On the Effects of High Pressure and Heating Rate during Coal

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On the Effects of High Pressure and Heating Rate during Coal Pyrolysis on Char Gasification Reactivity D. G. Roberts,*,† D. J. Harris,† and T. F. Wall‡ Cooperative Research Centre for Coal in Sustainable Development and CSIRO Energy Technology, P.O. Box 883, Kenmore, QLD, 4069, Australia, and Department of Chemical Engineering, University of Newcastle, Callaghan, NSW, 2308, Australia Received September 5, 2002. Revised Manuscript Received March 25, 2003

Effects of pyrolysis pressure on char reactivity remain a poorly understood aspect of the coal gasification process. In an attempt to address this problem, effects of pyrolysis pressure on char structure and reactivity are being investigated. In this paper, chemical reactivities to O2, CO2, and H2O of chars made from three Australian black coals were measured in a pressurized thermogravimetric analyzer, under conditions where chemical processes alone controlled reaction rates. These chars were prepared at a range of pyrolysis pressures, using pressurized flow reactors and an atmospheric pressure tube furnace, to ascertain any effects of devolatilization pressure and heating rate on the chemical reactivity of the resultant coal chars. It was found that while the apparent (as measured) reaction rate can be affected by pyrolysis pressure, the rate normalized to the char surface area (intrinsic rate) is much less affected, because of large effects of pyrolysis pressure on char micropore surface area. This finding was supported by measurements of char carbon crystallite dimensions that were unaffected by pyrolysis pressure increases. These results indicate that effects of pyrolysis pressure and heating rate on char gasification rates are more likely to be due to effects of structure and surface area and (depending on reaction conditions) the consequent effects on diffusion of reactants to the char surface, rather than on the intrinsic reactivity of the coal chars.

Introduction Background. In response to environmental demands, advanced power generation technologies are being developed and demonstrated worldwide. The leading systems use coal gasification to produce a fuel gas, which is cleaned and used in a combined-cycle gas turbine system. This produces electricity at high efficiencies and with significant reductions in the emissions of CO2, NOx, SOx, and particulates. These systems offer emission levels approaching those of natural gas combined-cycle plants, with the low fuel cost of coal. These technologies typically operate at elevated temperatures and with pressures much higher than those associated with conventional pulverized-fuel-fired boilers. Consequently, coal properties that are seen as favorable for use in pf boilers are often not directly relevant to coal assessment for use in advanced technologies. Therefore, an understanding of high-pressure coal conversion processes under gasification conditions (i.e., temperatures up to 1800 K and pressures of 2030 atm) is required. Performance data for coals under such conditions, however, are scarce. This is due in part to the difficulty in performing reliable and relevant measurements that represent coal behavior under these * Corresponding Author. Fax: +61 7 3327 4606. E-mail: [email protected]. † CRC for Coal in Sustainable Development and CSIRO Energy Technology. ‡ CRC for Coal in Sustainable Development and University of Newcastle.

conditions and which also produce data that are applicable to practical technologies. One approach, therefore, is to develop models of the important coal gasification processes that can utilize bench-scale pyrolysis and reactivity data (that are somewhat simpler to obtain) and to combine them with the appropriate modeling of mass- and heat-transfer processes to predict the gasification rates and conversion levels of coals and their chars under a given set of practical reaction conditions. In addition to coal pyrolysis and char gasification reactions, the behavior of mineral components of coal is a key selection and operating criterion. The slagging nature of coals for use in entrained-flow gasification applications, and the consequent implications this has for the use of coals in such technologies, is the subject of ongoing research.1 This paper focuses on the coal conversion aspect of gasification, and in particular, the conversion of coal char following pyrolysis. The development and application of char conversion models requires reliable experimental data. Coal devolatilization, char reaction kinetics, and physical structure are particularly important. Moreover, the data need to be applicable to pressures that are relevant to entrained-flow gasification applications. Recent work in this laboratory has investigated the effects of increased pressure on a range of important coal gasification performance parameters. Coal pyrolysis yields were found to decrease at elevated pressure but (1) Patterson, J. H.; Hurst, H. J. Fuel 2000, 79, 1671-1678.

