Effect of Temperature on Reduction of CaSO

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Ind. Eng. Chem. Res. 2008, 47, 8148–8159

Effect of Temperature on Reduction of CaSO4 Oxygen Carrier in Chemical-Looping Combustion of Simulated Coal Gas in a Fluidized Bed Reactor Qilei Song, Rui Xiao,* Zhongyi Deng, Laihong Shen, Jun Xiao, and Mingyao Zhang Thermoenergy Engineering Research Institute, School of Energy and EnVironment, Southeast UniVersity, Nanjing 210096, China

Chemical-looping combustion (CLC) is a promising combustion technology for gaseous and solid fuel with efficient use of energy and inherent separation of CO2. The concept of a coal-fueled CLC system using calcium sulfate (CaSO4) as oxygen carrier is proposed in this study. Reduction tests of CaSO4 oxygen carrier with simulated coal gas were performed in a laboratory-scale fluidized bed reactor in the temperature range of 890-950 °C. A high concentration of CO2 was obtained at the initial reduction period. CaSO4 oxygen carrier exhibited high reactivity initially and decreased gradually at the late period of reduction. The sulfur release during the reduction of CaSO4 as oxygen carrier was also observed and analyzed. H2 and CO conversions were greatly influenced by reduction temperature. The carbon deposition ratio was found to be quite low. The oxygen carrier conversion and mass-based reaction rates during the reduction at typical temperatures were compared. Higher temperatures would enhance reaction rates and result in high conversion of oxygen carrier. An XRD patterns study indicated that CaS was the dominant product of reduction and the variation of relative intensity with temperature is in agreement with the solid conversion. The slight content of CaO in reduced oxygen carrier at high temperatures was due to the formation of SO2 and H2S during the reduction period. ESEM analysis indicated that the surface structure of oxygen carrier particles changed significantly from impervious to porous after reduction. Slight agglomeration of small grains occurred for reduced particles at 950 °C. EDS analysis also demonstrated the transfer of oxygen from the oxygen carrier to the fuel gas and a certain amount of sulfur loss and CaO formation on the surface at higher temperatures. The reduction kinetics of CaSO4 oxygen carrier was explored with the shrinking unreacted-core model. The apparent kinetic parameters were obtained, and the kinetic equation well predicted the experimental data. Finally, some basic considerations on the use of CaSO4 oxygen carrier in a CLC system for solid fuels were discussed. 1. Introduction Chemical-looping combustion (CLC) has been proposed as a new technology that can be used to control the greenhouse gas emission with less energy loss.1 A CLC system is mainly composed of a fuel reactor and an air reactor. The oxygen is transferred from the air to the fuel by the oxygen carrier. The water in combustion products can be easily removed by condensation and pure CO2 is obtained without any loss of energy for separation. The CLC systems, even in a combined cycle application with gas turbine or some other type combined cycle system, would be more potentially efficient than systems with conventional combustion because of the inherent separation of CO2.2 The development of oxygen carrier is a key technology in the CLC system performance. A number of metal oxides have been extensively investigated as potential oxygen carriers. During the early stage of oxygen carrier development, Ishida and Jin initially performed fundamental studies on the oxygen carrier preparation and reactivity study mainly in TGA and demonstrated that the Nickel based oxygen carrier, for example, NiO/NiAl2O4, is a promising material for high reactivity.3-5 A research group led by Lyngfelt and Mattisson at the Chalmers University of Technology also compared a large number of different oxygen carriers in various reactors.6-11 Researchers at CSIC also investigated many potential oxygen carriers in different reactors and demonstrated good performance of Cubased oxygen carriers in the CLC system.12-16 Ryu et al. at the Korea Institute of Energy Research also performed many * To whom correspondence should be addressed. E-mail: ruixiao@ seu.edu.cn. Tel.: +86-25-83794744-803. Fax: +86-25-83795508.

investigations on Ni-based oxygen carriers.17 Recently Varma et al. at Purdue University investigated the reactivity of Nibased oxygen carriers using a solution combustion synthesis method.18,19 Most of the investigations on CLC focused on methane or natural gas as fuel. However, the majority of CO2 emission is from the combustion of solid fuels from coal-fueled power plants. Even though there are still many technical challenges,20 research on CLC of solid fuels is more attractive for clean coal conversion,21 especially in China where coal is the dominant energy supply. There are some publications on the CLC using the syngas and coal gasification gas,22-25 Jin et al. investigated the reactivity of Ni-based oxygen carriers in CLC in an elevated pressure fixed-bed reactor using hydrogen, natural gas, and coal gas.23,26,27 The results demonstrated that coal gas exhibited higher reactivity and stability than the natural gas. The results also suggested that the CLC system could be combined with IGCC and revealed promising potential of clean coal technology for high efficiency and greenhouse gas mitigation.23 Also some investigations on solid fuel application have been reported recently. Pan et al. proposed a design for a CLC system for solid fuels.28 Cao et al. presented an intensive process analysis on CLC for solid fuels and TGA experiments on the feasibility of CuO for solid fuels.20,29 Dennis et al. investigated the use of lignite char with Fe2O3 oxygen carriers using steam and CO2 as a gasification agent.30,31 Leion et al. has done extensive work on the use of petroleum coke using iron-based oxygen carriers.32,33 Mattisson et al. investigated a CLC system using syngas from coal gasification as fuel and focused on the development of an oxygen carrier reactor design and hot prototype CLC reactor construction and operation.24 A 10 kWth CLC reactor for solid

10.1021/ie8007264 CCC: $40.75  2008 American Chemical Society Published on Web 09/27/2008

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fuels has been designed and operated using petroleum coke and South African coal which provides promising results for CLC of solid fuels.34,35 In most of the above publications, except some recent studies on the use of ilmenite,33,34,36 the oxygen carriers are high cost metal oxides, which may be the primary limitation of use in CLC of solid fuels. Besides, these oxygen carriers also have some other disadvantages for solid fuels, for example, vulnerability to poisonous sulfur compounds from coal, loss with coal ash, and potential heavy-metal pollution to the environment. Therefore, it would be interesting to investigate low cost material, especially natural ores such as ilmenite33,36 and natural anhydrite.37 In this study the CaSO4-based oxygen carrier, natural anhydrite ore, is proposed for the CLC of solid fuels. CaSO4 is much more environmentally sound as a nonmetal oxide. CaSO4 is cheaper owing to vast gypsum resources all over the world. Compared with metal oxides, CaSO4 has a relatively higher oxygen capacity. As one of gypsum resources, natural anhydrite ore is of high purity of CaSO4 and has a relative higher mechanical strength compared to synthesized calcium-based oxygen carrier particles in our previous studies, therefore, it may be suitable for application in the fluidized bed reactor of CLC systems. The concept of a CLC system for solid fuels using CaSO4 as oxygen carrier is presented as follows. First, the solid fuels need to be gasified in the bubbling fluidized bed using H2O or CO2 to produce gasification gas. The main reactions for coal in the fuel reactor involve the gasification with H2O (eq 1), water-gas shift reaction (WGSR, eq 2), and Boudouard reaction (eq 3) as follows: C + H2O ) CO + H2

θ ∆H298K ) 131.30 kJ ⁄ mol (1)

CO + H2O ) CO2+H2

θ ∆H298K ) -41.14 kJ ⁄ mol (2)

C + CO2 ) 2CO

θ ∆H298K ) 172.42 kJ ⁄ mol

(3)

CaSO4 oxygen carrier is reduced by gasification gas to CaS simultaneously in the fuel reactor: CaSO4 + 4CO f CaS + 4CO2 CaSO4+4H2 f CaS + 4H2O

θ ∆H298K ) -170.96 kJ ⁄ mol (4) θ ∆H298K ) -6.41 kJ ⁄ mol

(5)

Then CaS is transported to the air reactor to be oxidized back to CaSO4 and generates a large amount of heat: CaS + 2O2 f CaSO4

θ ∆H298K ) -960.90 kJ ⁄ mol (6)

