Multicycle Study on Chemical-Looping ... - ACS Publications

Energy & Fuels 2008, 22, 3661–3672

3661

Multicycle Study on Chemical-Looping Combustion of Simulated Coal Gas with a CaSO4 Oxygen Carrier in a Fluidized Bed Reactor Qilei Song, Rui Xiao,* Zhongyi Deng, Wenguang Zheng, Laihong Shen, and Jun Xiao Thermoenergy Engineering Research Institute, School of Energy and EnVironment, Southeast UniVersity, Nanjing 210096, China ReceiVed April 22, 2008. ReVised Manuscript ReceiVed July 1, 2008

Chemical-looping combustion (CLC) is a promising technology for the combustion of gas and solid fuel with efficient use of energy and inherent separation of CO2. In this study, the cyclic test of a CaSO4-based oxygen carrier (natural anhydrite) in alternating reducing simulated coal gas and oxidizing conditions was performed at 950 °C in a fluidized bed reactor at atmospheric pressure. A high concentration of CO2 was obtained in the reduction. The H2 and CO conversions and CO2 yield increased initially and final decreased significantly. The release of SO2 and H2S during the cyclic test was found to be responsible for the decrease of reactivity of a CaSO4 oxygen carrier. The oxygen carrier conversion after the reduction reaction decreased gradually in the cyclic test. Through the comparison of mass-based reaction rates as a function of mass conversion at typical cycles, it was also evident that the reactivity of a CaSO4 oxygen carrier increased for the initial cycles but finally decreased after around 15 cycles. The mass conversion rate of a CaSO4 oxygen carrier was considerably lower than that of metal oxides. X-ray diffraction analysis revealed that the presence and intensity of the reduction sulfur species was in accordance with the results of gas conversion. The content of CaO was higher than expected, suggesting the formation of SO2 and H2S during the cycles. Surface morphology analysis demonstrates that the natural anhydrite particle surface varied from impervious to porous after the cyclic test. It was also observed that the small grains on the surface of the oxygen carrier sintered in the cyclic tests. Energy-dispersive spectrum analysis also demonstrated the decrease of oxygen intensity after reduction, and CaO became the main component after the 20th oxidation. Pore structure analysis suggested that the particles agglomerated or sintered in the cyclic tests. The possible method for sulfur mitigation is proposed. Finally, some basic consideration on the design criteria of a CLC system for solid fuels using a CaSO4 oxygen carrier is discussed by the references and provides direction for future work.

1. Introduction It is generally accepted that greenhouse gas emission has caused significant global climate change. Carbon dioxide from combustion of fossil fuels is considered as the main source for the greenhouse effect. Chemical-looping combustion (CLC) is a new combustion technology with inherent separation of CO2.1 In the CLC system, the fuel and the oxygen are never mixed. The fuel (natural gas, syngas, etc.) reacts with the oxygen carrier to CO2 and H2O. The reduced oxygen carrier is transferred to the air reactor, where it is oxidized. The oxygen carrier particles are recirculated between the two reactors. The flue gas from the air reactor is mainly N2 and unreacted O2. Almost pure CO2 is obtained after the condensation of water in the product gas from the fuel reactor without any loss of energy during separation. Most of the investigations on the CLC system focused on methane or natural gas.2-6 However, the major part of CO2 emission is from the combustion of solid fuels, e.g., coal, in power plants. Even though there are still many technical challenges,7 the research of CLC of solid fuels is quite attractive and deserves further study, especially in China, considering the dominant energy supply. The application of solid fuels involves mainly two approaches. First, the syngas is produced from coal gasification and used in CLC. Recently, there are some * To whom correspondence should be addressed. Telephone: +86-2583794744-803. Fax: +86-25-83795508. E-mail: [email protected]. (1) Richter, H. J.; Knoche, K. F. ACS Symp. Ser. 1983, 235, 71–86.

publications on CLC of syngas and coal gasification gas.8-11 Mattisson et al. investigated a CLC system using syngas from coal gasification as fuel and focused on the development of oxygen carrier, reactor design, and hot prototype CLC reactor construction and operation.10 The second reasonable approach is integrated gasification and CLC together in a fuel reactor as generally presented in the literature.7 Solid fuels are gasified with a high concentration of steam and/or CO2, and the oxygen carrier reacts with the gasification gas simultaneously. Some investigations on CLC of solid fuels have also been reported.7,12-17 On the basis of the investigations on a laboratory-scale fluidized bed reactor,15 a 10 kWth CLC reactor for solid fuels has been (2) Lyngfelt, A.; Leckner, B.; Mattisson, T. Chem. Eng. Sci. 2001, 56 (10), 3101–3113. (3) Cho, P.; Mattisson, T.; Lyngfelt, A. Fuel 2004, 83 (9), 1215–1225. (4) Johansson, M.; Mattisson, T.; Lyngfelt, A. Energy Fuels 2006, 20 (6), 2399–2407. (5) de Diego, L. F.; Garcia-Labiano, F.; Gayan, P.; Celaya, J.; Palacios, J. M.; Adanez, J. Fuel 2007, 86 (7-8), 1036–1045. (6) Ryu, H.; Jo, S.; Kim, J.; Park, M. Proceedings of the 6th International Symposium on Coal Combustion, Wuhan, China, 2007; pp 384-389. (7) Cao, Y.; Pan, W. P. Energy Fuels 2006, 20 (5), 1836–1844. (8) Abad, A.; Mattisson, T.; Lyngfelt, A.; Johansson, M. Fuel 2007, 86 (7-8), 1021–1035. (9) Jin, H.; Ishida, M. Fuel 2004, 83 (17-18), 2411–2417. (10) Mattisson, T.; Garcia-Labiano, F.; Kronberger, B.; Lyngfelt, A.; Adanez, J.; Hofbauer, H. Int. J. Greenhouse Gas Control 2007, 1 (2), 158– 169. (11) Abad, A.; Garcia-Labiano, F.; de Diego, L. F.; Gayan, P.; Adanez, J. Energy Fuels 2007, 21 (4), 1843–1853.

10.1021/ef800275a CCC: $40.75  2008 American Chemical Society Published on Web 09/06/2008

3662 Energy & Fuels, Vol. 22, No. 6, 2008

Song et al.

