Effect of Pressure on Char Formation during Pyrolysis of Pulverized Coal

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Energy & Fuels 2004, 18, 1346-1353

Effect of Pressure on Char Formation during Pyrolysis of Pulverized Coal Jianglong Yu,*,† David Harris,‡ John Lucas,† Daniel Roberts,‡ Hongwei Wu,§ and Terry Wall† Cooperative Research Centre for Coal in Sustainable Development (CCSD), Queensland Centre for Advanced Technologies, Technology Court, Pullenvale, QLD 4069, Australia Received January 21, 2003. Revised Manuscript Received October 15, 2003

Char samples prepared in a pressurized entrained flow reactor (PEFR) at a pressure of 2.0 MPa and in a drop tube furnace (DTF) at atmospheric pressure have been examined. Chars generated in the PEFR show characteristics (e.g., morphology and internal structure) similar to those produced in a pressurized drop tube furnace (PDTF) over the pressure range of 0.5-1.5 MPa but differ significantly from those of chars prepared in a DTF at atmospheric pressure. Consistent with previous work, high pressure favors the formation of a foam type of char structure while the number of both cenospheric and solid char particles decreases under pressurized conditions. Swollen chars with higher porosities are produced from pyrolysis at elevated pressures. These experimental measurements agree with the results that are predicted using a char formation model developed previously by the authors, demonstrating an optimum pressure range for maximum swelling.

1. Introduction Research interest on the effects of pressure on char formation has been driven by the development and implementation of high-intensity power generation technologies, such as pressurized fluidized bed combustion and entrained-flow gasification.1-6 These technologies provide several advantages over conventional coal firing processes, including a reduction in harmful emissions and an enhancement of the intensity of reactions (and, hence, coal throughput). Recent work on coal pyrolysis,7-15 coal swelling and char structure,9,16-20 and char reactivity3,13,15,21-23 has revealed that the operating pressure has a significant impact on coal swelling * Author to whom correspondence should be addressed. Current contact information: Department of Chemical Engineering, PO Box 36, Monash University, VIC 3800, Australia. Telephone: +61 3 99051961. Fax: +61 3 9905 5686. E-mail address: jianglong.yu@ eng.monash.edu.au. † Department of Chemical Engineering, University of Newcastle, Callaghan, NSW 2308, Australia. ‡ CSIRO Energy Technology, Queensland Centre for Advanced Technologies, Technology Court, Pullenvale, Qld 4069, Australia. § Centre for Fuels and Energy & Department of Chemical Engineering Curtin University of Technology, Perth, WA 6001, Australia. (1) Takematsu, T.; Maude, C. Coal Gasification for IGCC Power Generation; IEA Coal Research, Gemini House: London, 1991. (2) Harris, D. J.; Patterson, J. H. Aust. Inst. Energy J. 1995, 13, 22. (3) Benfell, K. E.; Liu, G.-S.; Roberts, D. G.; Harris, D. J.; Lucas, J. A.; Bailey, J. G.; Wall, T. F. Proc. Combust. Inst. 2000, 28, 2233. (4) Wu, H.; Bryant, G.; Wall, T. F. Energy Fuels 2000, 14, 745750. (5) Liu, G. S.; Rezaei, H. R.; Lucas, J. A.; Harris, D. J.; Wall, T. F. Fuel 2000, 79, 1767-1779. (6) Wang, A. L. T.; Stubington, J. F. In Proceedings of the 8th Australian Coal Science Conference, Australian Institute of Energy: Sydney, Australia; 1998; pp 261-266. (7) Mill, C. J. Pyrolysis of Fine Coal Particle at High Heating Rate and Pressure. Ph.D. Thesis, University of New South Wales, Australia, 2001. (8) Mill, C. J.; Harris, D. J.; Stubington, J. F. In Proceedings of the 8th Australian Coal Science Conference, Australian Institute of Energy: Sydney, Australia; 1998; pp 151-156.

