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Energy & Fuels 2006, 20, 964-978
Release to the Gas Phase of Inorganic Elements during Wood Combustion. Part 1: Development and Evaluation of Quantification Methods Simone C. van Lith,* Violeta Alonso-Ramı´rez, Peter A. Jensen, Flemming J. Frandsen, and Peter Glarborg CHEC Research Centre, Department of Chemical Engineering, Technical UniVersity of Denmark, Building 229, 2800 Kgs. Lyngby, Denmark ReceiVed May 4, 2005. ReVised Manuscript ReceiVed January 24, 2006
During wood combustion, inorganic elements such as alkali metals, sulfur, chlorine, and some heavy metals are partly released to the gas phase, which may cause problems in combustion facilities because of deposit formation and corrosion. Furthermore, it may cause harmful emissions of gases and particulate matter. The aim of this study is to obtain quantitative data on the release of inorganic elements during wood combustion, which will serve as input data for models aiming to address ash-related problems. Three quantification methods were developed. In all three methods, the release of inorganic elements was quantified by a mass balance based on the weights and inorganic compositions of the fuel and the ash residues obtained by high-temperature (500-1150 °C) treatment in a laboratory-scale tube reactor. However, method A involved the pyrolysis and combustion of a small fuel sample (∼30 g) in this reactor, whereas methods B and C involved initial pyrolysis and combustion, respectively, of a large fuel sample (∼5 kg) in a bench-scale fixed-bed reactor at 500 °C. The methods were evaluated by comparing the data on the release of Cl, S, K, Na, Zn, and Pb from fiber board obtained by the three methods. The release data were interpreted by use of literature information, equilibrium calculations, and scanning electron microscopy analysis of the ash samples. Large differences in the release trends (especially for S, Na, and Zn) were observed for the three methods because of the differences in sample size, oxidizing/reducing conditions, and the ash formation process. The combined results of the three methods provide a good understanding of the ash transformations and release of inorganic elements during wood combustion on a grate. Method A gives information on the local (or primary) release, whereas methods B and C provide insight into the influence of secondary reactions taking place in larger fuel beds.
Introduction The concern about global warming because of the emission of CO2 and other greenhouse gases and the limited availability of fossil fuels have increased the interest in using biomass as a fuel for energy production. Biomass is a renewable energy source and is considered to be CO2-neutral (i.e., during growth it accumulates the same amount of CO2 as is released during combustion). Woody biomass fuels are favorable because they often are waste products from forestry and numerous types of wood industry, and therefore they are often locally available and relatively cheap. However, the inorganic fraction of biomass (which is naturally present or sometimes added to the fuel during handling or processing) causes several problems during combustion. The most important problem is the formation of particulate matter (aerosols and fly ashes) during biomass combustion, causing deposit formation (slagging and fouling) on superheater tubes, which in turn leads to a reduction in the heat transfer efficiency to the water/steam system and may lead to corrosion of the superheater tubes. These problems may cause costly shutdowns of combustion units. In general, it has been observed that ash deposition and corrosion problems are more common during combustion of biomass fuels with high chlorine and alkali contents, such as straws and grasses. But even though woody * Corresponding author. Fax: +45 45 88 22 58. E-mail:
[email protected].
biomass fuels generally contain lower levels of chlorine and potassium than most other biomass fuels, corrosion of heat exchangers also occurs in wood-fired systems because of the deposition of Cl-rich aerosols or vapor species.1 Furthermore, because aerosol filters and precipitation devices are, because of their high costs, not applicable to small- and medium-scale wood combustion units,2 aerosols will be emitted to the atmosphere and may cause adverse health effects because aerosols have a high probability of penetrating into the alveolar regions of the lungs.3 In addition to aerosols, gases such as SO2 and HCl are also emitted during wood combustion, which contributes to the acidification of the environment. Investigations of aerosols and fly ashes in fixed-bed wood combustion systems have shown that the volatile alkali metals (1) Gunderson, J. R.; Folkedahl, B. C.; Schmidt, D. D.; Weber, G. F.; Zygarlicke, C. J. Barrier Issues to the Utilization of Biomass; Semiannual Technical Progress Report; Energy & Environmental Research Center, University of North Dakota: Grand Forks, ND, 2002. (2) Brunner, T.; Joeller, M.; Obernberger, I.; Frandsen, F. Aerosol and Fly Ash Formation in Fixed Bed Biomass Combustion Systems using Woody Biofuels. In Proceedings of the 12th European Conference and Technology Exhibition on Biomass for Energy, Industry and Climate Protection; Amsterdam, The Netherlands, June 17-21, 2002; ETA: Florence, Italy, 2002; pp 685-689. (3) Jokiniemi, J. K.; Lind, T.; Hokkinen, J.; Kurkela, J.; Kauppinen, E. I. Modelling and Experimental Results on Aerosol Formation, Deposition and Emissions in Fluidized Bed Combustion of Biomass. In Proceedings of the International IEA Seminar ‘Aerosols from Biomass Combustion’; Zurich, Switzerland, June 27, 2001; Verenum: Zurich, Switzerland, 2001; pp 31-35.
