Power plants – Combustion products used as motive fluid
Reexamination Certificate
2000-08-11
2002-04-16
Freay, Charles G. (Department: 3746)
Power plants
Combustion products used as motive fluid
C060S039550
Reexamination Certificate
active
06370862
ABSTRACT:
FIELD
This patent specification relates to the field of gas turbine engines, and more particularly to a steam injection nozzle system for the combustion liner of a gas turbine engine for enhancing the power output and efficiency of the gas turbine engine.
BACKGROUND
A gas turbine engine is a heat engine that is operated by a gas rather than being operated, for instance, by steam or water. The two major application areas of gas turbine engines are aircraft propulsion and electric power generation. A detailed description of gas turbines is provided in William W. Bathie,
Fundamentals of Gas Turbines
(John Wiley & Sons, Inc. 1996), which is hereby incorporated by reference.
The burner section of a gas turbine includes a combustion chamber which is designed to burn a mixture of fuel and air and to deliver the resulting gases to the turbine at a temperature not exceeding the allowable limit at the turbine inlet. The burners, within a very limited space, must add sufficient heat energy to the gases passing through the engine to accelerate their mass enough to produce the desired thrust for the engine and power for the turbine.
Combustion chambers are lined with combustion liners.
FIG. 1
illustrates a typical combustion liner. The holes supply primary, secondary, and cooling air for the gas turbine operations. As illustrated in
FIG. 1
, the fuel comes from one end of the combustion liner (in this case, from the left of the page) supplemented by a pre-mixed airflow. A plurality of jets, indicated by holes along the combustion liner, supply turbulence and a secondary air supply for the combustion regions. These jets are depicted by the holes of Sections 1, 2, and 3, which operate to satisfy various air requirements for achieving a quality combustion system.
Section 4 is usually a much larger area, with the air being supplied by higher-level jets to create turbulent mixing. In this process, the combustion flame products are mixed with the compressor air to reach a final homogeneous working fluid at designated operating turbine inlet temperatures (TIT). There are also many small cooling holes along the wall of the liner to keep the liner metal temperature down. The final flow of the combustion liner exhaust is directed into a transition piece which connects the air flow cross section of a typical combustion can (typically a cylinder-shaped burner) to a turbine nozzle bank segment. This design is typically used with all or nearly all modern, ground-based gas turbines.
Heavy duty gas turbines with long combustion liner designs frequently use reverse flow combustion liners to accommodate a long flame. Reverse flow combustion liners are used in other combustion liner variations as well. In general, to compensate for the length of the combustion liner, compressor discharge air ordinarily flows backwards into the envelope or combustion wrapper of the combustion cans, then reverses its direction, moving through the combustion liner and reaching a designated turbine inlet temperature at the first row of nozzles in the gas turbine. The gas turbine's performance is related to the turbine inlet temperature, and the first stage nozzle and blades have maximum metal temperature limitations. These temperature limitations are determined by the materials used. Even though today's turbines use single crystal metals, they must still employ various cooling means to keep the metal temperature down.
The combustion liner design has been carefully engineered over the years so that the flame can operate with a high ratio of turndowns to provide an efficient startup and to provide low load operating conditions. Using a diffusion flame, the fuel is mixed with an oxidizer before combustion. The oxidizer then diffuses to the flame envelope, allowing the oxidizer and the fuel to reach a stoicheometric ratio where the flame resides. A diffusion flame is the preferred method for increasing the diffusion rate of oxidants to the flame envelope. Because the flame always resides at the stoicheometric ratio surface, the gas turbine inlet temperature is controlled by running dilution air downstream along the length of the flame to reach an appropriate mixture and a designated homogenous design temperature for the gas turbine. The dilution holes are strategically located to provide an air jet which creates internal turbulence. Because this turbulence causes the pressure to drop, it also reduces the fluid working potential. The air jets must therefore attempt to provide turbulence at a level that will result in a minimal pressure drop across the combustion liner to avoid loss of the fluid working potential.
One area of gas turbine engines which needs improvement is the area of power output and efficiency. The Advanced Cheng Cycle, conceived by the Applicant of the present disclosure, is a massive steam-injected gas turbine that uses steam to augment its power output. Steam is injected into the gas turbine ahead of the first turbine nozzle bank just downstream of the combustion region. Steam injection has previously been employed as a power boost on some gas turbine engines. The injection, however, has been traditionally limited to about 5 to 9% of the air flow in order to avoid causing compressor stall and flame instability.
In previous systems, steam and air has been injected before the combustion liner; that is, steam was not injected directly into the combustion liner. The steam injection point has previously been the compressor exit plenum area. By this method, steam enters the combustor through all combustor liner admission areas: the primary zone, the dilution zone, and the cooling louvers. The combustor pressure drop increases with increased steam flow, depending on the steam-air ratio. Therefore, although the effect on combustion efficiency could be minimal, the additional mass going through the holes on the combustion can requires a higher pressure drop. Furthermore, steam carried in the air modifies the combustion of air by reducing the relative concentrations of both oxygen and nitrogen. Dilution of the oxygen in the composition lowers the combustion reaction rate.
Moreover, in recent years, air pollution has emerged as a major concern in the field of chemical engineering, and reducing air pollution is a secondary goal of the disclosure herein. The predominant emissions from gas turbines are the oxides of nitrogen, or NO
x
, which are one of today's leading components of air pollution. The most prevalent NO
x
emissions are nitric oxide, NO, and nitrogen dioxide, NO
2
. The diffusion flame temperature emanating from gas turbines produces these NO
x
emissions. If the flame front receives insufficient oxygen or turbulence, the resulting concentration of carbon monoxide can become an additional factor in highly polluted air.
In light of the air pollution problem, gas turbines now use a dry, low NO
x
, combustion liner. This type of combustion liner maintains both a lower pressure level and a higher turbulence level than ordinary combustion liners in order to achieve the low flame temperature necessary to reduce NO
x
emissions.
The Advanced Cheng Cycle demonstrates that NO
x
emissions have decreased substantially from previous simple cycle combustion liner designs. However, the reverse flow combustion liner differs somewhat from the aeroderivative gas turbine design by allowing steam to easily mix with the compressor discharge before it enters the combustion liner.
Some of the current gas turbine manufacturers have injected steam concentric to the fuel nozzle as a means of lowering the NO
x
emissions. The optimum steam injection using that particular method reaches a limit of flame stability at a steam-to-fuel ratio of approximately 1:1. However, a problem with this amount of steam is injection into a combustion liner is that power augmentation suffers.
A secondary method of steam injection involves injecting the steam at the plenum chamber. Here, the steam will mix with the compressor air and the mixture will flow around the combustion liner, entering the combustion liner through the various dil
Cheng Power Systems, Inc.
Cooper & Dunham LLP
Freay Charles G.
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