Energy Fuels 2010, 24, 1449–1463 Published on Web 12/30/2009
: DOI:10.1021/ef901070c
Pressurized Chemical-Looping Combustion of Chinese Bituminous Coal: Cyclic Performance and Characterization of Iron Ore-Based Oxygen Carrier Rui Xiao,* Qilei Song, Shuai Zhang, Wenguang Zheng, and Yichao Yang Thermal Engineering Institute and School of Energy and Environment, Southeast University, Sipailou No.2, Nanjing 210096, China Received September 22, 2009. Revised Manuscript Received December 9, 2009
A pressurized chemical-looping combustion combined cycle (PCLC) system is proposed for solid fuels combustion with potential high system efficiency, improving the fuel conversion and lowering the cost for CO2 sequestration. In this study, pressurized CLC of coal with Companhia Valedo Rio Doce (CVRD) iron ore was investigated in a laboratory fixed bed reactor focusing on cyclic performance. CVRD iron ore particles were exposed alternately for 20 cycles to reduction by 0.4 g of Chinese Xuzhou bituminous coal gasified with 87% steam/N2 mixture and oxidation with 5% O2 in N2 at 970 °C at atmospheric pressure (0.1 MPa) and a typical elevated pressure of 0.5 MPa. With increasing number of redox cycles, more pyrolysis gases are oxidized by the oxygen carrier. At elevated pressure, the char gasification is intensified with negligible gasification intermediate products released. The CO2 fraction increases from 80% to approximate 90% after 10 cycles at atmospheric pressure. At elevated pressures, the average CO2 fraction stabilizes at 95.75%, approximate to the equilibrium value. The carbon conversion at 0.1 MPa and 0.5 MPa is 76.48 and 84.65%, respectively, and maintains approximately the same during the cycles because excessive steam gasification agent used in this study. The oxygen carrier conversion determined from the oxygen mass balance verifies that reduction level increases with the cycle number. The physical and chemical properties of oxygen carrier particles were characterized. X-ray diffraction (XRD) analysis verifies the extent of reduction level increases with cycles. No detectable formation of compound of iron oxide and coal ash was observed. Scanning electron microscope (SEM) analyses show that the iron ore particles become porous and that more pores formed with cycles. Agglomeration of particles was not observed in all experiments at both pressures. Energy-dispersive X-ray spectroscopy (EDX) analysis show an increasing amount of coal ash on the oxygen carrier particles with increasing numbers of cycles. Pore size analyses show that the oxygen carrier particles maintained mesopores for both atmospheric and elevated pressure. The increase of both surface area and pore volume illustrates that the particles become more porous with redox cycles. This study show that pressurized CLC of coal is promising and a low-cost iron ore-based oxygen carrier may be suitable for pressurized CLC of coal.
