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Energy & Fuels 2006, 20, 2314-2320
A Kinetic Analysis of Coal Char Gasification Reactions at High Pressures D. G. Roberts* and D. J. Harris CooperatiVe Research Centre for Coal in Sustainable DeVelopment, CSIRO DiVision of Energy Technology, P. O. Box 883, Kenmore, Queensland 4069, Australia ReceiVed June 13, 2006. ReVised Manuscript ReceiVed July 16, 2006
A Langmuir-Hinshelwood (LH) rate equation is often used for the incorporation of gasification reaction kinetics data into gasification models, as it is applicable over a wider range of conditions than the nth-order rate equation. The use of a LH rate equation at high reactant partial pressures has been questioned, however, with some authors recommending extra terms based on additional reaction steps. Unfortunately, the lack of agreement on the details of these additional steps makes the incorporation of high-pressure gasification reactivity data into gasification models potentially a difficult task. This paper presents further analysis of previously published reactivity data for the reaction of reference chars with 0.1-3.0 MPa of CO2 and, separately, H2O. This analysis is done using LH-style rate equations well-established for use with up to 0.1 MPa of the reactant. It is shown that, in the absence of product gases, these established LH rate formulations can describe the measured high-pressure char-gas reaction kinetics. Furthermore, theoretical predictions of surface coverage phenomena made using these equations agree with experimental measurements of the relative amount of reaction intermediates present on the char surface as the reactant pressure increases. The effects of char surface area, and how reaction at high-pressure develops this surface, are highlighted as an area of investigation requiring more work.
Introduction The reactions of coal chars with O2, CO2, and H2O at pressures above atmospheric are the subject of research interest in response to the recent increase in the development and application of advanced gasification technologies for power generation from coal. Of particular interest for bituminous coals is entrained flow gasification, which reacts coal at high temperatures (∼1500-2200 K) and pressures (up to 3.0 MPa), producing a tappable slag and a syngas consisting predominantly of CO and H2. The slowest step in the conversion of coal to syngas is the gasification of char. Under the intense conditions present in entrained-flow gasification technologies, there are many chemical and physical processes that combine to influence the conversion rate of coal char. These include gas diffusion to the char particle and through the pores of the particle, reaction at the surface, and diffusion of the products away from the reaction site. These processes are associated with the consequential changes in pore structure and, in some cases, the chemical composition of the char that result from the gasification of the carbonaceous material. Any combination of these processes can have a controlling influence on the rate of char conversion; the result depends on a range of process and sample properties such as temperature, reactant and product gas composition, particle size, and char morphology. Char conversion data obtained at high temperatures are important in furthering the understanding of coal performance under gasification conditions. Intrinsic reaction rate data, however, are obtained under conditions where chemical processes alone control the gasification rates and are required for the successful * Author to whom correspondence should be addressed. Fax: +61 7 3327 4606. E-mail:
[email protected].
development and implementation of gasification models that are both accurate and transportable across different technologies and reaction conditions. Of particular importance is the rate of reaction of the coal char with CO2 and H2O. The relatively slow rate of these reactions compared with the rapid pyrolysis and char combustion processes makes them fundamentally important in determining overall coal conversion rates in a gasifier. Models that are designed to predict high-temperature coal gasification behavior usually have a chemical reaction component that accounts for the variation in intrinsic reaction rate with temperature and reactant pressure, as well as coal type. This component is usually combined with an estimate of the effects of pore diffusion limitations to arrive at an overall gasification rate over a wide range of temperatures, for example, refs 1 and 2. Attempts to accommodate changes in char pore structure and surface area during reaction have been made, although this is a complex issue that still requires considerable experimental measurements under relevant conditions. The focus of this paper will be the chemical reaction component of such models, and how it can be practically applied while still accurately describing the intrinsic reactivity behavior of coal chars over a wide range of reactant partial pressures. The generally accepted reaction scheme for the gasification of carbon with CO2 is
Cf + CO2 h C(O) + CO
k1, k2
(1)
(1) Hla, S. S.; Harris, D. J.; Roberts, D. G. A Coal Conversion Model for Interpretation and Application of Gasification Reactivity Data. Proceedings of International Conference on Coal Science and Technology, Okinawa, Japan, 2005. (2) Liu, G.-S.; Rezaei, H. R.; Lucas, J. A.; Harris, D. J.; Wall, T. F. Fuel 2000, 79, 1767-1779.
