8
Effect of Pressure on Char Formation during Pyrolysis of Pulverized Coal Jianglong Yu,* David Harris, John Lucas, ² Daniel Roberts, Hongwei Wu, § and Terry Wall ² 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 combus- tion and entrained-flow gasification. 1-6 These technolo- gies provide several advantages over conventional coal firing processes, including a reduction in harmful emis- sions 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 reactivity 3,13,15,21-23 has revealed that the operating pressure has a significant impact on coal swelling 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 re- viewed by Wall and co-workers. 24,25 The influence of pyrolysis pressure on char structure has been investigated recently, using bituminous coals 4,22,26 and coal maceral concentrates. 3 This work * 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. ² 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 Engineer- ing 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, 745- 750. (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. (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, 591- 601. (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, 957- 965. (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. 1346 Energy & Fuels 2004, 18, 1346-1353 10.1021/ef030019y CCC: $27.50 © 2004 American Chemical Society Published on Web 08/18/2004

Effect of Pressure on Char Formation during Pyrolysis of Pulverized Coal

  • Upload
    terry

  • View
    215

  • Download
    0

Embed Size (px)

Citation preview

Page 1: Effect of Pressure on Char Formation during Pyrolysis of Pulverized Coal

Effect of Pressure on Char Formation during Pyrolysisof Pulverized Coal

Jianglong Yu,*,† David Harris,‡ John Lucas,† Daniel Roberts,‡ Hongwei Wu,§ andTerry Wall†

Cooperative Research Centre for Coal in Sustainable Development (CCSD), Queensland Centrefor 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 2.0MPa and in a drop tube furnace (DTF) at atmospheric pressure have been examined. Charsgenerated in the PEFR show characteristics (e.g., morphology and internal structure) similar tothose produced in a pressurized drop tube furnace (PDTF) over the pressure range of 0.5-1.5MPa 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 structurewhile the number of both cenospheric and solid char particles decreases under pressurizedconditions. Swollen chars with higher porosities are produced from pyrolysis at elevated pressures.These experimental measurements agree with the results that are predicted using a charformation model developed previously by the authors, demonstrating an optimum pressure rangefor maximum swelling.

1. Introduction

Research interest on the effects of pressure on charformation has been driven by the development andimplementation of high-intensity power generationtechnologies, such as pressurized fluidized bed combus-tion and entrained-flow gasification.1-6 These technolo-gies provide several advantages over conventional coalfiring processes, including a reduction in harmful emis-sions and an enhancement of the intensity of reactions(and, hence, coal throughput). Recent work on coalpyrolysis,7-15 coal swelling and char structure,9,16-20 andchar reactivity3,13,15,21-23 has revealed that the operatingpressure has a significant impact on coal swelling

during devolatilization: char reactivity is enhanced athigh pressures, and pressure significantly influences theash formation mechanism through its effect on thestructure of chars formed. The effects of pressure on coalreactions and ash formation have been recently re-viewed by Wall and co-workers.24,25

The influence of pyrolysis pressure on char structurehas been investigated recently, using bituminouscoals4,22,26 and coal maceral concentrates.3 This work

* Author to whom correspondence should be addressed. Currentcontact information: Department of Chemical Engineering, PO Box36, Monash University, VIC 3800, Australia. Telephone: +61 399051961. Fax: +61 3 9905 5686. E-mail address: [email protected].

† Department of Chemical Engineering, University of Newcastle,Callaghan, NSW 2308, Australia.

‡ CSIRO Energy Technology, Queensland Centre for AdvancedTechnologies, Technology Court, Pullenvale, Qld 4069, Australia.

§ Centre for Fuels and Energy & Department of Chemical Engineer-ing Curtin University of Technology, Perth, WA 6001, Australia.

(1) Takematsu, T.; Maude, C. Coal Gasification for IGCC PowerGeneration; 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, 745-

750.(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 Rateand Pressure. Ph.D. Thesis, University of New South Wales, Australia,2001.

(8) Mill, C. J.; Harris, D. J.; Stubington, J. F. In Proceedings of the8th Australian Coal Science Conference, Australian Institute ofEnergy: Sydney, Australia; 1998; pp 151-156.

