Power plants – Combustion products used as motive fluid – With exhaust treatment
Reexamination Certificate
2000-05-31
2001-10-16
Freay, Charles G. (Department: 3746)
Power plants
Combustion products used as motive fluid
With exhaust treatment
C431S005000, C431S009000, C431S202000
Reexamination Certificate
active
06301875
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to heaters for raising the temperature of a gas flow, and in particular heaters for efficiently heating turbine exhaust gases in a non-polluting manner.
It is well known to use gas burners for raising the temperature of turbine exhaust gas (TEG) sufficiently (typically by between 100°-600° F.) so that the TEG can be used to generate steam, for example. Generating steam with TEG is efficient because the energy that would otherwise be needed for reaching the temperature of the incoming TEG is saved.
In the past, a variety of TEG heaters have been proposed, such as those disclosed in U.S. Pat. Nos. 4,767,319 and 4,462,795, for example.
A recurring problem with known TEG heaters is that they release pollutants, particularly CO. Significant amounts of CO are a byproduct of known TEG heaters because there is insufficient time to convert initially formed CO from combusting the heating gas into CO
2
during the former's residence time in the flame or combustion zone of the heater. As part of the overall effort to protect the environment, regulations have therefore been promulgated in the U.S. which now limit the release of CO from TEG heaters to 0.1 lb/million btu generated by the heater. This is a stringent requirement in and of itself. It has become more difficult to attain with increased turbine efficiencies, which resulted in a decrease in O
2
concentration (by volume) in the TEG. To alleviate this, it has been proposed to augment the TEG heater with additional air. Although this helps to reduce CO emissions, since more O
2
is made available to effect a complete combustion of the heating gas, it lowers the efficiency of the heater because the augmenting air must be heated from ambient to the temperature of the incoming TEG.
Achieving complete combustion of the CO generated by the TEG heater becomes still more difficult when steam is injected into the turbine, which in turn reduces the O
2
concentration in the TEG.
It has previously been recognized that CO emissions are reduced by increasing the residence time for the CO in the combustion zone of the TEG heater because this enhances the likelihood that CO will find an available O
2
molecule and be converted to CO
2
. Thus, for several years a TEG heater has been in use which consisted of a flame shield that extended across the TEG duct, had a gas supply pipe positioned on a center line of the duct, and had a flame shield defined by plates which diverged (in the downstream direction) from the gas pipe towards the walls of the duct. Spaced-apart slits were arranged in the plate through which TEG could flow into the combustion zone located downstream of the flame shield. Diverging heating gas jets were injected into the combustion zone to generate turbulence and effect a better mixing of heating gas with the TEG. Although this TEG heater worked well, it is unable to meet today's tightened CO emissions standards.
Other known TEG heaters have attempted a variety of different approaches to reduce CO emissions. These attempts principally concentrated on efforts to discharge the heating gas into the TEG flow to maximize turbulence and thereby a mixing of the TEG with the heating gas and/or augmenting the TEG with air to provide greater O
2
concentrations for oxidizing the heating gas. Still, the desired reduction in CO emissions to no more than 0.1 lb/10
6
btu in an energy efficient manner became difficult to attain.
SUMMARY OF THE INVENTION
In TEG heaters, the oxygen for burning the heating gas is obtained from the TEG. As turbines became more efficient, and more water was injected into them, the relative concentration of O
2
decreased, resulting in a corresponding increase in CO emissions due to its incomplete oxidation in the combustion zone of the heater. One way to achieve a greater conversion of CO to CO
2
is to use augmented combustion air. However, as mentioned above, this undesirably decreases the efficiency of the heater.
Detailed investigations demonstrated a link between CO formation, the local flame temperature distribution, and the residence time of the heating gas in the combustion zone. It was observed that CO formation resulted from a cooling of flame partial products by incoming TEG prior to complete oxidation. A reduction of CO discharge was observed when the residence time of the heating gas (and therewith the CO) in the combustion zone behind (downstream of) the flame shield was increased and the mixing of TEG with the heating gas in the combustion zone was limited.
Residence time could be increased by enlarging the flame shield, but that increases TEG velocities and leads to undesirable turbulence. Thus, the inventors set out to find ways to increase the residence time for the heating gas while reducing the flow of excess TEG into the combustion zone and keeping turbulence low. Excellent results were obtained by forming a relatively long, narrow combustion zone which kept the mixing of TEG with the heating gas to the minimum level needed for the complete oxidation of the gas during its residence time in the combustion zone of the heater.
A flame shield configuration was developed which resulted in the formation of two successive recirculation patterns in the combustion zone. This provides for an increased residence time in a narrow flame corridor without excessive blockage of the TEG flow or undesirable flame patterns. While typical residence times for earlier flame shields were approximately 50 msec in the recirculation zone, the flame shield incorporated in the TEG heater of the present invention achieves residence times which are as much as three times longer. Additionally, by diverting the bulk of the TEG flow towards the end of the flame or combustion zone, where the oxidation of heating gas is effectively complete, CO emissions from the TEG heater are further reduced.
Thus, a TEG heater constructed in accordance with the invention provides a reduction in CO emissions of up to about 50% over those attained with earlier burners, including the one installed by the assignee of the present invention some years ago.
In addition, the present invention assists in minimizing NO
x
generation and emissions. NO
x
in TEG duct heaters can be reduced by reburning incoming NO
x
from the TEG by reverse reactions from NO
x
to N
2
in UHC-rich flames. Such reverse reaction rates are relatively slow and, therefore, the extent of NO
x
reductions from reburning is a function of the residence time of the NO
x
in the reburn zone. For TEG duct burners, the reburn zone is effectively the combustion zone behind (downstream of) the flame shield.
The present invention therefore also reduces NO
x
emissions.
A TEG heater constructed in accordance with the invention is installed in a duct, bounded by duct walls, through which the TEG flows in a downstream direction and includes a flame shield that extends along a line, e.g. the line formed by the horizontal center plane of the duct (for simplicity hereinafter usually referred to as “line” or “center line”), at least partially across the duct. The shield has a plate the ends of which are spaced apart from and are substantially parallel to the center line. The plate has a plurality of spaced-apart slits, arranged substantially parallel to the center line and respective edges of the plate. The edges are spaced apart from the proximate duct walls. A gas supply conduit is connected with the plate and extends along the center line at least partially across the duct. The pipe has a plurality of spaced-apart orifices which face in a downstream direction, are in flow communication with a downstream side of the plate, and arc arranged for discharging heating gas jets parallel to the downstream direction (and therefore also parallel to the center plane of the duct). Baffle plates extend from the respective duct walls towards the center line and end in baffle edges which are spaced apart from and parallel to the edges of the flame shield plates.
This TEG heater forms an elongated combustion zone which has two recirculation
Ahn Kenneth Y.
Backlund Jonathan C.
Fiorenza Enrico E.
Coen Company, Inc.
Freay Charles G.
Townsend and Townsend and Crew
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