10.1021/ef060270o CCC: $33.50 Published 2006 by the American Chemical Society Published on Web 09/15/2006
Coal Char Gasification Reactions at High Pressures
C(O) f CO
k3
Energy & Fuels, Vol. 20, No. 6, 2006 2315
(2)
where Cf is a free active site and C(O) is an adsorbed surface complex. In reaction 1, k1 is the rate constant for the forward reaction and k2 is for the reverse reaction. If the rate of desorption of surface complexes (eq 2 above) gives the rate of reaction,3 then the corresponding Langmuir-Hinshelwood (LH) rate equation becomes
FCO2 )
[Ct]k1PCO2 k2 k1 1 + PCO + PCO2 k3 k3
(3)
C(O) f CO
k′3
k′1, k′2
Cf + 1/2H2 f C(H)
(4) (5)
where, in reaction 4, k′1 is the rate constant for the forward reaction and k′2 that for the reverse reaction. In this mechanism, H2 inhibits the reaction by decreasing the amount of reaction intermediates (the so-called oxygen-exchange mechanism) but also by direct adsorption onto active sites via associative (eq 6) or dissociative (eq 7) adsorption: (3) Laurendeau, N. M. Prog. Energy Combust. Sci. 1978, 4, 221-270. (4) Strange, J. F.; Walker, P. L. Carbon 1976, 14, 345-350. (5) Hu¨ttinger, K. J. Carbon 1990, 28, 453-456. (6) Hu¨ttinger, K. J.; Nill, J. S. Carbon 1990, 28, 457-465. (7) Hu¨ttinger, K. J.; Fritz, O. W. Carbon 1991, 29, 1113-1118. (8) Blackwood, J. D.; Ingeme, A. J. Aust. J. Chem. 1960, 13, 194-209. (9) Nozaki, T.; Adschiri, T.; Fujimoto, K. Fuel 1992, 71, 349-350.
k4 k5
(6) (7)
[The processes represented by eqs 6 and 7 need not be separate and exclusivesit might be expected, for example, that the weaker associative adsorption in eq 6 is the first step in the formation of the more strongly bound C(H) via eq 7.] Because both inhibition mechanisms can occur10-12 and the relative influence of each can vary,11 the rate equation must be written as
FH2O )
where [Ct] is the total concentration of active sites. ([Ct] represents the total concentration of active sites on the surface of a char sample. If none of these sites are being used for reaction, then [Cf] ) [Ct]; that is, the concentration of free sites is equal to the total concentration of active sites. For a reacting char sample, however, this is not the case, and [Ct] ) [Cf] + [C(O)].) Inhibition of this reaction by CO occurs via an oxygenexchange mechanism (reverse eq 1). Strange and Walker4 performed a thorough kinetic analysis of the graphite-CO2 reaction, in the presence of CO, confirming the validity of this equation at low partial pressures of CO2 (0.02-0.2 bar). Hu¨ttinger et al.5-7 successfully used this equation to interpret carbon-CO2 reaction kinetics generated at atmospheric pressure (CO2 concentrations between 10 and 100%). Their analysis showed that the LH formulation above describes the reaction of carbon with CO2 as a function of temperature and CO2 partial pressure well (up to 1 bar CO2). Furthermore, their work provided a direct link between the number of adsorbed surface complexes (formed from the dissociative adsorption of CO2 onto char active sites) and the overall reaction rate. Early work at high pressures (up to 4.0 MPa) by Blackwood and Ingeme8 found that extra reaction steps were required to explain their rate data at high pressures. Nozaki et al.9 modeled a char-CO2 system over a range of pressures (up to 1.0 MPa) and also reported that eq 3 was not able to adequately describe their measured rate of reaction as CO2 pressure increased. Nozaki et al.’s work also found that Blackwood and Ingeme’s extra equations were unsuitable for their data and proposed an alternative set of high-pressure reactions. The char-H2O reaction proceeds with a similar mechanism to the char-CO2 reaction:
Cf + H2O h C(O) + H2
Cf + H2 f C(H2)
[C′f]k′1PH2O k′1 1 + k*PH2 + PH2O k′3
(8)
where k* is not a rate constant in its own right but, rather, a combination of rate constants depending on the relative influence of the two inhibition processes. This is consistent with previous fundamental studies of the char-H2O reaction.10 The work presented in this paper, however, is based on data measured in a simplified system containing insignificant amounts of CO or H2. Therefore, the details of product inhibition, the mechanisms by which it occurs, and its effect on the reaction rates will not be discussed. These issues are being investigated, however, and will form the basis of future publications in this area. Mu¨hlen et al.13 successfully modeled a reaction system containing both CO2 and H2O, in the presence of product gases, using variations and combinations of eqs 3 and 8, as well as additional high-pressure reactions for both the C-CO2 and C-H2O reactions. The resulting equation was extremely complex, incorporating a large number of mixed and squared terms, but was able to describe the reaction kinetics of a German coal char that was the subject of their investigation. The coalspecific nature of their data, however, complicates a more general application of their results. To simplify the description of char-gas reaction kinetics as a function of temperature and reactant pressure, some gasification models developed for use at atmospheric pressure (e.g., ref 14) have used the generally accepted3,15-18 nth-order rate equation:
Fg ) SA‚Ai exp
( )
-Ea n P RT g
(9)
which is an overall representation of the individual rate equations discussed above, usually valid over a specific range of condi(10) Hu¨ttinger, K. J.; Merdes, W. F. Carbon 1992, 30, 883-894. (11) Lussier, M. G.; Zhang, Z.; Miller, D. J. Carbon 1998, 36, 13611369. (12) Smith, J. G. Effects of Hydrogen on the Reactivity of Petroleum Coke to Steam. MAppSci Thesis, NSW Institute of Technology, Sydney, Australia, 1987. (13) Mu¨hlen, H.-J.; van Heek, K. H.; Ju¨ntgen, H. Fuel 1985, 64, 944949. (14) Beath, A. C. Mathematical Modelling of Entrained Flow Coal Gasification. Ph.D. Thesis, University of Newcastle, New South Wales, Australia, 1996. (15) Miura, K.; Hashimoto, K.; Silveston, P. L. Fuel 1989, 68, 14611474. (16) Harris, D. J.; Smith, I. W. Intrinsic Reactivity of Coke and Char to Carbon Dioxide. Proceedings of 197th ACS National Meeting, Dallas, TX, 1989; American Chemical Society: Washington, DC, 1989; pp 94-101. (17) Harris, D. J.; Smith, I. W. Proc. Combust. Inst. 1990, 23, 11851190. (18) Salatino, P.; Senneca, O.; Masi, S. Carbon 1998, 36, 443-452.
