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14 Low-Rank Coal Hydropyrolysis C. S. WEN and T. P. KOBYLINSKI Gulf Research and Development Company, Pittsburgh, PA 15230
Recently, increased attention i s being directed toward the pyrolysis route of processing coal to produce l i q u i d and gaseous f u e l s , p a r t i c u l a r l y when coupled to the use of char by-product for power generation.(1) In view of the increased interest in coal p y r o l y s i s , a better understanding of the thermal response of coals as they are heated under various conditions i s needed. Conventional liquefaction processes developed for Eastern bituminous coals might not be the best choice for Western low-rank coals because of the substantial property and structural differences between them. In general, low-rank coals are more susceptible to reaction with H , CO, or H S. Coal hydropyrolysis i s defined as pyrolysis under hydrogen pressure and involves the thermal decomposition of coal macerals followed by evolution and cracking of v o l a t i l e s in the hydrogen. It is generally agreed that the presence of hydrogen during the pyrolysis increases overall coal conversion.(2,3) In the present study we investigated pyrolysis of various ranks of coals under different gaseous environments. Low-rank coals such as Wyoming subbituminous coal and North Dakota l i g n i t e were pyrolyzed and t h e i r results were compared with Kentucky and I l l i n o i s bituminous coals. 2
2
Experimental The coals used in this study included Wyoming subbituminous coal, North Dakota l i g n i t e , Kentucky bituminous c o a l , and I l l i n o i s No. 6 bituminous coal. Proximate and ultimate analyses of the coals studied are given in Table I . A l l pyrolysis experiments were carried out in the thermogravimetric apparatus (TGA) having a pressure capacity of up to 1000 p s i . A schematic of the experimental unit is shown in Figure 1. It consists of the DuPont 1090 Thermal Analyzer and the microbalance reactor. The l a t t e r was enclosed inside a pressure vessel with a controlled temperature programmer and a computer data storage system. The pressure vessel was custom manufactured by Autoclave Engineers. A similar set-up was used previously by others.(4_) 0097-6156/84/0264-0227$06.00/0 © 1984 American Chemical Society
6.1 39.9 45.6 8.4
Moisture Carbon Hydrogen Nitrogen Sulfur Ash Oxygen ( d i f f . )
6.1 64.6 4.3 0.9 0.5 8.4 15.1
Ultimate A n a l y s i s , wt%
Moisture Volatile Fixed Carbon Ash
Proximate Analysis , wt%
68.8 4.6 1.0 0.5 8.9 16.1
42.5 48.6 8.9
Wyoming Subbituminous Coal As Received Dry Basis
21.2 50.5 3.5 1.1 0.5 7.6 15.6
21.2 35.3 35.9 7.6
64.1 4.4 1.4 0.6 9.7 19.7
44.7 45.6 9.7
3.0 70.3 4.8 1.2 3.0 8.6 9.1
3.0 39.8 48.6 8.6
72.5 5.0 1.2 3.1 8.9 9.4
41.0 50.1 8.9
Kentucky Bituminous Coal As Received Dry Basis
Coal Analysis
North Dakota Lignite As Received Dry Basis
Table I .
3.9 67.5 4.5 1.3 3.4 11.2 8.2
3.9 38.1 46.8 11.2
70.3 4.7 1.4 3.5 11.7 8.4
39.6 48.7 11.7
11linois Bituminous Coal As Received Dry Basis
Schematic
Pressure Flow Control Meter
Figure 1.
®
(7)
Pressure Gauge
TGA
B. 0. Disk
apparatus.
