Environ. Sci. Technol. 1999, 33, 496-502
Volatilization of the Heavy Metals during Circulating Fluidized Bed Combustion of Forest Residue T E R T T A L I I S A L I N D , * ,† TUOMAS VALMARI,‡ ESKO I. KAUPPINEN,† GEORGE SFIRIS,§ KRISTINA NILSSON,§ AND WILLY MAENHAUT| VTT Chemical Technology, Aerosol Technology Group, P.O. Box 1401, FIN-02044 Espoo, Finland, VTT Energy, Aerosol Technology Group, Espoo, Finland, Vattenfall Utveckling AB, Stockholm, Sweden, and University of Gent, Gent, Belgium
The environmental impact of the heavy metals contained in the combustion product ash depends on the speciation of the heavy metals and the size distributions of the heavy metals in the ash. Therefore, the behavior of cadmium, lead, copper, and zinc was studied experimentally during circulating fluidized bed combustion (CFBC) of Swedish forest residue. The size distributions and concentrations of the heavy metals in the fly ash particles and in the gas phase were determined by low-pressure impactors and filters upstream of the convective back pass at 830 °C. Downstream of the convective back pass at T ) 150 °C, the size distributions were determined. The fly ash from CFBC was found to contain two separate particle classes. Fine particles (Dp < 0.5 µm) consisted mainly of KCl, and coarse particles (Dp > 0.5 µm) contained as major elements Ca and Si. Major fraction of all the studied heavy metals were found in the coarse fly ash particles at location 1 at 830 °C; 7-26% of Pb, 24-27% of Cu, 1-8% of Cd, and less than 1% of Zn were found in the gas phase. The gas-to-particle conversion route for Cd, Pb, and Cu was found by chemical surface reaction, probably with silicates. None of the studied heavy metals were enriched in the fine particles at the inlet of the electrostatic precipitator.
Introduction Increasing interest in using biomass for energy production has created a need to establish a method for its sustainable utilization. During combustion, the inorganic incombustible species in the biomass are converted into ash. To use biomass in a sustainable way, the ash that contains large quantities of nutrients should be returned to the soil to be used by growing plants. However, even though the amount of ashforming compounds in the biomass is generally low, some biomass fuels have a tendency to take up certain heavy metals from the soil and accumulate them in the various parts in the plant. Once the biomass is burned in the furnace, the * Corresponding author e-mail:
[email protected]; telephone: +358 9 456 6157; fax: +358 9 456 7021. † VTT Chemical Technology. ‡ VTT Energy. § Vattenfall Utveckling AB. | University of Gent. 496
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heavy metals are released from the fuel and transformed into ash, which results in high concentrations of heavy metals in certain ash fractions. Consequently, due to the high heavy metal concentrations, the ash cannot be circulated back to the soil unless the heavy metals can be separated into a small ash sidestream. The behavior of heavy metals in the combustion processes has been studied fairly extensively in the recent years (1). However, the work has mainly concentrated on coal combustion and waste incineration. Especially in the latter process, the fuel may contain significant amounts of heavy metals and chlorine, which are regulated compounds. Both during coal combustion and waste incineration, the combustion temperature, the gas atmosphere, and the amount of chlorine in the flue gas have been found to strongly affect the volatilization behavior of the ash-forming constituents. Especially high chlorine contents enhance volatilization of the heavy metals and the alkalis (2-5). Also chemical equilibrium calculations (6) show that in the presence of extensive amounts of Cl and S the volatilization temperatures of Cd, Cu, and Pb are lower than in the absence of these species. Absorption and chemical reactions of the heavy metals cadmium and lead with absorbent particles have been studied experimentally by Uberoi and Shadman (7, 8). They found that lead was effectively captured by silica, alumina, kaolinite, bauxite, and emathlite, and cadmium was effectively captured by alumina and bauxite. Kaolinite ceased to be effective for lead absorption when an extensive amount of chlorine was added (Cl/Pb molar ratio 10). It is not excluded that other species may also absorb the heavy metals. This may already happen inside the burning char particle and hence suppress volatilization of the heavy metals. In general, the amount of mineral particles in biomass is very small. However, in the fluidized bed combustion, a large surface area is provided by the bed sand particles, and they may act as absorbents for the gaseous ash-forming compounds, including heavy metals. Heavy metal capture by fluidized beds during wood pellet combustion has been studied by Ho and co-workers (9). It was found that the fluidized bed effectively captured lead at 700 °C. The capture efficiency depended on the sorbent material and the incineration conditions and ranged from 4.9% to 94.5%. It was postulated that at a lower temperature lead is not volatilized and is carried away with unburned carbon, but that it is released rapidly at higher temperatures. Despite the wide range of previous research in this area, no studies have addressed the volatilization of heavy metals during fluidized bed combustion of biomass in a real-scale combustion system by carrying out detailed aerosol and process measurements. In this work, we studied the behavior of Cd, Cu, Pb, and Zn during circulating fluidized bed combustion of biomass. The aim of the study was to assess the possibility of establishing a sustainable combustion cycle where the heavy metal levels in ash do not exceed the possible future regulated levels. If needed, then a heavy metal-rich ash fraction should be separated from the process, ensuring the possibility of circulating the major fraction of ash. For this aim, the volatilization behavior and the subsequent gasto-particle conversion mechanisms of heavy metals were studied. Heavy metal size distributions were determined since they determine the capture of heavy metals in the particle removal equipment and hence release into the atmosphere.
