Brendan Shaffer

Influence of Burner Material, Tip Temperature, and Geometrical Flame Configuration on Flashback Propensity of H2-Air Jet Flames

Flashback is a key operability issue for low emission premixed combustion systems operated on high hydrogen content fuels. Previous work investigated fuel composition impacts on flashback propensity and found that burner tip temperature was important in correlating flashback data in premixed jet flames. An enclosure around the jet flame was found to enhance the flame-burner rim interaction. The present study further addresses these issues using a jet burner with various geometric configurations and interchangeable materials. Systematic studies addressing the quantitative influence of various parameters such as tip temperature, burner material, enclosure size, and burner diameter on flashback propensity were carried out. A comprehensive overview of the flashback limits for all conditions tested in the current study as well as those published previously is given. The collective results indicate that the burner materials, tip temperature and flame confinement play significant roles for flashback propensity and thus help explain previous scatter in flashback data. Furthermore, the present work indicates that the upstream flame propagation during flashback is affected by the burner material. The material with lower thermal conductivity yields larger flashback propensity but slower flame regression inside the tube. These observations can be potentially exploited to minimize the negative impacts of flashback in practical applications.Copyright

Dynamic Model for Understanding Spatial Temperature and Species Distributions in Internal-Reforming Solid Oxide Fuel Cells

Direct internal reformation of methane in solid oxide fuel cells (SOFCs) leads to two major performance and longevity challenges: thermal stresses in the cell due to large temperature gradients and coke formation on the anode. A simplified quasi-two-dimensional direct internal reformation SOFC (DIR-SOFC) dynamic model was developed for investigation of the effects of various parameters and assumptions on the temperature gradients across the cell. The model consists of 64 nodes each of which contains four control volumes: the positive electrode, electrolyte, negative electrode (PEN); interconnect; anode gas; and cathode gas. Within each node the corresponding conservation, chemical, and electrochemical reaction equations are solved. The model simulates the counterflow configuration since previous research [8] has shown this configuration to yield the smallest temperature differentials. Steady state simulations revealed several results where the temperature difference across the cell was considerably affected by operating and cell design parameters. Increasing the performance of the cell through modifications to the electrochemical model to simulate modern cell performance produced significant changes in the cell temperature differential. Improved cell performance led to a maximum increase in the temperature differential across the cell of 31 K. An increase in the interconnect thickness also exhibits a considerable reduction in the temperature difference across the PEN. In particular, increasing the interconnect thickness from 3.5 to 4.5 mm can achieve about a 50 K reduction in the cross cell temperature difference. Variation of other physical parameters such as the thermal conductivity of the interconnect and the rib width also showed an effect on the temperature distribution. The sensitivity of temperature distribution to the adiabatic assumption was also performed and results showed a considerable effect near the fuel and air inlets. This resulted in severe temperature gradients approaching 160 K/cm. Copyright

Study of Fuel Composition Effects on Flashback Using a Confined Jet Flame Burner

Flashback is the main operability issue associated with converting lean, premixed combustion systems from operation on natural gas to operation on high hydrogen content fuels. Most syngas fuels contain some amount of hydrogen (15–100%) depending on the fuel processing scheme. With this variability in the composition of syngas, the question of how fuel composition impacts flashback propensity arises. To address this question, a jet burner configuration was used to develop systematic data for a wide range of compositions under turbulent flow conditions. The burner consisted of a quartz burner tube confined by a larger quartz tube. The use of quartz allowed visualization of the flashback processes occurring. Various fuel compositions of hydrogen, carbon monoxide, and natural gas were premixed with air at equivalence ratios corresponding to constant adiabatic flame temperatures (AFT) of 1700 K and 1900 K. Once a flame was stabilized on the burner, the air flow rate would be gradually reduced while holding the AFT constant via the equivalence ratio until flashback occurred. Schlieren and intensified OH* images captured at high speeds during flashback allowed some additional understanding of what is occurring during the highly dynamic process of flashback. Confined and unconfined flashback data were analyzed by comparing data collected in the present study with existing data in the literature. A statistically designed test matrix was used which allows analysis of variance of the results to be carried out, leading to correlation between fuel composition and flame temperature with (1) critical flashback velocity gradient and (2) burner tip temperature. Using the burner tip temperature as the unburned temperature in the laminar flame speed calculations showed increased correlation of the flashback data and laminar flame speed as opposed to when the actual unburned gas temperature was used.

