Energy & Fuels 1988,2,673-679
673
Temperature-Programmed Desorption and C02-Pulsed Gasification of Sodium- or Iron-Loaded Yallourn Coal Char Toshimitsu Suzuki," Kazuhiko Inoue, and Yoshihisa Watanabe Department of Hydrocarbon Chemistry, Faculty of Engineering, Kyoto University, Kyoto 606, Japan Received June 22, 1987. Revised Manuscript Received April 6, 1988
Temperature-programmed desorption (TPD) and pulsed-C02gasification studies of Yallourn coal char loaded with Fe(N03)3or Na2C03were carried out. From the TPD studies, it was concluded that the carbon monoxide desorption peak at 750-900 "C was closely related to the reduction of metal oxides by carbon corresponding to reactions b and d. For C02-pulsed gasification of Yallourn coal char loaded with Fe(N03)3or Na2C03,the following redox cycles were confirmed by using a 13C02 pulse:
-
+ C02 Fe,O,+l + C 2Na + C 0 2 NazO + C
Fe,O,
+ CO Fe,O, + CO NazO + CO 2Na + CO
Fe,O,+l
(a)
(b) (C)
(4
The apparent activation energies for reactions a and c were estimated as 77 and 44 kJ/mol, respectively, in the temperature range 500-700 "C. Those for reactions b and d were calculated as 1.1 X lo2 and 1.3 X lo2 kJ/mol, respectively, above 750 OC.
Introduction Much attention has been focused on catalyzed carbon gasification;' two issues of Fuel were devoted to papers on catalyzed gasifications.2 To understand the nature of the catalyst, a large number of studies about surface oxygen species were carried out by temperature-programmed desorption (TPD),3g4temperature-programmed reaction using 13C-labeledpotassium carbonate5 and sodium carbonate: and Auger electron spectroscopy (AES).' Adsorption of C02on K2C03-loaded charcoal has been reported by Yokoyama.s However, the mechanism of catalytic gasification is not fully understood. We have proposed the iron-sodium binary catalyst as a new and active catalyst for coal ga~ification.~Binary catalysts for steam gasification of graphite using transition metals and alkali metals were recently discussed by Carrazza et al.1° The binary effect of Ca and Fe on the steam gasification of coal was reported recently."
Iron-catalyzed carbon gasification in an HzO-Hz mixture was reported by Huttinger and co-workers. They proposed an oxidation reduction cycle of surface Fel-,O and Fe, while the bulk of the iron is kept in the reduced state under < 0.2).12 high hydrogen partial pressure (PH20/PH The pulse reaction technique, along witk the transient response technique,13has been widely used to understand a catalytic reaction on a solid surface.14 However, until now the pulse reaction technique has not been applied to carbon gasification except for very recent transient or step-response techniques by Cerfontain et al.15J6 and Freund.17 In this paper, we have attempted to determine the role of sodium and iron in the carbon dioxide gasification of a coal char. Pulsed gasification of metal-loaded Yallourn coal char followed by TPD was developed, and we have partially succeeded in proving the oxygen transfer reaction involving metal oxide and carbon. Experimental Section
(1)For a review, see: Wood, B. J.; Sancier, K. M. Catal. Rev.-Sci. Eng. 1984,26(2) 233. (2)Fuel 1983.62(2): Fuel 1986.65(10). (3)Freriks, I: L.'C.; van Wechem, H. M. H.; Stuirer, J. C. M.; Bauwman, R. Fuel 1981,60,463. (4)Wigmana, T.; van Doorn, J.; Moulijn, J. A. Fuel 1983,62, 190. (5)Saber, J. M.; Falconer, J. L.; Brown, L. F. Fuel 65, 1986,1356. (6)Saber, J. M.; Falconer, J. L.; Brown, L. F. J . Chem. Soc., Chem. Commun. 1987,445. (7)Kelemen, S. R.;Freund, H. Carbon 1985,23,723. (8)Yokoyama, S.;Miyahara, K.; Tanaka, K.; Tashiro, J.; Takakuwa, I. J . Chem. SOC. Jpn. 1980,6, 974. (9)Suzuki, T.: Miehima, M.; Watanabe, Y. Chem. Lett. 1982,985:Fuel 1985,64, 661. (IO) Carrazza, J.; Tyose, W. T.; Heinemann, H.; Somorjai, G. A. J. Catal. 1985,96,234. (11)Ohtsuka, Y.;Hosoda, K.; Nishiyama, Y. J. Fuel SOC.Jpn. 1987, 66,1031.