Pressure Vessel
Furnace
Book Pressura ^ Regulotor
setup o f the p r e s s u r e p y r o l y s i s
®
GC
230
THE CHEMISTRY O F LOW-RANK COALS
A chromel-alumel thermocouple was set in close proximity to the sample inside a reactor. The reactor was made of a quartz tube which was surrounded by a tubular furnace. In a t y p i c a l coal pyrolysis run, the coal sample (20-30 mg) was placed in a platinum boat which was suspended from the quartz beam of the TGA balance. The coal p a r t i c l e size used was 100-200 mesh. Samples were heated to desired temperatures at l i n e a r heating rates or heated i s o thermal ly under various gaseous environments. Fourier transform infrared spectroscopy (FTIR) was also used to monitor the degree of pyrolysis for various samples at different temperatures. The KBr (potassium bromide) p e l l e t of sample was prepared for FTIR a n a l y s i s . Results and Discussion Typical thermograms of Wyoming coal under hydrogen pressure are given in Figure 2. The TGA and the weight-loss rate thermograms show a major weight loss at temperatures ranging from 3 5 0 - 6 0 0 ° C . A secondary hydropyrolysis peak occurs below 600°C most l i k e l y due to the gas releases from the further decomposition of coal. Figure 3 shows a comparison of derivative thermograms for four different rank coals. The differences of d e v o l a t i l i z a t i o n rate are not large at temperatures up to 500°C; however, above 500°C, the secondary hydropyrolysis peak of low-rank coal becomes dominant. The effect of hydrogen on coal pyrolysis can further be i l l u s t r a t e d by Figure 4, where we compared derivative thermograms of Wyoming coal pyrolyzed at 200 psig of N 2 and 200 psig of H 2 at the same heating rates ( 2 0 ° C / m i n ) . The secondary hydropyrolysis peak observed at 580°C in the H 2 run was absent in the N 2 atmosphere. The influence of heating rate on coal hydropyrolysis was studied over a range of 5 - 1 0 0 ° C / m i n . As shown in Figure 5, two peaks were observed, the f i r s t of which we c a l l the primary v o l a t i l i z a t i o n , and the second, c h a r a c t e r i s t i c of local hydropyrolysis. The f i r s t peak increased rapidly with the increase of the heating r a t e . The second c h a r a c t e r i s t i c peak becomes r e l a t i v e l y dominant at lower heating rates. Although the p a r t i c l e size range (100-200 mesh) used was not small enough to eliminate p a r t i c l e size as a factor in the p y r o l y s i s , i t seems that the hydropyrolysis peak is favored by slow heating rates, i n d i c a t i n g a heat transfer and/or mass transfer limitations within the secondary hydropyrolysis region. In contrast, coal pyrolyzed under an inert nitrogen atmosphere results in an increase of weight-loss rate with increasing heating rates, but the shape of the curves remains the same (Figure 6). FTIR spectra of the o r i g i n a l coal and char from pyrolyzing Wyoming coal under H 2 pressure at various temperatures-are shown i n Figure 7 (only wave numbers between 1700 and 400 cm were shown here for- comparison). The strongest absorption band located at 1600 cm" begins to decrease in intensity at 380°C and shows a marked decrease at 4 7 0 ° C . This band has been assigned to aromatic ring C-C vibration associated with phenolic/phenoxy groups. (5_) S i m i l a r l y , the aromatic-oxygen vibration band near 1260 cm" shows equivalent decreases. These observations are in good agreement with the TGA weight-loss curve (as shown in Figure 2) where the onset 1
1
14.
WEN A N D KOBYLINSKI
Low-Rank Coal Hydropyrolysis
231
232
T H E CHEMISTRY OF LOW-RANK COALS
14.
WEN A N D KOBYLINSKI
Low-Rank Coal Hydropyrolysis
233
234
T H E CHEMISTRY OF LOW-RANK COALS
14.
Low-Rank Coal Hydropyrolysis
W E N A N D KOBYLINSKI
I
1455 O f ι
1
235
ASYMMETRIC C-CH3 • C-CH2 BENDINGS
1980 CM" SYMMETRIC C-CH3 BENDINGS 1
F i g u r e 7. FT-IR monitor o f Wyoming c o a l p y r o l y s i s under 500 p s i g H a t v a r i o u s temperatures. 9
236
T H E CHEMISTRY O F LOW-RANK COALS
temperature of d e v o l a t i l i z a t i o n occurs at around 350°C and the peak temperature of v o l a t i l i z a t i o n located at 467°C, suggesting the i n i t i a l coal d e v o l a t i l i z a t i o n involving the decomposition of oxygencontaining groups. The absorption bands between 720 to 870 cm" (which arise from the out-of-plane aromatic CH vibrations) increase markedly at 4 7 0 ° C , indicating a growth in size of aromatic clusters in the reacted coal. These aromatic out-of-plane peaks disappear as the secondary hydropyrolysis peak (shown in the TGA weight-loss curve) takes place. Therefore, at 650°C, most organic absorption bands .were diminished except for a broad band ranging from 1000-1090 cm" due to clay mineral absorption.(7_) An increase in the structureless background absorption is observed, suggesting a growth of graphitization in the residual c o a l . The kinetics of coal pyrolysis are complicated because of the numerous components or species which are simultaneously pyrolyzed and decomposed. The chemical expression in coal pyrolysis is represented b r i e f l y by an overall reaction: 1
1
W —>
χ V + (1-x)
S
(1)
where W, coal; V, total v o l a t i l e y i e l d ; S, char; and rate constant k = A exp(-E/RT); A , E , and x, frequency factor, activation energy, and stoichiometric c o e f f i c i e n t , respectively; R, gas constant; and T , temperature of coal p a r t i c l e . At any temperature, T , the rate of d e v o l a t i l i z a t i o n of coal may be written as:
or n
= A exp(-E/RT) w
(3)
For measuring k i n e t i c parameters, we treated data following the procedure of Coats and Redfern (8) and Mickelson and Einhorn (9) by taking logarithms from Equation (3) at temperatures, Τ · and T J : Ί
In
(-dw/dy-dw/dt.)
