal. The interaction between SO₂ and NO₂ in gas phase could not explain the effect of NO₂ on SO₂ removal under low-temperature and humid conditions. When Ca(NO₃)₂ and Ca(NO₂)2 as additive were added on the surface of sorbent, the desulfurization activity of sorbent decreased. However, the sorbent pretreated by NO₂ for a moment has higher SO₂ removal. The oxidization of SO₃²⁻ to SO₄²⁻ and the evolution of sorbent surface structure in the presence of NO₂ can explain the enhancement of SO₂ removal by the presence of NO₂. HSO₃⁻ and SO₃⁻ reacted with NO₂ to form sulfate, which can accelerate the hydrolysis of SO₂. The reaction between NO₂ and Ca(OH)₂ can make the unreacted sorbet under the SO₂ removal product exposed to the reactant gas.     

Activation energy variation for catalytic oxidation of aqueous SO2

SO₂ oxidation in aqueous solutions catalysed with manganous sulphate was studied to determine temperature dependencies of the reaction rate. The process was carried out at relatively high sulphuric acid (reaction product) concentrations with regard to its application for SO₂ removal from waste gases. Variation of the apparent activation energy has been linked with alteration of reaction rate determining steps.     

Shrinking-core model for systems with facile heterogeneous and homogeneous reactions

The shrinking-core equation for pore diffusion control has been extended to the case of a facile heterogeneous reaction coupled to a facile homogeneous reaction occurring within the pores of the product layer and in the bulk solution. The model considered is very general in that the simultaneous transport of all reacting species is included. The resulting equation is identical to the standard one for diffusional control by a single species except that the parabolic rate constant k_d is considerably more complex and contains useful information concerning the system. Analysis of its dependence on the system parameters yields important criteria that determine the identity of the rate-controlling species and the direction of the heterogeneous reaction. Depending upon the relative values of the equilibrium constants of the two reactions, the dependence of k_d on the bulk reactant concentration can vary significantly from the linearity expected from the standard model.     

Simulation and thermodynamic analysis of conventional and oxygen permeable CPO reactors

A one-dimensional steady-state heterogeneous model has been used to simulate the conventional CPO reactor. With the mechanism of O₂ permeable membrane, the model has been developed to simulate O₂ membrane reactor. The output temperature and the mole flow rates of different species in the tube side and the shell side can be calculated. They are the basis for the exergy analysis of the conventional CPO reactor with air, the conventional CPO reactor with pure O₂, and the O₂ permeable membrane CPO reactor. The simulation and exergy analysis results indicate that when the inlet conditions are the same, for a given methane conversion, the exergy efficiencies η₂ and η₁ of conventional CPO reactor with pure oxygen is lowest among the three reactors, because of the large amount of accumulative exergy required for obtaining pure oxygen.The exergy efficiencies η₁ and η₂ of membrane reactor are comparable with conventional CPO reactor with air and much higher than conventional CPO reactor with pure oxygen. As the membrane reactors can carry out simultaneous separation and reaction, in the mean time, removal of nitrogen from the product stream can be accomplished; the membrane reactor has advantages compared to other types of reactors.The operation of the membrane CPO reactor is more favourable when the inlet temperature is increased and the operation pressure is decreased from a thermodynamic point of view.     

Oxidation of Aqueous Sulphur Dioxide Catalysed by Poly-4-vinylpyridine–Cu(II) Complex. Part 2: Combined Homogeneous and Heterogeneous Phase Oxidation

Aqueous phase oxidation of sulphur dioxide at low concentrations catalysed by a PVP–Cu complex in the solid phase and dissolved Cu(II) in the liquid phase is studied in a rotating catalyst basket reactor (RCBR). The equilibrium adsorption of Cu(II) and S(VI) on PVP particles is found to be of the Langmuir-type. The diffusional effects of S(IV) species in PVP–Cu resin are found to be insignificant whereas that of product S(VI) are found to be significant. The intraparticle diffusivity of S(VI) is obtained from independent tracer experiments. In the oxidation reaction HSO₃⁻ is the reactive species. Both the S(IV) species in the solution, namely SO₂(aq) and HSO₃⁻, get adsorbed onto the active PVP–Cu sites of the catalyst, but only HSO₃⁻ undergoes oxidation. A kinetic mechanism is proposed based on this feature which shows that SO₂(aq) has a deactivating effect on the catalyst. A rate model is developed for the three-phase reaction system incorporating these factors along with the effect of concentration of H₂SO₄ on the solubility of SO₂ in the dilute aqueous solutions of Cu(II). Transient oxidation experiments are conducted at different conditions of concentration of SO₂ and O₂ in the gas phase and catalyst concentration, and the rate parameters are estimated from the data. The observed and calculated profiles are in very good agreement. This confirms the deactivating effect of non-reactive SO₂(aq) on the heterogeneous catalysis. © 1997 SCI.     

