湿式烟气脱硫喷淋塔内部流场数值模拟研究

以300MW机组湿法烟气脱硫喷淋塔为研究对象,利用计算流体力学通用软件对其内部两相流场进行模拟。气相湍流由标准k 模型描述,喷淋液滴由拉格朗日颗粒轨道模型描述。预测了无喷淋和有喷淋2种条件下的气相湍流流场分布、沿塔高方向不同截面上的气速分布以及喷淋液滴的轨迹。模拟结果表明,引入喷淋液后,出口截面气速分布明显均匀化,其最大值由无喷淋时的12m/s降至6m/s。该最大值出现在靠近塔壁处,是由塔壁附近喷淋密度较低造成的,可通过改进周边喷嘴的布置方式及喷嘴型式进行优化。     

湿式烟气脱硫喷淋塔内部流场数值模拟研究

以300MW机组湿法烟气脱硫喷淋塔为研究对象，利用计算流体力学通用软件对其内部两相流场进行模拟。气相湍流由标准κ-ε模型描述，喷淋液滴由拉格朗日颗粒轨道模型描述。预测了无喷淋和有喷淋2种条件下的气相湍流流场分布、沿塔高方向不同截面上的气速分布以及喷淋液滴的轨迹。模拟结果表明，引入喷淋液后，出口截面气速分布明显均匀化，其最大值由无喷淋时的12m／s降至6m／s。该最大值出现在靠近塔壁处，是由塔壁附近喷淋密度较低造成的，可通过改进周边喷嘴的布置方式及喷嘴型式进行优化。     

伞罩型除尘脱硫塔内除雾器性能研究

除雾器是湿法烟气脱硫(WFGD)系统内重要的设备之一,其性能对WFGD系统运行的可靠性有重要影响.利用Fluent6.2软件对新型伞罩型除尘脱硫塔内的三维两相流场进行数值模拟,气相采用RNG湍流模型,液相采用离散相模型,选择SIMPLE算法进行计算,分析塔内的折板除雾器和旋流板除雾器的速度场、压力场和液滴的分布情况.结果表明,烟气经过折板除雾器,产生了明显的压降,且在拐角区域湍流耗散强烈,是实现气液分离的关键区域;烟气经过旋流板除雾器,速度和压强分布具有良好的对称性,液滴被气流旋转抛向壁面实现气液分离.模拟结果对新型的WFGD除雾器的设计和运行具有一定的理论指导意义.     

新型湿法除尘除有害气体斜板塔的流场分析

研究对象为2种新型湿法除尘除有害气体的斜板塔:矩形斜板塔和伞罩形斜板塔.为了观察塔内流场分布规律,运用CFD(计算流体力学)软件,气相采用标准K-ε湍流模型描述,液相采用颗粒轨道模型描述,对2种新型塔内气液两相流动进行了数值模拟.预测了无喷淋和有喷淋两种情况下的气相湍流流场,不同空塔气速不同液气比的塔内压力损失.结果表明:采用中心出口的圆柱型塔可以有效地避免气体"死区"的产生;新型斜板塔能有效地增大气液接触面积,延长气体的塔内停留时间;加入喷淋液体以后,气相流场明显均匀化.该模拟也为塔体的进一步优化设计提供了依据.     

废水液滴在低温烟气中的蒸发特性数值研究

建立了液滴在低温低速烟气中运动和蒸发的数学模型,数值研究了液滴在不同的烟气速度和烟气温度环境中的蒸发特性,获得了液滴速度、温度和直径随时间的变化规律,以及烟气速度和温度对液滴蒸发特性的影响。计算结果表明：气相速度越大,液滴初速度对液滴运动速度的影响越小,液滴速度越容易达到烟气速度。对于初始直径和速度一定的液滴,气液初始相对速度越大,液滴达到临界蒸发温度所需的时间越短,完全蒸发时间也越短。在文中研究的烟气速度范围内,整体上气相速度的改变对液滴完全蒸发时间的影响很小。气相温度的改变对液滴达到临界蒸发温度的时间以及完全蒸发时间的影响很大。烟气从较高温度和较低温度分别下降相同幅度的温度时,液滴在烟气中的完全蒸发时间按不同的倍数在增加,烟气温度越低,液滴完全蒸发时间增加的越多。对于文中研究的液滴,无量纲完全蒸发时间随烟气温度变化呈指数递减规律。     