0887-0624/88/2502-0673$01.50/0
The coal sample used in this study is an Australian brown coal, Yallourn (C, 68.2 wt %; H, 4.5 wt % (daf); ash, 1.1wt %(db)). The catalyst was impregnated into the coal (0.3 mmol/g of dry coal) from aqueous solutions (Fe(N0J3 and Na2C03)or an ethanol solution of Na[HFe(C0)4]: and dried coal was heat-treated a t (12)Herman, G.;Huttinger, K. J. Carbon 1986,24, 429. (13)Kobayashi, H.; Kobayashi, M. Cat. Reu.-Sci. Eng. 1974,10,139. 1955, (14)Kokes, R.J.; Tobin, H.; Emmett, P. H. J. Am. Chem. SOC. 77,5869. (15) Cerfontain, M. B.; Moulijn, J. A. Proceedings-Znternational Conference on Coal Science; Center for Conference Management: Pittsburgh, PA, 1983;p 419. (16)Cerfontain, M. B.; Agalianos, D.; Moulijn, J. A. Carbon 1987,25, 351. (17) Freund, H.Fuel 1986,65, 63.
0 1988 American Chemical Society
674 Energy & Fuels, Vol. 2, No. 5, 1988
Suzuki et al.
Table I. Amounts of CO and C 0 2 Desorbed by T P D in Catalvst-Loaded Yallourn Coal Chars amt desorbed, mmol/g of char raw 10-30% 30-50% catalyst char" BOb BO none CO 0.53 0.64 (12%) 0.80 (30%) COZ 0.20 0.36 0.12 NaZCO3 CO 0.73 2.1 (25%) 2.2 (48%) COZ 0.52 0.58 0.52 Fe(N08h CO 0.52 1.2 (10%) COz 0.16 0.17 Na[HFe(CO)4] CO 1.2 1.9 (17%) 3.8 (50%) COB 0.70 0.64 1.0 Ca(OAc),, deashed coal CO 0.77 1.2 (18%) COz 0.14 0.32
3-way valve
Regulator
ll
Temperature-programmed desorption (TPD) and pulsed gasification were carried out with the apparatus shown in Figure 1. A quartz reactor (5 mm 0.d. X 300 mm) was placed in a n infrared reflection type furnace whose temperature was controlled by a temperature programmer and controller. A flow of carrier gas was controlled by a mass flow valve. A carrier gas line through an injection port of a gas chromatograph was connected to an inlet of the reactor. An outlet was fitted with an activated carbon column (3.00 mm i.d. X 3.0 m) and connected to a TCD detector and vented. A 50-mg coal char sample was placed in the center of the reactor with the aid of quartz glass wool plugs. The center of the reador tube was covered by stainless-steel bands fitted with chromel-alumel thermocouples. Isotope-labeled T O 2was prepared from Ba1%03 (99% isotope purity) by acidifying with perchloric acid and was stored in a mercury buret. Procedure. Under a flow of helium, the coal char was heat treated from ambient temperature to 1000 "C at a heating rate of 50 "C/min. Desorbed gases were separated with the activated carbon column directly connected to the outlet of the reactor and were analyzed by the TCD detector. Under selected reaction conditions, a n He flow rate was set at 30 mL/min, and elution times of CO and COz were measured without charging a char sample by injecting a CO and COPmixture as a pulse. They were estimated at 5 and 28 min respectively. The T P D run from 100 to 1000 "C, at the heating rate of 50 "C/min, requires 18 min. Therefore, CO and COz that desorbed during the T P D run eluted between 5 and 23 min (for CO) and 28 and 46 min (for Cob. From the known retention time of CO and COz, the desorption temperatures of the respective component were estimated. The
catalyst blankb none deashed char Ca(OAc),( Na,CO$
Injection port
Na2CO8 Fe(NW3 Fe(N08)sg Na[HFe(CO)4] Fe(N08)8 + Na2C08
7 4 W
Active carbon column
I '
0.3 0.15 0.3 0.6 0.3 0.15 0.3 0.6 0.3 0.3 0.3
9.2 13.4 16.6 14.4 16.3 18.8 20.3 30.9 20.8
Thermocouple
~
Infrared reflection'furnace
F i g u r e 1. Schematic diagram of the apparatus used for T P D and pulsed gasification. accuracy of estimated temperature of CO desorption is about 20 "C, due to the broadening by diffusion in the separating column. The amounts of CO and COz were determined by using calibration with known amounts of CO and COz (Table I). After reaching 1000 "C, the coal char was cooled to room temperature under helium flow. Again the char was heated to a certain temperature (usually 750 "C), the COz pulse was injected with an airtight gas syringe, and the product gases were analyzed as described above. In certain cases, three successive pulses (at 7-min intervals) were introduced and product gases were analyzed. As a blank run for the T P D after pulsed gasification, preheat-treated char was cooled to room temperature and was kept for 30 min under a stream of He. The T P D run was carried out. A small amount of CO was liberated, and the result is shown in Table 11. This is due to the following reasons: (i) During the first heat treatment stage all the metal oxides were not entirely reduced by char and a small amount of metal oxides were still left on the char surface. (ii) A small amount of oxygen contaminant in the carrier gas oxidized reduced metal even at lower temperature. (iii) Inevitably, very small amounts of air leaked into the system.