Study of Fuel Composition, Burner Material and Tip Temperature Effects on Flashback of Enclosed Jet Flame

Flashback is a key challenge for low NOx premixed combustion of high hydrogen content fuels. Previous work has systematically investigated the impact of fuel composition on flashback propensity, and noted that burner tip temperature played an important role on flashback, yet did not quantify any specific effect. The present work further investigates the coupling of flashback with burner tip temperature and leads to models for flashback propensity as a function of parameters studied. To achieve this, a jet burner configuration with interchangeable burner materials was developed along with automated flashback detection and rim temperature monitoring. An inline heater provides preheated air up to 810 K. Key observations include that for a given condition, tip temperature of a quartz burner at flashback is higher than that of a stainless burner. As a reasult, the flashback propensity of a quartz tube is about double of that of a stainless tube. A polynomial model based on analysis of variance is presented and shows that, if the tip temperature is introduced as a parameter, better correlations result. A physical model is developed and illustates that the critical velocity gradient is proportional to the laminar flame speed computed using the measured tip temperature. Addition of multiple parameters further refined the prediction of the flashback propensity, and the effects of materials are discussed qualitatively using a simple heat transfer analysis.Copyright

Establishing Operating Limits in a Commercial Lean Premixed Combustor Operating on Synthesis Gas Pertaining to Flashback and Blowout

Operability issues such as flashback and lean blow out are phenomena that must be addressed for successful commercial operation of stationary gas turbines. The present work focuses on flashback and lean blow out of premixed jet flames in a combustor from a commercially available gas turbine operating on synthesis gas compositions. The issue of flashback is exacerbated when operating on fuels with high hydrogen content due to the increased reactivity of hydrogen, thus increasing the propensity for flashback. Operating margins for mixtures of natural gas and carbon monoxide in hydrogen are reported. The results interestingly demonstrate reduced stability for mixtures of H2/NG than for H2/CO. Increasing H2 percentage from 0% to 100% reduced blowout equivalence ratios from Φ = 0.63 to Φ = 0.29 for H2/NG and Φ = 0.42 to Φ = 0.29 for H2/CO. In addition, results obtained for inlet temperatures of 300K and 623K are compared and show an upward shift of the stability limits for higher preheats. Modeling of the experimental data using a perfectly stirred reactor predicts the effect of the addition of H2 to natural gas on the blowout limits. With regards to flashback some key factors that dominate the characteristics are identified and attempts to correlate data are carried out. The results show that lean blowout and flashback occur at the same AFT, regardless of preheat temperatures. AFT at flashback and lean blowout are compared to a more fundamental burner [1] with results indicating reasonable scalability.Copyright

Benefits analysis of smart grid demonstration projects

Three example benefits analyses of smart grid demonstration projects are reported. Two U.S. projects are both in Irvine, CA. Southern California Edisons Irvine Smart Grid Demonstration project achieved excellent results for Volt-VAR control, but 9 demonstration zero net energy homes proved far from economic. A microgrid controller being developed for the U.C. Irvine campus promises valuable reliability benefits, and its combined heat and power plant delivers significant ongoing benefits. At the Tianjin Eco-city, multiple demonstrations have been effective technically, but fall short of economic viability.

Systematic Selection and Siting of Vehicle Fueling Infrastructure to Synergistically Meet Future Demands for Alternative Fuels

In order to meet the increasing demand for low carbon and renewable transportation fuels, a methodology for systematically establishing build-out scenarios is desirable. In an effort to minimize initial investment costs associated with the development of fueling infrastructure, the analytical hierarchy process (AHP) has been developed and applied, as an illustration, to the case of hydrogen fueling infrastructure deployment in the State of California. In this study, five parameters are selected in order to rank hydrogen transportation fuel generation locations within the State. In order to utilize meaningful weighting factors within the AHP, expert inputs were gathered and employed in the exercising of the models suite of weighting parameters. The analysis uses statewide geographic information and identifies both key energy infrastructure expansion locations and critical criteria that make the largest impact in the location of selected sites.

