US20100251975A1

US20100251975A1 - Economical use of air preheat

Info

Publication number: US20100251975A1
Application number: US12/416,498
Authority: US
Inventors: Glenn D. Mattison
Original assignee: Alstom Technology AG
Current assignee: General Electric Technology GmbH
Priority date: 2009-04-01
Filing date: 2009-04-01
Publication date: 2010-10-07
Also published as: CA2757021A1; KR20120003002A; TW201043877A; MX2011010426A; EP2414735A1; WO2010114647A1; JP2012522961A; CN102378879A

Abstract

An economical heat recovery system [100] is described for use in a boiler [26] includes an air preheater [150] that receives hot flue gasses [FG1] and inlet air and creates heated air [A2] and incremental air [A2′]. The incremental air [A2′] is provided to a regenerative heat capture and transfer (RHCT) system [300] positioned to receive the incremental air [A2′] from the air preheater [150]. The RHCT includes a heat exchanger [310] that preheats feed water [WF1] for the boiler [26]. Since a heat exchanger [310] receives clean air as opposed to those of the prior are, it may be made more efficient with more heat exchange units in closer proximity, since there is little chance of blockage. Also, there is less maintenance with the present invention.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to U.S. patent application “Reagent Drying Via Excess Air Preheat” by Kevin O'Boyle and incorporates this patent application by reference as if set forth in its entirety herein. The O'Boyle patent application is being filed on the same day as the present patent application and both applications have the same owner.

FIELD OF THE INVENTION

The present invention is directed to a system for efficiently capturing wasted heat from a flue gas output of a boiler. More particularly, the present invention is directed to a system for capturing wasted heat from a flue gas output of a boiler to preheat the feed water to the boiler.

BACKGROUND

Many power generation systems are powered by steam generated via coal or oil fired boilers. These power generation systems will often incorporate exhaust processing and heat recovery (EPHRS) systems to reduce flue gas emissions and/or recover heat energy expelled via the flue gas stream from the boiler.
A typical power generation system is generally depicted in the diagram shown as FIG. 1. FIG. 1 shows a power generation system 10 that includes a steam generation system 25 and an exhaust processing and heat recovery system (EPHRS) 15 and an exhaust stack 90. The steam generation system 25 includes a boiler 26.
The EPRS 15 includes an air preheater 50, a particulate removal system 70 and a flue gas scrubber system, shown here as a wet scrubber system 80. The particulate removal system 70 may be, for example, an electrostatic precipitator (ESP), a fabric filter system (Bag House) or the like. A forced draft (FD) fan 60 is provided to introduce air into the cold side of the air preheater 50.
The air preheater 50 is a device designed to heat air before it is introduced to another process such as, for example, combustion in the combustion chamber of a boiler 26. The air preheater heats the air stream input A2 to the boiler 26 capturing/recovering heat expelled from the boiler 26 via the flue gas stream from the boiler. By recovering heat from the flue gas (FG1) emitted from the combustion chamber of the boiler 26 the thermal efficiency of the boiler 26 can be increased and the amount of heat lost through the flue gas FG4 out of stack 90 is reduced.
In general, it is desirable to reduce the temperature of the flue gas FG2 leaving the air preheater 50 and before it is introduced to processing devices such as, for example, an electrostatic precipitator (ESP) used as particulate removal system 70. By increasing the airflow Al passing into the air preheater 50, it is possible to extract more heat from the flue gas stream FG1 and thereby further reduce the temperature of the flue gas stream FG2 that reaches the ESP 70.
However, this process also results in an increased volume of available heated air. It is often not feasible in a typical power generation system to direct the entire flow of heated air into the combustion chamber of the boiler 26 without negatively affecting the efficiency of the boiler 26.
One alternative for increasing the efficiency of the boiler 26 has been to introduce an “economizer” section 55 between the boiler 26 and the air preheater 50. This economizer section 55 is a type of heat exchanger used to capture heat from an air stream and transfer the heat into a fluid stream, such as, for example, water. Further, economizers are typically designed with finned tubes that improve the transfer of heat. In boilers, economizers are heat exchange devices that heat fluids, usually water, up to but not normally beyond the boiling point of that fluid. Economizers are so named because they can make use of the enthalpy in fluid streams that are hot, but not hot enough to be used in a boiler, thereby recovering more useful enthalpy and improving the boiler's efficiency. The economizer is a device that is coupled to boiler 26 which saves energy by using the exhaust flue gases FG from the boiler 26 to preheat/heat the feed water WF from a water supply 65.
FIG. 1 it shows that economizer 150 is configured to receive the flue gas stream FG from the boiler 26, and to pass the flue gas stream FG1 on to the air preheater 50. In this example, the economizer 55 acts to transfer heat from the flue gas stream FG to feed water WF that is provided to the boiler 25. This allows “pre-heated” water to be introduced into the boiler 25, thereby reducing the need for additional heat energy to heat the boiler water to a desired temperature.
The flue gas stream FG/FG1 will generally contain a substantial level of particulate matter. This particulate matter is typically removed from the flue gas stream after the flue gas stream FG2 has passed through the particulate removal system 70. However, until the flue gas stream is subjected to particulate removal operations, the presence of particulate matter in the flue gas stream is typically high. Since the economizer 55 receives the flue gas stream prior to it being subjected to dust removal operations, it is possible for particulate matter to get caught in between the heat exchange elements of the economizer 55 if the spacing between the heat exchange elements is not sufficient. To avoid having particulate get caught between the heat exchange elements, it is important that the spacing between heat transfer elements of the economizer be large enough to allow most, if not all, particulate matter to freely pass through the economizer 55. This large spacing leads to inefficiency.
If the spacing between heat transfer elements was smaller, particulate matter that are too large to pass between the heat exchange elements of the economizer will become caught and begin to accumulate within the economizer 55. This accumulation of particles will typically increase and eventually impede flow of the flue gas stream through the economizer 55 if steps are not taken to remove/clear the accumulations. The impeded flow of the flue gas stream reduces the effectiveness of the economizer 55. Further, it will be necessary to take steps to clear the accumulations from the economizer 55 in order to keep in operating properly. This leads to increased maintenance time and costs.
Currently, there is a need for an efficient heat exchanger in a boiler system which makes use of wasted heat and requires less maintenance than prior art systems.

