136
Energy & Fuels 1988,2, 136-141
that of our bituminous sample GM(C8), 4.8%. Their coal was ground to -400 mesh and suspended in water for 9 h while being agitated so that the handling was similar to our work except for a shorter suspension time. The iep for unoxidized vitrain shown in their Figure 7 was 5.8, in quantitative agreement with our work for sample GM(C8). Moreover, their slope (&"E/~H)iep appears to be identical with ours within experimental error. They also reported an iep of the same vitrain at pH 4.5 after 192 h of oxidation in pure oxygen at 80 "C. Casassa and Toor30 reported the iep of high Kaolin Splash Dam (17% ash) and lower Kittanning (8.7% ash) samples along with the iep of beneficiated samples, Splash Dam (1.6% ash) and lower Kittanning (2.3%). Both were bituminous coals that can be compared with our work. For the high-ash samples the iep was near pH 3, where kaolin has been reported to have a zero point of charge. The beneficiated samples had a positive mobility up to about pH 6, which they suggested "appeared to be the pattern of mobility associated with an unoxidized mainly carbonaceous surface." Casassa and Toor30cited a second paper by Wen and Sun31that included a study of unoxidized and oxidized vitrain. The oxidized vitrain showed no iep but the potential was always negative and ranged from -20 mV at pH 3 to -60 mV at pH 11.5. This is the same characteristic as reported by Laskowski for sieved samples, which might be expected to contain fines in a highly oxidized state. However, Wen and Sun also report that "unoxidized" vitrain had an iep at pH 4.5. The oxidized vitrain showed
an iep at pH 4.3 after treatment with M benzidene (4,4'-NH2C6H4C6H4NH2), which further increased to 6.2 on treatment with M benzidene. In order to reconcile their work with our results, Fuerstenau's itr rain:^ and the Splash Dam and Kittanning bituminous coals of Casassa and T o o P (the latter three of which suggest an iep at pH 5.8-61, we must conclude that Wen and Sun's "unoxidized"vitrain3' had acquired an acidic character perhaps through handling or sample history and that the treatment with benzidine removed the weakly associated acidic character. Marlow and Rowel12' have found the iep of GM sample C8 at pH 4.4 for a volume fraction of 0.4, a pronounced acid shift from the acoustophoreticvalue of 5.8 at a volume fraction of 0.04. We must consider that result as well in reconciliation of our work with the Laskowski results. Our discussion has been in terms of the isoelectric point (iep), which is the pH of zero net surface charge. What is measurable by electrophoresis is the point of zero mobility (pzm),which is zero net charge at the average surface of shear surrounding the moving particle. In the absence of specific adsorption, the iep and pzm are identical. The differences in the various samples compared may have been a consequence of differences in specific adsorption arising from handling and sample history. We prefer the term pzm because it is the measured quantity rather than the widely used term iep or the term pzr (point of { reversal) because the t potential is a derived quantity whose calculation depends on the assumption of a model of the charge distribution in the double layer.
(30)Casassa, E.Z.;Toor, E. W. Prepr. Pap.-Am. Chem. Soc., Diu. Fuel Chem. 1987,32,386-394. (31)Wen, W. W.; Sun, S. C. Sep. Sci. Technol. 1981,16,1491-1521.
Acknowledgment. This work was supported by a grant from the Atlantic Research Corp., Alexandria, VA. Technical assistance and instrument support was provided by Pen Kem, Inc., Bedford Hills, NY.
