Energy & Fuels 1992, 6,4Q-47
40
1. Supercritical water is a better fluid for hydrocarbon extraction and oxygen removal from coal than supercritical toluene. 2. The higher efficiency of supercritical water extraction may be due to the ability of water to participate in hydrolysis reactions leading to increased product formation. 3. Both oxygen removal and total hydrocarbon conversion reactions are second order in supercritical toluene and water. This does not, however, imply that similar
mechanisms are involved in net oxygen removal and hydrocarbon extraction from coal. 4. Chemical bonds of higher strengths are ruptured with higher degrees of total conversion or product formation.
Acknowledgment. Financial support for this work was provided by the Natural Sciences and Engineering Research Council (NSERC) of Canada and is gratefully acknowledged.
Structure and Reactivity of Char from Elevated Pressure Pyrolysis of Illinois No. 6 Bituminous Coal Chun Wai Lee,+Robert G. Jenkins,*J and Harold H. Schobert Fuel Science Program, Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802 Received November 15, 1990. Revised Manuscript Received September 30, 1991 Illinois No. 6 bituminous coal was pyrolyzed in entrained-flow reactors at 0,100,309, and 530 psig. Char reactivities were determined by thermogravimetry in air. Chars generated at 100 psig were more reactive than those produced at atmospheric or at higher pressures. The movement of carbon layers, and hence the generation of active sites, varies with pyrolysis pressure. At atmospheric pressure, the low fluidity of the coal generates few active sites. The improved fluidity at 100 psig increased the number of active sites; however, at even higher pressures the greater ordering of the char structures again reduces the number of active sites. The development of microporosity in the chars was also dependent on pressure. At 309 psig, the realignment of the carbon layers reduced the microporosity relative to that in char produced at 100 psig. The original mesopore system of the coal collapsed during the plastic stage of pyrolysis. The amount of new mesopores in the char decreased with increasing pyrolysis pressure. Thus the applied pyrolysis pressure affects the reactivity of the resulting char by influencing the number of active sites (via structural alignment of carbon layers) and by affecting the porous structure of the char.
Introduction The objective of this study was to investigate the influence of pressure on the reactivity and structure of a rapidly pyrolyzed caking bituminous coal. In two previous papers we have discussed the effects of pressure on devolatilization behavior' and on mechanisms and kinetics of pyrolysis2of an Illinois No. 6 high-volatile B bituminous coal pyrolyzed at heating rates of -lo4 K/s in a nitrogen atmosphere at pressures up to 530 psig and residence times up to 1.7 s. Increasing the applied pressure delayed devolatilization, lowered the asymptotic volatile yields, and promoted secondary reactions of the volatiles.' The effect of the enhanced secondary reactions was to decrease the tar yields and increase the gas yields. The effects of pressure on the secondary reactions of volatiles were different depending on whether the reactions occurred within the pyrolyzing coal particle or external to it. Pressure influences the devolatilization rate and the swelling rate in different ways. Enhanced char reactivity will result from reactions during coal pyrolysis that produce highly disordered chars, giving a greater number of active sites; more open pore Present address: U.S. Environmental Protection Agency, Research Triangle Park, NC 27711. Present address: College of Engineering, University of Cincinnati, Cincinnati, OH 45221-0018.
*
Qsa7-0624/92/2506-oo40$03.00/0
structures, giving a higher accessibility to the active sites; and better dispersion of the catalytic i m p ~ r i t i e s . The ~~ structure, and resultant reactivity, of the char are determined predominantly by the rank of the parent coal and the pyrolysis conditions, such as heating rate, final temperature, and soak time at t e m ~ e r a t u r e . ~Reactivities ?~ of bituminous coal chars had little correlation with micropore surface area7 Thermoplasticity reduces the microporosity development, while an insignificant weight loss during secondary devolatilization induces a large increase in microporosity.8 A rapid increase in C02 surface area is accompanied by a rapid drop in residual volatiles and reactivity during secondary devolatili~ation.~Residual volatiles may play a more important role than the microporosity in determining the reactivity of bituminous coal chars? Chars produced from bituminous coals of the same (1) Lee, C. W.; Jenkins, R. G.; Scaroni, A. W. Fuel 1991, 70, 957. (2) Lee, C.W. Jenkins, R. G.; Schobert, H. H. Energy Fueb 1991,5, 547. (3) Walker, P. L., Jr.; Rusinko, F., Jr.; Austin, L. G . In Advances in Catalysis; Eley, E. A., Ed.; Academic Press: New York, 1957;Vol. XI, p 133ff. (4) Laurendeau, N. M. B o g . Energy Combust. Sci. 1978,4, 221. (5)Essenhigh, R. H.In Chemistry of Coal Utilization; Elliott, M. A,, Ed.; Wiley: New York, 1981;2nd Suppl. Vol., pp 1153ff. (6)Walker, P.L., Jr. Fuel 1981,60, 801. (7)Maloney, D. J. Ph.D. Dissertation, The Pennsylvania State University, 1983. (8) Tsai, C. Y.; Scaroni, A. W. Fuel 1987,66, 200. (9)Tsai, C. Y.; Scaroni, A. W. Fuel 1987, 66, 1400.