SUMMARY OF THE INVENTION

The present invention may be embodied as an economical heat recovery system [100] for use with a boiler [26] that boils water feed from a water supply [125] supplied to it.
It includes an air preheater [150] for receiving heated flue gasses [FG1] produced by boiler [26], for receiving input air [A1] and for creating incremental air stream [A2′].
It also includes a regenerative heat capture and transfer (RHCT) system [300] adapted to receive the incremental air stream [A2′], said water feed [WF1], then transfer heat from the incremental air stream [A2′] to the water feed [WF1], create preheated water feed [WF2] and supplied preheated water feed [WF2] to boiler [26].
The RHCT uses a heat exchanger [310] to receive the incremental air stream [A2′] from the air preheater, receive the water feed [WF1] and transfer heat from the incremental air [A2′] to water feed [WF1] to create preheated water feed [WF2]. A pump 330 coupled to said water supply [125] and to the heat exchanger [310] pumps the feed water [WF1] from water supply [125] through the heat exchanger [310] and the preheated water feed [Wf2] to the boiler [26].
The placement of the RHCT after the air preheater [150] allows the RHCT to be designed in a much more efficient manner and require less maintenance.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention may be better understood and its numerous objects and advantages will become apparent to those skilled in the art by reference to the accompanying drawings in which:

FIG. 1 is a block diagram depicting a portion of a power generation system 10 according to the prior art.

FIG. 2 is a simplified block diagram depicting an embodiment of a power generation system 100 according to the present invention that incorporates a regenerative heat capture and transfer system (RHCT) 300.

FIG. 3 is a simplified block diagram depicting another embodiment of a power generation system 100 that incorporates a RHCT system 300 according to the present invention.

FIG. 4 is an enlarged block diagram depicting an embodiment of the RHCT system 300 of FIGS. 2 and 3.

FIG. 5 is a simplified block diagram depicting another embodiment of a power generation system 100 that incorporates a RHCT system 300 according to the present invention.

FIG. 6 is an enlarged block schematic diagram depicting the capture of heated leakage air from a rotary air preheater.

