Energy & Fuels 2007, 21, 1151-1160
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Effect of Wood Fuel on the Emissions from a Top-Feed Pellet Stove Olli Sippula,† Kati Hyto¨nen,† Jarkko Tissari,† Taisto Raunemaa,† and Jorma Jokiniemi*,†,‡ Fine Particle and Aerosol Technology Laboratory, Department of EnVironmental Sciences, UniVersity of Kuopio, Post Office Box 1627, FI-70211 Kuopio, Finland, and VTT Technical Research Centre of Finland, Fine Particles, Post Office Box 1000, FI-02044 VTT, Espoo, Finland ReceiVed June 21, 2006. ReVised Manuscript ReceiVed NoVember 17, 2006
Particle and gaseous emissions of a top-feed pellet stove were studied in laboratory conditions. Pellets made of separate stem and bark materials of five different wood species and a commercial pellet product were used as fuels. The study included the determination of the particle number concentration, size distribution, fineparticle mass (PM1.0), CO, CO2, NOx, and volatile organic compounds (VOC). The PM1.0 emission was analyzed for inorganic substances, organic carbon, and elemental carbon. Thermodynamic equilibrium calculations were performed to interpret the results from chemical analysis and to estimate the chemical composition of the PM1.0 mass emitted with various fuels. The bark fuels produced higher PM, VOC, and CO emissions than stem fuels. This was evidently related to the higher ash content of the bark fuels and was found to increase both the fly ash emission and the products of incomplete combustion. The fuel ash content correlated linearly with the PM1.0 emission. Among stem fuels, willow and alder produced higher PM1.0 emissions than birch, pine, spruce, and the commercial fuel. An exceptionally low PM1.0 emission was measured from pine bark combustion, which can be explained by the low ash content of the fuel. The main components in the PM1.0 were K2SO4, KCl, K2CO3, KOH, and organic material. Except birch fuels, around 60-80 mass % of potassium species were K2SO4 based on the equilibrium calculations. In the case of birch fuels, because of the high chlorine content and low S/Cl ratios, around half of the potassium was KCl.
Introduction The use of renewable biomass fuels is currently increasing rapidly worldwide because fossil fuel prices are rising and attempts to decrease greenhouse gas emissions are becoming implemented. However, the replacement of oil with biomass fuels in decentralized energy production may increase atmospheric emissions considerably. In Finland, the residential wood combustion has been estimated to be one of the major emission sources of fine particles and volatile organic compounds (VOC). High particle concentrations in the urban air have been found to increase respiratory and cardiovascular diseases and mortality.1 They may also have a significant influence on the air quality on a local scale and the climate system as a whole.2 VOC affect human health in several ways, and many of them have been categorized as hazardous air pollutants.3 VOC also participate in the formation of tropospheric ozone and secondary organic aerosols.4 In large wood combustion units, the majority of fine particles are formed from different ash constituents, especially from alkali metal compounds.5,6 Because of the efficient combustion process * To whom correspondence should be addressed. Telephone: +358-405050-668. Fax: +358-17-163-229. E-mail:
[email protected]. † University of Kuopio. ‡ VTT Technical Research Centre of Finland. (1) Dockery, D. W.; Pope, C. A.; Xu, X. P.; Spengler, J. D.; Ware, J. H.; Fay, M. E.; Ferris, B. G.; Speizer, F. E. N. Engl. J. Med. 1993, 329, 1753-1759. (2) Intergovernmental Panel on Climate Change. The Scientific Basis; Cambridge University Press: Cambridge, U.K., 2001. (3) Clean Air Act. Public Law 101-549; U.S. Federal Law, Nov 15, 1990. (4) Seinfeld, J. H.; Pandis, S. N. Atmospheric Chemistry and Physics; John Wiley and Sons: New York, 2001. (5) Valmari, T.; Kauppinen, E. I.; Kurkela, J.; Jokiniemi, J. K.; Sfiris, G.; Revitzer, H. J. Aerosol Sci. 1998, 29, 445-459. (6) Nussbaumer, T. Energy Fuels 2003, 17, 1510-1521.
