EXPERIMENTAL STUDY OF COAL PYROLYSIS AND GASIFICATION IN ASSOCIATION WITH SYNGAS COMBUSTION

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1 National Cheng Kung University From the SelectedWorks of Wei-Hsin Chen December, 27 EXPERIMENTAL STUDY OF COAL PYROLYSIS AND GASIFICATION IN ASSOCIATION WITH SYNGAS COMBUSTION Wei-Hsin Chen, National Cheng Kung University Available at:

2 EXPERIMENTAL STUDY OF COAL PYROLYSIS AND GASIFICATION IN ASSOCIATION WITH SYNGAS COMBUSTION W.-H. Chen * J.-C. Chen ** C.-D. Tsai *** Graduate Institute of Greenergy Technology National University of Tainan Tainan, Taiwan 75, R.O.C. Department of Environmental Engineering and Science Fooyin University Taliao, Kaohsiung Hsien, Taiwan 83, R.O.C. S.-W. Du **** Steel and Aluminum Research and Development Department China Steel Corporation Kaohsiung, Taiwan 833, R.O.C. ABSTRACT Coal pyrolysis and gasification incorporating synthesis gas (syngas) combustion are investigated experimentally in the present study. Two different coals are considered; one pertains to high-volatile bituminous and the other low-volatile one. For the pyrolysis, using thermogravimetry in association with mass spectrometry reveals that the concentrations of CO and CO 2 increase with increasing temperature, whereas those of H 2 and CH 4 undergo increase followed by decrease. Regarding the gasification, the formations of the four gases between the two different coals are similar. However, when the reaction temperature is relatively low such as 8 C, carbon reactivity of the low-volatile coal decays in a significant way. Furthermore, with the reaction temperature of 1 C the entire gasification histories of the two coals can be divided into five periods in accordance with syngas combustion. They are initiated, growing, rapidly decaying, progressively decaying, and frozen periods, sequentially. The flame is wrinkled in the growing and rapidly decaying periods where the reaction strength is much higher than that near extinction. When the reaction temperatures are 8 and 9 C, the growing and rapidly decaying periods tend to wither. Recognizing the syngas combustion characteristics, one is capable of figuring out the coal gasification process in more detail. Keywords : Pyrolysis, Gasification, Coal, Syngas combustion, Flame. 1. INTRODUCTION It is well known that energy utilization plays an important role in the progresses of industry and civilization. Before 185s, most of the energy relied on burning biomass such as woods [1]. After that, the employment of coal rose in a significant way for the purpose of getting heat and power. Subsequently, oil, natural gas, hydraulic power, and nuclear energy were sequentially found and developed in the earlier twentieth century. The applications of fossil fuels have pronouncedly promoted the levels of science and technology over the past one hundred years [2]; however, we nowadays are encountering the challenge of energy depletion. According to a recent study concerning energy resources and global develop [3], it was reported that crude oil, natural gas, and uranium are likely to be exhausted in the future 3 to 6 years, based on annual 2 energy consumption. In contrast, coal can still be utilized for two hundred years. Therefore, it is anticipated that the role played by coal in energy supply will become more and more compelling. Conventionally, most of the coal was pulverized and consumed via direct combustion in large-scale utility furnaces for generating electric power [4,5]. This method possesses the merits of well-developed technique and knowledge, but its efficiency is relatively low. Besides, on account of growing concerns upon emissions of combustion-generated air pollutants and greenhouse gas (viz., CO 2 ) from pulverized-coal power plants, it has greatly stimulated the study of clean coal technology [6]. Reviewing recently developed methods of the clean coal technology, since the Cool Water Coal Gasification Program was successfully performed in 1984, integrated gasification combined cycle (IGCC) has received a great deal of attention [7]. This arises from the fact that the efficiencies of the IGCC are much higher than those from coal-fired power plants. * Professor, corresponding author ** Associate Professor *** Graduate student **** Associate Scientist Journal of Mechanics, Vol. 23, No. 4, December