10.1021/ef050131r CCC: $33.50 © 2006 American Chemical Society Published on Web 03/11/2006
Quantifying Inorganic Release from Wood Combustion
K and Na, the volatile heavy metals Pb and Zn, as well as S and Cl are the most important aerosol-forming elements.4,5 During wood combustion, these elements are partly released from the fuel to the gas phase, where they may undergo chemical reactions and either instantly or during cooling of the flue gas form aerosols.6,7 Furthermore, it was observed that these elements are relevant for the formation of deposits and for deposit-induced corrosion in biomass-fired boilers.8,9 To be able to address ash-related problems during biomass combustion, it is important to understand the formation and behavior of aerosols and fly ashes. To predict the concentration and composition of aerosols and inorganic gases in the flue gas, quantitative data are needed on the release of inorganic elements from the fuel during combustion. Detailed information is available on the release of K, S, and Cl during combustion of annual biomass,10-12 but the ash contents and inorganic compositions of annual biomass fuels are significantly different from woody biomass fuels. Dayton and Milne13 investigated the release of alkali metals from biomass fuels under various combustion conditions and concluded that the initial feedstock composition had the most pronounced effect on the alkali metal release. It was concluded in the study of Dayton and Milne13 and in other studies14 that the Cl content of biomass is an important parameter that facilitates alkali release during combustion. According to Baxter et al.,14 the Cl concentration often dictates the amount of alkali vaporized during combustion more strongly than the alkali concentration in the fuel. Because of the relatively low contents of K and Cl in woody biomass fuels compared to other biomass fuels, other alkali release mechanisms may be dominant. Furthermore, the association of the inorganic elements in the fuel is important for the release behavior.11,15-17 Although several studies have been reported on the release of alkali metals (4) Brunner, T.; Obernberger, I.; Jo¨ller, M.; Arich, A.; Po¨lt, P. Behaviour of Ash Forming Compounds in Biomass Furnaces - Measurement and Analyses of Aerosols Formed during Fixed-Bed Biomass Combustion. In Proceedings of the International IEA Seminar ‘Aerosols from Biomass Combustion’; Zurich, Switzerland, June 27, 2001; Verenum: Zurich, Switzerland, 2001; pp 75-80. (5) Dahl, J.; Obernberger, I.; Brunner, T.; Biedermann, F. Results and Evaluation of a New Heavy Metal Fractionation Technology in Grate-Fired Biomass Combustion Plants as a Basis for Improved Ash Utilization. In Proceedings of the 12th European Conference and Technology Exhibition on Biomass for Energy, Industry and Climate Protection; Amsterdam, The Netherlands, June 17-21, 2002; ETA: Florence, Italy, 2002. (6) Friedlander, S. K. Smoke, Dust, and Haze; Wiley & Sons: New York, 1977. (7) Christensen, K. A. The Formation of Submicron Aerosol Particles from the Combustion of Straw. Ph.D. Thesis, Technical University of Denmark, Lyngby, Denmark, 1995. (8) Nielsen, H. P.; Frandsen, F. J.; Dam-Johansen, K.; Baxter, L. L. The Implications of Chlorine-Associated Corrosion on the Operation of BiomassFired Boilers. Prog. Energy Combust. Sci. 2000, 26, 283-298. (9) Hansen, L. A.; Nielsen, H. P.; Frandsen, F. J.; Dam-Johansen, K.; Hørlyck, S.; Karlsson, A. Influence of Deposit Formation on Corrosion at a Straw-Fired Boiler. Fuel Process. Technol. 2000, 64, 189-209. (10) Knudsen, J. N.; Jensen, P. A.; Lin, W.; Frandsen, F. J.; DamJohansen, K. Sulfur Transformations during Thermal Conversion of Herbaceous Biomass. Energy Fuels 2004, 18, 810-819. (11) Knudsen, J. N.; Jensen, P. A.; Dam-Johansen, K. Transformation and Release to the Gas Phase of Cl, K, and S during Combustion of Annual Biomass. Energy Fuels 2004, 18, 1385-1399. (12) Knudsen, J. N.; Jensen, P. A.; Lin, W.; Dam-Johansen, K. Secondary Capture of Chlorine and Sulfur during Thermal Conversion of Biomass. Energy Fuels 2005, 19, 606-617. (13) Dayton, D. C.; Milne, T. A. Laboratory Measurements of Alkali Metal Containing Vapors Released during Biomass Combustion. In Applications of AdVanced Technology to Ash-Related Problems in Boilers; Baxter, L., DeSollar, R., Eds.; Plenum Press: New York, 1996. (14) Baxter, L. L.; Miles, T. R.; Miles, T. R., Jr.; Jenkins, B. M.; Milne, T.; Dayton, D.; Bryers, R. W.; Oden, L. L. The Behavior of Inorganic Material in Biomass-Fired Power Boilers: Field and Laboratory Experiences. Fuel Process. Technol. 1998, 54, 47-78.