Methods Plant Description. The measurements were carried out at a full-scale circulating fluidized bed boiler in Na¨ssjo¨ Kraft10.1021/es9802596 CCC: $18.00
1999 American Chemical Society Published on Web 12/30/1998
FIGURE 1. Schematic picture of the Na1 ssjo1 CFBC unit with the aerosol measurement locations 1 and 2. va¨rmeverk, Sweden. The unit is a combined heat and power plant, fueled with coal and forest residues. The plant has been in operation since 1990. The capacity of the boiler is 26 MW heat and 9 MW electricity. The fuel during the experimental period was a Swedish forest residue (10). Forest residue contains tops and branches of (mostly pine) trees that are usually left in the forest when timber is taken to the pulp mills. The boiler conditions were kept as stable as possible during the measurements. During the experimental period, the boiler load was 80% of the full load. The bed temperature was about 780 °C, and the temperature at the top of the furnace about 880 °C. O2 concentration in the flue gas was approximately 3%, NO was 100 ppm, SO2 was 6 ppm, and CO varied from 20 to more than 500 ppm. The aerosol measurements were carried out at two locations in the flue gas channel (Figure 1). Measurement location 1 was downstream of the cyclone and upstream of the convective back pass, at the average flue gas temperature of 830 °C. Location 2 was downstream of the convective back pass and upstream of the electrostatic precipitator, at the average flue gas temperature of 150 °C. No soot blowing was carried out in the boiler during the aerosol measurements. Fuel, fly ash, and bottom ash samples were collected during the measurements and analyzed for the content of the matrix elements Ca, K, P, S, and Cl (inductively coupled plasma optical emission spectroscopy, ICP-OES) and the heavy metals Cd, Pb (graphite furnace atomic absorption spectroscopy, GFAAS), Cu, and Zn (inductively coupled plasma mass spectroscopy, ICP-MS). Fly ash was analyzed with X-ray diffraction (XRD) for the speciation of the main compounds.
Mass Size Distributions. The fly ash particle mass size distributions were determined by collecting size-classified fly ash samples with an 11-stage, multijet compressible flow Berner-type low-pressure impactor (BLPI) (11, 12) upstream and downstream of the convective back pass. The collections with the impactor upstream of the convective pass were carried out after the dilution with a probe. Downstream of the convective pass, the impactor was inserted inside the flue gas channel using a cyclone with aerodynamic cut diameter of about 8 µm as the precutter. The collection times were 9-10 and 2 min upstream and downstream of the convective back pass, respectively. In total, three BLPI samplings were carried out upstream of the convective pass, and four samplings were carried out downstream of the convective pass. More details on the measurement setup are given in ref 10. The size distributions of the cyclone-collected particles from measurement point 1 were measured using laserdiffraction method (Coulter LS 130). The particle size distribution obtained with laser diffraction was combined to the size distribution obtained with BLPI to cover the size range 0.01-100 µm. The elemental size distributions of the cyclone-collected particles were determined by multiplying the size distributions determined with laser diffraction with the average elemental concentrations. Filter Collections. Filter collections were carried out to determine the total mass concentration of the fly ash particles and the gaseous ash-forming species in the flue gas channel upstream of the convective back pass. The particles were collected by inserting a cyclone and a planar quartz fiber filter inside the flue gas channel. The aerodynamic cut diameter of the cyclone was 3 µm. After the quartz fiber filter, the particle-free gas was introduced into the dilution probe. The condensable vapor species formed new particles during the dilution, and these particles were collected on a 0.2-µm pore size polycarbonate (Nuclepore) filter. The collection time with the filters was 42-43 min (10). Elemental Analysis. The aerosol samples were analyzed for Ca, K, P, S, Cl, Cd, Pb, Cu, and Zn. The impactor samples, cyclone-collected samples, and Nuclepore filters were analyzed for Ca, K, and Cl by both instrumental neutron activation analysis (INAA) (13) and particle-induced X-ray emission analysis (PIXE) (14, 15); for Cd and Pb by GFAAS; and for P, S, Cu, and Zn by PIXE. The quartz fiber filters were analyzed by ICP-MS. For the ICP-MS analysis, the quartz fiber filters were dissolved in hydrofluoric and nitric acids at room temperature. The samples for the GFAAS were dissolved in nitric acid at room temperature. Samples for SEM. Particle samples were collected for scanning electron microscopy (SEM) from the flue gas downstream of the convective back pass after dilution with an ejector-based dilution system (16). The samples were collected on 47-mm diameter Nuclepore filters with a pore size of 0.2 µm. The filter samples were then sputter-coated with a thin layer of platinum and analyzed with a field emission scanning electron microscope (SEM, LEO 982 GEMINI) and energy-dispersive X-ray spectroscopy (EDS, NORAN, Voyager III).
