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Mechanism Investigation of Enhancing Reaction Performance with CaSO4/Fe2O3 Oxygen Carrier in Chemical-Looping Combustion of Coal Tao Song, Min Zheng, Laihong Shen,* Tao Zhang, Xin Niu, and Jun Xiao Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, School of Energy and Environment, Southeast University, Nanjing 210096, China ABSTRACT: CaSO4 as a promising oxygen carrier has a larger oxygen transport capacity. However, the formation of CaO in the CaSO4 reduction process by side reactions can cause sulfur species evolution and decrease its reactivity. An innovative method was investigated by means of adding hematite together with the CaSO4 to be combined oxygen carriers in CLC of coal. The objective was to decrease sulfur species evolution and increase the coal conversion. Experiments were performed in a batch fluidized-bed reactor at atmospheric pressure. Ten reduction/oxidation cycles confirmed the occurrences of side reactions toward sulfur species evolution, although the CaSO4 oxygen carrier showed an increasing reactivity in the initial cycles. With the addition of hematite, the gaseous sulfur species evolution was remarkably decreased in both reduction and oxidation processes during the cycles, while the coal conversion and CO2 capture were enhanced. The extensive physical and chemical properties of the oxygen carrier were characterized by X-ray diffractometer (XRD) analysis and scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (SEM-EDX) analysis to detect the mechanism. It was not the desulfurization capacity of iron oxide that lowered sulfur species emission but the suppression of the side reactions by adding hematite.

1. INTRODUCTION The global warming due to the emission of greenhouse gases especially for CO2 has been widely acknowledged. So far, carbon capture and sequestration (CCS) is believed to be the most efficient approach to restrict anthropogenic CO 2 emissions. One of the major concerns regarding CO2 capture technologies is its high cost, including the primary investment and operation cost. The solution is to improve the efficiency of current capture technologies and develop new cost-effective and environmentally friendly technologies. Among the different capture concepts, there is an increasing interest in the chemical-looping combustion (CLC) as a way to produce relatively pure CO2 that can be readily captured. The first tentative CLC design based on the circulating fluidized bed principle was proposed by Lyngfelt et al.,1 and it develops a well-accepted approach to conduct a chemical looping process in two fluidized-bed reactors (fuel reactor and air reactor) connected by solid transportation lines. Between these two reactors oxygen is transported by an oxygen carrier, thereby avoiding direct contact between fuel and air. In this way, the nitrogen from the air leaves the system from the air reactor, whereas the flue gas from the fuel reactor consists of only CO2 and water. After water condensation, almost pure CO2 can be obtained, and then compressed into liquid for storage. A schematic picture of the CLC process is shown in Figure 1. With the present development of CLC technology, more investigations have been turning to the use of solid fuels directly applicable to CLC. That is to introduce the coal directly to the fuel reactor where the oxygen carrier is reduced by the fuel, thus avoiding separate gasification and separation steps. When coal is pneumatically conveyed to the fuel reactor, coal particle gasification occurs, which is a very complex process. It consists of the devolatilization of coal particles, © 2013 American Chemical Society

Figure 1. Schematic picture of the CLC process.

reaction 1 (R1), and the gasification of the resultant char by R2 and R3. Meanwhile, a majority of homogeneous and heterogeneous chemistry is taking place, such as the water/ gas shift reaction R4, and so on. The gasification products are oxidized by the oxygen carriers with respect to the complete formation of CO2 and steam, R5. fast pyrolysis

Coal ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ Volatile matter + Char

(R1)

char(C) + H2O → CO + H2

(R2)

Received: Revised: Accepted: Published: 4059

January 7, 2013 February 21, 2013 February 22, 2013 February 22, 2013 dx.doi.org/10.1021/ie400070e | Ind. Eng. Chem. Res. 2013, 52, 4059−4071

Industrial & Engineering Chemistry Research

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char(C) + CO2 → 2CO

(R3)

CaS + 3H2O → CaO + SO2 + 3H2

(R11)

CO + H2O → CO2 + H2

(R4)

CaS + 3CO2 → CaO + SO2 + 3CO

(R12)

3 O2 → CaO + SO2 2

(R13)

CaS +

nMexOy + H2 , CO , Volatile matter → nMexOy − 1 + CO2 + H2O

(R5)

1 O2 + SO2 (R14) 2 To minimize the SO2 release is one of the significant issues for the application of CaSO4 oxygen carrier in CLC process. In order to minimize SO2 release, according to the previous study,22 the optimum temperature was determined to be 900− 950 °C in the fuel reactor and 1050−1150 °C in the air reactor. However, a high air reactor temperature pushes the sintering process of oxygen carrier, which can cause the deactivation of oxygen carriers. It is well-known that the mixed complex oxygen carriers may sometimes provide better properties than those of individual oxygen carrier.25,26 Also, it can resolve many of the shortcomings associated with the conventional oxygen carriers.26 In this work, chemical looping combustion with the CaSO4 oxygen carrier of coal was investigated in a batch fluidizedbed reactor at atmospheric pressure. This work was to experimentally evaluate the coal conversion performance and the reactivity of the CaSO4 oxygen carrier during multiple cycles. Also, with respect to checking for the eventual cooperative effects, a little hematite was mechanically mixed with the CaSO4 oxygen carrier into the bed. The effect of the blending ratio and multiple cycles on the product gas evolution and sulfur species evolution were investigated. Further, to explore the reaction mechanism, some technical analysis involving X-ray diffractometer (XRD) analysis and scanning electron microscopy (SEM) equipped with energy dispersive Xray spectroscopy (SEM-EDX) analysis was used in this work. CaSO4 → CaO +

