CaO固硫反应缩芯动力学模型

建立CaO固硫反应的未反应缩芯动力学模型,通过模型计算,考察了宏观反应、灰层扩散控制、气膜扩散控制和化学扩散控制条件下,反应温度、CaO粒径对固硫反应特性的影响.结果表明,提高反应温度、延长反应时间和减小CaO粒径,有利于提高CaO转化率,其中减小CaO粒径对于提高CaO固硫效果最为明显.模型计算结果与实验结果分析表明,未反应收缩芯模型可较好地描述CaO-SO2固硫反应的宏观动力学,其中影响CaO固硫反应的最主要因素是化学反应扩散控制,提高CaO固硫效率最有效的方法是减小CaO粒径.     

生料粉磨环境下石灰石低温半干法固硫特性研究

水泥生产中石灰石粉磨和烘干工序具有显著的SO2吸收特性,其属于半干法脱硫.采用固定床实验装置,设计了石英砂作为空白对照,研究了反应温度、石灰石含水率对石灰石低温半干法固硫的影响.结果表明,当石灰石含水率为20％,反应温度从50℃升高到100℃时,石灰石对SO2的吸附量从1.06 mg SO2/g石灰石下降至0.32 mg SO2/g石灰石,随温度升高而降低;其中SO2物理吸附比例从4.72％最高增加至17.81％,随温度升高而增大.当含水率为5％、10％和20％时,在50～100℃内石灰石颗粒对SO2的平均吸附量分别为0.14 mg SO2/g、0.25 mg SO2/g和0.60 mg SO2/g石灰石,其中SO2物理吸附比例分别为38.60％、17.66％和11.22％;SO2吸附量随含水率增加而显著增大,而物理吸附比例随含水率增加而显著降低.采用积分计算的石灰石SO3含量增加值与荧光分析结果相吻合,而与硫酸钡重量法检测结果不一致,表明石灰石低温半干法固硫产物以亚硫酸盐、亚硫酸氢盐等为主,而不是硫酸盐.     

流化床锅炉温度条件下钙基工业废弃物的固硫反应性能

在流化床锅炉温度条件下研究了赤泥、电石渣等钙基工业废弃物煅烧后的固硫特性,并与石灰石比较,同时研究了吸收剂在反应过程中的物相变化、微观结构特性。结果表明,在相同反应条件下,随反应时间增加,赤泥的钙转化率高于电石渣和石灰石,石灰石的钙转化率最小。赤泥和电石渣的最佳固硫温度分别为850～900℃和950～1000℃。随SO2浓度增加,在相同反应时间内赤泥的钙转化率和硫化反应速率也相应增大。粒径对赤泥的固硫性能影响不大。赤泥和电石渣中钙的主要化合物分别为Ca2SiO4和Ca(OH)2。它们煅烧后孔径主要分布在5～20nm内,这正是最有利于固硫的孔径区域,石灰石煅烧后孔径主要分布于45～420nm。钙基废弃物具有优良的孔隙结构,因而它们在流化床锅炉温度条件下具有良好的固硫性能。     

氧化铝厂赤泥剂的固硫反应动力学特性研究

采用热分析法研究了山东铝业公司赤泥和石灰石固硫反应过程,利用等效粒子模型计算分析了其固硫反应动力学参数,并在此基础上利用压汞仪、SEM分析了赤泥煅烧前后的微观结构,探讨了赤泥固硫机理,为利用氧化铝生产中排放的大量赤泥代替石灰石用作燃煤固硫剂提供了理论依据.结果表明,赤泥的钙利用率是相同条件下石灰石的2～3倍,赤泥煅烧后较石灰石具有更多的中孔和较大的比表面积,可提高化学反应速度和反应深度;另外,赤泥中较多的三氧化二铁和碱金属盐也可提高赤泥固硫反应速率常数和有效扩散系数,但温度过高会降低固体熔点而易导致烧结,削弱其固硫活性.     