DESCRIPTION OF THE INVENTION

FIG. 2 is a simplified block diagram depicting an embodiment of a power generation system 100 according to the present invention that incorporates a regenerative heat capture and transfer system (RHCT) 300. In this embodiment a power generation system 100 is provided that includes a steam generation system 25, an exhaust processing and heat recovery system (EPHRS) 15, a regenerative heat capture and transfer system (RHCT) 300, a water supply 125 and an exhaust stack 90.
Steam generation system 25 includes a boiler 26. The EPRS 15 includes a regenerative air preheater 50, a particulate removal system 70 and a wet scrubber system 80. A forced draft (FD) fan 60 is provided to introduce an air stream Al into the cold side input of the air preheater 50. In turn, air preheater 50 heats the air stream Al and outputs it as a heated air stream A2 that is fed to an air intake of the combustion chamber (not shown) of boiler 26 for combustion.
Exhaust gases FG1 expelled from the combustion chamber (not shown) of boiler 26 are received by a hot side input of the air preheater 50. These exhaust gases FG1 are cooled via the air preheater 50 and output as a cooler temperature exhaust gas stream FG2. Previously, gasses leaving air preheater 150 had to remain hot enough to prevent condensation of compounds in the flue gas. This reduced corrosion of the equipment downstream from the preheater 50.
Now, with the advent of corrosion reducing equipment and processes, corrosion is less of a problem. Therefore, there may be a greater amount of heat recovered that is fed back into the system. This results in higher boiler efficiency.
Exhaust gas stream FG2 is then processed to remove particulate matter via particulate removal system 70. The particulate removal system 70 may be, for example, an electrostatic precipitator (ESP), a fabric filter system (Bag House) or the like.
The processed exhaust stream FG3 may be further processed via, for example, a wet scrubber 80 to remove, for example, sulfuric oxide (SO₂). This processed stream FG4 is then output for introduction to the exhaust stack 90.
Regenerative heat capture and transfer system (RHCT) 300 is configured to receive an air stream A2′ and extract thermal energy therefrom. Air stream A2′ is a portion of air stream A2 expelled from the air preheater 50. In turn, the thermal energy extracted from air stream A2′ is transferred to a water feed supply WF1 which is then output as heated water feed WF2 and introduced to boiler 26. RHCT 300 is configured and positioned so as to transfer thermal energy from the input air stream A2′ to water feed WF1 without receiving contaminates. Air streams A2/A2′ are clean air stream that do not mix with the flue gas streams that have significant amount of particulate matter. Further, since no flue gas is used by the RHCT 300 to heat the water feed supply WF1, the RCHT 300 is not subjected to particulate matter that is often found in the flue gas stream FG.
Air preheater 150 can now be designed to be a high efficiency air preheater transferring a greater amount of heat. Also, air preheater 150 may be designed to output a greater volume of heated air than can be efficiently put to use by the steam generation system 25, creating excess heated air.
FIG. 3 is a simplified block diagram depicting another embodiment of a power generation system 100 that incorporates a RHCT system 300 according to the present invention. In this embodiment, air preheater 150 has one flue gas duct and two heated input air ducts. The output of one heated air duct releases heated air stream A2. This is provided to boiler 26. The second heated air duct provides incremental air stream A2′ that is passed to RHCT 300.
The remaining parts of FIG. 3 perform the same function as the parts of other figures having the same reference number.
FIG. 4 is an enlarged block diagram depicting an embodiment of the RHCT system 300 of FIGS. 2 and 3. In this embodiment, the RHCT 300 includes heat exchanger 310 and pump 330. Heat exchanger 310 is preferably configured to receive a portion A2′ of the heated air stream A2 from the air preheater 150.
Since the RCHT 300 is not subjected to the particulate matter typically found in the flue gas stream FG, it is possible for the heat exchange elements (not shown) used in the economizer to be placed in much closer proximity to each other and thereby provide for more surface area available to contact the air stream A2/A2′. In this way, the efficiency of the heat exchanger 310 can be significantly enhanced since the greater the surface area of the heat exchange elements that is provide, the more heat that can be captured for a given volume. Further, since the heat exchange elements are not subjected to much particulate matter, the threat of blockage due to accumulations of particulate matter in the economizer is greatly reduced, if not completely avoided.
In this particular case the finned tubes will not be exposed to coal ash (only preheated air); therefore, the fin density spacing can be reduced significantly from that of a typical economizer tube designed for exposure to flyash. Thus, the size of the economizer should be more efficient and smaller.
By coupling RHCT 300 to air preheater 150 instead of the boiler flue gas output, heat is more efficiently removed from the exhaust gases FG1, transferred to an air stream (A2′), introduced into the water feed [WF1/WF2] to supply boiler 26 than was previously possible in prior art systems.
FIG. 5 is a block diagram depicting another embodiment of a power generation system 100 that incorporates a RHCT system 300 according to the present invention.
Here incremental air stream [A2′] and/or leakage gasses 360 from exhaust conduits 361, 363 are provided to RHCT. Fan 367 facilitates the flow of leakage gasses 360.
FIG. 6 is an enlarged block schematic diagram depicting the capture of heated leakage gasses 360 from a rotary air preheater 150.
Hot flue gasses FG1 are passed into a hot side of an air preheater 150. A wheel 151 rotates on an axle 152. A motor causes rotation of wheel 151.
Wheel 151 has a plurality of air conduits passing through the wheel. Each of these has heating elements that heat us as flue gas FG1 passes through the conduits. These heating elements rotate to the cool side of the wheel where inlet air A1 is received. The inlet air comes in contact with the hot heating elements and is heated into preheated air A2 that exits the air preheater 150. Heating element cool as the input air A1 passes over them.
Wheel 151 continues to rotate and the heating elements come in contact with hot flue gasses FG1 again, absorbing heat. This process then continues.
There are outer seals 157, 158 that stop most of the leakage of hot flue gasses past the outer edge of wheel 151.
There are also inner seals that stop most of the flue gas leakage toward the inner hub section of wheel 151. However, some flue gas leaks past the seals and into inner plenums between the wheel and housing 154.
In this embodiment, a leakage outlet 325 is provided. This outlet may be implemented as an opening in the housing 154, which allows access to the plenum 159. An exhaust conduit 361 is provided for exhausting gas/air that may accumulate in the internal plenum 159. A fan device 367 may be provided to allow leakage gasses 360 to be exhausted from the internal plenum 159 more easily.
A further leakage outlet may also be provided so that leakage gases accumulating within the internal plenum 365 may be readily exhausted through another exhaust conduit 361.
Fan 367 also draws the leakage gasses 360 from exhaust conduit 363. The leakage gasses 360 and/or incremental air stream [A2′] are provided to the RHCT 300 to further heat the feed water [WF1]. Use of this wasted heat increases the efficiency of the boiler.
A separate fan may be employed for each exhaust conduit if so desired.
It should be emphasized that the above-described embodiments of the present invention, particularly, any “preferred” embodiments, are merely possible examples of implementations, merely set forth for a clear understanding of the principles of the invention. Many variations and modifications may be made to the above-described embodiment(s) of the invention without departing substantially from the spirit and principles of the invention. All such modifications and variations are intended to be included herein within the scope of this disclosure and the present invention and protected by the following claims.