Several CLC units for solid fuels using the configuration composed of two interconnected fluidized-bed reactors has been built and operated by Chalmers University of Technology of Sweden,2,3 ICB-CSIC of Spain,4,5 and Southeast University of China.6,7 Recently, Markström and Lyngfelt8,9 designed and operated the first 100 kWth CLC unit for coal. At Southeast University of China, a first pilot-scale unit for pressurized CLC of coal was constructed and operated.10 The 1 MW CLC plant was demonstrated in TU Darmstadt.11 Meanwhile some novel technologies to fulfill the CLC process by means of a moving bed has been developed in the National Taiwan University of Science and Technology,12 and The Ohio State University.13 Also, a rotary-bed reactor for CLC was designed and operated at the Massachusetts Institute of Technology.14 Nevertheless, the oxygen carrier is significant for fulfilling the CLC process and providing a reaction scheme to balance the endothermic gasification reactions. A high chemical reactivity in the two stages of reduction and oxidation of the oxygen carrier is needed. Except for this aspect, the manufacturing cost of oxygen carrier is another important factor affecting CLC industrial application. A calcium-based oxygen carrier, CaSO4, has been attractive as a potential oxygen carrier for CLC of solid fuels. Alstom Power Co. Ltd.15,16 proposed its hybrid combustion−gasification chemical looping system. Wang and Anthony17 also proposed a CO2-based gasification-coupled chemical looping process for the clean fuel combustion with the CaSO4 oxygen carrier. Ding et al.18 investigated a bindersupported CaSO4 oxygen carrier with CH4 as a fuel and obtained an optimal extrusion condition. In our research team, the reaction mechanism of CaSO4 as oxygen carrier using coal, syngas, CH4 as fuel was widely investigated.19−23 The redox system CaSO4/CaS for CLC has thermodynamic limitations in the conversion of H2 and CO.19,22 A detailed review for using CaSO4 as an oxygen carrier was given by Guo et al.24 When coal is gasified by steam, a series of reactions occurs: CO +

1 1 CaSO4 → CaS + CO2 4 4

(R6)

H2 +

1 1 CaSO4 → CaS + H2O 4 4

(R7)

CH4 + CaSO4 → CaS + CO2 + H2O

2. EXPERIMENTAL SECTION 2.1. Materials. An anhydrite ore was used as oxygen carrier particles supplied by Nanjing Anhydrite Ore Co., Ltd. The particles were sieved by pulverizer with a particle diameter of 0.3−0.5 mm. The natural anhydrite ore was composed largely of CaSO4 and a small proportion of other impurities, as presented in Table 1. The CaSO4 fraction in the natural anhydrite ore was 94.38 wt %. The specific density of the natural anhydrite ore was 2.9 g/cm3, and the bulk density was 1.509 g/cm3. The mechanically mixed samples were prepared by mixing anhydrite particles with the particles of hematite. The hematite particles were used as additives. The hematite consisted of 78 wt % Fe2O3 and 16.1 wt % SiO2 with a

(R8)

Table 1. Composition of Natural Anhydrite Ore (wt %)

A drawback to the use of CaSO4 as an oxygen carrier is the possible formation of CaO in the CaSO4 reduction process in the fuel reactor by side reactions of R9−R12, which can cause SO2 evolution. Also, in the air reactor, reactions of R13 and R14 are favored. Therefore, if CaO is formed either in the reduction process or the oxidation process, the oxygen transport capacity of CaSO4 oxygen carrier is eliminated, thus requiring the addition of fresh particles into the CLC system to replace the spent particles. CO + CaSO4 → CaO + SO2 + CO2

(R9)

H2 + CaSO4 → CaO + SO2 + H2O

(R10) 4060

compositions

value

CaSO4 MgO SiO2 Fe2O3 Al2O3 TiO2 K2O Na2O P2O5 other

94.38 1.68 0.63 0.094 0.12 0.014 0.044 0.055 0.028 2.955

dx.doi.org/10.1021/ie400070e | Ind. Eng. Chem. Res. 2013, 52, 4059−4071


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(MFC, Beijing Sevenstar Huachuang Electronics Co., Ltd.). The steam generator was composed of a TBP-50A constant flow-type pump and a cast aluminum heater, and the steam mass flow was controlled precisely by adjusting the value of deionized water. During the reduction process, after the temperature reached the desired reduction temperature of 900 °C and was kept stable, a certain amount of oxygen carrier particles was added to the bed under the N2 atmosphere with a flow rate of 5 L/min. The gas velocity at 900 °C was 0.42 m/s. Meanwhile, steam preheated at 170 °C was supplied to the bottom of the fluidized bed as a gasifying agent for the coal. The steam concentration introduced to the reactor was 35%. The pressure fluctuation could be observed according to the pressure measurement. Under the flow of N2 and steam, the particles in the reactor were well fluidized. When the desired temperature was stable again, the coal particles were quickly introduced to the bed under the pressurized effect of the instantaneous nitrogen supplement. Then, the reduction process began. All of the reduction reaction duration was set at 20 min. After that, the steam supplement was stopped. The product gases were blown away under the N2 flow for 5 min. After the reduction process was finished following the above procedure, the gas was switched to a gas mixture of 5% O2 in N2. The total flow rate was 1 L/min. During the oxidation process, the exothermic nature of the oxidation reaction means that there will be release of heat and therefore a subsequent temperature rise. To limit this temperature increase, a gas mixture with 5% O2 in N2 was used instead of air. Thus, large temperature increases were avoided because there is no possibility to cool the reactor in the present setup. The oxidation temperature for oxygen carrier regeneration was 950 °C. As soon as the reaction finished, the gas was switched to nitrogen to blow, and the heater was shut down. The oxygen carrier particles were cooled in the nitrogen flow. All of the flue gases leaving the reactor were directed to a drying tube filled with CaCl2 to remove the water in the exiting gas stream. Within the cooler, the steam could be condensed and removed without the adsorption of acid gas, and then be sampled by gas bags for offline analysis during each experiment. A fraction of the off gases was then pumped through an infrared analyzer (NGA 2000, Emerson) measuring 0−100 mol % CO, 0−100 mol % CO2, 0−100 mol % CH4, 0−25 mol % O2, and a Hydros 100 analyzer of 0−50 mol % H2. An Ecom-J2KN flue gas analyzer (RBR Company, Germany) was used to detect the dry concentrations of SO2 and H2S (0−10000 ppmv). An X-ray diffractometer (XRD, Rigaku Co.) using Cu Kα radiation was employed to analyze the oxygen carrier samples. The samples were scanned in a step-scan mode with a step size of 0.02° over the angular 2θ range of 10−90°. Also, the element distribution was characterized by an SEM-EDX system. 2.3. Experimental Conditions. The experimental conditions and procedures for these tests are described as follows. 2.3.1. CLC with CaSO4 Oxygen Carrier Particles. During the test, a sample of 50 g of CaSO4 oxygen carrier particles and another sample of 1.5 g of coal particles was introduced to the system following the above procedures. Since the molar amounts of carbon, hydrogen, and oxygen in 1 kg of Shenhua coal are 53.6, 39.1, and 12.9, respectively, the formula C53.6H39.1O12.9 is introduced to describe the given amount of coal directly. Then, if 1 kg Shenhua coal is fed into the fuel reactor, the natural anhydrite ore should be no less than 4.65 kg. In this study, the mass of coal was set as 1.5 g for all runs. Thus, to burn the coal completely, the theoretical amount of

