Study on the Poisoning Mechanism of Sulfur Dioxide for Perovskite La0.9Sr0.1CoO3 Model Catalysts

The reaction and poisoning mechanism of SO₂ with La_0.9Sr_0.1CoO₃ model catalysts have been investigated. The structure and the chemical states of the model catalysts have been studied by using AES, XPS and XRD techniques. The results indicated that SO₂ diffused into the La_0.9Sr_0.1CoO₃ film during poisoning. La₂(SO₄)₃ species was formed on the surface of the film and La₂(SO₄)₃, La₂(SO₃)₃, La₂O₂SO₄ and CoO species were found in the interior. The perovskite structure of La_0.9Sr_0.1CoO₃ was destroyed by invasion of SO₂. The concentration of sulfur in the film layer was related to the reaction temperature and time. After the sample was poisoned for a fairly long time, the distribution of sulfur in the La_0.9Sr_0.1CoO₃ layer became homogeneous, suggesting that a dynamic equilibrium was achieved between the poisoning reaction and the decomposition of the sulfates. XRD and catalytic activity test results proved that the destruction of perovskite structure and the formation of sulfates were the main causes of deactivation.     

Comprehensive sulfation model verified for T-T sorbent clusters during flue gas desulfurization at moderate temperatures

An empirical sulfation model for T-T sorbent clusters was developed based on amassed experimental results under moderate temperatures (300-800 °C). In the model, the reaction rate is a function of clusters mass, SO₂ concentration, CO₂ concentration, calcium conversion and temperature. The smaller pore volume partly results in a lower reaction rate at lower temperatures. The exponent on SO₂ concentration is 0.88 in the rapid reaction stage and then decreases gradually as reaction progresses. The exponent on the fraction of the unreacted calcium is 1/3 in the first stage and then increases significantly in the second stage. The CO₂ concentration has a negative influence on SO₂ removal, especially for the temperature range of 400-650 °C, which should be avoided to achieve a high effective calcium conversion. The sulfation model has been verified for the T-T sorbent clusters and has also been applied to CaO particles. Over extensive reaction conditions, the predictions agree well with experimental data.     

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.     

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.     

Kinetics of the reaction of iron blast furance slag/hydrated lime sorbents with SO2 at low temperatures: effects of sorbent preparation conditions

Sorbents highly reactive towards SO₂ have been prepared from iron blast furnace slag and hydrated lime under different hydration conditions. The reaction of the dry sorbents with SO₂ has been studied under the conditions similar to those in the bag filters in the spray-drying flue gas desulfurization system. The reaction was well described by a modified surface coverage model which assumes the reaction rate being controlled by chemical reaction on sorbent grain surface and takes into account the effect of sorbent Ca molar content and the surface coverage by product. The effects of sorbent preparation conditions on sorbent reactivity were entirely represented by the effects of the initial specific surface area (S_g0) and the Ca molar content (M⁻¹) of sorbent. The initial conversion rate of sorbent increased linearly with increasing S_g0, and the ultimate conversion increased linearly with increasing S_g0M⁻¹. The initial conversion rate and ultimate conversion of sorbent increased significantly with increasing relative humidity of the gas. Temperature and SO₂ concentration had mild effects on the initial conversion rate and negligible effects on the ultimate conversion.     

Carbonation reaction of lime, kinetics at ambient temperature

The uptake of carbon dioxide due to the carbonation reaction of Ca(OH)₂ in ambient temperature of approximately 20 °C has been studied. Different types of lime have been used and the CO₂ concentration has been varied to identify the influence of different variables on the kinetics of the reaction. A closed loop system has been developed and validated that allows measurement of the carbonation progress directly from monitoring CO₂ uptake. Thermal analysis (TA) was used to verify the degree of carbonation that reached up to 83%. Factor analysis on the data set has demonstrated that reaction speed is not dependent upon the CO₂ concentration within the limits tested. Carbonation speed depends on the specific surface of the lime. The results of this study contribute to research carried out on lime mortar carbonation models and on the carbonation process in general.     

Effect of initial droplet size distribution on sulfur removal efficiency in FGD/SDA

Dry scrubbing with lime slurry in a spray dryer [spray dryer absorber (SDA)] has been an important technology for flue gas desulfurization (FGD). Mathematical models based on the heat and mass balances are used to predict SO₂ removal in the SDA as a function of initial size distribution of slurry droplets. Since the existence of moisture in the droplets appreciably enhances the SO₂ removal, its removal efficiency depends on the rate of drying as well as that of SO₂ removal itself, both depending on droplet diameter. With the increase in the geometric standard deviation (GSD) of the initial droplet size distribution, the efficiency of SO₂ removal first increases and then decreases, showing a maximum at a certain value of GSD. This trend is altered by the sorbent content of the droplets, expressed as stoichiometric ratio (SR). The decrease in SR makes the maximum move to higher GSD and reduces the variation in the efficiency with respect to GSD. For SR<0.73, a minimum efficiency also appears, ahead of the maximum. The results are well explained by the specific rates of both drying and SO₂ removal of the droplets.     