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not in a predictable way with standard measures of coal type.2,3 Chemical reaction kinetics of gasification reactions of coal chars have also been investigated in some detail, in particular how they are affected by increases in reactant pressure4 and total pressure.5 It has also been shown that the pressure at which a coal is pyrolyzed affects the morphology of the char produced.6,7 Specifically, increasing the pyrolysis pressure increases the tendency for chars to be “spongy”, with large void areas and accessible porosity. Related work8 has shown how these morphological properties of chars can be incorporated into an overall char combustion model for use at increased pressures. The effect of high-pressure pyrolysis conditions on the gasification reactivity of the char, however, is not well defined. Consequently, this paper investigates the effects of pyrolysis pressure and heating rate on coal char reactivity in more detail. Effects of High-Pressure Pyrolysis on Char Reactivity. There exist in the literature some investigations examining how char reactivity is affected by pyrolysis at increased pressures. Sha et al.9 report a decrease in the reactivity of chars produced from pyrolysis at high pressure, postulating unspecified pressure effects on the pore structure of the char as the reason. Similar results for the steam reactivity of chars at 40 atm were reported by Chitsora et al.10 Both of these sets of results use char gasification rates that are probably controlled by a combination of chemical and diffusion processes, thus making effects of pyrolysis pressure on the chemical reaction rates difficult to extract from the data. A review by van Heek and Mu¨hlen11 reported that if pyrolysis is performed under inert conditions, then the steam gasification reactivity of chars is unaffected by pyrolysis pressure. Steam reactivities were decreased, however, if the pyrolysis was performed under a hydrogen atmosphere. Cai et al.12 found similar effects of hydropyrolysis pressure on maximum combustion rates of chars at 500 °C up to 40 atm, with increases in reactivity (attributed to surface area enhancement from partial gasification by hydrogen) at pressures higher than this. It is apparent from the small amount of published literature that any effect of pyrolysis pressure alone on the chemical reactivity of coal chars (as distinct from the char conversion rates under process conditions) (2) Mill, C. J. Pyrolysis of Fine Coal Particles at High Heating Rate and Pressure. Ph.D. Thesis, University of New South Wales, 2000. (3) Mill, C. J.; Harris, D. J.; Stubington, J. F. Pyrolysis of Fine Coal Particles at High Heating Rates and High Pressure. In Proceedings of 8th Australian Coal Science Conference, Sydney, 1998; pp 151-156. (4) Roberts, D. G.; Harris, D. J. Energy Fuels 2000, 14, 483-489. (5) Roberts, D. G.; Harris, D. J.; Wall, T. F. Fuel 2000, 79, 19971998. (6) Benfell, K. Ph.D. Thesis, University of Newcastle, 2000. (7) Wu, H.; Bryant, G.; Benfell, K.; Wall, T. Energy Fuels 2000, 14, 282-290. (8) 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. (9) Sha, X.-Z.; Chen, Y.-G.; Cao, J.; Yang, Y.-M.; Ren, D.-Q. Fuel 1990, 69, 656-659. (10) Chitsora, C. T.; Mu¨hlen, H.-J.; van Heek, K. H.; Juntgen, H. Fuel Process. Technol. 1987, 15, 17-29. (11) van Heek, K. H.; Mu¨hlen, H.-J. In Fundamental Issues in Control of Carbon Gasification Reactivity; Lahaye, J., Ehrburger, P., Eds.; Kluwer Academic Publishers: Dordrecht, The Netherlands. 1991; pp 1-34. (12) Cai, H.-Y.; Gu¨ell, A. J.; Chatzakis, I. N.; Lim, J.-Y.; Dugwell, D. R. Fuel 1996, 75, 15-24.

Roberts et al. Table 1. Proximate and Ultimate Analyses of the Coals Used To Make the Chars Examined in This Work Coal F

Coal D

Coal K

proximate analysis (%w/w, air-dried basis) moisture 2.8 3.4 ash 23.2 6.6 volatiles 27.6 38.6 fixed Carbon 46.4 51.4

7.8 9.4 41.2 41.6

ultimate analysis (%w/w, dry, ash-free basis) carbon 82.0 82.9 hydrogen 5.23 5.95 nitrogen 1.70 1.83 sulfur 0.27 0.88 oxygen (by difference) 10.8 8.4

76.0 5.85 1.15 0.49 16.5

gross specific energy (MJ kg-1, air-dried)