3 Meanwhile, the environmental impacts from the pollutant emissions of IGCC can also be reduced to a great extent, as a consequence of treating produced syngas prior to burning it. As a matter of fact, from the viewpoint of hydrogen economy [8,9], coal gasification will also become an important source of hydrogen in the near future in that the hydrogen contained in syngas can further be enriched through water-gas shift reaction [1,11], along with large size of available coal deposited []. Regarding the coal gasification techniques, the processes can be classified to fixed or moving bed, fluidized bed, entrained bed, and molten bath [13,14]. In the past, a number of efforts have been implemented to recognize the coal gasification characteristics in these gasifiers. For example, in the study Ohtsuka and Wu [15], the pyrolysis of a variety of coals in a fixed-bed gasifier filled with CO 2 and helium was carried out to realize nitrogen release behaviors from the coals. Sheth et al. [16] studied catalytic gasification of coal blended with eutectic salts in a fixed bed. It was reported that increases in gasification rates and conversion levels were characterized by increasing metal-tocarbon ratio. Meanwhile, the ternary eutectic catalyst provided better carbon conversions and gasification rates than the binary one. Ocampo et al. [17] explored coal gasification behavior in a fluidized bed by varying steam/coal and air/coal ratios, with emphasis on the heating values contained in the product gases. Lee et al. [18] investigated coal gasification in entrained flow by use of a drop tube reactor, revealing that H 2 /CO ratio decreased with increasing reaction temperature and a maximum distribution in syngas concentration was exhibited around ash fusion temperature. Instead of the experimental studies described above, Choi et al. [19] numerically predicted coal gasification in an entrained flow gasifier with slurry feed. In their study, the slurry coal gasification process was divided into several stages, consisting of slurry evaporation, coal devolatilization, and two-phase reactions. Skodras et al. [2] used a computational fluid dynamics (CFD) code to simulate coal gasification phenomena in a foaming molten slag gasification reactor, in which the models of foam (main phase) and bubbles (dispersed phase) were taken into account in detail. Because of the potential of coal gasification in both energy development and environmental concern, as mentioned above, the present study is intended to investigate pyrolysis (or devolatilization) and gasification characteristics of two different coals. To explore detailed pyrolysis dynamics of the coals, both thermogravimetry and mass spectrometry will be utilized to measure mass-loss and gas formation histories of the coals where they are placed in a gradually increasing temperature environment filled with inert gas. Then, the transient gasification characteristics of the coals with the supply of little amount oxygen are studied. Four gases, namely, H 2, CH 4, CO, and CO 2, formations will be detected by means of gas chromatography and gas analyzer. It is know that, once the syngas is ignited and burned, the flame length and structure are highly related to the gasification rate. A device possessing observing syngas combustion and measuring flame length is designed to qualitatively recognize the coal gasification processes. It will be illustrated later that the flame observation and measurement can provide a more comprehensive study in coal gasification. 2. EXPERIMENTAL METHOD The present studies are performed based on the pyrolysis and gasification of two bituminous coals which come from Australia. The coals are pulverized and used in blast furnaces to partially replace high-price cokes, for the purpose of producing hot metal from iron oxides. When the coal particles are injected into the blast furnaces, they will experience devolatilization, gasification, syngas combustion, and char reaction, etc [21]. In order to understand the role played by volatile matter upon coal reaction processes, two different coals, consisting of a high-volatile (Coal R) and a low-volatile (Coal J) bituminous coals, are considered. Their properties such as proximate and ultimate analyses as