Chemical Looping Combustion With Oxygen Uncoupling (CLOU) Using Novel Geopolymer Oxygen Carriers For Fluidized Bed

One of the best alternatives to reduce the economic cost of CO2 capture is represented by the chemical looping Combustion, a process which enables the inherent separation of carbon dioxide simply by water condensation, as the stream exiting the fuel reactor is only composed of CO2 and H2O. In Chemical Looping Combustion, with or without oxygen uncoupling (CLC and CLOU, respectively), the fuel is oxidized by a metal oxide, called the oxygen carrier (OC), avoiding a direct contact between the fuel and air. In CLOU processes, the metal oxides reduce by spontaneously releasing O2, which is directly used in the combustion of fuel, allowing also for the use of solid fuels. Both these technologies are generally carried out using two reactors: an air reactor, where the metal of the OC is oxidized by ambient air, and a fuel reactor where it reduces and the combustion of fuels occurs [1]. In both cases, the selection of suitable OCs is a key factor for the development of such technologies. Indeed, the limited stability at high temperatures and under continuous cycling conditions of most of the current OCs might represent a serious limitation, as well as the heavy environmental and economic impact of their production technologies [2]. In this work, synthetic OCs based on geopolymer composites for CLOU processes have been proposed as alternative to traditional ones, exploiting geopolymers' excellent thermal and mechanical stability, together with their easy, sustainable and low cost production process [3]. Suitable oxides, such as Cu and Mn oxides, also in combination with each other, have been used as the active phase and embedded within a geopolymer matrix. The performances of the produced OCS have been evaluated in terms of red-ox activity, oxygen transport and oxygen uncoupling ability. Laboratory experiments were carried out in a thermo-balance at 700 °C and 900°C, in order to measure the weight change of the sample in alternating inert and oxidizing atmospheres, then in a lab-scale reactor, under more representative conditions. Attrition tests were also carried out to evaluate the abrasion resistance of the materials and their suitability for the use in fluidized bed conditions. The conducted tests pointed out the good performance of the CuO-based OCs in terms of oxygen carrying capacity, exhibiting a better behavior compared to carriers based on mixed Cu-Mn oxides. However, both systems were able to release oxygen in inert atmosphere, demonstrating potentialities for CLOU applications, and were found to be stable to high temperatures and to repeated cycling, since no relevant modifications to their macro- and micro-structure have been detected.