SNCR的喷雾与混合过程及其对脱硝效率的影响

研究了SNCR(选择性无催化还原)工艺过程中还原剂液滴向炉内的喷射轨迹、以及还原剂在炉内高温空间的蒸发和混合扩散问题,得到了循环流化床锅炉不同截面的还原剂浓度分布。由研究结果可知,随着雾炬的扩散,还原剂浓度分布更加均匀,有效覆盖区域增加。根据还原剂有效覆盖范围,可以预测在合适的温度窗口处烟气的有效脱硝效率,在此基础上对喷枪喷射的液滴参数进行了优化分析。研究了不同液滴粒径分布对还原剂喷射和扩散的影响,结果表明:液滴雾化粒径的适当增大可以使蒸发时间延长,从而使得炉内还原剂与烟气中NO_x更好地混合、增加还原剂的有效覆盖面积;此外,根据还原剂覆盖浓度也可对还原剂的使用量进行预测。     

基于气液悬浮旋切掺混的气动旋流塔脱硫性能测试与分析

为了研制低运行能耗和高脱硫效率的新型脱硫塔,以满足国家最新环保超低排放标准,采用基于气液悬浮旋切掺混的气动旋流塔脱除燃煤烟气中的SO_2污染物,对其内部气动旋流单元的强化传质脱硫性能进行探究,考察了空塔喷淋段和气动旋流段的喷淋层位置和液气比对脱硫效率及系统阻力的影响,并对气动旋流单元的脱硫效率进行了理论计算模拟。结果表明:喷淋层距浆液池高度越高,液滴在吸收区停留的时间越长,脱硫效率越高,系统运行阻力也越大;增加液气比,可显著提高系统的脱硫效率,单层喷淋层阻力约为150 Pa;在低pH工况下,SO_2吸收过程为液膜控制,气动旋流单元的脱硫效率较低;随着pH的增大,SO_2吸收过程逐渐由液膜控制转变为双膜甚至气膜控制,气动旋流单元的脱硫效率逐渐增强;当pH=5时,液气比=25 L·m~(-3),5层喷淋层运行工况下的脱硫效率高达99.82%。气动旋流单元的脱硫效率模拟计算结果表明:在高pH下,气动旋流单元的脱硫效率更高;当pH=5.5时,气动脱硫单元的脱硫效率为62.56%,阻力为360 Pa,实验数据与理论计算曲线吻合较好。以上研究结果可为新型高效燃煤机组脱硫超低排放改造技术的开发及其在环境污染控制领域的应用提供参考。     

直筒型导叶直流式三相旋流器排气管上部内流场的实验研究

使用五孔球探针对一种用于分离锅炉净化烟气中含尘液滴的直筒型导叶直流式三相旋流器排气管上部的气相流场进行了实验测定。得到了这一区域的速度和静压分布。发现了排气管上部筒体中心存在强烈的内旋流;外层气体不断汇入内旋流,内旋流呈轴向加速运动;绝大部分气体不经过排气管外环形空间下行直接由排气管排出。主体段切向速度为Rankine组合涡。     

直筒型导叶直流式三相旋流器排气管上部内流场的实验研究

使用五孔球探针对一种用于分离锅炉净化烟气中含尘液滴的直筒型导叶直流式三相旋流器排气管上部的气相流场进行了实验测定。得到了这一区域的速度和静压分布。发现了排气管上部筒体中心存在强烈的内旋流；外层气体不断汇入内旋流，内旋流呈轴向加速运动；绝大部分气体不经过排气管外环形空间下行直接由排气管排出。主体段切向速度为Rankine组合涡。     

烟气脱硫卧式喷淋塔的数值模拟

卧式喷淋塔是北京科技大学环境中心研发的新型湿法烟气脱硫工艺,在一定程度上克服了立式喷淋塔的缺点,做到了高效率、低压损、低成本、易检修。但在工程实例中仍存在一些问题。为了研究不同喷淋布置方式对卧式喷淋塔的影响,构建了卧式喷淋塔的稳态三维的多相CFD模型三维,并使用Fluent软件进行模拟。连续相湍流模型采用了标准k-ε双方程模型,液滴离散相模型采用了Eulerian-Lagrangian模型。结果表明,喷淋布置方式的喷雾锥角为110°,安装高度控制在距顶面0.8 m,喷射方向与竖直方向成45°时,能显著减小压力损失、延长烟气停留时间、增强气液接触效率,最终提高脱硫效率,降低功率能耗。研究结果可为实验设计、工程应用提供指导。     

Effect of Diffusiophoresis and Thermophoresis on the Overall Particle Collection Efficiency of Spray Droplet Scrubbers

The overall particle collection efficiencies of spray scrubbers using monodisperse droplets of 100,500, and 1000 microns diameter were calculated for the cases of evaporating and condensing droplets. The properties of the gas at the inlet to the spray scrubber were maintained constant at 150°F, 100% relative humidity, and 1 atmosphere pressure. At the liquid entrance to the spray scrubber, the water droplet temperature was 50° F for the condensing case and 180° F for the evaporating case. The liquid to gas flow rate ratio for all the calculations was held constant at 4 gal/1000 acf. The gas velocity in the co-current spray tower was 1 ft/sec in the downwind direction. The calculation results show that for the particles in the 0.01 to 10 Mm diameter range, the overall spray scrubber particle collection efficiency is greater with the cooler 50°F water (condensing case) than with the warmer 180°F water (evaporating case). The effect of diffusiophoresis and thermophoresis is noticeable for all the water droplet sizes considered, but is more significant for the larger water droplets. This greater effect for the larger water droplets compared to the smaller droplets is due to the longer existence of the temperature and water vapor concentration gradients between the water droplets and the surrounding gas.     