This cycle can transfer the oxygen and heat from the air reactor to the fuel reactor, and the carbon dioxide is obtained in almost pure form without extra energy input. As for the application of calcium-based oxygen carriers, there are few publications. Alstom Power Inc. has started investigation on the feasibility of a CaSO4-based oxygen carrier in their hybrid combustion-gasification chemical looping coal power technology,38,39 but they still have not published some key data yet. Wang and Anthony et al. in CANMET has explored the use of CaSO4 as oxygen carrier in a process for gasification of solid fuels and, following a chemical-looping combustion process, the fundamental simulation result seems promising for the use of CaSO4 in CLC for solid fuels, especially high-sulfur fuels.21,40 In our previous studies, the feasibility of CaSO4 oxygen carrier has been investigated through thermodynamic analysis and

thermogravimetric analysis (TGA) with Fourier transform infrared spectroscopy.41 The results show that CaSO4/CaS can be an interesting alternative oxygen carrier for CLC systems. CaSO4 oxygen carrier also displayed high reduction reactivity and stability in a long-time reduction/oxidation test in a fixedbed reactor.37 However, thermodynamic calculations also show that there is thermodynamic limitation for CaSO4/CaS that caused the incomplete conversion of fuel gas and sulfur release. Thermodynamic calculations of CaSO4 oxygen carrier with CO and H2 were performed on the principle of the Gibbs free energy minimization via Aspen Plus software. The results show that the CO2 purity in the equilibrium component is estimated around 98.04% at 900 °C and 97.09% at 950 °C (The molar ratio of CO and CaSO4 is 4). A certain amount of unreacted CO and SO2 also exists in the equilibrium gas phase composition. The reaction trend for H2 and CH4 is similar to CO. Besides, the oxidation of CaS also accompanies side reactions. The sulfur release may be a limit for the practical use of CaSO4 oxygen carrier and requires intensive studies. The main purpose of this study was to investigate the reduction reactivity of the CaSO4 oxygen carrier with simulated coal gas in CLC. The effect of reduction temperature on the performance of CaSO4 for application in CLC was studied. Characterization analysis of the CaSO4 oxygen carrier particles was performed to investigate the reduction mechanism. The emission of sulfur in the oxygen carrier during the reduction was observed and analyzed. The apparent reduction kinetics was also investigated by reference to the literature. 2. Experimental Details 2.1. Material Preparation and Characterization. The oxygen carrier particle was natural anhydrite ore from Nanjing Anhydrite Ore Co. Ltd. The natural anhydrite ore was crushed to particles with a size range of 0.15-0.2 mm. CaSO4 is the main content in natural anhydrite ore (94.38%). The other components primarily consisted of MgO, SiO2, and Al2O3. The specific density of the natural anhydrite ore is 2.9 g/cm3 and the bulk density is 1.5 g/cm.3 The Mohs hardness is 3.0-3.5. The oxygen ratio of natural anhydrite, RO, is 0.444 as calculated by the purity of CaSO4. More information of the material is provided in the previous article.42 The fresh and reduced oxygen carriers were characterized by XRD for their crystal structure in a D/max 2500VL/PC system using Cu KR radiation with a step of 0.02° per second. The morphology of the fresh and reduced oxygen carriers was recorded by environmental scanning electron microscope (ESEM) in a microscope system (QUANTA 200,FEI,Holland) equipped with an energy-dispersive X-ray spectroscopy (EDS) system (INCA X-sight, OXFORD INSTRUMENTS, Britain). 2.2. Experimental Setup. The experiment was conducted in a bench-scale fluidized bed reactor under atmospheric pressure. The schematic diagram of the fluidized bed reactor for simulating chemical looping combustion has been presented in a recent article.42 The stainless steel tube reactor (i.d. ) 25 mm, length ) 950 mm) with 470 mm of preheating zone below the porous distributor plate was electrically heated in the furnace. The reaction temperature was controlled by two thermocouples, a Pt/Rh thermocouple between the tube and the heater and the other K-type thermocouple inside the tube. The inside thermocouple was adjustable to obtain the temperature along the bed height. The flow rates of calibration gas (N2) and reaction gas (H2, CO, CO2) were measured by mass flow controllers. The pressure drop of the fluidized bed was measured by a U-type

8150 Ind. Eng. Chem. Res., Vol. 47, No. 21, 2008 Table 1. Experimental Conditions oxygen carrier pressure reaction temperature particle size sample mass sample static height reduction gas reduction gas flow rate reduction duration inert gas inert gas flow rate

natural anhydrite ore 1 atm 890-950 °C 0.15-0.2 mm 30 g 40 mm H2/CO/CO2 ) 50/25/25% 600 mL/min 20 min N2 600 mL/min

pressure gauge to monitor the fluidization state. The product gas concentrations were measured by two online gas analyzers. The hot product gas concentrations were measured by an MRU SAE19 flue gas analyzer to detect the dry concentration of O2, SO2, H2S, and NOX. The other gas stream was sent to an ice-water cooler where the steam was condensed and removed. The dry product gas concentrations were measured by an Emerson multicomponent gas analyzer including a Rosemount NGA 2000 gas analyzer used to measure the concentrations of CO2, CH4, CO, and O2, and a Hydros 100 analyzer to detect the concentration of H2. During some experiments the gas was also collected by sample bag and examined by a gas chromatograph (GC, Agilent 6890N) equipped with a TCD detector. The moisture content was negligible as measured by GC, which verified the product gas with little water after cooling. The concentrations of all the other gases were consistent. 2.3. Experimental Procedure. Blank tests with quartz bed in reactor tube were performed to evaluate the influence of the metal tube with Ni content before other tests. The conversion of H2, CO was negligible which demonstrated that the interaction of reactor metal tube could be neglected. A certain amount of quartz sand (60 g, 80 mm height, particle size range ) 0.8-0.9 mm) was displayed on the distributor plate as preheating the feeding gas and was not fluidized during the test. The fresh oxygen carrier was added into the reactor above the quartz bed. Then the reactor was initially heated in an inert atmosphere to reaction temperature. When the desired reaction temperature was reached and became stable, the reaction gases were introduced to replace pure nitrogen. The concentration of product gas was recorded by the online gas analyzers and the average value for each two minute period was obtained, which may have decreased the influence of gas dispersion from the reactor to the gas analyzers. According to the literature, the technical approach of the CLC system for solid fuels would adopt steam gasification of coal.20 The simulated coal gasification gas ratio was determined by the results of steam gasification of a typical Chinese Shen-Hua bituminous coal within 900-950 °C. The overall dry basis concentration of H2, CO, CH4, and CO2 was 57.65%, 18.06%, 2.30%, and 21.99%, respectively. The coal gasification gas ratio at 950 °C was approximate to that at 900 °C. To simulate the gas concentration, a simplified ratio of H2/CO/CO2 at 50/25/ 25% was adopted in this study. The oxygen carrier mass was 30 g. The reduction time was set to 20 min. When the reduction was finished the gas was switched to nitrogen. The data in this stage were also recorded for 5 min until the product gas was cleared away. Then the heater was shut down. The reduced oxygen carrier particle was cooled in the nitrogen flow to room temperature and collected for further analysis. The particles were sealed in nitrogen atmosphere and kept dry to prevent oxidation. The inlet reduction gas flow rate was 600 mL/min. This corresponds to velocities of 6.56-8.15 umf for the reaction gas

in the temperature range of 890-950 °C. The umf was the minimum fluidization velocity and calculated using the equation described by Wen and Yu.43 The inert gas flow rate after reduction and oxidation period was 600 and 1200 mL/min, respectively. The terminal velocity ut was also calculated. More information concerning the fluidization properties can be seen in our recent accepted paper.42 The specific operation conditions of the reduction tests are shown in Table 1. 2.4. Data Evaluation. The conversion of oxygen carrier or degree of oxidation (X) is defined as m - mred mox - mred

X)

(7)

The conversion of H2 and CO as a function of time is defined as γCO,i )

γH2,i )