0 1992 American Chemical Society
Structure and Reactivity of Char ASTM ranks can exhibit remarkably different reactivities.1° For example, chars derived from bituminous coals of low vitrinite reflectance have more pore volume than those from coals of high vitrinite reflectance, the higher pore volume resulting in increasing acessibility for the reactant gas.'O The influence of pyrolysis conditions on the reactivity and structure of chars produced in pyrolysis at atmospheric pressure has been well studied. The reactivity of chars produced from the rapid pyrolysis of low- and high-volatile bituminous coals decreases by a factor of 2 as pyrolysis temperature is increased from 1073 to 1273 K (ref 11). Increasing the soak time at 1273 K from 0.23 to 3600 s caused a 7-fold decrease in char reactivity.'l The reactivities of bituminous coal chars are strongly influenced by the amount of hydrogen remaining in the char, since this property correlates well with the severity (temperature and soak time) of the pyrolysis conditions." Increasing the pyrolysis temperature from 1000 to 1600 K for three bituminous coals in a drop-tube reactor led to a decrease of factors of 2 to 2.5 in the H/C ratio of the resulting char.12 The distributions of pore volume and surface area changed from bimodal to trimodal with the creation of porosity in the transitional pore range. The oxidation rate of the char produced at 1600 K was decreased by 60%. Porosity generation resulting from pyrolysis is a balance between additional pore volume created by release of volatiles and loss of pore volume caused by enhanced alignment of the planar aromatic structures in the char.13 Rapid heating is more favorable for volatiles release than for alignment. Enhanced thermoplastic properties of caking coals can result from increased heating rate.14 Reduced crosslinking reactions favor thermoplasticity development at rapid heating ~0nditions.l~Although much has been accomplished in studying the influence of pyrolysis conditions at atmospheric pressure, there is a lack of data for elevated pressures.
Experimental Section Coal Sample. The coal used in this study was an Illinois No. 6 sample supplied by the Morgantown Energy Technology Center of the U.S.Department of Energy. It is a high-volatile B bituminous coal. Composition and properties of the coal were reported previously.2 Prior to receipt, the coal had been ground and sieved to obtain a 200 X 270 mesh fraction. After receipt in our laboratory, the coal was repackaged in 500-mL plastic bottles under nitrogen. High-pressure Microdilatometry. Thermoplastic properties at elevated pressures were investigated by high-pressure microdilatometry (HPMD). Detailed information on the design, construction, and operation of the apparatus has been published elsewhere.16 A 75mg coal sample is placed in a silica sample vial, and a displacement probe is placed on top of the sample. A furnace is placed around the sample holder and the entire assembly is then e n c l d within a pressure vessel. After the system is pressurized to the desired pressure, heating is begun using a temperature programmer/controller. The vertical displacement of the probe, resulting from changes in the sample volume, is recorded as a function of temperature on an X-Y plotter. For (10)Kim, S.T. M.S. Thesis, The Pennsylvania State University, 1988. (11)Maloney, D.J.; Jenkins, R. G. Fuel 1985,64,1415. (12) Sahu, R.;Levendis, Y.A.; Flagan, R. C.; Gavalas, G. R. Fuel 1988, 67. - , 275. - -
(13)Nsakala, N.; Essenhigh, R. H.; Walker, P. L., Jr. Fuel 1978,57, 605. (14)Loison, R.;Pey-hvy, A,; Boyer, A.; Grillot, R. In Chemistry of Coal Utilization;Lowry, H. H., Ed.; Wiley: New York, 1963;Suppl. Vol., pp
