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Energy & Fuels 1997, 11, 730-738

Experimental Observation and Image Analysis for Evaluation of Swelling and Fluidity of Single Coal Particles Heated with CO2 Laser Hong Gao, Satoru Murata, and Masakatsu Nomura* Department of Applied Chemistry, Faculty of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565, Japan

Masahiro Ishigaki, Mingchang Qu, and Masanori Tokuda Center for Interdisciplinary Research, Tohoku University, 2-2-1 Katahira Aoba-ku, Sendai 980, Japan Received October 2, 1996. Revised Manuscript Received February 2, 1997X

Swelling characteristics of three kinds of single coal particles heated with a well-characterized CO2 laser were directly observed and quantitatively evaluated by combined application of a highspeed video camera with an image analysis system. The results are as follows: (1) The interval time between swelling and shrinking of bubbles of coking coal particles (Blue Creek, Goonyella) could be used to express the ease of reforming bubbles after bursting and the growth of the bubbles before bursting. The distribution of the interval time of coking coals depends on laser intensity, particle size, and coal properties. (2) For the high-volatile weak coking coal (Witbank), there is no formation of bubbles during heating and the maximum swelling ratio is much smaller than that of coking coals. (3) The maximum swelling ratio and final swelling ratio evaluated with the relative projection area decrease with the increasing of laser intensity (or temperature and heating rate) and particle size. (4) There is a monotonously increasing relationship between the maximum relative projection area and the maximum Gieseler fluidity for all three kinds of coal, although the maximum relative projection area increases with increasing laser intensity. This result suggests that the relative projection area of coal particles on heating can be used for evaluating both the swelling and fluidity properties of coals. (5) The present results suggest a possibility that the “surface tension” of bubble film of the coking coal on heating seems to be attainable from the calculation of the rupture pressure of bubbles.

Introduction Thermoplastic properties, such as swelling, fluidity, and viscosity of coal particles, are strongly correlated with the chemical and physical aspects in the technical development of coal utilization processes, such as gasification, carbonization, and combustion processes. Since many bituminous coal particles, on heating to high temperature, pass through a transient plastic or fluid phase, the presence of the fluid phase can influence the processes mentioned above. The further understanding of the swelling, fluidity, and viscosity of coal under conditions of high heating rate is therefore essential to the development of new coal conversion and combustion processes. The importance of coal’s plastic behavior for many commercial processes is reflected in the large number of methods for measuring it. The coal’s swelling behavior accompanying the devolatilization of a single coal particle has been investigated by many workers, using different experimental techniques. Matsunaga et al.1 examined the influence of liquid ammonia treatment on the swelling behavior of a single coal particle heated rapidly to 870 K with an electrical heating plate in a Abstract published in Advance ACS Abstracts, April 1, 1997. (1) Matsunaga, T.; Nishiyama, Y.; Sawabe, H.; Tamai, Y. Fuel 1978, 57, 562-564. X

S0887-0624(96)00167-3 CCC: $14.00

nitrogen or a hydrogen atmosphere. Van Heek et al.2 studied the kinetics of swelling and plasticity of coal during pressurized rapid pyrolysis and hydropyrolysis by changing the heating rate in the range 50-900 K s-1 under 0.01-10 MPa hydrogen pressure in helium. A correlation between the extent of swelling of coal particles during heating and the product formation, especially tar formation, was also established there. Because of the unique ability of the laser to simulate the flame heating fluxes in conventional and highintensity combustors, it has been used as a means of studying heterogeneous combustion and the pyrolysis of coal particles.3-9 Recently, Masawadeh et al.10 developed a CO2 laser devolatilization GC/MS system (2) Lowental, G.; Wanzl, W.; van Heek, K. H. Fuel 1986, 65, 346353. (3) Hertzberg, M. Combust. Sci. Technol. 1970, 1, 449. (4) Hansen, R. L. Carbon 1978, 16, 159. (5) Hertzberg, M.; Zlochower, I. A. Twenty-third Symposium (Int.) on Combustion; The Combustion Institute: Pittsburgh, PA, 1990; pp 1247-1255. (6) Hertzberg, M.; Zlochower, I. A. Combust. Flame 1991, 84, 15. (7) Smith, M. W.; Malhotra, R. M. 1991 International Conference on Coal Science; Butterworth-Heinemann: Oxford, U.K., 1991; pp 496499. (8) Phuoc, T. X.; Maloney, D. J. Twenty-second Symposium (Int.) on Combustion; The Combustion Institute: Pittsburgh, PA, 1988; pp 125-134. (9) Phuoc, T. X.; Maloney, D. J.; Ekmann, J. M. Combust. Flame 1993, 93, 19. (10) Maswadeh, W.; Arnold, N. S.; McClennen, W. H.; Tripathi, A.; DuBow, J.; Meuzelaar, H. L. C. Energy Fuels 1993, 7, 1006-1012.