and long residence time, the flue gas contains only very small concentrations of hydrocarbons. In small-scale combustion however, because of incomplete combustion conditions and imperfect mixing, large amounts of hydrocarbons (tar and VOC) and elemental carbon (soot) can be formed. Hydrocarbons have been observed at relative high concentrations in both the particulate and gaseous phases.7-11 The emissions from residential wood combustion are still relatively unknown. Because of large variations in combustion technologies, fuel quality, and operating conditions, as well as different emission analysis methods, there are huge differences and uncertainties in the available emission factors. However, some trends can be concluded from the available literature. First, the batch-wise combusted appliances typically produce more emissions than continuously operated ones, and fan-assisted appliances produce less emissions than ones using natural draught. For example, Johansson et al.9 reported up to 180 times larger nominal particle mass emissions for an old wood boiler compared to a modern appliance using wood pellets. Second, the combustion conditions play an important role in the formation of particle and gaseous pollutants. The experimental findings indicate that the air/fuel ratio, combustion temperature, and mixing in the combustion chamber are important factors (7) Wierzbicka, A.; Lillieblad, L.; Pagels, J.; Strand, M.; Gudmunsson, A.; Gharibi, A.; Swietlicki, E.; Sanati, M.; Bohgard, M. Atmos. EnViron. 2005, 39, 139-150. (8) McDonald, J.; Zielinska, B.; Fujita, E.; Sagebiel, J.; Chow, J.; Watson, J. EnViron. Sci. Technol. 2000, 34, 2080-2091. (9) Johansson, L. S.; Leckner, B.; Gustavsson, L.; Cooper, D.; Tullin, C.; Potter, A. Atmos. EnViron. 2004, 38, 4183-4195. (10) Schauer, J.; Kleeman, M.; Cass, G.; Simoneit, B. EnViron. Sci. Technol. 2001, 34, 1716-1728. (11) Rogge, W. F.; Hildemann, L. M.; Mazurek, M. A.; Cass, G. R.; Simoneit, B. R. T. EnViron. Sci. Technol. 1998, 32, 13-22.
10.1021/ef060286e CCC: $37.00 © 2007 American Chemical Society Published on Web 02/01/2007
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influencing the overall emissions.12-14 Finally, the fuel quality, which has a close connection to the combustion conditions, may influence the emissions significantly. The effect of fuel quality factors (such as ash content, ash chemical composition, and moisture) becomes especially important in modern appliances, such as pellet-burning heaters, in which the combustion conditions and efficiency are optimized and emissions are at a relatively low level. One of the most important factors has been found to be the fuel ash content, which has been observed to correlate with the particle mass emission.15,16 In addition, the fuel alkali metal content has been seen to correlate with the fine-particle emissions.17 Furthermore, the chlorine and sulfur content in the fuel may influence particle formation considerably. The chlorine has been observed to increase volatilization of alkali metals and metals18-20 and, consequently, their enrichment in the fly ash, when on the other hand, there is some evidence that sulfur can inhibit the effect of chlorine on particle formation.18 The amount and composition of ash is also important in the slagging and corrosion of combustion units. The aim of this study was to evaluate the effect of wood species on the particle and gaseous emissions in small-scale combustion. To interpret the results, we looked for correlations between fuel quality factors, operating conditions, and emissions. The tests were performed in laboratory conditions using a new design top-feed pellet stove. Pellets made from bark and stem materials of five wood species were used as test fuels. The oxygen normalized flue gas and nominal emissions were determined for each fuel. The chemical composition of the particle emission was analyzed using several methods. To get information on the species composition and particle formation mechanisms, chemical equilibrium calculations were carried out. Experimental Section In the combustion experiments, a top-feed pellet stove was situated on a balance (Figure 1), allowing the fuel consumption to be monitored. To mimic a natural draught, the combustion gases were led into an insulated stack placed below a hood. The flow conditions in the hood were adjusted by a fan. The temperatures during the tests were monitored continuously in the outlet of the fire chamber, the dilution tunnel, the laboratory room air, and the flue gas at the sampling port. In addition, the pressure was measured in the stack and in the