中温条件下氧化铁对氧化钙脱硫的活化作用

运用热重分析仪对石灰与氧化铁混合物的固硫效果进行了分析.发现氧化铁在200～700 ℃和二氧化硫不发生反应.石灰的固硫效果随温度的提高而有所改善：500 ℃以下基本不能固硫,600 ℃、700 ℃有一定的硫化效果.加入氧化铁后,石灰固硫特性随温度变化的趋势没有改变,但石灰的钙利用率不仅大大提高,而且随氧化铁的混合配比单调递增.氧化铁对石灰中温烟气脱硫反应并没有传统意义上的催化作用.氧化铁所起的是改变石灰颗粒表层产物生成方式的活化作用,使反应生成物不会形成一个致密的外壳,而以氧化铁为活性中心进行硫化反应,形成了分散的硫酸钙产物堆积,改善了硫化过程中的表面孔隙结构,减少了石灰颗粒表面硫化产物层对反应气体向石灰颗粒内部扩散的阻力.     

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.     

Full-scale measurements of SO2 gas phase concentrations and slurry compositions in a wet flue gas desulphurisation spray absorber

Two measurement campaigns were carried out at ENERGI E2's Asnæs Power plant, unit 5. The unit has a capacity of 620 MW_e and is equipped with a wet flue gas desulphurisation (FGD) plant employing a counter-current spray absorber with five spray levels. In the first campaign, the power plant was firing Orimulsion^® with 2.85 wt% S resulting in a flue gas concentration of SO₂ exceeding 2000 ppmv. In the second campaign, the fuel applied was a low-S blended coal and the SO₂ concentration in the raw gas was around 400 ppmv. A novel probe for in situ sampling of gas phase concentrations in wet FGD spray absorbers was developed and applied for measuring axial profiles of the SO₂ gas phase concentrations in the absorber. The expected decrease in SO₂ concentrations along the height of the absorber was found in the spray section (from height 26.5 to 36.2 m) whereas the SO₂ concentration above the holding tank and below the gas inlet was quite low probably due to long local residence times in the region. Horizontal variations, due to somewhat different flow conditions near the column wall were investigated and the SO₂ concentrations were found to be higher near the wall. Measurements at different gross loads showed that the SO₂ gas phase concentration at a given position inside the absorber was roughly linearly related to the L/G ratio in the measuring interval. Turning off one of the lower spray levels, while burning coal with low S content, did not lower the overall removal efficiency of the absorber. However, the SO₂ gas phase concentration inside the lower part of the absorber was increased by a factor of 2-3. Measurements of slurry pH at different positions showed a decrease of approximately 0.5 units from the upper to the lower part of the absorber. The full-scale measurements provide a detailed set of experimental data for validation of mathematical models of a wet FGD spray absorber.     

Impurity control in the production of boric acid from colemanite in the presence of propionic acid

The most important problem in boric acid production from colemanite ores with H₂SO₄ is the formation of MgSO₄ impurity due to the partial decomposition of clay minerals in the reaction media. Increase of MgSO₄ concentration in solution may be balanced by the discharge of mother liquor which leads to decrease the efficiency of the process. Therefore, the intake rate of MgSO₄ should be lowered for obtaining high purity product in a high yield process. In order to control the intake rate of MgSO₄ impurity, propionic acid, which does not decompose the clay minerals, is used in an acid mixture with H₂SO₄. Batch wise laboratory experiments showed that the higher the propionic acid in reacting acid mixture, the lower the magnesium intake rate. When 10% of required H₂SO₄ replaced by propionic acid, magnesium intake concentration decreased to approximately half of the value obtained in the reaction with H₂SO₄.     

Absorption of NO by Aqueous Solutions of KMnO4/H2SO4

The absorption of NO in aqueous solutions of KMnO₄ and H₂SO₄ was carried out in a stirred tank reactor under atmosphere pressure. The results indicated that the absorption process was under a fast pseudo-m th reaction regime. The reaction between NO and aqueous solutions of KMnO₄/H₂SO₄ was found to be first-order with respect both to NO and to KMnO₄. The addition of H₂SO₄ to KMnO₄ solutions increased the absorption rate of NO and increasing reaction temperature was also favorable to the absorption of NO.     

SO2 and NO removal from flue gas over V2O5/AC at lower temperatures — role of V2O5 on SO2 removal

Supporting V₂O₅ onto an activated coke (AC) has been reported to significantly increase the AC's activity in simultaneous SO₂ and NO removal from flue gas. To understand the role of V₂O₅ on SO₂ removal, V₂O₅/AC is studied through SO₂ removal reaction, surface analysis, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) techniques. It is found that the main role of V₂O₅ in SO₂ removal over V₂O₅/AC is to catalyze SO₂ oxidation through a VOSO₄-like intermediate species, which reacts with O₂ to form SO₃ and V₂O₅. The SO₃ formed transfers from the V sites to AC sites and then reacts with H₂O to form H₂SO₄. At low V₂O₅ loadings, a V atom is able to catalyze as many as 8 SO₂ molecules to SO₃. At high V₂O₅ loadings, however, the number of SO₂ molecules catalyzed by a V atom is much less, due possibly to excessive amounts of V₂O₅ sites in comparison to the pores available for SO₃ and H₂SO₄ storage.     