© 1997 American Chemical Society

Swelling and Fluidity of Coal Particles

and investigated the CO2 laser pyrolysis products from single coal particles at heating rates in the range from 1 × 105 to 3 × 105 K s-1. Pyatenko et al.11 studied the devolatilization of single coal particles by a highintensity laser beam and referred to the characteristics of coal flash pyrolysis, such as time of devolatilization, specific molar yield, and total yield of volatile. The theoretical pattern of devolatilization through the coal particle pores is also discussed there. Dodoo et al.12 researched the swelling of coal particles irradiated with well-characterized laser pulses by using of an electrodynamic-balance (EDB) apparatus combined with the associated recording and analysis system. Maloney et al.13 studied the temperature histories and the size changes of single carbon and coal particles during the early stages of heating and the devolatilization at a heating rate on the order of 105 K s-1 using the system which incorporates an electrodynamic-balance and pulsed laser radiation source. On the other hand, the escape of tars from the Metaplast and out of the particle is an important process of devolatilization that needs to be understood and modeled correctly to interpret and predict the devolatilization behavior of coals.14 Softening coals are often treated as a liquid droplet during part of the devolatilization process, while in nonsoftening coals, the transport of volatile occurs within the pore structure of the particles.15 However, it is difficult to predict to what degree a coal will soften during devolatilization. Fluidity plays an important role in transport processes. Within a liquid coal particle, transport mechanisms include diffusion of the lighter fractions of tar precursors to the surface and convective transport of bubble.15 Transport of tars to bubbles with subsequent leakage of the bubbles is thought by some to be the main mode of escape of tars out of the softened coal particles.16 The pyrolysis and tar transport of softening coal is quantitatively modeled by Oh et al.17 and compared with the data obtained from packed-bed samples. Loison and collegues18 provide an interesting discussion of bubble rupture phenomenon in terms of a defined “surface tension” or exactly “film-forming properties” and show that this property affects the ease of re-forming bubbles after bursting and growth of bubbles before bursting. However, Loison and colleagues also stated that the properties affecting this “film-forming or surface tension” are fairly “difficult to define and measure”. Transportation of volatile matter is strongly influenced by the softening behavior of some coals; however, lack of understanding of the physical and chemical phenomena concerning the structural modifications leading to softening has limited model development. Up to now, there is little quantitative information about how the repeated behavior of swelling and rupture of softening coals is influenced by their chemical and (11) Pyatenko, A. T.; Bukhman, S. V.; Lebedinskii, V. S.; Nasarov, V. M.; Tolmachev, I. Ya. Fuel 1992, 71, 701-704. (12) Dodoo, J. N. D.; Ochran, R. A. Fuel 1994, 5, 773-778. (13) Maloney, D. J.; Monazam, E. R.; Woodruff, S. D.; Lawson, L. O. Combust. Flame 1991, 84, 210-220. (14) Smith, K. L.; Smoot, L. D.; Fletcher, T. H.; Pugmire, R. J. The Structure and Reaction Processes of Coal; Plenum Press: New York and London, 1994; pp 225-227. (15) Suuberg, E. M. Chemistry of Coal Conversion; Schloberg, R. H., Ed.; Plenum Press: New York, 1985; pp 67-119. (16) Gavalas, G. R. Coal Pyrolysis; Elsevier: Amsterdam, 1982. (17) Oh, M. S.; Peters, W. A.; Howard, J. B. AIChE J. 1989, 35, 775792. (18) Loison, R.; Foch, P.; Boyer, A. F. Coke: Quality and Production; Butterworth: London, 1989; pp 83-86.