carrier development, as summarized elsewhere.2-4 Continuous CLC units have been demonstrated from laboratory tests in 300 W to 140 kW with natural gas or syngas as fuel.5-8 Successful demonstration of 1000 h operation has recently been reported.9,10 However, CO2 from fossil fuel combustion accounts the dominant sources of greenhouse gases emissions.3 Therefore, it is more promising to apply the CLC process to solid fuels combustion. One technical approach for CLC of solid fuels is first gasifying the coal to syngas and supplying the syngas to the
1. Introduction Reducing and control the emission of greenhouse gases, mainly carbon dioxide, has become an urgent worldwide need.1 Chemical-looping combustion (CLC) is a new combustion technology with inherent separation of CO2. In the CLC process, the fuel is oxidized to CO2 and H2O by oxygen carrier particles that are usually metal oxides. The reduced oxygen carrier particles are oxidized in air for regeneration and producing N2 and depleted O2. The product gases during the reduction process are mainly CO2 and H2O. Almost pure CO2 is obtained after the condensation of water. After compression, the CO2 can be sequestrated for storage or industrial application. During the past two decades, extensive work has been done on the development of CLC of gaseous fuels in mainly three aspects: system analysis, reactor design, and oxygen
(4) Hossain, M. M.; de Lasa, H. I. Chem. Eng. Sci. 2008, 63 (18), 4433– 4451. (5) Abad, A.; Mattisson, T.; Lyngfelt, A.; Ryden, M. Fuel 2006, 85 (9), 1174–1185. (6) Lyngfelt, A.; Kronberger, B.; Adanez, J.; Morin, J.-X.; Hurst, P. In Seventh International Conference on Greenhouse Gas Control Technologies, Vancouver, Canada, Elsevier Science: Oxford, U.K., 2004. (7) Ryu, H. J.; Jin, G. T.; Yi, C. K. In Seventh International Conference on Greenhouse Gas Control Technologies, Vancouver, Canada, Elsevier Science: Oxford, U.K., 2004. (8) Kolbitsch, P.; Pr€ oll, T.; Bolhar-Nordenkampf, J.; Hofbauer, H. Energy Proc. 2009, 1 (1), 1465–1472. (9) Linderholm, C.; Mattisson, T.; Lyngfelt, A. Fuel 2009, 88 (11), 2083–2096. (10) Shulman, A.; Linderholm, C.; Mattisson, T.; Lyngfelt, A. Ind. Eng. Chem. Res. 2009, 48 (15), 7400–7405.
*To whom correspondence should be addressed. Telephone: þ86-258379 5726. Fax: þ86-25-5771 4489. E-mail:
[email protected]. (1) Intergovernmental Panel on Climate Change (IPCC), 2007. Working Group III Report: Mitigation of Climate Change; Cambridge University Press: Cambridge, UK, 2008. (2) Lyngfelt, A.; Johansson, M.; Mattisson, T. In 9th International Conference on Circulating Fluidized Beds (CFB-9), Hamburg, Germany, 2008. (3) Anthony, E. J. Ind. Eng. Chem. Res. 2008, 47 (6), 1747–1754. r 2009 American Chemical Society
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: DOI:10.1021/ef901070c
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11-14
Then, the reduced iron oxide (magnetite) is transported to the air reactor to be oxidized back to Fe2O3: 4Fe3 O4 þ O2 f 6Fe2 O3
CLC reactor. The weakness of this approach is that the addition of a gasifier and air separation unit would decrease the efficiency and make equipment facilities complicated. Another approach is integrating gasification of solid fuels and CLC in one reactor. The concept of CLC technology for solid fuels in two interconnected fluidized beds was patented in the 1950s.15 Dennis and Scott et al. proposed in situ gasification and chemical looping for solid fuels in one fluidized bed.16,17 Cao et al. proposed the CLC of solid fuels in two interconnected fluidized beds.18 This technical approach has been proposed and demonstrated in laboratory tests and a continuous 10 kW thermal CLC reactor with interconnected fluidized beds in Chalmers University of Technology18-22 and Southeast University.23-25 These successful demonstrations show that CLC of solid fuels is promising. The general reaction steps for CLC of solid fuels are as follows. First, coal pyrolysis generates tar; gases, including CO, H2, CO2, H2O, CH4, and other hydrocarbons; and residual char. The residual char is gasified with CO2 and H2O, and water gas shift reaction (WGSR) also occurs: C þ H2 O f CO þ H2 C þ CO2 f 2CO CO þ H2 O f CO2 þ H2
