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Energy & Fuels 2008, 22, 1598–1609
Release to the Gas Phase of Inorganic Elements during Wood Combustion. Part 2: Influence of Fuel Composition Simone C. van Lith,* 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 December 4, 2006. ReVised Manuscript ReceiVed January 28, 2008
Combustion of wood for heat and power production may cause problems such as ash deposition, corrosion, and harmful emissions of gases and particulate matter. These problems are all directly related to the release of inorganic elements (in particular Cl, S, K, Na, Zn, and Pb) from the fuel to the gas phase. The aims of this study are to obtain quantitative data on the release of inorganic elements during wood combustion and to investigate the influence of fuel composition. Quantitative release data were obtained by pyrolyzing and subsequently combusting small samples of wood (∼30 g) at various temperatures in the range of 500–1150 °C in a laboratory-scale tube reactor and by performing mass balance calculations based on the weight measurements and chemical analyses of the wood fuels and the residual ash samples. Four wood fuels with different ash contents and inorganic compositions were investigated, including wood chips from spruce and beech, bark, and fiber board. The results showed a high release of Cl (∼85–100%) and S (∼50–70%) already at 500 °C, so that only small variations in the release trends of Cl and S were seen between the different fuels in the range of 500–1150 °C. The release of the alkali metals K and Na was, however, strongly dependent on both the temperature and the fuel composition under the investigated conditions. The release of the heavy metals Zn and Pb started around 500 °C and increased sharply to more than 85% at 850 °C in the case of spruce, beech, and bark, and was therefore mainly dependent on the temperature. By comparing the data to literature data, and by using tools such as scanning electron microscopy, chemical fractionation analysis, and equilibrium calculations, a better understanding of the release mechanisms was obtained. Mechanisms for the release of Cl, S, K, Na, Zn, and Pb during wood combustion are proposed.
Introduction On basis of the Kyoto Protocol and the White Paper, the European Union has set the goal to considerably increase the use of biomass for energy production to reduce the emission of CO2, which is the most abundant greenhouse gas. To reach this goal, new biomass combustion plants will be established and fossil fuels will be (partly) replaced by biomass fuels in existing power plants. However, ash-related problems in biomass-fired boilers, such as deposit formation and corrosion, are still unsolved. Moreover, biomass combustion may lead to the formation of harmful gases (such as SO2 and HCl) and small particulates (aerosols). Deposition and corrosion have been observed to be the most severe when firing biomass fuels rich in K, Cl, and S, such as straws and grasses.1–3 However, even though wood fuels generally contain lower amounts of these “troublesome elements” compared to annual biomass fuels, fouling and significant corrosion of superheater tubes have also * Corresponding author. E-mail:
[email protected]. Fax: +45 45 88 22 58. Phone: +45 45 25 28 46. (1) Miles, T. R.; Miles, T. R., Jr.; Baxter, L. L.; Bryers, R. W.; Jenkins, B. M.; Oden, L. L. Alkali Deposits Found in Biomass Power Plants. A Preliminary InVestigation of Their Extent and Nature; Report NREL/TP433-8142 Sand 96-8225, National Renewable Energy Laboratory: Golden, CO, 1996. (2) Jensen, P. A.; Stenholm, M.; Hald, P. Deposition Investigation in Straw Fired Boilers. Energy Fuels 1997, 11, 1048–1055. (3) Michelsen, H. P.; Frandsen, F.; Dam-Johansen, K.; Larsen, O. H. Deposition and High Temperature Corrosion in a 10 MW Straw Fired Boiler. Fuel Process. Technol. 1998, 54, 95–108.
been reported for wood-fired boilers.4,5 Furthermore, it is important to understand ash formation and behavior in woodfired boilers, because of the steadily decreasing emission limits and the increasing desire to improve plant efficiencies. This work focuses on the release of inorganic elements to the gas phase during combustion of wood fuels, which is the first step in the inorganic aerosol formation process. To be able to model ash formation and behavior, quantitative data on the release of inorganic elements are needed. The volatile elements Cl, S, K, Na, Zn, and Pb are of particular interest, as they play a major role regarding gaseous and aerosol emissions, deposit formation, corrosion, and ash utilization/disposal. In part 1 of this article series,6 we presented and discussed three different methods to quantify the release of inorganic elements during wood combustion, at conditions that resemble grate-firing. The most important conclusion was that secondary reactions of Cl and S with the char take place during pyrolysis in larger fuel beds, which influence the net release of Cl, S, but also K and Na from the fuel bed. In the present article, the influence of fuel composition on the primary release of Cl, S, K, Na, Zn, and Pb is discussed. The (4) Henriksen, N.; Montgomery, M.; Larsen, O. H. High Temperature Corrosion in Biomass-Fired Boilers. VDI-Berichte 2002, 1680, 111–133. (5) Henderson, P.; Andersson, C.; Kassman, H. The Use of Fuel Additives in Wood and Waste Wood-Fired Boilers to Reduce Corrosion and Fouling Problems. VGB PowerTech 2004, 6, 58–62. (6) van Lith, S. C.; Alonso-Ramírez, V.; Jensen, P. A.; Frandsen, F. J.; Glarborg, P. Release of Inorganic Elements during Wood Combustion. Part 1: Development and Evaluation of Quantification Methods. Energy Fuels 2006, 20, 964–978.
10.1021/ef060613i CCC: $40.75 2008 American Chemical Society Published on Web 03/26/2008
Inorganic Element Release during Wood Combustion
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Table 1. Chemical Composition of the Fuelsa 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) P (mg/g db) Mn (mg/g db) Si (mg/g db) Al (mg/g db) Ti (mg/g db) Zn (mg/kg db) Pb (mg/kg db)
analyzing technique
spruce
beech
bark
fiber board
105 °C for 20 h 550 °C for 20 h elemental analyzer (ASTM D5373) elemental analyzer (ASTM D5373) elemental analyzer (ASTM D5373) calorimeter combustion, IC total digestion, ICP-OES total digestion, ICP-OES total digestion, ICP-OES total digestion, ICP-OES total digestion, ICP-OES dry fusion made with sodium carbonate-boric acid, ICP-OES dry fusion made with sodium carbonate-boric acid, ICP-OES total digestion, ICP-OES total digestion, ICP-OES total digestion, ICP-OES total digestion, ICP-OES total digestion, GF-AAS
6.6 0.95 50.2 6.3 0.13
