CO2 Gasification Rate Analysis of Datong Coal Using Slag Granules

Publication Date (Web): June 20, 2013 ..... the reaction degree across the temperature range suggests the rate-controlling step change during the enti...
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CO2 Gasification Rate Analysis of Datong Coal Using Slag Granules as Heat Carrier for Heat Recovery from Blast Furnace Slag by Using a Chemical Reaction Peng Li,* Qingbo Yu,* Huaqing Xie, Qin Qin, and Kun Wang School of Materials and Metallurgy, Northeastern University, No. 11, Lane 3, WenHua Road, HePing District, Shenyang, 110819 Liaoning, People’s Republic of China ABSTRACT: The kinetics of Datong coal gasification in solid BF (blast furnace) slag using carbon dioxide as gasifying agent was studied between 1223 and 1423 K. The relative mass change during the gasification reaction was continuously monitored using a high-resolution thermogravimetric system. The influence of reaction temperature and coal/slag mass ratio in the reaction rate was analyzed. The reaction rate has a strong dependence on reaction temperature and coal/slag ratio. With increasing reaction temperature, carbon conversion, the peak value of reaction rate, the intrinsic surface reaction rate, and the reactivity index increases, and the time for complete carbon conversion decreased. The activation energy decreases with an increasing coal/slag ratio. When the coal/slag ratio is 1:0, the intrinsic the activation energy is 112 kJ/mol; however, when the coal/slag is 1:3, it is 53 kJ/mol. This indicates that BF slag is an active catalyst for carbon gasification. Reaction model Am (volume reaction model as proposed by Avrami-Erofeev) has the best fit on coal gasification using BF slag as heat carrier. The kinetic parameters applicable to the Am model different coal/slag ratios were obtained. The global rate equation that includes these parameters was developed.

1. INTRODUCTION The resources and energy consumption of iron−steel enterprises is large; especially, energy consumption in the ironmaking process is ∼60% of the total energy consumption. Although many technologies, such as CDQ (coke dry quenching) and TRT (blast furnace top gas recovery turbine unit) were used, iron-making has achieved tremendous improvements in its energy efficiency, and the energy of molten blast furnace (BF) slag has only been used in only limited ways so far. Molten BF slag is normally discharged from the steelmaking industry as a byproduct at a higher temperature, up to 1823 K, bearing a substantial amount of highquality thermal energy.1 The energy recovery from molten BF slag is very difficult because of its low thermal conductivity and high viscosity. Although heat recovery from molten slag is very difficult, extensive studies2−11 have been carried out to resolve this problem. There are three methods for waste heat recovery from molten BF slag: a thermal energy recovery method, that is, transferring the waste heat of the molten slag to some heat carriers, such as hot air, steam, hot oil and so on; a chemical energy recovery method, that is, using the waste heat of the molten slag to produce fuel gas; and direct generation of electricity from the waste heat. Many researchers12−18 have studied and reported that the chemical energy recovery method becomes quite attractive in terms of energy storage without heat loss and have made a connection to other industries. Recently, we have proposed a new route to effectively utilize BF slag by using BF slag granules as the heat carrier of coal gasification, and preliminary studies have proved the new process as an environmentally friendly and highly energyefficient technology. This system can be divided into four parts: melting gasifier using molten BF slag as heat carrier, the rotary cup atomizer, moving bed gasifier using solid slag granules as © 2013 American Chemical Society

heat carrier, and boiler. In our previous work, we reported the feasibility of coal gasification using slag as the heat carrier,19−21 established the kinetics model of coal gasification using molten slag as heat carrier,22 and studied the granulation process of molten BF slag.23 In this paper, we study mainly the moving bed gasifier using slag granules as a heat carrier. The temperature of slag granules in moving bed gasifier ranges from 1473 to 1073 K. Coal gasification is a very complex process, which is basically composed of the devolatilization of coal particles and the gasification of the resultant char. The gasification reactivity of coal char is influenced by many factors, such as the coal type, pressure, gasification temperature, residence time, etc. Until now, coal gasification using slag granules as the heat carrier has not been reported, but many researchers have studied the kinetics of CO2 gasification, gotten the activation energy values,24−29 and developed some reaction models.30−32 To better understand the kinetics of coal gasification using slag granules as the heat carrier, we report the effects of temperature on carbon conversion, gasification reaction rate in the process, and determine the reaction rate equations according to the different models in this work.

