Chemical looping technologies for carbon capture and energy conversion

This Thesis is concerned with chemical looping technologies for carbon capture and energy conversion of fossil fuels. In chemical looping, solid oxygen carriers are cyclically reduced and oxidised to aid reactions such as combustion, air separation, thermochemical splitting etc. Chemical looping combustion has been proven to be a cost-competitive solution for producing low carbon electricity. The caveat to advance this technology lies in the further improvements in efficiency and development of carriers with efficient oxygen release capacity. The research in this Thesis has three major directions aimed at developing chemical looping technologies: (a) to develop realistic models of CLC powerplants with advanced steam cycles, (b) to model novel chemical looping coupled supercritical CO2 cycles for ultrahigh efficiency power generation, and (c) to synthesise and test novel synthetic ternary oxides phase of ferrites in chemical looping CO2 capture and oxygen release. The coupling of chemical looping combustion of solid fuels with advanced steam-based power cycles, viz. supercritical, ultra-supercritical and advanced ultra-supercritical Rankine cycles was investigated in Chapter 4. The energy and exergy efficiencies of the various chemical looping combustion power plant configurations are compared against the reference plants without carbon capture. This work incorporates practical considerations for reactor design. With an upper operating temperature limit of 950 °C, the maximum efficiencies achievable by integrated gasification combined cycle chemical looping combustion (IGCC–CLC) and in situ gasification chemical looping combustion power plants (iG-CLC) are 41.3% and 41.5%, respectively. Overall, iG-CLC emerges as the most efficient CLC configuration. Comparing to an integrated gasification combined cycle without carbon capture, the energy efficiency penalties for capturing CO2 from iG-CLC coupled with subcritical, supercritical, ultra-supercritical or advanced ultra-supercritical steam cycles are 5.1%, 5.0%, 5.2% or 13.0%, respectively. The biomass-fired chemical looping combustion power plants also show low energy efficiency penalties (<2.5%) compared to the reference biomass power plants without CO2 capture. The modelling results suggest that chemical looping combustion will remain an attractive carbon capture technology for solid fuel power plants, in a future when supercritical steam turbines become the norm. Chapter 5 proposes a novel power cycle coupling the Allam cycle, which is a class of oxy-fuel combustion power cycles using supercritical CO2 (s-CO2) as the thermal fluid, with chemical looping air separation (CLAS) to achieve power generation with inherent capture of CO2. Compared to other conventional CO2 capture techniques, the Allam cycle stands out owing to its high fuel-to-electricity conversion efficiency (~55-59%), the elimination of the Rankine cycle and reduced physical footprint. A key source of energy penalty of Allam cycle comes from the air separation unit (ASU), which supplies pure oxygen via an energy intensive cryogenic process. The integration of CLAS system with the Allam cycle show that the Allam-chemical looping air separation (Allam-CLAS) process can achieve 56.04% net electrical efficiency with a 100% CO2 capture rate, when a Co3O4-based oxygen carrier is used. This is about 6% efficiency points higher than the Allam cycle coupled to a cryogenic ASU. The exergetic efficiency of the Allam-CLAS system driven by the Co3O4-CoO redox cycle is 57.13%, also more favourable than a conventional Allam-ASU system (with reported exergetic efficiency of 53.4%). The newly proposed Allam-CLAS power cycle in this thesis presents a highly efficient, and cost-competitive solution to generate zero-carbon electricity from natural gas. The performance of the oxygen carriers is vital to the overall CLC plant. It is also of utmost importance to develop oxygen carriers with multifunctional characteristics and high oxygen release capacities. In Chapter 6, the performance of two ternary oxides – Ba3Fe2O6 and Ba5Fe2O8 were investigated for chemical looping oxygen release and carbon dioxide capture. The structures of both barium ferrites were characterised because the relevant structural information is lacking in the literature. Temperature swing experiments in a TGA showed that Ba3Fe2O6 exhibits excellent recyclability and satisfactory chemical looping oxygen uncoupling (CLOU) activity. Reversible oxygen release and uptake was observed over temperature swing cycles between 550 and 950 °C. In comparison, Ba5Fe2O8 was less active for CLOU. On the other hand, Ba5Fe2O8 showed excellent performance in reversibly taking up CO2, with a CO2 capture capacity of ~7 wt% consistently over multiple CO2 capture cycles at 1000 °C. The evolution of the phase compositions of the two barium ferrites during CLOU and CO2 capture cycles was studied by in situ XRD at high temperatures in reactive gas environments. In summary, the work presented in this thesis has offered advancements to chemical looping technologies in the context of designing, optimising and understanding novel chemical looping processes and exploration of novel materials for chemical looping applications.Doctor of Philosoph

Coupling chemical looping combustion of solid fuels with advanced steam cycles for CO₂ capture : a process modelling study

