O2 Supercritical Fluid - American

May 27, 2007 - Institute of Thermophysics, Siberian Branch of the Russian Academy of Sciences, LaVrentieV aV. 1,. 630090 NoVosibirsk, Russian Federati...
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Combustion of Coal Particles in H2O/O2 Supercritical Fluid Anatoli A. Vostrikov,* Dmitri Yu. Dubov, Sergey A. Psarov, and Mikhail Ya. Sokol Institute of Thermophysics, Siberian Branch of the Russian Academy of Sciences, LaVrentieV aV. 1, 630090 NoVosibirsk, Russian Federation

Combustion of single coal particles of 1-5 mm diameter in H2O/O2 supercritical fluid with a mass share of oxygen of 0-6.6%, pressure of 30 MPa, and temperature of 400-750 °C was studied in the semi-batch reactor. A decrease in coal mass was observed in two parallel processes: gasification by water and oxidation by oxygen. With an assumption of zero order in H2O concentration and Arrhenius dependence for the rate of gasification by water, the activation energy was estimated as 19 kJ/mol and the pre-exponential factor was determined as 1.02 × 10-2 s-1. It was found out that oxidation by oxygen under the temperature of above 500 °C is limited by the rate of O2 diffusion to combustible coal residue. Below 500 °C the rate of heterogeneous oxidation by oxygen is described by the first-order reaction in concentration of O2 and zero-order reaction in concentration of H2O with activation energy of 166 kJ/mol and pre-exponent of 1.5 × 108 cm3/(g‚s). Introduction The main factor that impedes the growth of coal consumption is the negative impact of coal power engineering on the environment and related expenses for environmental safety. This stimulates the search for new environmentally responsible and energy-saving methods of coal conversion aimed at production of high-enthalpy actuating fluids, syngas, and valuable hydrocarbons. In this respect, the interest to application of supercritical water, SCW (at temperature of T > 374 °C and pressure of P > 22.1 MPa), as a medium for conversion of low-grade fuels and utilization of harmful wastes increases drastically. The prospects of this SCW application are provided by its unique properties:1,2 high, almost unlimited solubility of organic substances and nonpolar gases (particularly, oxygen) in combination with low solubility of salts and acids; high density; high (in comparison with the liquid state) diffusivity; and low viscosity. Even at relatively low temperatures, SCW demonstrates high reactivity in radical reactions, typical for combustion. Thus, in pure SCW without addition of oxygen, carbon is oxidized at T > 600 °C in the reaction 〈C〉 + H2O ) CO + H2.3,4 The processes that seem to be promising for supercritical technologies of low-grade fuel conversion are as follows: partial oxidation,5-7 widening the conventional steam gasification to the range of supercritical parameters, and complete oxidation (combustion) in SCW8-10 for generation of high-enthalpy actuating fluids. During SCW conversion of coal efficient extraction of coal organic mass, separation of a mineral part and conversion products occurs. A high density of the reaction medium intensifies the combined interaction of water with carbon and suppresses ash and tar formation, which is observed during decomposition of heavy organic substances in gaseous media. The relatively low temperature of SCW conversion impedes formation of NOx and SOx,11 and closeness of this system excludes emissions of fine ashes. Energy-saving efficiency of SCW conversion of coal is provided by the fact that fuel is oxidized directly within a heat carrier. At that, complete combustion of fuel occurs without an excess of an oxidizer.12 It is also very important that watered fuel, including coal-water slurry, can be easily used. Recent * To whom correspondence should be addressed. Tel.: +7 383 330 80 94. Fax: +7 383 330 84 80. E-mail: [email protected].

