Patent Application
20060163065
This invention pertains to ion sensing, and more particularly to ion sensors for continuous combustion systems.
Continuous combustion systems such as gas turbine engines are used in a variety of industries. These industries include transportation, electric power generation, aircraft engines and afterburner applications, and process industries. During operation, the continuous combustion system produces energy by combusting fuels such as propane, natural gas, diesel, kerosene, or jet fuel. One of the byproducts of the combustion process is emission of pollutants into the atmosphere. The levels of pollutant emissions are regulated by government agencies. Despite significant reductions in the quantity of environmentally harmful gases emitted into the atmosphere, emission levels of gases such as NOx, CO, CO2 and hydrocarbon (HC) are regulated by the government to increasingly lower levels and in an ever increasing number of industries.
Industry developed various methods to reduce emission levels. For example, one method for gaseous fueled industrial turbines is lean premix combustion. In lean premix combustion, the ratio between fuel and air is kept low (lean) and the fuel is premixed with air before the combustion process. The temperature is then kept low enough to limit the formation of nitrous oxides (which occurs primarily at temperatures above 1850 K). The premixing also decreases the possibility of localized fuel rich areas where carbon monoxides and unburnt hydrocarbons are not fully oxidized.
Unfortunately, many of these systems have experienced problems associated with instabilities and flashback. Combustion instability can occur when the air/fuel ratio is too lean and the flame becomes unstable and creates pressure fluctuations, resulting in unsteady heat release of the burning fuel that can produce destructive pressure oscillations or acoustic oscillations. Flashback occurs when the flame normally contained to the combustion zone of the gas turbine combustion system, moves back into the fuel nozzle. When flashback occurs in the fuel nozzle, the temperatures inside the nozzle rise above the design temperature for the nozzle material causing costly damage, which can include fragments of the nozzle material, usually metal, tending to pass through the turbine system usually causing severe damage to the turbine blades. This type of failure, regardless of frequency, can be catastrophic in terms of down time, maintenance costs and lost revenue.
Another challenge facing both industrial and aircraft turbines in the detection of the flame and a phenomenon known as lean blow-off. Turbine control systems, and in particular industrial turbines, need to monitor all turbines for the presence of flames, including during ignition. Aircraft turbines encountering long descents will sometimes emit characteristics indicating the onset of lean blow-off where the fuel to air ratio is so lean that the flame cannot be sustained. Combustion sensing technologies are needed to detect these conditions.
U.S. Pat. No. 6,429,020 teaches an ion sensing apparatus that can survive the extreme temperatures of a flashback condition and provide the necessary response condition. U.S. patent application Ser. No. 10/411,167 teaches that an ion sensing apparatus such as that taught in the U.S. Pat. No. 6,429,020 patent can also be used to sense combustion instability.
The ion-sensing apparatus such as that taught in the U.S. Pat. No. 6,429,020 patent and the like are manufactured using machined electrodes, non-ferrous dielectric insulators, and mechanical fasteners for retention to the support structure (e.g., a gas turbine fuel nozzle). This construction technique limits the application of ion sensing due to the hardware complexity and space constraints required for the ion-sensing apparatus. Additionally, the mechanical fasteners carry the risk of losing preload over repeated thermal cycles, thereby allowing sensor components to fail and potentially causing downstream damage due to debris entering turbine blades or other exhaust system components.
The invention provides an ion sensor that uses thin-film coatings to form both the dielectric layer as well as the sensing electrode layer of the ion-sensing apparatus. The coating system can be applied to a fuel nozzle of a gas turbine combustor, premixer tube surrounding the nozzle, or other combustor surface.
The ion sensor consists of at least two coating layers. The first coating layer is a dielectric that electrically insulates the sensor from the nozzle body, which is grounded. The second coating layer, which is applied over the dielectric layer, forms the ion-sensing electrode.
The second coating layer will typically be a metallic material capable of withstanding high temperatures in the combustion environment without delamination from the dielectric layer and must also resist surface oxidation when exposed to the combustion flame.
In one embodiment, a shielded wire is secured to the nozzle body with the wire protruding through the dielectric layer to connect to the electrode layer. The wire connection allows a signal wire or conductor to be connected to the ion-sensing electrode layer for communication outside of the combustion zone (e.g., to a control module for signal processing).
In an alternate embodiment, the coating system layers (i.e., the dielectric coating layer and the electrode layer) are extended up the nozzle center body such that the mechanical connection to the wire is made further from the flame, thereby reducing thermal stress on the mechanical connection.
These and other aspects and advantages of the invention, as well as additional inventive features, will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.
a is a cross-sectional view along circle 3-3 of
b is a cross-sectional view along circle 3-3 of
While the invention will be described in connection with certain preferred embodiments, there is no intent to limit it to those embodiments. On the contrary, the intent is to cover all alternatives, modifications and equivalents as included within the spirit and scope of the invention as defined by the appended claims.
