Kelly J. Benson

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.

Acoustic and Ion Sensing of Lean Blowout in an Aircraft Combustor Simulator

** † ‡ § ** This paper describes work to develop practical, fast diagnostic techniques that can be used to monitor the proximity of a combustor to blowout using measurements of the flame’s acoustic and ion signatures. A novel ignition system with ion sensing capability is used for blowout detection in conjunction with acoustic sensing. Data was acquired from a commercial single nozzle combustor fueled with Jet-A. As lean blowout is approached, short duration, localized extinction and re-ignition events were observed. Several signal processing schemes were developed to detect these blowout precursors, including signal thresholding and spectral analysis. The analyses revealed changes in the low frequency acoustic and ion spectra and increased presence of intermittent precursor events in both acoustic and ion data closer to blowout.

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 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

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

Detection of Lean Blowout and Combustion Dynamics Using Flame Ionization

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. Of particular concern is the avoidance of lean blowout (LBO) and combustion dynamics. To minimize combustion temperature and NOx production, it is necessary to approach the LBO boundary. This paper describes continuing work on incipient lean blowout detection using flame ionization, investigating the impact of three different piloting and equivalence ratio reduction strategies applied in a pressurized, lean premixed combustor. This work builds upon previous research in the development of the Combustion Control and Diagnostic Sensor (CCADS). In previous papers, the detection of flashback, equivalence ratio, combustion dynamics, and lean blowout using CCADS has been investigated and described. Previous investigation of lean blowout, however, has been limited to a side pilot configuration. In this paper, lean blowout behavior for a side pilot and a centerbody tip pilot are compared. In addition, two different methods for decreasing equivalence ratio to approach LBO are investigated. These cases are found to have differing lean blowout behavior, and differing CCADS signatures. This paper also reports on the ion signal behavior due to combustion dynamics observed during the equivalence ratio sweeps, including passing through stability boundaries. Tests were performed at 5 atm using an industrial style, lean premixed combustor nozzle, equipped with CCADS electrodes, in a water-cooled, natural gas fueled, acoustically noisy combustor. Testing included sweeps of equivalence ratio from 0.65 to 0.45, crossing one or more stability boundaries. LBO was approached for configurations with a side pilot (on the inlet wall of the combustor, but set away from the premixer) and a centerbody tip pilot. The centerbody tip pilot and the side pilot both helped stabilize combustion, but combustion dynamics still occurred. Incipient LBO was apparent in all cases; however, the different flame structure encountered with each pilot configuration and fuel control strategy made the flame ionization signature differ for each case.Copyright