Effect of Inherent Moisture in Collie Coal during Pyrolysis Due to in

Energy & Fuels 2007, 21, 2883-2891

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Effect of Inherent Moisture in Collie Coal during Pyrolysis Due to in-Situ Steam Gasification Kongvui Yip,†,‡,§ Hongwei Wu,*,†,‡,§ and Dong-ke Zhang†,‡ CooperatiVe Research Centre for Coal in Sustainable DeVelopment, Centre for Fuels and Energy, and Department of Chemical Engineering, Curtin UniVersity of Technology, GPO Box U1987, Perth, Western Australia 6845, Australia ReceiVed May 15, 2007. ReVised Manuscript ReceiVed July 1, 2007

The influences of inherent moisture in Collie coal during pyrolysis has been studied using a series of Collie coal samples, including the raw coal, dilute acid-washed coal, and demineralized coal samples of both pulverized fuel and millimeter sizes. Coal without drying (moisture content ∼ 20%) and dried coal (moisture < 1%) were pyrolyzed under various conditions in a fixed-bed reactor, a drop-tube/fixed-bed reactor, and a fluidizedbed reactor. Measurement of char reactivity in air was carried out using a thermogravimetric analyzer. The results demonstrated that inherent moisture in Collie coal had a significant influence on the char yield and reactivity, depending on the pyrolysis conditions. Under slow-heating conditions in the fixed-bed reactor, moisture was swept out of the reactor during the long heating process, leading to little influence on char yield and reactivity. However, under fast-heating conditions with continuous coal feeding in the drop-tube/fixedbed or fluidized-bed reactor, significant interactions between steam produced in situ (from the coal inherent moisture) and the reacting coal/char occurred. The steam gasified the coal/char significantly at temperatures > 800 °C, leading to much lower char yields. Inorganic species in the coal/char also appeared to play an important role in such in-situ steam gasification. In the drop-tube reactor, where the interaction time was much shorter, a noticeable effect on the char yield was only observed at temperature > 1000 °C. Char reactivity data indicated that the steam-char interactions also led to considerable deactivation of the char at high pyrolysis temperatures. As the coal particle size increased, the effect of inherent moisture decreased. The results presented in this paper indicate that inherent moisture in low-rank coals can significantly change the properties of chars from the thermal upgrading of the coals, especially in processes where the coal/char particles experience long residence time or strong and prolonged interactions with steam produced in situ.

1. Introduction Collie coal is the only coal currently being mined in Western Australia and plays an important role in the energy supply of the state.1 It is a sub-bituminous coal of high moisture (2028%), high volatile (30-47% dry-ash-free basis), and low ash (4-12% dry-basis) contents.2,3 The high coal moisture content is well-known to adversely affect the energy efficiency of power generation and other coal utilization processes, as well as resulting in high propensity of spontaneous combustion4,5 and extra costs incurred in transport of the coal.5 * To whom the correspondence should be addressed. Department of Chemical Engineering, Curtin University of Technology, GPO Box U1987, Perth WA 6845, Australia. Fax: +61-8-92662681. E-mail: [email protected] (Dr. H. Wu). † Cooperative Research Centre for Coal in Sustainable Development. ‡ Centre for Fuels and Energy. § Department of Chemical Engineering. (1) DIOR, 2005-06 Western Australian Mineral and Petroleum STATISTICS DIGEST; Department of Industry and Resources, Government of Western Australia: Perth, Australia, 2006. (2) Clark, K.; Meakins, R.; Attalla, M.; Craig, K. Report on Project C3100: Agglomeration and Stabilisation of Collie Coals by Binderless Briquetting; Australian Coal Research Ltd.: Brisbane, Australia, 1997. (3) Esther, N. Investigation of Char Reactivity of Collie Coals. Ph.D. Thesis, Department of Chemical Engineering, Curtin University of Technology, Australia, 2006. (4) Wender, I.; Heredy L. A.; Neuworth M. B.; Dryden G. C. Chemical Reactions and the Constitution of Coal. In Chemistry of Coal Utilisation, 2nd Suppl. Vol.; Elliot, M. A., Ed.; John Wiley & Sons: New York, 1981. (5) Tsai, S. C. Fundamentals of Coal Beneficiation and Utilization; Elsevier Science: Amsterdam, The Netherlands, 1982.

Previous studies6,7 reported that the coal moisture or water added to the coal can stay in the coal matrix and act directly with the coal mass participating in thermochemical reactions, which changes the yield and compositions of pyrolysis products and/or carbon deposition rate, under slow-heating conditions. Hayashi and co-workers8 also found that part of the adsorbed water in low-rank coals studied, under very rapid heating, can take part in thermochemical reactions such as hydrolysis that converts the water into hydroxyls of liquids and/or char or suppresses the conversion of inherent hydroxyls into water, and the adsorbed water can suppress the conversion of carbon into liquids leading to higher char yield. Tyler9 carried out pyrolysis on a brown coal under continuous feeding conditions into a fluidized bed and studied the effect of coal moisture in pyrolysis up to only 750 °C and found little effect on pyrolysis. During the pyrolysis of large low-rank coal particles,10,11 coal moisture was found to delay devolatilization as a result of additional time as well as heat required for coal moisture evaporation. Coal moisture may also act as an inhibitor in the formation of tar/carbon deposits in pyrolysis either through (6) Butuzova, L.; Razvigorova, M.; Krzton, A.; Minkova, V. Fuel 1998, 77, 639. (7) Krebs, V.; Furdin, G.; Mareche, J-F.; Dumay, D. Fuel 1996, 75, 979. (8) Hayashi, J.; Norinaga, K.; Yamashita, T.; Chiba, T. Energy Fuels 1999, 13, 611. (9) Tyler, R. Fuel 1979, 58, 680. (10) Stubington, J. F.; Sasongko, D. Fuel, 1998, 77, 1021. (11) Heidenreich, C. A. Ph.D. Thesis. University of Adelaide, Australia, 1999.