Results and Discussion Temperature-ProgrammedDesorption of Iron- or Sodium-Loaded Char. Since we are interested in the catalytic gasification of coal, Yallourn coal char is employed as a carbon source though coal char contains inherent mineral matter that acts BS a catalyst for coal gasification.ls
Table 11. Results of t h e COI-Pulsed Gasification" mmolCof pmol of pmol of pmol of catalyst COz reacted CO(MP) CO(SP) 3.5 0.8 8.4 3.6
Detector 6-way valve
He
OAir was absorbed during handling of the char in the open air. bConversion of char by COz gasification is shown in parentheses.
800 "C under an argon atmosphere for 30 min. The yield of the char is approximately 50% in all cases.
Flow meter
5.2 0.03 12.5 4.0 8.9 21.1 23.4 17.1 23.0 26.5 30.3 46.4 27.6
1.8 0 0 0 0 0 2.6 8.8 10.4 14.5 14.1 7.0 10.9
desorption,d pmol CO' c0,e 4 0.5 7 4 7 6 14 17 15 7 7 8 7 18 9
1.0 0 0.9 0.6 0.8 0.8 0.7 0.6 0.4 0.4 0.6 0.2 1.0
"Conditions: char, 50 mg; CO,, 40.6 pmol pulse; 750 OC. Key: CO(MP), CO main peak; CO(SP), CO satellite peak. bTPD after heat treatment to 1000 "C. cPer gram of coal. dDesorption after three successive pulsed gasifications. eThe amounts of CO desorbed after pulsed gasification are 3-5 bmol larger than the amount COz reacted plus CO produced in the pulsed reaction. This descrepancy comes from incomplete removal of oxygenated surface species by the heat treatment before pulsed gasification or oxygen contaminant in the carrier gas. 'Deashed Yallourn coal was employed. #Gasification at 800 "C.
N a - or Fe-Loaded Yallourn Coal Char c a
Energy & Fuels, Vol. 2, N o . 5, 1988 675 a
0.1
0.0 5
0.3
0.05
1
-
0.1 \ 0
I
E
a a n
0
u 0.1
0
200
400
n
I1
t
600
Temperature
"'1
Oc
Figure 2. Temperature-programmed desorption of Yallourn coal char (dotted line for COz and solid line for CO): (a) Yalloum char; (b) Na2C03-loaded char; (c) Fe(N03)3-loadedchar; (d) Na[HFe(CO)4]-loadedchar (metal species 0.3 mmol/g of coal). The char was prepared a t 800 "C for 30 min under Ar and handled in open air before the T P D run.
In Figure 2 are shown the temperature programmed desorption patterns of Yallourn coal chars, loaded with different metal ions that were handled in open air. The desorption of CO, occurred at a lower temperature region irrespective of the metal ion loaded, without showing any specific temperature for the maximum desorption rate. However, the amount of C02 desorbed was larger when the sodium species were loaded. On the other hand, a sharp desorption peak of CO appeared at 750-800 "C in all cases. Carbon monoxide desorbed at 770-800 "C amounted to 40-60 pmol/g of char for both uncatalyzed and Na2C03-loadedchars. Chars loaded with NaHFe(CO), and Fe(N03)3+ Na2C03,however, liberated 0.2-0.6 mmol of CO/g of char in this temperature range. On the other hand, Yallourn coal char demineralized by 6 N HCl and loaded with Ca(OAc), or Na2C03did not show such a sharp CO desorption peak. The amount of C02 desorbed greatly increased with a loading of Na, while the amount of CO increased in the presence of Fe. These results seem to indicate that the CO desorption peak at the higher temperatures is closely related to the metal species contained in the char.l9 Loaded iron (oxidized during handling) or metal oxide in Yallourn coal char may possibly be reduced by carbon during TPD under helium flow. Direct reduction of iron oxide by carbon from residual oil has been reported by Hasegana et a1.20 TPD after COPGasification and C 0 2Adsorption. In order to determine the change in the surface species of various catalyst-loaded chars during gasification, TPD after COz gasification was carried out as follows: Yallourn coal char loaded with various catalysts was first heat treated as above to 1000 "C to eliminate oxygenated species during handling of the char. After the reaction was cooled down (18) Solano, A. L.;Mahajan, 0. P.; Walker, P. L., Jr. Fuel 1979, 58, 327. (19) Kyotani, T.; Karasawa, S.;Tomita, A. Fuel 1986,65, 1466. (20) Hasegawa, H.; Hachiya, A.; Koya, T.; Kunii, D. Kagaku Kogaku Ronbunshi 1981, 7, 278.
