Removal of fine particles by heterogeneous condensation in the double-alkali desulfurization process

Removal of fine particles by heterogeneous condensation in the rotating-stream-tray scrubber was investigated experimentally for the double-alkali desulfurization process in this paper. A supersaturated vapor phase, necessary for condensational growth of fine particles, was achieved in the SO₂ absorption zone and at the top of the scrubber by adding steam in the gas inlet, between the pieces of rotating-stream-tray and above the desulfurization liquid inlet of the scrubber, respectively. Fine particles grew in size by vapor heterogeneous condensation with the particles acting as nucleation centers. Then the condensational grown droplets were removed efficiently by the desulfurization liquid and a high-efficiency demister. In order to optimize the removal process, the influences of temperature of inlet flue gas and desulfurization liquid, steam addition method and amount of steam added on the particle removal efficiency were presented. The results show that a few fine particles could be removed in the rotating-stream-tray scrubber. The removal efficiency can be significantly improved for various steam addition cases, and the improve performance is related to the method and the amount of steam addition. Particle removal efficiency with steam added between the pieces of rotating-stream-tray is higher than that of else steam addition cases.     

洗涤塔脱除燃烧源超细颗粒的实验研究

在填料洗涤塔中进行了利用蒸汽相变原理促进燃煤和燃油超细颗粒凝结长大并高效脱除的实验研究;采用电称低压冲击器(ELPI)、SEM及XPS对两种燃烧源细颗粒凝结洗涤前后的数浓度、粒径分布、形貌和元素组分进行了分析测试,考察了洗涤塔进口气液温差、进口烟气含湿量及液气比等对脱除效率的影响。结果表明,燃煤和燃油产生的超细颗粒形貌和组分具有较大的差别,燃煤超细颗粒主要为硅铝矿物质,而燃油超细颗粒主要为含炭物质;在相同条件下,燃煤超细颗粒相变脱除效果优于燃油超细颗粒;脱除效率随洗涤塔进口气液温差的增大而提高,在相同进口气液温差下,增大进口烟气含湿量可显著提高超细颗粒的脱除效率;液气比的影响与填料洗涤塔内是否存在蒸汽相变有关;通过合理调节进口烟气含湿量及进口烟气与洗涤液的温差在填料塔内建立微粒凝结长大所需的过饱和水汽环境可有效脱除燃烧源超细颗粒。     

喷淋洗涤条件下水汽饱和度分布特性

为在脱硫喷淋塔中借助蒸汽相变原理促进PM_2.5凝结长大,需要在塔内建立适宜的过饱和水汽环境。基于此,建立喷淋洗涤条件下气液两相热质传递模型,系统研究了操作参数对塔内饱和度分布特性的影响规律。结果显示,通过降低喷淋液温度、增加塔进口烟气含湿量,以及在喷淋区上方添加蒸汽均能够在塔内建立过饱和水汽环境;采用较大的液气比、较小的喷淋液滴直径有利于水汽饱和度的提高。获得了改变操作参数在脱硫喷淋塔内建立过饱和水汽环境的措施,对实际的脱硫除尘工艺的设计和运行具有指导意义。     

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.     

洗涤塔内相变脱除燃煤超细颗粒的试验研究

为研究洗涤塔内相变对燃煤超细颗粒的脱除,在洗涤塔液相进口上方注入蒸汽使烟气达到过饱和,由高效除雾器脱除凝并长大的含尘液滴。系统研究了蒸汽添加量、洗涤塔入口气液温差、液气体积比、烟气在相变区域的停留时间等操作参数对燃煤超细颗粒脱除效率的影响规律。结果表明:在塔内添加少量蒸汽,可显著促进燃煤超细颗粒的脱除,蒸汽添加质量浓度为0.03 kg/m3时,数量浓度脱除效率由10%增至60%以上;液气体积比的提高有利于燃煤超细颗粒的脱除,特别是当洗涤塔入口气液温差较大时。研究结果表明利用洗涤塔内相变脱除燃煤细颗粒是一种有应用前景的新方法。     