28.0

28.2

26.4

remains ambiguous. Therefore, this work, as part of a broader study into the chemical reactivity of Australian coal chars under gasification conditions,13,14 investigates effects of pyrolysis pressure on chemical reaction rates of the resulting chars. The reaction rate data used here are of a form that allow the isolation of physical effects of diffusion from char-gas reactions at the sample surface, resulting in a true indication of how chemical reaction rates of chars are affected by pyrolysis pressure. Experimental Section Sample Preparation. Two batches of chars were used in this work. The first consisted of chars made from three Australian thermal coals, obtained for use in this study from concurrent research investigations.15 These parent coals have been labeled Coal D, Coal F, and Coal K. Proximate and ultimate analyses of these coals are given in Table 1. Chars made from these coals were obtained from related work using pressurized flow reactors. The reactors and the procedures used have been described in detail elsewhere.15-17 Coal D was devolatilized in a pressurized entrained-flow reactor (PEFR)15,16 at 5, 10, and 15 atm nitrogen pressure and with a temperature of 1100 °C. Coals F and K were devolatilized in a pressurized drop-tube furnace (PDTF)15,17 at pressures of 5, 10, and 15 atm and 1100 °C. In the latter case, stoichiometric amounts of oxygen were added to combust evolved volatile matter. Proximate analyses of the chars produced are presented in Table 2. The second batch of char consisted of a reference sample, the same as that used in previous investigations.4 This provided further information regarding the effect of highpressure pyrolysis on char reactivity, and provided data on the qualitative effects of heating rate on char reactivity. Chars made in this furnace also allowed a demonstration of how relevant previously reported effects of reactant pressure on char gasification kinetics4 are to chars made under more realistic conditions. This reference char was made from the same parent coal D as discussed earlier, but in a different apparatus. Coal was loaded in ceramic boats into a horizontal tube furnace (HTF) (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, 2000. (14) Harris, D. J.; Roberts, D. G.; Mill, C. J.; Kelly, M. D.; Otake, Y.; Wall, T. F. Development of Bench-Scale Techniques for Coal Reactivity Characterisation at Elevated Pressure. In Final Report for ACARP Project C6052, 1999. (15) Bryant, G. W.; Harris, D. J.; Tate, A. G.; Wall, T. F. Testing of Australian Black Coals in a Pressurised Drop Tube Furnace. In Final Report for ACARP Project C6051, 1999. (16) Reichelt, T.; Joustsenoja, T.; Spliethoff, H.; Hein, K. R. G.; Hernberg, R. Proc. Combust. Inst. 1998, 27, 2925-2932. (17) Ouyang, S.; Yeasmin, H.; Mathews, J. Rev. Sci. Instrum. 1998, 69, 3036-3041.

Pyrolysis Pressure and Char Gasification Reactivity

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Table 2. Proximate (as-received) Analyses of the High-Pressure Chars Produced in This Work parent coal Coal F Coal D Coal K

Table 3. N2 and CO2 Adsorption Surface Areas of Selected Chars Used in This Work

devol. press (atm)

moist. (%w/w)

ash (%w/w)

vol (%w/w)

FC (%w/w)

parent coal

pyrolysis reactora

preparation pressure (atm)

N2 BET (m2 g-1)

CO2 DR (m2 g-1)

5 10 15 5 10 15 5 10 15

5.0 6.0 5.4 2.1 2.0 1.9 7.4 6.6 6.7

31.2 31.1 29.1 18.0 19.6 18.4 17.9 17.2 19.9

2.6 2.6 0.6 2.8 1.8 1.4 3.8 3.4 1.8

61.3 60.3 64.9 77.1 76.7 78.3 70.9 72.9 71.7

Coal D Coal D Coal D Coal D Coal K Coal F

HTF PEFR PEFR PEFR PDTF PDTF

1 5 10 15 15 15

1.47 8.19 7.27 11.1 164 18.9

5.72 109 307 222 380 185

and heated slowly (10 °C min-1) in N2 to a final temperature of 1100 °Csthe same as that in the flow reactorsswhere it was held for 3 h. This technique was used to make (under consistent, well-defined conditions) the large amount of char required for the previous work. Char Characterization. Surface areas of the chars were measured using two techniques. One was the adsorption of N2 at 77 K and data analysis using the BET method. The second was CO2 adsorption at 273 K, with subsequent analysis using the Dubinin-Radushkevic (DR) method. As well as in the interpretation of reaction rate data, results from both methods were used to provide qualitative information regarding relative pore sizes.18 Earlier work investigating the crystalline nature of chars made from two coals (F and K) using X-ray diffraction measurements15 was extended in this work by examining chars made at different pressures from coal D. These analyses were performed on a JEOL JDX-8030 diffractometer. Some replicate measurements were performed on chars made from coals F and K in the present work to ensure consistency between the types of apparatus. Spectra were acquired over the range 5-90° 2θ, at intervals of 0.02° with a scan rate of 0.5° 2θ per minute, using a Cu KR X-ray source. These conditions are similar to those used on chars made from coals F and K in previous work. In addition to the relatively sharp peaks associated with mineral matter, carbonaceous materials such as coal chars produce two broad peaks in X-ray diffractograms. These correspond to the (002) and the (001) planes of the crystal lattice, or the stacking height (LC) and radial spread (LA) of crystalline structures, respectively. These dimensions can be calculated from X-ray spectra using eqs 1 and 2:19,20

LC )

0.89λ Å B cos(θ)

(1)

LA )

1.84λ Å B cos(θ)

