Coal Chemical Looping Gasification for Syngas Generation Using an

Dec 2, 2013 - Key Laboratory of Clean Chemical Processing Engineering in Universities of Shandong Province, Qingdao University of Science and Technolo...
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Coal Chemical Looping Gasification for Syngas Generation Using an Iron-Based Oxygen Carrier Qingjie Guo,*,† Yu Cheng,† Yongzhuo Liu,† Weihua Jia,† and Ho-Jung Ryu*,‡ †

Key Laboratory of Clean Chemical Processing Engineering in Universities of Shandong Province, Qingdao University of Science and Technology, Shandong Province, 266042, P. R. China ‡ Climate Change Technology Research Division, Korea Institute of Energy Research, Daejeon 305-343, Korea ABSTRACT: The chemical-looping gasification (CLG) of coal is a clean and effective technology for syngas generation. Sharing principles with chemical-looping combustion (CLC), CLG also uses oxygen carriers to transfer lattice oxygen to the fuel. Investigations into CLG with different O/C ratios are carried out in a fluidized bed reactor with steam used as the gasification− fluidization medium. The effect of the active component content of the oxygen carrier on the gas selectivity is performed, and reaction mechanisms between the Fe2O3 oxygen carrier and coal with steam as the gasification agent are discussed. Moreover, we also assessed the reactivity of the CaO-decorated iron-based oxygen carrier particles in multicycle reactions. The carbon conversion efficiency is increased from 55.74 to 81% with increasing O/C ratio, whereas the content of H2 first decreases and then increases. The addition of CaO can increase the carbon conversion efficiency and the gasification rate substantially and reduce the generation rate of H2S from 1.89 × 10−3 to 0.156 × 10−3 min−1. Furthermore, X-ray diffraction (XRD) images indicate that the CaO-decorated iron-based oxygen carrier particles were completely regenerated after six redox cycles. Finally, the peak fitting of gasification reaction rate curves is used to explore the reaction mechanism between coal char and the CaOdecorated iron-based oxygen carrier, indicating that the reactions in the CLG include three stages: the complex reactions involved an oxygen carrier, coal char, and steam; the gasification of coal char; and the reduction of Fe3O4 to FeO. The two-segment modified random pore model (MRPM) fits the experiment data well.

1. INTRODUCTION Synthesis gas, which is composed mainly of H2 and CO generated from natural gas or solid fuels, is an important raw material in various chemical industries, such as the ammonia and methanol manufacturing industries. The possibility of H2 as a future energy carrier in heating, electric power, and transportation sectors will cause an immense increase in H2 demand.1 At present, the steam reforming of natural gas, where the reforming occurs in reactor tubes packed with the catalyst, is the predominant method for synthesis gas production.2−4 In recent years, much attention has been paid to the generation of syngas using solid fuels, such as coal, biomass, and petcoke. Chemical-looping combustion (CLC), which is a novel combustion technology for gaseous fuels and solid fuels, has been extensively investigated.5−10 The chemical-looping reforming process (CLR) of methane, where the primary products are H2 and CO, has been proposed to be advantageous over the traditional methane reforming process.11 Similarly, chemicallooping gasification (CLG), which is investigated in this paper, shares the same basic principles with CLC. As illustrated in Figure 1, the oxygen carrier transfers oxygen as well as heat to the fuel in the fuel reactor. When inadequate oxygen is supplied to the solid fuel by the oxygen carrier, syngas consisting of CO and H2 will be generated. Otherwise, when oxygen supplied by the oxygen carrier is sufficient for the conversion of the solid fuel into CO2 and H2O, a considerable amount of heat will be produced for electricity generation. The oxygen carrier in this process can supply the desired heat of gasification, can serve as the catalyst for gasification, and can prevent the emission of SOx and NOx.12 Hatano et al.13 has conducted a low-temperature gasification of © 2013 American Chemical Society

Figure 1. Schematic illustration of the chemical-looping gasification process.

