SCR脱硝反应器烟道内部流场的数值模拟与优化

为优化无锡某电厂选择性催化还原脱硝反应器烟道内部流场,研究导流板和整流器对流场的影响,基于FLUENT软件和SIMPLE算法,采用RNG k-ε湍流模型和离散相模型数值模拟导流装置安装前、安装后和优化后3种情况下的烟道内部气流流动和固体颗粒流动.研究表明,导流板可以改善烟道内气流分布,同时保证气流进入催化剂层前的速度小于6 m/s,速度不均匀系数小于15%;整流器对改善气流流动、避免涡流存在具有重要作用,且对压降影响较小;合理的导流板结构和布置可以有效减小压降,导流板优化后压降降低至109.9 Pa.     

循环流化床烟气脱硫塔入口结构改进的数值模拟

采用商用Fluent软件提供的k-ε湍流模型对循环流化床脱硫塔内部流场进行了数值模拟,重点考察入口结构对脱硫塔内气相速度分布均匀性的影响,并用速度不均匀度定量描述了流场的均匀性。结果表明,当前使用90°弯管进气结构的脱硫塔内部流场存在明显的不均匀性,主要是入口处气流90°转向造成的。为此采用组合弯管进气结构代替90°弯管进气结构,以改善脱硫塔内流场不均匀性。入口结构改为组合弯管进气结构后,脱硫塔内流场不均匀性得到很大改善,速度分布的不均匀度明显减小,因此组合弯管进气结构有利于烟气与脱硫剂的充分混合反应,提高脱硫效率。     

喷嘴角度对脱硫塔内气液两相流场的影响

提出了采用数值模拟的方法研究喷嘴角度对脱硫塔内部气液两相流场的影响。由于实际脱硫塔尺寸庞大,给实验研究带来困难且成本很高,在数值模拟平台上,分别模拟了45°、75°和-30°3种喷嘴角度布置下脱硫塔内部速度场、温度场变化以及湍流强度的分布情况。结果表明,在角度为-30°布置时速度场变化不是很剧烈,脱硫塔进出口温差比较理想,湍流强度在脱硫塔底部较大随着塔高的增加缓慢降低,这样有助于气液两相均匀混合,并控制出口烟温,有利于提高脱硫效率。     

滤袋数目对翼形上进风袋式除尘器内流场影响的数值模拟研究

针对实际运行过程中,袋式除尘器滤袋使用寿命短,压力损失过大的问题,本文以翼形上进风袋式除尘器为研究对象,采用CFD(computational fluid dynamics)技术模拟分析不同滤袋数(分别为92、88、84、80、76和72)时袋式除尘器内气流分布和压力损失规律。主要考察了流量分配系数、最大流量不均幅值、气流迹线、滤袋表面速度分布与压降等指标。结果表明,滤袋数为76个时,气流分布最为均匀,各滤袋负载均衡;相同过滤速度下,装置的压降随滤袋数目的增加而上升,即压降大小顺序为9288847672;与72个滤袋相比,76个滤袋的可用过滤面积更大。综合考虑,袋式除尘器的最优滤袋数目为76个。模拟结果为袋式除尘器的设计和优化提供了依据。     

旋风-布袋复合除尘器优化和除尘效率的数值模拟

针对燃煤工业锅炉烟尘超低排放的要求,采用数值模拟方法,研究了旋风除尘器(A)和内部滤袋前无导流板(B)、有导流板(C)或开孔挡板(D)的几种旋风-布袋复合除尘器的流场。结果表明：除尘器C的流场较均匀,较高风速时压降低于除尘器B、D。进一步模拟了除尘器A、C的分级效率,发现对于粒径低于15 μm的颗粒,除尘器A去除率低于60%,除尘器C去除率高于97.8%。且复合除尘器占地小,滤袋寿命长,具有广阔的应用前景。上述数值模拟结果可为复合除尘器的优化设计提供参考。     

脱硝系统烟气流场的性能与优化

选择性催化还原(SCR)系统均匀喷氨后,空气预热器易出现堵塞,并且出口在线检测位置不具代表性。为此,根据国内典型SCR系统建立等尺寸的物理结构模型,利用计算流体力学(CFD)技术对烟道与装置内烟气分布、烟气与NH3的混合、压降分布及其原因进行分析,并对导流板的结构进行优化。同时,通过与现场实测入口速度、压降值的对比,论证了数值模型的准确性。结果表明,烟道缩放区域导流板促使烟气均匀分布的同时也使烟气产生倾斜的流动惯性,这使得喷氨格栅(AIG)区域的速度偏差过大,导致了烟气与NH3混和的不均匀,是氨逃逸率高与出口NOx不均匀的首要原因;优化的导流板结构消除了烟气倾斜的流动惯性,进一步促进了AIG区域与催化剂上层烟气的均匀分布以及烟气与NH3的均匀性混合。     

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.     

