【机械类毕业论文中英文对照文献翻译】利用静电增加对灰尘的沉积
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【机械类毕业论文中英文对照文献翻译】利用静电增加对灰尘的沉积,机械类毕业论文中英文对照文献翻译,机械类,毕业论文,中英文,对照,文献,翻译,利用,静电,增加,灰尘,沉积
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利用静电增加对灰尘的沉积概要：这项研究的基本思想是，利用静电沉淀来减少飞尘中重金属的浓度。当重金属的浓度很高时会妨碍利用飞尘作为肥料，并且这种浓度是可变化的。飞尘分馏法实验是通过在四个发电站利用静电沉淀装置进行的。基于实验结果，第一个收集装置中重金属的浓度处于它的最低值，而最后一个装置中的浓度则是最高。镉在作为肥料的飞尘中的浓度，可以通过应用静电沉淀分馏法，使其降低70%。对重金属的分馏情况相比之下没有对镉进行的分馏的效果明显。结果显示，静电沉淀分馏法在分馏将成为肥料飞尘或改善土壤方面是很合适的一种方法。导论：芬兰的热能工厂和发电厂每年都会产生大约400，000公吨的原始生物燃料灰烬。这种灰烬的产生量在将来会随着生物燃料使用量的增加而增加。木材和泥煤燃后的灰烬能够作为肥料或土壤改良物质而被应用在林地或耕地上。并且能够达到给土壤“补钙”的目的。对灰烬的利用一直受到它本身含尘量和重金属浓度这些因素的局限：后者在很多情况下已经超出了芬兰在对使用土壤改良剂所规定的最高允许标准。在2001年，燃煤灰烬的利用率（84%）相比泥煤和混合燃料灰烬的利用率（43%）高出相当多一部分。木材飞尘的利用率却相当低，在1997年大约是6%。从飞尘中提取出重金属能够使它更充分的被利用。通常，似乎操纵发电厂的燃料质量是达到这一目的唯一可行的方法。这就意味着我们必须知道这些易燃燃料的精确密度和性质，并且知道哪些物质提高灰尘含量中镉的浓度，进而筛选出含镉物质。 少量的飞尘在农业和林产方面作为肥料已被应用。一般情况下，各种类型的灰烬在农业生产中要比化学肥料更适合作为土壤的改良物质，因为在灰烬中可溶的植物营养物质量是很低的。泥煤主要作为一种磷酸盐肥料被应用，木材燃料灰烬对于种植在含石灰的矿物质土壤中的谷类粮食作物的生长中，主要作为一种基础的营养肥料。土壤中灰烬的石灰和肥料效应取决于灰烬中钙和营养物的浓度，灰烬中营养物质的可溶性，土壤，土壤的性质，如酸度，营养浓度。 表2显示了四种类型灰烬的重金属浓度。 灰烬中所含有的很多物质都是极其难溶解的。由于灰烬中的很多重金属（如镉，铅，镍）都很难溶解，可以假定灰烬肥料并不会受到重金属的显著影响。比如说，在水系中，短时期之内然后进行施肥。最后，一些有害的重金属可能就会被从灰烬中以一种可溶的形式释放出来，并且因此而转移到植被中去。灰烬中的石灰效应会降低土壤中重金属的溶解程度。灰烬最初会提高直立树木中的镉的浓度，但是一旦树木的生长提高了其中微量元素的浓度，重金属的浓度就会下降到甚至比最初还要低的水平。一些植物物种中的镉金属浓度的升高可以持续很长一段时间。镉被认为是所有重金属中危害最大的金属，因为它可以停留在土壤中，能够在食物链中逐渐富集，并且会使生物体中毒。图一 静电除尘器静电沉淀（图一）是当前用于分离发电站废气中的固体物质最普遍的一种方法，它的优势包括高效率收集（高达99.9%）和它在处理不同大小尺寸微粒（甚至大小小于1流明的粒子）的适用性，以及体积不定的废气。它的更进一步的优势是它的长期有效性，很好的使用可靠性和低廉的经营维护费用。静电沉淀的机能很显著的依赖于将要收集的飞尘的性质特征。将要被分离的微粒的数量和颗粒大小的分布情况对静电沉淀的机能有很大的影响。尽管静电沉淀收集的高效性或多或少可以持续忽略微粒的量，但是它的有效迁动速度在微粒颗粒小的情况下会有所下降。由于微粒不同的专有特性，对微粒的收集效率根据微粒的大小而有变化。从分离的角度看，最难分离的颗粒大小是在0.2到0.5流明之间。灰烬中的重金属浓度可以通过多层静电沉淀将废气中细小灰烬颗粒分馏出的方法来降低。沉淀的分馏工具会受到比如限制或振荡电流这些方法的影响。我们的研究结果已经显示出，重金属被浓缩聚集在细小灰尘颗粒中。根据Thun和Korhonen（1999）的结果，第三层电场的静电沉淀已经停止，依赖于它的运行条件，灰尘总量的84-95%在第一层次中，4-15%在第二层次中，大约1%的在最后一个层次中。灰尘中镉的浓度可以通过静电沉淀的分馏法，被降低至少15-25%。由于锅炉受到议论和怀疑，来自于以树皮为燃料和树木碎屑为燃料的发电站（大锅炉）的灰烬被划分为以下重量百分比级别，底部灰烬70-90%，气旋飞尘10-30%，静电沉淀飞尘2-8%和粉尘排放0.1-0.3%。在灰尘燃烧和流化床燃烧中，产生的飞尘所占份量为80-100%。多达75-90%的重金属存在于飞尘通过静电沉淀后的那些细颗粒中，图二展示了在底部灰尘，气旋灰尘和静电沉淀喉飞尘中锌，铅，和镉的含量（mg/kg and m-%）。基于表二种的数据。各种灰尘类型中的重金属浓度重金属mg/kg 煤灰 泥煤灰 木材燃灰 树皮燃灰砷 (As) 2.3200 2200 0.260 728镉(Cd) 0.01250 0.058 0.440 420铬 (Cr) 3.67400 15250 15250 4081铜 (Cu) 303000 20400 15300 57144汞 (Hg) 0.0180 0.0011 0.021 0.0120.4镍 (Ni) 1.8800 15200 20250 3652铅 (Pb) 3.11800 5150 151000 53140锌 (Zn) 1413,000 10600 1510,000 11005100表一土壤改良中允许的最大重金属浓度来自发电站A的飞尘中的物质元素 发电站A ：飞尘（mg/kg） 允许最大值 浓度（mg/kg） 2002年 2001年 1999年 汞(Hg) 2.5 0.31 2.0镉(Cd) 2.69 5.05 6.3 3.0砷(As) 18.34 19.73 35 50镍(Ni ) 100铅(Pb) 56.59 106.5 52.7 150铜(Cu ) 86.7 290.1 178 600锌(Zn) 189.7 376.9 706 1500可以这样确定，例如，静电沉淀飞尘含镉量要比气旋灰尘高，这是部分由于和气旋灰尘相比较，静电沉淀飞尘分离了含大部分重金属的细小颗粒。在这种情况下，一部分飞尘根据它在气旋灰尘的残余和它的浓度密度等情况适合于作为肥料。在一个机械分类器在灰尘到达沉淀器之前被连接的情况下，静电沉淀法比传统的方法更能有效地分馏飞尘。数据三显示了一个以燃烧生物燃料的发电站的基本规划设计图，并且其中在静电沉淀之前提供了多级旋风分离器装置。飞尘中所含的多达75-90%的重金属（镉和锌）一定要通过静电沉淀法分离出并会存在于微粒的一小部分中。合适的设计规划和调整过的静电沉淀装置在原则上，是能够将那部分微粒飞尘从废气中分离出来的，而那部分飞尘含有最大量的重金属，但是却只是全部飞尘量的一个小部分。飞尘中主要部分的重金属浓度也因而能够被降低到所允许的浓度最大值以下。2.原料和方法飞尘的分馏试验是在4个发电站进行的（A B C D ）。发电站的静电沉淀是在不同的电压水平下进行的，并且样品是来自于电压稳定程序。从静电沉淀装置中提取的所有样品都来自于在灰尘被放入筒仓之前安置在静电沉淀装置之下的灰尘进料装置中。通过石墨法分析样品中的铅，铜，锌，砷，和镉的存在情况，并且应用Malvern装置来决定粒子颗粒大小。发电站A使用泥煤，树木碎片和石油以及机械木材工业中的副产品作为燃料。发电站的锅炉有效燃料容量是150兆瓦。分馏试验室利用当前发电站的3层静电除尘装置进行的。发电站B使用两个锅炉，一个是泡沫焦性石墨循环流化床锅炉（燃料容量55兆瓦），另一个是单流化床锅炉（燃料容量42兆瓦）。试验的实施是利用单流化床锅炉进行。 该发电站的主要燃料是碾碎的泥煤块和木材燃料，煤烟和铝的氧化物的混合物。来自两个锅炉中的飞尘是通过2层的静电除尘被传送到共同的灰尘筒仓之中。发电站C 装备两个锅炉，锅炉1是一个单流化床，它的燃料容量是267兆瓦。锅炉2是一个泡沫焦性石墨循环流化床，燃料容量是315兆瓦。试验是通过利用泡沫焦性石墨循环流化床锅炉进行的。发电站所用的燃料主要是碾碎的泥煤块和各种木材燃料。两个锅炉都被安装了3层的静电沉淀装置，使锅炉中的飞尘能够由空气作用被吹到一个共同的灰尘筒仓。D发电站的流化床锅炉设备所产生的电能是77兆瓦，它的热容量是246兆瓦。流化床锅炉中所使用的燃料主要是碾碎的泥煤块和木材废物。废气中的飞尘通过一个3层的静电沉淀装置来进行分离。3.结果在A发电站的静电沉淀试验中（试验1-7），所使用的燃料是由49%的泥煤和61%的木材燃料构成。静电沉淀装置1-3层的灰尘过滤烟窗被取样并且进行了分析。