Patent Publication Number: US-2007119179-A1

Title: Opposed flow combustor

Description:
BACKGROUND  
      The present invention relates generally to gas turbine devices and, in particular, to a system and method for providing opposed flows of fuel and air in a combustor.  
      In traditional gas turbine devices, air is drawn from the environment, mixed with fuel and, subsequently, ignited to produce combustion gases, which may be used to drive a machine element or to generate power, for instance. Traditional gas turbine devices generally include three main systems: a compressor, a combustor and a turbine. The compressor pressurizes air and sends this air towards the combustor. The compressed air and a fuel are delivered to the combustor. The fuel and air delivered to the combustor are ignited, with the resulting combustion gases being employed to actuate a turbine or other mechanical device. When used to drive a turbine, the combustion gases flow across the turbine to drive a shaft that powers the compressor and produces output power for powering an electrical generator or for powering an aircraft, to name but few examples.  
      Gas turbine engines are typically operated for extended periods of time, and exhaust emissions from the combustion gases are a concern. For example, during combustion, nitrogen combines with oxygen to produce oxides of nitrogen (NOx), and these NOx emissions are often subject to regulatory limits and are generally undesired. Traditionally, gas turbine devices reduce the amount of NOx emissions by decreasing the fuel-to-air ratio, and these devices are often referred to as lean devices. Lean devices reduce the combustion temperature within the combustion chamber and, in turn, reduce the amount of NOx emissions produced during combustion.  
      Some regions of the United States require as little as three parts per million (ppm) N0x levels in natural gas operation. N0x emissions from a gas turbine have been significantly reduced using premixed combinations of natural gas. The degree of premixing has a strong impact on N0x reduction. However, highly premixed flames demonstrate increased instability and have proven difficult to anchor. Conventional premixed systems do not adequately reach N0x emission targets or theoretical limits so selective catalytic reduction (SCR) of N0x through ammonia injection has been employed. SCR is an expensive approach and improvements to the combustion system would reduce operating costs, such as the cost of electricity for operating the system. In systems powered by syngas or hydrogen, a diffusion flame has been used because high flame velocities associated with the hydrogen content may result in flashback into the premixer. Diluents are added at the injection tip to potentially reduce N0x emissions.  
      In addition to natural gas, combustors may employ other fuels, such as syngas (synthetic gas) or hydrogen. Syngas poses challenges to flame stabilization and emission reduction at high firing temperatures. Premixed hydrogen combustion may result in a risk of flashback and typically produces significant N0x without premixing. Thus, there exists a need to provide an improved system and method to reduce the temperature of combustion in gas turbine systems to facilitate a reduction in NOx emissions from such systems.  
     BRIEF DESCRIPTION  
      Briefly, in accordance with one embodiment of the present invention, a combustor is provided. The combustor comprises a combustion chamber, a first inlet adapted to provide a first air flow to the combustion chamber in a first direction, a fuel controller adapted to provide a fuel flow to the combustion chamber in the first direction, an opposing inlet adapted to provide an opposing air flow to the combustion chamber in a second direction generally in opposition to the first direction and wherein the first air flow and the fuel flow interact with the opposing air flow to form a stagnation zone in the combustion chamber.  
      A method of operating a combustion chamber in accordance with an exemplary embodiment of the present invention is also provided. The method comprises injecting a first air flow and a fuel flow into the combustion chamber in a first direction, and injecting an opposing air flow into the combustion chamber in opposition to the first air flow to form a stagnation zone in the combustion chamber. 
    
    
     DRAWINGS  
      These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:  
       FIG. 1  is a diagrammatic representation of a gas turbine device, in accordance with an exemplary embodiment of the present invention;  
       FIG. 2  is a partial and diagrammatic, cross-sectional view of a combustor assembly, in accordance with an exemplary embodiment of the present invention;  
       FIG. 3  is a partial and diagrammatic, cross-sectional view of a combustor assembly, in accordance with another exemplary embodiment of the present invention;  
       FIG. 4  is a partial and diagrammatic, cross-sectional view of a combustor assembly, in accordance with yet another exemplary embodiment of the present invention;  
       FIG. 5  is a partial and diagrammatic, cross-sectional view of a combustor assembly, in accordance with still another exemplary embodiment of the present invention; and  
       FIG. 6  is a flowchart illustrating an exemplary process for establishing an opposing flow in a combustor in accordance with an exemplary embodiment of the present invention.  
