Coal Gasification in Molten Blast Furnace Slag

Nov 26, 2012 - The coal/CO2 gasification reactions in molten BF (blast furnace) slag were studied kinetically by temperature-programmed thermogravimet...
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Kinetics of CO2/Coal Gasification in Molten Blast Furnace Slag Peng Li,* Qingbo Yu,* Qin Qin, and Wei Lei School of Materials and Metallurgy, Northeastern University, No. 11, Lane 3, WenHua Road, HePing District, Shenyang, 110819, Liaoning, P.R. China ABSTRACT: The coal/CO2 gasification reactions in molten BF (blast furnace) slag were studied kinetically by temperatureprogrammed thermogravimetry using a thermal analyzer. The effect of heating rates and molten BF slag on coal gasification were studied, and the activation energies, frequency factors, and most possibility mechanism functions were calculated. The results show that the order of reactivity sequence at these temperatures was DT (Datong) coal > FX (Fuxin) coal > coke. With the increase in heating rate, the carbon conversion, and the peak value of reaction rate increased at the same reaction time, the carbon conversion curve shifts to a higher temperature and the reaction rate curve shifts rightward systematically, both of the time required for the carbon conversion to reach nearly unity and the time necessary for reaction rate to reach its maximum decreased. The carbon conversion and reaction rates were sensitive to BF slag; at the same time, the carbon conversion and reaction rates of coal gasification with slag are higher than those without slag. The time required for the carbon conversion to reach nearly unity and the time required for the reaction rate to reach maximum with slag are both shorter than that without slag. In the presence of BF slag, the carbon conversion curve shifts to lower temperature, the peak value of reaction rate is higher than that without slag, and the reaction rate curve also shifts to lower temperature. The molten BF slag acts as a good catalyst to coal gasification. Without molten BF slag, the mechanism functions of coke and FX coal are a C1 model (phase boundary reaction (n = 2) model), while the mechanism function of DT coal is a C2 model (phase boundary reaction (n = 3/2) model). However, with molten BF slag, the mechanism function of coke is a D5 model (3-D diffusion (anti-Jander) model), the mechanism function of DT coal is a D4 model (3-D diffusion model), and the mechanism function of FX coal is a C2 model (phase boundary reaction (n = 3/2) model). The activation energies and frequency factors decrease as heating rates increase. The kinetic compensation effect of coal/CO2 gasification in molten BF slag exists.

1. INTRODUCTION Steelmaking contributes greatly to the environmental problem. The environmental problem is caused by the use of fossil fuel that emits CO2 and waste heat. The energy consumption of steelmaking is very large. Although it has achieved tremendous improvements in its energy efficiency in the past several decades, the energy efficiency is only about 60% and much waste heat has not been recovered, especially the waste heat of molten BF (blast furnace) slag. Molten slag is the byproduct of ironmaking at temperatures up to 1923 K, carrying a substantial amount of high quality thermal energy. The thermal conductivity of slag is very low,1 so the waste heat recovery is difficult. Over the past four decades, several processes2−11 have been proposed to recover the sensible heat of molten BF slag as heat, electricity, and fuel, while none has been commercialized. Akiyama12−14 pointed out that the limestone cracking, reforming of methane, and coal gasification were more suitable for the recycle of molten slag. Kasai15 proposed the concept of reforming of methane using molten slag. Shimada studied the rates of methane−steam reaction on the slag surface and the effects of the mass ratio of CaO and SiO2 on the reaction rate. Mizuochi16,17 proposed the industrial design based on the RCA (rotary cup atomizer) technology. Purwanto18 studied the possibility of hydrogen production from biogas using hot slag, in which the decomposition rate of CO2−CH4 in a pack bed of granulated slag was measured at constant flow rate and pressure. Li19 proposed the technical route of coal gasification using molten BF slag, and studied the possibility of coal gasification using molten BF slag. This © 2012 American Chemical Society

