Thesis (M.S.)--University of Hawaii at Manoa, 2007. / Includes bibliographical references (leaves 45-49). / v, 49 leaves, bound ill. 29 cm

It is widely accepted that climate change is a very serious environmental concern. Levels of carbon dioxide (CO2) and other emissions in the global atmosphere have increased substantially since the industrial revolution and now increasing faster than ever before. There is a thought that this has already led to dangerous warming in the Earth’s atmosphere and relevant changes around. Emissions legislations are going to be stringent as the years will pass. Hydro carbon fuel cost is also increasing substantially; more over this is non- renewable source of energy. There is an urgent need for novel combustor technologies for reducing emission as well as exploring alternative renewable fuels without effecting combustor performance. Development of novel combustors needs comprehensive understanding of conventional combustors. The design and development of gas turbine combustors is a crucial but uncertain part of an engine development process. At present, the design process relies upon a wealth of experimental data and correlations. Some major engine manufacturers have addressed the above problem by developing computer programs based on tests and empirical data to assist combustor designers, but such programs are proprietary. There is a need of developing design methodologies for combustors which would lead to substantial contribution to knowledge in field of combustors. Developed design methodologies would be useful for researchers for preliminary design assessments of a gas turbine combustor. In this study, step by step design methodologies of dual annular radial and axial combustor, triple annular combustor and reverse flow combustor have been developed. Design methodologies developed could be used to carry out preliminary design along with performance analysis for conventional combustion chambers. In this study the author has also proposed and undertaken preliminary studies of some novel combustor concepts. A novel concept of a dilution zone less combustor has been proposed in this study. According to this concept dilution air would be introduced through nozzle guide vanes to provide an optimum temperature traverse for turbine blades. Preliminary study on novel dilution zone less combustor predicts that the length of this combustor would be shorter compared to conventional case, resulting in reduced weight, fuel burn and vibrations. Reduced fuel burn eventually leads to lower emissions. Another novel concept of combustor with hydrogen synthesis from kerosene reformation has been proposed and a preliminary studies has been undertaken in this work. Addition of hydrogen as an additive in gas turbine combustor shows large benefits to the performance of gas turbine engines in addition to reduction in NOx levels. The novel combustor would have two stages, combustion of ~5% of the hydrocarbon fuel would occur in the first stage at higher equivalence ratios in the presence of a catalyst, which would eventually lead to the formation of hydrogen rich flue gases. In the subsequent stage the hydrogen rich flue gases from the first stage would act as an additive to combustion of the hydrocarbon fuel. It has been preliminary estimated that the mixture of the hydrocarbon fuel and air could subsequently be burned at much lower equivalence ratios than conventional cases, giving better temperature profiles, flame stability limits and lower NOx emissions. The effect of different geometrical parameters on the performance of vortex controlled hybrid diffuser has also been studied. It has been predicted that vortex chamber in vortex controlled hybrid diffuser does not play any role in altering the performance of diffuser. The overall contribution to knowledge of this study is development of combustor preliminary design methodologies with different variants. The other contribution to knowledge is related to novel combustors with a capability to produce low emissions. Study on novel combustor and diffuser has yielded application of two patent applications with several other publications which has resulted in a contribution to knowledge. A list of research articles, two patents, awards and achievements are presented in Appendix C.

It is widely accepted that climate change is a very serious environmental concern. Levels of carbon dioxide (CO2) and other emissions in the global atmosphere have increased substantially since the industrial revolution and now increasing faster than ever before. There is a thought that this has already led to dangerous warming in the Earth’s atmosphere and relevant changes around. Emissions legislations are going to be stringent as the years will pass. Hydro carbon fuel cost is also increasing substantially; more over this is non- renewable source of energy. There is an urgent need for novel combustor technologies for reducing emission as well as exploring alternative renewable fuels without effecting combustor performance. Development of novel combustors needs comprehensive understanding of conventional combustors. The design and development of gas turbine combustors is a crucial but uncertain part of an engine development process. At present, the design process relies upon a wealth of experimental data and correlations. Some major engine manufacturers have addressed the above problem by developing computer programs based on tests and empirical data to assist combustor designers, but such programs are proprietary. There is a need of developing design methodologies for combustors which would lead to substantial contribution to knowledge in field of combustors. Developed design methodologies would be useful for researchers for preliminary design assessments of a gas turbine combustor. In this study, step by step design methodologies of dual annular radial and axial combustor, triple annular combustor and reverse flow combustor have been developed. Design methodologies developed could be used to carry out preliminary design along with performance analysis for conventional combustion chambers. In this study the author has also proposed and undertaken preliminary studies of some novel combustor concepts. A novel concept of a dilution zone less combustor has been proposed in this study. According to this concept dilution air would be introduced through nozzle guide vanes to provide an optimum temperature traverse for turbine blades. Preliminary study on novel dilution zone less combustor predicts that the length of this combustor would be shorter compared to conventional case, resulting in reduced weight, fuel burn and vibrations. Reduced fuel burn eventually leads to lower emissions. Another novel concept of combustor with hydrogen synthesis from kerosene reformation has been proposed and a preliminary studies has been undertaken in this work. Addition of hydrogen as an additive in gas turbine combustor shows large benefits to the performance of gas turbine engines in addition to reduction in NOx levels. The novel combustor would have two stages, combustion of ~5% of the hydrocarbon fuel would occur in the first stage at higher equivalence ratios in the presence of a catalyst, which would eventually lead to the formation of hydrogen rich flue gases. In the subsequent stage the hydrogen rich flue gases from the first stage would act as an additive to combustion of the hydrocarbon fuel. It has been preliminary estimated that the mixture of the hydrocarbon fuel and air could subsequently be burned at much lower equivalence ratios than conventional cases, giving better temperature profiles, flame stability limits and lower NOx emissions. The effect of different geometrical parameters on the performance of vortex controlled hybrid diffuser has also been studied. It has been predicted that vortex chamber in vortex controlled hybrid diffuser does not play any role in altering the performance of diffuser. The overall contribution to knowledge of this study is development of combustor preliminary design methodologies with different variants. The other contribution to knowledge is related to novel combustors with a capability to produce low emissions. Study on novel combustor and diffuser has yielded application of two patent applications with several other publications which has resulted in a contribution to knowledge. A list of research articles, two patents, awards and achievements are presented in Appendix C.