polyethylene using lattice oxygen from NiO, Fe2O3, and TiO2 in a laboratory-scale fluidization bed. They found that the polyethylene can generate hydrogen when steam was used as the gasification medium. Gnanapragasam et al.14 have proposed a coal/iron-based direct chemical looping process for hydrogen generation and CO2 separation. Recently, Liu et al.15 have developed a CLG system in which the CaSO4−CaS system is used as both the oxygen and the heat source for coal gasification. They demonstrated the reactivity characteristics and mechanism of complex reactions involving CaSO4 and coal in the steam/ CO2 atmosphere. Received: Revised: Accepted: Published: 78

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The oxygen carrier plays an important role in the chemical looping process. For the CLC process, the oxygen carrier should have the following properties:16 (i) sufficient oxygen transport capacity; (ii) favorable thermodynamics properties regarding the fuel conversion to CO2 and H2O; (iii) high reactivity toward reduction and oxidation reactions; (iv) resistance to attrition to minimize losses of elutriated solids; (v) negligible carbon deposition; (vi) good fluidization properties; (vii) limited cost; and (viii) environmentally friendly characteristics. With respect to the CLG process, its catalytic properties for solid fuels gasification and reaction enthalpy appear to be of importance. Different types of oxygen carriers, including Ni-, Cu-, Fe-, Mn-, and Co-based oxides, have been examined. Among these oxides, Ni-, Cu-, and Fe-oxides are most widely used in CLC systems. In particular, Fe2O3, which exhibits high reactivity, low cost, and environmental friendliness, is considered as a promising oxygen carrier for the CLC despite its weak redox characteristics and poor oxygen transport capacity.17 For the CLG process, Fe2O3 is a good potential oxygen carrier. The gasification of solid fuel (char) is considered to be the rate-limiting process for the direct solid-fuel CLC process. Therefore, the enhancement of gasification rate draws wide attention. Yang et al.18 investigated coal CLC and showed that the addition of Ca and K elements to coal can reduce the gasification temperature. Yu et al.19 also reported that the addition of alkali metals to Fe2O3 can increase the reaction rate of solid fuels. Gu et al.20 found that a K2CO3-decorated iron-ore oxygen carrier exhibited stable catalysis for coal CLC. Calcium oxide21 exhibited not only catalytic activity but also the ability to capture sulfur and CO2. A compound oxygen carrier coupled with a catalytic component for coal CLG will overcome the disadvantage of the non-reusability of the catalysts and the required extra energy for the endothermic coal gasification reaction in the traditional coal gasification processes. However, few investigations into compound oxygen carriers coupled with a catalytic component for CLG have been reported in the literature. In this test, the performance of iron-based oxygen carriers was evaluated. The effects of the O/C ratio, multiredox reactions, and CaO-decorated oxygen carriers on the coal CLG process were investigated by fluidized-bed experiments. Furthermore, the mechanisms of the CaO-decorated Fe2O3 oxygen carrier in the coal CLG process were also examined.

Figure 2. Main routes of the chemical looping gasification process.

The reduction of Fe2O3 with coal involves multiple reactions. When the coal is fed into the fuel reactor, gasification occurs: (a) Gasification reactions C + H 2O → CO + H 2 θ ΔH1193

= +135.682 kJ/mol

C + CO2 → 2CO θ ΔH1193

(water−gas reaction)

(Boudouard reaction)

= +168.623 kJ/mol

CO + H 2O → CO2 + 2H 2 θ ΔH1193

(1)

(2)

(water‐gas shift reaction)

= −32.941 kJ/mol

(3)

Simultaneously, syngas is partially oxidized by the Fe2O3 oxygen carrier particles to CO2 and steam: (b) Combustion reactions H 2 + 3Fe2O3 → 2Fe3O4 + H 2O θ ΔH1193 = −8.254 kJ/mol

(4)

H 2 + Fe3O4 → 3FeO + H 2O θ ΔH1193 = +46.088 kJ/mol

(5)

