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Devolatilization Characteristics of Coal Particles Heated with CO2 Laser Controlled by Double Shutters. 1. An Experimental Investigation Hong Gao,*,† Mingchang Qu,‡ and Masahiro Ishigaki§ Department of Thermal Energy and Power Engineering, Vehicle and Energy College, Yanshan UniVersity, 438 West Hebei AVenue, Qinghuangdao, Hebei 066004, China, Nissei Chemical Engineering Co. Ltd., 24-13 Aramaki, Aza-kitanaganuma, Wakabayashi-ku, Sendai 984, Japan, and Center for Interdisciplinary Research, Tohoku UniVersity, 2-2-1 Aramaki, Aoba-ku, Sendai 980-77, Japan ReceiVed April 1, 2006. ReVised Manuscript ReceiVed June 1, 2006
A CO2 laser controlled well by specially designed double shutters was used to heat uniformly dispersed coal particles in nitrogen. The temperatures of the coal particles were measured using a highly accurate multiplepoint, two-color temperature measurement system. The average performance and characteristics of three kinds of coals with different volatile content were carefully investigated at various experimental conditions. The conclusions are as follows: (1) the proximate volatile matter content, particle size, and laser intensity are the three most important factors in the devolatilization process of coal particles under the laser heating condition; (2) the devolatilization processes can be separated into two parts by the proximate volatile matter contents of three coals, which suggests that two devolatilization schemes exist in each coal; (3) the effect of laser intensities on the devolatilization processes indicates that the devolatilization process cannot be well-characterized by only laser intensity; (4) the existence of cross points in the curves of weight loss and particle temperatures among the samples with different sizes are one of the important features of laser heating, which is distinctively different from other heating methods; (5) the variation in V/V0 values of all three kinds of coals with particle temperatures exponentially depends on coal particle temperatures, and can be expressed as V/V0 ) 9.0 × 10-4 exp(5.3 × 10-3 Tp) with R2 ) 0.8833; (6) the change in V/V0 of W coal with different particle sizes is highly dependent on particle temperatures and can be expressed as V/V0 ) 1.3 × 10-3 exp(5.2 × 10-3 Tp) with R2 ) 0.9169; (7) a comprehensive expression, which includes volatile matter content, in situ energy density per surface area of coal particle, final particle temperature, and in situ particle properties, such as swelling, shrinking, emissivity, etc., should be further investigated.
Introduction The yield, composition, and rate of volatile evolution during the early stages of coal gasification, carbonization, and combustion play an important role in coal conversion processes. Many studies were carried out for understanding this important process;1-8 however, large discrepancies exist in the observed temperature sensitivities for coal devolatilization, with variations of several orders of magnitude being common in rate constants reported at a given measurement temperature.9,10 The variations in rate constants can most likely be attributed to the experimental * To whom correspondence should be addressed. E-mail:
[email protected]. † Yanshan University. ‡ Nissei Chemical Engineering Co. Ltd. § Tohoku University. (1) Eddinger, R. T.; Friedman, L. D.; Rau, E. Fuel 1966, 45, 245. (2) Kimber, G. M.; Gray, M. D. Combust. Flame 1967, 11, 361. (3) Badzioch, S.; Hawksley, P. G. W. Ind. Eng. Chem. Process Des. DeV. 1970, 9, 521. (4) Suuberg, E. M.; Peters, W. A.; Howard, J. B. Prepr. Symp.-Am. Chem. Soc., DiV. Fuel Chem. 1977, 22, 112. (5) Solomon, P. R.; Colklt, M. B. Proceedings of the 17th International Symposium on Combustion; The Combustion Institute: Pittsburgh, PA, 1979; p 131. (6) Fu, W.; Zhang, Y.; Han, H.; Duan, Y. Combustion and Flame 1987, 70, 253. (7) Sorensen, L. H.; Biede, O.; Peck, O. E. Proceedings of the 25th International Symposium on Combustion; The Combustion Institute: Pittsburgh, PA, 1994; p 475. (8) Loison, R.; Chauvin, F. Chem. Ind. (Paris) 1964, 91, 269.