    
    
     DETAILED DESCRIPTION  
      As a preliminary matter, the definition of the term “or” for the purpose of the following discussion and the appended claims is intended to be an inclusive “or.” That is, the term “or” is not intended to differentiate between two mutually exclusive alternatives. Rather, the term “or” when employed as a conjunction between two elements is defined as including one element by itself, the other element itself, and combinations and permutations of the elements. For example, a discussion or recitation employing the terminology “A” or “B” includes: “A”, by itself “B” by itself and any combination thereof, such as “AB” and/or “BA.” 
      A flexible fuel combustor in accordance with an exemplary embodiment of the present invention is capable of burning hydrogen, natural gas or syngas in a gas turbine while maintaining low N0x emissions and low dynamics. Such systems may utilize opposed flows of fuel-air mixtures to create aerodynamic flame stabilization and to produce a highly stable flame zone in a combustor. Embodiments of the present invention may be compact in size and may provide low peak flame temperatures to help reduce undesirable N0x emissions.  
      In an exemplary embodiment of the present invention, the combustor flame stabilization zone is removed from the burner. Reactants from the combustion process entrained the diluent rich products in the combustor before reacting. The flame is aerodynamically stabilized by an opposed flow of an ultra-lean fuel-air mixture, which creates a stagnation zone. The opposed flow also cools the combustor wall close to the stagnation zone. The combustion products flow toward the burner and then flow into the turbine hot section.  
      Turning now to the drawings,  FIG. 1  is a diagrammatic representation of a gas turbine device in accordance with an exemplary embodiment of the present invention. In  FIG. 1 , the gas turbine device is generally referred to by the reference numeral  10 . The gas turbine device  10  comprises one or more compressor stages  12 , a combustor  14  and one or more turbine stages  16 . The compressor stages  12  provide a first air flow  20 , which is adapted by the shape of a first air flow chamber  18  to flow into the combustor  14  via a first inlet  22 . The combustor  14  also includes an opposing airflow chamber  24  to accommodate an opposing air flow  26 . Moreover, the output of the compressor stages  12  is split to form the first air flow  20  and the opposing air flow  26 . In the embodiment illustrated in  FIG. 1 , the opposing air flow  26  is delivered to the combustor  14  via an opposing inlet  28 .  
      A fuel source  30  provides fuel to a fuel controller  32 . The fuel controller  32  delivers a first fuel flow  34  to the combustor  14  via the first inlet  22 . The first airflow  20  and the first fuel flow  34  may be partially premixed. The first airflow  20  and the first fuel flow  34  are directed into the combustor  14  in a first direction, as indicated by the arrow  34  that represents the fuel flow. The opposing airflow  26  enters the combustor  14  in a second direction that is generally opposition to the first direction followed by the first airflow  20  and the first fuel flow  34 .  
      A perforated plate  36  may be disposed inside the combustor  14  between the first inlet  22  and the opposing inlet  28 . The opposition between the first air flow  20  and the opposing air flow  26  creates a stagnation zone in the combustor  14 . The stagnation zone, which may also be referred to as a reaction/combustion zone, is identified in  FIG. 1  by the reference numeral  38 . When combustion occurs, the combustion tends to happen near the stagnation zone  38 .  
      The air-fuel mixture in the combustor  14  is ignited to produce a combusted gas flow, as indicated by the arrow  40 . The combusted gas flow  40  exits the combustor  14  and is delivered to the turbine stages  16 . The reactants from the combustion process are directed toward the perforated plate  36  with an effusion flow of ultra-lean fuel-air. The jet thus created entrains hot products of combustion and the fuel does not ignite until the jet is very diluted with the hot combustion products. This action lowers peak flame temperatures and N0x production without requiring premixing. The stagnation zone  38  stabilizes the combustion process under lean conditions and reduces dynamic instabilities.  
      An exemplary embodiment of the present invention may employ syngas and hydrogen combustion without the use of diluents. This approach provides a stable combustion zone and reduces dynamics in the system. Reduced combustor cooling is enabled by reduced combustor size and lower peak gas temperatures.  
       FIG. 2  is a partial and diagrammatic cross-sectional view of a combustor assembly, in accordance with an exemplary embodiment of the present invention. In the embodiment illustrated in  FIG. 2 , the first air flow  20  and the first fuel flow  34  are delivered to the combustor  14  coaxially via the first inlet  22  from the left-hand side of  FIG. 2 . The first air flow  20  and the first fuel flow  34  form a jet that travels across the combustor  14 , entraining hot products from combustion. In the embodiment illustrated in  FIG. 2 , a second jet of fuel and air comprising the opposing air flow  26  and an opposing fuel flow  42  is desirably premixed and injected into the combustor  14  via the opposing inlet  28 , as shown at the right-hand side of  FIG. 2 . The premixing of the opposing air flow  26  and the opposing fuel flow  42  may be either full or partial. The opposing inlet  28  may comprise multiple openings in the right-hand side wall.  