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dilution of hydrocarbon vapor by steam or re-forming/cracking by steam.12 A recent study13 showed that the pyrolytic water formed from functional groups in coal during pyrolysis can play an important role in the re-forming of tar even under a short residence time inside a drop-tube reactor. External steam injected results in considerable gasification of the nascent char. An investigation on steam gasification of low-rank coals14 suggested that the inherent metallic species in the coal can play significant roles in the tar re-forming and the nascent char gasification. In a boiler or an entrained-flow reactor, pulverized fuel (pf) sized coal particles are fed into the reactor, evaporation of coal moisture occurs upon rapid particle heating, and the reacting coal/char particles are exposed to the steam produced in situ from either the coal inherent moisture or moisture generated from pyrolysis. A fluidized-bed gasifier takes large particles as feed. Due to the long residence time of char experienced in the fluidized-bed reactor, the nascent char continuously interacts with volatiles15 and/or steam produced from coal pyrolytic water.16 In a rotary kiln, which is often used for coal thermal upgrading,17-19 large coal/char particles are exposed to an atmosphere filled with coal moisture throughout its residence in the kiln. Although recent studies have made progress in understanding Collie coal behavior in various utilization processes,20-22 the problems presented by the high coal moisture as well as the possible effect of interaction between the moisture and coal/char need further studies, particularly in terms of the influences of inherent moisture during coal pyrolysis in relation to the application of Collie coal in various practical coal utilization processes. This study focuses on the effect of inherent moisture in the pyrolysis of Collie coal under both slow- and fast-heating conditions using reactor systems of different configurations. These reaction systems provide different reaction conditions and simulate conditions prevailing in various practical coal utilization processes. The char produced from pyrolysis is then subject to combustion reactivity measurements in order to probe the characteristics of the resultant char. 2. Experimental Section 2.1. Coal Samples. A Western Australian Collie coal was used in this study. The typical proximate and ultimate analysis of this coal is presented in Table 1. The coal was collected from the mine faces, crushed and sieved to obtain samples of pf-sized (90-106 µm) and millimeter (mm)-sized (0.8-1, 1-2, and 3.35-4 mm) fractions. These are the raw coal samples (with ash content of 4.6% db). To investigate the role of coal inorganic species, acid-washing was carried out on the pf-sized coal sample. The raw coal was (12) Nagata, M.; Mishioka, K.; Yoshida, S. Ironmaking Conference Proceedings AIME (the American Institute of Mining, Metallurgical, and Petroleum Engineers) 1985, 44, 355. (13) Hayashi, J.; Iwatsuki, M.; Morishita, K.; Tsutsumi, A.; Li, C.-Z.; Chiba, T. Fuel 2002, 81, 1977. (14) Hayashi, J.; Takahashi, H.; Iwatsuki, M.; Essaki, K.; Tsutsumi, A.; Chiba, T. Fuel 2000, 79, 439. (15) Wu, H.; Li, X.; Hayashi, J.; Chiba, T.; Li, C.-Z. Fuel 2005, 84, 1221. (16) Bayarsaikhan, B.; Sonoyama, N.; Hosokai, S.; Shinada, T.; Hayashi, J.; Li, C.-Z.; Chiba, T. Fuel 2006, 85, 340. (17) Patisson, F.; Lebas, E.; Hanrot, F.; Ablitzer, D.; Houzelot, J. Metall. Mater. Trans. B 2000, 21B, 381. (18) Barr, P. V.; Brimacomber, J. K.; Watkinson, A. P. Metall. Mater. Trans. B 1989, 20B, 403. (19) Wes, G. W. J.; Drinkenburg, A. A. H.; Stemerding, S. Powder Technol. 1976, 13, 185. (20) Wee, H. L.; Wu, H.; Zhang, D.-k.; French D. Proc. Combust. Inst. 2005, 30, 2981. (21) Yip, K.; Wu, H.; Zhang, D.-k. Energy Fuels 2007, 21, 419. (22) Wee, H. L.; Wu, H.; Zhang, D.-k. Energy Fuels 2007, 21, 441.