method uses the molten slag as heat sources to gasify CO2/ coal. The process has been demonstrated to possess at least the following advantages: (i) the ratio of H2/CO of synthesis gas is adjustable by adjusting the CO2/H2O/coal ratio, to suit the downstream synthesis processes, (ii) the gasification efficiency is more than 1 caused by using molten slag as heat sources, (iii) the carbon conversion rate is high because of the high gasification temperature, and (iv) the calorific capacity of synthesis gas is higher than other gasification systems.20,21 Coal gasification reactions are typically separated into coal pyrolysis and char gasification reactions. Pyrolysis involves the evaporation of moisture and devolatilization of compounds of condensable hydrocarbons and noncondensable gases. The char gasification reaction is a heterogeneous gas−solid reaction. Because the rate of char gasification is much slower than the rates of pyrolysis, char gasification is regarded as the ratedetermining process in the coal gasification process. As a result of a change in the char pore structure, the char gasification process is very complicated. Over the past decades, the effects of coal type and operation conditions (such as reaction temperature, partial pressure of the reactant gas, total system pressure, and particle size) with the use of different contacting equipments/reactors (fixed bed,22,23 fluidized bed,24,25 drop tube furnace,26−28 wire mesh, and thermo-gravimetric analReceived: Revised: Accepted: Published: 15872

June 25, 2012 October 7, 2012 November 26, 2012 November 26, 2012 dx.doi.org/10.1021/ie301678s | Ind. Eng. Chem. Res. 2012, 51, 15872−15883

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Table 1. Proximate Analysis of the Raw Coal and Char proximate analysis (%) CO (coke) DT (Datong) FX (Fuxin)

ash fusion point (K)

Mad

ash

V

FC

Tdef

Them

Tflow

0.07 9.05 6.59

12.5 14.15 34.41

1.59 38.38 26.05

85.84 38.42 32.95

>1773 1498

>1773 1578

>1773 1653

ysis29,30) have been extensively investigated. And the gasification mechanisms for O2/CO2 streams and the reaction rate models have been developed. Ye et al.25 explained the effect of different ranks of coals and pointed out that there is no general trend for the effect of coal rank on the reactivity but, generally, low-rank coals have high reactivities while high-rank coals have low reactivities. Ochoa et al.31 characterized the kinetics of CO2 gasification of chars prepared from two different types of lowrank coals from Argentinean minefields at different reaction temperatures ranging from 900 to 1160 °C and CO 2 concentrations ranging from 50 to 70% v/v. Liu et al.32,33 found the reaction rate of chars with low ash fusion temperatures leveled off, or even decreased a little as the temperature increased. It indicates that a temperature independence exists on char reactivity for low ash melting coals during gasification. Although many studies have been reported and many researchers have investigated the effects of coal type and operation conditions on the gasification reaction, few studies have been reported on coal gasification using molten BF slag as a heat carrier. The coal gasification process using molten BF slag as a heat carrier involves a very complex heterogeneous reaction system. In this process, molten BF slag acts as not only a heat carrier but also a reaction zone. Therefore, it is necessary to study the effect of operation conditions on the gasification reaction and the kinetic model. Among the char−O2/H2O/ CO2 reactions, char−CO2 gasification was considered in this study. We used a thermo-gravimetric analyzer in our study. Experiments were carried out at different heating rate (10, 20, and 30 K/min) and different reaction temperatures (1573, 1673, and 1723 K) using different coal types. The aim of the present work is to study the effects of molten slag and heating rate on the gasification reaction, the reaction mechanism, and build the kinetic model of coal gasification using molten BF slag as a heat carrier. Four structural models were applied to describe the experimental data, and the intrinsic kinetic parameters were evaluated using the model of best fit.

Figure 1. Schematic of the experimental apparatus.

analyzer (STA409PC, NETZSCH, working temperature up to 1773 K and heating rate up to 50 K/min) was used. The coal sample was placed in the crucible. The gas flow rates were controlled using flow meters. The N2 stream flowed through the balance at a flow rate of 30 mL·min−1 as the protective gas. As shown in Figure 2, char preparation and gasification processes were carried out consecutively in a thermo-

Figure 2. Diagram of experimental procedure.

gravimetric analyzer. In the char preparation process, a coal sample of 5 mg was placed in the crucible. Then, the coal sample was pyrolyzed by heating to 1223 K at a heating rate of 20 K/min under a N2 atmosphere. This temperature was maintained for 30 min to complete the pyrolysis. After the completion of the char preparation process, the char sample was continuously heated to the temperature beginning for the gasification process at a heating rate of 20 K/min. Then, this temperature was maintained for 3 min to make the system stable. After that, the flow of N2 (30 mL/min) was replaced by the flow of CO2 (30 mL/min) as the gasification agent. Then, the char gasification was conducted at the desired heating rate under atmospheric pressure. During the char preparation and char gasification processes, the change of the coal sample can be measured by the balance in thermo-gravimetric analyzer. And the measuring data can be real-time recorded by a computer. In this study, the weight loss over time during the gasification process was continuously measured at time intervals of 1 s. During the experimental process, the weight of the BF slag