(2)

where γ is the wavelength of the X-ray radiation, θ is the position of the (002) or (001) peak, and B is the angular width at half-maximum intensity of the peak. Reactivity Measurements. Reaction rates of chars were measured using a pressurized thermogravimetric analyzer (TGA). This apparatus has been described in detail previously.4 Verification experiments13 showed that under the conditions used here, reaction rates are sufficiently low to ensure that the temperature of the sample is not significantly different from that measured by a thermocouple situated directly below the sample in the reaction zone. Similar experiments also confirmed that experimental conditions were such that the (18) Anderson, R. B.; Bayer, J.; Hofer, L. J. E. Fuel 1965, 44, 443452. (19) Cullity, B. D. Elements of X-ray Diffraction, 2nd ed.; AddisonWesley: Reading, MA, 1978. (20) Lu, L.; Sahajwalla, V.; Harris, D. J. Energy Fuels 2000, 14, 869876.

a HTF ) Horizontal Tube Furnace; PEFR ) Pressurized Entrained-Flow Reactor; and PDTF ) Pressurized Drop-Tube Furnace.

reaction rate measurements were entirely within the regime where chemical processes alone control the reaction rate (i.e., regime I21), and where inhibition of the reaction rates by the reaction products was negligible. The apparent reaction rate (Fa) is the as-measured reaction rate, and was calculated from TGA data using the expression in eq 3:

Fa ) -

1 dw -1 -1 gg s w dt

(3)

where w is the sample mass (daf) remaining at reaction time t. Apparent reaction rates, while free from any effects of pore diffusion, are not free from the effects of inter-char variations in surface area. The surface areas discussed above can be used to normalize the apparent reaction rates to reduce the contribution of surface area variations to the measured rates. Normalization of the reaction rate calculated in eq 3 to the measured surface area results in the intrinsic reaction rate (Fi), in units of g m-2 s-1. Rates presented in this paper are initial reaction rates, that is, rates measured at a conversion of 0%. This initial rate was selected as the basis for comparison of the as-prepared chars because, at greater extents of reaction, the surface area and pore structure of the chars are products of the TGA reaction conditions. These conditions, while fundamentally valuable, do not necessarily reflect the practical conditions of interest here.

Results Char Surface Area. Surface areas of the chars using both N2 and CO2 adsorption are presented in Table 3. For samples produced at high pressures and with high heating rates, CO2 surface areas are consistently orders of magnitude larger than corresponding N2 surface areas. The only exception to this observation was char made from coal K, where the CO2 surface area was twice the surface area measured using N2 adsorption. The reference char, included for comparison, had CO2 and N2 surface areas that were both similar and low (1.47 and 5.72 m2 g-1). Data comparing effects of pyrolysis pressure on both CO2 and N2 adsorption surface areas using chars made from coal D are presented in Figure 1. There is a trend of increasing surface area with increasing pyrolysis pressure in data obtained using both CO2 and N2 adsorption; however, CO2 surface areas are more sensitive to pyrolysis pressure increases. Compared with the surface areas of char made at 1 atm, increasing the pyrolysis pressure increases the CO2 surface area by a (21) Walker, P. L., Jr.; Rusinko, F., Jr.; Austin, L. G. In Advances in Catalysis; Eley, D. D., Selwood, P. W., Weisz, P. B., Eds.; Academic Press: New York and London, 1959; Vol. 11, pp 133-221.

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Figure 1. Measured N2 and CO2 adsorption surface areas of chars made from coal D as a function of pyrolysis pressure.

factor of 20-50 times. N2 areas, over the same pyrolysis pressure interval, increase by a factor of 5-8 times. These results indicate that the chars produced in the PEFR and PDTF have significantly larger surface areas than those produced in the HTF, and that these surface areas are largely attributable to micropores. Furthermore, the microporous area of coal chars seems to be more affected by increases in pyrolysis pressure than the meso- and macropores. (This work refers to pores with diameters less than 2 nm as micropores and those with diameters larger than 50 nm as macropores. Those with intermediate diameters are referred to as mesopores.22) These results are important in the context of the reactivity data that are discussed in the following sections. Reactivity Comparisons with HTF Chars. Comparisons between chars produced at high pressures and heating rates and the “reference” HTF chars were also made to examine the effect of increased pyrolysis pressures and heating rates on char chemical reaction rates. Figure 2 shows the initial apparent and intrinsic rates of chars, made from coal D at 10 atm in the PEFR, to CO2, H2O, and O2, measured at 10 atm (100% CO2, 100% H2O, and 50% O2 in N2; with temperatures of 900 °C, 850 °C, and 350 °C, respectively). These rates are shown in comparison with those of chars made from coal D in the HTF under the same experimental conditions.4 For chars prepared in the PEFR, apparent reaction rates with CO2 and H2O were almost 30 times greater than those for chars prepared in the HTF. There was also a large difference in micropore surface area (Table 3), such that intrinsic reaction rates differed by less than a factor of 2. Results obtained for reaction with O2 were similar, but with less of an effect of normalization by CO2 surface area: the apparent rates of the chars made in the PEFR were 2 orders of magnitude greater than the chars made in the HTF; and the intrinsic rates differed by a factor of 5. These data indicate that the main contributing factor to the large difference in apparent reaction rates of the samples is the different surface areas produced, but that this is less applicable to oxygen reactivity data. This point will be expanded upon later in this section. It is also important that the overall trend of increasing reaction rate with reactant pressure that was found (22) Sing, K. S. W. Pure Appl. Chem. 1982, 52, 2201-2218.

