Energy & Fuels 2004, 18, 1385-1399
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Transformation and Release to the Gas Phase of Cl, K, and S during Combustion of Annual Biomass Jacob N. Knudsen, Peter A. Jensen,* and Kim Dam-Johansen CHEC Research Centre, Department of Chemical Engineering, Technical University of Denmark, Building 229, 2800 Kgs. Lyngby, Denmark Received March 3, 2004. Revised Manuscript Received May 26, 2004
The transformation of inorganic constituents in annual biomass was experimentally investigated at grate-combustion conditions. A laboratory fixed-bed reactor was applied to obtain quantitative information of the release of Cl, K, and S to the gas phase from six distinctively different annual biomass fuels. Samples of 4.0 g of biomass were combusted at well-controlled conditions at temperatures from 500 to 1150 °C. The elemental release was quantified by analysis of the residual ash and a mass balance on the system. The experimental results revealed that potassium was released to the gas phase in significant amounts at combustion above 700 °C. The potassium release increased with the applied combustion temperature for all biomass fuels; however, the quantity released was largely determined by the ash composition. At 1150 °C, between 50 and 90% of the total potassium was released to the gas phase. The biomass fuels with an appreciable content of silicate showed the lower release of potassium. Between 25 and 70% of the fuel chlorine was released below 500 °C; the residual chlorine was released by evaporation of KCl, mainly between 700 and 800 °C. Above 800 °C, the fuel chlorine was completely released to the gas phase for all of the samples. Between 30 and 55% of the fuel sulfur was released at 500 °C. The samples rich in K and Ca, but low in Si, displayed only a minor increase in the sulfur release as the combustion temperature was further increased. On the contrary, the sulfur release increased abruptly above 700-800 °C for the Si-rich samples. On the basis of the release quantification, the overall transformations of the ash-forming elements are discussed at grate-combustion conditions.
Introduction Annual biomass such as straw, stalks, and other agricultural residues is the more readily available renewable energy resource throughout the world.1 In Europe and North America, annual biomass is typically available in surplus from agricultural production. Disposal of annual biomass by combustion in heat- and power-producing boilers is an interesting option to make efficient use of a waste product and reduce the net CO2 emission. Biomass fuels are frequently combusted in grate-fired boilers ranging from a few megawatts up to more than 100 MWth. In Denmark, annual biomass such as straw has been fired in larger heat- and powerproducing grate boilers for more than a decade. However, the use of annual biomass as a fuel has proven to be a technical challenge. This is, among other things, because of the relatively high content of potassium, chlorine, and sulfur in annual biomass. Formation of acidic pollutants and high mass loadings of aerosols and deposition on heat-transfer surfaces of potentially corrosive components are among the encountered problems.2-5 During the combustion of annual biomass on a grate, Cl, K, and S are released to the gas phase * Corresponding author. E-mail:
[email protected]. Fax: +45 45 88 22 58. (1) Production Yearbook 2002; FAO Statistics Series; FAO: Rome, Italy, 2002; Vol. 56. (2) Michelsen, H. P.; Frandsen, F. J.; Dam-Johansen, K.; Larsen, O. H. Fuel Process. Technol. 1998, 54, 95-08.
but may also be partially retained in the bottom ash. The Cl, K, and S volatilized from the fuel are directly responsible for the mentioned problems. Thus, an assessment of the fraction of inorganic material that is released to the gas phase during combustion will improve our capability to predict the impacts of a given fuel in a combustion system. The release to the gas phase of alkali metals and other ash-forming elements from thermal conversion of biomass has been investigated in a limited number of publications.6-12 Molecular-beam mass spectroscopy (MBMS) was applied to directly detect the released (3) Sander, B.; Henriksen, N.; Larsen, O. H.; Skriver, A.; Ramsgaard-Nielsen, C.; Jensen, J. N.; Stærkind, K.; Livbjerg, H.; Thellefsen, M.; Dam-Johansen, K.; Frandsen, F. J.; van der Lans, R.; Hansen, J. Emissions, Corrosion and Alkali Chemistry in Straw-Fired Combined Heat and Power Plants. 1st World Conference on Biomass for Energy and Industry, Sevilla, Spain, June 2000. (4) Christensen, K. A.; Stenholm, M.; Livbjerg, H. J. Aerosol Sci. 1998, 29, 421-444. (5) Baxter, L. L. Fuel Process. Technol. 1998, 54, 47-78 (6) Dayton, D. C.; French, R. J.; Milne, T. A. Energy Fuels 1995, 9, 855-865. (7) Dayton, D. C.; Jenkins, B. M.; Turn, S. Q.; Bakker, R. R.; Williams, R. B.; Belle-Oudry, D.; Hill, L. M. Energy Fuels 1999, 13, 860-870. (8) Bjo¨rkman, E.; Stro¨mberg B. Energy Fuels 1997, 11, 1026-1032. (9) Olsson, J. G.; Ja¨glid, U.; Pettersson, J. B. C. Energy Fuels 1997, 11, 779-784. (10) Jensen, P. A.; Frandsen, F. J.; Dam-Johansen, K.; Sander, B. Energy Fuels 2000, 14, 1280-1285. (11) Zintl, F.; Stro¨mberg, B.; Bjo¨rkman, E. Release of Chlorine from Biomass at Gasification Conditions. 10th European Conference on Biomass for Energy and Industry, Wurzburg, Germany, June 1998.
