Abstract
Oxygen carriers play an important role in chemical looping processes to transport oxygen for fuel conversion. In this study, the defluidization phenomena of oxygen carriers were examined under highly reducing conditions in a fluidized-bed batch reactor. This is highly relevant to chemical-looping gasification, where oxygen carriers could be reduced to a significant extent compared to that in chemical-looping combustion. Only a few studies have reported the physical phenomena in a fluidized bed system under highly reducing conditions. Three iron- and two manganese-based oxygen carriers were investigated at 900 °C at several degrees of reduction in this study. Some oxygen carriers that have been exposed to several hours of operation in a 300 W chemical-looping reactor unit were also included in this study to provide a comparison to the fresh-calcined materials. Defluidization of particle beds occurred with the iron-based oxygen carriers, except for LD slag, which has a low content of iron. The defluidization was caused by the formation of elemental iron on the oxygen carriers' surface at high degree of reduction. All defluidizations occurred at a mass-based conversion (∆ω) between 3.2–5.0%. The manganese-based materials were found to be less prone to defluidization.
Highlights
In the last decades, carbon capture and storage (CCS) has emerged as one of the promising concepts to overcome climate change that leads to global warming and many other environmental issues
As the oxygen carriers were reduced, the mass-based conversions and gas yields changed over the reduction period, i.e. how long the particles were reduced in a cycle
The CO2-based gas yield declined as the oxygen carriers were reduced further
Summary
Carbon capture and storage (CCS) has emerged as one of the promising concepts to overcome climate change that leads to global warming and many other environmental issues. Carbon dioxide (CO2), which is considered as the main greenhouse gases that contribute to global warming, has been increasing rapidly in the earth's atmosphere due to various recent urban human activities [1]. One of the prominent ways to establish CCS is with chemical-looping technologies, in which direct contact between air and fuel is avoided, making the product gases nearly free from nitrogen [2]. Oxygen carriers circulating between reactors are used to avoid direct contact between air and fuel. The oxygen carriers are oxidized in the air reactor (AR) based on the reaction of
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