Energy & Fuels 1991,5,68-74
68 2599 0
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-
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-
; 599 -
I
II
I
I
U
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I
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Figure 13. Parity plot for S2correlation, eq 9. 60’ axial flow
impeller.
before the bed is expanded to the upper screen and particle carryover is not observed. If S2is less than S3the bed expands to the limit.
Conclusions A study of many different impeller variations identified two designs of interest. A radial flow design performed
well in liquid/solid applications but was unsuitable in gas/liquid/solid service. An axial flow design worked well in both applications. For the radial flow impeller, correlations were developed for (1) stirrer speed at incipient fluidization, SI,and (2) stirrer speed at particle carryover, S,. For the axial flow impeller, correlations were developed for (1)stirrer speed at incipient fluidization, SI,(2) stirrer speed at particle carryover, S2, and (3) stirrer speed corresponding to onset of cavitation. When S2 > SB,bed expansion is limited. These correlations are limited to reactors of this type and size and to the range of physical properties studied. Indications are that the “HaynesBorgialli” liquid fluidized bed microreactor will provide a promising new tool for screening coal liquefaction catalysts.
Acknowledgment. This work was supported in part by the US. Department of Energy (Grant DE-FG2288PC88942). Such support does not constitute an endorsement by DOE of the views expressed herein. Mr. Zhang’s support was provided by the Department of the Interior’s Mineral Institute program administered by the Bureau of Mines under allotment grant no. G1194156. Funds for the reactor were provided by the Industrial Fund of the University of Wyoming.
A Transient Kinetics Study of Char Gasification in Carbon Dioxide and Oxygen? Ljubisa R. Radovic,* Hong Jiang, and Anthony A. Lizzio Department of Materials Science and Engineering, Fuel Science Program, The Pennsylvania State University, University Park, Pennsylvania 16802 Received May 18, 1990. Revised Manuscript Received September 4, 1990
Transient (unsteady-state) kinetics, a relatively new technique for studying noncatalytic gas/solid reactions, has been used successfully to further our understanding of char (carbon) gasification. It provides the unique capability of separately determining the reaction rate constant (site reactivity or turnover frequency) and the number of active sites participating in the reaction (reactive surface area). Its application to the uncatalyzed gasification of coal-derived chars and polymer-derived carbons is illuatrated. In particular, the heretofore elusive quantitative understanding of their reactivity variations with conversion has been achieved for gasification in both carbon dioxide and oxygen.
Introduction The rate (R)of a heterogeneous gas/solid reaction (e.g., char gasification) at a given level of solid conversion (X) and at constant reactant gas pressure can be expressed in the following form:’v2
In eq 1, C, is the “concentration” of the solid. I t is conventionally identified with the total surface area (TSA) of the solid. Two parameters are of primary practical and fundamental interest here: the rate constant (k) and the Presented, in part, at the Symposium on the Fundamentals of Gasification, 197th National Meeting of the American Chemical Society, Dallas, April 9-14, 1989.
0887-0624/91/2505-08$02.50/0
time ( t )necessary to achieve complete solid (char) conversion. The rate constant for char (carbon) gasification expressed per unit TSA is very much dependent on the origin and/or thermal history of the ~ h a r . ~The ~ ~rate .~ constant expressed per unit active surface area (ASA) was found to be a function of temperature only, within a factor of 2-3.’~~ At constant reactant gas pressure, the complete char consumption time is determined from the following expression: (1) Radovic, L. R.; Walker, P. L., Jr.; Jenkins, R. G. Fuel 1983,62, 849-856. (2) Lizzio, A. A,; Jiang, H.; Radovic, L. R. Carbon 1990, 28, 7-19. (3) Radovic, L. R.; Steczko, K.;Walker, P. L., Jr.; Jenkins, R.G. Fuel Process. Technol. 1985, 10, 311-326. (4) Smith, I. W. Fuel 1978,57, 409-414.