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during pyrolysis and combustion of wood,13,15,18,19 no quantitative release data were provided in these studies. Also, very little is known about the quantity of heavy metals released during the combustion of wood and the mechanisms by which these elements are released to the gas phase. The aim of this work is to obtain quantitative data on the release of inorganic elements during wood combustion by performing experiments under conditions that resemble gratefiring. The release was quantified by performing a mass balance, based on the weights and inorganic compositions of the fuel and residual ash samples. Three different experimental methods were developed and tested, the main difference between the methods being the size of the wood sample used. Because the ash content of some woody biomass fuels, such as wood chips, is relatively low (usually less than 1% (w/w) dry basis, db), it is desirable to obtain a higher quantity of residual ash; this will improve the accuracy of the chemical analysis of the ash and thereby the release data. Moreover, a larger fuel sample is expected to be more homogeneous in chemistry and to provide a better reproducibility of the results. On the other hand, there is a risk of secondary reactions between released species and the char when using a larger fuel sample. In addition to the difference in sample size, the differences in residence times and inlet gas conditions may have an effect on the release data. This article describes the three different methods developed to quantify the release of inorganic elements during wood combustion and evaluates the methods by comparing and discussing the release data for Cl, S, K, Na, Zn, and Pb obtained by combusting fiber board according to the three methods in the temperature range of 500-1150 °C. To facilitate the interpretation of the results, chemical fractionation analysis, thermodynamic equilibrium calculations, and scanning electron microscopy were used. Details about the methods and the results of these studies can be found elsewhere.20 Experimental Section The release of inorganic elements during combustion of wood was quantified by a mass balance based on weight measurements and chemical analysis of the wood fuel and the ash obtained at various temperatures in the range of 500-1150 °C. To produce the high-temperature ash, three different methods were used, involving two different experimental setups: a laboratory-scale tube reactor setup and a bench-scale fixed-bed reactor setup. (1) Method A (wood combustion): this method involved pyrolysis and subsequent combustion of a small wood sample in the laboratory-scale tube reactor. (2) Method B (char combustion): in this method, a large amount of wood was first pyrolyzed (at 500 °C) in the bench-scale fixed(15) Dayton, D. C.; Jenkins, B. M.; Turn, S. Q.; Bakker, R. R.; Williams, R. B.; Belle-Oudry, D.; Hill, L. M. Release of Inorganic Constituents from Leached Biomass during Thermal Conversion. Energy Fuels 1999, 13, 860870. (16) Jensen, P. A.; Frandsen, F. J.; Dam-Johansen, K.; Sander, B. Experimental Investigation of the Transformation and Release to Gas Phase of Potassium and Chlorine during Straw Pyrolysis. Energy Fuels 2000, 14, 1280-1285. (17) Westberg, H. M.; Bystro¨m, M.; Leckner, B. Distribution of Potassium, Chlorine, and Sulfur between Solid and Vapor Phases during Combustion of Wood Chips and Coal. Energy Fuels 2003, 17, 18-28. (18) Davidsson, K. O.; Stojkova, B. J.; Pettersson, J. B. C. Alkali Emission from Birchwood Particles during Rapid Pyrolysis. Energy Fuels 2002, 16, 1033-1039. (19) Davidsson, K. O.; Korsgren, J. G.; Pettersson, J. B. C.; Ja¨glid, U. The Effects of Fuel Washing Techniques on Alkali Release from Biomass. Fuel 2002, 81, 137-142. (20) van Lith, S. C. Release of Inorganic Elements during Wood-Firing on a Grate. Ph.D. Thesis, Technical University of Denmark, Lyngby, Denmark, 2005.