well as heating values are summarized in Table 1. As shown in the table, the percentages of volatile matter in Coal R and Coal J are 34.3% and 15.24%, respectively, implying that the volatile matter of the former is much higher than that of the latter. On the other hand, the contents of fixed carbon in Coal R and Coal J are 53.18% and 73.91%, respectively. It will be found that these discrepancies in the volatile matter and fixed carbon will lead to substantial difference in syngas combustion. The pyrolysis characteristics of the coals are explored by means of thermogravimetry in combination with mass spectrometry (TG-MS: Netzsch skimmer system 43/5). In regard to the coal gasification and syngas combustion system, the experimental apparatus and procedure are sketched in Fig. 1. The entire system can roughly be divided into several units, composed of (1) a fixed-bed gasifier; (2) a burner; (3) a gas treatment unit; (4) gas measurement equipments; and (5) a reaction recorder. The gasifier (or reactor), which can be heated up to 11 C, is constructed from a tube of stainless steel with internal diameter and length of 32mm and 8mm, respectively. The reaction temperature is controlled by a control box in which PID (proportional band integral derivative) temperature controller and SCR (silicon controlled rectifier) power controller are employed. A hot spark is provided by the burner so as to check the ignitability of the product gas. The burner enables us to observe syngas combustion processes. Previous to measuring the product gas, it will undergo a series of unit operations, including cooling, water bath, filtration, and drying. The preceding procedure can remove tars, particular matters, and moisture contained in the syngas so as to avoid damaging the measurement equipments. Subsequently, the gas chromatography (GC: GOW-MAC Series 4) and 32 Journal of Mechanics, Vol. 23, No. 4, December 27

4 Table 1 Properties of investigated coals Coal Coal R Coal J High-volatile Classification (ASTM) bituminous Proximate analysis (wt %) Low-volatile bituminous Volatile matter Fixed carbon Moisture Ash Ultimate analysis (wt %, dry-ash-free) C H N S O (diff.) Heating value (kcal/kg) Fig. 1 Schematic of conducted coal gasification system gas analyzer (GA: Fuji ZRJF5Y23-AERYR-YKLY- YCY-A) are utilized to measure the volumetric fractions of H 2 as well as CO, CO 2, and CH 4, respectively. Three K-type thermocouples are mounted in the gasifier to provide reference temperatures of the control box. In the course of each experiment, the electric signals from the thermocouples and gas analyzer are sent into a computer where the signals are converted into temperatures and gas volumetric fractions. Then, the temperatures and volumetric fractions are displayed on the monitor and recorded in the computer. In each experimental run, 5g of coal particles serves as the basis and the particle sizes (mesh numbers) are in the range of.2cm to.4cm (1 to 5). The coals from hopper are charged into the reactor by a screw feeder. Following the feedings, the coals drop into the reactor bottom and the gasification reaction is initiated. Dry air (N 2 79% + O 2 21%) with volume flow rate of 3ml/min is sent into the reactor from the reactor bottom such that the product gas moves upward. In the experiments, three different reaction temperatures (8, 9, and 1 C) are individually performed. The experimental period of each case persists for 9 minutes. To ensure the measurement quality, prior to experiment air with fixed flow rate is blown into the gasification system and its flow rate is measured at the system exit, too. This guarantees that no gas leakage occurs. In addition, the calibrations of GC and GA are also carried out by means of standard gases. Syngas ignitability and flame observation are performed after the onset of coal gasification. However, in the initial experimental period soot and tar concentrations are very high so that it is inappropriate to measure gas concentrations. The measurements of the product gas volumetric fractions are thus performed 1 minutes later from the onset of coal charge. 