comparative analysis between energy efficiencies of the scheme with SCW coal combustion and traditional schemes of pressurized fluidized bed and pulverized coal boiler10 has shown the advantage of the SCW scheme by electric efficiency and specific emission of CO2, harmful oxides, and ashes in a wide range of temperatures and pressures. Now to develop the SCW technologies for solid fuels conversion, it is necessary to know the mechanisms and kinetic constants of combustion processes in SCW. By now the essential progress was reached only in investigation of homogeneous oxidation of organic substances.11,13-15 Simultaneously, heterogeneous conversion of carbonic substances was considered sparsely in studies. Gasification of an activated coal pack in pure SCW was studied by Matsumura et al.5 It is shown that the rate of gasification correlates well with extrapolated data on gasification under the atmospheric pressure. A change in size of single carbon particles at oxidation in the flow of supercritical H2O/O2 fluid under P ) 23-30 MPa and T ) 400-600 °C was observed recently.16-18 For the particles of activated coal, the oxidation rate was limited by oxygen diffusion to the surface.16 Under the same conditions the oxidation rate of synthetic graphite was limited by reactions at the particle surface.17 Simultaneously, the activation energy was significantly lower than that for graphite combustion under the atmospheric pressure. The rate of H2O/O2 oxidation of pyrolytic graphite (HOPG) was insignificant.17 Low-temperature (T < 420 °C) oxidation of the fossil coal pack was studied by Wang and Zhu.19,20 The authors have shown that sulfur of initial coal can be extracted only as the liquid products (sulfates and thiosulfates)19 and determined kinetic dependences of the coal mass loss.19 Hydrogen peroxide was used in studies16-20 as an oxidizing agent, and under the low-temperature it is not equivalent to oxidation by O2.21 Oxidation of a single coal particle in the flow of H2O/O2 fluid has been studied in the current paper. In contrast to SCW oxidation of model carbonic particles, the oxidation rate of fossil coal depends, probably, on the coal structure, organic matter composition, and share and composition of the mineral part of coal. For instant, the catalytic effect of Ca(OH)2 on the yield of volatiles and increasing portion of low-molecular products at autoclave gasification of a coal body in SCW was revealed.22 The oxidation rate of fossil coal can be limited by oxidizer diffusion through a porous ash layer, accumulating at the particle

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surface. We should note that SCW can effect rate of coal oxidation via different factors, whose relative importance a priori can be hardly determined. It is known23 that the combustion of a coal particle in the gaseous media under the standard and elevated pressure and moderate temperature is limited by chemical reactions and not diffusion of O2. However, with a rise of medium density, the diffusion coefficient decreases, and the limiting effect of diffusion on the rate of coal conversion in SCW may prevail even under relatively low temperatures. On the other hand, a high density of SCW increases concentration of HO2 and OH radicals, what increases oxidation.24 Simultaneously, water absorption at the particle surface may both accelerate oxidation at the expense of surface reactions between absorbed water and coal and hamper oxidation as a result of the blocking active centers at the coal surface. The role of active centers of a carbonic particle at gasification in gaseous media was discussed recently in a review.25 Experimental Section Oxidation of a single coal particle in the flow of H2O/O2 fluid is studied by the current work. We have used coal from Kuznetsk basin with the following characteristics: (wt %): Ad, 13.6; Wa, 5.3; Cdaf, 77.7; Hdaf, 5.5; Ndaf, 2.6; Sdaf, 0.5; Odaf, 13.6; and Vdaf, 40. Ash after coal burning was studied with an X-ray microanalysis technique which revealed the following composition: SiO2, 59.5%; Al2O3, 20.6%; Fe2O3, 6.7%; CaO, 3.9%; K2O, 3%; MgO, 2.7%; Na2O, 2%; TiO2, 0.9%; P2O3, 0.5%; and MnO2, 0.2%. A particle, mechanically treated to the shape of a sphere, was put on a flat porous stainless steel disc in the center of a vertical cylindrical reactor of 24-mm diameter.9 The reactor was filled with distilled degassed water and heated to the working temperature. Then, fluid H2O/O2 was pumped through the reactor under pressure P ) 30 MPa and the working temperature. In some certain time t, oxidation was fast (during e 5 s) quenched by cold water that was fed into the reactor. Experiments were carried out for T ) 400-750 °C, t ) 213-1200 s, mass ratio [O2]/[H2O] at the reactor inlet of 0-0.066, initial diameter of particle d0 ) 1-5 mm, and fluid velocity in the reactor of 0.6-3 mm/s. The Reynolds number Re, calculated by the particle diameter, did not exceed 15; that is, the particle was streamlined under the laminar mode. According to technical analysis of particles after the experiment, the moisture content was Wa ) 4-5% (by mass); that is, the moisture was close to the initial one. Therefore, mass of organic matter of coal (OMC) in the particle after the experiment, Mf, was determined by initial OMC mass, M0, and a decrease in the total mass of particle after its stay in the fluid flow. After the experiment the outer diameter of the particle was changed relative to d0, but not by more than 5%. It can be seen on a chip of an incompletely burned particle that in the center there is a coal residue surrounded by a shell of very porous ash. Results and Discussions While describing the mass losses of a particle, let us consider two parallel reactions, assuming that their channels are independent: coal conversion at interaction with H2O (let us term this process “coal gasification by water” (CGW)) and at interaction with O2 (“coal oxidation by oxygen” (COO)). Then, the total rate of OMC mass loss by a coal particle W ) dM/dt in H2O/O2 fluid equals the sum of the rates of mass losses in the CGW, WG, and COO, WO, processes:

