Power plants – Combustion products used as motive fluid – Combustion products generator
Reexamination Certificate
2001-09-27
2003-09-16
Casaregola, Louis J. (Department: 3746)
Power plants
Combustion products used as motive fluid
Combustion products generator
C431S170000
Reexamination Certificate
active
06619043
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to catalytic combustors. More specifically, the invention relates to an assembly within a catalytic combustor for supporting the catalyst.
2. Description of the Related Art
Combustion turbines, generally, have three main assemblies: a compressor assembly, a combustor assembly, and a turbine assembly. In operation, the compressor compresses ambient air. The compressed air flows into the combustor assembly where it is mixed with a fuel. The fuel and compressed air mixture is ignited creating a heated working gas. The heated working gas is expanded through the turbine assembly. The turbine assembly includes a plurality of stationary vanes and rotating blades. The rotating blades are coupled to a central shaft. The expansion of the working gas through the turbine section forces the blades, and therefore the shaft, to rotate. The shaft may be connected to a generator.
Typically, the combustor assembly creates a working gas at a temperature between 2,500° F. to 2,900° F. (1371° C. to 1593° C.). At high temperatures, particularly above about 1,500° C., the oxygen and nitrogen within the working gas combine to form the pollutants NO and NO
2
, collectively known as NO
x
. The formation rate of NO
x
increases exponentially with flame temperature. Thus, for a given engine working gas temperature, the minimum NO
X
will be created by the combustor assembly when the flame is at a uniform temperature, that is, there are no hot spots in the combustor assembly. This is accomplished by premixing all of the fuel with all of the air available for combustion (referred to as low NO
x
lean-premix combustion) so that the flame temperature within the combustor assembly is uniform and the NO
x
production is reduced.
Lean pre-mixed flames are generally less stabile than non-well-mixed flames, as the high temperature/fuel rich regions of non-well-mixed flames add to a flame's stability. One method of stabilizing lean premixed flames is to react some of the fuel/air mixture in conjunction with a catalyst prior to the combustion zone. To utilize the catalyst, a fuel/air mixture is passed over a catalyst material, or catalyst bed, causing a pre-reaction of a portion of the mixture and creating radicals which aid in stabilizing combustion at a downstream location within the combustor assembly.
Prior art catalytic combustors completely mix the fuel and the air prior to the catalyst. This provides a fuel lean mixture to the catalyst. However, with a fuel lean mixture, typical catalyst materials are not active at compressor discharge temperatures. As such, a preburner is required to heat the air prior to the catalyst adding cost and complexity to the design as well as generating NO
x
emissions. It is, therefore, desirable to have a combustor assembly that bums a fuel lean mixture, so that NO
x
is reduced, but passes a fuel rich mixture through the catalyst bed so that a preburner is not required. The preburner can be eliminated because the fuel rich mixture contains sufficient mixture strength, without being preheated, to activate the catalyst and create the necessary radicals to maintain a steady flame, when subjected to compressor discharge temperatures. One flow stream is mixed with fuel, as a fuel rich mixture, and passed over the catalyst bed. The other flow stream may be used to cool the catalyst bed.
One disadvantage of using a catalyst is that the catalyst is subject to degradation when exposed to high temperatures. High temperatures may be created by the reaction between the catalyst and the fuel, pre-ignition within the catalyst bed, and/or flashback ignition from the downstream combustion zone extending into the catalyst bed. Prior art catalyst beds included tubes. These tubes were susceptible to vibration because they were cantilevered, being connected to a tube sheet at their upstream ends. Support for these tubes may be provided by flaring the tube ends, which has the disadvantage of thinning the walls of the tubes by as much as 25%, or providing a baffle-type tube sheet at the downstream end of the tubes, which has the disadvantage of creating counterflow at the downstream end. The inner surface of the tubes were free of the catalyst material and allowed a portion of the compressed air to pass, unreacted, through the tubes. The fuel/air mixture passed over the exterior of the tubes, and reacted with, the catalyst bed. Then, the compressed air and the fuel/air mixture were combined. The compressed air absorbed heat created by the reaction of the fuel with the catalyst and/or any ignition or flashback within the catalyst bed.
The disadvantage of such systems is susceptibility of the tubular configuration to vibration damage resulting from: (1) flow of cooling air inside of the tubes, (2) flow of the fuel/air mixture passing over the tubes transverse and longitudinal to the tube bundle, and (3) other system/engine vibrations. Such vibration has caused problems in the power generation field, including degradation of the joint (e.g. braze) connecting the tubes to the tubesheet and degradation of the tubes themselves, both resulting from tube to tube and/or tube to support structure impacting.
Other proposed catalyst supporting structures include various corrugated plates, having a catalyst coating on one side. Such structures do not provide for the flexibility in gas flow patterns necessary to optimize the efficiency of the combustor.
Accordingly, there is a need for a support structure for a catalyst within a catalytic combustor having the necessary strength to withstand vibrations within the harsh environment of the combustor. Additionally, there is a need for a support structure for a catalyst within a catalytic combustor providing for multiple variations of gas flow patterns through the combustor.
SUMMARY OF THE INVENTION
The present invention is a catalyst supporting assembly for a catalytic combustor, providing increased structural support for the catalyst bearing surfaces, and increased flexibility in directing the air flow through the combuster.
The catalyst supporting structure includes a plurality of rectangular, tubular subassemblies. Each rectangular, tubular subassembly includes a pair of relatively wide surfaces hereinafter arbitrarily referred to as the top and bottom (although any orientation may be utilized). A pair of relatively narrow sides connects the top and bottom along their edges, thereby defining a tube between the top, bottom and pair of sides. The top and bottom may be slightly bent to add structural rigidity. One set of surfaces is coated with a catalyst. In one preferred embodiment, the outside surfaces of the top and bottom are coated with a catalyst. The sides, and the inside of the tube remain uncoated.
A plurality of such catalyst-coated subassemblies may be combined, with the top surface of one subassembly facing the bottom surface of an adjacent subassembly. A pair of support walls on either side of the subassemblies may include channels dimensioned and configured to receive the sides of the subassemblies, thereby supporting the subassemblies along their entire length. The resulting catalytic combustor includes a plurality of alternating catalyst coated and uncoated channels, with the uncoated channels preferably defined within each subassembly, and the coated channels preferably defined between adjacent subassemblies and the support walls.
The number of channels within a catalytic combustor may vary within a wide range, permitting a great deal of flexibility in selecting this number to meet a particular need. For example, a small number of subassemblies may fit within a rectangular catalytic combustor, or a larger number of small size subassemblies may fit within a trapezoidal combustor assembly section. A plurality of trapezoidal subassembly sections, for example, six sections, may surround the pilot nozzle of a combustor.
The flow of cooling air and fuel-air mixture through such a catalytic combustor assembly may be configured along several possible paths. For example
Alvin Mary Anne
Bachovchin Dennis M.
Bruck Gerald
Newburry Donald M.
Stampahar Maria E.
Casaregola Louis J.
Siemens Westinghouse Power Corporation
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