Optical interrogation sensors for combustion control

Certain embodiments of the invention may include systems and methods for providing optical interrogation sensors for combustion control. According to an example embodiment of the invention, a method for controlling combustion parameters associated with a gas turbine combustor is provided. The method can include providing an optical path through the gas turbine combustor, propagating light along the optical path, measuring absorption of the light within the gas turbine combustor, and controlling at least one of the combustion parameters based at least in part on the measured absorption.

FIELD OF THE INVENTION

This invention generally relates to sensors, and more particularly relates to optical interrogation sensors for combustion control.

BACKGROUND OF THE INVENTION

Modern industrial gas turbines are required to convert energy at a high efficiency while producing minimum polluting emissions. But these two requirements are at odds with each other since higher efficiencies are generally achieved by increasing overall gas temperature in the combustion chambers, while pollutants such as nitrogen oxide are typically reduced by lowering the maximum gas temperature. The maximum gas temperature can be reduced by maintaining a lean fuel-to-air ratio in the combustion chamber, but if the fuel/air mixture is too lean, incomplete fuel combustion can produce excessive carbon monoxide and unburned hydrocarbons. Other operational problems emerge when operating with lean combustion including unstable load transitions and combustion instability, also known as combustion dynamics. Therefore, the fuel/air mixture and the temperature in the reaction zone must be controlled to support complete combustion.

To balance the conflicting needs for increased efficiency and reduced emissions, extremely precise control is required to adjust the fuel/air mixture in the reaction zones of the combustors. Systems have been proposed for controlling the fuel/air mixture by monitoring various combustion parameters, and using the measured parameters as input to control the fuel system. For example, one conventional system includes a control system where fuel flow rates, pressure levels, and discharge exhaust temperature distributions are utilized as input for setting fuel trim control valves.

Other techniques for controlling combustion dynamics include measuring light emission from the combustion burner flame, and using the measured signal to control certain combustion parameters. For example, one conventional system uses a closed loop feedback system employing a silicon carbide photodiode to sense the combustion flame temperature via the measurement of ultraviolet radiation intensity. The sensed ultraviolet radiation is utilized to control the fuel/air ratio of the fuel mixture to keep the temperature of the flame below a predetermined level associated with a desired low level of nitrogen oxides.

Other conventional systems can use optical fibers for gathering and transmitting light from a combustion region to detectors. Yet other conventional systems can use a video camera to capture images of the flame primarily for monitoring the presence or absence of a flame.

Mass flux sensing techniques have been proposed for use in turbines. For example, laser-based Doppler-shift measurement systems may be used for determining air-flow in a turbine air inlet duct, and similar systems have been proposed for measuring the static temperature by comparing the absorption features from two light generators (lasers) of different frequency. A need remains for improved systems and methods for providing optical sensors.

BRIEF SUMMARY OF THE INVENTION

Some or all of the above needs may be addressed by certain embodiments of the invention. Certain embodiments of the invention may include systems and methods for providing optical interrogation sensors for combustion control.

According to an example embodiment of the invention, a method for controlling combustion parameters associated with a gas turbine combustor is provided. The method can include providing an optical path through the gas turbine combustor, propagating light along the optical path, measuring absorption of the light within the gas turbine combustor, and controlling at least one of the combustion parameters based at least in part on the measured absorption.

According to another example embodiment, a system for controlling combustion parameters associated with a gas turbine combustor is provided. The system can include at least one photodetector in communication with an optical path through the gas turbine combustor, a light source operable to propagate light along the optical path to the at least one photodetector, and a control device operable to control at least one of the combustion parameters based at least in part on one or more signals from the one or more photodetectors.

According to another example embodiment, a gas turbine is provided. The gas turbine can include a combustor, at least one photodetector in communication with an optical path through the combustor, a light source operable to propagate light along the optical path to the at least one photodetector, and at least one control device operable to control one or more combustion parameters based at least in part on one or more signals from the at least one photodetector, wherein the one or more signals comprise at least an absorption signal.

Other embodiments and aspects of the invention are described in detail herein and are considered a part of the claimed invention. Other embodiments and aspects can be understood with reference to the description and to the drawings.

