Time-resolved weight loss kinetics for the rapid devolatilization of a bituminous coal

Nineteenth Symposium (International) on Combustion/The Combustion Institute, 1982/pp. 1151-1157

TIME-RESOLVED WEIGHT DEVOLATILIZATION

LOSS KINETICS FOR THE OF A BITUMINOUS COAL

RAPID

STEPHEN NIKSA* W. B. RUSSEL D. A. SAVILLE Princeton University Department of Chemical Engineering Princeton, New Jersey 08544 U.S.A. The weight loss kinetics for the rapid devolatilization of a bituminous coal were measured over broad ranges of temperature (to 1000 C) and heating rate (102 to 104 K/s) in vacuum. The reaction period was resolved in 0.1 s time increments with rapid quenching. In addition, the effects of pressure (13.3 Pa-10 MPa) and particle size (50, 85, and 125 }xm) were studied. We found that the rate of generation of volatiles increases slowly with temperature and is relatively insensitive to pressure changes. However, the heating rate does affect the devolatilization rate and the total yield. Below 600 C, two stages of product evolution wei'e observed, one with and the other without coliberation of tarry liquids. At higher temperatures these two stages coalesced. Although the concept of simultaneous, parallel reactions can account for the low apparent activation energy of the first stage of the devolatilization process, a multiple reaction model with a Gaussian activation energy distribution does not adequately describe the total timetemperature history. In addition, extant models with competitive rate processes to describe the redeposition of volatiles and their escape from the particle seem incompatible with the effects of particle size and heating rate observed in vacuum.

Introduction In several analyses of rapid coal devolatilization, I-4 competition between the rates of formation, escape and redeposition of volatile matter and direct char formation is used to explain the influence of processing conditions and coal type. The details of these schemes differ and often are in conflict with changes observed in the physical structure of the coal particles during pyrolyses. This suggests, at the very least, a need for more thorough testing with detailed experimental data. Weight loss and product distributions for a range of conditions have been measured using captive sample devices, 5'6 but the presence of significant devolatilization during the slow cooling tends to obscure some kinetic processes. Entrained flow reactors can also be used to resolve kinetic processes on short time scales with rapid quenching but most studies involve very high heating rates, high temperatures

*Now at Sandia Laboratories, Livermore California 94550, USA.

and near atmospheric pressures. 7-1~ We have developed a captive sample device which can cover broad ranges of heating rate, temperature and pressure and still resolve events on a fine time scale. The device employs an electronic controller to monitor and control the sequence of events and is equipped with a system for quenching the sample rapidly. The experiments reported here provide data covering a range of conditions to help unravel the kinetics of devolatilization. The weight loss measurements from a HVA bituminous coal presented here include heating rates between 10~ and 104 K/s, temperatures to 1000 C, and reaction times resolved into 0.1 s increments. Analyses of escape rates based on diffusion or bulk flow indicate that the decomposition reactions should control in vacuum. Hence, heating rate, temperature, and reaction time were varied over wide ranges in vacuum to characterize the chemical kinetics. Mass transport limitations were evaluated by varying particle size between 50 and 125 p~m (in vacuum) and by examining ultimate yields between 13.3 Pa (0.1 torr) and 10 MPa (100 atm) in helium. Transient weight loss data were also obtained at 0.2 MPa and 3.1 MPa (2 and 31 atm).

1151

1152

COAL COMBUSTION MECHANISMS AND PYROLYSIS TABLE I Coal analyses, Pittsburgh seam no. 9 HVA bituminous (PSOC-102)

% % % %

Moisture Ash Vol. Matter Fixed Carbon

Proximate analysis*

Ultimate analyses

Particle diameter, ~m 50 85 125

Particle diameter, I~m 50 85 125

0.08 6, 77 28.08 65.07

<0.01 6.08 27.94 65.98

0.65 5.86 26.94 66.50

% % % % % % % %

Moisture Ash Carbon Hydrogen Nitrogen Sulphur Chlorine Oxygen (by diff.)

0.08 6.77 75.59 5.13 1.49 2.34 0.11 8.4

<0.01 6.08 76.99 5.30 1.38 2.18 0.07 8.0

0.65 5.86 75.87 4.84 1.51 1.88 0.04 9.3

*Prepared by Galbriath Laboratories, Knoxville, TN. The proximate volatile matter listed here differs by several percent from the analysis supplied with the sample from the Penn State Data Base.