I1
0.51
I
jI
0.1
___---.--/_...._____.___. 200
400
600
Temperature
800
io00
"C
Figure 3. Temperature-programmed desorption of Yalloum coal char after gasification by C02 a t 800 "C for 3-10 min (dotted line for COz and solid line for CO): (a) Yallourn char burn-off, 12%; (b) NazC03-loadedchar burn-off, 25%;(c) Fe(N03)3-loadedchar burn-off, 10%; (d) Na[HFe(CO)4]-loadedchar burn-off, 17% (metal species 0.3 mmol/g of coal).
to room temperature, the carrier gas was switched to CO, (30 mL/min). The char was gasified for a certain period under a flow of COz at 800 "C, the char was cooled to 300 "C (-50 "C/min) under a stream of COP, and then COPflow was switched to He flow. TPD patterns of selected samples and the amounts of CO and COz desorbed are shown in Figure 3 and Table I, respectively. The amounts of CO, desorbed did not change after C02 gasification except when noncatalyzed or Ca(OA~)~-loaded demineralized chars were employed. The amount of CO desorbed, on the other hand, increased with the addition of metal species to the coal after gasification. These tendencies were not altered when conversion of the char by CO, gasification increased to a higher level (to 50%). The amounts of CO and C02 desorbed and the peak temperature of desorption are not directly correlated to the gasification rates. A large number of studies on the TPD of alkali-metal carbonate loaded activated carbon or synthetic highpolymer chars have been reported. Most of them deal with the desorption profile and an amount of carbon dioxide at a lower temperature r e g i ~ n . ~From . ~ our results, the effects of metal oxides on the catalytic cycle seem to be more closely related to the desorption of carbon monoxide. The order of the CO, gasification rate of metal-loaded Yallourn coal measured with thermogravimetric method is the same order as the amount of CO desorbed in TPD after C02 gasification except for the NazC03-loadedcase (gasification rate at 800 "C: NaH[Fe(CO),] > Na2C03+ Fe(NO,), > Na2C03> Ca(OAc), > Fe(NO3I3 > none). Pulsed-C02Gasification of Yallourn Char Loaded with Various Catalysts and Desorption after Gasification. A large number of studies of the pulsed reaction have been carried out to understand the adsorption and desorption phenomena on the catalyst surface.21i22 Ap-
676 Energy & Fuels, Val. 2, No. 5, 1988
Suzuki et al. Table 111. Isotope Distribution in the 1*C02-Pulsed Gasification of Catalyst-Loaded Yallourn Coal Chars" main peak satellite peak catalyst amt, Wmol W O , % amt, pmol W O , % 11.9 81.0 0.3 22.0 Na2CO3
Fe(N0d3 Na[HFe(CO),]
12.9 28.6
88.0 88.0
3.5 3.4
2.0 4.0
"Conditions: char, 50 mg; metal, 30 pmol; COz (95.9% W),40.6
Wmol/pulse.
Assuming that carbon gasification by CO, occurs in the following ways, 1mol of reacted CO, produces 2 mol of CO.
5
10 30 Retention t i m e l m i n )
3:
Figure 4. Chromatograms of pulsed gasification of Yallourn char loaded with various catalysts: (a) Yallourn char at 750 "C; (b) NazC03-loaded Yallourn char at 750 "C; (c) Fe(NO&,-loaded Yallourn char at 750 "C; (d) Fe(N03)3-loadedYallourn char at 800 "C. Conditions: metal, 0.3 mmol/g of coal; COz, 40.6 wmol/pulse. plication of pulsed gasification of coal char or carbon has been reported only very recently.,, We have tried to examine the response of the transient reaction of COz on the char surface. To obtain a fresh surface, Yallourn coal char loaded with various catalysts was first heat treated to 1000 "C and was cooled to an ambient temperature under He. Into this sample was injected a pulse of C02 (1.0 mL) at a certain temperature. Typical chromatograms of pulsed gasification are shown in Figure 4. In all cases a certain amount of CO, was reduced to CO by passing through a char bed and eluting at a retention time of around 5 min. Unreacted CO, was eluted at 28-35 min with slight tailing. The most characteristic features in the pulsed COPgasification were observed when Fe(N03),- and Na[HFe(CO)&loaded chars were gasified. About 1-4 min after the distinct CO peak (hereafter denoted as the CO main peak), a small CO peak (identified by mass spectroscopy) appeared (hereafter the CO satellite peak). The pulsed gasification of Na[HFe(CO)4]-loaded char gave a pattern similar to that in Figure 4c, with a larger CO main peak and a smaller unreacted CO, peak. The amounts of CO produced by the pulsed COzgasification are shown in Table I1 (average of three successive reactions at intervals of 7 min). In general, the amount of CO produced (in the main peak) gradually decreased with repeated injections. However, the elution time of the CO satellite peak decreased, but the amount of CO in the satellite peak did not change. The amounts of CO, reacted increased when Na2C03,Fe(N03),, Na[HFe(C0I4],and Na2C03+ Fe(N03), were loaded to the coal char. The amount of COP reacted by the pulsed gasification also increased with increasing amounts of Na2C03and Fe(NO,), loaded to coal, as seen in Table 11. (21) Basset, D.W.;Habgood, H. W. J . Phys. Chem. 1960, 64, 769. (22) Murakami, Y. Shokubai 1970,12,1. (23) Kataoka, T.; Toyoda, K. J. Fuel SOC.Jpn. 1986, 65, 1027.