Table 1. Material Properties producing area particle size (mm) specific density (kg/m3) bulk density (kg/m3) BET surface area (m2/g) chemical composition (%) CaO SO3 MgO SiO2 Fe2O3 Al2O3 TiO2 K2O Na2O P2O5 crystallization water

Nanjing 0.15-0.2 2900 1500 0.5266 40.480 53.900 1.680 0.630 0.094 0.120 0.014 0.044 0.055 0.028 2.955

designed and operated using petroleum coke and South African coal, which provides promising results for CLC of solid fuels.18,19 A number of metal oxide oxygen carriers have been extensively investigated, such as Ni, Fe, Cu, Mn, Co, etc., as summarized in the literature.20,21 Even though traditional synthetic oxygen carriers exhibit high reactivity and stability, they also have shortcomings for applications in the CLC system, for example, high cost, low oxygen ratios, vulnerability to poisonous sulfur compound, and potential heavy-metal pollution to the environment. Recently, some iron-based oxygen carriers also attracted attention because of the benefits of low cost, especially natural ores, such as ilmenite.22,23 However, the oxygen ratio is relatively low. The CaSO4 oxygen carrier has many advantages that enable it interesting for use in the CLC system. It seems more environmentally benign than most of the proposed metal oxidized systems, and there are large amounts of natural gypsum available worldwide. CaSO4 has a relatively higher oxygen capacity compared to metal oxides. The oxygen ratio shows the maximum amount of oxygen that can be transferred between the air and fuel reactors. The theoretical oxygen ratio for CaSO4/ CaS is 0.4706, which is much higher than that of metal oxides. As one of the gypsum resources, natural anhydrite ore consists of a high purity of CaSO4, has a relatively higher mechanical strength compared to synthesized calcium-based oxygen carrier particles in our previous studies, and may be suitable for application in a fluidized bed reactor of CLC systems. 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: (12) Cao, Y.; Casenas, B.; Pan, W. P. Energy Fuels 2006, 20 (5), 1845– 1854. (13) Dennis, J. S.; Scott, S. A.; Hayhurst, A. N. J. Energy Inst. 2006, 79 (3), 187–190. (14) Leion, H.; Mattisson, T.; Lyngfelt, A. Fuel 2007, 86 (12-13), 1947– 1958. (15) Leion, H.; Mattisson, T.; Lyngfelt, A. Int. J. Greenhouse Gas Control 2008, 2 (2), 180–193. (16) Shen, L. H.; Zheng, M.; Xiao, J.; Zhang, H.; Xiao, R. Sci. China, Ser. E: Technol. Sci. 2007, 50 (2), 230–240. (17) Wang, J. S.; Anthony, E. J. Appl. Energy 2008, 85, 73–79. (18) Berguerand, N.; Lyngfelt, A. Fuel 2008, 87 (12), 2713–2726. (19) Berguerand, N.; Lyngfelt, A. Int. J. Greenhouse Gas Control 2008, 2 (2), 169–179. (20) Johansson, M.; Mattisson, T.; Rydeń, M.; Lyngfelt, A. International Seminar on Carbon Sequestion and Climate Change, Rio de Janeiro, Brazil, 2006. (21) Anthony, E. J. Ind. Eng. Chem. Res. 2008, 47 (6), 1747–1754. (22) Mattisson, T.; Lyngfelt, A.; Cho, P. Fuel 2001, 80 (13), 1953– 1962. (23) Leion, H.; Lyngfelt, A.; Johansson, M.; Jerndal, E.; Mattisson, T. Chem. Eng. Res. Des., in press.

θ C + H2O ) CO + H2 ∆H298 K ) 131.30 kJ/mol

(1)

θ CO + H2O ) CO2 + H2 ∆H298 K ) 41.14 kJ/mol

(2)

C + CO2 + 2CO

θ ∆H298 K ) 172.42

kJ/mol

(3)

Simultaneously, CaSO4 is reduced by gasification gas to CaS θ CaSO4 + 4CO f CaS + 4CO2 ∆H298 K ) 170.96 kJ/mol (4) θ CaSO4 + 4H2 f CaS + 4H2O ∆H298 K ) -6.41 kJ/mol (5)

Then, reduced oxygen carrier particles are transported to the air reactor and oxidized back to CaSO4 θ CaS + 2O2 f CaSO4 ∆H298 K ) -960.90 kJ/mol

(6)

Few studies have been investigated on the use of a CaSO4 oxygen carrier in CLC. Alstom Power, Inc. has started investigation on the feasibility of a CaSO4-based oxygen carrier in CLC of coal,24 but no information has been published. CANMET also explored the use of CaSO4 as an oxygen carrier in a clean combustion of solid fuels in CLC with ASPEN Plus software package, and the fundamental simulation results seem promising for the use of CaSO4 in CLC for solid fuels, especially high-sulfur fuels.17,21 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.25 The results show that CaSO4/CaS can be an interesting alternative oxygen carrier for CLC systems. The CaSO4 oxygen carrier (natural anhydrite) has been investigated under different operation conditions in a fixed bed reactor.26 The results showed that a CaSO4 oxygen carrier has high reduction reactivity and stability in a long-time reduction/oxidation test. However, thermodynamic calculations also show that there is thermodynamic limitation for CaSO4/ CaS that caused the incomplete conversion of fuel gas and sulfur release. According to our previous studies, the CO2 concentration is around 98.04% at 900 °C and decreases to 97.09% at 950 °C in the equilibrium gas-phase composition of the reduction of CaSO4 with pure CO (the molar ratio of CO and CaSO4 is 4). The other components are unreacted CO and SO2. The trend for H2 and CH4 conversion with temperature is similar. Besides, the oxidation of CaS also accompanies side reactions. The sulfur release may be a limit for the use of CaSO4 oxygen carrier and require intensive study. Therefore, to investigate the appropriate operation regime for CaSO4/CaS during periodic shifts between reduction and oxidation in CLC is essential. The purpose of this work is to investigate the reactivity and stability of a CaSO4-based oxygen carrier in a multicycle test of CLC of simulated coal gas in a laboratory fluidized bed reactor. The simulated coal gasification gas conversion and oxygen carrier conversion during the cyclic test was studied. The release of sulfur in the oxygen carrier was also studied. A series of characterization analyses were performed to investigate the phase change, surface morphology change, pore structure, and sulfur release. 2. Experimental Section 2.1. Material Preparation. The oxygen carrier particle used in the cyclic test was natural anhydrite ore from Nanjing Anhydrite (24) Andrus, H. E. J.; Chiu, J. H.; Stromberg, P. T.; Thibeault, P. R. 22nd Annual International Pittsburgh Coal Conference, Pittsburgh, PA, 2005. (25) Shen, L.; Zheng, M.; Xiao, J.; Xiao, R. Combust. Flame 2008, 154, 489–506. (26) Song, Q. L.; Xiao, R.; Deng, Z. Y.; Zhang, H. Y.; Shen, L. H.; Zhang, M. Y. Energy ConVers. Manage. 2008, doi: 10.1016/j.enconman.2008.05.020.

CLC of Simulated Coal Gas

Energy & Fuels, Vol. 22, No. 6, 2008 3663

Figure 1. Schematic diagram of the experimental setup.