CO + 3Fe2O3 → 2Fe3O4 + CO2

2. EXPERIMENTAL SECTION Typically, complex and competing reactions in the fuel reactor involved the coal gasification reactions, the reactions between coal−derived syngas and oxygen carrier, and the reaction between coal char and oxygen carrier. Mendiara et al.22 demonstrated that the solid−solid reactions between Fe2O3 and char should be considered in a fixed bed but are not of relevance in a fluidized bed. Thus, the solid−solid reactions were ignored in the CLG system. Subsequently, the reduced oxygen carrier is oxidized and regenerated with air as a reaction agent in the air reactor. The high exothermic oxidization reaction can provide heat for the endothermic coal gasification reactions. As shown in Figure 2, the complex and competing reactions involved in the system can be generally divided into three sections: (a) the coal gasification reaction, (b) coal-derived syngas combustion reactions, and (c) the regeneration of the asreduced oxygen carrier. The following reactions are expected to occur in the fuel reactor.

θ ΔH1193 = −41.195 kJ/mol

(6)

CO + Fe3O4 → 3FeO + CO2 θ ΔH1193 = +13.147 kJ/mol

(7)

Chemical reaction on CaO occurs as follows: CaO + H 2S → CaS + H 2O θ ΔH1193 = −14.867 kJ/mol

(8)

(c) Regeneration reactions 4Fe3O4 + O2 → 6Fe2O3 θ ΔH1193 = −481.233 kJ/mol

4FeO + O2 → 2Fe2O3

(9) θ ΔH1193 = −556.89 kJ/mol

(10) 79

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Table 1. Proximate and Ultimate Analysis of Coal proximate analysis (%) (mass, ad)

ultimate analysis (%) (mass, ad)

M

V

FC

A

C

H

O

N

S

LHV (MJ·kg−1)

3.38

38.27

53.55

4.80

74.00

4.96

9.13

1.29

2.44

29.92

2.1. Material Preparation. The active metal oxides are usually supported on different inert materials, such as Al2O3, MgAl2O4, SiO2, TiO2, ZrO2 or stabilized ZrO2, bentonite, sepiolite, etc. Given the reactivity, fragmentation, and avoidance of the interaction between Fe2O3 and inert supports, Al2O3 is recognized as one of the most promising supports. Two types of iron-based oxygen carriers, Fe2O3/Al2O3 and Fe2O3−CaO/ Al2O3, were prepared by the mechanical mixing method. The mass ratio of Fe2O3 to Al2O3 was 4:6 for the Fe2O3/Al2O3 oxygen carrier, whereas the Fe2O3 to Al2O3 to CaO ratio was 4:6:1 for the Fe2O3−CaO/Al2O3 oxygen carrier. After being calcined in air atmosphere at 1233 K for 3 h, the compound was crushed, sieved into 225−450 μm, and used as the as-received iron-based oxygen carrier. Beisu bituminous coal is selected as the fuel, whose proximate analysis and ultimate analysis results are listed in Table 1. The coal particles are sieved into the size range 225−450 μm for use. 2.2. Experimental Setup. The schematic diagram of the tubular fluidized bed reactor (fabricated with 1Cr18Ni9Ti stainless steel) is illustrated in Figure 3. The system consists of