C( ) + coz + C(0) + co (1) C ( 0 ) + C f + co + C ( ) (2) where C( ) indicates the vacant carbon active site and Cf means the potential carbon active site.l However, as seen in Table 11, chars loaded with various metal species gave smaller amounts of the CO main peak compared to 2 equiv of COz consumed. This seems to indicate that within the time scale in the elution of the CO main peak reaction 2 was not completed. From these results, the following reaction is supposed to occur in the pulse-CO, gasification of iron- or iron-sodium-loaded char: Fe,O, + CO, Fe,O,+l + CO (3) It was elucidated from X-ray diffraction study that part of iron was reduced to a-Fe after heat treatment to 1000 "C. Since we do not know the oxidation state of finely dispersed iron species on the char surface, not detected by X-ray diffraction, Fe,O, is used for the reduced iron species throughout this paper. In this reaction, carbon dioxide oxidizes iron oxide to a higher oxidation state,12and little or no carbon oxidation takes place. The iron oxide of a higher oxidation state can be reduced by carbon after a certain period to give a CO satellite peak (oxygen transfer mechanism). The reason why such a sharp peak was appeared is difficult to explain, but reaction 4 seems to be an autocatalytic process. Fe,O,+l + C Fe,O, + CO (4) When the pulsed gasification of CO, with Fe(NO,),loaded char was carried out at 800 OC, the interval between the CO main peak and a satellite peak reduced to 1 min, and a more distinct satellite peak was observed (see Figure 4d). The fact that a slightly larger amount of CO was produced at 800 "C than a half amount of reacted CO, indicated that in addition to reaction 3 carbon gasification did occur when a COz-pulsepassed through char bed. In general, in a blast furnace, reaction 4 is known to occur above 1000 "C. However, finely dispersed iron oxide, in this case, can be reduced at a temperature as low as 750 "C. Deashed Yallourn char gave a very small amount of the CO main peak on the COz-pulsereaction even at 850 or 900 "C. This strongly supports the assumption that the formation of the CO main peak in the pulse gasification is closely related to metal or metal oxide contained in char, and such metal or metal oxides in the char could be an active site for carbon gasification. 13COz-PulsedGasification. In order to confirm eq 3 and 4, l3C-labeled COz (95.9% 13C)was injected and the vent gases in the CO main peak and the satellite peak were collected. Since no satellite peak was observed for the use of NaZCO3catalyst, vent gas,in a center portion of the CO peak was collected as product corresponding to the main peak for iron-loaded case. Then vent gas of the tailing part of the CO peak, eluting between 7 and 8 min was collected.
-
-
N a - or Fe-Loaded Yallourn Coal Char
Energy & Fuels, Vol. 2, No. 5, 1988 677
Table IV. Effects of Pulsed Gasification Temperature on the Amounts of CO in the Main Peak, the Satellite Peak, and the Desorption after Gasification" TPD' pmol of CO(MP) pmol of CO(SP) pmol of COz reactb pmol of pmol of temp, O C pulse 2 pulse 3 pulse 1 pulse 2 pulse 3 pulse 1 pulse 2 pulse 3 pulse 1 Cod COZ 4OOe 500 550 600 650 700 750 800
2.0 3.5 6.7 8.1 10.9 25.5 63.8
4.0 6.4 9.6 12.8 16.0 29.0 64.0
1.5 2.9 5.5 6.3 9.1 27.9 64.0
3.3 4.5
6.8 4.2
7.2 4.0
9.0 11.2 12.6 12.2 15.8 16.6 35.7
4.0 3.6 8.8 7.0 11.6 19.2 34.8
19.5 13 16 16 17 19 15 12
1.9 1.8 6.8 6.8 10.1 16.4 34.1
14.5 7.2 4.4 1.5 1.3 1.2 0.7 0.5
"Yallourn char loaded with NaZCO3(0.3 mmol/g of coal, 50 mg), with 81.2 pmol of C02/pulse. bThe amount of COz reacted. 'The amounts of CO and COPdesorbed after three successive pulsed gasifications by TPD. dSee Table 11, footnote e. eAdsorptionunder COz flow (10 mL/min) for 2 h.
fication is reported to be the M-0-C type complexes by several g r o ~ p s . ~However, ~ , ~ ~ at present it is difficult to say whether (NaO), species are involved or not. If reaction 10 is involved, in order to regenerate an active form of sodium, reaction 6 or 9 must follow. Since we did not observe any distinct CO satellite peak when C02 gasification with NazC03-loadedchar (demineralized Yallourn coal) was carried out, this reaction might occur gradually. If reaction 9 containing Na213C03is followed by reaction 10, the CO evolved must contain 33% 13C0and 66% l2C0. However, we have obtained 22% 13C0in the tailing part of the CO in the main peak. Therefore, it seems reasonable to eliminate the possibility of reactions 9 and 10. This will be more clearly shown later.
