Energy & Fuels 1999, 13, 839-850
839
High-Temperature Melting Behavior of Urban Wood Fuel Ash P. Thy,*,† B. M. Jenkins,‡ and C. E. Lesher† Department of Geology and Department of Biological and Agricultural Engineering, One Shields Avenue, University of California, Davis, California 95616 Received November 10, 1998. Revised Manuscript Received March 30, 1999
Ashes from commercial wood fuel blends show wide compositional ranges that in part can be related to an admixed soil component. To evaluate agglomeration potentials and to formulate predictive models, an experimental study was conducted of an urban wood waste fuel ash from an operating power plant in California. The melting relations were investigated from the liquidus at 1276 °C to the near solidus at about 1130 °C. The liquidus phase is melilite composed of a solid solution dominated by åkermanite and sodium melilite. A phosphate phase appears at 1207 °C and can be described as a solid solution of calcium phosphate, sodium phosphate, and calcium disilicate. Garnet appears at 1159 °C and is composed of an andradite, pyrope, and grossular solid solution. The alkali metals are partially lost to the atmosphere during the experimental duration. Sodium loss is moderate and dependent on the liquid remaining. Potassium shows very low partitioning into the solid phases and is strongly lost from the slag. The experimental results have been used to formulate a calculation procedure for evaluating the high-temperature compositional behavior of urban wood fuel slag. The results predict strong potassium volatilization tendencies for relatively pure wood fuel ashes with less than about 47 wt % SiO2. Because the SiO2 content increases with increasing soil component, the effect of soil is to retain potassium in the solid residue. In addition, the effect of an admixed soil component on wood fuel slag behavior is principally to increase viscosity and has little effects on the surface tension. Because of the inverse effect of viscosity on agglomeration, the result is that an admixed soil to wood fuel will reduce alkali losses and agglomeration potential.
Introduction Slag and ash deposits in combustion furnaces and boilers fueled by biomass cause operating problems and reduction in efficiency and steam generation.1 Potassium and silica are present in large concentrations in most annual growth plant materials and are directly linked to the formation of tenacious surface deposits on firesides and heat transfer surfaces. During combustion, the organic structure of plant material is decomposed and the inorganic material is released and transported either in the form of solid particles or as vapor species in the combustion gas. Particulate adventitious materials undergo phase transformations and reactions with other inorganic components. Ash particles principally composed of refractory silicate minerals and liquids land on fireside surfaces and also agglomerate and sinter to form slag. Entrained particulate matter and potassium vapor species are carried to the low-temperature parts of the boiler were they may react with other combustion gas components and result in deposits composed of mixtures of refractory particles and potassium sulfates, carbonates, hydroxides, and chlorides.1-3 Such fouling * Author to whom correspondence should be addressed. † Department of Geology. ‡ Department of Biological and Agricultural Engineering. (1) Baxter, L. L. Biomass Bioenergy 1993, 4, 85-102. (2) Rindt, D. K.; Jones, M. L.; Schobert, H. H. In Fouling and Slagging Resulting from Impurities in Combustion Gases; Bryers, R. W., Ed.; Energy Foundation: New York, 1982; pp 17-35.
of heat exchangers prevents the use of some common types of biomass at many power plant facilities.4 While the major mechanisms leading to fouling have been investigated,1,5-7 our ability to predict and limit fouling and slag deposition is empirical and at best unreliable. The nature of the condensed phase dictates the severity of fouling, the tenacity of the deposits, and the ease of removal. Although predictive models utilizing equilibrium between gas and the condensed phases have been developed,8,9 little is known about the hightemperature phase relations and physical and chemical properties of the condensed phases.10-12 Empirical as(3) Raask, E. Mineral Impurities in Coal Combustion; Hemisphere Publishing: Washington, D.C., 1985. (4) Turnbull, J. H. Biomass Bioenergy 1993, 4, 75-84. (5) French, R. J.; Milne, T. A. Biomass Bioenergy 1994, 7, 1-6. (6) Jenkins, B. M.; Bakker, R. R.; Wei, J. B. Biomass Bioenergy 1996, 10, 177-200. (7) Miles, T. R.; Miles, T. R., Jr.; Baxter, L. L.; Bryers, R. W.; Jenkins, B. M.; Oden, L. L. Biomass Bioenergy 1996, 10, 125-138. (8) Hastie, J. W.; Plante, E. R.; Bonnell, D. W. In Metal Bonding and Interactions in High-Temperature Systems with Emphasis on Alkali Metals; Cole, J. L.; Stwalley, W. C., Eds.; American Chemical Society, Symposium Series 179: Washington, D.C., 1982; pp 543-600. (9) Cook, L. P.; Hastie, J. W. In Mineral Matter and Ash in Coal; Vorres, K. S., Ed.; American Chemical Society, Symposium Series 301: Washington, D. C., 1986; pp 277-286. (10) Huffman, G. P.; Huggins, F. E. In Mineral Matter and Ash in Coal; Vorres, K. S., Ed.; American Chemical Society, Symposium Series 301: Washington, D.C., 1986; pp 100-113. (11) Raask, E. In Mineral Matter and Ash in Coal; Vorres, K. S., Ed.; American Chemical Society, Symposium Series 301: Washington, D.C., 1986; pp 138-155.
