Energy & Fuels 2006, 20, 353-358
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Modeling on Pore Variation of Coal Chars during CO2 Gasification Associated with Their Submicropores and Closed Pores Tatsuya Morimoto,†,‡ Tetsuya Ochiai,† Sadao Wasaka,§ and Hirokazu Oda*,†,‡ Department of Chemical Engineering and High Technology Research Center (HRC), Faculty of Engineering, Kansai UniVersity: 3-3-35, Yamate-cho, Suita City, Osaka 564-8680 Japan, and EnVironment Technology DeVelopment Department, New Energy and Industrial Technology DeVelopment Organization (NEDO): 1310, Omiya-cho, Saiwai-ku, Kawasaki City, Kanagawa 212-8554 Japan ReceiVed July 25, 2005. ReVised Manuscript ReceiVed October 17, 2005
When predicting the variation of pore structure during CO2 gasification of coal chars using the random pore model (RPM), it is impossible to calculate exactly the ψ parameter from the pore characteristics, which were obtained by means of N2 adsorption, such as BET surface area (denoted as N2 pore characteristics), of the char prior to gasification. The values of ψ, which were calculated from the pore characteristics of chars at various carbon conversions, should be fundamentally constant, unaffected by the conversion of the char. However, this investigation exhibited a drastic decrease of ψ at the initial stage of the gasification reaction. This phenomenon is the result of a significant increase of N2 pore characteristics, of which the starting chars are extremely small. This increase might be explained by the widening of submicropores which are undetectable through the N2 adsorption method or by the reopening of closed pores inaccessible even to helium molecules, followed by the formation of new micropores exceeding the detection limit of N2. Consequently, this study introduced the volume of submicropores and closed pores into the ψ equation as correction terms. The value of ψ at the reaction starting point was close to that at the intermediate stage of reaction, indicating that the accuracy for ψ estimation was elevated and that the submicropores and closed pores should be taken into account when using RPM.
Introduction The coal gasification process is divided into two steps: the release of volatiles during pyrolysis and the gas-solid reaction of residual char that is the product of the pyrolysis. It is wellknown that the reaction rate of the latter, which determines the entire gasification process, is slower than that of the former. Therefore, it is important to clarify the factors that affect the gas-solid reaction of the char to analyze the gasification process. Coal and its char have an abundance of submicropores, of which the surface areas have been thought to be predominant in the char reactivity.1-14 The pore structures of a solid such as * Corresponding author. E-mail:
[email protected]. Phone: +816-6368-0808. Fax: +81-6-6388-8869. † Department of Chemical Engineering. ‡ High Technology Research Center (HRC). § Environment Technology Development Department. (1) Bhatia, S. K.; Perlmutter, D. D. A Random Pore Model for FluidSolid Reactions: I. Isothermal, Kinetic Control. AIChE J. 1980, 26, 379386. (2) Gavalas, G. R. A Random Capillary Model with Application to Char Gasification at Chemically Controlled Rates. AIChE J. 1980, 26, 577585. (3) Chi, W.; Perlmutter, D. D. The Effect of Pore Structure on the Charsteam Reaction. AIChE J. 1989, 33, 1791-1802. (4) Su, L. H.; Perlmutter, D. D. Effect of Pore Structure on Char Oxidation Kinetics. AIChE J. 1985, 31, 973-981. (5) Hashimoto, K.; Miura, K.; Mae, K.; Tsubota, J. Pyrolysis and Gasification of Coals by Use of Rapid Heating Method (in Japanese). J. Fuel Soc. Jpn. 1983, 62, 421-432. (6) Feng, B.; Bhatia, S. K. Variation of the Pore Structure of Coal Chars during Gasification. Carbon 2003, 41, 507-523. (7) Morimoto, T.; Arakita, K.; Kametani, T.; Oda, H. Variation Behavior of Pore Structure of Coal Chars during Gasification Process: Effect of Coal Grade, Pyrolysis Condition and Reactant Gases (in Japanese). Kagaku Kogaku Ronbunshu 2004, 30, 47-53.
char, which varies sequentially over the course of the gassolid reaction, should be expressed quantitatively. Several gassolid reaction models taking into account pore structures1,2 have already been proposed. Of these, the random pore model (RPM) developed by Bhatia and Perlmutter1 is supported as being appropriate for application. This model can represent the variation of nonuniform cylindrical pore structures within a solid during gasification as a function of conversion X using only one dimensionless parameter ψ.
