Experimental and Modeling Study of Char Gasification with Mixtures of

Dec 11, 2015 - Six chars, such as four coal chars of varying rank and two chars derived from acid-washed coals, were prepared by fast pyrolysis using ...
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Experimental and Modeling Study of Char Gasification with Mixtures of CO2 and H2O Chao Chen, Sen Zhang, Kai Xu, Guangqian Luo, and Hong Yao* State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan, Hubei 430074, People’s Republic of China ABSTRACT: Six chars, such as four coal chars of varying rank and two chars derived from acid-washed coals, were prepared by fast pyrolysis using a drop-tube furnace at 1323 K to study the gasification reaction mechanism when both CO2 and H2O were present using a thermogravimetric analyzer and a fluidized-bed reactor at the atmospheric condition at 1173−1273 K. It was found that the char gasification rate in the mixtures of CO2 and H2O was obviously lower than the sum of the two rates of the char individually reacting with CO2 or H2O but higher than the rate of each individual reaction. CO2 and H2O competed for the same active sites on the surface of char. The char−H2O reaction was independent of the char−CO2 reaction, while the char− CO2 reaction was inhibited by the char−H2O reaction. With the increase of the coal rank and temperature, the inhibition decreased. On the basis of the results, a new mechanism was suggested to describe the competition between the CO2 adsorption and the H2O adsorption. Moreover, a detailed mathematical model based on the derived mechanism was used to predict the performance of an axisymmetric, entrained-flow gasifier. Comparisons between values predicted by the model and experimental study of the literature had shown good agreement.

1. INTRODUCTION Gasification of coal offers several advantages; e.g., various types of coals ranging from lignite to anthracite can be used, and different syngas and byproducts can be obtained depending upon the operating conditions. Various types of reactors, such as fixed (moving) bed, fluidized bed, entrained bed, and molten bath, may be used along with autothermal or allothermal heat arrangements.1 Understanding the gasification reaction mechanism and reactivity is essential in relation to gasifier design, operation, and troubleshooting.2,3 Most published gasification reactivity investigations studied char gasification with a single CO2 or H2O.2−5 The surface reaction rate, especially the effect of gas partial pressures on that, has been successfully modeled using the Langmuir− Hinshelwood (LH) equation.6 The traditional LH rate equation can work well when the gas partial pressure is low.7 The mechanisms of the char−CO2 reaction and char−H2O reaction have been considered to be essentially the same as the following:

The mechanisms are also known as an oxygen-exchange mechanism. However, the mechanism of char gasification with the mixtures of CO2 and H2O is not clear. According to published results, a few possible surface reaction mechanisms were proposed. Some people proposed that char−H2O and char− CO2 reactions occurred at the separate active sites. The relationship between the CO2 adsorption and H2O adsorption was independent, and the gasification rate in mixtures was the sum of each individual gasification rate.8−10 On the other hand, other researchers found that the gasification rates in the mixtures were lower than the sum of each individual gasification rate and proposed that char−H2O and char−CO2 reactions occurred at the same active sites.11−14 The active sites for both the char−CO2 and char−H2O reactions were the edge sites, and the basal plane carbon atoms were usually less reactive.7 However, there is no consensus on the competition between CO2 adsorption and H2O adsorption. Mühlen et al.11 proposed that the relationship between the CO2 adsorption and H2O adsorption was parallel. The adsorption step was coupling. The CO2 adsorption had influence on the H2O adsorption, and the H2O adsorption also had influence on the CO2 adsorption. The gasification rate in mixtures was the coupled form of CO2 adsorption and H2O adsorption. However, Roberts and Harris13,15 proposed that the relationship between the CO2 adsorption and H2O adsorption was competitive. CO2 could be adsorbed preferentially at the active sites. Adsorption of H2O was blocked by pre-adsorbed CO2.

k1

C( ) + CO2 ↔ CO + C(O) k −1

k2

C(O) ↔ CO + C( )

(1) (2)

and k3

C( ) + H 2O ← → H 2 + C(O) k −3

k4

C(O) ↔ CO + C( )

(3) (4)

Special Issue: 5th Sino-Australian Symposium on Advanced Coal and Biomass Utilisation Technologies