2. MATERIALS AND METHODS 2.1. Raw Materials. In the present series of experiments, four types of Chinese industrial coal samples with different ranks were involved. The coal types were selected according to their volatile matter, ash content, and fixed carbon. The proximate analyses of these samples are summarized in Table 1. Sample DT is classified as bituminous coal, and sample FX is classified as lean coal. The coal samples were crushed and sieved to a size that is smaller than 0.125 mm. The chemical compositions of BF slag are 41.21 mass % CaO, 34.38 mass % of SiO2, 11.05 mass % of Al2O3, 8.22 mass % of MgO, 0.35 mass % of TiO2, and some minor constituents of iron, sulfur, and manganese as well as phosphor oxides. The gasification agent is CO2 with a purity of more than 99.9%. 2.2. Apparatus and Procedure. A schematic of the experimental setup is shown in Figure 1. A thermo-gravimetric 15873

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

reaction model

differential f(x)

integral F(x)

D1 D2 D3 D4 D5 D6 D7 D8 A1 A2 A3 A4 A5 A6 R1 R2 R3 R4 R5 C1 C2

dimensional diffusion two-dimensional diffusion three-dimensional diffusion three-dimensional diffusion 3-D diffusion (anti-Jander) 3-D diffusion (ZLT) 3-D diffusion (Jander) 2-D diffusion (Jander) nuclei production (n = 1) nuclei production (n = 1.5) nuclei production (n = 2) nuclei production (n = 3) nuclei production (n = 4) nuclei production (n = 3/4) shrinking core model shrinking core model shrinking core model (n = 2) shrinking core model (n = 3) shrinking core model (n = 4) phase boundary reaction (n = 2) phase boundary reaction (n = 3/2)

1/2x [−ln(1 − x)]−1 (3/2)[(1 − x)]−1/3 − 1]−1 (3/2)(1 − x)2/3[1 − (1 − x)2/3]−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 − x2/3[1 − (1 − x)1/3]1/2 (1 − x)1/2[1 − (1 − x)1/2]−1 1−x 1.5(1 − x)[−ln(1 − x)]1/3 2(1 − x)[−ln(1 − x)]1/2 3(1 − x)[−ln(1 − x)]2/3 4(1 − x)[−ln(1 − x)]3/4 (3/4)(1 − x)[−ln(1 − x)]1/4 2(1 − x)1/2 3(1 − x)2/3 (1/2)(1 − x)−1 (1/3)(1 − x)−2 (1/4)(1 − x)−3 (1 − x)2 2(1 − x)3/2

x2 x + (1 − x) ln(1 − x) [1 − (2/3)x] − (1 − x)2/3 [1 − (1 − x)1/3]2 [(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 −ln(1 − x) 3(1 − x)2/3[1 − (1 − x)1/3] [−ln(1 − x)]1/2 [−ln(1 − x)]1/3 [−ln(1 − x)]1/4 [−ln(1 − x)]3/4 1 − (1 − x)1/2 1 − (1 − x)1/3 1 − (1 − x)2 1 − (1 − x)3 1 − (1 − x)4 (1 − x)−1 − 1 (1 − x)−1/2

changed because of the evaporation of moisture in the slag. Thus, the carbon conversion calculation method of coal is different from the mixture sample of coal and slag. The carbon conversion x can be defined as the ratio of the gasified char at any time t to the initial char. The carbon conversion of coal without BF slag is expressed as x(t ) =

w0 − wt × 100% w0 − wash

r=

(1)

⎤ ⎥ 1 ⎥⎦

× (1 + m) × 100%

⎛ E ⎞ k = A exp⎜ − a ⎟ ⎝ RT ⎠

(2)

where wc0 is the initial mass of char and slag, wct is the instantaneous mass of char and slag at a reaction time t, wc∞ is the final mass of char and slag, ws0 is the initial mass of slag, wst is the instantaneous mass of slag at a reaction time t, ws∞ is the final mass of slag, and m is the ratio of slag to coal char.

