282
Energy & Fuels 2000, 14, 282-290
An Experimental Study on the Effect of System Pressure on Char Structure of an Australian Bituminous Coal Hongwei Wu,*,† Gary Bryant,† Kathy Benfell,‡ and Terry Wall† Cooperative Research Center for Black Coal Utilisation, Department of Chemical Engineering, The University of Newcastle, Callaghan, NSW 2308, Australia, and Department of Geology, The University of Newcastle, Callaghan, NSW 2308, Australia Received April 14, 1999
A 63-90 µm size fraction of an Australian bituminous coal sample was fed into a drop tube furnace (DTF) and a pressurized drop tube furnace (PDTF) to generate chars. A gas temperature of 1573 K and system pressures of 0.1, 0.5, 1.0, and 1.5 MPa were considered. Particle size analysis, scanning electron microscopy (SEM), and image analysis were employed to analyze the chars produced in order to investigate the effect of system pressure on the resultant char structure and morphology. The char character was found to be influenced significantly by the system pressure. Results obtained indicated that the coal and char fragmentation might have occurred during devolatilization at high pressure. The char size (as characterized by the swelling ratio) was also observed to increase with system pressure. The cross-sectional characterization of the char particles indicated that at high pressure the majority of the char particles are of high porosity with thin walls or network (the group I type) rather than of medium porosity with thicker walls or network (the group II type) and of low porosity (the group III type). Char particles generated at high pressure were also observed to have a higher internal and surface porosity.
Introduction A number of reviews on coal devolatilization can be found in the literature.1-4 During devolatilization, as a result of the chemical transformations the coal metaplast decomposes and volatiles are released in the form of tar and gases, leaving a char residue. At the same time, the particles may undergo complex structural and morphological transformations that determine the resulting physical properties of the char particles produced, such as internal and surface pore structure. The resultant physical structure and morphology of the char affects the char reactivity during the subsequent combustion or gasification process. The cross-sectional characterization of char particles has been conducted by a number of investigators.5-14 * Corresponding author. Current address: Cooperative Research Center for Clean Power from Lignite, Department of Chemical Engineering, Monash University, Clayton, Vic 3168, Australia. Tel: +61-3-9905 3431. Fax: +61-3-9905 5686. E-mail: eChemEng@ hotmail.com. † Department of Chemical Engineering. ‡ Department of Geology. (1) Howard, J. B. In Chemistry of Coal Utilisation; Elliott, M. A., Ed.; Wiley: New York, 1981. (2) Oh, M. S. Softening coal pyrolysis. PhD dissertation, Massachusetts Institute of Technology, Cambridge, 1985. (3) Oh, M. S.; Peters, W. A.; Howard, J. B. AIChE J. 1989, 35, 775. (4) Saxena, S. C. Prog. Energy Combust. Sci. 1990, 16, 55. (5) Jones, R. B.; McCourt, C. B.; King, K. Fuel 1985, 64, 1460. (6) Jones, R. B.; Morley, C.; McCourt, C. B. International Conference on Coal Science, Sydney 1985, p 669. (7) Oka, N. e. a. Fuel Process. Technol. 1987, 15, 213. (8) Goodarzi, F.; Vleeskens, J. M. J. Coal Quality 1988, 7, 80. (9) Bailey, J. G.; Tate, A.; Dissel, C. F. K.; Wall, T. F. Fuel 1990, 69, 225. (10) Menendez, R.; Vleeskens, J. M.; Marsh, H. Fuel 1993, 72, 611. (11) Vleeskens, J. M.; Menendez, R. M.; Roos, C. M.; Thomas, C. G. Fuel Process. Technol. 1993, 36, 91. (12) Rosenberg, P.; Petersen, H. I.; Thomsen, E. Fuel 1996, 75, 1071. (13) Alvarez, D.; Borrego, A. G.; Menendez, R. Fuel 1997, 76, 1241.
