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。     

基于非均匀入口条件SCR系统气固相组分的分布特性优化

采用实际工程中复杂的非均匀入口边界条件,对SCR系统气固两相流动特性及其优化进行了数值模拟。对烟道混合器下游截面上各测点处NO浓度进行实验测试,与数值模拟计算的NO浓度结果保持基本一致,模拟计算获得了SCR反应器入口导流板布置数量、倾角以及导流板间距的最优方案,催化剂首层入口截面烟气流速偏差系数Cx从18.13%降至12.38%。烟气中飞灰颗粒在反应器首层催化剂入口断面上呈现分布不均匀分布的特性,弯道C处无导流板时,截面中间区域的粉尘浓度一般在0.035 kg·m-3,最高值可达0.068 kg·m-3,弯道C加装导流板后首层催化剂入口烟道断面飞灰浓度不均匀偏差系数值由原有结构的34.21%降为26.18%,表明导流板结构有助于飞灰浓度均匀性分布。数值计算模型符合工程实际,具有很好的可靠性和实用性。     

伞罩型湿式脱硫除尘塔入口结构优化模拟

利用商用CFD软件Fluent，采用k-ε湍流模型和SIMPLE算法，对新型伞罩型湿式脱硫除尘塔内的三维两相流场进行数值模拟，发现了塔内烟气入口处流场所存在的不均匀性。为将其流场调节均匀从而提高脱硫除尘效率，在数值模拟的基础上提出在入口处加装直导流板、阶梯导流板和弯曲导流板，并分析塔内y=0截面速度的分布，以及z＝0.21 m截面上的颗粒浓度、速度和压力等参数的分布。结果表明：通过加装阶梯导流板和弯曲导流板均可以将流场调节均匀，达到较理想状态，从而实现高效净化气体的目的。模拟结果对设备的优化设计和实际运行有一定的指导作用。     

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。     

SCR脱硝超低排放工程改造流场优化

采用CFD数值模拟方法对某电厂400 MW机组烟气脱硝装置流场进行了诊断,分析了导致效率低、烟道积灰以及空预器磨损等问题的具体原因,之后对此脱硝装置流场进行优化。优化结果表明:在省煤器出口水平扩张段烟道增设导流板,消除了该区域的大范围涡流,有利于减轻烟道积灰,并使喷氨区域烟气速度分布均匀;将喷氨混合装置改成喷氨格栅+圆盘混合器型式,强化了氨氮混合,使首层催化剂入口氨浓度分布均匀性得到显著改善,有利于提高脱硝效率、降低氨逃逸率;在空预器上游烟道增设导流装置,提高了空预器烟气速度分布均匀性,有利于解决空预器磨损问题。通过流场优化后,提高了此脱硝装置脱硝效率,减轻和消除了烟道积灰及空预器磨损。     

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.     

垃圾焚烧电厂烟气降温模拟分析

采用fluent软件对江苏某垃圾焚烧厂的烟气2级喷水减温流程分别进行了模拟,并根据模拟结果分析了反应塔和冷却塔内烟气的温度、流速、湿度等在塔内的径向分布情况.结果表明,冷却塔和反应塔在不同进口烟气温度下将烟气温度降低到各自相应同一值时所需喷水量的模拟值和实际值均具有较好的一致性,模拟结果具有一定的可靠性;冷却塔内烟气温...     

加装导流板对立体循环一体化氧化沟(IODVC)流场的影响

流速分布不均导致立体循环一体化氧化沟(IODVC)出现污泥沉积等问题。利用计算流体力学(CFD)技术对IODVC进行了二维单相流模拟。研究中采用多重参考系模型来模拟曝气转刷的转动及沟内动力的来源,风扇模型来模拟水下推流器对水流的推动作用以及RNG k-ε模型来模拟氧化沟中流场的湍流变化,将模拟数据与实测数据进行拟合验证,再运用验证后的模型研究了加装导流板及其位置对IODVC内部流场的影响。结果表明,CFD模型能够较准确地模拟出IODVC沟道中液流速度分布及沿程变化,加装导流板及合理布设其位置可增加主沟内液流高速区所占比例,同时改善隔板背后的流速分布。因此,在弯道处加装导流板将有利于减少和防止沟道污泥沉积现象,对IODVC的优化设计及工程应用具有一定的指导意义。     