Ore Co. Ltd. The natural anhydrite ore was crushed and sieved to particles with a size range of 0.15-0.2 mm. The main properties are shown in Table 1. The oxygen ratio of natural anhydrite, RO, is 0.444, as calculated by the purity of CaSO4. The fresh and used oxygen carriers were analyzed by a series of characterization techniques. The particle structure properties, including BrunauerEmmett-Teller (BET) surface area, pore volume, and average pore diameter, were measured by nitrogen adsorption/desorption isotherms at 77 K with a Micrometritics instrument ASAP 2020. X-ray diffraction (XRD) for the crystal structure of fresh and used samples was performed in a D/max 2500VL/PC system, using Cu KR radiation with a step of 0.02°/s. The surface morphological features of the fresh and reacted samples were recorded by an environmental scanning electron microscope (ESEM) in a FEI Quanta 200 microscope system (Holland). The element distribution on the surface of samples was also characterized by an energy-dispersive X-ray spectroscopy (EDS) system (INCA X-sight, Oxford Instruments, U.K.) equipped with the ESEM system. 2.2. Experimental Setup. The experiment was conducted in a laboratory fluidized bed reactor under atmospheric pressure using natural anhydrite as an oxygen carrier. Figure 1 shows the schematic diagram of the fluidized bed reactor. It consists of a gas feeding system, reactor, electric heater, cooler, and gas analysis system. The reactor was a stainless-steel tube (i.d. ) 25 mm, length ) 950 mm) with 470 mm of preheating zone below the porous distributor plate. The reaction temperature was controlled by two thermocouples: a Pt/Rh thermocouple between the oven and the reactor and the other K-type thermocouple inside the tube. The inside thermocouple could be adjusted to obtain the temperature along the bed height. The flow rates of high-purity calibration gas and reaction gas (H2, CO, CO2, O2, and N2) provided by Nanjing Tongguang Gas Co. Ltd. were all measured by mass flow controllers (MFC, Beijing Sevenstar Huachuang Electronics Co., Ltd.). The pressure of the fluidized bed was measured by a U-type pressure gauge to monitor the fluidization state. One hot product gas stream concentration was measured by an online MRU SAE19 flue gas analyzer to detect the dry concentration of O2, SO2, H2S, and NOx. After the steam condensed and was removed, the other gas stream was measured by Emerson multicomponent gas analyzers, including

a Rosemount NGA 2000 gas analyzer used to measure the concentrations of CO2, CH4, CO, and O2 and a Rosemount Hydros 100 analyzer to measure the concentration of H2. The concentrations obtained were dry product gas because both gas analyzers were equipped with a water condenser and desiccators. During some experiments, the gas was also sampled and examined by a gas chromatograph (GC, Agilent 6890N) equipped with a thermal conductivity detector (TCD). The moisture content measured by GC was negligible. It should be also noted that this Rosemount NGA 2000 as a part of the Emerson multicomponent gas analyzer focused on coal gasification gas and the CH4 detector channel is not found to be interfered by SO2 or H2S. 2.3. Experimental Procedure. Before the cyclic test of reduction and oxidation was performed, blank tests with a quartz bed in the reactor with the same experimental operation conditions were performed to evaluate the influence of the stainless-steel tube (Ni). Because the conversion of H2, CO, and O2 was negligible, the interaction of the reactor metal tube could be neglected. A certain amount of quartz sand (60 g, 80 mm in height, particle size range ) 0.8-0.9 mm) was displayed on the distributor plate as the feeding gas was preheated and was not fluidized during the test. A sample of 30 g of fresh CaSO4 oxygen carrier particles was added above the quartz sand and was then initially heated in an inert atmosphere to the reaction temperature. The oxidation atmosphere is not needed considering that CaSO4 oxygen carrier particles do not decompose and are stable during the heating up period. 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 was obtained for each minute. The operation condition of the reduction/oxidation cycle test of a CaSO4 oxygen carrier in CLC was determined from the previous studies on a reduction test and thermodynamic analysis.25-27 The reduction test of a CaSO4 oxygen carrier with simulated coal gas has been performed in a temperature range of 890-950 °C. From (27) Song, Q. L.; Xiao, R.; Deng, Z. Y.; Zheng, W. G.; Shen, L. H.; Xiao, J.; Zhang, M. Y. Ind. Eng. Chem. Res. 2008, manuscript submitted.

3664 Energy & Fuels, Vol. 22, No. 6, 2008 Table 2. Experimental Conditions oxygen carrier pressure (atm) reaction temperature (°C) particle size (mm) sample height (mm) reduction gas reduction gas flow rate (mL/min) reduction duration (min) oxidation temperature (°C) oxidation gas oxidation gas flow rate (mL/min) oxidation duration (min) inert gas (calibration gas) inert gas flow rate (mL/min)

natural anhydrite ore 1 950 0.15-0.2 40 H2/CO/CO2 ) 50/25/25% 600 5 950 5% O2/N2 1200 30 N2 600 and 1200

γCO,i )

γH2,i )

X)

m - mred mox - mred

(7)

The conversion of H2 and CO as a function of time is defined as follows: (28) Wen, C. Y.; Yu, Y. H. AIChE J. 1996, 12 (3), 610–612. (29) Mattisson, T.; Johansson, M.; Lyngfelt, A. Energy Fuels 2004, 18 (3), 628–637. (30) Mattisson, T.; Johansson, M.; Lyngfelt, A. Fuel 2006, 85 (5-6), 736–747. (31) de Diego, L. F.; Gayan, P.; Garcia-Labiano, F.; Celaya, J.; Abad, M.; Adanez, J. Energy Fuels 2005, 19 (5), 1850–1856.

ti

outyCO,outdt

t0

ti

t0

inyCO,indt

∫ n˙ y 1∫ n˙ y ti

t0 ti

t0

out H2dt

in H2,indt

)

)

× 100%

(8)

× 100%

(9)

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

Xr,i ) Xr,i-1 the results of the reduction reactivity investigation, the suitable temperature for higher reduction reactivity in this fluidized bed reactor seems to be around 950 °C. Therefore, the reaction temperature was set to 950 °C for the cyclic test of reduction and oxidation. Because the CLC system for solid fuels focuses on the steam gasification, the simulated coal gasification gas ratio is determined by the results of steam gasification of 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 higher temperature 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 reduction duration was set to 5 min. When the reduction finished, the gas was switched to nitrogen to blow away the product gas. The oxidizing gas concentration was 5% O2 in N2 instead of air to avoid a large temperature increase because of heat generated from the intense exothermic oxidation. All of the oxidation duration was set to 30 min. Then, the gas was switched to N2, and the next cycle was started. The inlet reduction gas flow rate was 600 mL/min, and the inlet oxidizing gas flow rate was set to 1200 mL/min. This corresponds to velocities of 7.12-8.15umf and 15.6umf, respectively, for the inlet gas. The umf was calculated using the equation described by Wen and Yu.28 The inert gas flow rate after the reduction and oxidation period was 600 and 1200 mL/min, respectively. Table 2 shows the specific operation conditions concerning the cycle tests. The terminal velocity ut was also calculated. The fluidization properties are presented in Table 3. When reduction and cyclic tests were finished, the gas was switched to nitrogen and the heater was shut down. The oxygen carrier particle was cooled in the nitrogen flow to room temperature and collected for analysis. It should be noted that, to compare the reactivity of CaSO4 oxygen carrier to metal oxide, the system was designed similar to that from the Chalmers University of Technology in Sweden. Therefore, the operation condition and procedure were determined by reference to their investigations. Because of the limitation of our equipment and different reaction gas, the gas flow and oxygen carrier mass are different; however, the oxygen carrier conversion and stability can be compared. 2.4. Data Evaluation. The conversion of oxygen carrier or degree of oxidation is defined as

( (

∫ n˙ 1∫ n˙

Song et al.