particle diameter of 0.3−0.45 mm. This material is a proven oxygen carrier that has been used in our previous study.27 The bituminous Shenhua coal from Inner Mongolia, China, was selected as the coal sample. It was also sieved for the coal diameter to be 0.3−0.5 mm. The proximate and ultimate analyses of coal are given in Table 2. Table 2. Proximate, ultimate analyses of coal proximate analysis/wt %, ad moisture volatile ash fixed carbon

13.2 28.1 5.9 52.8

ultimate analysis/wt %, daf C H O N S

64.4 3.9 10.3 1.6 0.7

2.2. The Batch Fluidized-Bed Reactor System. Tests of coal CLC were conducted in a batch fluidized-bed reactor. The schematic diagram of the batch fluidized-bed reactor is shown in Figure 2. A straight stainless steel tube (i.d. = 32 mm, length

Figure 2. Schematic layout of the laboratory setup.

= 1340 mm) with a porous distributor plate located 450 mm from the bottom is used as the reaction chamber. The reactor is electrically heated by a furnace. The reaction temperature is controlled by two K-type thermocouples enclosed in the stainless steel, one between the tube and the heater and the other inside the tube. The reactor has pressure taps in order to measure the absolute pressure in the bed and the pressure drop. Agglomeration and defluidization problems are detected by a sharp decrease in the bed pressure drop during operation. It is important to inject coal particles in the dense phase to ensure a good contact between coal gasification products and oxygen carrier particles. In the tests, the coal particles are fed inside the fluidized bed by means of a fuel chute which ends 150 mm above the distributor plate. The upper part of the chute has a valve system that creates a reservoir in which the coal particles are placed and later pressurized with nitrogen to ensure quick coal particle feeding. In the batch tests, the N2 and steam were used as fluidizing gas and gasification agent, respectively. The flow rates of the high-purity gases (N2 and O2) provided by Nanjing Tongguang Gas Co., Ltd., were all measured by the mass flow controllers 4061



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Figure 3. Product gas concentrations from the cyclic test of CaSO4 oxygen carrier for the (a and b) 1st cycle, (c and d) 4th cycle, and (e and f) 10th cycle. Reduction temperature: 900 °C; oxidation temperature: 950 °C; coal fed: 1.5 g; steam concentration: 35%.

supplied for per mole Fe2O3 in the reduction process of Fe2O3 to Fe3O4. The blending ratio designed for hematite addition was based on the oxygen supplied. Five blending ratios of 97:3, 95:5, 93:7, 91:9 and 89:11 were investigated. 2.4. Data Evaluation. The molar flow rate of inlet N2 (NN2) was fixed, and the gas concentrations of the outlet gases (Xi, i = CO2, CO, CH4, H2, SO2, or H2S) were analyzed by the gas analyzer. Ni,out,t is defined as the molar amount of gaseous product species i within the time interval Δt in the gas bags, which could be calculated by the N2 mass balance method:

natural anhydrite ore should be 7 g for an ideal condition. Actually, the natural anhydrite ore introduced to the fuel reactor was set as 50 g. 2.3.2. CLC with Mixed CaSO4 and Hematite Oxygen Carriers. In theory, for every mole of CaSO4 oxygen carrier, 4 mol oxygen can be supplied for the coal conversion. When 50 g of CaSO4 oxygen carrier was used, the total amount oxygen of 1.388 mol can be supplied. For hematite as an oxygen carrier, the active phase is Fe2O3. The Fe2O3 to Fe3O4 is the favored step in the fuel reactor, due to a large equilibrium constant of Fe2O3 reduced to Fe3O4.28,29 Thus, 1/3 mol oxygen can be 4062


Industrial & Engineering Chemistry Research Ni , out , t =

Article

NN2 1−

t ∑t = 0 i X i

peak of CH4 was due to the fast pyrolysis of coal . Then the peaks of CO and CO2 were quickly followed as the char part of the fuel was being gasified to the intermediate products, which were subsequently oxidized by the oxygen carrier particles in the bed. The gas concentration profiles of the following cycles were similar. For the fourth reduction, as shown in Figure 3c, the peak of CO2 increased slightly from that for the first reduction, whereas the one for the tenth reduction decreased markedly. Because of some carbon remaining in the bed, there was some CO2 formation in the oxidation period following the reducing period. Also, according to the concentration profiles of b, d, and f of Figure 3, the CO2 evolution rate was the lowest one for the fourth oxidation in comparison with the ones for the first as well as tenth oxidation. The effect of cycles on the cumulative amounts of gases presented in Figure 4 show the increase of the cumulative

(1)

where t is the time variable from 0 to 20 min. The evolution rate of gaseous product ri is the molar ratio of the gas produced per unit time to the total amount of carbon introduced to the fuel reactor and is calculated as follows: ri =

Ni , out , t Δt × NC , Fuel

(2)

where NC,Fuel is the total molar amount of carbon introduced to the fuel reactor. The cumulative amount of gaseous product (Fi) is defined as: t

Fi =

∑ ri dt

(3)

t=0

The carbon conversion efficiency ηC is the ratio of carbon consumed to the carbon introduced to the fuel reactor, and the capture efficiency of CO2, ηCO2, is the ratio of carbon converted to CO2 to the carbon consumed in the fuel reactor. Both the parameters are defined below: t

ηC =

∑t = 0 (NCO2 , out , t + NCO , out , t + NCH4 , out , t ) NC , Fuel

(4)

t

ηCO = 2

∑t = 0 NCO2 , out , t NC , Fuel

(5)

During the experiments, some repeated tests were performed in order to give reliable data. The relative experimental uncertainty for these definitions is lower than 5%. Figure 4. Effect of cycles on the cumulative amounts of exit gases. Reduction temperature: 900 °C; oxidation temperature: 950 °C; coal fed: 1.5 g; steam concentration: 35%.