(9) Benfell, K. E.; Bailey, J. G. In Proceedings of the 8th AustralianCoal 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, 591-601.

(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, 957-

965.(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 withOxygen, 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.

1346 Energy & Fuels 2004, 18, 1346-1353

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

Page 2: Effect of Pressure on Char Formation during Pyrolysis of Pulverized Coal

showed that, as pressure increases, the overall propor-tion of Group I chars (with a porosity of >80% and smallwall thickness (<5 µm)) increases while that of GroupII and III chars (with relatively dense structures)

decreases. For instance, when the pressure increasesfrom 0.5 MPa to 1.5 MPa, the proportion of Group Ichars is increased from 38% to 72% for a coal samplecontaining a relatively high inertinite content.19

Chars with different structures have a tendency tobehave differently during combustion and gasifica-tion.5,22 Group I chars, because of their high porosity,are more readily fragmented, leading to the formationof finer ash particles. Therefore, the changes in thepopulation of the Group I chars have a significantinfluence on the final ash chemistry. Wu and co-workers4,26,27 concluded that a highly porous “foam” charstructure has a tendency to form during rapid heatingunder elevated pressures. They proposed a qualitativemechanism for the evolution of the foam structurethrough bubble generation on the basis of their observa-tions on the characteristics of chars prepared using apressurized drop tube furnace (PDTF). However, this

(26) Wu, H. W.; Bryant, G.; Benfell, K.; Wall, T. Energy Fuels 2000,14, 282-290.

(27) Wu, H.; Wall, T.; Liu, G.; Bryant, G. Energy Fuels 1999, 13,1197-1202.

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

Table 1. Properties of Raw Coals

Proximate Analysis (%, air dried) Ultimate Analysis (%, daf) Maceral Analysis (vol %, MMR)coal moisture ash VM FC C H O N S V L I

vitrinite reflectance,Rv,max (%)

A 6.70 12.10 40.20 41.00 78.60 6.10 13.70 1.07 0.48 62.00 24.90 13.10 0.44B 2.20 14.80 29.70 53.30 83.70 5.45 8.60 1.81 0.47 62.10 3.10 34.70 0.73

Table 2. Operating Conditions of Drop Tube FurnacesUsed To Produce Char Samples

reactorpressure

(MPa)wall temp

(K) gas flowresidence time,

tR (s)

DTFa 0.1 1573 N2 0.3-0.5PDTFb 0.5-1.5 1573 N2 ∼0.5

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

Table 3. Conditions of Preparation of SamplesGenerated in the Pressurized Entrained Flow Reactor

(PEFR) at 2.0 MPa

coalchar

samplewall

temp (K)[O2] inN2 (%)

C:Ostoichiometry

(%)residencetime, tR (s)

conversion,Xa (%)

A 020322b 1373 2.5 132 2.8 77B 010823c 1373 5.0 99.6 4.8 55B 010828c 1673 2.5 82.8 2.3 77

a Calculated using gas analysis and known coal feed rates.

Char Formation during Pyrolysis of Pulverized Coal Energy & Fuels, Vol. 18, No. 5, 2004 1347

Page 3: Effect of Pressure on Char Formation during Pyrolysis of Pulverized Coal

process was not quantitatively modeled. More recently,Yu et al.28 developed a mechanistic char structuremodel, which allowed quantitative predictions of theevolution of a char’s structure during rapid heating ofa pulverized coal. The model has also been applied toinvestigate the effect of pressure on the structure ofchar,29,30 and the results were consistent with publisheddata.

In this paper, previously published characteristics ofchars produced at atmospheric pressure and high pres-sures (0.5-1.5 MPa) are compared with chars recentlyproduced as a part of a wider research program in apressurized entrained flow reactor (PEFR) at 2.0 MPa.The qualitative observations reported here representpreliminary data on three char samples from thebroader gasification research program. The purpose ofthe optical and scanning electron microscopy (SEM)analyses is mainly to provide a preliminary investiga-tion into the structure of chars produced under PEFRconditions and to provide continuity between previouswork in this area using the drop tube furnaces (DTFs)and future high-pressure investigations. Some model

predictions under elevated pressure are presented andthe mechanism of char formation at pressure is dis-cussed.