techniques commonly used to study rapid rate devolatilization because the estimation and measurement of coal particle temperatures in these systems are difficult and frequently inaccurate. Freihaut and co-workers11,12 have demonstrated the complexities of making accurate temperature measurements during screen heater pyrolysis studies. In a comparison of published data, they noted that reported screen temperatures required to achieve half the potential tar evolution during transient heating range from as low as 575 K to as high as 1125 K. Published rate data for entrained flow pyrolysis show similar differences. Comparing with Solomon et al.13 and Fletcher,14 they noted that the range of reported rate constants implies an 800 K difference in temperature among different investigators. In addition, it has been shown 15 that, within a given reactor, order of magnitude differences in observed devolatilization rates can arise because of subtle differences in reactor operating characteristics. (9) Howard, J. B. Chemistry of Coal Utilization; Elliot, M. A., Ed.; Wiley: New York, 1981; 2nd Supplementary Volume, p 665. (10) Solomon, P. R.; Hamblen, D. G. Chemistry of Coal ConVersion; Schlosberg, R. H., Ed.; Plenum: New York, 1985; pp 121-251. (11) Freihaut, J. D.; Proscia, W. M. Energy Fuels 1989, 3, 625. (12) Freihaut, J. D.; Zabielski, M. F.; Seery, D. J. Proceedings of the 19th International Symposium on Combustion; The Combustion Institute: Pittsburgh, PA, 1982; p 1159. (13) Solomon, P. R.; Serio, M. A.; Carangelo, R. M.; Markham, J. R. Fuel 1986, 65, 182. (14) Fletcher, T. H. Combust. Sci. Technol. 1989, 63, 89.
10.1021/ef060139g CCC: $33.50 © 2006 American Chemical Society Published on Web 07/08/2006
DeVolatilization Characteristics of Laser-Heated Coal Particles
Attempts to overcome the limitations discussed above have concentrated on in situ temperature measurement or careful characterization of heat-transfer fields. Solomon and co-workers13,16 developed and applied an FTIR emission/transmission technique to measure average temperatures for clouds of devolatilizing coal particles in an entrained flow reactor. Fletcher14,17 reported using a two-color pyrometer to characterize the temperature histories of single devolatilizing particles in an entrained flow reactor. Freihaut and Proscia11 performed a detailed characterization of the temperature rise in a screen heater system. These investigations suggest that reported temperatures in many previous studies were likely to be overestimated and that coal devolatilization rates are significantly faster and have a stronger temperature sensitivity than implied in these earlier studies. Maloney and co-workers18 developed a novel system that incorporates an electrodynamics balance and a pulsed laser radiation source to isolate and rapidly heat individual particles for monitoring rapid changes in particle size and temperature during coal devolatilization at heating rates representative of high-intensity combustion environments. Measured temperature histories were compared with theoretical estimates of the temperature response of rapidly heated coal and carbon particles. Measurements and model predictions for 135 µm diameter carbon spheres were in excellent agreement using property data correlation commonly applied in modeling coal devolatilization and combustion behavior. However, model predictions for coal particles significantly underestimated (on the order of 50%) the observed heating rates for coal particles of 115 µm diameters using the same property correlation. Potential reasons for this may include inadequate understanding of relevant coal mass and heat transfer as well as failure to account for the change in particle size and shape, which lead to large errors in predicted temperature histories and associated devolatilization rates. Pyatenko et al.19 investigated devolatilization of a single coal particle under vacuum conditions (1 × 10-4 Torr) by laser heating and analyzing gaseous products experimentally. The characteristics of the time for volatile yield and total volatile yield were studied. The theoretical pattern of volatile yield through the coal particle pore was also discussed. However, the temperature of coal particles has not been measured in this work. Kobayashi et al.20,21 developed a new pyrometric technique for temperature measurement of packed beds of pulverized materials, platinum plates, oxidized stainless steel, pulverized graphite, and pulverized coals by processing the radiation spectrum on the basis of two-color pyrometry. Maswadeh et al.22 developed a laser devolatilization gas chromatography/mass spectrometry technique for single coal particles to identify substantial numbers of pyrolysis products from single coal particles in the range 50-150 µm. Reliable temperature/time profiles of single coal particles during rapid