A CO2 or H2O molecule is adsorbed at a carbon active site, releasing carbon monoxide or hydrogen and forming an oxidized surface complex. Then, the carbon−oxygen complex subsequently produces a molecule of CO and a new active site. © XXXX American Chemical Society

Received: September 30, 2015 Revised: December 11, 2015

A

DOI: 10.1021/acs.energyfuels.5b02294 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Therefore, the char−CO2 reaction was independent of the char−H2O reaction, while the char−H2O reaction was inhibited by char−CO2 reaction. Furthermore, Umemoto et al.16 also found that the gasification rates in the mixtures were lower than the sum of each individual gasification rate. However, the study proposed that both the separate active sites and the same active sites coexisted during char gasification in the mixtures of CO2 and H2O. CO2 gasification and H2O gasification could share active sites partially. The survey above indicates that the main differences among these mechanisms on char gasification with mixtures of CO2 and H2O are the relationship between the active sites for the char−CO2 reaction and those for the char−H2O reaction and the competition between the CO2 adsorption and the H2O adsorption. Previous work by the authors17 had found that gasification rates of the chars prepared by a lignite in the mixtures were obviously lower than each individual gasification rate at 1273 K. However, the char−H2O reaction was independent of the char−CO2 reaction, while the char−CO2 reaction was inhibited by the char−H2O reaction. To better understand the conditions applicable to various mechanisms, more experiments are needed to verify which mechanism is more suitable under certain conditions. In particular, the effects of coal type and temperature on the char gasification with mixtures of CO2 and H2O are needed to be understood, because the rank of parent coal is an important factor affecting the char active sites and the temperature is an important factor affecting the adsorption rate.18 Two chars derived from acidwashed coals have also been determined in this paper, avoiding the effect of the catalyst on the reaction mechanism. To study the competition between the CO2 adsorption and the H2O adsorption, the char gasification under different gas partial pressure conditions at different temperatures has been investigated.

Table 1. Properties of the Coals Used for Char Generation coal

CF

SH

Proximate Analysis moisture (%, ad) 4.47 4.90 ash (%, ad) 15.34 7.11 volatile matter (%, ad) 37.14 32.55 fixed carbon (%, ad) 43.05 55.44 Ultimate Analysis (%, daf) carbon 70.56 76.93 hydrogen 6.85 4.47 oxygen (by difference) 19.60 14.68 nitrogen 1.01 0.92 sulfur 1.98 3.00 Analysis of Ash (%) SiO2 46.66 29.17 Al2O3 15.94 14.18 Fe2O3 10.52 9.50 CaO 7.30 25.57 K2O 0.82 0.39 Na2O 0.83 2.19 MgO 2.65 1.13 MnO 0.26 0.33 P2O5 0.52 0.68 TiO2 0.42 0.46 SO3 13.44 16.07

PDS

YQ

4.31 35.15 28.51 32.03

3.14 16.57 11.95 68.35

73.77 5.52 13.68 1.22 5.81

90.45 3.46 3.64 1.26 1.20

58.38 28.75 3.02 2.26 1.21 1.16 1.67 0.05 0.82 0.82 1.60

49.90 34.36 3.14 2.84 0.98 1.16 1.24 0.02 0.75 1.00 4.30

H2O at atmospheric pressure were measured using a vertical TGA. The reactor diameter of the TGA was 70 mm. Each experiment used 3 NL/min of gas mixture and 300 mg of char sample. The conditions could ensure that the gasification rates were not affected by the gas flow rate, weight of sample, and particle size. The flow rates of CO2 and N2 were controlled by the mass flow meters. The flow rate of H2O was controlled by a peristaltic pump (0.8 mL/min of liquid water provides 1 NL/min of vapor). Specific gasification rates10,13 were calculated from the mass versus time profiles using eq 5