两相均流板对弯管中气固两相运动分布特征的影响

针对燃煤电厂烟气污染物脱除系统设备入口弯管内的气固两相分布不均的问题,提出一种两相均流板,采用CFD数值模拟的方法对比分析了安装两相均流板与常规导流装置的弯道内气流速度分布、颗粒质量浓度分布以及弯管进出口压差变化特征。结果表明:在弯管内安装两相均流板较安装导流板/三角翼挡板更优,既可以使气固两相均匀分布又可以有效降低系统的压阻,克服了常规均流装置气固两相均流效果不佳、阻力大等缺点,在较低的压阻下解决燃煤电厂烟气污染物脱除系统入口气固两相混合不均匀的问题,有效提高污染物脱除效率。     

大型燃煤机组SCR脱硝系统优化

江西某电厂660 MW燃煤机组SCR系统氨逃逸严重,催化剂层老化迅速,空预器运行短时间内会堵塞。为了改善SCR系统氨逃逸问题和机组运行可靠性,通过数值模拟的方法,结合物理模型速度场冷态实验及现场NO_x浓度测试结果,建立脱硝系统三维模型,模拟了不同圆盘导流板安装角度及不同喷氨方案下SCR系统流场分布。对模拟结果进行对比和分析,提出适当调大圆盘导流板倾角和合理差异化调整各喷口喷氨参数的优化方案,使第一层催化剂层入口处NH_3浓度、NO_x浓度和NH_3/NO_x分布都能很好地满足设计和运行要求,为大型燃煤机组SCR脱硝系统的优化调整和运行提供参考。     

Dioxin- and POP-contaminated sites—contemporary and future relevance and challenges

BACKGROUND, AIM AND SCOPE: Once they have been generated, polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs) and other persistent organic pollutants (POPs) can persist in soils and sediments and in waste repositories for periods extending from decades to centuries. In 1994, the US EPA concluded that contaminated sites and other reservoirs are likely to become the major source of contemporary pollution problems with these substances. With this in mind, this article is the first in a new series in ESPR under the title 'Case Studies on Dioxin and POP Contaminated Sites--Contemporary and Future Relevance and Challenges', which will address this important issue. The series will document various experiences from sites contaminated with PCDD/F and other POPs. This article provides an overview of the content of the articles comprising the series. In addition, it provides a review of the subject in its own right and identifies the key issues arising from dioxin/POP-contaminated sites. Additionally, it highlights the important conclusions that can be drawn from these examples. The key aim of this article and of the series as a whole is to provide a comprehensive overview of the types of PCDD/F contaminated sites that exist as a result of historical activities. It details the various processes whereby these sites became contaminated and attempts to evaluate their contemporary relevance as sources of PCDD/Fs and other POPs. It also details the various strategies used to assess these historical legacies of contamination and the concepts developed, or which are under development, to effect their remediation. MAIN FEATURES: Special sessions on 'Contaminated sites--Cases, remediation, risk and policy' were held at the DIOXIN conferences in 2006 and 2007, and this theme will be continued at DIOXIN 2008 to be held in Birmingham. Selected cases from the approximately 70 contributions made to these sessions, together with some additional invited case studies are outlined together with the key issues they raise. By evaluating these cases and adding details of experiences published in the current literature, an overview will be given of the different features and challenges of dioxin and POP-contaminated sites. RESULTS: This article provides a systematic categorisation of types of PCDD/F and POP-contaminated sites. These are categorised according to the chemical or manufacturing process, which generated the PCDD/Fs or POPs and also includes the use and disposal aspects of the product life cycle in question. The highest historical PCDD/F and dioxin-like polychlorinated biphenyl (PCB) contamination burdens have arisen as a result of the production of chlorine and of chlorinated organic chemicals. In particular, the production of chlorinated pesticides, PCBs and the related contaminated waste streams are identified being responsible for historical releases of toxic equivalents (TEQs) at a scale of many tonnes. Along with such releases, major PCDD/F contaminated sites have been created through the application or improper disposal of contaminated pesticides, PCBs and other organochlorine chemicals, as well through the recycling of wastes and their attempted destruction. In some extreme examples, PCDD/F contaminated sites have also resulted from thermal processes such as waste incinerators, secondary metal industries or from the recycling or deposition of specific waste (e.g. electronic waste or car shredder wastes), which often contain chlorinated or brominated organic chemicals. The examples of PCDD/F and dioxin-like PCB contamination of fish in European rivers or the impact of contaminated sites upon fishing grounds and upon other food resources demonstrate the relevance of these historical problems to current and future human generations. Many of the recent food contamination problems that have emerged in Europe and elsewhere demonstrate how PCDD/F and dioxin like PCBs from historical sources can directly contaminate human and animal feedstuffs and indeed highlight their considerable contemporary relevance in this respect. Accordingly, some key experiences and lessons learnt regarding the production, use, disposal and remediation of POPs from the contaminated sites are summarised. DISCUSSION: An important criterion for evaluating the significance and risks of PCDD/Fs and other POPs at contaminated sites is their present or future potential for mobility. This, in turn, determines to a large degree their propensity for off-site transport and environmental accessibility. The detailed evaluation of contaminated site cases reveals different site-specific factors, which influence the varied pathways through which poor water-soluble POPs can be mobilised. Co-contaminants with greater water solubility are also typically present at such sites. Hence, pumping of groundwater (pump and treat) is often required in addition to attempting to physically secure a site. At an increasing number of contaminated sites, securing measures are failing after relatively short time spans compared to the time horizon, which applies to persistent organic pollutant contamination. Due to the immense costs and challenges associated with remediation of contaminated sites 'monitored natural attenuation' is increasingly gaining purchase as a conceptual remediation approach. However, these concepts may well prove limited in their practical application to contaminated sites containing persistent organic pollutants and other key pollutants like heavy metals. CONCLUSIONS: It is inevitable, therefore, that dioxin/POP-contaminated sites will remain of contemporary and future relevance. They will continue to represent an environmental issue for future generations to address. The securing and/or remediation of dioxin/POP-contaminated sites is very costly, generally in the order of tens or hundreds of millions of dollars. Secured landfills and secured production sites need to be considered as constructions not made for 'eternity' but built for a finite time scale. Accordingly, they will need to be controlled, supervised and potentially repaired/renewed. Furthermore, the leachates and groundwater impacted by these sites will require ongoing monitoring and potential further remediation. These activities result in high maintenance costs, which are accrued for decades or centuries and should, therefore, be compared to the fully sustainable option of complete remediation. The contaminated site case studies highlight that, while extensive policies and established funds for remediation exist in most of the industrialised western countries, even these relatively well-regulated and wealthy countries face significant challenges in the implementation of a remediation strategy. This highlights the fact that ultimately only the prevention of contaminated sites represents a sustainable solution for the future and that the Polluter Pays Principle needs to be applied in a comprehensive way to current problems and those which may emerge in the future. RECOMMENDATIONS AND PERSPECTIVES: With the continuing shift of industrial activities in developing and transition economies, which often have poor regulation (and weak self-regulation of industries), additional global challenges regarding POPs and other contaminated sites may be expected. In this respect, a comprehensive application of the "polluter pays principle" in these countries will also be a key to facilitate the clean-up of contaminated areas and the prevention of future contaminated sites. The threats and challenges of contaminated sites and the high costs of securing/remediating the problems highlight the need for a comprehensive approach based upon integrated pollution prevention and control. If applied to all polluting (and potentially polluting) industrial sectors around the globe, such an approach will prove to be both the cheapest and most sustainable way to underpin the development of industries in developing and transition economies.     

