An experimental and modeling study of heating rate and particle size

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Energy & Fuels 1993,7, 291-305

297

An Experimental and Modeling Study of Heating Rate and Particle Size Effects in Bituminous Coal Pyrolysis Thomas P. Griffin,+Jack B. Howard,and William A. Peters* Department of Chemical Engineering and Energy Laboratory, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Received August 12, 1992. Revised Manuscript Received December 22, 1992

Heating rate (10-20 o00 K/s) and particle size (63-75 pm at 1073 K; 106125 pm at 1073 and 1273 K) effects on tar yields and total weight loss from pyrolysis of Pittsburgh No. 8 bituminous coal in an electricalscreen heater were measured under 1 atm of helium. Tar molecular weight distributions and gravity column elution behavior were also determined. With increasing heating rate, weight loss increased for both particle sizes. For the 106-125-pm particles, tar yield plateaued or maximized at about 1000-2000 K/s. Tar yields at 1073K were similar for both particle sizes at 10 K/s, but larger for the 63-75 pm particles at all other heating rates. Tar yields and intraparticle liquids inventories were mathematicallymodeled in terms of the kinetics of liquids generation by coal pyrolysis, depletion by intraparticle secondaryreactions, diffusionthrough molten coal, and release to ambient by diffusion of vaporized liquid (tar) through a circumambientgaseous boundary layer. Experimentaland predicted effects of heating rate, particle size, and temperature on tar yields were in reasonable agreement when the model allowed for liquids depletion by two parallel secondary reactions, e.g., cracking and repolymerization.

Introduction A significant body of experimental and analytical evidence supports the picture that there are ranges of operating conditions of scientific and practical interest, for which volatiles yields, compositions, and release rates in coal devolatilization are significantly impacted by volatiles transport and secondary reactions within the pyrolyzing coal. The present work was motivated by the need for better quantitative understanding of these socalled intraparticle secondary reactions of volatiles,during pyrolysis of softening coal. Anthony' and Suuberg2 observed that total pressure, particle diameter, and temperature-time history impact the rapid pyrolysis behavior of a softening coal. Gavalas and Wilkes3observed substantial changes in pore size distribution indicating loss of surface microporosity during bituminous coal devolatilization, and Tsai and Scaroni4recorded substantial changes in the morphology of softening coal during rapid pyrolysis under entrained flow conditions. Russel et al.6 modeled intraparticle transport and secondary reaction phenomena in terms of diffusion and hydrodynamic flow within coal particles assumed to have a wellcharacterized and time-invarient porosity. Using a transient devolatilizationmodel that accounted for intraparticle transport and secondary reactions, Bliek et al.'j predicted that, at least for nonsoftening coals, tar yield should pass through a maximum with increasing heating rate. Gibbins+ Present address: Molten Metal Technology, Inc., 421 Currant Road, Fall River, MA 02720. (1) Anthony, D. B. Ph.D. Thesis, Department of ChemicalEngineering, MIT, Cambridge, MA, 1974. (2) Suuberg,E. M. Sc.D.Thesis, Department of Chemical Engineering, MIT, Cambridge, MA, 1977. (3) Gavalas, G. R.; Wilks, K. A. AZChE J. 1980,26,201. (4) Tsai, C.-Y.; Scaroni, A. W. Energy Fuels 1987,1, 263. (5) Russel, W. B.; Saville, D. A.; Greene, M. I. AZChE J. 1979,25,65. (6) Bliek, A.; van Poelje, W. M.; van Swaaij, W. P. M.; van Beckum, F. P. H. AZChE J. 1985,31,1666.

Matham and Kandiyoti' observed that tar and total volatiles yields increased with increasing heating rate over the range 1-lo00 K/s, and Hamilton8 found that intraparticle inventories of bubbles in softened coal increased strongly with increasing heating rate. Fong et al.S12 provide time-resolved measurements of the plasticity of a softening bituminous coal undergoing pyrolysis during rapid heating. Mathematical models have been developed to predict the devolatilizationbehavior of softening coals. Suuberg and Sezen13 predicted pressure effects on tar yields and molecular weights using a model that described tar transport in terms of an effective diffusivityand tar release in terms of multicomponent evaporation. Niksal4 has modeled tar release rates in terms of equilibrium flash evaporation models. Some models have accounted for changes in coal particle morphology brought about by softening and resolidification. Lewellen16and Oh16et al.17 present bubble transport models in which tar movement through the softened particle is described in terms of the growth, coalescence, and escape of bubbles carrying tar vapor evaporated from molten coal. Haul8modeled effects (7) Gibbins-Matliam, J.; Kandiyoti, R. Energy Fuels 1988,2, 505. ( 8 ) Hamilton, L. H. Fuel 1981, 60,910. (9) Fong, W. S. Sc.D. Thesis, Department of Chemical Engineering, MIT, Cambridge, MA, 1986. (10) Fong, W. S.; Peters, W. A.; Howard, J. B. Rev. Sci. Znatrum. 1981, 56,586-591. (11) Fong, W. S.; Khalil, Y. F.; Peters, W. A,;Howard, J. B. Fuel 1986, 65, 195-201. (12) Fong, W. S.; Peters, W. A,; Howard, J. B. Fuel 1986,65,251-254. (13) Suuberg, E. M.; Sezen, Y. Proceedings 1985 International Conference on Coal Science; Pergamon Press: New York, 1985. (14) Niksa, S . AZChE J. 1988,34,790. (15) Lewellen, P. C. M.S. Thesis, Department of ChemicalEngineering, MIT, Cambridge, MA, 1975. (16) Oh, M. S. Sc.D. Thesis, Department of Chemical Engineering, MIT, Cambridge, MA, 1985. (17) Oh, M. S.;Peters, W. A.; Howard, J. B. AZChE J. 1989,35,775792. (18) Hsu, J. Ph.D. Thesis, Department of Chemical Engineering, MIT, Cambridge, MA, 1989.