∫

ti

ti-1

MO n˙ y - n˙inyCO2,in + n˙inyH2,in moxRO ( out CO2,out n˙outyH2,out + 2n˙outySO2,out)dt (10)

MO dXr ) n˙ y - n˙inyCO2,in + n˙inyH2,in - n˙outyH2,out + dt moxRO ( out CO2,out 2n˙outySO2,out) (11) Similarly, the oxygen carrier conversion during the oxidizing period is calculated on the basis of the mass balance of oxygen. Some oxygen in the air was consumed by the deposited carbon, and some oxygen was released in the form of SO2

Xo,i ) Xo,i-1 +

∫

ti

ti-1

2MO 1 n˙ y - n˙out yO2,out + yCO,out + moxRO in O2,in 2

(

(

))

yCO2,out + ySO2,out dt (12) 2MO dXo 1 ) n˙ y - n˙out yO2,out + yCO,out + yCO2,out + dt moxRO in O2,in 2

(

(

ySO2,out

)) (13)

A mass-based conversion is used to compare the reactivity of different oxygen carriers in the literature and defined as follows:

ω)

m ) 1 + Ro(X - 1) mox

(14)

The mass-based reduction rate was calculated from the reduction and oxidation rate as follows:

dX dω ) Ro dt dt

(15)

More information on the data evaluation can be found in the references.3,8,22,29-31

3. Results and Discussion 3.1. Concentration Profiles. Typical profiles of product gas concentrations as a function of reaction time for the reduction/ oxidation cycle test are shown in Figure 2. The reaction time started when N2 was turned off and reaction gas was introduced into the reactor. Parts a and b of Figure 2 show the product gas concentrations of reduction and oxidation during the first cycle. Natural anhydrite particles initially reacted with all of the incoming H2 and CO to form H2O and CO2; the concentration of CO2 reached the peak around 90% at 5 min. H2 was totally consumed within the reduction period. CO displayed a peak with the reaction time. There were also some content of SO2 and H2S in the product gas, which will be discussed in the next section. After switching to an inert atmosphere, the product gas was cleared up before the oxidation process was performed. In the oxidation


Energy & Fuels, Vol. 22, No. 6, 2008 3665 Table 3. Fluidization Behavior reduction

b

oxidation

species

umfa (m/s)

u (m/s)

u/umf

umf (m/s)

u (m/s)

u/umf

utb (m/s)

oxygen carrier quartz sand

0.0112-0.0128 0.238-0.273

0.0913 0.0913

7.12-8.15 0.33-0.38

0.0117 0.2480

0.1825 0.1825

15.6 0.73

1.82-2.14 8.36-9.83

a The lower value of u mf is calculated assuming the full conversion of CO and H2 to CO2 and H2O, while the higher value is based on no conversion. The lower value is obtained for the 5% air, while the higher value is for the simulated coal gas.

period, O2 was not released during the first 10 min and then quickly increased to 4.58%. This indicated that the reaction rate was initially quite fast because the rapid conversion at the early stage was ascribed to chemical control of the reaction. Then, the reaction slowed down, with most of the O2 being released without reacting. However, at the end of the oxidation period, the concentration of oxygen gradually increased to 4.98%, which indicated that the conversion of CaS to CaSO4 was nearly complete. During the early stages of oxidations, slight CO2 and SO2 also released. During some experiments, the inert period after reduction is prolonged for several minutes to observe the gas concentrations. The CO2 is negligible at 11 min. Therefore, we adopt 5 min for the inert period. In this case, the CO2 peak might indicate the possibility of carbon deposition during the reduction. On the other hand, this may not work for all cycles. The CO2 concentration observed at the early stages of oxidation might also be the remnant gas in the reduction. However, The CO2 itself is quite negligible. It is not the key issue in this study. It is now generally accepted that the carbon deposition did not appear to be a major problem if the oxidation reaction was carried out at a high enough temperature.21 The inert period should be carefully carried out in the future. Parts c and d of Figure 2 shows the product gas concentrations of reduction and oxidation during the 10th cycle. It is obvious that the CO2 peak value arrived 96.51% much higher than the first cycle, while less unreacting CO was released. This indicated that the fuel gas conversion increased after the initial cycles. During the reduction test, the particle became quite porous and uniform. This porosity reduced the diffusion resistance during the oxidation. The cyclic test of reduction and oxidation promoted the penetration of coal gas into the particle. After the reduction/oxidation cyclic test, the progress of the reaction was unaffected by the product layer until oxygen capacity decreased significantly. The CO and H2 peak increased in Figure 2e during the 15th cycle, which indicated that the fuel conversion began to decrease and the reactivity of CaSO4 oxygen carrier decreased after 15 cycles. This deactivation is evidently shown in Figure 2g. The CO and H2 significantly increased to around 20 and 15%. From the oxidation time shown in parts f and h of Figure 2, it can also be observed that the O2 consumption also decreased, indicating that the reduction degree decreased. Many investigations showed that the fact of incomplete oxidation was due to the CaSO4 product layer that increased the intraparticle diffusion resistance to oxygen and prevented the complete oxidation.32 The oxidation in this study was promoted with a fluidized bed reactor used. The reason might be attributed to the improved heat and mass transfer in the fluidization condition. In this study, the one cycle reduction degree (∆X) of CaSO4 was only around 0.1. Therefore, it is the surface film of the particles that participated in the reactions in the oxidation and next reduction cycle. Therefore, the mass transfer resistances in the gas film and in the CaSO4 product layer may not be important in this work. Through the comparison of product gas concentrations versus time at different cycles, it can be observed that the reactivity of (32) Qiu, K.; Lindqvist, O.; Mattisson, T. Fuel 1999, 78 (2), 225–231.