dilution tunnel. The pellets were combusted at the nominal output, with each test taking 3-6 h. A combination of gas analyzers (ABB Cemas gas-analyzing rack) was used to measure CO, CO2, O2, NO, and NO2. The particle and VOC samples were diluted in the dilution tunnel, according to the ISO 8178-1 standard,21 with average (12) Wiinikka, H.; Gebart, R. Energy Fuels 2004, 18, 897-907. (13) Tissari, J. M.; Hyto¨nen, K. H.; Sippula, O. M. J.; Turrek, T.; Jokiniemi, J. K. The effects of appliance type and operation on the emissions from the residential wood combustion. EnViron. Sci. Technol., manuscript submitted for publication. (14) Oser, M.; Nussbaumer, T.; Schweizer, B.; Mohr, M.; Figi, R. Aerosols from Biomass Combustion; International Seminar on June 27, 2001 in Zurich; International Energy Agency and the Swiss Federal Office of Energy, Switzerland, 2001; pp 59-64. (15) Johansson, L. S.; Tullin, C.; Leckner, B.; Sjo¨vall, P. Biomass Bioenergy 2003, 25, 435-446. (16) Wiinikka, H.; Gebart, R. Combust. Sci. Technol. 2005, 177, 741763. (17) Christensen, K. A.; Stenholm, M.; Livbjerg, H. J. Aerosol Sci. 1998, 29, 421-444. (18) Lind, T.; Kauppinen, E. I.; Hokkinen, J.; Jokiniemi, J. K.; Orjala, M.; Aurela, M.; Hillamo, R. Energy Fuels 2006, 20, 61-68. (19) Miller, B.; Dugwell, D. R.; Kandiyoti, R. Energy Fuels 2003, 17, 1382-1391. (20) Baxter, L. L.; Miles, T. R.; Miles, T. R., Jr.; Jenkins, B. M.; Milne, T.; Dayton, D.; Bryers, R. W.; Oden, L. L. Fuel Process. Technol. 1998, 54, 47-78.
Sippula et al.
Figure 1. Experimental setup.
dilution ratios of 73-86. The dilution air was filtered to remove particles, nitrogen oxides, and hydrocarbons. PM1.0 samples were collected on parallel polytetrafluoroethylene (PTFE) and quartz filters, with volume flow rates of 10 L/min and sampling times from 60 to 90 min. A cascade impactor (Dekati PM10 impactor) was used to remove particles larger than 1 µm. The particle-number concentrations and size distributions in the range of 0.029-10.2 µm were measured using an electrical low pressure impactor22 (ELPI, Dekati 30 L/min, Al-foil impactor plates). VOC samples were drawn through PTFE filters to remove particles and collected in Tenax GR (Supelco, mesh size 60/80) adsorbent resin tubes. The sample flow rate was about 160 mL min-1, and the sampling time was the same as in the PM1.0 measurements. Samples for electron microscopy were collected from the commercial pellet combustion test. The particles were collected on copper grids, from diluted and undiluted combustion gas, using an electrostatic particle sampler.23 The samples were viewed by scanning electron microscopy (SEM, Leo DSM 982 Gemini). Combustion Appliance. The combustion appliance, used in the experiments, was a commercially available top-feed pellet stove with a nominal output of 8 kW (Wodtke GmbH). A sketch of the stove is presented in Figure 2. The pellet stove has an internal fuel storage, from where the pellets are supplied, using an auger screw. The burning occurs in a pot, which has holes in the bottom. The combustion air is supplied through the holes under the pellet bed. The air supply is fan-assisted and depends upon the selected thermal output. A short cleaning period is programmed to occur once an hour in the stove. During cleaning, the fuel supply decreases and the air supply increases for a few minutes, removing the ash gathered on the pot. Fuels. The pellet fuels were prepared from separated bark and stem materials from five different Finnish wood species: birch, spruce, pine, alder, and willow. In addition, a commercial pellet fuel made of stem material was used in the tests. Thus, altogether 11 different pellet fuels were used in the measurements. The diameter of the pellets was 8 mm, and the length varied from 1 to 9 cm, being typically around 3 cm. The calorimetrical heating value, moisture, ignition residue at 550 °C, and concentrations of mineral species were analyzed for each fuel. Chlorine concentrations were analyzed using a titrimetric method, and other elemental analyses were performed applying inductively coupled plasma-optical spectrometry (ICP-OES). (21) ISO 8178-1: Reciprocating internal combustion enginessExhaust emission measurementsPart 1: Test bed measurement of gaseous and particulate emissions; International Organization for Standardization: Geneva, Switzerland, 1996. (22) Keskinen, J.; Pietarinen, K.; Lehtima¨ki, M. J. Aerosol Sci. 1992, 23, 353-360. (23) Yeh, H. Electrical Techniques. In Aerosol Measurement, Principles, Techniques and Applications; Willeke, K., Baron, P.A., Eds.; Van Nostrand Reinhold: New York, 1993; pp 410-426.
