Coal with Natural Iron Ore

Dec 21, 2010 - Chemical looping combustion (CLC) is a new innovative technology with inherent separation of CO2 without energy penalty. Experiments on...
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Energy Fuels 2011, 25, 446–455 Published on Web 12/21/2010

: DOI:10.1021/ef101318b

Chemical Looping Combustion of Biomass/Coal with Natural Iron Ore as Oxygen Carrier in a Continuous Reactor Haiming Gu, Laihong Shen,* Jun Xiao, Siwen Zhang, and Tao Song Thermoenergy Engineering Research Institute, Southeast University, Nanjing 210096, China Received September 28, 2010. Revised Manuscript Received December 5, 2010

Chemical looping combustion (CLC) is a new innovative technology with inherent separation of CO2 without energy penalty. Experiments on chemical looping combustion of biomass/coal were conducted in a 1 kWth continuous reactor, and an Australia iron ore was selected as oxygen carrier. Both biomass/coal mixture and biomass were used as fuels. The effect of temperature on gas composition of both the fuel reactor and the air reactor, conversion efficiency of carbonaceous gases, carbon capture efficiency, and oxide oxygen fraction was investigated. An increase in the fuel reactor temperature produced a higher CO2 concentration in the fuel reactor for biomass/coal mixture, whereas it produced a lower one for pure biomass. CO concentration in the fuel reactor increased in both fuel conditions. Due to the poor oxygen transport capacity and the thermodynamic constraint of the iron ore conversion from Fe2O3 to Fe3O4, a higher temperature would contribute to decreasing the conversion efficiency of carbonaceous gases for both biomass and biomass/coal mixture. Both carbon capture efficiency and oxide oxygen fraction were enhanced with increasing the fuel reactor temperature, and the deviation between them was caused by the combustible carbonaceous gases in the fuel reactor. Both the fresh and the used oxygen carrier particles were characterized. X-ray diffraction (XRD) results indicated that the iron ore as oxygen carrier possesses a good regenerable ability in the CLC process. This is attributed to the existence of quartz in the iron ore particles and its sintering inhibition. Reactions between SiO2 and Fe3O4 may occur at a high temperature under a reducing condition. Scanning electron microscope (SEM) analysis showed that as a consequence of accumulative effect of redox reaction and thermal stress, the used oxygen carrier particles obtained a porous structure facilitating the gas-solid reactions. Energy dispersive X-ray (EDX) results demonstrated the deposition of alkali metals on the particle surface of oxygen carrier during the CLC process of biomass. Blending biomass with coal and adding some additives might be effective measures to reduce the potential negative influence of biomass ash on oxygen carrier.

the fuel reactor consists of only CO2 and steam. After steam is condensed and removed, pure CO2 can be obtained thus realizing the inherent separation of CO2. Vital properties should be provided by the oxygen carrier: high reactivity, high redox reaction rate, complete fuel conversion to CO2 and H2O, and high resistance to attrition and thermal sintering. Also, the oxygen carrier should be environmentally benign and it will be advantageous if the price is low. Some metals such as Ni, Fe, Co, Cu, and Mn and some sulfates have been intensively tested as potential oxygen carriers in a thermo-gravimetric analysis or a singular batch bed using gaseous fuels, i.e., methane, hydrogen, or syngas from coal gasification.3-15 For a Ni-based oxygen carrier,

1. Introduction As we know, CO2 is the primary greenhouse gas causing global warming which has aroused increasing attention all over the world. Fossil fuels used for power generation account for a large amount of anthropogenic CO2 emission. It is a great challenge to capture CO2 from power generation. Chemical looping combustion (CLC)1 is a new technology with the characteristic of inherent separation of CO2 without energy penalty which may be a problem when using other techniques such as precombustion, oxy-fuel combustion, or postcombustion. In addition, based on two-step reactions, CLC realizes the cascade utilization of chemical energy.2 In the CLC process, a metal oxide, via recycle redox reaction, is often used to transport oxygen from air to fuel so that direct contact between air and fuel is avoided. As shown in Figure 1, metal oxide is reduced to metallic or suboxide MexOy-1 by a gas fuel in a fuel reactor and then the oxygen carrier particles are transported to an air reactor where it is oxidized to metal oxide MexOy by O2 (in air). Afterward, the oxygen carrier particles return to the fuel reactor for a new cycle. Flue gas leaving the air reactor contains nitrogen and some oxygen, whereas that leaving