（图表4）基于结果，静电沉淀装置第一层镉的浓度处于它的最低值，而在第三层中则是最高值。这是由于较大颗粒的飞尘微粒在第一层中积聚，而第三层中则容纳了含有最小微粒的飞尘。第一层中镉浓度的变化范围在2.2-3.6mg/kg之间，同时第三层中的镉浓度变化范围则是在7.2-12.4之间。这些浓度受很多种性质情况的影响，比如电子稳定程序，燃料质量、性质，和废气的流动率。几乎在每个静电沉淀装置第一层中镉的浓度都低于为灰尘作为肥料使用所规定的允许浓度的最大值极限（3.0mg/kg）在试验进展过程中，静电沉淀装置层中的运转中循环周期阻滞比率被控制在0-2之内。 0值是指被讨论的装置层所有的半循环周期在通常情况下都能正常活跃。2值是指只有半周期的三分之一起作用。因此运转循环周期阻滞的这个评价阐述了有多少连续的半周期已经关闭，也就是，分离器的补充电流的跳动频率是多少。在当前的研究中，运转循环周期阻滞价值由微观kraft（牛皮纸）控制器来操控，它的主要任务就是将电压保持在靠近击穿电压值附近。最重要的就是能够影响和改变静电沉淀装置第一层中的性能。第一层能够使飞尘产物中含有的重金属的浓度水平达到适合作为肥料的标准，例如，图表5显示了静电沉淀装置的运转循环周期阻滞比率在第一层中对飞尘中镉浓度水平的影响。在其他的情况之中，静电沉淀场的闪络数量的增加或减少则不在此控制之内。 有越多的半周期场发生无效，就会导致所产生的飞尘中重金属含量越低。镉的浓度变化也是由燃料质量和性质变化引起的，除了控制本身，还存在其他因素之中。图表6显示了静电沉淀装置的过滤器电压是如何影响在第一层的镉浓度水平重金属浓度水平随着电压的升高而升高。这是由于较高的电压能够更加有效的分离也含有重金属的优质微粒。过滤器电压水平比运转周期阻滞比率风有效的显示了电场的真实情况。在其他情况中，同时也要考虑任何一种电场中发生通电击穿的情况。表7显示了从试验5.6.7种得出的结果，它的一到三层电场中微粒的大小等级在直径10流明到50流明之间的情况。直径10流明大小是指关于样品的90%我微粒都较大，而10%的较小。直径50流明是一个二等分的微粒大小等级，或者说关于样品中的微粒大小较大的和较小的二者比例为1：1。根据这些数据，较小的微粒倾向于集中在静电沉淀装置的第三个电场中，而较大的微粒倾向于集中在第一个电场中。这些小微粒含有最高的重金属浓度（表8）。数据显示可作为肥料的镉浓度水平在微粒大小种类方面超过小于16流明的微粒。镉浓度在D发电站中处于最高值。这是由于在与其他的发电站所用燃料的比较中，该发电站的木材燃料占较大的比重。在分馏之后，静电沉淀装置第三层电场中的灰尘中镉的浓度逐渐累积，至少要为第一层电场中的浓度的5倍。除此之外，在这些试验中，在D发电站，有效地分馏灰尘是不可能的。根据得出的结果，铅，铜，和镍的浓度从分馏的角度看并没有什么问题。至于这些金属，飞尘不经过分馏而直接作为肥料应用是可以的。锌，砷和镉的浓度可能超过允许的最大极限时，如果将飞尘 当作肥料使用就会因此引起很多的问题。对于这些金属，将飞尘作为肥料应用时，就要取决于燃料的成分和分馏的效力。表10显示了对B，C，D发电站各种不同大小的微粒的分析。基于这些数据，可以这样说，在静电沉淀装置的第一层电场中仍有粗糙的灰尘。在发电站BCD，静电沉淀装置是通过调节电流值来控制的。然而，静电沉淀装置的生产力容量取决于它的电压水平。电压和电流的比例并不是均衡对称的。当电流大小被降低50%的时候，电压仅仅会降低10-20%。而且，当电流的给定值降低时，就会有相当的更少的击穿情况。在各种实验之间，平均电压值并没有改变并达到一个足够的值是有很多原因的，以致于影响到静电沉淀装置的分离效率。这就意味着对静电沉淀装置中的电流不同的调整并没有对实验期间的重金属浓度产生很大的影响。4.结论。燃烧后产生的灰烬，它的化学性质和物理性质，还有作为最后结果的灰烬的体积，都取决于易燃燃料的成分和性质。燃烧技术和参数（相关因素），例如温度，燃烧效率，补给空气体积，还有锅炉的燃烧环境以及以灰烬的恢复防御体系，都对最终得到的结果灰烬的性质有影响。从飞尘性质的角度来看，分离灰烬的装置尤其重要，因为在灰烬的构成上，它的废气中的细颗粒部分是至关重要的，飞尘中的微粒经常被重金属富集并浓缩强化。飞尘中重金属浓度水平的较大变化范围是飞尘在实际应用中受到质疑。飞尘中含的重金属浓度会超过可作肥料使用的法定要求局限。木材燃烧灰烬中最让人质疑的重金属就是镉。在芬兰，当前的有效法令规定的极限值是3mg/kg。而木材灰烬则超过了这个极限值。在若干情况中，重金属的发射性可以通过科技加工方法来降低。这些包括瓦斯泻出体积的最小值，废气收集，空气流通的利用，原料和能源的有效利用，以及含有最小重金属浓度的原料和燃料的利用。并且，飞尘的性质可以通过分馏而被改变。分馏的目的是将高密度和重金属浓度的细小微粒部分从适合实际应用的那部分中分离出来。分馏实验证明了在静电沉淀装置第一层电场中重金属的浓度处于最低值，而在第三层中则是最高值。重金属在飞尘微粒中会得到富集和增多。含有最大颗粒的飞尘微粒会在第一层电场中积累，而在第三层电场中则主要更多是细小含尘微粒。由于这样，第一层中积累的灰尘含有较少的重金属。灰尘中的重金属浓度会受到如电压稳定程序，燃料性质，和废气的流动率的影响。镍，铅和铜是重金属，他们的浓度不超过对木材和泥煤燃灰作为肥料使用所规定的极限值。至于这些金属，在考虑到当前许可条件要求的情况下，没有必要使用分馏法。然而，分馏明显的降低了飞尘中这些金属的浓度。基于测试得到的结果，可以说，静电沉淀装置可能会增加运转循环周期阻滞的比率的特性，这样为了好的性能将会降低在分离细微颗粒方面的效果。重金属的浓度的增长是与电压的增加相联系的。这是由于高电压能够更有效的分离那些含有重金属的细微颗粒，这就意味着静电沉淀装置的分离效果可能就要受使用的最大电压值和运转循环周期阻滞的时间的影响。这些调整可能会减少电场一的分离效果，这样就使得重金属浓度的最小值能够在最终的灰烬中积累增加。相对地，重金属在细微颗粒中具有的发射性将会由于通过加强静电沉淀装置最后一层电场的效果而被减弱。 对于静电沉淀法在分馏上的参数我们并不能给出全面评价和估计。使用后的燃料中的重金属浓度能够出现很大的差距和不同，并且很难决定所要求的合适分馏方法，除非使用的问题燃料已经被加以分析过。Increase the utilisation of fly ash with electrostatic precipitationOrava Hannea,*, Nordman Timob, Kuopanportti HannuaaYTI Research Centre, Mikkeli Polytechnic, P.O. Box 181, FI-50101 Mikkeli, FinlandbUniversity of Oulu, P.O. Box 4300, FI-90014 Oulun yliopisto, FinlandReceived 28 April 2006; accepted 7 July 2006Available online 11 September 2006AbstractThe basic idea in this study is to look into the possibilities of reducing the heavy metal concentrations of fly ash by means of elec-trostatic precipitation. The utilisation of fly ash as fertiliser is hampered by its high concentrations of heavy metals, which are highlyvariable. Fly ash fractionation experiments were done using electrostatic precipitators at four power plants. Based on the results, theconcentrations of heavy metal are at their lowest in the first collector chamber and highest in the last chamber. The concentration ofcadmium in fly ash used as fertiliser can be reduced by as much as 70% by applying electrostatic precipitation fractionation. The removalof other heavy metals is not as efficient as that of cadmium. The results show that electrostatic precipitation is an adequate method in thefractionating of fly ash to be used as a fertiliser or soil amendment.? 