W ) WG + WO

(1)

Note that in this study we did not carry out preliminary devolatilization of coal particles because our interest was in studying raw coal burning. Our tests were made at relatively low temperature (e.g., substantially lower than the temperature at which the Vdaf value was measured, 860 °C); thus, we would expect devolatilization of the particles, accompanying SCW conversion. Virtually, in this study we failed to separate devolatilization and gasification by water, and below both these processes are referred to as “gasification”. Moreover, analyzing the particle burning in SCW it is dufficult to use the value of Vdaf to consider the yield of a volatile matter: on the one hand, the yield may be enhanced by active extraction of OMC by SCW; on the other hand, there is some evidence that it should be considerably suppressed by the external pressure.26 Gasification of Coal Particles by SCW. Experiments on CGW were carried out under particle streamlining by pure SCW with t ) 213 s, T ) 552 and 700 °C, and varying d0 from 2.9 to 4.5 mm. Dependence of ∆M/M0 ) (M0 - Mf)/M0 on d0 is shown in Figure 1. The weak dependence allows averaging of ∆M/M0 measurements by d0. This averaging provides an error of not higher than 10%. Taking into accout that during COO the particle loses 20-50% of more mass, the contribution of averaging by d0 into the total error is even less. In the current study the fluid layer around a particle does not have enough time for saturation by gasification products. Under our conditions, the time of the saturation, estimated by the results from ref 27, is about 5 min, and the time of particle streamlining with d0 e 5 mm for the SCW velocity of ≈ 0.6 mm/s in the reactor does not exceed 10 s. Hence, in our experiments we can neglect the effect of mass transfer on CGW. As a result the rate of gasification WG can be assumed depending only on temperature T and conversion degree X(t) ) 1 - M(t)/M0, where M(t) is the current OMC mass of a particle. Recently we have studied28 the validity of different models of coal gasification29-31 for description of SCW conversion of coal pack and shown that dependences X(t) are described better by the model of random pores30 and a little bit worse by the model of surface gasification (“nonreacted core model”).31 However, the last model does not include adjustable parameters and provides more simple integration for determination of M(t). Using the model of surface gasification and assuming the Arrhenius type of temperature dependence WG(t), we will write

WG ) AG exp(-EG/RT)(1 - X)2/3M0

(2)

where EG is the activation energy of CGW and R is the universal gas constant. With least-squares fitting of experimental data by dependence (eq 2), we have obtained AG ) 1.02 × 10-2 s-1 and EG ) 19 kJ/mol. Oxidation of Coal Particles by Oxygen in H2O/O2. The COO process includes two successive stages: diffusion feed of oxygen to the particle’s OMC with rate wD and OMC oxidation with rate wO. Oxygen is fed to OMC by diffusion from the fluid to the outer particle surface and then through a porous ash residue. The flow of oxygen to the surface of a spherical particle with diameter d0 equals

Q ) ShπDd0(C∞ - CS0)

(3)

Here Sh is the Sherwood number, D is the coefficient of oxygen diffusion in H2O/O2 fluid, and C∞ and CS0 are oxygen concentration in the flow at an infinite distance from the particle and at the particle surface, correspondingly. Under the conditions

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WO )

12 + 0.85 (Q - nGWG) 32 × 1.2125

(8)

Substituting eqs 2, 6, and 8 into eq 1, we can easily derive the equation for the rate of particle’s OMC mass reduction during the processes of CGW and COO:

( )( )

EG M 2/3 dM ) AG exp M0 + dt RT M0 12.85 (πdC2kD(C∞ - CSC) - nGWG) (9) 32 × 1.2125

Figure 1. Relative loss of a particle mass after gasification in SCW depending on the initial particle diameter for the temperature T ) 700 °C (filled triangles) and 552 °C (open triangles).