CaSO4 oxygen carrier increased for the initial cycles, decreased after around 15 cycles, and deactivated after 20 cycles. The dry product gas flow behavior is also presented in Figure 2. The outlet dry gas flow rate was calculated relative to the gas analysis from the mass balance of nitrogen gas diluted in the outlet of the reactor before the gas analyzers. Because of the limitation of our experimental equipment, there was a distance between the reactor and the two gas analyzers. The gas concentration was measured that actually reaches the gas analyzers, while the time in the figures all start from the time the reaction gas was switched from N2 to coal gas and N2 to oxidizing gas. The concentrations and gas flow obtained are the transient data from the gas analyzers and not the concentration/flow behavior in the bed. Therefore, the delay time should be considered. The variation of dry product gas flow rate reflects the transient changes in gas concentration and is dependent upon the time delay caused by the resident time between the reactor and the condenser. The flow variation during the oxidation also verified the consumption of oxygen. A slight peak of flow at the switching time was because of the influence of nitrogen back mixing. The behavior is similar to the studies on a fluidized bed reactor from Chalmers University.29,33,34 3.2. Sulfur Release. The investigation of sulfur capture provides reference to the application of a CaSO4 oxygen carrier in CLC. However, the cycle of CaSO4/CaS is different from the previous extensive investigations on the reduction decomposition of CaSO4 in FBC systems performed by the Chalmers University of Technology in Sweden.35-41 As demonstrated in the literature by Anthony and Granatstein,42 CaSO4 is a thermodynamically stable product at typical FBC temperatures (800-950 °C) and overall oxidizing conditions. According to the extensive studies by Mattisson and Lyngfelt, CaSO4 is the stable compound at highly oxidizing conditions, CaS is the stable product at strongly reducing conditions and in the presence of SO2, while CaO is favored thermodynamically in intermediate conditions.36 This accounts for the SO2 release during the shifts between oxidizing and reducing conditions in a laboratory study.43 Recently, Shen proposed a detailed stability diagram of CaSO4/CaS/CaO species for oxygen carriers in CLC systems.25 The results are overall in accordance with the previous phase diagrams. In the fuel reactor of a CLC system, the high (33) Johansson, M.; Mattisson, T.; Lyngfelt, A.; Abad, A. Fuel 2008, 87 (6), 988–1001. (34) Cho, P.; Mattisson, T.; Lyngfelt, A. Ind. Eng. Chem. Res. 2006, 45 (3), 968–977. (35) Lyngfelt, A.; Leckner, B. Chem. Eng. Sci. 1989, 44 (2), 207–213. (36) Mattisson, T.; Lyngfelt, A. Energy Fuels 1998, 12 (5), 905–912. (37) Mattisson, T.; Lyngfelt, A. Thermochim. Acta 1999, 325 (1), 59– 67. (38) Lyngfelt, A.; Leckner, B. Chem. Eng. Sci. 1999, 54 (22), 5573– 5584. (39) Lyngfelt, A.; Leckner, B. Chem. Eng. J. 1989, 40 (2), 59–69. (40) Fernandez, M. J.; Lyngfelt, A.; Steenari, B. M. J. Inst. Energy 2000, 73 (495), 119–125. (41) Fernandez, M. J.; Lyngfelt, A.; Steenari, B. M. Energy Fuels 2000, 14 (3), 654–662. (42) Anthony, E. J.; Granatstein, D. L. Prog. Energy Combust. Sci. 2001, 27 (2), 215–236. (43) Hansen, P. F. B.; Dam-Johansen, K.; Ostergaard, K. Chem. Eng. Sci. 1993, 48 (7), 1325–1341.


Song et al.

Figure 2. Product gas concentrations from the fluidized bed reactor and dry gas flow during the cyclic test of a CaSO4 oxygen carrier for the (a and b) 1st cycle, (c and d) 10th cycle, (e and f) 15th cycle, and (g and h) 20th cycle. The dashed lines indicated the switch to an inert atmosphere.



concentrations of CO and H2 provide a stronger reducing condition for CaSO4 to CaS. Reduction tests within 890-950 °C demonstrated that CaS was the dominant product, with a slight content of CaO formed even though SO2 was not fed in the fuel gas. To obtain the stable product of CaS, a certain partial pressure of SO2 is needed. In this work, SO2 was not fed because we focused on the reactivity with CO and H2. While in the air reactor, a high velocity circulating fluidized bed reactor would enhance the oxidation of CaS to CaSO4. The release of SO2 involved with CaSO4 and CaS is varied with the operating conditions as illustrated in many investigations.37,41,44 SO2 formation is mainly due to the solid-solid reactions and also the partial oxidation at the initial oxidation period θ CaS + 3CaSO4 f 4CaO + 4SO2 ∆H298 K ) 1047.93 kJ/mol (16) θ CaS + 3/2O2 f CaO + SO2 ∆H298 K ) -458.69 kJ/mol (17)

The competing side reactions of CO and H2 with CaSO4 may also be feasible under reducing conditions θ CaSO4 + CO f CaO + CO2 + SO2 ∆H298 K ) 219.24 kJ/mol (18) θ CaSO4 + H2 f CaO + H2O + SO2 ∆H298 K ) 260.38 kJ/mol (19) θ CaSO4 + 4H2 f CaO + 3H2O + H2S ∆H298 K ) 53.04 kJ/mol (20)

The concentrations of SO2 and H2S in the product gas during reduction and oxidation at some typical cycles are displayed in parts a-h of Figure 2. As displayed in the figures, the sulfur fraction was quite high in both periods. However, when we consider the integration of gas concentration and flow rate with time, it is evident that the sulfur release during the oxidation period was much more. Take the 10th cycle (405-450 min) for example. The overall SO2 and H2S release accounts for 0.4654% of the total amount of sulfur moles. While the SO2 release during the oxidation amounts to 0.8562% of the total oxygen carrier. Besides, from the figure above, the release peak time can also be observed. SO2 was formed at the early stages of reduction with the peak emerging around 3 min. This indicates that the competing side reduction (eqs 18 and 19) occurs at the early stages and the target reduction (eqs 4 and 5) became dominant then. The peak time of H2S was a little later than that of SO2. In the oxidation phase, SO2 was significantly released at the early stages. This was attributed to the partial oxidation reaction (eq 17) that generates CaO and SO2. Also, because the strong exothermic nature of the oxidation and partial oxidation, the temperature increased by as high as 10 °C in some cycles during the early stages and soon decreased to 950 °C. Considering that eq 16 is endothermic, it would be enhanced with the temperature increased. Also, the partial oxidation (eq 17) may also be enhanced because of the low concentration of oxygen in the oxidizing gas. Figure 3 also shows the variation of SO2 and H2S concentrations in the dry product gas as a function of time of the 20 cycles. It is clear that the SO2 concentration increased for the initial cycles of reduction and oxidation, then became stable for several cycles, and significantly increased after 16 cycles. (44) Sohn, H. Y.; Kim, B. S. Ind. Eng. Chem. Res. 2002, 41 (13), 3081– 3086.

Figure 3. Outlet SO2 and H2S concentrations as a function of time during the 20 cyclic tests.

