Benjamin T. Chorpening

Electronic Structures, Bonding Configurations, and Band-Gap-Opening Properties of Graphene Binding with Low-Concentration Fluorine

To better understand the effects of low-level fluorine in graphene-based sensors, first-principles density functional theory (DFT) with van der Waals dispersion interactions has been employed to investigate the structure and impact of fluorine defects on the electrical properties of single-layer graphene films. The results show that both graphite-2 H and graphene have zero band gaps. When fluorine bonds to a carbon atom, the carbon atom is pulled slightly above the graphene plane, creating what is referred to as a CF defect. The lowest-binding energy state is found to correspond to two CF defects on nearest neighbor sites, with one fluorine above the carbon plane and the other below the plane. Overall this has the effect of buckling the graphene. The results further show that the addition of fluorine to graphene leads to the formation of an energy band (BF) near the Fermi level, contributed mainly from the 2p orbitals of fluorine with a small contribution from the p orbitals of the carbon. Among the 11 binding configurations studied, our results show that only in two cases does the BF serve as a conduction band and open a band gap of 0.37 eV and 0.24 eV respectively. The binding energy decreases with decreasing fluorine concentration due to the interaction between neighboring fluorine atoms. The obtained results are useful for sensor development and nanoelectronics.

Theoretical and experimental investigation of evanescent-wave absorption sensors for extreme temperature applications

Recently, significant developments in evanescent wave absorption sensors have been demonstrated for high temperature sensing applications based upon the optical responses of advanced thin film materials. We will demonstrate how such sensors can be utilized in a mode that allows for chemical or temperature sensing starting from basic theoretical considerations. We will also present experimental high temperature sensing results for fabricated sensors. Potential applications of the sensors to be discussed include a range of high temperature systems relevant for fossil energy and combustion monitoring such as industrial combustors or reaction vessels, solid oxide fuel cells, and gas turbines. In these applications, even a small increase in operating efficiency realized via careful observation of in-process parameters and implementation of real-time process controls can result in dramatic savings across the energy industry, illustrating the necessity of pursuing such techniques. It is hoped that sensors of the type described here will allow for unprecedented measurement-access to processes which present challenging high-temperature and chemically reactive environments.

Plasmon resonance at extreme temperatures in sputtered Au nanoparticle incorporated TiO2 films

Sensor technologies that can operate under extreme conditions including high temperatures, high pressures, highly reducing and oxidizing environments, and corrosive gases are needed for process monitoring and control in advanced fossil energy applications. Au nanoparticle incorporated metal oxide thin films have recently been demonstrated to show a useful optical response to changing ambient gases at high temperatures as a result of modifications to the localized surface plasmon resonance (LSPR) of the Au nanoparticles. Au nanoparticle incorporated TiO2 films were prepared through sputter deposition techniques followed by high temperature oxidation treatments. Upon exposure to a 4% H2/N2 gas atmosphere at elevated temperatures, a shift of the absorption resonance associated with Au nanoparticles to shorter wavelengths is observed, as demonstrated in the literature previously. In this work, we also demonstrate that there is a shift of similar magnitude in the scattering resonance associated with Au. The LSPR absorption peak was monitored as a function of temperature up to 850oC demonstrating a broadening and a decrease in the maximum peak absorptance. Calculations performed in the quasistatic approximation are also presented to explain observed changes in LSPR as a function of temperature and to illustrate the effects on sensitivity of Au – based LSPR sensor materials for extreme temperature applications.

A Combustion Control and Diagnostics Sensor for Gas Turbines

The implementation of sophisticated combustion control schemes in modern gas turbines is motivated by the desire to maximize thermodynamic efficiency while meeting NOx emission restrictions. To achieve target NOx levels, modern turbine combustors must operate with a finely controlled fuel-air ratio near the fuel-lean flame extinction limit, where the combustor is most susceptible to instabilities. In turbine configurations with multiple combustors arranged around the annulus, differences in flow splits caused by manufacturing variations or engine wear can compromise engine performance. Optimal combustion control is also complicated by changes in environmental conditions, fuel-quality, or fuel-type. As a consequence, engines must be commissioned in the field with adequate stability margin such that manufacturing tolerances, normally expected component wear, fuel-quality, and environmental conditions will not cause unstable combustion. A lack of robust combustion in-situ monitoring has limited the ability of modern turbines to achieve stable ultra-low emission performance over the entire load range. This paper describes a combustion control and diagnostics sensor (CCADS) that can potentially revolutionize the manner in which modern gas turbines are controlled. This robust sensor uses the electrical properties of the flame to detect key events and monitor critical operating parameters within the combustor. The CCADS is integrated into the fuel nozzle such that low cost and long life are achieved. Tests conducted at turbine conditions in laboratory combustors instrumented with CCADS have demonstrated the following potential capabilities: 1) detection of incipient flashback and autoignition 2) detection of incipient lean blowout 3) detection of dynamic pressure oscillations 4) and a qualitative measure of equivalence ratio within the combustor. Many of these capabilities have been reported in other publications with data from an atmospheric combustion rig. This paper will summarize each of the capabilities with recent data at turbine conditions. The expectation is that CCADS will provide the key in-situ monitoring for diagnostics and control of modern gas turbines, allowing them to achieve stable ultra-low emissions performance.Copyright