The present invention provides an apparatus to sense ion current in a combustion region of a continuous combustion system such as a gas turbine, industrial burner, industrial boiler, or afterburner utilizing ionization signals. The invention may be used with any hydrocarbon fuels, such as liquid or gaseous fuels, that produce free ions in the flame when the fuel is burned. The magnitude of the free ions in the flame is proportional to the concentration of hydrocarbons, and therefore the measured ion current is also proportional to the magnitude of free ions. The invention eliminates many of the mechanical components comprising the sensing elements of prior designs where the mechanical components are retained to the mounting structure by mechanical fasteners and over repeated thermal stress cycles and mechanical vibration, the mechanical components can become loose, thereby exposing the turbine or engine to downstream damage from flying debris. The dielectric coating layer of the present invention can be much thinner than the mechanical dielectric washers of prior sensors, resulting in much improved heat transfer from the nozzle tip to the cooler nozzle structure. The resulting lower surface temperatures should reduce the tendency of the flame to attach itself to the nozzle, which could lead to catastrophic failure. Additionally, the coating system of the present invention is typically less expensive to manufacture than a mechanical assembly. For the purpose of describing the invention, a gas turbine fuel nozzle application will be described.
Turning to the drawings, wherein like reference numerals refer to like elements, the invention is illustrated as being implemented in a suitable combustion environment.
With reference to
The electronic module 102 may be a separate module, part of an ignition control module or part of an engine control module. The electronic module 102 includes a power supply 130 for providing a controlled ac or dc voltage signal to the electrodes 120, 122 when commanded by processor 132. Processor 132 commands the power supply to provide power to the electrodes 120, 122, receives ion current signals from electrodes 120, 122 via conditioning module 136, performs computational tasks required to analyze the ion signals to determine the onset of combustion instability and combustion instability, and communicates with other modules such as an engine control module through interface 134. Conditioning module 136 receives signals from the electrodes 120, 122 via lines 138 and performs any required filtering or amplification.
The present invention uses thin-film coatings to form both the dielectric layer as well as the sensing electrode layer. In the description that follows, a grounded nozzle body shall be used to describe the invention. It is noted that other structures may be used. For example, the coating system can be applied to a premixer tube surrounding the nozzle, the nozzle shield (see
The second coating layer (applied over top of the dielectric) is the ion-sensing electrode which will be a metallic material capable of withstanding high temperatures in a combustion environment without delamination from the dielectric layer. The metallic material must also resist surface oxidation when exposed to the combustion flame.
Turning now to
a also illustrates a method of connecting a wire from inside the nozzle body to the ion-sensing electrode. In this embodiment a metal-shielded thermocouple wire assembly 126 is secured to the nozzle body while the conducting wire 128 protrudes through the dielectric layer 124 and conducting electrode 120, 122 and is secured to the electrode 120, 122. Care must be taken to avoid touching the conductive coating 120, 122 with the shield 130 of the thermocouple wire assembly 126. The wire assembly 126 is routed outside the combustion zone to electronic module 102. The conducting wire 128 does not contact any grounded surface. During operation, the electrodes 120, 122 are electrically charged via conducting wires 128. Note that other types of wire may also be used. For example, high temperature coaxial cables may be used.
b shows an alternate embodiment of the electrodes 120, 122 where the surface area of electrode 120 is maximized by using the entire tip of the center body 112. In this embodiment, the dielectric coating (e.g., alumina) is first applied over the entire area of the nozzle tip. Secondly, two thin film ion-sensing elements are precisely coated over the top of the dielectric layer 124 to form the electrodes 120, 122.
Turning now to
It can therefore be seen that an ion sensing apparatus using thin-film coatings and methods to connect to the ion sensing apparatus has been described. The invention eliminates mechanical components (e.g., sensing elements and dielectric insulators) of prior sensors that can become loose over repeated thermal stress and mechanical vibrations, thereby exposing the turbine or engine to downstream damage from flying debris. The use of coatings results in reduced manufacturing costs compared to a mechanical component system.
All references, including publications, patent applications, and patents, cited herein are hereby incorporated by reference to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and were set forth in its entirety herein.
The use of the terms “a” and “an” and “the” and similar referents in the context of describing the invention (especially in the context of the following claims) is to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms “comprising,” “having,” “including,” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to,”) unless otherwise noted. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention.
Preferred embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. Variations of those preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. For example, the ion-sensing coating system can be applied to the dome area of a cylinder head, fuel injector tip, or other available surface of a reciprocating gas or diesel engine. The inventors expect skilled artisans to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context.