ΔH θ1223K ¼ 135:6 kJ=mol ΔH θ1223K ¼ 168:3 kJ=mol ΔH θ1223K ¼ 32:66 kJ=mol
ΔH θ1273K ¼ -478:82 kJ=mol
Therefore, the indirect combustion of coal with inherent CO2 separation can be achieved via the cyclic reduction and oxidation of an oxygen carrier. For the above technical approach for CLC of solid fuels, one challenge is that the operating temperature for CLC of solid fuels is limited at ∼1000 °C by low melting points of coal ash and oxygen carrier particles. The gas turbine inlet temperature will be lowered and the system efficiency would also be affected.12,26-28 Another important concern is that the slow coal gasification process limits the overall coal combustion efficiency.17,21,29 These two major challenges limit the coal-fueled CLC to compete with other nearly commercial available CO2 capture technologies. One possible solution is increasing the system operating pressure. The pressurized CLC process could be coupled with advanced power generation systems, which will also enable the recovery of high-pressure CO2 and lower the cost for sequestration and storage of CO2.30 The pressurized chemical looping combustion combined cycle (PCLC-CC) system is tentatively proposed as shown in Figure 1. The PCLC system consists of mainly two pressurized fluidized beds (PFB), where the fuel reactor could be a pressurized spout-fluidized bed composed of a mixer and a riser, and the air reactor could be a moving bed reactor. The spout-fluid bed provides strong solids mixing and long residence time for coal gasification and reaction with oxygen carrier. The high-temperature high-pressure flue gases could drive a gas turbine (GT), followed by a heat-recovery steam generator (HRSG) for a steam turbine (ST), similar to the pressurized fluidized bed combustion (PFBC) system. There are currently no studies on CLC of solid fuels at elevated pressures, which is quite necessary to investigate.3,31 Coal gasification in a pressurized fluidized bed has become an established technology.32-34 A research group at Southeast University has done extensive research work on PFBC-CC technology and successfully developed a 15 MW electrical pilot scale PFBC-CC power plant with 1000 h operation and also research on a second PFBC technology.32-35 Many technologies in the PFBC-CC system could be referred to develop the PCLC-CC system. Similar to the PFB gasifier,
ð1Þ ð2Þ ð3Þ
Simultaneously, the oxygen carrier is reduced by pyrolysis gases and gasification intermediate products in the fuel reactor, generating a stream of CO2 and steam that results in a pure stream of CO2 after condensation of steam. Here the oxygen carrier is using Fe2O3/Fe3O4 as an example. 12Fe2 O3 þ CH4 f 8Fe3 O4 þ CO2 þ 2H2 O ΔH θ1223K ¼ 37:74 kJ=mol
ð4Þ
3Fe2 O3 þ CO f 2Fe3 O4 þ CO2 ΔH θ1223K ¼ -41:47 kJ=mol
ð5Þ
3Fe2 O3 þ H2 f 2Fe3 O4 þ H2 O ΔH θ1223K ¼ -8:81 kJ=mol
ð7Þ
ð6Þ
(11) Abad, A.; Mattisson, T.; Lyngfelt, A.; Johansson, M. Fuel 2007, 86 (7-8), 1021–1035. (12) Jin, H.; Ishida, M. Fuel 2004, 83 (17-18), 2411–2417. (13) Mattisson, T.; Garcı´ a-Labiano, F.; Kronberger, B.; Lyngfelt, A.; Adanez, J.; Hofbauer, H. Int. J. Greenhouse Gas Control 2007, 1 (2), 158– 169. (14) Ryu, H. J.; Shun, D.; Bae, D. H.; Park, M. H. Kor. J. Chem. Eng. 2009, 26 (2), 523–527. (15) Lewis, W. K.; Gilliland, E. R. Production of pure carbon dioxide. US Patent No: 2665972, 1954. (16) Dennis, J. S.; Scott, S. A.; Hayhurst, A. N. J. Energy Inst. 2006, 79 (3), 187–190. (17) Scott, S. A.; Dennis, J. S.; Hayhurst, A. N.; Brown, T. AIChE J. 2006, 52 (9), 3325–3328. (18) Cao, Y.; Pan, W. P. Energy Fuels 2006, 20 (5), 1836–1844. (19) Pan, W. P.; Cao, Y.; Liu, K. L.; Wu, W. Y.; Riley, J. T. Abstr. Pap. Am. Chem. Soc. 2004, 228, U676–U676. (20) Berguerand, N.; Lyngfelt, A. Fuel 2008, 87 (12), 2713–2726. (21) Leion, H.; Mattisson, T.; Lyngfelt, A. Int. J. Greenhouse Gas Control 2008, 2 (2), 180–193. (22) Wang, J. S.; Anthony, E. J. Applied Energy 2008, 85, 73–79. (23) Shen, L. H.; Wu, J. H.; Xiao, J. Combust. Flame 2009, 156 (3), 721–728. (24) Shen, L. H.; Wu, J. H.; Gao, Z. P.; Xiao, J. Combust. Flame 2009, 156 (7), 1377–1385. (25) Shen, L. H.; Wu, J. H.; Xiao, J.; Song, Q. L.; Xiao, R. Energy Fuels 2009, 23, 2498–2505.
