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Energy & Fuels 2007, 21, 2840-2845
Kinetics of Coal Conversion in Supercritical Water Anatoli A. Vostrikov,* Sergey A. Psarov, Dmitri Yu. Dubov, Oxana N. Fedyaeva, and Mikhail Ya. Sokol Institute of Thermophysics, Siberian Branch of the Russian Academy of Sciences, LaVrentieV aV. 1, 630090 NoVosibirsk, Russian Federation ReceiVed March 12, 2007. ReVised Manuscript ReceiVed May 24, 2007
Conversion of the coal particle pack in supercritical water (SCW) was studied in the semibatch reactor under the pressure of P ) 30 MPa, in the temperature range of T ) 500-750 °C, and in the reaction time of t ) 60-720 s. The experimental results were analyzed within the framework of homogeneous, nonreacted core, and random pore models. The quantitative composition of conversion products was determined. Dependences of the conversion rate on the degree of coal conversion, reaction time, and temperature were described in an assumption of the first-order reaction and Arrhenius dependence. It was found that activation energy of conversion is E ) 103 kJ/mol and the pre-exponential factor is A0 ) 1.3 × 103.1 s-1. It was revealed that coal gasification in SCW without oxidants is the weakly endothermic process. The addition of CO2 into SCW decreases the conversion rate and increases the CO yield.
Introduction
〈C〉 + H2O ) CO + H2 ∆H ) +132 kJ/mol
(1)
The main factor that restrains the development of the coal market is a negative effect of coal utilization on the environment and related expenses for environmental safety. This fact stimulates the search for new ecologically responsible methods of coal conversion into the desired products. At that, ecological cleanness should have a low cost. The present-day methods of coal conversion, such as liquefaction, gasification, and pyrolysis, require highly expensive pretreatment of coal aimed at emission reduction and are characterized by a high prime cost of the end products. The most promising method of coal conversion is steam gasification.1 This is caused by the fact that, in comparison to other substances, water is the cheapest and most environmentally clean donor of hydrogen and oxygen, as well as a heat carrier. In the past few years, the prospects of steam gasification for all kinds of low-grade fuels are connected with the usage of water under the supercritical parameters (T > 374 °C and P > 22.1 MPa).2 The supercritical water (SCW) is the universal solvent of organic substances,3 and a high density of SCW provides high specific productivity of low-temperature conversion of lowgrade fuels.4 In addition, SCW becomes a hydrogen and oxygen donor at a temperature as low as 600 °C.5 The main reactions of steam gasification are
CO + H2O ) CO2 + H2 ∆H ) -41 kJ/mol
(2)
〈C〉 + 2H2O ) CO2 + 2H2 ∆H ) +91 kJ/mol
(3)
〈C〉 + 2H2 ) CH4 ∆H ) -87.4 kJ/mol
(4)
〈C〉 + CO2 ) 2CO ∆H ) +159.7 kJ/mol
(5)
〈C〉 + O2 ) CO2 ∆H ) -405.9 kJ/mol
(6)
* To whom correspondence should be addressed. Fax: +7-383-3308094. E-mail:
[email protected]. (1) Khodakov, G. S.; Gorlov, E. G.; Golovin, G. S. Solid Fuel Chem. 2005, (6), 15-32. (2) Savage, Ph. E.; Gopalan, S.; Mizan, T. I.; Martino, C. J.; Brock E. E. AIChE J. 1995, 41 (7), 1723-1778. (3) Savage, Ph. E. Chem. ReV. 1999, 99 (2), 603-615. (4) Vostrikov, A. A.; Dubov, D. Yu.; Psarov, S. A. Collection of Proceedings of the Workshop “Physics of an Oil Stratum” (in Russian); NK “YuKOS-EP”: Novosibirsk, Russia, 2002; pp 31-39. (5) Vostrikov, A. A.; Dubov, D. Yu.; Psarov, S. A. Russ. Chem. Bull. 2001, 50 (8), 1481-1485.