基于数值模拟的电袋复合除尘器入口流速和管道结构优化

利用计算流体动力学(CFD)软件对电袋复合除尘器进行数值模拟,通过改变气流入口流速、调整电凝并区前水平管道长度和增设入口管道内导流板等方法进行优化。结果表明,气流入口流速对改善电袋复合除尘器的除尘效果影响不大,而入口管道增设导流板影响很大。当气流入口流速为18.00m/s、电凝并区前水平管道长度为0.50m、入口管道后一弯管处增设置3块导流板时,电凝并区入口断面、电除尘区入口断面气流分布均匀,相对均方根分别为0.17、0.14,涡流区域基本消失。     

活性焦联合脱硫脱硝脱汞塔入口气流均布与喷氨优化

某新建活性焦联合脱硫脱硝脱汞反应塔在试运行过程中运行阻力大,且脱硫脱硝效率较低。为提升反应塔的性能,以原设备为几何模型,采用CFD方法进行模拟优化。针对影响设备压降和脱硫性能的入口气流均匀性问题,模拟对比了4种导流板方案,考察板间距和圆弧板半径对气流均布的影响。同时为提高喷氨均匀性,结合烟气在过渡气室的流动情况提出了2种喷氨格栅加密方案。模拟结果表明：板间距和圆弧板半径都相同的导流板方案均布气流的效果最好,烟气在活性焦脱硫层的气流均匀性提升了11.48%,同时设备的压降降低了451 Pa。合理的喷氨格栅加密方案使氨气分布均匀性提升了34.88%。反应塔的压降模拟结果与现场实测数据吻合较好,模拟结果对设备的优化设计和实际运行有指导作用。     

湍流器对湿法脱硫塔内热态流场影响的数值模拟

为了达到燃煤电厂超低排放的要求,运用CFD技术对装有3种不同孔隙率与不同湍流单元直径的湍流器的脱硫塔内热态流场进行数值模拟,分析速度、温度和压力分布随孔隙率和湍流单元直径的变化规律,揭示湍流器的作用机理。气、液两相分别选用RNG k-ε湍流模型和拉格朗日颗粒轨道模型,并结合SIMPLE算法进行数值模拟。模拟结果表明：安装湍流器可明显改善脱硫塔内烟气流场的均匀性,使横截面的速度标准方差减小到1.0以下,并有效延长浆液驻留时间,提高吸收区的气液接触概率及浆液利用率;在综合考虑流场分布、气液掺混程度与能量损失的情况下,安装孔隙率为50%、湍流单元直径为1.2 m的湍流器效果最佳。研究结果可为大型电厂脱硫塔中湍流器的优化及选用提供依据。     

Chloroaromatic pollutants in mussels incubated in two finnish watercourses polluted by industry

Concentrations of different chlorinated compounds were measured in mussels incubated in two polluted watercourses, a river (the River Kymijoki) and a lake (Lake Vanaja) for four weeks in summer 1995. The sum concentrations of polychlorinated phenols (PCP) and biphenyls (PCB) were both about 1 μg/g lipid weight (lw) in Lake Vanaja mussels, while in the River Kymijoki mussels PCPs were non-detectable and PCBs were measured 120 ng/g lIw. The concentrations of toxic polychlorinated dibenzo-p-dioxin (PCDD) and dibenzofuran (PCDF) congeners ranged between <17 and 370 pg/g Iw in Lake Vanaja mussels and between <38 and 11,000 pg/g lw in the River Kymijoki mussels. Polychlorinated diphenyl ethers (PCDE) were detected in the mussels incubated in the River Kymijoki (0.4–1.1 ng/g Iw), but not in those incubated in Lake Vanaja. Polychlorinated phenoxyanisoles (PCPA) were measured 33 ng/g lw and polychlorinated phenoxyphenols (PCPP) 300 ng/g lw in the mussels incubated in the River Kymijoki. PCPAs were also detected in reference samples, which were sediment and pike from the River Kymijoki and Baltic salmon, seal and white-tailed sea eagle.     