2316 Energy & Fuels, Vol. 20, No. 6, 2006
tions. In such an application of this equation, the pre-exponential factor contains effects of other variables known to influence char-gas reactions, for example, carbon crystallinity, catalysis by mineral matter, and so forth. The intrinsic pre-exponential factor, Ai, does not include effects of surface area and would therefore be used in conjunction with a measured or predicted value of surface area, SA. Recent work by the authors19 reported effects of reactant pressure (up to 3.0 MPa) on the chemical reaction kinetics of chars reacting with CO2 and H2O. When analyzed using eq 9, the data increased with reactant partial pressures in a manner consistent with data previously obtained at CO2 partial pressures up to 0.1 MPa.16,17 However, at pressures greater than ∼1.5 MPa of CO2, further increases in reactant pressure had much less of an effect; that is, the “pressure order” (n in eq 9) of the reaction was not constant and decreased with increasing pressure. This finding is consistent with work summarized by Dutta et al.,20 who compared their own CO2 gasification data with data from other investigations generated over a range of temperatures and pressures, and with a variety of carbons. It has also been reported in recent work by the authors for reactions with CO2, H2O, and O219 and by others for reactions with O221 that the true activation energy of the reactions, that measured in the absence of all diffusion and mass-transfer limitations, was not affected by the reactant or total pressure. The value of A obtained with specific rate data showed some effects of pressure. These effects were significantly reduced upon normalization to the surface area of the char (i.e., when based on intrinsic data).19 This suggests a significant effect of reactant pressure on the development of char surface area during reactionsthe implication of this is discussed further in subsequent sections of this paper. It is therefore apparent that a single, overall nth-order rate equation is unsuitable for use in describing the rate of the char gasification reactions at increased pressures, because n (and, to a lesser extent, Ai) is not constant with pressure. (Should the pressure effects on n and Ai be predictable, however, modifications to this equation may increase the pressure range over which it is practically applicable.) Analyses of high-pressure CO2 and H2O reactivity data9,13 have recognized the limitations of the nth-order rate equation and have used rate equations based on a Langmuir-Hinshelwood reaction scheme. While it has been shown that the LH equations (with the possibility of additional high-pressure equations) can be used to describe the rate of the char gasification reactions, there is a lack of a good agreement as to how LH-based rate equations should be used to describe the chemical reaction rate component in gasification models. Liu et al.2,22 have shown how a simplified version of eq 5 (with simplified “rate constants” and the use of an overall “activation energy”) can be used at high pressures to incorporate C-CO2 kinetics into a simple char gasification model. However, it is still unclear what level of complexity is required in the LH reaction equation (and the consequent need for specific reaction rate data) to describe coal char reaction rates at high pressures for application to practical gasification systems. This paper analyzes some high-pressure char gasification kinetics previously reported by the authors19 to ascertain the (19) Roberts, D. G.; Harris, D. J. Energy Fuels 2000, 14, 483-489. (20) Dutta, S.; Wen, C. Y.; Belt, R. J. Ind. Eng. Chem. Process Des. DeV. 1977, 16, 20-30. (21) Hecker, W. C.; Madsen, P. M.; Sherman, M. R.; Allen, J. W.; Sawaya, R. J.; Fletcher, T. H. Energy Fuels 2003, 17, 427-432. (22) Liu, G.-S.; Tate, A. G.; Bryant, G. W.; Wall, T. F. Fuel 2000, 79, 1145-1154.
Roberts and Harris Table 1. Proximate and Ultimate Analyses of the Parent Coals and Their Chars Used in This Work coal B
char B
coal D
char D
moisture ash volatiles fixed carbon
9.1 6.8 26.7 57.4
Proximate Analysis (% ad) 0.8 3.4 0.2 11.2 6.6 14.0 0.0 38.6 0.0 87.6 51.4 85.9
carbon hydrogen nitrogen sulfur oxygen
83.5 4.84 1.84 0.35 9.5
Ultimate Analysis (% daf) 97.0 82.9 98.6 0.1 5.95 0.17 0.74 1.83 0.92 0.88 0.62 8.4