( (

∫ n˙ 1∫ n˙ ti

outyco,outdt

t0

ti

t0

∫ n˙ 1∫ n˙ ti

inyco,indt

outyH2,outdt

t0

ti

t0

inyH2,indt

) )

× 100%

(8)

× 100%

(9)

The CO2 capture yield as a function of time is defined as the amount of CO2 in the product gas divided by the total amount of carbon introduced during the reduction period and defined as γCO2,i )

∫

ti

t0

∫

ti

n˙outyco2,outdt

n˙in(yco,in + yco2,in)dt

t0

× 100%

(10)

The mass balance of carbon is defined as the carbon balance ratio (Cout/Cin), as follows: Cout ⁄ Cin )

∫

ti

t0

n˙out(yco2,out + yCO,out + yCH4,out)dt

∫

ti

t0

n˙in(yco,in + yco2,in)dt

× 100% (11)

The carbon deposition ratio, Cdep/Cin, is the amount of carbon formed during the reduction period. The definitions are referenced from the literature.44 Cdep ⁄ Cin ) 100 - Cout ⁄ Cin

(12)

The conversion of the CaSO4 oxygen carrier as a function of time for the reducing period can be calculated assuming the mass balance of oxygen from the oxygen carrier to the product gas. In this study the formula is revised to facilitate the calculation of kinetic data considering the long hold-up time. Xi ) Xi-1 -

∫

ti

ti-1

MO (n˙ y - n˙outyco,out + n˙inyH2,in moxRO in co,in n˙outyH2,out + 2n˙outySO2,out)dt (13)

MO dX ) (n˙ y - n˙outyco,out + n˙inyH2,in dt moxRO in co,in n˙outyH2,out + 2n˙outySO2,out) (14) To compare the reactivity of different oxygen carriers, a massbased conversion was often applied as follows: ω)

m ) 1 + RO(X - 1) mox

(15)


The mass-based reduction rate was calculated as follows: dω dX ) RO dt dt

(16)

More information on the data evaluation can be found in the references.6-11,22,45 3. Results and Discussion 3.1. Reactivity Investigation. 3.1.1. Gas Concentration Profiles. The product gas concentration profiles as a function of time at typical reaction temperature of 900 and 950 °C are presented in Figure 1. The time started when N2 was turned off and reaction gas was introduced into the reactor. For the reduction at 900 °C as shown in Figure 1a, the concentration of CO2 first increased to peak at 5 min and decreased gradually. The CO and H2 concentration increased to 34.4% and 25.2% at the end of reduction, respectively. whereas, for the reduction at 950 °C as shown in Figure 1b, the concentration of CO2 was as high as 94.1% at around 6 min. The concentration of CO slightly increased to 10% at 14 min and H2 was not observed until 16 min. At the late reduction period the concentration of CO2 decreased while the CO and H2 concentrations increased. CH4 was not observed at 900 °C, while at 950 °C it increased to 0.18% at the initial reduction period but soon decreased to 0 after 10 min. The results demonstrate that the methanation (CO + H2 f CH4 + H2O) was negligible in this study. This is different from the results in the literature where metal served as a catalyst.23 As examined by a gas chromatograph there were no other hydrocarbons detected from the sample gas. The results of product gas concentration illustrated that CaSO4 oxygen carrier exhibited high reduction reactivity with coal gas during the initial reduction period and increased at higher temperature. The dry product gas flow behavior is also presented in Figure 1. The gas flow was calculated relative to the gas analysis from the balance of nitrogen gas diluted in the outlet of reactor before the gas analyzers. The variation of dry product gas flow rate reflects the transient changes in gas concentration and dependent on the time delay caused by the resident time between the reactor and the condenser. Peak of flow at the switching time was also observed because of the influence of nitrogen. The behavior is similar to the previous study on a fluidized bed reactor of Chalmers University of Technology in Sweden.7,9,46 3.1.2. Sulfur Release Profiles. The emission of SO2 and H2S during the reduction test at different temperatures was observed as shown in Figure 2. Because this study focused on the reactivity of CO and H2 with CaSO4 oxygen carrier, therefore, the influence of SO2 was not performed in this study. It is evident that temperature has significant influence on the emission of SO2 and H2S during the reduction period. At 890 °C, the SO2 concentration was relatively low. The maximum SO2 concentration increased as the temperature increased. This trend was in accordance with our previous study on TGA. As shown in Figure 2b, the variation of H2S as a function of time was also largely dependent on the reaction temperature. The maximum and total value of H2S was much lower than that of SO2. The peak time of H2S was also much later than that of SO2 which indicated that the H2 was converted to H2O with CaSO4 through the target reaction at the early stages. The competing reaction that generates H2S occurred at the middle stage of reduction and decreased at the end of reduction. According to the previous study on sulfur capture,47-52 the release of SO2 involved with CaSO4 and CaS is a complex process. In this study, as illustrated above, SO2 formation may

Figure 1. Product gas concentrations as a function of time at typical reaction temperature of (a) 900 and (b) 950 °C. The dash line indicates the switch to N2.

be mainly due to the solid-solid reactions during the reduction period (eq 17). The competing side reactions of CO and H2 with CaSO4 also occur under reducing conditions: CaS + 3CaSO4 f 4CaO + 4SO2 θ ∆H298K ) 1047.93 kJ/mol (17)

CaSO4 + CO f CaO + CO2 + SO2 θ ∆H298K ) 219.24 kJ/mol (18)

CaSO4+H2 f CaO + H2O + SO2 θ ∆H298K ) 260.38 kJ/mol (19)

CaSO4 + 4H2 f CaO + 3H2O + H2S θ ∆H298K ) 53.04 kJ/mol (20)

The emission of sulfur in this work is only for the reduction of fresh oxygen carrier, further studies on formation of sulfur in oxidation process and cyclic test of reduction and oxidation in CLC system are reported in another article. 3.1.3. Fuel Gas Conversion. The effect of reaction temperature on the overall conversion of H2 and CO during each reduction run is shown in Figure 3. Clearly, the reaction

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Figure 2. The outlet (a) SO2 and (b) H2S concentrations as a function of time during reduction.

Figure 4. CO2 capture yield (γCO2) as a function of time for the reduction in the temperature range of 890-950 °C.

Figure 3. Gas conversion as a function of reaction temperature during reduction.

temperature had a strong effect on the reduction reaction. The conversion of H2 was around 70% when the temperature was between 890 and 900 °C. It increased to 83.3% at 910 °C and 96.5% at 950 °C. The variation of CO conversion was similar but much lower than that of H2. The difference might reflect that the reactivity of H2 with CaSO4 oxygen carrier was much higher than that of CO. This is in accordance to the previous thermodynamic analysis and experimental studies.20,22,25 Previous studies of chemical looping combustion using metal oxide as oxygen carrier also demonstrated that H2 exhibited higher reactivity than CO.22,25 In this study at low temperatures the product gas concentrations from the reactor may be changed by the reverse water-gas shift (RWGS) reaction under the experimental conditions, which causes the faster consumption of H2 and lower conversion of CO. When the RWGS equilibrium is attained in the entire reactor, the amount of oxygen transfer is determined. In the further study, a gas mixture of CO/H2 and CO/H2/N2 may be more reasonable to obtain the conversion of CO and H2 because the reactivity is important for reactor design.53 3.1.4. The CO2 Capture Yield. Figure 4 shows the variation of the CO2 capture yield during the reduction period at different

temperatures. Because of the residence time and measure time which has been accounted for as the flow behavior above, the variation of CO2 capture with time is similar and increased at higher temperatures. It was 57.2% at 890 °C and increased to 84.8% at 950 °C. The variation of CO2 capture at different temperatures is in line with the results of gas conversion. The curve at 900 °C seems much different from the other temperatures at the early stage of reduction. This may be due to the flow behavior. The high CO2 capture yield during the reduction that could be obtained at high temperatures also illustrated the effect of temperature on reactivity of the CaSO4 oxygen carrier. 3.1.5. Carbon Deposition. Previous studies have investigated the carbon formation on nickel-based oxygen carrier particles for chemical looping combustion of methane and coal gas.23,44,46 Carbon formation could occur through catalytic methane decomposition (CH4 f C + 2H2) or the Boudouard reaction (2CO f C + CO2). In this study the carbon deposition on CaSO4 oxygen carrier during reduction is investigated from the mass balance of carbon. As presented in Figure 5, the carbon balance ratio is overall high in the temperature range of 890-950 °C. It decreased a little to 99.02% with the reduction temperature increased to 930 °C and then the carbon balance ratio increased to 99.50% at 950 °C. The extent of carbon formation changed accordingly. It is evident that the carbon formation ratio was quite low in this study. The CaSO4 oxygen carrier was sufficient in oxygen