Energy &Fuels, Vol. 6, No. 1, 1992 41 the work reported here, HPMD measurements were performed at pressures of 0,100,178,309,530, and 900 psig in He and H2 atmospheres at 80 and 160 OC/min heating rates. Entrained-Flow Pyrolysis. The entrained flow pyrolysis was conducted in two reactors, one operatingat atmospheric pressure, and the other at elevated pressure. Except for the differences necessary to accommodate operation at high pressure, the fundamental design and principles of operation were the same for both reactors. Char sampleswere prepared by pyrolyzing the coal in a N2 atmosphere at 1189 K. Pressures of 100,309, and 530 psig were used in the high-pressure entrained-flow reactor, and 0 psig in the atmospheric-pressure reactor. The apparatus and methods of recovery of char samples were described in previous papers.'S2 Scanning Electron Microscopy. The morphologies of chars were examined by electron microscopy. The instrument used as an Internahonal Scientific Instruments Model SX-40, with a 15-kV electron beam. The particle surfaces were coated with a 20-nm gold film by vacuum deposition. Surface Area and Density. Macropore and transitional pore surface areas were determined by nitrogen adsorption at 77 K using a Carlo Erba Sorptomatic 1800gas adsorption apparatus. The char sample was outgassed at 383 K ovemight at 1"a. At least five points were taken on each isotherm over a relative pressure range of 0.05-0.35. Surface areas were calculated using the BET equation. Micropore surface areas were determined from C02adsorption at 298 K using the same instrument. More than 10 points were taken on each isotherm over a relative pressure range from 0.0002 to 0.15. Micropore surface areas were calculated from the isotherm using the Dubinin-Polanyi equation. The densities of char samples were determined by measurement of volumes in the presence of helium, using a Quantachrome stereopycnometer. Char Reactivities. Char reactivities were determined in air using a thermogravimetric method. A Perkin-Elmer TGS-2 thermogravimetric analyzer (TGA) was used. The system was purged with Nzfor 10 min and then heated to the reaction temperature, 683 K. The sample was held at temperature until the weight was constant, which usually required less than 15min. The gas flow through the instrument was then switched from N2 to air at a flow rate of 100 mL/min, and changes in sample weight were monitored over a period of time. The char reactivities were expressed in two forms. One is an averagedreactivity parameter, R , (ref 9), and the other is T ~ . the ~ , time required for 50% burn-off." For R,, the burn-off curve was divided into several arbitrary time segments and R , was calculated from RW% = C ( R u A w ) / C A ~ where R , is the instantaneous reaction rate with respect to unreacted char and Aw is the weight loss occurring within each interval. R, is calculated from R , = (l/w,)(dw/dt) where w, is the weight (daf basis) char unreacted at time t and dw/dt is the slope of the bum-off curve at t. The subscript x % indicates the burn-off range over which R, is determined. In this study x was taken as 50%; that is, the values of R , were summed from 0 to 50% bum-off. Thus the values of thisreactivityreported are expressed as R1450W and are a measure of the early part of the burn-off curve.
Results and Discussion Characterization of Thermoplasticity. The characteristic thermoplastic properties of the Illinois No. 6 coal, measured in helium, are reported in Table I. Comparable data obtained in a hydrogen atmosphere are reported in Table 11. Although the pyrolysis work reported previ0us1y'*~was performed in nitrogen, helium was used as an inert gas in the HPMD because He and H2 have similar thermal diffusivities. Reproducible linear heating rates at high pressures can only be attained in the HPMD apparatus using He or H2 as the gas; the high thermal dif(17)Mahajan, 0.P.; Yanab, R.; Walker, P. L., Jr. Fuel 1978,57,643.
Lee et al.
