The Evolution of Char Surface Area along Pulverized Coal Combustion


the textural changes of the chars along pulverized coal combustion, with special care to isolate the variations of surface area attributable to char c...
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Energy & Fuels 2007, 21, 1085-1091

1085

The Evolution of Char Surface Area along Pulverized Coal Combustion Diego Alvarez* and Angeles G. Borrego Instituto Nacional del Carbo´ n, CSIC, P. O. Box 73, 33080 OViedo, Spain ReceiVed NoVember 13, 2006. ReVised Manuscript ReceiVed January 18, 2007

Pulverized (36-75 µm) samples from three coals were fed to a drop tube furnace (DTF) operated at 1300 °C under different O2/N2 mixtures. From the obtained set of char samples, one low burnout char from each parent coal was re-entered into the DTF under varying gas compositions. The aim of this work is to monitor the textural changes of the chars along pulverized coal combustion, with special care to isolate the variations of surface area attributable to char combustion from those arising from early char/oxygen interactions during the pyrolysis stage. The burnout, helium density, and CO2 and BET surface areas of the combusted and refired chars were determined, and the patterns of variation of the textural parameters were established for these two types of unburned material. It was found that the presence of oxygen in the vicinity of pyrolyzing coal particles affected the development of surface area of the newly formed chars and that this effect was both rank- and maceral-dependent.

Introduction The early stages of pulverized coal combustion comprise a number of physicochemical processes of great complexity. First, the coal particles, on entering the boiler, undergo a sudden heating which provokes the release of the volatile fraction of the coal, and then both the volatiles and the carbon-rich char particles resulting from the pyrolysis stage are consumed by the oxygen present in the boiler. There is not a universal consensus about the environmental conditions under which these two processes take place simultaneously or consecutively,1,2 but it is well-known that the degree of overlap between pyrolysis and combustion depends on the amount of volatiles released by the coal particles, as these volatiles will, to varying extent, prevent the oxygen from reaching the surface of the particles before the pyrolysis processes cease. The evolution of volatiles, in turn, mainly depends on the coal chemical composition and particle size, as well as other process variables such as the temperature and heating rate.3 For a given set of operating conditions, the cloud of volatiles will more effectively screen the oxygen the bigger the particle size (more volatiles per unit surface area of the particles) and the lower the rank of the coal (more volatiles per unit volume of the coal). For the range of particle sizes prevailing in pulverized coal combustion (typically, less than 75 µm diameter), it is generally assumed that the bigger particles will pyrolyze in an essentially oxygen-free environment, whereas for the smaller particles, the processes of pyrolysis and combustion will take place with considerable overlap. Much research work has been devoted to studying the interactions of the oxygen with the char and the volatiles, in order to elucidate whether the ignition takes place in a homogeneous (gas-gas) or a heterogeneous phase (gas-solid), * Corresponding author phone: +34 985119090; fax: +34 985297662; e-mail: [email protected] (1) Essenhigh, R. H.; Misra, M. K.; Shaw, D. W. Combust. Flame 1989, 77, 3-30. (2) Wall, T. F.; Gupta, R. P.; Gururajan, V. S.; Zhang, D. Fuel 1991, 70, 1011-1016. (3) Fletcher, T. H. Combust. Flame 1989, 78, 223-236.

as this is a critical issue for those involved in modeling work.4 Even when there is limited access of oxygen to the surface of the pyrolyzing coal particles, too low to promote their heterogeneous ignition, it has been reported that some surface oxidation can still take place, which notably reduces the fluidity of the molten carbonaceous matter.5 This is attributed to the formation of oxygenated cross-links on the surface of the particles, with the net effect of reducing the mobility of the polyaromatic rings which make up the molten ground mass, and therefore preventing their rearrangement into larger units.6 Both the surface area and the intrinsic reactivity of the resolidified char shall be modified accordingly: more active sites shall remain in the char, and its surface area shall be higher than in a char obtained in an oxygen-free environment.7,8 All these early interactions between char and oxygen can substantially modify the combustion behavior of a coal, as, later in the combustion process, once the coal particles have evolved their volatile matter and modified their morphology and chemical structure accordingly, the course of the carbon-oxygen reaction will be mainly conditioned by the surface area and/or chemical structure of the chars.9-11 The relative importance of these two features is dictated mainly by the combustion temperature and the particle size of the char, and three combustion regimes are defined according to the rate-limiting step in the process: (I) the kinetics of the reaction, (II) the (4) 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; pp 131-293. (5) Street, P. J.; Weight, R. P.; Lightman, P. Fuel 1969, 48, 343-365. (6) Alvarez, D.; Borrego, A. G.; Menendez, R. Procceedings of the 12th International Conference on Coal Science; The Australian Institute of Energy: Toukley, New South Wales, Australia, 2003; p CD-6. (7) Hurt, R. H.; Dudek, D. R.; Longwell, J. P.; Sarofim, A. F. Carbon 1988, 26, 433-449. (8) Gale, T. K.; Fletcher, T. H.; Bartholomew, C. H. Energy Fuels 1995, 9, 513-524. (9) Zolin, A.; Jensen, A.; Dam-Johansen, K. Combust. Flame 2001, 125, 1341-1360. (10) Bar-Ziv, E.; Kantorovich, I. I. Prog. Energy Combust. Sci. 2001, 27, 667-697. (11) Feng, B.; Bhatia, S. K. Carbon 2003, 41, 507-523.

