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Energy & Fuels 2001, 15, 1110-1122
The Influence of Inorganic Materials on the Thermal Deactivation of Fuel Chars Alfredo Zolin, Anker Jensen,* Peter Arendt Jensen, Flemming Frandsen, and Kim Dam-Johansen Department of Chemical Engineering, Technical University of Denmark, Building 229, DK-2800 Lyngby, Denmark Received December 30, 2000. Revised Manuscript Received June 5, 2001
The influence of inorganic material on the thermal deactivation of a biomass fuel and a coal was investigated. Reactivity experiments were performed for chars from wheat and leached wheat straw pyrolyzed at a wide range of heat treatment temperatures (HTT) in a thermogravimetric analyzer (TGA) and entrained flow reactor (EFR). The HTT were varied from 973 to 1673 K in the TGA, and 973 to 1613 K in the EFR. Reactivity experiments were also carried out for chars from the low rank coal, Dietz and demineralized Dietz, prepared in the TGA at the same conditions as the biomass chars. From the TGA data, the biomass chars show at any HTT a higher reactivity than the coal chars, but owing to thermal deactivation the reactivity of all fuel chars converge at the highest HTT. The influence of catalysts for char oxidation in both the biomass and the coal is significant up to HTT ) 1273 K. Likely mechanisms for the loss of catalytic activity at high HTT in the TGA are discussed. The straw chars prepared in the EFR are on average a factor 30 to 40 times more reactive than the leached straw chars prepared at the same conditions in this reactor. SEM analysis of the EFR chars reveal that the main active catalyst, potassium, is found both in the organic matrix and in Si-rich regions of the char. Straw and leached straw produce chars that may have undergone a plastic stage during pyrolysis, similar to what is observed in coal chars. Using a common activation energy, the reactivity of the leached straw char prepared in the EFR at 1613 K is slightly higher than that of a typical char from a coal used in utility boilers (Cerrejon), suggesting that leached straw can be used in co-firing applications.
1. Introduction The utilization of biomass in co-firing represents an important alternative for reduction of net CO2 levels from boiler units in power stations. Biomass fuels are considered CO2 neutral and consequently their use in the power plant industry is gaining interest. In Denmark for example, the government, apart from having imposed a stop in the utilization of coal in future power units, passed a legislation that forced the power industry to burn 1.0 to 1.2 million tons of straw per year by the year 2000. Co-firing of coal with biomass, and particularly wheat straw in existing utility boiler units is an appealing possibility for reducing net CO2 emissions and embraces other environmental benefits, such as the potential reduction of NOx and SOx levels from the plants.1 Although straw-fired boilers have suffered from severe operational problems due to slagging and fouling,2-4 the use of a relatively low straw-to-coal ratio in co-firing * Corresponding author: Phone: (45) 45 25 28 00 (Denmark). Fax: (45) 45 88 22 58. E-mail:
[email protected]. (1) Pedersen, L. S.; Morgan, D. J.; van de Kamp, W. L.; Christensen, J.; Jespersen, P.; Dam-Johansen, K. Energy Fuels 1997, 11, 439-446. (2) Nielsen, H. P.; Frandsen, F. J.; Dam-Johansen, K.; Baxter, L. L. Prog. Energy Combust. Sci. 2000, 26, 283-298. (3) Hansen, L. A.; Nielsen, H. P.; Frandsen, F. J.; Dam-Johansen, K.; Hørlyck, S.; Karlsson, A. Fuel Process. Technol. 2000, 64, 189209. (4) Nielsen, H. P.; Baxter, L. L.; Sclippab, G.; Morey, C.; Frandsen, F. J.; Dam-Johansen, K. Fuel 2000, 79, 131-139.
applications has not shown negative effects in terms of deposits formation.5 A likely explanation to this is the interaction of K in the straw with Si and Al-Si species present in the coal ash, generating components with a relatively high melting temperature that do not easily form deposits.5 Wheat straw, however, being characterized by its high content of potassium and chlorine may still cause serious operational problems to the plant, for example the deactivation of selective catalytic reduction (SCR) catalysts6 and a reduction in the quality of the fly ash,7 thus preventing its use in the cement industry. During combustion, the inorganic materials participate not only in ash formation, but also as catalysts for the char-oxygen reaction. The way in which the inorganic material is distributed in the solid fuels is, however, different. In coals, most of the inorganic material is present as minerals, whereas in straw the inorganic materials can also exist as salts or are organically bound.8 As a result, in coal combustion only a small fraction of the inorganic material vaporize (up (5) Andersen, K. H.; Frandsen, F. J.; Hansen, P. F. B.; WieckHansen, K.; Rasmussen, I.; Overgaard, P.; Dam-Johansen, K. Energy Fuels 2000, 14, 765-780. (6) Hjalmarson, A. Technical report, IEA Coal Research. IEACR/ 24, 1990. (7) Sander, B.; Nielsen, B. Proceedings of the International Energy Agency Bioenergy Agreement Seminar; Mitchell, C. P., Bridgewater, A. V., Eds.; Snekkersten: Denmark, 1993. (8) Jensen, P. A.; Frandsen, F. J.; Dam-Johansen, K.; Sander, B. Energy Fuels 2000, 14, 1280-1285.