of our experiment, the value of Sh can be determined by the empirical dependence of the rate of mass transfer to the sphere surface in a laminar flow:32

Sh ) 2 + 0.6Re1/2 Sc1/3

(4)

Here Sc ) ν/D is the Schmidt number and ν is the kinematic viscosity of the fluid. The coefficient of oxygen diffusion in H2O/O2 fluid was calculated by the empirical formula suggested in ref 33: D (cm2/s) ) 2.24 × 10-6T (K)0.792/F (g/cm3), where F is the density of fluid. Many works are devoted to the theory of a porous structure of a carbonic particle and penetration dynamics of gas reagents through it (e.g., see ref 34 and references therein). In our case the flow of oxygen from the surface of ash residue to the surface of the unreacted part of OMC is retained, and with an assumption of one-dimensional radial mass transfer, it can be writen as35

(

Q ) 2πDeff

)

d0dC (C - CSC) d0 + dC S0

(5)

Here, Deff is the effective coefficient of oxygen diffusion in H2O/ O2 fluid through a porous ash layer, dC is the diameter of the unburnt core (OMC residue), and CSC is oxygen concentration at the residue surface. The coefficient Deff is related to ash porosity θ via the expression35 Deff ) Dθ2.5, where θ ) 1 Faf/Fan, and Faf and Fan are ash density in the porous and pressed states. Excluding CS0 from eqs 3 and 5, we obtain

Q ) πdC2kD(C∞ - CSC)

(6)

Here, kD ) ShDd0θ2.5/(ShδdC + θ2.5dC2) and δ ) (d0 - dC)/2 is the thickness of the porous ash layer. It should be also taken into account that a part of the oxygen flow to a particle is spent for combustion of combustible products of CGW; therefore, only the residue of O2 participates in particle oxidation. Oxygen consumption for combustion of CGW products equaled QOG ) nGWG, where coefficient nG, calculated by the composition of CGW products (H2, CH4, CO),7 was 2.12. The oxygen residue is spent for COO in the crossreaction:

C1H0.85 + 1.2125O2 f 0.425H2O + CO2

(7)

Considering eq 7, for the rate of COO with wO . wD we obtain

The oxygen concentration CSC in eq 9 can be determined from the condition of equality between the oxygen flow to the residue surface, Q - QOG, and oxygen consumption in reaction 7. Let us consider two limit modes of COO: wO , wD and wO . wD. For wO , wD, the COO process is limited by the chemical heterogeneous reactions of oxidation and it is characterized by a strong temperature dependence between the rate of mass loss and volumetric burnout of a particle.23 Under the normal pressure such a mode of COO is observed for T < 800 °C. However, in our experiments the rate of OMC losses did not depend on T even for T g 500 °C, and the unburnt particle had a dense carbonic core. Most likely, that for T g 500 °C COO in SCW is limited by diffusion of oxygen, that is, wO . wD. In this case, it can be assumed that CSC ) 0 in eqs 6 and 9. Dependence of the measured OMC loss ∆M/M0 on calculated (∆M/M0)calc by formula 9 for CSC ) 0 is shown in Figure 2. In calculation, value of dC was determined by the current mass M and initial density of a particle. Density of ash Fan is taken 3.14 g/cm3 from its chemical composition, and ash density Faf is determined with the assumption of the constant outer diameter of a particle Faf ) 6M0/(πd03)Ad. The data points in Figure 2 are grouped together into three temperature ranges. It is obvious that (excluding points at T e 500 °C, see the next section) combustion of a coal particle in SCW is described well by diffusion approximation. Primarily, this is explained by higher density (and lower diffusivity) of fluid in comparison with the gaseous media: with a rise of pressure from 0.1 to 30 MPa, the coefficient of O2 diffusion in H2O decreases by the factor of 280 for T ) 750 °C and by the factor of 1000 for T ) 400 °C.33,36 Oxidation of a Particle at Temperature T e 500 °C. It is seen in Figure 2 that in this temperature range the experimental mass loss differs noticeably from the calculated one. This difference is more evident in Figure 3 where the value of ∆ ) ∆M/M0 - (∆M/M0)calc is shown depending on the process temperature. One can see that for T g 500 °C, value ∆ is distributed relative to zero quite symmetrically, but for T < 500 °C it exhibits a regular trend to negative magnitudes. Perhaps this is caused by a violation of the condition wO . wD; that is, the rates of oxygen diffusion and oxidation become comparable, at least. Hence, it cannot be assumed in eqs 6 and 9 that CSC ) 0. To estimate wO, we again use the model of surface gasification31 and Arrhenius dependence of wO on T. Then, the rate of oxidation can be writen as