0 1993 American Chemical Society

298 Energy & Fuels, Vol. 7, No. 2, 1993

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Tar Collection Apparatus

K+-€z

Suction Tubing

Aluminum Foils

-SS Mesh (Sample) I +-- Electrode

Microfilter

f-

Shielded Connector

4-

“Suction

Tubing

Figure 1. Setup of funnels, foils, and microfilters for forcedflow collection of tar.

of temperature, heating rate, and ambient pressure on tar yields, and total weight loss from rapid pyrolysis of softening coal, by treating intraparticle transport and secondary reactions of metaplast, as well as modifications in coal particle morphology during pyrolysis. Griffinlget aL20 developed a mathematical model to predict pressure and temperature effects on tar and extractables yields from rapid pyrolysis of a softening coal. Their model treats intraparticle transport and reaction of newly formed liquids and well captures important features of tar and extractables yield data. The present work was undertaken to provide better quantitative understanding of intraparticle transport and reactions of volatiles in coal pyrolysis. Changes in particle size and heating rate can modify the residence time of volatiles within the coal and thus the time available for intraparticle secondaryreactions of liquids. Study of how systematic variations in these two parameters impact the rapid pyrolysis behavior of a softening coal was therefore viewed as a useful experimental method for investigating internal secondary reactions. This paper provides new data on how systematic variations in particle size (63-75, 106-125 pm), and in heating rate (10-20 OOO K/s) affect yields of tar and total volatiles, and global indices of tar molecular structure, from pyrolysis of Pittsburgh No. 8 bituminous coal at final temperatures of 1073and 1273K. A mathematical model for predicting heating rate and particlea size effects on tar yields is also presented.

Experimental Section Coal particles were pyrolyzed in an electrical screen heater reactor19 similar to those successfully wed by Hajaligol2I and Oh,”j among others, for systematic studies of the rapid devolatilization behavior of solid fuels. In this apparatus (Figure l ) , a thin layer of powdered coal is supported on a325-mesh stainless steel screen and heated according to a preselected temperaturetime profile by passing electricity through the screen. Volatile5 that escape the coal are rapidly diluted and cooled by the ambient reactor gas. To increase the precision of tar yield determinations, (19) Griffin, T. P. Ph.D. Thesis, Department of ChemicalEngineering, MIT, Cambridge, MA, 1989. (20)Griffin, T. P.; Peters, W. A.; Howard, J. B. Pressure and Temperature Effects in Bituminous Coal Pyrolysis: Experimental Observations and a Transient Lumped-Parameter Model. Submitted for publication in Fuel. (21) Hajaligol, M. R. Ph.D. Thesis, Department of Chemical Engineering, MIT, Cambridge, MA, 1980.

the reactor was modified to achieve better tar separation from the hot environment of the screen, to reduce extraparticle secondary reactions, and to improve tar collection efficiency. Following K O , funnels ~~ lined with removable aluminum foils and venting into small microfilterswere positioned directly above and below the heater screen (Figure 1). The top and bottom plates of the reactor, and rubber tubing that connected the funnels, microfilters, and aspiration lines, were also covered with aluminum foil. The funnels were aspirated during the entire heating time and for several seconds immediately before and after heating. Typically, more than 70% of all the tar collected was on the microfilters,which contributed to good reproducibility in the measured tar yields, i.e., typically 5 1 0 % relative. Procedures for heating the screens were also modified in some runs. Above 200 K/s, essentially constant heating rates were readily obtained by setting two variable transformers to fixed output voltages prior to beginning a pyrolysis run. This method was inadequate for heatup rates below 200 K/s, because as the screen temperature increased, rates of screen heat loss became comparable to the screen input power, thus deceleratingthe initial heatup rate. Constant heating rates below 200 K/s were achieved by manually adjusting the output voltage of one or both heating transformers virtually continuously during the heatup period. Ground Pittsburgh No. 8 bituminous coal (PSOC1451-D)was sieved to give samples in the 63-75- and 106-125-pm range of particle size. Smaller size cuts were unsuitable for studies of particle size effects alone since their maceral composition varied with particle size. Similar behavior was found by Tsai and S ~ a r o n i .In ~ a typical experiment, a known weight of coal was heated at a preselected contant rate to a prescribed final temperature, held there for afixed time (typically 15 s),and then allowed to cool to ambient temperature by radiation and natural convection, at initial rates of about 500 K/s. After a run, gases and condensablevolatiles were recovered for yield determinations and further characterizati~n.~~ The mass of coal used and the mass of char formed were respectively obtained as the difference in weight between an empty screen and the screen plus coal or char, using a Mettler H20 analytical balance with a precision of hO.01 mg. Tar yields were also determined gravimetrically as the weight increase of foils, filters, and other collection surfaces. Intraparticle liquid inventories were determined by Soxhlet extraction of cooled char samples with pyridine, using procedures similar to those described by Oh,16Fong,S and Hajalig01.~~ Approximate molecular weight distributions (MWD) of dichloromethane-solubletar samples were determined by size exclusion chromatography using two Microstyragel 500 and 100A columns in series, and a Waters ALC/GPC 201 gel permeation chromatography (GPC) system consisting of an M-45 positive-displacement pump, a U6K sample injector, and an ultraviolet absorbance detector (Waters UV Model 440) with a 280-nm aperture. The mobile phase was dichlormethane. Using procedures described by Unger and S ~ u b e r gand ~ ~by Oh,16 the GPC system was calibrated by comparingretention volumes for different fractions of coal pyrolysis tar fractions eluted from a preparative scale GPC, to the average molecular weight of each fraction measured by vapor pressure osmometry. Molecular weight was related to elution volume using Griffin’sl9revision of Oh’s16 mathematical method. Softening during pyrolysis can drastically change coal particle morphology‘and thus affect intraparticle mass transfer and the time available for volatiles secondary reactions within the coal. To learn more about particle morphology changes we examined selected samples of cooled chars from the screen heater, using optical microscopy. It was assumed that rapid quenching caused these chars to preserve important structural features of the pyrolyzing coal, e.g., overall form and shape as well as size and number density of bubbles. Char particles were bonded to the (22) KO,G. H. Ph.D. Thesis, Department of Chemical Engineering, MIT, Cambridge, MA, 1988. (23) Unger, P. E.; Suuberg, E. M. Fuel 1984,63, 806. (24) Lafleur, A. L.; Monchamp, P. A.; Chaw, N. T.; Plummer, E. F.; Wornat, M. J. J. Chromatog. Sci. 1988,26,337.