Experimental Conditions A Pittsburgh Seam No. 9 HVA bituminous from the Penn State Data Base (PSOC-102) was used in each experiment. The original sample was ground in a N z atmosphere in a disk grinder installed within a glove box using a few grams of sample and several ounces of liquid nitrogen to cool and blanket the ambient surfaces during grinding. Narrow size fractions were prepared in a RO-TAP with standard sieves. Moisture was removed by holding the coal in an evacuated oven at 100-104 C for several hours. The bulk of the sample was kept in a dessicator and samples for the experiments were dispensed under nitrogen. Ultimate and proximate analyses are shown in Table I. The analyses for the three different particle sizes are essentially the same, and the ultimate analyses are consistent with both the general rank of the coal and with the Penn State analysis. The proximate volatile matter contents of 27-28%, however, are lower than the analysis from the Penn State Data Base and differ by at least 5% from the typical level for all HVA bituminous coal. No explanation for this discrepancy is yet available. The microsample strip furnace used in this study features precise temperature programming, rapid quenching, a flowing gas stream, and reliable thermometry from a fixed point on the wire mesh heating element. The system and the associated analyses and procedures are described elsewhere 11 12 so only a general outline is given here. The reactor cell appears in Fig. la. A small sample of coal (ca. 15 mg) is spread evenly within a folded piece of stainless steel wire mesh and sealed in a pressure vessel. The sample is then subjected to a precise temperature program by heating the support electrically. Reaction gas, preheated to 400 C, carries volatile reaction products away from

the hot surfaces. Electrical power is supplied first at constant current to ensure nearly uniform heatup and then at constant voltage to maintain an independent reaction temperature. Switchover between modes is accomplished electronically, eliminating overshoot after heat-up. After the isothermal ,S,~4PLEELEW~NT

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RAPID DEVOLATILIZATION OF BITUMINOUS COAL reaction period the support is quenched in less than 0.1 s by spraying with cold nitrogen from below. The quench stream flows against no back-pressure because the system depressurizes rapidly through a large-orifice solenoid syncronized with a valve in the quench line. A synchronous programmer controls the duration of the heating cycle and activates all valves in the gas handling systems to coordinate preheating of the reaction gas, depressurization, and quenching. Temperature programs are measured with thermocouples (Type K, 50 Ixm wire) welded to the support and recorded on a storage oscilloscope. These readings accurately indicate the support temperature when the thermocouple is welded to the mesh and the heating period is much longer than the junction response time and the pressure is near atmospheric or above. In vacuum, however, the thermocouple signal can lag the mesh temperature by several hundred degrees during heat-up even at relatively low heating rates (ca. 1000 K/s). Theoretical analysis11 shows that heat transfer from the mesh to the thermocouple junction is slow in vacuum but rapid at atmospheric pressure due to conduction through the gas. Thus it is possible to set the time-temperature history for each run on the synchronous programmer by using a blank calibration at 1 atm. of He. The programmer is then tuned to furnish small corrections for convective losses from the mesh that are present at pressure but absent in vacuum. Then voltage and current traces recorded during the actual run, together with the correctly measured steady state temperature, provide a check of the settings for the run. The thermocouple traces in Fig. lb illustrate the precision of the temperature programs and the

time-resolution for this device. This series of ten runs all begin with heatup at 103 K/s but have isothermal reaction periods (IRP) increasing in 0.1 s increments from 0.1 s to 1 s IRP. Heating rates and reaction t e m p e r a t u r e s are unambiguous and quenching is sufficiently rapid to eliminate decomposition during cooling. Variations in the reaction temperature at low pressures are usually less than 25 K; at high pressures, however, the signal fluctuates about the mean (by up to 100 K at 10 MPa) due to natural convection. Multiple runs with different supports are used to develop complete weight loss transients, making variations in both heating rate and reaction temperature at different isothermal reaction periods the prime source of experimental uncertainty. The statistics in Table II show that uncertainties in both heating rate and reaction temperature are relatively small. Heating rates tend to be below levels set in calibration trials but rarely by more than the uncertainty in the rate itself. Most importantly, the variations in heating rate over all these studies are sufficiently small to have little effect on reactivity. Likewise the reaction temperatures were controlled within close tolerances at 525, 600, 750, and 900 C. The initial measure of weight loss from the dry 60

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TABLE II Uncertainties in heating rate and reaction temperature for the transient and reaction temperature studies in the experimental program

1153

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GBV HTV LTV 5T10P 6T10P 7T10P 9T10P T500P PBU LQT BQT

18 7 12 12 8 10 5 14 19 10 7

Heating rate K/s 947 988 981 1016 976 974 1014 947 930 99 1002

-+ 34 • 22 • 46 • 95 • 34 • 31 --- 20 • 72 • 68 • 4 • 31

Reaction temperature, C 730 888 581 529 600 750 906 737 750

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FIC. 2. Effect of temperature on ultimate primary devolatilization yields at 13.3 Pa (9 and 0.19M Pa (+).

