Geo. A. Richards

Passive Control of Combustion Dynamics in Stationary Gas Turbines

This paper summarizes passive methods used to improve the stability of low-emission combustors in stationary power gas turbines. Common passive methods are reviewed, including discussion of control model concepts, application of simple time-lag models, and a review of acoustic dampers. Applications of time-lag modie cations are presented, and limitations of this approach are discussed. Nyquist analysis is used to show how changing the time lag can be confounded by the presence of multiple acoustic modes. Experimental results demonstrating the frequency shifts predicted by Nyquist analysis are also shown. Stabilizing effects of distributed time lags are discussed, along with some e eld applications. A review of acousticdampers showsthat these devices are not widely applied in stationary engines compared to rocket or afterburner combustors, but have shown good results where applied. Nomenclature ¤ AB;C; D, = combustor sections and the acoustic E; F; G transfer matrices that represent these sections; elements of these 2 £2 matrices are identie ed using subscripts Ai;j where i D0;1 and j D0;1 c

Effect of Fuel Nozzle Configuration on Premix Combustion Dynamics

Combustion dynamics (or combustion oscillations) have emerged as a significant consideration in the development of low-emission gas turbines. To date, the effect of premix fuel nozzle geometry on combustion dynamics has not been well-documented. This paper presents experimental stability data from several different fuel nozzle geometries (i.e., changing the axial position of fuel injection in the premixer, and considering simultaneous injection from two axial positions). Tests are conducted in a can-style combustor designed specifically to study combustion dynamics. The operating pressure is fixed at 7.5 atmospheres and the inlet air temperature is fixed at 588K (600F). Tests are conducted with a nominal heat input of 1MWth (3MBTUH). Equivalence ratio and nozzle reference velocity are varied over the ranges typical of premix combustor design. The fuel is natural gas. Results show that observed dynamics can be understood from a time-lag model for oscillations, but the presence of multiple acoustic modes in this combustor makes it difficult to achieve stable combustion by simply re-locating the point of fuel injection. In contrast, reduced oscillating pressure amplitude was observed at most test conditions using simultaneous fuel injection from two axial positions.Copyright

Effect of Axial Swirl Vane Location on Combustion Dynamics

This paper reports the effect of changing the location of axial swirl vanes on premix combustion dynamics. Tests are conducted in a specially designed single-injector combustor operating at a pressure of 7.5 atmospheres and an inlet air temperature of 588K (600F). All of the tests are conducted using natural gas as the fuel. The air velocity and equivalence ratio are varied over an operating map for four different axial swirl vane positions in the premix nozzle. In contrast to earlier studies reported from this combustor, the fuel injection location is fixed. The results confirm the importance of the convective fuel time lag for the different swirl vane locations, but also show that changing the vane location at a fixed time lag can significantly affect the magnitude of the combustion oscillations.Copyright

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.

Application of Macrolamination Technology to Lean, Premixed Combustion

Macrolamination, a novel manufacturing technique, is used to develop a dual-fuel premixer. A spatially distributed injection strategy is used to enhance fuel placement, distribution, and mixing inside the premixer. Parametric studies are conducted with different configurations of the premixer to determine the effects of residence time and nozzle configuration on pollutant emissions and flame stability. Diesel fuel (DF-2) and natural gas are used as fuels. Tests are conducted at a pressure of 400 kPa (5 atmospheres), and an inlet air temperature of 533 K. The pollutant emissions and RMS pressure levels are presented for a relatively wide range of nozzle velocities (50-80 m/s) and equivalence ratios (0.54-0.75). These results indicate very good pollutant emissions for a prototype design. These results also indicate that the time-lag model, previously associated with combustion oscillations for gaseous-fuel applications, also applies to liquid-fuel operation.

