環(huán)保鋼結(jié)構(gòu)的設(shè)計與制造——袋式除塵器成套設(shè)備【說明書+CAD】

環(huán)保鋼結(jié)構(gòu)的設(shè)計與制造——袋式除塵器成套設(shè)備【說明書+CAD】,說明書+CAD,環(huán)保鋼結(jié)構(gòu)的設(shè)計與制造——袋式除塵器成套設(shè)備【說明書+CAD】,環(huán)保,鋼結(jié)構(gòu),設(shè)計,制造,除塵器,成套設(shè)備,說明書,仿單,cad 11 concentrations of heavy metal are at their lowest in the first collector chamber and highest in the last chamber. The concentration of cadmium in fly ash used as fertiliser can be reduced by as much as 70% by applying electrostatic precipitation fractionation. The removal of other heavy metals is not as ecient as that of cadmium. The results show that electrostatic precipitation is an adequate method in the as increasing amounts of biofuels are used. Wood and peat ash 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 the soil, and on soil properties, e.g. acidity and nutrient con- centrations (Orava et al., 2004; Silfverberg, 1996). Table 2 shows the heavy metal concentrations of four ash types. Many substances contained in ash are in extremely poorly 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 purpose of adding calcium to the soil. The use of ash has been con- strained by factors such as its dust content and heavy metal concentrations; the latter having in many cases exceeded the maximum permitted levels imposed in Finland on soil improvement 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 in agriculture and in forestry. Generally, various ash types are more suitable as a soil improvement material than fertilis- ers in agriculture because the amounts of soluble plant nutrients in ash are fairly low. Peat ash is used mainly as a phosphate fertiliser and wood ash in liming of mineral soils and as a basic and support fertiliser in the growing of cereal crops. The liming and fertiliser eects of ash in the 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; Recycling 1. Introduction Heating-energy plants and power plants in Finland gen- erate approx. 400,000 tonnes of ash of biofuel origin per year. The amounts of such ash will increase in the future The extraction of heavy metals from fly ash could enable its more ecient utilisation. Currently, it appears that manipulating the power plant fuel quality is the only method available for this purpose. This means that we must know the combustible fuels exact consistency, and which Increase the utilisation of fly ash Orava Hanne a, * , Nordman Timo a YTI Research Centre, Mikkeli Polytechnic, b University of Oulu, P.O. Box 4300, Received 28 April 2006; Available online Abstract The basic idea in this study is to look into the possibilities of reducing trostatic precipitation. The utilisation of fly ash as fertiliser is hampered variable. Fly ash fractionation experiments were done using electrostatic 0892-6875/$ - see front matter C211 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2006.07.002 with electrostatic precipitation b , Kuopanportti Hannu a P.O. Box 181, FI-50101 Mikkeli, Finland FI-90014 Oulun yliopisto, Finland accepted 7 July 2006 September 2006 the heavy metal concentrations of fly ash by means of elec- by its high concentrations of heavy metals, which are highly precipitators at four power plants. Based on the results, the This article is also available online at: 15961602 lead, nickel) in ash are in very poorly soluble forms, it can Table 2 Heavy metal concentrations of various ash types (Palola, 1998) Heavy metals (mg/kg) Coal ash Peat ash Wood ash Bark ash Arsenic (As) 2.3200 2200 0.260 728 Cadmium (Cd) 0.01250 0.058 0.440 420 Chrome (Cr) 3.67400 15250 15250 4081 Copper (Cu) 303000 20400 15300 57144 Mercury (Hg) 0.0180 0.0011 0.021 0.0120.4 Nickel (Ni) 1.8800 15200 20250 3652 Lead (Pb) 3.11800 5150 151000 53140 Zinc (Zn) 1413,000 10600 1510,000 11005100 Table 1 Maximum permitted concentrations of heavy metals in soil improvement materials and in fly ash from power plant A (Orava, 2003) Element Power plant A: fly ash (mg/kg) Maximum permitted concentration (mg/kg) Year 2002 Year 2001 Year 1999 Mercury (Hg) 2.5 0.31 2.0 Cadmium (Cd) 2.69 5.05 6.3 3.0 Arsenic (As) 18.34 19.73 35 50 Nickel (Ni) 100 Lead (Pb) 56.59 106.5 52.7 150 Copper (Cu) 86.7 290.1 178 600 Zinc (Zn) 189.7 376.9 706 1500 O. Hanne et al. / Minerals Engineerin be assumed that ash fertilisation will not result in signifi- cant heavy-metal impacts, e.g. in water systems, within a short period of time following fertilisation. In the long