Simultaneous absorption of mercury and chlorine in sulfite solutions

Simultaneous absorption of mercury (Hg) and chlorine (Cl₂) into aqueous sulfite/bisulfite (0 to S(IV)) at pH 4.7 and 5.7 was measured in a wetted wall column. Experiments were performed at ambient temperature and pressure using 5- Cl₂ and 46 ppb Hg. Absorption was modeled using the theory of mass transfer with chemical reaction. At the gas/liquid interface, chlorine oxidizes the elemental Hg to a more soluble form. The rate constant for the reaction of mercury and chlorine was determined to be . Mercuric chloride and sodium hypochlorite also enhanced Hg absorption. The addition of sodium chloride did not affect the rate of Hg/Cl₂ absorption with S(IV). When no S(IV) was present, the chloride significantly enhanced Hg absorption. Possible reaction pathways are discussed. These results are relevant in the simultaneous removal of chlorine, sulfur dioxide, and elemental mercury from flue gas. A model was developed to predict the expected Hg removal in a limestone slurry scrubber. Mercury removal decreases as the S(IV) concentration increases. The process feasibility will depend on the SO₂/S(IV) concentration of the scrubber, the desired Hg removal, and the amount of Cl₂ which can be tolerated.     

喷淋塔内可吸入颗粒物的脱除与凝结增长特性

引言 可吸入颗粒物是一种重要的大气污染物,其对人类健康和大气环境带来极大危害[1-2].在当前以燃用化石燃料为主的能源结构和与日俱增的能源消耗形势下,燃烧源的排放成为可吸入颗粒物的一个重要来源[3],主要原因是目前电站及工业生产中的除尘设备,如电除尘器、过滤除尘器等虽然已达到很高的水平,但对可吸入颗粒物的脱除效率却较低,使得大量颗粒排放到大气中.     

Catalytic combustion assisted methane steam reforming in a catalytic plate reactor

A theoretical study of methane steam reforming coupled with methane catalytic combustion in a catalytic plate reactor (CPR) based on a two-dimensional model is presented. Plates with coated catalyst layers of order of micrometers at distances of order of millimetres offer a high degree of compactness and minimise heat and mass transport resistances. Choosing similar operating conditions in terms of inlet composition and temperature as in industrial reformer allows a direct comparison of CPRs with the latter. It is shown that short distance between heat source and heat sink increases the efficiency of heat exchange. Transverse temperature gradients do not exceed across the wall and across the gas-phase, in contrast to difference in temperature of outside wall and mean gas phase temperature inside the tube usually observed in conventional reformers. The effectiveness factors for the reforming chemical reactions are about one order of magnitude higher than in conventional processes. Minimisation of heat and mass transfer resistances results in reduction of reactor volume and catalyst weight by two orders of magnitude as compared to industrial reformer. Alteration of distance between plates in the range 1- does not result in significant difference in reactor performance, if made at constant inlet flowrates. However, if such modifications are made at constant inlet velocities, conversion and temperature profiles are considerably affected. Similar effects are observed when catalyst layer thicknesses are increased.     

应用润湿剂促进WFGD系统脱除细颗粒物的性能

采用旋流板塔双碱法脱硫工艺,进行了脱硫液中分别添加聚醚硅油、Compound No.3、XHG-248润湿剂促进湿法烟气脱硫(WFGD)系统脱除细颗粒物的试验研究,考察了脱硫液气比、脱硫液温度等对润湿剂作用性能的影响,并进行了协同利用蒸汽相变原理和添加润湿剂促进细颗粒物脱除的试验.结果表明:脱硫液温度对添加润湿剂的作用...     