10.1021/ef000288d CCC: $20.00 © 2001 American Chemical Society Published on Web 08/17/2001
Inorganic Materials and Thermal Deactivation of Fuel Chars
to 4 wt %), whereas in straw the fraction of the inorganic material vaporized can be up to 75 wt % at 1473 K.5 In coals, and especially low rank coals, which contain up to 2 wt % of calcium minerals, the catalyst action in the char-oxygen reaction is caused mainly by CaO, which can be highly dispersed in the carbonaceous matrix.9-12 At high temperatures, however, the catalytic activity of CaO is reduced, since the CaO crystallites may vaporize, or sinter, thus reducing the available CaO surface area and probably also blocking the micropores in the char.12 In straw, although being relatively rich in calcium, the main inorganic catalyst is potassium (K). Alkali metals such as potassium and sodium (Na) are known to be active catalysts in the reaction with O-containing species.13 The reason for the high catalytic activity appears also to be the dispersion of the alkali metal. K has a lower propensity to form carbonate crystallites and thereby to lose dispersion compared to other alkali metals such as Na. The role of alkali metals in straw is not only confined to char combustion. Even tiny amounts of K can alter the pyrolysis behavior, and hence give rise to chars of different morphological characteristics than when the salts are not present in the raw fuel.14 Because of the deteriorating effect of potassium and chlorine on the activity of SCR catalysts and the quality of fly ash in co-firing applications, a pretreatment process in which the straw is leached is highly attractive, since most of the potassium, chlorine, and other salts can be removed from the fuel by this procedure.15,16 It is known, that this has implications on the reduction of nitrogen oxides over char,17 but it still remains to investigate how the removal of K affects the combustion reactivity of the char at a wide range of temperatures. In solid fuel combustion, both pyrolysis and char combustion are important steps. Char combustion in particular is directly related to the reactivity of the char (e.g., mass-based reactivity measured by thermogravimetry under kinetically controlled conditions) and this in turn is severely influenced by the heat treatment. The pyrolysis and combustion of leached (washed) and wheat straw has been studied to some extent.14,18-24 However, (9) Radovic, L. R.; Walker, P. L., Jr.; Jenkins, R. G. Fuel 1983, 62, 209-212. (10) Yamashita, H.; Nomura, M.; Tomita, A. Energy Fuels 1992, 6, 656-661. (11) Gopalakrishnan, R.; Fullwood, M. J.; Bartholomew, C. H. Energy Fuels 1994, 8, 984-989. (12) Gopalakrishnan, R.; Bartholomew, C. H. Energy Fuels 1996, 10, 689-695. (13) Moulijn, J. A.; Kapteijn, F. Carbon 1995, 33, 1155-1165. (14) Raveendran, K.; Ganesh, A. Fuel 1998, 77, 769-781. (15) Jensen, P. A.; Dam-Johansen, K.; Sander, B. Proceedings of the 2nd Olle Lindstro¨ m Symposium; Stockholm, Sweden, 9-11 June, 1999. (16) 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. (17) Sørensen, C. O.; Johnsson, J. E.; Jensen, A. Reduction of NO over wheat straw char. Energy Fuels, submitted. (18) Antal, M. J., Jr.; Varhegyi, G. Ind. Eng. Chem. Res. 1995, 34, 703-717. (19) Jensen, A.; Dam-Johansen, K.; Wo´jtowicz, M. A.; Serio, M. A. Energy Fuels 1998, 12, 929-938. (20) Ganga Devi, T.; Kannan, M. P. Energy Fuels 2000, 14, 127130. (21) Enrich, E.; Bu¨rkle, S.; Meza-Renken, Z. I.; Rumpel, S. J. Anal. Appl. Pyrolysis 1999, 49, 221-241. (22) Di Blasi, C.; Buonanno, F.; Branca, C. Carbon 1999, 37, 12271238. (23) Wornat, M. J.; Hurt, R. H.; Yang, N. Y. C.; Headley, T. Combust. Flame 1995, 100, 131-143.