Yip et al. Table 1. Proximate, Ultimate,and Ash Analysis of the Collie Coal Sample Used in This Study proximate analysis (wt % as-received) fixed carbon volatile matter ash moisture

ultimate analysis (wt % daf)

45.6 30.7 3.7 20.0

C H O N S

76.14 4.56 16.61 1.41 1.28

Table 2. Inorganic Species Contents in the Raw Coal, Dilute Acid-Washed Coal, and Demineralized Coals inorganic species

content in raw coal (wt % db)

content in dilute acid- washed coal (wt % db)

content in demineralized coal (wt % db)

ash Al Ba Ca Fe K Mg Na P S Si Sr Ti

4.6 0.5282 0.0115 0.0553 0.5321 0.0136 0.0183 0.0187 0.0461 0.1014 1.0482 0.0205 0.0501

3.4 0.3514 0.0105 0.0270 0.2936 0.0111 0.0022 0.0011 0.0350 0.0425 0.9097 0.0161 0.0501

0.4 0.0597 0.0018 0.0108 0.0306 0.0014 0.0021 0.0011 0.0008 0.0130 0.0600 0.0005 0.0158

treated with 0.1 M H2SO4 for 24 h and filtered, and the treatment was repeated four times, before being repeatedly washed with double-distilled water until the pH of the filtered solution became constant and no SO42- could be detected in the filtered solution.23 This is the dilute acid-washed coal (with ash content of 3.4% db), with metallic cations organically bound to the coal structure and a small portion of other minerals like carbonates being removed. This sample was further subject to demineralization. The dilute acidwashed coal was treated with 5 M HCl for 24 h at 55-60 °C and filtered. The HCl-washed coal was then treated with 11 M HF for 3 h at 55-60 °C, filtered again, and treated with another batch of 5 M HCl for 3 h at 55-60 °C. The HF washed coal was repeatedly washed with double-distilled water until the pH of the filtered solution became constant and no Cl- was detected upon addition of silver nitrate. These treatments remove most of the remaining discrete mineral matter.24,25 The ash content of the demineralized coal is reduced to 0.4% db. All these samples (with moisture content of ∼20% for the raw samples, ∼19% for the dilute acid-washed sample, and ∼12% for the demineralized sample) are hereafter referred to as raw coal, dilute acid-washed coal, and demineralized coal, without drying, respectively. The inorganic species in the coal samples were analyzed by first ashing the samples using the procedure suggested by Sathe et al.26 that minimizes ash loss, followed by borate fusion of the ash (using a method modified from Australian Standard AS 1038.14.1 27) and inductively coupled plasma-atomic emission spectroscopy (ICP-AES) analysis. The contents of inorganic species in these coals are detailed in Table 2. To study the effect of moisture, the raw coal, dilute acid-washed coal, and demineralized coal samples were dried at 120 °C in N2 until the moisture content was typically about or less than 1% before being used in any experiment. These samples are hereafter referred to as “dried” coal. In summary, the eight coal samples used in this study are as follows: (a) pf-sized particles of the raw coal without drying and dried raw coal; (b) mm-sized particles of the raw coal without drying and dried raw coal; (c) pf-sized particles of the dilute acid-washed coal without drying and dried dilute acid-washed coal; (23) Li, C-Z.; Sathe, C.; Kershaw, J. R.; Pang, Y. Fuel 2000, 79, 427. (24) Otake, Y.; Walker, P. L. Fuel 1993, 72, 139. (25) Feng, B. Ph.D. Thesis. University of Queensland, Australia. (26) Sathe, C.; Pang, Y.; Li, C.-Z. Energy Fuels 1999, 13, 748. (27) Australian Standards. AS1038.14.1, Higher rank coal ash and coke ashsMajor and minor elementssBorate fusion/flame atomic absorption spectrometric method, 2003.

Effect of Inherent Moisture on Coal Pyrolysis

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Figure 1. Reactors for pyrolysis: (a) Fixed-bed reactor or drop-tube/fixed-bed reactor; (b) fluidized-bed reactor; (c) drop-tube reactor.

(d) pf-sized particles of the demineralized coal without drying and dried demineralized coal. 2.2. Pyrolysis. Four types of reactors are considered in this study, a fixed-bed reactor, a drop-tube/fixed-bed reactor, a fluidized-bed reactor, and a drop-tube reactor (all made of quartz, with internal diameter of 28 mm), depending on the pyrolysis conditions required. The char yield was calculated from the total amount of coal fed into the reactor and the char produced after the experiment. All pyrolysis experiments were carried out in high-purity nitrogen (purity > 99.99%). To confirm the effect of the coal inherent moisture, in some experiments with the dried coal samples appropriate amounts of steam were also introduced into the nitrogen stream externally by feeding double-distilled water using an HPLC pump model 526. The experimental setups are shown in the Figure 1. Slow-heating pyrolysis was carried out using the fixed-bed reactor configuration shown in Figure 1a. The reactor preloaded with a coal sample was heated at a rate of 10 K min-1 until the pyrolysis temperature, held for another 30 min before being lifted out of the furnace, and cooled naturally to room temperature. Fast-heating (∼103 K s-1) pyrolysis of the pf-sized coal particles was conducted using the drop-tube/fixed-bed reactor configuration (also Figure 1a) similar to that used by Tan and Li.28 The reactor is preheated in a furnace until the desired pyrolysis temperature (28) Tan, L. L.; Li, C.-Z. Fuel 2000, 79, 1883.