laser heating were obtained by using a specially designed two(15) Maloney, D. J.; Jenkins, R. G. Proceedings of the 20th International Symposium on Combustion: The Combustion Institute: Pittsburgh, PA, 1984; p 1435. (16) Best, P. E.; Carangelo, R. M.; Markham, J. R.; Solomon, P. R. Combust. Flame 1986, 66, 47. (17) Fletcher, T. H. Combust. Flame 1989, 78, 223. (18) Maloney, D. J.; Monazam, E. R.; Woodruff, S. D.; Lawson, L. O. Combust. Flame 1991, 84, 220. (19) Pyatenko, A. T.; Bukhman, S. V.; Lebedinskii, V. S.; Nasarov, V. M.; Tolmachev, I. Ya. Fuel 1991, 71, 701. (20) Kobayashi, S.; Tokuda, M. Res. Inst. Min. Dress. Met. Rep. 1987, 43, 205. (21) Qu, M.; Kobayashi, S.; Tokuda, M. Res. Inst. Min. Dress. Met. Rep. 1991, 47, 49. (22) Maswadeh, W.; Arnold, N. S.; McClennen, W. H.; Tripathi, J. D.; Meuzelaar, H. L. C. Energy Fuels 1993, 7, 100.
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wavelength radiation thermometer module with integral video microscope. Comparison of an EDB (electrodynamic balance) type particle leviator with an EM (electron microscopy) grid type particle support system revealed considerable advantages of the EM grid approach with respect to tar collection efficiency, particle position stability, particle visualization, and recoverability of residual char particles. However, it should be noted that the coal tar yields obtained in this work exhibit quite a bit of scatter ((20% uncertainty at a 95% confidence level). This is to a large extent due to the particle size and shape variations and also the considerable amount of heterogeneity between different coal particles. Joseph et al.23 investigated the timeresolved swelling of coal particles irradiated with laser pulses inside an EDB apparatus using high-speed cinematography. Neither the heating flux intensity nor the initial particle diameter appeared to influence the final swelling ratio significantly. Gao et al.24 developed a novel method of studying the preliminary temperature profiles and surface textural change of single particles heated by a well-characterized CO2 laser system with double shutters by combined application single-wavelength infrared pyrometer and SEM observation. Gao et al.25 quantitatively studied the swelling characteristics (time-resolved swelling ratio, interval time between swelling and shrinking of bubble, etc.) of coal particles heated using the same laser system24 by combined application of high-speed video camera with an image analysis system under atmospheric pressure in nitrogen. They observed that the swelling characteristics of coal particles were dependent on laser intensity, particle size, and coal properties. Qu et al.26,27 made experimental investigations and numeric simulations of the ignition and combustion of laserheated coal and char particles. Godoy and Lockwood28 developed a two-color pyrometer operating in the infrared for coal particle temperature measurements during devolatilization. Bhattacharya and Wall29 investigated the development of the emittance of coal particles during devolatilization and char particles (45-125 µm) at temperatures between 473 and 1273 K and discussed the magnitude of potential errors in pyrometric temperature measurement. Recently, Tripathi et al.30,31 measured and modeled temperature profiles of individual carbonaceous particles and coal particles from three kinds of different rank coals using a CO2 laser pyrolysis system with two-color micropyrometry. They predicted that intraparticle gradients between surface temperature and bulk average radius temperature fall within the estimated error range (100 K) of two-color pyrometry measurements and thus can be ignored for particle sizes of 80-120 µm. They found that the larger the particle size, the higher the temperature reached by the particles. They indicated that using measured temperature histories was more reliable than using predicted average bulk temperatures for kinetics calculation. From the above brief review, it is clear that both the experimental investigations and theory studies on the devolatilization of coal particles are needed for more accurately describing this important process. In the present paper, a CO2 (23) Joseph, N. D. D.; Richard, A. O. Fuel 1994, 73, 773. (24) Gao, H.; Murata, S.; Nomura, M.; Ishigaki, M.; Tokuda, M. Energy Fuels 1996, 10, 730. (25) Gao, H.; Murata, S.; Nomura, M.; Ishigaki, M.; Qu, M.; Tokuda, M. Energy Fuels 1997, 11, 730. (26) Qu, M.; Ishigaki, M.; Tokuda, M. Fuel 1996, 75, 1155. (27) Ishigaki, M.; Qu, M.; Tokuda, M. ISIJ 1997, 37, 729. (28) Godoy, S. M.; Lockwood, F. Fuel 1998, 77, 995. (29) Bhattacharya, S. P.; Wall, T. F. Fuel 1999, 78, 511. (30) Tripathi, A.; Vaughn, C. L.; Maswadeh, W.; Meuzelaar, H. L. C. Thermochim. Acta 2002, 388, 183. (31) Tripathi, A.; Vaughn, C. L.; Maswadeh, W.; Meuzelaar, H. L. C. Thermochim. Acta 2002, 388, 199.
