Energy & Fuels 2004, 18, 611-618
611
Relation between the Petrographic Composition of Coal and the Morphology of Pyrolysis Char Produced in Fluidized Bed B. Valentim,† M. J. Lemos de Sousa,† P. Abelha,‡ D. Boavida,*,‡ and I. Gulyurtlu‡ Centro de Geologia da Universidade do Porto, Faculdade de Cieˆ ncias, Prac¸ a de Gomes Teixeira, 4099-002, Porto, Portugal, and Departamento de Engenharia Energe´ tica e Controlo Ambiental (DEECA), Instituto Nacional de Engenharia e Tecnologia Industrial (INETI), Estrada do Pac¸ o do Lumiar, 22, edif. J, 1649-038, Lisboa, Portugal Received June 3, 2003. Revised Manuscript Received January 19, 2004
Several previous studies have already established, for pulverized coal combustion conditions, global correlations between petrographic composition of the coal and those of char produced from the same coal. However, for fluidized bed combustion, there has not been much new work since the eighties. The results presented in this paper include the petrographic characterization of seven different coals from several origins and also of their respective chars produced at 700 °C, 800 °C, 900 °C, and 1000 °C in a laboratory fluidized bed reactor. The results show a marked predominance of tenuispheres as the trial temperatures increase. While vitrinite-rich coals essentially produced highly porous chars, the inertinite-rich coals produced large amounts of medium- and low-porous chars. Semi-anthracite vitrinite produced high-porous chars and thermal affected coal particles originated low-porous and angular char morphotypes. The analysis of the data obtained revealed that vitrinite + liptinite related well with the high-porous char (sum of cenospheres and tenuinetworks), classified as Group 1. The same trend, but with a weaker relation, was also observed between vitrinite and liptinite rich microlithotypes and Group 1.
Introduction The early work related to char properties dates back to 1924-19301,2 and is basically focused on coke properties. However, if it is true that the coke petrography has a long history and pulverized fuel (PF) char petrographic studies are well established, the same does not apply to chars produced during fluidized bed combustion (FBC). It has been suspected for some time that macerals reactivity that has been observed to have an important role during cokefication could also be applied to combustion conditions. For example, vitrinite and liptinite from “coking coals” (84-91 wt % C, daf) rapidly soften during cokefication3 and that the majority of inertinite macerals do not soften, with the exception of low reflectance semifusinite.4 However, only after 1950/60, following the use of the reflected light microscopy for char particles from mine explosions and PF combustion residues, the research work to study the effect of rank and maceral composition * Correspondingauthor.Fax: +351217166569.E-mail: dulce.boavida@ mail.ineti.pt. † Centro de Geologia da Universidade do Porto. ‡ Instituto Nacional de Engenharia e Tecnologia Industrial (INETI). (1) Alpern, B. Centre d’E Ä tudes et Recherches des Charbonnages de France. Verneuil-en-Halatte. (Document Inte´rieur CHERCHAR N°. 1562), 1965. (2) Mene´ndez, R.; A Ä lvarez, D.; Go´mez, A.; Vleeskens, J.; Roos, M. Proc. Int. Conf. Coal Sci.: Alberta, Canada, 1993; Vol. I, pp 39-42. (3) Varma, A. K. Int. J. Coal Geol. 1996, 30, 337-347. (4) Taylor, G. H.; Teichmu¨ller, M.; Davis, A.; Diessel, C. F. K.; Littke, R.; Robert, P. Organic Petrology; Gebru¨der Borntraeger: Berlin, Stuttgart, 1998; p 704.