was reached. Coal feeding commenced, and the sample was fed at a rate of about 120 mg min-1 (dry basis) into the reactor continuously, for 11 min. The char particles were accumulated on the frit inside the reactor during the experimental run until the feeding ended. The reactor was then held for a further 30 min before being lifted out from the furnace. This provides basic information for pyrolysis of the pf-sized particle under fast heating with strong interactions between the nascent char and coal moisture simulating the conditions in processes such as rotary kilns where coal is continuously fed and experiences long residence time. The fluidized-bed setup, as shown in Figure 1b, similar to that used by Bayarsaikhan and co-workers,16 was employed for fastheating pyrolysis of the mm-sized coal samples. Silica sand (250355 µm) was used as the bed material. A vertical and downward nozzle is immersed into the bed, and coal particles are fed continuously through the nozzle into the bed. A secondary stream of nitrogen is introduced as shown in Figure 1b to ensure that no volatiles from pyrolysis would flow into the coal feeder through the feeding nozzle. The pyrolysis and feeding conditions were similar to that for the drop-tube/fixed-bed setup, to enable comparison with pyrolysis of the pf-sized particles. This is aimed at studying the pyrolysis of large coal particles in relation to the conditions in fluidized-bed reactors. The drop-tube reactor configuration, shown in Figure 1c, was used to pyrolyze the pf-sized coal particles under fast-heating


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Table 3. Main Features for Different Reactor Configurations for Pyrolysis reactor

coal particle size

fixed-bed reactor drop-tube/fixed-bed reactora

pf-sized pf-sized

fluidized-bed reactorb

mm-sized

drop-tube reactor

pf-sized

pyrolysis mode slow-heating fast-heating, char particles accumulating inside reactor and experiencing interactions with volatiles, steam produced in situ from coal moisture and pyrolytic water fast-heating, char particles accumulating in the reactor and experiencing interactions with volatiles, steam produced in situ from coal moisture and pyrolytic water fast-heating, short (∼2 s) particle residence time

a Similar to the one used in a previous study.28 used in a previous study.16

b

Similar to the one

conditions but with much shorter residence time, as in a practical pf boiler or an entrained-flow gasifier. The gas flow rate was adjusted to give a similar residence time of coal/char particles inside the reactor for experiments of different pyrolysis temperatures. The coal feeding rate was again ∼120 mg min-1 (dry basis). The char particles, after being “dropped” through the reactor, were collected on a quartz frit situated well outside the furnace. The residence time of the particles inside the reactor was estimated to be ∼2 s. The main features for the different reactor configurations are summarized in Table 3. The weight loss, therefore char yield, during pyrolysis was determined by direct measurement of the reactor weights before and after each experiment. All char yield data were repeatable within an error of 0.2-2% on the dry-ash-free (daf) basis. Collection of char samples was straightforward for the experiments in the fixed-bed reactor, drop-tube/fixed bed reactor, and the droptube reactor by emptying the chars out of the reactor. For the fluidized bed reactor, after each experiment, a high gas flow rate was passed through the reactor so that char particles were segregated from the bed materials due to the difference in density and particle size between the char and the bed materials. The majority of chars were then recovered. A small proportion of char remaining in the bed was recovered by sieving as the char particles (typically >800 µm) were much bigger than the silica sand (250-355 µm). After sieving, careful visual inspection and manual separation aided in recovering any remaining char. In all experiments, compared to char yield data, char recovery of 95-99% was achieved. It should also be noted that, in the drop-tube/fixed-bed reactor and the fluidized-bed reactor, the residence/holding time of each “batch” of char on the frit is different, as a result of continuous feeding. Hence, each batch of char would have experienced a different period of steam gasification. Therefore, the char yield of an experiment using the drop-tube/fixed-bed reactor and the fluidized-bed reactor was the averaged value of the overall char sample. The char sample collected was ground and well-mixed before being used for subsequent reactivity and other measurements, during which repeated analyses were carried out and reproducible data were obtained. 2.3. Char Reactivity. Char reactivity was measured using a thermogravimetric analyzer (TGA, model TA SDT-Q600), under conditions with minimized mass-transfer limitations. For reactivity measurements of chars from large-particle pyrolysis, the char obtained was ground to fines before being loaded into the TGA. The tests were conducted in an atmosphere of 5% O2 in nitrogen, instead of air, in order to minimize the effect of chemisorption of oxygen on reactivity measurement.29,30 About 6 mg of char (giving (29) Feng, B.; Bhatia, S. K. Chem. Eng. Sci. 2002, 57, 2907. (30) Esther, N.; Wu, H.; Zhang, D.-k. Distribution of Unburnt Carbon in Fly Ash Collected from a Utility Boiler Firing a Sub-bituminous Coal (32nd Australian Chemical Engineering Conference). CHEMECA, Sydney, Australia [CD-ROM], 2004.

Figure 2. Char yields from the slow-heating pyrolysis of pf-sized particles of raw coal, with and without drying, in the fixed-bed reactor at various temperatures.

a thin layer of bed in the crucible) was heated at 10 °C min-1 in pure nitrogen to 120 °C, and the char moisture was taken from the leveling-off weight. The char was then further heated to an appropriate temperature at which the reactivity measurement was desired and the gas was switched from nitrogen to 5% O2 in nitrogen. The temperature chosen was sufficiently low so that the reactivity was measured under kinetic control regime. The temperature for reactivity measurement for 800 °C char (char prepared from 800 °C pyrolysis) and 900 °C char was 430 °C, whereas that for 1000 and 1100 °C char was 470 °C. The specific reactivity (R) of a char at any instant was calculated from the differential mass loss data (dW/dt) according to R)-

1 dW W dt

where W is the mass (daf) of the char at any time t. At the end of a reactivity measurement run, the temperature was further increased at 10 °C min-1 to 830 °C, to completely burn off any carbonaceous residue so as to determine the ash content of the char, being the final mass remaining in the TGA. In this study, repeated reactivity measurements were carried out and the reactivity data for all char samples were reproducible. The reactivity was plotted versus coal conversion instead of char conversion or time to show the changes of char structure during pyrolysis. To examine the structural difference between char samples prepared from coals with and without drying prior to their pyrolysis, surface area analysis was carried out on the chars, using the N2 adsorption technique (Micromeritics Gemini III 2375).