Particle flow, mixing, and chemical reaction in circulating fluidized bed absorbers

A mixing model has been developed to simulate the particle residence time distribution (RTD) in a circulating fluidized bed absorber (CFBA). Also, a gas/solid reaction model for sulfur dioxide (SO₂) removal by lime has been developed. For the reaction model that considers RTD distribution inside the core and annulus regions of a CFBA, a macrochemical reaction can be simulated based on microchemical reaction dynamics. The presented model can predict SO₂ and lime concentration distributions inside the CFBA, and give the amount of lime needed to remove a given percentage of SO₂. It is found that SO₂ concentration decreases with the increase of CFBA distance from the bottom in the core region. However, lime concentration exhibits a very slight variation in the core region. This means that lime is efficiently utilized to remove SO₂. The model also predicts that SO₂ partial pressure at the exit of the CFBA decreases with the increase in the percentage of fresh lime injected in the CFBA.     

Sulfation diffusion model for SO2 capture on the T-T sorbent at moderate temperatures

A sulfation model was developed for dry flue gas desulfurization (FGD) at moderate temperatures to describe the reaction characteristics of the T-T sorbent clusters and the fine CaO particles that fall off the sorbent grains in a circulating fluidized bed (CFB) reactor. The cluster model describes the calcium conversion and reaction rate for various size sorbent clusters. The sulfation reaction is first order with respect to the SO₂ concentration above 973 K. The calcium conversion and reaction rate for the CaO particles were obtained by extrapolation. In the model for CaO particle, the reaction rate is linearly related to the calcium conversion and the SO₂ concentration in the rapid reaction stage and linearly related only with the calcium conversion after the product layer forms. The sulfation model accurately describes the sulfation of the T-T sorbent flowing through a CFB reactor. This work was presented at the 7 ^th China-Korea Workshop on Clean Energy Technology held at Taiyuan, Shanxi, China, June 26–28, 2008.     

AFM investigation of solid product layers of MgSO4 generated on MgO surfaces for the reaction of MgO with SO2 and O2

The nucleation, growth and arrangement of MgSO₄ product layers during the reaction of single-crystal MgO with SO₂ and O₂ were investigated via atomic force microscopy (AFM). The morphology of the single-crystal MgO surface after reacting with SO₂ and O₂ consisted of three-dimensional cone-shaped MgSO₄ product islands. Most of the islands appeared at locations with terraces, steps and kinks. With increasing reaction time, small product islands grew to large islands, finally coalescing into continuous islands. The nucleation, growth and arrangement of the solid products on the reactant surface are controlled by thermodynamics and kinetics. The morphology of the MgSO₄ product islands is governed by the competition between the intrinsic chemical reaction rate and the product molecular diffusion rate, which are both significantly affected by the reaction temperature. As the reaction temperature increases, the mean size of the MgSO₄ islands increases while the island density decreases. The nucleation, growth and arrangement of the solid product layers and the product layer morphology on the solid reactant surface directly affect the mechanism of the gas–solid reaction.     

Mathematical modeling of CO<Subscript>2</Subscript> removal using carbonation with CaO: The grain model

CaO carbonation with CO₂ is potentially a very important reaction for CO₂ removal from exhaust gas produced in power plants and other metallurgical plants and for hydrogen production by promoting water gas shift reaction in fossil fuel gasification. A mathematical model based on the grain model was applied for modeling of this reaction. Diffusion of gaseous phase through the product layer and structural change of the grains were considered in the model. The modeling results show that ignoring the reaction kinetics controlling regime in the early stage of the reaction and replacing it with a regime considering both the reaction kinetics and diffusion can generate good simulation results. The frequency factor of the reaction rate equation and the diffusivity of CO₂ through the CaCO₃ layer were justified to get the best fit at different temperature range from 400 to 750 °C with respect to experimental data in the literature. The mathematical model switches to a pure diffusion controlling regime at final stage of reaction.     

Sulfur release from coal in fluidized-bed reactor through pyrolysis and partial oxidation with low concentration of oxygen

Yima (YM) and Datong (DT) raw coal were pyrolyzed in a fluidized bed reactor under 0.6%O₂-N₂, 1.1%O₂-N₂ and 2.1%O₂-N₂ atmosphere, and a flue gas analyzer was used to check the SO₂ in pyrolysis gas. The product of sulfur removal and char yield is suggested to measure the efficiency of sulfur removal. For YM coal, sulfur removal generally has increasing trend with the increase of oxygen concentration in atmosphere. The char yield of YM coal has no remarkable decrease when the oxygen content is lower than 1.1%. However, in 2.1%O₂-N₂ less char yield is obtained. For DT coal more sulfur is removed in 0.6%O₂-N₂ than in N₂, and at the same temperature more SO₂ is released with increasing oxygen content. It is suggested that the atmospheres used selectively break the C-S bonds other than C-C bonds. Pyrolysis of coal in fluidized-bed reactor under low concentration of oxygen atmosphere is a promising method to greatly remove sulfur, and not remarkably decrease the char yield.