COAL COMBUSTION MECHANISMS AND PYROLYSIS

1154

sample is converted to the dry, mineral-matter-free basis using the mineral matter content for PSOC102 from the Penn State Data Base (8.3% mm). Supports were weighed before and after each experiment, and changes in the support weight were factored into the calculated yield. With only a few exceptions, the weight loss measurements at identical conditions agree to within 2%. In vacuum the pressure was held below 6 Pa by continuous pumping. Although none of the volatile products were collected in this study, tars and oils condense on the walls of the flow channel around the support in vacuum, The reaction times at constant temperature or the analogous temperatures at constant reaction time where the condensate first appears are noted. In runs under pressure the gas stream sweeps the products out of the reactor. The mass flowrate of helium was fixed at 2.6 • 10-4 kg/s at all pressures. Changes in the physical appearance of the bulk char were recorded as the extent of agglomeration. Base cases for particle size and heating rate were maintained throughout the kinetic studies, whereas the other conditions were varied over broad ranges. When not stated explicitly, the particle size was 125 I~m and the heating rate was 1000 K/s. Ultimate yield refers to the DMMF weight loss after 30 s, well beyond the end o f primary devolatilization. Shorter, isothermal reaction periods included under this heading pertain to higher temperatures where the asymptotic yield is reached much sooner.

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Fie. 4. Time-resolved weight loss after heating in He at O.19M Pa at 10a K/s to 530 (+), 600 (.), 750 (O), and 906 (~) C. to temperature between 475 and 700 C. The yields at the two pressures appear to diverge at a temperature between 550 C and 600 C. This temperature is close to that at which liquids are first deposited within the reactor. The weight loss in vacuum exceeds that at 0.19 MPa (28 psi) after the appearance of liquids, with the difference approximately constant above 700 C. The particles did not

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The data in Figure 2 demonstrate the temperature dependence of the yield after 30 s at 13.3 Pa (0.1 torr) and at 0.19 MPa (1.9 atm), This represents the ultimate yield for primary devolatilization, with subsequent decomposition occurring on much longer time scales. The yield is especially sensitive 5O

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FIG. 3. Time-resolved weight loss after heating at 103 K/s to 580 (+), 730 (O), and 890 (.) at 13.3 Pa, showing the time when liquids first appeared (T).

FIG. 5. Weight loss during heating at 10a K/s and vacuum (O) and 0.189M Pa (+) versus temperature, showing the temperature at which liquids first appeared in vacuum.

RAPID DEVOLATILIZATION OF BITUMINOUS COAL agglomerate in vacuum but did at 0.19 MPa above 800 C. Weight loss is plotted as a function of isothermal reaction period in Figure 3 for 13.3 Pa pressure (0.1 torr) and three temperatures. The asymptotes reached after 1-2 s at the higher temperatures coincide with the ultimate yields in Figure 2 but at 580 C the yield increases slowly from 20% at 4 s to 24% at 8 s before asymptoting to 29% after 30 s. The initial slopes of each curve indicate a reaction rate which increases slowly with temperature with an apparent activation energy of roughly 63 KJ/mole (15 kcal/mole). At 580 C a distinct increase in reaction rate appears after 1.5 s along with the first deposition of liquids. At the higher temperatures liquids appear either early in the isothermal reaction period or during heatup. The transients at 0.19 MPa (28 psi) shown in Figure 4 have a similar appearance, differing only 60

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FIG. 6. Yield versus temperature after heating at different rates in vacuum. The isothermal contact time was 30 s after heating at 10a K/s (O) and 104 K/s (+), but at 10~ K/s it was 0.5 (,) or between 1.5 and 5.2 s (~). There was some uncertainty as to whether the ultimate yield was reached at the low temperatures; at the highest heating rate the uncertainty involves whether or not the actual rate was 104 K/s. Data for those two regions are identified by dashed lines.