3. Results and Discussion 3.1. Char Yields. Figure 2 shows the char yield for slowheating pyrolysis of the raw coal using the fixed-bed reactor. The char yield decreases with pyrolysis temperature as expected and levels off at temperatures > 1000 °C. It can be seen that, even at the high temperatures (1000 and 1100 °C), the char yields from the pyrolysis of coals with and without drying are similar. For slow heating in the fixed-bed reactor, any volatiles released from the coal will be purged away immediately by the N2 flow and the interaction between the volatile and the solid residue sitting on the frit is minimal. Hence, any moisture in the coal would have been evaporated during the long period of the heating process and carried away from the reactor, leaving apparently no effect on the subsequent pyrolysis process. Figure 3 shows the char yield from pyrolysis of the pf-sized raw coal particle in the drop-tube/fixed-bed reactor. In this setup, the coal was continuously fed into the reactor and the char produced accumulates on the frit, in contact with the incoming volatiles released from subsequent coal particles being pyrolyzed. Figure 3 shows that there is a remarkable difference that


Figure 3. Char yields from the fast-heating pyrolysis of pf-sized particles of raw coals, with and without drying, in the drop-tube/fixedbed reactor at various temperatures.

can be observed in the trends of char yield between pyrolysis of coals with and without drying. For the dried coal, the char yield levels off at temperature > 900 °C, as expected. For the coal without drying, at 700 °C or lower temperature, the char yield is almost identical to that of the dried coal. However, above 750 °C, the char yield of the coal without drying deviates from that of the dried coal, continues to decrease drastically until 1000 °C, and levels off thereafter. A significant difference in the char yields was observed; for instance, at 1000 and 1100 °C, the char yields for the coal are 12-13% lower than those for the respective dried coal. A set of experiments with steam added to the N2 stream were also carried out on the dried coal. The amount of steam added was determined as such that the steam concentration in the reactor atmosphere was similar to what would have been produced from the moisture evaporated from coals without drying if the experiment had been carried out using these coals. Figure 3 indicates that the char yields for this set of experiments match the trends for the coals without drying very well. It is therefore reasonable to conclude that the steam produced in situ from the inherent moisture of the coals without drying during pyrolysis has interacted with the vulnerable nascent char sitting on the frit and gasified the char to a noticeable extent, leading to the much lower char yields. In other words, each batch of nascent char, once produced and sitting on the frit, would be exposed to an atmosphere containing steam, until the end of feeding. Nevertheless, the gasification effect seems to reach a limit after 1000 °C. This observation, together with a calculation of the amount of steam that could be produced from the coal moisture content, indicates that the in situ steam availability seems to have become the limiting factor for gasification at pyrolysis temperatures above 1000 °C. It is worthwhile to note that the higher char yields for the pyrolysis of the dried coals were unlikely due to possible changes induced by the coal drying process. In fact, when the coal was dried at 40 °C for a long period of time (coal moisture was reduced to ∼3%), a similar trend of char yield was also observed. Additionally, compared to 1 min of holding time, a 30 min holding time at a pyrolysis temperature of 1000 °C led to little variations in char yields. Comparing the data in Figures 2 and 3, it can be seen that, at 900-1100 °C, the char yields from the pyrolysis of dried coals under fast heating are about 5-6% lower than those from the slow-heating pyrolysis. Apart from the effect of the heating rate itself (slow-heating conditions lead to a longer residence time of volatile precursors within the particles and hence enhance intraparticle volatile cracking and deposition in the char,


Figure 4. Char yields from the fast-heating pyrolysis of pf-sized particles of raw coal in the drop-tub/fixed-bed reactor at 1000 °C, for particles of various moisture contents.

Figure 5. Char yields from the fast-heating pyrolysis of mm-sized particles of raw coal, with and without drying, in the fluidized-bed reactor at various temperatures.

thus generally giving higher char yields31,32), similar to inherent coal moisture, pyrolytic water might also have played a role in gasifying part of the char produced under fast-heating pyrolysis of the dried coal, considering the high content of oxygen functional groups in the char structure of low-rank coals33 such as Collie coal and hence a higher probability of forming products of high water content during pyrolysis. Further experiments were then carried out to investigate the char yields for the pyrolysis of the same coal with various moisture contents in the same reactor at 1000 °C. Figure 4 shows that the char yield decreases with increasing coal moisture content, further confirming that the in-situ gasification between the nascent char and steam produced in situ greatly depends on the steam partial pressure in the reactor. A coal of higher moisture content produces more in-situ steam, while the N2 flow remains the same, hence, giving a higher steam partial pressure. When large coal particles (mm-sized particles) were carried out using the fluidized-bed reactor, similar effects of in-situ steam was also observed. Figure 5 shows the pyrolysis char yield for 0.8-1 mm coal particles. At 1000 °C, the char yield for the raw coal pyrolysis is again considerably lower than that for the dried coal (∼10% difference). This difference, however, is slightly lower as compared to that from the pf-sized particle pyrolysis (which is 12-13%; see Figure 3). The generally higher char yield for the pyrolysis of the large coal particles as (31) Li, C.-Z.; Bartie, K. D.; Kandiyoti, R. Fuel 1993, 72, 3. (32) Kershaw, J. R.; Sathe, C.; Hayashi, J.; Li, C.-Z.; Chiba, T. Energy Fuels 2000, 14, 476. (33) Howard, J. B. Fundamentals of Coal Pyrolysis and Hydropyrolysis. In hemistry of Coal Utilisation, 2nd Suppl. Vol.; Elliot, M. A., Ed.; John Wiley & Sons: New York, 1981.


