140
Energy & Fuels 2003, 17, 140-149
Use of Spot Heater Apparatus for Investigation in Rapid Coal Devolatilization Xiaoxun Ma,†,‡ Tadashi Yoshida,‡ Michiaki Harada,§ Shohei Takeda,‡ Guangwen Xu‡ and Hiroshi Nagaishi*,‡ National Institute of Advanced Industrial Science & Technology (AIST), 2-17 Tsukisamu-Higashi, Toyohira-ku, Sapporo 062-8517, Japan, and Center for Coal Utilization, Japan, 24 Daikyocho, Shinjuku-ku, Tokyo 160-0015, Japan Received May 22, 2002
The rapid devolatilization experiments of single coal particles with direct particle temperature measurements by a thin thermocouple were performed using a spot heater apparatus. Fairly large coal particles ranging from 600 to 1300 µm were used in the present investigation. During the devolatilization, the initial stages and micro phenomena of the devolatilization were in-situ observed by recording the images of released volatiles from the coal sample with a CCD digital video camera. The utility of this apparatus for rapid coal devolatilization was verified experimentally under various conditions, such as coal type, inert gas pressure, temperature, heating rate, particle size, and inert atmosphere (N2, He, and Ar). Experimental results showed that temperature history and devolatilization behavior of the coal sample depend on the devolatilization conditions. It was found in this apparatus that pressure has a negative impact on the heating rate and final temperature of the coal sample; the effect of coal particle size on both temperature history and weight loss can be neglected for Tatong coal smaller than 700 µm. Particularly, inert gas properties have a significant impact on coal temperature history and weight loss. Coal samples in the inert gas with smaller thermal conductivity/specific heat were heated more quickly and up to a higher final temperature, thus they decomposed more extensively and quickly. This is contrary to the results obtained in almost all of other devolatilization apparatus in which coal samples are heated by means of forced thermal convection/thermal conduction rather than thermal radiation. In addition, on the basis of analysis of the data of weight-loss versus time/temperature, kinetic parameters of the distributed activation energy model (DAEM) for rapid coal devolatilization were estimated.
Introduction Coal is one of the important energy sources because of its large reserves and even distribution throughout the world. As a fundamental process of most coal utilizations and a means to recover valuable chemicals under rather mild conditions, rapid coal devolatilization has been attracting the attention of both engineers and researchers. Extensive investigations on rapid coal devolatilization have so far been made from various aspects of the subject, involving the impact of operating conditions1 and coal type2 on conversion yields, product distribution and its control,3 devolatilization mechanism and evaluation of devolatilization products,4 modeling of coal devolatilization and analysis of the devolatilization rate,5-10 etc. * Corresponding author. Fax: +81-11-857-8986. E-mail: h.nagaishi@ aist.go.jp. † Industrial Technology Researcher of NEDO. ‡ National Institute of Advanced Industrial Science & Technology (AIST). § Center for Coal Utilization. (1) Howard, J. B. Fundamentals of Coal Pyrolysis and Hydropyrolysis. Chemistry of Coal Utilization, 2nd Suppl. Vol.; Elliott, M. A., Ed.; John Wiley and Sons: New York, 1981; Chapter 12, pp 665-784. (2) Xu, W.-C.; Tomita, A. Effect of Coal Type on the Flash Pyrolysis of Various Coals. Fuel 1987, 66, 627-631. (3) Miura, K.; Mae, K.; Murata, A.; Sato, A.; Sakurada, K.; Hashimoto, K. Flash pyrolysis of coal in solvent vapor for controlling product distribution. Energy Fuels 1992, 6, 179-184.