2. EXPERIMENTAL SECTION Because the char reactivity is generally proportional to the active sites and the active sites for char−CO2 and char−H2O reactions in the mixtures were difficult to measure directly, most researchers usually study the relationship between the active sites for the char−CO2 reaction and those for the char−H2O reaction by the measurement of the gasification rate. In this study, a common experimental reactor of the thermogravimetric analyzer (TGA) was used to measure the rates by changes of the char weight. The condition approximated a commercial moving bed gasifier, but it was different from that in a fluidized bed or entrained flow gasifier.6 Thus, a fluidized-bed reactor (FBR) was also used to measure the rates by evolved gas analysis, strengthening the collision frequency between the gas and the solid. 2.1. Char Preparation. Four coals were used in this work with sizes of 106−180 μm. They range from lignite to anthracite. The properties of the pulverized coals are presented in Table 1. With the increase of the coal rank, the volatile content and oxygen content decrease. To study the effect of mineral matter on the char gasification, two coals were subjected to acid washing to remove the inorganic matter. About 5 g of coal sample was mixed with 48% concentrated hydrofluoric acid for 24 h at room temperature, then treated with 6 N hydrochloric acid, and kept for 12 h at 333 K. At last, the sample was washed with excess demineralized water. Most of the ash could be removed by the demineralization process.19 Four raw coals and two acid washed coals (AW-SH and AW-PDS) were used to prepare the chars. The pyrolysis was carried out in a drop-tube furnace at atmosphere pressure under a N2 atmosphere at 1323 K, with a high heating rate. 2.2. Reaction Rate Measurements in the TGA. The rates of the char gasification with CO2 or in H2O and the mixtures of CO2 and

r=

1 dX 1 − X dt

(g s−1 g −1)

(5)

where X is the char conversion at reaction time t. The kinetic parameters and adsorption and desorption rates were derived by the LH rate equation. 2.3. Fluidized-Bed Gasification Experiment. The experimental conditions in the FBR were designed to inhibit the fluid dynamics on the intrinsic reaction kinetics. The reactor is a one-stage fluidized bed with 40 mm diameter and 120 mm height, as shown in Figure 1, which is similar to the study by Xie et al.20 The Al2O3 particles with a size of 74−106 μm were used as the fluidization medium. The critical fluidization velocities of the two type of particles were closed, avoiding the separation of particles in the bed. A total of 600 mL/min of gas mixture and 60 mg of char sample were used for each experiment to ensure a high excess gasifying agent ratio, to reduce the diffusion resistance between the emulsion phase and bubble phase. The char sample was injected into the fluidization medium at the holding temperature in one shot within 2 s to cause the fast mixing, heating, and reaction. The evolved gas was then passed through concentrated sulfuric acid to remove vapor, and the dried gas was analyzed by an online gas analyzer. CO2, CO, CH4, and CnHm were measured by a non-dispersive infrared (NDIR) method with an accuracy of ±0.1%. H2 was measured by an electrochemical detector with an accuracy of ±0.4%, and O 2 was measured by an electrochemical detector with an accuracy of ±0.2%. The gasification rates could be obtained through the production rate of evolved gas based on the mass balance calculations. B

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curves of other chars have the same trend, which are not shown here. Specific reaction rates at the char conversion of 50% for the six chars reacting in all atmospheres investigated in the TGA are all shown in Table 2. With the increase of the coal rank, the gasification rate decreased under the same conditions. The gasification rate of raw coal char was faster than that of acid-washed coal char. The char−H2O reaction rate was higher than the char−CO2 reaction rate in the same gasification agent partial pressure. Because the experiments were carried out at a lower pressure, the reaction rate was controlled by the adsorption step. It meant than H2O might be more easily absorbed on the surface of char than CO2. The CO2 gasification rate of CF char was about 10 times higher than that of YQ char, while the H2O gasification rate of CF char was about 2 times higher than that of YQ char in the TGA. The coal rank had greater impact on the char−CO2 reaction than on the char− H2O reaction. The char−mixture reaction rates were a little higher than the individual char−H2O reaction rate but lower than the sum of each individual reaction rate for all chars. Figures 3 and 4 show the variations of the production rates of syngas in different atmospheres during SH char and AW-SH char gasification in the FBR. Char conversion curves in the FBR can be obtained, which are also shown in Figure 2, by the following equations:17

Figure 1. Schematic diagram of the one-stage FBR.