Figure 5. Carbon balance ratio (Cout/Cin) and carbon deposition ratio (Cdep/ Cin) as a function of the reduction temperature.

due to its high oxygen ratio (Ro) and the degree of oxidation (X) was still quite high during reduction. This verified the explanation as reported by the literature that the carbon formation was negligible as long as sufficient oxygen was present in the particles to oxidize the carbon.44 The reason might also be ascribed to the suppressing of the Boudouard reaction from the viewpoint of stoichiometry because of the influence of CO2 concentration. Overall, the carbon deposition on CaSO4 oxygen carrier in this study was believed to be suppressed in reduction with simulated coal gas. Carbon deposition is not the key issue in this study. It is now generally accepted that the carbon deposition does not appear to be a major problem if the oxidation reaction is carried out at a high enough temperature. 3.1.6. The Oxygen Carrier Conversion and Reaction Rate. The effect of reaction temperature on the conversion of CaSO4 during the 20 min reduction is shown in Figure 6. The decrease of degree of oxidation Xi as a function of time after each reduction period indicates the increase of extent of reduction and that more oxygen in the oxygen carrier was transferred to the fuel gas or sulfur release. The oxygen carrier was not completely reduced. The degree of oxidation decreased from 1.0 to 0.5544 after the reduction at 950 °C; that is, around 44.56% of mass in the natural anhydrite was transferred. The conversion was also dependent on the reduction time. For a longer reduction time the conversion was higher. From the slope of the curves, it can be observed that at higher temperatures the degree of oxidation decreased much faster. Figure 6b shows the comparison of the mass-based reaction rates for different temperatures. The reaction rate was evidently high at the initial reduction period and increased at higher temperatures. This result also indicates that higher temperatures would enhance reaction rates and result in high conversion of oxygen carrier, which is in accordance with the gas conversion and some investigations on the reduction of calcium sulfate.54,55 The maximum value of the mass-based conversion rate at 950 °C was very stable around 1.7 × 10-4 s-1, which is considerably lower than that of the Fe2O3-based oxygen carrier in the literature.7 The mass balance calculated for 950 °C shows that the oxygen mass transferred to fuel gas amounts to 5.8847 g, accounting for 19.62 mass % of the fresh particles. The SO2 release is around 0.04536 g, which means that 0.15 mass % is lost mainly due to side reactions. The residue mass is 80.23%, which is in accordance with the change of mass conversion ∆ω in Figure 6b, which also well demonstrates the application of eq 13.

From the results of reduction reactivity investigation, it can be concluded that temperature has important influence on the reduction reaction. Based on the experimental results, the suitable temperature for higher reduction reactivity in the fuel reactor seems to be around 950 °C. 3.2. Analysis of CaSO4 Oxygen Carrier. 3.2.1. Phase Characterization. Samples of fresh and reduced oxygen carrier particles were extracted for X-ray powder diffraction studies. The respective powder XRD patterns of the fresh and reduced CaSO4 oxygen carrier at different temperatures are shown in Figure 7a-e. To simplify the comparison, the crystalline phases are all substituted with chemical compounds for compound identification by JADE 7.0; for example, CaSO4 is substituted for anhydrite, and CaO for lime. The number marked above the peak is the relative intensity. It is evident that temperature has significant influence on the product crystalline phase of the oxygen carrier. The presence of anhydrite (CaSO4) as the main crystalline phase in the fresh oxygen carrier is clearly evidenced in Figure 7a, which is consistent with the result of chemical composition analysis. The most intense reflection is located at 2θ ) 25.8° marked as CaSO4. Some other weak reflection peaks are also identified as CaSO4. In Figure 7b-e, the intense reflection of the CaS phase at 2θ ) 31.3°, 44.9°, 55.8°, 65.4°, 74.3°, and 82.7° strongly demonstrated that the reduction product was CaS rather than CaO. The peak of CaS at 950 °C was comparable to that of CaSO4. The increase of relative intensity of CaS with the increase of reduction temperature was in accordance to the results of solid conversion. It can be observed that CaO is identified at 2θ ) 32.1°, 37.3°, and 53.84°. The presence of CaO at low temperatures might be partly attributed to a calcined product of MgCa(CO3)2 identified at 2θ ) 31.3° and 37.3° in the fresh oxygen carrier. The relative intensity of CaO slightly increased with the increase of reduction temperature, which suggests the formation of SO2 and H2S during the reduction. In summary, the XRD analysis of fresh and reduced oxygen carriers verified that CaS may be the dominant reduced product at strong reducing conditions and that it is strongly influenced by the reduction temperature. At higher temperatures the slight formation of CaO also took place because of side reactions. To obtain total conversion to CaS, a certain partial pressure of SO2 is needed. This will be studied in the future. 3.2.2. Surface Morphology and Element Distribution. The particles of CaSO4 oxygen carrier were examined using the scanning electron microscope before and after reduction. Figure 8 panels a and b show the surface morphology of the oxygen carrier particle before reduction, which display that the fresh oxygen carrier is impervious. After reduction at 900 °C as shown in Figure 8c,d, the surfaces of reduced oxygen carrier are clearly much rougher and more porous. Small grains of a size around 1 µm appeared as a consequence of the reduction reaction on the surface of CaSO4 oxygen carrier particles. While for particles reduced at 950 °C as shown in Figure 8e,f, the appearance of the surface was basically similar to that shown in Figure 8c,d. However, one difference from the particle at 900 °C was that the size of crystal grains became a little larger in a compact agglomerated state. It seemed that the small grains on the surface of oxygen carrier sintered at higher temperatures. This result was possibly due to the solid-solid reaction. The overall results are in agreement with the previous studies.55 Besides, the uniform porous surface of oxygen carrier also verified the full fluidization state during the reduction experiment period. In our previous studies, the surface of reduced oxygen carrier particles in a fixed bed reactor was much different.


Figure 6. (a) Oxygen carrier conversion (X) as a function of time, and (b) mass-based reaction rates (dω/dt) as a function of mass conversion (ω) for the reduction period in the temperature range of 890-950 °C.

Figure 7. XRD patterns of fresh CaSO4 oxygen carrier particles and reduced oxygen carrier at typical temperatures.