2. Experimental Section

2.1. Coal Samples. Two Australian thermal coals wereused in this work. The properties of these coals are shown inTable 1. Coal A is a high volatile sub-bituminous coal high inliptinite (24.9%). The volatile matter (VM) content of coal A is40.2%, and the mean maximum vitrinite reflectance is 0.44%.Coal B is a bituminous coal high in inertinite (34.7%). Its VMcontent is 29.7%, and its mean maximum vitrinite reflectanceis 0.73%.

2.2. Char Preparation. Char samples were prepared intwo apparatuses: a DTF that has been previously described30

and a pressurized entrained flow reactor (PEFR).31 Theconditions used to make char samples are summarized inTables 2 and 3. The PEFR is a facility that is designed toinvestigate the reactions of coals and their chars under high-pressure gasification conditions. Unlike chars collected in theDTF, chars collected in the PEFR have undergone bothpyrolysis and gasification reactions with oxygen, steam, andcarbon dioxide (CO2). The extent of influence of the gasificationreactions is dependent on the reaction conditions and theresidence time of the sample, which is presented in Table 3,along with other relevant characteristics. Furthermore, sampleswere generated under a range of stoichiometries (the C:O ratioin the feed), which are also listed in Table 3. In this work, astoichiometry of 100% means there is enough oxygen to convertall carbon in the feed to CO. It follows that stoichiometries of<100% are truly gasifying conditions and stoichiometries of>100% represent more combustion-like conditions. The feedcoal for the PEFR was prepared for a size fraction of +45-180 µm.

A size cut of +63-95 µm of the raw coal sample was usedin pyrolysis experiments in the DTF. Char samples werecollected at 1573 K and atmospheric pressure in a nitrogengas flow. The experimental procedure and the setup of the DTFhave been detailed elsewhere.28,30,32 Tables 2 and 3 also providea comparison of the experimental conditions for PEFR andDTF for the preparation of chars with the PDTF reactor usedin the previous work.33

2.3. Char Characterization. Proximate and ultimateanalyses of char samples produced were performed. Theparticle size distributions of coal and char samples weredetermined using a Malvern laser sizer. The swelling ratio wasobtained by comparing the mean particle size of chars to thatof the feed coal. The morphology and cross-section structureof chars were observed via scanning electronic microscopy(SEM). All the images were taken using the same accelerationvoltage of 15 kV. The two-dimensional macroporosities of charswere measured through SEM image analysis using the Im-ageTools (v2.00) software,34 and the approach of the measure-ments has been detailed elsewhere.30,32

3. Results and Discussions

3.1. Morphology and Structures of PEFR Chars.Figure 1 shows SEM images of chars produced fromcoals A and B, prepared in the PEFR at 1373 K and

(28) Yu, J.; Strezov, V.; Lucas, J.; Liu, G. S.; Wall, T. Proc. Combust.Inst. 2002, 30, 467-473.

(29) Yu, J.; Lucas, J. A.; Liu, G.; Sheng, C.; Wall, T. F. Combust.Flame 2004, 136, 519-532.

(30) Yu, J. L. A Mechanistic Study of Coal Swelling and CharStructure Evolution during PyrolysissExperiments and Model Predic-tions; Ph.D. Thesis, University of Newcastle, Australia, 2002.

(31) Harris, D. J.; Roberts, D. G.; Henderson, D. G. Presented atthe 12th International Conference on Coal Science, Cairns, Queen-sland, Australia, 2003.

(32) Yu, J.; Lucas, J. A.; Strezov, V.; Wall, T. F. Energy Fuels 2003,17, 1160-1174.

(33) Wu, H. Ash Formation during Pulverised Coal Combustion andGasification at Pressure. Ph.D. Thesis, University of Newcastle,Australia, 2000.

(34) ImageTool Application Version 3.0 Final. UTHSCSA DentalDiagnostic Science, San Antonio, TX, 2002.

Figure 2. (a) Metaplast content, (b) viscosity of coal melt(during heating), and (c) transient swelling ratio of coal B, eachas a function of pressure using the char structure modeldeveloped by the authors..