Demonstration of a reheat combustor for power production with CO2 sequestration

Concerns about climate change have encouraged significant interest in concepts for ultralow or “zero”-emissions power generation systems. In a concept proposed by Clean Energy Systems, Inc., nitrogen is removed from the combustion air and replaced with steam diluent. In this way, formation of nitrogen oxides is prevented, and the exhaust stream can be separated into concentrated CO2 and water streams. The concentrated CO2 stream could then serve as input to a CO2 sequestration process. In this study, experimental data are reported from a full-scale combustion test using steam as the diluent in oxy-fuel combustion. This combustor represents the “reheat” combustion system in a steam cycle that uses a high and low-pressure steam expansion. The reheat combustor serves to raise the temperature of the low-pressure steam turbine inlet, similar to the reheat stage of a conventional steam power cycle. Unlike a conventional steam cycle, the reheat enthalpy is actually generated by oxy-fuel combustion in the steam flow. This paper reports on the unique design aspects of this combustor, as well as initial emissions and operating performance.

Control of Combustion Dynamics Using Fuel System Impedance

Combustion oscillations (dynamics) have become a major challenge in the development of low-emission premix combustors. In this paper, a variable impedance fuel system is used to modulate the phase and magnitude of the combustion response in a laboratory scale 30 kW combustor. With the proper choice of design parameters, this technique demonstrates significant attenuation of dynamics pressures, over a wide range of operating conditions. The technique is similar to active control, but does not require high frequency actuators. The paper will report on the key design variables that should be considered when using this concept to improve dynamic stability.Copyright

Effect of Fuel System Impedance Mismatch on Combustion Dynamics

Combustion dynamics are a challenging problem in the design and operation of premixed gas turbine combustors. In premixed combustors, pressure oscillations created by the flame dynamic response can lead to damaging pressure oscillations. These dynamics are typically controlled by designing the combustor to achieve stable operation for planned conditions, but dynamics may still occur with minor changes in ambient operating conditions, or fuel composition. In these situations, pilot flames, or adjustment to fuel flow splits can be used to stabilize the combustor, but often with a compromise in emissions performance. As an alternative to purely passive design changes, prior studies have demonstrated that adjustment to the fuel system impedance can be used to stabilize combustion. Prior studies have considered just the response of individual fuel injector and combustor. However, in practical combustion systems, multiple fuel injectors are used. In this situation, individual injector impedance can be modified to produce a different dynamic response from individual flames. The resulting impedance mismatch prevents all injectors from strongly coupling to the same acoustic mode. In principle, this mismatch should reduce the amplitude of dynamics, and may expand the operating margin for stable combustion conditions. In this paper, a 30 kW laboratory combustor with two premixed fuel injectors is used to study the effect of impedance mismatch on combustion stability. The two fuel injectors are equipped with variable geometry resonators that allow a survey of dynamic stability while changing the impedance of the individual fuel systems. Results demonstrate that a wide variation in dynamic response can be achieved by combining different impedence fuel injectors. A baseline 7% RMS pressure oscillation was reduced to less than 3% by mismatching the fuel impedance.Copyright

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

Assessment of RQL Trapped Vortex Combustor for Stationary Gas Turbines

This paper describes the evaluation of an alternative combustion approach to achieve low emissions for a wide range of fuel-types. This approach combines the potential advantages of a staged Rich-burn, Quick-mix, Lean-burn (RQL) combustor with the revolutionary Trapped Vortex Combustor (TVC) concept. Although RQL combustors have been proposed for low-BTU fuels, this paper considers the application of an RQL combustor for high-BTU natural gas applications. This paper will describe the RQL/TVC concept and experimental results conducted at 10 atmospheres (1013 kPa or 147 psia) and an inlet-air temperature of 644K (700°F). The results from a simple network reactor model using detailed kinetics are compared to the experimental observations. Neglecting mixing limitations, the simplified model suggests that NOx and CO performance below 10 parts-per-million could be achieved in an RQL approach. The CO levels predicted by the model are reasonably close to the experimental results over a wide range of operating conditions. The predicted NOx levels are reasonably close for some operating conditions, however, as the rich-stage equivalence ratio increases, the discrepancy between the experiment and the model increases. Mixing limitations are critical in any RQL combustor, and the mixing limitations for this RQL/TVC design are discussed.Copyright