10.1021/ef980249q CCC: $18.00 © 1999 American Chemical Society Published on Web 05/13/1999
840 Energy & Fuels, Vol. 13, No. 4, 1999
sessment techniques, such as the standard ASTM cone test for determining fusibility of ash,13 are unsuccessful in predicting the initial melting properties of biomass ash or the tenacity of deposits upon formation. Attempts have been made to use ternary equilibrium phase diagrams to predict melting behavior,14 but their significance is uncertain because of the multicomponent nature of ashes. A better approach is offered by direct experimental melting studies of natural ashes and has led to a more reliable understanding of the phase equilibria and melting behavior of coal slags.11,15-17 These studies have been extended to biomass fuel systems, but so far generally below melting such as by Misra et al.18 and Olanders and Steenari.12 The principal reason for performing high-temperature melting experiments on coal and biomass ashes is the desire to apply the results to ashes for which only the composition is known and to be able to predict their melting and slagging behavior. This paper is the first in a series of experimental determinations of phase equilibria and phase compositions of typical biomass ashes as a function of temperature. Here we report the high-temperature experimental characterizations of an urban wood fuel ash selected from considerations of the compositional variation of ashes of urban wood waste fuel typically received at commercial power plant facilities. The composition of approximately 60 ashes19 is illustrated in Figure 1 on binary variation diagrams with SiO2 (wt %) as the abscissa. The observed variation is ascribed to binary mixing between a high CaO and low Al2O3 and SiO2 component and a low CaO and high Al2O3 and SiO2 component. This trend reflects admission of a soil component high in Si and Al to the fuel mixture as is typical for commercial fuels. The fuel selected as starting material for the melting experiments is a relatively pure low SiO2 and high CaO urban wood fuel (Figure 1; Table 1) obtained from an operating power plant in California (Woodland Biomass, Woodland, California). The results are used to develop and evaluate predictive models for slag formation and its physical and compositional properties. These models are tested on the commercial range of urban wood fuel ashes shown in Figure 1. Experimental and Analytical Methods The ash content of the wood fuel is determined over a range of temperatures from 575 to 1000 °C and is constant at about 3.2% (575 °C, 3.39%; 750 °C, 3.02%; 900° C, 3.09%; 1000 °C, 3.16%). The ash produced at 575 °C was used in this study. The ash was ground in an agate mortar, mixed with an organic binder (PVA), and formed into a pellet. The pellet was broken into 20-30 mg pieces that were sintered to 4 mil platinum (12) Olanders, B.; Steenari, B.-M. Biomass Bioenergy 1995, 8, 105115. (13) ASTM. Fusibility of coal and coke ash. D1857-68, 1986. (14) Huggins, F. E.; Kosmack, D. A.; Huffman, G. P. Fuel 1981, 60, 577-584. (15) Kalmanovitch, D. P.; Williamson, J. In Mineral Matter and Ash in Coal; Vorres, K. S., Ed.; American Chemical Society, Symposium Series 301: Washington, D.C., 1986; pp 234-255. (16) Huffman, G. P.; Huggins, F. E.; Dunmyre, G. R. Fuel 1981, 60, 585-597. (17) Biggs, D. L.; Lindsay, C. G. In Mineral Matter and Ash in Coal; Vorres, K. S., Ed.; American Chemical Society, Symposium Series 301: Washington, D.C., 1986; pp 128-137. (18) Misra, M. K.; Ragland, K. W.; Baker, A. J. Biomass Bioenergy 1993, 4, 103-116. (19) B. M. Jenkins, unpublished compilation, 1998.
Thy et al.