(3) Cho, P.; Mattisson, T.; Lyngfelt, A. Fuel 2004, 83, 1215–1225. (4) Zafar, Q.; Mattisson, T.; Gevert, B. Energy Fuels 2006, 20, 34–44. (5) Abad, A.; Adanez, J.; Garia-Labı´ ano, F.; de Diego, L. F.; Gayan, P.; Celaya, J. Chem. Eng. Sci. 2007, 62 (1-2), 533–549. (6) Wolf, J.; Anheden, M.; Yan, J. Y. Fuel 2005, 84, 993–1006. (7) de Diego, L. F.; Garcı´ a-Labiano, F.; Adanez, J.; Gayan, P.; Abad, A.; Corbella, B. M.; Palacios, J. M. Fuel 2004, 83 (13), 1749–1757. (8) Lyngfelt, A; Kronberger, B.; Adanez, J.; Morin, J. X.; Hurst, P. In Proceedings of the 7th International Conference on Greenhouse Gas Control Technologies, Vancouver, Canada. 2004. (9) Mattisson, T.; Lyngfelt, A.; Cho, P. Fuel 2001, 80 (13), 1953–1962. (10) Jin, H. G.; Ishida, M. Int. J. Hydrogen Energy 2001, 268, 889–894. (11) Ishida, M.; Yamamoto, M.; Ohba, T. Energy Convers. Manage. 2002, 43 (9-12), 1469–1478. (12) Adanez, J.; de Diego, L. F.; Garcı´ a-Labiano, F.; Gayan, P.; Abad, A.; Palacios, J. M. Energy Fuels 2004, 18 (2), 371–377.

*To whom correspondence should be addressed. Telephone: þ86258379 5598. Fax: þ86-25-5771 4489. E-mail: [email protected]. (1) Richter, H.; Knoche, K. Symp. Ser. 1983, 235 (1), 71–86. (2) Jin, H. G.; Wang, B. Q. J. Eng. Thermophys. 2004, 25 (2), 181–184. r 2010 American Chemical Society

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: DOI:10.1021/ef101318b

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solid fuels. As for solid fuels used in CLC, there are often two reaction paths between fuel and oxygen carrier particles.29,30 Path 1 is direct reaction between oxygen carrier particles and solid fuels. Path 2 is reaction between oxygen carrier particles and gaseous intermediates from the gasification of solid fuels. Path 2 is the main reaction method because of the low rate of solid-solid reaction in path 1. Additionally, gasification rate can be enhanced under a pressurized condition31-35 which maybe used in CLC. Since 2001, a number of continuous reactors of 300 Wth to 120 kWth have been proposed for the CLC process.8,36-40 A 10 kWth interconnected fluidized bed reactor for chemical looping combustion was built at Southeast University, China in 2008. Successful operations with coal and biomass as fuels indicate that the use of solid fuels in chemical looping combustion is feasible.41,42 There were two compartments in the configuration of the fuel reactor, i.e., a reaction chamber and an inner seal. However, the inner seal could not prevent the bypassing of flue gas from the fuel reactor to the air reactor, thus decreasing the CO2 capture efficiency. Then, an improved prototype configuration for chemical looping combustion of solid fuels was made.43 Primarily different from the 10 kWth reactor,41 the new prototype configuration had an external loop-seal connecting the fuel reactor with the air reactor at their bottoms. Fluidized by steam, the loop-seal can effectively prevent gas leakage between the two reactors. Abundant biomass in our agricultural country is a large energy resource. Recently, biomass and biomass/coal mixture have received extensive investigation from the view of economy and energy utilization. The purpose of present work was to investigate the utilization of biomass in chemical looping combustion at atmospheric pressure. The experiments were conducted in the 1 kWth reactor43 with an Australia natural iron ore as oxygen carrier. Both biomass and biomass/coal mixture were used as solid fuels. The effect of fuel reactor temperature on the gas compositions of both the fuel reactor and the air reactor, conversion efficiency of carbonaceous gases in the fuel reactor, carbon capture efficiency, and oxide oxygen fraction was experimentally investigated. X-ray diffraction (XRD) and scanning electron microscopy and energy dispersive X-ray (SEM-EDX) were used to characterize the