The EDS analysis of the surface shown in Figure 8 was also performed. The spectrum and average accurate quantitative analysis of atomic percentage of major elements are shown in Figure 9 and Table 2, respectively. The peak of Ca and S was not fully displayed but the relative intensity can be clearly viewed. The average value of O/Ca and S/Ca of fresh and used oxygen carrier particles were obtained. For the fresh CaSO4 particles shown in Figure 8a, the O, S, and Ca peaks were the dominant ones in Figure 9a. There was a slight peak of Mg observed. The S and Ca peaks were comparable with a S/Ca ratio of 1.146, and the O/Ca ratio was 4.211, which verified CaSO4 as the main component of the oxygen carrier particles. This result is in agreement with the literature; that is, the S/Ca ratio of 1.1 is expected for both CaS and CaSO4. After reduction at 900 °C, the intensity of O decreased significantly as shown in Figure 9b, while the peak of S was still comparable to that of Ca. The results demonstrated the transfer of oxygen from the CaSO4 oxygen carrier to the fuel gas. However, the atomic ratio of S/Ca also decreased slightly to 0.9653 which suggested that the loss of sulfur and formation of CaO on the surface. As for the particles reduced at 950 °C shown in Figure 8f, the element distribution spectrum in Figure 9c was much different from that in Figure 9b. The S peaks decreased evidently while the O peak slightly increased. The S/Ca and O/Ca decreased to 0.5251 and 0.8832, respectively. The results suggested that there

was a certain amount of sulfur loss and CaO formed on the surface. This demonstrated that both reduction and side reactions were enhanced at higher temperatures. The ESEM-EDS study demonstrated the variation of surface morphology at different temperatures and the element transfer on the surface. The results were in agreement with the reactivity studies of CaSO4 oxygen carrier and XRD analysis. 3.3. Reduction Kinetics. Many investigations have successfully applied the shrinking unreacted-core model (SCM) to describe the reaction mechanisms of reduction of CaSO4 and oxidation of CaS.56-58 The shrinking core model has also been applied for kinetic modeling in the CLC system.25,59-61 There are few publications on the reduction kinetics study on chemical looping combustion in a fluidized bed reactor.62 As presented in the literature, the reaction of metal oxides in CLC was so fast within a well-mixed fluidized bed that it is difficult to obtain the intrinsic kinetic parameters from fluidized bed experiments. However, these data may still be valid to judge the reactivity of different oxygen carriers. On the basis of the data obtained in the reactivity tests and the above characterization analysis, the reduction kinetics of CaSO4 oxygen carrier with simulated coal gas may be tentatively obtained using the shrinking core model. The oxygen carrier solid particles are assumed spherical as shown in Figure 8a. During the reduction of CaSO4, H2 and CO diffuse to the CaSO4 core through the film surrounding the particle and react with the CaSO4 on the surface to form CaS, H2O, and CO2. A small amount of CaO was also possibly formed via the side reactions. The particle size was approximately the same as before reduction as demonstrated in the ESEM study. The inner core is CaSO4 and the outer layer is mainly CaS. CaS is much more porous than CaSO4 as observed from the ESEM-EDS study. The porous product layer provides less resistance for the diffusion of CO and H2 through the film surrounding the particles, and the reaction can proceed further to the core. The gaseous products, mainly H2O and CO2 diffuse from the solid reaction surface into the surrounding gas stream. Besides, the high fluidization velocity may also eliminate the external diffusion resistance, and the small particle size applied in the present work may also decrease the internal mass transfer resistance. Also the temperature within the bed was stable and the particles could be considered isothermal. The reaction rate for CaSO4 oxygen carrier is much lower which indicates the reduction kinetics may be different from the condition in the literature.62 Therefore,


Figure 8. SEM micrographs of the fresh and reduced oxygen carrier particles at typical temperatures.

Figure 9. Spectrums collected from the oxygen carrier particles for (a) fresh, (b) reduced at 900 °C, and (c) reduced at 950 °C.

the chemical reaction could be the only limiting mechanism that controls the reduction process in this work.

The kinetic equation for the shrinking core model for spherical grain with chemical control is

8156 Ind. Eng. Chem. Res., Vol. 47, No. 21, 2008 Table 2. Atomic Ratio of Major Elements on the Surface of Fresh and Reduced CaSO4 Oxygen Carrier Particles species

fresh

reduced at 900 °C

reduced at 950 °C

S/Ca O/Ca

1.146 4.211

0.9653 0.3056

0.5251 0.8832

1 - (1 - Xr)1⁄3 )

t τ

1 bkCn kapp ) ) τ Fmrg

(21)

To be in line with the literature and more readable, here the conversion of oxygen carrier Xr is defined as Xr ) 1 - X. The oxygen carrier conversions for CO and H2 were recalculated separately with the same method in eq 13. The coefficient of the SO2 term is 1 for both H2 and CO considering the SO2 release from eqs 18 and 19. Xr,H2 )

∫

ti

t0

MO (n˙ y - n˙outyH2,out + n˙outySO2,out)dt (22) moxRO in H2,in

ti MO (n˙inyco,in - n˙outyco,out + n˙outySO2,out)dt (23) Xr,CO ) t 0 m R ox O

∫

1

The stoiometric factor b for eqs 4 and 5 is /4 in this work. The reaction order n is 1 in this work; the reaction order of H2 and CO with CaSO4 has been proved to be first order in the literature and our recent kinetic study in a differential reactor.55,56 With the oxygen carrier conversion data as a function of time at different temperatures, the curves of 1 - (1 - Xr)1/3 with time t can be plotted and kapp can be obtained through linearization. Then the kinetic constant k was obtained through eq 21, and it follows an Arrhenius type of expression: k ) k0 exp(-Ea ⁄ RT)

(24)

Figure 10 displays the Arrhenius plot for the chemical reaction rate constant of the reduction for different gas in the temperature range of 890-950 °C. The parameters k0, Ea, and correlation coefficient R2 for the linearization plot in Figure 10 were calculated and displayed in Table 3. The fitting data using eq 24 are shown in the solid line in Figure 11a and Figure 11b. As shown in Figure 11, at low temperatures, the fitting data deviates from the experimental data especially for the oxygen carrier conversion reacted by CO. The reason may be attributed to side reactions at low temperatures. The reverse water-gas shift reaction occurs in the feeding gas at low temperatures which may greatly influence the gas conversion. For the kinetic data within the temperatures of practical interest (910-950 °C), the kinetic equation was in good agreement with the experimental results. The results also demonstrated that the reactivity of H2 was higher than that of CO judging from the apparent activation energies. This interference of a side reaction has been also been observed by Abad et al.25 By reference to the method they applied, for reduction reaction using CO and H2 mixtures, the addition of the reaction rates for H2 and CO can be summed together by using a value for τ obtained as τ)

(

1 1 + τH2 τCO

)

-1

(25)

Therefore, the overall apparent rate constant k for fuel gas can be calculated using the following equation kapp ) kapp, H2 + kapp, CO )

bkCCO+H2 Fmrg

(26)

The Arrhenius plot of kapp for the H2 and CO mixture is also presented in Figure 10, and the parameters are listed in Table

Figure 10. Arrhenius plot for the chemical reaction rate constant of the reduction for different gas in the temperature range of 890-950 °C.

3. The fitting data using eq 26 is shown in the solid line in Figure 12. The fitting line well predicted the experimental results of the oxygen carrier conversion. As referenced in the literature, some other kinetic models were also calculated, such as the product layer diffusion control62 and nucleation and growth model;55 the linearization of the kinetic equation seems not as good as the chemical reaction control. This study focused on the reactivity of CaSO4 oxygen carrier with simulated coal gas; the kinetic data obtained may only be used to judge the reactivity of the CaSO4 oxygen carrier. Further extensive kinetic study is needed to determine the intrinsic reduction and oxidation kinetics of CaSO4/CaS oxygen carrier. Recently some preliminary experiments on the reduction of CaSO4 with CO and H2 and CaS oxidation have been done in a differential fixed bed reactor referring to the literature,63 and the overall results seems to be well in agreement with the shrinking core model. 3.4. Some Considerations for Use in CLC of Solid Fuels. There are several significant technical issues that should be considered for chemical looping combustion of solid fuels.20 As for the CaSO4 oxygen carrier, extensive considerations for the design criteria of a CLC system of solid fuels using CaSO4 oxygen carrier has been discussed in another article.42 As presented in this study, the reaction rates of the CaSO4 oxygen carrier obtained in this study was much lower. Some investigations on mixed oxygen carriers have been performed and different synergy effects were created.4,9,64 Also nickel and iron catalyst may promote the reduction and regeneration of a CaSO4 oxygen carrier.54,55 Considering the advantages of high oxygen transport capacity and low cost of CaSO4, it would be promising if the CaSO4 oxygen carrier can be mixed with some cheap active metal oxides, which may create a synergy effect on enhancing the reactivity and avoiding the side reactions and secondary pollutants. In a CLC system for solid fuels, when the coal is gasified or burned, products such as unreacted char and ash would be produced. These solid products should be separated from the oxygen carrier particles. Our recent experiments on the use of solid fuels such as coal and biomass in a bench scale CLC system using nickel-based and iron-based oxygen carriers demonstrate that this separation may not be a problem. During the experiments small fuel particles were fed into the fuel reactor. After gasification the coal particles generated smaller particles. These smaller particles have different fluidizing properties from the oxygen carrier particles, considering their