Early work5-11 focused on describing the observed char structure and morphology on the basis of optical microscopy. The observed structures and morphologies were then attempted to be correlated to the parent coal petrography and the combustion behavior. A number of authors suggested morphological classification schemes.15 Bailey’s classification system9 has been accepted and used in previous investigations.12 Simplified systems based on this system were also proposed in previous studies13,14 in order to reduce the complexity. Zygourakis16 applied optical microscopy with digital image analysis to quantify the total volume and surface area of macropores, in char cross sections, with arbitrary geometrical shapes with sizes limited to 1-2 µm. Surface characterization has received little attention in the open literature. It has been reported that the char particle surface irregularities, such as surface pores (termed “blow-holes”), can affect the particle burning characteristics17 as well as the measured burning area under diffusion-limited conditions.18 Digital image analysis was employed for the determination of the surface porosity of char particles.18 The system pressures of up to 40 atm have been applied in advanced technologies such as pressurized fluidized bed combustion/gasification and pressurized entrained flow gasification.19 Advantages associated with the use of these technologies include a reduction (14) Benfell, K. E.; Bailey, J. G. AIE 8th Australian Coal Science Conference, Sydney, 1998; p 157. (15) Cloke, M.; Lester, E. Fuel 1994, 73, 315. (16) Zygourakis, K. Energy Fuels 1993, 7, 33. (17) Loewenberg, M.; Levendis, Y. A. Combust. Flame 1991, 84, 47. (18) Bayless, D. J.; Schroeder, A. R.; Peters, J. E.; Buckius, R. O. Combust. Flame 1997, 108, 187. (19) Takematsu, T. i.; Maude, C. Coal gasification for IGCC power generation; IEA Coal Research, 1991.
10.1021/ef990066j CCC: $19.00 © 2000 American Chemical Society Published on Web 02/08/2000
Effect of System Pressure on Char Structure
in the capital cost for the unit, an increase in the coal throughput, a reduction in pollutant emission, an enhancement in the intensity of reaction, and an ability to satisfy the pressure requirements for gas turbines.20 Pressure plays an important role in the devolatilization of pulverized coal. Under high-pressure conditions, it is believed that pyrolysis is dominated by intraparticle mass transfer.1,21-24 Bubble movement in the char particle dominates the volatile transport of softening coals,2,3,25-28 which may be influenced by atmosphere,29-32 heating rate,28,32 and pressure.24,26,27 Bituminous coal particles exhibited quite different devolatilization and swelling behavior at elevated system pressures under both low heating rates in a high-pressure microdilatometer33-35 and high heating rates in a highpressure entrained flow reactor.24 This implies that atmospheric pressure data may not predict high-pressure devolatilization and swelling behavior. Quantitative data, describing both the surface and cross-sectional structure of residual char particles at high pressure is currently unavailable for use in mathematical burnout and ash formation models. This paper presents a quantitative description of the char surface and cross-sectional morphology for chars obtained from devolatilization of an Australian bituminous coal. The determination of quantitative information as a basis for inputs into mathematical models will assist in developing a better understanding of char combustion and ash formation mechanisms at high pressure. The work