0 1991 American Chemical Society
Energy & Fuels, Vol. 5, No.1, 1991 69
Char Gasification in C02 and O2
I t is seen that the integration of this expression requires the knowledge of the conversion dependence of the char concentration term, i.e., C, = f(X).&' Equation 1can be rewritten as2
RSA - k(RSA) TSA ASA
R = k(TSA)-
(3)
Very few, if any, studies of carbon and char gasification kinetics have attempted the direct determination of the reactive surface area (RSA). In practically all existing kinetic models, for example, the assumption is made that RSA/TSA is constant and the gasification reactivity profiles (reactivity variations with conversion) are thus interpreted on the basis of TSA variations with conversion. More often than not, however, this assumption is not valid? In our experience, the useful simplifying assumption that RSA/ASA is constant is most often valid to within a factor of 2-3.1*8 When a more precise comparison of reactivities of different chars is needed or when eq 2 needs to be solved for a given char, this assumption needs to be relaxed and tested. The technique of transient kinetics has often been used in heterogeneous catalysis! It has the virtue of being able to provide separately the rate constant and the (re)active site density (RSA). Only recently has its use been suggested1° and reported"J2 for providing new insights into carbon (char) gasification kinetics. In this paper, we discuss ita application to char gasification in carbon dioxide and oxygen. Theory Gasification in Carbon Dioxide. The well-known Langmuir-Hinshelwood mechanism for the C-C02 reaction was considered to be applicable13 Cf + c02 C(0) + co (4)
-
C(0) 2% co + Cf (5) where Cf is a free (re)active site and C ( 0 ) is the reactive intermediate on the carbon surface; k2 is the rate constant for the irreversible reaction (eq 5). Irrespective of the rate-determining step for this reaction, at steady state the reaction rate is equal to the rate of desorption of the reactive intermediate. At steady state, Le., with the reactant gas (CO,)being continuously added to the flowing feed stream, a mass balance on the surface intermediate, C(O), gives the following: d[C(O)l - rate of production of C(0) -
--rlt --
rate of consumption of C ( 0 ) = 0 (6)
(5)Laine, N. R.;Vastola, F. J.; Walker, P. L., Jr. J. Phys. Chem. 1963, 67,2030-2034. (6)Taylor, R.L.;Walker, P. L., Jr. Extended Abstracts, 15th Biennial Conference on Carbon, Philadelphia,PA, 1981;pp 437-438. (7)Radovic, L. R.;Lizzio, A. A. Proc. 4th Annu. Pittsburgh Coal Conf. 1987,440-444. (8)Garcia, X.;Radovic, L. R. Fuel 1986,65, 292-294. (9)Biloen, P. J. Mol. Catal. 1983,21,17-24. (10)Kapteijn, F.; Moulijn, J. A. In Carbon and Coal Gasification; Figuieredo, J. L., Moulijn, J. A., Ede.;NATO AS1 Series E 105; Martinus Nijhoff: Dordrecht, 1986; pp 291-360. (11)Freund, H.Fuel 1986,65,63-66. (12)Adschiri, T.; Zhu, 2.-B.; Furusawa, T. Proceedings of the International Conference on Coal Science: Elsevier: New York. 1987:. DD 515-518. (13)Walker, P. L., Jr.; Rusinko, F., Jr.; Austin, L. G. Adu. Catal. 19159, 11, 133-221.
__
Table I. Coal Analysis Data Proximate Analysis (As Received) 4.19 volatile matter, % moisture, % ash, % 15.20 fixed carbon, % ash, % carbon, % hydrogen, % nitrogen, %
Ultimate Analysis (DryBasis) 15.85 sulfur, % 67.48 chlorine, % 4.82 oxygen, %
33.67 46.94 3.95
0.05 6.63
1.22
In the transient kinetics experiment (see Experimental Section), the steady state was interrupted and the unsteady-state behavior (i.e., a decay in the appearance of CO) was monitored after abruptly changing the gaseous atmosphere from the reactive one (Cod . ". to an inert one (e.g., N2). Under these conditions1° rate of production of C ( 0 ) = 0
(F ) ~
= -rate of consumption of
c(0)
From eq 5 and neglecting the reverse reaction in rate of consumption of C(0) = k,[C(O)j, and therefore
(F),
= -IZ,[C(O)]tr
With removal of C 0 2 taking place at t = 0 [C(O)I,, = [C(O)l, exp(-k2t) where the subscript ss stands for "steady state" and tr stands for "transient". The slope of the resulting straight line on a semilog plot of CO concentration vs time should give the gasification (desorption) rate constant (k2 or turnover frequency, i.e., reactivity per site) and the y intercept gives the concentration of reactive sites (RSA) at the conversion level at which the steady-state reaction was interrupted. Gasification in Oxygen. The mechanism of the carbon-oxygen reaction is more complex than that of the C / C 0 2 rea~ti0n.l~For example, Marsh15 presented an extensive set of postulated elementary steps for this reaction which takes into account the variable CO/CO2 ratio of the reaction products and mobility of oxygen on the carbon surface. LaurendeaulS summarized the principal kinetic expressions that have been proposed to describe the C-02 reaction and concluded that the rate-determining step is primarily a function of the reaction temperature. It is well-known that CO and C02 are both primary products in the C-02 reaction.15 In light of these findings, it is assumed in the present study that both CO and C02 evolved in a TK experiment originate from the reactive carbon atoms. Experimental Section Sample Preparation. A high-volatile A Illinois No. 6 bituminous coal (PSOC-1098)"was used as a precursor for a typical coal char whose inorganic constituents possess no significant catalytic effects. Its thorough characterization is on file at the Penn State/DOE Sample and Data Bank." The proximate and ultimate analyses are given in Table I. The as-received coal was (14)Essenhigh, R. H.In Chemistry of Coal Utilization, 2nd Suppl. Vol.; Elliott, M. A., Ed.; Wiley: New York, 1981;pp 1153-1312. (15) Marsh, H. Chem. SOC.Spec. Publ. 1978;No. 32, pp 133-174. (16) Laurendeau, N. M. Prog. Energy Combust. Sci. 1978,4,221-270. (17)Penn State/DOE Coal Data Base, The Pennsylvania State University.
Radovic et al.
70 Energy & Fuels, Vol. 5, No. 1, 1991
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,--Switching Valve
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Reactive
,e‘
ControUcr
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Gas
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Figure 1. Schematic representation of the transient kinetics apparatus. ground by using an agate mortar and pestle and the -100 mesh and/or -250 mesh (