S ) S0x1 - ψln(1 - X)
(1)
where S and S0 represent the pore surface area per unit weight at conversions of X and 0 (at the starting point of gasification), (8) Morimoto, T.; Ochiai, T.; Arakita, K.; Kametani, T.; Oda, H. Effects of Coal Grade and Gasification Condition on the Pore Characteristic Parameter of Coal Chars (in Japanese). Kagaku Kogaku Ronbunshu 2005, 31, 56-61. (9) Ballal, G.; Zygourakis, K. Evolution of Pore Surface Area during Noncatalytic Gas-Solid Reactions. 2. Experimental Results and Model Validation. Ind. Eng. Chem. Res. 1987, 26, 1787-1796. (10) Hurt, R. H.; Sarofim, A. F.; Longwell, J. P. The Role of Microporous Surface Area in the Gasification of Chars from A Sub-bituminous Coal. Fuel 1991, 70, 1079-1082. (11) Hurt, R. H.; Sarofim, A. F.; Longwell, J. P. Role of Microporous Surface Area in Uncatalyzed Carbon Gasification. Energy Fuels 1991, 5, 290-299. (12) Alvarez, T.; Fuertes, A. B.; Pis, J. J.; Ehrburger, P. Influence of Coal Oxidation upon Char Gasification Reactivity. Fuel 1995, 74, 729735. (13) Dutta, S.; Wen, C. Y.; Belt, R. J. Reactivity of Coal and Char. 1. In Carbon Dioxide Atmosphere. 1977, 16, 20-30. (14) Dutta, S.; Wen, C. Y.; Belt, R. J. Reactivity of Coal and Char. 2. In Oxygen-Nitrogen Atmosphere. Ind. Eng. Chem. Proc. Des. DeV. 1977, 16, 31-37.
10.1021/ef0502296 CCC: $33.50 © 2006 American Chemical Society Published on Web 11/23/2005
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respectively. Parameter ψ can be obtained as follows using the pore characteristics of a substrate solid.3,4
ψ)
4πL0 F0S0
2
Table 1. Proximate and Ultimate Analyses of the Raw Coals Used
Experiment Details Coal Samples and Carbonization. This investigation used three coal samples, A, B, and C, with carbon contents of 85, 79, and 74%, respectively. Table 1 shows proximate and ultimate analyses (15) Kajitani, S.; Hara, S.; Matsuda, H. Gasification Rate Analysis of Coal Char with a Pressurized Drop Tube Furnace. Fuel 2002, 81, 539546. (16) Ochoa, J.; Cassanello, M. C.; Bonelli, P. R.; Cukinerman, A. L. CO2 Gasification of Argentinean Coal Chars: A Kinetic Characterization. Fuel Process. Technol. 2001, 74, 161-176. (17) Liu, H.; Luo, C.; Kaneko, M.; Kato, S.; Kojima, T. Unification of Gasification Kinetics of Char in CO2 at Elevated Temperatures with a Modified Random Pore Model. Energy Fuels 2003, 17, 961-970.
