Energy & Fuels 1991,5, 610-611
610 150r
:i
+ blank
I
--
8 O-melhylaled
+ O-butylated
+ O-oclylated
100
mg/g
80
I
100
+
2
50
*
20
Whole
t
,
::LYlated
I
O-butylated
+ O-octylated
0
50
100
150
200
TIME (minutes)
Figure 2. Desorption of benzene by O-alkylated Illinois No. 6 Coals at 30
O C
under vacuum (CO.1 Torr).
Table 11. Sorption Data for Extracted, O-Alkylated Illinois No. 6 Coals and Benzene at 30 OC ( p / p o= 0.59) wt % equilib equilib benzene time, amt,' percentage coal extractable' min mg/g desorbed untreated 2 3000 124 67 blank 6 300 217 O-methylated 20 30 226 88 0-butylated 18 30 191 98 O-octylated 23 30 173 99 dmmf basis.
methylation. The enhanced rate of benzene sorption observed for the O-butylated and O-octylated coals may be related to the inherent flexibility of the added alkyl groups themselves. That is, some regions of these coals may be somewhat fluid in nature, allowing benzene even more rapid penetration. Another factor that may be important is the "dilution" by the larger alkyl groups. The 0octylated coal is nearly half octyl groups by weight. If secondary forces other than hydrogen bonding must be overcome to get penetration by benzene, the O-octylated coal possesses fewer of these forces per unit weight of coal than the O-methylated or untreated coals. Desorption curves were also obtained for the coals, which are shown in Figure 2. A large fraction of benzene could not be desorbed from the blank coal after pumping under vacuum for 24 h (see Table I). The blank coal apparently collapses and traps a substantial amount of benzene. In contrast, most of the benzene could be desorbed from the O-alkylated coals. These coals apparently maintain more open structures which allows for more complete desorption of residual benzene. The coals studied above were not extracted prior to the sorption experiment. In order to assess the effect of the extractable material on the sorption process, all coals were exhaustively Soxhlet-extracted with benzene and then dried prior to exposure to benzene vapor. The results are summarized in Figure 3 and Table 11. A major effect of the benzene extraction is to increase the rate of sorption for the untreated, blank, and O-methylated coals. The effect is most dramatic for the blank coal. The sorption curves for the extracted, O-alkylated coals are all very similar as shown in Figure 3, with equilibrium achieved in about 30 min for each coal. The reason for the more rapid attainment of equilibrium for the extracted coals might be attributed to an increase in surface area created by extraction with benzene. We have no direct evidence for increased surface area, but Peterson et al. have noted that the surface area of a Roland seam coal increases by almost a factor of 2 by extraction
0
20
40
60
80
100
120
140
160
180
200
Time (minutes)
Figure 3. Sorption of benzene by extracted, 0-alkylated Illinois No. 6 coals at 30 O C and relative pressure of 0.59.
of benzene at 150 O C . l S Increased surface area should result in more rapid penetration of benzene. All extracted coals, with the exception of the untreated coal, sorb more benzene than the corresponding unextracted coal. This result may also be explained by an increase in surface area caused by extraction (more adsorbed benzene). The sorption results obtained on this single coal suggest that the network of hydrogen bonds in the coal may control the rate of diffusion of some solvents/reagents into it. If so, simple O-methylation of the coal may be a convenient way of increasing the reaction rate of many coal reactions, particularly if the reaction is thought to be diffusioncontrolled. Acknowledgment. The generous support of the U.S. Department of Energy, Grant No. DE-FG22-88PC88924, and the Research Corporation is acknowledged. (15)Medeiros, D.;Peterson, E. E. Fuel 1979,58, 531-533.
Thomas K. Green,* James E.Ball, Kevin Conkright Department of Chemistry, Center for Coal Science Western Kentucky University Bowling Green, Kentucky 42101 Received June 13,1990 Revised Manuscript Received March 11, 1991 Comparison of Steam Gasification Rate and COz Gasification Rate through the Surface Oxide Complexes Sir: It is generally recognized that coal gasification proceeds through the surface oxide complex, C(O), as an intermediate species.' Thus the gasification of coal by steam and C02 can be analyzed by the evaluation of both the amount of surface oxide complex and the rate of its formation and desorption. Freund introduced the transient kinetic method to evaluate the desorption rate of surface oxide complexa2 Other workers have applied it for the evaluation of the amount of surface oxide complex,3f)in (1) Marsh, H.; OHair, E.; Wynne-Jonea,W. F. K. T".Faraday Soc.
1965,61, 274. (2) Freund, H. Fuel 1986, 65, 63.
(3)Zhu, Z.-B.;Adschiri, T.; Furuaawa, T. R o c . 1987 Znt. Conf. Coal
Sei. 1987, 515.