Bituminous Coal Pyrolysis

Energy & Fuels, Vol. 7, No. 2,1993 299

0.6 (A) Volatiles

1

0 5 t Yield. Weight 0.4 Fraction of Coal (DAF)

Yield, Weight Fraction of Coal (OAF)

Yield. Weight Fraction 01 Coal (DAF)

0.1

3 10

10‘ 10’ Heating Rate, K/s

104

Figure 2. Tar and volatiles yields as a function of heating rate. Holding time at maximum temperature,15 s;cooling rate (initial) approximately 500 K/s; pressure, 1atm of helium. (A) 1073 K; particle size, 63-75 pm. (B)1073 K; particle size, 106-125 pm. (C)1273 K; particle size, 106-125 pm. heater screens by resolidification of softened coal. To avoid disturbingand potentiallydamagingthese chars,the screenswere cut into approximately 2 X 3 mm rectangularpieces,which were then set in polyurethane resin using cylindrical molds (-2.5 cm i.d. X -4 cm long) and a methyl ethyl ketone hardener. To fill the different size voids in the chair particles with optically contrasting material, dye resin was applied to the molds at least five times, by intermittent repressurization. Using alumina particles in distilled water the hardened molds were polished until nonuniformitieswere below 0.05 pm. The molds were then examined and photographed, typically at 250X magnification, using a Leitz optical microscope.

Results and Discussion Changes in heating rate, particle size, or temperature can accentuate or inhibit volatiles secondary reactions within a coal particle. For example, for the same final temperature, lower heating rates provide more time for reactions to occur as the coal temperature rises. Increasing particle size or ambient p r e s ~ u r ecan ~ ~increase ~ ~ ~ the holdup of volatiles within the particle. Temperature increases will preferentially accelerate rate processes with larger activation energies. Pyrolysis of 63-75-pm particles was studied at six different heating rates for conditions given in Figure 2A. For heating rate increases from 10 to 20 OOO K/s, yields of tar and total volatiles respectively increased from 26 to 33 wt % and 44 to 52 wt % (daf) (Figure 2A). For 106125-pmparticles (Figure 2B), totalvolatiles increased from 42.5 to 52 wt % when heating rate was increased from 10

to 20 OOO K/s, while tar yield increased modestly when heating rate was increased from 10 to about 1OOO K/s but then remained constant or declined slightly as heating rate was further increased to 20000 K/s. Competitive intraparticle rate processes,lge.g., tar generation, destruction, and transport, could cause tar yield to maximize with increasing heating rate, if their activation energies are sufficiently different, although the 1073K temperature of Figure 2B may be too low to induce such a maximum. To intentionally promote a maximum in tar yield with increasing heating rate, 106-125-pm coal particles were pyrolyzed at a higher final temperature (1273 K). A more distinct maximum in tar yield resulted at about lo00 K/s (Figure 2C). Decreasing heating rate from 200 to 10 K/s decreases tar and total volatiles by 3 and 2 wt % (Figure 2A) and thus increases gas and char yields by 1 and 2 wt %, respectively 1% char = (100 - % total volatiles); % total volatiles = (% tar + % gas)]. Heating to the same final temperature at a lower rate provides more time during heatup for the coal to decompose to gas and liquids. The decrease in tar and increase in char are therefore ascribed to secondary reactions of newly formed tar, since a major change in the coal decomposition mechanism with heating rate is less plausible.25 Further, the geometry and operation of the devolatilization apparatus (Figure 1)are such that only secondary reactions occurring on or within the coal particles are likely to increase char yield, since products of reaction in the small volume of hot gas near the heater screen are rapidly diluted, cooled, and swept away from the coal particles. The facts that, for the stated heating rate reduction, the char increase is double the gas increase and the coal spends a much longer time at low temperatures suggest that tar polymerization rather than tar cracking is the dominant secondary reaction pathway. An alternative explanation for the decline in tar yield with decreasing heating rate (Figure 2) is increased retention of liquids within the char residue. To further examine this possibility, six char samples generated by heating 63-75- and 106-125-pm particles at 10 or 50 K/s to 1073 K, holding there for different times, and then cooling to room temperature were separately extracted with pyridine according to procedures noted above. Liquid extractable5 were not detected in any of these chars, reinforcing our conclusion that intraparticle secondary reactions are responsible for the lower tar yield at lower heating rates. Increasing heating rate from 200 to 20 000 K/s for 6375-pm particles increased tar and total volatiles yields at 1073K by 4 and 6.5 wt % of daf coal, respectively (Figure 2A), implying that gas and char yields respectively increase by 2.5 wt 9% and decrease by6.5 wt % These yield changes are also attributed to competitive rate processes within the particle. Higher heating rates provide less time for volatiles generation, transport, and reactions during heatup, and as a corollary, a greater fraction of the total experimental time is expended at higher temperatures. One possible effect is intensified gas generation rates which result in accelerated transport of tar through the particle. Further, tarthermal cracking would be expected to increase in importance because ita activation energy exceeds that for polymerization. In this temperature range, gas rather

.