钙基吸收剂碳酸化实验研究

对CaO吸收CO2反应的特性进行了实验研究,采用未反应收缩核模型分析碳酸化反应动力学特性.结果表明,化学反应速率常数在650℃～750℃范围内基本为一常数,产物层扩散系数随着温度的增加而增大.化学反应控制段的活化能Ea=29.70 kJ/moL,产物控制段的活化能Ea=92.80 kJ/moL.温度一定时,随着CO2体积分数增加,碳酸化反应速率加快,转化率增大.     

石灰石在燃煤流化床中固硫的数学模拟进展

石灰石在燃煤流化床中的固硫属于非催化气－固反应，其复杂性在于它是由反应气体在氧化钙颗粒孔内扩散、反应气体在产物层内扩散以及反应气体和氧化钙进行表面化学反应等三个过程的耦合，而且固体反应物的结构随着反应进行而变化。本文评述了固硫过程模拟工作的历史发展及最新趋势，并基于大量的实验事实和偏微分方程数值解的可能性，提出在建立模型时应考虑石灰石中惰性物含量和固硫反应热的影响。     

石灰石固硫反应模型

在燃烧固硫试验系统和LCT－2型热天平上对石灰石固硫反应特性进行了试验研究．建立了石灰石煅烧与固硫反应同时进行时的数学模型,并通过试验结果的计算机处理,得到CO_2通过固硫剂颗粒的扩散系数D_(co_2)与传质系数a_(co_2).模型的合理性通过理论计算与试验结果的比较得到了验证．     

循环流化床锅炉内石灰石同时煅烧/硫化反应模型研究

循环流化床锅炉内石灰石分解、脱硫发生的是同时煅烧/硫化反应。建立了石灰石同时煅烧/硫化反应的随机孔模型,综合考虑石灰石的分解、烧结和硫化,且CaO硫化反应基于CaSO_4产物层固态离子扩散模式。模型计算结果与实验测试结果吻合良好,并采用该模型研究了同时煅烧/硫化反应的特性。石灰石的同时煅烧/硫化反应包含连续的质量下降阶段和质量上升阶段,且质量最低点随着SO_2浓度的增加而升高。石灰石颗粒的煅烧反应发生在颗粒的壳层内,煅烧反应更符合区域反应模型而不是均相反应或缩核反应模型。煅烧环境中的SO_2与CaO层反应生成CaSO_4,导致CaO层内孔径和孔隙率减小,CO_2外扩散阻力增大,从而导致煅烧反应减慢。颗粒硫化反应速度随时间减慢,主要是颗粒内SO_2耗尽导致的,颗粒外层不断积累的CaSO_4减小了SO_2扩散通道,增加了SO_2孔内扩散阻力,使颗粒内SO_2耗尽区不断增大,颗粒的硫化反应速度不断下降。     

分形多孔介质的孔隙特性对气体扩散的影响

采用随机行走方法建立了分形多孔介质生成模型,生成的颗粒在形貌上与真实多孔颗粒接近,且能够反映其固有分形特征。在此模型基础上,根据经典分子动理论建立扩散控制方程,对气体在多孔介质中的扩散进行数值模拟。分析了比表面积、平均孔径、孔隙率等孔隙特性参数对扩散的影响,获得了分形多孔介质中气体扩散系数与平均孔径的函数关系。结果表明,扩散系数随平均孔径的增大以幂函数形式增大,相应的指数表征扩散系数对平均孔径的敏感度,其值随孔隙率的增大呈线性减小。     