= {=- (Tj - T . J / T . T j - η l n f y / W j )
(4)
At a constant r a t i o of w.'/w^, a plot of
In
(-dw/dt^-dw/dt.)
vs (T
- T.J/T.Tj
(5)
should show the straight l i n e plot with the data points where Ε and η can be determined. The frequency factor can be further computed, based on the known values of η and Ε and on an approximate integral solution to Equation (3).
14.
WEN AND KOBYLINSKI
Low-Rank Coal Hydropyrolysis
237
The k i n e t i c parameters for four different coals heated under H 2 pressure are presented in Table I I . A reaction order equal 2.3 to 2.9 was observed for the primary hydropyrolysis peaks. A high reaction order was obtained for the secondary reaction peak under hydrogen pressure. The k i n e t i c parameters for four different coals heated under No atmosphere compared with data presented in the l i t e r a t u r e (6,10-12) are l i s t e d in Table I I I . Kinetic parameters for η (reaction order) and Ε (activation energy) in the l i t e r a t u r e show s i g n i f i c a n t variation for different techniques and coal (Table I I I ) . By considering the complexity of coal thermal degradation, many authors have contended that a simple, f i r s t - o r d e r reaction is inadequate. Wiser et a l . (10) found that n=2 gave the best f i t to t h e i r data, while Skylar et a l . (11) observed that values of η above 2 were required to f i t nonisothermal d e v o l a t i l i z a t i o n data for different coals. The k i n e t i c parameters obtained in this study also show a non-integral reaction order. The thermal decomposition of coals are complex because of the numerous components or species which are simultaneously decomposed and recondensed. Figure 8 demonstrates the influence of CO on d e v o l a t i l i z a t i o n for different ranks of c o a l . For the higher rank coals (i.e., Kentucky and I l l i n o i s bituminous coals), pyrolysis in the presence of CO plus H 2 or H 2 alone follows the same path. However, l i g n i t e showed a marked increase of pyrolysis rate in the run where CO was added. A higher content of reactive oxygenated bonds (i.e., carboxyl or ether linkages) in low-rank coals could be the reason for the high r e a c t i v i t y in the presence of CO. The k i n e t i c parameters determined for coal pyrolyzed in syngas (C0/H mixture) are l i s t e d in Table IV. As shown in the t a b l e , a high reaction order was obtained for the Hp and CO/H2 runs, p a r t i c u l a r l y for the low-rank coals which showed a secondary reaction occurring at temperatures above 5 0 0 ° C . 2
Conclusions Laboratory microscale studies have demonstrated that the coal pyrolysis i n a hydrogen atmosphere gave higher degree of d e v o l a t i l i z a t i o n in low-rank coals than pyrolysis in an inert atmosphere. In hydrogen atmosphere two d i s t i n c t steps in coal d e v o l a t i l i z a t i o n were observed as shown by the double peak of the d e v o l a t i l i z a t i o n rate. Only one step was observed under nitrogen atmosphere. In a compari son of k i n e t i c parameters, a high reaction order and a low a c t i v a tion energy were also obtained in the coal hydropyrolysis. Application of data and observations from t h i s study could lead to a better understanding of chemical and physical changes during the coal hydropyrolysis and seek alternative coal conversion routes.
23.9 23.4 29.2
4.7 — —
2.9 2.3 2.4
Kentucky
11linois
1.9xl0
--
4.2xl0
1.6xl0
8
6
7
5
1st Peak
3.1xl0
b
—
70.4
64.5
2nd Peak
1
--
--
1.2xl0
1.3xl0
18
15
2nd Peak
Frequency-Factor (min" )
Samples were heated at 50°C/min under 500 psig H 2 . Hydropyrolysis c h a r a c t e r i s t i c peak occurred in low-rank c o a l s .
18.9
North Dakota Lignite
3.4
2.5
1st Peak
Wyomi n g
b
1st Peak
Coal
2nd Peak
Activation Energy (kcal/mole)
Kinetic Parameters of Coal Hydropyrolysis
Reaction Order
Table I I .
b
14.
