Chemical-looping combustion (CLC) is a combustion technology for clean and efficient utilization of fossil
fuels for energy production. This process which produces sequestration ready CO2 systems is a promising
technology to be utilized with coal gasification systems. In the present work, chemical-looping combustion
has been studied with an oxygen carrier, NiO/bentonite (60 wt % NiO) for the gasification systems utilizing
simulated synthesis gas. Global reaction rates of reduction and oxidation as a function of conversion were
calculated for oxidation−reduction cycles utilizing the thermogravimetric analysis (TGA) data on multicycle
tests conducted with NiO/bentonite at atmospheric pressure between 700 and 900 °C. The rate of reduction
increased slightly with an increase in temperature, while the rate of oxidation decreased at 900 °C. The effect
of particle size of the oxygen carrier on CLC was studied for the particle size between 20 and 200 mesh. The
rates of reactions depended on the particle size of the oxygen carrier. The smaller the particle size, the higher
the reaction rates. The multicycle CLC tests conducted in a high-pressure flow reactor showed stable reactivity
for the production of CO2 from fuel gas at 800 and 900 °C and full consumption of hydrogen during the
reaction. The data from a one cycle test on the effect of the pressure on the performance with NiO/bentonite
utilizing the tapered element oscillating microbalance (TEOM) showed a positive effect of the pressure on the
global rates of reduction−oxidation reactions at higher fractional conversions. The X-ray diffraction (XRD)
analysis confirmed the presence of the NiO phase in NiO/bentonite with the oxidized sample in the high-pressure reactor and Ni phase with the reduced sample. The presence of a small amount of NiO in the reduced
sample detected by X-ray photoelectron spectroscopy (XPS) may be due to its exposure to air during sample
transfer from the reactor to XPS. Scanning electron microscopy (SEM) analysis showed no significant changes
in morphology of NiO/bentonite reacted in the temperature range 700−800 °C in an atmospheric TGA for 10
oxidation−reduction cycles, but some loss of surface area and porosity was observed at 900 °C. This effect
was found to be greater with increase in the particle size of the oxygen carrier.
CuO/bentonite and CuO-BHA nanocomposites were studied as oxygen carriers in chemical-looping combustion (CLC) of simulated synthesis gas. Global reaction rates of reduction and oxidation, as the function of reaction conversion, were calculated from 10-cycle oxidation/reduction tests utilizing thermogravimetric analysis at atmospheric pressure between 700 and 900 °C. It was found that the reduction reactions are always faster than oxidation reactions; reaction temperature and particle size do not significantly affect the reaction performance of CuO/bentonite. Multicycle CLC tests conducted in a high-pressure flow reactor showed stable reactivity for production of CO 2 from fuel gas at 800 and 900 °C and full consumption of hydrogen during the reaction. Results of the tapered element oscillating microbalance showed a negative effect of pressure on the global rates of reduction-oxidation reactions at higher fractional conversions. X-ray diffraction patterns confirmed the presence of CuO in the bulk phase of the oxidized sample. Electron microanalysis showed significant morphology changes of reacted CuO/bentonite samples after the 10 oxidation-reduction cycles above 700 °C in an atmospheric thermogravimetric analyzer. The nanostructured CuO-BHA carrier also showed excellent stability and, in comparison to the CuO/bentonite system, slightly accelerated redox kinetics albeit at the expense of significantly increased complexity of manufacturing. Overall, both types of CuO carriers exhibited excellent reaction performance and thermal stability for the CLC process at 700-900 °C.
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