2. EXPERIMENTAL SECTION A Chinese Datong (DT) bituminous coal was used in this study. The coal was crushed and pulverized to create particles with diameter sizes of < 0.15 mm. Its composition and properties are shown in Table 1. The solid reactant is a mixture of DT coal and BF slag at certain proportions. The gases used were CO2 99.99% purity and N2 99.99% purity. The chemical compositions of BF slag and the schematic of the experimental setup can be seen in our previous paper.22 Received: February 26, 2013 Published: June 20, 2013 4810

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atmosphere. After that, the solid sample was continuously heated to the desired temperature. When the desired gasification temperature was reached and steady, the flow of N2 (30 mL/min) was replaced by a flow of CO2 (30 mL/min) as the gasification agent, then the coal gasification happened. The mass ratios of coal to slag were 1:0, 1:1, 1:2, and 1:3. The reaction temperatures are 1223, 1323, 1373, and 1423 K. The kinetic studies were done isothermally at different temperatures (1223−1423 K). Mass change occurring during char gasification was continuously monitored at regular intervals of 1 s. Using the carbon conversion calculation formulas in our previous work,22 we can get the carbon conversion with or without BF slag. Then we can get the reaction rate. The reaction rate, r, can be expressed as

Table 1. Properties of Coal item

value Proximate Analysis

moisture (wt %) volatile matter (wt %) fixed carbon (wt %) ash (wt %) Ultimate Analysis (wt %) carbon hydrogen nitrogen sulfur oxygen Fusion Point (K) deformation temperature soften temperature fluid temperature

9.05 38.38 38.42 14.15 64.53 3.746 0.956 0.561 7.007

r=

dx = k(T , pCO ) f (x) 2 dt

(1)

where k is the intrinsic surface reaction rate, which is a function of the temperature and reactant gas concentration, and f(x) is a function that describes the change in the surface area during the reaction. The different mechanism functions were shown in Table 2.33 In our previous work,22 the molten BF slag could affect the mechanism function of coal gasification, and it catalyzed coal gasification and affected the activation energy values. Thus, we should do further study to get the most probable mechanism function of coal gasification using slag granules as the heat carrier and calculate the activation energy.

1498 1578 1653

Figure 1 shows the diagram of the experimental procedure. First, the solid sample was pyrolyzed by heating to 1223 K under N2

3. RESULTS AND DISCUSSION 3.1. Effects of Temperature. Author: The effect of temperature on the gasification rate of DT coal was analyzed in this study. Figure 2 show the effect of temperature on the gasification rate at different coal/slag ratios. It can be observed that within the same reaction time, the carbon conversion increased with increasing reaction temperature at different coal/slag ratios, and with increasing reaction temperature, the time for complete carbon conversion decreased. For example, when the reaction temperature is 1223 K, the time for complete carbon conversion is ∼25 min with a coal/slag ratio of 1:0; however, when the reaction temperature is 1423 K, the time for

Figure 1. Diagram of experimental procedure.