966 Energy & Fuels, Vol. 20, No. 3, 2006
Figure 1. Schematic drawing of the laboratory-scale tube reactor with size specifications of the reactor tubes (Do ) outer diameter, Di ) inner diameter, L ) length).
Figure 2. Temperature profiles of the laboratory-scale tube reactor at 500, 700, and 1000 °C measured under gas conditions similar to the experimental conditions, but using an empty sample boat. The positions of the sample and thermocouple in the combustion experiments are indicated.
bed reactor, and a small sample of the obtained char was then combusted in the laboratory-scale tube reactor. (3) Method C (ash heating): in this method, a large amount of wood was completely combusted (at 500 °C) in the bench-scale fixed-bed reactor, after which a small sample of the obtained ash was heated in the laboratory-scale tube reactor. The methods resembled grate-fired conditions because of the pyrolysis and subsequent char burnout phases that the fuel particles underwent. Furthermore, the temperature on the grate in a woodfired boiler was expected to be in the range of 500-1150 °C. Before explaining the methods in further details, a description of the two different experimental setups will be given. The fuel used in this study was fiber board. This fuel was chosen because its content of ash (1.24% (w/w) db) and of the elements Cl, S, K, Na, Zn, and Pb was expected to be sufficient for accurate determination of the release of these elements by all three methods, so that a good comparison of the methods could be made. Laboratory-Scale Tube Reactor Setup. To study the release of inorganic elements during combustion of a small sample of woody biomass, a laboratory-scale tube reactor setup was designed and assembled. A schematic drawing of the setup can be found elsewhere.20 The setup included a gas mixing system, a reactor, a gas conditioning system, gas analyzers, a thermocouple, and a data acquisition system. The reactor consisted of a two-zone electrically heated oven, in which a cylindrical alumina tube was mounted horizontally, having water-cooled flanges at both ends. Inside the tube, a removable alumina tube was placed, in which a sample boat could be inserted. The configuration and dimensions of the reactor tubes are shown in Figure 1. Figure 2 shows that the temperature profiles under the experimental conditions were reasonably flat at the position of the sample in the reactor.
Van Lith et al. Two different types of sample boats were used in the study: a boat made out of an alumina tube with one end closed and a smaller boat made of a platinum/gold alloy (95% Pt). In the case of experiments up to 850 °C, the fuel was inserted into the alumina boat, whereas in the case of experiments above 850 °C, the fuel was inserted into two Pt/Au boats, which were both placed into the alumina boat to facilitate the insertion and positioning of the sample. The reason for using the Pt/Au boats at temperatures above 850 °C was to eliminate possible reactions between alkali compounds present in the ash and the alumina boat at elevated temperature, which would affect the results of the chemical analysis of the ash and thereby the release data. To prevent breakage of the alumina tubes as a result of thermal shock when a cold sample boat was inserted into the hot reactor, a layer of ceramic fiber insulation material was glued (using ceramic glue) inside the inner reactor tube, covering the bottom half. The front of the reactor can be sealed with a stainless steel plate, which contained openings for the primary gas inlet and a thermocouple that could be placed inside the fuel sample. Secondary air was added through a probe inserted at the back of the reactor. The composition and flow rate of the primary and secondary air were controlled by a series of mass flow controllers. At the back of the reactor, an alumina outlet tube was placed, followed by a Teflon tube leading to an Erlenmeyer flask to collect the