Combustion – Fuel disperser installed in furnace – Disperser feeds into permeable mass – e.g. – checkerwork – etc.
Reexamination Certificate
2001-07-20
2002-05-28
Yeung, James C. (Department: 3743)
Combustion
Fuel disperser installed in furnace
Disperser feeds into permeable mass, e.g., checkerwork, etc.
C431S326000, C431S353000, C431S007000
Reexamination Certificate
active
06394791
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a catalytic reactor that may be employed in a variety of uses, such as for gas turbine engine combustion, or for other combustion systems. More particularly, the present invention is directed to a method that creates a product stream and a heat of reaction from a fuel-rich fuel/air mixture and then contacts the product stream with a sufficient quantity of additional air to completely combust all of the fuel and to which a portion of the heat of reaction has been transferred.
2. Brief Description of the Related Art
At high temperature, particularly above approximately 2800 degrees F., the oxygen and nitrogen present in air combine to form the pollutants NO and NO
2
, collectively known as NOx. As flame temperatures of most fuels reacting with air can easily exceed this value, a goal of modern combustion systems is to operate at reduced temperatures, so that such thermal formation of NOx is limited.
Reduced-temperature combustion is typically accomplished by premixing the fuel with sufficient excess air that the flame temperature is reduced to a value at which thermal NOx production is minimal (typically a temperature below approximately 2800 degrees F., At these lower flame temperatures, however, the rate of combustion may be insufficient to prevent localized or global blow-off or extinction, particularly under conditions of turbulent flow. Flame anchoring and flame stability thus become problematic at the lower flame temperatures required for truly low-NOx lean-premixed combustion. Thus, achievable NOx reduction is limited.
A commonly-employed solution to the problems of flame anchoring and flame stability is to react a portion of the fuel at a higher temperature, and to then use the resulting high-temperature gases to initiate, sustain, and stabilize (“pilot”) the lower-temperature combustion of the main fuel/air mixture. The higher-temperature “pilot” combustion zone can take various forms, and can be fuel-lean or fuel-rich (the fuel-rich pilot case is typically known as Rich-burn/Quench/Lean-bum or RQL combustion). In either case, undesirable NOx generation results from operation of the pilot. For the fuel-lean case, NOx generation occurs within the pilot flame as a result of the high flame temperature required for stable pilot operation. For the fuel-rich case, the oxygen-deficiency of the pilot's fuel-rich environment is not favorable to NOx formation within the pilot; however, NOx generation occurs when the high-temperature highly-reactive mixture exiting the pilot contacts (and reacts with) the additional air required to complete combustion of the fuel.
An alternative method of stabilizing combustion, without high-temperature piloting, is to employ a catalyst. Because a catalyst allows stable low-temperature reaction of fuel and air, all or a portion of the fuel can be reacted at a moderate temperature without NOx generation. The pre-reaction of a portion of the fuel stabilizes the main combustion process by providing preheat, reactive species from fuel partial oxidation or fragmentation, or both. This type of system is referred to herein as “catalytically stabilized combustion,” or simply “catalytic combustion.”
While the effectiveness (stability and low pollutant emissions) of catalytic combustion is well documented and well known in the art, commercial development of catalytic combustion requires resolution of unique new design issues introduced by the catalytic reactor. Dominant among these issues is the need to operate the catalyst at a safe temperature.
For example, in gas turbine engine applications having high turbine inlet temperatures, the adiabatic flame temperature of the final fuel/air mixture generally exceeds catalyst and/or substrate material temperature limits. Accordingly, there is a need to control and limit catalyst operating temperature to a value below the final adiabatic flame temperature. Thus, only a portion of the total fuel can be reacted in the catalyst bed. For high thermal efficiency in a heat engine, such as a gas turbine engine, such control and limitation of catalyst operating temperature must occur without net heat extraction from the engine's working fluid.
While it is possible to stage the fuel injection, so that only a portion of the fuel passes through the catalyst bed (with the remainder being injected downstream), issues arise of fuel injection and mixing in the hot-gas path downstream of the catalyst. Thus, it is generally considered preferable to pass all of the fuel through the catalyst bed. Such use of a single fuel stage, however, requires some means of ensuring only partial combustion of the fuel passing through the catalyst bed. Depending upon catalyst operating conditions, additional means of limiting the catalyst operating temperature may also be required, such as catalyst cooling by combustion air or fuel or both.
One approach to limiting combustion of the fuel/air mixture in the catalyst bed is presented in U.S. Pat. No. 5,235,804 (to Colket et al.). The '804 patent teaches partially reacting the fuel in a fuel-rich fuel/air mixture in the catalyst bed, the reaction being limited by an insufficiency of oxygen to completely convert all the fuel to CO
2
and H
2
O. In the '804 patent, the catalytic reaction is intended to provide both flame stability enhancement to the primary (gas-phase) combustion zone and a means for thermal management. Thermal management means that a portion of the heat of catalytic reaction is extracted from the combustion system, permitting a reduction of the flame temperature in the primary combustion zone, and consequently a reduction in NOx formation.
Because a primary goal of the '804 patent's system is to reduce flame temperature in the primary combustion zone by extracting a portion of the heat of reaction before the primary combustion zone, a key feature of this system is the use of a bypass air stream to provide the oxygen required for combustion completion in the primary combustion zone. This bypass air stream does not obtain heat from the heat of reaction within the catalytic oxidation stage. Thus, a third stream is required for catalyst cooling.
The need for separate cooling and combustion air streams introduces several disadvantages. For example, in a gas turbine engine operating with a turbine inlet temperature at or only slightly lower than the maximum combustion temperature for low NOx emissions, low NOx operation requires that virtually all the compressor air enter the primary combustion zone to limit combustion temperature, with little or no dilution air added to the combustor effluent before the turbine. Thus, little or no compressor air would be available for catalyst cooling in the '804 system. If sufficient cooling air were made available to the catalyst, the turbine inlet temperature would be limited to a value significantly lower than the maximum low-NOx combustion temperature by addition of this cooling air downstream of combustion. Alternatively, catalyst cooling air could exit the system without passing through the turbine, resulting in a system loss of the heat of reaction extracted by the catalyst cooling stream, and a loss of engine efficiency.
In either case, the catalyst cooling air, which will be in close contact with the catalyzed fuel/air mixture during cooling, must be directed around the primary combustion zone while the catalyzed fuel/air mixture is directed into the primary combustion zone. The bypass air must be directed around the catalyst bed and then into the primary combustion zone. While this routing is not prohibitive, it does introduce hardware complexity, space requirements, and design challenges to the overall combustion system.
It has now been found that a system employing a fuel-rich catalytic reaction, with transfer of a portion of the heat of reaction to the ultimate combustion air stream (not to a separate cooling stream), can provide low-NOx combustion with enhanced combustion stability along with well-moderated catalyst oper
Castaldi Marco J.
Etemad Shahrokh
Karim Hasan Ul
Pfefferle William C.
Smith Lance L.
McCormick Paulding & Huber LLP
Precision Combustion Inc.
Yeung James C.
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