Wood Fuel on Emissions from a Pellet StoVe
Figure 2. Sketch of the pellet stove used in this study.
Particle Analysis. The total PM1.0 mass was determined from the filter samples using a microbalance after which the samples were analyzed for organic, elemental, and carbonate carbon and several inorganic ions. Results from inorganic analysis were interpreted by chemical equilibrium model calculations. Using the results of the chemical analysis together with the flue gas equilibrium data, chemical compositions for the PM1.0 mass were calculated. The inorganic constituents (except carbonates) were analyzed from PTFE filters by ion chromatography. The organic, elemental, and carbonate carbon fractions were determined from quartz-filter samples by a thermal optical method,24 using a carbon analyzer from Sunset Laboratories. The analyses were performed according to the National Institute for Occupational Safety and Health (NIOSH) procedure.25 The carbonate carbon (CC) content of the samples was determined indirectly by performing two runs of each filter sample and exposing the second sample punch to HCl vapor, which is presumed to break down the carbonates and, consequently, release the CC as CO2. Thus, the difference between total carbon results gives an estimate of the CC content in the sample.25 Because this method is not widely used, its functionality was checked in separate tests with specially prepared glucose/potassium carbonate samples. The method was found to result in 6-37% errors in the carbonate content. To convert the organic carbon (OC) to total organic matter (OM), one needs to know the average molecular weight per carbon weight of the organic material. Because of the lack of this information, an OM/OC ratio of 2.0 was adopted, which is typical for wood smoke containing aerosols.26 The quantity of PM1.0 ash was calculated on the basis of the inorganic analyses made from the filter samples and on the equilibrium calculations. The following ions were included in the PM1.0 ash: K, Na, Ca, Mg, SO4, Cl, CO3, NO3, OH, and oxides. The calculation method is described in the Results. Model Calculations. Using the results from inorganic analyses of particle samples, the concentrations of different alkali metal compounds were assessed by thermodynamic equilibrium model calculations. The calculations were carried out using the FactSage 5.3 software,27 which identifies the most probable species in a multiphase system by a method of Gibbs energy minimization. Ideal solutions were used in the analysis. The following elements were included in the final calculations: C, O, H, N, S, Cl, Na, and K. (24) Turpin, J. T.; Saxena, P.; Andrews, E. Atmos. EnViron. 2000, 34, 2983-3013. (25) NIOSH Elemental Carbon (Diesel Particulate) Method 5040. In NIOSH Manual of Analytical Methods (NMAM), 4th ed.; The National Institute for Occupational Safety and Health: Atlanta, GA, 1999. (26) Turpin, J. T.; Lim, H. Aerosol Sci. Technol. 2001, 35, 602-610. (27) Bale, C. W.; Chartrand, P.; Degterov, S. A.; Eriksson, G.; Hack, K.; Mahfoud, R. B.; Melancon, J.; Pelton, A. D.; Petersen, S. CALPHAD: Comput. Coupling Phase Diagrams Thermochem. 2002, 26, 189-228.
