Energy & Fuels 1998, 12, 849-855
849
An Unexpected Trend in the Combustion Behavior of hvBb Coals As Shown by the Study of Their Chars Diego Alvarez,* Angeles G. Borrego, and Rosa Mene´ndez Instituto Nacional del Carbo´ n, CSIC, Apartado 73, 33080 Oviedo, Spain
Judith G. Bailey Department of Geology, University of Newcastle, Newcastle, N.S.W. 2308, Australia Received October 16, 1997
This study is an attempt to find useful relationships between the petrographic characteristics of coals and their efficiencies during pulverized fuel combustion through the study of their pyrolysis chars. Specifically, the effects of coal rank have been investigated using a series of vitrinite-rich coals ranging from subbituminous to low volatile bituminous coal. The behavior of these coals during combustion at 500 and 1100 °C and the petrographic characteristics of their pyrolysis chars (1000 °C) were investigated. In a foregoing paper (Borrego et al.10), unexpectedly poor combustion performances were found for some hvBb coals, which were attributed to rank rather than maceral effect. A general pattern of variation of coal burnout with rank was found in this study, providing additional data in which those unexpected results could be fitted. The observed trend was explained in terms of the changes undergone by coal both during coalification and pyrolysis, which make the number and accessibility of active sites vary in opposite directions, thus leading to a nonuniform variation of coal combustion efficiency with rank.
Introduction There is general agreement that as coal rank increases, its reactivity to oxygen decreases, accompanied by a parallel increase in heat-generating capacity during combustion. It is also generally accepted that the progressive formation of aromatic rings, at the expense of aliphatic chains, and the condensation of these rings into larger polyaromatic units brought about by coalification produce a higher stability in the structure of coal (i.e., reduced reactivity)1,2 and an accordingly higher energy release resulting from the cleavage of these bonds (increased calorific value).3 The above outlined effects can explain the general trends observed in pulverized coal combustion, provided that the rank of the coal is accurately determined. However, deviations from these trends are common, as has been reported by numerous research groups4,5 studying coal combustion under a variety of experimental conditions, from small bench-scale reactors to fullscale plants. If coal combustion is to be studied from a mechanistic point of view, i.e., with the emphasis put on the interactions between the carbonaceous matter and the reacting oxygen, it is convenient to divide the whole process into an initial pyrolysis stage and a * To whom correspondence should be addressed. (1) van Krevelen, D. W. Coal; Elsevier: Amsterdam, 1993. (2) Oberlin, A.; Villey, M.; Combaz, A. Carbon 1980, 18, 347-355. (3) Ward, C. R. Coal Geology and Coal Technology; Blackell Scientific Publications: Cambridge, MA, 1984. (4) Bengtsson, M. Fuel Process. Technol. 1987, 15, 201-212. (5) Carpenter, A.; Skorupska, N. Coal combustion-Analysis and testing. IEACR/64 Final Report; London, 1993.
subsequent gasification stage.6 The pyrolysis char resulting from the first stage of combustion, after the drastic changes undergone by coal particles during their sudden heating, constitutes an excellent starting point from which the differences observed in the combustion behavior of coals could be explained. The plastic properties of the different coal components (macerals) and their degrees of association will determine the morphology of chars,7 i.e., increased diameter due to swelling, sphericity, thickness of walls, properties which could have a major influence on the efficiency with which these chars will be gasified. Although there is general agreement on vitrinite particles generating cenospheric char during pyrolysis,8,9 even this relationship is too simplistic, as it is limited to a definite rank interval within which vitrinite tends to swell to a much higher extent than the accompanying macerals. In a previous paper10 focused on the study of inertinite behavior under the conditions of pulverized fuel combustion, the existence of a minimum in combustion efficiency at vitrinite reflectances close to 0.65% was suggested. This was attributed to a specially unfavorable arrangement of the polyaromatic units in the structure of coals at this particular rank, resulting in a (6) Smith, K. L.; Smoot, L. D.; Fletcher, T. H. In Fundamentals of coal combustion for clean and efficient use; Smoot, L. D., Ed.; Elsevier: Amsterdam, 1993; 131-298. (7) Rosenberg, P.; Petersen, H. J.; Thomsen, E. Fuel 1996, 75, 10711082. (8) Lightman, P.; Street, P. J. Fuel 1968, 47, 7-28. (9) Bailey, J.; Tate, A.; Diessel, C. F. K.; Wall, T. Fuel 1990, 4, 225239. (10) Borrego, A. G.; Alvarez, D.; Mene´ndez, R. Energy Fuels 1997, 11, 702-708.