during devolatilization: char reactivity is enhanced at high pressures, and pressure significantly influences the ash formation mechanism through its effect on the structure of chars formed. The effects of pressure on coal reactions and ash formation have been recently reviewed by Wall and co-workers.24,25 The influence of pyrolysis pressure on char structure has been investigated recently, using bituminous coals4,22,26 and coal maceral concentrates.3 This work (9) Benfell, K. E.; Bailey, J. G. In Proceedings of the 8th Australian Coal Science Conference, Australian Institute of Energy: Sydney, Australia; 1998; pp 157-162. (10) Griffin, T. P.; Howard, J. B.; Peters, W. A. Fuel 1994, 73, 591601. (11) Cai, H. Y.; Guell, A. J.; Dugwell, D. R.; Kandiyoti, R. Fuel 1993, 72, 321-327. (12) Megaritis, A.; Messenbock, R. C.; Chatzakis, I. N.; Dugwell, D. R.; Kandiyoti, R. Fuel 1999, 78, 871-882. (13) Cai, H. Y.; Megaritis, A.; Messenbock, R.; Vasanthakumar, L.; Dugwell, D. R.; Kandiyoti, R. Fuel 1996, 75, 15-24. (14) Lee, C. W.; Jenkins, R. G.; Schobert, H. H. Energy Fuels 1991, 5, 547-555. (15) Lee, C. W. Energy Fuels 1992, 6, 40-47. (16) Khan, M. R.; Walker, P. L., Jr.; Jenkins, R. G. Fuel 1988, 67, 693-699. (17) Khan, M. R.; Jenkins, R. G. Fuel 1986, 65, 725-731. (18) Lee, C. W.; Scaroni, A. W.; Jenkins, R. G. Fuel 1991, 70, 957965. (19) Benfell, K. E. Assessment of Char Morphology in High-Pressure Pyrolysis and Combustion. Ph.D. Thesis, University of Newcastle, Australia, 2001. (20) Matsuoka, K.; Akiho, H.; Xu, W.; Gupta, R.; Wall, T.; Tomita, A. Energy Fuels, in press. (21) Roberts, D. G. Intrinsic Reaction Kinetics of Coal Chars with Oxygen, Carbon Dioxide and Steam at Elevated Pressures; Ph.D. Thesis, University of Newcastle, Australia, 2000. (22) Liu, G.-S.; Tate, A. G.; Bryant, G. W.; Wall, T. F. Fuel 2000, 79, 1145-1154. (23) Gadiou, R.; Bouzidi, Y.; Prado, G. Fuel 2002, 81, 2121-2130. (24) Wall, T. F.; Liu, G. S.; Wu, H.-W.; Roberts, D. G.; Benfell, K. E.; Lucas, J. A.; Harris, D. J. Prog. Energy Combust. Sci. 2002, 28, 405-433. (25) Wall, T. F.; Yu, J.; Wu, H.; Liu, G.; Lucas, J. A.; Harris, D. J. Prepr. Pap.sAm. Chem. Soc. Div. Fuel Chem. 2002, 47, 801-806.

10.1021/ef030019y CCC: $27.50 © 2004 American Chemical Society Published on Web 08/18/2004

Char Formation during Pyrolysis of Pulverized Coal

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Figure 1. Scanning electron microscopy (SEM) images of pressurized entrained flow reactor (PEFR) chars from coals A and B prepared at 1373 K and under a pressure of 2.0 MPa: (a) sample 020322b, low magnification (stoichiometry of Ψ ) 132%, residence time of tR ) 2.8 s, conversion of X ) 77%), coal A; (b) sample 020322b, high magnification, coal A; (c) sample 010823c, low magnification (Ψ ) 99%, tR ) 4.8 s, X ) 55%), coal B; and (d) sample 010823c, high magnification, coal B. Table 1. Properties of Raw Coals coal

Proximate Analysis (%, air dried) moisture ash VM FC

A B

6.70 2.20

12.10 14.80

40.20 29.70

41.00 53.30

C

Ultimate Analysis (%, daf) H O N S

78.60 83.70

6.10 5.45

13.70 8.60

Table 2. Operating Conditions of Drop Tube Furnaces Used To Produce Char Samples reactor

pressure (MPa)

wall temp (K)

gas flow

residence time, tR (s)

DTFa PDTFb

0.1 0.5-1.5

1573 1573

N2 N2

0.3-0.5 ∼0.5

a Drop tube furnace. b Pressurized drop tube furnace (data taken from ref 33).

