Solar Energy Driven Chemical Looping Air Separation

Abstract

Chemical looping is emerging as a feasible alternative to carry out reduction and oxidation processes under different process conditions. This technology proves especially useful when reduction and oxidation processes proceed with different time constants. With the possibility of incorporation of solar energy to the endothermic end of the process, chemical looping technology has recently become more popular. Chemical looping air separation (CLAS) is an alternative to cryogenic separation, by utilizing solar thermal energy. In this process a reducible metal oxide is heated to a temperature such that the metal releases its lattice oxygen. In the second step, the metal oxide is exposed to air and oxygen is captured by the oxide while pure nitrogen is released at the outlet and the loop is closed. Mn2O3 is selected as the oxygen carrier to perform chemical looping cycles. Oxygen mobility of the metal oxide and reversibility through redox cycles are tested with thermogravimetric analysis (TGA). The redox cycles are designed such that the air oxidizes, and the steam reduces the material. The reduction behavior of manganese (III) oxide under inert atmosphere is tested in TGA. It is proved that steam acts as an inert gas under the reaction conditions. The use of steam at the reduction stage results in a more convenient separation of the sweep gas (steam) from the oxygen released from the oxide. It is demonstrated that a redox cycle between Mn2O3 and Mn3O4 can be performed isothermally. The capability of the system to be coupled with solar energy makes it more alluring for environmentally friendly option seekers. The use of solar irradiation is tested with parabolic mirrors to observe the power output. Overall, CLAS process works on milder conditions which is crucial in reducing the energy and equipment costs, and its advantages regarding energy efficiency increase even more when solar energy is incorporated into the system.