Energy & Fuels, Vol. 15, No. 5, 2001 1111
no explicit account has been made on the effect of leaching of straw on the reactivity of chars pyrolyzed at well-defined conditions and at a wide range of heat treatment temperatures, particularly above 1273 K. An understanding of the relation between char reactivity and heat treatment temperature is important for accurate predictions of carbon burnout in combustion processes. Apart from shedding light on the role of potential catalysts in char combustion at a wide range of temperatures, the data can be used to expand current databases on char thermal deactivation, used in annealing sub-models of char combustion codes.25-28 In the present paper, we investigate the influence of inorganic materials on the loss of char reactivity with heat treatment temperature. This is based on experiments performed on leached and raw wheat straw, which represent the main fuels of this work. For comparison, the analysis is complemented with equivalent experiments performed on a low rank coal Dietz and its demineralized counterpart. Chars from Dietz coal and demineralized Dietz were prepared in a thermogravimetric analyzer (TGA), whereas straw and leached straw chars were prepared in both the TGA and an entrained flow reactor (EFR). Additionally, for the straw and leached straw chars prepared in the EFR, scanning electronic microscope (SEM) combined with energy-dispersive X-ray (EDX) analysis were performed to gain information on the fate of potassium during pyrolysis at different HTT. 2. Experimental Section 2.1. Samples. The raw fuels used were a Danish wheat straw (Straw), and a low rank coal Dietz from the United States.. The leached straw (L. Straw) was prepared by repeated washing in distilled water at 80 °C29 until the potassium content was reduced to 5% of its original content in the raw fuel. The demineralized coal (Dem. Dietz) was produced following the procedure of Radovic et al.30 Table 1 shows the fuel properties and ash analysis. 2.2. Char Pyrolysis and Char Oxidation. The description of the reactors used for the preparation of the chars as well as for char reactivity experiments is presented in previous communications.31,32 Briefly, an entrained flow reactor (EFR) and a thermogravimetric apparatus Netzsch STA 409C (TGA) operating at atmospheric pressure were used to produce the chars in an inert atmosphere. In the TGA, the experimental runs were divided in pyrolysis and char oxidation steps. The chars were prepared in nitrogen at a heating rate of 45 K/min (0.75 K/s) to different heat treatment temperatures (HTT) with (24) Sørensen, L. H. Fuel reactivity as a function of temperature, pressure and conversion. Ph.D. Thesis, Risø National Laboratory, Denmark, 1994. (25) Hurt, R.; Sun, J. K.; Lunden, M. Combust. Flame 1998, 113, 181-197. (26) Sun, J. K.; Hurt, R. H. Proc. Combust. Inst. 2000, 28, 22052213. (27) Zolin, A.; Jensen, A.; Dam-Johansen, K. Proc. Combust. Inst. 2000, 28, 2181-2188. (28) Zolin, A.; Jensen, A.; Dam-Johansen, K. Coupling thermal deactivation with oxidation for predicting the combustion of a solid fuel. Combust. Flame. in press. (29) Varhegyi, G.; Szabo, P.; Antal, M. J., Jr. Advances in Thermochemical Biomass Conversion; Bridgewater, T., Ed.; Chapman and Hall: London, 1984; Vol. 2, pp 760-771. (30) Radovic, L.; Walker, P. L., Jr.; Jenkins, R. G. Fuel 1983, 62, 849-856. (31) Tørslev, P.; Mitchell, R. E. Energy Research Project No. 1323/ 87-16. Ministry of the Environment, Denmark, 1993. (32) Zolin, A.; Jensen, A.; Pedersen, L. S.; Dam-Johansen, K.; Tørslev, P. Energy Fuels 1998, 12, 268-276.
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Energy & Fuels, Vol. 15, No. 5, 2001
Zolin et al.
Figure 1. TGA pyrolysis profiles of Dietz and demineralized Dietz (Dem. Dietz). (a) Mass of char left, HTT ) 973 K. (b) Rate of mass loss, HTT ) 973 K. (c) Mass of char left, HTT ) 1673 K. (d) Rate of mass loss, HTT ) 1673 K. Table 1: Composition of Fuels dry basis (wt %) volatiles % ash % C (%) H (%) O (%) N (%) S (%)
74.9 6.7 45.5 6.0 0.3 0.14
K (%) Cl (%)
1.2 0.41
0.41
Denmark
Subbit. U.S.A.
ASTM rank origin (state) SiO2 (%) Al2O3 (%) Fe2O3 (%) CaO (%) MgO (%) TiO2 (%) Li2O (%) Na2O (%) K2O (%) SO3 (%) P2O5 (%) a
wheat straw
Dietza PSOC 1488
Ash Analysis 57