Figure 11. Kinetic model fitting of oxygen carrier conversion Xr as a function of time in the temperature range of 890-950 °C for (a) H2 and (b) CO. Solid symbols, experimental results obtained by eqs 22 and 23; continuous lines, fitting data using eq 24. Table 3. Kinetic Model Parameters and Correlation Coefficient (R2) for the Reduction of CaSO4 Oxygen Carrier parameters

H2

CO

H2 + CO

890-910 °C

Ea, kJ/mol ko, m/s R2

160.19 1.52 × 103 0.95

769.06 8.76 × 10-29 0.99

222.44 7.97 × 10-5 0.93

910-950 °C

Ea, kJ/mol ko, m/s R2

21.75 1.20 × 10-3 0.99

61.76 4.87 × 10-2 0.96

35.84 4.78 × 10-3 0.99

differences in particle density and particle size distribution, therefore, they were easily separated and elutriated with the fluidizing steam and CO2 and separated in the cyclone. This separation was also effective for CaSO4/CaS in our preliminary work using the CaSO4 oxygen carrier. According to the literature, the remaining unburned carbon can be recollected and fed to the fuel reactor or directly combusted.20 However, it was also observed that some smaller oxygen carrier particles were inevitably formed because of cracking and fragmentation in long-time fluidization and lost with ash together. Oxygen carriers may also suffer from sulfur compounding and reaction with ash components, and loss of material during the separation with ash. All these findings are in agreement with the discussion in the literature.20,33,40 The oxygen carriers for solid fuels may have to be replaced with fresh particles, therefore, low cost natural ore like ilmenite, anhydrite ore, and iron ore would be more economical and feasible in practical use. Further study would be performed to investigate the combination of these natural ores in the CLC system for solid fuels. 4. Conclusions In this study chemical-looping combustion of simulated coal gasification gas with CaSO4 as oxygen carrier was conducted in a laboratory-scale fluidized bed reactor. The effect of reaction temperature on reduction reactivity was investigated in the range of 890-950 °C. A high concentration of CO2 was obtained at 950 °C. The H2 and CO conversions were greatly influenced by the reaction temperature. CaSO4 oxygen carrier exhibited high reactivity with simulated coal gas during the initial reduction period and decreased gradually at the late period of reduction. The sulfur release during reduction was also analyzed. The SO2 and H2S concentrations displayed different peak time and were significantly influenced by temperature. Carbon deposition during the reduction period was found quite low due to the abundant oxygen provided. Through the comparison of

Figure 12. Kinetic model fitting of oxygen carrier conversion as a function of time in the temperature range of 890-950 °C for H2 and CO mixtures. Solid symbols: experimental results, continuous lines: fitting data using eq 26.

mass-based reaction rates during the reduction at typical temperatures, it is evident that higher temperatures would enhance reaction rates and result in a high conversion of oxygen carrier. XRD patterns study indicated that CaS was the dominant product of reduction, and the relative intensity is in agreement with the solid conversion. The relatively higher content of CaO at 950 °C suggested the formation of SO2 and H2S at high temperatures. ESEM analysis of the fresh and reduced oxygen carrier particles indicated that the surface structure changed significantly from impervious to porous after reduction. The slight agglomeration of small grains occurred for reduced particles at 950 °C. EDS analysis demonstrated the transfer of oxygen from the CaSO4 oxygen carrier to the fuel gas. The EDS results also suggested that there was certain amount of sulfur loss and CaO formed on the surface due to side reactions at higher temperatures. The reduction kinetics of CaSO4 oxygen carrier was explored with the shrinking unreacted-core model. The apparent kinetic parameters were obtained and the fitting data well predicts the experimental data. Finally, some basic considerations on the use of CaSO4 oxygen carrier in a CLC system for solid fuels were discussed.


Acknowledgment

Literature Cited

The support from National Natural Science Foundation of China (Grant Nos. 50606006, 90610016) for this study is gratefully acknowledged. The authors thank Xiao Ban for her assistance in carrying out the experimental work. We also acknowledge Dr. Marcus Johnson of Chalmers University of Technology in Sweden for providing valuable suggestions for data evaluation. Special gratitude is extended to Prof. E. J. Anthony for valuable comments on CaSO4 oxygen carrier which contributed to our further study. We also sincerely acknowledge Dr. Alberto Abad in CSIC in Spain for valuable advice on kinetic study on chemical looping combustion.

(1) Lyngfelt, A.; Leckner, B.; Mattisson, T. A Fluidized-Bed Combustion Process with Inherent CO2 Separation; Application of Chemical-Looping Combustion. Chem. Eng. Sci. 2001, 56 (10), 3101–3113. (2) Xiang, W.; Wang, S.; Di, T. Investigation of Gasification Chemical Looping Combustion Combined Cycle Performance. Energy Fuels 2008, 22 (2), 961–966. (3) Ishida, M.; Jin, H.; Okamoto, T. A Fundamental Study of a New Kind of Medium Material for Chemical-Looping Combustion. Energy Fuels 1996, 10 (4), 958–963. (4) Jin, H.; Okamoto, T.; Ishida, M. Development of a Novel ChemicalLooping Combustion: Synthesis of a Looping Material with a Double Metal Oxide of CoO-NiO. Energy Fuels 1998, 12 (6), 1272–1277. (5) Jin, H. G.; Okamoto, T.; Ishida, M. Development of a Novel Chemical-Looping Combustion: Synthesis of a Solid Looping Material of NiO/NiAl2O4. Ind. Eng. Chem. Res. 1999, 38 (1), 126–132. (6) Mattisson, T.; Jardnas, A.; Lyngfelt, A. Reactivity of Some Metal Oxides Supported on Alumina with Alternating Methane and OxygenApplication for Chemical-Looping Combustion. Energy Fuels 2003, 17 (3), 643–651. (7) Mattisson, T.; Johansson, M.; Lyngfelt, A. Multicycle Reduction and Oxidation of Different Types of Iron Oxide Particles - Application to Chemical-Looping Combustion. Energy Fuels 2004, 18 (3), 628–637. (8) Cho, P.; Mattisson, T.; Lyngfelt, A. Comparison of Iron-, Nickel-, Copper- and Manganese-Based Oxygen Carriers for Chemical-Looping Combustion. Fuel 2004, 83 (9), 1215–1225. (9) Johansson, M.; Mattisson, T.; Lyngfelt, A. Creating a Synergy Effect by Using Mixed Oxides of Iron- and Nickel Oxides in the Combustion of Methane in a Chemical-Looping Combustion Reactor. Energy Fuels 2006, 20 (6), 2399–2407. (10) Johansson, M.; Mattisson, T.; Lyngfelt, A. Use of NiO/NiAl2O4 Particles in a 10 kW Chemical-Looping Combustor. Ind. Eng. Chem. Res. 2006, 45 (17), 5911–5919. (11) Mattisson, T.; Johansson, M.; Lyngfelt, A. The Use of NiO as an Oxygen Carrier in Chemical-Looping Combustion. Fuel 2006, 85 (5-6), 736–747. (12) Adańez, J.; de Diego, L. F.; Garcıá-Labiano, F.; Gayań, P.; Abad, A.; Palacios, J. M. Selection of Oxygen Carriers for Chemical-Looping Combustion. Energy Fuels 2004, 18 (2), 371–377. (13) de Diego, L. F.; Garcıá-Labiano, F.; Adańez, J.; Gayań, P.; Abad, A.; Corbella, B. M.; Maria Palacios, J. Development of Cu-Based Oxygen Carriers for Chemical-Looping Combustion. Fuel 2004, 83 (13), 1749– 1757. (14) de Diego, L. F.; Garcıá-Labiano, F.; Gayań, P.; Celaya, J.; Palacios, J. M.; Adańez, J. Operation of a 10 kWth Chemical-Looping Combustor During 200 h with a CuO-Al2O3 Oxygen Carrier. Fuel 2007, 86 (7-8), 1036–1045. (15) de Diego, L. F.; Gayań, P.; Garcıá-Labiano, F.; Celaya, J.; Abad, M.; Adańez, J. Impregnated CuO/Al2O3 Oxygen Carriers for ChemicalLooping Combustion: Avoiding Fluidized Bed Agglomeration. Energy Fuels 2005, 19 (5), 1850–1856. (16) Adańez, J.; Gayań, P.; Celaya, J.; de Diego, L. F.; Garcıá-Labiano, F.; Abad, A. Chemical Looping Combustion in a 10 kW(th) Prototype Using a CuO-Al2O3 Oxygen Carrier: Effect of Operating Conditions on Methane Combustion. Ind. Eng. Chem. Res. 2006, 45 (17), 6075–6080. (17) Ryu, H. J.; Bae, D. H.; Jin, G. T. Effect of Temperature on Reduction Reactivity of Oxygen Carrier Particles in a Fixed Bed ChemicalLooping Combustor. Korean J. Chem. Eng. 2003, 20 (5), 960-966. (18) Varma, A.; Diokov, V.; Shafirovich, E. Heterogeneous Combustion: Recent Developments and New Opportunities for Chemical Engineers. AIChE J. 2005, 51 (11), 2876–2884. (19) Erri, P.; Varma, A. Spinel-Supported Oxygen Carriers for Inherent CO2 Separation During Power Generation. Ind. Eng. Chem. Res. 2007, 46, 8597–8601. (20) Cao, Y.; Pan, W. P. Investigation of Chemical Looping Combustion by Solid Fuels. 1. Process Analysis. Energy Fuels 2006, 20 (5), 1836– 1844. (21) Anthony, E. J. Solid Looping Cycles: A New Technology for Coal Conversion. Ind. Eng. Chem. Res. 2008, 47 (6), 1747–1754. (22) Abad, A.; Mattisson, T.; Lyngfelt, A.; Johansson, M. The Use of Iron Oxide as Oxygen Carrier in a Chemical-Looping Reactor. Fuel 2007, 86 (7-8), 1021–1035. (23) Jin, H.; Ishida, M. A New Type of Coal Gas Fueled ChemicalLooping Combustion. Fuel 2004, 83 (17-18), 2411–2417. (24) Mattisson, T.; Garcıá-Labiano, F.; Kronberger, B.; Lyngfelt, A.; Adanez, J.; Hofbauer, H. Chemical-Looping Combustion Using Syngas as Fuel. Int. J. Greenhouse Gas Control 2007, 1 (2), 158–169.