循环流化床锅炉内石灰石同时煅烧/硫化反应模型研究

循环流化床锅炉内石灰石分解、脱硫发生的是同时煅烧/硫化反应。建立了石灰石同时煅烧/硫化反应的随机孔模型,综合考虑石灰石的分解、烧结和硫化,且CaO硫化反应基于CaSO₄产物层固态离子扩散模式。模型计算结果与实验测试结果吻合良好,并采用该模型研究了同时煅烧/硫化反应的特性。石灰石的同时煅烧/硫化反应包含连续的质量下降阶段和质量上升阶段,且质量最低点随着SO₂浓度的增加而升高。石灰石颗粒的煅烧反应发生在颗粒的壳层内,煅烧反应更符合区域反应模型而不是均相反应或缩核反应模型。煅烧环境中的SO₂与CaO层反应生成CaSO₄,导致CaO层内孔径和孔隙率减小,CO₂外扩散阻力增大,从而导致煅烧反应减慢。颗粒硫化反应速度随时间减慢,主要是颗粒内SO₂耗尽导致的,颗粒外层不断积累的CaSO₄减小了SO₂扩散通道,增加了SO₂孔内扩散阻力,使颗粒内SO₂耗尽区不断增大,颗粒的硫化反应速度不断下降。     

The Degree of Desulphurization of a Limestone/Gypsum Wet FGD Spray Tower using Response Surface Methodology

The degree of desulphurization was studied using response surface methodology (RSM), which enables effect examinations of parameters with a moderate number of experiments. All experiments were conducted in a lab‐scale spray tower for limestone/gypsum wet flue gas desulphurization (FGD). The model flue gas was prepared from air and SO₂ gas. The SO₂ concentrations in the gas phase were determined by a multi‐method analyzer. The degree of desulphurization correlated well with operating parameters, including pH, L/G, T, and v, with a determination coefficient R–Sq of 0.964. Effect tests indicate that L/G has the most significant influence on the degree of desulphurization. The interactions of L/G with pH, and with v, both play important roles. The result indicates that the evolutive response surface model is helpful to describe the degree of desulphurization of the limestone/gypsum wet FGD spray tower.     

Modeling and Simulation of a Bubbling SO2 Absorber with Granular Limestone Slurry and an Organic Acid Additive

The absorption of SO₂ into limestone slurry containing suspended reactive particles was performed in a bubble reactor with continuous feeding of both gas and liquid phases at a constant pH and high temperature (50 °C). An absorption model with a reaction plane based on the film model was developed. The effect of limestone particle size, concentration, acetic acid additives, and inlet SO₂ concentration on the concentration distribution of chemical species in the liquid film and SO₂ absorption rate were simulated. Increasing the concentration of limestone slurry, adding acetic acid additives into the system or decreasing the limestone particle size or inlet SO₂ concentration caused the reaction plane in the liquid film to shift towards the gas‐liquid interface. Model and experimental results were compared, and it was shown that the model fits the experimental data well.     

Effective diffusivity and optimum apparent density of vanadia catalysts for sulfur dioxide oxidation

The effective diffusivities of air and SO₂ in four industrial vanadium pentoxide catalysts were measured at steady-state using helium as the counter diffusing gas. An improved catalyst mounting technique and diffusion cell were employed. The nonsurface component of diffusion was successfully correlated using Bruggeman's model for tortuosity. and ¯a based on pore size distribution data or calculated from specific pore volume and surface. However, it was necessary to use flow porosity in place of open porosity. Since the same pore model can be used for the catalytic oxidation So₂, non-reacting flow measurements can be employed to predict effective diffusivities under reaction conditions in this case.With models for the effective diffusivity and the kinetics of the catalytic oxidation of SO₂, an optimum apparent density of the catalyst may be determined which gives the maximum rate of reaction per unit volume of catalyst. Calculations are given for the SVD catalyst.     