on coal combustion was initiated.1,5-13 More recently, the development of diverse laboratory combustion techniques allowed the research on the behavior of different coals according to rank, petrographic composition, reactivity, heating rate, pyrolysis, formation and oxidation of the char, pollutant emissions, etc.4,14 These studies have led to the conclusion that, in addition to the conditions prevailing during devolatilization such as temperature, heating rate, and pressure,15,16 char morphology could also be influenced by the particle size and coal rank, and that the specific nature of char morphotypes is related to the behavior of specific macerals during devolatilization or later during oxidation. Basically, vitrinite macerals are found to generate thin- and (5) Alpern, B. Centre d’E Ä tudes et Recherches des Charbonnages de France. Verneuil-en-Halatte. (Document Inte´rieur CHERCHAR N°. 1185), 1961. (6) Alpern, B.; Chauvin, R. Rev. Ind. Mine´ r., Nr. Spe´ c. 1958, 15, 210-218. (7) Alpern, B.; Courbon, P.; Plateau, G.; Tissandier, G. Essais sur le Me´ canisme de Combustion du Charbon - 1e Se´ rie (CMC1), Ijmuiden 1957. 9 pp. (Document N° F17/b/5). (8) Goodarzi, F.; Murchison, D. G. Fuel 1972, 51, 322-328. (9) Lightman, P.; Street, P. J. Fuel 1968, 47, 7-28. (10) Nandi, B. N.; Brown, T. D.; Lee, G. K. Fuel 1977, 56, 125-130. (11) Sanyal, A. J. Inst. Energy 1983 (June). (12) Tsai, C.; Scaroni, A. W. Fuel 1987, 66, 200-206. (13) Street, P. J.; Weight, R. P.; Lightman, P. Fuel 1969, 48, 343365. (14) Diessel, C. F. K. Organic Petrology; Gebru¨der Borntraeger: Berlin, Stuttgart, 1998; pp 519-614. (15) Bengtsson, M. Ph.D. Thesis. The Royal Institute of Technology. Department of Heat and Furnace Technology, Stockholm, Sweden, 1986. (16) Cai, H.-Y.; Gu¨ell, A. J.; Chatzakis, I. N.; Lim, J.-Y.; Dugwell, D. R.; Kandiyoti, R. Fuel 1996, 75, 15-24.
10.1021/ef034016x CCC: $27.50 © 2004 American Chemical Society Published on Web 05/01/2004
612
Energy & Fuels, Vol. 18, No. 3, 2004
thick-walled cenospheres (tenuispheres and crassispheres, respectively) and thin-walled networks (tenuinetworks); inertinite macerals tend to form angular dense forms, mixed forms (fused and unfused) and eventually cenospheres and networks in the case of low reflectance semifusinite, while liptinite is only important during devolatilization and does not contribute significantly to the nature of char.14 Between the authors that correlated high-temperature devolatilization char morphotypes and pulverized coal combustion residues with corresponding coal microlithotypes,14,17-21 it was concluded that some microlithotypes are the main precursors of some char morphotypes: highly porous chars (cenospheres and tenuinetworks) resulting from vitrinite-rich microlithotypes (vitrite, clarite, vitrinertite-V, trimacerite-V-E); medium porous chars (crassinetwork and mixed char forms) originating from a heterogeneous group of complex bi- and tri-maceralic microlithotypes (durite-I+E, vitrinertite-I, trimacerite I, and some semifusite); and, low porous chars (inertoids, solids, and fusinoids) obtained from a homogeneous fraction inertinite-rich (fusite, inertodetrite, and some semifusite). Regarding what concerns FBC, cenospheric char was detected due to smaller or lighter particles elutriated from the bed to the freeboard where temperature is usually lower and char combustion is not enhanced. These authors concluded the following: high volatile bituminous coal developed tenuispheres and basic anisotropy; medium volatile bituminous coal developed crassispheres and semianthracitic vitrinite cenospheres and sub-angular char; in all cases “non-reactive” inertinite remains unaltered, thus keeping its original morphological identity while “reactive” macerals developed cenospheres and became rounded.22-28 From these FBC studies, it could be concluded that, as in pulverized coal combustion, char morphotypes depend on the rank and petrographic composition of coal and, to some degree, follow the same trends with rank and petrographic