A model for performance optimization of wet flue gas desulfurization systems of power plants

In this paper, a model of limestone/gypsum wet flue gas desulfurization (WFGD) system was developed based on unsteady theory. The models of processes of absorption section and oxidation section were developed and incorporated into the WFGD integral model. The sub-model of motion of slurry drops, absorption of SO₂, dissolution of limestone and crystallization of gypsum were included in the model of absorption section, while the model of oxidation section was developed by the population balance theory. The calculation results of the desulfurization system for 300 MW utility in China by this model was compared to that of corresponding measured results. The simulation results agreed with the measurement results very well. The operation mode of boilers of power plant in China is different to that of other countries since variable coal property and unstable loads of boilers. The differences of operation mode lead to the variation of process parameters of WFGD system. The influences of liquid-to-gas ratio, SO₂ concentration of inlet flue gas, and combination mode of different spray levels to the desulfurization efficiency were analyzed. Based on the analysis, some advices of performance optimization of flue gas desulfurization systems in China were suggested.     

Optimum Values of Process Parameters of the “Wet Limestone Flue Gas Desulfurization System”

In this article the method of cost optimization of the “Wet Limestone Flue Gas Desulfurization System” is presented. The optimization calculations include process and cost models. The process model describes the most important stage of SO₂ removal that runs in the absorber and in the holding tank. It includes absorption of sulfur dioxide, oxidation of SO^2–₃, dissolution of limestone, and crystallization of gypsum. The model was applied to calculate indispensable parameters for estimating costs and then to minimize capital and operating costs. Costs of all important equipment were estimated, such as SO₂ removal systems with the absorber and the holding tank, reagent feed system with the ball mill and dewatering gypsum slurry system. Optimum values of the process parameters for different conditions of running flue gas desulfurization system were found. The process and cost model can be useful when designing the wet limestone FGD systems and carrying out economic analysis of the flue gas desulfurization plants.     

Removal of fine particles in wet flue gas desulfurization system by heterogeneous condensation

A novel process to remove fine particles with high efficiency by heterogeneous condensation in a wet flue gas desulfurization (WFGD) system is presented. A supersaturated vapor phase, necessary for condensational growth of fine particles, was achieved in the SO₂ absorption zone and at the top of the wet FGD scrubber by adding steam in the gas inlet and above the scrubbing liquid inlet of the scrubber, respectively. The condensational grown droplets were then removed by the scrubbing liquid and a high-efficiency demister. The results show that the effectiveness of the WFGD system for removal of fine particles is related to the SO₂ absorbent employed. When using CaCO₃ and NH₃·H₂O to remove SO₂ from flue gas, the fine particle removal efficiencies are lower than those for Na₂CO₃ and water, and the morphology and elemental composition of fine particles are changed. This effect can be attributed to the formation of aerosol particles in the limestone and ammonia-based FGD processes. The performance of the WFGD system for removal of fine particles can be significantly improved for both steam addition cases, for which the removal efficiency increases with increasing amount of added steam. A high liquid to gas ratio is beneficial for efficient removal of fine particles by heterogeneous condensation of water vapor.     

氨法烟气脱硫工艺过程模拟与优化

为了进一步研究氨法烟气脱硫工艺过程,以非平衡级传质理论为依据,利用化工流程模拟软件Aspen Plus建立填料吸收塔脱除SO2过程的数值计算模型,分析和优化操作参数对脱硫效果的影响,模拟结果与文献值吻合较好。模型计算结果表明:脱硫率随着吸收液pH值和液气比的增大而增大;随着进口SO2质量浓度和进口烟气流量的增大而降低,并得出吸收液pH值及液气比在氨法烟气脱硫效率影响因素中占主要位置。所建立的氨法烟气脱硫工艺过程模型准确合理,模拟结果可为脱硫系统实际运行调节各操作参数对脱硫效果的影响提供参考和借鉴。     