B
C
proximate (% db)
volatiles fixed carbon ash
24.69 60.52 14.78
41.12 52.95 5.93
38.68 48.10 13.22
ultimate (% daf)
carbon hydrogen oxygen sulfur nitrogen
84.38 4.67 8.54 0.46 1.95
79.10 5.71 13.75 1.27 0.17
74.72 5.58 17.85 0.27 1.59
(2)
where L0 and F0 represent the pore length per unit weight and bulk density of the reaction starting material, respectively. Parameter ψ should be calculated using eq 2 from the view of chemical engineering. The right side of eq 2, pore characteristics, such as pore surface area, is considered to be determined with a conventional method such as N2 adsorption.1 RPM may be able to estimate gas-solid reaction rates, in addition to modeling the variation of pore structure. Therefore, it can be applied to the analysis of coal gasification rates.5,15-17 However, a number of research studies have used another method to calculate ψ. For example, Hashimoto et al.,5 in coal pyrolysis and gasification experiments, calculated ψ by the minimum-square method with pore surface areas at carbon conversions of X > 0.15, assuming that the variation of the pore surface area during gasification was subjected to eq 1. Ochoa et al.16 calculated ψ from conversion X at the maximum reaction rate. Liu et al.17 regarded ψ to be 2.5 regardless of the coal grade to determine the reaction rate for arbitrary conversion of fluid-bed gasification by applying RPM. Thus, in none of these cases was the value of ψ calculated by using eq 2. On the other hand, Feng and Bhatia6 calculated ψ using eq 2 based on the pore characteristics of several partially gasified chars produced from Australian semianthracite coal at a conversion range of X ) 0-0.75. The results showed that the values of ψ determined from the pore characteristics over the intermediate stage of gasification were consistent with those calculated by a fitting method from a surface area-conversion curve. They stated that the consistency of both ψ values may be caused by the absence of inaccessible pores. The pores without exits, existing in a carbon substance, are generally called closed pores. The formation of closed pores could be attributed to the coking of the carbon matrix in the carbonization of coal or organic compounds or to the shrinkage of micropores by the rearrangement of a graphene layer at a high-temperature heat treatment (>1473 K). Such closed pores would be reopened during a gas-carbon reaction (activation or gasification) and consequently be evaluated as new pores. RPM was developed with an assumption that no new pores are formed during the reaction. Therefore, the correction on the closed pore should be essential for the determination of ψ and the simulation of the variation of the pore structure. Consequently, this investigation attempted to modify RPM from the perspective of closed pores, for three different grades of coals: the volume of closed pores was introduced to the ψ calculation as a correction factor, and the applicability of the corrected pore model was examined.
A
of these coals. The coal samples, 2.0 g pulverized to a particle size of less than 0.15 mm, were carbonized by heating to 1273 K at a rate of 10 K/min and by holding the temperature for 30 min in a horizontal tube reactor under a nitrogen flow. After they were cooled to room temperature, char samples were obtained from the coked materials. Gasification of the Produced Chars. Gasification reactivity of the char samples was measured by using a thermogravimetric analyzer (TGA-50H; Shimadzu Co., Ltd.). About 10 mg of the sample was heated to 1273 K under a nitrogen flow, and subsequently, CO2 gas was supplied after the temperature had been held for 10 min at 1273 K. The reaction was continued until more than 95% of the carbon in the char was gasified. Thus, carbon conversion, X, was calculated as follows X)
m0 - m m0(1 - R)
(3)
where m0 and m represent the mass of char at the initiation of reaction and that at a reaction time t, respectively, and R represents ash content of the char on a dry basis. These results (X-t profile) were used to gasify 0.20 g of the char sample in a horizontal tube reactor to prepare four kinds of chars with different carbon conversions (partially gasified chars), namely, X ) 0 (before feeding the reactant gas), initial, intermediate, and later stage. Characterization of the Chars. (1) N2 and CO2 Adsorption Isotherm. An N2 adsorption isotherm at 77 K was measured using an adsorption apparatus of constant volume (BELSORP 28, BEL Japan Co., Ltd.). Prior to the measurement, the samples were degassed at 383 K. The char surface areas were calculated by the BET method. The pore volumes calculated from the amount adsorbed at relative pressures of P/P0 ) 0.1 and 0.95 were defined as micropore and total pore volumes, respectively,18 and the difference between these two volumes was defined as the mesopore volume. N2 adsorption was also used to calculate mesopore length and a surface area of 2.0-20 nm with the Dollimore and Heal (DH)19 method. The standard isotherm to apply the DH method was measured with heat-treated carbon black (M11-02). On the other hand, a CO2 adsorption isotherm was measured to calculate the volume of narrow micropores, inaccessible to N2 molecules at 77 K,21,22 by applying the Dubinin-Astakhov (DA) equation (n ) 2). (2) Measurement of Density by Helium Intrusion. A helium intrusion method (AccuPyc 1330, Micromeritics Co. Ltd.) was used to determine the char bulk density (by excluding the pore volume accessible to helium). The samples were degassed at 383 K prior to the measurement under an intrusion pressure of 134 kPa. The (18) Rodriguez-Reinoso, F.; Lopez-Gonzalez, J. d. D.; Berenguer, C. Activated Carbons from Almond Shells - I: Preparation and Characterization by Nitrogen Adsorption. Carbon 1982, 20, 513-518. (19) Dollimore, D.; Heal, G. R. An Improved Method for the Calculation of Pore Size Distribution from Adsorption Data. J. Appl. Chem. 1964, 14, 109-114. (20) Dubinin, M. M.; Astakhov, V. A. Description of Adsorption Equilibria of Vapors on Zeolites over Wide Ranges of Temperature and Pressure. AdV. Chem. Ser. 1970, 102, 69-85. (21) Gan, H.; Nandi, S. P.; Walker, P. L. Nature of the Porosity in American Coals. Fuel 1972, 51, 272-277. (22) Mahajan, O. P. Coal Porosity. Coal Structure; Academic Press: New York, 1982; pp 51-86.