Experimental investigation of a pilot-scale jet bubbling reactor for wet flue gas desulphurisation

In the present work, an experimental parameter study was conducted in a pilot-scale jet bubbling reactor for wet flue gas desulphurisation (FGD). The pilot plant is downscaled from a limestone-based, gypsum producing full-scale wet FGD plant. Important process parameters, such as slurry pH, inlet flue gas concentration of SO₂, reactor temperature, and slurry concentration of Cl⁻ have been varied. The degree of desulphurisation, residual limestone content of the gypsum, liquid phase concentrations, and solids content of the slurry were measured during the experimental series.The SO₂ removal efficiency increased from 66.1% to 71.5% when the reactor slurry pH was changed from 3.5 to 5.5. Addition of Cl⁻ (in the form of CaCl₂·2H₂O) to the slurry increased the degree of desulphurisation to above 99%, due to the onset of extensive foaming, which substantially increased the gas-liquid contact area. An increase in the inlet flue gas SO₂ concentration from 502 to led to a decrease in the SO₂ removal efficiency from 80.1% to 69.4%. A temperature increase from 296 to caused a reduction in the degree of desulphurisation from 69.4% to 68.1%, but this result is almost within the experimental uncertainty. The residual limestone level in the gypsum formed increased with increasing values of reactor slurry pH, inlet flue gas SO₂ concentration, and slurry concentration of Cl⁻.     

Solid circulation rate in a circulating fluidized bed in the presence of fine powders

Fine powders (Geldart's group C) are added to a circulating fluidized bed (CFB) of coarse particles (Geldart's group A) and the solid circulation rate (SCR) is investigated with addition of fine powders of different sizes and different fractions (different hold-ups) to the bed. Experiments were carried out in a CFB of 2 m in height and 0.052 m in diameter, using FCC catalyst particles of as the coarse particles and cohesive aluminum hydroxide powders of 0.5- as the fine powders. The effects of hold-up of fine powders in the bed, fine powders size, and superficial gas velocity on the SCR were investigated.The SCR strongly depended on the hold-up of fine powders of 0.5- in size and noticeably decreased with increasing the hold-up of fine powders under constant gas velocity. This dependency disappeared when the size of fine powders was larger than . Thus, depending on the size of fine powders added to the CFB, two distinct regions for the changes of SCR could be clearly identified.     

Development and performance test of a thermo-denuder for separation of volatile matter from submicron aerosol particles

In this study we designed and evaluated a home-made thermo-denuder (TD) both experimentally and numerically. Sodium chloride (NaCl) particles, toluene gas, and carbon black particles were used for the performance evaluation of the TD. The TD was evaluated for various set-point air temperatures and particle sizes using the following three parameters: the temperature profile, penetration efficiency, and gas adsorption efficiency. At , the temperature was nearly uniform, remaining within of the set-point temperature, in the heating section and decreased to the temperature of ambient air in the cooling section. The particle penetration efficiencies were 93–96% at and 58–67% at for particle sizes of 20–60 nm. The gas adsorption efficiency was nearly unity until the breakthrough time of 65 h, and the total amount of toluene adsorbed by activated carbon particles was 72 mg-toluene/g-activated carbon particles. From size distribution measurements of dry carbon black and toluene enriched carbon black particles, the mode diameter measured at the set-point temperature of was found to be 48.6 nm, which agreed with the one obtained from the dry particle measurements. The overall number concentration obtained after particle losses were compensated was lower than that measured without using the TD by 35.6%, which was caused by gas adsorption in the TD.     

SIMULTANEOUS ABSORPTION OF AN ACID GAS (H2S) AND A BASIC GAS (NH3) IN A SCRUBBING DEVICE

H₂S and NH₃ are major contaminants in many synfuel process gas streams. The H₂S is frequently removed by dissociation and reaction in scrubbers using alkaline scrubbing liquids. Some of the NH₃ is removed by simple dissolution in the same unit; the remaining NH₃ can be removed in a second scrubber using a mildly acidic liquid. The first scrubbing stage in such a system is novel in that an acidic gas (H₂S) and a basic gas (NH₃) are absorbed simultaneously. In this paper, the interesting behavior of this simultaneous acid gas-basic gas scrubbing process is described and discussed. The percentage H₂S and NH₃, absorption as a function of the injected liquid pH, the liquid-to-gas ratio, and the NH₃ content of the gas stream have been determined.     