Figure 2. Apparent and intrinsic reaction rates of chars made from coal D at 10 atm pyrolysis pressure reacting with 10 atm of (a) CO2 at 900 °C, (b) H2O, and (c) 50% O2 in N2 at 350 °C.

in previous work,4 using chars made from coal D in the HTF, be tested on chars made at elevated pressures and heating rates. A sample of char from coal D, devolatilized at 10 atm and 1100 °C in the PEFR, was prepared to determine reaction rate vs reactant pressure data to enable such a comparison. The experiments were performed using the same method as that used to examine the reference chars.4 Initial reaction rates of the PEFR chars were measured for a range of O2, CO2, and H2O partial pressures. Figures 3, 4, and 5 compare these results with initial reaction rate data for the HTF reference char made from the same parent coal. (The previously published HTF data4 are included on the same scale as the PEFR data, to demonstrate the difference in magnitude of the apparent and intrinsic rates.) For reaction of the char with O2, the reaction rate increases with increasing reactant pressure (Figure 5) consistent with previous data.4 For the reaction of the char with CO2 and H2O, the effect of pressure decreased at reactant pressures above 20-30 atm (Figures 3 and


Figure 3. Effects of CO2 pressure on the initial (a) apparent and (b) intrinsic reaction rates of coal D devolatilized in a pressurized entrained-flow reactor (PEFR) at 10 atm and 1100 °C, and coal D devolatilized in a tube furnace at 1 atm and 1100 °C.4

Figure 4. Effects of H2O pressure on the initial (a) apparent and (b) intrinsic reaction rates of coal D devolatilized in a pressurized entrained-flow reactor (PEFR) at 10 atm and 1100 °C, and coal D devolatilized in a tube furnace at 1 atm and 1100 °C.4

4). That is, as reactant pressure increased, the reaction order decreased. These results are all consistent with those obtained previously using laboratory-produced reference chars4 and furthermore, indicate that such


Figure 5. Effects of O2 pressure on the initial (a) apparent and (b) intrinsic reaction rates of coal D devolatilized in a pressurized entrained-flow reactor (PEFR) at 10 atm and 1100 °C, and coal D devolatilized in a tube furnace at 1 atm and 1100 °C.4

results are also applicable to chars made under more realistic, high-pressure gasification conditions. In addition to demonstrating this important continuity between reactivity data for reference and more “practical” char samples, these data also highlight a difference in the effects of normalization to surface areas of CO2 and H2O rate data and O2 rate data. It is apparent from data presented in Figures 2 and 5 that a difference exists in the effectiveness of normalization to char surface area between data generated for chars reacting in H2O or CO2 and chars reacting in O2. Figures 2-4 show small differences (typically factors of 1.5-2) in the intrinsic (normalized) reaction rates between chars made in the HTF and in the PEFR. That is, CO2 surface areas account for much of the variation in reactivity to CO2 and H2O for char made from coal D under the different pyrolysis conditions. For the same chars reacting in O2, however, we find that CO2 surface areas account for much less of the difference between the chars made in the different reactors, in some cases up to an order of magnitude difference remains. This observation is consistent with the data of Salatino et al.,23 who performed detailed analyses of char porosity development during reaction with CO2 and O2. They found that the development of char microporosity was much more extensive during reaction with CO2 than with O2. These observations of different extents of micropore development for chars reacting with different reactants is a possible explanation for the difference observed in the normalization of O2 rates and CO2/ H2O rates to CO2 surface areas. Reaction of chars with CO2 and H2O uses and develops the micropore surface area; thus normalization to this area accounts quite (23) Salatino, P.; Senneca, O.; Masi, S. Carbon 1998, 36, 443-452.