(26) Jin, H. G.; Ishida, M. Int. J. Hydrogen Energy 2000, 25 (12), 1209–1215. (27) Wolf, J.; Anheden, M.; Yan, J. Fuel 2005, 84 (7-8), 993–1006. (28) Naqvi, R.; Bolland, O. Int. J. Greenhouse Gas Control 2007, 1 (1), 19–30. (29) Gao, Z. P.; Shen, L. H.; Xiao, J.; Qing, C. J.; Song, Q. L. Ind. Eng. Chem. Res. 2008, 47 (23), 9279–9287. (30) Garcı´ a-Labiano, F.; Adanez, J.; de Diego, L. F.; Gayan, P.; Abad, A. Energy Fuels 2006, 20 (1), 26–33. (31) Liu, K.; Downs, W.; Vecci, S. J.; Bonk, D. L. In The 31st International Technical Conference on Coal Utilization & Fuel Systems, Clearwater, Florida; Coal Technology Association: Gaithersburg, MD, 2006. (32) Xiao, R.; Zhang, M. Y.; Jin, B. S.; Huang, Y. J.; Zhou, H. C. Energy Fuels 2006, 20 (2), 715–720. (33) Xiao, R.; Zhang, M. Y.; Jin, B. S.; Xiong, Y. Q.; Zhou, H. C.; Duan, Y. F.; Zhong, Z. P.; Chen, X. P.; Shen, L. H.; Huang, Y. Fuel 2007, 86 (10-11), 1631–1640. (34) Deng, Z.; Xiao, R.; Jin, B.; Huang, H.; Shen, L.; Song, Q.; Li, Q. Energy Fuels 2008, 22 (3), 1560–1569. (35) Zhang, M. Y. Project of a 15 MWe Scale PFBC-CC pilot Power Plant, In Proceedings of the 12th International Conference on Fluidized Bed Combustion; ASME: New York, 1993; p 431.
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Figure 1. Schematic diagram of a coal-fueled PCLC-CC for power generation and CO2 capture.
third solution is finding low-cost minerals or ores as oxygen carrier.20,45-49 Recently, low-cost natural iron-based ores such as ilmenite and iron ores have attracted significant interest because they are much cheaper and easily obtained from the steel industry. Leion et al. performed extensive work on iron-based oxygen carriers, such as ilmenite mineral (FeTiO3/Fe2TiO5) with syngas and different solid fuels,21,45 and a number of iron, manganese ore, and iron scales with syngas and solid fuels.50,51 Ilmenite particles were further tested in a continuous 10 kW thermal CLC combustor with coal and petroleum coke as fuel by Berguerand et al.20,48 The Mt. Wright iron ore and oxide scale also showed good performance. All these findings show that iron-based oxygen carrier particles are promising in CLC. One interesting phenomenon in all these studies is that the reactivity of natural particles increases with redox cycles.45,52 The mechanism for this activation process is still not clearly understood, especially the behavior of particles under elevated pressure.