湿式逆流喷淋脱硫塔中SO2吸收特性的研究

根据双模吸收理论及SO2在溶液中电离特性，建立了逆流喷淋塔的SO2吸收模型，在考虑浆液飞溅到塔壁的影响后，模拟结果与实验值吻合较好。根据吸收模型，对塔内液气比和浆液的含固率等因素进行了分析。研究表明：减少浆液飞溅到塔壁可提高浆液利用率及脱硫装置性能；根据烟气中SO2的初始浓度及最终脱硫效率，可合理选择液气比及吸收时问（塔的高度）；浆液中的含固率直接影响到SO2的吸收速率、循环浆液量、脱硫效率及浆液中SO2浓度等，在液气比较小时，含固率对脱硫效率的影响尤其明显。     

湿式逆流喷淋脱硫塔中SO_2吸收特性的研究

根据双模吸收理论及SO2在溶液中电离特性,建立了逆流喷淋塔的SO2吸收模型,在考虑浆液飞溅到塔壁的影响后,模拟结果与实验值吻合较好。根据吸收模型,对塔内液气比和浆液的含固率等因素进行了分析。研究表明:减少浆液飞溅到塔壁可提高浆液利用率及脱硫装置性能;根据烟气中SO2的初始浓度及最终脱硫效率,可合理选择液气比及吸收时间(塔的高度);浆液中的含固率直接影响到SO2的吸收速率、循环浆液量、脱硫效率及浆液中SO2浓度等,在液气比较小时,含固率对脱硫效率的影响尤其明显。     

石灰石湿法脱硫过程中SO2吸收数学模型

为揭示石灰石湿法脱硫体系中喷淋塔内SO2的浓度和脱硫效率的变化情况,针对喷淋塔内石灰石在气膜控制、气液膜控制和固体溶解控制的3个不同阶段,以双膜理论为基础,以单个石灰石颗粒为研究对象,通过石灰石在不同阶段的转化率和粒径变化,得到SO2在不同阶段脱硫效率随时间的变化规律,建立SO2吸收的数学模型.模型计算结果表明,在烟气行程上,脱硫效率受SO2气膜传质阻力和石灰石溶解速率限制.在吸收塔底部和上端SO2吸收速率较低,在SO2和石灰石摩尔比在适宜条件下,有效吸收段高度为2 m左右.理论模型揭示的规律对喷淋塔的设计和运行参数选取有一定借鉴意义.     

湍球塔和喷淋塔的海水脱硫冷态实验对比

通过湍球塔和喷淋塔的海水脱硫冷态实验对比,研究海水脱硫过程中烟气和海水参数对湍球塔和喷淋塔脱硫的不同影响。实验结果表明,SO2分压力增大,脱硫效率和尾水pH值减小;海水碱度、pH值和液气比增大,脱硫效率和尾水pH值也随之增大;湍球塔的脱硫效率和尾水pH值与液气比改变方式无关,实验用湍球塔的合适液气比值为2.3 L/m3;湍球塔脱硫实验中,塔内气速为1.58 m/s,SO2分压力为20 Pa,水温为10.2℃,液气比为1.1~2.8 L/m3时,尾水pH值在2.4~2.8之间;增大液气比时,喷淋塔改变海水流量的脱硫效果要比改变气体流量的脱硫效果明显;塔内气速1 m/s以上时,一级喷淋塔的脱硫效率要比湍球塔小很多,有时只有湍球塔的1/2左右。     

气液两相旋流喷嘴雾化特性

为提高烟尘净化用喷嘴的雾化性能，提出一种气液两相旋流喷嘴，用CFX软件对该喷嘴内部流场模拟的结果表明，强烈的气旋作用使液流呈环状，使气液混合流在达到喷嘴末端时易于破碎成雾。在实验室条件下，利用Winner313型喷雾激光粒度分析仪对喷嘴进行了雾化粒度测定，分析发现雾滴中位径与气液比的关系呈幂函数衰减，这一规律将有助于在工程应用时确定喷嘴工况。     