(25) Howard,J.B. Fundamentalsof Coal Pyrolysis and Hydropyrolysis, Chapter 12. In Chemistry of Coal Utilization Second Supplementary Volume; Elliott, M. A., Ed.; J. Wiley and Sons: New York, 1981.

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300 Energy &Fuels, Vol. 7,No.2, 1993

Average

350

150

L ! I

1

10

T

10' 101 Heating Rate, Us

104

1

10s

Figure 3. Number-averagemolecular weight (MWN)and weightaverage molecular weight (MWw) as functions of heating rate and particle size. (A)MWN,63-75 pm; (0)MWw, 63-75 pm; (0) MWN,106-125 pm; (0) MWw, 106-125 pm. Other conditions same as in Figures 2A and 2B.

than coke or soot, as well as lower molecular weight tars (see Figure 3),would be the expected tar cracking products, as was observed. Figure 2,A and B, also shows that particle size effects on tar yields depend on heating rate. Increasing particle diameter from 63-75 to 106-125 pm decreases tar yields by only about 0.5 wt % at 10 K/s, but by 3-4 w t % and 5-6 wt % at 1000 and 20 000 K/s, respectively. Competition between intraparticle transport and secondary reactions of tar is again a reasonable explanation of particle size effects. Larger particles expose newly-formed volatiles to greater average transport lengths and hence longer intraparticle residence times, thus increasing the opportunity for intraparticle secondary reactions. Heating rate may in turn modify particle size effects for any of the following reactions: (1)larger heating rates may increase the driving force for internal mass transfer by increasing intraparticle concentration gradients for ambulatory species; (2)changes in heating rate may change coal morphology and hence the accessibility, surface area, and chemistryof reactive interfaces,as well as the characteristic length scales for physical transport-softening and volatiles generation may temporarily transform relatively porous solid coal into a molten material consisting of bubbles, mineral matter, and unsoftened macerals dispersed in a liquid continuum, which, before resolidification, may swell into a cenosphere or other shape drastically different from that of the original coal; one effect can be a major decrease in the distance tar must travel to escape the substrate intemals; (3)increasing heatingrate increases contributions from chemical reactions with higher activation energies-tar yields would be affected when their generation and depletion reactions have significantly different activation e n e r g i e ~ . l ~ y ~ ~ Another potential explanation for heating rate effects is thermal lag of the coal particle internals during rapid surface heating. The magnitude and duration of intraparticle spatial temperature gradients depend upon the thermal physical properties of the medium, the thermochemistry of coal pyrolysis and secondary reactions, particle size, and the surface heating rate or heat flux density.27 For sufficiently rapid surface heating, thermoneutral or exothermic pyrolysis, and no thermal lensing effects analogousto those discussed by Hottel and Williams for heavy fuel regions of the substrate near ita surface (26) Darivakis, G.S.;Peters, W. A.; Howard, J. B. AIChE J. 1990,36, 1189. (27) Hajaligol, M. R.;Peters, W. A.; Howard, J. B. Energy Fuels 1988, 2,430-437. (28) Hottel, H. C.; Williams, 111, C. C. Ind. Eng. Chem. 1955,47,1136.

may become much hotter than regions closer to the particle center. Consequently, if volatiles holdup times within the particle are much less than the particle thermal relaxation times, volatiles generated deep within the particle, will need to traverse a zone of elevated temperature before escaping the substrate. This will in turn increase their opportunity for intraparticle secondary reactions, if the characteristic times for these reactions are small compared to volatiles transport times. In the present work, we view these effects as minor even for our highest heating rate, 20000 "C/s, and largest particle size, 125 pm. Heattransfer calculations imply that, above 600-650 "C, at a surface heating rate of 20000 "Us the space mean temperature for 125-pmcoal particles could lag the particle surface temperature by about 70 "C, but only for about 4 ms. Comparison of Figure 2, B and C, shows that at much lower heating rates of 10 "C/s where the corresponding intraparticle spatial temperature gradient is negligible &e., 0.0035 "C, 1/2000of the above) tar yields change by only about 3 wt % daf coal for a 200 "C temperature increase, i.e., from 1073 to 1273 K, after heating for 15 s at final temperature. Thus, a temperature gradient of 70 "C for 0.004 s should have little effect on tar yields. Bliek et al.'s6 mathematical model for devolatilization of nonsoftening coals predicts that total volatiles yield (Le., weight loss) will maximize with increasing heating rate. Their explanation is that, with increasing heating rate, volatiles yields will increase at low heating rates due to enhanced intraparticle transport and decrease at high heating rates because of augmented secondary reactions. Bliek et al.'s analysis was for nonsoftening, spherical coal particles. Here, for 63-75-pmparticles of softening coal (Figure 2A) tar yield at 1073 K may pass through a maximum with increasing heating rate, above 20 000K/s. For 106-125-pm particles, maxima in tar yield with increasing heating rate are indicated a t about 2000 and 1000K/s, respectively, a t 1073 and 1273 K (Figure 2B,C). A higher temperature would be expected to give a maximum a t a lower heating rate if tar depletion reactions have high activation energies as we believe is true in this range of heating rates and final temperatures due to a preponderance of cracking over polymerization. Niksa et al.34and studied heating rate effects on rapid pyrolysis of a Pittsburgh No. 9 and an Illinois No. 6 bituminous coal, with a mean particle size of 125 pm. For the Illinois No. 6 they found that when heating rate was increased from 100 to lo00 K/s, weight loss above about 950 K increased under vacuum, but not under a pressure of 0.19MPaeUGibbons-Matham and Kandiyot? also report a reduced heating rate effect in coal pyrolysis with an increase in pressure. Pressure and temperature (29) Serio, M. A. Ph.D. Thesis, Department of Chemical Engineering, MIT, Cambridge, MA,1984. (30) Suuberg, E. M.; Unger, P. E. Molecular Weight Distributions of TarsProduced byFlaahPyrolyaieofCoala. Paperpresentedatthe AIChE Meeting, Los Angeles, CA, November 1982. (31) Wolfs, P. M. J.; Van Krevelen, D. W.; Waterman, H. I. Fuel 1960, 39, 25. (32) Solomon, P. R.;Hamblen, D. G.; Carangelo, R. M.; Serio, M. A.; Deshpande, G . V. Energy Fueb 1988,2,405-422. (33) Anthony, D. B.;Howard, J. B.; Hottel, H. C.; Meissner, H. P. Symp. (Int.) Combust., [Proc.], 15,1975, 1976, 1303-1317. (34) Niksa, S.;Heyd, L. E.; Russel, W. G.; Saville, D. A. Symp. (Int.) Combust. [Proc.], 20,1985, 1985,1445-1463. (35) Heyd, L.E. M.S. Theais, Department of Chemical Engineering, Princeton University, Princeton, NJ, 1982. (36) Hajaligol, M. R., aa referenced by T. P. Griffin. PbD. Thesis, Department of Chemical Engineering, MIT, Cambridge, MA,1989, p 107.