2006 Elsevier Ltd. All rights reserved.Keywords: Electrostatic separation; Sizing; Classification; Flue dusts; Recycling1. IntroductionHeating-energy plants and power plants in Finland gen-erate approx. 400,000 tonnes of ash of biofuel origin peryear. The amounts of such ash will increase in the futureas increasing amounts of biofuels are used. Wood and peatash can be spread onto forest lands or arable land as fertil-iser or as soil improvement material, and with the purposeof adding calcium to the soil. The use of ash has been con-strained by factors such as its dust content and heavy metalconcentrations; the latter having in many cases exceededthe maximum permitted levels imposed in Finland on soilimprovement substances (Table 1).In 2001, the utilisation rate of coal ash (84%) was con-siderably higher than that of peat and mixed fuel ash(43%). Wood fly ash utilisation rates have been consider-ably lower, being about 6% in 1997.The extraction of heavy metals from fly ash could enableits more efficient utilisation. Currently, it appears thatmanipulating the power plant fuel quality is the onlymethod available for this purpose. This means that we mustknow the combustible fuels exact consistency, and whichmaterials increase the ash-contained Cd concentrations,and then screen out the cadmium containing materials.Small amounts of fly ash are used as a fertiliser both inagriculture and in forestry. Generally, various ash types aremore suitable as a soil improvement material than fertilis-ers in agriculture because the amounts of soluble plantnutrients in ash are fairly low. Peat ash is used mainly asa phosphate fertiliser and wood ash in liming of mineralsoils and as a basic and support fertiliser in the growingof cereal crops. The liming and fertiliser effects of ash inthe soil depend on the concentrations of calcium and nutri-ents in ash, on the solubility of nutrients in the ash and thesoil, and on soil properties, e.g. acidity and nutrient con-centrations (Orava et al., 2004; Silfverberg, 1996). Table2 shows the heavy metal concentrations of four ash types.Many substances contained in ash are in extremelypoorly soluble forms. As the heavy metals (e.g. cadmium,0892-6875/$ - see front matter ? 2006 Elsevier Ltd. All rights reserved.doi:10.1016/j.mineng.2006.07.002*Corresponding author.E-mail addresses: hanne.oravamikkeliamk.fi(O. Hanne), timo.nord-manoulu.fi(N. Timo), hannu.kuopanporttimikkeliamk.fi(K. Hannu).This article is also available online at:/locate/minengMinerals Engineering 19 (2006) 15961602lead, nickel) in ash are in very poorly soluble forms, it canbe assumed that ash fertilisation will not result in signifi-cant heavy-metal impacts, e.g. in water systems, within ashort period of time following fertilisation. In the longrun, harmful heavy metals may, however, be released fromash in soluble forms and be thereby translocated into thevegetation (Nieminen, 2003). The liming effect of ash low-ers the solubility of heavy metals in the soil. Ash may atfirst raise the cadmium concentration in tree stands, butonce tree growth has improved the concentrations of traceelements and heavy metals may fall even below the initiallevel. The rise in the Cd concentration in some plant speciescan last for a long time (Moilanen, 2003). Cadmium is con-sidered to be the most harmful of all heavy metals becauseit remains in the soil, it becomes enriched in food chains,and it is toxic to organisms.Electrostatic precipitation (Fig. 1) is currently the mostcommon method used in separating the solid matter frompower plant flue gases. The advantages of electrostatic pre-cipitation include high