Energy Fuels 2010, 24, 100–107 Published on Web 08/31/2009
: DOI:10.1021/ef900503x
The Significance of Char Morphology to the Analysis of High-Temperature Char-CO2 Reaction Rates† Elizabeth. M. Hodge,‡,§ Daniel. G. Roberts,*,‡ 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 May 22, 2009. Revised Manuscript Received August 2, 2009
The reaction rates of coal chars with CO2 is an important aspect of coal performance under gasification conditions. Most studies of the char-CO2 reaction are undertaken at temperatures (typically ∼1200 K or below) significantly lower than those found in entrained flow applications, to allow detailed investigations into the surface reaction processes. Application of these data to high temperatures therefore requires consideration of how these reaction rates are affected by gas diffusion through the pore structure of reacting particles, yet there are very few char-CO2 rate data at high temperatures and pressures against which such applications can be tested or verified. This paper presents results of measurements of the char-CO2 reaction rate at high pressures (2.0 MPa) and high temperatures (up to 1673 K) using an entrained-flow reactor and also presents analyses of the morphology of the chars sampled during reaction. Using an effectiveness-factor-based approach, the effects of pore-diffusion and surface reaction (intrinsic reactivity) are separated, allowing determination of the high temperature intrinsic (surface) reaction rate. By accounting for the char morphology that arises specifically from high-pressure, high-temperature pyrolysis, these high-temperature intrinsic rates can be shown to be consistent with an extrapolation to high temperatures of intrinsic rates directly measured at low temperatures. This demonstrates the importance of accounting for the morphology of high-pressure chars, and shows that traditional techniques where char particles are considered as reacting porous solids are often unsuitable. Using laboratory-scale techniques, such as thermogravimetric analysers, to measure high-temperature char gasification rates produces data that are difficult to analyze and apply. Competing influences of heat transfer and gas diffusion to and through the samples make the data difficult to interpret and apply more generally to practical (non-TGA) reaction systems. High-temperature flow reactors are commonly used to simulate the important aspects of entrained flow gasification while still allowing sufficient control of key parameters to allow measurement and interpretation of gasification rates. Drop tube furnaces have been useful in the past for combustion reactivity tests (e.g., refs 1-3) and in the preparation of char under reasonably well-controlled conditions (e.g., refs 4-7). Pressurized drop tube furnaces have also been used8-11 to
Introduction Char gasification reactions are the topic of research interest due to their significance to the rate of coal conversion in a gasifier, and consequently the performance and efficiency of gasification-based systems. Significant progress has been made over the past decade or two developing greater understanding of the mechanisms and kinetics of the reaction of coal chars with CO2 and H2O. Mathematical and process models are also being developed to facilitate the application of this knowledge to practical systems. To ensure the generation of reliable, transportable experimental rate data, it is necessary to conduct “intrinsic” reactivity measurements at relatively low temperatures, so that reaction mechanisms and kinetics can be studied free from physical limitations of diffusion and experimental conditions. The industrial systems of interest to gasification researchers, however, involve temperatures significantly greater than those used in these fundamental laboratory measurements. At practical gasification temperatures there is inevitably an interaction between the rates of the surface gasification reactions and the rates of gas diffusion through the pores of reacting char particles;so-called “Regime II” conditions. Understanding these interactions can present significant practical and scientific challenges; however, their quantification is important in order to understand the high-temperature gasification system with the detail required to support an industry transition to new gasification-based energy systems.
(1) Abd El-Samed, A. K.; Hampartsoumian, E.; Farag, T. M.; Williams, A. Fuel 1990, 69, 1029–1035. (2) Rubiera, F.; Arenillas, A.; Arias, B.; Pis, J. J.; Suarez-Ruiz, I.; Steel, K. M.; Patrick, J. W. Fuel 2003, 82 (15-17), 2145–2151. (3) Stanmore, B. R.; Choi, Y. C.; Gadiou, R.; Charon, O.; Gilot, P. Combust. Sci. Technol. 2000, 159, 237–253. (4) Liu, G.-S.; Benyon, P.; Benfell, K. E.; Bryant, G. W.; Tate, A. G.; Boyd, R. K.; Harris, D. J.; Wall, T. F. Fuel 2000, 79 (6), 617–626. (5) Matsuoka, K.; Ma, Z.-x.; Akiho, H.; Zhang, Z.-g.; Tomita, A.; Fletcher, T. H.; W ojtowicz, M. A.; Niksa, S. Energy Fuels 2003, 17 (4), 984–990. (6) Yu, J.; Lucas, J. A.; Strezov, V.; Wall, T. F. Energy Fuels 2003, 17 (5), 1160–1174. (7) Yoshizawa, N.; Maruyama, K.; Yamashita, T.; Akimoto, A. Fuel 2006, 85 (14-15), 2064–2070. (8) Kajitani, S.; Hara, S.; Matsuda, H. Fuel 2002, 81 (5), 539–546. (9) Kajitani, S.; Suzuki, N.; Ashizawa, M.; Hara, S. Fuel 2006, 85, 163–169. (10) Ahn, D. H.; Gibbs, B. M.; Ko, K. H.; Kim, J. J. Fuel 2001, 80, 1651–1658. (11) Shin, Y.; Choi, S.; Ahn, D. H. Int. J. Energy Res. 2000, 24, 749– 758.