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Energy & Fuels, Vol. 20, No. 1, 2006 355
Table 2. Pore Characteristics of the Coal Chars Prior to Gasification (X ) 0) characteristics SBET-0 SDH-0 VT-0 Vmi-0 Vme-0 VDA-0 FHe-0
A
B
C
units
7 6 0.016 0.003 0.013 0.030 2.05
44 17 0.036 0.017 0.019 0.079 2.07
39 12 0.030 0.015 0.015 0.159 2.02
m2/g m2/g cm3/g cm3/g cm3/g cm3/g g/cm3
pore characteristics of the above-mentioned chars at the starting point of gasification are listed in Table 2.
Figure 1. Variation of BET surface area during char gasification. Symbols represent the measured values, and lines represent the values estimated using ψ and SBET-0.
Results and Discussion Inconsistency of RPM Parameter ψ through Char Gasification. In our previous work,8 the pore characteristics of coal chars, required to determine the RPM dimensionless parameter, were obtained in terms of N2 adsorption. The characteristics mentioned above, which indicate the BET surface area and the corresponding pore length, cannot be measured directly. Consequently, the authors indirectly calculated the BET pore length using the mesopore surface area and length in a pore diameter range of Dp ) 2.0-20 nm obtained by the Dollimore-Heal19 method, with an assumption of the following equation
LBET/Lme ) SBET/Sme
(4)
The pore surface area can be determined when the pore diameter and length are obtained. Of which the pore diameter is enough smaller than the pore length, therefore the pore surface area might be significantly dependent on the pore length. For the above reason, it was proposed that eq 4 could be applied to calculate LBET. This investigation calculated ψ using eq 2 from the pore characteristics of chars prior to gasification, such as LBET-0, SBET-0, and FHe-0. The pore surface area at a conversion of X, SBET-X, was estimated in terms of eq 1, of which the comparison with the measured area was shown in Figure 1. The measured areas of SBET-X, which exhibited a drastic increase, were significantly larger than those estimated. This is attributed to a number of narrow micropores (submicropores) below the detection limit of the N2 adsorption method, as reported previously.21,22 Such submicropores are widened during char gasification into micropores, accessible to N2 molecules and to the determination of the BET surface area. The porous materials that cause the phenomena mentioned above are unlikely to permit one to predict the variation of pore characteristics during gasification by using only one parameter ψ. To confirm this assertion, the RPM parameter was determined from the pore surface areas and lengths of chars at various conversions, denoted as ψX, by applying eq 2 to conversions other than X ) 0
ψX )
4πLX FXSX2
(5)
Figure 2 shows a plot of respective ψX values versus X including X ) 0. It seems that ψX decreased remarkably during the initial stage of the reaction (X ) 0-0.3) and then remained approximately constant throughout the conversion from the intermediate stage. The drastic increase in the BET surface area attributed to the enlargement of narrow micropores can be regarded as the formation of new pores from the view of characterization by N2 adsorption. It is suggested, therefore, that
Figure 2. RPM parameters at the respective conversions determined using eq 5 from the pore characteristics of chars (denoted as ψX).