Figure 1. Schematic of chemical looping combustion.

potential sulphide16 may have a poisonous effect on the reactivity of the oxygen carrier. Also, NiO with low toxicity is very expensive. Because of the low melting point, Cu-based oxygen carriers are limited to use at a relatively low temperature. Fe-based oxygen carriers with different compound formations all have a high melting point17 and complete fuel conversion is only possible when Fe2O3 is partially reduced to Fe3O4.18,19 As an oxygen carrier, sulfate itself has the problem of sulfur emission under conditions which may be encountered in CLC.15 To obtain better mechanical strength and more surface area, oxygen carriers are often supported on inert materials such as SiO2, Al2O3, or YSZ with several synthetic methods.20-24 However, these synthetically manufactured oxygen carriers would increase the cost of chemical looping combustion, thus increasing the cost for CO2 capture. Recently, several investigations25-28 have focused on the use of natural iron ores as potential oxygen carriers because they are much cheaper compared with the synthetic Cu- and Nibased oxygen carriers. Due to the low cost and the abundance of solid fuels, it will be an advantage to apply chemical looping combustion to (13) Abad, A.; Mattisson, T.; Lyngfelt, A.; Ryden, M. Fuel 2006, 85 (9), 1174–1185. (14) Son, S. R.; Kim, S. D. Ind. Eng. Chem. Res. 2006, 45 (8), 2689– 2696. (15) Song, Q. L.; Xiao, R.; Deng, Z. Y.; Zhang, H. Y.; Shen, L. H.; Xiao, J.; Zhang, M. Y. Energy Convers. Manage. 2008, 49 (11), 3178– 3187. (16) Jerndal, E.; Mattisson, T.; Lyngfelt, A. Chem. Eng. Res. Des. 2006, 84 (9), 795–806. (17) Corbella, B. M.; Palacios, J. M. Fuel 2007, 86 (1-2), 113–122. (18) Mattisson, T.; Lyngfelt, A. Capture of CO2 Using chemicallooping combustion. In First Biennial Meeting of the ScandinavianNordic Section of the Combustion Institute, G€ oteborg, Sweden, 2001; pp 163-168. (19) Scott, S. A.; Dennis, J. S.; Hayhurst, A. N.; Brown, T. AIChE J. 2006, 529, 3325–3328. (20) Chuang, S. Y.; Dennis, J. S.; Hayhurst, A. N.; Scott, S. A. Combust. Flame 2008, 154 (1-2), 109–121. (21) Tian, H. J.; Chaudhari, K.; Simonyi, T.; Poston, J.; Liu, T. F.; Sanders, T.; Veser, G.; Siriwardane, R. Energy Fuels 2008, 22 (6), 3744– 3755. (22) Jerndal, E.; Mattisson, T.; Lyngfelt, A. Energy Fuels 2009, 23 (1), 665–676. (23) Ad anez, J.; Dueso, C.; de Diego, L. F.; Garcı´ a-Labiano, F.; Gay an, P.; Abad, A. Energy Fuels 2009, 23 (1), 130–142. (24) Gay an, P.; Dueso, C.; Abad, A.; Adanez, J.; de Diego, L. F.; Garcı´ a-Labiano, F. Fuel 2009, 88 (6), 1016–1023. (25) Berguerand, N.; Lyngfelt, A. Fuel 2008, 87 (12), 2731–2726. (26) Leion, H.; Lyngfelt, A.; Johansson, M.; Jerndal, E.; Mattisson, T. Chem. Eng. Res. Des. 2008, 86 (9), 1017–1026. (27) Berguerand, N.; Lyngfelt, A. Int. J. Greenhouse Gas Control 2008, 2 (2), 169–179. (28) Gu, H. M.; Wu, J. H.; Hao, J. G.; Shen, L. H.; Xiao, J. Pro. C. S. E. E. 2010, 30 (17), 51–56.