10.1021/ef049944q CCC: $27.50 © 2004 American Chemical Society Published on Web 07/24/2004
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vapor species from combustion of annual biomass samples in a laboratory reactor.6,7 The investigation revealed that HCl, SO2, and minor amounts of K were volatilized during the devolatilization, whereas alkali was mainly volatilized during the char burnout, predominantly in the form of KCl. The amount of KCl released to the gas phase was found to increase when the combustion temperature was increased from 800 to 1100 °C. Likewise, SO2 was detected during char burnout at the higher temperature. Further investigations9,10 have revealed that a substantial fraction of the biomass K can be released as KCl during pyrolysis. Quantification of the pyrolysis release of Cl and K from wheat straw10 showed that Cl was released in two steps. Approximately 60% of the Cl was released when the pyrolysis temperature was increased from 200 to 400 °C. Between 400 and 700 °C, almost no Cl was released, and finally the residual Cl was released between 700 and 900 °C. No significant amount of K was volatilized below 700 °C, whereas above 700 °C, the volatilization of K increased progressively with temperature and approximately 25% was volatilized at 1050 °C. The release of alkali species to the gas phase during fuel devolatilization below 500 °C has been quantified by an online surface ionization technique.9,12 It was shown that the initial release accompanied the release of volatiles and was independent of the fuel Cl content. However, the low-temperature release constituted only 0.1-0.2% of the total K content. Most Cl and K and some S found in annual biomass can be removed by aqueous leaching.7,13 Direct detection of the gas-phase release of inorganic matter from combustion of leached biomass samples has confirmed that very little alkali vapor and HCl are released.7 On the other hand, the release of SO2 during devolatilization could not be eliminated by leaching.7 Global equilibrium calculations have also been applied in the literature to predict the distribution of ashforming elements between the condensed phase and the gas phase.6,10,14-18 On a qualitative scale, good agreements have been found between equilibrium predictions and experimental observations.6,10 However, quantitative predictions of the gas-phase release of inorganic constituents suffer from the assumptions made in the global equilibrium approach; i.e., all reactions are in equilibrium, and no temperature and compositional gradients appear. These assumptions are typically not satisfactorily fulfilled in thermal conversion systems. In grate-fired furnaces, the local atmosphere and temperature vary considerably along the grate; likewise, the gas-solid contact and mass-transfer limitations need (12) Davidsson, K. O.; Korsgren, J. G.; Pettersson, J. B. C.; Ja¨glid, U. Fuel 2002, 81, 137-142. (13) Jenkins, B. B.; Bakker, R. R.; Wei, J. B. Biomass Bioenergy 1995, 10, 177-200. (14) Hansen, L. A.; Nielsen, H. P.; Frandsen, F. J.; Dam-Johansen, K.; Hørlyck, S.; Karlsson, A. Fuel Process. Technol. 2000, 64, 189209. (15) Blander, M.; Pelton, A. D. Biomass Bioenergy 1997, 12, 295298. (16) Blander, M.; Milne, T. A.; Dayton, D. C.; Backman, R.; Blake, D.; Ku¨hnel, V.; Linak, W.; Nordin, A.; Ljung, A. Energy Fuels 2001, 15, 344-349. (17) Wei, X.; Lopez, C.; von Puttkamer, T.; Schnell, U.; Unterberger, S.; Hein, R. G. Energy Fuels 2002, 16, 1095-1108. (18) Furimsky, E.; Zheng, L. Fuel Process. Technol. 2003, 81, 7-21.