Chemical looping combustion is a cost-competitive solution for producing low carbon electricity. In this paper, we investigate by means of a process modelling study, the coupling of chemical looping combustion of solid fuels with advanced steam-based power cycles, viz. supercritical, ultra-supercritical and advanced ultra-supercritical Rankine cycles. The energy and exergy efficiencies of the various chemical looping combustion power plant configurations are compared against the reference plants without carbon capture. Our models incorporate practical considerations for reactor design. With an upper operating temperature limit of 950 °C, the maximum efficiencies achievable by integrated gasification combined cycle chemical looping combustion (IGCC–CLC) and in situ gasification chemical looping combustion power plants (iG-CLC) are 41.3% and 41.5%, respectively. Overall, iG-CLC emerges as the most efficient CLC configuration. Comparing to an integrated gasification combined cycle without carbon capture, the energy efficiency penalties for capturing CO2 from iG-CLC coupled with subcritical, supercritical, ultra-supercritical or advanced ultra-supercritical steam cycles are 5.1%, 5.0%, 5.2% or 13.0%, respectively. The biomass-fired chemical looping combustion power plants also show low energy efficiency penalties (<2.5%) compared to the reference biomass power plants without CO2 capture. Our modelling results suggest that chemical looping combustion will remain an attractive carbon capture technology for solid fuel power plants, in a future when supercritical steam turbines become the norm.Ministry of Education (MOE)Nanyang Technological UniversityNational Research Foundation (NRF)Submitted/Accepted versionThe authors wish to acknowledge the financial support by the Start-Up Grant from Nanyang Technological University and Academic Research Fund Tier 1 (Grant No. RG112/18) from the Singapore Ministry of Education. The work is also funded by National Research Foundation (NRF), Prime Minister’s Office, Singapore under its Campus for Research Excellence and Technological Enterprise (CREATE) programme

Coupling chemical looping air separation with the Allam cycle – a thermodynamic analysis

The Allam cycle is a class of oxy-fuel combustion power cycles using supercritical CO2 (s-CO2) as the thermal fluid to achieve power generation with inherent capture of CO2. Compared to other conventional CO2 capture techniques, the Allam cycle stands out owing to its high fuel-to-electricity conversion efficiency (55–59%), the elimination of the Rankine cycle and reduced physical footprint. A key source of energy penalty of Allam cycle comes from the air separation unit (ASU), which supplies pure oxygen via an energy intensive cryogenic process. This paper presents a thermodynamic analysis of a novel supercritical CO2-based power generation scheme, in which a natural gas fuelled Allam cycle is integrated with a chemical looping air separation (CLAS) system, which supplies oxygen to the combustor. The modelling results show that the Allam-chemical looping air separation (Allam-CLAS) process can achieve 56.04% net electrical efficiency with a 100% CO2 capture rate, when a Co3O4-based oxygen carrier is used. This is 6% higher than the Allam cycle coupled to a cryogenic ASU. The exergetic efficiency of the Allam-CLAS system driven by the Co3O4-CoO redox cycle is 57.13%, also more favourable than a conventional Allam-ASU system (with reported exergetic efficiency of 53.4%). This newly proposed Allam-CLAS power cycle presents a highly efficient, and simple solution to generate zero-carbon electricity from natural gas.Nanyang Technological UniversityThe authors wish to acknowledge financial support by Nanyang Technological University

High performance Ni catalysts prepared by freeze drying for efficient dry reforming of methane

For supported metal catalysts, the architectures of the supports are as important as the chemical compositions of the catalysts and the morphologies of the supported metal particles. In this work, we report a simple and versatile method for preparing Ni nanoparticle catalysts supported on Mg-Al mixed oxides for the dry reforming of methane. The catalysts were prepared by freeze drying of Ni-Mg-Al layered double hydroxide precursors, followed by calcination and H2 reduction. Compared to Ni/Mg-Al-O catalysts prepared by oven drying, the freeze-dried catalysts retain a unique, loosely packed platelet structure with high macroporosity, which gives rise to high dispersion, high DRM activity and high resistance against deactivation. At 800 °C, the catalyst consistently achieves equilibrium CH4 conversion of 95% over 100 h time on stream with a of 40 L-CH4 h−1 g-cat−1. The DRM activity seen in the present study supersedes other Ni-based catalysts reported in the literature.National Research Foundation (NRF)Accepted versionThis work is funded by the National Research Foundation (NRF), Prime Minister’s Office, Singapore under its Campus for Research Excellence and Technological Enterprise (CREATE) programme

Breaking the stoichiometric limit in oxygen-carrying capacity of Fe-based oxygen carriers for chemical looping combustion using the Mg-Fe-O solid solution system

The performance of oxygen carriers contributes significantly to the efficiency of chemical looping combustion (CLC), an emerging carbon capture technology. Despite their low cost, Fe2O3-based oxygen carriers suffer from sintering-induced deactivation and low oxygen-carrying capacity (OCC) during CLC operations. Here, we report the development of a sintering-resistant MgO-doped Fe2O3oxygen carrier with an optimal composition of 5MgO·MgFe2O4, which exhibits superior cyclic stability and an OCC of 0.45 mol O/mol Fe (2.25 mmol O/gsolid), exceeding the widely accepted OCC limit of 0.167 mol O/mol Fe (2.08 mmol O/gsolid) of unmodified commercial Fe2O3. This result distinguishes this report from all past studies, in which efforts to enhance the cyclic stability of Fe-based oxygen carriers would always result in dilution of the OCC. The capacity enhancement by MgO is attributed to the unique mixtures of MgxFe1-xO (halite) and Mg1-yFe2+yO4(spinel) solid solutions, which effectively reduce the exergonicity for the reduction from Fe3+to Fe2+, while preventing any irreversible structural transformations during the redox process. This hypothesis-driven oxygen carrier design approach provides a new avenue for tailoring the lattice oxygen activities of oxygen carriers for chemical looping applications.Ministry of Education (MOE)National Research Foundation (NRF)Submitted/Accepted versionThe authors acknowledge financial support by the Ministry of Education Singapore’s Academic Research Fund Tier 1 (RT03/19 and RG112/18) and the National Research Foundation (NRF), Prime Minister’s Office, Singapore, under its Campus for Research Excellence and Technological Enterprise (CREATE) programme. Y.D. is grateful for financial support from the National Natural Science Foundation of China (21802070)