The H2S concentration is relatively stable, with a maximum value around 2000 ppmv. As illustrated above, the side reactions including solid-solid reaction (eq 16), competing oxidation (eq 17), and the competing reduction (eqs 18-20) all played vital roles in the sulfur release in the cyclic test. The release of SO2 was mainly due to the high temperatures of reduction and oxidation, especially the low oxygen concentration (5%). It was applied to avoid a large temperature increase, which was not favorable for the target oxidation reaction. The influence of SO2 was not performed in this study. On the basis of the eqs 18 and 19, the SO2 component in the fuel gas can help to prevent the decomposition of CaSO4 to CaO. Besides, the oxidation might be promoted if a high concentration of oxygen was adopted. In future work, the operation condition of oxidation should be determined with caution. 3.3. Gas Conversion and CO2 Yield versus Cycles. The variation of H2 and CO conversion versus the cyclic test is shown in Figure 4a. The H2 conversion was 100% during the first 11 cycles and then decreased to around 95% until the 17th cycle. The variation of CO was very different from that of H2. The overall conversion of CO increased from 75 to 90% at the second cycle and fluctuated around that until the 16th cycle. Both H2 and CO conversion significantly decreased after the 17th cycle to 70 and 45% at the 20th cycle, respectively. The fact that the conversion of H2 was higher than that of CO may indicate that H2 was a better reducing agent than CO for oxygen carriers. This is in accordance with the previous thermodynamic analysis and experimental studies.7,8,11 The gas conversion may also be changed by the reverse water-gas shift (RWGS) reaction under this experimental condition. The CO2 yield during the reduction is evaluated in the form of a transient outlet CO2 divided by total inlet carbon, that is, CO2,out/(CO + CO2)in. The CO2 yield was calculated to display the fraction of CO2 released from the total inlet carbon, which is straightforward to show the mass balance of carbon rather than subtracting the inlet CO2. Figure 4b shows the variation of the CO2 yield during the typical cycle as a function of time at typical cycles. Because of the residence time and measure time, which has been accounted for as the flow behavior above, the CO2 yield relatively increased during the reduction period. The overall CO2 yield was 86.5% at the first cycle and became stable at 95% between the 2nd and 15th cycle, and then it decreased to 77.3% at the 18th cycle and finally to 74.1% at the 20th cycle. The variation of the CO2 yield at a typical cycle


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Figure 4. (a) Gas conversion as a function of the number of cycles and (b) CO2 yield as a function of time for the reduction during the multicycle test.

Figure 5. Variation of the oxygen carrier conversion as a function of the number of cycles.

is in line with the results of gas conversion. The initial increase and final decrease of gas conversion and CO2 yield during the reduction in the multicycle test demonstrates the variation of reactivity of the CaSO4 oxygen carrier. 3.4. Oxygen Carrier Conversion. The oxygen carrier conversion or the degree of oxidation during the 20 cyclic process of the reduction/oxidation test is shown in Figure 5. It is evident that the oxygen carrier conversion after the reduction reaction decreased gradually in the cyclic test. The decrease of conversion after each reduction period indicates the increase of the extent of reduction, and more oxygen in the oxygen carrier was used as the number of cycles increased. It was also observed that after each run of oxidation the oxygen carrier conversion cannot restore the same level as that before reduction. The mass balance calculated for the first cycle is 98.52%, which means 1.48% of oxygen carrier loss mainly because of side reactions after one cycle. The decrease of the oxygen transport capacity was ascribed to the release of sulfur species during the cyclic test of reduction and oxidation. There was less oxygen available for reaction with coal gasification gas and consequently resulted in the change in conversion. During the last several cycles (15-20th), the solid conversion value dramatically decreased. A large quantity of SO2 emission and slight temperature increase during oxidation were observed during these cycles, as shown in Figure 3. The partial oxidation reaction (eq 17) was intensified, and this exothermic reaction aggravates the endothermic solid-solid reaction (eq 16) and the loss of sulfur and oxygen in the CaSO4 oxygen carrier. It was also entirely possible

that the oxygen carrier particle sintered after many cycles because of the temperature increase and side reactions. Parts a and b of Figure 6 show that mass-based reaction rates (dω/dt) as a function of the mass-based degree of conversion (ω) during the reduction and oxidation at typical cycles. As shown in Figure 6a, the variation of the reaction rate is quite different from previous investigations on methane combustion using metal oxide as a oxygen carrier.29,45 The maximum value of the mass-based conversion rate is much stable around 1.7 × 10-4 s-1, which is considerably lower than that of a Fe2O3based oxygen carrier in the literature.29 The reduction rate increased for the initial cycles, then became stable, and finally decreased significantly for the 20th cycle, which is in agreement with the gas concentration in Figure 2 and the sulfur release trend in Figure 3. The mass conversion decreased significantly after each cycle. As displayed in Figure 6b, the comparison of the mass-based conversion rate as a function of mass conversion for the oxidation period at typical cycles is also evident. It is clear that the reaction rate was relatively higher at the early stages of oxidation. This is attributed to the chemical reaction control at the early stage. It was also demonstrated that the oxidation rate was low at the first cycle because of the possible product layer. However, this resistance seemed to decrease in the cyclic tests. The oxidation rate increased for the next few cycles and finally decreased significantly at the end of cycles, suggesting the decrease of oxygen capacity. The breakthrough at the early stages illustrates the release of SO2 because of the competing oxidation reaction. The mass conversion gradually decreased with the cycles. The results of a mass-based oxygen carrier conversion demonstrate that the natural anhydrite particles in this study displayed low reactivity and tendency to lose reactivity because of the sulfur release. This indicates that the reactivity of the CaSO4 oxygen carrier needed to be improved. Sulphated dolomite may be used as an oxygen carrier. Also, pelleted CaSO4 with a superior reactivity and stability may also be considered.17 3.5. Characterization Analysis. 3.5.1. Phase Characterization. The results of XRD analysis of a fresh and reacted CaSO4 oxygen carrier of different cycles are shown in Figure 7. The presence of anhydrite (CaSO4) as the main crystalline phase in the fresh oxygen carrier is clearly evidenced. Figure 7b shows the powder XRD pattern of the sample after the reduction stage in the 15th cycle of the redox test. It is clear that CaSO4 was still the main component. The presence and intensity of CaS (45) Johansson, M.; Mattisson, T.; Lyngfelt, A. Ind. Eng. Chem. Res. 2004, 43 (22), 6978–6987.



Figure 6. Mass-based rate, dω/dt, as a function of the mass conversion, ω, for the (a) reduction and (b) oxidation at typical cycles.

Figure 7. XRD patterns of a CaSO4 oxygen carrier: (a) fresh, (b) used after the 15th cycle reduction, and (c) used after the 20th cycle oxidation.