chromatograph (PerkinElmer) equipped with a thermal conductivity detector (TCD) to detect CO2, CO, H2, and CH4 with argon as the carrier gas. The other stream is measured with a SP-3420A gas chromatograph (Beijing Beifen-Ruili Analytical Instrument (Group)) to detect the concentrations of H2S (dry basis) with hydrogen and air as the ignition gases. The surface morphological features of the solid residues and the fresh samples are examined using a scanning electron microscope (JEOL JSM-6700F, Japan) operated at an accelerating potential of 8.0 kV. The crystalline structures of the Fe2O3/Al2O3 oxygen carrier are analyzed with an X-ray diffractometer (Rigaku D/MAX-2500, Japan) equipped with a Cu Kα radiation source; samples are scanned over the 2θ range of 5−80° with a step of 0.02°/s. All X-ray diffraction patterns are analyzed using the Jade 5.0 software package (Materials Data, Inc. (MDI)). 2.3. Experimental Procedure. Experiments were conducted in the fluidized bed with 6 g of pulverized coal samples and 110 g of iron-based oxygen carrier (here O/C = 1) as bed materials. To determine the effect of O/C ratio, four O/C ratios, 0:1, 0.5:1, 1:1 ,and 2:1, were studied. Herein, the O/C ratio is defined as the mole ratio of the mole content of oxygen in the oxygen carrier, which can be transported to the coal, to the mole content of fixed carbon in the coal. In the present investigation, the Fe2O3 was just considered to be reduced to Fe3O4; the mole content of oxygen in 1 mol of Fe2O3 is 1/3. Chemically pure Al2O3 powder was used instead of an oxygen carrier in the blank experiments. The particle size of all the bed materials was in the range 225−450 μm. The mixture of coal and oxygen carrier was fed into the reactor in advance. After the reactor was heated to 920 °C under the argon atmosphere at atmospheric pressure, the steam was introduced into the reactor at a flow rate of 2.5 g/min to reach the predetermined steam partial pressures. Meanwhile, the experiment was timed. Here, the volatile matter was considered to release completely before the feeding of steam. All reaction durations were 100 min. As soon as the reaction finished, the heater was shut down, and the oxygen carrier particles were cooled down in the argon flow to room temperature and then collected for further analysis. To examine the cycle characteristic of the oxygen carrier, after 100 min of reaction of the oxygen carrier and coal char, we switched the steam to air and raised the temperature to 960 °C, keeping one redox cycle for 30 min. The flow rate of argon was kept at 0.3 L/min to fluidize the bed materials. 2.4. Data Evaluation. (1). Carbon Conversion, XC. The carbon conversion is calculated according to the concentrations of carbonaceous gases (CO, CO2, and CH4) and the total amount of carbon contained in the coal:

Figure 3. Schematic layout of the fluidization bed laboratory setup.

a reactor, a coal feeding unit, a temperature control unit, a gas feeding system, a steam generator unit, a steam cooler, a wet-type gas flow meter, and a gas analysis system. Also, the reactor in the electric heater is 700 mm in height and 50 mm in diameter equipped with a porous steel plate. The steam generator is composed of a constant flow pump and a cast aluminum heater, and the steam mass flow is controlled precisely through adjustments to the flow rate of deionized water. Air is provided by an air pump. The flow rate of the argon gas is measured by mass flow controller. The hot product-gas stream from the fuel reactor is successively introduced into a cyclone separator and a cooler. Then, the steam is condensed within the cooler, and finally, the product gas is sampled using gas bags for offline analysis. The gas product is divided into two streams to determine the product concentrations (dry basis) by gas chromatography (GC). One gas stream is measured with a Clarus 500 gas

t

XC =

∫0 nout × (XCO + XCO2 + XCH4) dt nC,Fuel

(11)

where XH2, XCO, and XCO2 are the molar fractions of H2, CO, and CO2 in the outlet gas from the reactor, respectively, nout is the 80

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total molar amount of gas outlet from reactor, and nC,Fuel is the molar amount of fixed carbon contained in coal. (2). Gas Composition, Ci. The gas composition is the ratio of the carbon converted to H2, CO, and CO2 to the carbon introduced into the fuel reactor; it is given by t

Ci =

∫0 nout × Xi dt nC,Fuel

(i = H 2 , CO, CO2 )

(12)

(3). Selectivity of Carbonaceous Gases, Si. The syngas selectivity formation SCO refers to the fraction of CO in the carbon-containing species CO2 and CO leaving the fuel reactor, which is defined as t

Si =

∫0 nout × Xi dt t

∫0 nout × (XCO + XCO2) dt

(i = CO, CO2 ) (13)

(4). Syngas Yield, Syn. The syngas yield is the fraction of the effective gas (CO, H2, and CH4) in the total amount of gas during a certain period; it is expressed as t

Syn =

∫t i (VCO + VH2 + VCH4) dt 0

Vtotal

× 100%

(14)

where VCO, VH2, and VCH4 are the volume of CO, H2, and CH4 in the outlet gas from the reactor, respectively. Vtotal is the total volume of generated gas from the reactor.