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effects on the pyrolysis behavior of 63-75-pmparticles of the present PSOC 1451-Dcoal a t a 1000 K/s heating rate, are discussed in detail e 1 s e ~ h e r e . l ~ ~ ~ ~ Figure 3 displays number-average and weight-average molecular weights, MN and Mw, respectively, as affected by heating rate and particle size. For both particle sizes MWand MN decrease gradually as heating rate is increased from 10 to 20 000 K/s. These observations support the above reasoning that cracking reactions dominate intraparticle secondary reactions at higher heating rates, since cracking would reduce the average molecular sizes of tars from decomposition of the coal. The dispersion factor ( M w / M N declines ) with increasing heating rate, implying that cracking reactions homogenize the tar molecular weight distribution. A similar trend was observed with increasing pressure and peak t e m p e r a t ~ r e . ~ ~ ~ ~ ~ Secondary cracking of tar will not necessarily result in major reductions in the recorded tar yield, e.g., when high molecular weight moleculesare converted into two or more lower molecular weight species, which are still sufficiently nonvolatile to be trapped in the tar collection system. At high thermal severities, cracking reactions will eliminate peripheral functional groups to produce gases, and lower molecular weight tars.29Suuberg and Unger30postulated that tar release during bituminous coal pyrolysis is largely governed by mass transfer of tar molecules from the coal surface to the ambient gas. Niska14modeled tar evolution during coal devolatilization in terms of an equilibrium flash vaporization process. For purely evaporativecontrol, the average molecular weight of the product tar is expected to increase with increasing coal temperatures and heating rates. The decline in tar molecular weight with increasing heating rate observed here for pyrolysis at 1073 K substantiates the importance of nonevaporative processes, e.g., intraparticle mass transfer and secondary reactions, in tar release during softening coal pyrolysis at higher temperatures. Our data on effects of pressure and peak temperature on pyridine extractable5 yields, reported e l s e ~ h e r eimply , ~ ~ ~a ~dominant ~ role for evaporation in tar release below about 973 K. Solomon et alas2present a general model of coal devolatilization that includes coal depolymerizationand metaplast repolymerization, as well as intraparticle and extraparticle transport of lower molecular weight molecules, in modeling tar formation. Tar molecular weight shows little or no effect of increasing particle size from 63-75 to'106-125 pm, for heating rates of 10 and 1000 K/s, but on average declines about 10 Da at 20 000 K/s (Figure 3). A slight increase in tar secondary reactions within the larger particles would explain these findings. The gravity column elution behavior of tars from pyrolysis at 1073 K and 15 s holding time was studied using a protocol developed by Lafleur et al.24 The procedure is designed to separate complex organic mixtures according to similarities in chemical functionality and/or polarity, by successivegravity elution from a cyano-bonded phase column, with four solvents of increasing polarity. Figure 4 displays, for different heating rates, the mass of material measured in each eluant fraction, normalized against the total mass of material eluted from the column, typically 87-113 % of the tar charged. The elution data were virtually the same for the 63-75- and 106-125-pm particles and are therefore combined in Figure 4. This technique does not separate a mixture as complex as coal tar perfectly by chemical functionality or polarity. How-

re o'

Fraction Weight 0'4

Methanol

4:

-4

Dirhloromethane Hexane

0

10

10' 10' Heating Rate, K/r

104

Figure 4. Fractionation of tar soluble in different solvents as a function of heating rate at which tar is produced: normalized sequential column chromatographyresults. Particle size range: 63-125 Wm. Other conditions as given in Figure 2A.