3. RESULTS AND DISCUSSION 3.1. Performance of CaSO4 Oxygen Carrier Particles. 3.1.1. Effect of Temperature and Multiple Cycles. Thermodynamic studies have been performed in order to detect the possible reactions when CaSO4 oxygen carrier was used in the CLC process.19,22 In our previous study,22 the effect of temperature and species of gasification agents on both the coal gasification and CLC were experimentally evaluated. The increase of the reaction temperature favored an enhancement of the CO2-generating efficiency at the low temperatures (less than 950 °C). The reaction rate and carbon conversion increase monotonously with the increasing reaction temperature. The CO2 selectivity formation also increased with the temperature, but it decreased when the temperature exceeded 950 °C due to the sintering effect of oxygen carrier particles. One of the benefits in using the oxygen carrier for CO2 capture during coal conversion is its ability to be regenerated and thus cyclically used. It is therefore of interest to know the physical and chemical properties of the CaSO4 oxygen carrier particles. Few investigations have been performed for the cyclical performance of this oxygen carrier when using coal as fuel. Solid samples were collected from the reactor at the end of each test. Ten reduction/oxidation cycles were carried out when the particles were cyclically used. The time started when the coal was inserted into the bed. Figure 3 illustrates evolution rates of gases during the first cycle, fourth cycle, and the tenth cycle as a function of time for reducing periods (Figure 3a, c, e) and oxidizing periods (Figure 3b, d, f) with the CaSO4 oxygen carrier particles. The initial

amount of CO2 after the initial four cycles and the decrease in the next cycles. Meanwhile, the ones for the unconverted coal gasification products of CO, H2, and CH4 increased after the four cycles. This indicates that the conversion of the coal gasification products increased during the initial four cycles. The reactivity of the CaSO4 oxygen carrier became activated. The same results were obtained by Song et al.30 using the CaSO4 oxygen carrier with the simulated fuel gases of 50% H2, 25% CO, and 25% CO2. During the reduction process of the oxygen carrier, the particles became quite porous and uniform, which promoted the penetration of coal gas into the particles. After the reduction/oxidation cyclic tests, the reaction progress was unaffected by the product layer until oxygen capacity decreased significantly. 3.1.2. SO2 and H2S Evolution during Multiple Cycles. The sulfur release was mainly the result of the side reactions. The sulfur release from the coal was much lower in comparison with that from side reactions.22 As indicated in Figure 3a and c, the SO2 evolution increased with time even though the combined process of coal gasification and CaSO4 reduction with coal syngas at 900 °C had finished, and this phenomena was not obvious in Figure 3e since the SO2 in this reduction process became much less. Note that the maximum SO2 evolution was thermodynamically favored at a high reaction temperature and poor reducing condition.19,31,32 When coal syngas was oxidized by the oxygen carrier, it favored SO2 formation. The effect of 4063



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during the continuous cycles. The particles during the initial four cycles were composed of loose surfaces. However, the porosity increased in the case of the used particles (particle after 10 cycles) due to the oxygen transfer during the redox operation, and then the surface became much rougher. Further, the particles after 10 cycles were characterized with four magnifications of 900×, 3000×, 10000×, and 20000×. Except for some large pores, it can also be seen that the filaments clearly extended to the surrounding of the grains and connected with the grains. Some irregular smaller crystal lattices were formed and filled the void of the grains. This structure of the particles can also facilitate the diffusion of reactant gases into the core of oxygen carrier particles, enhancing the reactions between gas and oxygen carrier particles. However, during the initial cycles, the sulfur loss was much higher, leading to the significant loss of oxygen transfer capacity. Therefore, in the 10th cycle, the CO2 peak (0.11) as shown in Figure 3e is much lower than the one (0.18) in the fourth cycle in Figure 3c. Figure 7 shows the results of the XRD analysis of the used particles of multiple tests after the first, fourth, and tenth reductions. It can be seen that CaSO4 is still the main component after the first reduction. The presence and intensity of CaS suggest the reduction of the oxygen carriers. However, the intensity of CaO became higher than expected, especially for the particles after 10 cycles, which suggests the occurrence of a much higher degree of side reactions. This result demonstrates the decrease of the gas conversion due to the loss of the reactivity of CaSO4 oxygen carrier particles. Also, it confirms that the SO2 evolution was much less in the tenth reduction (as shown in Figure 3e) which was ascribed to the significant loss of its reactivity during the above cycles. As illustrated in Figure 7, although CaSO4 as an oxygen carrier has the merit of large oxygen transport capacity, the sulfur species emission was rather considerable. Xiao et al.33 summarized the solid circulation rate of the CaSO4/CaS oxygen carrier in comparison with the rates of some metal oxide-based systems using the method described by Abad et al.34 The results showed that the high oxygen-carrying capacity of the CaSO4/CaS-based oxygen carrier requires a much lower circulation rate compared to that for the metal oxide-based oxygen carrier. It should also be mentioned that the price of CaSO4 is much lower than that of metal oxides. However, if the sulfur release were controlled to maintain the oxygen-carrying capacity over the redox cycles, CaSO4 would still be a promising oxygen carrier. 3.2. Role of Hematite Addition. The effect of hematite addition in the coal CLC process was evaluated by comparing the gas product distributions obtained using CaSO4 as the oxygen carrier with or without hematite addition. The evaluation of the role of hematite addition was focused on the ratio of hematite addition, the sulfur species evolution, and the effect of cycles. 3.2.1. Effect of Blending Ratio. Figure 8 shows the CO, CH4, and CO2 evolution rate with the ratio of the hematite addition of 0−11%. H2 in the exit gas was much less with hematite addition, indicating that H2 as the coal gasification gas reacts fast with hematite. To add some hematite increased the H2 conversion. According to the Figure 8, it is obvious that the CO and CH4 evolution rates decrease when hematite is added. Meanwhile, the CO2 evolution rate increased with the ratio of hematite addition, indicating that more coal gas reacts with the oxygen carrier to form CO2 and H2O. The carbon conversion efficiency