the gasification rate in the PCLC reactor may be enhanced with pressure.36-39 As simply expected now, the PCLC-CC system would potentially achieve much higher coal combustion efficiency and system efficiency for power generation with CO2 capture compared to the current CLC system.31 The development of oxygen carrier is a key issue for the CLC process. A number of metal oxide oxygen carriers have been investigated as summarized elsewhere.2-4 Many important factors should be considered for use of oxygen carrier with solid fuels, such as: low cost, high oxygen capacity and reactivity with CO and H2, high mechanical strength, resistant to thermal sintering and agglomeration, and environmentally benign. One method is to develop synthetic particles of high performance and stability, such as an NiO-based oxygen carrier.24,40-42 However, Ni is very expensive for solid fuels combustion and is possibly deactivated by sulfur.42 Another solution is to synthesize a low-cost oxygen carrier.43,44 The (36) Chen, H.; Luo, Z.; Yang, H.; Ju, F.; Zhang, S. Energy Fuels 2008, 22 (2), 1136–1141. (37) Yang, H.; Chen, H.; Ju, F.; Yan, R.; Zhang, S. Energy Fuels 2007, 21 (6), 3165–3170. (38) Roberts, D. G.; Harris, D. J. Energy Fuels 2006, 20 (6), 2314– 2320. (39) Roberts, D. G.; Harris, D. J. Energy Fuels 2000, 14 (2), 483–489. (40) Solunke, R. D.; Veser, G., Energy Fuels 2009, doi:10.1021/ ef900280m. (41) Zhao, H.; Liu, L.; Wang, B.; Xu, D.; Jiang, L.; Zheng, C. Energy Fuels 2008, 22 (2), 898–905. (42) Leion, H.; Lyngfelt, A.; Mattisson, T. Chem. Eng. Res. Des. 2009, 87 (11), 1543–1550. (43) Chuang, S. Y.; Dennis, J. S.; Hayhurst, A. N.; Scott, S. A. Combust. Flame 2008, 154 (1-2), 109–121. (44) Chuang, S. Y.; Dennis, J. S.; Hayhurst, A. N.; Scott, S. A. Proc. Combust. Inst. 2009, 32 (2), 2633–2640. (45) Leion, H.; Lyngfelt, A.; Johansson, M.; Jerndal, E.; Mattisson, T. Chem. Eng. Res. Des. 2008, 86 (9), 1017–1026.
(46) Mattisson, T.; Lyngfelt, A.; Cho, P. Fuel 2001, 80 (13), 1953– 1962. (47) Leion, H.; Mattisson, T.; Lyngfelt, A. Fuel 2007, 86 (12-13), 1947–1958. (48) Berguerand, N.; Lyngfelt, A. Int. J. Greenhouse Gas Control 2008, 2 (2), 169–179. (49) Pr€ oll, T.; Mayer, K.; Bolhar-Nordenkampf, J.; Kolbitsch, P.; Mattisson, T.; Lyngfelt, A.; Hofbauer, H. Energy Proc. 2009, 1 (1), 27–34. (50) Leion, H.; Mattisson, T.; Lyngfelt, A. Energy Fuels 2009, 23, 2307–2315. (51) Leion, H.; Jerndal, E.; Steenari, B.-M.; Hermansson, S.; Israelsson, M.; Jansson, E.; Johnsson, M.; Thunberg, R.; Vadenbo, A.; Mattisson, T.; Lyngfelt, A. Fuel 2009, 88 (10), 1945–1954. (52) Cuadrat, A.; Abad, A.; Adanez, J.; Diego, L. d.; Garcı´ a-Labiano, F.; Gayan, P. In 4th European Combustion Meeting, Vienna, Austria, 2009.