Spray-dry desulfurization of flue gas from heavy oil combustion

An experimental investigation on sulfur dioxide removal in a pilot-scale spray dryer from the flue gas generated by combustion of low-sulfur (S) heavy oil is reported. A limewater slurry was sprayed through an ultrasonic two-fluid atomizer in the spray-dry chamber, and the spent sorbent was collected downstream in a pulse-jet baghouse together with fly ash. Flue gas was sampled at different points to measure the desulfurization efficiency after both the spray-dry chamber and the baghouse. Parametric tests were performed to study the effect of the following variables: gas inlet temperature, difference between gas outlet temperature and adiabatic saturation temperature, lime-to-S ratio, and average size of lime particles in the slurry. Results indicated that spray drying is an effective technology for the desulfurization of low-S fuel oil flue gas, provided operating conditions are chosen carefully. In particular, the lowest gas inlet and outlet temperatures compatible with baghouse operation should be selected, as should a sufficiently high lime-to-S ratio. The attainment of a small lime particle size in the slurry is critical for obtaining a high desulfurization efficiency. A previously presented spray-dry flue gas desulfurization model was used to simulate the pilot-scale desulfurization tests, to check the ability of the model to predict the S capture data and its usefulness as a design tool, minimizing the need for pilot-scale experimentation. Comparison between model and experimental results was fairly good for the whole range of calcium/S ratios considered.     

基于正交实验的颗粒移动床烟气脱硫工艺参数优化

石灰石颗粒移动床脱硫工艺参数是影响脱硫效率和操作压降变化的重要因素,考虑到工艺参数对脱硫过程的影响,基于正交实验方法,通过直观分析和方差分析得到喷淋密度、空床气速、SO2浓度、烟气温度和床层下移速度对脱硫效率影响程度的主次顺序和脱硫效率与各因素之间的关系,并且分析了各因素对脱硫效率的影响规律。研究发现,各因素对脱硫效率的影响程度依次是空床气速>喷淋密度>SO2浓度>烟气温度>床层下移速度。喷淋密度对脱硫效率的影响高度显著,呈正相关;空床气速对脱硫效率的影响高度显著,呈负相关;SO2浓度和烟气温度对脱硫效率有影响但不显著,呈负相关。床层下移速度与脱硫效率呈正相关。     

Agrochemical spray drift; assessment and mitigation--a review

During application of agrochemicals spray droplets can drift beyond the intended target to non-target receptors, including water, plants and animals. Factors affecting this spray drift include mode of application, droplet size, which can be modified by the nozzle types, formulation adjuvants, wind direction, wind speed, air stability, relative humidity, temperature and height of released spray relative to the crop canopy. The rate of fall of spray droplets depends upon the size of the droplets but is modified by entrainment in a mobile air mass and is also influenced by the rate of evaporation of the liquid constituting the aerosol. The longer the aerosol remains in the air before falling to the ground (or alternatively striking an object above ground) the greater the opportunity for it to be carried away from its intended target. In general, all size classes of droplets are capable of movement off target, but the smallest are likely to move the farthest before depositing on the ground or a non-target receptor. It is not possible to avoid spray drift completely but it can be minimized by using best-management practices. These include using appropriate nozzle types, shields, spray pressure, volumes per area sprayed, tractor speed and only spraying when climatic conditions are suitable. Field layout can also influence spray drift, whilst crop-free and spray-free buffer zones and windbreak crops can also have a mitigating effect. Various models are available to estimate the environmental exposure from spray drift at the time of application.     

Atmospheric loss of pesticides above an artificial vineyard during air-assisted spraying

A procedure to assess pesticide emission to the air and characterise possible air pollution sources was carried out using a tracer dye and 2 mm PVC lines during air-assisted spraying of an artificial vineyard. Three experiments were performed to evaluate the method feasibility, quantify upward movements of sprayed droplets and investigate the influence of microclimatic variables on pesticide emission. During each experiment two test series were carried out with two droplet size distributions (very fine and fine spray, according to the BCPC classification). The amount of sprayed liquid collected at 2.5 m above ground varied between 9.0% and 10.7% of the total dose applied for very fine spray and between 5.6% and 7.3% for fine spray. In stable atmospheric conditions the spray drifted along the mean wind direction over the crop whereas in unstable conditions the sprayed liquid plume was larger, with a greater amount of material sent to higher levels. A statistical model based on a simple multiple regression featuring droplet characteristics and microclimatic variables (wind speed, temperature, stability parameter and relative humidity) provided a robust estimate of spray loss just above the crop, with an acceptable determination coefficient (R²=0.84). This method is therefore suitable for quantifying spray drift and provides a way to study the influence of several variables on the amount of pesticide released into the atmosphere by air-assisted spraying, with suitable accuracy.