(4) −1

where A is the pre-exponential factor (min ), Ea is the activation energy (kJ·mol−1), and R is the universal gas constant (8.314 J·(mol·K)−1). Thus, we can get r=

3. KINETIC ANALYSIS In the present experiments, the mass loss was mainly because of the char gasification with CO2 and ignition loss of BF slag. The char gasification is shown as follows. C + CO2 = 2CO

(3)

where k is the rate constant based on the reaction temperature T, pCO2 is the partial pressure of CO2, and f(x) is the mechanism function. The activation of char with oxidizing gas is a heterogeneous gas−solid reaction that pore structure and surface area of the solid particle are changing due to the reaction. In the reaction mechanism function, these structural variations and other phenomena such as film mass transfer, pore diffusion, and chemical reaction have to be considered.34 Thus, many different mechanism functions were established, as shown in Table 2. In our study, the molten BF slag can affect the mechanism function of coal gasification, so we should analyze which one is best and get the most probable mechanism function. In this experiment, the partial pressure of CO2 remains constant during gasification. The gasification reaction rate can be represented by the Arrhenius equation as

where x(t) is the carbon conversion at a reaction time t, w0 is the initial mass of char, wt is the instantaneous char mass at a reaction time t, and wash is the mass of ash in the char. The carbon conversion rate in the presence of BF slag is expressed as ⎡⎛ w − w ⎞ ⎛ w − w ⎞ m ct st x(t ) = ⎢⎜ c0 ⎟ − ⎜ s0 ⎟× ⎢⎣⎝ wc0 − wc ∞ ⎠ ⎝ ws0 − ws ∞ ⎠ m +

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

⎛ E ⎞ dx = A exp⎜ − a ⎟f (x) ⎝ RT ⎠ dt

(5)

The experiments were conducted by non-isothermal thermogravimetry. The heating rate is shown as eq 6. β = dT /dt

0 ΔH298K = + 159.7 kJ·mol−1

(6)

Then, we can get the kinetic equation under non-isothermal and heterogeneous conditions. It is expressed as

On the basis of the measurements of mass loss versus time, the carbon conversion can be defined as eqs 1 and 2. The gasification reaction rate r could be derived and expressed by the variation of the carbon conversion x versus time:

⎛ E ⎞ dx ⎟f (x) = (1/β)A exp⎜ − ⎝ RT ⎠ dT 15874

(7)

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In this study, we use the integration method to analyze the coal gasification in molten BF slag. We can get eq 8. F (x ) =

∫0

x

dx A = β f (x )

∫T

T

e−Ea / RT dT

(8)

0

To eq 8, we use the approximation of temperature integral of Li Chung-Hsiung integration35 to conduct it. Thus, we can get ⎤ ⎡ 1 E AR − a ln⎢ 2 F(x)⎥ = ln ⎦ ⎣T RT βEa

(9) 2

To analyze the linear relationship between ln[F(x)/T ] and 1/ T using different F(x), we can get the most probable mechanism function. Using this mechanism function, we can get the activation energy and frequency factor. The differential and integral expressions of common gas−solid reaction mechanism functions are shown in Table 2.36−38 The equation of regression correlation coefficient is shown as follows.

Figure 4. Carbon conversion versus time during CO2 gasification of coke with and without BF slag at different heating rates.

n

r=

Figure 5 shows carbon conversion versus time during CO2 gasification of FX coal. It can be seen that they show the same

∑i = 1 (xi − x ̅ )(yi − y ̅ ) n

∑i = 1 (xi − x ̅ )2 ×

n

∑i = 1 (yi − y ̅ )2

(10)

where xi and yi are experiment points and x̅ and y ̅ are the average values of xi and yi, respectively.

4. RESULTS AND DISCUSSION 4.1. Effects of Heating Rates. Experiments were performed to determine the rates of CO2 gasification of the coal char with or without the presence of BF slag at different heating rates (10, 20, and 30 K/min). Figure 3 shows carbon

Figure 5. Carbon conversion versus time during CO2 gasification of FX coal with and without BF slag at different heating rates.

tendency as well as DT coal, but there are also some differences. The time for the complete carbon conversion of coke when the heating rate is 30 K/min was almost a quarter of the time when the heating rate is 10 K/min, and the difference between 10 and 20 K/min is very large. We can also observe that, with or without BF slag, the tendency of carbon conversion has no difference. The total time required to complete gasification increases in the order of 30 K min−1 < 20 K min−1 < 10 K min−1. Figures 6, 7, and 8 show carbon conversion versus temperature during CO2 gasification of DT, coke, and FX coal separately with and without BF slag at different heating rates. It can be observed that, with increasing heating rate, the carbon conversion curve shifts to higher temperature zone. At the same reaction temperature, the higher the heating rate is, the lower the carbon conversion is. As the temperature reaches the final point (1723 K), the carbon conversion of all samples with or without BF slag are the same at different conditions. It indicates the an increase in heating rate only shortens the time approaching the final temperature but has little influence on its gasification mechanism. Figures 9, 10, and 11 show reaction rate versus time during CO2 gasification of DT, coke, and FX coal separately with and without BF slag at different heating rates. It can be observed

