Energy Fuels 2010, 24, 5300–5308 Published on Web 09/22/2010
: DOI:10.1021/ef100980h
Kinetics of Char Gasification with CO2 under Regime II Conditions: Effects of Temperature, Reactant, and Total Pressure Daniel G. Roberts,*,† Elizabeth M. Hodge,†,‡ David J. Harris,† and John F. Stubington‡ †
CSIRO Energy Technology, PO Box 883, Kenmore, QLD 4069 Australia, and ‡School of Chemical Sciences and Engineering, University of New South Wales, NSW 2052 Australia Received July 29, 2010. Revised Manuscript Received August 29, 2010
There are relatively few studies reported in the literature characterizing the kinetics of high-pressure, high-temperature char gasification reactions. Of particular interest to the application of kinetic data to gasification models are studies that provide links between reaction rate data relevant to high-temperature conditions, where some extent of pore diffusion limitation may apply, and lower-temperature, intrinsic gasification reactivity data. This work describes the effects of temperature, reactant partial pressure, and total pressure on the kinetics of the char-CO2 reaction under conditions where reactant diffusion through the pores of the reacting char has been shown to impact the overall conversion rate. A global nth-order rate equation was used to describe these kinetics, in particular, the effect of temperature (via the activation energy) and reactant pressure (via the reaction order). As expected, activation energies at high temperatures (g∼1300 K) were consistently less than those obtained under conditions free of diffusion limitations; the extent to which the measured activation energies were less than the “true” activation energy was dependent on the extent of diffusion limitation. The effect of CO2 partial pressure was less than expected, based on measurements of the intrinsic reaction orders, although increasing the CO2 partial pressure did lead to an increase in the measured reaction rate. Total pressure was not found to have a significant and systematic effect on the measured kinetics;this is consistent with Knudsen diffusion being the dominant mode of diffusion in these char samples, which have relatively small pore size (∼15-30 A˚).
the pore structure of the reacting char particles becomes significant, and an accurate understanding of char microstructure and morphology is required to interpret and analyze the overall gasification rates effectively.3 Recent work has demonstrated that (1) the rate of reaction of char with CO2 at high temperatures (1373-1673 K) and pressures (2.0 MPa) is controlled by a combination of Regime I reaction kinetics and the rate of diffusion of reactants through the char pores, and (2) these two processes can be reliably separated and quantified with an adequate knowledge of the char-gas reaction kinetics and the morphology and porosity of the reacting particle.3 Knowledge of Regime I reaction kinetics, and how these kinetics are affected by temperature, reactant, and total pressure, is reasonably well-advanced. The char-CO2 reaction is considered to follow a Langmuir-Hinshelwood (LH) reaction mechanism, and a rate equation derived from this mechanism describes the effects of reactant partial pressure and temperature adequately over a relevant partial pressure range.4-7 Because of the relative complexity of a LH approach, however, effects of temperature and pressure on char gasification rates are commonly quantified using the nth order (so-called “global”) rate equation, with global activation energy and reaction orders applicable under the conditions of interest. The temperatures used in these Regime I studies are, by necessity, lower than many practical applications of the results.
Introduction Gasification-based energy systems are expected to play an important role in the future energy mix. Research into the fundamental aspects of the gasification process has clearly identified the rate of gasification of char as a significant factor controlling coal or biomass gasification behavior. This is due primarily to the relatively slow kinetics of the reactions of chars with H2O and CO2 under gasification conditions. The importance of the rates of these reactions to gasifier performance and efficiency has been the subject of extensive research, and data on the intrinsic reaction kinetics of a wide range of carbonaceous chars has been developed and reviewed.1 Recent emphasis has been on the effects of high pressure on these kinetics.2 To appropriately measure intrinsic rate data, most of these investigations have been performed at relatively low temperatures, where the rate of diffusion through the pores of the reacting chars plays no role in determining the overall rate of reaction: these are usually referred to as “Regime I”, or kinetically controlled conditions. Such work provides important and useful data regarding the kinetics of the char-gas reactions. Under realistic industrial gasification conditions, in particular, those experienced in entrained-flow gasification technologies, temperatures are significantly higher than the conditions under which these fundamental experiments are performed. Under such conditions, the rate of diffusion of reactants through
(3) Hodge, E. M.; Roberts, D. G.; Harris, D. J.; Stubington, J. F. Energy Fuels 2010, 24, 100–107. (4) Roberts, D. G.; Harris, D. J. Energy Fuels 2006, 20 (6), 2314–2320. (5) H€ uttinger, K. J. Carbon 1990, 28 (4), 453–456. (6) H€ uttinger, K. J.; Nill, J. S. Carbon 1990, 28 (4), 457–465. (7) M€ uhlen, H.-J.; van Heek, K. H.; J€ untgen, H. Fuel 1985, 64, 944– 949.