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Table 1. Ultimate and Proximate Analysis of Coals ultimate analysis (wt %, daf) coal K W P a
C
H
N
S
Oa
87.6 77.8 81.2
3.9 4.6 5.9
1.5 1.9 1.3
0.5 0.7 0.4
6.5 7.7 11.2
proximate analysis (wt %, db) VM
FC
ash
18.70 32.70 43.00
71.70 59.90 53.50
9.60 7.40 3.50
By difference.
Figure 2. Multiple-point, two-color temperature measurement and calibration system.
Figure 1. Schematic diagram of experimental apparatus for the devolatilization of coal particles heated by CO2 laser with double shutters.
laser system with specially designed double shutters was used to obtain highly accurate control of heating time and heating temperature. A highly accurate multiple-point, two-color temperature measurement system was used to measure particle temperature. Considering the heterogeneity of coal particles and measurement accuracy of the yield of released products and char, we carried out measurements more than 30 times for one given sample at given laser intensity and heating time, and the average performance of the sample was finally determined.
Figure 3. Surface temperatures of heated-up cylindrical tube of stainless steel measured by the pyrometer and a thermocouple (PtPtRh13%, diameter of thermocouple bead ) 0.30 mm).
Experimental Section Coal Samples. A South Africa highly volatile weak cooking coal, Witbank (W), a Rosier bituminous coal, K, and an Indonesia bituminous coal, Prima (P), were used as raw coals. The coals were crushed and sieved to give samples with three ranges of particle size: 125-149, 180-210, and 250-350 µm. The proximate and ultimate analyses are shown in Table 1. To avoid the obvious difference in size, shape, and mineral matters, we carefully selected experimental samples using a clean stainless steel needle under microscope. Experimental Apparatus. As shown in Figure 1, the experimental apparatus consists of a CO2 laser-heating system with two shutters controlled by a computer, a multiple-point, two-color temperature measurement system, a specially designed reactor, a stereomicroscope with CCD camera and VTR system, and a gas supply system. The reactor was made from a brass chamber with an inner diameter of 30 mm. The sample holder was made of a transparent quartz plate 1 mm thick and 10 mm in diameter that was supported by a brass tube. The chamber was cooled with water, and all inlets and outlets (sample holder, laser beam, optical fiber, gas tube) of the chamber were sealed with O-rings. A multiple-point, two-color pyrometer with a spectral sensitivity range of 682.1-837.5 nm and its calibration system are shown in Figure 2. The pyrometer system was optimized for the temperature range of 873-2073 K with a system error in the range 1-3%. Some examples for proving temperature measurement are shown in Figures 3-5. Figure 3 shows the measured temperatures of a heatedup cylindrical tube of stainless steel by the pyrometer and a Pt-
Figure 4. Relationship between pyrometer temperatures (Tpy) and thermocouple temperatures (Ttc) from Figure 3.