( )

WO ) AO exp -

EO R β C C (1 - X)2/3M0 RT W SC

(10)

where CW is the concentration of SCW and R and β are the orders of oxidation reactions on concentrations of water and oxygen, correspondingly. Substituting eqs 6 and 10 into eq 8, we obtain the expression for determination of CSC:

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( )

AO exp -

EO R β C C (1 - X)2/3M0 ) RT W SC 12.85 (πd 2k (C - CSC) - nGWG) (11) 32 × 1.2125 C D ∞

The value of CSC, obtained from eq 11 together with eq 9, determines the rate of coal particle oxidation by oxygen WO. Unfortunately, eq 11 is transcendental, and for the general case it can be solved only numerically. However, the problem can be simplified, taking the reaction order for oxygen β ) 1, which corresponds to the common kinetic models.37 In this case, eq 11 is reduced to a simple algebraic equation. Considering a high excess of SCW and the fact that water does not participate in the reaction as an oxidizer in this temperature range,3,4 it can be assumed in eq 10 that R ) 0. With these assumptions, the value (∆M)calc was calculated by integration of eq 9. It was found out that EO ) 150 kJ/mol and AO ) 4 × 107 cm3/(g‚s) gave the best fit of calculated values (∆M/M0)calc to experimental data for T ) 400-500 °C. One can see the quality of fitting in Figure 3 where the difference between experimental mass loss and results of calculations with these parameters is shown by open diamonds. Let us take into account that the oxidation of the coal particle is accompanied by heat release, which gives rise to the temperature difference ∆T between the particle and SCW fluid. Koda et al. estimated this difference using direct computational fluid dynamics modeling17 and a simplified heat balance equation.18 They found that the value of ∆T for the activated carbon particle may be as high as hundreds of kelvins. Following their equations for heat transfer in laminar flow18 we have estimated the particle temperatures in our experiments and found under similar conditions a much lower value of ∆T: for example, in experiments on oxidation at T e 500 °C, when the average mass share of oxygen was 2.5%, the average increase of particle temperature was only 37 K. The closer view revealed that the main reasons for discrepancy are both a lower reaction rate of natural coal and a smaller particle size: an average diameter of our particles was 1.8 mm (in contrast to 4 mm in ref 18). As a result, taking into account the particle heating shifted the values of EO and AO to 166 kJ/mol and 1.5 × 108 cm3/(g‚s), correspondingly. The heating of coal becomes more affective with the increase of particle diameter, fluid temperature, and especially O2 share. In this case heat transfer is greatly intensified by buoyancy-driven flow and needs more precise analysis which is currently under study. Comparing the value of EO with available activation energies for coal char oxidation, for example, those compiled by Backreedy et al.,38 we should keep in mind their “enormous variation” from 25 to above 160 kJ/mol (see Figure 6 in ref 38). One can see that our value of energy EO is near the upper limit of this range and close to 130 kJ/mol, obtained recently for oxidation of a coke residue by oxygen under the normal pressure at the low temperature of 440-560 °C.31 Extrapolation to Higher Reaction Rates. To organize slow combustion of a particle, the mass share of O2 in current experiments did not exceed 6.6%. However, for the real schemes of SCW combustion of coal, it should be significantly higher. Actually, for water heating from initial T ) 25 °C and P ) 0.1 MPa to 650 °C, 30 MPa, we need 3.3 MJ/kg. If the caloricity of our coal is 39.5 MJ/kg of OMC, such energy can be produced with complete combustion of 84 g of OMC. Coal oxidation in the H2O/O2 fluid does not require excessive oxygen;10 therefore, the mass ratio [O2]/[H2O], corresponding to reaction 7, will be near to 0.25. Using the obtained kinetic constants, we can estimate the typical time of oxidation for the given case. Time

Figure 2. Relative loss of a particle mass, observed in experiment, depending on the relative mass loss, calculated by the suggested model.

Figure 3. Difference between experimental and calculated relative mass loss of a coal particle depending on the temperature. Diamonds provide calculation results with consideration of a finite value of surface oxidation rate.