      In the embodiment illustrated in  FIG. 2 , the first air flow  20  and the first fuel flow  34  exhibit a flammability that is greater than the lean flammability limit for the system. The opposing airflow  26  and opposing fuel flow  42  exhibit a flammability that is less than the lean flammability limit. The jet entering the combustor  14  from the left hand side is at a relatively high velocity. A stagnation control pressure p sc  is defined to be the pressure in the combustion chamber  14  in the region around the entry point of the opposing airflow  26 , but prior to the point where the opposing airflow  26  encounters the perforated plate  36 . As illustrated in  FIG. 2 , the stagnation control pressure p sc  is greater than a stagnation zone pressure p stagnation . Inside the combustor  14 , the flame stabilizes in the stagnation region between the two flows and hot products flow back to the opening on the left hand side, as illustrated by the arrow representing the combusted gas flow  40 . In the exemplary embodiment illustrated in  FIG. 2 , the combusted gas flow  40  exits the combustion chamber  14  upstream relative to the first direction (as indicated by the arrow  34 ) from the stagnation zone  38 . Moreover, the exit of the combusted gas flow  40  is not coaxial with the first direction (as indicated by the arrow  34 ) in the exemplary embodiment illustrated in  FIG. 2 .  
      The opposing fuel flow may be provided by a fuel controller (see  FIG. 1 ). The amount and velocity of fuel injected into the combustor  14  via the opposing fuel flow  42  is desirably variable. It may be controlled in by fluidic means or the like to cause a uniform temperature distribution within the combustor  14 . In fluidic control, an area of a flow, which is proportional to its velocity, is changed by introducing a second flow in the general region such as through the same inlet. In such a manner, the magnitude of the opposing airflow  26  and/or the opposing fuel flow  42  may be adjusted to move the stagnation zone leftward in the combustion chamber  14 . By moving the stagnation zone away from the right-hand side of  FIG. 2  in this manner, the temperature of the right-hand wall of the combustor  14  adjacent to the opposing inlet  28  may be desirably reduced.  
       FIG. 3  is a partial and diagrammatic, cross-sectional view of a combustor assembly in accordance with another exemplary embodiment of the present invention. In the exemplary embodiment illustrated in  FIG. 3 , the first fuel flow  34  and opposing fuel flow  42  are omitted for clarity. The combustor  14  is disposed at an angle θ relative to the horizontal. The first inlet  22  and the opposing inlet  28  extend outwardly into the combustion chamber  14 .  
      The value of θ may be in the range of 0 degrees to 90 degrees depending on design criteria for the combustor  14 . At a θ of 0 degrees, the combustor may extend too far into space to be practical. At a θ value of 90 degrees, escaping gases may have a more direct path to the turbine stages  16 .  
       FIG. 4  is a partial and diagrammatic, cross-sectional view of a combustor assembly in accordance with yet another exemplary embodiment of the present invention. In the embodiment illustrated in  FIG. 4 , the first airflow  20  enters the combustor  14  from a relatively central location. The combusted gas flow  40  exits the combustor  14  from a radially removed location relative to the position of the first inlet  22 , as indicated by the arrow  40 .  
       FIG. 5  is a partial and diagrammatic, cross-sectional view of a combustor assembly in accordance with still another exemplary embodiment of the present invention. In the embodiment illustrated in  FIG. 5 , the combustor  14  is connected to the turbine stages (not shown) via a horn seal  44 . Those of ordinary skill in the art will appreciate that the horn seal  44  facilitates the detachment of the combustion chamber  14  for maintenance.  
      With  FIG. 1  in mind,  FIG. 6  is a flowchart illustrating an exemplary process for establishing an opposing flow in a combustor in accordance with an exemplary embodiment of the present invention. The process is generally referred to by the reference numeral  44 . At block  46 , a first air flow and a fuel flow are injected into a combustion chamber in a first direction. At block  48 , an opposing air flow is injected into the combustion chamber in opposition to the first air flow and the fuel flow to form a stagnation zone in the combustion chamber. Further, the first air flow and the fuel flow interact with the opposing air flow to form a vortex flow inside the combustion chamber.  
      While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.