Figure 3. Carbon conversion versus time during CO2 gasification of DT coal with and without BF slag at different heating rates.

conversion versus time during CO2 gasification of DT coal with and without BF slag at different heating rates. It can be observed that the carbon conversion was sensitive to heating rates. At the same reaction time, the carbon conversion increased with increasing heating rate and the carbon conversion of DT coal when the heating rate is 30 K/min is much higher than that when the heating rates are 10 and 20 K/ min. The difference of carbon conversion at the same time between 10 and 20 K/min is little. And the time required for the carbon conversion to reach nearly unity decreased as the heating rates increased. The time for the complete carbon conversion when the heating rate is 30 K/min was almost half of the time when the heating rate is 10 K/min. Figure 4 shows carbon conversion versus time during CO2 gasification of coke. 15875

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Figure 9. Reaction rate versus time during CO2 gasification of DT coal with and without BF slag at different heating rates.

Figure 6. Carbon conversion versus temperature during CO2 gasification of DT coal with and without BF slag at different heating rates.

Figure 10. Reaction rate versus time during CO2 gasification of coke with and without BF slag at different heating rates.

Figure 7. Carbon conversion versus temperature during CO2 gasification of coke with and without BF slag at different heating rates.

Figure 11. Reaction rate versus time during CO2 gasification of FX coal with and without BF slag at different heating rates. Figure 8. Carbon conversion versus temperature during CO2 gasification of FX coal with and without BF slag at different heating rates.

min, whereas the rate at 10 K/min presented a peak at a time of 6 min. At the higher heating rate, the reaction rate increased faster and also decreased faster. And in contrast, at the lower heating rate, the reaction rate increased slowly and also decreased slowly. Figures 12, 13, and 14 show reaction rate versus temperature during CO2 gasification of DT, coke, and FX coal separately with and without BF slag at different heating rates. It can be observed that, with the increase in heating rate, the reaction rate curve shifts rightward systematically and the peak value of

that the reaction rate increased first and then decreased with time. The peak value of reaction rate increased with increasing heating rate, and the time necessary for the reaction rate to reach its maximum decreased with increasing heating rate. For example, for the gasification of FX coal without slag, the peak value of reaction rate is 0.24 at a heating rate of 30 K/min, but the peak value of reaction rate is only 0.14 at 10 K/min. And the reaction rate at 30 K/min presented a peak at a time of 3 15876

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Figure 15. Reaction rate versus carbon conversion during CO2 gasification of DT coal with and without BF slag at different heating rates.

Figure 12. Reaction rate versus temperature during CO2 gasification of DT coal with and without BF slag at different heating rates.

Figure 13. Reaction rate versus temperature during CO2 gasification of coke with and without BF slag at different heating rates.

Figure 16. Reaction rate versus carbon conversion during CO2 gasification of coke with and without BF slag at different heating rates.

Figure 14. Reaction rate versus temperature during CO2 gasification of FX coal with and without BF slag at different heating rates.

Figure 17. Reaction rate versus carbon conversion during CO2 gasification of FX coal with and without BF slag at different heating rates.

the reaction rate increases and shifts to a higher temperature zone. For the gasification of DT coal without slag, as the heating rate increases from 10 to 20 and 30 K/min, the maximum of the reaction rate increases from 0.21 to 0.28 to 0.35 with corresponding temperature increases from 1304 to 1345 to 1405 K. Figures 15, 16, and 17 show reaction rate versus carbon conversion during CO2 gasification of DT, coke, and FX coal separately with and without BF slag at different heating rates. It can be seen that the reaction rate increased first and then decreased with carbon conversion. At the same carbon

conversion, the higher the heating rate is, the higher the reaction rate is. And the peak value of reaction rate increased with increasing heating rate. 4.2. Effects of BF Slag. From Figures 3 to 17, we also can see the effect of BF slag on coal gasification. It can be observed that, at the same time, the carbon conversion of coal gasification with BF slag is higher than that without slag. The time required for the carbon conversion to reach nearly unity 15877