Sorption and Desorption Behavior of Chloroanilines and Chlorophenols on Montmorillonite and Kaolinite

The bioavailability of pollutants, pesticides and/or their degradation products in soil depends on the strength of their sorption by the different soil components, particularly by the clay minerals. This study reports the sorption-desorption behavior of the environmentally hazardous industrial pollutants and certain pesticides degradation products, 3-chloroaniline, 3,4-dichloroaniline, 2,4,6-trichloroaniline, 4-chlorophenol, 2,4-dichlorophenol and 2,4,6-trichlorophenol on the reference clays kaolinite KGa-1 and Na-montmorillonite SWy-l. In batch studies, 2.0 g of clay were equilibrated with 100.0 mL solutions of each chemical at concentrations ranging from 10.0 to 200.0 mg/L. The uptake of the compounds was deduced from the results of HPLC-UV-Vis analysis. The lipophilic species were best retained by both clay materials. The most lipophilic chemical used in the study, 2,4,6-trichloroaniline, was also the most strongly retained, with sorption of up to 8 mg/g. In desorption experiments, which also relied on HPLC-UV-Vis technique, 2,4,6-trichloroaniline was the least desorbed from montmorillonite. However, on kaolinite all of the compounds under study were irreversibly retained. The experimental data have been modelled according to the Langmuir and Freundlich isotherms. A hypothesis is proposed concerning the sorption mechanism and potential applications of the findings in remediation strategies have been suggested.     