3. RESULTS AND DISCUSSION 3.1. Effect of Coal Types on the Char Gasification with CO2, H2O, and Their Mixture. Char conversion curves during gasification in the TGA were obtained by the direct measure of weight changes in the controlled gasification atmosphere. The typical results of SH char and AW-SH char conversion are shown in Figure 2 in 1/3 atm CO2, 1/3 atm H2O, and their mixture of 1/3 atm CO2 and 1/3 atm H2O. These data were all presented at 1273 K. The reaction time was shorter for the char−mixture reaction than for the char−H2O reaction and much shorter than that for the char−CO2 reaction. Conversion

nC = 0.5(nCO + n H2)

∫0

m = MC

(6)

t

∫0

nC dt = 0.5MC

t

(nCO + n H2) dt

(7)

t

0.5MC∫ (nCO + n H2) dt m 0 X= = ∞ m∞ 0.5MC∫ (nCO + n H2) dt 0

(8)

where nC is the conversion rate of carbon (mol/min), nCO and nH2 are the production rates of carbon monoxide and hydrogen (mol/min), respectively, m is the weight conversion of carbon (g), and MC is the molecular weight of carbon (g/mol). The fixed carbon content of 60 mg of SH char particles was about 53.18 mg, and that of 60 mg AW-SH char particles was 60 mg in theory. The weight conversion of SH char during the FBR gasification was 53.61, 53.40, and 51.84 mg, respectively, by eq 7, under different gasification conditions. The weight conversion of AW-SH char during the FBR gasification was about 58.31, 59.11, and 58.94 mg, respectively. The results showed that the two quartz frits had a good effect on the gas− solid separation, and the mass balance for the one-stage FBR was good to the kinetics tests. The obtained specific gasification rates are shown in Table 2. The results of equilibrium calculation are also shown in Figures 3 and 4 on the basis of the assumption that the gas reaction was local instantaneous equilibrium. It was found that the experimental syngas production rates were very close to the results of equilibrium calculation. It means that the water-gas shift reaction always tends to the equilibrium in the FBR. Thus, the compositions of syngas were very different between the char−H2O and char−mixture gasifications, even if the gasification rate of the char−mixture reaction was a little higher than that of the char−H2O reaction. The char−H2O reaction produced more hydrogen, while the char−mixture reaction produced more carbon monoxide as a result of the high concentration of CO2.

Figure 2. (a) SH char and (b) AW-SH char conversion curves reacting in the TGA and the FBR at 1273 K in 1/3 atm CO2, 1/3 atm H2O, and their mixture of 1/3 atm CO2 and 1/3 atm H2O. C

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Table 2. Specific Reaction Rates at the Char Conversion of 50% for the Six Chars Reacting in All Atmospheres Investigated in the TGA and the FBR specific gasification rate (×103, g s−1 g−1) reactor TGA

FBR

partial pressure

CF

SH

PDS

YQ

AW-SH

AW-PDS

/3 atm CO2 2 /3 atm CO2 1 atm CO2 1 /5 atm H2O 1 /3 atm H2O 2 /3 atm H2O 1 /3 atm H2O + 1/3 atm CO2 1 /3 atm H2O + 2/3 atm CO2 1 /3 atm CO2 1 /3 atm H2O 1 /3 atm H2O + 1/3 atm CO2