run, harmful heavy metals may, however, be released from ash in soluble forms and be thereby translocated into the vegetation (Nieminen, 2003). The liming eect of ash low- ers the solubility of heavy metals in the soil. Ash may at first raise the cadmium concentration in tree stands, but once tree growth has improved the concentrations of trace elements and heavy metals may fall even below the initial level. The rise in the Cd concentration in some plant species can last for a long time (Moilanen, 2003). Cadmium is con- sidered to be the most harmful of all heavy metals because it remains in the soil, it becomes enriched in food chains, and it is toxic to organisms. Electrostatic precipitation (Fig. 1) is currently the most common method used in separating the solid matter from power plant flue gases. The advantages of electrostatic pre- cipitation include high collection eciency (as high as 99.9%) and its suitability for dealing with particles of dier- ent sizes (even particle sizes below 1 lm) and variable flue gas 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 to be removed have a significant impact on the functioning of the electrostatic precipitator. Although the collection e- ciency of electrostatic precipitator is more or less constant irrespective of the particle mass, the eective migration velocity is lower in the case of small particles. Due to the dierent charging properties of the particles, the collection eciency of the particles varies as a function of particle size. The most dicult particle size from the point of view of separation is 0.20.5 lm(Nykanen, 1993; Kouvo, 2003). The concentrations of heavy metal in ash can be reduced by fractionation of the finest ash particles from flue gases by means of multi-chamber electrostatic precipitators. The fractionating properties of the precipitator can be influenced by actions such as restricting and pulsating the current. Our research results have shown that heavy metals Fig. 1. Alstom Finland Oys electrostatic precipitator (Jalovaara et al., 2003). g 19 (2006) 15961602 1597 are 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 was stopped, depending on the operating conditions, 8495% of the overall amount of ash in the first field, 415% in the second field, and approx. 1% in the last field. The cad- mium concentration of ash can be reduced at least by 15 25% by means of fractionating the ash using electrostatic precipitators (Orava et al., 2004; Thun and Korhonen, 1999). Depending on the boiler in question, ashes from bark fuelled and wood chip fuelled power plants (grate boilers) are divided into weight percentage categories as follows: bottom ash 7090%, cyclone fly ash 1030%, electrostatic precipitator 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 is 80100%. As much as 7590% of the heavy metals (Cd and Zn) are contained in the fine particle fraction of the fly ash, which is separated by electrostatic precipitators (Dahl et al.). Fig. 2 sets out the zinc, lead and cadmium contents (mg/kg and m-%) in bottom ash, cyclone ash and electrostatic precipitator fly ash. Based on the figure, ash can thus be reduced to below the maximum permitted concentrations. 2. Materials and methods The fractionating trials with fly ash were performed at four power plants (A, B, C and D). The electrostatic pre- cipitators were operated at the power plants at dierent voltage 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 under the electrostatic precipitators before the ash was fed into the silo. The samples were analysed for the presence of Pb, Cu, Zn, Ni, As and Cd using the graphite method 1598 O. Hanne et al. / Minerals Engineering 19 (2006) 15961602 it may be stated, for example, that electrostatic precipitator fly ash has a higher Cd content than cyclone ash, which is partly due to the fact that, compared to cyclones, electro- static precipitators separate smaller particles that contain the majority of heavy metals. In this case, the portion of the fly ash that is suitable for use as a fertiliser, in terms of its consistency, remains at the cyclone (Obernberger and Biedermann, 1997). The electrostatic precipitator can more eectively frac- tionate fly ash than the traditional methods when a mechanical classifier (cyclone) is connected before the ash reaches the precipitator. Fig. 3 shows a basic layout draw- ing of a power plant fired by using biofuels and which is