Persistence and dissipation kinetics of trifloxystrobin and tebuconazole in onion and soil

The persistence and dissipation kinetics of trifloxystrobin and tebuconazole on onion were studied after application of their combination formulation at a standard and double dose of 75 + 150 and 150 + 300 g a.i. ha^?1. The fungicides were extracted with acetone, cleaned-up using activated charcoal (trifloxystrobin) and neutral alumina (tebuconazole). Analysis was carried out by gas chromatograph (GC) and confirmed by gas chromatograph mass spectrometry (GC-MS). The recovery was above 80% and limit of quantification (LOQ) 0.05 mg kg^?1 for both fungicides. Initial residue deposits of trifloxystrobin were 0.68 and 1.01 mg kg^?1 and tebuconazole 0.673 and 1.95 mg kg^?1 from standard and double dose treatments, respectively. Dissipation of the fungicides followed first-order kinetics and the half life of degradation was 6–6.6 days. Matured onion bulb (and field soil) harvested after 30 days was free from fungicide residues. These findings suggest recommended safe pre-harvest interval (PHI) of 14 and 25 days for spring onion consumption after treatment of Nativo 75 WG at the standard and double doses, respectively. Matured onion bulbs at harvest were free from fungicide residues.     

Field and laboratory studies of the fate and enantiomeric enrichment of venlafaxine and O-desmethylvenlafaxine under aerobic and anaerobic conditions

The stereoselectivity of R,S-venlafaxine and its metabolites R,S-O-desmethylvenlafaxine, N-desmethylvenlafaxine, O,N-didesmethylvenlafaxine, N,N-didesmethylvenlafaxine and tridesmethylvenlafaxine was studied in three processes: (i) anaerobic and aerobic laboratory scale tests; (ii) six wastewater treatment plants (WWTPs) operating under different conditions; and (iii) a variety of wastewater treatments including conventional activated sludge, natural attenuation along a receiving river stream and storage in operational and seasonal reservoirs. In the laboratory and field studies, the degradation of the venlafaxine yielded O-desmethylvenalfaxine as the dominant metabolite under aerobic and anaerobic conditions. Venlafaxine was almost exclusively converted to O-desmethylvenlafaxine under anaerobic conditions, but only a fraction of the drug was transformed to O-desmethylvenlafaxine under aerobic conditions. Degradation of venlafaxine involved only small stereoisomeric selectivity. In contrast, the degradation of O-desmethylvenlafaxine yielded remarkable S to R enrichment under aerobic conditions but none under anaerobic conditions. Determination of venlafaxine and its metabolites in the WWTPs agreed well with the stereoselectivity observed in the laboratory studies. Our results suggest that the levels of the drug and its metabolites and the stereoisomeric enrichment of the metabolite and its parent drug can be used for source tracking and for discrimination between domestic and nondomestic wastewater pollution. This was indeed demonstrated in the investigations carried out at the Jerusalem WWTP.     

Synthesis and characterization of m- and p-methylbenzyl-mercapturic acids derived from m- and p-xylenes

Chemical synthesis and physical properties of two mercapturic acids suggested as urinary metabolites of m- and p-xylenes ace described. These compounds may be used for the identification and quantivative determination by high-performance liquid chromatography of the corresponding mercapturic acids in urine.     

Levels of PCDDs and PCDFs in products and effluent from the Swedish pulp and paper industry and chloralkali process

Within the Swedish Dioxin Survey various samples from the pulp and paper industry and the chloralkali process have been analyzed by congener specific analytical methods. In addition to the generally discussed “bleaching pattern” of the tetrachlorinated congeners, these samples also contained higher chlorinated congeners like hexa-CDDs, hepta-CDFs, octa-CDD and octa-CDF. Consequently it is recommended that samples from the pulp industry should be analyzed for all PCDDs and PCDFs (tetra- through octa-). Counted as Nordic Toxic Equivalents (NTEQ), the recycled pulp samples had the highest contamination level followed by TMP, unbleached sulfite and bleached softwood and hardwood. In addition to the bleaching process, various chemicals used in the pulping, bleaching and wastewater treatment can contribute to the contamination.     