Flame ionization sensor testing in a pressurized combustor

The National Energy Technology Laboratory (NETL) and Woodward Industrial Controls are developing a combustion control and diagnostics sensor (CCADS), an ionization-based sensor for monitoring hydrocarbon combustion in turbines. The patented CCADS design offers a unique multifunction sensor capability. This unique capability coupled with a durable and simple design has increased commercial interest in CCADS for gas turbine applications. A commercial prototype CCADS has been developed and tested under a cooperative research and development agreement (CRADA) between NETL and Woodward. Tests in an industrial-scale high pressure combustor demonstrated detection of flashback, combustion dynamics monitoring, and detection of lean blowout precursors. With realtime monitoring of the combustion process that CCADS can provide, turbine controls are envisioned which would allow sustained low-emissions performance

Design and industrial testing of ultra-fast multi-gas Raman spectrometer

We previously reported the use of hollow metal and dielectric lined waveguides as gas cells used in real-time Raman spectroscopy of gas mixtures. Our team has constructed a multi-gas Raman sensor system capable of measuring molecular components in most gas mixtures with sub-percent accuracy and a sub-second sampling rate. This combination of speed and accuracy is enabled by the novel combination of optimized sample-cell collection and appropriate gas-stream configuration. Here, we discuss the new state-of-the-art in Raman process-gas analysis and share relevant testing data on our optimized system for potential industrial end-users. We conclude that a paradigm shift in technology for gas measurement applications could result from the instrumentation developed herein.

Azimuthal polarization for Raman enhancement in capillary waveguides

Abstract. Hollow, metal-lined capillary waveguides have recently been utilized in spontaneous gas-Raman spectroscopy to improve signal strength and response time. The hollow waveguide is used to contain the sample gases, efficiently propagate a pump beam, and efficiently collect Raman scattering from those gases. Transmission losses in the waveguide may be reduced by using an azimuthally polarized pump beam instead of a linearly or radially polarized pump. This will lead to improved Raman signal strength, accuracy, and response time in waveguide-based Raman gas-composition sensors. A linearly polarized laser beam is azimuthally polarized using passive components including a spiral phase plate and an azimuthal-type linear analyzer element. Half-wave plates are then used to switch between the azimuthally polarized beam and the radially polarized beam with no change in input pump power. The collected Raman signal strength and laser throughput are improved when the azimuthally polarized pump is used. Optimization of the hollow waveguide Raman gas sensor is discussed with respect to incident pump polarization.

Field testing the Raman gas composition sensor for gas turbine operation

A gas composition sensor based on Raman spectroscopy using reflective metal lined capillary waveguides is tested under field conditions for feed-forward applications in gas turbine control. The capillary waveguide enables effective use of low powered lasers and rapid composition determination, for computation of required parameters to pre-adjust burner control based on incoming fuel. Tests on high pressure fuel streams show sub-second time response and better than one percent accuracy on natural gas fuel mixtures. Fuel composition and Wobbe constant values are provided at one second intervals or faster. The sensor, designed and constructed at NETL, is packaged for Class I Division 2 operations typical of gas turbine environments, and samples gas at up to 800 psig. Simultaneous determination of the hydrocarbons methane, ethane, and propane plus CO, CO2, H2O, H2, N2, and O2 are realized. The capillary waveguide permits use of miniature spectrometers and laser power of less than 100 mW. The capillary dimensions of 1 m length and 300 μm ID also enable a full sample exchange in 0.4 s or less at 5 psig pressure differential, which allows a fast response to changes in sample composition. Sensor operation under field operation conditions will be reported.

Field tests of the Raman gas composition sensor

A gas composition sensor based on Raman spectroscopy using reflective metal lined capillary waveguides is tested under field conditions for feedforward applications in combustion control. The capillary waveguide enables effective use of low powered lasers and rapid composition determination, for computation of required parameters to pre-adjust burner control based on incoming fuel. Tests on high pressure fuel streams show sub-second time response and better than one percent accuracy on natural gas fuel mixtures. Fuel composition and Wobbe constant values are provided at one second intervals or faster. The sensor, designed and constructed at NETL, is packaged for Class I Division 2 operations typical of gas turbine and boiler environments, and samples gas at up to 800 psig. Simultaneous determination of the hydrocarbons methane, ethane, and propane plus CO, CO2, H2O, H2, N2, and O2 are realized. The capillary waveguide permits use of miniature spectrometers and laser power of less than 100 mW. The capillary dimensions of 1 m length and 300 μm ID also enable a full sample exchange in 0.4 s or less at 5 psig pressure differential, which allows a fast response to changes in sample composition. Sensor operation under field operation conditions will be reported.

Optical waveguide modeling of conducting metal oxide enabled evanescent wave absorption spectroscopy sensors

Recent work has demonstrated significant promise for high temperature optical gas sensing based upon optical property responses in a class of high electronic conductivity metal oxides. In this work, we theoretically simulate the response of aluminum-doped zinc-oxide (an exemplary conducting metal oxide) in optical fiber evanescent wave absorption spectroscopy sensor devices through the application of a general model of the optical constants for this class of materials in conjunction with prior published material-specific constants for the systems under investigation. Theoretical simulations are compared with recently published experimental results for Al-doped ZnO thin films and the various factors responsible for optimizing sensing responses in this class of materials will be discussed.