ISSN 1990-7931, Russian Journal of Physical Chemistry B, 2016, Vol. 10, No. 4, pp. 576–581. © Pleiades Publishing, Ltd., 2016. Original Russian Text © A.G. Korotkikh, K.V. Slyusarskiy, A.A. Ditts, 2016, published in Khimicheskaya Fizika, 2016, Vol. 35, No. 7, pp. 16–22.
KINETICS AND MECHANISM OF CHEMICAL REACTIONS. CATALYSIS
Kinetics of Coal Char Gasification in a Carbon Dioxide Medium A. G. Korotkikha, b, *, K. V. Slyusarskiya, and A. A. Dittsa aNational
Research Tomsk Polytechnic University, Tomsk, Russia b Tomsk State University, Tomsk, Russia *e-mail:
[email protected] Received May 27, 2015
Abstract—Solid fuel samples with different carbon contents are gasified by successively subjecting to pyrolysis in argon and oxidation in carbon dioxide at various temperatures to determine the rate of the chemical reactions and the activation energy required for simulating and optimizing the operation of gas generators. The samples were prepared from bituminous coal, lignite, and anthracite of the Kuznetsk and Kansk-Achinsk coal basins. The gasification of coal char samples in a carbon dioxide medium at 900–1200°C is analyzed by thermogravimetry. The temperature dependences of the weight change rate and gasification time of coal char samples are measured and used to calculate the preexponential factor and activation energy of the carbon oxidation reaction. It is found that, with increasing oxidizing medium temperature from 900 to 1200°C, the gasification time of the coal char samples obtained from anthracite and bituminous coal decrease 8- and 22-fold, respectively. A physicomathematical model of coal char gasification in a fixed bed, with the oxidizing gas diffusing through the ash layer formed, is proposed. Keywords: solid fuel, coal char, pyrolysis, gasification, reaction rate, thermal analysis, carbon dioxide, temperature DOI: 10.1134/S1990793116040059
INTRODUCTION At present, special attention is paid to the gasification of solid fuels, in particular bituminous coal and anthracite, in order to replace traditional fossil fuels for burning in thermal power plants generating heat and electricity and to produce syngas and hydrogen for the chemical and metallurgical industries [1, 2]. The use of solid fuel gasification technology in thermal power plants enables to reduce emissions of pollutants into the environment due to a partial recirculation of carbon dioxide. In addition, the use of gas-producing facilities at thermal power plants makes it possible to use the most modern technologies of flue gas purification from CO2, H2S, and SO2 [3, 4] more efficiently and economically than in the case of burning of traditional fossil fuels [4]. The thermochemical gasification of solid fuels to produce combustible gases (H2, CO, and CnHm) is carried out by reacting carbon fuels with different oxidants. The oxidants used are oxygen (air), steam, carbon dioxide, or mixtures thereof [5, 6]. Depending on the ratio of the starting components of the gas mixture, temperature of the environment, reaction time and other factors, it is possible synthesize gases of different compositions with desired characteristics (for example, heat of combustion). Solid fuel gasification is performed in gas-generating facilities fed with gaseous oxidants, wherein the
main heterogeneous chemical reactions of carbon with oxygen and steam occur, yielding carbon monoxide and dioxide, and of carbon with carbon dioxide, yielding CO [7], C + CO2 = 2CO. Experimental studies of the gasification of solid fuels and of the heterogeneous reactions of carbon with carbon dioxide are of considerable importance, since the reactions are closely associated with heterogeneous reactions of carbon with steam and oxygen [8]: С + О2 = СО2, С + H2O = CO + H2, С + 2H2O = CO2 + 2H2. This paper reports the results of a thermal analysis for three samples of coal char produced from coals of the Kuznetsk and Kansk-Achinsk basins in a carbon dioxide medium at temperatures of 900–1200°C. These results were used to determine the rate of weight change of the samples, gasification time, and activation energy for the heterogeneous oxidation of carbon. A physicomathematical model of coal char gasification was developed, which enabled to determine the rate and duration of the heterogeneous chemical reaction as functions of the ambient temperature and coal grade.
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(a)
577
(b)
50 μm
50 μm (c)
50 μm Fig. 1. Microphotographs of the studied solid fuel samples: (a) anthracite (b) bituminous coal, and (c) lignite.