ever, useful information on global or average structures in the tar can be obtained by assuming that each tar fraction eluted by a particular solvent has similar polar and chemical character to that assigned by Lafleur et al.,24Le., hexane, dominated by aliphatic compounds; benzene, mainly condensed aromatic and small, monosubstituted polar compounds; dichlormethane, large (up to 10 rings) polycyclic aromatic hydrocarbons and polar compounds; and methanol, primarily very polar compounds with multiple polar substituents, e.g., hydroxyls and carboxyls. For all cases in Figure 4 typically 85 wt % or more of the eluted tar was in the benzene and methanol eluates. Increasing heating rate from 10 to 1000 K/s reduced the normalized yield of benzene eluate from 0.58 to 0.51 and increased the methanol eluate from 0.28 to 0.34. Figure 4 thus implies that polars decline and aromatics increase as heating rate is lowered from lo00 to 10 K/s. This behavior is consistent with our hypothesis that at lower heating rates tar cracking is less important than tar repolymerization and supports the association of polar oxygen functional groups with cr~ss-linking.~~ Figure 4 shows little or no effect on the individual eluant fractions of increasing heating rate from lo00 to 20 OOO K/s. Thus, the data of Figure 4apparently have insufficient sensitivity to definitivelytest our hypothesis that cracking dominates secondary reactions at higher heating rates. Eighteen different char samples consisting of several distinct particles generated by coal pyrolysis at 1073 K for the ranges of heating rates and particle size fractions specified in Figure 2A, were examined by optical microscopy. As depicted schematically in Figure 5, the number of cavities per unit volume of particle was found to be significantly larger for chars generated under more rapid heating. The particles generated at 10-50 K/s approximated simple cenospheres and typically consisted of one major cavity and a few additional small cavities. At 2001000 K/s the chars generally contained 10-30 cavities of roughly similar dimensions,more or less evenly distributed throughout the entire volume of the char particle. The intraparticle number density of cavities rose about a factor of 10 for chars generated at heating rates of 5000-20 OOO K/s, with individual cavity sizes being rather narrowly distributed. The existence of a narrow size distribution for bubbles (cavities upon resolidification of the molten coal) was predicted theoretically by Oh16, et al." using a mathematical model for the rapid devolatilizationkinetics of softening coal that accounted for intraparticle transport of tar vapor and gases, by the growth and coalescence of bubbles. As expectable from geometric considerations, the micrographsshow (Griffin'g) that the apparent average

Griffinet al.

302 Energy & Fuels, Vol. 7, No. 2, 1993 lntrapanicle Bubble Dynamics

(1)

Bubble Nucleation Swell Ratio - 1 4

N L

-- 5-10 1-3 pm

-

N 10-30 L -2-3 pm

-

N 100-300 L -0 5 - 1 pm

Figure 6. Schematic illustration of effect of heating rate on char morphology. N , number of bubbles in cross section; L, approximate transport distance. Particle size range: 63-125 pm; other conditions as given in Figure 2A.

distance of separation between the bubbles declines roughly in proportion to the square of the bubble number density. The observations summarized in Figure 5 are consistent with electron microscopy results from Hamilton8 who found that over the range 0.1-10 OOO K/s,faster heating of vitrinites from most coal types ranging from subbitiuminous to anthracite produced coal pyrolysis chars more intensely permeated with isolated and interconnected cavities and with thinner walls between adjacent cavities. Hamilton8 and the present studies both found the morphology changes to be larger in the lower end of the heating rate range. In the present work, too few particles were examined to relate char morphology to initial particle size with high statistical reliability. Qualitatively however, the halfthickness of the cenosphere wall scaled approximately linearly with the particle diameter of the unpyrolyzed coal. This wall half-thickness can be associated with a critical length scale for tar transport within molten coal, as discussed in the next section.

Theory

A mathematical model to predict tar yields and intraparticle inventories of liquid extractables from pyrolysis of a softening coal, as affected by temperature and total external pressure has been d e v e l ~ p e d . l ~This > ~ ~paper extends this model to predict tar yields as affected by heating rate and particle size. The model assumes a spatially isothermal coal particle throughout which liquids are generated by pyrolysis of the parent coal, consumed by secondary reactions, transported to the particle surface by diffusion through molten coal, vaporized instantaneously, and released to the ambient as tar by transport through a circumambient gaseous boundary layer. Pyrolysis liquids which survive secondary reactions but do not escape the particle are defined as metaplast or as Uextractables", since most of these liquids can usually be recovered by instantaneously quenching the pyrolyzing coal and then extracting the resulting char with a suitable solvent. The model was developed from a material balance on liquids concentration CT, in a differential spherical shell of a pyrolyzing spherical coal particle

where NT is the local net flux density of tar (mol/(cm*.s) or g/(cm2-s))and R T , and ~ RT,~ respectively denote rates of liquids production by pyrolysis of the coal and of liquids depletion by secondary reactions. Following Anthony et pyrolysis of the parent coal was assumed to occur by multiple independent parallel first-order reactions, sufficient in number that their activation energies can be mathematically described by a continuous probability distribution function. The kinetics of liquids secondary reactions within the coal were described by either one firstorder reaction with an Arrhenius rate constant = k2CT(x,t)

(2)

or, extending our earlier by two parallel reactions assumed to be repolymerization and cracking, with Arrhenius rate constants k2 and k3, respectively. Extractables and tars from pyrolysis are typically complex mixtures of many organic compounds. Here they are respectivelyoperationally defined as material recovered by solvent extraction of the cooled char and detected at tar collection stationsoutside the particle. Thus "cracking" and 'repolymerization" are also defined operationally, since they denote global chemical reactions which, by different pathways, reduce the mass yields of liquid products of pyrolysis. "Cracking" refers to molecular weight (MW) reduction reactions that transform liquids mass into nonliquid products, i.e., gas or materials not solvent extractable by our procedures, including char. For example, conversion to gas of a side chain on a liquid molecule would be counted as cracking, even if another product of this reaction is a liquid (of lower MW than the parent liquid). However, scission of one higher MW liquid into two lower MW liquids would not register as cracking unless the reaction also produced a detectable loss of liquids mass. Repolymerization means MW growth reactions that transform liquids mass into gas or inextractables. Thus, if several liquid molecules chemically combined to form a higher MW, but still liquid, product, there would be attributed to repolymerization only the net weight of liquids converted to nonliquid products, Le., char and other inextractables, and gas. To implement the two-reaction description, intraparticle liquids were assumed to consist of two lumps, one susceptible only to cracking, the other subject to both cracking and repolymerization. The latter lump was assumed to represent a fixed fraction 7 of the total amount of liquids generated up to any time t, so that

where D g is the time-dependent total amount of liquids depleted by repolymerization, and the amount of liquids subject to repolymerization and to cracking is the product of the totalamount of liquids generated up to a given time [V = V(t)l and y. To clarify the meaning and use of the parameter y, one can note the following. The two-reaction model for liquids secondary reactions assumes that all of the intraparticle liquids are susceptible to secondary cracking at high enough thermal severity but that only a fixed fraction, y, of the liquids, is susceptible to repolymerization. The model assumes that repolymerization (k2) and cracking