3. RESULTS AND DISCUSSION 3.1 Thermal Decomposition Characteristic of Coal Figure 2 first demonstrates the thermogravimetric analyses (TGA) and derivative thermogravimetric (DTG) analyses of the two different coals to recognize the thermal decomposition characteristics of the coals in the absence of oxidizer. In the analyses, the coals were exposed to an environment of pure helium while the heating rate of the TGA was 1 C / min. In the figure, W and W represent the instantaneous and initial coal weights, respectively. In examining the TGA distributions of the two coals, it is clear that the majority of the volatile matter is released when the environmental temperature ranges from 4 C to 85 C. For example, the percentages of the volatile matters of Coal R and Coal J are 34.3% and 15.24%, respectively. Corresponding to these data, when the TGA values of Coal R and Coal J respectively reach 65.7% and 84.76% which can be thought of as the depletion points of the volatile matter, the temperatures are 858 C and 82 C, respectively. That is to say, the devolatilization processes are almost achieved before the reaction temperature reaches 85 C. With regard to the distributions of DTG, Fig. 2 indicates that the maximum decomposition rate of Coal R occurs at the temperature of 452 C where the pyrolysis rate is.254 %/ C. In contrast, the maximum reaction rate of Coal J is exhibited at 5 C where the value is.72 %/ C. These results evidently reveal that the decomposition process of the high-volatile coal (Coal R) is excited at lower reaction temperature and its pyrolysis intensity is much large when compared to that of the low-volatile coal (Coal J). Specifically, the maximum pyrolysis rate of Coal R is larger than that of Coal J by a factor of 3.5. The TG-mass spectrometry of Coal R and Coal J are displayed in Figs. 3 and 4, respectively. Upon inspection of the mass spectrometry of Coal R, merely a bit of H 2 O and CO are emitted from the coal (see Fig. 3(b)) when the reaction temperature is low. Once the temperature rises to a certain extent such as 4 C, the formations of CH 4, CO, and CO 2 become significant. Journal of Mechanics, Vol. 23, No. 4, December

5 11 1 DTG -.5 TGA (%) TGA (=W/W ) DTG (%/ C) % -.25 T=452 o C Temperature ( o C) (a) Coal R 11 (a) 15 1 DTG -.2 TGA (%) TGA (=W/W ) DTG (%/ C) 8 75 T=5 o C Temperature ( o C) 82.1% -.8 (b) Coal J Fig. 2 Thermogravimetric analyses (TGA) and derivative thermogravimetric (DTG) analyses of (a) Coal R and (b) Coal J (b) Fig. 3 Thermogravimetric mass analysis (TG-MS) of Coal R (a) Fig. 4 (b) Thermogravimetric mass analysis (TG-MS) of Coal J Meanwhile, Fig. 3(a) indicates that H 2 will not be elicited as the environmental temperature is low; only when the temperature is as high as 4 C, H 2 tends to appear. With further increasing the temperature, CH 4 concentration turns to decrease whereas the concentrations of H 2, CO, and CO 2 keep growing. The generation of hydrogen reaches a maximum value at a certain temperature and then decreases. It is known that the formation of hydrogen is partially due to bond break of hydrogen from the coals [13]. On the other hand, because the coals contain moisture, it is inferred that the hydrogen also partially comes from carbon-moisture reaction. That is C + H2O H2 + CO H = kj/mol (1) The above reaction is an endothermic reaction in nature. Consequently, a higher reaction temperature favors to elicit more hydrogen. Alternatively, the growths of CO and CO 2 with increasing temperature suggest that a large portion of energy is used for reactions between carbon and oxygen. Because the oxygen is insufficient and the formation of CO is easier than CO 2, the concentration of the former is always higher than that of the latter. When the mass spectrometry of Coal J is examined, basically, the formation trends of the four gases are similar to those of Coal R, as shown in Fig. 4. However, on account of less volatile matter contained in Coal J, the concentrations of H 2, CH 4, CO, and CO 2 are relatively low compared with the results of Coal R. 322 Journal of Mechanics, Vol. 23, No. 4, December 27