1348 Energy & Fuels, Vol. 18, No. 5, 2004 Yu et al.

Page 4: Effect of Pressure on Char Formation during Pyrolysis of Pulverized Coal

under a pressure of 2.0 MPa. The images in Figure 1afor coal A and Figure 1c for coal B were obtained underlow magnification. In these images, it is apparent thatthe particles have a closed and smooth surface androunded contours. The particle shape is irregular, withmany projections and “noses”. These observations differfrom those for DTF chars produced at atmosphericpressure. The irregular shape and the formation of theprojections and noses may be attributed to the followingfactors:

(1) The possibility of a higher heating rate in thePEFR, combined with the larger particle size of the feedcoal, could lead to a temperature gradient not onlybetween the surface and the inside of the particle, butalso among the different locations at the surface,because of the irregular shape of the raw coal particles;

(2) Particles of larger size have a greater chance ofcontaining different maceral constituents in one par-ticle. Therefore, the decomposition and thermoplasticproperties of different parts in the same particle maybe different;

(3) Coagulations of softened and sticky coal particlesduring pyrolysis could also lead to the irregular andlarge char particles that are observed via SEM.

The smooth and closed surface is more clearly dem-onstrated at high magnification under SEM, as shownin Figure 1b for coal A and Figure 1d for coal B. Thesurface texture has some obvious similarity to that ofchars prepared in the PDTF.33 This is more apparentfor coal B, probably because coal B has a rank that issimilar to the coal used in Wu’s work. The images

suggest that these PEFR chars have an internal struc-ture resembling a honeycomb-like cellular configurationwith thin outer shells.

These pictures also show that the number of bubblesin char particles created at high pressure is large. Thisobservation supports previous model predictions of thepressure effect on the structure of the char: both theporosity and the number of bubbles increase as thepressure increases.29,30 The bubble sizes are not signifi-cantly different in the entire particle, as shown in theimages.

The highly smooth surface is considered as evidenceof the low viscosity of the coal melt under elevatedpressures. The char morphology also suggests that thecontraction of a char particle is small during the plasticstage. This is also consistent with model predictions.Panels a, b, and c in Figure 2 respectively show themetaplast content, viscosity, and swelling of coal B, asa function of pressure predicted using our char structuremodel.

The cross-sectional images observed via SEM, inFigure 3, reinforce the aforementioned observation thatchars from both coals prepared at elevated pressure arehighly porous. Consistent with observations in theliterature4,26 and previous model predictions by theauthors,29 the “foam” structure (a honeycomb-typeinternal configuration, when viewed in cross section) isa typical structure of high-pressure chars. This is alsoconsistent with the previously mentioned observationson the surface morphology. The wall thickness is verysmall for most of the particles. Some cenospheric chars

Figure 3. SEM cross-sectional images of chars prepared in the PEFR at a temperature of 1373 K and under a pressure of 2.0MPa: (a) coal A (sample 020322b, Ψ ) 132%, tR ) 2.8 s, X ) 77%) and (b) coal B (sample 010823c, Ψ ) 99%, tR ) 4.8 s, X ) 55%).

Char Formation during Pyrolysis of Pulverized Coal Energy & Fuels, Vol. 18, No. 5, 2004 1349

Page 5: Effect of Pressure on Char Formation during Pyrolysis of Pulverized Coal

can be seen, but the population is smaller than that ofchars prepared in the DTF from the same coal. Solidchar particles (with dense structures) are rare, which

means the population of Group III chars is small. Figure4 quantitatively shows the high porosity of the PEFRchar particles, in panels a and c, and Group I chars aredominant in char samples from PEFR for both coals (asshown in panels b and d). Compared with DTF charsfrom the same coal, char samples prepared in the PEFRhave a significantly larger population of Group I charsand a much smaller population of Group III chars.Furthermore, the average porosity of Group II chars is>70%. The results imply that coal develops higherfluidity when a high pressure is applied during pyroly-sis. Some particles that do not develop fluidity atatmospheric pressure may undergo softening and swell-ing under elevated pressures.

Figure 4. Macroporosity and distribution of structures of chars from coal A and B, measured through image analysis: (a) porosityof chars from coal A, (b) char type distribution of coal A, (c) porosity of chars from coal B, and (d) char type distribution of coalB.

