Novel co-precipitated oxygen carriers for chemical looping combustion of gaseous fuel

Ekpe, Ngozi Chinwe (2017) Novel co-precipitated oxygen carriers for chemical looping combustion of gaseous fuel. PhD thesis, University of Nottingham.

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Abstract

Carbon Capture and Storage (CCS) is one option to meet the increasing energy demand as well as reduce net CO2 emissions to the atmosphere. Chemical Looping Combustion (CLC) is a promising CCS technology proposed to meet the challenge of mitigating the carbon dioxide (CO2) emissions. CLC process can be based on interconnected fluidized beds, consisting of air reactor, fuel reactor and oxygen carrier (OC) which undergoes redox reactions while it circulates between the reactors. The main products are CO2 and water, thus eliminating the need of an additional energy intensive CO2 separation.

The feasibility of CLC depends on the oxygen carrier's (OC) ability to transfer O2 from air reactor to fuel reactor and have sufficient oxygen capacity, high reactivity and withstand a high number of redox cycles without significant loss in performance. OCs based on transition metal oxides of Cu, Co, Fe, Mn and Ni has been explored. Nevertheless, research is focused on improving the OCs performance with the aim to overcome their various practical limitations. Mechanical mixing and impregnation which fails to provide a high degree of dispersion and high metal loading respectively are commonly used for OC synthesis. Very few works have been reported for Mn-oxide and co-precipitated oxygen carriers. The few studies on co-precipitated OCs mainly use strong base as precipitants. One drawback to this is the repetitive washing of precipitate to remove excess alkali ions and controlled loading of active components cannot be easily obtained. In this study, weak base instead of strong base was used in the synthesis of OCs. This is the first time this controlled approach has been applied to prepare oxygen carrier in CLC for manganese and iron.

This thesis is a novel research on development and detailed investigation of co-precipitated Mn-oxide and Fe-oxide OCs with ZrO2 and combined ZrO2–CeO2 support. The reaction kinetics, stability and oxygen transfer capacity (OTC) of the OCs were studied by TGA up to 1173 K in H2, CO and CH4. Characterization of physical and chemical structures of particles was obtained by SEM-EDX, XRD, BET and pycnometer.

The result reveals that regardless of the composition of the co-precipitated oxygen carriers, there was no interaction of the metal oxides with the support material which could have altered the thermodynamics of the redox system. Furthermore, co-precipitated Mn/Zr and Fe/Zr OCs were more reactive than their counterpart prepared by impregnation and mechanical mixing. Also, changes in reactivity and OTC suggest that the synergistic effect varies with ratios of the single oxides in the bimetallic OCs. Co-precipitated Mn-rich oxygen carriers were more reactive than Fe-rich OCs. Interestingly, OCs with zirconia-ceria support exhibited activation tendency behaviour. Moreover, the use of combined zirconia-ceria for bimetallic Mn-Fe oxide reversed the characteristic progressive decrease in the performance of the OC with equimolar composition. For co-precipitated Mn-Fe Oxide oxygen carriers, zirconia content of 44 wt. % is sufficient to maintain the mechanical integrity of the particles during redox reactions compared to a zirconia content of 20 wt. %. This research has resulted in the development of highly reactive and stable oxygen carriers, which are promising for CLC. Mn/Zr OC reached full conversion in less than 48 secs and bimetallic Mn-Fe OCs reached 30% conversion in less than 43 secs in CH4 and maintained stability in a thirty multicycle test.

The redox reaction kinetics of the most reactive oxygen carrier using CH4, H2, CO and air was investigated at isothermal conditions (973-1173 K) to determine the kinetic parameters. Models of the reduction and oxidation reactions were selected by using a model fitting method. The nucleation model was the most statistically significant and suitable model for describing the reduction and oxidation behaviour of the oxygen carrier. The values of activation energy obtained for the reduction reaction in CH4, H2 and CO were 142.8 KJ/mol, 32.95 KJ/mol and 26.37 KJ/mol respectively. Whereas, for the oxidation reaction, the activation energy obtained using air was 28.83 KJ/mol.

In the application of co-precipitation technique for the synthesis of multicomponent materials, a heterogeneous product could be obtained from using improper preparation conditions. Results from this research have demonstrated that, the application of the well-designed co-precipitation procedure effectively produced composite materials (up to four co-precipitated mixed metal oxides) with controlled compositions and homogeneous dispersion. Furthermore, this study provides insight into the fundamental behaviours of co-precipitated manganese and iron based oxygen carriers to aid the design and optimization of future materials development.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: Snape, Colin E.
Sun, Cheng-Gong
Liu, Hao
Keywords: Chemical Looping Combustion, Co-precipitation, Oxygen Carriers, Manganese oxide, Iron oxide, Zirconia, Ceria, Kinetics, Carbon capture.
Subjects: T Technology > TD Environmental technology. Sanitary engineering
Faculties/Schools: UK Campuses > Faculty of Engineering
Item ID: 39557
Depositing User: Ekpe, Ngozi
Date Deposited: 13 Jul 2017 04:40
Last Modified: 12 Oct 2017 14:28
URI: http://eprints.nottingham.ac.uk/id/eprint/39557

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