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Figure 6. (a) Apparent and (b) intrinsic reaction rates as a function of pyrolysis pressure for chars made from (b) coal D, (0) coal F, and (+) coal K reacting in CO2 at 900 °C and 10 atm (coal D chars) and 15 atm (coal F and K chars).

successfully for the differences in reaction rates between HTF and PEFR chars. Reaction of chars with O2, however, neither uses nor develops this micropore surface area to the same extent. Therefore, normalization of O2 rates to micropore surface areas cannot be expected to be as successful in removing effects of pyrolysis heating rate and pressure. Subsequent sections deal with the effect of pyrolysis pressure alone in more detail. Effects of Pyrolysis Pressure on Reaction Rates. Chars from all three coals were prepared at pyrolysis pressures of 5, 10, and 15 atm; the surface areas of these chars have already been discussed. The reaction rates of these chars to CO2, H2O, and O2 are shown in Figures 6, 7, and 8, respectively. Rates were measured at 10 atm (coal D chars) and at 15 atm pressure (chars made from coals F and K). Figure 6a shows apparent rate data for the chars reacting in CO2. Chars made from coals D and K show generally increasing reaction rates as pyrolysis pressure increases, whereas chars made from coal F tend to have apparent rates that decrease with increasing pyrolysis pressure. When these data are normalized to the measured surface area of the chars (Figure 6b), the result is intrinsic rate data that are essentially independent of pyrolysis pressure. The same chars reacting in H2O show similar results (Figure 7). Again, chars made from coal D have apparent reaction rates increasing as pyrolysis pressure increases. The rates of chars made from coal K also increase as pyrolysis pressure increases; however, this effect is much less than that observed with coal D chars. Chars made from coal F still have apparent rates that decrease as the pyrolysis pressure increases. As was the case with CO2 rate data, normalization of these rates

Roberts et al.

Figure 7. (a) Apparent and (b) intrinsic reaction rates as a function of pyrolysis pressure for chars made from (b) coal D, (0) coal F, and (+) coal K reacting in H2O at 800 °C and 10 atm (coal D chars) and 15 atm (coal F and K chars).

Figure 8. (a) Apparent and (b) intrinsic reaction rates as a function of pyrolysis pressure for chars made from (b) coal D, (0) coal F, and (+) coal K reacting in 50% O2 in N2 at 350 °C and 10 atm (coal D chars) and 15 atm (coal F and K chars).

to the char surface area (Figure 7b) produces intrinsic rate data that show little variation with increasing pyrolysis pressure. Figure 8a shows apparent rate data for the chars reacting in O2. Chars made from coal D again have rates that increase as pyrolysis pressure increases. The effects


Energy & Fuels, Vol. 17, No. 4, 2003 893 Table 4. Relative Intrinsic Reaction Rates (at 15 atm) of Chars Made at 15 atm to CO2, H2O, and O2 at 800 °C, Compared with Relative Rate Data Generated in Previous Work at Atmospheric Pressure reactant

Char F

Char D

Char K

Harris and Smith25

Walker21

CO2 H2O O2

1 4 5 × 105

1 15 5 × 105

1 3 3 × 105

1 2 0.1 × 105

1 3 1 × 105

a

Rates normalized to those measured in CO2 for each sam-

ple.

Figure 9. Comparing intrinsic data calculated using N2 surface areas with those calculated using CO2 surface areas. The samples are chars made from coal D reacting in (b) CO2 at 900 °C, (+) H2O at 850 °C, and (0) 50% O2 in N2 at 350 °C.

of pyrolysis pressure on the apparent rates of chars made from coals F and K, however, are less obvious for reactions in O2. The point to be made, though, is that the effects of pyrolysis pressure on the apparent rate for coal D chars, and the variation in rates for the other coals’ chars, are largely removed upon normalization to the char surface area (Figure 8b). The intrinsic rate data discussed above are calculated using CO2 surface areas. The different effects of pyrolysis pressure on N2 and CO2 surface areas have been presented in the Results section. To compare the use of CO2 and N2 surface areas in normalizing the apparent rate data, intrinsic rates for chars made from coal D were also examined using N2 surface areas. These results are presented in Figure 9, along with the apparent data and the intrinsic data calculated using CO2 surface areas. The effects of pyrolysis pressure on the apparent rate of chars made from coal D reacting in all three reactants is partially reduced upon