Book review

The Pesticide Manual ‐ A World Compendium, 8th Edition, C.R. Worthing, Editor and S.B. Walker, Assistant Editor, British Crop Protection Council, BCPC Publications Sales, Bear Farm, Binfield, Bracknell, Berkshire RG12 5QE, England. 1987, 1100 pp., UK £50; Overseas £56. ISBN 0–948404–01–9.     

Organochlorine compounds in a three-step terrestrial food chain

The concentrations of 15 organochlorine chemicals (PCBs and pesticides) were studied in a Central European oak wood food chain system: Great tit (Parus major), caterpillars (Tortrix viridana, Operophtera brumata, Erannis defoliaria), and oak-leaves (Quercus robur). Juvenile tits receive organochlorines from the mother via egg transfer and, eventually to a greater extent, from the caterpillar food source during nestling period. The concentrations of PCB 153 (2,2′,4,4′,5,5′-hexachlorobiphenyl, the most abundant in this study) was found in leaf material at ca. 1 ng/g, in caterpillars 10 ng/g, and in bird eggs 170 ng/g on an average and on a dry mass basis.     

The chemical stability of formulations of some hydrolyzable insecticides in aqueous mixtures with hydrolysis catalysts

Abstract

The active ingredients in commercial formulations of malathion, oxamyl, carbaryl, diazinon, and chlorpyrifos diluted to “spray tank”; concentrations with buffered distilled or natural water of pH 4–9 were stable for at least 24 hr. Formulations of trichlorfon were not stable at pH 7 or above but disappearance rates were slower than for the pure chemical in homogeneous solution. Cupric ion was observed to be an effective catalyst for the hydrolysis of a variety of pure organophosphorus insecticides but did not catalyze hydrolysis of the active ingredients of the formulations examined. Increasing the dilution of the formulation increased the susceptibility of malathion, oxamyl, and carbaryl to hydrolysis.     

Observations on the influence of water and soil pH on the persistence of insecticides

Abstract

The pH‐disappearance rate profiles were determined at ca. 25°C for 24 insecticides at 4 or 5 pH values over the range 4.5 to 8.0 in sterile phosphate buffers prepared in water‐ethanol (99: 1 v/v). Half‐lives measured at pH 8 were generally smaller than at lower pH values. Changes in half lives between pH 8.0 and 4.5 were largest (>1000x) for the aryl carbamates, carbofuran and carbaryl, the oxime carbamate, oxamyl, and the organophosphorus insecticide, trichlorfon. In contrast, half lives of phorate, terbufos, heptachlor, fensulfothion and aldicarb were affected only slightly by pH changes. Under the experimental conditions described half lives at pH8 varied from 1–2 days for trichlorfon and oxamyl to >1 year for fensulfothion and cyper‐methrin. Insecticide persistence on alumina (acid, neutral and basic), mineral soils amended with aluminum sulfate or calcium hydroxide to different pH values and four natural soils of different pH was examined. No correlation was observed between the measured pH of these solids and the rate of disappearance of selected insecticides applied to them. These observations demonstrate the difficulty of extrapolating the pH dependent disappearance behaviour observed in homogeneous solution to partially solid heterogeneous systems such as soil.     

Radionuclides in pinon pine (Pinus edulis) nuts from los alamos national laboratory lands and the dose from consumption

Abstract

One of the dominant tree species growing within and around the eastern portion of Los Alamos National Laboratory (LANL), Los Alamos, NM, lands is the pinon pine (Pinus edulis). Pinon pine is used for firewood, fence posts, and building materials and is a source of nuts for food—the seeds are consumed by a wide variety of animals and are also gathered by people in the area and eaten raw or roasted. This study investigated the (1) concentration of ³H, ¹³⁷Cs, ⁹⁰Sr, ^totU, ²³⁸Pu, ^{239, 240}Pu, and²⁴¹ Am in soils (0‐ to 12‐in. [31 cm] depth underneath the tree), pinon pine shoots (PPS), and pinon pine nuts (PPN) collected from LANL lands and regional background (BG) locations, (2) committed effective dose equivalent (CEDE) from the ingestion of nuts, and (3) soil to PPS to PPN concentration ratios (CRs). Most radionuclides, with the exception of ³H in soils, were not significantly higher (p < 0.10) in soils, PPS, and PPN collected from LANL as compared to BG locations, and concentrations of most radionuclides in PPN from LANL have decreased over time. The maximum net CEDE (the CEDE plus two sigma minus BG) at the most conservative ingestion rate (10 lb [4.5 kg]) was 0.0018 mrem (0.018 μSv); this is far below the International Commission on Radiological Protection (all pathway) permissible dose limit of 100 mrem (1000 μSv). Soil‐to‐nut CRs for most radionuclides were within the range of default values in the literature for common fruits and vegetables.     