6 3.2 Gas Formation from Coal Gasification Subsequently, emphasis is placed on the transient gasification reactions of the two coals under three different gasification temperatures (i.e., 8, 9, and 1 C) and the volumetric fraction distributions of H 2, CH 4, CO, and CO 2 originating from Coal R and Coal J reactions are presented in Figs. 5 and 6, respectively. Following the onset of coal reaction, seeing that the pyrolysis reaction is violent, it can be seen that the initial volumetric fractions of H 2 and CH 4 are relatively high. In addition to bond break from volatile matter, the formation of H 2 can also come from the interaction between carbon and moisture, as illustrated in Eq. 1. It should be mentioned that the higher the gasification temperature, the richer the initial H 2 volumetric fraction; however, with increasing time it decays more rapidly. Unlike hydrogen formation, a lower reaction temperature such as 8 C is conducive to the generation of CH 4 compared to 1 C. This is because that the methane formation is related to the following exothermic reaction C+ 2H2 CH 4 H = 91.6kJ/mol (2) The foregoing reaction tends to reverse when the reaction temperature is 1 C. Whatever the gasification temperature is, when the reaction time is long to some extent, say, to 5min, Figs. 5 and 6 depict that the volumetric fractions of H 2 and CH 4 become quite low. It implies, in turn, that the devolatilization reactions tend to diminish. Thereafter, the volumetric fractions of CO and CO 2 are relatively high in comparison with the other two gases. It follows that the gasification turns to be dominated by the reaction between char and oxygen. In fact, the char not only reacts with oxygen, it also interacts with carbon dioxide in accordance with the following chemical equations 8 (a) Hydrogen 8 (a) Hydrogen H 2 (Vol. %) o C 9 o C 1 o C H 2 (Vol. %) o C 9 o C 1 o C (b) Methane 24 (b) Methane 2 2 CH 4 (Vol. %) 16 8 CH 4 (Vol. %) (c) Carbon monoxide 2 (c) Carbon monoxide CO (Vol. %) CO (Vol. %) (d) Carbon dioxide 15 (d) Carbon dioxide CO 2 (Vol.%) 9 6 CO 2 (Vol. %) Fig. 5 Transient volumetric fraction variations of (a) H 2, (b) CH 4, (c) CO, and (d) CO 2 at three different gasification temperatures from Coal R gasification Fig. 6 Transient volumetric fraction variations of (a) H 2, (b) CH 4, (c) CO, and (d) CO 2 at three different gasification temperatures from Coal J gasification Journal of Mechanics, Vol. 23, No. 4, December

7 C + 1/ 2O2 CO H = 111 kj/mol (3) CO + 1/ 2O2 CO 2 H = 283 kj/mol (4) C + CO2 2CO H = 172 kj/mol (5) From the viewpoint of thermodynamics, increasing reaction temperature is conducive to the backward reaction of the first two equations (i.e., exothermic reaction), whereas it will facilitate the forward reaction in the third equation (i.e., endothermic reaction). Examining Figs. 5 and 6 indicates that the volumetric fraction of CO at 1 C is higher than that at 8 and 9 C. This depicts that the third reaction is somewhat more important than the other two reactions as the reaction temperature rises. On the other hand, it is noted that the volumetric fractions of CO and CO 2 shown in Figs. 6(c) and 6(d) are low when the reaction temperature is 8 C, in contrast to that at 9 and 1 C. It is thus concluded that the carbon reactivity of Coal J is poor under such a reaction temperature Synthesis Gas Combustion Figure 7 sketches the flame length profile due to syngas combustion from Coal R gasification at the reaction temperature of 1 C where the flame length is defined as the distance from burner exit to flame tip. Flame shapes at various reaction times are demonstrated in the figure as well. In the experiment, it was observed that, following the feeding of the coal into the reactor, just a bit of gas was emitted from the burner. This is attributed to the fact that heat uptake by the coal for pyrolysis or devolatilization was the dominant mechanism in the initial reaction. Meanwhile, some soot was entrained and injected from the nozzle. When the reaction time reached 2 minutes, the color of the syngas was dark-yellow, resulting from the emission of sulfide combined with soot. At this moment, the syngas was combustible, as shown in Fig. 7. Afterward, by virtue of much gas release along with soot, it was seen that the flame stretched markedly in a short time (viz., the time between 2 and 4min) and it was luminous and bright. This implies that the gasification reaction was enhanced significantly. It was also noted that the flames were wrinkled for the cases of 4 and 6min (see Fig. 7), reflecting that the syngas combustion converted the laminar flame into the turbulent flame at a certain time. The flame length