cycles on the cumulative amounts of SO2 and H2S is shown in Figure 5. SO2 emission was dominant, and its cumulative

Figure 5. Effect of cycles on the sulfur species evolution during the reduction period (a) and oxidation period (b). Reduction temperature: 900 °C; oxidation temperature: 950 °C; coal fed: 1.5 g; steam concentration: 35%.

amount was much higher than that of H2S. The cumulative amount of SO2 increased till the fourth cycle of 0.18, and it remarkably decreased to a low level during the later cycles. As illustrated in section 3.1.1, after three cycles the CaSO4 oxygen carrier became activated, indicating more gasification products gradually penetrated into the internal spaces of the oxygen carriers to be oxidized. It would have a strong positive effect on SO2 formation. In the oxidation process, the SO2 was the main sulfur species, as shown in Figure 5b, which came from the CaS oxidation, R13, and CaSO4 decomposition, R14. Also, in the oxidation process, there was almost no H2S evolution. 3.1.3. Characterization of Oxygen Carrier. A morphological characterization of several samples was performed to observe changes in the solid structure throughout consecutive multiple cycles. Figure 6 shows SEM images of the CaSO4 particles after the first, fourth, and tenth cycles. As shown in Figure 6, it can be observed that some sharp-edged margin of the particles continuously disappeared and the particles began to conform to a granular shape. The crystalline structure of the particles during the initial cycles was absent from the used one after 10 cycles, indicating the increased size of oxygen carrier particles 4064



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Figure 6. SEM images for the reduced CaSO4 oxygen carriers (without hematite addition) after 1st cycle, 4th cycle, and 10th cycle.

and CO2 capture efficiency are shown in Figure 9. Both of the two efficiencies increase with the hematite addition. Further, when the ratio of hematite addition reaches 7%, there is no obvious increase for the two efficiencies. Then, the carbon conversion efficiency and CO2 capture efficiency were equal to 0.90 and 0.91, respectively, higher than those without hematite addition of 0.79 and 0.88, respectively. The SO2 evolution rate with the ratio of hematite addition is shown in Figure 10. At the early stage of the process, there is no obvious trend for the SO2 evolution rate when hematite is added. However, after 5 min, the remarkable decrease for the SO2 evolution can be observed. When the ratio increases more than 5%, the trend of the SO2 evolutionary rate is not obvious. To summarize, improved reaction performance is observed when the combined oxygen carriers are used. Among the five ratios investigated, there is no obvious increase for the carbon conversion efficiency and CO2 capture efficiency when the ratio of hematite addition reaches 7%. Meanwhile, the remarkably

Figure 7. XRD analysis of the used CaSO4 oxygen carrier particles after 1st reduction, 4th reduction, and 10th reduction.

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Figure 8. Effect of oxygen-supplied ratio by Fe2O3 on the exit gas distribution during the reduction period. (a) CO; (b) CH4; (c) CO2. Reduction temperature: 900 °C; coal fed: 1.5 g; steam concentration: 35%.

Figure 10. Effect of oxygen-supplied ratio by Fe2O3 on the SO2 evolution during the reduction period. Reduction temperature: 900 °C; coal fed: 1.5 g; steam concentration: 35%.

Figure 9. Effect of oxygen-supplied ratio by Fe2O3 on the carbon conversion efficiency and CO2 capture efficiency.

decreasing trend of SO2 evolution is observed. This is analyzed and discussed in detail in section 3.2.5. 3.2.2. Effect of Multiple Cycles. Tests of 10 reduction/ oxidation cycles of the coal CLC process were carried out using 93% CaSO4/7% Fe2O3 as combined oxygen carriers. The chemical and physical characterization were investigated after the multiple cycles. The amounts of gases accumulated with the cycles are shown in Figure 11. Meanwhile, the ones for the CaSO4 oxygen carrier used individually are also summarized and shown for

comparison. At the same experimental temperature of 900 °C and in comparison with the investigation for the individual CaSO4 oxygen carrier used above, it is obvious that the amounts of unconverted gases were much lower for the combined oxygen carriers. It indicates that more coal conversion gases were oxidized by the combined oxygen carriers, leading to a greater amount of CO2 accumulation. Also, when the combination of oxygen carriers was used, the accumulated amounts of CO and CH4 were more or less 4066