1. MEASUREMENT PROCEDURES 1.1. Solid Fuel Samples The solid fuel samples were of different degrees of enrichment, with a particle size of less than 80 μm. The microphotographs of samples of powdered solid fuels taken on a JCM-6000 scanning electron microscope are displayed in Fig. 1. The values of the density of the solid fuel samples, determined pycnometrically from three to four parallel measurements, are presented in Table 1. The carbon content in the starting solid fuel was determined from an elemental analysis performed using an energy-dispersive spectrometer with a nitrogen-free silicon drift detector operating at accelerating Table 1. Density of the solid fuel samples Solid fuel sample Anthracite Bituminous coal Lignite
Specific density, kg/m3
Bulk density, kg/m3
1753 ± 56 1927 ± 15 1425 ± 10
339 ± 2 340 ± 6 347 ± 13
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voltages of 10 and 15 kV, which is built in the JCM-6000 scanning electron microscope. The elemental analysis results are listed in Table 2, which are highly accurate for heavy elements in the solid fuel samples, such as Al, Si, Ca, Fe, with a relative error of less than 1%. The content of carbon in the anthracite, bituminous coal, and lignite samples was 89, 74, and 66 wt %, respectively. 1.2. Thermal Analysis Before thermal analysis, the solid fuel samples were first subjected to pyrolysis in argon in a SNOL 30/1100 oven by heating to 1000°C at a rate of 50°C/min to produce coal char and remove moisture and volatile components of the starting fuel. A 25- to 30-g solid fuel samples were placed in a ceramic crucible, kept in an oven for 1 h, and then cooled to 400°C. The ash content in the coal char sample was determined from the ratio of the final weight of the sample after differential thermal analysis (DTA) in carbon dioxide under isothermal conditions to the initial weight of the sample. The ash contents in the coal char samples obtained from anthracite, bituminous coal, and lignite were found to be 14, 34, and 16 wt %, respectively.
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Thermal analysis of a ~20-mg coal char sample was conducted on a Netzsch STA 449 F3 Jupiter TG– DSC combined analyzer. The coal char samples were first heated in the oven to 900, 1000, 1100 or 1200°C at a rate of 50°C/min in a 10 mL/min argon flow. When the desired temperature was reached, argon was replaced by a 24 : 1 carbon dioxide–argon mixture and the flow rate was increased to 250 mL/min. In studying the oxidation of carbon, the time of isothermal exposure of coal char samples was varied from 60 to 120 min, depending on the oven temperature. 2. EXPERIMENTAL RESULTS 2.1. Effect of the Ambient Temperature For each coal char sample, three parallel TG measurements at the different oven temperatures were performed; the relevant thermograms are shown in Fig. 2. During heating a coal char sample to a predetermined temperature in the argon flow, the weight of the sample decreased by 2 to 4%, depending on the fuel grade. When carbon dioxide was fed into the heated oven, the endothermic oxidation of carbon occurred, decreasing the sample weight by 63 and 82% for the coal char samples prepared from bituminous coal and from anthracite and lignite, respectively. The time of (a)
m, wt %
T, °C
Т
100 80 60
1200
600 40
2
400
1
0
200
20
40
60 t, min
C
N
O
Al
Si Ca Fe Others
Anthracite 89 Bituminous coal 74 Lignite 66
– 14 15
6 8 14
– 1 –
2 1 –
0
80
With increasing temperature of the carbon dioxide in the oven, the gasification time of the sample decreases, while the weight change rate and carbon oxidation rate increase. The rate of fuel weight change for the heterogeneous oxidation of carbon is described by the Arrhenius equation [9, 10],
v = A0 exp ( −E a RT ) ,
m, wt %
(1)
T, °C
Т
100
1100 1000
3
80
800
60
600
TG 2
40 1 20 20
40
60 t, min
m, wt % 100
80
800
80
60
600
60
400
40
80
200
20
Т
0
TG
20
40
60
0
T, °C
3 2
2
40 1
400 200
0
1000
20
1 2 1
(d) T, °C
Т
100
1 – –
gasification of the coal char sample was defined as the period from the start of supply of carbon dioxide into the oven chamber of the TG–DTA combined analyzer to the moment the sample weight stopped changing. The gasification times of the studied coal char samples at various temperatures are listed in Table 3.