PtRh%13 thermocouple. Figure 4 gives the relationship between pyrometer temperatures and thermocouple temperatures (Tpy ) 1.0061Ttc, R2 ) 0.9931). Figure 5 shows the measured pyrometric temperatures of graphite particles with different particle number under the conditions of a laser intensity of 1.85 M Wm-2, particle size of 180-210 µm, and a nitrogen atmosphere. All measured temperature differences were within the system error, indicating that particle number does not affect temperature measurements. These results show that the reliability and reproducibility of the heating system and the temperature measurement system were welldesigned. The heating system is the same as in our previous works.21,24-26 The heating source was a 125 W CO2 laser (Japan Laser Automation, NEC, CL112C) with a 10.6 µm wavelength and 6 mm beam
DeVolatilization Characteristics of Laser-Heated Coal Particles
Figure 5. Effect of particle number on particle temperature (Tp) measurement (laser intensity ) 1.85 MW m-2; graphite size ) 200 µm; particle number: 0 5, 4 10, O 20, ] 20).
diameter. The reliable output of the laser was within (3%. The laser beam was focused by a 70 mm ZnSe focus lens to provide a cross-section 8 mm in diameter. The incident output of the laser was determined using a calibrated optical power meter. The heating time was controlled with a specially designed novel double-shutter system controlled by a computer. By means of this novel method, the minimum heating period could be controlled on the order of 1 ms. The expected laser intensity could be obtained by adjusting both the gas flow rate and the focus length of the laser beam. The swelling and shrinking behaviors of the coal particles were monitored and recorded by a stereomicroscope (Japan Olympus Co. Ltd., model SZ-CTV) with CCD camera and VTR (Japan Olympus Co. Ltd., model CS220). The swelling and shrinking of coal particles were quantitatively evaluated with an image analysis system (Japan Nireco Co. Ltd., LUZEX-3). The experiments were carried out under nitrogen atmospheric pressure, and dried, high-purity nitrogen gas (99.9995%) was used. The flow rates of nitrogen were controlled with gas-flow controllers. The weight loss of coal particles under the given conditions was determined by measuring the weight of samples before and after heating using a high-sensitivity electronic balance. Experimental Procedure. The experiments were carried out under various laser intensities, heating times, and particle sizes in nitrogen at atmospheric pressure. First, 0.3 mg samples of a given size were placed uniformly on the surface of the sample plate within a 3 mm diameter cross-section area. Second, the gas controllers were opened and high-purity nitrogen gas was introduced into the reactor for at least 30 min to exclude the air in the chamber. The coal particles were then heated to the expected heating time or expected temperature under a given laser intensity in a nitrogen atmosphere at a constant gas flow rate (N2, 140 mL/min). After heating, the samples were cooled rapidly to ambient temperature for weighting and further examination. The same procedure was done at least 30 times to obtain about 10 mg samples. Finally, the samples were weighed using a high-sensitivity electronic balance, and the weight loss was determined for the experimental conditions.
Results and Discussion Some studies of devolatilization were carried out using laserEDB and laser-EM-Py-GC/MS and other related methods. However, there were not sufficient data that could be used to establish a general description for the devolatilization processes under laser-heating conditions. The yield, composition, and rate of volatile evolution of coal particles are dependent on physical and chemical properties of the coal, heating conditions, and atmosphere. In this study, the most important factors, volatile content, laser intensity per area of coal particles, and particle sizes, and their effect on the particle temperatures, heating rates, relative volatile yields,
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Figure 6. Weight-loss changes of coal particles with heating times (laser intensity ) 1.85 MW m-2, particle size ) 200 µm).
