Power plants – Combustion products used as motive fluid – Process
Reexamination Certificate
2000-06-12
2002-07-16
Kim, Ted (Department: 3746)
Power plants
Combustion products used as motive fluid
Process
C060S776000, C060S039550, C060S742000, C060S039260, C431S162000, C431S163000
Reexamination Certificate
active
06418724
ABSTRACT:
FIELD
The disclosure herein relates to the field of combustion systems, and more particularly, to a system for reducing emissions in combustion systems.
BACKGROUND
The reduction of harmful emissions has been a longstanding goal in the design of combustion systems, particularly power plants. The predominant emissions from gas turbine power plants are the oxides of nitrogen, or NO
x
. The most prevalent NO
x
emissions are nitric oxide, NO, and nitrogen dioxide, NO
2
.
Although many combustion systems use natural gas, which is one of the cleanest-burning fuels, the NOx levels of these combustion systems remain relatively high. For example, in the standard household kitchen stove, the burner flame releases NOx emissions at about 48 parts per million (ppm). Other devices such as gas barbecue stands, hot water heaters, and Bunsen Burners also release NOx emissions at approximately that level. Therefore, there is a need to further reduce NOx emissions for combustion systems, particularly in power plants, but also in other combustion systems. Although electricity is the cleanest energy option, NOx emissions still occur, concentrated at the source where electricity is generated (i.e., at the power plants).
NOx emissions are produced by a high-temperature reaction of the nitrogen and oxygen contained in air. Reducing the combustion temperature reduces the level of NOx emissions. However, a reduction of the combustion temperature generally slows down the chemical reaction of carbon combustion, thereby generating high levels of carbon monoxide. For this reason, gas turbine combustion systems and natural gas burning power plants usually use a diluent such as steam or water spray in order to reduce the flame temperature.
Mixing steam and water creates turbulence, effectively increasing the diffusivity of the oxygen to be mixed with the fuel for combustion. Water droplets in the flame front evaporate rapidly, creating a phenomena known as “microexplosions.” While the injection of steam or water creates turbulence and reduces the flame temperature, the water vapor becomes an additional inert gas (other than nitrogen) with a high heat capacity. It has been shown that the use of such diluents in a gas turbine significantly reduces NOx emission levels, for example, to lower than 25 ppm.
NOx reduction improvements have stagnated, and a need remains to add further reduction means to combustion systems to reduce the NOx emission levels even more. Existing devices can be expensive and difficult to operate, and sometimes even create other emissions themselves. One such device is a selective catalytic reduction system (SCR), which uses ammonia and a catalyst to reduce the NOx emissions. A selective catalytic reduction system can normally reduce NOx emissions by 90% in the flue gas. However, ammonia itself can be a dangerous substance, and under high temperature conditions, ammonia can react violently with water, causing bums and eye injuries. Ammonia also decomposes into nitrogen and hydrogen, which is an undesired and unproductive result. Therefore, there is a need to further reduce NOx emissions of combustion systems through more practical and effective means.
FIG. 1
shows the structure of a typical diffusion flame. The gaseous fuel enters through a nozzle
10
and is supported by a diffusion flame such as a fuel injector or a candle. The flame structure can be simplified into a paralysis zone
12
(shown cross-hatched in the middle), a fuel diffusion zone
14
, and a flame surface
16
. Oxygen is diffused from the surrounding area toward the flame surface. Under the diffusion flame structure, the combustion reaction can only take place on the flame surface
16
when the fuel and oxidizer reach the stoichiometric ratio. The temperature at the flame surface therefore remains substantially constant and independent of the rate at which fuel is emitted into the nozzle
10
. The change to a higher fuel emission rate would cause a larger flame surface.
The heat from the flame surface transfers back to the center of the fuel supply, causing the fuel to be paralyzed into smaller chemical elements such as carbon and hydrogen. These smaller elements diffuse toward the flame surface to support the combustion process. The combustion heat is divided between the combustion products and ambient inert gas. If the surrounding gas is air, then nitrogen will remove some of the heat without participating in the chemical reaction, thereby lowering the overall flame surface temperature. However, if the gas is pure oxygen, the flame surface will reach its highest possible combustion temperature. A gas that does not react with oxygen also can act as an inert gas, removing heat from the flame temperature without participating in the chemical reaction and thereby further lowering the flame temperature.
FIG. 2
a
illustrates a typical mutual diffusion profile of fuel and oxidizer without combustion. That is,
FIG. 2
a
represents a diffusion phenomena of fuel and oxidizer as a concentration profile with respect to distance from the centerline (i.e., from the source of the fuel or the middle of the paralysis zone) without combustion. The x-axis represents the distance from the source of the fuel. No chemical reaction has taken place in
FIG. 2
a
. In
FIG. 2
b
, when the chemical reaction occurs, in the form of combustion, the concentrations of fuel and oxidizer both approach zero at the flame surface. The concentration of the combustion products is highest at the flame surface. Despite the disappearance of fuel and oxidizer, however, the flame maintains the diffusion rate present when the concentrations of fuel and oxidizer are at the stoichiometric ratio, as illustrated in
FIG. 2
a.
FIG. 3
illustrates the flame height as a function of turbulence level with an increasing fuel nozzle jet velocity. The left side shows a very long flame having a height that increases along with the fuel jet velocity. The flame is a laminar flame. The right side shows the flame as the fuel jet velocity increases. Although the height of the flame decreases at first, an increase of the fuel jet velocity eventually keeps the turbulent diffusion flame at a constant height. With the laminar flame on the left side, the flame diffusion is strictly molecular. Therefore, the surface area of the flame remains proportional to the fuel ejection rate from the fuel nozzle. When the velocity continues to increase, it induces turbulent mixing which greatly increases the molecular diffusivity. The jet of the fuel nozzle finally reaches a condition known as a similarity flow, which means that the flame is at a constant flame height. The similarity flow occurs when the turbulent mixing profile becomes independent of the magnitude of the velocity.
When the chemical reaction rate is slower than the turbulent diffusion rate, the flame will be lifted from the fuel nozzle, creating a blowout condition.
FIG. 4
illustrates combustion flame profiles with respect to blowout conditions.
FIG. 4
a
illustrates the condition of fuel with an extremely high jet velocity. In order to improve the chemical reaction rates and stabilize the flame, some of the combustion products are recirculated through turbulent mixing as chemical reaction seed material. The bell-shaped profile in
FIG. 4
a
illustrates the root of the flame, and the cone-shaped region represents the turbulent combustion of fuel and air. When the velocities of both fuel and air increase, the root of the flame lifts away from the nozzle, leaving certain recirculation chemical species to support the combustion. As illustrated in
FIG. 4
b
, when the velocity increases, the recirculation is reduced, causing the flame to lift away from the nozzle and creating a neutral condition.
FIG. 4
c
illustrates the results of a maximum increase in the velocities of both the jet and air. Chemical species can no longer recirculate, and the flame completely lifts from the nozzle, creating a blowout condition. Candles illustrate this phenomena well: when one blows gently on a candle, the combustion rate of the candle increas
Cheng Power Systems, Inc.
Kim Ted
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