(c) m, wt %
1 – 4
800
TG
20
Content of elements, wt %
Solid fuel sample
(b)
1000
3
Table 2. Elemental composition of the solid fuel samples
3 80 100 t, min
0
0
40
60
600 400
TG
1
20
900 800
200 80 100 120 140 160 t, min
0
Fig. 2. TG curves of the coal char samples at temperatures of (a) 1200°C, (b) 1100°C, (c) 1000°C, and (d) 900°C; the sample were prepared from (1) anthracite, (2) bituminous coal, and (3) lignite. RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY B
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2.2. Effect of the Carbon Content in the Solid Fuel Sample The rate of weight change of the coal char sample increases with the carbon content in the staring solid fuel, which may be associated with the porosity and reactive surface area of the sample. The reactive surface area of coal particles is directly proportional to their porosity [14–16]. According to [16], the porosity of the sample is related to its pour density as
Table 3. Gasification time of the coal char samples Gasification time, min Coal char from Anthracite Bituminous coal Lignite
900°С
1000°С
1100°С
1200°С
80 378 396
32 77 137
17 26 68
10 18 54
where v is the weight change rate, kg/s; А0 is the preexponential factor, kg/s; Еа is the activation energy, J/mol; R is the universal gas constant, J/(mol K); and T is the temperature, K. Based on the TG analysis data, we determined the average values of the rate of weight change of the coal char samples over a certain gasification period. The dependences of the rate of weight change of the studied coal char samples on the carbon dioxide temperature are presented in Fig. 3. The gasification of coal char is controlled by the diffusion mechanism [11–13] and results in the formation of ash on the surface of the reaction layer of the sample. The diffusion coefficient D (in m2/s), which influences the intensity of the gasification process, depends on the ambient temperature as n
⎛ ⎞ p D = D0 ⎜ T ⎟ 0 , ⎝T0 ⎠ p where D0, T0, and p0 are the diffusion coefficient, temperature, and pressure of the mixture under normal conditions, (in m2/s, K, and Pa, respectively); p is the medium pressure (Pa); and n is an exponent, which, depending on the temperature, ranges from 1.5 to 2.5. In the experiments performed, the gaseous medium temperature was substantially greater than the Sutherland coefficient [9] for CO and CO2, and consequently, n = 1.5.
80
v, μg/s 1
70
579
60 50
ε =1−
where ρb and ρsp are, respectively, the bulk density and specific density of the samples (kg/m3). Since the specific density and pour density of the samples differ insignificantly, it can be assumed that the reactive surface areas of the samples differ only slightly. Note that, at the ambient temperature 1100°C, for the coal char prepared from anthracite, with a carbon content of 89 wt %, the rate of change of the sample weight was 1.5–2.0 times greater than that for the coal char prepared from lignite and bituminous coal, containing, respectively, 66 and 74 wt % carbon (Fig. 3). The obtained parameters of Eq. (1) for the oxidation of the coal char samples in a carbon dioxide medium at 900–1200°C are listed in Table 4. The values obtained of the activation energy and preexponential factor are consistent with the available data for high- and low-grade coals [17, 18]. An analysis of the kinetic constants of the reaction showed that the activation energy decreases with the ash content in the coal char sample. The latter finding suggests that the rate of gasification of high-grade coals is more strongly dependent on the gaseous medium temperature. 3. MATHEMATICAL MODEL OF THE GASIFICATION PROCESS For a mathematical description of the process of gasification of coal char in a ceramic crucible of the oven (Fig. 4), we developed a model based on a onedimensional heat conduction equation with a moving boundary between the coal char and the ash formed during sample gasification:
ρCc p C ∂ T = λ C ∂ T2 at x ∈ ( 0; x gr ) , ∂τ ∂x 2 ∂ ∂ T ρ A c pA = λ A T2 at x ∈ ( x gr ; x 0 ) , ∂τ ∂x where τ is the time, s; ρ is the density, kg/m3; cp is the specific heat, J/(kg K); λ is the thermal conductivity coefficient. The indices “C” and “A” designate the coal char and ash phases, respectively. The second-order (symmetry) conditions were set at the lower boundary (crucible bottom) and the thirdorder conditions at the top. At the boundary between 2
40
2
30 20
3
10 0 1150
1200
1250
1300
1350
1400
1450 1500 T, K
Fig. 3. Dependence of the weight change rate for the examined coal char samples on the oxidizing environment temperature. The designations are the same as in Fig. 2. RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY B
ρb , ρ sp
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the media, the fourth-order boundary condition, with heat release by the gasification reactions, holds:
x = 0; ∂ T = 0, ∂x ∂ T x = x 0; − λ A = α (T − Te ) , ∂x x = x gr ; − λ C ∂ T = −λ A ∂ T + q r , ∂x ∂x where q r = 14.2 A0 exp(−E a RT ) is the thermal effect of the gasification reaction, W/m2 [19, 20]; α is the heat transfer coefficient, W/(m2 K); and Te is the ambient temperature, K. The boundary x0 is set to be immobile, whereas xgr moves over the sites of the spatial grid at time steps given by
τ i = hS (ρ C − ρ A ) v , where h is the spatial grid step, m; S is the cross-sectional area of the crucible, m2. The time of coal char gasification was calculated by adding up all the time steps at each stage of the calculation. The calculation was terminated when the mobile boundary reached the crucible bottom (lower boundary). The physicomathematical model was based on the following assumptions: (1) The thermophysical properties of the coal char and ash are time invariant. The porosity of the coal particles is disregarded. (2) The heat of reaction is released only at the interphase. (3) The initial temperature distribution in the coal char sample is uniform, with the temperature of the coal char being equal to the isothermal exposure temperature. The TG data obtained at the isothermal exposure temperature of 900°C (Fig. 2d) show the gradual reduction of the rate of gasification of the sample with time because of the formation of ash on the surface of the reaction layer of the coal char. To assess the impact of changes in the partial pressure of carbon dioxide over the surface of the reaction layer, the mathematical model takes into account gas diffusion through the porous layer of ash by solving the one-dimensional unsteady differential diffusion equation:
∂ pCO2 ∂ 2 pCO2 =D , 2 ∂τ ∂x where pCO2 is the carbon dioxide partial pressure (in Pa). At the interface between the coal char and the ash, the second-order boundary condition was applied, whereas at the lower boundary, the first-order condition. The decrease in the carbon dioxide concentration at the coal char–ash interface was determined from chemical kinetics equations [21]:
X
cpA λA
x0
ρA xgr cpC λC
0
ρC
Fig. 4. Computational diagram of the mathematical model.