a correction of RPM associated with such new pore formation should be required for such porous materials. Modification of RPM (1): Associated with Narrow Submicropores. (1) Transformation of the Model Equation. The CO2 adsorption measurement was also carried out for coal chars during gasification. Since the adsorption measurement is performed at room temperature, CO2 gas can access narrow submicropores into which N2 at 77 K can scarcely intrude. However, the CO2 adsorption in this study cannot assess wider micropores and mesopores because the measurable relative pressure is limited to a very low range (103 kPa of equilibrium pressure). Consequently, the assessment of pore structure in terms of CO2 adsorption for the micropores was calculated using the Dubinin-Astakhov equation. As suggested by Marsh,23 there is a method for determining the pore surface area by assessing the limited adsorption volume, VDA, as it corresponds to the saturated monolayer adsorption from BET theory. It is pointed out, however, that there is a problem in applying Marsh’s method to submicropores with pore widths close to the widths of molecules. It seems problematic, therefore, to determine ψ using eqs 2 or 5 for the micropores evaluated by CO2 adsorption. This study introduced a method to calculate ψ using pore volumes as follows. First, assuming that all the pores within coal chars are cylindrical,1 pore surface area Sp and volume Vp are given in terms of average pore diameter Dp and pore length Lp as
Sp ) πDpLp
(6)
Vp ) πDp2Lp/4
(7)
By substituting the transformation of eqs 6 and 7 into eq 2, when S0 ) Sp, one obtains
ψ)
1 4 ) FSpDp FVp
(8)
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Figure 3. Comparison of the determined RPM parameters ψX between from eq 5 and from eq 9.
Figure 5. RPM parameters determined from the pore characteristics VDA and Vme of chars at the respective conversions (denoted as ψC1 X ).
Figure 4. Comparison of the variation of micropore volume between VDA and Vmi during gasification.
In this investigation, the ψX parameters were calculated by replacing Vp with VT-X and F with FHe-X as follows
ψX )
1 FHe-XVTX
(9)
Figure 3 represents the consistency between ψX obtained from eq 5 and that from eq 9. The latter ψX tended to be slightly larger than the former ψX except for a portion of these data (for considerably large ψX). Although it can hardly be judged which ψX is more reliable, the ψX determined by eq 5 is unlikely to include all of the pores (neglecting macropores). On the other hand, a considerably large ψX, namely, tedious BET surface area (a value of one order) or pore volume, might be attributed to the unreliability of data and the lack of reproducibility. As mentioned above, the use of eq 5 might be problematic with regard to calculating the surface area of the submicropores; therefore, eq 9 will be used in the following discussion with regard to the determination of a corrected RPM parameter. (2) Determination of ψX Parameter using VDA. For coal sample B, Figure 4 compares the micropore volumes, VDA and Vmi, at individual conversions, obtained by CO2 and N2 adsorptions, respectively. One can see VDA > Vmi at the beginning and initial stage of char gasification and that VDA < Vmi beyond the intermediate stage. This result suggests that the widening of narrow micropores progresses during the reaction. Clearly, N2 adsorption gives an underestimated total pore volume as is shown by VDA > Vmi up to the initial stage of gasification. Consequently, the total pore volume of chars with (23) Marsh, H.; Simieniewska, T. The Surface Areas of Coals as Evaluated from the Adsorption Isotherms of Carbon Dioxide Using the Dubinin-Polanyi Equation. Fuel 1965, 44, 355-365.
Figure 6. Variation of closed pore volumes of chars during gasification.
VDA > Vmi can be represented as VDA + Vme, which replaces VT in eq 9, to determine a modified RPM parameter, ψC1 X as 1
ψXC )
1 FHe-X(VDA + Vme)
(10)
As shown in Figure 5, the decline of parameter ψC1 X up to the initial stage of the reaction slowed compared to that of ψX but was, however, inconsistent throughout gasification. The results point to the need for further modification through the use of factors other than the volume of narrow submicropores such as VDA. This investigation focused on gas-inaccessible pores, namely closed pores, discussed by Feng and Bhatia,6 and their variation during gasification. Modification of RPM (2): Associated with Inaccessible Closed Pores. (1) Closed Pore Volume of Chars and its Variation during Reaction. Feng and Bhatia6 defined pores inaccessible to helium molecules as closed pores and expressed their volumes as VC in terms of the following equation
VC )
(
1-R R 1 + FHe FC FA
)
(11)
where FC and FA represent the true density of a carbon matrix and ash, respectively. According to Feng and Bhatia, FC ) 2.20 g/cm3 and FA ) 2.75 g/cm3. This investigation measured the closed pore volumes, VC, of coal chars during gasification using Feng and Bhatia’s method. Figure 6 shows the relationship between VC and conversion X. For each of the three coal samples, VC was the highest at X ) 0 and subsequently
Pore Variation of Coal Chars
Energy & Fuels, Vol. 20, No. 1, 2006 357
Figure 7. RPM parameters at the respective conversions determined from the pore characteristics of chars containing their closed pores C1 (denoted as ψC2 X ). Dashed lines indicate the corresponding ψX values.