氨法烟气脱硫SO2吸收传质系数研究

喷淋塔氨法脱硫技术被广泛应用于净化烟气的SO₂,传质系数是喷淋吸收塔重要的设计和运行参数,但目前文献中有关氨法脱硫传质系数的报道很少,还有待进一步研究。在喷淋塔中对氨法脱硫SO₂吸收传质过程进行了实验研究,结合对液滴和塔壁液膜运动的计算,得到不同实验条件下SO₂的吸收传质速率,并建立了氨法脱硫SO₂吸收传质系数表达式。该传质系数包含浆液pH、烟气流速u_g和液气比L/G等主要参数,能够反映不同pH、u_g和L/G条件下SO₂在单位气液接触面积上的传质速率。对比验证结果表明,该传质系数计算得到的SO₂吸收传质速率与实验值之间的相对误差小于±12%,二者能够较好地吻合。建立的传质系数表达式能够为喷淋塔氨法脱硫的优化设计和运行提供理论参考。     

The formation and removal characteristics of aerosols in ammonia-based wet flue gas desulfurization

The characteristics of aerosol generation were studied experimentally in an ammonia-based wet flue gas desulfurization process. Particle size distributions and concentrations, morphologies and compositions before and after desulfurization were measured using an electrical low pressure impactor and scanning electron microscopy, respectively. The results show that aerosols can be generated between ammonia and sulfur dioxide resulting in gas-phase reaction and the aerosol concentration at the outlet of scrubber is significantly higher than at the inlet. Before desulfurization the particles are primarily silica-alumina minerals including O, Al, Si and C, while after ammonia-based desulfurization aerosol particles have smooth surfaces with regular structures, such as cubic and prismatic crystals, and contain principally O, S and N. Particle sizes up to 10 μm were measured, but the majority of the aerosol particles are in the submicron range. Separation of submicron particles from flue gas is difficult by conventional desulfurization scrubber; however high removal efficiency can be achieved based on the enlargement of the particles by heterogeneous condensation. The influence of the supersaturation degree and liquid–gas ratio on the removal efficiencies of the particles are demonstrated additionally. It indicates that aerosols generated in ammonia-based desulfurization process can be deliquesced and gradually enlarged in a high humidity flue gas, but the supersaturation degree higher than 1.2 is required for heterogeneous nucleation of water vapor with ash particles from coal combustion.     

氨法烟气脱硫过程中气溶胶颗粒生成特性

利用自行搭建的模拟氨法烟气脱硫实验平台,针对氨法脱硫中脱硫浆液液滴夹带蒸发和非均相反应两种气溶胶生成途径,采用电称低压冲击器(ELPI)、PM_2.5/10采样器、场发射扫描电子显微镜(FESEM)等手段对两种途径下生成的气溶胶浓度、粒度分布以及形貌特征进行了测试分析,实验考察了氨法脱硫工艺参数对两种途径下生成的气溶胶特性的影响。结果表明,氨法脱硫中形成的气溶胶颗粒从数浓度角度而言,主要为亚微米级颗粒,从质量浓度分析,则以微米级及更大颗粒为主;脱硫浆液中挥发逸出的气态NH₃和烟气中水汽、SO₂间的非均相反应是氨法脱硫气溶胶的主要来源,其形成量主要取决于脱硫浆液pH、脱硫塔进口烟温等;其次来自于脱硫液滴的夹带蒸发作用,其形成量主要取决于脱硫浆液浓度、空塔气速等。     

Removal of SO2 and NOX Using Microwave Swing Adsorption over Activated Carbon Carried Catalyst

SO₂ and NO are the main precursors for acid precipitation. Experimental studies on desulfurization and denitrification were carried out using microwave irradiation over activated carbon carried catalyst. The results show that adsorption capacities and removal efficiencies of activated carbon carried Cu‐based catalyst were higher than Mn‐based or Zn‐based ones. The adsorption capacity of SO₂ improved with the increasing moisture in flue gas, but the adsorption capacity of NO had a peak at 6.23 mg g^–1 and then began to drop. The desulfurization efficiency increased with O₂ content in flue gas, but no noticeable change of denitrification efficiency was observed from the experimental data. The desulfurization efficiency descended with the increase of moisture in flue gas, while the denitrification efficiency augmented earlier and reached a plateau later with the addition of the water steam. In addition, characterization of activated carbon confirmed that the main active component of Cu‐based catalyst is CuO.     