Table 2. Differential and Integral Expressions of Common Gas-Solid Reaction Mechanism Functions code

reaction model

differential f(x)

integral G(x)

Am A1 A2 A3 A4 Rm R1/2 R1/3 R1/4 R2 R3 Dm D1 D2 D3 D4 D5 D6 D7 D8 Cn C1 C2

Avrami-Erofeev m=1 m=2 m=3 m=4 shrinking core model m = 1/2 m = 1/3 m = 1/4 m=2 m=3 diffusion one-dimensional diffusion two-dimensional diffusion three-dimensional diffusion three-dimensional diffusion 3-D (anti-Jander) 3-D (ZLT) 3-D (Jander) 2-D (Jander) chemical reaction reaction order: n = 2 reaction order: n = 3/2

m(1 − x)[−ln(1 − x)]m−1/m 1−x 2(1 − x)[−ln(1 − x)]1/2 3(1 − x)[−ln(1 − x)]2/3 4(1 − x)[−ln(1 − x)]1/4 m(1 − x)m−1/m (1/2)(1 − x)−1 (1/3)(1 − x)−2 (1/4)(1 − x)−3 2(1 − x)1/2 3(1 − x)2/3

[−ln(1 − x)]1/m −ln(1 − x) [−ln(1 − x)]1/2 [−ln(1 − x)]1/3 [−ln(1 − x)]1/4 1 − (1 − x)1/m 1 − (1 − x)2 1 − (1 − x)3 1 − (1 − x)4 1 − (1 − x)1/2 1 − (1 − x)1/3

1/2x−1 [−ln(1 − x)]−1 (3/2)(1 − x)2/3[1 − (1 − x)1/3]−1 (3/2)[(1 − x)−1/3 − 1]−1 (3/2)(1 + x)2/3[(1 + x)1/3 − 1]−1 (3/2)(1 − x)4/3[(1 − x)−1/3 − 1]−1 6(1 − x)2/3[1 − (1 − x)1/3]1/2 (1 − x)1/2[1 − (1 − x)1/2]−1 (1 − x)n (1 − x)2 2(1 − x)3/2

x2 x + (1 − x) ln(1 − x) [1 − (1 − x)1/3]2 1−2/3x − (1 − x)2/3 [(1 + x)1/3 − 1]2 [(1 − x)−1/3 − 1]2 [1 − (1 − x)1/3]1/2 [1 − (1 − x)1/2]2 (1 − (1 − x)1−n)/(1 − n) (1 − x)−1 − 1 (1 − x)−1/2 − 1

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Figure 2. Effect of temperature on carbon conversion at different coal/slag ratios (a) coal/slag ratio = 1:0, (b) coal/slag ratio = 1:1, (c) coal/slag ratio = 1:2, (d) coal/slag ratio = 1:3.

Figure 3. Effect of temperature on coal gasification rate at different coal/slag ratios (a) coal/slag ratio = 1:0, (b) coal/slag ratio = 1:1, (c) coal/slag ratio = 1:2, (d) coal/slag ratio = 1:3.

complete carbon conversion is only ∼5 min with a coal/slag ratio of 1:0. The same phenomenon can be observed with other coal/slag ratios. In addition, it can be observed that when the coal/slag ratio is lower than 1:2 and the reaction temperature is 1423 K, 100% carbon conversion cannot be achieved. This is because in the mixed solid sample, the BF slag is too abundant and influences the mass transfer, and at high temperature, the action of the chemical reaction becomes small, and the action of the diffusion becomes large, which we found interesting. This also indicated that the mass transfer in coal gasification using BF slag as the heat carrier is very important. We also found that in the presence of BF slag, the difference in carbon conversion

within the same reaction time between 1373 and 1423 K is small. Figure 3 shows the effect of the reaction temperature on the gasification rate with different coal/slag ratios. It can be observed that no matter whether it is with or without BF slag, within the same reaction time, the peak value of the gasification rate increased with increasing reaction temperature. When the reaction temperature was 1223 K, the peak value of the reaction rate was about 0.1 min−1 with coal/slag = 1:0; however, when the reaction temperature was 1423 K, the reaction rate was ∼0.37 min−1 with a coal/slag ratio of 1:0. In addition, the gasification reaction rate curve shifts to the left with increasing reaction temperature. 4812