42 Energy & Fuels, Vol. 6,No. 1, 1992 Table I. Characteristic Thermoplastic Parameters of Illinois No. 6 Coal Measured in He Atmosphere 100 178 309 0 530 900 applied press., psig 80 80 80 80 160 160 80 80 160 160 160 heating rate, "C/min 386 372 351 450 428 451 417 326 320 402 415 softening temp, "C 518 520 516 582 578 575 508 513 563 610 579 contraction temp, O C 550 544 549 551 550 616 625 N.D." 627 630 665 maximum swelling temp, "C 574 572 569 565 N.D. 632 651 N.D. 641 642 685 resolidification temp, "C 43 48 46 49 29 30 46 48 33 34 48 initial contraction volume, % 33 58 92 114 43 64 26 115 90 133 0 maximum swelling volume, % 5 42 10 26 -15 61 43 -32 78 96 -16 volume change on resolidification, %
160 352 540 607 628 40 124 84
"N.D. = not defined.
Table 11. Characteristic Thermoplastic Parameters of Illinois No. 6 Coal Measured in H2Atmosphere 0 100 178 309 530 900 applied press., psig 160 80 160 80 160 80 160 80 80 160 80 heating rate, "C/min 429 375 421 368 405 364 400 350 394 440 382 softening temn "C 585 534 574 516 582 524 585 514 543 589 536 contraction t e i p , "C 619 560 616 551 648 568 620 563 625 564 maximum swelling temp, " C N.D." 651 585 616 551 668 568 620 563 543 625 584 resolidification temp, OC 31 41 34 46 44 32 46 36 30 initial contraction volume, % 40 44 65 0 12 30 46 56 65 116 maximum swelling volume, % 58 118 56 12 34 -40 -34 -14 2 25 80 22 92 volume change on resolidification, % 14
160 378 561 611 611 48 64 64
" N.D. = not defined. fusivity of the gas is necessary for heating rates >50 "C/min. Heating rate has a significant effect on thermoplastic properties. In general, increasing the heating rate shifts the plastic transformations, expressed as percentage changes in volume of the original sample, to higher temperatures and to greater magnitudes. For example, the maximum swelling (V,) in 309 psig of H2 is 65% at a heating rate of 80 OC/min; however, increasing the heating rate to 160 "C/min increases V, to 116%. These observations with Illinois No. 6 coal are consistent with earlier HPMD results.18 The increase in resolidification temperature and decrease in softening temperature as applied pressure increases results in a net increase in the plastic range. For example, at 100 psig He and a heating rate of 160 "C/min, a plastic range of 182 "C is observed, but this value increases to 270 "C when the pressure is increased to 530 psig. An increase in plastic range with increasing pressure has been noted by other investigators for heating rates of 1OC/s (refs 18 and 19). Increased plastic ranges with increasing pressure for very high heating rates (103 "C/s) have also been reported.20 Pyrolysis under elevated pressures increases the residence time of the plasticizing materials generated by the pyrolytic reactions and causes the resolidification reactions to occur at higher temperatures. The effects of pressure and gas atmosphere are shown in Figures 1and 2 for heating rates of 80 and 160 "C/min, respectively. Applied pressure has a significant effect on softening temperature (T,).This effect is particularly noteworthy at the lower heating rate in inert atmosphere. In this case T, drops from about 400 to about 320 "C when the applied pressure is increased from 0 to 900 psig. The corresponding change in hydrogen atmosphere is much less, from 395 to 350 "C. The retention of the hydrogen-donating materials important for plasticity development can be influenced by the applied pressure.z1 The bitumens22or mobile phasez3 (18) Khan, M. R. Ph.D. Dissertation, The Pennsylvania State University, 1985. (19) Kaiho, M.; Toda, Y. Fuel 1979, 58, 397. (20) Lowenthal, G.; Wand, W.; van Heek, K. H. Fuel 1986, 65, 346. (21) Neavel, R. C. In Coal Science; Gorbaty, M. L., Larsen, J. W., Wender, I., Eds.; Academic Press: New York, 1982; Vol. 1, pp Iff. (22) Dryden, I. G. C. In Chemistry of Coal Utilization;Lowry, H. H., Ed.; Wiley: New York, 1963; Suppl. Vol, p 232ff.
400
\
He Atmosphere 320
3001 0
200
400
800
Mx)
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Applied Ressure (psig)
Figure 1, Effect of pressure on softening temperature (T,) of Illinois No. 6 coal measured at 80 "C/min heating rate, in hydrogen and helium atmospheres.
3401 0
,
,
,
, 200
,
.
,
, 400
,
,
,
, 6M)
.
.
,
,
.
,
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.