S0887-0624(97)00200-4 CCC: $15.00 © 1998 American Chemical Society Published on Web 07/23/1998
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poor accessibility of oxygen to the edge carbons (generally recognized as active sites) of the resulting chars. It was also observed, in agreement with some recent studies carried out on inertinite reactivity,11-14 that coals yielding high amounts of dense isotropic inertinitederived chars showed very good combustion performances, as good as those from isotropic chars derived from low reflecting vitrinite chars. With the aim of confirming these results as well as improving the understanding of coal behavior during combustion, a series of vitrinite-rich coals with ranks ranging from subbituminous to low volatile bituminous has been selected. Their combustion efficiencies and the reactivities to oxygen of their chars will be discussed in the framework of the existing knowledge on the structure of coals, its evolution with coalification, and the transformations occurring during pyrolysis. Experimental Section Eight vitrinite-rich coals with ranks ranging from subbituminous to low volatile bituminous were used in this study. These coals were selected from different parts of the world: Illinois (IL) from the U.S.A.; Kellingly (KE), Bentinck (BE), and Taff Merthyr (TM) from the U.K.; Candı´n (CA) and Santa Ba´rbara (SB) from Spain; and Heinrich Robert (HR) and Emil Mayrisch (EM) from Germany. Representative coal samples were ground and sieved to a size range of 36-75 µm diameter. These size fractions were checked using a Coulter Multisizer Accucomp II, which revealed similar particle size distributions for all the studied coals, with sharp peaks around 53 µm and spread within the limits 36-75 µm. Ultimate, proximate, and standard petrographic analyses were carried out on these size fractions. Five grams of each coal was pyrolyzed in an entrained flow reactor (EFR) operating at 1000 °C under nitrogen with an estimated heating rate of 104-105 °C min-1. Coals were fed at a rate of 3 g h-1 and the gas flow rate was maintained at 37.5 L min-1, which ensured the existence of laminar flow conditions and the required residence time of 1 s. The char particles left the reactor through a water-cooled probe and were collected in a cyclone. Morphological characterization of the individual char structures was performed by reflected light microscopy using a Zeiss microscope attached to an image analysis system (Vidas 2.1, Kontron). The selection of particles was performed by automatic point-counting on polished surfaces, individual particles being chosen when the crosswire fell on their carbonaceous material. Porosities, surface-to-volume ratios, equivalent (circle) diameters, and shape factors were determined on digitized binary images of these sections.15,16 The optical texture of the carbonaceous matter was also examined under the microscope using crossed polars and a 1 λ retarding plate. Char combustion profiles were obtained in a thermobalance SETARAM TAG24. Ten milligrams of each char sample was heated to 700 °C at 50 °C min-1 under nitrogen. Once the weight of the sample was stabilized, the temperature was lowered to 500 °C and the gas was changed from nitrogen to (11) Thomas, C. G.; Gosnell, M. E.; Gawronski, E.; Phong-anant, D.; Shibaoka, M. Org. Geochem. 1993, 20, 779-788. (12) Thomas, C. G.; Gosnell, M. E.; Gawronski, E.; Nicholls, P. M. CSIRO Internal Report CET/IR 487; 1996. (13) Thomas, C. G.; Shibaoka, M.; Gawronski, E.; Gosnell, M. E.; Phong-anant, D. Fuel 1993, 72, 913-919. (14) Bend, S. L.; Edwards, I. A. S.; Mash, H. Fuel 1992, 72, 493501. (15) Alvarez, D. Influence of char structure on coal combustion behaviour. Ph.D. Thesis, The University of Oviedo, 1996 (in Spanish). (16) Alvarez, D.; Borrego, A. G.; Mene´ndez, R. Fuel 1997, 76, 12411248.