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with slag is shorter than that without slag. For example, in Figure 3, when the heating rate is 10 K/min, the time for the complete carbon conversion with slag is about 4.7 min, whereas that without slag is about 7.5 min. And at the same time, the reaction rate with slag is also higher than that without slag, and the time required for the reaction rate to reach maximum with slag is shorter than that without slag. In Figure 9, when the heating rate is 30 K/min, the maximum of reaction rate with slag is 0.53 with a corresponding time of 1.2 min, whereas that without slag is only 0.35 with a corresponding time of 2.6 min. In the presence of BF slag, at the same reaction temperature, the carbon conversion is higher than that without slag, and the carbon conversion curve shifts to lower temperature; the peak value of reaction rate is also higher than that without slag, and the reaction rate curve also shifts to lower temperature. For example, in Figure 12, when the heating rate is 30 K/min, the peak value of reaction rate with slag is 0.53 with a corresponding temperature of 1372 K, whereas that without slag is only 0.35 with a corresponding temperature of 1405 K. These indicate that BF slag acts as a catalyst for the gasification reaction rate. This is very important because coal gasification with CO2 is a high-temperature process, but from an economical point of view, low temperature is desired. At the same temperature, the gasification reaction rate with slag is higher than that without slag, so the BF slag can be used for low temperature gasification to overcome slow reaction of carbon with CO2. The reason BF slag can act as a catalyst for coal gasification is that the Mg and Ca contents of BF slag are high. The 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 so on. Numerous researchers39,40 in the area of catalytic carbon gasification have pointed out that group VIII metals as well as alkali and alkaline earth catalytic are mostly used and found effective. 4.3. Effects of Coal Type. In general, the gasification reactivity of a coal species is affected by the nature of its properties such as volatile matter, ash content, and so on. Figures 18 and 19 show the variation of the carbon conversion with time during CO2 gasification of different coal types without and with BF slag at different heating rates. It can be observed that, no matter with or without BF slag, at the same time, the carbon conversion of DT coal is highest, whereas the carbon conversion of coke is lowest. At the same heating rate, the order time for complete carbon conversion was DT < FX
FX > CO.

Figure 22. Reaction rate versus time during CO2 gasification of different coal types without BF slag at different heating rates.

5. THE STUDY OF KINETIC MODELS 5.1. The Most Probable Mechanism Function. On the basis of the kinetic analysis, the correlation coefficient was calculated; the mechanism functions under different conditions are shown in Table 3. As shown in Table 3, it can be seen that the mechanism functions of coal gasification when different types of coal were employed are different. The mechanism functions of coke and FX coal are a C1 model (phase boundary reaction (n = 2) model), and the mechanism functions of DT coal are a C2 model (phase boundary reaction (n = 3/2) model). When there is molten BF slag, the mechanism function is different from that with no molten BF slag. The mechanism function of a mixture sample of coke and slag is a D5 model (3D diffusion (anti-Jander) model). The mechanism function of a mixture sample of DT coal and slag is a D4 model (3-D diffusion model). The mechanism function of a mixture sample of DT coal and slag is a C2 model (phase boundary reaction (n = 3/2) model). 5.2. The Calculation of Ea and A. The date of the nonisothermal gas−solid kinetic reaction can be conducted using derivation, integration, the bad minus differential method, and the maximum reaction rate method. In this paper, we use Satava−Sestak integration to calculate the activation energy Ea and the frequency factor A of coal gasification in molten BF slag. The expression of the Satava−Sestak integration method is shown as follows.

Figure 23. Reaction rate versus time during CO2 gasification of different coal types with BF slag at different heating rates.

⎛ AE ⎞ E lg F(x) = lg⎜ a ⎟ − 2.315 − 0.4567 a RT ⎝ Rβ ⎠

Figure 24. Reaction rate versus temperature during CO2 gasification of different coal types without BF slag at different heating rates.