Figure 6. Effect of particle size on char yield for fast-heating pyrolysis at 1000 °C (experiments of pf-sized and mm-sized raw coal samples were carried out in the drop-tube/fixed-bed reactor and the fluidizedbed reactor, respectively).

Figure 7. Char yields from the fast-heating pyrolysis of the pf-sized particles of raw coal, with and without drying, in the drop-tube reactor at 1000 °C and 1100 °C.

compared to pf-sized particle pyrolysis is attributed to the transport effects (heat and mass transport) in the pyrolysis process, leading to greater intraparticle tar/carbon deposition and hence higher char yield. Figure 6 reveals the effect of particle size on the extent of in-situ steam gasification. Note that in Figure 6 the char yield data for pyrolysis of pf-sized particles and 0.8-1 mm particles are reproduced from Figures 3 and 5, respectively. For particles of 3.35-4 mm, the difference in char yield between the pyrolysis of coals with and without drying is reduced to 6-7%. This may be attributed to first the slower heating rate inherent to the large particles and second the reduced moisture evaporation rate due to the pore diffusion effect in the large particles. When the large particles are fed into the reactor, it takes a longer period of time to heat the particles until the temperature is high enough for steam gasification. The reduced particle heating rate and large particle size also lead to a slow evaporation rate of moisture from the coal particles due to transport effects within the pores.34 Compared to pf-sized particles, both shorten the effective period of time (for gasification) for which the particles are exposed to the steam-containing atmosphere. Results in Figure 6 indicate show that the in-situ gasification effect is not only limited to pf-sized particles but can also be important in coal utilization processes employing larger particles such as in the fluidized-bed gasifier or a kiln for coal thermal upgrading. Figure 7 shows the char yield from pyrolysis of pf-sized particles of the raw coal in a drop-tube reactor. At 1000 °C, there is little difference between the char yields from the coals (34) Laurendeau, N. M. Prog. Energy Combust. Sci. 1978, 4, 221.

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Figure 8. Char yields from the fast-heating pyrolysis of pf-sized particles of raw, dilute acid-washed, and demineralized coals, with and without drying, in the drop-tube/fixed-bed reactor at 1000 °C.

with and without drying, as the duration of interaction between in-situ steam and char is much shorter (2 s) in this case than that in the drop-tube/fixed-bed furnace. However, at 1100 °C, a more noticeable difference is observed; that is, the char yield from the coal without drying is about 5% lower than that from the dried coal, apparently due to a much faster gasification rate at the higher temperature. This implies that the gasification effect of the steam produced in situ from coal moisture should still be an important factor to be considered for the designing and operation of reactors (e.g., boilers or entrained-flow gasifiers) utilizing high-moisture coals at high temperatures. Pyrolysis was carried out on the dilute acid-washed and demineralized coals at 1000 °C in the drop-tube/fixed-bed reactor, to examine the influence of coal inorganic species on the effect of inherent moisture in pyrolysis. Figure 8 shows the comparison between char yields from pyrolysis of the raw coal, dilute acid-washed coal, and demineralized coal, with and without drying. Comparing the char yields for the pyrolysis of dried coals (in the absence of inherent moisture), no significant difference is observed for the three coal samples, in general agreement with the observations in the literature24,26 for pyrolysis of raw and acid-washed lignite samples at high temperatures. Nonetheless, the slightly lower char yield for the dried dilute acid-washed coal compared to the dried raw coal is most probably due to the enhanced cross-linking of the metallic species in the raw coal during pyrolysis.35-37 The char yield for the dried demineralized Coal would be expected to be similar to, if not lower than, that of the dried dilute acid-washed coal, yet it is even slightly higher than that of the dried raw coal. As aforementioned, even for pyrolysis of the dried raw coal, insitu (catalytic) gasification, by pyrolytic water might have occurred to some extent. On the other hand, for the demineralized coal, the (noncatalytic) gasification by any pyrolytic water would be expected to be much slower relatively. This may account for the slightly higher char yield for the dried demineralized coal compared to that of the dried raw coal as well as the dried dilute acid-washed coal. Comparing the char yields between pyrolysis of the coals with and without drying, it can be seen that the extent of insitu gasification by the coal inherent moisture is similar for the raw and dilute acid-washed coals (the char yields of the coals without drying are 10-13% lower than those of the dried coals), whereas it is not significant for demineralized coal (merely about (35) Shibaoka, M.; Ohtsuka, Y.; Wornatt, M. J.; Thomas, C. G.; Bennett, A. J. R. Fuel 1995, 74, 1648. (36) Hayashi, J.; Takahashi, H.; Doi, S.; Kumagai, H.; Chiba, T.; Yoshida, T.; Tustsumi, A. Energy Fuels 2000, 14, 400. (37) Wornat, M. J.; Sakurovs, R. Fuel 1996, 75, 867.