Nomenclature b ) stoichiometric factor in the reduction, mol of CaSO4 per mole of fuel gas, b)1/4 in this work Ci ) molar concentration of gas species i in the inlet gas, mol m-3 Cin) total amount of carbon in the fuel gas, mol Cout ) total amount of carbon in the product gas, mol Cdep ) the amount of deposited carbon, mol Ea ) apparent activation energy, kJ mol-1 k ) kinetic rate constant, m s-1 kapp ) apparent rate constant, s-1 k0 ) preexponential factor rate constant of the reaction, m s-1 m ) actual mass of oxygen carrier sample in the reactor, kg mox ) mass of the sample when fully oxidized, kg mred ) mass of the sample in the reduced form, kg MO ) atomic mass of oxygen, 16 g mol-1 n ) reaction order, n ) 1 in this work n˙in ) molar flow of the gas entering the reactor, mol s-1 n˙out ) molar flow of the gas exiting the reactor after the water has been removed, mol s-1 no ) amount of oxygen in the oxygen carrier that can be removed from the fully oxidized oxygen carrier, mol rg ) grain radius, m R ) Constant of the ideal gases, 8.314 J mol-1 K-1 Ro ) oxygen transport capacity, or oxygen ratio of the oxygen carrier, defined as Ro ) (mox - mred)/mox t ) time, s T ) absolute temperature, K u ) the mean velocity, m s-1 umf ) minimum fluidization velocity, m s-1 ut ) terminal velocity, m s-1 dXr/dt ) the reduction rate of oxygen carrier, s-1 X ) oxygen carrier conversion, or degree of oxidation, X ) (m mred)/(mox - mred) Xr ) oxygen carrier conversion during reduction period, Xr ) (mox - m)/(mox - mred), Xr ) 1 - X yi,in ) inlet molar fraction of the gas species i entering the reactor yi,out ) outlet moloar fraction of the gas species i exiting the reactor after the water removed Greek letters ∆P ) pressure difference, kPa ∆X) variation of the conversion of oxygen carrier ∆ω ) variation of the mass conversion of oxygen carrier γCO ) the conversion of CO during reduction period, % γCO2 ) CO2 capture yield during the reduction period, % γH2 ) the conversion of H2 during reduction period, % Fm ) the molar density of CaSO4 in the solid, mol m-3 τ ) time for complete solid conversion, s ω ) mass based conversion of oxygen carrier

Ind. Eng. Chem. Res., Vol. 47, No. 21, 2008 8159 (25) Abad, A.; Garcıá-Labiano, F.; de Diego, L. F.; Gayań, P.; Adańez, J. Reduction Kinetics of Cu-, Ni-, and Fe-Based Oxygen Carriers Using Syngas (CO + H2) for Chemical-Looping Combustion. Energy Fuels 2007, 21 (4), 1843–1853. (26) Jin, H. G.; Ishida, M. Reactivity Study on a Novel Hydrogen Fueled Chemical-Looping Combustion. Int. J. Hydrogen Energy 2001, 26 (8), 889– 894. (27) Jin, H. G.; Ishida, M. Reactivity Study on Natural-Gas-Fueled Chemical-Looping Combustion by a Fixed-Bed Reactor. Ind. Eng. Chem. Res. 2002, 41 (16), 4004–4007. (28) Pan, W. P.; Cao, Y.; Liu, K. L.; Wu, W. Y.; Riley, J. T. Application of a Circulating Fluidized-Bed Process for the Chemical Looping Combustion of Solid Fuels. Abstr. Pap.sAm. Chem. Soc. 2004, 228, U676–U676. (29) Cao, Y.; Casenas, B.; Pan, W. P. Investigation of Chemical Looping Combustion by Solid Fuels. 2. Redox Reaction Kinetics and Product Characterization with Coal, Biomass, and Solid Waste as Solid Fuels and CuO as an Oxygen Carrier. Energy Fuels 2006, 20 (5), 1845–1854. (30) Dennis, J. S.; Scott, S. A.; Hayhurst, A. N. In Situ Gasification of Coal Using Steam with Chemical Looping: A Technique for Isolating CO2 from Burning a Solid Fuel. J. Energy Inst. 2006, 79 (3), 187–190. (31) Scott, S. A.; Dennis, J. S.; Hayhurst, A. N.; Brown, T. In Situ Gasification of a Solid Fuel and CO2 Separation Using Chemical Looping. AIChE J. 2006, 52 (9), 3325–3328. (32) Leion, H.; Mattisson, T.; Lyngfelt, A. The Use of Petroleum Coke as Fuel in Chemical-Looping Combustion. Fuel 2007, 86 (12-13), 1947– 1958. (33) Leion, H.; Mattisson, T.; Lyngfelt, A. Solid Fuels in ChemicalLooping Combustion. Int. J. Greenhouse Gas Control 2008, 2 (2), 180– 193. (34) Berguerand, N.; Lyngfelt, A. Design and Operation of a 10 kWth Chemical-Looping Combustor for Solid Fuels - Testing with South African Coal. Fuel 2008, 87 (12), 2713–2726. (35) Berguerand, N.; Lyngfelt, A. The Use of Petroleum Coke as Fuel in a 10 kWth Chemical-Looping Combustor. Int. J. Greenhouse Gas Control 2008, 2 (2), 169–179. (36) Leion, H.; Lyngfelt, A.; Johansson, M.; Jerndal, E.; Mattisson, T. The Use of Ilmenite as an Oxygen Carrier in Chemical-Looping Combustion. Chem. Eng. Res. Des., in press. (37) Song, Q. L.; Xiao, R.; Deng, Z. Y.; Zhang, H. Y.; Shen, L. H.; Zhang, M. Y. Chemical-Looping Combustion of Methane with CaSO4 Oxygen Carrier in a Fixed Bed Reactor. Energy ConVers. Manage. 2008, 49 (11), 3178–3187. (38) Carl, B.; Nancy, M. EnVironmentally AdVanced Clean Coal Plants; 19th World Energy Congress; World Energy Council: Sydney, Australia, 2004. (39) Andrus, H. E. J.; Chiu, J. H.; Stromberg, P. T.; Thibeault, P. R. Alstom’s Hybrid Combustion-Gasification Chemcial Looping Coal Power Technology DeVelopment; Twenty-Second Annual International Pittsburgh Coal Conference, Pittsburgh, PA, 2005. (40) Wang, J. S.; Anthony, E. J. Clean Combustion of Solid Fuels. Appl. Energy 2008, 85, 73–79. (41) Shen, L.; Zheng, M.; Xiao, J.; Xiao, R. A Mechanistic Investigation of a Calcium-Based Oxygen Carrier for Chemical Looping Combustion. Combust. Flame 2008, 154 (3), 489–506. (42) 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, doi: 10.1021/ef800275a. (43) Wen, C. Y.; Yu, Y. H. A Generalized Method for Predicting the Minimum Fluidization Velocity. AIChE J. 1996, 12 (3), 610–612. (44) Cho, P.; Mattisson, T.; Lyngfelt, A. Carbon Formation on Nickel and Iron Oxide-Containing Oxygen Carriers for Chemical-Looping Combustion. Ind. Eng. Chem. Res. 2005, 44 (4), 668–676. (45) Mattisson, T.; Lyngfelt, A.; Cho, P. The Use of Iron Oxide as an Oxygen Carrier in Chemical-Looping Combustion of Methane with Inherent Separation of CO2. Fuel 2001, 80 (13), 1953–1962. (46) Cho, P.; Mattisson, T.; Lyngfelt, A. Defluidization Conditions for a Fluidized Bed of Iron Oxide-, Nickel Oxide-, and Manganese Oxide-

