Energy & Fuels 1994,8, 705-713
705
Oxygen and C02 Gasification of Chars from Wood Treated with Iron(I1) and Iron(II1) Sulfates Glenn R. Ponder* and Geoffrey N. Richards Shafizadeh Center for Wood and Carbohydrate Chemistry, University of Montana, Missoula, Montana 59812 Received November 3, 1993. Revised Manuscript Received February 21, 1994"
The present work extends previous investigations, relating to production of useful chemicals (especially levoglucosenone) from pyrolysis of wood treated with iron sulfate, by investigating the possibility of utilizing the resulting high-yield chars as gasification substrates. Gasification rates were measured by thermogravimetry after pyrolysis to form the chars in situ. The results show that sorbed ferrous and ferric sulfates are superior to indigenous inorganics in cottonwood as catalysts in 0 2 gasification of low-HTT (heat treatment temperature) chars (e.g., HTT 400 "C) but are less effective than these indigenous species in high-HTT chars (e.g.,HTT 850 "C). In the case of C02 gasification of the high-HTT chars, the added iron sulfates are superior to the indigenous species. For both types of gasification, ferric sulfate appears to be superior to ferrous sulfate as a catalyst. Apparent activation energies are calculated and reported. On the basis of this and other work, we conclude that the high-yield, low-HTT chars can be effectivelyutilized as substrates for 0 2 gasification, and that high-HTT char has potential as a substrate for 0 2 gasification and especially for C02 gasification.
Introduction Levoglucosan (LG) and especially levoglucosenone (LGO) are among the useful volatile products whose formation is catalyzed in the pyrolysis of wood treated with iron sulfate Thus, up to 4.9% yield of LGO and up to 7.5% yield of LG were obtained from pyrolysis of finely ground cottonwood sorbed with ferrous sulfate such that the wood contained 1.7% Fe.l Comparable amounts of these chemicals have been obtained from cottonwoodsorbed with ferric sulfate.3 The chars resulting from such pyrolyses are obtained in high yield (e.g.,47% at 300 "C) and are expected to be suitable as gasification substrates since they already contain species likely to function as catalysts. Thus, it is known that transition metals, such as iron, and their salts can act as good catalysts in the gasification of chars, using 02: C02,4or H205as the reactive gas. Since the gasification temperature (GT) does not normally exceed the heat treatment temperature (HTT) of the char, and since C02 and H2O gasifications require relatively high temperatures, even when catalyzed, we are constrained to consider only 0 2 gasification of the high-yield, low-HTT chars. Another possibility for these chars, which will be detailed in a subsequent publication, is pyrolytic gasification, i.e., the formation of additional volatile products by heating at higher temperatures in an inert atmosphere. Investigations in our laboratory have shown that pyrolytic gasificationof the types of chars under consideration produces significant yields of a variety of useful gases, including carbon oxides and light hydrocarbons such as methane, ethane, and ethene.3 In the event
that this approach is taken for utilizing the low-HTT char, the nonvolatile product will be a high-HTT char (e.g., HTT 800-900 "C) with potential as a substrate for all three types of gasification mentioned above. In this study, the 0 2 and CO2 gasifications of the high-HTT chars are investigated, as is the 0 2 gasification of the low-HTT char. (For reviews on biomass pyrolysis, see ref 6. For a review of biomass gasification processes, see ref 7. For a review of the present understanding of mechanisms of oxidative gasification of carbon, see ref 8.)
*Abstract published in Advance ACS Abstracts, April 1, 1994. (1) Richards,G. N.; Zheng,G. J.Anal. Appl. Pyrol. 1991,21,133-146. (2)Edye, L. A.; Richards, G. N.; Zheng, G. Transition Metals as Catalysta for Pyrolysis and Gasification of Biomass. In Clean Energy from Waste and Coal;Kahn, R. Ed.;ACS Books: Washington, DC, 1992; pp 90-101. (3) Zheng, G. Ph.D. Thesis, University of Montana, 1994. (4) Gallagher, J. T.; Harker, H. Carbon 1964,2,163-173. (5) Hermann, G.; Huttinger, K. J. Carbon 1986, 24, 429-435.