Reactions 1 and 5, which are of particular importance for coal gasification, are endothermic. Because the gross process (reaction 3) is also endothermic, to provide autothermal conditions, the partial oxidation of coal in reaction 6 is required. Experimental Section In this work, we have studied the conversion of the coal particle pack in SCW for P ) 30 MPa in the range of T ) 500-750 °C. Coal of Yakutsk coalfield with the following characteristics was explored (wt %): Ad, 14.8; Wa, 6.8; Cdaf, 74.0; Hdaf, 4.8; Ndaf, 2.1; Sdaf, 0.4; Odaf, 18.7. Coal was grinded in a vortex chamber. The size of coal particles varied from 0.1 to 1.5 mm. It is known that the size of coal particles considerably effects the gasification rate only if this size is higher than 2 mm.6 Thus, we can assume that, in our experiments, the rate of coal conversion did not depend upon the particle size. Experiments were carried out in the semibatch tube reactors 1 (Figure 1). We used the reactors with different inner diameters from 10 to 24 mm and lengths from 150 to 900 mm made of stainless steel (12Cr18Ni10Ti, an analogue of the AISI 321). The temperature of the reactor was measured by six K-type thermocouples 7. The reactor temperature was kept with the accuracy of (4 °C. The pressure was measured by strain gauges 6, whose accuracy in the (6) Hanson, S.; Patric, J. W.; Walker, A. Fuel 2002, 81, 531-537.
10.1021/ef070127a CCC: $37.00 © 2007 American Chemical Society Published on Web 07/19/2007
Kinetics of Coal ConVersion in SCW
Figure 1. Scheme of setup: 1, reactor; 2, heat exchanger; 3, flow meter; 4, high-pressure pump; 5, coal bin; 6, strain gauge; 7, thermocouples; 8, collection vessel; 9, samplers; 10, fore chamber for mass spectrometric analysis; 11, vacuum pumps; 12, high-vacuum chamber with mass analyzer; 13, mass spectrometer control unit; 14, PC; 15, vacuum meters.
range of 1-40 MPa was 0.1 MPa. To prevent coal particle caking, anticaking agents (Al2O3 powder or SiO2 sand) with the mass ratio of coal/agent equal to 1 were used. At first, the mixture was fed into the reactor under room temperature; the sealed reactor was heated to a temperature of 400 °C; and distilled water was added to make the pressure in the reactor 30 ( 1 MPa for the given temperature T. When the reaction temperature was reached, upward SCW pumping started through the coal layer. A serious disadvantage of this scheme was the long time for coal heating up to the working temperature. The start-up heating was accompanied by an intensive mass loss of organic matter of coal (OMC) (see below). Moreover, the rate of heating is known to effect significantly the gasification rate (see, e.g., ref 7). Therefore, to determine the rate of initial coal conversion, coal should be supplied into the reactor under the working temperature. For this purpose, bin 5 was placed at the reactor top and coal was fed into the hot reactor by a screw. This scheme allowed for the determination of the coal conversion rate and product composition for conversion degree X > 0.06. Here, conversion degree X is (MOMC - MOMC)/MOMC , where MOMC and 0 0 0 MOMC are the initial and current OMC masses. The conversion products from the reactor were fed into prevacuumized vessel 8. In the course of the experiment, gaseous products were analyzed by the use of special samplers 9. Besides, the final composition of gaseous products collected in the vessel 8 was also analyzed. During analysis of gaseous product composition, the probe passed through prevacuumized vessel 10 and got a vacuum chamber 12 under the free-molecular mode. The central part of the flow was collimated by the coaxial conical diaphragms and passed an ion source of the quadrupole mass spectrometer MS-7303. The product composition was determined from the mass spectra with the use of mass spectrometer calibration by reference mixtures, and then the amount of each component in the products of coal conversion was calculated.8 After the experiment, the mass of coal residue in the reactor was measured by the gravimetric method, and its composition was determined by the ultimate analysis.