Alvarez et al. air. The samples were kept under these conditions for 2 h, sufficient to achieve complete combustion. Mass losses were monitored as a function of residence time, and the reactivity of the studied chars was evaluated at 50% conversion as
R50% ) (1/mo)(dm/dt)50% Combustion experiments were performed in a drop-tube furnace (DTF) operating at 1100 °C under air, the oxygen being 50% in excess of the stoichiometric amount required. The residence time in the active length (33 cm) of the reactor was about 0.3 s, with a coal feed rate of 6 g h-1. The combustion efficiency of coals was estimated using a derivation of the ashtracer technique. Fuel losses on ignition are commonly estimated by means of a mass balance on the ash entering and leaving the reactor. It assumes that there is no selective retention of ash in the reactor and that mineral matter undergoes the same transformations during coal ashing (proximate analysis) and combustion. This is reliable for temperatures under 1200 °C where no further ash devolatilization is expected compared with the standard assay at 800-840 °C.17 Conversion (X) can then be calculated using the ash percentages in the coal and the combustion residue as follows:
[ (
X) 1-
)(
)]
Ashcoal 100 - Ashresidue 100 - Ashcoal Ashresidue
× 100
Although this approach offers a good estimate of the percentage of fuel which has burnt away during combustion, it has to be kept in mind that these fuel losses are due to two separate processes, pyrolysis and gasification, and thus the obtained values depend on both the volatile yield upon pyrolysis and the combustion reactivity itself. Moreover, this ash-tracer analysis assumes that all the volatiles released will be burnt to completion, which is not necessarily true. As our study deals mainly with the char characteristics impacting the combustion performance of coals, the calculations were modified so that both weight losses could be roughly separated. Hence, volatiles were assumed to be released, regardless the efficiency with which they will be burnt, in equal amounts as they are in the standard volatile matter test, and the burning rates were calculated as a percentage of the solid pyrolysis product formed. That is, fixed carbon (ash-and-volatile-matterfree coal) instead of pure coal (ash-free) was considered as the starting material, using the equation
[ (
Xcor ) 1 -
)(
)]
Ashchar 100 - Ashresidue 100 - Ashchar Ashresidue
× 100
where the ash content in the char was estimated as
Ashchar ≈ Ashcoal[100/(100 - Volatile matter)] Jones and co-workers18 have demonstrated that volatiles production under proximate analysis and DTF conditions are only slightly different, the small increase observed for DTF conditions being almost constant (about 5%) for a set of 18 coals of widely varying ranks. The combustion experiments were performed under both highly oxygen-rich conditions (50% excess air) and using very low feed rates (6 g h-1), with coal particles forming an extremely diluted phase with virtually no interparticle effects. It can then be assumed that the variations in the amounts of oxygen consumed in the combustion of volatiles from the (17) Man, C. K.; Gibbins, J. R.; Skorupska, N. M.; Seitz, M. H. Proc.Int. Conf. Coal Sci. 1997. (18) Jones, R.; McCourt, C.; Morley, C.; King, K. Fuel 1985, 64, 1460-1467.
Combustion Behavior of HvBb Coals
Energy & Fuels, Vol. 12, No. 5, 1998 851
Table 1. Maceral Analyses and Mean Random Vitrinite Reflectance (VRr) of Coals
IL KE CA BE SB HR EM TM
VRr (%)
vitrinite (vol %)
liptinite (vol %)
inertinite (vol %)
0.44 0.73 0.74 0.76 1.04 1.14 1.54 1.82
78.9 66.4 90.2 81.9 93.4 77.6 73.8 76.7
8.3 16.9 4.6 7.7 3.0 4.6 0.9 0.0
12.8 16.7 5.2 10.4 3.6 17.8 25.3 23.3
different coals will not introduce major differences in the oxygen levels during the subsequent gasification of their chars.
Results Chemical and Petrographic Characterization of Coals. Vitrinite-rich (>75%) coals with low mineral matter content (