(11)

If we define a = lg(AEa/βR) − 2.315, b = −0.4567Ea/R, x = 1/ T, and y = ln F(x), we can get

23, 24, and 25, we can see that, at the same conditions, the reaction rate of DT coal is highest, whereas the reaction rate of coke is lowest. The time of DT coal required to get the peak value of reaction rate is shortest, and the time of coke required to get the peak value of reaction rate is longest. For example,

y = a + bx

(12)

With regression analysis, we can use the linear regression model and the activation energy and frequency factor can be calculated. 15879

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Table 3. The Kinetic Mechanism Functions of Different Coal Samples coal sample

temperature range (K)

coke coke and slag DT coal DT coal and slag FX coal FX coal and slag

1273−1723 1273−1723 1273−1723 1273−1723 1273−1723 1273−1723

heating rate (K/min) 10, 10, 10, 10, 10, 10,

20, 20, 20, 20, 20, 20,

30 30 30 30 30 30

Figures 26 and 27 show the curves of linear regression of gasification reaction. Table 4 shows the kinetic parameters of

reaction model

mechanism function f(x)

C1 D5 C2 D4 C1 C2

(1 − x)2 (3/2)(1 + x)2/3[(1 + x)1/3 − 1]−1 2(1 − x)3/2 (3/2)(1 − x)2/3[1 − (1 − x)2/3]−1 (1 − x)2 2(1 − x)3/2

Table 4. The Kinetic Parameters of Coal Gasification under Different Conditions coal sample coke

coke and slag

DT coal

DT coal and slag

Figure 26. Curves of linear regression of coal gasification (coke).

FX coal

FX coal and slag

heating rate (K·min−1)

activation energy (kJ·mol−1)

frequency factor, ln A

related coefficient

10 20 30 10

426.2004 350.5756 303.5787 584.5138

33.4709 26.7301 23.0412 45.0061

0.9657 0.9623 0.9917 0.9698

20 30 10 20 30 10

567.1586 559.4309 203.6043 184.0905 169.4831 155.4190

42.6439 41.4470 18.2202 16.0794 15.3611 13.1373

0.9811 0.9991 0.9846 0.9694 0.9687 0.9936

20 30 10 20 30 10

126.9627 122.5459 261.2952 211.6360 187.9408 186.5873

11.2789 11.4019 22.5596 18.0766 16.1807 15.6893

0.9324 0.9416 0.9923 0.9843 0.9762 0.9439

20 30

159.8175 152.6617

14.1634 13.4009

0.9816 0.9926

Figure 27. Curves of linear regression of coal gasification (coke and slag).

coal gasification in molten BF slag. It can be seen that the activation energy and frequency factor decreased with increasing heating rate. The activation energy of coke without slag is higher than that of coke with slag. However, the frequency factor of coke without slag is lower than that of coke with slag. Otherwise, both of the activation energies and frequency factors of DT coal and FX coal without slag are higher than those of DT coal and FX coal with slag. Figures 28 and 29 show the relationship between activation energy and frequency factor. It can be seen that the frequency factor is linearly related to the activation energy. This indicates that the kinetic compensation effect of coal/CO2 gasification in molten BF slag existed. For example, the kinetic compensation

Figure 28. The kinetic compensation of activation energy and preexperimental (coke).

effect of coke/CO2 without molten slag is shown as eq 13, and the kinetic compensation effect of coke/CO2 with molten slag is shown as eq 14.

15880

ln A = 0.0854Ea − 3.0213

(13)

ln A = 0.1409Ea − 37.3423

(14)

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possibility mechanism function for coal/CO2 gasification in molten BF slag were determined. The results show that, with the increase in heating rate, the carbon conversion and the peak value of reaction rate increased at the same reaction time, the carbon conversion curve shifted to higher temperature and the reaction rate curve shifted rightward systematically, and both the time required for the carbon conversion to reach nearly unity and the time necessary for the reaction rate to reach its maximum decreased. The carbon conversion and reaction rate were sensitive to BF slag; at the same time, the carbon conversion and reaction rate of coal gasification with slag are higher than that without slag. The time required for the carbon conversion to reach nearly unity and the time required for the reaction rate to reach a maximum with slag are both shorter than that without slag. In the presence of BF slag, at the same reaction temperature, the carbon conversion is higher than that without slag and the carbon conversion curve shifts to lower temperature; the peak value of reaction rate is also higher than that without slag, and the reaction rate curve also shifts to lower temperature. At the same temperature, the gasification reaction rate with slag is higher than that without slag, so the BF slag can be used for low temperature gasification to overcome slow reaction of carbon with CO2. The gasification reactivity of different coal types is different. In this paper, the order of reactivity sequence at these temperatures was DT > FX > CO. The activation energies, mechanism function, and frequency factors of coal gasification were much different when different gasification conditions were employed. The molten BF slag can change the mechanism function of coal gasification. Without molten BF slag, the mechanism functions of coke and FX coal are a C1 model (phase boundary reaction (n = 2) model), while the mechanism functions of DT coal are a C2 model (phase

Figure 29. The kinetic compensation of activation energy and preexperimental (DT and FX).