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decreased with increasing BF slag content. That is because in this system, BF slag granules serve not only as heat carriers but also as a reaction zone. The main chemical compositions of BF slag are 41.21 mass % CaO, 34.38 mass % SiO2, 11.05 mass % Al2O3, 8.22 mass % MgO, 0.35 mass % TiO2, and some minor constituents of iron. It has been reported that group VIII metals as well as alkali and alkaline earth are mostly used as catalysts and found effective in coal gasification. Thus, the BF slag content in coal gasification is very important. Figure 4 shows the variation of reactivity index with the coal/slag ratio. It can be observed that the reactivity index increased with increasing BF slag. For example, when the reaction temperature was 1423 K, the reactivity index of the DT coal was 0.23 with a coal/slag ratio of 1:0; however, the reactivity index of the DT coal was 0.35 with a coal/slag ration of 1:3. This indicates that BF slag can accelerate reaction rates, and it is an active catalyst for carbon gasification. It has been well recognized that Ca, Mg, and Fe species in coals may exhibit catalytic activity for char gasification, whereas Si and Al generally act as retardants.34,35 For the iron catalyst, the following oxygen transfer mechanism was proposed.36

Figure 4. Variation of reactivity index with reaction temperature and coal/slag ratio.

Figure 4 shows the variation of the reactivity index with reaction temperature and coal/slag ratio. The reactivity index of coal gasification, R0.5, is expressed aswhere τ0.5 is the time R 0.5 =

0.5 τ0.5

(2)

necessary for the carbon conversion rate to reach 0.5. It can be observed that with increasing reaction temperature, the reactivity index increased. For example when the coal/slag ratio was 1:0, the reactivity index increased two times from 1223 to 1423 K. 3.2. Effects of BF Slag. When comparing parts a−d in Figures 2 and 3, it can be observed that the influence of BF slag on coal gasification is very large. Both the carbon conversion and the peak value of the reaction rate increased with increasing slag content. The time for complete carbon conversion

FemOn + CO2 → FemOn + 1 + CO

(3)

FemOn + 1 + C → FemOn + CO

(4)

Calcium-catalyzed carbon gasification may be represented as37 CaO + (1/2)O2 → CaO2

(5)

CaO2 + C → CaO + CO

(6)

Table 3. Correlation Coefficients Calculated Using Different Mechanism Functions DT coal gasif w/ CO2; mass ratio of coal/slag 1:0

1:1

1:2

1:3

reaction temp, K code

1223

1323

1373

1423

1223

1323

1373

1423

1223

1323

correlation coefficients R Am A1 A2 A3 A4 Rm R1/2 R1/3 R1/4 R2 R3 Dm D1 D2 D3 D4 D5 D6 D7 D8 Cn C1 C2