decreased with conversion to zero at the intermediate stage of reaction. The result indicates that closed pores reopen with the consumption of carbon from the starting point of the reaction. Consequently, these would be assessed as new open pores. The following discussion represents the modification of RPM associated with such closed pores. (2) Determination of Parameter ψX using VC. Assuming that gas-inaccessible closed pores are cylindrical, another corrected RPM parameter, ψC2 X , was calculated by replacing VT with VT + VC and 1/FHe with [(1 - R)/FC + R/FA] () 1/Fs; the inverse of true density calculated under the assumption that helium can access the whole pores)
1 FS-X(VT-X + VC-X)
(12)
1 FS-X(VDA-X + Vme-X + VCX )
(13)
ψC2 X ) ψC2 X )
Figure 7 shows the plots of ψC2 X versus X. It can be seen that up to the initial stage of gasification each coal sample ψC2 X decreased compared to the corresponding ψC1 X . However, the RPM parameter, modified by the term of closed pore volumes, did not remain constant throughout the entire conversion process. C1 Interestingly, the difference between ψC2 X and ψX at the intermediate stage seemed to depend on the grade of raw coal. Such difference in ψCX is likely to correspond to the thickness of the graphene layers inserted between the tip of open pores and closed pores, namely, the location of closed pores in the substrate of the char. The closed pores in close proximity to open pore surfaces are unlikely to factor into the above results because of the coalescence with the widening of the open pores. Figure 8 describes the models of closed pores within the three coal chars prior to gasification, as proposed from the results shown in Figures 6 and 7. The following supposition can be mentioned: coal sample C tends to have a high portion of closed, but easily reopened pores (type i), while coal sample B’s pores can hardly be reopened (type ii); on the other hand, coal sample A’s termini of open pores have to be excavated to reopen the closed pores (type iii). The results obtained in this investigation suggest that the formation of new pores associated with the excavation of the termini of open pores would be the subject for the modification of RPM. Conclusions To predict the variation of pore characteristics for carbon materials such as coal char, the RPM parameter ψ should be constant during gasification, independent of the conversion of the char. As shown in this study, however, ψ determined from only N2 adsorption, decreased drastically at the initial stage of
Figure 8. Proposed schematic model of closed pores according to the results of Figures 6 and 7.
carbon conversion because of the enlargement of submicropores below the detection limit of N2 adsorption and the reopening of closed pores inaccessible even to helium. The introduction of these anomalous pores into the ψ equation relaxed the variation of ψ at the initial stage of reaction dramatically, confirming an appropriate correction of RPM. However, depending on the coal grade, up to a 3-fold difference between both ψ values was observed, suggesting that the correction seemed to be associated with the excavation of the termini of open pores. Acknowledgment. This study was carried out with the support of the New Energy and Industrial Technology Development Organization (NEDO) and the Japan Coal Energy Center (JCOAL). The authors would like to express their cordial thanks to NEDO and JCOAL.
Nomenclature D ) pore diameter (nm) L ) pore length per unit weight (m/g) m ) mass of char sample (mg) S ) pore surface area per unit weight (m2/g) V ) pore volume per unit weight (cm3/g) X ) gasification conversion (-)
358 Energy & Fuels, Vol. 20, No. 1, 2006 t ) reaction time (min) R ) ash content (-) F ) bulk density of the solid (g/cm3) ψ ) pore characteristic parameter in the random pore model Superscript C ) closed pores C1 ) corrected in terms of narrow micropores C2 ) corrected in terms of closed pores Subscript -0 ) the starting point of char gasification A ) ash
Morimoto et al. BET ) corresponding to BET method C ) carbon DA ) in terms of Dubinin-Astakhov equation He ) in terms of helium intrusion mi ) micropore me ) mesopore p ) pore (in a general sense) s ) an ideal scenario where the fluid can access all the pores T ) total pores -X ) A conversion of X EF0502296