(6) Antal, M. J., Jr. A d a Solar Energy 1983,1,61-111; 1985,2,175255. (7) Reed, T. B. Principles and technology of biomass gasification. In Advances in Solar Energy; Plenum: New York, 1986; Vol. 2. (8) Chen, S. G.; Yang, R. T; Kapteijn, F.; Moulijn, J. A. Ind. Eng. Chem, Res. 1993,32,2835-2840. (9) Kannan, M. P.; Richards, G. N. Fuel 1990,69,999-1006. (10) Ganga Devi, T.; Kannan, M. P.; Richards, G. N. Fuel 1990,69, 1440-1447.
Experimental Section The samples of cottonwood sapwood (Populustrichocarpa), milled to -80 mesh and sorbed with salt solutions, were prepared as described previously for salt-sorbed cellulose.@ Metal ion contents were measured by atomic absorption. Samples were further ground to -60 mesh before being converted to chars, this being found necessary to ensure sample homogeneity and reproducibility of results. C-H analyses of chars were accomplished with a Perkin-Elmer 2400 CHN elemental analyzer. The thermogravimetry system used to obtain thermograms and to measure gasification rates has been described previously.1° Gasification experiments were carried out as follows. The sample (4-12 mg), in dry nitrogen (80 mL/min), was heated from 50 to 110 OC a t 50 deg/min, held a t 110 "C for 10 min (to dry the sample and measure dry weight), heated to the heat treatment temperature (HTT) at 50 deg/min, and allowed to pyrolyze for 15 min, thus forming the char in situ. The char (2-5 mg) was then cooled (50 deg/min) to the gasification temperature (GT) and held for 10 min before the reactive gas was admitted. This allowed time for the char to stabilize (Le., for the weight-loss rate
0887-0624/94/2508-0705$04.50/0 0 1994 American Chemical Society
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Ponder and Richards
706 Energy & Fuels, Vol. 8, No. 3, 1994 r
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Figure 1. Thermogram of untreated cottonwood.
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400
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Figure 3. Thermogram of cottonwoodtreated with ferric sulfate (4.4% Fe).
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Figure 2. Thermogram of cottonwood treated with ferrous sulfate (3.6% Fe).
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300
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due to pyrolysis to approach zero). Gasification in air (80 mL/ min, 22% 02), 5% oxygen (80mL/min, 4.8% 0 2 in Nz), or C02 (80 mL/min) was allowed to proceed for 30 min, after which nitrogen (80 mL/min) was again allowed to flow through the system for 10 min in order to reestablish the base line. The TG balance was purged with helium (20 mL/min) which was first passed through an oxygen trap. All gases were obtained from Liquid Air Corp. except for C02, which was obtained from Matheson Gas Products, Inc. The CO2 was 99.995% pure and contained less than 1 ppm of 0 2 .
Results and Discussion Formation of Chars. A thermogram of the pyrolysis of finely ground (-60 mesh) cottonwood sapwood (CW, 2200 ppm total metal ions by atomic absorption (a.a.)) is shown in Figure 1. The DTG curve shows a maximum rate of weight loss occurring at 383 "C. The TG curve shows that the char yield after this major thermal event is about 20% (measured at the point of inflection in the DTG curve), and at 700 "C the char yield is 15%. When the cottonwood is first acid-washed (CW/AW,60 ppm total metal ions by a.a.1, the DTG maximum occurs at 386 "C, and at 700 "C the char yield is 1076. The difference between 383 "C (DTG maximum of untreated wood) and 386 "C (DTG maximum of acid-washed wood) is a minor but reproducible effect of indigenous metal ions, which tend to lower the temperature a t which the main DTG maximum occurs. A more conspicuous effect is seen in the final char yield at 700 "C, which decreases from 15% to 10% when the indigenous ions are removed by acid washing. Figures 2-4 showthe correspondingthermograms for cottonwood treated with ferrous sulfate (CW/FeS04), with ferric sulfate (CW/Fe2(SO&, and with ferrous acetate (CW/Fe(OAc)Z) (included for comparison). These salttreated samples were shown by atomic absorption to
Figure 4. Thermogram of cottonwood treated with ferrous acetate (3.2% Fe). Table 1. CW/FeSOd Pyrolyses in Nz (Heating Rate: 10 deg/min) Fe (ppm)b 1410 2600 60oO
20100 36200
DTG maxima ("C) 1st 2nd 3rd 371 -d 339 302 298 378 547 292 368 532
char yield ( 5% )a initial0 700OC 19 13 25 15 34 18 49 26 54 28
aD ry basis; not corrected for added FeSO4. Iron sorbed onto cottonwood (daf). At point of inflection in DTG curve after first DTG maximum. d - = not detected.