W E N A N D KOBYLINSKI
Table I I I .
Coal
Activation Energy (kcal/mole)
Frequency Factor (min )
Reference
Wiser et a l .
Utah Bitumi nous
2
15.0
2.9xl0
J
(10)
Stone et a l .
Pittsburgh Seam Bitumi nous
l
a
27.3
3.2xl0
8
(12)
Ciuryla et a l .
Pittsburgh Seam Bituminous
l
a
39.5
1.7xl0
North Dakota Li gnite
l
a
53.6
1.7x10 15
(6)
Illinois Bitumi nous
l
a
52.3
1.7x10 15
(6)
Skylar et a l .
This Work
b
A Comparison of K i n e t i c Parameters i n Coal Pyrolysis
Reaction Order
Investigators
a
239
Low-Rank Coal Hydropyrolysis
b
-1
U
(6)
Soviet Coal
2.3
10.0
1.3x10°
(11)
Soviet Gas Coal
2.1
14.6
5.1x10°
(11)
Wyoming Subbituminous
2.1
25.1
6.1x10°
North Dakota Li gnite
2.2
23.2
1.2x10°
Kentucky Bitumi nous
1.9
26.9
4.2x10'.8
Illinois Bitumi nous
1.8
30.9
4.8xl0
Based on a series of f i r s t - o r d e r reactions. Samples were heated at 50°C/min under 50 cc/min ambient N
2
8
flow.
240
THE CHEMISTRY OF LOW-RANK COALS
F i g u r e 8. The i n f l u e n c e of CO on c o a l d e v o l a t i l i z a t i o n , where i s 500 p s i g C0/H , and i s 500 p s i g H . 2
9
25.4
2.3
11linois Coal
a
22.9
2.3
2
32.6
57.5
3
7
6
6
7
--
1.8xl0
7.1xl0
mole r a t i o ) .
3.5xl0
2.3xl0
2.6xl0
l.lxlO
12
1 4
Frequency Factor (min ) 1st Peak 2nd Peak
Samples were heated at 50°C/min under 500 psig H 0/C0 (3/1
20.8
Kentucky Coal
2.8
2.6
North Dakota Lignite
23.1
Activation Energy (kcal/mole) 1st Peak 2nd Peak
2.8
3.6
Reaction Order 1st Peak 2nd Peak
Kinetic Parameters of Coal Pyrolysis Under Syngas
Wyoming Coal
Coal
Table IV.
T H E CHEMISTRY O F L O W - R A N K COALS
242
Literature Cited 1. 2. 3. 4. 5.
6. 7.
8. 9. 10. 11. 12.
Parker, H. W. "Liquid Synfuels Via Pyrolysis of Coal in Association with Electric Power Generation," Energy Progress 1982, 2(1), 4. Belt, R. J.; Bissett, L. A. "Assessment of Flash Pyrolysis and Hydropyrolysis," DOE Report METC/RI-79/2, 1978. Arendt, P.; Van Heek, Κ. H. Fuel 1981, 60(9), 779. Dobner, S.; Kan, G.; Graff, R. Α.; Squires, A. M. "Modifica tion of a Thermobalance for High Pressure Process Studies," AIChE Meeting, Los Angeles, November 16-20, 1975. Speight, J . G. "Assessment of Structures in Coal by Spectro scopic Techniques"; "In Analytical Methods for Coal and Coal Products"; Karr, C . , Jr., Ed.; Academic Press: New York, 1978; Vol II, Chapter 22, pp. 75-101. Ciuryla, V. T.; Weimer, R. F . ; Bivans, D. Α.; Motika, S. A. Fuel 1979, 58(10), 748. Painter, P. C.; Coleman, M. M. "The Application of Fourier Transform Infrared Spectroscopy to the Characterization of Coal and Its Derived Products," DIGILAB FTS/IR Notes No. 31, January 1980. Coats, A. W.; Redfern, J . P. "Kinetic Parameters from Thermogravimetric Data," Nature 1964, 201, 68. Mickelson, R. W.; Einhorn, I. N. Thermochim. Acta 1970, 1, 147. Wiser, W. H. Fuel 1968, 47, 475, and Wiser, W. H . ; Hill, G. R.; Kertamus, N. J . I&EC Process Design and Develop. 1967, 6(1), 133. Skylar, M. G.; Shustikov, V. I.; Virozub, I. V. Intern. Chem. Eng. 1969, 9, 595. Stone, H. N.; Batchelor, J . D.; Johnstone, H. F. I&EC 1954, 46, 274.
RECEIVED March 5, 1984