1.93 3.33 4.00 2.46 3.25 4.78 4.04 4.63 10.9 16.3 22.8

1.66 2.30 2.79 2.10 3.17 4.93 4.14 4.59 10.2 14.7 20.1

1.33 1.84 2.15 2.42 3.26 5.18 3.91 4.16 7.37 13.9 17.7

0.190 0.264 0.327 1.03 1.46 2.38 1.59 1.65 0.587 3.22 3.80

1.11 1.51 1.80 2.34 3.08 4.46 3.56 3.73 5.15 12.4 15.1

1.02 1.42 1.76 2.05 2.72 4.39 3.39 3.59 3.69 8.79 10.5

1

competition between the CO2 adsorption and the H2O adsorption 3.3. Effect of the Temperature on the Char Gasification with CO2, H2O, and Their Mixture. Figures 6 and 7 show the conversion of SH char in 1/3 atm CO2, 1/3 atm H2O, and their mixture at 1173, 1223, and 1273 K. With the increase of the temperature, the coal reaction rate increased rapidly. The char−mixture reaction rate was always obviously lower than the sum of the two rates of the char individually reacting with CO2 and H2O at different temperatures. On the experimental hypothesis that the char−H2O reaction was independent of the char−CO2 reaction, while the char−CO2 reaction was inhibited by the char−H2O reaction, the coefficient of rH2O was 1 and the coefficient of rCO2 could be obtained by calculation, as shown in the Figures 6 and 7. The obtained coefficient of rCO2 increased with the increase of the temperature. It indicated that the inhibition decreased with the increase of the temperature 3.4. Possible Rate Equation of the Char−Mixture Reaction. It was found that char−H2O and char−CO2 reactions might not occur at the completely separate active sites for all of the investigated chars. Thus, a new possible rate equation of the char−mixture reaction can be proposed assuming that CO2 and H2O compete for the same active sites, and H2O can be adsorbed preferentially at the active sites. CO2 does not significantly retard the rate of the char−H2O reaction. The empty sites for H2O adsoprtion are not affected by CO2. The rate of H2O adsorption should be proportional to the empty sites. Considering the retardation effect of H2 on the char−H2O reaction, the steady-state reaction equation (overall reaction rate is equal to the adsorption and desorption step rates) based on the LH mechanism can be expressed

The rate of char gasification in the FBR was about 3−5 times higher than that in the TGA at the same atmosphere because of the better collision between the char and the gasification agents, increasing the frequency factor in chemical kinetics. The changes in the reactor seemed to have a similar effect on the char−CO2, char−H2O, and char−mixture reactions. The gasification rates in mixtures were also lower than the sum of each individual gasification rate for all chars in the FBR, just like in the TGA. 3.2. H2O “Inhibition” of the Char−CO2 Reaction. As shown in Table 2, char−mixture reaction rates are obviously higher than each individual reaction rate but lower than the sum of the two rates of the char individually reacting with CO2 and H2O. At the fixed partial pressure of H2O, the char gasification rate increased with the increase of the CO2 partial pressure. It indicated that the char−H2O and char−CO2 reactions might compete for the same active sites on the surface of char. Figure 5 shows the variation of rmix (char−mixture reaction rate in the TGA) with rCO2 (char−CO2 reaction rate in the TGA), indicating a good linear relationship between the two rates for all of the gasification cases at the fixed partial pressure of H2O. Therefore, linear regression can be employed to derive the correlation coefficients, and the results are shown in Table 3. The regressed coefficients of rH2O were all close to 1, while those of rCO2 were all lower than 1. This might be due to the bond energy of H−O of H2O being lower than that of CO of CO2, and the chemical adsorption of H2O on the surface of the char was preferential. The coefficient of rCO2 decreased with the increase of the coal rank because the lower H2O adsorption rate of high-rank char decreased the H2O inhibition of the char− CO2 reaction. The raw coal char−mixture reaction and acid-washed coal char−mixture reaction had a similar mechanism. It might be due to the mineral matter having no impact on the chemical bond of the gas molecule. The competition between the CO2 adsorption and the H2O adsorption was mainly affected by the gas properties. The rate expression shown in Table 3 for char gasification in the mixture of CO2 and H2O derived from the TGA experiments can also well represent the char gasification process in the FBR, even if there is better contact between char and gasification agents in the FBR. The coupling mode between the gas and solid phase might had little impact on the

k 3(1 − θH2O)PH2O − k −3θH2OPH2 = k4θH2O

rH2O‐mix = k4θH2O = rH2O

(9) (10)

where θH2O is the fractional coverage of H2O at the active sites and rH2O‑mix is the rate of H2O gasification in the mixture. However, adsorption of CO2 is blocked by pre-adsorbed H2O; therefore, the rate of the char−CO2 reaction is reduced by the presence of H2O. The empty sites for CO2 adsorption are affected by pre-adsorbed H2O. Considering the retardation effect of CO on the char−CO2 reaction, the steady-state reaction equation can be expressed D