† Presented at the 2009 Sino-Australian Symposium on Advanced Coal and Biomass Utilisation Technologies. *To whom correspondence should be addressed. E-mail: Daniel.
[email protected].
r 2009 American Chemical Society
100
pubs.acs.org/EF
Energy Fuels 2010, 24, 100–107
: DOI:10.1021/ef900503x
Hodge et al. 12-15
measure char gasification reaction rates at temperatures up to 1773 K and pressures up to 20 bar. Drop-tube furnaces, however, in contrast to true entrained-flow systems, do not allow stoichiometric conditions (i.e., carbon to oxygen ratios) that are relevant to practical coal gasification systems. A drop tube furnace was used by Kajitani et al.8,9 to measure the rate of char reacting with CO2 for several different coal chars over the temperature range 1373-1773 K, the pressure range 0.2-2.0 MPa, and with 0.05-0.7 MPa CO2 partial pressure. This work has shown that the gasification rate varies strongly with coal type and that the reaction most likely occurs in Regime II at temperatures higher than about 1573 K, based on observed discontinuities in the Arrhenius plot of the rate data. Their work is strongly aligned with the development of models and interrogative tools for the design and construction of an air-blown, two-stage entrained flow gasifier. Ahn et al.10 also used a pressurized drop tube furnace to measure the char-CO2 reaction rate over the temperature range 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 also showed that, for that coal char under the conditions considered, the reaction most likely occurred in Regime II at temperatures higher than about 1273 K. As only one coal was used, with no detailed analysis of the char properties, it is difficult to interpret the published data with the view to applying them more generally to gasification systems involving other coal types and reaction conditions. The difficulties in applying high-temperature (Regime II) data more generally are due to the complex and coal-specific impacts that char morphological and structural properties are likely to have on the char gasification rates. It is known that char properties such as surface area and carbon crystallinity can significantly influence the char gasification reaction rates at low temperatures. On the basis of these studies and our understanding of the theoretical nature of porous solids reacting with gases, it is expected that properties such as particle size and pore size distribution would also be significant at high temperature. Investigations that quantify these effects under high temperature gasification conditions, however, are rare. Kajitani et al.8 measured char-CO2 reaction rates of a single coal char at high temperature and pressure and characterized the char using SEM, particle size, and surface area techniques. The char particles were observed to be, in part, swollen and thin-walled, with a noticeable decrease in particle size around 50% conversion, consistent with fragmentation. Surface area was seen to increase with conversion and peak around 30%. More recently, Kajitani et al.9 measured char-CO2 gasification rates at high temperature and pressure, for four different coals. However, in this paper, the surface area change with conversion was only reported for two of the coals, again showing a peak around 30% char conversion. Characterization of char morphology, including particle size, pore structure, and surface area, and quantifying its effect on high-temperature reaction rates is crucial for accurate modeling of gasification systems. Previous work on the relationship between coal properties, devolatilization conditions, and char morphology has demonstrated how chars produced with high heating rates at high temperatures and pressures are more likely to consist of a large proportion of swollen, thin-walled chars, with large amounts of voidage and
This work provides an excellent complex microporosity. basis for our understanding of high-temperature char gasification rate data; however, the quantitative impact of these properties on gasification rates, and how these char morphologies change with conversion, must be understood in more detail. In order to understand the impact of char morphology on high temperature reaction rates, quantification of the relative impacts of pore diffusion and intrinsic reactivity is required. In studies of a high temperature char combustion system, Valix et al.16 were able to separate the effects of pore diffusion and chemical reactivity, and present data on intrinsic char combustion rates obtained from reaction rate measurements made at high temperatures (nominally Regime 2 conditions). As with many other studies of porous solid-gas reaction systems, an effectiveness factor was used in their study to mathematically describe the limitations imposed on the surface chemical reactions by diffusion through pores; their work coupled this technique with measurements of pore diffusion coefficients. The effectiveness factor (η) is defined as the ratio of the measured reaction rate to that which would occur if the reacting gas concentration was uniform throughout the sample.17-19 It has been used extensively in catalyst and combustion literature as a useful tool for analyzing gas-solid reaction