collection efficiency (as high as99.9%) and its suitability for dealing with particles of differ-ent sizes (even particle sizes below 1 lm) and variable fluegas volumes. Its further advantages are long service life,good operational reliability, and low operating and mainte-nance costs (Walsh, 1997; Immonen, 1987).The functioning of the electrostatic precipitator is signif-icantly dependent on the properties of the fly ash to be col-lected. The amount and size distribution of the particles tobe removed have a significant impact on the functioning ofthe electrostatic precipitator. Although the collection effi-ciency of electrostatic precipitator is more or less constantirrespective of the particle mass, the effective migrationvelocity is lower in the case of small particles. Due to thedifferent charging properties of the particles, the collectionefficiency of the particles varies as a function of particlesize. The most difficult particle size from the point of viewof separation is 0.20.5 lm (Nyka nen, 1993; Kouvo, 2003).The concentrations of heavy metal in ash can be reducedby fractionation of the finest ash particles from flue gasesby means of multi-chamber electrostatic precipitators.The fractionating properties of the precipitator can beinfluenced by actions such as restricting and pulsating thecurrent. Our research results have shown that heavy metalsare concentrated in fine ash particles (Orava et al., 2004;Orava, 2003). According to the results of Thun and Korho-nen(1999),the3-fieldelectrostaticprecipitatorwasstopped, depending on the operating conditions, 8495%of the overall amount of ash in the first field, 415% inthe second field, and approx. 1% in the last field. The cad-mium concentration of ash can be reduced at least by 1525% by means of fractionating the ash using electrostaticprecipitators (Orava et al., 2004; Thun and Korhonen,1999).Depending on the boiler in question, ashes from barkfuelled and wood chip fuelled power plants (grate boilers)are divided into weight percentage categories as follows:bottom ash 7090%, cyclone fly ash 1030%, electrostaticprecipitator fly ash 28% and dust emissions 0.13.0%(Agarwal and Agarwal, 1999). In dust combustion and flu-idized-bed combustion, the share of fly ash generation is80100%. As much as 7590% of the heavy metals (Cdand Zn) are contained in the fine particle fraction of thefly ash, which is separated by electrostatic precipitators(Dahl et al.). Fig. 2 sets out the zinc, lead and cadmiumcontents (mg/kg and m-%) in bottom ash, cyclone ashand electrostatic precipitator fly ash. Based on the figure,Table 2Heavy metal concentrations of various ash types (Palola, 1998)Heavy metals (mg/kg)Coal ashPeat ashWood ashBark ashArsenic (As)2.320022000.260728Cadmium (Cd)0.012500.0580.440420Chrome (Cr)3.6740015250152504081Copper (Cu)303000204001530057144Mercury (Hg)0.01800.00110.0210.0120.4Nickel (Ni)1.880015200202503652Lead (Pb)3.11800515015100053140Zinc (Zn)1413,000106001510,00011005100Fig. 1. Alstom Finland Oys electrostatic precipitator (Jalovaara et al.,2003).Table 1Maximum permitted concentrations of heavy metals in soil improvementmaterials and in fly ash from power plant A (Orava, 2003)ElementPower plant A: fly ash (mg/kg)Maximumpermittedconcentration(mg/kg)Year 2002Year 2001Year 1999Mercury (Hg)2.50.312.0Cadmium (Cd)2.695.056.33.0Arsenic (As)18.3419.733550Nickel (Ni)100Lead (Pb)56.59106.552.7150Copper (Cu)86.7290.1178600Zinc (Zn)189.7376.97061500O. Hanne et al. / Minerals Engineering 19 (2006) 159616021597it may be stated, for example, that electrostatic precipitatorfly ash has a higher Cd content than cyclone ash, which ispartly due to the fact that, compared to cyclones, electro-static precipitators separate smaller particles that containthe