Figure 5. (a) Char morphology and (b) cross-sectional image of chars of coal A prepared in the ordinary drop tube furnace (DTF)under the following conditions: temperature, 1573 K; gas flow, N2 gas; pressure, 0.1 MPa; and feed coal particle size, +90-105µm).

Table 4. Macroporosity and Char-Type Distribution ofCoals A and Ba

Coal A Coal B

charPEFRchar

DTFcharb

PEFRchar

DTFchar

Group I char (%) 50.0 24.6 73.1 41.97Group II char (%) 40.9 42.0 19.2 24.01Group III char (%) 9.1 33.4 7.7 34.02average porosity (%) 77.41 52.83 80.46 61.9

a Chars prepared at a wall temperature of 1373 K and under apressure of 2.0 MPa. b Data for a size fraction of +75-90 µm.

1350 Energy & Fuels, Vol. 18, No. 5, 2004 Yu et al.

Page 6: Effect of Pressure on Char Formation during Pyrolysis of Pulverized Coal

3.2. Comparison of Chars from Different Reac-tors. Figures 5 and 6 show cross-sectional images ofchar samples from coals A and B prepared in the DTFat 1373 K at atmospheric pressure with a feed coalparticle size of +90-105 µm. Apparent distinctions inboth morphology and structure exist between charsprepared in the PEFR under elevated pressure and DTFunder atmospheric pressure. Blowholes and cracksobserved for the DTF char do not appear in chars fromthe PEFR. The wall thickness of DTF chars is largerthan that of chars from the PEFR. A Tenui-networkstructure,35 has not been observed in these PEFR charsfrom the same coal. Quantitative comparisons in Table

4 demonstratethat the population of Group I chars fromcoal A at high pressure is ∼25% higher than that of DTFchars, whereas the population of Group I chars preparedfrom coal B in the PEFR is ∼30% higher than that ofDTF chars. The population of Group III chars underelevated pressure is small: 9.1% for coal A and 8% forcoal B. This further suggests that a large number ofparticles that generate solid chars (Group III) underconditions in a DTF will develop significant fluidity andswelling, producing porous chars at high pressure. Someparticles 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.

Figure 6. (a) Char morphology and (b) cross-sectional image of chars from coal B prepared in the DTF under the followingconditions: 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 pressureof 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 of1.6 × 104 K/s and a temperature of 1573 K. Model data taken from ref 29.

Char Formation during Pyrolysis of Pulverized Coal Energy & Fuels, Vol. 18, No. 5, 2004 1351

Page 7: Effect of Pressure on Char Formation during Pyrolysis of Pulverized Coal

develop greater porosity and contribute to the popula-tion of Group I char. The results qualitatively agree withmeasurements in the previous work.19,33 The averagemacroporosity measured through image analysis onPEFR chars from coal A is 24% higher than that of theDTF char sample from the same coal. Similarly, theaverage 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 samepressure (2.0 MPa) but at a lower C:O stoichiometry anda shorter residence time in the reaction zone. Table 3shows that the conversion of this sample is ∼12% higherthan that of the chars collected at 1373 K. The picturesshow that the thin carbon films have been partiallygasified. However, the honeycomb-like cellular struc-tures are more clearly revealed.

3.3. DiscussionsChar Formation at Pressure. Ithas been established in the literature that high pressurehas a major role in the formation of chars in pressurizedreactor systems. It has been suggested that this effectis due to the increase in resistance to the volatile escapeandpromotionofsecondaryreactionsathighpressure.36-38

Previous predictions using the char structure modeldeveloped by the authors30 showed that the liquidfraction (i.e., metaplast, which is an intermediateproduct during pyrolysis) increases at high pressures,as shown in Figure 2a. The liquid may further promotethe destruction of the coal macromolecular structureduring devolatilization. With the increase in the meta-plast content, the apparent fluidity of the entire mate-rial increases. This effect has been reported in theliterature39 and has been predicted by the char structuremodel, as shown in Figure 2b.30 On one hand, underhigh pressures, the increase in fluidity and higher yieldsof light gases due to secondary reactions increase thegrowth rate of bubbles, therefore enhancing the particleswelling. On the other hand, the high external pressurereduces the growth rate of bubbles (hence, the swelling).This leads to an optimum pressure range for a maxi-mum char swelling, a trend which has been predictedin previous work,28,30 as shown in Figure 8a. Cor-respondingly, the change in the char porosity followsthe same trend of the swelling ratio when the pressureincreases.