TPD Study on H20-Gasified and 02-Chemisorbed Coal Chars? Takashi Kyotani, Zhan-Guo Zhang, Shigetoshi Hayashi, and Akira Tomita* Chemical Research Institute of Non-Aqueous Solutions, Tohoku University, Sendai 980, Japan Received August 4, 1987. Revised Manuscript Received November 25, 1987 Temperature-programmed desorption (TPD) profiles of H20-gasifiedand 02-chemisorbedcoal chars were investigated with five coals of different ranks. Coals were devolatilized in Nz and then gasified in HzO at 1100 K. Initially, TPD of the partially gasified char was determined to 1100 K in vacuo. Later the TPD experiment was repeated after the 02-chemisorptionat 420 K on the above heat-treated sample. The sharp peaks in TPD patterns, corresponding to HzO, COP,and CO evolution, all resulted from the presence of mineral matter, since no sharp peak was observed in the TPD patterns of the demineralized coal chars. The gas evolution pattern from the demineralized char was almost independent of coal type, and all of the demineralized coal chars exhibited almost the same gasification reactivities. The relationship between the gasification reactivity and the total amount of C02 and CO evolution during TPD of gasified chars and 02-chemisorbed chars is discussed. Introduction It is generally accepted that the first step of the steam gasification reaction is the chemisorption of steam on a carbon substrate to form surface oxygen complexes and
that such complexes function as intermediates in the gasification reaction.'p2 In the reaction of carbon with oxygen, some investigators carried out oxygen chemisorption studies to estimate the amount of surface oxygen com-
Presented at the Symposium on the Surface Chemistry of Coals, 193rd National Meeting of the American Chemical Society, Denver CO, April 5-10, 1987.
(1) Walker, P. L., Jr.; Rusinko, F., Jr.; Austin, L. G.; Adu. Catal. 1959, 11, 133. (2)Ergun, S.; Mentser, M. Chem. Phys. Carbon 1965,1, 203.
088~-0S24/88/2502-0l36$01.50/0 0 1988 American Chemical Society
TPD Study on Coal Chars
coal Morwell Yallourn Wandoan Taiheiyo Grose Valley
" Volatile matter. Table 11. Metal coal Si MW 0.04 YL 0.04 WD 4.21 2.58 TH 4.97 GV
Energy & Fuels, Vol. 2, No. 2, 1988 137
code MW YL WD TH GV
Table I. Ultimate and Proximate Analyses of t h e Raw Coal ultimate anal., wt % (daf) proximate anal., wt % C H N S 0 moisture VM" ash 67.9 5.0 0.5 0.3 26.3 25.9 38.2 1.1 66.1 5.3 0.6 0.3 27.7 14.3 47.3 0.8 75.8 6.8 1.0 0.3 16.1 9.1 47.3 7.3 77.0 6.3 1.5 0.3 14.9 5.8 46.5 10.8 81.7 5.1 1.4 0.4 11.4 3.4 29.7 16.6
FCb 34.8 37.6 36.3 36.9 50.3
*Fixed carbon. Content A1 0.01 0.02 3.24 1.42 3.27
in t h e Raw Coal (wt %. Dry) Fe Ca Na K 0.10 0.30 0.05 0.00 0.26 0.08 0.08 0.00 0.20 0.60 0.12 0.16 0.34 0.65 0.10 0.16 0.46 0.01 0.02 0.11 ~
~~~~
~
p l e ~ e s .They ~ ~ attempted to correlate the amount of the oxygen complex with the oxidation reactivity and to clarify the role of the complex in the oxidation reaction. However, detailed features of the complex are not yet well understood. A temperature-programmed desorption (TPD) technique has successfully been used in examining the surface oxygen complex.&'" However, most studies were done by using pure carbons, and essentially no work has been done on the surface complexes formed on coal char during steam gasification. In the case of coal char, the role of the surface complex becomes more complicated due to the presence of inherent mineral matter. Our previous paper described a preliminary study on this subject." In that study, however, the gasified chars were exposed to air prior to the TPD run and also the reactant gas was contaminated by a small amount of air. Therefore, the history of the TPD sample was rather ill-defined. The present study was undertaken to clarify the nature of surface complexes under more strictly controlled conditions. After H 2 0 gasification, coal chars were cooled down and then their TPD patterns were recorded without the chars being exposed to air. After this TPD experiment, the sample was exposed to pure O2 and the TPD pattern was determined again in order to check the difference between surface complexes formed in H 2 0 and in 02.The effects of coal type and mineral matter on TPD pattern were also investigated. In addition, the gasification reactivity is discussed in relation to TPD patterns of char.