Energy & Fuels, Vol. 21, No. 2, 2007 1153 The chemical equilibriums were calculated at temperatures of 3001200 °C. The analyzed amounts of K, Na, S, and Cl in the PM1.0 fraction were given as input in the calculations. This is done because the calculation on the volatilization of mineral species has large uncertainties. In our cases, the input feeds of sulfur and chlorine in the fuel roughly correspond to the PM1.0 concentrations. In addition, the fuels were rich enough in alkali compounds to bind all of the sulfur and chlorine. On the basis of these facts, we suggested that no significant amount of gaseous sulfur and chlorine remained in the flue gas. VOC Analysis. The hydrocarbon concentrations in the VOC samples were determined by a thermal-desorption cold-trap injector-gas chromatography-mass spectrometry (TCT-GC-MS) (operated in scan mode, mass range m/z 30-300). The analysis equipment and the temperature program were almost identical to those described by Vuorinen et al.28 A HP-5 silica capillary column (50 m × 0.2 mm i.d., 0.5 µm film thickness) was used in the gas chromatography. All mass numbers between 30 and 300 m/z were recorded using the silica-coated aluminum nitride (SCAN) technique. The compounds were identified by comparing their mass spectra with the Wiley data library. An estimation of the total VOC emission (TVOC) was calculated as a toluene equivalent by comparing the area response of all compounds to that of a known amount of toluene. Compounds that had retention times from approximately 9.2 min (benzene) to 40.7 min (4-hydroxy-3methoxy-benzeneacetic acid) were included in the TVOC. Parameter Calculations. The emission concentrations were normalized to 10% oxygen in the dry flue gas. In addition, the particle emission values were dilution-corrected. The dilution ratio (DR) was calculated by dividing the total flow in the dilution tunnel by the sample flow. The DR was checked before and after measurements by measuring the sample flow entering the dilution tunnel. Because the volume flow rate stays nearly constant in the sample line and the sampling temperatures vary during the measurements, a temperature correction was made for the dilution ratio. The uncertainty of the dilution ratio was estimated to be 20%. The nominal emission values were calculated in relation to energy input to the burning process,29 as well as in relation to the amount of fuel used. The effective heating values of the fuels were calculated by a generally used equation based on the elemental composition of the fuel. The elemental composition values for different wood fuels were taken from the Phyllis database.30 The uncertainty in the emission values was estimated by taking uncertainties of the air/fuel and dilution ratios into account. The uncertainty in the air/fuel ratio was calculated on the basis of the standard deviation of the flue gas oxygen concentration. The efficiency of the combustion appliance was determined using a standardized indirect method31 that takes the thermal and chemical heat losses into consideration. The value expresses the ratio of the total heat output to the total heat input. The thermal heat loss is calculated on the basis of the difference between the temperature of the flue gas and the room temperature and the specific heat of the flue gas. The chemical heat loss is calculated from the CO concentration of the flue gas.
Results The results include data from altogether 22 measurement cycles using 11 different fuels. The length of a single cycle varied from 1 to 1.5 h, depending upon the timing of the filter sample collection. The emission values from continuous measurements are presented as an average of all measured data. (28) Vuorinen, T.; Nerg, A-M.; Vapaavuori, E.; Holopainen, J. K. Atmos. EnViron. 2005, 39, 1185-1197. (29) SFS 5624, Air quality. Stationary source emissions. Determination of flue gas conditions; Finnish Standards Association SFS, Helsinki, Finland, 1990. (30) Phyllis, database for biomass and waste. http://www.ecn.nl/phyllis; Energy Research Centre of the Netherlands, Petten, The Netherlands, 2004. (31) SFS-EN 13240, Room heaters fired by solid fuel. Requirements and test methods; Finnish Standards Association SFS, Helsinki, Finland, 2001.