10.1021/ef0605697 CCC: $37.00 © 2007 American Chemical Society Published on Web 03/01/2007

1086 Energy & Fuels, Vol. 21, No. 2, 2007

AlVarez and Borrego

Table 1. Proximate, Ultimate and Petrographic Analysis of the Coalsa wt % db

wt % daf

%

vol %, mmf

coal

ash

VM

C

H

N

S

O

Rr

V

L

I

PT WA FM

19.7 6.6 10.6

38.4 36.9 25.6

82.3 80.8 86.5

5.0 5.1 4.6

1.7 1.9 1.3

0.6 0.3 0.2

10.4 11.9 7.4

0.66 0.66 1.07

65.6 57.2 55.6

13.0 3.6 0.4

21.4 39.2 44.0

a db ) dry basis; daf ) dry, ash-free basis; mmf ) mineral-matter-free basis; vol ) volume; wt ) weight; VM ) volatile matter; Rr ) random reflectance; V ) vitrinite; L ) liptinite; I ) inertinite.

diffusion of oxygen into the pore network of the char, and (III) the diffusion of oxygen across a boundary layer surrounding the outer surface of the particles.12 Again, the identification of the combustion regime is essential for the modeling of the reaction. As in any basic research work, the list of references provided here cannot pretend to be comprehensive. Rather, an attempt was made to select, from the vast literature existing in the field, those studies where the points raised in this paper are more specifically highlighted. The present paper addresses the influence of the pyrolysis environment on the texture and reactivity of the chars obtained, with an emphasis in the early interactions between the newly formed char and the oxygen present in the gas phase. Three coals with different ranks and maceral compositions were fed to a drop-tube furnace (DTF) operated at 1300 °C and simulating the time-temperature histories prevailing in pulverized coal combustion, and under oxygen concentrations varying from substoichiometric to highly superstoichiometric. Combustion chars with varying degrees of burnout were obtained by different routes, and the evolution of surface area and reactivity along combustion was determined for the different combustion environments tested. Experimental Section Three steam coals have been selected for this study on the basis of their variable rank and maceral composition. A Medium volatile bituminous coal was obtained from the Fording River mine (FM) in British Columbia (Canada). This coal forms part of the large coal measures available in the Elk Valley region, which produces both metallurgical and thermal coals. A High volatile bituminous coal was taken at Emma mine, in Puertollano (PT), one of the mining localities in south-central Spain, and is mostly consumed in a power plant nearby. The third coal is named Bayswater (WA) and is a High volatile bituminous coal from New South Wales (Australia). The mine is part of the huge resources of Hunter Valley from which most of the coal is sold overseas. The coal characterization comprised (i) ultimate analyses performed using a LECO CHN600 for carbon, nitrogen, and hydrogen; a LECO SC132 for sulfur; and a LECO VTF900 for oxygen; (ii) proximate analyses carried out following the standard procedures described in UNE 32-019-84 for volatile matter and ISO-1171/ 1981 for ash contents; and (iii) petrographic analyses (maceral ISO 7404-3, 1994, and random reflectance ISO 7404-5, 1994). The chemical and petrographic characterization data of the three coals selected for this study are given in Table 1. PT and WA are High volatile bituminous coals with the same rank, as indicated by their vitrinite reflectance (0.66%) and different inertinite contents (21.4 and 39.2 vol %, respectively). FM is a Medium volatile bituminous coal with a high inertinite content (44.0 vol %). The drop-tube furnace used for the combustion experiments has been described elsewhere.13 Size-graded (36-75 µm) samples of the coals described above were used in the combustion experiments, (12) Roberts P. T.; Morley C. In Fundamentals of the Physical-Chemistry of PulVerised Fuel Combustion; Lahaye, J., Prado, G., Eds.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1987; NATO ASI Series 137, pp 452-463. (13) Milenkova, K. S.; Borrego, A. G.; Alvarez, D.; Xiberta, J.; Mene´ndez, R. Fuel 2003, 82, 1883-1891.

