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11concentrations 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 ecient as that of cadmium. The results show that electrostatic precipitation is an adequate method in theas increasing amounts of biofuels are used. Wood and peatash can be spread onto forest lands or arable land as fertil-ably lower, being about 6% in 1997.materials increase the ash-contained Cd concentrations,and then screen out the cadmium containing materials.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,*Corresponding author.E-mail addresses: hanne.oravamikkeliamk.fi (O. Hanne), timo.nord-manoulu.fi (N. Timo), hannu.kuopanporttimikkeliamk.fi (K. Hannu).Minerals Engineering 19 (2006)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-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 eects of ash inthe soil depend on the concentrations of calcium and nutri-fractionating of fly ash to be used as a fertiliser or soil amendment.C211 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 futureThe extraction of heavy metals from fly ash could enableits more ecient 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 whichIncrease the utilisation of fly ashOrava Hannea,*, Nordman TimoaYTI Research Centre, Mikkeli Polytechnic,bUniversity of Oulu, P.O. Box 4300,Received 28 April 2006;Available onlineAbstractThe basic idea in this study is to look into the possibilities of reducingtrostatic precipitation. The utilisation of fly ash as fertiliser is hamperedvariable. Fly ash fractionation experiments were done using electrostatic0892-6875/$ - see front matter C211 2006 Elsevier Ltd. All rights reserved.doi:10.1016/j.mineng.2006.07.002with electrostatic precipitationb, Kuopanportti HannuaP.O. Box 181, FI-50101 Mikkeli, FinlandFI-90014 Oulun yliopisto, Finlandaccepted 7 July 2006September 2006the heavy metal concentrations of fly ash by means of elec-by its high concentrations of heavy metals, which are highlyprecipitators at four power plants. Based on the results, theThis article is also available online at:/locate/mineng15961602lead, nickel) in ash are in very poorly soluble forms, it canTable 2Heavy metal concentrations of various ash types (Palola, 1998)Heavy metals (mg/kg) Coal ash Peat ash Wood ash Bark ashArsenic (As) 2.3200 2200 0.260 728Cadmium (Cd) 0.01250 0.058 0.440 420Chrome (Cr) 3.67400 15250 15250 4081Copper (Cu) 303000 20400 15300 57144Mercury (Hg) 0.0180 0.0011 0.021 0.0120.4Nickel (Ni) 1.8800 15200 20250 3652Lead (Pb) 3.11800 5150 151000 53140Zinc (Zn) 1413,000 10600 1510,000 11005100Table 1Maximum permitted concentrations of heavy metals in soil improvementmaterials and in fly ash from power plant A (Orava, 2003)Element Power plant A: fly ash (mg/kg) Maximumpermittedconcentration(mg/kg)Year 2002 Year 2001 Year 1999Mercury (Hg) 2.5 0.31 2.0Cadmium (Cd) 2.69 5.05 6.3 3.0Arsenic (As) 18.34 19.73 35 50Nickel (Ni) 100Lead (Pb) 56.59 106.5 52.7 150Copper (Cu) 86.7 290.1 178 600Zinc (Zn) 189.7 376.9 706 1500O. Hanne et al. / Minerals Engineerinbe 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 eect 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 eciency (as high as99.9%) and its suitability for dealing with particles of dier-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 e-ciency of electrostatic precipitator is more or less constantirrespective of the particle mass, the eective migrationvelocity is lower in the case of small particles. Due to thedierent charging properties of the particles, the collectioneciency of the particles varies as a function of particlesize. The most dicult particle size from the point of viewof separation is 0.20.5 lm(Nykanen, 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 metalsFig. 1. Alstom Finland Oys electrostatic precipitator (Jalovaara et al.,2003).g 19 (2006) 15961602 1597are concentrated in fine ash particles (Orava et al., 2004;Orava, 2003). According to the results of Thun and Korho-nen (1999), the 3-field electrostatic precipitator wasstopped, 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,ash 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 dierentvoltage 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 method1598 O. Hanne et al. / Minerals Engineering 19 (2006) 15961602it 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 eectively 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-Fig. 2. Heavy metal concentrations and their division as bulk percentagefigures in bottom ash, cyclone fly ash and filter fly ash (Agarwal andAgarwal).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 theFig. 3. The ash fractions produced by a biofuel-firedand 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 arepower plant (Agarwal and Agarwal).mainly 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 theparticles in the first field of the electrostatic precipitator.2.833.23.4024681012CBO-ratioCd mg/kgFig. 5. The eect of the CBO ratio from electrostatic precipitator field 1on fly ash Cd concentrations (mg/kg) during trial runs with peat fuel.2.83.8425 30 35 40 45 50Voltage kVCd mg/kgFig. 6. The eect of the filter voltage level (kV) from electrostaticO. Hanne et al. / Minerals Engineering 19 (2006) 15961602 1599smallest 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 areaected 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 aects the fly ash Cd concentrationlevels. Among other things, the increasing or decreasing024681012141234567Cd 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.number 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 aects 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 eectively separate fine particles thatalso contain heavy metals. The filter voltage level indicatesthe fields actual status more eectively 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-sizedprecipitator field 1 on fly ash Cd concentrations (mg/kg) during trial runswith peat fuel.These 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 1.051015202530D10 D50 D10 D50 D10 D50mField 1Field 2Field 3Fig. 7. The particle size classes (lm) D10 and D50 of the fly ash in theelectrostatic precipitators fields 13.1600 O. Hanne et al. / Minerals Engineerin23456786 11162126Cd mg/kgThe particle size class D50 mFig. 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.Despite this, in these tests it was not possible to eectivelyfractionate 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 theeectiveness of fractionation.Fig. 10 shows the analysed particle sizes from vario