Energy & Fuels, Vol. 11, No. 3, 1997 731

physical properties such as rate of devolatilization, density, specific heat, thermal conductivity, emissivity, structural properties, and so on. On the other hand, there are a number of studies to examine the fluidity of coals using different approaches under various conditions. Lloyd et al.19-21 have studied the Gieseler plasticity and correlated the Gieseler plasticity with analytical pyrolysis data of coals, mainly under the conditions pertinent to metallurgical coke-making, namely, slow carbonization of packed-bed samples. Clemens et al.22 studied the effect of pretreatment, such as the addition of solvent extracts, preheating, quenching, addition of radical stabilizer, and blending on the development of Gieseler fluidity. A number of methods have been used by other workers in attempt to modify the Gieseler method. Fong et al.23 developed a fastresponse plastometer, with which heating rates, final temperatures, and sample residence times can be separately selected and controlled over the ranges 40800 K s-1, 600-1250 K, and 0-40 s, where hydrogen or inert gas pressures can be adjusted up to 10 MPa. The results show that, at high heating rates (40-800 K s-1), the duration of plasticity and its rate of disappearance depend strongly on pressure, temperature, and heating rate. Chan et al.24 studied the thermoplastic properties of three kinds of coal using a high-pressure dilatometer and a high-pressure constant shear rate plastometer, the results showing that some measurements of thermoplastic behavior, for example, dilatation and coke intensity, are very dependent on heating rate and pressure. In contrast, the plastometric caking intensity only varies slightly with heating rate and pressure. The optical anisotropy of the carbonized residues from the high-pressure thermoplasticity experiments increases markedly with increasing heating rate. The relations among dilatometer, plastometer, and optical anisotropy under the experimental conditions are complex. There are also some chemometric studies25-28 on the thermoplastic properties, using mass spectrometer, solvent-swelling, and solvent-extraction methods. However, there are still many unclear points on the mechanism of coal plasticity, and there is no systematic method for evaluating plasticity (such as swelling, fluidity, and viscosity) of single coal particles accompanying the devolatilization at very high heating rate. The new method for evaluation of swelling, fluidity, and viscosity of coal particles during rapid heating is important for the development of new coal conversion and combustion processes. The objective of the present study is to directly observe the swelling and rupture behavior of the coal particles heated with a well-characterized CO2 laser, (19) Lloyd, W. G.; Reasoner, J. W.; Hower, J. C.; Yates, L. P. Fuel 1990, 69, 1257-1270. (20) Reasoner, J. W.; Hower, J. C.; Yates, L. P.; Lloyd, W. G. Fuel 1985, 64, 1269. (21) Lloyd, W. G.; Reasoner, J. W.; Hower, J. C.; Yates, L. P. Energy Fuels 1989, 3, 585. (22) Clemens, A. H.; Matheson, T. W. Fuel 1995, 74, 57-62. (23) Fong, W. S.; Khalil, Y. F.; Peters, W. A.; Howard, J. B. Fuel 1986, 65, 195-201. (24) Chan, M. L.; Parkyns, N. D.; Thomas, K. M. Fuel 1991, 70, 447453. (25) Marzec, A.; Czajkowska, S.; Moszynski, J.; Schulten, H.-R. Energy Fuels 1992, 6, 97-103. (26) Schulten, H.-R.; Marzec, A.; Czajkowska, S. Energy Fuels 1992, 6, 104-108. (27) Marzec, A.; Czajkowska, S. Energy Fuels 1994, 8, 360-368. (28) Ouchi, K.; Itoh, S.; Makabe, M.; Itoh, H. Fuel 1989, 68, 735740.

732 Energy & Fuels, Vol. 11, No. 3, 1997

Gao et al. Table 1. Properties of the Coals

elemental analysis (d.b. %)

proximate analysis (d.b. %)

thermoplasticitya

optical property

coal

C

H

N

S

O (diff.)

VM

ash

M (wt %)

log(Fmax)

b

Witbank Goonyella Blue Creek

77.8 82.1 80.2

4.6 4.4 4.9

1.9 1.8 1.8

0.7 0.6 0.6

7.7 2.8 3.8

32.4 24.6 25.7

7.3 8.3 8.7

3.3 3.0 3.7

1.2 2.9 3.7

0.79 0.79 0.73-0.77

a

Gieseler thermoplasticities. b Emissivity of the coals measured in an electric furnace. Table 2. Experimental Conditions particle size (µm) temperature (°C) laser intensity (MW m-2) heating time (ms) heating rate (°C s-1)

Figure 1. Schematic diagram of the apparatus for observing and evaluating the plasticity of single coal particles heated with a CO2 laser.

then quantitatively evaluate the changes of swelling ratio and interval time between swelling and shrink of the bubbles by combined application of a high-speed video camera with an image analysis system, and finally discuss the relations among the swelling, the fluidity, and the viscosity of the coal particles. Experimental Section Coal Samples. A South African high-volatile weak coking coal (Witbank), an Australian medium-volatile coking coal (Goonyella), and an American medium-volatile coking coal (Blue Creek) were used as raw coals. The raw coals were crushed and sieved to give the samples with particle size of approximately 100-355 µm. The characteristics of the coals such as ultimate, proximate, Gieseler thermoplastic properties, and optical properties are shown in Table 1. As it is known that macerals of coals show great differences in their physical and chemical behaviors during pyrolysis, it is necessary to analyze vitrain, durain, and fusain particles separately. Because of its inert nature, the fusain shows no change in particle size during pyrolysis. Therefore, fusain is not regarded in the following part of the paper. To perform the experiments, single particles were selected by hand using a microscope. However, for comparison of results, it should be noted that the measured particles differ in size, shape, mineral content, and maceral content. Experimental Apparatus and Procedure. As shown in Figure 1, the experimental apparatus consists of a CO2 laser (λ ) 10.6 µm) heating system with two shutters controlled by a computer, a temperature measurement system of coal particles, a high-speed camera system for monitoring and recording the behavior of coal particles on heating, an image analysis system for quantitatively evaluating plasticities of coals, a transparent silica tube reactor, and a gas supply system. The reactor was made of a 200 mm long transparent silica tube with an inner diameter of 16 mm. The sample plate was made of a 1 mm thick transparent silica with a diameter of 10 mm. To exclude air and prevent volatile matters decomposed from coal from adhering to the focus lens, the high-purity nitrogen gas was supplied from both the top and bottom of the reactor.