Removal of SO2 from flue gas using a new semidry flue gas desulfurization process with a powder-particle spouted bed

A spouted bed of binary particle mixture was applied to a low temperature desulfurization process in order to develop a new type of semidry flue gas desulfurization (FGD) technology. We investigated the effects of operating parameters, such as type of SO₂ sorbent, diameter of SO₂ sorbent particles, apparent residence time of gas in the bed, approach to saturation temperature and Ca/S molar ratio, on SO₂ removal in a bench-scale powder-particle spouted bed. We also investigated the utilization rate of SO₂ sorbent and ways to enhance the efficiency of SO₂ removal and SO₂ sorbent utilization. The experimental results showed that SO₂ removal is significantly affected by the approach to saturation temperature and Ca/S molar ratio, and that a high SO₂ removal efficiency and effective sorbent utilization can be obtained under appropriate operating conditions. Thus, this new simple process of flue gas desulfurization is highly efficient and has little impact on the environment.     

Mass transfer and reaction process of the wet desulfurization reactor with falling film by cross-flow scrubbing

In the present study, a series of wet flue gas desulfurization experiments have been carried out in comparison with different slurry feeding ways, i.e., by series connection and by parallel connection, by means of cross-flow scrubbing with falling film. The experiment results show that there is optimal desulfurization performance for the slurry feeding way by series connection. A liquid side mass transfer-reaction model and desulfurization mass transfer by cross-flow scrubbing model have been developed. The pH values of the outlet slurry inside the reactor and the ion concentration distributions of H₂SO₃, HSO₃⁻ and SO₃²⁻ along the axial direction of the tubes were obtained by analyzing and calculating the models. The calculation values agree well with the experimental values. It shows that the models can predict well the ion concentration distributions along the axial direction of the tubes. This work was presented at the 6 ^th Korea-China Workshop on Clean Energy Technology held at Busan, Korea, July 4–7, 2006.     

Simulation of the operation of an industrial wet flue gas desulfurization system

In this work the simulation of a wet flue gas desulfurization (FGD) unit with spray tower of a power plant is presented, aiming at an efficient follow-up and the optimization of the FGD system operation. The dynamic model developed to simulate the performance of the system has been validated with operation data collected over a long period of time. All the partaking physical and chemical processes like the limestone dissolution, the crystallization of calcium sulfite and gypsum and the oxidation of sulfite ions have been taken into account for the development of the simulation model while the gas absorption by the liquid droplets was based on the two-film theory. The effect of the mean diameter of the slurry droplets on the performance of the system was examined, as it was used as an index factor of the normal operation of the system. The operation limits of the system were investigated on the basis of the model developed. It is concluded that the model is capable of simulating the system for significantly different SO₂ loads and that the absorption rate of SO₂ is strongly affected by the liquid dispersion in the tower.     

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.     

湿法烟气脱硫系统对细颗粒脱除性能的实验研究

针对石灰石-石膏法、双碱法、氨法等典型湿法烟气脱硫（WFGD）工艺,在喷淋脱硫塔中实验研究了WFGD系统对细颗粒的脱除作用;采用电称低压冲击器、场发射扫描电镜、X射线衍射仪等对WFGD系统前后烟气中细颗粒的浓度、粒径分布、形态、元素及物相组成进行了测试分析,考察了脱硫剂、液气比对WFGD系统脱除细颗粒性能的影响,并进行了利用蒸汽相变原理促进细颗粒凝结长大并高效脱除的实验研究。结果表明,脱硫剂对WFGD系统脱除细颗粒的性能具有重要影响,由于形成无机盐气溶胶细颗粒,采用CaCO₃、NH₃·H₂O脱硫剂时,WFGD系统对细颗粒的脱除效果明显不及Na₂CO₃脱硫剂和水洗涤,且颗粒形貌特征及元素组成发生明显变化;除NH₃·H₂O脱硫剂外,液气比对WFGD系统脱除细颗粒的影响不明显;在脱硫塔进口烟气、塔内脱硫液进口上方添加适量蒸汽建立蒸汽相变所需的过饱和水汽环境可显著促进细颗粒的脱除。     