Degradation and sorption of imidacloprid in dissimilar surface and subsurface soils

Degradation and sorption/desorption are important processes affecting the leaching of pesticides through soil. This research characterized the degradation and sorption of imidacloprid (1-[(6-chloro-3-pyridinyl)-methyl]-N-nitro-2-imidazolidinimine) in Drummer (silty clay loam) and Exeter (sandy loam) surface soils and their corresponding subsurface soils using sequential extraction methods over 400 days. By the end of the incubation, approximately 55% of imidacloprid applied at a rate of 1.0 mg kg^?1 degraded in the Exeter sandy loam surface and subsurface soils, compared to 40% of applied imidacloprid within 300 days in Drummer surface and subsurface soils. At the 0.1 mg kg^?1 application rate, dissipation was slower for all four soils. Water-extractable imidacloprid in Exeter surface soil decreased from 98% of applied at day 1 to > 70% of the imidacloprid remaining after 400 d, as compared to 55% in the Drummer surface soil at day 1 and 12% at day 400. These data suggest that imidacloprid was bioavailable to degrading soil microorganisms and sorption/desorption was not the limiting factor for biodegradation. In subsurface soils > 40% of ¹⁴C-benzoic acid was mineralized over 21 days, demonstrating an active microbial community. In contrast, cumulative ¹⁴CO₂ was less than 1.5% of applied ¹⁴C-imidacloprid in all soils over 400 d. Qualitative differences in the microbial communities appear to limit the degradation of imidacloprid in the subsurface soils.     

Evaluation of trichloroethene recovery processes in heterogeneous aquifer cells flushed with biodegradable surfactants

The ability of two biodegradable surfactants, polyoxyethylene (20) sorbitan monooleate (Tween 80) and sodium dihexyl sulfosuccinate (Aerosol MA), to recover a representative dense non-aqueous-phase liquid (DNAPL), trichloroethene (TCE), from heterogeneous porous media was evaluated through a combination of batch and aquifer cell experiments. An aqueous solution containing 3.3% Aerosol MA, 8% 2-propanol and 6 g/l CaCl(2) yielded a weight solubilization ratio (WSR) of 1.21 g TCE/g surfactant, with a corresponding liquid-liquid interfacial tension (IFT) of 0.19 dyn/cm. Flushing of aquifer cells containing a TCE-DNAPL source zone with approximately two pore volumes of the AMA formulation resulted in substantial (>30%) mobilization of TCE-DNAPL. However, a TCE mass recovery of 81% was achieved when the aqueous-phase flow rate was sufficient to displace the mobile TCE-DNAPL toward the effluent well. Aqueous solutions of Tween 80 exhibited a greater capacity to solubilize TCE (WSR=1.74 g TCE/g surfactant) and exerted markedly less reduction in IFT (10.4 dyn/cm). These data contradict an accepted empirical correlation used to estimate IFT values from solubilization capacity, and indicate a unique capacity of T80 to form concentrated TCE emulsions. Flushing of aquifer cells with less than 2.5 pore volumes of a 4% T80 solution achieved TCE mass recoveries ranging from 66 to 85%, with only slight TCE-DNAPL mobilization (<5%) occurring when the total trapping number exceeded 2 x 10(-5). These findings demonstrate the ability of Tween 80 and Aerosol MA solutions to efficiently recover TCE from a heterogeneous DNAPL source zone, and the utility of the total trapping number as a design parameter for a priori prediction of DNAPL mobilization and bank angle formation when flushing with low-IFT solutions. Given their potential to stimulate microbial reductive dechlorination at low concentrations, these surfactants are well-suited for remedial action plans that couple aggressive mass removal followed by enhanced bioremediation to treat chlorinated solvent source zones.     

Biochemical interaction of six op delayed neurotoxicants with several neurotargets

Abstract

Five organophosphorous insecticides: Leptophos, EPN, Cyano‐fenphos, trichloronate and salithion proved to cause irreversible ataxia not only to chicken but also to mice and sheep. TOCP was included as a reference. Cyanofenphos blocked the catecholamine B‐receptor binding activity with ³H‐norepinephrine at a level similar to that of the specific inhibitor propranolol in the mouse heart preparation. In the lamb heart preparation, the B‐receptor was more sensitive to Leptophos, salithion and TOCP than to propranolol. The six compounds and their oxons were screened for their in‐vitro inhibition to monamine oxidase (MAO), acetyl cholinesterase (AChE) and neurotoxic esterase (NTE) in the brain of either mouse, lamb or chicken. It is believed that their AChE inhibition stands for their acute toxicity, while NTE inhibition is responsible for their paralytic ataxia.     

