Ind. Eng. Chem. Res. 2008, 47, 7147–7153
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Experimental Study of O2-CO2 Production for the Oxyfuel Combustion Using a Co-Based Oxygen Carrier Zhen-shan Li, Teng Zhang, and Ning-sheng Cai* Key Laboratory for Thermal Science and Power Engineering of MOE, Tsinghua UniVersity, Beijing 100084, China
Production of O2-CO2 mixed gases for the oxyfuel combustion using a Co-based oxygen carrier packed in a fixed bed reactor was investigated. The reaction kinetics of CoO with O2 and the decomposition kinetics of Co3O4 in CO2 atmosphere at different temperatures were studied using thermogravimetric analysis (TGA). Both desorption and sorption processes exhibit a high reaction rate. Multiple sorption and desorption cycles indicated that Co-based oxygen carrier has high reactivity and cyclic stability. The results of X-ray diffraction indicated that Co-based oxygen carrier does not react with CO2 during the desorption stage, and this is especially important for oxyfuel combustion. The high temperature sorption process for production of O2-CO2 gas mixtures in a fixed bed reactor packed with Co-based oxygen carrier particles through air separation with carbon dioxide as the purge gas is investigated. Oxygen is absorbed, and heat is stored by the Co-based oxygen carrier particles with air being fed. An O2-CO2 stream can be obtained when the fixed bed is regenerated with carbon dioxide as the desorption gas. O2 fraction in the O2-CO2 gas mixtures can be controlled by adjusting the flow rate of CO2 regeneration gas. This Co-based oxygen carrier offers potential for further study in the O2-CO2 production for the oxyfuel coal combustion process. 1. Introduction It is now well accepted that the release of CO2 from fossil fuel combustion contributes to the enhanced greenhouse effect with possible disastrous effect on climate change. The emissions of CO2 into the atmosphere have been reported to account for half of the greenhouse effect that causes global warming.1 An approach to reduce CO2 release from large point sources is CO2 (or carbon) capture and storage (CCS). The estimated costs for CO2 transportation (US$1-3 per ton per 100 km)2 and sequestration (US$4-8 per ton of CO2)3 are small compared to the cost for CO2 capture, estimated at US$35-55 per ton of CO2 capture.4 Therefore, reducing the cost of CO2 capture is absolutely necessary to make CCS more economically attractive. The high cost of CO2 capture stems from the considerable amount of energy required in the separation process. 4 There are three main options for capturing CO2 from flue gases: (1) postcombustion capture (e.g., chemical absorption where CO2 is separated essentially from nitrogen in the flue gas stream), (2) precombustion capture (typically, coal gasification with CO2 capture followed by combusting the hydrogen produced in combined cycles), and (3) oxyfuel combustion where nitrogen is removed prior to combustion. Oxyfuel combustion is recently proposed technology that is also named as the O2/CO2 combustion.5-10 In the process of oxyfuel combustion, high purity of O2 is mixed with recycled flue gas, and then the mixed O2-CO2 gas is fed to furnace for combustion. The remaining flue gas, consisting mainly of CO2, water vapor, and small quantities of NOx and SOx, is then ready for flue gas treatment. Oxyfuel combustion has the following advantages:8 (1) concentrations of CO2 up to 95% in the dry flue gas, (2) reduced flue gas volume, lower gas energy loss, and higher plant efficiency, and (3) reduced NOx and SOx volume. Although oxyfuel combustion has the above benefits, the requirement for pure oxygen is its major challenge and limitation. Significant reduction in the cost of oxygen production is a key requirement to making the oxyfuel combustion power * To whom correspondence should be addressed. Tel.: 86-1062789955. Fax: 86-10-62770209. E-mail:
[email protected].