majority of heavy metals. In this case, the portion ofthe fly ash that is suitable for use as a fertiliser, in termsof its consistency, remains at the cyclone (Obernbergerand Biedermann, 1997).The electrostatic precipitator can more effectively frac-tionate fly ash than the traditional methods when amechanical classifier (cyclone) is connected before the ashreaches the precipitator. Fig. 3 shows a basic layout draw-ing of a power plant fired by using biofuels and which isprovided with a multi-cyclone before the electrostatic pre-cipitator. As much as 7590% of the heavy metals (Cdand Zn) contained in fly ash are bound to the fine fly ashfraction separated by the electrostatic precipitator (Dahlet al., 2002).Properly designed and adjusted electrostatic precipita-tion is in principle, capable of separating that fraction ofthe flue gases, which contains the greatest amount of heavymetals but only a fraction of the overall amount of ash.The heavy metal concentrations in the main part of theash can thus be reduced to below the maximum permittedconcentrations.2. Materials and methodsThe fractionating trials with fly ash were performed atfour power plants (A, B, C and D). The electrostatic pre-cipitators were operated at the power plants at differentvoltage levels and samples were taken from the ESPs var-ious fields. All the samples taken from the electrostatic pre-cipitators were taken from the ash feeders located underthe electrostatic precipitators before the ash was fed intothe silo. The samples were analysed for the presence ofPb, Cu, Zn, Ni, As and Cd using the graphite methodand particle size determination was done using a Malverndevice.Power plant A uses peat, forest chip and oil and the by-products of the mechanical wood processing industry as itsfuels. The boiler capacity available to the power plant is150 MW. The fractionating trials were performed withthe power plants current 3-field electrostatic precipitator.Power plant B uses two boilers, one a Pyroflow circulat-ing fluidized-bed boiler (capacity 55 MW) and the other afluidized-bed boiler (capacity 42 MW). The trials were car-ried out using the fluidized-bed boiler. The power plantsprincipal fuel is milled peat with wood fuels, soot and alu-minium oxide mixed in with it. The fly ash from both boil-ers is conveyed via 2-field electrostatic precipitators to acommon ash silo.Power plant C is equipped with two power plant boilers.Boiler 1 is a fluidized-bed boiler with a fuel capacity of267 MW. Boiler 2 is a Pyroflow circulating fluidized-bedboiler with a fuel capacity of 315 MW. The tests were per-formed using the Pyroflow circulating fluidized-bed boiler.The fuels used at the power plant were mainly milled peatand various wood fuels. Both boilers are equipped with 3-field electrostatic precipitators from which the boilers flyash is blown pneumatically to a common ash silo.The electrical power generated by power plant Ds fluid-ized-bed boiler plant is 77 MW and its heating capacity is246 MW. The fuels used in the fluidized-bed boiler areFig. 2. Heavy metal concentrations and their division as bulk percentagefigures in bottom ash, cyclone fly ash and filter fly ash (Agarwal andAgarwal).Fig. 3. The ash fractions produced by a biofuel-fired power plant (Agarwal and Agarwal).1598O. Hanne et al. / Minerals Engineering 19 (2006) 15961602mainly milled peat and wood waste. The fly ash is sepa-rated from the flue gases by means of a 3-field electrostaticprecipitator.3. ResultsDuring trials with power plant As electrostatic precipi-tator (trials 17) the fuel used was composed 49% peat and51% wood fuels. The ash funnels of fields 13 of the elec-trostatic precipitator were sampled and analysed (Fig. 4).On the basis of the results, the Cd concentration was