Dynamic model for understanding spatial temperature and species distributions in internal-reforming solid oxide fuel cells

Direct internal reformation of methane in solid oxide fuel cells (SOFCs) leads to two major performance and longevity challenges: thermal stresses in the cell due to large temperature gradients and coke formation on the anode. A simplified quasi-two-dimensional direct internal reformation SOFC (DIR-SOFC) dynamic model was developed for investigation of the effects of various parameters and assumptions on the temperature gradients across the cell. The model consists of 64 nodes each of which contains four control volumes: the positive electrode, electrolyte, negative electrode (PEN); interconnect; anode gas; and cathode gas. Within each node the corresponding conservation, chemical, and electrochemical reaction equations are solved. The model simulates the counterflow configuration since previous research [8] has shown this configuration to yield the smallest temperature differentials. Steady state simulations revealed several results where the temperature difference across the cell was considerably affected by operating and cell design parameters. Increasing the performance of the cell through modifications to the electrochemical model to simulate modern cell performance produced significant changes in the cell temperature differential. Improved cell performance led to a maximum increase in the temperature differential across the cell of 31 K. An increase in the interconnect thickness also exhibits a considerable reduction in the temperature difference across the PEN. In particular, increasing the interconnect thickness from 3.5 to 4.5 mm can achieve about a 50 K reduction in the cross cell temperature difference. Variation of other physical parameters such as the thermal conductivity of the interconnect and the rib width also showed an effect on the temperature distribution. The sensitivity of temperature distribution to the adiabatic assumption was also performed and results showed a considerable effect near the fuel and air inlets. This resulted in severe temperature gradients approaching 160 K/cm. Copyright

A planning tool to assess advanced vehicle sensor technologies on traffic flow, fuel economy, and emissions

Author(s): Van Wifvat, Van Thomas | Advisor(s): Samuelsen, Scott | Abstract: Light-duty vehicles are responsible for over 16% greenhouse gas (GHG) emissions in the United States. Human driving behavior has a significant impact on vehicle efficiency, the emission of GHG and primary pollutants, and safety. With environmental health in mind, both academia and industry have the opportunity to develop advanced sensor and complementary control technologies to manage the human role. To explore this hypothesis, the research reported herein began with a comprehensive study of demonstration projects and academic publications which test and evaluate modern technologies to mitigate threats associated with safety and efficiency. The research identified the environmental signals to detect, the corresponding sensors to detect these signals, and the sensor technologies to study in greater depth. Of all the sensor technologies, vehicle-to-vehicle (V2V) and vehicle-to-infrastructure (V2I) communications technologies emerged as the most promising. A major requirement identified is a planning tool designed to assess advanced vehicle sensor technologies on traffic flow, fuel economy, and emissions.In response, a major focus of the research was then directed to (1) developing the Fuel Economy and Traffic of Connected Hybrids (FETCH) planning tool, and (2) evaluating the utility of FETCH for a simple V2V-enabled automatic re-routing control on a custom roadway. The major outcomes of the thesis are (1) the FETCH tool, (2) a research plan for utilizing FETCH to explore the variety of scenarios evolving for the advanced control of hybrid vehicles, and (3) an overall perspective for the evolution of advanced technologies to enable safer and cleaner light duty transportation.

Deployment of Fuel Cell Electric Buses in Transit Agencies: Hydrogen Fueling Infrastructure Scenarios

For transit agencies looking to implement Zero Emission Vehicles (ZEV), Fuel Cell Electric Buses (FCEBs) represent an opportunity because of the similar range and refueling times compared to conventional buses, but with improved fuel economy. To assure an environmentally sensitive hydrogen infrastructure that can respond to the wide range of needs and limitations of transit agencies, a systematic evaluation of options is essential. This paper illustrates the systematic evaluation of different hydrogen infrastructure scenarios for a transit agency. The Orange County Transportation Authority (OCTA) in California was selected for the study.Three different hydrogen infrastructure configurations are evaluated and compared to the existing paradigm of compressed natural gas buses and diesel buses. One additional scenario is analyzed in order to compare feasibility and environmental benefits of FCEBs with Plug-in Electric Buses. Each scenario represents (1) a specific mix and percentage of contribution from the various hydrogen generation technologies (e.g., on-site electrolysis, central SMR, and on-site SMR), (2) defined paths to obtain the corresponding feedstock for each generation process (e.g., biogas, natural gas, renewable energies), (3) detailed hydrogen distribution system (e.g., mix of gaseous/liquid truck delivery), and (4) the spatial allocation of the generation location and fueling locations (e.g., on-site / off-site refueling station) while also accounting for constraints specific to the OCTA bases.This systematic evaluation provides Well-to-Wheel (WTW) impacts of energy and water consumption, greenhouse gases and criteria pollutant emissions of the processes and infrastructure required to deploy FCEBs and Plug-in Electric Buses at OCTA. In addition, this evaluation includes a detailed analysis of the space requirements and operations modifications that may be necessary, but yet feasible, for the placement of such infrastructure.Copyright