Power plants – Combustion products used as motive fluid – Combustion products generator
Reexamination Certificate
2002-02-11
2004-12-07
Kim, Ted (Department: 3746)
Power plants
Combustion products used as motive fluid
Combustion products generator
C060S760000, C060S806000
Reexamination Certificate
active
06826912
ABSTRACT:
FIELD AND BACKGROUND OF THE INVENTION
The present invention relates to NO
x
emission reduction in power plants without loss of thermal efficiency, and in particular, to the utilization of flameless oxidation to achieve NO
x
emissions reduction in adiabatic combustors such as those used in gas turbine engines.
Awareness and sensitivity to environmental issues have been increasing around the world, and in their wake, environmental legislation has dictated increasingly strict standards for stationary, propulsive and vehicular power-plant emissions, including the emission of NO
x
gases. NO
x
gases are formed mainly at high temperatures and contribute to smog and acid rain at low levels of the atmosphere, and to stratospheric ozone depletion. Carbon dioxide (CO
2
), another emitted pollutant, is directly linked to the greenhouse effect. Because CO
2
is a natural product of efficient hydrocarbon combustion, there is no way of avoiding CO
2
production in a combustor using conventional fuels. Hence, a reduction in CO
2
emissions from the various kinds of power plants operating with fossil fuels can be obtained only by improvements in the overall thermal efficiency of the system.
Although increased combustor temperatures and pressure ratios improve gas turbine power rating and efficiency, these conditions in conventional combustors tend to promote NO
x
formation, such that there is a natural conflict between energy savings and combustion performance on one hand and reduction of pollutant emission on the other hand. Thus, in order to improve gas turbine efficiency, it has been necessary to develop low-NO
x
combustion systems. These systems can be divided into two groups of methodologies, one based on post-treatment of flue gases to reduce NO
x
levels, and the other based on modification of the internal combustion process. This category can be further divided into two main groups; “dry” techniques in which no additives to the fuel and air supply are applied, and “non-dry” techniques using steam or water injection for flame cooling. The present invention is concerned with the dry techniques of the second category.
The dry techniques include the following main methods:
1. Staged Combustion
Both in the classic technology of fuel staging, which has actually been implemented in commercial service, and in variable geometry (air-staging) technology, the designs introduce additional mechanical complexity and control problems, e.g., moving parts in the case of variable geometry and multi-fuel injection system in the case of fuel-staging. In addition, the pollutant reduction potential is only moderate. In the pilot diffusion flame of a staged combustor, a large amount of NO
x
is still produced. Moreover, radially-staged combustors of current design have pattern factors at the turbine inlet which are far from uniform, such that the potential reduction in NO
x
emissions is limited.
2. Lean Pre-vaporized Premixed Combustion (LPP)
LPP technologies are based on the combustion of a Lean Pre-vaporized and Pre-mixed mixture to reduce the maximum flame temperature. LPP requires operation of a pre-mixer, which can be damaged by flashback or by auto-ignition of the air-fuel mixture. In addition, leakage of fuel or gases from the pre-mixer into the hot section of the combustor may result in severe failures and even explosion of the engine casing. These safety problems appear to be even more pronounced when using liquid fuels, because of the longer time required for complete pre-evaporation. In addition, LPP can not be used at high air inlet temperatures because under such conditions, the mixture is even more susceptible to early auto-ignition. Moreover, it is known that pre-mixed combustion can lead to combustion instabilities that shorten combustor lifetime. In addition, in order to be fully effective under a wide range of operating conditions and to avoid blow-off at idle or partial loading, the LPP system must be coupled to a variable geometry system.
3. Rich Quench Lean (RQL) Combustion
The Rich-Quench-Lean (RQL) combustion methods are based on a rich combustion phase in a reducing combustion environment followed by a lean combustion to complete the burnout. The main advantage of the rich zone is that it allows reduction of NO
x
emissions from Fuel Bound Nitrogen and avoids thermal-NO formation by remaining far in excess of the stoichiometric fuel to air ratio. However, RQL requires physical separation of the combustor into two chambers, rich and lean, as well as an intermediate transition passage known as the quenching zone. RQL technologies also require a special form of cooling for the rich combustion zone. In addition, the primary zone generates a large amount of soot, which radiates heat to the walls, thereby aggravating the cooling problem. The RQL method is limited by the practical difficulty in realizing an effective and uniform quenching between the rich zone and the lean zone. This is due to the fact that in the quenching zone, the stoichiometric ratio is reduced below unity. The requisite degree of complexity to achieve the careful balance between the rich-burn zone and the lean-burn zone over the full range of operation of a gas turbine combustor is subtantial, and consequently, such a balance has yet to be fully realized to date.
4. Catalytic Combustion
Catalytic combustion allows fuel oxidation to take place at temperatures well below the lean flammability limit of the fuel/air pre-mixture. Catalytic combustion can decrease the NO
x
emissions by several orders of magnitude. However, the concept is not easily applicable to non-stationary power-plants and has several drawbacks: catalytic combustion requires relatively high inlet temperatures (depending on the catalyst), and therefore requires a control system for inlet conditions. Because of the premixing, there is also risk of auto-ignition of the premixed mixture before the catalytic bed and consequent flashback, which can lead to catastrophic failures. The catalytic bed increases engine weight and pressure losses. In addition, the catalytic beds of today still reduce drastically the reliability and lifetime of the combustor. Therefore, catalytic combustion is not yet a viable technology, particularly for aircraft applications.
5. Exhaust Gas Recirculation (EGR)
Exhaust gas recycling, whether it is internal or external, is an effective method to reduce flame temperature and, thereby, nitrogen oxide emissions. Unfortunately, the efficiency of this method is limited by the maximal available quantity of recirculated exhaust gas since flame instabilities and ultimately blowout can occur if the burner is operated at overly-high recirculation rates. External recirculation is feasible only if the temperature of the recirculated exhaust gas is relatively low, typically about 850° K, as is the case for industrial furnaces. Recirculation at higher temperatures is impractical, mainly due to external piping limitations and thermodynamic losses. In addition, external recirculation is viable specifically in furnace-type applications because such applications are essentially free of geometrical constraints and weight considerations.
The deficiencies in these alternative combustion technologies are particularly manifest in renewable energy applications, such as the combustion of synthesis gas produced from the gasification or pyrolysis of biomass, including municipal waste. Although renewable energy utilization has become an integral part of the energy policies of the European community and the United States, the efficient exploitation of synthetic gas is not widespread because of various technological difficulties. These technological difficulties mainly arise from the LHV (Low calorific Heat Value) of such fuels, which requires operation at super-adiabatic temperatures. In addition, the relatively-high laminar flame speed makes premixing systems using synthesis gas susceptible to combustion instabilities, including auto ignition and flash-back, both of which have an extremely deleterious impact on safety and on NO
x
emissions.
Consequently
Arfi Patrick
Levy Yeshayahou
Friedman Mark M.
Kim Ted
Yeshayahou Levy
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