4
3
C
m m
0
2
0 V
C
-
1
0.9
1.0
1.1 1.2 1 I T x 10 % l / K )
1.3
Figure 5. Logarithms of CO produced against 1/T in the COz-pulsed gasifications of NazC03-loaded(A) and Fe(N03)3Yallourn char. The dotted line indicates an alternate loaded (0) fitting of the Arrhenius plot for Fe(NO&-loaded char in a higher temperature region.
The mass spectroscopic analyses of collected gases are shown in Table 111. In all cases CO in the main peak consisted mainly of 13C0,indicating that only reaction 3 occurred. Reduction of iron oxide by carbon did occur to give ' T O (reaction 4) in the satellite peak. As evidenced by very little 12COzin the unreacted COz, the carbon exchange reaction shown below hardly occurs: 13C02 C * W O 2 13C (5)
+
+
Redox cycles involving NazC03,9NazO,Sor (Na0)26were proposed for the NazC03-catalyzedcarbon gasification by COz and HzO. Two distinct oxidation and reduction cycles are not separately observed in this pulse gasification. Reduction of Na20 by carbon (eq 7) seems to slightly occur almost simultaneously with oxidation. This is partly supported by the lower concentration of 13C0in the main peak as compared with Fe(N03)3-loadedchar. However, as compared to Fe(NO,),-loaded or uncatalyzed chars, larger amounts of CO were recovered by the TPD runs after pulsed gasification (Table 11). This seems to indicate that the formation of oxide and the reduction of oxide species by carbon are not as simple as is shown in reactions 7 and 8. Very recently, oxidation of NazO by COz (eq 11 and 12) has been proposed by the gasification of 13Cwith C 0 2 by a temperature-programmed reaction technique.6 This mechanism seems to be allowable from our pulsed reaction. The active form of alkali metal in carbon gasi-
Na2C03-,Na20 + C02
(6)
Na20 + C
(7)
2Na
+ + + + + -
+ CO C02 -,Na20 + CO
Na2C03 2C 2Na
C02
Na20
COz
2Na
(8)
+ 3CO Na20 + CO
(8)
Na2C03
(10)
2Na
(9)
+ CO (NaO)2+ C -,Na20 + CO
Na20
C02
(NaO),
(11) (12)
Effect of Temperature on the Pulsed Gasification. An Na2C03-loadedchar (15 pmol to 50 mg of char) was pretreated by heating up to 1000 "C under He flow, and the C02 was absorbed at a flow rate of 10 mL/min at 400 "C for 2 h. After COPadsorption, TPD desorbed 19.5 hmol of CO and 14.5 pmol of COz,and these values corresponded closely to the amount of maximum surface oxygen (30 pmol of Na in the char theoretically evolves as 15 pmol of COP and 15 pmol of CO; see eq 6 and 7). Three successive C02-pulse (2.0 mL, 81.2 pmol) gasifications at an interval of 7 min were carried out in the temperature range of 500-800 "C with a 50 "C increment. Ten minutes after the third pulse gasification,the char was cooled to 300 "C at a cooling rate of 50 "C/min. Again, TPD was carried out up to 1000 "C, and the amounts of CO and C02 desorbed were determined. Even at temperature as low as 500 "C, a small amount of CO was obtained by the pulse gasification. The amount of CO in the main peak gradually increased up to 700 "C and abruptly increased above 750 "C (Table IV). ~~
~
(24) Hashimoto, K.; Miura, K.; Xu, J. J.; Watanabe, A.; Masukami, H., Fuel 1986, 65, 489. (25) Mims, C. A.; Pabst, J . K. Fuel 1983, 62, 176.
678 Energy & Fuels, Vol. 2, No. 5, 1988
Suzuki et al.
Table V. Effects of Pulsed Gasification Temperature-on the Amounts of CO in the Main Peak, the Satellite Peak, and the Desorption after Gasification" TPDc pmol of CO(MP) pmol of CO(SP) pmol of C 0 2 reactb pmol of pmol of temu. OC Dulse 1.2 Dulse 2 Dulse 3 Dulse 1 Dulse 2 Dulse 3 Dulse 1 Dulse 2 Dulse 3 Cod co, ~
500 550 600 650 700 750 775 800
1.9 3.0 6.8 12.9 14.4 24.9 27.7 49.4
1.5 3.2 3.9 7.9 9.3 48.1 65.2 76.4
1.4 2.7 2.9 5.5 7.0 31.8 55.3 73.2
12.6 14.4 15.1
10.5 13.1 14.6
12.9 13.6 16.6
1.8 6.0 9.0 14.5 24.9 27.4 26.2 34.7
1.9 5.1 7.2 10.5 17.9 35.6 40.8 48.1
1.7 5.2 8.5 11.0 18.5 30.9 36.7 41.5
10 12 14 18 15 6 4 4
1.7 2.3 3.4 3.7 2.3 0.3 0.3 0.3
aYallourn char loaded with Fe(N03)3(0.3 mmol/g of coal, 50 mg),with 81.2 pmol of COz/pulse. bThe amount of C 0 2 reacted. cThe amounts of CO and COz desorbed after three successive pulsed gasification by TPD. See Table 11, footnote e.