provided with a multi-cyclone before the electrostatic pre- cipitator. As much as 7590% of the heavy metals (Cd and Zn) contained in fly ash are bound to the fine fly ash fraction separated by the electrostatic precipitator (Dahl et al., 2002). Properly designed and adjusted electrostatic precipita- Fig. 2. Heavy metal concentrations and their division as bulk percentage figures in bottom ash, cyclone fly ash and filter fly ash (Agarwal and Agarwal). tion is in principle, capable of separating that fraction of the flue gases, which contains the greatest amount of heavy metals but only a fraction of the overall amount of ash. The heavy metal concentrations in the main part of the Fig. 3. The ash fractions produced by a biofuel-fired and particle size determination was done using a Malvern device. Power plant A uses peat, forest chip and oil and the by- products of the mechanical wood processing industry as its fuels. The boiler capacity available to the power plant is 150 MW. The fractionating trials were performed with the 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 a fluidized-bed boiler (capacity 42 MW). The trials were car- ried out using the fluidized-bed boiler. The power plants principal 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 a common ash silo. Power plant C is equipped with two power plant boilers. Boiler 1 is a fluidized-bed boiler with a fuel capacity of 267 MW. Boiler 2 is a Pyroflow circulating fluidized-bed boiler 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 peat and various wood fuels. Both boilers are equipped with 3- field electrostatic precipitators from which the boilers fly ash 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 is 246 MW. The fuels used in the fluidized-bed boiler are power 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 electrostatic precipitator. 3. Results During trials with power plant As electrostatic precipi- tator (trials 17) the fuel used was composed 49% peat and 51% 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 at its lowest in field 1 of the electrostatic precipitator and at its highest in field 3. This is due to the bigger fly ash particles accumulating in field 1 and field 3 containing ash with the particles in the first field of the electrostatic precipitator. 2 2.2 2.4 2.6 2.8 3 3.2 3.4 024681012 CBO-ratio Cd mg/kg Fig. 5. The eect of the CBO ratio from electrostatic precipitator field 1 on fly ash Cd concentrations (mg/kg) during trial runs with peat fuel. 2 2.2 2.4 2.6 2.8 3 3.2 3.4 3.6 3.8 4 25 30 35 40 45 50 Voltage kV Cd mg/kg Fig. 6. The eect of the filter voltage level (kV) from electrostatic O. Hanne et al. / Minerals Engineering 19 (2006) 15961602 1599 smallest particles. The Cd concentration in field 1 varies within the range of 2.23.6 mg/kg and in the last field within the range of 7.212.4 mg/kg. Concentrations are aected 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 permitted maximum limit (3.0 mg/kg) set down for ash intended for fertiliser use. During trial runs, the electrostatic precipitator fields CBO ratio (cycle block in operation) was controlled within the range 012. The value 0 means that all the half-cycles of the field in question are currently active, and for exam- ple, the value 2 means that only a third of the half-cycles are active. Thereby the CBO value declares how many sequential 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 the breakdown voltage. The most important thing is to be able to influence and change the properties of field 1 in the electrostatic precipi- tator. The first field enables the production of fly ash with heavy metal concentration levels that make it suitable as a fertiliser, for example. Fig. 5 shows how the electrostatic precipitators CBO ratio control for field 1 aects the fly ash Cd concentration levels. Among other things, the increasing or decreasing 0 2 4 6 8 10 12 14 1234567 Cd mg/kg Field 1 Field 2 Field 3 Fig. 4. Cd concentrations (mg/kg) of the fly ash in fields (13) during trial runs (17) with peat fuel when using the electrostatic precipitator. number of electrostatic precipitator field flashovers is excluded from this control. The more field half-cycles there are deactivated, the lower the heavy metal content of the fly ash being gener- ated. Cd concentration variations are also caused by fuel quality variations, in addition to the control itself, among