Alumina ceramic is widely used as a thermocouple protector or engineering ceramic in harsh environments due to its high-temperature stability, chemical inertness, and low thermal conductivity. Because alumina is prevalent in high-temperature equipment, it is useful to understand its Raman properties to enable temperature measurement via Raman spectroscopy in these harsh-environment systems. In this paper, we report temperature mapping on the surface of high-temperature alumina ceramics via Raman spectroscopy. The ceramics studied were as cast (unpolished) to approximate use in high-temperature industrial applications. Temperature calibration equations were generated covering the range from room temperature to ~1000°C. The most accurate mathematical temperature relation used the Raman line shift of the most intense Stokes A<sub>1g</sub> peak at 418cm<sup>-1</sup>. The average standard deviation of the Raman temperature measurement was less than 5°C over the entire experimental temperature range. This method will provide accurate non-contact surface temperature measurments in harsh environments where thermal radiation and variable surface emissivity contributes significant error to simpler infrared thermometry.
The modern energy production base in the U.S. is increasingly incorporating opportunity fuels such as biogas, coalbed methane, coal syngas, solar-derived hydrogen, and others. In many cases, suppliers operate turbine-based generation systems to efficiently utilize these diverse fuels. Unfortunately, turbine engines are difficult to control given the varying energy content of these fuels, combined with the need for a backup natural gas supply to provide continuous operation. Here, we study the use of a specially designed Raman Gas Analyzer based on capillary waveguide technology with sub-second response time for turbine control applications. The NETL Raman Gas Analyzer utilizes a low-power visible pump laser, and a capillary waveguide gas-cell to integrate large spontaneous Raman signals, and fast gas-transfer piping to facilitate quick measurements of fuel-gas components. A U.S. Department of Energy turbine facility known as HYPER (hybrid performance system) serves as a platform for apriori fuel composition measurements for turbine speed or power control. A fuel-dilution system is used to simulate a compositional upset while simultaneously measuring the resultant fuel composition and turbine response functions in real-time. The feasibility and efficacy of system control using the spontaneous Raman-based measurement system is then explored with the goal of illustrating the ability to control a turbine system using available fuel composition as an input process variable.
Opportunities exist to improve on-line process control in energy applications with a fast, non-destructive measurement of gas composition. Here, we demonstrate a Raman sensing system which is capable of reporting the concentrations of numerous species simultaneously with sub-percent accuracy and sampling times below one-second for process control applications in energy or chemical production. The sensor is based upon a hollow-core capillary waveguide with a 300 micron bore with reflective thin-film metal and dielectric linings. The effect of using such a waveguide in a Raman process is to integrate Raman photons along the length of the sample-filled waveguide, thus permitting the acquisition of very large Raman signals for low-density gases in a short time. The resultant integrated Raman signals can then be used for quick and accurate analysis of a gaseous mixture. The sensor is currently being tested for energy applications such as coal gasification, turbine control, well-head monitoring for exploration or production, and non-conventional gas utilization. In conjunction with an ongoing commercialization effort, the researchers have recently completed two prototype instruments suitable for hazardous area operation and testing. Here, we report pre-commercialization testing of those field prototypes for control applications in gasification or similar processes. Results will be discussed with respect to accuracy, calibration requirements, gas sampling techniques, and possible control strategies of industrial significance.
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.
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.
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.
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, CO<sub>2</sub>, H<sub>2</sub>O, H<sub>2</sub>, N<sub>2</sub>, and O<sub>2</sub> 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.
Researchers have long sought to improve collection efficiencies in scattered-light sensing applications. Herein, we
demonstrate efficient collection of Raman scattered light from gaseous samples. This enables the accurate, real-time,
simultaneous measurement of otherwise difficult to distinguish molecular gasses or hydrocarbons. Hollow capillary
waveguides, lined with a metal and dielectric over-coating, have often been used to deliver IR laser light to a target. We
show that these waveguides can be used as both a sample holder for Raman gasses and as a laser-pumped optical cell
which can collect Raman scattered light from these gasses. We extend existing low mode-order capillary waveguide
analysis to treat higher order modes. This extension allows a robust computer simulation to accurately predict the
spontaneous Raman scattering power that can be collected by the waveguide. We verify our new theoretical models
with experimental measurements of Raman signals from a nitrogen filled waveguide. We demonstrate a cutback
experiment which verifies our new theoretical predictions of the variation of scattering collection efficiency with guide
dimensions. The prediction accuracy of our simulations allows us to design spectrometers and detectors to maximize
Raman-light throughput in a high-sensitivity gas detection system.