Characterization of the reactivity of limestones with SO2 in a fluidized bed reactor

The reaction between SO₂ and calcined limestone particles has been studied in a fluidized bed combustor. Measurements of sorbent reactivity with SO₂ were made for small batches of limestone injected into the combustor. Simultaneous continuous combustion of bituminous coal provided conditions like those of a boiler for study of the sulphation reaction. A semi-empirical rate model of the CaO-SO₂ reaction has been developed. External mass transfer of SO₂, diffusion within the particles and chemical reaction are taken into account. The limestone reactivity with SO₂ is characterized by two parameters which are dependent on the temperature and sorbent particle size. A model for predicting the limestone requirements in a fluidized bed boiler has been developed. Parameters from the batch experiments are included. The predictions for sulfur retention agree with the experimental results. In addition, effects of operating conditions (gas velocity, recycle, limestone particle size) on the retention of SO₂ were simulated using the model.     

Calcium-based sorbents behaviour during sulphation at oxy-fuel fluidised bed combustion conditions

Sulphur capture by calcium-based sorbents is a process highly dependent on the temperature and CO₂ concentration. In oxy-fuel combustion in fluidised beds (FB), CO₂ concentration in the flue gas may be enriched up to 95%. Under so high CO₂ concentration, different from that in conventional coal combustion with air, the calcination and sulphation behaviour of the sorbent must be defined to determine the optimum operating temperature in the FB combustors.In this work, the SO₂ retention capacity of two different limestones was tested by thermogravimetric analysis at typical oxy-fuel conditions in FB combustors. The effect of the main operating variables affecting calcination and sulphation reactions, like CO₂ and SO₂ concentrations, temperature, and sorbent particle size, was analysed.It was observed a clear difference in the sulphation conversion reached by the sorbent whether the sulphation takes place under indirect or direct sulphation, being much higher under indirect sulphation. But, in spite of this difference, for a given condition and temperature, the CO₂ concentration did not affect to the sulphation conversion, being its major effect to delay the CaCO₃ decomposition to a higher temperature.For the typical operating conditions and sorbent particle sizes used in oxy-fuel FB combustors, the maximum sorbent sulphation conversions were reached at temperatures of about 900 °C. At these conditions, limestone sulphation took place in two steps. The first one was controlled by diffusion through porous system of the particles until pore plugging, and the second controlled by the diffusion through product layer. As a consequence, the maximum sulphation conversion increased with decreasing the particle size and increasing the SO₂ concentration.     

A simplified model of SO2 capture by limestone in 71 MWe pressurized fluidized bed combustor

A mathematical model of SO₂ capture by uncalcined limestone particles with solid attrition under pressurized fluidized bed combustion conditions was developed based on the shrinking unreacted-core model. Since the thickness of the product layer is sufficiently much smaller than the particle size, a flat surface model was employed. The difference in SO₂ capture behavior between continuous solid attrition and intermittent attrition was investigated. The reaction rate for intermittent solid attrition was found to be lower than that for continuous attrition mode under low SO₂ concentration conditions. A simple mathematical expression to calculate reaction rate of SO₂ capture per unit external surface area of limestone is proposed.The present simplified mathematical model of SO₂ capture by single limestone particle under periodical attrition conditions was applied to the analysis of a large-scale pressurized fluidized bed combustor. By giving the period of attrition as a parameter, the experimental results agreed well with the model results. From the vertical concentration profile of SO₂ concentration, the emission of SO₂ was found to be governed by the balance between SO₂ formation rate from char and SO₂ capture by limestone at the upper surface of the dense bed. A simplified expression to estimate SO₂ emission from pressurized fluidized bed combustors was proposed.     

The combined effect of H2O and SO2 on the simultaneous calcination/sulfation reaction in CFBs