plant a viable future option when carbon dioxide capture becomes a necessity.11 A novel high-temperature sorption-based technology referred to as ceramic autothermal recovery (CAR) for oxygen production and supply to oxyfuel boilers with flue gas recycle was developed by BOC,12 with first invention reported by Lin et al.13 The CAR process is based on sorption and storage of oxygen in a fixed bed containing ionic and electronic conducting materials operated at high temperature and increased pressure. The stored oxygen is then released by pressure reduction using a sweeping gas, such as recycled flue gas. An important feature of the CAR process is that it can be tailored to produce lowpressure oxygen at the concentration required for oxyfuel combustion by using recycled cleaned flue gas as a sweep gas. Fundamental studies on perovskite-type oxides consist of selection and syntheses of materials and determination of their O2 sorption equilibrium, structural identity and stability, thermal properties, as well as O2 sorption kinetics.14-17 In addition, studies of overall process performance were investigated. 12 Relative slow sorption and desorption rates in this process may be a major drawback of this type of sorbents. This issue may cause challenges with regard to achieving a high O2 product purity in industrial air separation, and also ensuring a sufficiently high efficiency of sorbent regeneration.18 Another major challenge of the CAR process using perovskite material is that perovskite material undergoes a reversible reaction can be expressed by the following formulas 18,19 La0.1Sr0.9Co0.5Fe0.5O2.6 + 0.9CO2 S 0.9SrCO3 + 0.05La2O3 + 0.5CoO + 0.25Fe2O3 + 0.15O2(1) Sr0.5Ca0.5Co0.5Fe0.5O2.47+CO2 S 0.5SrCO3 + 0.5CaO3 + 0.5CoO + 0.25Fe2O3 + 0.11O2(2) This reversible process makes perovskite material react with CO2 to form carbonate during the CO2 regeneration step. In the next step, oxygen sorption, the carbonate formed in the regeneration step decompose and CO2 is released into air, diluted by nitrogen. Zeng et al.20 reported their experimental result using perovskite material as oxygen sorbent. In their experiment,
10.1021/ie071527o CCC: $40.75 2008 American Chemical Society Published on Web 08/27/2008
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La0.2Sr0.8Co0.6Fe0.4O3-δ perovskite extrudates were packed in a tubular reactor at 825 °C. An air stream at 7.6 slpm and a CO2 stream at 4.7 slpm were alternately fed into reactor for every 30 s in a counter-current fashion. The average product composition during the CO2-regeneration step was 27.8% O2, 67.1% CO2, and 7.4% N2, and the gas stream during air sorption step contained 2.3% O2, 12.5% CO2, and 83.5% N2.20 This results in a failure to capture the CO2 from flue gas. Thus an oxygen carrier with a high reactivity, reaction rate, and stability and that does not react with CO2 is critical for the CAR oxygen production process. In this study, the reversible gas-solid reaction 3CoO + 0.5O2 S Co3O4 was used to produce O2-CO2 gas mixtures for oxyfuel combustion. A Co-based oxygen carrier has been tested. Results indicate that it has high reactivity and stability under the conditions of the multiple sorption/desorption cycles. More importantly, the Co-based oxygen carrier did not react with CO2 from the recycled flue gas during the desorption step. On the basis of the results, the Co-based oxygen carrier particle, Co3O4/ Al2CoO4, was synthesized with the integration of a new binder, Al2CoO4. The O2 sorption/desorption characteristic and evolution of the O2 fraction with reaction times at different conditions based on a fixed-bed packed with Co-based oxygen carrier particles were investigated. 2. Experimental Description 2.1. Sorption and Desorption of Co-Based Oxygen Carrier during Thermogravimetric Analysis (TGA). A Dupont 951 TGA (TA Instrument 1200) was used to study the oxygen sorption process of the Co-based oxygen carrier. The TGA consists of a quartz tube placed in an oven that can be operated at temperatures up to 1200 °C. A computer continuously records the reaction temperature and sample weight. The reacting gas contains O2, N2, and CO2, and their proportion can be adjusted with mass flow controllers. A small amount of the Co-based oxygen carrier (about