A CO satellite peak due to mineral matter in the ash (probably Fe,O,) appeared above 750 "C. it is noted that decreases in the amount of CO produced were observed during three successive pulse injections in the temperature range 500-700 "C. Furthermore, the amounts of CO produced were considerably smaller than the amounts of COPreacted at 500-600 "C. Sodium carbonate was reduced to its metallic state in the first TPD run.26p27By the first pulse of C02,reaction 8 occurs to give 1equiv of CO consuming 1equiv of C02, and then reaction 10 follows without producing CO. Thus the amounts of CO liberated are smaller than the amount of C02 consumed in the temperature range 500-600 OC. Above this temperature, it is reported that Na2C03tends to decompose to give C02 and NazO in the absence of C02.26,27Our observation is in agreement with the reported results. Decreases in the amount of CO produced by the series of pulse reaction can be interpreted by the accumulation of Na2C03below 600 "C and the Na20 species above 650 "C on the char surface. Above 750 "C, reaction 7, reduction of Na20 by carbon, proceeds to give CO (carbon gasification) within the time scale of CO elution. The logarithms of the amounts of CO produced in the series of pulse gasification are plotted against the inverse of the reaction temperature. The result is shown in Figure 5. Two distinct lines with different slopes are obtained. The apparent activation energies are estimated as 46 kJ/mol and 1.1 X lo2 kJ/mol for the low- and high-temperature regions, respectively. Similar phenomena were reported by Cheng and Harriot in the oxidation of activated carbon by oxygen.28 According to them, lower activation energy is observed solely for the adsorption-controlled region, and higher activation energy is due to the desorption controlled reaction. In our case, these results indicate that reaction 8 occurs with lower activation energy, and that reaction 7 requires a large activation energy. After three successive pulse gasifications, the amounts of oxygen trapped by char were estimated by TPD. The results are shown in Figure 6. The amounts of C02 desorbed monotonously decreased with an increase in the gasification temperature. Above 600 "C, small amounts of COz were desorbed. The amounts of C02 desorbed by TPD and the amounts of C02adsorbed during pulse gasification are in good agreement for the reactions at 500 and 550 "C.As described above, this clearly indicates that reaction 10 did occur at a lower temperature. On the other hand, the amounts of CO desorbed gradually increased up to gasification at 700 "C, indicating the formation of Na20 during C02 pulse gasification (eq 8). Above 700 "C, the amounts of CO desorbed decreased due (26) McKee, D. W. Carbon 1979, 17,419. (27) McKee, D. W.; Lamby, E. J. Fuel 1983, 62, 217. (28) Cheng, A.; Hariott, P. Carbon 1986,24, 143.
t 15
1
co
i
*\
-I5
0
-10
E
2 N
0
u
--X-
0 400
500
600 Temperature
700
0 800
I'C)
Figure 6. Amounts of CO and COz desorbed by the TPD after three successive pulsed gasifications of NazC03-loadedYallourn char at different temperatures. Condition: COz,81.2 pmol/pulse.
to the reduction of Na20 by carbon (eq 7) during gasification. All these results clearly reveal that, in the presence of Na, metal oxide and metal redox cycles are the principal reactions. As shown above, if alkali-metal-loaded char were treated with COz below 600 "C, the reaction of C02 with Na20 occurs to give Na2C03. If we consider the catalytic cycle involving reactions 11and 12, reaction 11occurs only above 600 "C, and below this temperature NazO reacts with COP to give Na2C03. This mechanism is also allowable at present, if reaction 7 does not or rarely occurs below 1000 "C. As mentioned above, after adsorption of COz at 400 "C, Na-loaded char liberated CO and COz corresponding to reactions 6 and 7. Therefore, we assume that redox cycles involving reactions 7 and 8 are more reasonable. Three successive pulsed gasifications of Fe(NO&-loaded Yallourn char were examined between 500 and 800 "C with a temperature increment of 25 or 50 "C. The results are shown in Table V. A small amount of CO was produced between 500 and 600 "C, and the amount of CO formed with the first pulse of COz increased at 650 and 700 "C, indicating that reaction 3 occurs extensively at these temperatures, while FenOm+lcannot be reduced during the period when the second pulse passes through the char bed (no satellite peak). Above 750 "C a further increase in the CO main peak was observed in addition to an appearance of the CO satellite peak. The CO formed by the second pulse increased greatly as compared to the cases reacted below 700 "C. After three successive pulsed gasifications, the TPD desorbed considerable amounts of CO and C02 in the temperature range 500-700 "C. In this temperature range, Fe,O,+l cannot be reduced by carbon in the course of pulsed gasification. However, above 750 OC,smaller
Energy & Fuels 1988,2, 679-684 amounts of CO and C02 were desorbed in the TPD runs, by liberating CO as a satellite peak during pulsed gasification. The logarithms of CO produced (including the satellite peak above 750 OC) in the first pulsed gasification were plotted against the inverse of temperature to give a straight line.29 The apparent activation energy was estimated at 77 kJ/mol, indicating difficulty in reaction 3 as compared to reaction 8. Similar to the Na2C03-loadedcase above 700 "C (as shown by the dotted line in Figure 5 ) , a straight line having a different slope may also be drawn. The apparent activation energy was estimated as 1.3 X lo2 kJ/mol. This indicates that activation energy for the reduction of iron oxide by carbon is the same order of magnitude as that for the reduction of Na20 by carbon. At 700 "C, the amount of COP reacted exceeded the amount of CO produced, and repeated experiments gave similar results. The reason for this cannot be clarified yet, but the formation of cementite (Fe3C) is one plausible reason:30 5Fe + COz 2Fe0 + Fe3C (13)
-
(29)Since a freah surface of iron-loaded char was prepared by the heat treatment up to lo00 "C, the results obtained from the first C02pulse offered significant physical meaning.