Figure 8a also compares swelling ratios predicted bythe model29 with experimental measurements under twopressures: 0.1 and 2.0 MPa. At 0.1 MPa, the predictedswelling ratio is 1.39, whereas the experimental resultis 1.27. At 2.0 MPa, the model predicts a swelling ratioof 2.51 and the experimental result is 2.01. High-pressure reduces the rate of bubble ruptures; therefore,the number of bubbles in the resulting char residues athigh pressures increases significantly, as predicted bythe model, and are shown in Figure 8b. This explainswhy high pressure favors the formation of foam char

structures and a decrease in the population of ceno-spheric chars, as observed in the present experiments.Characteristics of chars collected in the PEFR and charsprepared in PDTF suggest that the coalescence ofbubbles may not have a significant role in the formationof char structures, because of the high viscosity of thecoal melt during coal pyrolysis in gas flow reactorswhere the heating rate is high (∼104 K/s). Otherwise,the number of bubbles in chars produced under highpressures would not be significantly different from thatof chars prepared in the DTF.

Figure 9 compares the typical surface texture of achar with a porous structure from coal B collected fromthe PEFR with that from the DTF in this study and aPDTF char from a previous study by Wu et al.33 The

(36) Howard, J. B. Fundamentals of Coal Pyrolysis and Hydropy-rolysis. In Chemistry of Coal Utilization; Elliott, M. A., Ed.; Wiley: NewYork, 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, 463-474.

(39) Lancet, M. S.; Sim, F. A. Prepr. Pap.sAm. Chem. Soc. Div. FuelChem. 1981, 26, 167-173.

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

1352 Energy & Fuels, Vol. 18, No. 5, 2004 Yu et al.

Page 8: Effect of Pressure on Char Formation during Pyrolysis of Pulverized Coal

bubble size of the PEFR char, shown in Figure 9a, issmaller than that of the DTF char (see Figure 9b),whereas that of the PDTF char (see Figure 9c) is moresimilar to the PEFR char. Only the DTF char has a largeblowhole (which is also clearly visible in Figures 6a and7a, which is believed to be evidence of the release ofvolatile matter (VM)) at the surface, whereas the PEFRchar and PDTF char have a closed and smooth surface.The regular honeycomb-like cellular structure and ribsare typical for these PEFR and PDTF chars, instead ofthe irregular flow patterns with the DTF chars.

Conclusions

Compared to chars produced in a drop tube furnace(DTF) at atmospheric pressure, chars prepared underelevated pressure in the pressurized entrained flowreactor (PEFR) have a smooth surface texture and aclosed surface with smaller bubble sizes. The averagemacroporosity of PEFR chars from coal A is 25% higher(20% higher for coal B) than that of the DTF charscollected under atmospheric pressure. Observationsusing scanning electron microscopy (SEM) shows thatPEFR chars have a larger number of bubbles withsmaller sizes, compared to DTF chars. The similarity

of PEFR chars to pressurized drop tube furnace (PDTF)chars and the significant distinctions between thecharacters of chars from pressurized reactors and theDTF reinforce previous findings that pressure has amajor role in the formation of char structures duringpyrolysis in pressurized systems. Model predictionsconsistently suggest that the number of bubbles in-creases significantly as pressure increases, and that, inpressurized reactors, the formation of the foam-typechar structures with a high porosity is favorable. Highpressure leads to a decrease in the population ofboth cenospheric chars (Group I) and solid chars (GroupIII).

Acknowledgment. The authors wish to acknowl-edge the support provided by the Cooperative ResearchCentre for Coal in Sustainable Development (CCSD),which is funded in part by the CRC Program ofAustralia. Mr. D. Henderson is gratefully acknowledgedfor his experimental assistance in the generation ofPEFR char samples. We thank Dr. V. Strezov and Dr.R. Gupta at the University of Newcastle for helpfuldiscussion.

EF030019Y

Char Formation during Pyrolysis of Pulverized Coal Energy & Fuels, Vol. 18, No. 5, 2004 1353