100-355 200-1000 0.80-2.96 0-35000 2500-15000

The heating system includes a 125 W CO2 laser source (Japan Laser Automation, NEC, CL112C) and attendant optics to direct the CO2 laser beam into the reactor. The laser beam can be configured for continuous wave (for laser intensity of >0.80 MW m-2) or periodical pulse (for laser intensity of e0.80 MW m-2) programmed internally. The laser beam is focused by a 70 mm ZnSe focal lens to provide a 8 mm diameter cross section. The intensity of incident laser was determined using a calibrated optical power meter. The heating time was controlled by a specially designed double-shutter system which was controlled by a computer, with which the minimum heating interval can be controlled on the order of 1 ms. The expected laser intensity could be obtained by adjusting the both laser power and focus length. The particle temperatures were measured with a singlewavelength infrared pyrometer (Japan Sensor Co. Ltd., Modified TTS-F1 A3000) which incorporated a thermoelectrically cooled PbS detector with a spectral sensitivity range 2.0-2.6 µm. The system was optimized for particle temperature in the range 200-1000 °C with a response time of 2 ms. The average emissivity of the coals required for temperature measurement of the coals was measured in an electrical furnace with the infrared pyrometer and thermocouples. The behavior of the coals on heating, e.g. devolatilization, swelling, and shrinking, was monitored and recorded with a high-speed video camera (Japan Kodak Co. Ltd., High-Speed Motion Analyzer, Model 4540). The swelling of single coal particles was quantitatively evaluated with an image analysis system (Japan Nireco Co. Ltd., LUZEX-3). As shown in Table 2, the experiments were carried out with various laser intensities, heating times, and particle sizes in an atmospheric pressure of nitrogen. Each experiment was done at least three times for reproducibility.

Results and Discussion The Temperature History, Volatile Matter Yield, and Heating Rate of Single Coal Particles. The temperature history, volatile matter yield, and heating rate of a single coal particle heated with the CO2 laser were modeled with the following assumptions: (1) Coal particles were heated with the CO2 laser from one side with πDc cross section. (2) The coal particle is taken as blackbody. This means that energy is absorbed at the particle surface with an efficiency  which is equal to the emissivity of coals and the internal heating in coal particle is by conduction. (3) Convection and radiation heat losses are taken on the entire particle surface (πDc2) for a sphere. (4) The pyrolysis process is assumed to be thermally neutral. Under these assumptions, the transient temperature can be described with the energy conservation eq 1 (for

Swelling and Fluidity of Coal Particles

Energy & Fuels, Vol. 11, No. 3, 1997 733

Figure 2. Temperature history, volatile matter yield, and heating rate of Witbank coal particle heated with 2.22 MW m-2 CO2 laser.

definitions of following variables, refer to the Glossary given later in this paper).

dTc/dt ) 6[I/4 - hc(Tc - Tg) - σ(Tc4 - Tg4)]/ (FcDcCpc) (1) The kinetics equation29 of devolatilization of coal particle is shown in eq 2.

dVm/dt ) Ko(V∞ - Vm) exp(-E/RTc)

(2)

The expression30 for the final yield of volatile matter is shown in eq 3.

V∞ ) 1.2(Vdaf)0.8 exp{(-2 × 106)/[R(T∞ - T0)2]} (3) The density of coal particles on heating is described with eq 4.

Fc ) F0(1 - Vm)/[(St/S0)1.5]

(4)

Equations 1 and 2 were solved numerically using RKG scheme. The heat transfer coefficient at the particle surface was calculated by assuming a Nusselt number of 2. In the variable-size calculations, measured particle cross-sectional area was converted to equivalent particle radii from which changes in the geometric absorption cross section and particle density were incorporated into the particle energy balance equation. The temperaturedependent heat capacity of coals is described using the Merrick model.31-33 Calculations were performed using the particle size history measured as input to the model. A simulated example (temperature history, volatile yield, and heating rate) is shown in Figure 2, where the calculated temperature agrees well with the measured one. Microscopy Images of Three Kinds of Coal Particles on Heating. Photographs of Blue Creek, Goonyella, and Witbank coal particles heated by CO2 laser with intensity from 0.80 to 2.22 MW m-2 are shown in Figures 3, 4, and 5, respectively. In general, for the medium-volatile softening coals (Blue Creek and Goonyella), the swelling occurs through formation of bubbles by the evolving gas, and shrinking occurs through the rupture of the bubbles. The swelling and shrinking of the bubbles repeat with certain interval time or fre(29) Fu, W.; Zhang, Y.; Han, H.; Wang, D. Fuel 1989, 68 , 505-510. (30) Zhang, Y.; Mou, J.; Fu, W. Fuel 1990, 69, 401-403. (31) Merrick, D. Fuel 1983, 62, 534-539. (32) Merrick, D. Fuel 1983, 62, 540-546. (33) Merrick, D. Fuel 1983, 62, 547-552.