(4) Cerfontain, M. B.; Meijer, R.; Kapteijn,F.; Moulijn, J. A. J. Catal. 1987,107, 173. (5)Lizzio, A. A,; Jiang, H.; Radovic, L. R. Carbon 1990,28, 7. (6)
Zhu, Z.-B.;Furusawa, T.; Adachiri, T.; Nozaki, T. R e p r . Pap,-
Am. Chem. SOC.,Diu. Fuel Chem. 1989, 34(1), 87.
0887-062419112505-0610%02.50/0 0 1991 American Chemical Societv
Energy & Fuels, Vol. 5, No. 4, 1991 611
Communications 3,
o
1123K PHzo=25kPa terminated after 74% burn-off
c
II
0 co
A CO,
-
I
1123K at 50 % burn-off
I1
gasification reagent
I
co
//’
9
///
, 100
n
I
200
20 30 50 100
0
/
\A
0
A
0
” ,/’ 2
[ 10 mol
I03 25
I
mol
Figure 2. Relationship between the amount of surface oxide complex and the gasification rate.
Time [ sec 1
Figure 1. A typical response after terminating steam gasification.
COz gasification. Lizzio5 and Zhus reported that the rate of C02gasification is proportional to the amount of surface oxide complex. So far, all of these experiments were conducted for COz gasification. In this study, we evaluated the amount of surface oxide complex formed through steam gasification by using the same technique. The result suggests that the desorption rate of the surface oxide complex formed through steam gasification is almost the same as that of the surface oxide complex formed through carbon dioxide gasification. Baiduri coal char was employed as a sample. Char was prepared with a fluidized bed pyrolyzer at 1173 K.’ About 20 mg of char (particle size 0.5-0.59 mm) was loaded in a quartz tube reactor (3.5 mm i.d., 5 mm 0.d.). Steam diluted by nitrogen (10-25%) was fed to the reactor at 10 mL/s and periodically switched over to and from NZ.All lines of the apparatus were heated at about 500 K to prevent water condensation. The effluent gas was sampled intermittently at intervals of several seconds after water was removed in a cold trap. Hydrogen concentration in the product gas was measured by using a MS-13X column and a TCD detector. Concentrations of CO and COzwere measured by using a Porapack N column and a FID detector with a methanator. Other compounds were not detected. During gasification, hydrogen concentration, [C,,], was equal to [Cco + 2Ccoz]. Carbon conversion rate, dX/dt, and carbon conversion level, X,were calculated as dX (CCO + CCOJ -= (1 \ \-/
Figure 1shows a typical response just after steam was switched over to nitrogen. Hydrogen was not detected in the transient response. A stepwise response experiment of CO which was conducted to evaluate the time constant of our system proved that the time constant of the system was several seconds at 1123 K. The decay curve of CO in Figure 1 is not exponential. The response in C02 gasification reported by Lizzio is not exponential, either.6 A (7) Adschiri, T.; Shiraha, T.; Kojima, T.; Furusawa, T. Fuel 1986,65, 1688. (8) Johnson, J. L. Chemistry of Coal Utilization, Second Supplementary Volume; Wiley-Interscience: New York, 1981; pp 1579-1592.
detailed discussion will be given in a later paper. Concentrations of CO and COz were monitored for about 30 min, until they reduced to the undetectable level. The total amount of CO produced was calculated as
1
“CCOF
nco =
0.012 dt
0.025 w,
(3)
where Cco is carbon monoxide concentration, F is gas flow rate [m3/s], and W,, is initial amount of carbon [kg]. The total amount of COP,ncq,, was also evaluated in the same way. In this study the total amount of C02was neglected and the amount of surface oxide complex was defined as the total amount of CO evaluated by eq 3, since COz disappeared quickly and ncOzwas 1 order of magnitude smaller than nco. A widely accepted mechanism8 of steam gasification is shown below. C + HzO C(0) + H2 (4) C(0) co (5) Carbon monoxide formation shown in Figure 1is attributed to reaction 2. Carbon dioxide was also observed in this response; it cannot be explained by the above-mentioned mechanism. It may be due to a reaction between surface oxide complexes (2C(O) Cf COz). Figure 2 shows the relation between the gasification rate and the amount of surface oxide complexes at X = 0.5. The previous result obtained for C02 gasification is also shown in this figure. A linear relationship between gasification rate and the amount of surface oxide complexes was obtained. Furthermore, the results of both steam gasification and C02gasification fell on the same line. The ratio of gasification rate to the amount of surface oxide complex is the desorption rate of surface oxide complex, the value of which is 0.05 s-l. Thus this result suggests that the desorption rate of surface oxide complex in HzO gasification is equal to that in COz gasification. Furthermore, the result obtained here demonstrated the validity of the method to evaluate the amount of surface oxide complex.
--
-
+
‘Department of Biochemistry and Chemical Engineering, Tohoku University, Aoba-ku, Sendai, 980 Japan.
Takao Nozaki,* Tadafumi Adschiri,’ Kaoru Fujimoto
Department of Synthetic Chemistry The University of Tokyo 7-3-1Hongo, Bunkyo-ku, Tokyo, 113 Japan Received December 18, 1990 Revised Manuscript Received March 11, 1991