Results and Discussion Nonisothermal SCW Coal Conversion. Conversion of preloaded coal heated in SCW was studied by the following method. The reactor was filled with coal (∼1/2 of the reactor volume) and water in such an amount that, for T ) 550 °C, the pressure reached 30 MPa. The rate of reactor heating was ∼2 °C/min. Three samples were taken at temperatures T ) 550, (7) Senneca, O.; Russo, P.; Salatino, P.; Masi, S. Carbon 1997, 35 (1), 141-151. (8) Galichin, V. A.; Drozdov, S. V.; Dubov, D. Yu.; Mikhailin, V. V.; Psarov, S. A.; Vostrikov A. A. Actual Problems of Thermal Physics and Physical Hydrogasdynamics. Proceedings of the 5th International Conference of Young Scientists; IT SB RAS: Novosibirsk, Russia, 1998; pp 268276.
Energy & Fuels, Vol. 21, No. 5, 2007 2841
Figure 2. Temperature dependences for the gaseous products yield from coal in SCW under P ) 30 MPa.
650, and 750 °C. Sampling was accompanied by reactor pressure reduction, which was compensated by the SCW supply. The mass of fed water did not exceed 10% of the initial water amount. Temperature dependences of emitted volatile masses, reduced to initial OMC, are shown in Figure 2. The results are given as the cumulative sums; i.e., every point in Figure 2 corresponds to the total amount of volatiles, emitted from the beginning of heating. One can see that, with an increase in T, the yields of CO2, CH4, and BTX (benzene, toluene, and xylene) increase drastically. In the products, there is almost no CO and the amount of oxygen in CO2 at the temperature of 650 and 750 °C makes up 12.9 and 17.2%, correspondingly (for the initial mass share of oxygen in coal of 13.7%). Excess oxygen in products, observed at a high temperature, is a result of water dissociation.4,5,9 The content of hydrogen in the products of coal conversion in SCW at T ) 750 °C was 1.85%; i.e., 26.5% of the initial amount of hydrogen in coal was removed. The share of hydrogen, formed at water dissociation, was 7.1% of the initial amount of hydrogen in coal. It was derived from the molar balance of oxygen in initial coal and conversion products. It corresponds to the dissociation of 14.4% of initial water. In general, our findings are in agreement with the earlier data of Adschiri et al., who observed that, for coal with an atomic ratio H/C ) 0.99, heated in water up to 380 °C, the OMC mass loss is as high as 50%.10 Isothermal SCW Coal Conversion. While studying isothermal SCW conversion of coal, the mixture of coal with an anticaking agent was fed into a hot reactor within 10 s. The initial amount of coal was varied from 20 to 300 g with a mixture filling up to half of the reactor volume. The mass flow of SCW through the reactor did not exceed 20 g/min. For the given values of P and T, the SCW flow rate and the amount of coal in the reactor determined the time of reaction t (the time of SCW interaction with coal), which was changed from 60 to 720 s. Coal conversion was explored by sampling and analyzing the effluent. Assuming a plug-flow approximation, the current amount of products in the effluent allowed for the determination of the current conversion degree X of coal in the reactor. The correctness of this approximation was verified by proximate and ultimate analyses of the coal residue after the experiment. For five values of T ranging from 500 to 750 °C and P ) 30 MPa, several tests were performed that gave a set of data on product concentration in SCW depending upon T, t, and X. Let us first discuss the results obtained at the highest temperature T ) 750 °C. Conversion Kinetics at T ) 750 °C. The mass of converted carbon MC per 1 g of inlet water is shown in Figure 3 versus (9) Vostrikov, A. A.; Dubov, D. Yu.; Psarov, S. A. Russ. Chem. Bull. 2001, 50 (8), 1478-1481. (10) Adschiri, T.; Sato, T.; Shibuichi, H.; Fang, Z.; Okazaki, S.; Arai, K. Fuel 2000, 79, 243-248.