Uptake and translocation of metformin, ciprofloxacin and narasin in forage- and crop plants

Transfer of bioactive organic compounds from soil to plants might represent animal and human health risks. Sewage sludge and manure are potential sources for bioactive compounds such as human- and veterinary drugs. In the present study, uptake of the anti-diabetic compound, metformin, the antibiotic agent ciprofloxacin and the anti-coccidial narasin in carrot (Daucuscarota ssp. sativus cvs. Napoli) and barley (Hordeumvulgare) were investigated. The pharmaceuticals were selected in order to cover various chemical properties, in addition to their presence in relevant environmental matrixes. The root concentration factors (RCF) found in the present study were higher than the corresponding leaf concentration factors (LCF) for the three test pharmaceuticals. The uptake of metformin was higher compared with ciprofloxacin and narasin for all plant compartments analyzed. Metformin was studied more explicitly with regard to uptake and translocation in meadow fescue (Festucapratense), three other carrot cultivars (D.carota ssp. sativus cvs. Amager, Rothild and Nutri Red), wheat cereal (Triticumaestivum) and turnip rape seed (Brassicacampestris). Uptake of metformin in meadow fescue was comparable with uptake in the four carrot cultivars (RCF 2-10, LCF approximately 1.5), uptake in wheat cereals were comparable with barley cereals (seed concentration factors, SCF, 0.02-0.04) while the accumulation in turnip rape seeds was as high as 1.5. All three pharmaceuticals produced negative effects on growth and development of carrots when grown in soil concentration of 6-10 mg kg⁻¹ dry weight.     

Copper-catalyzed chlorination and condensation of acetylene and dichloroacetylene

The chlorination and condensation of acetylene at low temperatures is demonstrated using copper chlorides as chlorinated agents coated to model borosilicate surfaces. Experiments with and without both a chlorine source and borosilicate surfaces indicate the absence of gas-phase and gas-surface reactions. Chlorination and condensation occur only in the presence of the copper catalyst. C₂ through C₈ organic products were observed in the effluent; PCDD/F were only observed from extraction of the borosilicate surfaces. A global reaction model is proposed that is consistent with the observed product distributions. Similar experiments with dichloroacetylene indicate greater reactivity in the absence of the copper catalyst. Reaction is observed in the gas-phase and in the presence of borosilicate surfaces at low temperatures. The formation of hexachlorobenzene is only observed in the presence of a copper catalyst. PCDD/F were only observed from extraction of the borosilicate surfaces. A global reaction model is proposed for the formation of hexachlorobenzene from dichloroacetylene.     

Aerobic and anaerobic degradation of profluralin and trifluralin

Abstract

The degradation of profluralin [N‐(cyclopropylmethyl)‐α,α,α‐trifluoro‐2,6‐dinitro‐N‐propyl‐]p‐toluidine] and trifluralin (α,α,α‐trifluoro‐2,6‐dinitro‐N,N‐dipropyl‐p‐toluidine) was studied under aerobic and anaerobic soil conditions. Three soils (Goldsboro loamy sand, Cecil loamy sand, Drummer clay loam) were each treated with 1 ppmw herbicide; anaerobic conditions were maintained by flooding. Soil samples were extracted monthly and subjected to TLC analysis. No degradation was detected in sterile controls. Aerobic degradation of both herbicides was greatest in the Cecil loamy sand soil over the entire incubation period. Degradation of profluralin in Cecil soil under aerobic conditions was 86 percent after 4 months with three products detected; 83 percent of the trifluralin was degraded with two products detected. Anaerobic degradation accounted for 72 percent of the profluralin and 78 percent of the trifluralin after 4 months. Degradation of both herbicides increased with incubation time for the first 3 months and decreased slightly thereafter. Generally there was more extensive degradation (percent and in number of products formed) of profluralin than trifluralin under the conditions tested. More degradation products were detected for both herbicides under aerobic conditions than under anaerobic conditions.     

Sources and distribution of polychlorinated-dibenzo-p-dioxins and -dibenzofurans in soil and sediment from the Yellow Sea region of China and Korea

Polychlorinated-dibenzo-p-dioxins and -dibenzofurans (PCDD/Fs) were measured in soils and sediments from the Yellow Sea region. Korean soils and sediments mostly contained detectable PCDD/Fs and showed a widespread distribution among locations. Soil and sedimentary PCDD/Fs from China were comparable to or less than those in Korea. The patterns of relative concentrations of individual congeners in soils were different between the two countries, but similar in sediments. Sources of PCDD/Fs in China and Korea were found to be independent of each other and their distributions reflected matrix-dependent accumulation. Spatial distribution indicated some point sources in Korea while Chinese sources were more widespread and diffuse. PCDD/Fs measured in the coastal areas of the Yellow Sea were comparable to or less than those previously reported in for eastern Asia. However, ∑TEQs in soils and sediments were near to or, in some cases exceeded environmental quality guidelines.