0.95 0.99 0.95 0.93

0.98 0.98 0.95 0.93

0.94 0.99 0.97 0.97

0.97 0.97 0.94 0.92

0.99 0.98 0.95 0.92

0.90 0.86 0.85 0.98 0.88

0.91 0.88 0.87 0.97 0.95

0.94 0.92 0.90 0.93 0.89

0.89 0.87 0.86 0.96 0.96

0.90 0.87 0.86 0.98 0.95

0.98 0.99 0.98 0.94 0.98 0.17 0.96 0.97

0.95 0.97 0.97 0.96 0.96 0.84 0.96 0.97

0.96 0.95 0.94 0.91 0.96 0.78 0.98 0.93

0.94 0.95 0.96 0.96 0.94 0.92 0.94 0.96

0.97 0.98 0.98 0.96 0.97 0.72 0.96 0.98

0.07 0.45

0.82 0.93

0.77 0.86

0.92 0.96

0.70 0.88

1373

1423

Volume Reaction Model, f(x) = m(1 − x)[−ln(1 − x)]m−1/m 0.80 0.87 0.93 0.99 0.88 0.79 0.85 0.98 0.99 0.99 0.97 0.98 0.96 0.96 0.99 0.95 0.98 0.94 0.99 0.96 0.99 0.98 0.99 0.97 0.92 0.98 0.99 0.99 Shrinking Core Model, f(x) = m(1 − x)m−1/m 0.96 0.94 0.94 0.88 0.95 0.94 0.95 0.95 0.93 0.92 0.86 0.93 0.93 0.95 0.94 0.92 0.91 0.85 0.92 0.92 0.95 0.83 0.88 0.93 0.97 0.91 0.86 0.80 0.65 0.78 0.88 0.98 0.81 0.72 0.74 Diffusion 0.92 0.93 0.94 0.95 0.95 0.92 0.86 0.88 0.90 0.94 0.97 0.93 0.89 0.82 0.84 0.88 0.94 0.97 0.91 0.87 0.80 0.73 0.83 0.90 0.98 0.85 0.79 0.76 0.94 0.94 0.95 0.94 0.95 0.93 0.87 0.20 0.31 0.54 0.88 0.44 0.23 0.59 0.99 0.99 0.98 0.94 0.99 0.99 0.98 0.81 0.87 0.93 0.98 0.90 0.85 0.79 Chemical Reaction, f(x) = (1 − x)n 0.15 0.22 0.45 0.87 0.36 0.15 0.64 0.39 0.55 0.75 0.95 0.59 0.36 0.75 4813

1223

1323

1373

1423

2

0.68 0.93 0.87 0.99

0.89 0.96 0.97 0.96

0.78 0.94 0.98 0.99

0.82 0.95 0.98 0.99

0.91 0.91 0.89 0.66 0.52

0.94 0.93 0.93 0.90 0.84

0.94 0.94 0.94 0.81 0.68

0.93 0.94 0.94 0.77 0.71

0.78 0.72 0.67 0.57 0.81 0.26 0.96 0.64

0.94 0.92 0.90 0.86 0.94 0.61 0.97 0.89

0.88 0.85 0.82 0.74 0.01 0.33 0.97 0.80

0.83 0.80 0.78 0.73 0.85 0.55 0.96 0.76

0.26 0.43

0.55 0.74

0.29 0.48

0.60 0.72

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Figure 5. Linera fitting of the linearized expression of the reaction model for DT coal with different coal/slag ratios.

3.3. Mechanism Function Determination. As expressed earlier, the rate of a heterogeneous solid−gas reaction can be expressed by eq 1. During gasification, the gasification agent is CO2, so the partial pressure of CO2 remains constant. The reaction rate can be represented by the equation as r=

dx = k(T ) f (x) dt

Am model was adopted to derive the reaction rates based on the time variation results from experiments. Author: To the authors’ best knowledge, no coal gasification using BF slag granules as the heat carrier have been published. However, our results can be compared with available data for coal gasification. Ye et al.38 reported that their results closely followed the VRM during CO2 gasification, and Zhang et al.39 stated that their results for anthracite chars during steam gasification were well described by both the SCM and VRM. In addition, Vyazovkin33 reported that the reaction models can be reduced to three major types: accelerating models, decelerating models, and autocatalytic models. The autocatalytic models represent processes whose rate increases first and then decreases with an increase in the extent of conversion; the most common example is Am (Avrami-Erofeev) models. Thus, from Figures 2 and 3, we can see that the reaction rate of coal gasification using solid BF slag granules as the heat carrier increases first and then decreases with time. Based in Table 2, the reaction rate of coal gasification using BF slag granules as heat carrier can be represented by the equation as:

(7)

By rearranging eq 7 and integrating,

∫0

x1

dx = G(x) = f (x )

G(x) = k(T ) t

∫0

t1

k (T ) d t

(8) (9)

where, G(x) is the mechanism function integral as shown in Table 2. To analyze the linear relationship between G(x) and t using different G(x)’s, we can get the most probable mechanism function, f(x). The correlation coefficients (R2) of all plots for different ratios of coal to slag and different reaction temperatures are listed in Table 3. From Table 3, the correlation coefficients calculated using different mechanism function is difference can be observed. But for convenience of use in industry, we need to find a mechanism function to describe coal gasification under the same coal/slag ratios and different reaction temperatures, so we need to comprehensively consider the four correlation coefficients under the same coal/slag ratio and different reaction temperatures. Although a mechanism function under a constant reaction temperature is better than other mechanism functions, we cannot choose it, and we should ensure the comprehensive index of the mechanism function is better than others. On the basis of that principle, in this study, when the coal/slag ratio was 1:0 and 1:1, the overall fitting extent of the A2 model was slightly better than other reaction models. When the coal/slag ratio was 1:2, the overall fitting extent of the A3 model was slightly better than other reaction models. When the coal/slag ratio was 1:3, the overall fitting extent of the A4 model was slightly better than other reaction models. Therefore, the