Removal of SO2 Using a Magnetically Fluidized Bed in the Semi‐Dry Flue Gas Desulfurization Process: Roles of Ferromagnetic Particles and Applied Magnetic Field in the Desulfurization Reaction

A new semidry flue gas desulfurization (FGD) process is proposed. The process uses a magnetically fluidized bed (MFB) as the reactor in which ferromagnetic particles are fluidized with simulated flue gas under the influence of an external magnetic field. A slurry of lime is continuously sprayed into the reactor by an atomizer fixed at the top of the bed. As a consequence, the desulfurization reaction and slurry drying take place simultaneously in a same reactor. Experiments with a laboratory‐scale apparatus were carried out to investigate the roles of the ferromagnetic particles and the magnetic field applied in the desulfurization reaction. The results show that when ferromagnetic particles are used as the fluidization material, both sulfite (SO₃^2–) salts and sulfate (SO₄^2–) salts are found in the desulfurization products. When quartz particles are used, only sulfite (SO₃^2–) salts are found. This suggests that the Fe(III) ions and Fe(II) ions result from the ferromagnetic particles dissolving in the liquid phase. In addition, the ions act as catalysts in the oxidation of S(IV) to S(VI) and react with SO₂ producing FeSO₃ and Fe₂(SO₄)₃ as the products. On the other hand, the level of the sulfate (SO₄^2–) salts in the products increases with increasing intensity of applied field intensity, which suggests that the oxidation of S(IV) can be enhanced by the applied magnetic field. The oxidation of S(IV) can increase the solubility of SO₂, and therefore, intensify the reaction between SO₂ and Ca(OH)₂, leading to an increased SO₂ removal efficiency.     

Quantification of gypsum crystal nucleation,growth, and breakage rates in a wet flue gas desulfurization pilot plant

The aim of this work is to study the influence of nucleation, growth and breakage on the particle size distribution (PSD) of gypsum crystals produced by the wet flue gas desulfurization (FGD) process. The steady state PSD, obtained in a falling film wet FGD pilot plant during desulfurization of a 1000 ppm(v) SO₂ gas stream, displayed a strong nonlinear behaviour (in a ln(n(l)) vs. l plot) at the lower end of the particle size range, compared to the well‐known linear mixed suspension mixed product removal model. A transient population balance breakage model, fitted to experimental data, was able to model an increase in the fraction of small particles, but not to the extent observed experimentally. A three‐parameter, size‐dependent growth model, previously used for sodium sulphate decahydrate and potash alum, was able to describe the experimental data, indicating either size‐dependent integration kinetics or growth rate dispersion. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Dissolution of a South African calcium based material using urea: An optimized process

The rate at which limestone dissolves is very important in wet flue gas desulfurization process (FGD). High dissolution rates provide better alkalinity, which is important for sulfur dioxide (SO₂) absorption. This study investigates the use of urea to improve the dissolution rate of limestone. The dissolution characteristics have been studied by using a pH-Stat method. The dissolution rate constant was measured according to the shrinking core model with surface control, i.e. (1−(1−X)^1/3)=k _r t. The effect of experimental variables such as temperature, amount of urea, solid to liquid ratio and stirring speed on the dissolution rate of limestone were investigated. Using a central composite design (CCD) of experiments variables, a mathematical model was developed to correlate the experimental variables to the dissolution rate constant. The experimental value was found to agree satisfactorily with predicted dissolution rate constant. The model shows that high temperature and low solid to liquid ratio improves the dissolution rate. The dissolution rate increased slightly with increase in the stirring speed. In the presence of urea the dissolution rate constant increased by 122%. The dissolution reaction follows a shrinking-core model with the chemical reaction control as the rate-controlling step.     