Nevertheless, it is very difficult to accurately interpret chemical and transport mechanisms in rapid coal devolatilization because of coal’s heterogeneous chemical constitution, irregular macromolecular configuration, complex physical structure, and especially the changes in its physical and chemical properties along with rise in its temperature during rapid heating. Furthermore, a variety of reactors have been widely used to perform experiments of rapid coal devolatilization, depending on the aim of the experiment, such as a drop tube furnace, (4) Solomon, P. R.; Serio, M. A.; Despande, G. V.; Kroo, E. CrossLinking Reactions During Coal Conversion. Energy Fuels 1990, 4, 4254. (5) Anthony, D. B.; Howard, J. B. Coal Devolatilization and Hydrogasification. AIChE J. 1976, 22, 625-656. (6) Solomon, P. R.; Hamblen, D. G.; Carangelo, R. M.; Serio, M. A.; Deshpande, G. V. General Model of Coal Devolatilization. Energy Fuels 1988, 2, 405-422. (7) Grant, D. M.; Pugmire, R. J.; Fletcher, T. H.; Kerstein, A. R. Chemical Model of Coal Devolatilization Using Percolation Lattice Statistics. Energy Fuels 1989, 3, 175-186. (8) Niksa, S.; Kerstein, A. R. Flashchain Theory for Rapid Coal Devolatilization Kinetics. 1. Formulation. Energy Fuels 1991, 5, 647665. (9) Miura, K.; Maki, T. A Simple Method for Estimating f(E) and k0(E) in the Distributed Activation Energy Model. Energy Fuels 1998, 12, 864-869. (10) Wiktorsson, L.-P.; Wanzl, W. Kinetic parameters for coal pyrolysis at low and high heating rates a comparison of data from different laboratory equipment. Fuel 2000, 79 (6), 701-716.
10.1021/ef020115j CCC: $25.00 © 2003 American Chemical Society Published on Web 11/01/2002
Use of Spot Heater Apparatus in Coal Devolatilization
Energy & Fuels, Vol. 17, No. 1, 2003 141
Table 1. Proximate and Ultimate Analyses of Coals proximate analysis [wt %]
ultimate analysis [d.b. %]
atomic ratio
coal type
moist.
ash
V.M.
F.C.
C
H
O
N
S
d-ash
H/C
O/C
Tatong Skyline Adaro
3.16 4.21 12.58
9.68 6.80 1.58
28.76 41.51 40.65
58.40 47.48 45.19
74.40 73.49 71.73
4.21 5.31 5.02
9.78 12.24 20.38
0.95 1.45 1.06
0.66 0.41 0.00
10.00 7.10 1.81
0.68 0.87 0.84
0.10 0.12 0.21
shock tubes, heated grid reactors, curie-point pyrolyzer, fluidized beds, and so on. With these apparatus, particle temperature during rapid devolatilization cannot be measured directly, but can be calculated indirectly from knowledge of the surrounding ambient temperatures on the basis of some assumptions. In most cases, however, the assumptions regarding heat transfer (or particle temperature) were not verified with direct particle temperature measurements.11 Solomon et al. evaluated the kinetics of rapid coal devolatilization and concluded that the accuracy of coal particle temperature determinations (assumed, calculated, or measured) was the main cause of several orders of magnitude variation in the rate constant.11,12 This is because product evolution rates during devolatilization are very sensitive to temperature, suggesting a major role for chemical kinetics. Very few technologies have been devoted to direct temperature measurement of coal particles in rapid devolatilization experiments, e.g., laser heating with direct particle temperature measurements by two-color pyrometry,13 heating by contact with hot gases with particle temperature measurements by FT-IR emission/ transmission spectroscopy14,15 or two-color pyrometry.16 Recently, a novel method for direct temperature measurements of samples has been developed by using a so-called spot heater apparatus in our institute to investigate coal ignition,17 heavy oil vaporization,18 and rapid coal pyrolysis.19 Such a heating device as a spot heater apparatus is effective for rapid measurements of the temperature of single objects (e.g., a single coal particle and heavy oil droplet, etc.) and short-lived events. The objective of the present work is to explore the possibility of using the simple and inexpensive spot heater apparatus for investigation in rapid coal devola(11) Solomon, P. R.; Serio, M. A.; Suubergy, E. M. Coal Pyrolysis: Experiment, Kinetic Rates and Mechanisms. Prog. Energy Combust. Sci. 1992, 18, 133-200. (12) Solomon, P. R.; Serio, M. A. Evaluation of coal pyrolysis kinetics. Fundamentals of the physical chemistry of pulverized coal combustion; Lahaye, J., Prado, G., Eds.; Martinus Nijhoff (c/o Kluwer Academic Publishers): Hingham, MA, 1987; p 126. (13) Witte, A. B.; Gat, N. Effect of rapid heating on coal nitrogen and sulfur release. DOE Direct Utilization AR & TD Contractors’ Meeting, Pittsburgh, PA, 1983. (14) Solomon, P. R.; Serio, M. A.; Carangelo, R. M.; Markham, J. R. Very rapid coal pyrolysis. Fuel 1986, 65, 182-194. (15) Serio, M. A.; Hamblen, D. G.; Markham, J. R.; Solomon, P. R. Kinetics of volatile product evolution in coal pyrolysis: Experiment and Theory. Energy Fuel 1987, 1, 138-152. (16) Fletcher, T. H. Time-resolved particle temperature measurements and mass loss measurements of a bituminous coal during devolatilization. Combust. Flame 1989, 78, 223-269. (17) Katalambula, H.; Kitano, K.; Ikeda, K.; Chiba, T. Mechanism of Ignition of Single Coal Particle: Effect of Heating Rate on ParticleSize Dependence of Ignition Temperature. J. Chem. Eng. Jpn. 1996, 29, 523-530. (18) Nagaishi, H.; Ikeda, K.; Kitano, K.; Chan, E. W.; Gray, M. R. Observation of Heavy Oil Vaporization under Rapid Heating. Energy Fuels 1998, 12, 1174-1180. (19) Ma, X.; Nagaishi, H.; Yoshida, T.; Harada, M. Evaluation of kinetic parameters for rapid coal pyrolysis by analyzing two-dimensional images of releasing volatile matter. J. Jpn. Inst. Energy 2001, 80 (8), 736-746.