679
Conclusion By the use of pulse gasification and the TPD technique, the oxidation-reduction cycle between metal and metal oxide species on the char surface is elucidated. Oxidation of iron or reduced-state iron oxide after heat treatment of char under an He flow can be observed by pulsed C02 gasification as the formation of CO (CO main peak in the on-line gas chromatograph). Reduction of iron oxide of higher valency by carbon can be observed by the CO satellite peak followed by the CO main peak in the COz gasification of Fe(N03)3-loadedcoal char. The active form of Na2C03-catalyzedchar gasification seems to involve the following redox cycle: Na + COz Na20 + CO Na20 C Na + CO
+
-
-
Acknowledgment. This work was financially supported by a grant for "Special project for energy" from the Ministry of Education, Science and Culture of Japan. Critical comments from reviewers of this manuscript are also appreciated. Registry No. Na2C03,497-19-8;Fe(CO&, 10421-48-4; Na20, 1313-59-3;COz, 124-38-9;Fenom,1332-37-2;CO, 630-08-0. (30)Ohtsuka, Y.;Kuroda, Y.; Tamai, Y.; Tomita, A. Fuel 1986,65, 1476.
TPD Study on Coal Chars Chemisorbed with Oxygen-Containing Gases Zhan-Guo Zhang, Takashi Kyotani," and Akira Tomita Chemical Research Institute of Non-Aqueous Solutions, Tohoku University, Sendai 980, Japan Received March 1, 1988. Revised Manuscript Received April 14, 1988
The surface complexes formed by the chemisorption of oxygen-containing gases (02,COz,and H20) on Morwell coal char were investigated by using a temperature-programmed desorption (TPD) technique. TPD patterns of the chemisorbed chars indicated that there are many types of oxygen complexes both on carbon and mineral matter (Ca and Fe), and the amount and feature of these complexes strongly depends on the adsorption conditions. The study on Ca-derived surface complexes revealed that oxygen easily moved around over carbon and Ca. The Ca-catalyzed gasification mechanism is discussed in relation to the present TPD results. The solid-state reaction between CaC03 and C does not occur during the gasification, but Ca catalyst functions as the medium for oxygen transfer.
Introduction The mechanism of gasification of carbonaceous materials with oxygen-containing gases (02,C02,and H20) has been widely studied. It is generally accepted that the first step in the reaction is the chemisorption of the gases on carbon to form surface oxygen complexes and that such complexes act as reaction intermediates. The oxygen-chemisorption study was carried out to estimate the amount of surface active sites. Walker and his co-workers' first determined this amount and termed it active surface area (ASA). They correlated it with the reactivity of carbon black in 0 2 and (1) Laine, N. R.; Vastola, F. J.; Walker, P. L., Jr. J. Phys. Chem. 1963, 67,2030-2034.
0887-0624/88/2502-0679$01.50/0
emphasized the importance of this amount in the reaction kinetics. For other carbonaceous materials, the correlation between the rate and the ASA has also been demonstrated by many worker^.^-^ The temperature-programmed desorption (TPD) technique gives useful information about the surface complexes on carbon.&1° Surprisingly,however, most of these studies (2)Chen, C.-J.; Back, M. 'H. Carbon 1979,17, 495-503. (3)Radovic, L R.; Walker, P. L., Jr.; Jenkins, R. G. Fuel 1983,62, 849-856. (4)Ahmed, S.; Back, M. H. Carbon 1985,23,513-524. (5)Su,J.-L.; Perlmutter, D. D. AlChE J. 1985,31, 1725-1727. (6)Wigmans, T.;van Doorn, J.; Moulijn, J. A. Fuel 1983,62,190-195. (7)Causton, P.; McEnaney, B. Fuel 1985,64, 1447-1452.
0 1988 American Chemical Society