Figure 4. Variation of the composition of the generated gas as a function of time without (a) and with (b) Fe2O3 as an oxygen carrier.

3. RESULTS AND DISCUSSION 3.1. Effect of Oxygen Carrier on Syngas Generation. The effects of the oxygen carrier on product gas composition, syngas yield, and coal conversion in the CLG process were evaluated. Figure 4 illustrated the experimental results of the coal gasification in the cases without and with oxygen carrier at 920 °C with O/C mole ratio 1:1. As shown in Figure 4a, in the absence of an oxygen carrier, the concentrations of CO2, CO, and H2 reached maxima of 0.93, 0.605, and 1.957% at 9 min, respectively, and then decreased. The main reaction stage was maintained for approximately 40 min. During 100 min, the carbon conversion rate reached 55.73%, and syngas accounted for approximately 70.8% of all the gas generated. An experiment with Fe2O3 as the oxygen carrier was also conducted. As illustrated in Figure 4b, the carbon conversion rate and the syngas content reached 64.163 and 80.623%, respectively; such values were both higher than those achieved in the absence of the oxygen carrier. The concentrations of CO2, CO, and H2 exhibited similar trends with that in the absence of the Fe2O3 oxygen carrier. However, the relative concentration of each gas including CO2, CO, and H2 showed significant differences when the Fe2O3 oxygen carrier was employed. The maximum concentration of CO2 increased and achieved its maximum value earlier, at 7 min, in the presence of the oxygen carrier, whereas the maximum concentration of CO decreased. Conversely, the required time to approach the maximum concentration of H2 was delayed at the presence of the oxygen carrier, although the maximum concentration of H2 was greater. The peak of H2 appeared at 12 min, 3 min later than its appearance in the absence of the Fe2O3 oxygen carrier. Despite the fact that the second peak of CO2 and H2 appeared in 20−50 min, the main reaction finished within the initial 20 min. These results indicated that the presence of the Fe2O3 oxygen carrier

Figure 5. Effect of O/C ratios on the gasification product.

enhanced the chemical reaction rate and facilitated the syngas generation, which would positively influence the coal conversion. The difference between two conditions can be explained by the involved reactions. In Figure 4a, the increase and subsequent decrease in the concentrations of CO2, CO, and H2 were mainly caused by the gasification of the coal char by the steam and the consumption of coal char with the time. When Fe2O3 was added into the coal, the reaction between gaseous products (mainly CO, H 2 , and CH 4 ) and the oxygen carrier occurred simultaneously. Due to the higher reaction rate of H2 with the oxygen carrier than CO, the relative composition of CO2 except for the generated steam reached its maximum value quickly. Subsequently, the composition of H2 became to increase with the depletion of the oxygen carrier. Moreover, the water-gas shift reaction R3 in conjunction with the consumption of H2 by the 81

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Figure 6. Effect of O/C ratios on carbon conversion and syngas production.

Figure 8. Effect of CaO on syngas composition during the process of coal gasification: (a) syngas composition of different Fe2O3 oxygen carriers; (b) concentration of H2S.

Figure 7. Variation of the concentration of syngas as a function of time with a CaO-decorated Fe2O3 oxygen carrier.