presented in this paper employed an image analysis procedure which combines scanning electron microscopy (SEM) with digital image analysis. Experimental Section An Australian bituminous coal was used in the current investigation. The coal was ground and sieved to a size fraction 63-90 µm. The chemical and petrographic analyses for the sample are presented in Table 1. The atmospheric and high-pressure devolatilization experiments were conducted in a laminar flow drop tube furnace (DTF) and a laminar flow pressurized drop tube furnace (PDTF). Detailed descriptions of the two facilities can be found elsewhere.36,37 The principles employed in the two facilities are similar. A schematic diagram of the DTF and PDTF systems is shown in Figure 1. (20) Harris, D. J.; Patterson, J. H. Aust. Inst. Energy. J. 1994, 13, 22. (21) Wanger, R.; Wanzl, W.; van Heek, K. H. Fuel 1985, 64, 571. (22) Bleik, A.; van Poelje, W. M.; van Swaaij, W. P. M. e. a. AIChE J. 1985, 31, 1666. (23) Bautistu, J. R.; Russel, W. B.; Saville, D. A. Ind. Eng. Chem. Fundam. 1986, 25, 536. (24) Lee, C. W.; Scaroni, A. W.; Jenkins, R. G. Fuel 1991, 70, 957. (25) Lewellen, P. C. Product decomposition effects in coal pyrolysis. M. S. thesis, Massachusetts Institute of Technology, Cambridge, 1975. (26) Gibbins, J.; Kandiyoti, R. Energy Fuels 1989, 3, 670. (27) Griffin, T. P.; Howard, J. B.; Peters, W. A. Fuel 1994, 73, 591. (28) Gale, T. K.; Bartholomew, C. H.; Fletcher, T. H. Combust. Flame 1995, 100, 94. (29) Lightman, P.; Street, P. J. Fuel 1968, 47, 7. (30) Street, P. J.; Weight, R. P.; Lightman, P. Fuel 1969, 48, 343. (31) Tsai, C.; Scaroni, A. W. Fuel 1987, 66, 200. (32) Fletcher, T. H. Fuel 1993, 72, 1485. (33) Khan, M. R.; Jenkins, R. G. Fuel 1984, 63, 109. (34) Khan, M. R.; Jenkins, R. G. Fuel 1985, 64, 487. (35) Khan, M. R.; Jenkins, R. G. Fuel 1986, 65, 725. (36) Wu, H. Ash formation during pulverised combustion at pressure. PhD dissertation, The University of Newcastle, Newcastle, Australia, 2000. (37) Ouyang, S.; Yeasmin, H.; Mathews, J. Rev. Sci. Instrum. 1998, 69, 3036.
Energy & Fuels, Vol. 14, No. 2, 2000 283 Table 1. Properties of Coal Used in the Experiments proximate analysis
(wt %, a.d.)
moisture ash volatile matter fixed carbon
2.30 13.00 28.00 56.70
ultimate analysis
(wt %, daf)
C H N S O
84.00 5.11 1.75 0.24 8.90
petrographic analysis
(vol %, mmf)
vitrinite liptinite inertinite
37.60 4.60 57.80
crucible test swelling index
1
The DTF employs an entrained flow coal feeder while the PDTF has a fluidized coal feeder. The feeding rate was 0.0050.020 kg/h for the devolatilization experiments. The residence time of the coal particles in the furnace was estimated to range from 2 to 5 s. The initial heating rate for the particles was ∼104-105 K/s. The char samples were collected by a cyclone with size cut around 2 µm. The gas temperature in the furnace is controlled electronically. Pressure is also controlled automatically in the PDTF. Char samples were generated in a N2 atmosphere at a gas temperature of 1573 K and pressures of 0.1, 0.5, 1.0, and 1.5 MPa. An amount of oxygen, required to burn out 75% of the released volatiles, was added to the N2 atmosphere to reduce soot formation inside or on the surface of the char particles. The amount of required oxygen was calculated using the chemical percolation devolatilization (CPD) model.38-40 The O2 concentrations of the furnace outlet were approximately 110 ppm, analyzed using a gas chromatography (GC). The char yeilds of the devolatilization at various pressures ranged from 63%-74%, determined using ash tracer.