Thesis (Ph. D.)--Ohio State University, 2007. / Full text release at OhioLINK's ETD Center delayed at author's request

Hydrogen, synthesized through water splitting, is poised to play a pivotal role in the transition to a carbon neutral economy. While traditional photovoltaic-electrolysis remains the most market ready technology, generation costs still significantly exceed the U.S. Department of Energy target of 1 kg⁻¹. In contrast, photocatalytic water splitting, which uses nano- or micro- sized particles to simultaneously absorb sunlight and drive electrochemical reactions, offers a simpler and potentially low-cost approach to scalable production of hydrogen using renewable energy. Two major challenges in the commercialization of photocatalytic water splitting have been material stability and low solar to hydrogen (STH) conversion efficiencies. Recent studies have demonstrated some progress in achieving long-term stability or improved efficiency, with the latter breakthrough made possible by operating under conditions of concentrated sunlight, elevated temperature and reduced pressure. However, both demonstrations involved co-evolution of oxygen and hydrogen in the same reactor vessel, which poses significant safety risks and increases operational costs due to the need for downstream gas separation. An alternative approach is nature-inspired Z-scheme photocatalysis, where the oxygen and hydrogen evolution reactions occur on two different particles. This strategy offers an intrinsically safe approach to water splitting when the two particles operate in separate reactor compartments. However, Z-scheme photocatalysis introduces additional challenges particularly related to the use of a redox mediator required to shuttle charge between the two compartments. Notably, the prevalence of undesired back reactions of the mediator species leads to reduced STH efficiencies. Thus, designing both oxidation and reduction reaction sites which are more selective towards the desired forward reactions while minimizing undesired back reactions is crucial. One common strategy to enhance reaction selectivity is to apply permselective nanoscopic oxide overlayers to the surface of reaction sites. Although previous studies have overwhelmingly involved the use of chromium oxide overlayers, these studies have had mixed success. To enhance the efficacy of overlayers for Z-scheme water splitting, it is necessary to establish the fundamental design rules governing the transport and kinetic effects of overlayers on photocatalyst performance. Towards that end, this dissertation explores the development of oxide overlayers using model electrocatalytic systems and correlative microscopy, with a particular emphasis on the use and development of scanning electrochemical microscopy (SECM) methods. Chapter 2 focuses on quantifying defects in silicon oxide (SiOx)-overlayers deposited on Pt thin film electrocatalysts, examining their electrochemical selectivity for hydrogen evolution over ferric ion reduction. By correlating SECM activity maps with other characterization techniques, the influence of defects was determined to have a significant impact on the quantified transport properties of the oxide material. A transport model was developed to account for defect contributions, revealing the ferric ion permeability within SiOx overlayers was over an order of magnitude lower than permeabilities determined from analyses that neglected defect contributions. Chapter 3 explores how ionic conductivity, electronic conductivity, and overlayer thickness influence the location of catalytic active sites on oxide-coated electrocatalysts. Titanium oxide (TiOx) or SiOx overlayers were evaluated using a combination of electroanalytical methods and molecular dynamics simulations. This study decoupled the influences of ionic and electronic conductivity on reaction rates towards the oxygen and ferrous oxidation reactions at varying overlayer thicknesses. While both overlayers were permeable to water and oxygen at all thicknesses, TiOx and SiOx overlayers exhibited differing behaviors for ferrous oxidation at increased thicknesses, attributed to the difference in electronic conductivity. These differences dictated the ferrous oxidation occurred on outer surface of the TiOx overlayer but at the catalyst-overlayer interface for SiOx overlayers. Chapter 4 explores the need for a selective overlayer deposition scheme in photocatalytic systems, as coating both oxidation and reduction reaction sites would reduce the overall activity. An area selective atomic layer deposition (AS-ALD) technique was developed on planar model electrocatalysts, achieving both the desired reactivity and area selectivity within a specific thickness range. This technique was subsequentially applied to a model dual-site electrocatalyst system designed to simulate the undesired diffusional coupling of redox reactions occurring between neighboring particles. Selective overlayers deposited by AS-ALD were shown to reduce the undesired diffusional back reactions by over an order of magnitude while maintaining the desired reactivity at both reaction sites. Finally, concluding remarks and future extensions of research described in Chapters 2-4 are presented in Chapter 5, which includes additional use cases for SECM within the development of photocatalysts such as single particle measurements and exploring the role of oxide overlayers on the separation of photo-generated charge carriers.

