Power plants – Combustion products used as motive fluid – Process
Reexamination Certificate
2001-10-26
2004-04-13
Yu, Justine R. (Department: 3746)
Power plants
Combustion products used as motive fluid
Process
Reexamination Certificate
active
06718772
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to methods and apparatus, both devices and systems, for control of NOx in catalytic combustion systems, and more particularly the control of NOx produced downstream of the catalytic reaction zone of a combustor, while at the same time maintaining the same power output yet low CO, by reducing combustion residence time, inter alia, through control of the location of the homogeneous combustion wave.
BACKGROUND
Gas turbines are used for a variety of purposes, among them: motive power; gas compression; and generation of electricity. The use of gas turbines for electrical generation is of particular and growing interest due to a number of factors, among them being modularity of design, generation output capacity to size and weight, portability, scalability, and efficiency. In addition, gas turbines generally use low sulfur hydrocarbon fuels, principally natural gas, which offers the promise of lower sulfur oxides or SOx pollutant output. This is particularly important in urban areas that use, or can use, gas turbines for power generation, as they are attractive for power-grid supply in-fill to cover growing power needs as urban densification occurs.
Gas turbines tend to operate with a high turbine inlet 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 combustion system must produce a somewhat higher temperature, generally 1200 to 1600° C. as a result of some air addition due to seal leakage or the purposeful addition of air for cooling of portions of the gas turbine structure. At these temperatures, the combustion system will produce NOx. The amount of NOx produced increases as the temperature increases. However, to meet ever more stringent emissions standards, turbine operating conditions must be controlled so that the amount of NOx produced does not increase.
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 located just downstream of the combustor.
FIG. 1
shows such a prior art system employing a catalytic combustion system in 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 NOx 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 type of drive turbine has a designed inlet temperature, called the design temperature. For proper operation of a gas turbine at high efficiency, the system or operator must control the outlet temperature of the combustor section to keep the temperature at the design-temperature of the drive turbine. This can be a very high temperature, in the range of 1100° C. for moderate efficiency gas turbines and as high as 1400 to 1600° C. for modern high efficiency engines. As shown in
FIG. 1
, at these high temperatures, NOx forms in the “Post catalyst reaction zone” of the combustor section. Although the NOx level produced in the post catalytic combustion zone 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.
FIG. 2
shows the relationship between the temperature in the post catalyst reaction zone and the amount of NOx produced, for a catalytic combustion system of the type shown in FIG.
1
. At temperatures below about 1450° C., identified in the figure as Region A, the level of NOx produced is below 1 ppm. As seen in
FIG. 2
, at temperatures above about 1450° C., the Region B lower boundary, the NOx level rises rapidly, with 5 ppm produced at 1550° C., and even higher levels above that temperature, on the order of 9-10 ppm or higher.
The formation of NOx at a high temperature is a kinetically controlled process. A portion of the NOx, called “Prompt NOx,” or “Fennimore NOx,” forms in the region of the combustor where rapid reactions occur. The amount of Prompt NOx formed depends on the fuel-to-air ratio and final flame temperature, but this Prompt NOx stops forming once the flame-front has consumed most of the fuel. A second pathway to the formation of NOx is the “Thermal NOx” or “Zeldovich pathway,” in which NOx is formed continuously at high temperatures and in quantities dependant only on time and temperature. In typical gas turbine systems with residence times in the range of 10 to 20 ms (milliseconds), the prompt and thermal pathways produce roughly the same amount of NOx.
In most combustion processes, reaction of the fuel occurs in a flame that is fixed in location by a flame holder. The flame holder can be either a physical object or an aerodynamic process to anchor or stabilize the flame. Physical elements include bluff bodies, v-gutters, or other such mechanical parts that recirculate the gas stream to stabilize the flame. Aerodynamic stabilizers include physical elements such as swirlers and vanes and such modifications as expanded flow area to stabilize the flame. Flame temperature, temperature profile, physical dimensions of the combustor, and other such features determine the thermal NOx formation. For example, the designer cannot change thermal NOx levels without changing the volume or length of the combustor or the position at which the combustor design anchors the flame.
In the case of a catalytic combustion system using the technology described in the above-identified U S Patents, and other references, only a portion of the fuel is combusted within the catalyst and a significant portion of the fuel is combusted down stream of the catalyst in a post catalyst homogeneous combustion (HC) zone.
FIG. 3
schematically illustrates the downstream HC zone.
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 catalytic 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 unit
Dalla Betta Ralph A.
Nickolas Sarento G.
Velasco Marco A.
Yee David K.
Belena John F.
Catalytica Energy Systems, Inc.
Morrison & Foerster / LLP
Yu Justine R.
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