in the asymptotic yields above 600 C. under these conditions the ultimate yields obtain after only four seconds of isothermal reaction time. In contrast to the distinct char particles resulting from reaction in vacuum, those produced at 0.19 MPa were usually agglomerated. At 530 and 600 C agglomeration became evident after the apparent increase in devolatilization rates (i.e. 2 and 1 s isothermal reaction times, respectively); at 750 C the effect was present at the end of the heating period. The data in Figure 5 for yield during heatup at 10a K/s without an isothermal reaction period complements the time-resolved measurements in Figures 3 and 4. Liquids appear only above 850 C and pressure has a negligible effect. Clearly, at heating rates of 103 K/s, measurement of isothermal kinetics is feasible only below 750 C, but the constant heating rate and rapid quench possible with the apparatus facilitates nonisothermal analyses as proposed by Juntgen and van Heek. 14 The influence of heating rate on the ultimate yield, even at vacuum, is apparent from the results in Figure 6. At high temperatures, increasing the heating rate from 102 K/s to 10a K/s clearly increases the yield by 8-10 wt%. At higher heating rates or lower temperatures the situation is less

COAL COMBUSTION MECHANISMS AND PYROLYSIS

1156

clear for the following reasons. While the system can heat the support at 104 K/s, our heat transfer analyses indicate that the particle heating rate may be limited to ~3 x 103 K/s, which is consistent with the small difference observed. At the lower temperatures an isothermal reaction period of 30 s followed the heatup at 103 K/s but the experiments at 102 K/s were terminated after heatup. In three instances (denoted by ~) an isothermal reaction period of a few seconds was added but produced little or no additional weight loss; however, thirty seconds additional reaction time at the lower temperatures could increase the weight loss measureably. A s shown in Figure 7, the ultimate yields in vacuum also depend on the particle size. At intermediate temperatures the yield for the 85 p~m particles exceeds that for the 125 txm particles by as much as 5 wt%. Again the curves diverge near the temperature at which liquids first appear. The data for the 50 txm particles contains considerably more scatter, due to the greater chance of losing coal through the mesh. All three sizes are sufficiently small for thermal lags within the particle to be negligible at 103 K/s. The ultimate yield is acutely sensitive to pressure below 2-3 MPa (20-30 atm), as illustrated by the data at 750 C and 103 K/s with 125 p~m particles in Figure 8. The 8 wt% decrease between 13.3 Pa and 0.19 MPa noted in Figure 2 accounts for over half of the total difference observed between high and low pressure yields. The true vacuum limit remains uncertain due to the steepness of the curve and our limitation to a single subambient pressure. Weight loss transients at 750 C and three different pressures are compared in Figure 9. The overall rates are roughly the same while the asymptotes reflect the pressure dependence of the ultimate yields as noted above. The higher yields after heat45--

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Discussion

The weight loss data reported herein for the flash pyrolysis of Pittsburgh No. 9 HVA bituminous coal (PSOC-102) reveals several features of the process not evident from previous studies: 1'5-I~ (1) a significant dependence of the yield in vacuum on the particle size and heating rate, (2) transients which rapidly reach the ultimate yield for primary devolatilization at temperatures above 700 C but exhibit two distinct reaction time scales at 600 C or below, and (3) a correlation between the temperature above which liquids appear and that above which the ultimate yield depends on pressure, particle size, and heating rate.

(1) a low apparent activation energy for the rate of primary devolatilization, (2) a substantial increase in the ultimate yield from primary devolatilization with increasing temperature, and (3) a decrease in ultimate yield with increasing pressure but little or no effect on the yields during heatup or early in the isothermal reaction period.

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In addition, our data reinforce several observations made during earlier studies:

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FIG. 8. Ultimate yield at 750 C for different pressures for 30 s contact time after heating at 103 K/s.

The sensitivity of the yield to heating rate and to temperature during the heating period illustrates the importance of careful control of the time-temperature history. Any non-uniformity in the heating rate, overshoot of the reaction temperature at the end of the heating period, or delay in quenching the sample at the end of the isothermal reaction