tar produced during the experiments. After this flask, the major part of the flue gas left the system to the exhaust, but a small sample of the flue gas was pumped through a gas conditioning system. Here, the gas was cooled and cleaned using a universal filter and an aerosol filter and subsequently led to O2, CO, and CO2 analyzers. Measurements of the sample temperature and flue gas composition were acquired continuously and were monitored online during the experiments. Bench-Scale Fixed-Bed Reactor Setup. The bench-scale fixedbed reactor setup (used in methods B and C) consisted of a gas supply system, a large oven, gas analyzers, two thermocouples, and a data acquisition system. The oven was electrically heated and contained a removable stainless steel container of 64 L in which the fuel sample was inserted. Inside this container, a stainless steel grid was placed, which served to distribute the gas evenly through the fuel sample. The gas was introduced through a tube below the grid, and a sample of the flue gas was directed to CO and CO2 analyzers. The container with the fuel sample was shaken frequently to obtain a good mixing between the fuel and the air. Method A (Wood Combustion). Table 1 shows a comparison of the parameter settings applied during pyrolysis and combustion in methods A, B, and C. In method A (wood combustion), the laboratory-scale tube reactor setup was used to pyrolyze and subsequently combust a fuel sample. This was done as follows. When the desired temperature inside the reactor was reached and stabilized, the reactor was purged with nitrogen. A sample boat containing ∼30 g of fuel was then inserted, and as soon as the reactor was closed, a primary gas flow (5 NL/min) of pure nitrogen was added to create a pyrolysis atmosphere, and a secondary gas flow (5 NL/min) consisting of a mixture of 20% (v/v) oxygen and 80% (v/v) nitrogen was added at the end of the reactor to combust the evolved volatiles. The initial heating rate of the sample was ∼30 °C/min at an oven set-point of 500 °C and ∼170 °C/min at an oven set-point of 850 °C. After 1 h, 1% (v/v) of oxygen was introduced with the primary air. The oxygen content of the primary air was then increased stepwise to 20% (v/v) according to the time schedule given in Table 2. This was done to minimize the sample overshoot temperature during combustion (a typical overshoot temperature observed during the experiments performed in this way was ∼50 °C). Figure 3 shows an example of the change in sample temperature during an experiment at 700 °C. After the total combustion stage of 140 min, or when the CO and CO2 concentrations had dropped below 50 ppm, the sample boat was removed from the reactor and was cooled to room temperature under ambient conditions. Thereafter, the sample boat containing the ash residue was weighed, and the ash residue was removed and later chemically analyzed by inductively coupled
Quantifying Inorganic Release from Wood Combustion
Energy & Fuels, Vol. 20, No. 3, 2006 967
Table 1. Comparison of Parameter Settings during Pyrolysis and Combustion by Methods A, B, and C method A (wood combustion) Pyrolysis Stage reactor type temperature (range) (°C) amount of fuel (g) primary air flow (NL/min) time for pyrolysis (min) Low-Temperature Combustion Stage (Bench-Scale Fixed-Bed Reactor) temperature (°C) primary air flow (NL/min) oxygen content (% v/v) time for combustion (days) High-Temperature Combustion/Heating Stage (Laboratory-Scale Tube Reactor) temperature range (°C) type of sample amount of sample (g) a
laboratory-scale tube reactor 500-1150 30 5 60
method B (char combustion)
method C (ash heating)
bench-scale fixed-bed reactor 500 5370.6 3 960
bench-scale fixed-bed reactor 500 5334.8 3 960 500 50.7 1.4 11
500-1150 char from pyrolysis stage amount of char from 30-g wood
500-1150 char from pyrolysis stage 15/20a
500-1150 low-temperature ash 0.5/1a
Second value is amount used at 1000 and 1150 °C.