Figure 3. Frame photographs of Blue Creek coal particles heated at different laser intensities: (a) 0.80 MW m-2, (b) 0.86 MW m-2, (c) 2.22 MW m-2.

quency, which depends on the laser intensity (temperature and heating rate), particle size, and coal properties. Obviously, the devolatilization during the thermoplastic stage is not continuous but intermittent. For the high-volatile weak coking coal (Witbank), there is no formation of bubble film during heating and the maximum swelling ratio is much smaller than that of coking coals. The idea that swelling of softening coal occurs through formation of a bubble by the evolving gas is quite well-known and is discussed in several standard works18,34-38 having chapters on coal swelling and fluidity. It has been known for at least 70 years that the devolatilization of bituminous coals occurs in at least three stages or regions, but at the relatively slow heating rates used in most early work, these stages overlapped. Discussions were given by Berkowitz,34 Habermehl et al.,36 and van Krevelen.38 However, van Krevelen38 shows a very interesting graph of total gas evolution of packed-bed vitrinite of a medium-volatile bituminous coal samples heated at 1.8 °C min-1, in which quite distinct local maxima are apparent, although van Krevelen did not interpret the reason. One of the reasons for this phenomenon seems to be that, even for packed-bed samples heated at lower heating rate, due to the formation of bubbles and the rupture of the bubbles with some interval time, the gas evolution (34) Berkowitz, N. An Introduction to Coal Technology; Academic Press: New York, 1979; pp 131-157. (35) Lowry, H. H. Chemistry of Coal Utilization, Supplementary Volume; Wiley: New York, 1963; pp 150-191. (36) Elliott, M. A. Chemistry of Coal Utilization, Supplementary Volume; Wiley: New York, 1981; pp 319-357. (37) Gorbaty, M.; Larsen, J. W.; Wender, I. Coal Science; Academic Press: New York, 1982; pp 32-39. (38) Van Krevelen, D. W. Coal, 3rd ed.; Academic Press: New York, 1993; p 684.

734 Energy & Fuels, Vol. 11, No. 3, 1997

Gao et al.

Figure 5. Frame photographs of Witbank coal particles heated at different laser intensities: (a) 0.86 MW m-2, (b) 2.22 MW m-2.

Figure 4. Frame photographs of Goonyella coal particles heated at different laser intensities: (a) 0.80 MW m-2, (b) 0.86 MW m-2, (c) 1.48 MW m-2, (d) 2.22 MW m-2.

could be retarded, so that the local maxima of gas evolution are apparent, then being followed by following the rupture of bubbles. The similar phenomena can be clearly observed in our experiments (Figures 3a and 6a, Blue Creek coal heated by 0.80 MW m-2 laser intensity) in which the onset of the bubble swelling is at about 167 ms, and this bubble ruptures at 2930 ms accompanying evolution of much gas where the coal

particle almost shrinks to the initial state of the bubble, then starts swelling until ruptures at 4829 ms and resolidifies with a relatively high swelling ratio (4.9). On the other hand, volatile products of the coal pyrolysis are generated through the coal matrix, and the distribution and yield of pyrolysis products can be greatly influenced by transport and chemical reactions of the volatile within and outside the coal particle. Quantitative modeling of pyrolysis, therefore, requires analyses of intraparticle-transport, particle-surface-transport, extra-particle-transport, and chemical reaction. Using the present method, we can obtain more correct quantitative data (distribution of swelling ratio, distribution of interval time of swelling and shrinking, softening point, resolidified point, and so on) of single coal particles under a wide range of heating conditions. The utilization of high-speed camera allows us to observe not just a continuous curve with apparent local maxima but also the actual resolution of a curve of this type into distinct, discontinuous, or intermittent peaks. We believe that the present data and method are useful in improving the kinetic model of coal devolatilization processes and thermoplastic development processes more reasonably. The Effect of Laser Intensity on Swelling Features of Single Coal Particles. The changes of swelling ratio evaluated with the relative projection area (St/S0) and the temperature history of Blue Creek, Goonyella, and Witbank coal particles heated by CO2 laser with intensity from 0.80 to 2.22 MW m-2 are shown in Figure 6. The effect of laser intensity on maximum swelling ratio for three kinds of coal is shown in Figure 7. The effect of laser intensity on the plastic parameters of coal particle on heating is summarized in Table 3. With increasing of laser intensity (0.80 to 2.22 MW m-2 for Blue Creek and Goonyella coals, 0.86 to 2.22 MW m-2 for Witbank coal), we can see that the maximum swelling ratio (SRmax) decreases from 6.6 to 4.5 for Blue Creek, 5.0 to 3.0 for Goonyella, and 1.8 to 1.5 for Witbank and that the final swelling ratio (SRf) changes from 4.9 to 0.9 for Blue Creek, 4.0 to 1.1 for Goonyella, and 1.2 to 1.3 for Witbank. The temperature at maximum swelling (Tmax) changes from 460 to 620 °C for Blue Creek, from 460 to 660 °C for Goonyella, and from 474 to 690 °C for Witbank. Under high laser intensity, the devolatilization velocity of volatile matter is considered to be fast, and the viscosity of swelling

Swelling and Fluidity of Coal Particles

Energy & Fuels, Vol. 11, No. 3, 1997 735

Table 3. Effect of Laser Intensity on the Plasticities of Three Kinds of Coal Particles (L 127 µm) laser intensity (MW m-2)

coal

HRmaxa (°C s-1)

SRmaxb (-)

Tmaxc (°C)

SRfd (-)

tie (ms)

Ff (per 100 ms)