composition. In the work presented in this paper, the results of char morphotypes evolution with temperature are reported and the relations between the petrographic composition are determined on seven commercial coals from different origins and the chars resulting from the devolatilization of these coals under conditions similar to those encountered during coal combustion in FBCs. (17) Bailey, J. G. 46th Annu. Meet. Int. Com. Coal Org. Petrol. (ICCP): Oviedo (Spain), 1994: pp 17-18. (18) Bailey, J. G.; Tate, A.; Diessel, C. F. K.; Wall, T. F. Fuel 1990, 96, 225-239. (19) Bend, S. L. Ph.D. Thesis. The University of Newcastle upon Tyne, 1989. (20) Rosenberg, P.; Petersen, H. I.; Sorensen, H. S.; Thomsen, E.; Guvad, C. Final Report. Danmarks and Gronlands Geologiske Undersogelse Rapport, 1996/54, Copenhagen, 1996. (21) Rosenberg, P.; Petersen, H. I.; Thomsen, E. Fuel 1996, 75, 1071-1082. (22) Gay, A. J.; Littlejohn, R. F.; van Duin, P. J. Fuel 1983, 62, 1224-1226. (23) Gay, A. J.; Littlejohn, R. F.; van Duin, P. J. Sci. Total Environ. 1984, 36, 339-246. (24) Goodarzi, F.; Vleeskens, J. M. Stichting Energieonderzoek Centrum Nederland (Publ. ref. ECN-85-097), 1985. (25) Goodarzi, F.; Vleeskens, J. M. J. Coal Quality 1988, 7, 80-85. (26) Vleeskens, J. M. Netherlands Energy Research Foundation (Publ. ref. ECN-133), 1983. (27) Vleeskens, J. M.; van Haasteren, T. W.; Roos, M.; Gerrits, J. Fuel 1988, 67, 426-430. (28) Vleeskens, J. M.; Nandi, N. Fuel 1986, 65, 797-802.
Valentim et al.
Additionally, and due to the size of the particles devolatilized (500-1000 µm), the microlithotype analysis needed no modification as it happens in PF conditions.18 Therefore, direct correlations were made between the microlithotype composition and the char morphotypes. Experimental Section Seven commercial coals from different origins (Spain, Colombia, South Africa, and the United States) were chosen for this study on the basis of their chemical properties (Table 1). After the preparation of the classical coal particulate block (ISO Standard 7404-2), coal petrographic analyses (vitrinite reflectance, maceral group composition, and microlithotypes, carbominerites, and minerite composition, in accordance with ISO 7404-5, 7404-3, 7404-4, respectively) were performed in a MPVC Leitz microscope with magnifications up to 500× using the MPVGEOR computer program. Chars were obtained from coal devolatilization in a fluidized bed reactor with 80 mm of internal diameter and 500 mm of height. The inert carrier gas used in all tests was N2. During the volatile release, CO and CO2 amounts were measured with nondispersive infrared analyzers and, when the analyzers could no longer detect these gases, the heating was switched off while maintaining the N2 flow. Chars were produced at four temperatures (700, 800, 900, and 1000 °C), with a heating rate of ca. 104 K/s and coal particle sizes of 500-1000 µm. Char samples for petrographic analysis were prepared according to the technique presented by AÄ lvarez.30 Char petrographic analyses were performed in a light reflection Nikon microscope, with 80× magnification and coupled with a Swift F 415C semiautomatic point-counter. The procedure used for point counting was as follows: at each location, if the crosswire was positioned on the carbonaceous material of a char particle, the particle was classified and counted, as described below:17,31 • tenuisphere - spherical to angular, porosity > 80%, and 75% wall thickness < 30 µm (Figure 1a); • crassisphere - spherical to angular, porosity > 60%, and 75% wall thickness > 30 µm (Figure 1b); • tenuinetwork - internal network structure, porosity > 70%, and 75% wall thickness < 30 µm (Figure 1c); • crassinetwork - internal network structure, porosity 4070%, and 75% wall thickness > 30 µm (Figure 1d); • mixed - char in part fused and unfused, porosity 40-70%; fused or unfused parts never less than 25% (Figure 1e); • solid - dense, porosity 5-40%, 75% wall thickness > 30 µm; inertinitic structures still present; and solids with
