Jimmy D. Thornton

Flame Ionization Sensor Integrated Into a Gas Turbine Fuel Nozzle

Recent advances in lean premix gas turbine combustion have focused primarily on increasing thermodynamic efficiency, reducing emissions, and minimizing combustion dynamics. The practical limitation on increasing efficiency at lower emissions is the onset of combustion instability, which is known to occur near the lean flammability limit. In a laboratory environment there are many sensors available that provide the combustion engineer with adequate information about flame stability, but those sensors are generally too expensive or unreliable for widespread application in the field. As a consequence, engines must be commissioned in the field with adequate stability margin such that normally expected component wear, fuel quality, and environmental conditions will not cause the turbine to experience unstable combustion. Woodward Industrial Controls, in cooperation with the National Energy Technology Laboratory, is developing a novel combustion sensor that is integrated into the fuel nozzle such that low cost and long life are achieved. The sensor monitors flame ionization, which is indicative of air-fuel ratio and most importantly flame stability.

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

In-Situ O Sensor Demonstration in a Pressurized Lean Premixed Combustion Rig 2

State-of-the-art gas turbines engines continuously strive for higher cycle efficiencies and lower pollutant emissions. The gas turbine combustor is critical for achieving these goals, and operation near the lean extinction limit is necessary. Slight changes in the flow splits (fuel and/or air) can lead to unexpected flame extinction, or other operational issues such as combustion oscillations. Operating these gas turbine combustors near the flame extinction limit could be improved if acceptable in-situ sensors for the combustion section were commercially available. Although exhaust gas oxygen sensors are commonly used in other combustion processes including large utility boilers and automobiles, the use of in-situ oxygen sensors in gas turbine engines has been limited. This paper will describe the results from rig tests in a pressurized lean premixed combustor. The O2 sensor technology used in these tests is commercially available for industrial boiler applications which typically operate near atmospheric pressure and oxygen levels that range from 0–3% of the effluent. In modern gas turbines, however, the amount of excess oxygen is considerably higher. These high levels of excess oxygen result in low level signals from the O2 sensor, which creates concern for in-situ monitoring in gas turbines. The results indicate that this sensor technology will operate at elevated pressure and at high levels of excess oxygen in the process gas suggesting possible application as an operational and diagnostic tool. Data will be presented to show the effects of different operating variables such as pressure, inlet-air temperature, heat-release, and fuel-air ratio.Copyright

Flame Ionization Sensor Integrated Into Gas Turbine Fuel Nozzle

Recent advances in lean premix gas turbine combustion have focused primarily on increasing thermodynamic efficiency, reducing emissions, and minimizing combustion dynamics. The practical limitation on increasing efficiency at lower emissions is the onset of combustion instability, which is known to occur near the lean flammability limit. In a laboratory environment there are many sensors available which provide the combustion engineer with adequate information about flame stability, but those sensors are generally too expensive or unreliable for widespread application in the field. As a consequence, engines must be commissioned in the field with adequate stability margin such that normally expected component wear, fuel quality, and environmental conditions will not cause the turbine to experience unstable combustion. Woodward, in cooperation with NETL, is developing a novel combustion sensor which is integrated into the fuel nozzle such that low cost and long life are achieved. The sensor monitors flame ionization, which is indicative of air-fuel ratio, and most importantly flame stability.Copyright

Fabrication and Preliminary Testing of a Novel Piezoelectrically Actuated Microvalve

This paper presents the fabrication and preliminary testing of a novel piezoelectric microvalve. Fabrication has three steps, which are the actuator fabrication, valve body fabrication and assembly of the microvalve. Fabricating an actuator involves cutting piezoelectric and brass beams, gluing the brass and piezoceramic beams into a trimorph sandwich structure, and curing them under pressure at elevated temperatures. Actuators are then wired either by using conductive epoxy or soldering. Valve body parts are constructed from single crystal silicon substrates using deep reactive ion etching (DRIE). DRIE is a subtractive process, whereby a mask is created on the surface of the stock, which will shield the parts that are not to be machined. Refinements in the actuator manufacturing process are made to increase the quality and decrease the fabrication time. Using a photonic probe, tip deflections of the actuators have been tested at various temperature and voltage levels. Currently, the valves are being assembled. Once assembled, multiple microvalves will undergo cold flow testing with air followed by extensive flow extensive flow testing at elevated temperatures with humidified hydrogen.© 2003 ASME

Time-Varying Flame Ionization Sensing Applied to Natural Gas and Propane Blends in a Pressurized Lean Premixed (LPM) Combustor