0.80

B G W B G G W B G

2600 2600 5260 4730 5200 8560 12000 12000 12000

6.6 5.0 1.8 5.7 5.2 4.3 1.5 4.5 3.4

460 460 474 510 600 650 690 620 660

4.9 4.0 1.2 1.8 1.8 1.8 1.3 0.9 1.1

1800-2200 60-750

0.3-0.5 1.3-1.5

80-570 140-450 5-35

0.1-1.3 0.2-0.7 2.9-20.0

2-10 3-10

1.0-50.0 10.0-83.3

0.86 1.48 2.22

a Maximum heating rate. b Maximum swelling ratio. c Temperature at maximum swelling ratio. between swelling and shrinking of bubbles. f Frequency of swelling and shrinking of bubbles.

d

Final swelling ratio. e Interval time

Figure 7. Effect of laser intensity on the maximum relative projection area of three kinds of coal.

Figure 6. Effect of laser intensity on the relative projection area and temperature profile: (a) Blue Creek, (b) Goonyella, (c) Witbank.

bubble film is to be low before the formation of resolidified shells; therefore, the rupture of the bubble is easy. As a consequence, relatively large amounts of the evolution of gas and metaplast might prevent the bubble from swelling further under high laser intensity. The observation that a higher heating rate reduces swelling is contradictory to what is customarily thought to be

true; namely, an increase in heating rate enhances the various plastic properties of coal (this phenomenon was well discussed by Berkowitz and Loison et al.34). However, most of the increase in plastic properties attributed to “higher” heating rates was measured at heating rates that, while higher than the conventional parameters used to measure plastic properties, were nonetheless much lower than those used by the present study. In our previous work,39 the SEM observable changes of surface texture were not observed up to 485 °C at heating rates above 6300 °C s-1, even with coking coals. However, the surface changes were observed at 440 °C at heating rates lower than 1800 °C s-1, even with weak coking coals. These results suggest that the effect of heating rate on increasing plasticity has a limited range. High heating rates can substantially reduce, and perhaps destroy completely, the plastic properties. The effect of laser intensity on the distribution of interval time of bubble’s swelling and shrinking of Blue Creek and Goonyella coal is shown in Figure 8. Under the condition of pulse mode with 0.80 MW m-2 laser intensity (Figure 8a), the interval time of Blue Creek coal changes from 2200 to 1800 ms, and the bubble ruptures only two times. However, the bubbles of Goonyella coal rupture 10 times and the interval time decreases from 750 to 60 ms until resolidification. This difference in the interval time of bubble swelling and shrinking could be interpreted using the Gieseler maximum fluidity because the Gieseler maximum fluidity of Blue Creek coal and Goonyella coal are 3.7 and 2.9, respectively: it seems that the high Gieseler fluidity can reduce rupture. We can see that the interval time of Blue Creek coal is 3-30 times higher than that of Goonyella. In the case of continuous-wave mode with (39) Gao, H.; Murata, S.; Nomura, M.; Ishigaki, M; Tokuda, M. Energy Fuels 1996, 10, 1227-1234.

736 Energy & Fuels, Vol. 11, No. 3, 1997

Gao et al.

Figure 9. Effect of particle size on the relative projection area and temperature profile of coal particles: (a) Blue Creek, (b) Goonyella. Table 4. Effect of Particle Size on the Plasticities of Coking Coals (Laser Intensity ) 2.22 MW m-2) particle size HRmaxa SRmaxb Tmaxc SRfd coal (µm) (°C s-1) (-) (°C) (-) B G

Figure 8. Distribution of interval time between swelling and shrink of bubbles of Blue Creek coal particles on heating: (a) 0.80 MW m-2, (b) 0.86 MW m-2, (c) 2.22 MW m-2.

0.86 laser intensity (Figure 8b), with increasing of heating time, the interval time of Blue Creek decreases from 300 to 80 ms then increases to 570 ms, while that of Goonyella decreases from 300 to 140 ms then increases to 450 ms. As for the time of the first rupture, the shortest interval and resolidification between Blue Creek and Goonyella coals in Figure 8, following time delay was observed. In the case of 0.86 MW m-2 laser intensity (Figure 8b), the times are delayed about 150, 100, and 250 ms, respectively. In the case of 2.22 MW m-2 laser intensity (Figure 8c), the interval times of Blue Creek and Goonyella coals are almost the same (about 2-10 ms); however, the retention time of plasticity and the numbers of rupture of Goonyella coal bubble are about 4.5 times and 2.5 times that of Blue Creek coal, respectively. Because the data of elemental analysis and proximate analysis of two kinds of coal are very similar to each other (Table 1), the difference in the behavior of bubble rupture suggests that the chemical structure and structural change related to viscosity of

149 355 149 355

11000 8400 11000 8400

4.5 3.3 3.4 2.7

700 830 710 860

0.8 1.7 1.1 1.6

tie (ms)

Ff (per 100 ms)

3-10 10-33.3 4-65 1.5-25.0 1-14 25.0-100.0 2-10 10.0-50.0

a Maximum heating rate. b Maximum swelling ratio. c Temperature at maximum swelling ratio. d Final swelling ratio. e Interval time between swelling and shrinking of bubbles. f Frequency of swelling and shrinking of bubbles.