normalization to the N2 surface area, yet some effects remain. As already discussed, these effects are mostly removed if the CO2 surface area of the char is used. This reinforces the theme of this paper so far; that is, that pyrolysis pressure affects the microporosity of coal chars, without seemingly having an effect on the inherent chemical reactivity. These data also concur with previous studies of microporosity of chars23 which reported CO2 areas of microporous samples typically larger than N2 areas. During subsequent carbon conversion, the values of surface area measured using both techniques approached each other as the pore structure of the sample developed. Relative Reaction Rates. It has been shown at atmospheric pressure21,24,25 and recently at increased pressure4 that, when normalized to a constant temperature and reactant partial pressure, char-O2 reactions are typically several orders of magnitude faster than char-CO2 and char-H2O reactions. The latter two reactions have been shown to have similar rates, with char-H2O reaction rates on the average slightly higher than those of char-CO2 reactions. A selection of the data presented above has been adjusted to a common temperature and reactant partial pressure using activation energies and reaction orders previously measured at high pressures.4 This allows relative reaction rates to be determined for chars made at high pressures and with high heating rates. These data are presented in Table 4, normalized to 15 atm pressure (100% reactant) and 850 °C, then presented relative to the reaction rate in CO2 for each sample. Included in this table are data from previously published investigations using chars made at atmospheric pressure. It can be seen that, consistent with previous work, char-O2 reaction rates are approximately 5 orders of magnitude faster than char-CO2 rates, and char-H2O rates are slightly faster than char-CO2 rates, but typically within an order of magnitude. The results presented so far indicate that pyrolysis pressure influences char reaction rates through physical effects, reflected largely in the (microporous) surface area of the char. There is little indication that the inherent chemical nature of the chars changes with increasing pyrolysis pressuresthis point is expanded upon below. Discussion The apparent reaction rates of chars (with O2, CO2, and H2O) made from the same parent coal are strongly (24) Laurendeau, N. M. Prog. Energy Combust. Sci. 1978, 4, 221270. (25) Harris, D. J.; Smith, I. W. Proc. Combust. Inst. 1990, 23, 11851190.

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affected by increasing pyrolysis pressure and heating rate. Chars made under such conditions are orders of magnitude more reactive than “reference” chars made from the same coal at the same temperature but with slower heating rates and at atmospheric pressure. Effects of pyrolysis pressure alone (determined by reacting samples produced at different pressures) appear to be different for different coal chars; in this work chars from coal D and K have apparent rates that increased with increasing pyrolysis pressure, while chars from coal F are less affected. These differences, however, are largely attributable to the microporous surface area of the char, such that the variations in the apparent reaction rates are reduced from over an order of magnitude to a factor of approximately two in the corresponding intrinsic rates. Furthermore, effects of increases in reactant pressure on the reaction rates of reference chars previously reported are similar when reacting chars made at increased pressures and heating rates. Similarly unaffected by pyrolysis pressure is the relative reactivity of chars in O2, CO2, and H2O, with char-H2O reactions of magnitude similar to char-CO2 reactions (on the average char-H2O rates are slightly higher) and char-O2 reactions 5 orders of magnitude faster than char-CO2 reactions. Not all inter-char variation in reactivity is attributable to surface area alone, however. In the past, the reactivity of chars to O2, CO2, and H2O has been associated with the degree of ordering, or crystallization, of the carbon in the char (see, for example, refs 262826-28). This is particularly relevant for chars having high-rank coal precursors, where the potential for catalysis by mineral matter is less than for lower-rank coals (which typically have catalytic elements present in more functional forms). Highly ordered coal chars, such as those made at high temperatures and with long residence times, usually demonstrate reaction rates (on an area basis) less than those for chars with less ordered crystalline structures (for example, from coal pyrolysis at lower temperatures). X-ray diffraction provides information regarding the extent of char crystallinity, in particular regarding the size of the carbon crystallites. Carbon crystal structure is known to be affected by heat treatment,29 with X-ray diffraction measurements showing increases in crystallite size as the heat treatment temperature of the char increases.20,26,30 Furthermore, the proportion of char in a crystalline form has been shown to increase during isothermal reaction in oxygen.31 This is believed to be due to the preferential removal of the noncrystalline, more reactive components of the char matrix during reaction. (26) Marsh, H.; Taylor, D. A.; Lander, J. R. Carbon 1981, 19, 375381. (27) Senneca, O.; Salatino, P.; Masi, S. Fuel 1998, 77, 1483-1493. (28) Russell, N. V.; Gibbins, J. R.; Williamson, J. Fuel 1999, 78, 803807. (29) Marsh, H.; Diez, M. A.; Kuo, K. In Fundamental Issues in Control of Carbon Gasification Reactivity; Lahaye, J., Ehrburger, P., Eds.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1991; pp 205-220. (30) Marsh, H.; Menendez, R. In Introduction to Carbon Science; Marsh, H., Ed.; Butterworths: London, 1989; pp 37-74. (31) Lu, L.; Sahajwalla, V.; Harris, D. J. Metall. Mater. Trans. B 2001, 32, 811-820.

Roberts et al.

Figure 10. XRD Spectra for chars used in this work made from coal D at (a) 1 atm with a slow heating rate and a long residence time; and (b) 5 atm (c) 10 atm, and (d) 15 atm with fast heating rates and shorter residence times. All chars had a maximum temperature of 1100 °C. The peak labeled “SiO2” is produced from the silica sample holder.