atits lowest in field 1 of the electrostatic precipitator and at itshighest in field 3. This is due to the bigger fly ash particlesaccumulating in field 1 and field 3 containing ash with thesmallest particles. The Cd concentration in field 1 varieswithin the range of 2.23.6 mg/kg and in the last fieldwithin the range of 7.212.4 mg/kg. Concentrations areaffected by properties such as the ESP voltage, fuel quality,and flue gas flow rate. In almost every electrostatic precip-itators field 1 the Cd concentration is below the permittedmaximum limit (3.0 mg/kg) set down for ash intended forfertiliser use.During trial runs, the electrostatic precipitator fieldsCBO ratio (cycle block in operation) was controlled withinthe range 012. The value 0 means that all the half-cycles ofthe field in question are currently active, and for exam-ple, the value 2 means that only a third of the half-cyclesare active. Thereby the CBO value declares how manysequential half-cycles are closed, that is, how often the sep-arators supply current is pulsated. In the present research,the CBO value was controlled by the Micro-Kraft control-ler whose main task is to keep the voltage near to thebreakdown voltage.The most important thing is to be able to influence andchange the properties of field 1 in the electrostatic precipi-tator. The first field enables the production of fly ash withheavy metal concentration levels that make it suitable as afertiliser, for example.Fig. 5 shows how the electrostatic precipitators CBOratio control for field 1 affects the fly ash Cd concentrationlevels. Among other things, the increasing or decreasingnumber of electrostatic precipitator field flashovers isexcluded from this control.The more field half-cycles there are deactivated, thelower the heavy metal content of the fly ash being gener-ated. Cd concentration variations are also caused by fuelquality variations, in addition to the control itself, amongother factors.Fig. 6 shows how the electrostatic precipitators filtervoltage affects the Cd concentration level of field 1. Theheavy metal concentration level increases in accordancewith the rising voltage. This is due to the fact that highervoltages can more effectively separate fine particles thatalso contain heavy metals. The filter voltage level indicatesthe fields actual status more effectively than does the CBOratio. Among other things, it also takes into account anyelectric breakdowns that occur within the field.Fig. 7 shows the particle size classes (lm) D10 and D50of fields 13 resulting from trials 5, 6 and 7. D10 is a par-ticle size with respect to which 90% of the samples particlesare larger and 10% are smaller. D50 is a halving particlesize class, or the particle size with respect to which samplesparticles are larger and smaller in the ratio of 50/50.On the basis of this figure, the smaller-sized particlestend to be concentrated in the last field and the bigger-sizedparticles in the first field of the electrostatic precipitator.024681012141234567Cd mg/kgField 1Field 2Field 3Fig. 4. Cd concentrations (mg/kg) of the fly ash in fields (13) during trialruns (17) with peat fuel when using the electrostatic precipitator.2.833.23.4024681012CBO-ratioCd mg/kgFig. 5. The effect of the CBO ratio from electrostatic precipitator field 1on fly ash Cd concentrations (mg/kg) during trial runs with peat fuel.2.83.84253035404550Voltage kVCd mg/kgFig. 