Figure 1. Schematic diagram of experimental setup.
tilization. For this purpose, a comprehensive experimental investigation of rapid coal devolatilization was made using the apparatus. This paper described the temperature history of the coal sample, the initial stages, and micro phenomena of the devolatilization illustrated with video images, as well as the devolatilization behavior of a single coal particle during rapid heating under various conditions, such as coal type, pressure, temperature, heating rate, particle size, and inert atmosphere. In addition, an alternative experimental technique was devised to give weight loss versus time/temperature data due to the features of spot heater apparatus. On the basis of analysis of the weight-loss data, kinetic parameters for rapid coal devolatilization were estimated. Experimental Section Coal Sample Preparation. Three types of coal, i.e., Tatong, Skyline, and Adaro, were used in this work. Their proximate and ultimate analyses are given in Table 1. Tatong and Skyline are bituminous coal, differing largely in volatile matter content, whereas Adaro is a brown coal. Coal particles were arranged for shape and sieved to obtain different particle size ranges. Only those close to the globular shape were used as samples in the experiments. Before experiment, coal samples were dried at 380 K for 4 h. Experimental Setup. The experimental setup was composed of a spot heater apparatus, chamber, spot heater controller (HC2403A, PHOENIX Electric Co., Ltd), high-speed data logger (L840, UNIPULSE), timer (H3CA-A-306, OMRON) and video imaging system (3CCD digital video camera (SONY DCR-TRV900), digital video cassette recorder (SONY DHR1000)), as shown schematically in Figure 1. The spot heater apparatus consists of four spot heaters mounted every 90 degrees on a supporting frame with a 3D manipulator. Each spot heater has a halogen lamp (24 V, 75 W, λ ) 0.5-3.5 µm) with gold mirror surface, through which a focus of 5 mm diameter at a focal distance of 15 mm can be formed. A platinum wire (φ0.15 mm) was coiled in its one tip to form a one-turn (about 0.4 mm) supporter for coal sample loading. Another tip of the wire was inserted in a ceramic tube (1.4 mm outside diameter) and fixed on the manipulator together with the tube. A coal sample, loaded on the supporter, can be placed at a common focus point of four spot heaters by the manipulator.
142
Energy & Fuels, Vol. 17, No. 1, 2003
The spot heater controller was used to control heating rate and final temperature of the coal sample by a PID or by adjusting the heating voltage. With the controller and spot heater apparatus, the heating operation, at a heating rate of the order of 1000 K/s and with final sample temperature up to 1500 K, could be performed. The timer was used to control the heating time of the coal sample in the experiments. It allows a minimum heating time of 0.1 s. A 0.05 mm Pt/Pt‚Rh13% thermocouple was used to determine coal particle temperature. The thermocouple was connected with a data logger through a thermocouple amplifier (D2000A, UNIPULSE). The electromotive force changes of the thermocouple during heating were recorded on a memory card inserted in the data logger. The data logger allows a minimum sampling interval of 0.001 s. The steel chamber is a 16-dm3 circular cylinder that has four quartz glass windows (8.3 cm i.d., 2 cm thickness) every 90° for visual observation. This chamber can be sealed so as to remove air by filling it with inert gas, and performing the experiments at high pressure, atmospheric pressure, and reduced pressure, respectively. Experimental Procedure. In the experiment, a coal particle on the supporter was placed at the focal point of the spot heaters and the thermocouple tip was tightly positioned on the surface of the particle by the manipulator. After that, the spot heater apparatus was enclosed in the chamber. The pressure in the chamber was reduced to