Claims

1. An economical flue gas heat recovery system for use with a boiler [26] comprising:

a regenerative heat capture and transfer (RHCT) system [300] configured to receive a heated air stream [A2′] from an air preheater [50], to receive a water feed [WF1] from a water supply [65], to transfer heat from the heated air stream [A2′] to the water feed [WF1] to create a heated water supply feed, and to output a heated water supply feed [WF2] to a boiler [26].

2. The system of claim 1 wherein the RHCT system [300] comprises a heat exchanger 310 configured to receive the water feed WF1 and the heated air stream A2′.

3. The system of claim 2 wherein the heat exchanger [310] is further configured to transfer heat from the incremental air stream [A2′] to the water supply feed WF1.

4. The system of claim 2 further comprising an air preheater 150 configured to provide the incremental air stream [A2′] to the RHCT system 300.

5. The system of claim 4 wherein the air preheater 150 is configured to receive flue gases [FG1] from said boiler [26] and to transfer heat from the flue gases [FG1] to an air stream input [A1].

6. The system of claim 3 wherein the heated air stream [A2′] is substantially free of post combustion particulate matter.

7. The system of claim 4 wherein the RHCT system [300] is configured to receive a portion of an input air stream [A2] from the air preheater [150] as incremental air [A2′] and to transfer heat from the incremental air [A2′] to feed water [WF1] for said boiler [26].

8. The system of claim 7 further comprising a pump [340] for pumping feed water [WF1] through the heat exchanger [310].

9. An economical heat recovery system [100] for use with a boiler [26] that boils water feed from a water supply [125] supplied to it, comprising:

an air preheater [150] for receiving heated flue gasses [FG1] produced by boiler [26], for receiving input air [A1] and for creating incremental air [A2′];

a regenerative heat capture and transfer (RHCT) system [300] adapted to receive the incremental air [A2′], said water feed [WF1], then transfer heat from the incremental air [A2′] to the water feed [WF1], create preheated water feed [WF2] and supplied preheated water feed [WF2] to boiler [26].

10. The economical heat recovery system [100] of claim 9, wherein the RHCT system comprises:

an heat exchanger [310] adapted to receive the incremental air [A2′] from the air preheater [150], receive the water feed [WF1] and transfer heat from the incremental air [A2′] to water feed [WF1] to create preheated water feed [WF2], and

a pump coupled to said water supply [125] and to the heat exchanger [310] for pumping the feed water [WF1] from water supply [125] through the heat exchanger [310] and the preheated water feed [WF2] to the boiler [26].

11. The economical heat recovery system [100] of claim 9, wherein at least some incremental air [A2′] from air preheater [150] is provided to the RHCT [300].

12. The economical heat recovery system [100] of claim 9, wherein leakage gasses [360] from the air preheater [150] are provided to the RHCT [300].

13. The economical heat recovery system [100] of claim 10, wherein:

the heat exchanger [310] is designed with tolerances that do not include additional internal space for particulate buildup.