suggests the reduction of the sulfur species was in accordance with the results of gas conversion. However, the intensity of CaO was higher than expected, which suggested the formation of SO2 and H2S during the cycles. As presented above, the main sulfur release was in the oxidation period. The most remarkable change was that CaO became the main crystalline phase after the oxidation stage of the 20th cycle. This result demonstrated the decrease of the reaction rate and gas conversion. 3.5.2. Surface Morphology. The particles of the CaSO4 oxygen carrier were examined using the environmental scanning electron microscope before and after reduction. Parts a and b of Figure 8 show the surface morphology of the oxygen carrier particle before reduction, which displays that the fresh oxygen carrier is impervious. After the 15th cycle reduction at 950 °C, as shown in parts c and d of Figure 8, the surfaces of the reduced oxygen carrier are clearly rougher and quite porous. Small grains of a size around 2-4 µm appeared on the surface of CaSO4 oxygen carrier particles. The crystal grains seemed to be formed as a consequence of the reduction reaction, with the oxygen released leaving interstices among the grains. Figure 8d also indicates that some large grains were agglomerated. While for particles after the 20th cycle oxidation as shown in parts e and f of Figure 8, the appearance of the surface was basically similar to that shown in parts c and d of Figure 8 with porosity and interstice. However, a difference from the particle after the 15th reduction 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 the oxygen carrier sintered

after the 20th oxidation. This result was possibly due to the cyclic test of reduction and oxidation and side reactions. The overall results of ESEM analysis were in agreement with the previous studies.46 Besides, in comparison to the surface of reduced oxygen carrier particles in a fixed bed reactor in our previous studies, the more uniform porous surface in this study also verified the full fluidization state during the reduction experiment period. The EDS analysis spectrum and average accurate quantitative analysis of the atomic percentage of major elements are shown in Figure 9 and Table 4, respectively. The average value of O/Ca and S/Ca of fresh and used oxygen carrier particles was obtained. For the fresh CaSO4 particles shown in Figure 8b, the S and Ca peaks were comparable in Figure 9a. The S/Ca ratio was 1.146, and the O/Ca ratio was 4.211, which verified the main component of CaSO4 in the particles. This result is in agreement with the literature; that is, the S/Ca ratio of 1.1 is expected for both CaS and CaSO4.47 After the 15th cycle reduction at 950 °C, both O and S intensity decreased, with the S/Ca ratio decreasing to 0.0992 and O/Ca decreasing to 1.801, which suggested the formation of CaO on the surface. The decrease of O/Ca also indicates the release of oxygen after cyclic reduction, including the transfer to fuel gas and side reactions. As for the particles shown in Figure 8f, the S and O peaks decreased evidently shown in Figure 9c, with the S/Ca and O/Ca decreasing to 0.065 and 1.144, respectively. The results suggested that the grains on the surface were mainly composed of CaO. The results of the ESEM-EDS study verified the variation of reactivity of the CaSO4 oxygen carrier. It also provided information concerning the reaction steps; that is, the coal gasification gas reacted with the surface and gradually diffused to the kernel with the cycles. 3.5.3. BET Analysis. The BET surface area, pore structure, and pore size distribution of fresh and used catalysts are illustrated in Figure 10 and Table 5. The porous properties had a significant change after 20 cyclic tests. The surface area decreased from 0.5266 to 0.4426 m2/g, while the total pore volume, micropore volume, and average pore diameter of the reacted oxygen carrier all increased. The decrease of the BET surface area might be ascribed to the agglomerated grains after sintering in the cyclic tests. The increase of the pore volume may be due to the crack and interstice of particles, which can be obviously observed in parts c and e of Figure 8. (46) Kim, B. S.; Sohn, H. Y. Ind. Eng. Chem. Res. 2002, 41 (13), 3092– 3096. (47) Marbań, G.; Garcıá-Calzada, M.; Fuertes, A. B. Chem. Eng. Sci. 1999, 54 (4), 495–506.


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Figure 8. ESEM micrographs of the fresh and used oxygen carriers.

Figure 9. Spectrums collected from the oxygen carrier particles from (a) fresh, (b) used after the 15th reduction, and (c) used after the 20th oxidation.

3.6. Sulfur Reduction Method. As presented above, the sulfur release is a complicated process and should be reduced.

Limestone sorbent may be a reasonable method to capture the sulfur from the coal gasification and decrease the sulfur release



Table 4. Atomic Ratio of Major Elements on the Surface of Fresh and Used CaSO4 Oxygen Carrier Particles species

fresh sample

15th reduction

20th oxidation

S/Ca O/Ca

1.146 4.211

0.1424 1.614

0.065 1.144

in the reduction/oxidation process. Fresh limestone and CaO sorbent particles are fed with coal into the fuel reactor and react via the following reactions: CaCO3 f CaO + CO2

(21)

CaO + H2S f CaS + H2O

(22)

CaCO3 + H2S f CaS + H2O + CO2

(23)

CaO + SO2 f CaSO3

(24)

If the partial pressure of CO2 in the fuel reactor is lower than the equilibrium partial pressure of calcination of CaCO3, then the reactions 21, 22, and 24 work. On the contrary, the reaction 23 would be the main desulphurization reaction for coal gasification gas. The oxygen carrier particles, desulphurization products (CaS and CaSO3), and unreacted CaO are all transported to the air reactor and expected to be oxidized to CaSO4. The SO2 emission in the air reactor may also be captured with the unreacted CaO. θ CaO + SO2 + 1/2O2 f CaSO4 ∆H298 K ) -502.21 kJ/mol (25)

Therefore, the desulphurization sorbent may also serve as an oxygen carrier after the cycle regeneration. It should be noted that the regeneration of CaS has been extensively studied under atmospheric pressure and pressurized conditions. Cheaper lime sorbent may serve the function of desulphurization of the coal gasification and alleviate the emission of sulfur in the reduction/ oxidation cycle. Therefore, the additional costs for sulfur capture, including capital, operation, and maintenance, might be reduced greatly. However, further work is essential to test the effect of CaO on the sulfur reduction. Besides, as suggested by Anthony,17 the sulfur released from the fuel, such as petroleum coke and high-sulfur coal, may be captured with CO2 together for sequestration. If this is possible, it would eliminate the need for desulphurization equipment. Also, the high content of sulfur from the petroleum coke plays a positive role to limit the formation of CaO. 3.7. Consideration about Design Criteria for the CLC System. There are many aspects that should be considered for the development of a CLC system, including the performance of an oxygen carrier, heat balance, mass balance, circulation rate, solid inventory, pressure, etc.2,29,48 Currently, we focus on the investigation on the reactivity and stability of the new CaSO4/CaS oxygen carrier with coal gasification gas. The reduction of a CaSO4 oxygen carrier with coal gasification gas is exothermic, and the oxidation is a strong exothermic reaction. Therefore, the CaSO4 oxygen carrier can transfer heat from the air reactor to the fuel reactor, and the solid circulation rate is not limited by the heat balance. This behavior is similar to the combustion of nickel-, iron-, and copper-based oxygen carriers with syngas.48 As for the CLC system for solid fuels using a CaSO4 oxygen carrier, the operation conditions for the CaSO4 oxygen carrier would be much more complicated. The major reaction involved in coal gasification, eqs 1 and 3, are all intensive endothermic (48) Abad, A.; Adanez, J.; Garcia-Labiano, F.; de Diego, L. F.; Gayan, P.; Celaya, J. Chem. Eng. Sci. 2007, 62 (1-2), 533–549.