Fe2O3 oxygen carrier would provide an additional pathway for coal conversion. In addition, the decrease in the CO concentration improved the carbon conversion by reaction R1. Because the rapid gasification reaction partially accelerated the CLC reaction during the initial 20 min, the concentrations of H2O and CO2 increased during this period. It resulted in a second, less-intense peak during the 20−50 min period, and the reaction between Fe3O4 and the coal enhanced the gasification reaction further. 3.2. Effect of the O/C Ratios on Gas Selectivity and Coal Conversion. To investigate the influence of the O/C ratio on the product gas composition, the yield, and the coal conversion, experiments with four O/C ratios (0:1, 0.5:1, 1:1, and 2:1) were performed. Figure 5 revealed the effect of the O/C ratio ratios on the concentration of H2, the CO2 selectivity (SCO2), and the CO selectivity (SCO). The CO selectivity decreased from 17.1% at 0:1 (O/C) to 1.7% at 2:1 (O/C). However, the SCO2 increased from 82.9 to 98.3%. The concentration of H2 decreased from 72.01% at 0:1 (O/C) to 62.57% at 0.5:1 (O/C) and then increased to 73.16% at 2:1 (O/C). Previous researchers23 have found that an increase in the O2 supply resulted in a decrease in the H2 and CO contents and an increase in the CO2 concentration. The contradicting data, concerning the increase in the H2 content from 0.5:1 to 2:1 (O/C), resulted from the enhanced water-gas

Figure 9. Effect of redox times on syngas composition.

shift reaction. The decreasing content of CO was consistent with the increase of CO2 due to the water-gas shift reaction. Figure 6 shows the effect of the O/C ratio ratios on the carbon conversion and syngas production. The fact that carbon conversion increased with the increasing O/C ratio was mainly due to the promotion of the reaction between the oxygen carrier and the syngas from coal gasification, while the trend that the concentration of syngas first increased and then decreased was most likely due to the change in the H2 and CO2 contents. 3.3. Effect of a CaO-Decorated Iron-Based Oxygen Carrier on Syngas Generation. Because of the catalytic and 82

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Figure 10. XRD patterns of iron-based oxygen carriers after six redox reaction cycles.

sulfur-capturing properties of CaO, the iron-based oxygen carrier was loaded with 10% CaO to boost its performance. The effect of CaO on the concentration of the generated gas, including sulfur species, as well as on the gas selectivity was evaluated in these experiments. Figure 7 showed the concentrations of outlet-gas as a function of time when Fe2O3−CaO/Al2O3 was used as an oxygen carrier. The concentration of each gas for the CaO-decorated iron-based oxygen carrier exhibited trends similar to those for the original oxygen carrier, as illustrated in Figure 4b. As displayed in Figure 7, all of the gas concentrations were higher than those achieved with the original oxygen carrier. The concentration of hydrogen increased to as high as 8.2% from 3.5%. In addition, the main reaction time was shortened to the initial 18 min. We concluded that the CaO-decorated oxygen carrier can accelerate the coal gasification more significantly than the original oxygen carrier. Due to the concentration of hydrogen between 18 and 40 min, the reaction of Fe3O4 with char and H2O was highly promoted, which will be discussed later. A significant change in the carbon conversion, from 60.5% with the CaO-decorated iron-based oxygen carrier to 81.2% with the original oxygen carrier, was observed. Due to the catalytic properties of CaO, the CaOdecorated iron-based oxygen carrier significantly improved the carbon conversion in the fast reaction stage. Figure 8 revealed the effect of the CaO-decorated oxygen carrier on the gas composition. The gas compositions without the oxygen carrier and that with the Fe2O3 oxygen carrier are also displayed for comparison in Figure 8. In the absence of the oxygen carrier, the volume concentration of H2 was 72.15%, whereas that of CO was 4.65%. When the Fe2O3 oxygen carrier was added into the coal gasification system, the concentration of H2 and CO decreased while that of CO2 increased. Obviously, the oxygen carrier consumed part of the generated H2 and CO. When the Fe2O3 oxygen carrier was decorated with CaO, the volume concentration of H2 and the CO content significantly increased. The volume concentration of H2 was even higher than that observed without an oxygen carrier. The catalytic property of CaO can explain this phenomenon. With the CaO added, the carbon conversion has increased to 81.2% from 60.5%. Therefore, the total volume of generated gas, the volume concentration of H2, and the CO content significantly increased. As shown in Figure 8b, the addition of CaO suppressed the H2S

Figure 11. Peak fitting curves for the reaction rate data: □, original data points; , fitting curve; ---, splitting peak curve.