6. The effect of the filter voltage level (kV) from electrostaticprecipitator field 1 on fly ash Cd concentrations (mg/kg) during trial runswith peat fuel.O. Hanne et al. / Minerals Engineering 19 (2006) 159616021599These small particles have the highest concentrations ofheavy metals (Fig. 8). The figure shows that the permissibleCd concentration level for use as a fertiliser is exceeded inparticle size category 16 lm.The cadmium concentrations were at their highest atpower plant D (Fig. 9). This was due to the bigger shareof wood fuel in the fuel when compared to the other powerplants. Following fractionation, the cadmium concentra-tion of the ash accumulated in field 3 of the electrostaticprecipitator was at best five fold compared to field 1.Despite this, in these tests it was not possible to effectivelyfractionate the ash at power plant D.According to the results the Pb, Cu and Ni concentra-tions are not problematic from the viewpoint of fraction-ation. With respect to these metals, fly ash can be used asa fertiliser without fractionation being necessary. Zn, Asand Cd concentrations may exceed the permitted limit val-ues and thereby cause problems for the fertiliser use of flyash. With respect to these metals, the use of the fly ash as afertiliser depends on the composition of the fuel and on theeffectiveness of fractionation.Fig. 10 shows the analysed particle sizes from variousblocks at power plants B, C and D. Based on this figure,it may be stated the coarsest dust remains in electrostaticprecipitator field 1.At power plant B, C and D, the electrostatic precipitatorwas controlled by adjusting the current value. Nevertheless,the electrostatic precipitators productive capacity dependson its voltage level. The proportion between the voltageand the current is not symmetric. When the current levelwas reduced by 50%, the voltage level merely decreasedby 1020%. In addition, there were considerably fewerbreakdowns when the currents setpoint value was reduced.These were the reasons why the average voltage value didnot change to a sufficient degree between the various tests,so as to affect the electrostatic precipitators separation effi-ciency. This means that the various current adjustmentsmade to the electrostatic precipitator did not have a signif-icant effect on heavy metal concentrations during the trialruns.4. ConclusionsThe chemical and physical properties of combustion-generated ash, as well as the resulting ash volumes, dependon the combustible fuels composition and quality. Thecombustion technology and the parameters applied, suchas the temperature, combustion rate and air supply vol-umes, plus the boiler condition and the ash recovery sys-tems also contribute to the resulting ash quality. In viewof the fly ash properties, the ash separation equipment is0102030405060BBBCCCDDDD50 m Field 1Field 2Field 3SiloFig. 10. Fly ash particle sizes in various blocks of the electrostaticprecipitator, particle size category (lm) D50.051015202530D10D50D10D50D10D50mField 1Field 2Field 3Fig. 7. The particle size classes (lm) D10 and D50 of the fly ash in theelectrostatic precipitators fields 13.2345678611162126The particle size class D50 mCd mg/kgFig. 8. Cd concentration levels (mg/kg) and particle size (lm) D50 duringtrial runs with peat fuel.0510152025BBBCCCDDDCd mg/kgField 1Field 2Field 3SiloFig. 9. Cd concentrations of the fly ash in the various fields of theelectrostatic precipitator (limit value 3 mg/kg) at power plant B, C and D.1600O. Hanne et al. / Minerals Engineering 19 (2006) 15961602of particular importance since its flue gas borne finest frac-tion is the crucial one, with regards to the ash composition.The fine particles of fly ash are frequently enrichedby heavy metals. The extensive variation of its heavymetal concentration level makes fly ash problematic touse in practice. Fly ash may contain heavy metal concen-trations that exceed the statutory limits for its use as a fer-tiliser. The most problematic heavy metal in wood-basedfly ash is cadmium. In Finland, its currently valid statutorylimit value is 3 mg/kg. Wood-based ash exceeds this