DT coal gasification without BF slag: r=

dx = 2 × k(T )(1‐x)[‐ln(1‐x)]1/2 dt

(10)

In the presence of BF slag, we define m=

mcoal + mslag mcoal

(11)

where mcoal is the initial mass of DT coal in the mixture sample and mslag is the initial mass of BF slag in mixture sample. Thus, we can get the reaction rate of the coal gasification rate using BF slag as the heat carrier: r=

dx = k(T ) f (x)A m dt

= k(T )m (1 − x)[ − ln(1 − x)]m − 1/ m 4814

(12)

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3.4. Kinetic Parameters and Reaction Modeling. In Sections 3.1 and 3.2, those figures show x−t curves for coal gasification between 1223 and 1423K with different coal/slag ratios. A strong dependence of the gasification rate on temperature and coal/slag ratio can be seen. As expressed earlier, the rate of coal gasification using BF slag granules as heat carriers can be expressed by eqs 10 and 12. Integration of eqs 10 and 12 leads to

The results are shown in Figure 6 for coal/slag ratios of 1:0, 1:1, 1:2, and 1:3. From the slope of the straight lines, the intrinsic activation energy of 112 kJ/mol with a coal/slag ratio of 1:0, 94 kJ/mol with coal/slag ratio of 1:1, 87 kJ/mol with coal/slag ratio of 1:2, and 53 kJ/mol with coal/slag ratio of 1:3 were determined. From the intercept of the straight lines, the following were determined: a pre-exponential factor of 4806 min−1 with a coal/slag ratio of 1:0, 1625 min−1 with coal/slag ratio of 1:1, 965 min−1 with coal/slag ratioof 1:2, and 52 min−1 with coal/slag ratio of 1:3. Both the activation energy and preexponential factor decreased with increasing BF slag. The intrinsic activation energy with a coal/slag ratio of 1:3 is lower than others. The changed value of Ea obtained as a function of the reaction degree across the temperature range suggests the rate-controlling step change during the entire reaction. To the authors’ knowledge, no coal gasification using BF slag as the heat carrier studies have been published. However, our results can be compared with available data for different coal types. Ahn et al.40 reported that at a CO2 partial pressure of 0.2 MPa for the temperature range of 1373−1673 K, the activation energies, Ea, and pre-exponential factor, k0, of subbituminous coal char in a pressurized drop tube furnace reactor were 71.5 kJ/mol and 174.1 s−1, respectively. On the other hand, at a low temperature range of 1173−1273 K, the value of Ea was 144 kJ/ mol. Liu et al.41 reported that because the activation energy value is lower in the high-temperature range than in the low temperature range, it can be stated that the pore diffusion resistance of reactant gas in char increases as the reaction temperature increases. On the basis of these results, a global rate equation that includes these parameters has been proposed, as shown in Table 5. Finally, in Figure 7 the carbon conversion-vs-time curves for the gasification of DT coal are presented together with the calculated carbon conversion with Am models. It was found that, the gasification reaction rate equation shown in Table 5 gives good description of the coal gasification using BF slag as heat carrier.