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heterogeneous conversion reactions are considered as the homogeneous ones, occurring in the whole volume of the coal particle. At that, the conversion rate can be written as
k(X, T) ) A(T)(1 - X)
(9)
The “nonreacted core” model differs from the previous one by the assumption that conversion occurs only on the particle surface. Because the surface area decreases proportional to (1 - X)2/3, we get Figure 3. Mass of carbon, transformed into gas-phase products, in 1 g of incoming water depending upon the reaction time: (1) T ) 750 °C, X ) 0.06-0.12; (2) T ) 750 °C, X ) 0.65-0.75; (3) T ) 500 °C, X ) 0.11-0.23; (4) T ) 500 °C, X ) 0.63-0.74.
k′(X, T) ) A′(T)(1 - X)2/3
A change in the structure of the coal particle is considered, for instance, by the “random pore” model,14 which assumes a random distribution of pores inside a particle. It supposes that
k′′(X, T) ) A′′(T)(1 - X)x1 - ψ ln(1 - X)
Figure 4. Experimental (solid points 1, T ) 750 °C; open points 2, T ) 500 °C) and calculated dependences of the coal conversion rate on the conversion degree in SCW for P ) 30 MPa. Line 3, calculation by eq 9; line 4, calculation by eq 10; line 5, calculation by eq 11.
time t of water residence in the coal layer for two groups of X values. Strictly speaking, the value of X is changed during the interaction of water with coal. However, when a large surplus of coal is taken into account with regard to SCW and relatively low concentrations of products, we can neglect that change. It can be seen in Figure 3 that at T ) 750 °C the mass of carbon, converted from coal into gaseous products at different X, tends to a limit value M∞C with an increase in t. At the same time, the rate of water saturation by the conversion products significantly depends upon X. Assuming that the reaction of gasification has the pseudofirst order on MC, for dependence MC(t), one can write
dMC/dt ) k(T, X)(M∞C - MC)
(7)
We suggest in eq 7 that, at T ) constant, the rate constant k depends only upon X and the SCW concentration is nearconstant. The last assumption is justified by a small concentration of products in SCW. As a result of the integration of eq 7, we obtain the following expression for the mass of coal, converted into volatile products:
MC(t) ) M∞C (1 - exp( - k(T, X)t))
(8)
In Figure 3, dependences MC(t), fitting experimental results at single M∞C ) 210 mg/g, are shown by the solid lines. From dependences MC(t) for different values of X, we can obtain dependence k(X) (Figure 4). Conversion Kinetics Modeling. To analyze k(X) dependence, we should use one of the models suggested before.7,11-14 The simplest models do not consider a change in the structure of coal particles at conversion. Thus, in the “homogeneous” model, (11) Molina, A.; Mondragon, F. Fuel 1998, 77 (15), 1831-1839. (12) Lee, J. M.; Kim, Y. J.; Lee, W. J.; Kim S. D. Energy 1998, 23 (6), 475-488. (13) Sohn, H. Y.; Szekely, L. J. Chem. Eng. Sci. 1972, 27, 763-772. (14) Bhatia, S. K.; Perlmutter, D. S. AIChE J. 1980, 26, 379-392.
(10)
(11)
where ψ is a parameter depending upon geometrical characteristics of pores. Substituting successively eqs 9-11 into eq 8, we obtained three equations for the determination of MC. For T ) 750 °C, the best agreement between calculation and measurement values of MC is observed at
k(X) ) 8.3(1 - X) (mg g-1 s-1)
(12)
k′(X) )7.0(1 - X)2/3 (mg g-1 s-1)
(13)
k′′(X) ) 6.3(1 - X)x1 - 1.59 ln(1 - X) (mg g-1 s-1) (14) Calculated dependences k(X), k′(X), and k′′(X) are shown in Figure 4 by lines. It can be seen that eqs 10 (curve 4) and 11 (curve 5) describe experimental data better than eq 9 (curve 3). We believe that the obtained parameters in eqs 12-14 will be valid for coals of different ranges, because it is known that, for coals containing more than 80% of carbon in OMC, the difference in rates k(X) is insignificant.11,15 The rate k(X) of these coals differs from the average one by a factor not higher than 1.3. Remember that raw coal, used in our work, contains 74.0% of carbon; under conversion, the share of carbon increases. The value of the coefficient ψ ) 1.59 in eq 14 is rather low. While describing the conversion rate of different coals by the random pore model, values of ψ range from 1 to 20.11,16 The higher values correspond to the case when a sharp maximum of dependence k(X) is observed for X ) 0.2-0.4. This maximum is a result of increasing the efficient gasification surface because of an increase in the porosity of coal particles. The obtained value ψ ) 1.59 indicates that, at X < 0.4 in our tests, the rate of carbon conversion into the gas-phase products is the sum of pyrolysis and gasification rates (it is supported by data on the separate product yield given in the next section). Note that in this study we did not carry out any preliminary devolatilization of coal because our interest was in studying raw coal conversion. Our tests were made at a relatively low temperature (e.g., it was substantially lower than the temperature at which the Vdaf value is commonly measured, 860 °C); thus, we would expect pyrolysis of the particles, accompanying SCW conversion. Virtually, in this study, we failed to separate pyrolysis and gasification by water, and both of these processes are referred to as “conversion”. Moreover, when the conversion of coal is analyzed in SCW, it is difficult to use the value of Vdaf to (15) Miura, K.; Hashimoto, K.; Silveston, P. Fuel 1989, 68 (11), 14611475. (16) Kajitani, S.; Hara, S.; Matsuda, H. Fuel 2002, 81 (5), 539-546.