1
loo0
Applied Ressun @sig)
Figure 2. Effect of pressure on softening temperature (T,) of Illinois No. 6 coal measured at 160 OC/min heating rate, in hydrogen and helium atmospheres.
probably serve as the initial vehicle that solvates and "lubricates" the aromatic units as they become loosened thermally. A comparatively high hydrogen content allows (23) Given, P. H. B o g . Energ. Combust. Sci. 1984, 10, 149.
Structure and Reactivity of Char
Energy & Fuels, Vol. 6, No. 1, 1992 43
120,
I
.-1
0
0
.
0
Applied Pressure (psig)
i
.
.
.
, 200
.
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,
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,
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.
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.
.
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.
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.
50
.
.
,
60
Weight Loss (8daf Coal)
Figure 3. Effect of pressure on maximum swelling volume (V,) of Illinois No. 6 coal measured at 80 “C/min heating rate, in hydrogen and helium atmospheres.
0, 0
I
.
.
.
, 800
.
.
, loo0
Figure 4. Effect of pressure on maximum swelling volume (V,) of Illinois No. 6 coal measured at 160 “C/min heating rate, in hydrogen and helium atmospheres. the bitumens to serve as mobile hydrogen donors to stabilize thermally generated free radicals before plasticity begins. Once the coal begins to soften as a general mobility of the aromatic units starts, free radicals generated by thermal cleavage of the macromolecular network of the coal structure may, at that time, have a higher probability to accept available hydrogen from elsewhere on the macromolecular network, so that the original bitumen or mobile phase would be of less consequence. Without the initial vehicle, however, there would have been no solvating and hydrogen-donating substance to stabilize the highly reactive free radicals generated at the very beginning of thermal decomposition of the coal. If in fact there had been no solvating or donating material initially present, any bond cleavage at temperatures preceding a general softening of the coal would have led to repolymerization by radical recombination, such that plasticity could never begin. The retardation of the bitumen loss by increased applied pressure enhances the development of fluidity and the lowering of the softening temperature via the retention of bitumens in the coal. The effect of pressure on maximum swelling is illustrated in Figures 3 and 4. At 80 OC/min, V,increases from zero to a maximum of 110% when the applied He pressure increased from 0 to 530 psig. Swelling then decreases at higher pressures. In Hz, swelling reaches a maximum of 60% at 309 psig and remains relatively constant at higher pressures. V,values in the two different atmospheres are
Figure 5. N2surface areas of samples of pyrolyzed Illinois No. 6 coal as a function of weight loss at 1189 K. The sample at 0% weight loss is the unreacted coal. almost identical up to 200 psig. In the higher heating rate case, V, in Hz and He are similar to -300 psig and are similar to values measured at the lower heating rate. However, at higher pressures, V, at the higher heating rate is greater than that measured at 80 “C/min. Swelling is a maximum at 530 psig and decreases slightly when the pressure is raised. An initial increase of applied pressure (i.e., from 0 psig) improves the fluidity of the coal. This pressure increase increases the retention of volatiles, which act as plasticizer to enhance the fluidity of the plastic mms. Consequently, swelling increases when the applied pressure is increased to certain levels. However, as pressure increases further, the counterbalance to the internal swelling pressure by the external applied pressure then becomes more significant than the effect of pressure on improving fluidity, and swelling will decrease. There appears to be an optimum fluidity associated with the maximum swelling at any given condition. If the fluidity is low, the plastic mass is too viscous for swelling to occur. On the other hand, if fluidity is high, the volatiles can easily pass through the plastic mass without generating significant swelling pressure. The swelling being lower in H2than in He may be due to an “overfluidity” resulting from hydrogenation. Not only would a high pressure of hydrogen prevent the escape of primary volatiles, but it might also react with them to produce a greater quantity of fluid products or to improve the fluidity of the existing liquids. The more profound effect of H2 vis-a-vis He at the lower heating rate suggests that more hydrogenation is occurring due to the longer heating time (e.g., Figure 3). As the heating rate increases, hydrogenation decreases because the total heating time is reduced and thus the effect of gas atmosphere can be observed only at very high pressure (e.g., Figure 4). Internal Structure of Chars. An SEM study reported previously illustrated the variation of char morphology as a function of pyrolysis conditions.’ Optical image analysis, also reported previously,’ showed that the change in size and shape of the char particles is also determined by pyrolysis conditions. These changes are closely associated with fluidity development during pyrolysis. In addition, the internal structure of the chars is also influenced by pyrolysis conditions. The importance of the internal structure is that it is a key parameter in determining the subsequent gasification potential of the chars. Figure 5 shows the effect of weight loss on Nzsurface area of the chars. Pyrolysis at any pressure results in a reduction in Nz surface area. However, as weight loss increases, there is a slight increase in area occupied by the
Lee et al.