DETAILED DESCRIPTION OF THE INVENTION

An embodiment of the invention may enable combustion parameters to be measured in a turbine combustor by probing or interrogating the combustor with light to detect the temporal and/or spectral attenuation of the light after it has passed through the regions of interest. According to embodiments of the invention, the measured combustion parameters may in turn be utilized to control various parameters of the combustor, including, but not limited to fuel flow rates, fuel/air ratios, and fuel flow distributions to optimize nitrous oxide emissions, dynamic pressure oscillations, and fuel efficiencies.

According to example embodiments, specific chemical species may be monitored and controlled within the combustor by utilizing the principle of light absorption. According to an example embodiment, light that is launched through a combustor may be measured to determine the presence and concentration of certain chemical species within the combustor via the spectral and/or temporal attenuation of the light. According to example embodiments, the spectrum-resolved light absorption may be used to identify chemical species including H2O, CH4, CO, CO2, C2, CH, OH and NO. The measured signals may be correlated with the fuel-to-air ratio, heat release rate, and temperature. According to example embodiments, the time-resolved output from optical detectors may be analyzed to reveal unsteady phenomena associated with the combustion, and may be used to indicate combustion-acoustic oscillations (combustion dynamics). In addition, the output signals may be used as feedback for use in a closed-loop combustion control system. Various sensor options and configurations for combustion control applications, according to embodiments of the invention, will now be described with reference to the accompanying figures.

FIG. 1illustrates an example can combustor with interrogation sensors and control system100for controlling combustion parameters associated with a gas turbine combustor102, according to example embodiments of the invention. The interrogation sensor components may be placed or mounted adjacent to the can combustor102and may selectively interrogate regions within the can combustor102, for example, at or near the air/fuel region108, or within the post-flame (or exhaust) region110of the can combustor102.FIG. 1indicates two example placements and embodiments of the interrogation system, one near the air/fuel region108and one near the post flame region110. One or more such systems may be placed at any suitable location within the combustor system without departing from the scope of the invention.

According to an example embodiment of the invention, a light source111under control of a light source controller112may generate light for interrogating the combustor. The generated light may propagate through an inner portion of the combustor102chamber via a series of optical components. According to an example embodiment, the light generated by the light source111may be coupled into a waveguide114, such as an optical fiber, for convenient routing to an appropriate entry region of the combustor102. According to an example embodiment, the light propagating out of the waveguide114may undergo divergence and may result in a spreading or diverging optical beam117that may be collimated by a lens116or concave mirror to produce a collimated optical beam119. According to another example embodiment, the light produced by the light source (particularly if it is already collimated by the light source) may travel through free-space and may reach the appropriate entry region of the combustor102either directly, or via reflecting mirrors or intervening optics. The collimated optical beam119may enter the combustor102via an input optical port118. The input optical port118and an output optical port120may be provided in the body of the turbine can combustor102to allow the optical energy to pass through the combustor102. The input optical port118and an output optical port120may be constructed from high temperature-resistant, optically transparent material such as quartz, sapphire, or other suitable materials with low loss and a transmission bandwidth appropriate for the wavelengths of interest.

According to example embodiments of the invention, two or more optical ports118120may be positioned on the combustor102at various locations for measuring combustion species at different points along the air/fuel-flame-exhaust path. According to example embodiments, the collimated light119propagating within the combustor102may interact with combustion species, and as a result of the path-averaged interaction with the species, may undergo wavelength-specific spectral attenuation that may correlate with the concentration of the particular chemical species within the combustor102.

According to an example embodiment of the invention, the spectrally attenuated light exiting the combustor102through the output optical port120may pass through a lens122or concave mirror124to produce a converging optical beam123for sensing with one or more detectors126. According to example embodiments of the invention, the optical detector(s)126may be selected for response within certain wavelength spectra windows of interest. For example, a silicon carbide (SiC) photo detector may be selected because of its sensitivity to the ultra violet portion of the wavelength spectrum. According to another embodiment, a silicon (Si) photo detector may be utilized for monitoring the emission from chemical species in the about 400 to about 1000 nm spectrum. According to another example embodiment, Indium gallium arsenide (InGaAs) photodiodes may be selected for measuring infra-red wavelengths in the about 1000 to about 1700 nm spectrum. The optical signals detected by the detectors126may be converted by the detectors126into electronic signals that may be further processed (filtered, amplified, etc.) by the detector electronics128. The output electronic signals from the detector electronics128may be utilized by the combustor control system130to dynamically adjust combustor parameters (air/fuel ratios, fuel distribution, mass flow fuel nozzle acoustic impedance, air flow distribution, etc.) to optimize the parameters of the combustor.