Computational modeling of furnace sorbent injection for SO2 removal from coal-fired utility boilers

Furnace sorbent injection (FSI) is used to remove SO₂ formed during coal combustion by injecting sorbent into the high temperature zone of a furnace above the fireball. FSI is cost effective for older coal-fired boilers, especially when space or capital budgets are limited. To optimize the design and performance of FSI, an SO₂/sorbent modeling scheme that simultaneously considers calcination (or dehydration), sintering, and sulfation has been developed and implemented. It is coupled with a three-dimensional combustion model based on computational fluid dynamics to determine the most desirable locations for sorbent injection and to optimize the amount of sorbent needed to achieve a targeted SO₂ removal efficiency. A sensitivity analysis was conducted to determine the effect of flue gas temperature, particle diameter, and SO₂ concentration on the extent of sulfation. This SO₂/sorbent sub-model was applied to a 126-MW front-wall fired boiler firing eastern bituminous coal. The SO₂ removal efficiencies predicted by the model agreed well with those measured in the field. The modeling results indicated that sorbent injected directly into the furnace through boosted over-fired air ports is more effective at removing SO₂, due to longer residence time and better mixing, relative to ports higher in the furnace with poor mixing. This modeling approach is optimized for full-furnace application to facilitate the design process.     

Enhancement of SO2 sorption on lime by structure modifiers

The presence of certain inert compounds in lime can improve the rate and the final limiting conversion significantly during the lime SO₂ reaction. This physical effect is primarily due to modification of porosity. In particular, the enhancement appears to he due to increase in macro-porosity and macro-pore diffusivity; this in turn increases the sulfation rate and decreases pore plugging. In certain lime application processes, these compounds (structure modifiers) can be added to lime as beneficial additives to promote the reaction. Experimental data on the effect of kaolin, silica and bauxite as structure modifiers are presented. A theoretical model is developed which explains the observed effects both qualitatively and quantitatively.     

DECOMPOSITION BEHAVIOR AND MECHANISM OF CALCIUM SULFATE UNDER THE CONDITION OF O2/CO2 PULVERIZED COAL COMBUSTION

The decomposition behavior and mechanism of calcium sulfate in O₂/CO₂ pulverized coal combustion were studied in an entrained flow reactor. A reaction rate expression correlating the influence of various factors was proposed for CaS0₄ decomposition and it is able to predict CaS0₄ decomposition satisfactorily. Under the conditions investigated, the decomposition of CaS0₄ was found to be a regime of chemically controlled shrinking core reaction. A CO₂-rich atmosphere enhances CaSO₄ decomposition in absence of oxygen. CaSO₄ particles have catalytic effect on formation of CO from CO₂. A high SO₂ concentration inhibits CaSO₄ decomposition. The kinetics of CaSO₄decomposition has obvious dependence on experimental facilities and conditions, whereas the activation energy has much lower dependence. The kinetics derived in this work is more appropriate for investigating desulfurization in O₂/CO₂ pulverized coal combustion because an entrained flow reactor has a much closer condition to that in O₂/CO₂ pulverized coal combustion than a TGA.     

Tentative modelling of spray-dry scrubbing of SO2

A 0.5 MW spray-dry scrubbing FGD pilot plant was used in the study of spray dryer performance over a wide range of operating conditions. Experimental findings were compared with a spray dryer model. During operation with large excesses of lime, the SO₂ absorption was limited by gas phase diffusion. At operation with a shortage of lime, the rate limiting step was the dissolution rate of lime. In addition, the flow regime in a spray dryer can be best described as well mixed. The SO₂ level in the flue gas was found to exert no direct effect on the efficiency of SO₂ removal. The observed effects are attributed solely to the changes in the drying process, due to the inter-dependence of slurry composition and SO₂ concentration.     

Kinetic Studies on NO2 Absorption into Ammonium Sulfite Solutions

The absorption reactions of NO₂ into (NH₄)₂SO₃ solution were investigated in a stirred tank reactor. The kinetic regime of the absorption reaction was identified and the effects of various experimental parameters were studied. The experimental results indicated that the absorption of NO₂ into (NH₄)₂SO₃ solution is accompanied by a fast pseudo-first-order reaction. It was found that the NO₂ absorption rates were enhanced by the increasing concentration of (NH₄)₂SO₃ but nearly remained constant if the concentration is greater than 0.1 mol/L. The absorption rates also increased with increasing the reaction temperature and the concentration of NO₂ in inlet gas, but decreased as the oxygen concentration increased.