Bulk monolithic Ce–Zr–Fe–O/Al2O3 oxygen carriers for a fixed bed scheme of the chemical looping combustion: Reactivity of oxygen carrier

Abstract

The size and geometry of oxygen carriers are one of the key factors to determine the efficiency of a large-scale chemical looping combustion (CLC) system in fixed bed reactors, because they strongly affect the dynamic conditions of the gas–solid reactions, such as the intra particle mass transfer limitation for reactants, the pressure drop and the flow distribution. In the present work, we describe for the first time the utilization of bulk monolithic oxygen carriers for chemical looping combustion of methane in a fixed bed reactor. The comparison on the structure and reactivity of the monolithic Ce–Zr–Fe–O/Al2O3 oxygen carrier with the powder one is investigated in detail. The successive CH4-reduction/air-oxidation redox testing of the monolithic oxygen carrier is also performed. It is found that the Ce–Zr–Fe–O/Al2O3 oxygen carriers own high activity for methane complete oxidation due to the strong active component (i.e., Ce–Zr–Fe–O) to support (i.e., Al2O3) interaction. The powder and monolithic oxygen carriers show similar reduction behaviors either in hydrogen or in methane atmosphere. This indicates that the utilization of organic binders and additives in the fabrication procedures of the monolith has no significant effect on the reducibility of the oxygen carrier. The monolithic oxygen carriers used in the chemical looping combustion of methane in its natural form (4.5cm long, 6.0cm in diameter, square cell size of 2.0mm, and wall thickness of 0.9mm) represent high activity in a high gas hourly space velocity (GHSV, 6000h−1). This can be attributed to the special geometric structure and layered microstructure. The activity of the monolithic oxygen carrier is also very stable in the successive redox process. On the other hand, the requirement on the mechanical strength of the monolithic oxygen carrier is much lower than that toward the pellets, which allows the oxygen carrier to have relatively high specific surface area in a large-scale CLC system. The monolith reveals very high structure stability in both macro and micro aspect during the chemical looping process.