44 Energy & Fuels, Vol. 6, No. 1, 1992 Table 111. Weight Loss, Helium Densities, and Surface Areas of Char Samples helium density, N2 surface COBsurface Dress. residence wt loss.. -, e/cm3 area, m2/e-* area, m2 psig time, s 50' (daf) (daf) (daf cha;) (daf char) 0.0 raw coal 1.29 16.6 170 6.1 270 54 1.56 0.0 0.1 6.8 380 51 1.58 0.0 0.3 4.8 230 32 1.45 100.0 0.3 5.5 294 43 1.46 100.0 0.8 5.8 332 42 1.49 100.0 2.0 6.0 365 45 1.55 100.0 1.7 0.6 370 22 1.44 309.9 0.5 2.3 302 38 1.45 309.0 0.8 280 4.2 39 1.46 309.0 1.0 2.6 332 47 1.50 309.0 1.7 2.8 228 35 1.45 530.0 1.0 .
+
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25
3
30
35
40 O 45
50
55
60
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Weight Loss (B dnf Coal)
I .8
I
I
1 0 0 0
0 0
Figure 7. Changes in C02 surface area as a function of weight loss for pyrolysis of Illinois No. 6 coal at 1189 K in a nitrogen atmosphere. The sample at 0% weight loss is the unreacted coal.
inhibition mechanism which retards the evolution of volatiles or via secondary reactions. Thus in the case of a given residence time, as pressure is increased the chars become more hydro-carbonaceous, leading to a lower helium density. The change in surface area as a function of weight loss is shown in Figure 7. The correlation between COzsurface area and weight loss indicates that the major development of microporosity occurs at the later stages of pyrolysis, when more than half the devolatilization has been completed. A rapid rise in COz surface area between 25 and 45% weight loss has been observed for lignitez4 and high-volatile bituminous ~ o a l s . ~ ~ ~ ~ Comparison of the microporosity development and swelling behavior indicates that microporosity development is influenced by applied pressure. For atmospheric pressure pyrolysis, COz surface area increases rapidly between 0.1 and 0.3 s when devolatilization and swelling are not significant. At 100 psig, a rapid increase of particle diameters is observed between 0.3 and 1.0 s, but the increase in COz surface area over the same residence time range is moderate. A large increase in volume due to swelling may be expected to cause a corresponding increase in surface area; however, most of the volume increase on dilation results from pores formed by large gas bubbles. Since most of this porosity is in the form of large pores, increases in surface area are relatively small. At 309 psig, a rapid increase in C02 surface area occurred between 0 and 0.5 s. As the residence time increases further, the coal becomes sufficiently fluid to suppress swelling under an external applied force. The high fluidity also enables the aromatic layers formed in the char to achieve better alignment, with a consequence of reducing surface area. Char Reactivity. Reactivity of the char samples was determined in 1 atm of air. The change in weight of the char as a function of time on exposure to air at 683 K was followed. The reactivity measurements were performed at relatively low temperatures in air because of the relatively high gasification rates of carbons in air. At a temperature as low as this, the heterogeneous char gasification rate is maintained below the region where the reaction is dominated by mass transfer. Representative chemical reactivities of the chars can therefore be determined. The
1
1.2 0
10
20
M
40
50
60
Weight Loss (% daf Coal)
Helium densities of samples of pyrolyzed Illinois NO. 6 coal as a function of weight loss at 1189 K. The sample at 0% weight loss is the unreacted coal. Figure 6.