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Table 1. Proximate and Ultimate Analyses of Coal Samples proximate analysis (ad, wt %)a
ultimate analysis (ad, wt %)
low heating value (MJ/kg)
sample
moisture
volatile
fixed carbon
ash
Cad
Had
Oadb
Nad
Sad
LHV
Xuzhou coal
1.4
29.87
42.86
25.87
64.76
4.16
1.74
1.16
0.91
26.12
a
ad: air dried basis. b Calculated by difference.
Table 2. Xuzhou Bituminous Coal Ash Analysis ash composition of coal sample (wt%)
ash fusion temperature (°C)
SiO2
Al2O3
Fe2O3
CaO
MgO
TiO2
SO3
K2O
Na2O
P2O5
DT
ST
FT
54.18
32.84
5.35
1.57
0.53
2.27
1.47
1.39
0.41
0
1150
1310
1420
There are some publications on oxygen carrier conversion in gaseous fuel experiments of pressurized CLC. However, the effect of the pressurized condition on the reduction and oxidation of oxygen carrier are still unclear as explored in pressurized thermogravimetric analyzers (PTGA) or fixed beds,12,30,53-56 as summarized in a review by Anthony.3 According to a representative paper by Garcı´ a-Labiano et al.,30 the reaction rates of both reduction and oxidation of oxygen carrier are negatively affected by the total pressure, and higher solid inventories would be necessary for pressurized CLC conditions. However, it should be noted that all these experiments were performed with gaseous fuels. In previous work, the effect of pressure on the CLC of coal within short-term cycles has been examined in a laboratory fixed bed reactor.57 The limited results show that the overall combustion efficiency of coal at elevated pressure would be determined by coal gasification rate and reduction of oxygen carrier together. The results show that the char conversion rate during reduction period increases with pressure but slows down above 0.3 MPa and reaches a maximum at around 0.4-0.5 MPa, then decreases at 0.6 MPa. The oxygen carrier conversion level is higher at elevated pressure because more gasification intermediate products were produced. However, the increased gasification agents (steam or CO2) partial pressures in pressurized CLC of coal may have detrimental effect on the reduction reaction, thus affecting the conversion of gasification products. The Companhia Valedo Rio Doce (CVRD) iron orebased oxygen carrier particles did not show evident change after short cycles at different pressures. Therefore, long-term reduction and oxidation cycles at atmospheric pressure and an optimal pressure of 0.5 MPa are performed in this work. The purpose of this work was to evaluate the long-term performance of pressurized CLC of coal with iron ore-based oxygen carrier both at atmospheric and elevated pressure. The oxygen carrier was exposed to reduction by coal/steam mixture and oxidation by diluted oxygen both at atmospheric and at typical elevated pressure for 20 cycles, respectively. The variation of gas concentration, overall gas composition, carbon conversion, and oxygen carrier conversion with cycles are investigated. The oxygen carrier particles sampled in the reduction cycles were characterized by several physical and chemical techniques such as X-ray diffraction (XRD) analysis,
scanning electron microscope and energy-dispersive X-ray spectroscopy (SEM-EDX), and pore size analyzer. 2. Experimental Section 2.1. Material Preparation. A typical Chinese low rank, high ash, bituminous coal produced from Xuzhou, Jiangsu Province was used in the experiments. The proximate analysis and ultimate analysis of the coal samples are listed in Table 1. The ash content of the coal sample is shown in Table 2. The coal particles were sieved to a size range of 0.125-0.180 mm for use in this study. The oxygen carrier particle used in this study was CVRD iron ore produced from Brazil, which is imported and provided by the Nanjing Meishan Steel, Shanghai Baosteel Co. Ltd. The CVRD iron ore particles were heat treated in a fluidized bed in air (600 mL/min) for 6 h at 1100 °C to reach its maximum oxidized state. According to XRD and EDX analysis, only hematite (Fe2O3), SiO2, and slight Al2O3 are detected in the calcined particles. The calcined CVRD iron ore consists of 94.79% Fe2O3, 4.31% SiO2, and 0.91% Al2O3. The particle size range was within 0.09-0.125 mm. The oxygen transport capacity, or oxygen ratio of the oxygen carrier, calculated by Ro= (mox-mred)/mox, of the calcined oxygen carrier is 0.0316. The true density (FS) and skeleton apparent density (FP) of the calcined particles are 5130 and 5100 kg/m3, respectively. The porosity of the calcined particles is below 0.01 (0.5%). The bulk or packing density (Fb) of the particles is 2000 kg/m3. 