FIG. 2adepicts an end view an optical interrogation system, in accordance with example embodiments of the invention, the light source emitter111may comprise a tunable laser. In another example embodiment, light source emitter111may be a fixed wavelength laser. In yet another example embodiment, light source emitters111202may comprise multiple lasers or multiple line lasers. According to another example embodiment, the light source emitter111may comprise a wide-band light source such as an Amplified Stimulated Emission (ASE) source, supercontinuum source, or super luminescent light emitting diode (SLED).

The design details of the measurement system for interrogating the chemical species within the combustor102may depend upon on the chemical species of interest, and may range in complexity from a single line laser light source111, with a single detector126, to a tunable laser or ASE source, with additional optical components to enable portions of the spectrum to be resolved and measured. Various example optical systems and detection schemes, according to example embodiments of the invention will now be discussed with reference toFIGS. 2a-c.

Basic Fixed Laser Interrogation Embodiments

According to one example embodiment, and as mentioned above, a single line laser or laser diode may be utilized as the light source111. The narrowband emission from the laser may be adjusted to match an absorption band of a chemical species of interest. For example, H2O has absorption bands near 1.45 μm, 1.95 μm, and 2.5 μm; CH4 has a absorption band near 1.65 μm; CO has an absorption band near 1.55 μm; C2 has an absorption band near 518 nm; CH has an absorption band near 530 nm; OH has an absorption band near 310 nm, and NO has an absorption band near 226 nm. By matching the light source's111emission wavelength to one or more of these absorption wavelengths, and by selecting the proper optical detector126, a ratio of input to output optical energy can be measured and correlated with the relative concentration of the combustion species of interest.

Multiple Laser Interrogation Embodiments

According to an example embodiment, multiple laser sources111202and multiple corresponding detectors126214may be utilized for measuring multiple combustion species simultaneously, or for more accurately measuring a single combustion species by employing normalizing methods, as will be described subsequently. In one example embodiment, one or more light sources111202may couple into, and may be routed via corresponding optical waveguides114214to a common lens116and input port118, and may utilize co-linear (or roughly parallel) but spatially separated optical paths204205and may exit a common output port120and may be detected with corresponding optical detectors126214by virtue of the optical path separations or launch angles. In another example embodiment (not shown inFIG. 2a, but alluded to inFIG. 1) the multiple light sources may follow individual paths and may utilize dedicated optics (lenses, mirrors, input and output ports, detectors, etc.).

Tunable Laser Interrogation Embodiments

According to an example embodiment of the invention, the light source202may comprise a tunable laser capable of tuning over a spectrum of wavelengths. The tunable laser may enable measuring the absorption spectra of one or more chemical species within the combustor. According to example embodiments, and depicted inFIGS. 2band2c, a transmission regime grating208or reflection regime grating210may be utilized to angularly separate the spectral components (λ1, λ2. . . λn) of the tuned laser light after it has travelled through and interacted with the chemical species within the combustor102. The angularly separated light may then be resolved and detected by multiple (spatially separated) detectors in a detector array216. The resulting detected signals may be sampled in time and related to the known tuning wavelength of the tunable laser to produce a combined representation of the chemical species' absorption spectra within the combustor102. The measured absorption spectra may then be related to the relative concentrations of the chemical species of interest, and may be utilized for controlling the parameters of the combustor102.

According to another example embodiment of the system employing a tunable laser, such as a chirped laser, a single detector may be utilized to measure the light that has passed through the combustor102. By relating the tunable laser wavelength change with the detected signal in the time domain, the time-domain signal may be utilized to measure the absorption spectra of the chemical species within the combustor112over the wavelength band of interest without utilizing multiple detectors or gratings.

