(60c) Effects of Fluidizing Gas on Copper-Manganese Mixed Oxide’s Reactivity for Chemical Looping Combustion of CH4

Chemical looping combustion (CLC) is a promising carbon capture technology because CO₂ is inherently separated without the need of gas separation. In CLC units, instead of direct contact between the fuel and air, like conventional combustion, oxygen is supplied to the fuel by means of a metal oxide, called the oxygen carrier. A variant of CLC is chemical looping with oxygen uncoupling (CLOU) that is most suitable for solid fuel combustion. In CLOU, the fuel reacts with gas-phase oxygen released by the decomposition of the metal oxide at suitable temperatures and oxygen partial pressures, unlike the CLC where the fuel reacts with the solid metal oxide to access the lattice oxygen. Since solid fuel oxidation with gas phase oxygen is kinetically favorable compared to oxidation on a solid metal oxide, CLOU is considered more effective than CLC for coal combustion. CLC/CLOU processes are usually operated in two interconnected fluidized bed reactors with either steam or CO₂ or a mixture of both being used as the fluidizing gas. However, research focusing on the effect of fluidizing gas on oxygen carriers’ reactivity is scarce.

The purpose of this study is to investigate the effect of CO₂ fluidizing gas on copper-manganese mixed oxide's reactivity with CH₄. As an oxygen carrier the oxide has received much attention for both CLC and CLOU process because of its significant oxygen release capacity and high fuel conversion efficiency. The mixed-oxide (Cu34Mn66) is synthesized with 34 wt.% CuO and 66 wt.% Mn₃O₄ by incipient wetness impregnation and tested in a batch fluidized bed reactor. Length of reducing period of Cu34Mn66 is found to be an important parameter while determining CO₂ fluidizing effects. For short reducing period, as occurs in CLOU, no significant outcome of CO₂ fluidization is found. However, if the oxygen carrier is exposed for a longer reduction period (characteristic of CLC), the Cu and MnO reduced phases catalyze CH₄ dry reforming and reverse water gas shift reactions (RWGS). The extent of the RWGS reaction depends on the feed CO₂/CH₄ ratio. At CO₂/CH₄ ratio greater than 1, the RWGS probability is high and CO/H₂ ratio becomes on average greater than 1. When the feed ratio is 1 or less, the ratio of CO/H₂ becomes roughly equal to 1. These side reactions as a result of CO₂ fluidization should be avoided to achieve complete combustion of CH₄ in CLC.