Energy & Fuels 1987,1, 308-309
308
The range of Mcobtained by this method compares well with estimates based on a rigid-chain model applied to comparable ranks of coal,13in the range X = 0.3-0.5. It is seen that the extractable and transportable fragments of the coal have a number average molecular mass that is comparable to or smaller than the number average molecular mass between cross-links, for any reasonable value of
2. The downward shift in molecular mass distributions of pyrolysis tars with increasing temperature from a lignite and a low-volatile bituminous coal is consistent with occurrence of polycondensation during pyrolysis. 3. Pyrolysis fragments appear to be, on the average, comparable in size to or smaller than the number average molecular mass between cross-links in a Bruceton bituminous coal.
Conclusion
Acknowledgment. E.M.S. gratefully acknowledges the support of the US. DOE (Grants DE-FG22-81PC40803 and DE-AC18084FC1061),and J.W.L. thanks the Exxon Education Foundation and the Gas Research Institute for support of this work.
x.
1. A lignite is seen to cross-link at much lower pyrolysis temperatures than high-volatile bituminous coals. A lowvolatile bituminous coal is already too highly cross-linked to use solvent swelling to track pyrolysis behavior.
/y
9
9
Communications New Utilization of NaCl as a Catalyst Precursor for Catalytic Gasification of Low-Rank Coal Sir: Among many gasification catalysts, alkali-metal carbonates have been generally accepted to be the most promising ones. Although alkali-metal chlorides are cheaper than carbonates, they have two intrinsic disadvantages: (1)The affinity between cation and anion is so strong that the alkali-metal cation can not favorably interact with char; therefore, the catalytic effectiveness is small. (2) The C1 may muse corrosive problems on various parts of materials once introduced into the gasification system. Several successful attempts have been made to overcome the first demerit,ld3but none of them have been satisfactory with respect to the removal of chlorine. In the present paper, a new method utilizing NaCl as the catalyst raw material in the steam gasification of brown coal is presented. Not only high catalytic activity but also complete removal of C1 was easily achieved. Yallourn coal (C, 67.1; H, 4.8; N, 0.8; S, 0.3; C1,0.09; 0, 26.9 w t % (daf)), an Australian brown coal, was used. It was received in the form of briquettes and was crushed up to 100-200 mesh. Carboxyl and hydroxyl groups in the coal were determined by ion exchange with barium acetate and by acetylation? the amounts of these groups being 1.7 and 6.0 mequiv/g (daf) of coal, respectively. The oxygen-containing functional groups in such a low-rank coal act as cation-exchange sites. The method of catalyst addition essentially consists of a cation-exchange step and a water-washing step. Thus, 10 g of coal powder was soaked in 200 mL of NaCl solution in the concentration range 0.3-5 N, and the pH of the solution was adjusted by adding an adequate amount of NH,. As the ion exchange progressed, the pH of the solution decreased. The ion exchange was allowed to continue until there was no further change in pH. The exchanged coal was separated from the solution by fitration, washed twice with deionized water (100 mL each), and then dried at 380 K in N2. The amount of Na incorporated into the coal was determined (1)Huttinger, K. J.; Minges, R. Fuel 1984, 63, 9-12. (2) Lang, R. J. Fuel 1986,65, 1324-1329. (3) Huttinger, K. J.; Minges, R. Fuel 1985, 64, 486-490. (4) Zhou, P.; Dermer, 0. C.; Crynes, B. L. In Coal Science; Gorbaty, M. L., Larsen, J. W., Wender, I., Eds.; Academic: London, 1984; Vol. 3, pp 253-300.
0887-062418712501-0308$01.50/0
Table I. Catalyst Content and Reactivity in Steam at 923 K content, w t %
(dry) catalyst none NaCl NaCl NaCl NaCl NaCl NaCl NaCl Na2CO8 Na2C03
addn method
final pH
Na
ion exchange ion exchange ion exchange ion exchange ion exchange ion exchange impregnation impregnation impregnation
2.6 5.5 6.2 9.2 10.3 11.1
0.3O (O.l)b 1.7 (1.6) 2.6 (2.4) 3.5 (3.4) 4.4 (4.2) 5.3 (5.3) 2.7 (4.4) 3.6 (3.8) 5.3 (5.6)
C1 0.09 0.10 0.10
0.09 3.7
specific rate, h-' 0.15 0.27 1.6 1.8 2.2 2.1 2.6 0.21 2.0 2.4
Determined by the flame spectrochemical analysis. bEstimated from the ash content in the gasification residue.
