Design Theory of a CLC Air Reactor with Oxygen Carrier Recirculation and Its Application to a 3 MWth Pilot

Abstract

In chemical looping combustion (CLC), oxidation of a reduced oxygen carrier with air occurs in the air reactor and some of the oxygen carrier separated by a cyclone is returned to the air reactor for reoxidation. However, previously developed reactor models did not consider the effect of oxygen carrier recirculation on the oxidation kinetics and could not reasonably predict the oxidation behavior that occurs in a CLC air reactor. This work proposes a design theory of an air reactor with oxygen carrier recirculation to evaluate the effect of recirculation on the oxidation degree of the oxygen carrier. The developed reactor model includes submodels of the reaction kinetics, hydrodynamics, oxygen conversion, and mass and heat balance. Three oxide materials, including ilmenite, manganese ore, and perovskite (CaMn0.375Ti0.5Fe0.125O3-δ) with different oxidation kinetics, are selected as the oxygen carriers for investigating the recirculation effect on kinetic behaviors. The results indicate that only perovskite achieves its full oxidation, while manganese ore and ilmenite cannot achieve the full oxidation even with recirculation. Increasing the recirculation ratio is found to reduce the conversion of the oxygen carrier at the air reactor inlet (Xin,OC) and increase the conversion difference of the oxygen carrier (ΔXOC), which is not beneficial to the oxidation behaviors in the air reactor. The value of the recirculation ratio should be zero, i.e., all of the oxygen carriers from the air reactor should be transported to the fuel reactor. For the oxygen carriers with two-stage kinetic behavior, the initial fast stage should be fully used, while the slower second stage is not suitable for full oxidation because of very slow kinetics. This finding provides a reference for the selection of a suitable oxygen carrier and the design and optimization of the air reactor in chemical looping combustion.