DOI: 10.1021/acs.energyfuels.5b02294 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 3. Variations of production rates during SH char gasification in the FBR at 1273 K in (a) 1/3 atm CO2, (b) 1/3 atm H2O, and (c) their mixture of 1/3 atm CO2 and 1/3 atm H2O. (■ and △) Equilibrium calculation results of evolved gas.

k1(1 − θH2O − θCO2)PCO2 − k −1θCO2PCO = k 2θCO2

Figure 4. Variations of production rates during AW-SH char gasification in the FBR at 1273 K in (a) 1/3 atm CO2, (b) 1/3 atm H2O, and (c) their mixture of 1/3 atm CO2 and 1/3 atm H2O. (■ and △) Equilibrium results of evolved gas.

Because of the slower H2O adsorption rate k3 of high-rank coal char, the inhibition decreases with the increase of the raw coal rank. Because of the lower active energy of k3 lower than k4, the ratio of k3/k4, the inhibition decreases with the increase of the temperature in the regime of adsorption control. These conclusions are consistent with the experimental results.

(11)

⎛ ⎞ k 3PH2O ⎟rCO rCO2‐mix = k 2θCO2 = ⎜⎜1 − k4 + k −3PH2 + k 3PH2O ⎟⎠ 2 ⎝ (12)

where θCO2 is the fractional coverage of CO2 at the active sites and rCO2‑mix is the rate of CO2 gasification in the mixture. Thus, a combined rate expression of eq 13 can be derived to calculate the char−mixture reaction rate. H2O inhibition of the char−CO2 reaction is affect by the partial pressure of H2 and H2O and the adsorption and desorption rates of the char−H2O reaction. With the increase of PH2O and the decrease of PH2, the coefficient of rCO2 decreases and the inhibition increases.

rmix = rCO2‐mix + rH2O‐mix ⎡ ⎤ (k 3/k4)PH2O ⎥rCO + rH O = ⎢1 − 2 1 + (k −3/k4)PH2 + (k 3/k4)PH2O ⎥⎦ 2 ⎢⎣ (13)

Figure 8 shows the comparison of experimental rates of the char−mixture reaction and calculated rates using eq 13. The derived rate equation is appropriate to predict the char E

DOI: 10.1021/acs.energyfuels.5b02294 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 5. Linear relationship between rmix (char gasification rate with the mixture of CO2 and H2O in the TGA) and rCO2 (individual char gasification rate with CO2 in the TGA).

Table 3. Linear Fit Equation between rmix (Char Gasification Rate with the Mixture of CO2 and H2O in the TGA) and rCO2 (Individual Char Gasification Rate with CO2 in the TGA) char CF

SH

PDS

YQ

AW-SH

AW-PDS

partial pressure 1

/3 1 /3 1 /3 1 /3 1 /3 1 /3 1 /3 1 /3 1 /3 1 /3 1 /3 1 /3 1 /3 1 /3 1 /3 1 /3 1 /3 1 /3

atm atm atm atm atm atm atm atm atm atm atm atm atm atm atm atm atm atm

H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O

reaction rate equation rmix = 0.414rCO2 + 1.000rH2O

+ 1/3 atm CO2 + 2/3 atm CO2 rmix = 0.611rCO2 + 1.001rH2O + 1/3 atm CO2 + 2/3 atm CO2 rmix = 0.489rCO2 + 1.000rH2O + 1/3 atm CO2 + 2/3 atm CO2 rmix = 0.713rCO2 + 1.000rH2O + 1/3 atm CO2 + 2/3 atm CO2 rmix = 0.431rCO2 + 1.000rH2O + 1/3 atm CO2 + 2/3 atm CO2 rmix = 0.621rCO2 + 1.003rH2O + 1/3 atm CO2 + 2/3 atm CO2

Figure 6. SH char conversion curves reacting in the TGA in 1/3 atm CO2, 1/3 atm H2O, and their mixture of 1/3 atm CO2 and 1/3 atm H2O, at (a) 1173 K, (b) 1223 K, and (c) 1273 K.

gasification rates with the mixtures in both the TGA and the FBR.