maximum volatile yields, and devolatilization rates were systematically investigated and discussed. The results and discussions will also become the base of a second paper in this series, which concerns a model investigation of coal devolatilization under laser-heating conditions. Effect of Volatile Content on the Devolatilization of the Coal Particles. To investigate the effect of coal chemical properties, especially volatile content, on the devolatilization process, we selected three kinds of coals with different volatile content (K coal, 18.7%; W coal, 32.7%; and P coal, 43.0%). The changes in the weight loss of coal particles with heating time are shown in Figure 6. For comparison, the proximate volatile contents of the coals are also shown with point lines in the figure. The devolatilization rates sharply increase after 100 ms up to proximate values at about 300 ms for P coal and W coal, but 400 ms for K coal; the devolatilization rates increase with increasing volatile content. In addition, the final volatile yields (Vf) are 1.2 and 1.07 times the proximate values for K coal and W coal, respectively; for P coal, the Vf (0.97V0) does not reach the proximate value of the coal at a heating time of 500 ms. This result suggests that the final relative volatile yields decrease with an increase in volatile content, and higher volatile content needs more time to release from relatively large particles under laser-heating conditions. It should be noted that an interpretation of the above results should consider the different temperature histories among the coal samples caused by their different coal properties, such as thermoplasticity, swelling of particles, and the optical properties. The temperature profiles of the three coals with particle size 200 µm and laser intensity 1.85 MW m-2 are shown in Figure 7. The maximum heating rates of the three coals are 1 × 104 K s-1 (K coal), 9.5 × 103 K s-1 (P coal), and 9.0 × 103 K s-1 (W coal). P coal and W coal have the same final temperature of 1240 K; K coal has a higher final temperature of 1340 K. These results imply that temperature histories of different kinds of coals aren’t affected greatly by the volatile content and its release. In general, weight loss and devolatilization rates become the fastest in the 670720 K range with low heating rates (8000 K s-1). The values of the final volatile yield of the three coals seem to be a reflection for the different features of the volatile releasing of coal particles with a relatively large size heated rapidly by laser. In addition, the
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Figure 7. Particle temperatures (Tp) of three kinds of coals at different heating times (laser intensity )1.85 MW m-2, particle size ) 200 µm).
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Figure 9. Effect of volatile contents on final particle temperatures (Tpf) and final relative volatile yields (Vf/V0) of three coals (laser intensity ) 1.85 MW m-2, particle size ) 200 µm).
Figure 8. Variation of relative volatile yields (V/V0) of three coals with particle temperatures (4 K coal, O W coal, 0 P coal).
Figure 10. Weight-loss changes of W coal particles heated with different laser intensities (particle size ) 200 µm).
heating rates of the three coals are in the order K > P > W, which is the same as the order of swelling of the three coals. It is easy to interpret this phenomenon as the larger the swelling ratio of coal particles, the more the received energy per mass unit, therefore the heating rate and final temperature also become higher. Now it is clear that the higher ratio of final volatile yield with its proximate values (1.2V0) of the K coal is caused by its higher heating rate, higher final temperature, and higher thermoplasticity. The variation in relative volatile yield (V/V0) with particle temperatures of the three coals is shown in Figure 8. The values of V/V0 increase rapidly after 900 K; more than 60% V0 was released at 1150 K for W coal, 54% V0 at 1240 K for P coal, and 63% V0 at 1340 K for K coal. In addition, the curve of V/ V0 of K coal shifts about 100 K to right, which may be caused by its low volatile content and high swelling feature. Contradictory to K coal, the final temperatures of W coal and P coal are almost identical to each other. The important result is that the change in V/V0 with temperatures of coal particles for all three coals is exponentially related (V/V0 ) 9.0 × 10-4 exp(5.3 × 10-3 Tp), R2 ) 0.8833). This result indicates that V/V0 is highly related to particle temperature but not to volatile content of coals. The effects of proximate volatile matter content (V0) on final particle temperature (Tpf) and final relative volatile yield (Vf/ V0) are shown in Figure 9. The conclusion is clear: Vf/V0 and Tpf decrease almost linearly with an increase in V0. These results