Simultaneous desulfurization and denitrification of flue gas by pre-ozonation combined with ammonia absorption

A process of simultaneous desulfurization and denitrification of flue gas was conducted in this study. The flue gas containing 200 mg·m⁻³ NO, 1000–4000 mg·m⁻³ SO₂, 3%–9% O₂, and 10%–20% CO₂ was first oxidized by O₃ and then absorbed by ammonia in a bubbling reactor. Increasing the ammonia concentration or the SO₂ content in flue gas can promote the absorption of NO_X and extend the effective absorption time. On the contrary, both increasing the absorbent temperature or the O₂ content shorten the effective absorption time of NO_X. The change of solution pH had substantial influence on NO_X absorption. In the presence of CO₂, the NO_X removal efficiency reached 89.2% when the absorbent temperature was raised to 60 °C, and the effective absorption time can be maintained for 8 h, which attribute to the buffering effect in the absorbent. Besides, both the addition of Na₂S₂O₃ and urea can promote the NO_X removal efficiency when the absorbent temperature is 25 °C, and the addition of Na₂S₂O₃ had achieved better results. The advantage of adding Na₂S₂O₃ became less evident at higher absorbent temperature and coexistence of CO₂. In all experiments, SO₂ removal efficiency was always above 99%, and it was basically not affected by the above factors.     

Modeling study on the impaction and humidification process in desulfurization activation reactor

In a semi-industrial desulfurization activation reactor, the high-CaO coal ash is directly used as desulfurization sorbent and activated by spray water. As a result of the inertial impactions between ash particles and water droplets, concentrated slurry droplets are formed. In order to numerically investigate the process, a new model of particle-flow-passage is deduced to simulate the impaction humidification process between particles and droplets. Computation results show that the spray water flow rate and droplets size have obvious influence on droplets catching efficiency, and the ash particles are caught primarily in the vicinity of the atomizer nozzle. With the models of gas phase turbulence, heat and mass transfer, and chemical reactions being considered, a comprehensive 3D model for FGD process (Flue Gas Desulfurization) is established, and several operation conditions are simulated by the model. The predicted results such as SO₂ absorption efficiency and the flue gas temperature at the outlet are in good agreement with experimental results.     

Removal of SO2 in Semi‐Dry Flue Gas Desulfurization Process with a Powder‐Particle Spouted Bed

Semi‐dry flue gas desulfurization was investigated with several kinds of SO₂ sorbents, such as slaked lime, limestone, Mg(OH)₂ and concrete pile sludge, in a powder‐particle spouted bed. Slurry droplets including sorbent fine particles were fed to a spouted bed of coarse inert particles spouted with hot gas containing SO₂. SO₂ removal efficiency was strongly affected by the approach to saturation temperature, Ca/S molar ratio and particle size of sorbent. Slaked lime showed the highest desulfurization efficiency. In this process, despite very short gas residence time, more than 90% SO₂ removal was easily achieved by choosing appropriate conditions.     

Optimization of a Pilot‐Scale Amine Scrubber to Remove SO2: Higher Selectivity and Lower Solvent Consumption

A pilot‐scale study of flue gas desulfurization based on an amine‐based solvent using applicable industrial values was carried out for sulfur dioxide (SO₂) removal. The plant consisting of absorption and desorption columns was operated with different working parameters such as solvent flow rate, inlet concentration of SO₂, temperature of desorption column, and pH of absorption agent. The Taguchi method was utilized to obtain the best combination of working parameters for the most efficient reduction of SO₂ outlet concentration. The industrial gas‐to‐liquid ratio could be optimized by applying a defined SO₂ concentration, stripper temperature, and solvent pH value. The achieved efficiency is much better compared to our previous study while the gas‐to‐liquid ratio is higher in this work.