Figure 10. Pore size distribution changes of CaSO4 oxygen carrier particles before and after 20 cyclic tests. Table 5. Surface Properties of Fresh and Used CaSO4 Oxygen Carrier Particles species

fresh sample

20th oxidation

BET surface area (m2/g) total pore volume (cm3/g) micropore volume (cm3/g) average pore width (nm) average pore diameter (nm)

0.5266 0.001868 0.000026 14.1883 28.9794

0.4426 0.004537 0.000286 40.9999 37.6913

processes. The WGSR (eq 2) and reduction of CaSO4 with CO and H2 (eqs 4 and 5) are all exothermic reactions. As already indicated in some publications,14,15 solid fuel gasification is the time-limiting step because it is much slower than the reaction of the gasification gas with an oxygen carrier. The heat balance between the fuel reactor and air reactor depends upon solid circulation and temperature difference between two reactors. It also relies on CaSO4 conversion in the fuel reactor, CaS conversion in the air reactor, and coal gasification. A large temperature drop would inhibit the gasification process and the following reduction with CaSO4. With the decrease of the oxygen carrier reduction, the oxidation in the air reactor would generate less heat. Because less heat is transferred to the fuel reactor, the temperature in the fuel reactor would decrease more and finally cause the temperature decrease and low performance of the CLC system.48 To prevent a larger temperature drop in the fuel reactor, a high recirculation rate is necessary to exchange the heat from the air reactor to the fuel reactor. In our previous studies, Shen has presented the concept of a CLC of coal in the interconnected fluidized beds using NiO and CaSO4 oxygen carriers.16 The coal gasification and the reduction of the oxygen carrier with syngas were analyzed in the rector by using Aspen Plus software. The recirculation of CaSO4 oxygen carrier particles was influenced by the air reactor temperature, fuel reactor temperature, and ratio of water/coal on the composition of fuel gas. The suitable temperature for high conversion of solid fuels in the fuel reactor is 900-950 °C. For the same amount of heat to transfer from the air reactor to the fuel reactor, the required recirculation of oxygen carrier particles decrease exponentially, with the air reactor temperature increasing. For the fuel reactor temperature at 900 °C and the ratio of water/coal at 0.3, to maintain the heat balance, the air reactor temperature should be between 1000 and 1050 °C, with a temperature difference of around 100 °C. Accordingly, the required recirculation of CaSO4 oxygen carrier particles is above 30 kg/kg of coal, which is about half that of the NiO oxygen


carrier because of the high oxygen transport capacity of the CaSO4 oxygen carrier.16 The above simulation results provide useful information for the use of a CaSO4 oxygen carrier in CLC of solid fuels. Further experimental studies on this issue are necessary. A large solid inventory is possibly needed to compromise the relative low reactivity obtained in this study. Also, a certain fresh CaSO4 oxygen carrier may be necessary to make up the deactivated particles because of sulfur release, and a reject flow is withdrawn.17 Finally, the current experiment on solid fuel gasification and CLC is conducted under atmospheric pressure. A pressurized CLC system for solid fuels would be investigated in future work because it has many benefits in improving the fuel conversion, system efficiency, and lowering the cost for sequestrate CO2. Also, the pressurized condition in the oxidation reactor increases the conversion of CaS and minimizes the emission of SO2. Also, increasing pressure may suppress the competing decomposition of CaSO4 to CaO and SO2 (eq 16). However, the effect of pressure on the oxidation process at high temperatures has been insufficient.49 Further works would be needed. 4. Conclusions In this study, the cyclic test of a CaSO4 oxygen carrier in alternating reducing simulated coal gas and oxidizing conditions was performed at 950 °C in a fluidized bed reactor at atmospheric pressure. A high concentration of CO2 could be obtained in the reduction. The H2 and CO conversions and CO2 yield initially increased and finally decreased during the reduction in the multicycle test. The release of SO2 and H2S during the cyclic test was found to be responsible for the decrease of reactivity of the CaSO4 oxygen carrier. The oxygen carrier conversion after the reduction reaction decreased gradually in the cyclic test. Through the comparison of mass-based reaction rates as a function of mass conversion at typical cycles, it is evident that the reactivity of the CaSO4 oxygen carrier increased for the initial cycles but finally decreased after around 15 cycles. The mass conversion rate of the CaSO4 oxygen carrier was considerably lower than that of metal oxides. XRD analysis revealed that the presence and intensity of the reduction sulfur species was in accordance with the result of gas conversion. The content of CaO was higher than expected, suggesting the formation of SO2 and H2S during the cycles. ESEM analysis demonstrates the surface morphology variation from impervious to porous after the cyclic test. It was also observed that the small grains on the surface of the oxygen carrier sintered in the cyclic tests. EDS analysis also demonstrated the decrease of the oxygen intensity after reduction, and CaO became the main component after 20th oxidation. BET analysis suggested that the particles agglomerated or sintered in the cyclic tests. The possible method (49) Qiu, K.; Anthony, E. J.; Jia, L. Fuel 2001, 80 (4), 549–558.

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for sulfur mitigation is proposed. Finally, some basic consideration on the design criteria of a CLC system for solid fuels using a CaSO4 oxygen carrier is discussed and provides direction for the future work. Therefore, further extensive work would be required for the use of a CaSO4-based oxygen carrier in CLC. Acknowledgment. The support from the National Natural Science Foundation of China (Grants 50606006 and 90610016) for this study is gratefully acknowledged. We also acknowledge Dr. Marcus Johnson of Chalmers University of Technology in Sweden for providing valuable suggestions for data evaluation. The authors also acknowledge Xiao Ban for her assistance in carrying out the experimental work. We also sincerely acknowledge the valuable comments and suggestions of the reviewers. The authors also express sincere gratitude to Prof. E. J. Anthony and Dr. Jinsheng Wang for valuable comments, which contributed to our further study.

Nomenclature dXo/dt ) oxidation rate of the oxygen carrier, s-1 dXr/dt ) reduction rate of the oxygen carrier, s-1 m ) actual mass of the 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, kg/mol n˙in ) molar flow of the gas entering the reactor, mol/s n˙out ) molar flow of the gas exiting the reactor after the water has been removed, mol/s nO ) amount of oxygen in the oxygen carrier that can be removed from the fully oxidized oxygen carrier, mol RO ) oxygen transport capacity or oxygen ratio of the oxygen carrier, defined as Ro ) (mox - mred)/mox t ) time, s T ) temperature, °C u ) mean velocity, m/s umf ) minimum fluidization velocity, m/s ut ) terminal velocity, m/s X ) oxygen carrier conversion or the degree of oxidation Xo ) oxygen carrier conversion during the oxidation reaction Xr ) oxygen carrier conversion during the reduction reaction yi,in ) inlet molar fraction of the gas species i entering the reactor yi,out ) outlet molar fraction of the gas species i exiting the reactor after the water has been removed Greek Letters ∆P ) pressure difference, kPa ∆X ) variation of the conversion of the oxygen carrier ∆ω ) variation of the mass conversion of the oxygen carrier γCO ) conversion of CO during the reduction period γH2 ) conversion of H2 during the reduction period ω ) mass-based conversion of the oxygen carrier Subscripts in ) inlet out ) outlet EF800275A

Multicycle Study on Chemical-Looping ... - ACS Publications

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