Kinetics of Coal ConVersion in SCW
Energy & Fuels, Vol. 21, No. 5, 2007 2843
Figure 6. Thermal effect of coal gasification ∆H, number of hydrogen atoms in the solid residue of coal m, share of hydrogen q in products from decomposed water depending upon the degree of coal conversion X for P ) 30 MPa and T ) 750 °C.
heteroatoms is less than 3% of OMC, the gross reaction of coal conversion at X > 0.2 can be written as
(a + b + c)〈CHm〉 + (a + 2b)H2O f aCO + bCO2 + cCH4 + dH2 (16)
Figure 5. Specific (per 1 g of H2O) formation rate of gaseous products at SCW coal conversion at T ) 750 °C depending upon the conversion degree.
consider the yield of a volatile matter by pyrolysis: the yield may be both enhanced by intense SCW extraction of OMC and suppressed considerably by the elevated pressure.17 Nevertheless, it is reasonable to assume that the rate of pyrolysis decreases drastically with a rise of X and reaches zero with X ≈ 0.5, i.e., by the moment of almost complete conversion of elemental hydrogen from coal. Dynamics of Separate Products Formation. The rates of separate product formation are shown in Figure 5. The dependences ki(X) were obtained by the formula
ki(X) ) k′′(X)
Mi (12/16)MCH4 + (12/28)MCO + (12/44)MCO2 (15)
where Mi is the mass of the ith product (H2, CH4, CO, and CO2). It can be seen in Figure 5 that, with an increase in X, the rate of H2 and CO2 formation reaches its maximum in the range of X ) 0.2-0.4 and then it decreases constantly. Perhaps, this maximum is a sequence of an increase in porosity and reactivity of the particles, for instance, during the removal of oxygenbearing substances from OMC. The rate of methane formation has no maximum. This may be explained by the fact that methane is removed most intensively during coal pyrolysis at X < 0.4. For X > 0.4, the main source of methane is the reaction of the hydrogen and water interaction (see reactions 1 and 4) with the carbon residue. The contribution of the methane formation rate into the total rate of coal conversion k(X) is the crucial one; thus, it is the pyrolysis that is the main reason for the lack of a maximum on k(X) dependence. Hydrogenation of Products by Water Decomposition. The contribution of water into hydrogenation of the products of SCW conversion of coal can be determined from the oxygen balance. Considering that, for T > 650 °C and X > 0.2, coal in SCW looses oxygen almost completely and the mass of other (17) 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, 405433.