Effect of gas temperature on the capture of charged particles by oppositely charged water droplets

This work reports experimental results on the effects of temperature (25, 45, and 65°C at different relative humidity) on the scrubbing of charged submicron particles by means of cold (25°C) droplets charged with opposite polarity. The aim of the study is to experiment how the capture of particles is influenced by the simultaneous presence of electrostatic and phoretic forces related to the occurrence of thermal and water vapor gradients close to the droplet surface. This information plays an important role in the development of wet electrostatic scrubbing (WES), an emerging technology for submicron and ultrafine particle capture. Tests were performed in a lab-scale system in which the particle laden-gas was scrubbed by a train of identic droplets. Particles were charged by a corona source while droplets are generated by electrospraying. Experiments revealed that for particles larger than about 250–300 nm, there were higher removal efficiencies in nonisothermal conditions, with limited differences between 45 and 65°C tests. For particles finer than about 150 nm, we sometimes observed lower removal efficiencies for higher gas temperatures, probably due to the difficulties in controlling particle charging for these particles. The experiments were interpreted with a consolidated stochastic model that predicted successfully the data at isothermal conditions, but was less effective for tests at higher gas temperatures. In our opinion, this discrepancy relies on synergies among the fluid dynamic field induced by droplet evaporation/condensation, the phoretic and the electrostatic forces, which are not considered in the model.

Copyright © 2016 American Association for Aerosol Research     

Effect of cohesive powders on the elutriation of particles from a fluid bed

The elutriation of fine particles (Group C or A particles in Geldart's classification) from a fluid bed of mixed fine and coarse particles is investigated in a steady state. Al(OH)₃ and alumina and TiO₂ powder of 0.5- were used as fines. FCC, alumina, Al(OH)₃, limestone, silica sand, SiC particles of 44- were used as coarse particles. The paper investigates the effect on the elutriation rate constant of both fine powders and coarse particles (i) of the weight fraction of Geldart C powders in the bed, (ii) of the superficial gas velocity, and (iii) of the size of C powder and size of coarse particles in the bed.The elutriation rate constant of group C or group A particles is not only affected by the properties of the elutriated powders or particles and gas velocity, but also by both the weight fraction and size of C powder in the bed. This finding differs from the elutriation result of A or B particles from a fluidized bed.     

基于湿法脱硫技术的钢渣脱硫剂性能研究

开发了以钢渣作为一种新型的脱硫剂,利用旋流板塔作为吸收器的湿法脱硫技术。实验研究了钢渣中的主要成分对脱硫效果的作用;讨论了液气体积比、吸收浆液的pH值、气体中SO2的进口体积分数等主要操作参数对脱硫效率的影响。实验结果表明,钢渣浆液质量分数为2%,进口气体温度为20℃,液气体积比大于4.85的条件下,脱硫率可达到85%以上,钢渣中的MgO,Fe2O3对于脱硫效果具有促进作用。因此,钢渣在旋流板塔湿法脱硫过程中,是一种有效的脱硫剂。     

SELECTIVE GAS ABSORPTION UNDER PRESSURE

For selective removal of H2S from much larger quantities of CO₂ under pressure, an industrial prototype spray column has been constructed. Sodium hydroxide solution was atomized by a pressure nozzle of special design and entered the scrubber as fine spray to contact the sour gases.

Several operating variables were examined in order to indicate optimal operating conditions for maximum selectivity of H₂S over CO₂. Fine mist and short contact time favor this selective absorption process. An optimum inlet reactant concentration was found dependent upon the H₂S content relative to CO₂ in the inlet sour gas mixture. A special nozzle/shield configuration to avoid contact of sour gas with highly turbulent liquid during droplet formation significantly improved the selectivity.