Figure 4. Time dependences of particle COM calculated for different temperatures. Other conditions of SCW conversion are presented in the text.

dependence of OMC mass M(t) for the particle with d0 ) 2 mm and M0 ) 5.7 mg and for the fluid velocity of 1 mm/s is shown in Figure 4 for several values of T. We should emphasize good linearity of dependence M(t) for T g 500 °C, and this proves constant heat release during SCW combustion. It is also obvious that because of diffusion limitation of COO, an increase in temperature for T > 500 °C does not significantly effect the rate of oxidation. Also, one can hardly increase the rate of combustion because of an increase in the fluid velocity.

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Under the CGW process (accompanied by coal devolatilization) the gasification rate is varied through 2 orders of magnitude with the coal sort;26 therefore, we can assume that the obtained constants of gasification rate are hardly suitable for other coals, whereas mass transfer limited kinetics of COO at T g 500 °C seems to be obeyed. This is evident from the fact that the rate wO of devolatilized char oxidation depends on coal sort to a smaller extent, while the rate wD of oxygen supply to the surface does not depend on it at all. Conclusions

Figure 5. Calculated time of complete combustion of a particle depending on temperature. Diameters of particles are 1 mm (dotted line) and 2 mm (solid line).

Figure 6. Ratio of O2 concentrations at the surface of carbonic residue and in the initial fluid (1) and rates of gasification (2) and oxidation (3) of a particle depending on temperature. Calculation for X ) 0 (solid lines) and 0.9 (dashed lines).

According to calculation with T ) 650 °C, with a rise of velocity from 1 to 10 mm/s, the time of complete combustion of a particle decreases by 23%. Perhaps this is due to a weak dependence of Sh on Re (see formula 4). Dispersion of coal affects the rate of combustion more significantly. Temperature dependences of the time of complete combustion of particles with d0 ) 1 and 2 mm are shown in Figure 5. It is obvious that for the double reduction in particle diameter, the time of complete combustion for T ) 650 °C decreases by the factor of 3.7. This provides high application efficiency for coal-water slurry in technologies of SCW combustion. In the current experiments, the condition WG ≈ WO was met for the mass share of O2, equal to 2-3%. For higher concentrations of O2, the effect of CGW on the conversion rate will be negligible. This can be seen in Figure 6, where the temperature dependences of CGW and COO rates, calculated for the particle with d0 ) 2 mm for [O2]/[H2O] ) 0.25 and two degrees of COM conversion X ) 0 and 0.9, are shown. Despite significantly different values of activation energy for CGW and COO (see above), ratio WO/WG for T e 530 °C stays almost constant. This is explained by diffusion hampering of oxidation due to a drastic decrease in concentration of O2 at the surface of COM residue relative to initial concentration C∞ (see curve 1 in Figure 6). We should note that for P ) 30 MPa and [O2]/[H2O] ) 0.25, the partial pressure of water P(H2O) ) 23.7 MPa; that is, it remains supercritical. The obtained rate constants raise an important question of whether they are applicable for SCW conversion of other coals.

Combustion of single coal particles of 1-5 mm diameter in the semi-batch reactor was studied in the flow of H2O/O2 supercritical fluid with an oxygen mass share of 0-6.6% under the pressure of 30 MPa and temperature ranging from 400 to 750 °C. Both gasification by water and oxidation by oxygen have been considered. It is shown that under the studied conditions, the rate of gasification causes a loss of particle mass, comparable with such a loss at oxidation by oxygen, if the mass share of oxygen in the fluid is 2-3%. With the assumption of zero order of H2O concentration, the activation energy and preexponential factor for the rate of gasification by water were estimated as 19 kJ/mol and 1.02 × 10-2 s-1, correspondingly. It is determined that for the temperature of 500-750 °C, the process of oxidation is limited by the rate of O2 mass transfer to the particle surface; therefore, with a rise of temperature, the time of particle combustion decreases insignificantly. Below 500 °C the rate of heterogeneous oxidation by oxygen is described by the first-order reaction in concentration of O2 and zero-order reaction in concentration of H2O with activation energy of 150 kJ/mol and pre-exponent of 4 × 107 cm3/(g‚s). As a whole the rates of gasification and oxidation of coal in supercritical fluid H2O/O2 are high enough for generation of actuating fluids for vapor-gas power devices with the high energetic and ecological efficiency. Acknowledgment The work was financially supported by the Russian Foundation for Basic Research (Project Nos. 05-08-17982, 06-0800717, and 07-03-00698). Nomenclature AG ) pre-exponential factor of Arrhenius dependence for gasification rate, 1/s AO ) pre-exponential factor of Arrhenius dependence for oxidation rate, m3/(kg s) C∞ ) oxygen mass concentration in the bulk of the fluid, kg/ m3 CS0 ) oxygen mass concentration at the surface of particle, kg/ m3 CSC ) oxygen mass concentration at the surface of particle’s carbon residue, kg/m3 CW ) water concentration, kg/m3 D ) diffusion constant of O2 in water, m2/s Deff ) effective diffusion constant of O2 through porous shell of ash, m2/s d0 ) initial diameter of the particle, m dC ) diameter of the particle’s carbon residue, m EG ) activation energy of the gasification rate, J/mol EO ) activation energy of the oxidation rate, J/mol kD ) mass-transfer rate coefficient (introduced in eq 6), m/s M ) mass of coal organic matter (COM) in the particle, kg M0 ) initial COM mass in the particle, kg