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and particle emission components. The particle-number emission (NELPI) and the geometric mean diameter (dg) are based on the ELPI data. In addition to the total fine particle mass (PM1.0), different chemical fractions of the PM1.0 mass are also presented. In the case of alder stem, the single elemental components together yielded a higher mass than weighed on the balance, and for this reason, the elemental analysis results were rejected. The OC, elemental carbon (EC), and CC were analyzed from quartz filters, while other PM1.0 values were analyzed from PTFE filters. Particle-mass concentrations obtained from quartz were on average 5% higher (varying from 0 to 13%), which is assumed to be due to the adsorption of vapors by the filter material. This issue has been discussed in more detail in the literature.24,32 In general, the bark fuels produced higher emissions than stem fuels. The PM1.0 emissions for bark fuels were 4-8 times higher than those of stem fuels. The highest emissions were measured from willow, spruce, and alder bark fuels. Among stem fuels, willow and alder produced the highest PM1.0 emissions. The lowest PM1.0 emissions from stem and bark fuel combustion were measured from pine fuels. With some exceptions, CO emission levels followed PM1.0 when comparing different fuels. The commercial wood pellet had similar emissions to stem pellets from pine, spruce, and birch. The PM1.0 mass emission varied from 46 mg/MJ (pine stem) to 652 mg/MJ (willow bark). The particle-number emissions varied from 4.6 × 1013 particles/MJ (spruce stem) to 1.5 × 1014 particles/MJ (alder bark). The lowest CO emissions were measured for commercial, pine stem, and spruce stem pellets (101-201 mg/MJ). Slightly higher emissions were determined from stem pellets made of alder, birch, and willow (207-355 mg/MJ). For bark pellets, except pine bark, the CO emissions were considerably higher, ranging from 604 to 3076 mg/MJ. Altogether over 140 individual VOC were found and identified in the samples. Table 4 shows the emission values of (a) 13 individual volatile organic compounds, (b) all analyzed and identified VOC, (c) all analyzed but not identified VOC, and (d) TVOC (b plus c). The bark fuels produced higher emissions than stem fuels. Among stem fuels, the TVOC emission was highest for willow, whose TVOC emission was equal to those of alder bark and pine bark pellets. The emissions of most of the individual VOC were lowest for birch stem combustion and highest for spruce bark combustion. The TVOC emissions varied from 4 mg of toluene equivalents/MJ (birch stem) to 96 mg of toluene equivalents/MJ (spruce bark). The proportion of the unidentified VOC varied from 24% (pine bark) to 49% (willow stem).
Table 1. Combustion and Sampling Parameters of the Measurement Cycles cycle
pellet fuel
length efficiency air T0 TS TD (min) (%) ratio (°C) (°C) (°C) DR
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
alder stem spruce bark spruce stem spruce stem pine stem pine stem alder stem alder stem willow stem willow bark willow bark birch bark alder bark birch stem birch stem alder stem pine bark commercial commercial willow stem alder bark alder bark
90 91 90 91 90 60 91 90 90 90 60 81 93 90 90 90 90 90 60 90 90 60
85 76 85 83 83 82 85 83 86 74 77 86 80 81 81 84 81 81 81 81 78 81
4.5 7.2 4.4 3.8 3.4 3.4 4.2 4.1 4.7 6.8 6.0 5.3 5.4 4.5 5.0 4.2 3.9 4.3 4.7 4.9 6.3 5.3
414 392 315 346 351 283 354 425 346 324 354 379 397 415 388 427 442 451 287 307 344 350
125 110 112 146 153 165 122 138 104 114 118 102 134 99 115 131 159 149 143 110 124 128
24 22 23 24 23 25 22 23 23 23 23 22 23 22 23 22 23 23 24 22 22 23
78 75 76 82 84 86 78 81 74 76 77 74 80 73 76 80 85 83 82 75 78 79
Combustion conditions during the measurements were stable, and the variation in emission values was generally low. Some changes in the combustion process and emission levels were caused by cleaning periods, which occur automatically in the stove once in an hour. Thus, each measurement cycle contains one or two cleaning periods. During cleaning, combustion air flow increases and fuel supply decreases for 2 min. This was found to increase the CO emission substantially. The particle emissions clearly increased also during the cleaning period, but the increase in the overall emission was negligible. Table 1 shows the average values of the fire chamber outlet temperature (T0), sampling temperature (TS), diluted sample temperature (TD), DR, efficiency of the combustion appliance, and air/fuel ratio (λ) for each measurement cycle. Fuels. Moisture, ignition residue, lower heating values (LHV), and the results of the elemental analysis of each pellet fuel are presented in Table 2. The ignition residue (at 550 °C) is considered here to be the ash content of the fuel. The bark fuels had noticeably higher ash contents than the stem fuels. However, the elemental distributions between stem and bark fuels of the same species were roughly equal. When the birch fuels were compared to other fuels, they had high chlorine contents. The (Cl + 2S)/(K + Na) molar ratio was for all of the fuels less than 1 (from 0.3 to 0.9). Particulate and Gaseous Emissions. Table 3 presents nominal emission values for gaseous (CO, NOx, and TVOC)
Table 2. Properties of the Fuels Useda pellet fuel
commercial
birch bark
birch stem
spruce bark
spruce stem
alder bark
alder stem
pine bark
pine stem
willow bark
willow stem
moisture (%) ash (%) LHV (MJ/kg) Na Mg P S Cl K Ca Zn Cd
5.9 0.3 17.7