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one for CaSO4/CaS.25 A higher equilibrium constant represents a higher conversion of the reducing gas. That is, with Fe2O3 addition, the coal gasification gases are oxidized by Fe2O3, faster than those by CaSO4/CaS. On the other hand, the equilibrium of the water/gas reaction may be shifted. The water/gas reaction shift has an influence on the final gas composition. The lower H2 evolution indicates that the faster H2 consumption with the combined oxygen carriers may shift the water/gas reaction equilibrium toward the formation of more H2 and CO2, thus also increasing the CO consumption. When coal is used as fuel, the use of the oxygen carrier improves the coal gasification at the expense of the products of coal gasification, H2 and CO, which are well-known to be inhibitors of gasification. This mechanism would be able to explain both the lower CO and H2 concentrations and also the more rapid coal conversion with the combined oxygen carriers. 3.2.3. SO2 and H2S Evolution during Multiple Cycles. It was noted that H2S release in CLC of coal can be oxidized by hematite to form SO2, which can cause SO2 evolution. However, in the CLC process of coal with the CaSO4 oxygen carrier, the sulfur release was mainly the result of the side reactions, and the sulfur contained in the coal may be neglected.19 Thus, the influence of hematite addition involving the sulfur evolution was not considered. Figure 12 shows the effect of cycles on the accumulated amounts of sulfur species with the combined oxygen carriers and the individual CaSO4 oxygen carrier during the reduction period and oxidation period. As shown in Figure 12a, when hematite is added during the reduction period of the multiple cycles, the trend for the release of SO2 and H2S is lower. Also, the SO2 release is much lower for the combined oxygen carriers used during the oxidation period, as Figure 12b shows. Related to the data from Figures 11 and 12, the average values for the accumulated amounts of the gases during multiple cycles were used for evaluation. It could be calculated that in the reduction process, the dry gas concentrations of CO2, CO, CH4, SO2, and H2S when adding 7% hematite were 95.4, 0.8, 0.2, 3.6, and 0%, respectively. Although to add some amounts of hematite together with the CaSO4 oxygen carrier, the SO2 and H2S release were remarkably suppressed, the SO2 evolution was still much higher. With respect to achieving this process in a continuously operating plant, some control factors, such as the reactor design and operation parameters can also affect the final sulfur evolution. However, adding some amounts of hematite together with the CaSO4 oxygen carrier provides a potential method to use. 3.2.4. Characterization of Oxygen Carrier. When the coal CLC process with the compound oxygen carrier was finished, some powerful magnets were employed to separate the hematite and CaSO4 particles. Figure 13 shows the XRD analysis of the used CaSO4 oxygen carrier particles after the 1st, 4th, and 10th reductions. It is obvious that the intensity of CaSO4 is still higher even after the 10th reduction. Also, the diffraction peak of CaO is much smaller, which is in accordance with the results of the coal conversion as demonstrated in section 3.2.2. In comparison with Figure 7, it can be concluded that the CaO formation was suppressed and CaS was the dominant product after adding the hematite with CaSO4 as a combined oxygen carrier. The XRD results indicate that the side reaction was suppressed, leading to less sulfur release, which is in accordance with the results of section 3.2.3. Figure 14 shows the EDX spectra of CaSO4 particles. Two different areas (A and B) were selected to detect the element

Figure 11. Effect of cycles on the accumulated amounts of gases for the CaSO4 oxygen carrier and combined oxygen carriers (91:7). N: CaSO4 oxygen carrier; C: combined oxygen carrier (91:7).

stable for the 10 cycles, indicating a stable reactivity for the oxygen carrier combination used. Table 3 gives a summary of the CO2 capture efficiency and carbon conversion efficiency at different reduction/oxidation Table 3. CO2 Capture Efficiency and Carbon Conversion Efficiency at Different Cycles cycle

CO2 capture efficiency

Combined Oxygen Carrier (91:7) 1st 0.92 4th 0.93 7th 0.91 10th 0.90 CaSO4 When Individually Used 1st 0.79 4th 0.85 7th 0.83 10th 0.73

carbon conversion efficiency 0.93 0.93 0.92 0.92 0.88 0.91 0.89 0.88

cycles. In comparison with the individual CaSO4 oxygen carrier used, both the carbon conversion efficiency and CO2 capture efficiency increased with each cycle. Further, if we use the average value of carbon conversion efficiency during these cycles, it is equal to 0.925. Compared with the carbon conversion efficiency (0.93) obtained by our previous work24 at the same operation conditions with a fuel reactor temperature of 950 °C, it can be concluded that the temperature needed to reach the same carbon conversion was lowered to 900 °C when adding 7% Fe2O3 together with CaSO4 as the combined oxygen carriers. As we know, in the practical industrial application of CLC, the energy needed for the fuel conversion in the fuel reactor is supplied by means of the flows of the circulating solid particles of the oxygen carrier. Therefore, less energy is needed for the fuel conversion in the fuel reactor in the case of the combined oxygen carriers used. As for the combined oxygen carrier used in CLC of coal, the improved coal conversion with the addition of hematite was ascribed to the following explanations. On the one hand, Fe2O3 works as an additional oxygen carrier between oxidizing and reducing periods. According to the thermodynamic analysis, the equilibrium constant of Fe2O3/Fe3O4 for the reduction reaction with H2 and CO at any temperature is much higher than the 4067



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above experiments (section 3.1) are also analyzed and summarized. For the fresh CaSO4 particles, the S/Ca ratio was 1.134, and the O/Ca ratio was 4.209. After the first reduction at 900 °C, both O and S intensities decreased, with the S/Ca ratio decreasing to 0.895 and O/Ca decreasing to 0, which suggests the formation of CaS on the surface and that it is the dominant product. It also indicates the absence of side reactions leading to CaO formation. After the 10th reduction, the S/Ca and O/ Ca decreased to the average values of 0.62 and 0.49, respectively, indicating that the grains on the surface of particles were still mainly composed of CaS. However, for comparing with the results obtained when the CaSO4 oxygen carrier was individually used, after the 10th reduction the S/Ca and O/Ca decreased to the values of 0 and 0.877, respectively, which indicated that the grains on the surface were all composed of CaO. Therefore, it can be concluded that the side reaction leading to CaO formation was suppressed when hematite was added, which is in accordance with the conclusions of XRD analysis. 3.2.5. Sulfur Release Suppressed Mechanism. According to the gas distributions and XRD and EDX analyses of the used particles, it can be obviously concluded that sulfur release (H2S and SO2) is suppressed when adding some hematite with CaSO4 as oxygen carriers. The H2S release is mainly the result of the side reactions of the CaSO4 reductive decomposition by H2 (R15) and the CaS decomposition with steam (R16). 4H2 + CaSO4 → CaO + H2S + 3H2O

(R15)

H2O + CaS → CaO + H2S

(R16)