Bituminous Coal Pyrolysis The term (Vy - DR)/VTin eq 3 allows the total liquids concentration variable C&,t) to be used directly in the rate expressions for repolymerization and for cracking. Since all of the liquids at any time are susceptible to cracking, the term C T ( X , can ~ ) be used directly to represent the concentration driving force for cracking. However, at any time t, only is capable of participating a particular fraction of CT(Z,~) in repolymerization, and this fraction depends on the time. Therefore, to use CT(X,~)in the rate expression for repolymerization, CT(X,~)must be corrected by this fraction. The correction factor is the ratio of the absolute amount of liquids that can participate in repolymerization at that particular time to the total amount of repolymerizable liquids that have been formed up to that time. The latter quantity is the total amount of “primary” liquids generated by pyrolysis up to that time V(t), multiplied by the constant reactant fraction, y, that can repolymerize, i.e., rV(t). The numerator in this ratio is yV(t) less the total amount of liquids already depleted by repolymerization up to that time, Le., DR. Thus, the required ratio is (rv(t) - DR)/?’V(t). Intraparticle transport of liquids was modeled as liquidphase diffusion of tar through a continuum of softened ~ 0 a l . ~Substitution ~9~~ in eq 1 of a dimensionless intraparticle liquids concentration CT,a dimensionless position coordinate z = x/L, the expression for tar mass transfer, and either eq 2 or 3 for the rates of secondary reactions, gives

Energy & Fuels, Vol. 7, No. 2,1993 303 Transport Model Conceptualization

(k3) proceed independently and in parallel.

idealized Cenorphere

Actual Cenorphere 2L

Vapor Phase

Cartesian Approximation

Cr (x. t) +dM+

2L + d ~ *

Figure 6. Geometrical conceptualizationof transport model. L, transport distance (shell half-thickness);dM,vapor-phase boundintraparticle ary layer thickness for tar transport; CT(~C,~), concentration profile of tar precursor as a function of position and time.

CT = 0

(all z; t = 0) (initial condition)

(6)

acT --0

az

(all t; z = 0) (symmetry at the particle center) (7)

(flux continuity at the particle surface) (8) where C, is the dimensionless concentration of tar vapor in the ambient gas. Griffinlg assumed that this quantity is zero and that boundary condition (8)can be expressed as evaporation of tar at the particle surface followed by diffusion of the tar vapor through a gaseous boundary layer surrounding the particle

-@eT

aCT/az = (9) where @ is a surface evaporation coefficient given by respectively for the one- and two-reaction models of secondary reactions, where V* is the maximum or ultimate yield of liquids generated by pyrolysis of the coal, ki is the Arrhenius rate constant for reaction i, f(E) is a normalized Gaussian probability density function for the activation energies of thermal decomposition of the c0al,1133having a mean and standard deviation of EOand 6,respectively, DL is the liquid-phase diffusivity of tar in the molten coal, and L is the half-thickness of the shell of molten coal. The quantity DL is an apparent overall effective diffusivity for tar molecules in molten coal. Since it is estimated by best fitting the model to experimental data on tar and extractables yields, it reflects the cumulative effect of various mechanisms for transport of tar within the coal particle, including growth and coalescence of bubbles, as well as the effects of the complex composition of these liquids on their intraparticle transport. Optical microscopy datalg provide qualitative insights on the probable geometry for tar transport within particles of molten coal. Figure 6 displays a Cartesian idealization of this geometry used here to simplify the mathematical modeling. Initial and boundary conditions for eqs 4 and 5 appropriate to this idealization are

and D,is the diffusivity of tar vapor in a boundary layer of thickness i& P, , is the vapor pressure of pure tar at the particle surface, MWLis the molecular weight of the liquids at the particle surface, DL is the diffusivity of pyrolysis liquids in molten coal, R is the gas constant, T is the absolute temperature, and p~ is the mass density of the pyrolysis liquids. Griffin19discusses procedures for calculating parameters needed to implement eq 10 for coal liquids, and shows that eqs 9 and 10 can be combined to give aC,/az(z=i,t) = -b exp((E, - 8108)/TjP 2 s P C T (11) where b is a fitted constant, P is total pressure, and ED (cal/(g.mol) is related to an activation energy for tar diffusion within molten coal. The cumulative tar yield YT,from pyrolysis of the coal from time 0 up to a time t’, is computed as the time integral of the tar flux at the particle surface

We assumed that the pyridine extractables of cooled char

Griffin et al.

304 Energy &Fuels, Vol. 7, No. 2, 1993

63-75 Microns 03 Yield, Weight Fraction of Coal (OAF)

Tar Yield, Weight Fraction of Coal (OAF)

0.1

0 10

0 3 c / /

I

\\

10' 10' Heating Rate, Kis

106

Figure 8. Model predictions with single-reaction secondary reaction option (curves) and averaged tar yield data (points): heating rate and particle size effects at 1073K peak temperature. Other conditions as given in Figure 2 A and B.

