U.S. patent application number 10/085715 was filed with the patent office on 2003-08-28 for system for controlling flue gas exit temperature for optimal scr operations.
Invention is credited to Albrecht, Melvin J., Rogan, John B..
Application Number | 20030159662 10/085715 |
Document ID | / |
Family ID | 27753705 |
Filed Date | 2003-08-28 |
United States Patent
Application |
20030159662 |
Kind Code |
A1 |
Albrecht, Melvin J. ; et
al. |
August 28, 2003 |
SYSTEM FOR CONTROLLING FLUE GAS EXIT TEMPERATURE FOR OPTIMAL SCR
OPERATIONS
Abstract
A system for maintaining an optimal flue gas inlet to a boiler
mounted SCR assembly in the flue of the boiler is accomplished by
mixing the normal inlet feedwater to an economizer of the boiler
with near saturation water from downcomers of the boiler to thereby
raise the temperature of the flue gas passing across the economizer
and raising the SCR inlet to the desired optimal SCR operation
temperature.
Inventors: |
Albrecht, Melvin J.;
(Homeworth, OH) ; Rogan, John B.; (Roswell,
GA) |
Correspondence
Address: |
Eric Marich
Patent Department
The Babcock & Wilcox Company
20 S. Van Buren Avenue
Barberton
OH
44203
US
|
Family ID: |
27753705 |
Appl. No.: |
10/085715 |
Filed: |
February 27, 2002 |
Current U.S.
Class: |
122/468 ;
122/155.1; 122/166.2; 122/467; 122/479.2 |
Current CPC
Class: |
F22B 37/008
20130101 |
Class at
Publication: |
122/468 ;
122/467; 122/479.2; 122/155.1; 122/166.2 |
International
Class: |
F22G 007/00 |
Claims
We claim:
1. A system for maintaining an optimal flue gas temperature to the
inlet of an SCR assembly mounted therein comprising; a boiler
having an economizer mounted in the flue thereof; a boiler
downcomer having water therein near saturation temperature; an
economizer water inlet providing a mixture of normal feedwater and
water from said downcomer; and a control system for mixing the
water inlet to insure that the flue temperature of the inlet to the
SCR is optional.
2. A system as set forth in claim 1 wherein said control system
includes a temperature sensor mounted at the inlet of the SCR to
monitor flue temperature and a controller to vary the quantity of
water from said downcomer to said economizer water inlet in
response to a difference between optimal SCR inlet temperature and
actual inlet temperature.
3. A system as set forth in claim 3 including a controller
connected to said comparator for controlling a valve varying the
water flow from said downcomer to said economizer inlet.
4. A system as set forth in claim 3 including a controller
connected to said comparator for controlling a valve varying the
water flow from said downcomer to said economizer inlet.
5. A system as set forth in claim 4 including a bypass for
connecting the water flow from said downcomer to the water outlet
of said economizer when the SCR inlet temperature was optimal.
6. A boiler water recirculation system comprising; a boiler having
downcomers and an economizer connected to a boiler drum; and said
economizer having an inlet for mixing normal feedwater with water
from said downcomers.
7. A boiler recirculation system as set forth in claim 6 wherein
the mixing of water from said downcomer in said inlet is done with
a fixed feedwater flow.
8. A boiler recirculation system as set forth in claim 6 wherein
the amount of water from said downcomers is proportional to the
difference between actual and desired flue gas temperature from the
boiler.
Description
FIELD AND BACKGROUND OF THE INVENTION
[0001] The present invention is generally drawn to boilers using
SCR (Selective Catalyst Reduction) systems at the flue exhaust to
clean the exhaust gas thereby and more particularly to the
optimized temperature operation of same.
[0002] In operating a boiler with a Selective Catalytic Reduction
system, or SCR, at the flue gas exhaust, the reactiveness of the
catalyst is dependent upon the flue gas temperature entering the
catalyst reactor. A given catalyst will have maximum performance
when it is operated at the temperature of peak performance (TPP).
As an example, in a typical SCR for NO.sub.X removal, the
temperature of peak performance (typically 650.degree. F.) at the
reaction of ammonia with NO.sub.X present in the flue gas is
optimized and the amount of the ammonia needed for the catalytic
reaction is minimized. Therefore, for economic reasons the desired
gas temperature entering the catalyst reactor should be maintained
at the TPP at all loads. Also, maintaining the desired flue gas
temperature reduces the formation of ammonia and/or sulfate salts
within the ammonia injection grid (AIG) and the catalyst.
[0003] However, as boiler load decreases, the boiler exit gas
temperature will drop below the TPP. To increase the gas
temperature to TPP, current practice has been to use an economizer
gas bypass. The economizer gas bypass is used to bypass the hotter
gases upstream of the economizer to the cooler gas that leaves the
economizer and mixes with the flue gas. By controlling the amount
of gas that passes through the bypass system, a boiler exit flue
gas temperature of approaching the TPP can be maintained at the
lower boiler loads which normally results in the flue gas
temperature below TPP.
[0004] Also, systems for mixing economizer feedwater with hot water
at the inlet of the economizer are known. These systems were known
as the Off Line Circulation System and were developed in the mid
1980s. However, this system was not designed for increasing the
flue gas temperature from the economizer. This system's main
purpose was to reduce the economizer inlet headers thermal shock
that occurs during boiler start up and shut down and to eliminate
the stratification/subcooling temperature effects that occur in the
furnace walls of the boiler when the boiler is off line and put
into hot standby.