Figure 12. Five fitting models of experimental data as a function of carbon conversion.

generation rate from 1.89 × 10−3 to 0.156 × 10−3 min−1. In conclusion, the added CaO can reduce sulfide emissions. 83

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Table 2. Reaction Kinetic Equations model

equations

model

equations

uniform reaction (UR)

dx = K (1 − x) dt

random pore model (RPM)

dx = A 0(1 − x) 1 − Ψ|n(1 − x) dt

integrated model (IM)

dx = K (1 − x)m dt

modified volumetric model (MVM)

dx = a1/ bb[− ln(1 − x)](b − 1)/ b (1 − x) dt

modified random pore model (MRPM)

dx = A 0 exp[x(c − x)](1 − x) 1 − Ψ|n(1 − x) dt

Table 3. Parameters of Shenmu Char Fitted by Five Kinetics Models uniform reaction 0.0147

integrated model R2

K

K

0.6532 0.01739 modified volumetric model

random pore model

m

R2

1.6016

0.8731

Ψ

A0

0.0136 −0.5265 modified random pore model

R2 0.8083

a

b

R2

A0

Ψ

C

R2

0.0087

1.0000

0.8112

0.0068

100.5267

−2.5991

0.9739

based oxygen carrier was the Fe2O3 → Fe3O4 process during the chemical looping combustion and gasification processes. After a complete contact with air, the oxygen carrier was regenerated completely to Fe2O3. The iron-based oxygen carrier exhibited good cycling performance and is therefore suitable for use in the chemical looping gasification process for coal. 3.5. Reaction Mechanism. To explore the previously discussed complex reactions, we conducted experiments with and without two kinds of oxygen carriers, iron-based oxygen carrier and CaO-decorated iron-based oxygen carrier, in a fluidized bed as well as experiments. Figure 11 shows the variations in the carbon conversion rate with time and its peak fitting curves. Figure 11a indicates one reaction peak, while Figures 11b and c show two reaction peaks, corresponding to the use of the iron-based oxygen carrier and the CaO-decorated ironbased oxygen carrier, respectively. Given the reactions between the Fe2O3 and coal, the reaction rate curve might be the superposition of several reaction peaks that correspond to different conditions with time. Consequently, a peak fitting method was used to fit the complex reaction rate curve with Gaussian function as the peak function type. As shown in Figure 11a, the gasification rate of coal reached its maximum at 20 min. Figure 11b highlights two peaks, at 5 and 20 min, with an obvious increase in the reaction rate. The fitting curves labeled as II appeared separately in Figure 11a and in Figure 11b at the same time of 20 min. It can be concluded that the gasification reaction of coal with water (C + H2O → CO, CO2, H2) occurred at 20 min. In contrast, the fitting curve labeled as I in Figure 11b might be the CLC reaction of Fe2O3 with syngas, demonstrating that the oxygen carrier can promote the gasification reaction. The rate of the gasification reaction shown in Figure 11c increased sharply to 2.5 times greater than that shown in Figure 11a, and the rapid reaction stage was shortened from 30 min in Figure 11b to 17 min. The reaction rate of the consumption of coal in the initial reaction stage apparently decreased. A comparison of these results with those in Figure 11b reveals that the border between the fitting curves I (CLC) and II (CLG) disappeared and that they combined into a more intense CLG reaction peak. Moreover, another curve fitting in Figure 11c means that CaO facilitates the CLG reaction and can promote the conversion of Fe3O4 into FeO. These results indicate that the complex reactions of CLG include three stages: the reactions

Table 4. Parameters for Shenmu Char Fitted by the TwoSegment MRPM A stage B stage

A0

Ψ

C

R2

0.0069 0.0022

70.7586 306.9646

−7.5363 −3.4895

0.9712 0.9646

Figure 13. Two-segment MRPM fitting models of experimental data.