rates under conditions where both surface chemical reactions and pore diffusion contribute to the overall rate of reaction. It has not been used in a comprehensive fashion in a description of the high-temperature, high-pressure char gasification system. Any application of an effectiveness factor to such conditions needs to account for particular effects of char morphology from high-pressure pyrolysis and the role of char gasification kinetics, and importantly, consider these in the context of appropriate measurements of gasification rates under process conditions. This paper presents some results of high-temperature, highpressure measurements of the rate of the char-CO2 reaction. These results are analyzed in terms of the structural and morphological characteristics of the reacting chars. The data are then used, in conjunction with an effectiveness factor approach, to provide an intrinsic kinetic characterization of the high-temperature char-CO2 reaction system. Experimental Section Coal Samples. Three Australian thermal coals are used in this study, sourced from mines in New South Wales and Queensland. Analyses of these coals are given in Table 1. These coals were chosen in order to cover a range of properties of Australian thermal coals, while maintaining some consistency with previous, related gasification research. (12) Benfell, K. E.; Bailey, J. G. In Comparison of Combustion and High Pressure Pyrolysis Chars from Australian Black Coals, 8th Australian Coal Science Conference, Sydney, 1998. (13) Benfell, K.; Liu, G.-S.; Roberts, D. G.; Harris, D. J.; Lucas, J.; Bailey, J.; Wall, T. F. Proc. Combust. Inst. 2000, 28, 2233–2241. (14) Wu, H.; Bryant, G.; Benfell, K.; Wall, T. Energy Fuels 2000, 14 (2), 282–290. (15) Yu, J.; Harris, D. J.; Lucas, J. A.; Roberts, D. G.; Wu, H.; Wall, T. F. Energy Fuels 2004, 18 (5), 1346–1353. (16) Valix, M. G.; Trimm, D. L.; Smith, I. W.; Harris, D. J. Chem. Eng. Sci. 1992, 47 (7), 1607–1617. (17) Walker, P. L., Jr.; Rusinko, F., Jr.; Austin, L. G., Gas Reactions of Carbon. In Advances in Catalysis, Eley, D. D.; Selwood, P. W.; Weisz, P. B., Eds.; Academic Press: NY and London, 1959; Vol. 11, pp 133-221. (18) Satterfield, C. N.; Sherwood, T. K., The Role of Diffusion in Catalysis; Addison-Wesley Publishing Company, Inc.: Reading, Massachusetts, USA, 1963; p i-viii, 1-118. (19) Walker, P. L., Jr; Geller, I. Nature 1956, 178, 1001.
101
Energy Fuels 2010, 24, 100–107
: DOI:10.1021/ef900503x
Hodge et al.
Rate Measurements. The rate measurements discussed in this work were made using a high-pressure entrained flow reactor (PEFR). This facility has been described previously.20 It is designed to allow the coal gasification process to be studied under conditions of pressure, heating rate, temperature, and gas composition relevant to entrained-flow coal gasification applications. The reactor was slightly modified for the work program presented here, allowing reactant gas mixtures of CO2 in N2 to be used, in order that the char-CO2 reaction be isolated and investigated in some detail. The experiments reported in this paper were performed with nominal reactor wall temperatures of 1273, 1373, 1473, 1573, and 1673 K, a coal feed rate of 1.5 kg/h, and a total reactor pressure of 2.0 MPa. The CO2 partial pressures (in a balance of N2) were nominally 0.25, 0.50, and 0.75 MPa. Sufficient air was fed to the reactor in order to combust the volatile matter produced from coal devolatilisation. The oxygen requirement to achieve this was calculated on the basis of the reactor conditions and separate high-pressure volatile yield measurements made using a heated grid reactor21 and confirmed experimentally. This resulted in a required O2 molar flow of about 1-2% of the total gas flow, depending on the coal being fed. Partially reacted char and gas samples were sampled isokinetically from the reaction tube using an oil-cooled sampling probe. By adjusting the height of the probe, samples could be collected at various levels of conversion, at residence times from approximately 0.5 to 3.0 s. Solid samples were removed from the sampled stream using a sintered stainless steel filter, allowing up to 100 g of char to be sampled at a given steady-state condition. The gas sample stream was then dried and passed to a gas analysis system. Char conversion is calculated from the measured CO concentration in the reactor using the following equation: X ¼ n_ CO MC ð1Þ 2m_ C %
Table 1. Coal Samples Used in This Work CRC252
fixed carbon volatile matter ash moisture C H N S O MVRa (%) vitrinite liptinite inertinite mineral a
CRC272
CRC281
Proximate Analysis (%, db) 42.4 52.6 41.3 35.0 10.2 10.1 6.1 2.3
81.4 8.7 8.4 1.5
Ultimate Analysis (% daf basis) 78.1 82.6 5.9 5.36 1.1 1.68 0.5 1.04 14.5 9.4
90.8 3.71 1.93 0.95 2.6
Coal Rank 0.48
0.67
2.50
Maceral Analysis (% v/v) 88.0 47.2 1.4 2.8 4.5 39.9 6.1 10.1
61.2 0 30.7 8.1
Mean vitrinite reflectance.
Low-temperature (Regime 1) rate measurements were made using an atmospheric pressure fixed bed reactor23 and a highpressure thermogravimetric analyzer,24 using established, previously published techniques. These facilities are known to generate reliable measurements of intrinsic, Regime I reactivity data at low (