(29) Cao, Y.; Pan, W. P. Energy Fuels 2006, 20 (5), 1836–1844. (30) Cao, Y.; Casenas, B.; Pan, W. P. Energy Fuels 2006, 20 (5), 1845–1854. (31) Chen, H.; Luo, Z.; Yang, H.; Ju, F.; Zhang, S. Energy Fuels 2008, 22 (2), 1136–1141. (32) Yang, H.; Chen, H.; Ju, F.; Yan, R.; Zhang, S. Energy Fuels 2007, 21 (6), 3165–3170. (33) Roberts, D. G.; Harris, D. J. Energy Fuels 2006, 20 (6), 2314– 2320. (34) Roberts, D. G.; Harris, D. J. Energy Fuels 2000, 14 (2), 483–489. (35) Xiao, R.; Song, Q. L.; Zhang, S.; Zheng, W. G.; Yang., Y. C. Energy Fuels 2010, 24, 1449–1463. (36) Adanez, J.; Gayan, P.; Celaya, J.; de Diego, L. F.; Garcı´ a-Labiano, F.; Abad, A. Ind. Eng. Chem. Res. 2006, 45 (17), 6075–6080. (37) Johansson, E.; Mattisson, T.; Lyngfelt, A.; Thunman, H. Chem. Eng. Res. Des. 2006, 84 (A9), 819–827. (38) Son, S. R.; Kim, S. D. Ind. Eng. Chem. Res. 2006, 45 (8), 2689– 2696. (39) Ryu, H. J.; Jin, G. T.; Yi, C. K. In Proceedings of the 7th International Conference on Greenhouse Gas Control Technologies, Vancouver, Canada, 2004. (40) Kolbitsch, P.; Proll, T.; Bolhar-Nordenkampf, J.; Hofbauer, H. Energy Fuels 2009, 23, 1450–1455. (41) Shen, L. H.; Wu, J. H; Xiao, J.; Song, Q. L.; Xiao, R. Energy Fuels 2009, 23, 2498–2505. (42) Shen, L. H.; Wu, J. H.; Xiao, J. Combust. Flame 2009, 156, 721–728. (43) Shen, L. H.; Wu, J. H; Gao, Z. P; Xiao, J. Combust. Flame 2010, 157, 934–942. (44) Hacker, V.; Vallant, R.; Thaler, M. Ind. Eng. Chem. Res. 2007, 46 (26), 8993–8999.

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: DOI:10.1021/ef101318b

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oxygen carrier particles. Issues of biomass ash in chemical looping combustion were discussed. This work would be significant to the development of CLC and energy utilization.