Table 3. Conditions of Preparation of Samples Generated in the Pressurized Entrained Flow Reactor (PEFR) at 2.0 MPa C:O char wall [O2] in stoichiometry residence conversion, coal sample temp (K) N2 (%) (%) time, tR (s) Xa (%) A B B a

020322b 010823c 010828c

1373 1373 1673

2.5 5.0 2.5

132 99.6 82.8

2.8 4.8 2.3

77 55 77

Calculated using gas analysis and known coal feed rates.

showed that, as pressure increases, the overall proportion of Group I chars (with a porosity of >80% and small wall thickness (70%. The results imply that coal develops higher fluidity when a high pressure is applied during pyrolysis. Some particles that do not develop fluidity at atmospheric pressure may undergo softening and swelling under elevated pressures.

Char Formation during Pyrolysis of Pulverized Coal

Energy & Fuels, Vol. 18, No. 5, 2004 1351

Figure 6. (a) Char morphology and (b) cross-sectional image of chars from coal B prepared in the DTF under the following conditions: temperature, 1573 K; gas flow, N2 gas; pressure, 0.1 MPa; and feed coal particle size, +90-105 µm.

Figure 7. Morphology of PEFR chars from coal B (sample 010828c) prepared at a temperature of 1673 K and under a pressure of 2.0 MPa.

Figure 8. (a) Swelling ratios and (b) the final/initial number of bubbles in a char, as a function of pressure, at a heating rate of 1.6 × 104 K/s and a temperature of 1573 K. Model data taken from ref 29.

3.2. Comparison of Chars from Different Reactors. Figures 5 and 6 show cross-sectional images of char samples from coals A and B prepared in the DTF at 1373 K at atmospheric pressure with a feed coal particle size of +90-105 µm. Apparent distinctions in both morphology and structure exist between chars prepared in the PEFR under elevated pressure and DTF under atmospheric pressure. Blowholes and cracks observed for the DTF char do not appear in chars from the PEFR. The wall thickness of DTF chars is larger than that of chars from the PEFR. A Tenui-network structure,35 has not been observed in these PEFR chars from the same coal. Quantitative comparisons in Table

4 demonstratethat the population of Group I chars from coal A at high pressure is ∼25% higher than that of DTF chars, whereas the population of Group I chars prepared from coal B in the PEFR is ∼30% higher than that of DTF chars. The population of Group III chars under elevated pressure is small: 9.1% for coal A and 8% for coal B. This further suggests that a large number of particles that generate solid chars (Group III) under conditions in a DTF will develop significant fluidity and swelling, producing porous chars at high pressure. Some particles producing Group II chars in the DTF may (35) Bailey, J. G.; Tate, A.; Diessel, C. F. K.; Wall, T. F. Fuel 1990, 69, 225-239.

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develop greater porosity and contribute to the population of Group I char. The results qualitatively agree with measurements in the previous work.19,33 The average macroporosity measured through image analysis on PEFR chars from coal A is 24% higher than that of the DTF char sample from the same coal. Similarly, the average macroporosity of PEFR chars from coal B is ∼18% higher than that of the corresponding DTF chars. Figure 7presents the morphology of PEFR chars (sample 010828c) prepared at a higher temperature (1673 K). This sample was produced at the same pressure (2.0 MPa) but at a lower C:O stoichiometry and a shorter residence time in the reaction zone. Table 3 shows that the conversion of this sample is ∼12% higher than that of the chars collected at 1373 K. The pictures show that the thin carbon films have been partially gasified. However, the honeycomb-like cellular structures are more clearly revealed. 3.3. DiscussionsChar Formation at Pressure. It has been established in the literature that high pressure has a major role in the formation of chars in pressurized reactor systems. It has been suggested that this effect is due to the increase in resistance to the volatile escape and promotion ofsecondary reactions at high pressure.36-38 Previous predictions using the char structure model developed by the authors30 showed that the liquid fraction (i.e., metaplast, which is an intermediate product during pyrolysis) increases at high pressures, as shown in Figure 2a. The liquid may further promote the destruction of the coal macromolecular structure during devolatilization. With the increase in the metaplast content, the apparent fluidity of the entire material increases. This effect has been reported in the literature39 and has been predicted by the char structure model, as shown in Figure 2b.30 On one hand, under high pressures, the increase in fluidity and higher yields of light gases due to secondary reactions increase the growth rate of bubbles, therefore enhancing the particle swelling. On the other hand, the high external pressure reduces the growth rate of bubbles (hence, the swelling). This leads to an optimum pressure range for a maximum char swelling, a trend which has been predicted in previous work,28,30 as shown in Figure 8a. Correspondingly, the change in the char porosity follows the same trend of the swelling ratio when the pressure increases. Figure 8a also compares swelling ratios predicted by the model29 with experimental measurements under two pressures: 0.1 and 2.0 MPa. At 0.1 MPa, the predicted swelling ratio is 1.39, whereas the experimental result is 1.27. At 2.0 MPa, the model predicts a swelling ratio of 2.51 and the experimental result is 2.01. Highpressure reduces the rate of bubble ruptures; therefore, the number of bubbles in the resulting char residues at high pressures increases significantly, as predicted by the model, and are shown in Figure 8b. This explains why high pressure favors the formation of foam char (36) Howard, J. B. Fundamentals of Coal Pyrolysis and Hydropyrolysis. In Chemistry of Coal Utilization; Elliott, M. A., Ed.; Wiley: New York, 1981; p 665. (37) Smith, K. L. The Structure and Reaction Processes of Coal; Plenum Press: New York, 1994. (38) Solomon, P. R.; Fletcher, T. H. Proc. Combust. Inst. 1994, 463474. (39) Lancet, M. S.; Sim, F. A. Prepr. Pap.sAm. Chem. Soc. Div. Fuel Chem. 1981, 26, 167-173.