G(x) = k(T )t = [ − ln(1 − x)]1/2 (gasification without BF slag)

(13)

G(x) = k(T )t = [ − ln(1 − x)]1/ m (gasification with BF slag)

(14)

To determine the kinetic parameters, two procedures were applied. First, the intrinsic surface reaction rate, k(T), for the gasification of DT coal with or without BF slag was calculated from the slope of the G(x)-vs-t plots (Figure 5). Table 4 shows Table 4. The Intrinsic Surface Reaction Rate of DT Coal with Different Temperatures and Different Coal/Slag Ratios temp, K coal/slag mass ratio

1223

1323

1373

1423

1:0 1:1 1:2 1:3

0.083 0.15 0.18 0.292

0.176 0.360 0.405 0.427

0.286 0.471 0.494 0.520

0.379 0.523 0.585 0.600

the intrinsic surface reaction rate of DT coal with different reaction temperatures and different coal/slag ratios. It can be observed that the intrinsic surface reaction rate increased with increasing temperature and amounts of BF slag. Second, using the data of Table 4 and eq 16, the activation energy (Ea) and pre-exponential factor (k0) can be calculated from the slope of the ln k(T)-vs-1/T plots (Figure 6). ⎛ E ⎞ k(T ) = k 0 exp⎜ − a ⎟ ⎝ RT ⎠

4. CONCLUSIONS The kinetics of the gasification of coal from the Datong coal mine using solid BF slag as the heat carrier in the temperature range of 1223−1423 K was studied by isothermal thermogravimetry. Char−CO2 gasification reactivity was affected by many different factors. The gasification temperature had the greatest influence on reactivity. With increasing reaction temperature, the carbon conversion, the peak value of the reaction rate increases, the intrinsic surface reaction rate and the reactivity index increase, and the time for complete carbon conversion decreased. When the reaction temperature is 1223 K, the peak value of the reaction rate is ∼0.1 min−1 when the coal/slag ratio is 1:0; however, when the reaction temperature is 1423 K, the reaction rate is ∼0.37 min−1 when the coal/slag ratio is 1:0. The reaction rate of coal gasification using BF slag as the heat carrier has a strong dependence on the coal/slag ratio. BF slag is an active catalyst for carbon gasification. Within the same reaction time, the carbon conversion, the peak value of the reaction rate, and the intrinsic surface reaction rate increase as the coal/slag ratio increases. The activation energy decreases with increasing coal/slag ratio. When the reaction temperature is 1423 K, the reactivity index of DT coal is 0.23 when the coal/ slag ratio is 1:0; however, the reactivity index of DT coal is 0.35 when the coal/slag ratio is 1:3.

(15)

Figure 6. Linear fitting for calculation of the activation energy and preexponential factor with Am models.

By taking the logarithm of both sides of eq 11, one obtains Ea 1 × (16) R T Equation 16 allows one to obtain the activation energy and pre-exponential factor from the slope and intercept of the plot ln k(T) vs 1/T. ln k(T ) = ln k 0 −

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Table 5. Gasification Rate Equation and Kinetic Parameter for Coal Gasification with Carbon Dioxide Using BF slag As Heat Carrier

Figure 7. Carbon conversion-vs-time curves for the gasification of DT coal with or without BF slag and calculated carbon conversion with Am models.

■

Reaction model A2 has the best fit for coal gasification without BF slag. Reaction model A2 has the best fit for coal gasification with a coal/slag ratio of 1:1, reaction model A3 has the best fit for coal gasification with a coal/slag ratio of 1:2, and reaction model A4 has the best fit for coal gasification with a coal/slag ratio of 1:3. From the slope of the straight lines, an intrinsic activation energy of 112 kJ/mol with acoal/slag ratio of 1:0, 94 kJ/mol with a coal/slag ratio of 1:1, 87 kJ/mol with a coal/slag ratio of 1:2, and 53 kJ/mol with acoal/slag ratio of 1:3 were determined. The kinetic expression for char gasification using BF slag granules as the heat carrier was obtained.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*Telephone/Fax: +86-024-83672216. E-mail: [email protected]. edu.cn, [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supposed by the National Natural Science Fund (51274066), Fundamental Research Funds for the Central Universities (N110602002), and Academic New Artist Ministry of Education Doctoral Post Graduate. 4816

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Article

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