limitvalue.In several cases, heavy metal emissions may be reducedby process technological methods. These include the mini-misation of gas emission volumes, waste gas collection, theuse of air recirculation, efficient use of raw materials andenergy, and the use of raw materials and fuels with minimalheavy metal concentrations. In addition, the quality of flyash may be manipulated through fractionation. Fraction-ation is geared towards separating the fractions with highfine particle densities and high heavy metal concentrationsfrom the portion that is suitable for practical applications.The fractionation trials demonstrated that the heavymetal concentrations are at their lowest in field 1 of theelectrostatic precipitator and at their highest in field 3.The heavy metals are enriched in the small particles ofthe ash. The fly ash particles having the biggest particle sizeaccumulate in the first field while the last field has more ofthe ash-containing small particles. Due to this, the dustaccumulated in field 1 contains less heavy metal. The con-centrations of heavy metals in the ash are affected by prop-erties such as the ESP voltage, fuel quality, and flue gasflow rate.Ni, Pb and Cu are the heavy metals, whose concentra-tions do not exceed the limit values imposed on woodand peat ash intended to be used as fertiliser. Fractionationis not necessary with respect to these metals considering thepresent permit conditions. However, fractionation clearlylowered the fly ash concentrations of these metals.Zn, Cd and As are the metals, whose concentrationsmay exceed the limit values. Cd and Zn can cause problemswhen burning wood whereas the problem in peat burning isAs. However, the concentrations of these metals signifi-cantly depend on the fuel being used. Especially in the caseof wood, Cd and Zn concentrations vary a great deal. Inregard to Zn, fractionation always resulted in concentra-tions below the permit limit values. Cd was the most prob-lematic metal of all and it was not always possible to bringits concentrations below maximum permitted concentra-tions even after making adjustments to the electrostaticprecipitators settings. The cadmium concentrations ofash due to be used as a fertiliser can be reduced by as muchas 70% by fractionating the ash using an electrostatic pre-cipitator. The concentrations of the other heavy metals donot diminish quite as much. Arsenic concentrations werealways brought to levels below the maximum permittedlimits, but making adjustments to the electrostatic precipi-tator may be necessary if the arsenic concentration of thepeat is high. If various by-products formed in industry orwastes are used as fuels, there is no single overall assess-ment available regarding the concentrations of heavymetals.Based on the test results obtained, it may be stated thatelectrostatic precipitators increasing CBO ratio values willdecrease the separation efficiency in the case of fine parti-cles. The heavy metal concentration level increases inaccordance with the rising voltage. This is due to the factthat higher voltages can more effectively separate fineparticles that also contain heavy metals. This means thatelectrostatic precipitators separation efficiency may beinfluenced by adjusting their maximum voltage setpointvalues and CBO ratio values. These adjustments maydecrease the efficiency of field 1, thus enabling the minimi-sation of heavy metal concentrations accumulating in theresulting ash. Correspondingly, heavy metal emissions withfine particle concentrations may be reduced by making theelectrostatic precipitators final field more effective.No overall values can be given for the parameters ofelectrostatic precipitators used in fract
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