In-situ monitoring of combustion phenomena is a critical need for optimal operation and control of advanced gas turbine combustion systems. The concept described in this paper is based on naturally occurring flame ionization processes that accompany the combustion of hydrocarbon fuels. Previous work has shown that flame ionization techniques may be applied to detect flashback, lean blowout, and some aspects of thermo-acoustic combustion instabilities. Previous work has focused on application of DC electric fields. By application of time-varying electric fields, significant improvements to sensor capabilities have been observed. These data have been collected in a lean premixed combustion test rig operating at 0.51–0.76 MPa (5–7.5 atm) with air preheated to 588 K (600°F). Five percent of the total fuel flow is injected through the centerbody tip as a diffusion pilot. The fuel composition is varied independently by blending approximately 5% (volume) propane with the pipeline natural gas. The reference velocity through the premixing annulus is kept constant for all conditions at a nominal value of 70 m/s. The fuel-air equivalence ratio is varied independently from 0.46 – 0.58. Relative to the DC field version, the time-varying combustion control and diagnostic sensor (TV-CCADS) shows a significant improvement in the correlation between the measured flame ionization current and local fuel-air equivalence ratio. In testing with different fuel compositions, the triangle wave data show the most distinct change in flame ionization current in response to an increase in propane content. Continued development of this sensor technology will improve the capability to control advanced gas turbine combustion systems, and help address issues associated with variations in fuel supplies.

Flame Ionization Distribution and Dynamics Monitoring in a Turbulent Premixed Combustor

To achieve very low NOx emission levels, lean-premixed gas turbine combustors have been commercially implemented which operate near the fuel-lean flame extinction limit. Near the lean limit, however, flashback, lean blowoff, and combustion dynamics have appeared as problems during operation. To help address these operational problems, a combustion control and diagnostics sensor (CCADS) for gas turbine combustors is being developed. CCADS uses the electrical properties of the flame to detect key events and monitor critical operating parameters within the combustor. Previous development efforts have shown the capability of CCADS to monitor flashback and equivalence ratio, and progress has been made on detecting and measuring combustion instabilities. In support of this development, a highly instrumented atmospheric combustor has been used to measure the pressure oscillations in the combustor, the ultraviolet flame emission, and the flame ion field at the premix injector outlet and along the walls of the combustor. This instrumentation allows examination of the downstream extent of the combustion field using both the ultraviolet (mostly OH*) emission and the corresponding electron and ion distribution near the walls of the combustor. During testing the combustion dynamics were controlled using a fuel feed impedance control technique. This provided flame ionization measurements for both steady and unsteady combustion, without changing the operating parameters of the combustor. Previous testing in this combustor had fewer data acquisition channels, and did not include a full implementation of a CCADS centerbody. This testing included both the guard and sense CCADS electrodes installed on the nozzle centerbody, and an array of 14 wall mounted spark plugs to monitor the flame ionization downstream along the walls of the combustor. This paper reports the results of this testing, focusing on the relationship between the flame ionization, ultraviolet flame emission, and pressure oscillations. Tests were run over a matrix of equivalence ratios from 0.6 to 0.8, with inlet reference velocities of 20 and 25 m/s. The acoustics of the fuel system for the combustor were tuned using an active-passive technique with an adjustable quarter-wave resonator. Data processing included computing the logarithm of the real-time current signal from the guard electrode, to compensate for the exponential decay of the potential field from the electrode. The data show the standard deviation of the guard current to be the most promising statistic investigated for correlation with the standard deviation of the chamber pressure. This correlation could expand the capabilities of CCADS to allow for dynamic pressure monitoring on commercial gas turbines without a pressure transducer.Copyright

Piezoelectric Microvalve for Flow Control in Polymer Electrolyte Fuel Cells

Maldistribution of fuel across the cells of a fuel cell stack is an issue that can contribute to poor cell performance and possible cell failure. It has been proposed that an array of microvalves could promote even distribution of fuel across a fuel cell stack. A piezoelectric microvalve has been developed for this purpose. This valve can be tuned to a nominal flow rate (and failure position) from which the actuator would either increase or decrease the flow rate and fuel. The valve can successfully regulate the flow of fuel from 0.7 to 1.1 slpm of hydrogen in the range of temperatures from 80° to 100°C and has been tested over pressure drops from 0.5 to 1 psi. A bank of these valves is currently being tested in a four-cell stack at the U.S. Department of Energy National Energy Technology Laboratory.Copyright

Fuel Cell Performance Improvements Using Cell-to-Cell Flow Distribution Control

Balanced flow distribution to each cell in a fuel cell stack plays a significant role in the stack being able to operate at maximum capability and efficiency. This paper discusses the performance improvements in proton exchange membrane fuel cell stacks that can be obtained by using cell-to-cell flow distribution control. In a specially instrumented four-cell stack that employs needle valves to externally control the air and fuel flows to each cell, fuel to a single cell was reduced. The V-I curves collected under these conditions (unbalanced) are compared to curves collected when the fuel flow to each cell was equal (balanced). Reducing the fuel flow to a single cell by 30% decreased the V-I curve cutoff load by 8.5% — demonstrating the negative effect that unbalanced fuel flows can have on stack performance. Typical fuel cell stacks have no dynamic means to keep flows in the stack balanced between the cells, but this work indicates that flow balancing among cells can extend the V-I curve for a fuel cell to higher current values, allowing fuel cell stacks to operate reliably at higher loading and fuel utilizations. Plans to use novel, custom-built micro-valves to dynamically balance flow to individual cells in a fuel cell stack are being pursued as a result of this work, and the status of this development effort is provided.Copyright