RAPID DEVOLATILIZATION OF BITUMINOUS COAL period tends to obscure these features. In this system the electronically controlled power supply and the rapid liquid nitrogen quench minimize these anamolies. The low apparent activation energy and the dependence of ultimate yield on temperature observed previously led to the multiple, parallel reaction model} '5 To test its ability to describe our time-resolved kinetics we used the data on the temperature dependence of the ultimate yield in vacuum (Figure 3) to fix the parameters in the model with a Gaussian distribution of activation energies, viz: ultimate yield (V~ = 50% DMMF), pre-exponential factor (ko = 5 • 109 s-I), mean activation energy (Eo = 180 KJ/mole) and standard deviation (or = 17 KJ/mole). Comparisons of the yields predicted at the end of the heating period and at different isothermal reaction times with the experimental data then serves to test the model. In all instances the agreement was poor. At 730 and 890 C the model predicted significant devolatilization between 4 and 30 s, contrary to the data in Figure 3, while at low temperatures the parallel first order reactions could not, of course, explain the increase in rate with the appearance of liquids evident both at vacuum and at 0.19 MPa (1.9 atm). The effects of the heating rate and particle size on the ultimate yield in vacuum also pose problems for existing mass transfer models that involve secondary cracking in the gas phase, a process which becomes unimportant in vacuum. 2'4'5 Hence extant models can explain neither the time-temperature effects nor the reduction of yield in vacuum observed with larger particles or slower heating. In a qualitative sense the current knowledge of coal chemistry provides some rationale for our observations. The yields during heatup at high temperatures and up to the first plateau at low temperatures can be associated with light gases arising from the elimination of peripheral groups attached to the larger skeletal units within the coal molecule. 15A6 Moreover, the second stage at low temperature clearly represent the simultaneous liberation of liquids and elimination of additional peripheral groups as bridges cleave to free nuclei. Since multiple bonds must break to free nuclei, 3 this stage follows the primary gas evolution and resembles a two-step reaction. At high temperatures the two stages coalesce, The dependence on heating rate and particle size in vacuum, together with the pressure dependence, suggest that the secondary reactions leading to char occur in the liquid phase. Similarly, the transient weight loss data, with the rapid approach to the temperature-dependent asymptotic yield and the stepwise behavior at low temperatures, indicate a competitive, multistep set of reactions. Since this study is limited to weightloss data on a single coal further time-resolved data on product distributions and physical transforma-

1157

tions of the coal particles on this, and other coals will be necessary for the development of a comprehensive model. REFERENCES 1. ANTHONY,D. B., HOWARD, J. B., HOTFEL, H. C., A N D MEISSNER, H. P,, "Rapid Devolatilization of Pulverized Coal," Fifteenth Symposium (International) on Combustion, p. 1303, The Combustion Institute, Pittsburgh, PA, 1975. 2. RUSSEL, W. B,, SAVILLE,D. A., AND GREENE, M. I., AIChE J. 25, 65 (1979). 3. CHEONG, P. H., "A Modelling Study of Coal Pyrolysis," PhD Thesis, California Institute of Technology, Pasadena, CA, 1977. 4. GAVALAS,G. R., AND WILKS, K. A., AIChE J. 26(2), 201 (1980). 5. ANTHONY, D. B., "Rapid Devolatilization and Hydrogasification of Pulverized Coal," Sc.D. thesis, Massachusetts Institute of Technology, Cambridge, MA, 1974. 6. SUUBERG, E. M., "'Rapid Pyrolysis and Hydropyrolysis of Coal," Sc.D. thesis, Massachusetts Institute of Technology, Cambridge, MA, 1977. 7. KOBAYASHI,H., HOWARD, J, B., AND SAROFIM, A. F., Sixteenth Symposium (International) on Combustion, p. 411, The Combustion Institute, Pittsburgh, PA, 1977. 8. UBHAYAKAR,S. K., STICKLER, D. B., VON RoSENBERG, JR., C. W., AND GANNON, R. E., Sixteenth Symposium (International) on Combustion, p. 427, The Combustion Institute, Pittsburgh, PA, 1977. 9. NSAKALA, Y. M., ESSENHIGH, R. H., AND WALKER, JR., P. L., Comb. Sci. Tech. 16, 153 (1977). 10. SCARONI,A. W., WALKER, JR., P. L., AND ESSENHIGH, R. H., Fuel 60, 71 (1981). 11. NIKSA, S., "Time-Resolved Kinetics of Rapid Coal Devolatilization," Ph.D. thesis, Princeton University, Princeton, NJ. 1982. 12. NIKSA,S., RUSSEL,W. B., AND SAVILLE,D. A., Fuel (to appear 1982). 13. PETERS, W., AND BERTLING, H., Fuel 44, 317 (1965). 14. JUNTGEN, H., AND VAN HEEK, K. H., "Reaktionablaufe unter Nicht Isothermen Bedingungen," Fortshritte der Chemischen Forschung, Vol. 13, Springer-Verlag, Berlin, 1970. 15. CAMPBELL,J. n., Fuel 57, 217 (1978). 16. SOLOMON, P. R., Am. Chem. Soc. Div. Fuel Chem. Pre. 24(3), 154 (1979). 17. SOLOMON, P. R., AND COLLET, M. B., Fuel 157, 749 (1978). 18. BLAIR,D. W., WENDT, J. L., AND BARTOK,W., Sixteenth Symposium (International) on Combustion, p. 475, The Combustion Institute, Pittsburgh, PA, 1977.