Power plants – Combustion products used as motive fluid – Process
Reexamination Certificate
2001-08-31
2003-07-22
Freay, Charles G. (Department: 3746)
Power plants
Combustion products used as motive fluid
Process
C060S778000, C060S039550, C060S723000
Reexamination Certificate
active
06595003
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to methods and apparatus, both devices and systems, for control of NO
x
in catalytic combustion systems, and more particularly to control of thermal or/and prompt NO
x
produced during combustion of liquid or gaseous fuels in the combustor sections of catalytic combustor-type gas turbines, by controlled injection of water in liquid or vapor form at selected locations, orientations, amounts, rates, temperatures, phases, forms and manners in the combustor and/or compressor sections of gas turbines. The ratio of NO
x
ppm reduction to water addition, in weight %, is on the order of 4-20, with % NO
x
reduction on the order of up to about 50-80%, or more, and NO
x
of below 2 ppm being achievable by the inventive process.
BACKGROUND OF THE FIELD
Gas turbines are used for a variety of purposes, among them being motive power, gas compression and generation of electricity. The use of gas turbines for electrical generation is of particular growing interest due to a number of factors, among them being modularity of design, good ratio of generation output capacity to size and weight, portability, scalability, and efficiency. They also generally use low sulfur hydrocarbon fuels, principally natural gas, which offers the promise of lower sulfur oxides (SO
X
) pollutant output. This is particularly important in the case of use of gas turbines for power generation in urban areas, where they are attractive for grid in-fill to cover growing power needs as urban densification occurs.
However, gas turbines operate at high temperature, in the range of from about 1100° C. for moderate efficiency turbines, to 1500° C. for modern high efficiency engines. To achieve these temperatures at the turbine inlet, the upstream combustor section must produce a somewhat higher temperature, generally 1200 to 1600° C. to compensate for air infiltration as a result of seal leakage or the purposeful addition of air for cooling of the metal walls. At these temperatures, the combustion system will produce NO
x
, in amounts increasing as the temperature increases. The increased amounts of NO
x
need to be reduced to meet increasingly stringent emissions requirements.
Current Gas Turbine Systems
A typical gas turbine system comprises a compressor upstream of, and feeding compressed air to, a combustor section in which fuel is injected and burned to provide hot gases to the drive turbine which is located just downstream of the combustor section.
FIG. 1
shows a conventional system of the type described in U.S. Pat. No. 5,183,401 by Dalla Betta et al., U.S. Pat. No. 5,232,357 by Dalla Betta et al., U.S. Pat. No. 5,250,489 by Dalla Betta et al., U.S. Pat. No. 5,281,128 by Dalla Betta et al., and U.S. Pat. No. 5,425,632 by Tsurumi et al. These types of turbines employ an integrated catalytic combustion system in the combustor section. Note the combustor section comprises the apparatus system between the compressor and the drive turbine.
As shown in
FIG. 1
the illustrative combustor section comprises: a housing in which is disposed a preburner; fuel source inlets; catalyst fuel injector and mixer; one or more catalyst sections; and a post catalyst reaction zone. The preburner burns a portion of the total fuel to raise the temperature of the gas mixture entering the catalyst, and some NO
x
is formed there. Additional fuel is introduced downstream of the preburner and upstream of the catalyst and is mixed with the process air by an injector mixer to provide a fuel/air mixture (F/A mixture). The F/A mixture is introduced into the catalyst where a portion of the F/A mixture is oxidized by the catalyst, further raising the temperature. This partially combusted F/A mixture then flows into the post catalyst reaction zone wherein auto-ignition takes place a spaced distance downstream of the outlet end of the catalyst module. The remaining unburned F/A mixture combusts in what is called the homogeneous combustion (HC) zone (within the post catalyst reaction zone), raising the process gases to the temperature required to efficiently operate the turbine. Note that in this catalytic combustion technology, only a portion of the fuel is combusted within the catalyst module and a significant portion of the fuel is combusted downstream of the catalyst in the HC zone.
Each model and type of drive turbine has a required inlet temperature, called the design temperature or turbine inlet temperature. In addition, because cooling air is injected just upstream of the drive turbine, the outlet temperature of the combustor must in fact be higher then the turbine inlet temperature. For proper operation of a gas turbine at high efficiency, the combustor section outlet temperature must be continuously controlled to be maintained at the desired combustor outlet temperature. Typically, the turbine inlet temperature ranges from about 900° C. to about 1250° C. and the required combustor outlet temperature can be as high as 1500° C. to 1600° C. At these high temperatures, additional NO
x
is formed in the post catalyst reaction zone of the combustor section of FIG.
1
. Although the NO
x
level produced in the catalytic combustor is typically low for natural gas and similar fuels, it is still desirable to reduce this level even further to meet increasingly stringent emissions requirements.
The relationship between temperature in the turbine combustor section and NO
x
produced therein is shown in FIG.
2
.
FIG. 2
shows the level of NO
x
that ordinarily is produced in a combustor of the type shown in FIG.
1
. At temperatures below about 1450° C., identified in the figure as Region A, the level of NO
x
produced is below 1 ppm. As seen in
FIG. 2
, at temperatures above about 1450° C., the Region B lower boundary, the NO
x
level rises rapidly, with 5 ppm produced at 1550° C., and even higher levels, 9 to 10 or more ppm, above that temperature. For gas turbines that require combustor outlet temperatures in Region B to achieve the drive turbine design (inlet) temperatures, and where emissions requirements demand emissions levels below 2 ppm, it becomes necessary to further modify the combustion system, including combustion process, apparatus and controls, to maintain the NO
x
level produced in the combustion section of a gas turbine system at lower NO
x
levels, for example, 2 ppm or less.
The top portion of
FIG. 3
is an enlarged schematic of a portion of
FIG. 1
showing the major components of a catalytic combustion system
12
located downstream of the preburner. The cataltic combustion system includes a catalyst fuel injector
11
, one or more catalyst sections
13
and the post catalyst reaction zone
14
in which is located the HC (homogeneous combustion) zone
15
. The bottom portion of
FIG. 3
illustrates the temperature profile and fuel composition of the combustion gases as they flow through the combustor section described above. Temperature profile
17
shows gas temperature rise through the catalyst as a portion of the fuel is combusted. After a delay, called the ignition delay time 16, the remaining fuel reacts to give the full temperature rise. In addition, the corresponding drop in the concentration of the fuel
18
along the same path is shown as a dotted line.
Water Addition in Non-Catalytic Systems
A. Bhargava, et. al, in ASME 99-GT-8, 1999 reports on the addition of water to a fuel-air mixture, combusting it in a flame combustor and measuring the NO
x
level. That work was not done on a catalytic combustion system, but rather was done in a premixed combustion system in which the fuel and air is premixed prior to combustion. Further, the system tested by Bhargava, et. al. was a flame combustor system, as compared to a flameless catalytic system. The Bhargava combustion process relies on recirculation and other mechanisms to stabilize the flame combustion process. The Bhargava et al. report does show that water addition to the premixed fuel air mixture introduced into a flame combustor does reduce the NO
x
level produced by the flame combustor. Flame combustors may be used upstream
Caron Timothy J.
Corr, II Robert A.
Dalla Betta Ralph A.
McCarty Jon G.
Nickolas Sarento G.
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