x = 0; pCO2 = patm , ∂p x = x gr ; D CO2 = qСО2 , ∂x where patm is the atmospheric pressure (10000 Pa) and qCO2 is the mass flow rate due to CO2 absorption during the gasification reaction, Pa m/s. The differential equations of the mathematical model were solved by the sweep method using an implicit finite-difference scheme. The movement of the interface between coal char and the ash was determined by adjusting the time step so as to make it coincide with a spatial grid point. Figure 5 shows the dependences of the calculated and measured values of the gasification time of the samples on the carbon dioxide temperature. With increasing temperature of the oxidizing environment from 900 to 1200°C, the gasification times of the coal char samples prepared from anthracite, bituminous coal, and lignite decrease, respectively, from 80 to 10, from 378 to 18, and from 396 to 54 min. As can be seen, the calculated and measured values of the solid fuel gasification time are in close agreement. The greatest discrepancy between the calculated and measured gasification times (up to 22–25%) was observed for the coal char samples prepared from bituminous coal and lignite at the oxidizing environment temperature of 900°C, which is probably associated with the porosity of the particles and the low rates of oxidant diffusion and of gaseous product release, as well as with the accuracy of experimental determination of the gasification time. Table 4. Activation energy and preexponential factor of the carbon oxidation reaction for the tested coal char samples Constants of Eq. (1) Coal char from Anthracite Bituminous coal Lignite
Ea, kJ/mol 100 150 97
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A0, μg/s 258332 7936635 44712
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at an oxidizing medium temperature of 900°C, which is possibly associated with the porosity of the particles and the low rate of outflow of the gaseous reaction products.
t, min
100 3 2 10
1 1150
1
1200
581
1250
1300
1350
1400
1450 1500 T, K
REFERENCES
Fig. 5. Dependence of the duration of gasification of the coal char samples on the carbon dioxide temperature: the line and symbols represented the computational and experimental results, respectively. The designations are the same as in Fig. 2.
An analysis of the calculation results showed that the distribution of temperature and partial pressure of carbon dioxide in the coal char sample is insignificant. For example, at 1200°C, the maximum deviation from the isothermal exposure temperature is ~0.3°C. At the lower isothermal exposure temperatures, the temperature deviation is even smaller. The maximum deviation of the partial pressure of CO2 in the sample from that in the oxidizing medium is 4 Pa. CONCLUSIONS (1) A study of the gasification of solid fuel samples with different carbon contents, during which the samples were sequentially subjected to pyrolysis in an argon medium and to oxidation in a carbon dioxide medium, made it possible to determine the rates of the chemical reactions involved and the gasification times at temperatures of 900 to 1200°C. It was found that, as the oxidizing ambient temperature increases from 900 to 1200°C, the times of gasification of the anthracite coal char and lignite coal char decrease 8- and 22-fold, respectively. (2) The values of the activation energy and preexponential factor for the carbon oxidation reaction were determined. It has been established that the activation energy decreases with reduction of the ash content in the coal char sample. The latter suggests that the rate of gasification of high-grade coals is largely dependent on the oxidation medium temperature. (3) A mathematical model of the gasification of solid fuels with consideration of the diffusion of gaseous oxidizer through the ash layer was developed. The numerical results were found to be in good agreement with the experimental data. The greatest discrepancy between the simulation and experimental data on the gasification time of the samples was observed for the coal char samples prepared from bituminous coal and lignite RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY B
ACKNOWLEDGMENTS The work was performed at the Federal State Autonomous Educational Institution of Higher Education “National Research Tomsk Polytechnic University” in the framework of the federal target program “Research and Development in Priority Directions of Scientific-Technological Complex of the Russian Federation for 2014–2020” (applied research and experimental development unique identifier PNIER RFMEFI58114X0001).
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