Blue Creek coal on heating is much different from that of Goonyella coal. The explanation on this phenomenon could be related to the structural change during the coal devolatilization process. The further study on the structural variations, such as phenomena of metaplast formation, cross-link formation, rearrangement of aromatic units, and mesophase formation during carbonization of the coal particles at very high heating rates, is now in progress in our laboratory. The Effect of Coal Particle Size on the Swelling Features of Single Coal Particles. The effect of coal particle size on swelling is shown in Figure 9. The effect of particle size on the plastic parameters of the coal particles is summarized in Table 4. As they show, the maximum swelling ratio decreases from 4.5 to 3.3 for Blue Creek and from 3.4 to 2.7 for Goonyella with the increasing of coal particle size from 149 to 355 µm, respectively. The temperature of maximum swelling changes from 700 to 830 °C for Blue Creek and from 710 to 860 °C for Goonyella. However, the final swelling ratio increases from 0.8 to 1.7 for Blue Creek and from

Swelling and Fluidity of Coal Particles

Energy & Fuels, Vol. 11, No. 3, 1997 737

Figure 11. Relationship between maximum relative projection area and maximum Gieseler fluidity of three kinds of coal.

Figure 10. Effect of particle size on the interval time between swelling and shrinking of bubbles: (a) Blue Creek, (b) Goonyella.

1.1 to 1.6 for Goonyella, respectively . These results suggest that the swelling process under the conditions of high heating rates (8400-11000 °C s-1) and relative large particle sizes (L 149-355 µm) is controlled by the transfer of gas and metaplast in the particles. Before the resolidified shells were formed, the diffusion of gas and metaplast is easier in small particle than that in the large one. Therefore, in the case of small coal particles, the maximum swelling is enhanced, however, the final swelling is weakened, this being consistent with the analysis made by Loison et al.:18 “Within the particle, principally the largest, bubbles of more or less spherical form arise and grow because the gases that form at the center of the material do not have an easy exit; the larger the particle, the more difficult it becomes.” These results also imply that the relative evolution of gas and metaplast decreases with increasing of particle size, this being qualitatively consistent with the data by Suuberg et al.40 and the data predicted by Oh et al.17 The effect of coal particle size on the interval time of swelling and shrinking of the bubbles is shown in Figure 10. For Blue Creek coal, in the case of large particle size, the local maxima were largely increased, especially during the initial stage, although the minimum interval time is almost same for both size. The retention time of plasticity is elongated about twice. For Goonyella coal, the increase of duration time of plasticity is the same as that for Blue Creek coal; however, the interval time decreases with increasing of particle size, this being different from the Blue Creek. These results suggest that the developments of viscosity or “surface tension” in two kinds of coal particle are (40) Suuberg. E. M.; Peters, W. A.; Howard, J. B. Int. Symp. Combust. 1978, 117.

greatly different from each other. It is generally believed that the smaller the particle size, the lower the plasticity coal will retain, or that fine grinding of coal decreases all of the physical phenomena that are associated with plasticity. However, these results were almost obtained under the conditions of lower heating rates and packed-bed samples. The present results, therefore, supply us the knowledge that, at least, the results of packed-bed samples at lower heating rate cannot be simply used to interpret the phenomena of single coal particles heated at very high heating rates. The effect of particle size on plasticity strongly depends on heat and mass transfer in particles and the surrounding conditions of the coal particles. The Relations Among Swelling, Fluidity, and Viscosity of the Coals. The relationship between maximum swelling ratio and maximum Gieseler fluidity is shown in Figure 11. Under the conditions that the laser intensity changes from 0.8 to 2.96 MW m-2 and particle size is 127 µm, there is a monotonously increasing relationship between maximum relative projection area and the maximum Gieseler fluidity for all three kinds of coal, although maximum swelling ratio changes from 1.5 to 1.8 for Witbank coal, from 2.2 to 5.0 for Goonyella, and from 3.8 to 6.6 for Blue Creek coal, respectively . The larger the Gieseler fluidity, the larger the relative projection area of coal particles will become. This suggests that the relative projection area of coal particles on heating seems to be used in evaluating both the swelling properties and fluidity of coals. By using this method, both the swelling properties and fluidity of single coal particles are obtainable at the same time. In addition, the viscosity or “surface tension” of coking coals on heating seems to be evaluated indirectly by the swelling ratio and the interval time, because the endurance of bubble film on the pressure caused by devolatilization of volatile matter could be used to present the viscosity or “surface tension” of the bubble on heating, although we now cannot yet establish the direct relation between viscosity and rupture pressure. Because the viscosity is a function of “surface tension” or rupture pressure of the bubble, it is obvious that the higher the swelling ratio, the higher the viscosity or surface tension the coal will retain. Van Krevelen38 and Elliott36 in their monographs indicated that “it is possible to express plasticity in terms of an apparent viscosity by comparing the results for coal with those model substances such as pitches. Such apparent viscosity depends upon the foam structure within the