other factors. Fig. 6 shows how the electrostatic precipitators filter voltage aects the Cd concentration level of field 1. The heavy metal concentration level increases in accordance with the rising voltage. This is due to the fact that higher voltages can more eectively separate fine particles that also contain heavy metals. The filter voltage level indicates the fields actual status more eectively than does the CBO ratio. Among other things, it also takes into account any electric breakdowns that occur within the field. Fig. 7 shows the particle size classes (lm) D10 and D50 of fields 13 resulting from trials 5, 6 and 7. D10 is a par- ticle size with respect to which 90% of the samples particles are larger and 10% are smaller. D50 is a halving particle size class, or the particle size with respect to which samples particles are larger and smaller in the ratio of 50/50. On the basis of this figure, the smaller-sized particles tend to be concentrated in the last field and the bigger-sized precipitator field 1 on fly ash Cd concentrations (mg/kg) during trial runs with peat fuel. These small particles have the highest concentrations of heavy metals (Fig. 8). The figure shows that the permissible Cd concentration level for use as a fertiliser is exceeded in particle size category 16 lm. The cadmium concentrations were at their highest at power plant D (Fig. 9). This was due to the bigger share of wood fuel in the fuel when compared to the other power plants. Following fractionation, the cadmium concentra- tion of the ash accumulated in field 3 of the electrostatic precipitator was at best five fold compared to field 1. 0 5 10 15 20 25 30 D10 D50 D10 D50 D10 D50 m Field 1 Field 2 Field 3 Fig. 7. The particle size classes (lm) D10 and D50 of the fly ash in the electrostatic precipitators fields 13. 1600 O. Hanne et al. / Minerals Engineerin 2 3 4 5 6 7 8 6 11162126 Cd mg/kg The particle size class D50 m Fig. 8. Cd concentration levels (mg/kg) and particle size (lm) D50 during trial runs with peat fuel. 0 5 10 15 20 25 BBBCCCDDD Cd mg/kg Field 1 Field 2 Field 3 Silo Fig. 9. Cd concentrations of the fly ash in the various fields of the electrostatic precipitator (limit value 3 mg/kg) at power plant B, C and D. Despite this, in these tests it was not possible to eectively fractionate 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 as a fertiliser without fractionation being necessary. Zn, As and Cd concentrations may exceed the permitted limit val- ues and thereby cause problems for the fertiliser use of fly ash. With respect to these metals, the use of the fly ash as a fertiliser depends on the composition of the fuel and on the eectiveness of fractionation. Fig. 10 shows the analysed particle sizes from various blocks at power plants B, C and D. Based on this figure, it may be stated the coarsest dust remains in electrostatic precipitator field 1. At power plant B, C and D, the electrostatic precipitator was controlled by adjusting the current value. Nevertheless, the electrostatic precipitators productive capacity depends on its voltage level. The proportion between the voltage and the current is not symmetric. When the current level was reduced by 50%, the voltage level merely decreased by 1020%. In addition, there were considerably fewer breakdowns when the currents setpoint value was reduced. 0 10 20 30 40 50 60 BBBCCCDDD D50 m Field 1 Field 2 Field 3 Silo Fig. 10. Fly ash particle sizes in various blocks of the electrostatic precipitator, particle size category (lm) D50. g 19 (2006) 15961602 These were the reasons why the average voltage value did not change to a sucient degree between the various tests, so as to aect the electrostatic precipitators separation e- ciency. This means that the various current adjustments made to the electrostatic precipitator did not have a signif- icant eect on heavy metal concentrations during the trial runs. 4. Conclusions The chemical and physical properties of combustion- generated ash, as well as the resulting ash volumes, depend on the combustible fuels composition and quality. The combustion technology and the parameters applied, such as 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 view of the fly ash properties, the ash separation equipment is of particular importance since its flue gas borne finest frac- tion is the crucial one, with regards to the ash composition. The fin