Phytoextraction of lead-contaminated soil using vetivergrass (<Emphasis Type="Italic">Vetiveria zizanioides</Emphasis> L.), cogongrass (<Emphasis Type="Italic">Imperata cylindrica</Emphasis> L.) and carabaograss (<Emphasis Type="Italic">Paspalum conjugatum</Emphasis> L.)

Background, Aims and Scope The global problem concerning contamination of the environment as a consequence of human activities is increasing. Most of the environmental contaminants are chemical by-products and heavy metals such as lead (Pb). Lead released into the environment makes its way into the air, soil and water. Lead contributes to a variety of health effects such as decline in mental, cognitive and physical health of the individual. An alternative way of reducing Pb concentration from the soil is through phytoremediation. Phytoremediation is an alternative method that uses plants to clean up a contaminated area. The objectives of this study were: (1) to determine the survival rate and vegetative characteristics of three grass species such as vetivergrass, cogongrass and carabaograss grown in soils with different Pb levels; and (2) to determine and compare the ability of the three grass species as potential phytoremediators in terms of Pb accumulation by plants. Methods The three test plants: vetivergrass (Vetiveria zizanioides L.); cogongrass (Imperata cylindrica L.); and carabaograss (Paspalum conjugatum L.) were grown in individual plastic bags containing soils with 75 mg kg⁻¹ (37.5 kg ha⁻¹) and 150 mg kg⁻¹ (75 kg ha⁻¹) of Pb, respectively. The Pb contents of the test plants and the soil were analyzed before and after experimental treatments using an atomic absorption spectrophotometer. This study was laid out following a 3 × 2 factorial experiment in a completely randomized design. Results On the vegetative characteristics of the test plants, vetivergrass registered the highest whole plant dry matter weight (33.85–39.39 Mg ha⁻¹). Carabaograss had the lowest herbage mass production of 4.12 Mg ha⁻¹ and 5.72 Mg ha⁻¹ from soils added with 75 and 150 mg Pb kg⁻¹, respectively. Vetivergrass also had the highest percent plant survival which meant it best tolerated the Pb contamination in soils. Vetivergrass registered the highest rate of Pb absorption (10.16 ± 2.81 mg kg⁻¹). This was followed by cogongrass (2.34 ± 0.52 mg kg⁻¹) and carabaograss with a mean Pb level of 0.49 ± 0.56 mg kg⁻¹. Levels of Pb among the three grasses (shoots + roots) did not vary significantly with the amount of Pb added (75 and 150 mg kg⁻¹) to the soil. Discussion Vetivergrass yielded the highest biomass; it also has the greatest amount of Pb absorbed (roots + shoots). This can be attributed to the highly extensive root system of vetivergrass with the presence of an enormous amount of root hairs. Extensive root system denotes more contact to nutrients in soils, therefore more likelihood of nutrient absorption and Pb uptake. The efficiency of plants as phytoremediators could be correlated with the plants’ total biomass. This implies that the higher the biomass, the greater the Pb uptake. Plants characteristically exhibit remarkable capacity to absorb what they need and exclude what they do not need. Some plants utilize exclusion mechanisms, where there is a reduced uptake by the roots or a restricted transport of the metals from root to shoots. Combination of high metal accumulation and high biomass production results in the most metal removal from the soil. Conclusions The present study indicated that vetivergrass possessed many beneficial characteristics to uptake Pb from contaminated soil. It was the most tolerant and could grow in soil contaminated with high Pb concentration. Cogongrass and carabaograss are also potential phytoremediators since they can absorb small amount of Pb in soils, although cogongrass is more tolerant to Pb-contaminated soil compared with carabaograss. The important implication of our findings is that vetivergrass can be used for phytoextraction on sites contaminated with high levels of heavy metals; particularly Pb. Recommendations and Perspectives High levels of Pb in localized areas are still a concern especially in urban areas with high levels of traffic, near Pb smelters, battery plants, or industrial facilities that burn fuel ending up in water and soils. The grasses used in the study, and particularly vetivergrass, can be used to phytoremediate urban soil with various contaminations by planting these grasses in lawns and public parks. ESS-Submission Editor: Dr. Willie Peijnenburg (wjgm.peijnenburg@rivm.nl)