*Author to whom correspondence should be addressed. E-mail:
[email protected]. (1) Laurendeau, N. M. Prog. Energy Combust. Sci. 1978, 4, 221–270. (2) Wall, T. F.; Liu, G.; Wu, H.; Roberts, D. G.; Benfell, K. E.; Gupta, S.; Lucas, J. A.; Harris, D. J. Prog. Energy Combust. Sci. 2002, 28 (5), 405–433. r 2010 American Chemical Society
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Energy Fuels 2010, 24, 5300–5308
: DOI:10.1021/ef100980h
Roberts et al. 15,16
reactors. Kajitani et al. have measured the char-CO2 reaction rate for several different samples at temperatures of 13731773 K, pressures of 0.2-2.0 MPa, and with CO2 partial pressures of 0.05-0.70 MPa in a pressurized drop tube furnace. This experimental program and the analysis of the data have shown that the gasification rate varies strongly according to the type of coal, with the reaction occurring in Regime II at temperatures higher than ∼1573 K. Their work is strongly aligned with the development of models and interrogative tools for the design and construction of an air-blown entrained-flow gasifier and, therefore, is targeted specifically at that application. Ahn et al.17 also used a pressurized drop tube furnace to measure the char-CO2 reaction rate at 1173-1673 K, total pressures of 0.5-1.5 MPa, and CO2 partial pressures of 0.1-0.5 MPa for a single coal char. This work showed that, for the conditions considered, the reaction occurred in Regime II at temperatures higher than ∼1273 K. Because only one coal was used, with no detailed analysis of the char properties, it is difficult to apply these results more generally to gasification systems that involve other coal types and reaction conditions. Tables 1 and 2 show a summary of these high-temperature gasification kinetics measurements. Clearly, only limited data exist for rates of char gasification with CO2 and H2O at temperatures and pressures relevant to entrained-flow gasification. While there do exist some good reports of high-temperature and high-pressure conversion experiments, most of the analyses are related only to a certain coal sample under the specific experimental conditions used, rather than understanding the conversion rates in light of important char properties;this makes it difficult to apply the results more widely to char gasification in a range of potential gasification technologies. The work presented in this manuscript is part of a wider program of work that measures, analyses, and interprets char gasification kinetics under Regime II conditions. This paper builds on previous work by the authors,3 which quantified the effects of char morphology on Regime II reaction rates, and describes, in more detail, the effects of CO2 partial pressure, total system pressure, and gasification temperature on measured char-CO2 reaction rates at temperatures where both surface reaction and pore diffusion are significant.
Reliable and accurate measurements made at high temperatures and pressures are required to allow testing of reaction models and to clarify our understanding of the relative importance of reactant and total pressures, diffusion processes, and temperature on char conversion kinetics under gasification conditions. There are a few research groups around the world actively studying char gasification kinetics under these significantly more-complex conditions. At atmospheric pressure and high temperatures (1073-1673 K), Kasaoka et al.8 measured the char-CO2 and char-H2O reaction rates in a thermogravimetric analysis (TGA) device. While it is difficult to overcome the apparatus-specific effects in high-temperature TGA measurements of gas-solid reactions effectively, evidence of the onset of diffusion limitations was observed for both reactions at ∼1273 K. Kasaoka et al.8 hypothesized that the rates measured in this work were influenced by ash melting and subsequent filling of the pores in the particles at the higher temperatures. Hampartsoumian et al.9 measured the intrinsic reaction rate of pulverized coal chars with CO2 directly, at temperatures of 1300-1800 K, using a methane flame at atmospheric pressure. These high-temperature results were consistent with extrapolations from low-temperature measurements of intrinsic reaction rates made by the same authors, using TGA at 900-1233 K. Bench scale