0.1

n 700

n@i 900 1100 Peak Temperature, K

A 0 1300

Figure 7. Model predictions (curves) and yield data (points): pressure and temperature effects. Heating rate, lo00 K/s, hold time at peak temperature, none; cooling rate (initial), approximately 200 K/s; particle size, 63-75 pm; helium atmosphere. (A) Tar product. (B) Intraparticle liquids (extracted from particles with pyridine).

well represent the inventory of liquids within the coal particle, M. The model computes M by integrating the instantaneous liquids concentration over the entire particle volume, giving, in dimensionless form

Tar Yield, Weight Fraction of Coal (DAF)

106-125 Microns 02

0

Tar Yield, Weight 02 Fraction of Coal (DAF)

-

Without secondary reactions, liquids generated by pyrolysis of the coal, so-called primary liquids, consist of tar released to ambient plus liquids retained within the coal particle, i.e., extractables. Kinetics parameters for primary liquids production were obtained by best fitting eqs 12 and 13, respectively, to data on the combined yield of tar and extractables from experiments a t 1and 10 atm pressure, and peak temperatures below 873 K,l9s2Owhere secondary reactions are relatively unimportant. The procedure computed CT using eq 4 without its secondary reactions term, and assumed that the preexponential factor for reaction 1,klo could be fixed at 1013s-l. The resulting best fit values for the parameters EO,u, and V* are given by Griffinlg et dZo Griffinlgdescribes several cases where these parameters were used to obtain kinetics data for intraparticle transport and secondary reactions of liquids, by best fitting eqs 4 and 11 (or (5) and (11) for the two-reaction model) to various combinations of our experimental data on yields of extractables and/or tar, as affected by pyrolysis temperature, heating rate, particle size, and pressure. For a heating rate of lo00 K/s, Figure 719.20 illustrates the ability of the model to well predict pressure and peak temperature effects on yields of tar (Figure 7A), and of extractables (Figure 7B) up to about 900 K. In this case, the model was p a r a m e t r i ~ e d by ~ ~best p ~ ~fitting eqs 4 and 11to the data in Figure 7. For the one-reaction model of secondary reactions, eq 2, Figure 8 compares model prections of effects of particle size and heating rate on tar yield at a final temperature of 1073 K and 1 atm pressure. For this case, the model was parametri~ed'~ using data on tar and extractables yields a t all peak temperatures and 1and 10atm pressure,

01-

1

I

I

1

Bituminous Coal Pyrolysis

In these circumstances, the model would be made more reliable by explicitly accountingfor the spatial dependence of the coal temperature during pyrolysis. The hoped for improvements in predictive capability would need to be sufficiently valuable to justify the increase in model complexity and computation time to obtain results. Case No. 2 was parametrized by fitting eqs 5 and 11to the same data set as was used in Figure 8.19 The resulting best-fitted values for the activation energies for secondary reactions E2 and& (49.9 and63.5 kcal/mol) are reasonable for repolymerizationand cracking,respectively. The bestfitted value of y (0.44) suggests that almost half of the liquids formed by primary pyrolysis of the coal are susceptible to repolymerization. This is consistent with Fong et al.’s observation that roughly one-half of the maximum amount of intraparticle extractable5 formed by rapid pyrolysis (heating rate about 600 K/s) of this same coal type, was transformed to nonextractable material upon continued heating for a few s e c o n d ~ . ~ JThis ~ J ~case used a fixed EDvalue, 2.8 X lo4K, which is a best-fitted value obtained in the fit to the data for Figure 7 noted above. This value corresponds to an apparent activation energy for effective diffusion of about 56 kcal/mol, which is much higher than activation energies typical of approximating molecular diffusion as an activated process. It may arise because, for mathematical simplicity, the model forces all processes contributing to volatilestransport within molten coal, e.g., bubble growth and coalescence, see Oh16et al.,17 to be described by one global diffusion coefficient.

Conclusions Heating rate and particle size can affect yields, compositions, and release rates of volatiles from pyrolysis of softening bituminous coal. The sign and magnitude of specific effects depend upon the particular value of both

Energy & Fuels, Vol. 7, No. 2, 1993 305

of thesevariables, as well as on temperature. For example, for 63-75-rm particles, tar yield from 1073 K pyrolysis increased steadily as the heating rate was increased over more than 3 orders of magnitude (from 10 to 20 000 K/s), but for 106-125-pm particles pyrolyzed to 1073or 1273K, tar yield plateaued or maximized a t about lo00 to 2000 K/s. At 1073 K, tar yields were virtually the same at 10 K/s for both size ranges, but larger for the 63-75-pm particles at all higher heating rates up to 20 000 K/s. Thus, experimental and mathematical analyses of heating rate and particle size effects in coal pyrolysis will be more informative when interactive effects of these two variables, as well as of temperature, are studied. Mechanistically, heating rate and particle size effects on coal pyrolysis arise from their ability at certain temperatures to facilitate or inhibit intraparticle transport and secondary reactions of thermally labile liquids generated by thermal decomposition of the coal. The reliability of mathematical models for predicting these effects may depend strongly on the sophistication with which the model treats intraparticle secondary reaction kinetics as well as on the data sets and adjustable constants used to parameterize the m0de1.l~

Acknowledgment. We thank the U.S.Department of Energy, Morgantown Energy Technology Center, for financial support of this work under Contract No. DERA21-85MC-22049, and Drs. Justin Beesom, Richard Johnson, and James Longanbach for serving as our technical project officers. We also thank Drs. James Freihaut, Mohammad Hajaligol, Arthur L d e u r , Thomas McKinnon, and Costas Tsonopoulosfor valuable technical inputs. Fellowship support for T.P.G. from the MIT Department of Chemical Engineering and the MIT School of Chemical Engineering Practice is gratefully acknowledged.