contain 3.6%,4.4%,and 3.2%Fe, respectively (daf). These thermograms all show an initial thermal event preceding, and leading into, the main thermal event (i.e.,a hump on the left side of the main DTG peak). This initial event is generally assumed to be the pyrolysis of hemicelluloses and, to some extent, of lignin.l' Reactions which form char from celluloseare responsible for the main DTG peak in all cases, and Figure 1shows that this is the last thermal event of any significance for the untreated sample. The thermograms of cottonwood sorbed with iron sulfate salts (Figures 2 and 3) show the major DTG maximum at a lower temperature than that for untreated cottonwood (Figure 1). This promotion of the principal pyrolysis reactions increases with increasing concentration of these salts sorbed onto the wood (Tables 1and 21, possibly due to acid dissociation of the hydrated metal ions (e.g., coordinated with lignin phenolic groups), thus effectively (11) DeGroot, W.F.; Pan,W.-P.; Rahman, M.D.; Richards, G. N. J.
Anal. Appl. Pyrol. 1988, 13, 221-231.
02 and COP Gasification of Chars
Energy & Fuels, Vol. 8, No. 3, 1994 707
Table 2. CW/Fez(SO4)sPyrolyses in Nz (Heating Rate: 10 deg/min) Fe (ppm)b
1st
DTG maxima ("C) 2nd 3rd 4th
char yield ( % )a initialC 700°C
290 271 250
356 364 370
-d
460
tJ
-
50 24 58 30 70 34 a Dry basis; not corrected for added Fez(SO4)s. Iron sorbed onto cottonwood (daf). At point of inflection in DTG curve after first DTG maximum. - = not detected. 5310 16700 43600
12
640
generating sulfuric acid, which catalyzes charring.12 This idea is supported by the thermogram of C W / F ~ ( O A C ) ~ (Figure 4), in which the acetate is lost as acetic acid at a relatively low temperature, and hence the DTG maximum is little different from that for the untreated sample (Figure 1). For a given iron concentration, the ferric salt catalyzes this major thermal event at a lower temperature than the ferrous salt, possibly due to the greater acidity of the Initial char yields, measured at the hydrated ferric point of inflection of the DTG curve after the major thermal event, increased from ca. 30 % to 50 ?4 over the concentration range of ca. 0.5-3.5% Fe for the ferrous salt (Table 1) and from ca. 50 75 to 65 % over approximately the same concentration range for the ferric salt (Table 2), in accordance with the above concept. Figure 2 shows that there are two significant thermal events involving weight loss beyond the main DTG peak for the CW/FeS04 sample. These involve a 12% weight loss from 340 to 420 "C and a 4 % weight loss from 520 to 570 "C. The temperatures for these events, as well as the magnitudes of weight loss corresponding to them, are dependent upon the amount of sorbed salt. To investigate the possibility that one or both of these events may involve the formation of a ferromagnetic species, a TG run was performed during which a magnetic field existed beneath the sample pan, so that the weight of the sample would increase in the event that any such species were formed. This modification of the TG instrument into a crude Gouy balance was achieved simply by placing opposite poles of a permanent magnet on either side of the furnace such that the center line of the two poles was about 15 mm beneath the level of the pan. The low sensitivity of this arrangement was such that a relatively high Fe content was needed in the char in order for ferromagnetic species to make a detectable difference in the thermogram, and it was not expected that formation of paramagnetic species (e.g.,FezO3) would result in any such differences. Qualitative information about ferromagnetic species can be obtained by noting the temperature at which magnetism is lost, ie., the Curie point temperature. Some possible iron species and their corresponding Curie points include cementite (Fe3C), 215 "C; magnetite (FesOr), 584 "C; and elemental iron, 780 "C. Figure 5 shows another thermogram of the CW/FeS04 sample obtained under the same conditions as in Figure 2 except that a magnet was present as described above. The two thermograms are identical until the thermal event from 520 to 570 "C, which appears as a weight loss in Figure 2 but as a weight gain in Figure 5, indicating that this event is a chemical reaction forming both gaseous products and a ferromagnetic species. The latter appears to lose its magnetism gradually from 580 to 650 "C, suggestive of magnetite. The unusual gradual (12) Antal, M. J., Jr., et el. Energy Fuels 1990, 4, 221-225. (13) Cotton, A. F.;Wilkinson,F. R. S. Aduancedlnorganic Chemistry, 2nd ed.; Interscience: New York, 1966; p 868.