The combined effect of H₂O and SO₂ on the reaction kinetics and pore structure of limestone during simultaneous calcination/sulfation reactions under circulating fluidized bed (CFB) conditions was first studied in a constant-temperature reactor. H₂O can accelerate the sulfation reaction rate in the slow-sulfation stage significantly but has a smaller effect in the fast-sulfation stage. H₂O can also accelerate the calcination of CaCO₃, and should be considered as a catalyst, as the activation energy for the calcination reaction was lower in the presence of H₂O. When the limestone particles are calcining, SO₂ in the flue gas can react with CaO on the outer particle layer and the resulting CaSO₄ blocks the CaO pores, increases the diffusion resistance of CO₂, and, in consequence, decreases the calcination rate of CaCO₃. Here, gases containing 15% H₂O and 0.3% SO₂ are shown to increase the calcination rate. This means that the accelerating effect of 15% H₂O on CaCO₃ decomposition is stronger than the impeding effect caused by 0.3% SO₂. The calcination rate of limestone particles was controlled by both the intrinsic reaction and the CO₂ diffusion rate in the pores, but the intrinsic reaction rate played a major role as indicated by the effectiveness factors determined in this work. This may explain the synergic effect of H₂O and SO₂ on CaCO₃ decomposition observed here. Finally, the effect of H₂O and SO₂ on sulfur capture in a 600 MWe CFB boiler burning petroleum coke is also analyzed. The sulfation performance of limestone evaluated by simultaneous calcination/sulfation is shown to be much higher than that by sulfation of CaO. Based on our calculations, a novel use of the wet flue gas recycle method was put forward to improve the sulfur capture performance for high-sulfur low-moisture fuels such as petroleum coke. © 2019 American Institute of Chemical Engineers AIChE J, 65: 1256–1268, 2019     

Breakthrough and Sulfate Conversion Analysis during Removal of Sulfur Dioxide by Calcium Oxide Sorbents

Sorption of sulfur dioxide (SO₂) was carried out on calcium‐based sorbents under dynamic conditions in a fixed bed. The experimental conditions were reaction temperature (700 to 1000°C), SO₂ concentration (1000‐10 000 ppm), sorbent particles size (1 to 2 mm) and the types of sorbents (hydroxide or carbonate). The sorption process was found to be effective at low concentration levels (less than 10 000 ppm) as the breakthrough time significantly decreased with increase in concentration. The maximum removal of SO₂ was observed at a reaction temperature of 950°C. The hydroxide‐based sorbents of relatively smaller particle size were found to exhibit superior sorption performance in terms of longer breakthrough time and higher sulfate conversion. A mathematical model developed, assuming a porous structure of the sorbent materials, attributed the low sulfation conversion during SO₂ sorption due to a pore diffusion mechanism.     

添加有机酸强化粗颗粒石灰石烟气脱硫

针对传统石灰石湿法烟气脱硫成本高的问题,提出了一种新型石灰石-石膏法烟气脱硫方法,即通过在石灰石浆液中加入有机酸,采用大尺寸石灰石165～200 μm（-80+100目）代替传统的47 μm（325目）以下的细颗粒石灰石进行脱硫,同时在鼓泡搅拌吸收反应器脱硫装置上与传统石灰石湿法烟气脱硫进行了对比实验.研究结果表明,只采用165～200 μm石灰石浆液直接脱硫其脱硫率和石灰石利用率分别为60.7%和44%,但当石灰石（165～200 μm）浆液中乙酸浓度达到10～30 mmol•L^-1,其脱硫率和石灰石利用率分别为95%和93.5%,都达到甚至优于传统石灰石脱硫中的结果,新型石灰石湿法脱硫系统的pH值在正常的脱硫区间波动也较小.在此基础上,提出了添加乙酸促进SO₂吸收的机理.本文提出的新型烟气脱硫方法具有很好的工业应用前景.     

Absorption of sulfur dioxide into limestone slurry in a stirred tank

The absorption rate of sulfur dioxide into limestone slurries has been observed under various experimental conditions, using a stirred tank absorber which had a plane surface. The effects of the interfacial concentration of SO₂, the slurry concentration, the concentration of SO^2?₂ ion, and the limestone particle size on the rate have been examined. Taking the effect of SO^2?₂ ion on the rate into consideration, a mathematical model for SO₂absorption into limestone slurries based on the film concept has been developed. The model can explain the mechanism of SO₂ absorption into limestone slurries.