profile indicates that the maximum gasification reaction was obtained at 4min where the flame was approximately 3cm. Later, the flame withered rapidly. In the meantime, it should be addressed that, in the early period of gasification, the fluctuation of flame length was pronounced as a result of intensified reaction. Once the reaction time reached 1min, the flame has been shortened to about 6.5cm. At this moment, the flame color was light-yellow around its tip and it was light-blue in the vicinity of nozzle. Hence it was recognized that the soot emission stemming from gasification disappeared and the flame Fig. 7 Time variation of flame length and observations of syngas combustion from Coal R gasification at 1 C was purely contributed by the burning of syngas and methane, as observed in Fig. 5. After that, the flame length decreased slowly with time and the whole flame color evolved into light-blue. When the reaction time was 26min, because both syngas amount and concentration were low while heat generation stemming from combustion could not keep up with energy loss to the environment, extinction occurred. According to the syngas combustion observed above, the entire gasification process can be partitioned into five periods; they are initiated ( ~ 2min), growing (2 ~ 4min), rapidly decaying (4 ~ 1min), progressively decaying (1 ~ 26min), and frozen (> 26min) periods, sequentially. Moreover, seeing that the flame length at t = 4min was about 3 times of that at t = 26min, it suggests the gasification strength in the initial period was much higher that that near extinction. The syngas combustion phenomena from Coal R gasification under the three different reaction temperatures (i.e., 8, 9, and 1 C) are displayed and compared with each other in Fig. 8. When the reaction temperatures were 8 and 9 C, it is apparent that the growing characteristics of flame were not as significant as that at 1 C. However, it is of interest that the flame length in the case of 8 C was higher than the other two cases as the reaction time was between 9 and 19min, implying that more time was required to induce volatile matter from Coal R. For the cases of 9 and 1 C, the product gases were ignitable when the reaction time was 2min, whereas it occurred at 4min for 8 C. These results elucidate that the volatile matter of the coal at 1 C favored to release in the initial period, whereas when the temperature was 8 C, the intensified reaction proceeded lately. In regard to 9 C, its behavior was between 8 C and 1 C. When the extinction was examined, it can be found that the occurring time was insensitive to the reaction temperature, as shown in Fig Journal of Mechanics, Vol. 23, No. 4, December 27

8 35 Flame length (cm) ignition Coal R 8 o C 9 o C 1 o C extinction Fig. 8 Time variations of flame length from Coal R gasification at (a) 8 C, (b) 9 C, and (c) 1 C For the low-volatile bituminous, the flame length profile and flame observations for Coal J at 1 C are plotted in Fig. 9. A comparison with Fig. 7 reveals that the syngas ignition of Coal J took placed somewhat later than that of Coal R, but the entire combustion processes between the two coals were fairly similar. In other words, the coal gasification was also characterized by the five periods. Because of less amount of volatile matter contained in Coal J, it is not surprised to find that the maximum flame length sketched in Fig. 9 is much shorter than that in Fig. 7, approximately less 7cm. Though the flame was also skewed at certain times such as 6 and 8min, their extents of skewness were not as obvious as that shown in Fig. 7. Figure 1 presents the flame length profiles for Coal J reaction at the three different reaction temperatures. It is noteworthy that within the initial 1min the distributions among the three gasification temperature were quite different each other. Specifically, the growing and rapidly periods withered gradually when the gasification temperature declined. However, the syngas combustion could sustain longer with decreasing temperature. For instance, the extinction of the reaction temperatures of 1, 9, and 8 C occurred at 24, 3, and 35min, respectively. Accordingly, it is known that the gasification processes of the low-volatile coal are more dependent on the gasification temperature, in contrast