bottom of the spout-fluid bed. The tube was used to introduce the spouting flow and fuel particles into the fuel reactor. By means of a variable-speed screw feeder, the fuel particles were pneumatically conveyed to the bottom of the spout-fluid bed. The transmission medium N2 stream was controlled by a mass flow controller. The spout-fluid bed was connected with the fast fluidized bed at their bottom parts by a loop-seal, which was also a rectangular bed with a cross section of 34  30 mm2 and a height of 370 mm. The loop-seal was aerated by steam allowing only particles to move through, thus efficiently preventing the contamination of flue gas between the two reactors. Because of the small scale, the system was placed in an oven supplying heat for the experimental setup and for compensating heat loss during operation. The electric heater temperature was controlled by PID temperature controllers with Pt/Rh thermocouples. Along with the height both of the air reactor and the fuel reactor, three K-type thermocouples and pressure gauges were installed to detect the temperature and pressure, monitoring the fluidizing condition in the system. The fuel reactor, the loop-seal, and the air reactor were fluidized by N2 together with steam, steam, and air, respectively. In the fuel reactor, the steam also functioned as a gasifying medium for solid fuels. The steam producer was composed of a TBP-50A type constant flow pump and a cast aluminum heater, and the steam flow was controlled precisely by adjusting the value of water. In this work, the gases used were all provided by Tongguang Gas Co., Ltd.. Gases from both reactors were first conducted to a condenser for water removal and then collected in bags for offline analysis. Therefore, the obtained concentration was based on dry gas. The dry gases after water removal were sent to Emerson gas analyzers including a Rosemount NGA 2000 gas analyzer to measure the concentrations of CO2, CH4, CO, and O2 and a Hydros 100 analyzer to detect H2 concentration (concentration denotes volume percentage). The detailed specifications and accuracies of all measurement devices are displayed in Table 1. The uncertainties of measured parameters were obtained based on the full scale and the results are also listed in Table 1. An X-ray diffractometer (XRD, Rigaku Co.) using Cu Ka radiation (60 kV, 200 mA) was employed to analyze the oxygen carrier particles. The samples were scanned in a step-scan mode with a step of 0.02° over the angular range of 10 - 90° . The morphological features and spectroscopy features of oxygen carrier particles were characterized by field-emission scanning electron microscope (SEM) and energy diffraction X-ray (EDX), respectively. 2.2. Oxygen Carrier and Fuel. In this work, the iron ore used as oxygen carrier was from the Rio Tinto company of Australia provided by Nanjing steel manufacturing company. The iron ore particles after a thermal treatment were used as fresh oxygen carrier in this work. The calcination was performed in a muffle oven at 950 °C for 3 h, and it was considered to obtain the oxygen carrier in its most oxidized state and to improve the properties. Based on X-ray fluorescence (XRF) measurement,

2. Experimental Section 2.1. Experimental Setup. The experiments on chemical looping combustion of biomass/coal were conducted in a 1 kWth (corresponding to fuel feeding and thermal value) continuous reactor of interconnected fluidized beds as shown in Figure 2. It consisted of a high-velocity fluidized bed as an air reactor, a cyclone, a spout-fluid bed as a fuel reactor, and a loop-seal. The high-velocity fluidized bed was a circular column of 18 mm i.d. and 1600 mm height, having a perforated plate as air distributor. The spout-fluid bed was a rectangular bed, with a cross section of 50  30 mm2 and a height of 1000 mm. A 60o conical distributor connected with a tube of 10 mm i.d. was mounted at the

Figure 2. Configuration for chemical looping combustion of biomass/coal in interconnected fluidized beds.

Table 1. Precision of Measurement Devices and Uncertainty of Direct Measured Parameters parameters

devices

full scale

precision

uncertainty

air flow N2 flow steam flow control temperature test temperature pressure gas concentration CO2 CO CH4 O2 H2

mass flow controller mass flow controller constant flow pump TBP-50A Pt/Rh thermocouple K-type thermocouple CYR-2D pressure transmitter

30 L/min 15 L/min 50 mL/min 1600 °C 1100 °C 16 kPa

(2.5% (2.5% < ( 1% (1 °C (1 °C (0.5%

(0.75 L/min (0.375 L/min < ( 0.5 mL/min (1 °C (1 °C (0.08 kPa

Rosemount NGA 2000 Rosemount NGA 2000 Rosemount NGA 2000 Rosemount NGA 2000 Rosemount Hydros 100

100% 100% 100% 25% 50%