Table 2. Parameter Settings Applied in the Laboratory-Scale Tube Reactor (Similar for Methods A, B, and C) primary air
secondary air total residence time (min)
flow (NL/min) oxygen addition pattern during combustion stage (min)
flow (NL/min) oxygen content (%) pyrolysis combustion
0% (v/v) O2 1% (v/v) O2 3% (v/v) O2 6% (v/v) O2 10% (v/v) O2 20% (v/v) O2
5 60 10 10 30 60 30 5 20 60 140
plasma-optical emission spectroscopy (ICP-OES) (see below for details). Because the Pt/Au sample boats used in the experiments at temperatures above 850 °C Pt/Au can only contain ∼15 g of fuel, the experiments were repeated at these temperatures and the ash samples from both experiments were combined to obtain a sufficient quantity of ash to perform accurate chemical analysis. Method B (Char Combustion). The first part of the char combustion method consists of the production of a large amount of char. This was done by inserting the container with a preweighed fuel sample (∼5 kg) and a nitrogen flow of 3 NL/min into the bench-scale fixed-bed reactor. The sample temperature was increased from room temperature to 500 °C using a heating rate of 1 °C/min and then maintained at this temperature for 8 h. Afterward, the container was removed from the reactor and weighed to determine the mass loss during the pyrolysis. A small sample (15 g at temperatures up to 850 °C and 20 g at higher temperatures) of the char remains was then combusted in the laboratory-scale tube reactor setup. The conditions and procedure for this part of the method are the same as for the fuel combustion method described above (see also Table 2). Method C (Ash Heating). The ash heating method involves the production of a large amount of low-temperature (500 °C) ash in the bench-scale fixed-bed reactor setup. This was also done in two stages. (1) First, a large amount of fuel was pyrolyzed in exactly the same way as in the first part of the char combustion method. (2) During the second stage, the obtained char was subsequently combusted in the same setup, by changing the gas flow to 50 NL/ min of nitrogen and 0.7 NL/min of oxygen (1.4% (v/v) O2) but maintaining the temperature at 500 °C. Under these conditions it would take about 6 days to combust the sample, but because of a temporary shortage in the nitrogen supply, the experiment lasted 11 days. The reason for the low oxygen concentration (1.4% (v/v)) was to minimize the temperature increase of the sample during combustion. When low values of
Figure 3. Temperature measurement inside the sample (see Figure 2 for the position of the thermocouple) during pyrolysis and combustion at 700 °C according to method A (wood combustion) in the laboratoryscale tube reactor. The length of the pyrolysis and combustion stages, as well as the oxygen concentration of the primary air added during each combustion step (see also Table 2), are indicated. The initial heating rate of the sample was ∼1.5 °C/s in this experiment.
CO and CO2 were measured, the gas flow was switched to normal air to ensure complete combustion of the sample. Afterward, the container with the ash remains was removed from the reactor and weighed to determine the weight loss during the total experiment. A small sample (0.5 g at temperatures up to 850 °C and 1 g at higher temperatures) of ash was then heated in the laboratory-scale tube reactor setup. The conditions and procedure for this part of the method are also the same as for the fuel combustion method described above (see also Table 1). Fuel and Ash Analyzing Techniques. Chemical analyses of the fuel and ashes were performed by Elsam A/S Laboratory at Ensted Power Station in Denmark. The moisture content of the fuel was measured after drying ∼10 g of sample for 20 h in a N2 atmosphere at 105 °C. The ash content of the fuel was determined by combustion at 550 °C for 20 h. The contents of C, H, and N in the fuel were determined by an elemental analyzer (according to ASTM D5373). The concentrations of the inorganic elements in the fuel and ash samples were determined by ICP-OES, graphite furnace-atomic absorption spectrophotometry (GF-AAS), and ion chromatography (IC) (see Table 3). Before elemental analysis of the fuel, the sample was first ground using a 1-mm screen and afterward pulverized in a ball mill using steel beakers. The ash samples were ground and homogenized in agate mortars. Samples for ICP-OES and GF-AAS analyses were
Van Lith et al.
968 Energy & Fuels, Vol. 20, No. 3, 2006 Table 3. Overview of Analyzing Techniques for Determining Inorganic Elements in the Fuel and Ash Samples element
analyzing technique
S metals (except Pb) Pb
calorimeter combustion, IC (fuel analysis) ICP-OES (axial) (ash analysis) ICP-OES (axial) ICP-OES (radial) GF-AAS
Cl
Table 4. Characteristics of the Fiber Board Fuel Used in the Experimental Study and Wood Chips from Spruce for Comparison (db ) dry basis)
moisture (% (w/w)) ash (% (w/w) db) C (% (w/w) db) H (% (w/w) db) N (% (w/w) db) Cl (% (w/w) db) S (mg/g db) K (mg/g db) Na (mg/g db) Ca (mg/g db) Mg (mg/g db) Ti (mg/g db) Si (mg/g db) Al (mg/g db) Zn (mg/kg db) Pb (mg/kg db) K/Cl K/S K/Si K/Al K/Ti a
fiber board
spruce
6.8 1.24 48.2 6.2 3.6 0.05 0.3 0.64 0.18 1.5 0.21 4.5 0.55 0.35 27 16 1.2 1.8 0.84 1.3 0.17
6.6 0.95 50.2 6.3 0.13
