Chemical-looping combustion : investigation of Cu-based oxygen carriers
Author(s)
Okpoko, Egwono Kelvin
Type
Thesis
Abstract
Chemical looping combustion (CLC) is a process developed to efficiently combust hydrocarbon fuels and at the same time capture CO2 without an additional energy intensive separation unit. CLC is a two-stage process which involves supplying oxygen to the fuel for combustion with an oxygen carrier and the same oxygen carrier is regenerated back to its original state with air in a subsequent reactor, prior to recycling. CLC is potentially economically and financially viable, and depending on the design, a variety of products can be obtained. Hydrogen and syngas are two very important products useful for power generation and intermediates in the chemical industry. This project focuses on studying the CLC process, with issues related to the development of suitable oxygen carriers for complete fuel combustion as well as assessing their performances under different process conditions.
A 500 W laboratory-scale fixed-bed reactor was designed and built, and was used to investigate the reaction kinetics and performance of suitable oxygen carriers. Particles composed of 40, 60 and 80 wt.% CuO supported on Al2O3 were prepared by co-precipitation methods with Na2CO3 and NH4OH. Ideal pH conditions ranged between 8.5 – 9.7 and 4.4 – 6.0 for precipitation with Na2CO3 and NH4OH respectively. X-ray diffraction analyses (XRD) revealed the presence of CuO and CuAl2O4, and both species were reducible as observed from the CO/CH4-TPR analysis. The reduction and oxidation kinetics were investigated in a Thermal Gravimetric Analyser (TGA) for particle sizes < 355 μm. Reaction orders with CO, CH4 and O2 were ~ 2, 1.5 and 0.7 respectively and activation energies were in the order O2 > CO > CH4 at T ≤ 700 oC. The reaction mechanism for the reduction and oxidation was investigated, as well as the rate-limiting steps. The reactivities of the Cu-based oxygen carriers over 20 redox cycles were investigated in a TGA and also in a fixed-bed reactor at temperatures ≤ 800 oC. SEM analysis revealed no sign of sintering with the carriers prepared with Na2CO3, but sintering effects were observed with carriers prepared with NH4OH. Overall, the conducted experiments suggest that Cu-based oxygen carriers prepared by co-precipitation are potentially suitable for a CLC system.
A 500 W laboratory-scale fixed-bed reactor was designed and built, and was used to investigate the reaction kinetics and performance of suitable oxygen carriers. Particles composed of 40, 60 and 80 wt.% CuO supported on Al2O3 were prepared by co-precipitation methods with Na2CO3 and NH4OH. Ideal pH conditions ranged between 8.5 – 9.7 and 4.4 – 6.0 for precipitation with Na2CO3 and NH4OH respectively. X-ray diffraction analyses (XRD) revealed the presence of CuO and CuAl2O4, and both species were reducible as observed from the CO/CH4-TPR analysis. The reduction and oxidation kinetics were investigated in a Thermal Gravimetric Analyser (TGA) for particle sizes < 355 μm. Reaction orders with CO, CH4 and O2 were ~ 2, 1.5 and 0.7 respectively and activation energies were in the order O2 > CO > CH4 at T ≤ 700 oC. The reaction mechanism for the reduction and oxidation was investigated, as well as the rate-limiting steps. The reactivities of the Cu-based oxygen carriers over 20 redox cycles were investigated in a TGA and also in a fixed-bed reactor at temperatures ≤ 800 oC. SEM analysis revealed no sign of sintering with the carriers prepared with Na2CO3, but sintering effects were observed with carriers prepared with NH4OH. Overall, the conducted experiments suggest that Cu-based oxygen carriers prepared by co-precipitation are potentially suitable for a CLC system.
Date Issued
2012-10
Date Awarded
2012-11
Copyright Statement
Attribution NoDerivatives 4.0 International Licence (CC BY-ND)
License URL
Advisor
Fennell, Paul
Hellgardt, Klaus
Publisher Department
Chemical Engineering
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)