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Mf ) final COM mass in the particle, kg nG ) ratio of QOG to WG P ) pressure, Pa Q ) O2 mass flow, kg/s QOG ) O2 mass flow consumed in the oxidation of gasification products, kg/s R ) gas constant, J/(mol K) T ) temperature, °C t ) time of H2O/O2 fluid supply, s W ) rate of particle’s COM mass loss, kg/s WG ) rate of COM mass loss due to gasification of particle by water, kg/s WO ) rate of COM mass loss due to oxidation of particle by O2, kg/s wD ) rate of O2 diffusion to the particle’s carbon residue, kg/ (m2 s) wO ) oxidation rate of the particle’s carbon residue, kg/(m2 s) X ) carbon conversion coefficient Re ) Reynolds number Sc ) Schmidt number Sh ) Sherwood number Coal Characteristics Ad ) ash content (dry basis), wt% Cdaf ) carbon content (dry ash free basis), wt% Hdaf ) hydrogen content (dry ash free basis), wt% Ndaf ) nitrogen content (dry ash free basis), wt% Sdaf ) sulfur content (dry ash free basis), wt% Odaf ) oxygen content (dry ash free basis), wt% Wa ) moisture (as-received), wt% Greek Letters ∆M ) difference between COM mass before and after test, kg R ) reaction order for water concentration β ) reaction order for oxygen concentration ∆ ) difference between experimental and calculated COM specific mass δ ) thickness of porous shell of ash, m ν ) fluid kinematic viscosity, m2/s θ ) ash porosity F ) fluid density, kg/m3 Faf ) ash density in porous state, kg/m3 Fan ) ash density in pressed state, kg/m3 Glossary CGW ) coal gasification by water COO ) coal oxidation by oxygen HOPG ) highly ordered pyrolytic graphite OMC ) organic matter of coal SCW ) supercritical water Literature Cited (1) Galkin, A. A.; Lunin, V. V. Subcritical and supercritical water: a universal medium for chemical reactions. Russ. Chem. ReV. 2005, 74, 21. (2) Abdulagatov, I. M.; Bazaev, A. R.; Bazaev, E. A.; Saidakhmedova, M. B.; Ramazanova, A. E. PVTx measurements and partial molar volumes for water-hydrocarbon mixtures in the near-critical and supercritical conditions. Fluid Phase Equilibr. 1998, 150-151, 537. (3) Vostrikov, A. A.; Dubov, D. Yu.; Psarov, S. A. Pyrolysis of eicosane in supercritical water. Russ. Chem. Bull. 2001, 50, 1478. (4) Vostrikov, A. A.; Dubov, D. Yu.; Psarov, S. A. Naphthalene oxidation in supercritical water. Russ. Chem. Bull. 2001, 50, 1481. (5) Matsumura, Y.; Xu, X.; Antal, M. J., Jr. Gasification characteristics of an activated carbon in supercritical water. Carbon 1997, 35, 819. (6) Scott, D. S.; Radlein, D.; Piskorz, J.; Majerski, P.; deBruijn, Th. J. W. Upgrading of bitumen in supercritical fluids. Fuel 2001, 80, 1087.

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ReceiVed for reView March 12, 2007 ReVised manuscript receiVed May 11, 2007 Accepted May 16, 2007 IE0703686