where 〈CHm〉 is the solid residue of OMC. Coefficients a, b, c, and d in reaction 16 can be easily obtained from the data on gas product yields. Then, using the balance of oxygen and hydrogen atoms in reaction 16, we can recover the amount of hydrogen atoms in the solid residue of coal as m ) 4 + 2(d a)/(a + b + c), the rate of water decomposition kH2O, which, for instance, for X ) 0.3, equaled 67 mg g-1 s-1, the share of hydrogen in conversion products, formed at water decomposition, q ) (a + 2b)/(2c + d), and enthalpy ∆H of SCW coal conversion. Calculations of ∆H were carried out using the known thermodynamic data on the heats of formation and combustion.18,19 The enthalpy of coal formation from elements was calculated by an empirical formula.20 Simultaneously, a change in coal composition during conversion was taken into account: an increase in carbon content in coal and a reduction of other components. Dependences of q, m, and ratios H/C for combustible gases (H2, CH4, and CO) on X for X > 0.2 are shown in Figure 6. Because of a wide scattering of q, m, and ∆H, only the averaged curves are given. It is clear that, with an increase in the degree of SCW conversion of coal, continuous reduction of m to m ≈ 0 is observed for X ≈ 0.7. An increase in m for X > 0.7 might be explained by an error in m determination. However, results of the ultimate analysis of the coal residue, having undergone SCW conversion, demonstrated an increase in m for X > 0.7. We have also observed this effect during SCW conversion of tar.4 A change in the structure of the carbon residue may be the reason. It can be seen in Figure 6 that, with a rise of a degree of SCW coal conversion, heat consumption increases and, for X g 0.6, reaches a limit value of ∆H ≈ 0.09QC, where QC ) 39.5 kJ/g is a heat value of OMC, estimated from coal composition using a formula developed by the Institute of Gas Technology.20 The average value of heat consumption for SCW conversion of coal equals 〈∆H(X)〉 ≈ 6.5 × 10-2QC. Hence, the maximal efficiency of the process is above 93%. Coal Conversion in CO2/H2O Fluid. For industrial technologies of SCW conversion of coal for the provision of the autothermal character of the process, the addition of an oxidant (18) Glushko, V. P. Thermodynamic Properties of Separate Substances, Handbook (in Russian); Nauka: Moscow, Russia, 1978. (19) Tatevsky, V. M. Physical-Chemical Properties of Separate Hydrocarbons (in Russian); Gostoptekhizdat: Moscow, Russia, 1960. (20) Perry’s Chemical Engineers’ Handbook, 7th ed.; Perry, R. H., Green, D. W., Eds.; McGraw-Hill: New York, 1999; p. 5, Sect. 27.
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Figure 8. Rate of SCW coal conversion at P ) 30 MPa depending upon the temperature.
Figure 7. Specific (per 1 g of H2O) formation rates ki of gaseous products and the rate of coal conversion k for X ) 0, T ) 750 °C, and P ) 30 MPa depending upon the concentration of CO2.
(air or pure oxygen) into the SCW flow is inevitable. At that, an additional amount of CO2 is formed. In the current paper, we have studied the effect of CO2 additions in SCW on coal conversion. Dependences of the complete conversion rate k(X, T) and the rate of separate product formation ki(X, T) on concentrations of CO2 are shown in Figure 7 for X ) 0. One can see that, with an increase in the CO2 concentration, the formation of all products, excluding CO, decreases. The difference between the rates of CO2 production at the expense of initial oxygen in coal and decay of CO2 in reaction 5 decreases to zero for a concentration of [CO2] ≈ 1.5 mol/L. This value corresponds to an equilibrium concentration of CO2 and a minimal rate of SCW conversion of coal. The following increase in the CO2 concentration leads to the fact that the rate of gasification slightly increases. Thus, under the autothermal mode of SCW conversion of coal, the formation of additional CO2 in the process of fuel combustion affects the conversion rate and the CO/H2 ratio in the products. The rate of coal conversion in pure H2O and CO2 fluids is equal to 6.3 and 3.3 mg g-1 s-1, correspondingly. We should note that, depending upon the degree of coal