Research On Combustion Synthesis Of Fe₂O₃-based Oxygen Carrier And Its Performance Used In Chemical Looping Combustion

Fossil fuels, especially coal, are used as fuel to generate electricity and meet energy demand, but they also emits a large amount of CO₂ and causes the great increase of CO₂ concentration in the atmosphere along with ever-increasing effect from the CO₂-related greenhouse effect. Therefore, it is of great necessity to decrease CO₂ emission into the atmosphere from fossil combustion. Compared to various kinds of present existing technologies to combat CO₂ emission from fossil combustion, chemical looping combustion receives great attraction and intensive research for its three distinguished advantages, such as high combustion efficiency by decreasing the combustion irreversibility, eradication of NOx formation as well as CO₂ inherent separation without extra instrument and energy consumption. Obviously, it would be greatly meaningful and advantageous if fossil fuels, especially coal were adopted as the fuel for chemical looping combustion. Under the project of the related research cooperation between State Key Laboratory of Coal Combustion in Huazhong University of Science and Technology and Institute of Environmental Science and Engineering in Nanyang Technological University (Singapore), a detailed research on chemical looping combustion with coal as the fuel and Fe₂O₃-based oxides as the oxygen carriers are performed. Meanwhile, the chemical utilization of CO₂ emitted from the system of chemical looping combustion were also innovatively explored. Based on all the related researches, the meaningful results are summarized as follows:The reaction of active metal oxides (the main component for oxygen carriers) with syngas derived from coal gasification are simulated by means of thermodynamics on the principle of the minimization of Gibbs energy, with focus on the various influencing factors on the carbon deposition and the formation of solid sulfur compounds. The simulation results indicated that under the fixed condition, the increase of pressure would lead to more carbon and solid sulfur compounds deposited on the oxygen carriers. On the contrast, the increase of temperature can inhibit carbon deposition and produce more SO₂ in gas state. In terms of H₂O and CO₂ function in the syngas, similar to the function of temperature, the increase of the two gas fractions will decease the formation of solid carbon and produce more gas SO₂, though the inhibition function of carbon deposition and the oxidization capacity of the two fractions in the syngas are different. The investigation of the effect on the temperature of fuel reactor (FR) from the reaction between different oxygen carriers (including various active metal oxides and inert supports) and syngas using mass and energy balances demonstrates, NiO and CoO based oxygen carriers lead to the decrease of FR temperature, while the Fe₂O₃ and Mn₃O₄ based oxygen carriers are beneficial to maintain the FR temperature, but due to the exothermal specialty, the increase of CuO content in CuO based oxygen carriers will cause dramatical increase of FR temperature. With regard to the supports' effect on FR temperature, the optimal selection option in descending order should be: MgAl₂O₄ > Al₂O₃> SiO₂ > TiO₂ >ZrO₂.Oxygen carrier preparation is the basis of the investigation and application of chemical looping combustion. The novel sol-gel combustion synthesis method (SGCS) for the preparation of Fe₂O₃/Al₂O₃ oxygen carrier is designed and optimized. Different mass ratios of Fe₂O₃ to Al₂O₃ oxygen carriers are produced and experimentally researched, indicating the mass ratio of 8 to 2 for Fe₂O₃/Al₂O₃ is the best whether for the reactivity or the resistance to sintering and carbon deposition. Furthermore, the CuO and MgO stabilized Fe₂O₃/Al₂O₃ oxygen carrier were synthesized and indicated that the addition of CuO was conductive to the improvement of the reactivity and resistance to carbon deposition, while MgO-stabilized oxygen carrier is better in its resistance to sintering and carbon deposition.The reaction of variety of coals with Fe₂O₃ based oxygen carriers performed on TGA indicates that the product of the reduction of Fe₂O₃ with coals is oxides with valence no lower than Fe₃O₄.Oxygen carriers actually react with the products from the pyrolysis and gasification of coal instead of coal in itself when coal is directly used as the fuel. Furthermore, CuO-Fe₂O₃ mixed oxygen carriers really demonstrate the synergistic effect on the oxidization of coal along with less heat demand for the CuO exothermal specialty.Finally, preliminary experimental and thermodynamic research of reduction between the hydrogen-activated Fe₂O₃ and CuFe₂O₃ with CO₂ emitted from the chemical looping combustion to produce CO are performed. The feasibility of producing CO from such process is validated to reach the decrease the goal of emission of CO₂.