5.3. The Kinetic Model of Coal Gasification in Molten BF Slag. Using the most possibility mechanism function, the activation energy, and frequency factor which we have gotten, we can get the kinetic model of coal gasification in molten BF slag. Table 5 show the kinetic models of different coal samples under different gasification conditions.

6. CONCLUSIONS The char−CO2 gasification reactions in molten BF slag were studied kinetically by temperature-programmed thermogravimetry using an STA409PC thermal analyzer. The effect of heating rates and molten slag on coal gasification were studied, and the activation energy, frequency factor, and the most

Table 5. The Kinetic Model of Coal Gasification in Molten BF Slag coal sample coke

coke and slag

DT coal

DT coal and slag

FX coal

FX coal and slag

heating rate (K·min−1)

kinetic model of coal gasification in molten BF slag

10

dx /dT = 3.4 × 1013 exp(− 5.1 × 104 /T )(1 − x)2

20

dx /dT = 2.0 × 1010 exp(− 4.2 × 104 /T )(1 − x)2

30

dx /dT = 3.4 × 108 exp(− 3.6 × 104 /T )(1 − x)2

10

dx /dT = 3.5 × 1018 exp(− 7.0 × 104 /T )(3/2)(1 + x)2/3 [(1 + x)1/3 − 1]−1

20

dx /dT = 1.6 × 1017 exp(− 6.8 × 104 /T )(3/2)(1 + x)2/3 [(1 + x)1/3 − 1]−1

30

dx /dT = 3.3 × 1016 exp(− 6.7 × 104 /T )(3/2)(1 + x)2/3 [(1 + x)1/3 − 1]−1

10

dx /dT = 8.2 × 107 exp(− 2.4 × 104 /T ) × 2(1 − x)3/2

20

dx /dT = 4.8 × 105 exp(− 2.2 × 104 /T ) × 2(1 − x)3/2

30

dx /dT = 1.6 × 105 exp(− 2.0 × 104 /T ) × 2(1 − x)3/2

10

dx /dT = 5.1 × 104 exp(− 1.9 × 104 /T )(3/2)(1 + x)2/3 [1 − (1 − x)2/3 ]−1

20

dx /dT = 4.0 × 103 exp(− 1.5 × 104 /T )(3/2)(1 + x)2/3 [1 − (1 − x)2/3 ]−1

30

dx /dT = 3.0 × 103 exp(− 1.5 × 104 /T )(3/2)(1 + x)2/3 [1 − (1 − x)2/3 ]−1

10

dx /dT = 2.0 × 107 exp(− 2.7 × 104 /T ) × 2(1 − x)3/2

20

dx /dT = 3.5 × 106 exp(− 2.5 × 104 /T ) × 2(1 − x)3/2

30

dx /dT = 3.5 × 105 exp(− 2.3 × 104 /T ) × 2(1 − x)3/2

10

dx /dT = 6.5 × 105 exp(− 2.2 × 104 /T ) × 2(1 − x)3/2

20

dx /dT = 7.1 × 104 exp(− 1.9 × 104 /T ) × 2(1 − x)3/2

30

dx /dT = 2.2 × 104 exp(− 1.8 × 104 /T ) × 2(1 − x)3/2 15881

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boundary reaction (n = 3/2) model). However, with molten BF slag, the mechanism function of coke is a D5 model (3-D diffusion (anti-Jander) model), the mechanism function of DT coal is a D4 model (3-D diffusion model), and the mechanism function of FX coal is a C2 model (phase boundary reaction (n = 3/2) model). The activation energies and frequency factors decrease as the heating rates increase. The activation energy of coke without slag is higher than that of coke with slag. However, the frequency factor of coke without slag is lower than that of coke with slag. On the contrary, both the activation energies and frequency factors of DT coal and FX coal without slag are higher than those of DT coal and FX coal with slag. Besides, the kinetic compensation effect of coal/CO2 gasification in molten BF slag existed.

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AUTHOR INFORMATION

Corresponding Author

*Tel./Fax: +86-024-83672216. E-mail: [email protected] (Q.B.Y.); [email protected] (P.L.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by National Natural Science Fund (51274066), National High-tech R&D Program (2013BAA03B03), Fundamental Research Funds for the Central Universities (N110602002), and Academic New Artist Ministry of Education Doctoral Post Graduate.

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