methods have also been used to measure gasification reaction rates at high temperature and high pressure. Peralta, Wang, and others10,11 used a wire mesh reactor (WMR) to measure the char-CO2 reaction rate for 14 coals at temperatures up to 1773 K and pressures of 0.25-2.0 MPa, with 100% CO2 concentration. Moors12 also used a WMR to measure the char-CO2 and char-H2O reaction at temperatures of 1573-1973 K and pressures up to 2.5 MPa for one coal char. Temporal resolution of rapid conversion measurements is difficult using wire-mesh techniques, so detailed quantification of reaction rates is not possible. While Moors did observe data consistent with Kasaoka,8 no low-temperature (Regime I) rate measurements were made to allow quantification of any effects of pore-diffusion limitations. Chars produced in WMRs may have different morphology and pore structures than those produced under more-realistic flow conditions.13 This has been shown to be an insignificant influence for rates measured at low temperatures (Regime I), where the entire char surface is available for reaction.14 However, the difference in pore structure may influence the conversion rate measured under conditions where pore diffusion processes are important. Therefore, rates measured in WMRs must be considered in conjunction with detailed analyses of char morphology and structure, so that they can be reliably applied to entrained-flow gasification systems. There exist some investigations into gasification reaction rates at high temperature and pressure using larger-scale flow
Experimental Section Coal Samples. Three Australian thermal coals were used in this work. These coals cover the range from high-volatile bituminous coal to semianthracite, and they are consistent with previous high-temperature work3 and fundamental4,20 work that has been conducted by this laboratory. Coals were obtained directly from the coal mines and crushed, sieved, and classified to -180 þ 45 μm; they were stored in a N2 atmosphere at 4 °C. Table 3 gives proximate, ultimate, and petrographic analyses of the samples. Reaction Rate Measurements. High-Temperature (Regime II) Rate Measurements. Rate measurements at high temperatures
(8) Kasaoka, S.; Sakata, Y.; Tong, C. Int. Chem. Eng. 1985, 25 (1), 160–175. (9) Hampartsoumian, E.; Murdoch, P.; Pourkashanian, M.; Trangmar, D. T.; Williams, A. Combust. Sci. Technol. 1993, 22, 105–121. (10) Peralta, D.; Paterson, N.; Dugwell, D. R.; Kandiyoti, R. Energy Fuels 2005, 19, 532–537. (11) Wang, B.; Li, X.; Xu, S.; Paterson, N.; Dugwell, D. R.; Kandiyoti, R. Energy Fuels 2005, 19 (5), 2006–2013. (12) Moors, J. H. J. Pulverised Char Combustion and Gasification at High Temperatures and Pressures, Ph.D. Thesis, Tecnische Universiteit Eindhoven, Amsterdam, 1999. (13) Roberts, D. G. Intrinsic Reaction Kinetics of Coal Chars with Oxygen, Carbon Dioxide and Steam at Elevated Pressures, Ph.D. Thesis, University of Newcastle, Newcastle, Australia, 2000. (14) Roberts, D. G.; Harris, D. J.; Wall, T. F. Energy Fuels 2003, 17 (4), 887–895.
(15) Kajitani, S.; Hara, S.; Matsuda, H. Fuel 2002, 81 (5), 539–546. (16) Kajitani, S.; Suzuki, N.; Ashizawa, M.; Hara, S. Fuel 2006, 85, 163–169. (17) Ahn, D. H.; Gibbs, B. M.; Ko, K. H.; Kim, J. J. Fuel 2001, 80, 1651–1658. (18) Kasaoka, S.; Sakata, Y.; Kayano, S.; Masuoka, Y. Int. Chem. Eng. 1983, 23 (3), 477–485. (19) Peng, F. F.; Lee, I. C.; Yang, R. Y. K. Fuel Process. Technol. 1995, 41 (3), 233–251. (20) Roberts, D. G.; Harris, D. J. Fuel 2007, 86 (17-18), 2672– 2678.
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Energy Fuels 2010, 24, 5300–5308
: DOI:10.1021/ef100980h
Roberts et al.
Table 1. Temperature of Transition between Kinetic and Diffusion Regimes (Ttrans) for Char Gasification Reactions at High Temperature and Various Pressures Activation Energy (kJ mol-1) apparatus
particle size (μm)
reactant
Ptot (MPa)
∼Ttrans (K)
Ea (meas)
Ea (true)
8,18
Kasaoka et al. Kasaoka et al.8,18 Kasaoka et al.8,18 Kasaoka et al.8,18
TGA TGA TGA TGA
1000 1000 1000 1000
CO2 CO2 CO2 CO2
0.1 0.1 0.1 0.1
1373 1373 1373 1373
100 80 70 60
197 310 260 243
Kasaoka et al.8,18 Kasaoka et al.8,18 Kasaoka et al.8,18
TGA TGA TGA
1000 1000 1000
H2O H2O H2O
0.1 0.1 0.1
1373 1373 1373
75 70 80
196 197 159
Peng et al.19 Peng et al.19
TGA WMR
149-210