Figure 9. Specific reactivity versus coal conversion for chars from pyrolysis of pf-sized particles of raw coal, with and without drying, in the drop-tube/fixed-bed reactor: (a) 800, (b) 900, (c) 1000, and (d) 1100 °C char.

2% difference). Since the inherent moisture content of the demineralized coal is only 12%, another pyrolysis experiment was carried out on the dried demineralized coal with additional steam added externally, such that the amount of steam fed is similar to that as if produced from the raw coal (with ca. 20% moisture). Again, the char yield is only about 3% lower than that of the dried demineralized coal. The data suggest that the great extent of in-situ gasification in the raw and dilute acidwashed coal pyrolysis is mainly due to catalytic gasification13,14,16,38-40 of the nascent char in the presence of some inorganic species in the coal. Moreover, it seems that the inorganic species removed in the dilute acid-washing has little influence on the gasification, as opposed to the profound influences of inorganic matter (mainly metallic species organically bounded to the coal structure) in the brown coal gasification reactivity.13,14,16,38-48 Therefore, it appears that some minerals (only removed in the demineralized coal) present in the raw and dilute acid-washed coals act as catalysts for the in-situ steam gasification (see Table 2 for the inorganic species contents of the different coal samples). (38) Miura, K.; Hashimoto, K.; Silveston, L. Fuel 1989, 68, 1461. (39) Bayarsaikhan, B.; Hayashi, J.; Shinada, T.; Sathe, C.; Li, C.-Z.; Tsutsumi, A.; Chiba, T. Fuel 2005, 84, 1612. (40) Kitsuka, T.; Bayarsaikhan, B.; Sonoyama, N.; Hosokai, S.; Li, C.Z.; Norinaga, K.; Hayashi, J.-I.; Energy Fuels 2007, 21, 387 (41) Quyn, D. M.; Wu, H.; Li, C.-Z. Fuel 2002, 81, 143. (42) Quyn, D. M.; Wu, H.; Li, C.-Z. Fuel 2002, 81, 151. (43) Wu, H.; Quyn, D. M.; Li, C.-Z. Fuel 2002, 81, 1033. (44) Quyn, D. M.; Wu, H.; Hayashi, J.-I.; Li, C.-Z. Fuel 2003, 82, 587. (45) Wu, H.; Hayashi, J.-I.; Chiba, T.; Takarada, T.; Li, C.-Z. Fuel 2004, 83, 23. (46) Li, C.-Z. AdVances in the Science of Victorian Brown Coal; Elsevier: Oxford, U.K., 2004. (47) Yu, J.; Tian, F.-J.; Chow, M. C.; McKenzie, L. J.; Li, C.-Z. Fuel 2006, 85, 127. (48) Li, C.-Z. Fuel 2007, 86, 1664.

3.2. Char Reactivity. The influence of the in-situ gasification on char reactivity was also investigated. The specific reactivity of char samples collected after pyrolysis of the various coal samples is plotted against coal conversion, instead of char conversion. This aims to enable comparisons between the pyrolysis of coals with and without drying, since part of the char has been reacted by the in-situ steam during the pyrolysis of coals without drying. Such a plot of reactivity versus coal conversion takes into account any changes of the char properties, which may result in a reactivity change. 3.2.1. Influence of Pyrolysis Temperature. Figure 9 shows the reactivity profiles for chars prepared at different temperatures for the raw pf-sized coal pyrolysis in the drop-tube/fixed-bed reactor. At 800 °C (Figure 9a), the reactivity profiles between the chars from the raw and dried coal showed very little difference. Starting from 900 °C, as shown in Figure 9b, the char reactivity from the raw coal becomes lower than that from the dried coal. Subsequently, from Figure 9c,d, at 1000 and 1100 °C, the inherent moisture from coal leads to a considerable decrease in the char reactivity, at the same coal conversion level. These are also the temperatures at which the gasification of char by in-situ steam becomes significant. If the presence of inherent coal moisture had little effect on the properties of char formed during pyrolysis, similar char reactivity should be observed. Therefore, it seems logical to state that char deactivation has occurred during pyrolysis due to the presence of inherent coal moisture. At 1100 °C, the deactivation effect appears to be slightly lower than that at 1000 °C. The increased char ordering and hence deactivation caused due to heat treatment during pyrolysis itself at the high temperatures could have made the deactivation effect by steam-char interactions less apparent. Also, from Figure 9a-d, feeding external steam simultaneously


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Figure 11. Specific reactivity versus coal conversion for chars from pyrolysis of the pf-sized raw, dilute acid-washed, and demineralized particles, with and without drying, in the drop-tube/fixed-bed reactor at 1000 °C.