738 Energy & Fuels, Vol. 11, No. 3, 1997

experimental capsule as well as upon the true coal plasticity and has, as a consequence, no simple physical significance.” However, up to now, there is little quantitative data of fluidity and viscosity at very high heating rates, especially for single coal particles. Loison et al.18 tried to use “surface tension or bubble forming properties ” to characterize the ease of re-forming bubbles after bursting and growth of bubbles before bursting. However, Loison and colleagues stated that the properties affecting this “surface tension” and bubble formation and bursting are “difficult to define and measure”. In the present study, we avoid this difficult problem and try to use new parameters: interval time (ti) or frequency (F) of the swelling and shrinking of bubble films to express the ease of reforming bubbles after bursting and the growth of the bubbles before bursting. Moreover, using the swelling ratios (St/S0) at rupture points, we can indirectly characterize the viscosity or “surface tension” of bubble film on heating. If we had known the volumetric swelling ratio, weight loss of each rupture of bubble, average molecular weight distribution of devolatilized products, and coal particle temperature, we could theoretically calculate the pressure in the bubbles: the pressure is a good expression of the endurance of bubble film or “surface tension”. Obviously, the calculation and measurement on the pressure in the bubbles is an interesting but a challenging work. Conclusions From the high-speed video camera observation and the image analysis of the swelling behavior of coking coals and weak coking coal heated with a well-characterized CO2 laser, we found the following: (1) The interval time between swelling and shrinking of bubbles of coking coal particles on heating seems to be used to express the ease of reforming bubbles after bursting and the growth of the bubbles before bursting. The distribution of the interval time depends on laser intensity, particle size, and coal properties. In the case of a continuous laser heating mode, the interval time of swelling and shrinking of bubbles of Blue Creek and Goonyella coals changes from 2 to 10 ms at 2.22 MW m-2 laser intensity, from 5 to 35 ms at 1.5 MW m-2 laser intensity, and from 80 to 570 ms at 0.86 MW m-2 laser intensity, respectively. However, in the case of the pulse heating mode with a laser intensity of 0.8 MW m-2, the interval time changes from 2200 to 1800 ms for Blue Creek coal (rupture two times) and from 750 to 60 ms for Goonyella coal (rupture 10 times). (2) In the case of high-volatile weak coking coal, there is no formation of bubble of the plasticizer and the maximum swelling ratio is much smaller than that of the coking coals. (3) The maximum swelling ratio and final swelling ratio evaluated with relative projection area is strongly dependent on the laser intensity (or heating rate and temperature): with increasing of laser intensity from 0.80 to 2.22 MW m-2, the maximum swelling ratio changes from 5.0 to 3.0 for Goonyella and from 6.6 to

Gao et al.

4.5 for Blue Creek coal. The final swelling ratio changes from 4.9 to 0.9 for Blue Creek and from 4.0 to 1.1 for Goonyella. With increasing of laser intensity from 0.86 to 2.22 MW m-2, the maximum swelling ratio changes from 1.8 to 1.5 and the final swelling ratio changes from 1.3 to 1.2 for Witbank. (4) There is a monotonously increasing relation between maximum relative projection area and maximum Gieseler fluidity for all three kinds of coal at high heating rates, although the maximum relative projection area increases with increasing of laser intensity. This means that the relative projection area of coal particles on heating at high heating rate seems to be used in evaluating both the swelling and fluidity. By using this method, both swelling properties and fluidity can be obtained at the same time. (5) The present results suggest a possibility that the viscosity or “surface tension” of bubble film of the coking coals on heating seems to be attainable from the calculation of the rupture pressure of the bubbles. Glossary Cpc Dc E hc I Ko R S0 St Tc Tg T0 T∞ t Vm V∞ Vdaf σ  F0 Fc

specific heat of coal (J kg-1 K-1) diameter of coal particle (m) equivalent activation energy of the coal devolatilization for the final temperature (J mol-1) convective heat transfer coefficient at coal particle surface (J s-1 m-2 K-1) intensity of laser beam (J m-2) equivalent frequency factor for the final temperature (J mol-1 K-1) general gas constant (J mol-1 K-1) initial projection area (m2) projection area at time t (m2) transient temperature of coal particle during devolatilization (K) temperature of ambient gas (K) initial temperature of coal particles (K) final temperature of coal particles (K) time (s) yield of volatile matter (%) final yield of volatile matter (%) volatile matter fraction of proximate analysis, daf basis (%) Stefen-Boltzman constant (J s-1 m-2 K-4) emissivity of coal particles (-) initial density of coal particles (kg m-3) density of coal particles during devolatilization (kg m-3)

Acknowledgment. The experiments were performed at the Institute for Advanced Materials Processing of Tohoku University. We thank very sincerely Prof. Oba (Institute of Fluid Science of Tohoku University, Japan) for providing the high-speed video camera and constructive comments, Mr. M. Sakawa (Nippon Steel Corp.) for providing the coal samples, and Mr. S. Asada (Research Laboratory, Kansai Coke and Chemicals Co., Ltd., Japan) for the analyses of coal samples. We also grateful for the careful review and valuable suggestions for this paper by Dr. A. Levent (University of Cukurova, Turkey). EF9601677