[0005] Thus, what was needed was a simpler system that required
less physical space to obtain the desired flue gas temperature to
the SCR at various boiler loads. With the known flue gas bypass
systems currently used for SCR application, static mixing devices,
pressure reducing vanes/plates and thermal mixing devices were
required to make the different temperature flue gases mix before
the gas mixture reaches the inlet of the catalyst reactor. In most
applications, obtaining the strict mixing requirements for flow,
temperature and the mixing of the reagent (if received) before the
catalyst reactor was often difficult.
SUMMARY OF THE INVENTION
[0006] The present invention solves the problems associated with
prior art devices as well as others by providing a boiler water
recirculation system where the variation in the gas flow and
temperature at the economizer outlet is less severe than with a
flue gas bypass system, making it easier to meet the gas mixing
requirement for the catalyst reactor at the optimal inlet
temperature.
[0007] To accomplish this, the invention uses the economizer to
increase the outlet temperature of the flue gases to the desired
temperature at the lower boiler loads by using a boiler
recirculation system to provide higher temperature water from the
circulation system that is used to cool the furnace walls. The
recirculation system supplies near saturation water from the
downcomers of drum circulation boiler applications, or for
once-through boiler applications, the fluid is obtained from a
fluid mix location in the upper region of the lower furnace. In
either a drum or once through boiler application, the higher
temperature water is transferred to the economizer inlet and mixed
with the boiler's economizer normal feedwater inlet flow. The
mixture of the two fluid streams results in a higher temperature
fluid in the economizer that can be used to increase the flue gas
temperature leaving the economizer. With proper adjustment of the
different fluid streams to the economizer, the desired flue gas
temperature can be obtained for any boiler load. The amount of near
saturation water (or higher temperature furnace wall water for a
once-through boiler) from the boiler recirculation system is
controlled throughout the load range. Calculations have shown that
no catastrophic effects (critical heat flux or tube failures) on
the cooling of the boiler's furnace walls will occur in the use of
this system.
[0008] In view of the foregoing it is seen that one aspect of the
present invention is to provide stable flue gas temperature control
system based on economizer water inlet temperature.
[0009] Yet another aspect of the present invention is to provide an
increased temperature economizer gas outlet responsive to increased
economizer water inlet temperature.
[0010] These and other aspects of the present invention will be
more fully understood upon a review of the following description of
the preferred embodiment when considered in conjunction with the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] In the drawings:
[0012] FIG. 1 is a schematic of a boiler water/steam recirculation
system utilizing the increased temperature economizer water inlet
of the present invention.
[0013] FIG. 2 is a schematic of the control system used to increase
flue gas temperature in response to increased economizer water
inlet temperature.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0014] Referring to FIGS. 1 and 2 of the drawings, the present
invention uses a different approach to obtaining a TPP boiler exit
flue gas temperature. In a normal boiler application, the water
side of the economizer is used to cool the flue gas that flows over
the surface that is installed in the boiler. Here, the boiler
recirculation system (10) is modified to have higher temperature
water near saturation from downcomers (12) connected by a bypass
line (14) to an inlet (16) of an economizer (18). The inlet (16) is
a tee inlet with the other inlet of the tee providing normal
feedwater flow from line (20). The flow through line (14) is
provided by a pump (22) which has monitoring, flow F and pressure P
sensors mounted on both sides of the pump (22). An economizer (18)
bypass line (24) is provided as shown in dotted lines on FIG. 1, to
recirculate the downcomers (12) saturated water back thereto from
drum (26) when no increased water temperature is needed for mixing
with the normal economizer (18) feedwater from line (20).
[0015] With particular reference to FIG. 2, it will be seen that
the operation of this invention is as follows. An SCR (28) located
on an outlet (30) of a boiler flue (32) needs the optimum flue gas
temperature supplied to the inlet thereof for optimal operation as
was described earlier. To accomplish this end, a temperature sensor
(34) is mounted in a flue (32) near the entrance to the SCR (28)
ton monitor the flue gas temperature. A signal indicative of the
actual flue gas is transmitted along line (36) to comparator
station (38) having a set point signal of the optimum temperature
inputting thereto along line (40). Any difference in these two
signals develops an error signal e along line (42) to a controller
(44) which controls the opening of a gate valve (46) to control the
quantity of saturation temperature water sent along lone (14) to
the tee (16) to be mixed with the normal temperature feedwater from
line (20) and supplied to the economizer (18).
[0016] The bypass line (24) is closed by normally closed valve (48)
being maintained closed by the error signal e being transmitted
along line (50) to a NAND gate (52). As long as there is a positive
signal from comparator (38) to the NAND gate (52), there will be no
control signal passed therefrom along line (54) to the valve (48)
and it will remain shut. When the error e signal becomes O
indicating a flue gas temperature is at the optimum, a O signal
will enter NAND gate (52) along line (50) and a O signal will enter
the NAND gate (52) along line (56)from the controller (44). This
will cause an output control signal to be transmitted along line
(54) to normally closed valve (48) to open and a control signal
along line (58) to the normally open valve (46) to close. This
establishes flow back to the downcomers (12) bypassing the
economizer (18) until the flue (32) temperature falls below
650.degree. F. and saturated water will again be mixed with normal
feedwater to the economizer (18) inlet.
[0017] Clearly as more saturated water in inputted to the inlet of
the economizer (18) the flue temperature across the economizer (18)
will rise and, when mixed with normal flue gas, will raise the
temperature to the temperature of peak performance at the SCR (28)
inlet.
[0018] Certain modifications and construction details have been
deleted herein since they are obvious to those of ordinary skill in
the art area and for the sake of conciseness and readability but
are properly within the scope of the following claim.
* * * * *