Figure 12. Specific reactivity versus coal conversion for chars from pyrolysis of pf-sized raw coal particles, with and without drying, in the drop-tube reactor at 1100 °C. Table 4. BET Surface Area of Chars from Pyrolysis of Pf-Sized Raw Coal in the Drop-Tube/Fixed-Bed Reactor at 1000 °C BET surface area char from the raw coal without drying char from the dried raw coal

Figure 10. Specific reactivity versus coal conversion for char from the pyrolysis of the mm-sized raw coal (0.8-1 mm), with and without drying, in the fluidized-bed reactor: (a) 800, (b) 900, and (c) 1000 °C char.

with the dried coal in pyrolysis generally has effects rather similar to that of the coal without drying on char reactivity. Figure 10 shows the reactivity profiles of chars from pyrolysis of the raw large (0.8-1 mm) coal particles in the fluidizedbed reactor. Generally, the reactivity trends are similar to those observed for pf-sized particles. At 1000 °C (Figure 10c), the in-situ steam deactivation effect for large-particle pyrolysis appears to be lower than that for pf-sized-particle pyrolysis. Surface area analysis was carried out on the chars from pyrolysis of pf-sized coal samples, with and without drying, in the drop-tube/fixed-bed reactor operating at 1000 °C. The reactivity profiles for these two chars have been shown in Figure 9c. Table 4 shows the BET surface areas of the two chars and reveals that the char from pyrolysis of the coal without drying has a slightly higher surface area than that from the dried coal. This may be due to the widening of pores or generation of new

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pores leading to the increase in surface area by the in-situ gasification.49 Hence, the lower reactivity of char from the raw coal may have resulted from some intrinsic chemical changes in the char, which have far surpassed any possible gasificationinduced physical activation as reflected in the surface area analysis. One possible change is a change in the char carbonaceous structure. It is very likely that the gasification of char by in-situ steam has preferentially consumed the more reactive sites with low activation energies, hence leading to a more condensed char structure with higher overall activation energies. Another probable reason for the decrease in char reactivity is the changes in catalytic activity of coal inorganic species following steam-char interaction. This will be examined in more detail in the following section. 3.2.2. Influence of Coal Inorganic Species. Figure 11 compares the reactivity of chars from pyrolysis of the raw, dilute acid-washed, and demineralized coals, with and without drying, in the drop-tube/fixed-bed reactor. Comparing the reactivity of chars from pyrolysis of coals without drying, it is seen that the dilute acid-washing has not led to any significant changes in the char reactivity, while demineralization has resulted in a substantial decrease in char reactivity. This may again be attributed to the arguments as presented under (49) Petrov, N.; Gergova, K.; Eser, S. Fuel 1994, 73, 1197.


section 3.1 regarding the catalytic effect of inorganic species on the gasification reactivity. Yet another important observation here is that the steam-char interaction during pyrolysis of the demineralized coals has not led to any decrease in char reactivity; that is, the chars from the dried dimineralized coal and dried demineralized coal with external steam added to the N2 stream during pyrolysis have similar reactivity. From this, it is deduced that the decrease in char reactivity due to the inherent moisture in the raw and dilute acid-washed coals is, at least partly, due to some changes in catalytic activity of the coal inorganic species (for example deactivation of the catalysts38) as a consequence of steam-char interaction, apart from the possible reason of changes in char structure following steam-char interaction as discussed earlier. 3.2.3. Influence of Extent of Interaction between in-Situ Steam and Char. Figure 12 shows the reactivity profiles of char from pyrolysis of the pf-sized raw coal particles in the drop-tube reactor at 1100 °C. The reactivity is much higher than that from the drop-tube/fixed-bed reactor as shown in Figure 9d, because of the much shorter char residence time in the drop-tube reactor thus less thermal annealing and less volatile-char interactions. Little difference is observed between the chars from the raw coal with and without drying, despite the fact that there is some extent of gasification (∼5%) by the in-situ steam at 1100 °C (see Figure 7) under the short residence time. Therefore, the interactions between the in-situ steam and char only lead to insignificant steam deactivation due to the short char residence time, resulting in little noticeable reactivity loss. 4. Conclusions Strong interactions between the coal moisture and coal matter, believed to be in the form of char gasification by the in-situ


steam, have been observed during the pyrolysis of various Collie coal samples without predrying in the drop-tube/fixedbed or the fluidized-bed reactors, where the residence time of char is relatively long. The in-situ steam gasification can convert a significant proportion of the char at temperatures > 800 °C, resulting in much lower char yields. Inorganic species in the coal seems to play an important role in the in-situ gasification. However, in the drop-tube reactor, where the residence time of char is much shorter, the effect of the in-situ steam gasification is only noticeable at temperatures greater than 1000 °C. The interactions between the in-situ steam and char can considerably deactivate the char at high pyrolysis temperatures. The effect of inherent moisture decreases as the coal particle size increases. The results from this study imply that inherent moisture in lowrank coals should be an important consideration in several existing coal utilization processes as well as in various technologies being developed, especially those deploying reactors in which the coal/char particles experience long residence time. Acknowledgment. The authors gratefully acknowledge the financial and other support received for this research from the Cooperative Research Centre for Coal in Sustainable Development (CCSD), which is established and supported under the Australian Government’s Cooperative Research Centre program. Associate Professor Ron Watkins of Curtin University of Technology assisted in coal inorganic species analysis. Partial support from Australia’s Department of Education, Science and Training through AustraliaChina Special Fund for S&T Cooperation (CH050013) is also acknowledged. EF7002443

Effect of Inherent Moisture in Collie Coal during Pyrolysis Due to in

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