As for H2S suppressed by hematite addition in the above finding, the traditional hot gas desulfurization (HGD) process can be used for analysis and discussion. Syngas produced by the gasification process of solid fuels should require a deep desulfurization to meet stringent evolution standards. Due to the demand of better HGD sorbents in terms of cost, efficiency, and environmentally friendly end products, some investigations have been carried out to use Fe2O3 as a sorbent for H2S removal in HGD desulfurization.35 Tsukada et al.36 investigated the use of composite iron oxide sorbents for the simultaneous removal of H2S from gasifier products. A considerable desulfurization rate of iron oxide to remove H2S was found. Slimane and Abbasian37 investigated the desulfurization performance of iron oxide from waste materials of metal processing operations. It was observed that sorbents based on iron waste were the most reactive and exhibited the highest effective capacities both in reducing and oxidizing atmospheres. However, according to the previous study, the temperature needed for all the research using Fe2O3 for H2S desulfurization is lower than 600 °C. The reason is that, at a temperature higher than 600 °C in a reducing atmosphere, the most stable forms of iron were FeO and Fe which showed less favorable sulfidation equilibrium and led to a reduction in sulfurremoving capacity.38 Although an investigation involved in HGD using an iron oxide sorbent with a hot gas containing H2, CO and H2S over a temperature range of 600−900 °C was performed by Tamhankar et al.,39 the authors found that the initial fast reduction of Fe2O3 led to spongy iron formation, which reacted slowly with the H2S to form iron sulfide. Therefore, with respect to the previous HGD results using Fe2O3 as sorbent, the iron sulfide was believed unfavorable in

Figure 12. Effect of cycles on the accumulation amounts of sulfur gases during the reduction and oxidation periods. (a) Reduction period; (b) oxidation period. N: CaSO4 oxygen carrier; C: combined oxygen carrier (91:7).

Figure 13. XRD analysis of the used CaSO4 oxygen carrier particles after 1st, 4th, and 10th reductions. The CaSO4 particles are from the combined oxygen carrier (hematite addition).

distribution of the particles. The used particles consisted mainly of Ca and S. The presence of Mg belonged to the particle itself. The average accurate quantitative analyses of the atomic percentage of the major elements are shown in Table 4. The average values of O/Ca and S/Ca of fresh and used oxygen carrier particles are presented. Also, in order to compare the results with those of using the CaSO4 oxygen carrier individually, the corresponding atomic distribution in the 4068



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Figure 14. EDX spectra for the used CaSO4 oxygen carrier at 900 °C after 10 cycles. A and B represent two different regions of the particle. The CaSO4 particles are from the combined oxygen carrier (hematite addition).

conditions, part of the released SO2 would be recaptured by the formed CaO as CaS,40 which could be reoxidized to CaO by R13 in the oxidizing period. However, in the presence of hematite fewer of these reactions take place.

Table 4. Atomic Percent of Major Elements on the Surface of Particles of the CaSO4 Oxygen Carrier reduction A1a B1a A2b B2b

species

fresh sample

1st

4th

10th

S/Ca O/Ca S/Ca O/Ca

1.134 4.209 1.134 4.209

0.895 0 0.916 0

0.975 0.375 0.235 0.580

(A)0.55 (B)0.68 (A)0.72 (B)0.25 0 0.877

4. CONCLUSIONS An experimental study to evaluate the coal conversion with the CaSO4 oxygen carrier in CLC was carried out. The effect of operating parameters such as temperature and multiple reduction and oxidation cycles on carbon conversion and CO2 capture as well as sulfur species evolution were investigated. Also, an innovative method to increase carbon conversion and lower sulfur species evolution was experimentally proposed and confirmed. The main conclusions that can be extracted from the study are the following: (a) At a reduction temperature of 900 °C, the natural anhydrite ore (CaSO4) as oxygen carrier increased its reactivity during the initial redox cycles. However, the sulfur species emission was rather considerable due to the occurrence of side reactions. After 10 reduction/ oxidation cycles, SEM analysis results showed that, except for some large pores, there were also some filaments clearly extended to the surrounding grains and connected with the grains. Some irregular smaller crystal lattices formed and filled the void of the grains. EDX analysis results indicated that the grains on the surface of the particles were all of CaO. XRD analysis results suggested that the CaO became the main component

a

A1 and B1: analysis results for the particle of CaSO4 from the compound oxygen carriers. bA2 and B2: analysis results for the particle of CaSO4 when individually used.

the present CLC process. Also, in the presence of a high concentration of steam in the fuel reactor, the iron sulfide could be oxidized by steam to H2S. The particles for the used hematite were selected for XRD analysis. Results showed that the main components of the used hematite particles were Fe3O4 and Fe2O3. There was no iron sulfide formation for the combined oxygen carriers in the CLC process. Thus, it can be concluded that the lower release of H2S when the combined oxygen carriers were used is a result of the side reactions of R15 and R16 being suppressed. It is not due to the desulfurization capacity of iron oxide. On the basis of the above information, it is clear from the experimental results that, when hematite is added, there is effectively a higher oxygen capacity and this lowers significantly the level of reducing species (CO and H2) in the reducing period. Therefore, less reductive decomposition of CaSO4 through reactions R9 and R10 takes place. Under the reducing 4069