4. SIMULATION MODELING 4.1. Model Description. A number of models have been developed to simulate the coal gasification process, many of them using simple reaction kinetics. The aim of this paper is to report the development gasifier model using the derived char− mixture reaction mechanism based on the LH equation. A laboratory-scale, axisymmetric, entrained-flow gasifier at Brighham Yong Unversity (BYU) was simulated. Brown et al.21 had given detailed experimental methods and test conditions. The one-dimensional steady calculation was used in the numerical simulation of gasification. The temperature and gas compositions were assumed to be uniformly dispersed across the reactor and only change along the reactor. Gas turbulence was neglected. The model includes the following components: particle heating, devolatilization, char combustion, char gasification, gas reaction and equilibrium, internal and external diffusion, and heat balance between the wall, gas, and particles.

These model components can be found in the paper by Liu and Niksa.12 The original gasification model was provided by Liu and Niksa12 and was not repeated here. 4.2. Modeling Results. As shown in Figure 9, the predicted gas species in the axial direction agree reasonably well with the experimental tests. The effluent gas property composition (dry and inert free basis) of the modeling result was 49.7% CO, 26.0% H2, 24.1% CO2, and 0.0025% CH4 compared to the experimental result of 51.1% CO, 24.7% H2, 23.6% CO2, and 0.3% CH4. The carbon conversion of the modeling result was 81.2% compared to the experimental result of 82%. The exit gas temperature of the modeling result was 1344 K, which is also close to the experimental result of about 1350 K. The mean deviations between the modeling result and the experimental result were less than 2%. The results indicated that the derived F

DOI: 10.1021/acs.energyfuels.5b02294 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 9. Modeling predictions and experimental data at the reactor centerline.

Figure 7. SH char conversion curves reacting in the FBR in 1/3 atm CO2, 1/3 atm H2O, and their mixture of 1/3 atm CO2 and 1/3 atm H2O, at (a) 1173 K, (b) 1223 K, and (c) 1273 K.

char−mixture rate equation based on the LH equation gave a reasonable prediction. Figure 10 shows the sensitivity analysis of different char− mixture rate equations on the char conversion. The results showed that the deviation of predicated carbon conversion was about 3% and the gas components were similar because the gas component was mainly controlled by the local instantaneous equilibrium. The results indicated that the simplified kinetic equations in the gasification simulation were also acceptable, because gasification was generally carried out at a high temperature and the inhibition of the char−H2O reaction and the char−CO2 reaction was not strong.

5. CONCLUSION Four chars produced by four raw coals range from lignite through bituminous to anthracite and two chars produced by two acid-washed coals were used to investigate the mechanism of the char gasification reaction mechanism with mixtures of

Figure 8. Comparison of experimental rates of the char−mixture reaction and calculated rates using eq 13.

G

DOI: 10.1021/acs.energyfuels.5b02294 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

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Figure 10. Sensitivity analysis of different char−mixture rate equations on the coal conversion.

CO2 and H2O using both the TGA and the FBR. It was found that CO2 and H2O competed for the same active sites and H2O could be adsorbed preferentially at the active sites. The char− H2O reaction was independent of the char−CO2 reaction, while the char−CO2 reaction was inhibited by preferentially adsorbed H2O. With the increase of the coal rank and temperature, the inhibition decreased. A new possible mechanism equation was proposed, which was appropriate to predict the char−mixture reaction rates. Future work will focus on the effect of the pressure on the inhibition between the CO2 adsorption and H2O adsorption and using the derived mechanism to simulate some three-dimensional gasifiers at elevated temperatures and pressures.

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AUTHOR INFORMATION

Corresponding Author

*Telephone/Fax: +86-27-87545526. E-mail: [email protected]. edu.cn. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation (51476065), the International Science & Technology Cooperation Program of China (2015DFA60410), and the National Major Scientific Instruments Development Project of China (2011YQ120039). The authors also thank Dr. Liu GuiSu for the initial development of the gasification model and the Analytical and Testing Center of Huazhong University of Science and Technology for providing the facilities for the experimental measurements.

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REFERENCES

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DOI: 10.1021/acs.energyfuels.5b02294 Energy Fuels XXXX, XXX, XXX−XXX