Clostridium thermocellum is a gram-positive, acetogenic, thermophilic, anaerobic bacterium that degrades cellulose and carries out mixed product fermentation, catabolising glucose to acetate, lactate, and ethanol under various growth conditions, with the concomitant release of H, and CO2. We have begun to investigate H2-production by C. thermocellum ATCC 27405 cultured in media containing different carbon sources, including glucose, cellobiose, crystalline cellulose, a-cellulose, paper, and delignified wood fibres. We have detected formate synthesis by C. thermocellum ATCC 27405 cultured on both cellobiose and a-cellulose. While formate synthesis has been reported for one strain of Clostridium thermocellum (strain I-1-B), numerous fermentation studies of C. thermocellum 27405 have failed to detect the presence of formate. Formate production was detected throughout growth, and pyruvate:formate lyase (PFL) enzyme activity was detected in late log and stationary phase in extracts of C. thermocellum cultured on cellobiose. Formate synthesis competes with the production of hydrogen (H2) as a fermentation end-product, and thus negatively impacts H2 yields. Bioinformatic analyses of the C. thermocellum genome identified genes encoding key enzymes in pyruvate catabolism pathways, including two putative lactate dehydrogenases (LDH), one PFL. four pyruvate:formate lyase activating enzymes, and at least three putative pyruvate:ferredoxin oxidoreductase (POR) or POR-like enzymes. Our data suggests that hydrogen may be generated through the action of either a Ferredoxin (Fd)-dependent NiFe hydrogenase, often referred to as "Energy-converting Hydrogenases" (Ech), or via NAD(P)H-dependent Fe-only hydrogenases which would permit H2 production from NADH generated during the glyeeraldehyde-3-phosphate dehydrogenase reaction. Furthermore, our findings show the presence of multiple genes putatively encoding NADH:Fd oxidoreductase; suggesting a possible mechanism in which electrons could be transferred from NADH to ferredoxin. The elucidation of pyruvate catabolism pathways and mechanisms of H2 synthesis is the first step in developing strategies to increase hydrogen yields from biomass. My studies have outlined the likely pathways leading to hydrogen synthesis in C, thermocellum ATCC 27405. The actual functional roles of these gene products during pyruvate catabolism and in H2 synthesis remain to be elucidated and will need to be confirmed using both expression analysis and protein characterization.

Clostridium thermocellum is a gram-positive, acetogenic, thermophilic, anaerobic bacterium that degrades cellulose and carries out mixed product fermentation, catabolising glucose to acetate, lactate, and ethanol under various growth conditions, with the concomitant release of H, and CO2. We have begun to investigate H2-production by C. thermocellum ATCC 27405 cultured in media containing different carbon sources, including glucose, cellobiose, crystalline cellulose, a-cellulose, paper, and delignified wood fibres. We have detected formate synthesis by C. thermocellum ATCC 27405 cultured on both cellobiose and a-cellulose. While formate synthesis has been reported for one strain of Clostridium thermocellum (strain I-1-B), numerous fermentation studies of C. thermocellum 27405 have failed to detect the presence of formate. Formate production was detected throughout growth, and pyruvate:formate lyase (PFL) enzyme activity was detected in late log and stationary phase in extracts of C. thermocellum cultured on cellobiose. Formate synthesis competes with the production of hydrogen (H2) as a fermentation end-product, and thus negatively impacts H2 yields. Bioinformatic analyses of the C. thermocellum genome identified genes encoding key enzymes in pyruvate catabolism pathways, including two putative lactate dehydrogenases (LDH), one PFL. four pyruvate:formate lyase activating enzymes, and at least three putative pyruvate:ferredoxin oxidoreductase (POR) or POR-like enzymes. Our data suggests that hydrogen may be generated through the action of either a Ferredoxin (Fd)-dependent NiFe hydrogenase, often referred to as "Energy-converting Hydrogenases" (Ech), or via NAD(P)H-dependent Fe-only hydrogenases which would permit H2 production from NADH generated during the glyeeraldehyde-3-phosphate dehydrogenase reaction. Furthermore, our findings show the presence of multiple genes putatively encoding NADH:Fd oxidoreductase; suggesting a possible mechanism in which electrons could be transferred from NADH to ferredoxin. The elucidation of pyruvate catabolism pathways and mechanisms of H2 synthesis is the first step in developing strategies to increase hydrogen yields from biomass. My studies have outlined the likely pathways leading to hydrogen synthesis in C, thermocellum ATCC 27405. The actual functional roles of these gene products during pyruvate catabolism and in H2 synthesis remain to be elucidated and will need to be confirmed using both expression analysis and protein characterization.