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Figure 5. Thermogram of cottonwood treated with ferrous sulfate (3.6% Fe), with magnetic field beneath sample pan.
Table 3. C-H Analyses of Chars. char
%C
76 H
H/C (atomic)
HTT 320 OC CW/FeS04(2.0) HTT 850 OC CW/FeS04(2.0) HTT 320 OC CW/Fez(SO&(l.7) HTT 850 "C CW/Fez(S0&(1.7)
59.2 87.5 60.9 89.0
4.1 0.7 4.0 0.9
0.83 0.10 0.79 0.12
Values are averages of duplicate measurements and are corrected for Fe content. Balance is expected to be mostly oxygen, with smaller amounts of sulfur species.
rather than abrupt weight loss may be associated with some overlap between formation and demagnetization of the ferromagnetic species. Figure 3 shows that there are three significant thermal events involving weight loss beyond the main DTG peak for the CW/Fe2(S04)3sample. These involve 14% weight loss from 320 to 440 "C, 5% weight loss from 440 to 500 "C and 3% weight loss from 600 to 650 "C. When another sample of CW/Fe2(S04)3was heated in the presence of the magnet, the same thermogram was obtained without any variation, indicating that none of the above thermal events coincide with formation of a ferromagnetic species. The ultimate fate of the iron in the char is thus seen to be dependent upon its initial oxidation state. Figure 4 shows that there is one significant thermal event beyond the main DTG peak for the CW/Fe(OAc)2 sample, involving 2.5 % weight loss from 610 to 625 "C. When another sample of CW/Fe(OAc)2was heated in the presence of the magnet, this thermal event was seen to coincide with the formation of a ferromagnetic species, which appears to gradually lose its magnetism above the Curie point of iron. Among the main chars used in gasification experiments in this study were the HTT 320 "C and HTT 850 "C chars from the cottonwood sample sorbed with 2.0% Fe in the form of FeS04 (CW/FeS04 (2.01, fourth entry in Table l), and the HTT 320 "C and HTT 850 "C chars from the cottonwood sample sorbed with 1.7% Fe in the form of F e ~ ( s 0(CW/Fe&04)3(1.7), ~)~ second entry in Table 2). Results of C-H analyses of these chars (Table 3) indicate an unusually high aliphatic/aromatic ratio in the low-HTT chars. This result supports the expectation that these chars can serve as excellent gasification substrates.14 Gasificationin Oxygen. Table 4 shows results of four air gasification experiments designed to compare the catalytic effectiveness of the iron salts with each other, with indigenous ions, and with no catalyst. (Numbers in (14) Snow, C. W.; Wallace, D. R.; Lyon, L. L.; Crocker, G. R. h o c . 4th Conf. on Carbon; Pergamon Press: New York, 1960; p 79.
Ponder and Richards
708 Energy &Fuels, Vol. 8, No. 3, 1994
Table 5. Gasifications of CW/FeS04(2.0) in Air HTT 320 OC (46% Char Yield, 4.3% Fe in Char) rate X lo00 (min-1)
rate X lo00 (min-1) GT("C) 240 250 260 20
60
40
00
% Ga
6.5 11.1 16.5
avgb 2.2 3.7 5.5
mine -4 -2 -4
GT('C) 270 280 290
%GO avgb 21.6 7.2 27.7 9.2 39.0 13.0
mine
a Percentage of char gasifiedduring 30min. Average rate of weight loss during 30 min. e Initial weight gain due to chemisorption of 02.
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Figure 6. Temperature program and weight loss curve for pyrolysis (HTTIOOO C ) of CW/Fe2(SO4)3(1.7)and air gasification
(GT300 "C) of resulting char.
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Figure 7. Differential thermal gravimetric (DTG)curve for gasification stage of Figure 6. Table 4. Gasifications in Air (HTT 400 'C, GT 300 "C) char yield sample CWIAW
cw
CW/FeS04(2.0) CW/Fe2(SO4)3(1.7)
(%)
17.1 20.1 31.7 37.4
Fe in char (%) -d
6.3 4.4
% Ga
rate X lo00 (min-1) avgb mine
4.3 6.2 39.8 52.3
1.4 2.1 13.3 17.4
-17 -23 -15 -17
* Percentage of char gasifiedduring 30min. Average rate of weight loss during 30 min. c Initial weight gain due to chemisorptionof 02. d - =