Results Mass Size Distributions. The fly ash mass size distributions upstream (location 1) and downstream (location 2) of the convective back pass were found to be bimodal (Figure 2). At location 1, the mean aerodynamic size of the coarse mode was 12 µm, and the fine mode was 0.1 µm. The total particle mass concentration in the flue gas at location 1 was 10001300 mg/Nm3. Concentration of the fine particles (diameter < 0.5 µm) was 23-25 mg/Nm3, and it represented 1.9-2.2% of the total particle concentration. The total particle mass concentrations obtained from the filter and impactor colVOL. 33, NO. 3, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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was 3 µm. The fine particle concentration was approximately the same as at location 1, where the fine particle mode was mainly the result of particle formation during sampling. The larger size of the fine mode at location 2 was due to the slower cooling of the vapors in the heat exchangers than in the dilution at location 1. Particle Morphology with SEM. Supermicron (>1 µm) fly ash particles (Figure 3) were found to be irregular agglomerates consisting of up to thousands of primary particles. Primary particles in one agglomerate had varying compositions. The main compound in large fly ash particles was calcite, with smaller amounts of anhydrite, feldspar, and quartz. Due to the agglomerate structure of the particles, specific surface area of the coarse fly ash particles was fairly high (8 m2/g) as determined with nitrogen absorption. FIGURE 2. Fly ash particle mass size distributions determined upstream (location 1) and downstream (location 2) of the convective back pass. lections were consistent with each other. Approximately 90% of the particles were collected in the precutter cyclone (coarser than 3 µm). The vapor concentrations obtained from the filter collections were lower than the fine particle concentrations derived from the impactors. This is explained by the fact that the filter collected only vapors that were condensed during dilution whereas the impactor collected also fine particles that were formed in-duct prior to dilution. Also, gaseous species may have reacted with the quartz fiber filters or ash particles collected on the filters, hence reducing their concentrations in the vapor phase. The total particle concentration at the location 2 as obtained from the impactors was 230-320 mg/Nm3, and the fine mode concentration was 14-23 mg/Nm3. The total particle concentration was thus about 30% of the concentration at location 1. This was due to the deposition of the particles in the convective back pass (10). The mean diameter of fine particles was 0.2 µm, and that of the coarse particles
In fine particle size range (Dae < 0.5 µm; Figure 4), the particles seemed mainly to be single, close to spherical particles. The fine particles consisted mainly of potassium chloride. The fine particle morphology was found to be quite similar to that in CFBC of willow (Salix) (17, 18), but in the latter case, the fine particles consisted mainly of alkali sulfates. Elemental Mass Balances. The mass balances for matrix elements Ca, K, P, S, and Cl as well as for the heavy metals Cd, Pb, Cu, and Zn were derived from the aerosol measurements carried out upstream of the convective pass and the fuel and bottom ash analyses. The mass balances were used to calculate the fraction of each element that was removed from the furnace with the bottom ash (Table 1). Approximately 6 ppm of SO2 that was found in the flue gas is included in Table 1. The in-going amount of the elements was generally lower than the out-coming amount. This was probably due to the heterogeneity of the fuel. Approximately 30-40% of the total ash was removed from the furnace with the bottom ash. The removal percentages for the elements showed quite some variability. Sulfur, chlorine, and cadmium were nearly absent in the bottom ash, and also lead was somewhat depleted. In contrast,
FIGURE 3. Scanning electron micrograph of a supermicron-sized (diameter >1 µm) fly ash particle from CFBC of forest residue. Supermicron particles seem mainly to be composed of smaller primary particles that have agglomerated to form large clusters. 498
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FIGURE 4. Scanning electron micrograph of the fine fly ash particles formed in the CFBC of forest residue. The ultrafine particles are mostly single particles. Some of them show indications of rectangular shape suggesting crystallinity.
TABLE 1. Mass Balances and the Amount of the Matrix Elements and Heavy Metals Retained in the Bottom Ash Based on the Fuel, Bottom Ash, and Bed Sand Analyses and the Aerosol Measurements Upstream of the Convective Pass at 830 °C element
element in/out (%)
removed with bottom ash (%)
total ash
72-90
31-40
Ca K P S Cl
Matrix Elements 76-101 90-93 69-79 39-72 330-333
31-41 45-57 31 5