Hollow core fiber optics enable gas phase Raman spectroscopy with relatively low power laser excitation sources. A
Raman sensor for gaseous fuel analysis is demonstrated using silver coated capillary optical fiber as the sample cell and
as the signal collection optic. Using laser powers with as little as a few milliwatts excitation power, the majority species
of natural gas and syngas are readily detected, as well as oxygen and nitrogen in a single sensor system. Exchange rates
in the capillary optical fiber are high enough to enable optical analysis in sub-second response time for real time sensing
and control. Because this one sensor system simultaneously detects and resolves all the component species, real time
feedback to the combustion control system of fuel content and properties is enabled.
Typical control systems that are found in modern power plants must control the many physical aspects of the complex
processes that occur inside the various components of the power plant. As detection and monitoring of pollutants
becomes increasingly important to plant operation, these control systems will become increasingly complex, and will
depend upon accurate monitoring of the concentration levels of the various chemical species that are found in the gas
streams. In many cases this monitoring can be done optically. Optical access can also be used to measure thermal
emissions and the particulate loading levels in the fluid streams. Some typical environments were optical access is
needed are combustion chambers, reactor vessels, the gas and solid flows in fluidized beds, hot gas filters and heat
exchangers. These applications all have harsh environments that are at high temperatures and pressures. They are often
laden with products of combustion and other fine particulate matter which is destructive to any optical window that could
be used to monitor the processes in these environments in order to apply some control scheme over the process. The dust
and char that normally collects on the optical surfaces reduces the optical quality and thus impairs the ability of the
optical surface to transmit data. Once this has occurred, there is generally no way to clean the optical surface during
operation. The probe must be dismounted from the vessel, disassembled and cleaned or replaced, then remounted. This
would require the shutdown of the particular component of the plant where optical monitoring is required. This renders
the probe ineffective to be used as the monitoring part of any control system application.
The components of optical monitoring equipment are usually built in supporting structures that require precise
alignment. This is almost always accomplished using fine scale adjustments to specialized mounting hardware that is
attached to the reactor vessel. When the temperature of these supporting structures increases due to the high temperature
process that is occurring inside the vessel, the optical alignment can often suffer due to the thermal expansion of the
mounting structure. This can render them useless especially for gas velocity measurements or other situations where
precise optical alignment is required.
What is needed is an optical probe that can be inserted into any hazardous environment that will not suffer alignment
problems or other failure modes that are related to high temperature dirty environments, and at the same time maintain a
clean optical surface through the lifetime of the devise so that it may be continually used for optical inspection or for
control system applications. This paper describes details of the construction and the use of a transpiration purged optical
probe which mitigates the problems that are outlined above. The transpiration probe may be used as either an emitter or a
detector. The probe is implemented in the harsh high temperature environment of the NETL pulsed combustion system
where products of combustion and particulate matter have been shown to degrade the performance of a normal optical
window. Assessments of combustion heat release are made by monitoring the ultraviolet signatures that are produced by
the concentration of OH during a pulsed combustion process. It is shown that these measurements are directly correlated
with the pressure within the pulsed combustor. Probe temperature measurements are also presented to show how the
probe and its mounting hardware remain at constant temperatures well below the high temperature environment which
We describe the design and lasing characteristics of a miniaturized, chip-scale Nd:YAG slab laser. The Nd:YAG laser slab utilizes a retroreflecting corner to achieve a 17-cm optical path length with slab geometry of 10×12×2 mm. The laser is constructed, tested, and shown to successfully lase at 1064 nm under edge pumping with an AlGaAs diode laser at 808 nm. The slab laser is operated in a pulsed-pump excitation mode and shown to support a TEM10 single output mode. The fluence achievable from the edge pumping of this initial cavity geometry results in a 6.0% experimentally measured lasing efficiency.
Deregulation of the power industry and increasingly tight emission controls are pushing gas turbine manufacturers to develop engines operating at high pressure for efficiency and lean fuel mixtures to control NO<SUB>x</SUB>. This combination also gives rise to combustion instabilities which threaten engine integrity through acoustic pressure oscillations and flashback. High speed imaging and OH emission sensors have been demonstrated to be invaluable tools in characterizing and monitoring unstable combustion processes. Asynchronous imaging technique permit detailed viewing of cyclic flame structure in an acoustic environment which may be modeled or utilized in burner design . The response of the flame front to the acoustic pressure cycle may be tracked with an OH emission monitor using a sapphire light pipe for optical access. The OH optical emission can be correlated to pressure sensor data for better understanding of the acoustical coupling of the flame. Active control f the combustion cycle can be implemented using an OH emission sensor for feedback.