Figure 1. The compositional variation of CaO, Al2O3, and SiO2 in a compilation of urban wood waste fuel ashes obtained from various sources.19 All analyses have been normalized to 100% SiO2, TiO2, Al2O3, Fe2O3, MnO, MgO, CaO, Na2O, K2O, and P2O5 (wt %). All iron is calculated as Fe2O3. The composition of the superliquidus liquid from Table 4 is indicated by boxed symbols. Table 1. Wood Ash Composition SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO a
30.18 0.72 7.47 4.68 0.26 4.97 19.23
Na2O K2O P2O5 SO3 Cl LOIa total
3.58 5.90 2.42 3.26 0.74 17.12 100.45
LOI represents loss on ignition after burning in air at 575 °C.
wire loops20 by heating in a H2-O2 gas torch. The experimental charges were suspended in an isothermally controlled vertical quench furnace21 in air at atmospheric pressure. The furnace temperature was kept constant during the duration of the experiments and was monitored by a Pt100/Pt90Rh10 (S type) thermocouple calibrated against the melting point of gold.22 The experimental temperatures were chosen from above the liquidus to the near solidus in approximate steps of 10 °C, and duration was varied from 50 min near the liquidus to 5500 min near the solidus. All experiments were terminated by quenching in air. The experimental conditions are summarized in Table 2. The experimental products were mounted in epoxy, sectioned, and surface polished to allow examination with reflected light and scanning electron microscopes. The glass and coexisting mineral phases were analyzed using a CAMECA SX50 electron microprobe operated at 15 kV voltage, a beam current of 10 nA, and counting times between 10 and 30 s. Natural minerals and synthetic oxides were used as standards. The potential volatile elements, Na and K, were analyzed first with a 10 µm beam size and counting times of 10 s in order to minimize volatilization.23,24 A fused international rock (20) Donaldson, C. H.; Williams, R. J.; Lofgren, G. Am. Mineral. 1975, 60, 324-326. (21) Williams, R. J.; Mullins, O. NASA Technical Memorandum 58234, 1981. (22) Biggar, G. M. Mineral. Mag. 1972, 38, 741-770. (23) Spray, J. G.; Rae, D. A. Can. Mineral. 1995, 33, 323-332. (24) Reed, S. J. B. Electron Microprobe Analysis, 2nd ed.; Cambridge University Press: Cambridge, 1996.
High-Temperature Melting Behavior of Urban Wood Fuel Ash
Energy & Fuels, Vol. 13, No. 4, 1999 841
Table 2. Experimental Conditions and Resultsa losses (wt % oxides) run no.
temp. (°C)
duration (min)
glass
36 28 27 26 25 24 23 15 17 5 4 3 1 8 9
1299 1289 1280 1271 1258 1249 1239 1228 1210 1197 1187 1178 1168 1149 1139
50 1560 1560 1375 1435 1487 1480 1415 1255 1340 1430 1295 1460 4210 5555
1.000 1.000 1.000 0.907 0.892 0.734 0.794 0.745 0.753 0.609 0.532 0.461 0.367 0.145 0.143
melilite
0.093 0.108 0.266 0.206 0.255 0.247 0.370 0.443 0.485 0.566 0.621 0.652
phosphate
0.021 0.025 0.054 0.067 0.093 0.091
garnet
0.143 0.114
Na2O
K 2O
P2O5
∑R2
2.20 2.00 1.70 1.80 1.20 1.50 1.30 1.20
4.00 3.90 3.60 3.50 2.60 3.60 3.70 3.20 3.40 3.10 2.20 3.30 4.40 4.40
1.00 0.70 0.50 0.50 0.50 0.50 0.20
0.617 0.495 0.247 0.272 0.193 0.338 0.327 0.282 0.223 0.111 0.253 0.405 0.419 0.276
0.50 0.70 0.50 0.30 0.40
a Phase proportions (wt %) have been estimated by least-squares mixing calculations and the determined phase compositions of Table 4. Na2O, K2O, and P2O5 losses have been estimated by including these oxides in the mixing calculations. ∑R2 is the sum of the square of the residuals of the mixing calculations. The calculations use interpolated phosphate compositions for run nos. 3 and 5 since this phase was not analyzed for these.
Table 3. Analytical Precision and Accuracya rhyolite Glass SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 total
Experimental Results
basaltic Glass
N ) 10
1 SD
rec.
N ) 50
1 SD
rec.
76.26 0.09 12.18 1.30 0.02 0.03 0.43 3.70 4.63 0.00 98.64
0.30 0.04 0.13 0.11 0.02 0.02 0.03 0.07 0.14 0.00
76.71 0.12 12.06 1.37 0.03