Experimental Section Materials. Five raw coals (32 X 60 mesh) and their demineralized coals were used in this study. The proximate and ultimate analyses of the raw coals are presented in Table I. Their names are abbreviated in this paper as listed in Table I. The metal contents of the raw coals are given in Table II. Demineralization of the raw coal was made by washing with a mixture of HCl and H F solutions. For TH coal, the above method did not sufficiently remove the inorganic matter. A portion of the TH coal was (3) Laine, N. R.; Vastola, F. J.; Walker, P. L., Jr. J.Phys. Chem. 1963, 67, 2030. (4) Ahmed, S.; Back, M. H. Carbon 1985,23, 513. (5) Radovic, L. R.; Walker, P. L., Jr.; Jenkins, R. G. Fuel 1983,62,849. (6) Freriks, I. L. C.; van Wechem, H. M. H.; Stuiver, J. C. M.; Bouwman, R. Fuel 1981,60,463. (7) Wigmans, T.; van Doorn, J.; Moulijn, J. A. Fuel 1983, 62, 190. (8) Causton, P.; McEnaney, B. Fuel 1985, 64, 1447. (9) Kapteijn, F.; Porre, H.; Moulijn, J. A. AZChE J. 1986, 32, 691. (10) Hermann, G.; Huttinger, K. J. Fuel 1986, 65, 1410. (11)Kyotani, T.; Karasawa, S.; Tomita, A. Fuel 1986, 65, 1466.
k w WD
.--
I TH
py 400
, , , 800
cob'
,_"A
400 6 0 0 SO0 1000 Temperature (K)
(a) H,O-gasified
char
(b)
800
0,-adsorbed
1000
char
F i g u r e 1. T P D patterns of Raw coal chars. subjected to pretreatment with a dilute H N 0 3 solution before being washed with a HCl/HF solution. This treatment resulted in the complete removal of mineral matter and a considerable increase in nitrogen and oxygen contents (N, 5.2%; S + 0,31.5%). The raw (denoted as "Raw") and demineralized ("Dem") coals were devolatilized in N2at 1100 K for 30 min in a small fluidized bed reactor as described in the previous paper." The ash contents in the resultant chars (not in coals) are listed in Table 111, which also includes the BET surface area of chars determined by Nz adsorption at 77 K. The areas of chars from brown coals (MW, YL) are much higher than other chars. The areas generally decreased upon demineralization. A p p a r a t u s and Procedure. Steam gasification, oxygen chemisorption, and T P D were sequentially carried out in the same quartz tube reactor, making it possible to carry out a T P D experiment without removing a sample to an ambient atmosphere after gasification. About 50 mg of devolatilized char was held in a quartz basket that was hung by a quartz spring. The sample was heated to 1100 K under N2 flow (40 mL/min (STP)), with the O2in the Nz being removed by a deoxygenator. Then the sample was gasified with 50 kPa of HzO vapor (diluted with N2) a t a total flow rate of 100 mL/min (STP). It was confirmed that no diffusional problem was present with this flow rate. The weight change during gasification was monitored by a spring balance. The gasification rate per unit weight of remaining char was almost constant and no peculiar rate profde was observed. After the char conversion reached 50 wt % (daf), the sample was cooled to 570 K in HzO, followed by cooling in Nz to 420 K. This partially gasified char is referred to as "G char" in this paper. The G char was outgassed to 0.1 Pa a t 420 K and then subjected to T P D at this pressure. The temperature was raised a t a linear rate of 10 K/min up to 1100 K. A large portion of the evolved gas was evacuated with a rotary pump, and a small portion of the gas was introduced to a gas analysis system. The gases (COz,CO, and HzO)were analyzed with a quadrupole mass spectrometer. After the T P D experiment of the G char, the temperature was lowered to 420 K in vacuo, and O2at 0.1 MPa was allowed to contact the
138 Energy &Fuels, Vol. 2, No. 2, 1988
Kyotani et al.
Table 111. Amount of Gas Desorption and Reactivity Raw char Dem char gas desorptionb gas desorption* SA,” m2/g G char 0 char reactivity: h-’ ash, wt % SA,” m2/g G char 0 char reactivity: h-’
coal MW YL WD TH
ash, wt % 3.6 2.3 13.1 25.2
293 213 3 8
43 33 23 29
96 46 72 77
2.1 1.2 1.5 0.96
GV
21.3
12
10
27
0.13
0.2 0.1 1.5 O.ld (3.9e 0.6
84 127
11 5 7 7 25