2.2. Material Characterization. The calcined and reacted oxygen carriers were analyzed by a series of characterization techniques. The pore structure properties including BET surface area, pore volume and average pore diameter were measured by nitrogen adsorption/desorption isotherms at 77 K with a BELSORP-miniII surface area and pore size analyzer, BEL Japan, Inc. X-ray diffraction (XRD) for the crystal structure of fresh and used samples was performed in a D/max 2500 VL/PC X-ray diffractometer using Cu KR radiation (40 kV, 200 mA) from 5 to 85° with a step of 0.02°/s. The surface morphological features of the fresh and reacted samples were recorded by a scanning electron microscope (SEM) 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 (EDX) system equipped with the SEM system. 2.3. Experimental Setup and Procedure. The experiment was conducted in a laboratory scale fixed bed reactor system that includes mainly coal feeding unit, water/gas feeding and steam generator units, reactor, temperature control unit, back pressure regulator, steam cooler, filters, and gas analysis system. The schematic flow diagram of the system is shown in Figure 2. A straight stainless steel tube (I.D. = 30 mm, length = 950 mm) with a porous distributor plate located at 450 mm from the bottom was used. The reactor is able to work at a maximum pressure of 2 MPa at 1273 K. The reactor was heated in an electric furnace and the furnace temperature was controlled by a K thermocouple between the tube and the heater (T3) while the
(53) Jin, H. G.; Ishida, M. Int. J. Hydrogen Energy 2001, 26 (8), 889– 894. (54) Jin, H. G.; Ishida, M. Ind. Eng. Chem. Res. 2002, 41 (16), 4004– 4007. (55) Siriwardane, R.; Poston, J.; Chaudhari, K.; Zinn, A.; Simonyi, T.; Robinson, C. Energy Fuels 2007, 21 (3), 1582–1591. (56) Tian, H.; Chaudhari, K.; Simonyi, T.; Poston, J.; Liu, T.; Sanders, T.; Veser, G.; Siriwardane, R. Energy Fuels 2008, 22 (6), 3744–3755. (57) Xiao, R.; Song, Q. L., Combust. Flame, Submitted.
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of 5.0% O2 in N2 with total flow rate 1000 mL/min. The low O2 fraction was adopted to avoid an excessive temperature increase due to heat generated from the intense exothermic oxidation of the oxygen carrier. All the oxidation duration was set to 40 min, enough to maximize the oxidation conversion. Then, the gas was switched to N2 at a flow rate of 1000 mL/min for 5 min to purge the O2 in the system. Then, the inlet gas was switched to the steam/N2 mixture again. With the above consecutive stages, the iron ore particles were exposed alternately to coal and 5% O2/N2 for 20 redox cycles. When the cyclic tests were finished the inlet 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. In this work the gas analysis was semicontinuous in order to minimize the gas dispersion during the long distance from the reactor to the gas analyzers. The dry sample gas was measured by an Emerson multicomponent gas analyzer including a Rosemount NGA 2000 gas analyzer. The volumetric fractions of CO2, CH4, and CO were measured by nondispersive infrared analysis (NDIR) with a detecting limit at 0.01%. The O2 fraction was measured with a paramagnetic analyzer with a detecting limit at 0.01%. The fraction of H2 was measured by a Rosemount Hydros 100 analyzer based on gas thermal conductivity with a detecting limit at 0.1%. The gas analyzer was calibrated with standard gas, providing high measurement accuracy (