suggest that, besides temperature and heating rate, V0 is another indispensable parameter for describing the devolatilization process. Effect of Laser Intensity and Heating rate on the Devolatilization of the Coals. To obtain a quantitative understanding about how laser intensity, heating rate, and final temperature influence the devolatilization process, we used a series of laser intensities, and the results are shown in Figures 10-13. As expected, the heating rates, devolatilization rates, final temperatures, and final volatile yields increase with increasing laser intensity. The ratios of final volatile yield with the proximate analysis value (Vf/V0) are 1.09, 1.14, and 1.20 for laser intensities 1.85, 2.59, and 3.70 MW m-2, respectively. The interesting results are the patterns of the weight loss shown in Figure 10, in which the curves seem to be separated into two parts with different devolatilization rates by the V0 line. The times to reach the V0 line are 300, 270, and 150 ms for laser intensities 1.85, 2.59, and 3.70 MW m-2, respectively, and the temperatures at these times are 1280, 1480, and 1620 K, respectively. These results suggest that the increased volatile release above V0 has a different releasing mechanism. It should be noted that the devolatilization processes after the V0 line are conducted at almost constant temperature until the final stage. As shown in Figure 11, for the three laser intensities, the maximum heating rates are 9.0 × 103, 1.2 × 104, and 2.0 × 104 K s-1, respectively; the final temperatures are 1265, 1495, and 1700 K, respectively. Therefore, the higher the heating rates and final temperatures,
DeVolatilization Characteristics of Laser-Heated Coal Particles
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Figure 14. Weight-loss changes of W coal particles with different sizes heated with a laser intensity of 1.85M Wm-2. Figure 11. Particle temperatures (Tp) of W coal particles heated with different laser intensities (particle size ) 200 µm).
Figure 12. Variation of relative volatile yields (V/V0) of W coal with particle temperatures (Tp) under different laser intensities (particle size ) 200 µm; laser intensity: 4 1.85 MW m-2, O 2.59 MW m-2, 0 3.70 MW m-2).
Figure 13. Effect of laser intensities on final particle temperatures (Tpf) and final relative volatile yields (Vf/V0) of W coal (particle size ) 200 µm).
the higher the devolatilization rates and final relative volatile yields. As shown in Figure 12, the relative volatile yields (V/ V0) are very low (0.1-0.25) before 900 K for all three coals. In addition, at the same temperature, the values of V/V0 under lower laser intensity are much greater than those under high laser intensity. For example, at 1280 K, the value of V/V0 reached 0.5 under the 2.59 MW m-2 condition; however, the value of V/V0 had already raised to 1.1 under 1.85 MW m-2. These important results indicate that the devolatilization process cannot be characterized well by only laser intensity; therefore, a comprehensive parameter including heat and mass transfer
Figure 15. Effect of coal particle sizes on particle temperatures (Tp) (W coal, laser intensity ) 1.85 MW m-2).
should be introduced. Nevertheless, the exponential relationship between V/V0 and Tp still exists (V/V0 ) 1.84 × 10-2 exp(2.7 × 10-3 Tp), R2 ) 0.6726). The effects of laser intensity on Tpf and Vf/V0 are shown in Figure 13. Both Vf/V0 and Tpf increase linearly with increasing laser intensity. Effect of Particle Size on the Devolatilization of the Coals. The weight-loss changes and temperature histories of W coal with different particle sizes are shown in Figures 14 and 15. At 100 ms, the particle temperatures have reached 1000, 900, and 800 K for particles with sizes 140, 200, and 300 µm, respectively. The initial heating rates for these sizes are 1 × 104, 9.0 × 103, and 8.0 × 103 K s-1, respectively. Before 175 ms, the devolatilization rates and particle temperatures increase with a decrease in particle size. After 200 ms, the devolatilization rates of 140 µm samples become much lower than those of large samples (200 and 300 µm). The final volatile yield is only 0.75V0, and the final temperature is lower than that of the large samples by 150-250 K. The heating rate of the largest samples (300 µm) is lower than those of small samples. However, its duration time of a high heating rate is the longest, and the final temperature is also the highest. This result is consistent with the conclusion by Tripathi et al.30-31 The existence of crosspoints in weight loss and temperature histories among the samples with different particle sizes is one of the important features of coal particles under laser-heating conditions, which is distinctively different from other heating systems. Because the area per mass unit of small particles is larger than that of large particles, when particle temperatures rise to some extent,
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Figure 16. Effect of coal particle sizes on final particle temperatures (Tpf) and final relative volatile yields (Vf/V0) (W coal, laser intensity )1.85 MW m-2). Figure 18. Relationship between final relative volatile yields (Vf/V0) and final particle temperatures (Tpf) of three kinds of coals.