metamorphism, the rate of coal gasification by steam can vary from 0.1 to 8 mg g-1 s-1 for T ) 850 °C and P ) 2/7 MPa and the rate of gasification by CO2 varies from 5 × 10-2 to 4.7 mg g-1 s-1.17 Therefore, even for the lower temperature of T ) 750 °C, the rates of coal conversion in H2O and CO2 obtained in the current work for P ) 30 MPa were close to the maximal rates of coal gasification, observed at the lower pressure and higher temperatures. Temperature Effect on Conversion. In closing, let us briefly discuss the results obtained at the lower temperatures. A decrease in temperature was found to have a strong effect on the SCW coal conversion. At T e 700 °C, only a linear increase in the MC(t) dependence was observed for the reaction time t up to 720 s, and that is the maximal time for these experiments. This corresponds to k(T, X)t , 1 limit in eq 8 (see, also, in Figure 3 data for T ) 500 °C) and does not allow us to obtain M∞C and k(T, X) values separately. We can try to estimate the rate constant k(T, X) assuming a temperature-independent value of M∞C . (Realizing high roughness of this assumption, we believe that this analysis may be useful for the estimation of conversion degrees under similar conditions.) The fitting of experimental k(T, X) at T ) 500 °C by three above-discussed models is shown in Figure 4; the fitting parameters in eqs 9-11
are the following: A ) 0.2 mg g-1 s-1, A′ ) 0.16 mg g-1 s-1, A′′ ) 0.14 mg g-1 s-1, and ψ ) 1.59. It can be seen that the “nonreacted core” and “random pore” models are equally good for approximation and the value of ψ ) 1.59 is found to be appropriate for both temperatures T ) 500 and 750 °C. Then, assuming in eq 11 that A′′(T) ) A0 exp(-E/RT), we have obtained that A0 ) 103.1 ( 0.5 s-1 and E ) 103 ( 9 kJ/mol are best suited for the fitting of experimental k(T). At coal gasification in steam12 in the range of T ) 750-900 °C and P ) 0.025-0.08 MPa, the first order of reaction in the water concentration and activation energy E ) 165 kJ/mol have been obtained. For the higher values of T ) 1100-1500 °C and P ) 0.2-2 MPa, it was found16 that E ) 214 kJ/mol and the reaction order in water is 0.86. Conclusions Kinetics of coal particle pack conversion in SCW at P ) 30 MPa, T ) 500-750 °C, and t ) 60-720 s with and without the addition of CO2 was investigated. A high hydrogenation degree of SCW conversion products caused by water decomposition (up to 30% of SCW flow rate at T ) 750 °C and P ) 30 MPa) was found. Therefore, the main products of gasification were H2, CH4, CO, and CO2. For the temperature of T ) 750 °C, the rates of coal conversion in both H2O and CO2 obtained in the current work for P ) 30 MPa were close to the maximal rates of coal gasification, observed at the lower pressure and higher temperature. The “random pore” and “nonreactive core” models are suitable for describing the rate of coal conversion dependent upon the coal conversion degree. SCW conversion is a highly efficient process of low-grade fuel transformation into burning gases with a specific calorific value, exceeding the calorific value of initial coal by 6.5%. The efficiency of total conversion is observed to be no less than 93.5%. The autothermal character of the process can be provided by direct combustion of some part of fuel in SCW. Acknowledgment. This work was supported by the Russian Foundation for Basic Research (project numbers 05-08-17982, 0608-00717, and 07-03-00698).
Nomenclature P ) Reaction pressure, MPa T ) Reaction temperature, °C t ) Reaction time, s k ) Coal conversion rate, mg g-1 s-1 MOMC ) Mass of coal organic matter, g ) Initial mass of coal organic matter, g MOMC 0 X ) Coal conversion degree
Kinetics of Coal ConVersion in SCW MC ) Mass of converted coal, g/g of SCW M∞C ) Maximum mass of carbon converted from coal into gaseous products, g/g of SCW ψ ) Coal pores structural parameter a, b, c, and d ) Stoichiometric coefficients m ) Number of hydrogen atoms in the coal residue
Energy & Fuels, Vol. 21, No. 5, 2007 2845 E ) Activation energy, kJ/mol A0 ) Pre-exponential factor, s-1 ∆H ) Enthalpy of the reaction QC ) Heat value of the organic coal mass, kJ/g EF070127A