Figure 1. Diagram showing the various sets of char samples obtained and the nomenclature used.

which were carried out at 1300 °C, with a gas flow rate of 20 L min-1 and under different O2/N2 gas mixtures. Three different strategies were designed in order to obtain partly burnt chars at varying burnouts. Figure 1 shows a sequential scheme of the char samples obtained along this study. In the first series of experiments, the combustion tests were carried out under different oxygen concentrations. The three coals were burnt under 0, 2.5, 5, 10, 15, and 21% O2 in N2. The samples thus obtained will hereafter be referred to as XXB0, XXB2, XXB5, XXB10, XXB15, and XXB21, respectively, where XX is the corresponding parent coal, FM, WA, or PT. The second series consisted of a sequence of refiring experiments where the coals were fed to the drop tube operating under 2.5% O2 in N2, and then the collected chars were successively re-entered in the reactor three more times using the same gas composition. Subsamples obtained after every passage through the reactor were reserved for their characterization. The refired samples will be identified here as XXR2, XXR2-2, and XXR2-3 where, again, XX is to be substituted by FM, WA, or PT, the parent coals. The third series of combustion experiments also took the char samples obtained from every coal under 2.5% O2 in N2 (XXB2) as a starting material, and these chars were subsequently refired under 2.5, 5, 10, and 15% O2 in N2. These refired chars are denoted XXR2 (equivalent to the first refired sample of series II), XXR5, XXR10, and XXR15. Up to eight runs in the DTF were necessary in order to obtain a sufficient amount of the XXB2 samples for the subsequent refiring experiments of series II and III. The burnout of the samples was estimated using the ash-tracer technique

[ (

Conversion (%) ) 1 -

)(

)]

ashcoal 100 - ashchar 100 - ashcoal ashchar

× 100 (1)

Two widely used methods to determine the pore surface area of carbon from gas adsorption isotherms were applied in this study, using CO2 at 273 K and N2 at 77 K as adsorptives. The equipment used was a Micromeritics ASAP 2020. Prior to gas adsorption experiments, the chars were heated under a vacuum at 5 °C min-1 and holding temperatures of 90 °C (1 h) and 350 °C (4 h). CO2 adsorption isotherms were performed at 0 °C at the interval of pressure 0.035-0.0001 Torr, and the Dubinin-Radushkevich (DR) equation14 was applied to the adsorption data. The Brunauer(14) Dubinin, M.; Radushkevich, L. Proc. Acad. Sci. USSR 1947, 55, 331-335.

The EVolution of Char Surface Area

Energy & Fuels, Vol. 21, No. 2, 2007 1087

Emmett-Teller (BET) theory was applied to the N2 adsorption data to obtain the surface area.15 These two methods can be regarded as complementary, given the difficulties of CO2 to fill large micropores and the slow diffusion of N2 in the small micropores,11 and therefore the combination of both methods is considered to describe appropriately the surface area of micropores (CO2 D-R) and mesopores (N2 BET). As some of the samples contained large amounts of mineral matter with different adsorption properties than the organic fraction, the isotherms were corrected for mineral effects. An extensively burned char from every coal was ashed, and the corresponding CO2 and N2 isotherm was obtained and subtracted from the corresponding sample isotherm before calculating the surface area. The isotherms were analyzed using the Micromeritics density functional theory (DFT) software package DFT plus. The pore size distribution was obtained in the size range of 4-10 Å for CO2 adsorption and in the range 4-2500 Å for N2 adsorption. The helium densities of the chars were measured in a Micromeritics AccuPyc 1330. The raw data obtained were recalculated to an ash-free basis using 2.4 g cm-3 as the helium density of the ashes. Due to the generally limited amount of sample available, combined with the diluting effect of the ashes, the helium densities of the high-burnout chars are subject to big uncertainties. Thus, the determination of the carbon density of a char with more than 40 wt % ash is subject to a (5% error, which goes up to (26% for ash contents higher than 90 wt %. For this reason, the densities of high-ash samples will not be reported in this paper. The apparent density (Fap) of the char material was estimated through the pore volume (VP), as given by DFT, and the true density (FHe), using

(

Fap ) VP +

1 FHe

)

Figure 2. SEM micrographs of vitrinite char from (A) PT coal pyrolyzed under N2, (B and C) PT coal burned under 2% O2 in N2, (D) FM coal pyrolyzed under N2, (E and F) FM coal burned under 2% O2 in N2.

-1

(2)

This is not the apparent density of the char particles, considered as the weight of the sample divided by the total volume enclosed by the char particle outer surfaces, including the large devolatilization voids. Rather, the density values given by eq 2 refer to the volume of char material plus the volume of pores smaller than 1 µm in diameter. In this work, it has been preferred to use these density values, on the basis of the observation of the morphologies commonly encountered in bituminous coal chars, with porosities due to large devolatilization voids typically higher than 50%. For such highly swollen particles, it is unlikely that the oxygen transfer from the outer area of the particles to the surface of the char walls through these large devolatilization pores might play an essential role in the control of the reaction. From the perspective of the oxygen molecules, the char material is seen as a solid with a true density FHe and a pore network formed of pores