Figure 17. Variation of relative volatile yields (V/V0) of W coal with particle temperatures (Tp) (particle size: 4 140 µm, O 200 µm, 0 300 µm).
the radiation term becomes important, the thermal loss of the smaller particles is much greater than that of larger ones; therefore, the thermal-balance temperature is also low. The effects of particle size on final particle temperatures (Tpf) and ratios of final volatile yield (Vf) with V0 of W coal heated with a laser intensity of 1.85 MW m-2 are shown in Figure 16. The increase in Tpf with increasing particle size is nearly linear; however, an increase in Vf/V0 with increasing particle size is not linear. On the other hand, as shown in Figure 17, the variation in V/V0 with particle temperatures for all three sample sizes can be expressed by V/V0 ) 1.3 × 10-3 exp(5.2 × 10-3 Tp), R2 ) 0.9169. This result suggests that particle size does not influence the exponential dependence of V/V0 on particle temperatures. Final Volatile Yield of the Coals. The relationship between Tpf and Vf/V0 of three kinds of coals with different particle sizes, volatile contents, and laser intensities are summarized in Figure 18. It is clear that the values of Vf/V0 increase with increasing Tpf; Vf/V0 is polynomially related to Tpf. The change in Vf/V0 with particle size and laser intensity is linear, but it is not linear with final particle temperature and volatile content. Because the patterns of the relationships cannot be described with a simple expression, a comprehensive expression, considering coal volatile matter content, in situ energy density per surface area of coal particle, particle size, and coal properties should be investigated under laser-heating conditions. Conclusions Considering the heterogeneity of coal particles, to overcome the shortage of laser heating and obtain overall understanding
and reliable devolatilization data for three kinds of coals, we used a CO2 laser controlled well by a specially designed doubleshutter system to uniformly heat dispersed particles in nitrogen at atmospheric pressure. A highly accurate multiple-point, twocolor temperature measurement system was used to measure temperatures of coal particles. For a given sample, the measurement was carried out more than 30 times at a given temperature or heating time, particle size, laser intensity, and atmosphere. The average performance and characterization of three coals with different volatile contents were carefully investigated. The experimental results indicate that the apparatus and methodology used in this study are effective and useful. The important conclusions obtained from this study are as follows: (1) Volatile content, particle size, and laser intensity are the three most important parameters in the devolatilization process of coal particles in nitrogen under laser-heating conditions. (2) The devolatilization processes can be separated into two parts by the proximate volatile matter content of the three coals. This result suggests that two devolatilization schemes exist in each coal. (3) The effect of laser intensity on the devolatilization process indicates that the devolatilization process cannot be characterized well by only laser intensity. The particle temperature is more important for the process description. (4) The existence of cross-points in the curves of weight loss and particle temperatures among the samples with different particle sizes is one of the important features of laser heating, which is distinctively different from other heating systems. (5) The change in V/V0 of the W coal with particle temperatures for all three sample sizes is highly dependent on particle temperatures and can be expressed as V/V0 ) 1.3 × 10-3 exp(5.2 × 10-3 Tp) with R2 ) 0.9169. (6) The variation in V/V0 of all three kinds of coals with particle temperatures exponentially depends on particle temperatures and can be expressed as V/V0 ) 9.0 × 10-4 exp(5.3 × 10-3 Tp) with R2 ) 0.8833. (7) To obtain a comprehensive expression for devolatilization process, we should further investigate in situ energy density per surface area of coal particle and in situ coal properties, such as swelling, shrinking, emissivity, and so on. Acknowledgment. This work was performed in the Institute for Advanced Materials Processing at the Tohoku University of Japan. We sincerely thank the Institute for its support in both finance and experimental apparatus. EF060139G