Analytical Techniques Malvern Laser Size Analyzer. A Malvern Master Sizer Analyzer, which uses a laser diffraction technique, was employed to analyze the PSD of the coal and char samples. Scanning Electron Microscopy (SEM). A JEOL JSM-840 scanning electron microscope with an attached KEVEX Si(Li) energy-dispersive X-ray detection system, was used to examine the coal and char samples for both cross-sectional and surface characterization. The SEM operated at 15 kV and a working distance of 15 mm. Pellets for the cross-sectional characterization were prepared by mounting the samples in resin. A vacuum desiccator was used to remove entrapped air and force the resin into hollow particles with openings on the surface. After setting, the pellets were ground and polished. An ∼20 nm carbon coating was applied on the cross section prior to the SEM examination. (38) Grant, D. M.; Pugmire, R. J.; Fletcher, T. H. Energy Fuels 1989, 3, 175. (39) Fletcher, T. H.; Kerstern, A. R.; Pugmire, R. J.; Grant, D. M. Energy Fuels 1990, 4, 54. (40) Fletcher, T. H.; Kerstein, A. R.; Pugmire, R. J.; Solomon, M. S.; Grant, D. M. Energy Fuels 1992, 6, 414.
284
Energy & Fuels, Vol. 14, No. 2, 2000
Wu et al.
Figure 1. The configuration of the drop-tube furnaces used in experiments.
Pellets for surface characterization were prepared by mounting the untreated samples on double-sided adhesive carbon tape on 10 mm diameter Bakelite mounts. The particles were spread out on the carbon tape so that most of the particles could be observed individually. Samples were carbon coated prior to the SEM analysis. The representative sets of particles from the samples were obtained by applying a conventional point-counting procedure, which is widely used in optical microscopy.9,12-14,16 Once a point was counted, the twodimensional image was digitized and stored for future examination by image analysis software. A minimum of 250 points were counted for each sample. Image Analysis. Analysis of the SEM images was performed on a Pentium computer using an image analysis program. Compared to the previous studies9,10,12-14,16 which principally employed optical microscopy with image analysis, the current study combines SEM with digital image analysis. The combination of SEM and image analysis allows analysis of not only the cross-sectional but also the surface character for the sample. Surface characterization of the sample allows easier recognition of fragments and agglomerates in the char sample. Surface characterization also gives the true particle size and sphericity. The electron beam in the SEM produces a gray scale image which is able to distinguish ash in the char particle.
Reflected visible light microscopy is unable to do surface analysis due to its limitation in depth of field.41 Fragmentation and agglomerates were not counted during the image analysis, ensuring only whole particles were analyzed. The image analysis program is able to determine the characteristics of objects in an image using an edge detection technique.42 The program detects all objects and calculates the object characteristics automatically. In surface characterization the objects can be the whole coal particle or surface pores. For cross-sectional analysis the whole char particle, char matter, and mineral matter can be analyzed. The program calculates several parameters for each object including area, maximum axis, minimum axis, sphericity, etc. In this study, the analysis gives reliable results for sizes greater than 1 µm, determined from the magnification of the image. The procedure for image analysis is shown in Figure 2. Cross-sectional image analysis provides the char group classification and internal macroporosity. Stereological correction is applied for the two-dimensional cross-sectional analysis. From the viewpoint of combustion and ash formation, the simplified classification (41) Goodhew, P. J.; Humphreys, F. J. Electron microscopy and analysis; Taylor & Francis: Philadelphia, 1988. (42) Parker, J. R. Algoriths for image processing and computer vision; Wiley Computer Publishing: New York, 1997.
Effect of System Pressure on Char Structure
Energy & Fuels, Vol. 14, No. 2, 2000 285
Figure 2. Procedure for cross-sectional (left) and surface (right) analysis of char particles, showing the image (top), the derived image used for the particle dimension (middle), and the derived images used for char cross-section char matter estimates (bottom) which leads to a cross-sectional porosity estimates, and surface porosity estimates. Table 2. Simplified Char Classification System and Its General Characterization group type
types included in the classification system of Bailey et al.9 and at ICCP meeting in Chania, Crete, 1993
typical characterization
I II III
Tenuisphere, Tenuinetwork Crassisphere, Crassinetwork, Mesosphere, Mixed Porous Inertoid, Solid, Fusinoid, Mixed Dense
High-porosity (>70%), thin wall thickness Medium-porosity (40%-70%), medium wall thickness low-porosity (