Wideband Light Source Interrogation Embodiments

According to example embodiments of the invention, the light source202, may comprise a wideband optical source, such as an amplified stimulated emission (ASE) source, supercontinuum source, or super luminescent light emitting diode (SLED) source. In these embodiments, photons covering a spectrum of wavelengths within the emission bandwidth of the source may simultaneously interrogate the chemical species within the combustor102to produce a combined absorption spectra. According to example embodiments, and with reference toFIGS. 2band2c, a transmission grating208or reflection grating210may be utilized to angularly separate the spectral components (λ1, λ2. . . λn) of the wideband light after it has travelled through and interacted with the chemical species within the combustor102. The angularly separated light may then be resolved and detected by multiple (spatially separated) detectors in a detector array216. The resulting detected signals may represent a combined chemical species' absorption spectra within the combustor102. The measured absorption spectra may then be related to the relative concentrations of the chemical species of interest, and may be utilized for controlling the parameters of the combustor102. According to another example embodiment, one or more optical filters215, (dichroic, Fabry Perot, etc.) may be placed in the optical path to limit the measured spectrum to a wavelength band of interest. Pre- or post-combustor filtering of the light (prior to reaching the detector) may simplify the detector arrangement, and may serve to eliminate the need for a grating or multiple detectors. Placing a filter215over the detector may also be used to reduce unwanted stray light, for example, from the flame region. Many combinations and variations of the above-mentioned embodiments may be employed without departing from the scope of the invention. Therefore, it is to be understood that the invention is not to be limited to the specific embodiments disclosed.

Also shown inFIGS. 1 and 2aare blocks representing the detector electronics128and the combustion control system130. According to an example embodiment, the detector electronics128may be operable to condition, amplify, filter, and process the signals from the optical detectors126214216. The detector electronics128may also provide control for automatically adjusting the position of any of the corresponding optical components. The output signal from the detector electronics may be used as a control signal for the combustion control system130. For example, according to an embodiment of the invention, the measured concentration of CH4, or the measured ratio of CH4 to CO2 may be utilized as feedback in the combustion control system130, and may provide a control to dynamically adjust the fuel/air ratio.

An example method for measuring chemical species within a combustor102, and for controlling combustion characteristics based on the measurements will now be described with reference to the flowchart ofFIG. 3. Beginning in block302and according to an example embodiment of the invention, a light source111and light source controller112may be provided. At least one optical path, including an optical input port118and output port120, may be provided in the body of the turbine can combustor102adjacent to a region of interest106110to allow light from the light source111to propagate through the combustor102for monitoring the chemical species present in the combustor102via optical absorption. The optical ports118120may be constructed from high temperature resistant, optically transparent material such as quartz, sapphire, or other suitable materials with low loss and a transmission bandwidth appropriate for the wavelengths of interest. In optional block304, a collimator116may be provided adjacent to the input port118, if necessary, to correct any beam divergence of the light from the light source111and to collimate the beam119. Adjacent to the output port120, a focusing device may be provided to concentrate the modified light exiting the combustor102. According to example embodiments, the focusing device may be a lens122or a concave mirror124. In block306, a terminating photodetector126may be provided adjacent to the output port120and may be operable to accept the focused or concentrated light123provided by the output lens122or mirror124.

Blocks308,310, and312indicate that the absorption signal measurement may be normalized to increase the measurement accuracy and sensitivity by measuring the absorption signal and dividing by a normalizing signal. In block312, according to an example embodiment, the normalizing signal may be produced by measuring a portion of the light from the light source111prior to the propagation of the light through the combustor102. Such a signal may already be available at the light source controller112since the typical optical source controllers utilize an internal detector for monitoring the optical power of the light source111for feedback control. According to other example embodiments of the invention, the normalizing signal may be obtained using external light splitters and separate detectors (not shown) to capture and detect a portion of the light prior to propagating through the combustor102. Block310indicates that the absorption signal is obtained by directing light through the optical path in the combustor102, and by measuring the wavelength-varying and/or time-varying absorption signal at one or terminating photodetectors126214216. Block312indicates that the normalized measurement signal may be obtained by dividing the absorption signal (numerator) by the normalizing signal (denominator). According to an example embodiment, since the normalizing method may be optional, if the absorption signal is not normalized, then the denominator can be set to 1.

In block316, the extracted absorption spectra and/or time varying measurement information may be utilized to control and optimize the combustion characteristics of the combustor102. For example, the extracted combustion parameters may be utilized in a feedback control loop for adjusting the fuel flow, fuel-to-air ratio, fuel distribution among the burners, etc.