Yu et al.

Figure 9. Comparison of the surface texture of a PEFR char with chars prepared in DTF and PDTF: (a) PEFR char, (b) DTF char, and (c) PDTF char (after Wu and co-workers26,33).

structures and a decrease in the population of cenospheric chars, as observed in the present experiments. Characteristics of chars collected in the PEFR and chars prepared in PDTF suggest that the coalescence of bubbles may not have a significant role in the formation of char structures, because of the high viscosity of the coal melt during coal pyrolysis in gas flow reactors where the heating rate is high (∼104 K/s). Otherwise, the number of bubbles in chars produced under high pressures would not be significantly different from that of chars prepared in the DTF. Figure 9 compares the typical surface texture of a char with a porous structure from coal B collected from the PEFR with that from the DTF in this study and a PDTF char from a previous study by Wu et al.33 The

Char Formation during Pyrolysis of Pulverized Coal

bubble size of the PEFR char, shown in Figure 9a, is smaller than that of the DTF char (see Figure 9b), whereas that of the PDTF char (see Figure 9c) is more similar to the PEFR char. Only the DTF char has a large blowhole (which is also clearly visible in Figures 6a and 7a, which is believed to be evidence of the release of volatile matter (VM)) at the surface, whereas the PEFR char and PDTF char have a closed and smooth surface. The regular honeycomb-like cellular structure and ribs are typical for these PEFR and PDTF chars, instead of the irregular flow patterns with the DTF chars. Conclusions Compared to chars produced in a drop tube furnace (DTF) at atmospheric pressure, chars prepared under elevated pressure in the pressurized entrained flow reactor (PEFR) have a smooth surface texture and a closed surface with smaller bubble sizes. The average macroporosity of PEFR chars from coal A is 25% higher (20% higher for coal B) than that of the DTF chars collected under atmospheric pressure. Observations using scanning electron microscopy (SEM) shows that PEFR chars have a larger number of bubbles with smaller sizes, compared to DTF chars. The similarity

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of PEFR chars to pressurized drop tube furnace (PDTF) chars and the significant distinctions between the characters of chars from pressurized reactors and the DTF reinforce previous findings that pressure has a major role in the formation of char structures during pyrolysis in pressurized systems. Model predictions consistently suggest that the number of bubbles increases significantly as pressure increases, and that, in pressurized reactors, the formation of the foam-type char structures with a high porosity is favorable. High pressure leads to a decrease in the population of both cenospheric chars (Group I) and solid chars (Group III). Acknowledgment. The authors wish to acknowledge the support provided by the Cooperative Research Centre for Coal in Sustainable Development (CCSD), which is funded in part by the CRC Program of Australia. Mr. D. Henderson is gratefully acknowledged for his experimental assistance in the generation of PEFR char samples. We thank Dr. V. Strezov and Dr. R. Gupta at the University of Newcastle for helpful discussion. EF030019Y