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(4) Cuadrat, A.; Abad, A.; García-Labiano, F.; Gayán, P.; de Diego, L. F.; Adánez, J. Effect of Operating Conditions in Chemical-Looping Combustion of Coal in a 500 Wth Unit. Int. J. Greenhouse Gas Control 2012, 6, 153−163. (5) Cuadrat, A.; Abad, A.; García-Labiano, F.; Gayán, P.; de Diego, L. F.; Adánez, J. The Use of Ilmenite As Oxygen-Carrier in a 500 Wth Chemical-Looping Coal Combustion Unit. Int. J. Greenhouse Gas Control 2011, 5, 1630−1642. (6) Shen, L.; Wu, J.; Gao, Z.; Xiao, J. Characterization of Chemical Looping Combustion of Coal in a 1 kWth Reactor with a Nickel-Based Oxygen Carrier. Combust. Flame 2010, 157, 934−942. (7) Shen, L.; Wu, J.; Xiao, J. Experiments on Chemical Looping Combustion of Coal with a NiO Based Oxygen Carrier. Combust. Flame 2009, 156, 721−728. (8) Markström, P.; Lyngfelt, A. Designing and Operating a ColdFlow Model of a 100 kW Chemical-Looping Combustor. Powder Technol. 2012, 222, 182−192. (9) Markström, P.; Linderholm, C.; Lyngfelt, A. Operation of a 100 kW Chemical-Looping Combustor with Mexican Petroleum and Cerrjón Coal; International Conference on Chemical Looping 2012 - A Concept for Efficient and Clean Use of Fossil Resources, Darmstadt, Germany, 26th to 28th of September, 2012. (10) Xiao, R.; Chen, L.; Saha, C.; Zhang, S.; Bhattacharya, S. Pressurized Chemical-Looping Combustion of Coal Using an Iron Ore As Oxygen Carrier in a Pilot-Scale Unit. Int. J. of Greenhouse Gas Control 2012, 10, 363−373. (11) Orth, M.; Ströhle, J.; Epple, B., Design and Operation of a 1 MWth Chemical Looping Plant; International Conference on Chemical Looping 2012 - A Concept for Efficient and Clean Use of Fossil Resources, Darmstadt, Germany, 26th to 28th of September, 2012. (12) Wu, H. C.; Ku, Y.; Chiu, P. C.; Shiu, S. H.; Kuo, Y. L.; Tseng, Y. H., Methane Combustion by Moving Bed Fuel Reactor with Fe-Based Oxygen Carrier; International Conference on Chemical Looping 2012 A Concept for Efficient and Clean Use of Fossil Resources, Darmstadt, Germany, 26−28 September, 2012. (13) Fan, L.; Li, F.; Ramkumar, S. Utilization of Chemical Looping Strategy in Coal Gasification Processes. Particuology 2008, 6, 131−142. (14) Zhao, Z.; Chen, T.; Ghoniem, A. F. Rotary Bed Reactor for Chemical-Looping Combustion with Carbon Capture. Part 1: Reactor Design and Model Development. Energy Fuels 2013, 27, 327−343. (15) Andrus, H. E.; Chiu, J. H.; Thiebeault, P. R.; Brautsch, A., Alstom’s Calcium Oxide Chemical Looping Combustion Coal Power Technology Development; Coal: World Energy Security: 34th International Technical Conference on Clean Coal and Fuel Systems, Clearwater, FL, 31 May to 4 June , 2009. (16) Andrus, H. E.; Chiu, J. H.; Thiebeault, P. R., Alstom’s Chemical Looping Combustion Coal Power Technology Development Prototype; International Conference on Chemical Looping 2010, Lyon, France, 2010. (17) Wang, J.; Anthony, E. J. Clean Combustion of Solid Fuels. Appl. Energy 2008, 85, 73−79. (18) Ding, N.; Zheng, Y.; Luo, C.; Wu, Q.; Fu, P.; Zheng, C. Development and Performance of Binder-Supported CaSO4 Oxygen Carriers for Chemical Looping Combustion. Chem. Eng. J. 2011, 171, 1018−1026. (19) Zheng, M.; Shen, L.; Xiao, J. Reduction of CaSO4 Oxygen Carrier with Coal in Chemical-Looping Combustion: Effects of Temperature and Gasification Intermediate. Int. J. of Greenhouse Gas Control 2010, 4, 716−728. (20) Song, Q.; Xiao, R.; Deng, Z.; Zheng, W.; Shen, L.; Xiao, J. Multicycle Study on Chemical-Looping Combustion of Simulated Coal Gas with a CaSO4 Oxygen Carrier in a Fluidized Bed Reactor. Energy Fuels 2008, 22, 3661−3672. (21) Song, Q.; Xiao, R.; Deng, Z.; Zhang, H.; Shen, L.; Xiao, J.; Zhang, M. Chemical-Looping Combustion of Methane with CaSO4 Oxygen Carrier in a Fixed Bed Reactor. Energy Convers. Manage. 2008, 49, 3178−3187.

after 10 cycles, which suggested the occurrence of a much higher degree of side reactions than expected. (b) When hematite was added, the CO and CH4 evolution rates decreased, and the CO2 evolution rate increased. As a result, both the carbon conversion efficiency and the CO2 capture efficiency increased. Further, when the ratio of hematite addition reached 7%, there was no obvious increase for the two efficiencies. At that point, the carbon conversion efficiency and CO2 capture efficiency were equal to 0.90 and 0.91, respectively, higher than the ones without hematite addition of 0.79 and 0.88, respectively. Also, the SO2 and H2S release were remarkably suppressed. (c) Ten reduction/oxidation test cycles of the coal CLC process were carried out using 93% CaSO4/7% Fe2O3 as oxygen carriers. Results showed that the amount of unconverted gases was much lower for the combined oxygen carriers. The amounts of accumulated CO and CH4 were more or less stable for the 10 cycles, indicating a stable reactivity for the combined oxygen carrier. XRD results indicated that the side reactions were suppressed, leading to less sulfur release when hematite was added. EDX analysis results showed that the grains on the surface of particles were still mainly composed of CaS. All of the findings suggest that the sulfur species evolution was suppressed when hematite was added. (d) The suppressed sulfur release mechanism was discussed. It was revealed that the suppression of the sulfur release mechanism was not due to the desulfurization capacity of iron oxide. The reason was that less reductive decomposition of CaSO4 through the side reactions took place when adding hematite. The side reactions toward sulfur evolution were suppressed by means of adding some hematite not only in the reduction period but also in the oxidation period.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-25-8379 5598; Fax: +86-25-5771 4489; E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51276037), the Key Technology Research and Development Program of Jiangsu Province of China (BE2012166), the Foundation of Graduate Creative Program of Jiangsu Province (CXZZ12-0098), and the Scientific Research Foundation of Graduate School of Southeast University (YBJJ1214).

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