3.4. Multiredox Reactions of the CaO-Decorated IronBased Oxygen Carrier. Multiredox experiments were conducted to investigate the stability of the CaO-decorated iron-based oxygen carrier. Figure 9 showed the effect of cycling on the concentration of syngas during the initial 40 min after six redox experiments. The activity of the iron-based oxygen carrier was basically stable during the six cycles. The concentration of CO stabilized at 5%, and the concentration of CO2 slightly decreased from 30.3 to 25%; correspondingly, the concentration of H2 fluctuated at approximately 65% and slightly increased from 64.77 to 70.15% during the six cycles, which was due to the decreased concentration of CO2. The XRD results of the fresh oxygen carriers (see Figure 10) depicted the composition of reduced oxygen carriers and the oxidation oxygen carriers after six redox reaction cycles. The phase of the iron-based oxygen carrier was complex, due to the mixed valency of the iron in Fe2O3 induced by the reduction reaction. After the reduction reaction of the iron-based oxygen carrier, Fe2O3 was mainly converted to Fe3O4 and a small amount of FeO without Fe observed. The main conversion of the iron84

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Natural Science Funds for Distinguished Young Scholar in Shandong Province (JQ200904) is also greatly appreciated.

involving Fe2O3, coal char, and steam; the gasification of coal char; and the reaction of Fe3O4 to FeO. Five models, whose kinetic equation is listed in Table 2, including uniform reaction (UR), integrated model (IM), modified volumetric model (MVM), random pore model (RPM), and modified random pore model (MRPM) are employed for fitting the reaction steam-char gasification with CaO-decorated iron-based oxygen carrier. The fitting curves and the corresponding parameters of five reaction models are illustrated in Figure 12 and Table 3, respectively. The fitting results demonstrate that the MRPM model fitted the experimental data well. Considering the complex reactions occurring in the process, the fitting curve of reaction rate versus carbon conversion is divided into two segments. As illustrated in Table 44 and Figure 13, the segmented MRPM model is more suitable to model the experimental data. As the reactions proceed, the pores of coal char develope and overlap, which causes a decrease of specific surface area of coal char. Therefore, when carbon conversion exceeds 0.39, the structure parameter ψ increases significantly. The corresponding time to carbon conversion 0.39 is 20 min, when the fitting curves I and II in Figure 11 has already finished. When XC < 0.39, the reactions involving Fe2O3, coal char, and steam and the gasification of coal char occur with the existence of Fe2O3 and the catalysis of CaO, while the reduction Fe3O4 → FeO takes place with low reaction rate.

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4. CONCLUSIONS On the basis of experiments in a fluidized bed reactor, the following conclusions were obtained: (1) The presence of the Fe2O3 oxygen carrier increased the chemical reaction rate and enhanced the generation of syngas. The optimal O/C ratio for the chemical looping process for syngas generation might be 1:1. (2) The CaO-decorated iron-based oxygen carrier efficiently improved the carbon conversion in the fast reaction stage and shortened the time required to achieve the same conversion. The addition of CaO into the oxygen carrier can reduce sulfide emissions. The Fe2O3 oxygen carriers exhibited excellent performance in the chemical-looping gasification of coal after multiple cycles. (3) The reactions in the CLG are complex and include three stages: the complex reactions involving an oxygen carrier, coal char, and steam; the gasification of coal char; and the reaction of Fe3O4 to FeO. The iron-based oxygen carrier participated directly in complex reactions in the fuel reactor of CLG, while CaO was the catalytic effect on the coal gasification reaction. The two-segment MRPM model fitted the experimental data well.

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was conducted under the framework of Research and Development Program of the Korea Institute of Energy Research (KIER) (B3-2421-06). The financial support from the Natural Science Foundation of China (21276129, 20876079) and 85

dx.doi.org/10.1021/ie401568x | Ind. Eng. Chem. Res. 2014, 53, 78−86

Industrial & Engineering Chemistry Research

Article

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