larger pores. Table I11 indicates no real trend with pressure. The reduction in Nz surface area from the unreacted coal to the char indicates that the mesopores of the coal particle collapse as a result of plastic deformation during pyrolysis. The BET surface area is considered to be associated mainly with the mesopores and larger micropores, the macropores having relatively little significant contribution.l8 A new mesopore system with negligible surface area is developed during the release of volatiles. Changes in the mesopore system during pyrolysis are determined by whether the coal has entered a plastic stage. The results of helium density measurements, reported on a dry, ash-free basis, are shown in Table 111. Two general trends are evident: the density of char increases with increasing residence time and decreases with increasing pressure. The density of the unreacted coal is significantly lower than those of the chars, reflecting the higher carbon content and more condensed aromatic structure of the chars. The decrease in density with increasing pressure is attributed to the pressure dependence of the realignment of carbon layers during pyrolysis. The fluidity of the plastic coal improves as pressure increases; consequently the carbon layers have more mobility to align and produce a more ordered char structure. The helium densities are shown as a function of weight loss in Figure 6. A t any given pressure the influence of residence time (and, of course, the influence of increasing weight loss, which depends on residence time) is to make the chars more carbonaceous,and more ordered, thus increasing the helium density. On the other hand, as the pressure increases, pyrolysis is apparently retarded, either via an
(24) Reuther, R. B. Ph.D. Dissertation, The Pennsylvania State University, 1988. (25) Tsai, C. Y. Ph.D. Dissertation, The Pennsylvania State University, 1985.
Structure and Reactivity of Char
Energy & Fuels, Vol. 6, No. 1, 1992 45
Table IV. Comparison of Air Reactivity Parameter Char SamDles Measured at 683 K pressure, psig residence time, s min
of
~~
0.0 0.0 100.0 100.0 100.0 100.0 309.0 309.0 309.0 309.0 530.0
0.1 0.3 0.3 0.8 1.0 1.7 0.5 0.8 1.0 1.7 1.0
21.2 26.3 10.8 12.1 14.7 20.5 13.8 14.3 15.3 20.2 16.5
low temperature can also minimize changes in char structure, such as thermal annealing of active sites, which could alter the gasification behavior. It is well known that char structure changes continuously during TGA reactivity measurements, and the burn-off vs time curves for most chars derived from coals of a wide range of ranks exhibit similar shapes.26 The initial slow increase in the slope of the TGA plot is due to the opening of closed pores and enlarging of existing pores to increase the accessibility of reactant gas during the induction period. The slope increases as burn-off increases until a maximum is reached. Then the slope starts to decline, slowly decreasing to complete burn-off. The decrease in slope results from decreases of amount of gasifiable material and from lower reactive area due to the coalescence of pores as burn-off proceeds. Because the structure is changing during the reactivity measurement, some assumptions are necessary to define char reactivity. The time required to achieve a 50% burn-off, 70.5,is a reasonable parameter to correlate reactivity data for gasification rates in air, COz, H2, and steam." The reactivity parameters, 70.5, are presented in Table IV. The dependency of reactivity on pyrolysis residence time and pressure is indicated by these data. Chars produced at atmospheric pressure have much longer burn-off times than those generated a t elevated pressures, and bum-off times increase with increasing pyrolysis residence times for most chars. For char produced at atmospheric pressure 70.5 increases from 21.2 to 26.3 min as pyrolysis residence time increases from 0.1 to 0.3 s; i.e., the chars become less reactive as pyrolysis proceeds. Chars produced at 100 psig have significantly lower 70.5 than those of chars generated at atmospheric or at >lo0 psig pressures. At comparable pyrolysis residence times, chars produced a t 100 pig are more than twice as reactive as those produced at atmospheric pressure. Pyrolysis at 309 psig also produces chars of lower reactivity. At any pressure, as pyrolysis residence time increases the reactivity, 70.5, decreases. That is, the char becomes less reactive as pyrolysis proceeds. At a constant residence time (0.8 or 1.0 s) there is a slight decrease in reactivity with increasing pressure, although for residence times of 1.7 s the reactivity is unchanged as pressure increases. Figure 8 shows a comparison of the values of Ra,501 for the chars generated a t different pyrolysis conditions. Chars generated at atmoepheric pressure have significantly lower Ra,50%than those of chars generated a t elevated pressures. The Ra,5046of char produced a t 100 psig decreases with increasing residence time. The char produced at 0.5 s residence time and 309 psig has a of 3.0 mg/(mg.h),but the reactivity decreases for chars produced at longer residence times. For the short residence time chars (