by back-exchange with a dilute HC1 solution followed by flame spectrochemical analysis. The C1 content in the resulting sample was determined by the Eschka method. The steam gasification was conducted in a thermobalance at 923 K. Gasification was allowed to continue for 2 h, and the remaining char was completely burnt to check the total amount of ash and catalyst. Details of the gasification procedure have been described el~ewhere.~ The reaction consisted of the devolatilization stage and the following char gasification stage. The reactivities of Na2C03-impregnated and NaC1-impregnated coals were determined for comparison. These samples were prepared by evaporating Na2C03or NaCl solutions to dryness in the presence of coal. The results are summarized in Table I. The amount of exchanged Na was not dependent on the NaCl concentration but on the final pH of the solution; the extent of exchange increases with an increase in the pH. At pH 9.2, the Na content was 3.5 wt %, which corresponds to 1.6 mequiv/g (daf) of coal. It was very close to the amount of carboxyl groups in the raw coal, 1.7 mequiv/g (daf). This result suggests that only carboxyl groups are exchangeable sites for Na ion at pH less than 9. A t a high pH of 11, the Na loading increased to 5.3 wt % (2.4 mequiv/g). This suggests that Na ion was exchanged not only on carboxyl groups but also on phenolic hydroxyl groups ( 5 ) Tomita, A,; Takarada, T.; Tamai, Y. Fuel 1983, 62, 62-68.
Q - 1987 American Chemical Societv
Communications in this pH region. A similar phenomenon was observed by Schafer.6 The C1 content in washed coal was found to be as low as 0.1 wt %I ,which is nearly equal to that for the raw coal. This result is very important, since such a low content of C1 would cause little problem in the subsequent processing stages. The reactivity of Na-loaded coal is also shown in Table I. The specific rate of char, that is the gasification rate per unit weight of residual char, was independent of the char conversion up to about 7070, and thus the rates obtained were used to compare the catalytic effectiveness of Na. The reactivity of Na-exchanged coal increased with an increase in Na loading. The rate for the sample with 5.3 w t %I Na was 2.6 h-l, which was approximately 20 times that of raw coal. The rate for the Na2C03-impregnated coal with 5.3 wt 9% Na was 2.4 h-l. Thus the catalytic effect of Na prepared by the present method was comparable to that of Na2C03. On the other hand, the reactivity for the NaC1-impregnated coal was very low. This low activity is due to the high stability of the Na-C1 bond.2 A considerable amount of C1 was retained in this sample in contrast to the ion-exchanged samples. Since ion exchange initially provides essentially atomic dispersion, the ion-exchanged Na catalyst may form well-dispersed species on char surface after decarboxylation? It has been reported that alkali-metal catalysts loaded on coal are vaporized during the gasifi~ation.8~~ In order (6) Schafer, H.N.S. Fuel 1970,49, 197-213. (7) Yuh, S.J.; Wolf, E. E. Fuel 1984, 63, 1604-1609. (8)McKee, D.W.; Chatterji, D. Carbon 1978,16,53-57. (9) Same,D.A.;Talverdian,T.; Shadman, F. Fuel 1988,64,1208-1214.
Energy & Fuels, Vol. 1, No. 3, 1987 309 to clarify whether or not catalyst loss takes place in this gasification, the Na content in the ash obtained by burning the gasification residue was estimated. It was assumed for this estimation that all the Na in the ash was present as Na2C03,which was the only Na species identified by an X-ray diffraction analysis. The Na content is converted to a Na-loaded coal basis, and these values are shown in parentheses in Table I. The Na contents estimated from the amount of the ash are in fairly good agreement with the Na contents determined by the back-exchange of loaded coals. This suggests that the catalyst loss is not significant under the present conditions. This may be due to the lower gasification temperature of 923 K. The complete retention of Na as Na2C03shows that the recovery of Na from the ash may be feasible. In conclusion, C1-free Na catalyst can be prepared from NaCl solution by using the ion-exchange technique. This method is very effective for the activation of NaCl as a catalyst for steam gasification of low-rank coals. Registry No. Na, 7440-23-5; NaCl, 7647-14-5. (10) Present address: Department of Chemical Engineering, Gunma University, Tenjin-Cho, Kiryu 376, Japan. (11) To whom correspondence should be addressed.
Takayuki Takarada,lO Toshihide Nabatame Yasuo Ohtsuka, Akira Tomita**l Chemical Research Institute of Non-Aqueous Solutions Tohoku University, Katahira, Sendai 980, Japan Received November 10, 1986 Revised Manuscript Received February 28, 1987