Containing Oxygen Carriers for Chemical-Looping Combustion. Ind. Eng. Chem. Res. 2006, 45 (3), 968–977. (47) Mattisson, T.; Lyngfelt, A. Reaction between Sulfur Dioxide and Limestone under Periodically Changing Oxidizing and Reducing ConditionsEffect of Cycle Time. Energy Fuels 1998, 12 (5), 905–912. (48) Lyngfelt, A.; Leckner, B. Sulphur Capture in Fluidized Bed Boilers: The Effect of Reductive Decomposition of CaSO4. Chem. Eng. J. 1989, 40 (2), 59–69. (49) Fernandez, M. J.; Lyngfelt, A.; Steenari, B. M. Reaction between Limestone and SO2 under Conditions Alternating between Oxidizing and Reducing. The Effect of Short Cycle Times. Energy Fuels 2000, 14 (3), 654–662. (50) Anthony, E. J.; Granatstein, D. L. Sulfation Phenomena in Fluidized Bed Combustion Systems. Prog. Energy Combust. Sci. 2001, 27 (2), 215– 236. (51) Qiu, K.; Lindqvist, O.; Mattisson, T. Regeneration of Calcium Sulfide under Alternating Oxidizing and Inert Conditions: Kinetics and Mechanism. Ind. Eng. Chem. Res. 1998, 37 (3), 923–928. (52) Qiu, K.; Anthony, E. J.; Jia, L. Oxidation of Sulfided Limestone under the Conditions of Pressurized Fluidized Bed Combustion. Fuel 2001, 80 (4), 549–558. (53) Abad, A.; Adańez, J.; Garcıá-Labiano, F.; de Diego, L. F.; Gayań, P.; Celaya, J. Mapping of the Range of Operational Conditions for Cu-, Fe-, and Ni-Based Oxygen Carriers in Chemical-Looping Combustion. Chem. Eng. Sci. 2007, 62 (1-2), 533–549. (54) Sohn, H. Y.; Kim, B. S. A Novel Cyclic Reaction System Involving CaS and CaSO4 for Converting Sulfur Dioxide to Elemental Sulfur without Generating Secondary Pollutants. 1. Determination of Process Feasibility. Ind. Eng. Chem. Res. 2002, 41 (13), 3081–3086. (55) Kim, B. S.; Sohn, H. Y. A Novel Cyclic Reaction System Involving CaS and CaSO4 for Converting Sulfur Dioxide to Elemental Sulfur without Generating Secondary Pollutants. 3. Kinetics of the Hydrogen Reduction of the Calcium Sulfate Powder to Calcium Sulfide. Ind. Eng. Chem. Res. 2002, 41 (13), 3092–3096. (56) Talukdar, J.; Basu, P.; Greenblatt, J. H. Reduction of Calcium Sulfate in a Coal-Fired Circulating Fluidized Bed Furnace. Fuel 1996, 75 (9), 1115–1123. (57) Qiu, K.; Mattisson, T.; Steenari, B. M.; Lindqvist, O. Thermogravimetric Combined with Mass Spectrometric Studies on the Oxidation of Calcium Sulfide. Thermochim. Acta 1997, 298 (1-2), 87–93. (58) Marba˘n, G.; Garcıá-Calzada, M.; Fuertes, A. B. Kinetics of Oxidation of Cas Particles in the Regime of Low SO2 Release. Chem. Eng. Sci. 1999, 54 (1), 77–90. (59) Garcıá-Labiano, F.; de Diego, L. F.; Adańez, J.; Abad, A.; Gayan, P. Reduction and Oxidation Kinetics of a Copper-Based Oxygen Carrier Prepared by Impregnation for Chemical-Looping Combustion. Ind. Eng. Chem. Res. 2004, 43 (26), 8168–8177. (60) Zafar, Q.; Abad, A.; Mattisson, T.; Gevert, B. Reaction Kinetics of Freeze-Granulated NiO/MgAl2O4 Oxygen Carrier Particles for ChemicalLooping Combustion. Energy Fuels 2007, 21 (2), 610–618. (61) Zafar, Q.; Abad, A.; Mattisson, T.; Gevert, B.; Strand, M. Reduction and Oxidation Kinetics of Mn3O4/Mg-ZrO2 Oxygen Carrier Particles for Chemical-Looping Combustion. Chem. Eng. Sci. 2007, 62 (23), 6556–6567. (62) Brandvoll, O.; Kolbeinsen, L.; Olsen, N.; Bolland, O. Chemical Looping Combustion - Reduction of Nickel Oxide/Nickel Aluminate with Hydrogen. Chemical Engineering Transactions 2003, 3, 105–110. (63) Adańez, J.; Abad, A.; de Diego, L. F.; Garcıá-Labiano, F.; Gayań, P. Direct Sulfidation of Half-Calcined Dolomite under Pressurized Conditions. Ind. Eng. Chem. Res. 2004, 43 (15), 4132–4139. (64) Adańez, J.; Garcıá-Labiano, F.; de Diego, L. F.; Gayań, P.; Celaya, J.; Abad, A. Nickel-Copper Oxygen Carriers to Reach Zero CO and H2 Emissions in Chemical-Looping Combustion. Ind. Eng. Chem. Res. 2006, 45 (8), 2617–2625.

ReceiVed for reView May 5, 2008 ReVised manuscript receiVed August 8, 2008 Accepted August 12, 2008 IE8007264

Effect of Temperature on Reduction of CaSO

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