Knudsen et al.
to be considered. Hence, more detailed modeling is required to make reliable predictions of the gas-phase release of Cl, K, and S. As suggested by several authors,6,7,9,10,19 the mode of occurrence of ash-forming elements in the biomass structure is important for the release behavior. The majority of the ash-forming elements found in annual biomass are nutrients required for plant growth. However, inorganic impurities can also occur as a result of contamination of the biomass with soil during harvest or handling. In annual biomass, the seven most abundant ash-forming elements are typically Ca, K, Mg, Cl, P, S, and Si. K and Cl are usually present in high amounts in annual biomass. Cl and K are not metabolized by the plant but remain in ionic form.20 The ionic association of K and Cl is well in agreement with the fact that typically more than 90% can be removed by leaching.7,13 The main function of K+ and Cl- in plants is to maintain the pH and charge neutrality, regulate the osmotic pressure, and stimulate enzyme activity. The K+ and Cl- ions are characterized by a high mobility at all levels of the plant. In Cl-rich plants, Cl- is typically the main counteranion for K+ in the vacuole where K+ is accumulated. In Cl-lean plants, the malate(2-) ion is the main accompanying anion for K+. K and Cl are mainly found as free ions in solution in the plant fluids.20 During drying of the plant material, Cl and K in solution is likely to precipitate as salts. However, the exact association and distribution of K and Cl in plants will be dependent on plant species, growth conditions, and handling after harvest. S is one of the ash-forming elements that is metabolized and incorporated into the organic structure of plants. S is assimilated as SO42-, which can be directly esterified through a hydroxyl group of an organic compound; however, S is mainly reduced to the level of S2-. Reduced S is predominantly incorporated into the amino acids, cysteine and methionine, which are used for synthesis of proteins.20 Silicon is usually present in relatively high concentrations in annual biomass, depending on the plant species. Si is assimilated as monosilicic acid, Si(OH)4, and in most cereal species, it forms a silicate network structure in the cell walls.20 In the cereal species with a significant Si content, Si is mainly present as a silicate skeleton on the external surface of the straw, which provides structural strength and protection against microorganisms.20 Si may also occur in biomass as discrete particles of SiO2 and clay minerals as a result of soil contamination. The alkaline-earth cations, Ca2+ and Mg2+, are also required for plant growth and observed in most biomass fuels in moderate amounts. Ca and, in particular, Mg do to a large extent form complexes with organic counterions.20 Phosphor is required in minor amounts. Phosphor is assimilated as phosphate and remains as inorganic phosphate or is esterified through a hydroxyl group of an organic compound.20 (19) Westberg, H. M.; Bystro¨m, M.; Leckner, B. Energy Fuels 2003, 17, 18-28. (20) Marschner, H. Mineral Nutrients in higher plants, 2nd ed.; Academic Press: London, 2002; Chapter 8.
Combustion of Annual Biomass
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Table 1. Characteristics of the Six Annual Biomass Fuels Applied in the Experimental Investigation property
SR1 rice
SR2 barley
SR3 wheat
SL1 oat
moisture (wt %) 7.4 8.5 8.4 7.8 ash (wt %, dry) 7.6 6.9 4.8 3.8 C (wt %, dry) 44 45 46 48 H (wt %, dry) 5.9 6.0 6.1 6.3 Cl (wt %, dry) 0.71 0.79 0.27 0.05 P (wt %, dry) 0.079 0.055 0.053 0.11 S (wt %, dry) 0.17 0.20 0.17 0.14 Ca (wt %, dry) 0.49 0.34 0.35 0.72 K (wt %, dry) 1.5 2.3 1.2 0.55 Na (wt %, dry) 0.055 0.26 0.019 0.13 Mg (wt %, dry) 0.25 0.091 0.069 0.074 Si (wt %, dry) 1.7 0.81 0.79 0.27 K/Si (mol/mol) 0.63 2.0 1.1 1.5 (Ca + Mg)/Si 0.37 0.42 0.41 2.2 Cl/K 0.52 0.38 0.25 0.1 qa 2.5 2.5 2.6 4.0
SL2 carinata
SL3 rape
7.3 4.9 45 6.0 0.05 0.14 0.26 0.60 1.4 0.01 0.063