The XRD spectra for chars made from coal D are presented in Figure 10. Consider first the chars made in the pressurized entrained-flow reactor at various pressures (traces b, c, and d). Each spectrum has a strong, narrow signal at 26.6° 2θ which is from the single-crystal SiO2 sample holder. The char made at 5 atm (trace b) shows a number of weak narrow signals, resulting from mineral matter in the carbon matrix. The features of interest in this work, however, are the two broad bands centered at approximately 25 and 44° 2θ. The major peak is that centered at approximately 25° 2θ which corresponds to the LC (stacking height) of the crystal lattice. The minor band is centered at approximately 44° 2θ and this is associated with the LA (radial spread) dimension. The scattering angle for both of these bands is the same for each of the three chars, which suggests a consistency in the main crystal structures of the chars as pyrolysis pressure is changed. Trace (a) is also of a char made from coal D, devolatilized at the same temperature, but at 1 atm pressure and with a slow heating rate (10 °C min-1) and long residence time (3 h). It also demonstrates the broad bands in the same positions, indicating a crystal structure similar to the chars made at increased pressures. The major difference, however, lies in the breadth of the major peak. It is narrower and taller than the chars made at increased pressures. This suggests a slightly more crystalline carbon matrix, perhaps arising from the longer residence time at the same pyrolysis temperature. Analyses of these peaks yield quantitative data regarding the extent of crystallization of the samples. Figure 11 shows these data, with the previously performed analyses for chars made from coals F and K included.15 Consistent with the above analysis, the char made with a very long residence time has a slightly more developed crystal structure; however, it is apparent that this crystal structure is not systematically affected by pyrolysis pressure, a finding in support of the reactivity data presented earlier in the paper.



°C). It is understood that final temperature has a significant effect on the intrinsic reactivity of chars29s future work at increased pressures and temperatures will extend the data presented and discussed here. Conclusions

Figure 11. Crystalline dimensions of the carbon in the chars studied in this work, as determined by X-ray diffraction spectroscopy; (a) stacking heights, LC; and (b) radial extents, LA.

The findings presented and discussed in this paper indicate that the major effects of pyrolysis pressure on observed char conversion rates are most likely due to effects of pyrolysis conditions on char morphology and surface area, rather than on the chemical reactivity of the char itself. Such effects on char structure would influence the rate of diffusion of reactant gases through the pore structure of a particle at high temperatures, and play a role in determining the conversion rates under practical combustion and gasification conditions. They would not, however, affect the “intrinsic” reactivity of the char surface. The results presented in this work simplify the potential development of models that combine intrinsic rate data with char structural submodels, by removing the need to account for the effect of pyrolysis pressure and heating rate on the intrinsic reactivity of the sample. Indeed, a pressurized combustion model has recently been demonstrated8 that combines high-pressure intrinsic oxidation kinetics generated in this laboratory with char structural submodels. It remains for the gasification data generated in this work to be applied in a similar fashion. It needs to be emphasized that the data in this paper have all been generated using char samples made at only one temperature (1100

The data presented in this paper have led to the following conclusions: 1. Chars made at high pressures and with high heating rates have apparent reaction rates in CO2, H2O, and O2 that are orders of magnitude faster than those of char made from the same coal at atmospheric pressure and slow heating rate conditions. These effects, however, are largely due to the increased surface area of chars made at high pressure (this effect is reduced for char-O2 reactions). 2. The reaction rates of chars made at high pressures and with high heating rates vary with CO2, H2O, and O2 pressure in a way similar to those of chars made under controlled laboratory conditions. 3. The effects of pyrolysis pressure on char reaction rates arise mostly from physical effects, reflected in this work in the microporous surface area of the char, and not the intrinsic reactivity of the char itself. 4. The notion that pyrolysis pressure has little effect on inherent char chemistry is supported by char carbon crystal structure measurements that are not affected by pyrolysis pressure. These findings make incorporation of chemical (intrinsic) reaction rate data with measured and predicted structural parameters a more readily achievable task, in particular, under the wide range of high-intensity conditions that prevail in advanced coal utilization technologies. Acknowledgment. The authors acknowledge the financial support provided by the Cooperative Research Centre for Coal in Sustainable Development, which is funded in part by the Cooperative Research Centres Program of the Commonwealth Government of Australia. This work was also supported in part by the Australian Coal Association Research Program (ACARP). The authors also acknowledge the funds provided by the Australian Commonwealth’s Department of Industry, Science and Resources’ Technology Diffusion Program which supported in part a postdoctoral study visit of Dr. Roberts to the laboratories of the Central Research Institute of the Electric Power Industry (CRIEPI) in Japan. Thanks are extended to Mr. Shiro Kajitani (CRIEPI, Japan) for his role in the generation of XRD data for the coal D chars in this work, and to Drs. Kathy Benfell and Hongwei Wu (University of Newcastle) for the provision of char samples made from coals K and F. EF020199W

On the Effects of High Pressure and Heating Rate during Coal

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