to those of the highvolatile coal. Eventually, the pictures of scanning electron microscope (SEM) of Coal R and Coal J feed coals as well as their char particles (9 C) are demonstrated in Fig.11. When Coal R and Coal J feed coals were examined, Figs. 11(a) and 11(b) show that the particle surfaces were smooth and no holes were found. After they experienced gasification reaction with the temperature of 9 C, the volatiles would be emitted from the coal Fig. 9 Time variation of flame length and observations of syngas combustion from Coal J gasification at 1 C Flame length (cm) ignition Coal J 8 o C 9 o C 1 o C extinction Fig. 1 Time variations of flame length from Coal J gasification at (a) 8 C, (b) 9 C, and (c) 1 C interiors and it might result in break of coal particles. For this reason, as shown in Figs. 11(c) and 11(d), the char particles became smaller and they agglomerated each other. In particular, it was observed that a large number of pores developed along the char surfaces from Coal R reaction (see Fig. 11(c)), resulting from high volatile matter contained. 4. CONCLUSIONS By employing TGA, TG-mass, GC, and GA in association with the conducted coal gasification system, experimental studies on pyrolysis, gasification, and Journal of Mechanics, Vol. 23, No. 4, December

9 (a) Coal R feed coal (b) Coal J feed coal (c) Coal R char (9 C) (d) Coal J char (9 C) Fig. 11 Scanning electron microscope pictures ( 25) of (a) Coal R feed coal, (b) Coal J feed coal, (c) Coal R char at 9 C, and (d) Coal J char at 9 C syngas combustion from two different ranks of coals have been achieved. The results from TGA and TGmass suggest that the devolatilization reaction of the high-volatile coal (Coal R) is intensified at lower temperature and its maximum decomposition rate is much higher than that of the low-volatile coal (Coal J). For the two coals, the mole fractions of H 2 and CH 4 undergo increase followed by decrease, whereas the formations of CO and CO 2 rise with increasing temperature. With regard to the coal gasification, in view of the intensified pyrolysis reaction initially, the initial mole fractions of H 2 and CH 4 are high, but they decline rapidly. When the time is sufficiently long, char reaction becomes the dominant mechanism so that the mole fractions of CO and CO 2 are relatively high. Furthermore, when the syngas combustion of the two coals with the reaction temperature of 1 C is examined, the reaction is obviously characterized by the five-period gasification process. That is, the entire reaction process can be decomposed into initiated, growing, rapidly decaying, progressively decaying, and frozen periods. However, as long as the gasification temperatures are relatively lower such as 8 and 9 C, the growing and rapidly decaying periods wither. From the flame length distributions, the transient strength of coal gasification has been figured out clearly. Therefore, coal reaction in association with flame observation and measurement enable us to qualitatively recognize the gasification process characteristics. ACKNOWLEDGEMENTS The authors wish to express their gratitude to the National Science Council and Bureau of Energy, Ministry of Economic Affairs, Taiwan, ROC, for Grant Numbers NSC E and NSC ET, which supported this research. REFERENCES 1. Ristinen, R. A. and Kraushaar, J. J., Energy and the Environment, John Wiley and Sons, New York (26). 2. Hinrichs, R. A. and Kleinbach M, Energy: Its Use and the Environment, Harcourt, Orlando (22). 3. Chow, J., Kopp, R. J. and Portney, P. R., Energy Resources and Global Development, Science, 32, pp (23). 4. Smoot, L. D. and Smith, P. J., Coal Combustion and Gasification, Plenum Press, New York (1985). 5. Smoot, L. D., Fundamentals for Coal Combustion: for Clean and Efficient Use, Elsevier, New York (1993). 6. Cheng, J., Zhou, J., Liu, J., Zhou, Z., Huang, Z., Cao, X., Zhao, X. and Cen K., Sulfur Removal at High Temperature During Coal Combustion in Furnaces: A Review, Progress in Energy and Combustion Science, 29, pp (23). 7. Chen, W. H., Chen, J. C., Tsai, C. D. and Jiang, T. L., Transient Gasification and Syngas Formation for Coal 326 Journal of Mechanics, Vol. 23, No. 4, December 27

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11 328 Journal of Mechanics, Vol. 23, No. 4, December 27