Split pass economizer bank with integrated water coil air heating and feedwater biasing

Abstract

An apparatus for using a water coil air heater with a single bank economizer. A boiler economizer arrangement includes an economizer bank which has separate hot pass bank and cold pass bank economizer portions in a parallel arrangement, each facing the same flow of hot flue gas. Feedwater enters the cold pass bank economizer where it is heated by the hot flue gas, and then flows to a water coil air heater away from the hot flue gas. The feedwater dissipates heat energy in the water coil air heater which may be used to heat air bound for combustion. The feedwater continues into the hot pass bank economizer portion of the economizer arrangement where it absorbs additional heat from the flue gas. The heated feedwater flows out of the economizer arrangement and may be subject to additional heating by a boiler or other heat exchanger.

Claims

1. A boiler economizer arrangement comprising: a single cavity for conveying heated flue gas flow, the single cavity including a first economizer side wall and a second economizer side wall, wherein the first and second economizer side walls are opposite each other to define the single cavity therebetween; the single cavity having an upstream direction which receives a stream of heated flue gas flow, and a downstream direction for exiting flue gas; an economizer bank stretching substantially from the first economizer side wall to the second economizer side wall, the economizer bank comprising a cold pass bank economizer and a hot pass bank economizer, and wherein the cold pass bank and the hot pass bank are positioned in a parallel arrangement relative to the stream of heated flue gas such that each bank receives a different portion of the stream of heated flue gas; a split collection header comprising a hot pass split collection header for the hot pass bank economizer, a cold pass split collection header for the cold pass bank economizer, the cold pass split collection header extending between a first end and a second end, the hot pass split collection header extending between a third end and a fourth end, and a split at a location between the second end and the third end cold pass split collection header and the hot pass split collection header, said location being within the single cavity such that a portion of the stream of heated flue gas passes through the split; a water coil air heater positioned outside of the cavity and adapted for transferring heat from a flow of feedwater flowing inside the water coil air heater to a stream of air outside of the water coil air heater; a feedwater inlet for receiving the flow of feedwater into the economizer arrangement from outside the cavity; a feedwater outlet for the flow of feedwater exiting the economizer arrangement; and at least one valve adapted for controlling the flow of feedwater between the cold pass bank economizer and the water coil air heater; wherein the economizer arrangement is adapted to route the flow of feedwater from the feedwater inlet, then to the cold pass bank economizer, then outside the cavity to the water coil air heater, then back into the cavity into the hot pass bank economizer, and then to the feedwater outlet and out of the economizer arrangement; wherein the flow of feedwater is routed away from the economizer bank to the water coil air heater at the split between the cold pass split collection header and the hot pass split collection header; and wherein the cold pass bank economizer and the hot pass bank economizer are connected in series relative to the flow of feedwater.

2. The economizer arrangement of claim 1, wherein the cold pass bank economizer is adjacent to the first economizer side wall and the hot pass bank economizer is adjacent to the second economizer side wall.

3. An economizer arrangement comprising: a single cavity for conveying heated gas flow, the single cavity having a first economizer side wall and a second economizer side wall to define the single cavity therebetween; the single cavity having an upstream direction which receives a stream of heated gas and a downstream direction for exiting gas flow; an economizer bank stretching substantially from the first economizer side wall to the second economizer side wall, the economizer bank comprising a plurality of sections including at least a cold pass bank economizer and a hot pass bank economizer, and wherein the cold pass bank and the hot pass bank are positioned in a parallel arrangement such that each bank receives a different portion of the stream of heated gas; a split collection header comprising a hot pass split collection header for the hot pass bank economizer, a cold pass split collection header for the cold pass bank economizer, and a split at a location between the cold pass split collection header and the hot pass split collection header, said location being within the single cavity such that a portion of the stream of heated gas passes through the split; a water coil air heater positioned outside of the cavity and adapted for transferring heat from a flow of feedwater flowing inside the water coil air heater to a stream of air outside of the water coil air heater; wherein the economizer arrangement is adapted to route the flow of feedwater into the cold pass bank economizer, then outside the cavity to the water coil air heater, then back into the cavity into the hot pass bank economizer, and then out of the economizer arrangement; wherein the flow of feedwater is routed away from the economizer bank to the water coil air heater at the split between the cold pass split collection header and the hot pass split collection header; and wherein the cold pass bank economizer and the hot pass bank economizer are connected in series relative to the flow of feedwater.

4. The economizer arrangement of claim 3, further comprising: a feedwater inlet for receiving the flow of feedwater entering the economizer arrangement; and a feedwater outlet for the flow of feedwater exiting the economizer arrangement.

5. The economizer arrangement of claim 3, wherein the water coil air heater is adapted to warm the stream of air on its way to a furnace.

6. The economizer arrangement of claim 3, further comprising at least one valve adapted for controlling the flow of feedwater including between the cold pass bank economizer and the water coil air heater.

7. The economizer arrangement of claim 3, wherein the single cavity is part of a boiler, and wherein the first economizer side wall and the second economizer side wall are directly opposite each other.

8. The economizer arrangement of claim 7, wherein the cold pass bank economizer is adjacent to the first side wall and the hot pass bank economizer is adjacent to the second side wall.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The following is a brief description of the drawings, which are presented for the purposes of illustrating the exemplary embodiments disclosed herein, where like reference numbers are used to refer to the same or functionally similar elements and not for the purposes of limiting the same.

(2) FIG. 1 is a schematic view of a prior art arrangement comprising separate hot and cold economizer banks in series and a water coil air heater;

(3) FIG. 2 is a perspective drawing of a bottom portion of a prior art economizer bank;

(4) FIG. 3 is a plan view diagram of a prior art economizer cold bank;

(5) FIG. 4 is a schematic view of a split bank economizer arrangement in parallel of the present arrangement; and

(6) FIG. 5 is a plan view of a split bank economizer bank of the present arrangement.

DESCRIPTION OF THE INVENTION

(7) A more complete understanding of the processes and apparatuses disclosed herein can be obtained by reference to the accompanying drawings. These figures are merely schematic representations based on convenience and the ease of demonstrating the existing art and/or the present development, and are, therefore, not intended to indicate relative size and dimensions of the assemblies or components thereof.

(8) Although specific terms are used in the following description for the sake of clarity, these terms are intended to refer only to the particular structure of the embodiments selected for illustration in the drawings, and are not intended to define or limit the scope of the disclosure. In the drawings and the following description below, it is to be understood that like numeric designations refer to components of like function.

(9) It should be noted that many of the terms used herein are relative terms. For example, the terms inlet and outlet are relative to a direction of flow, and should not be construed as requiring a particular orientation or location of the structure.

(10) To the extent that explanations of certain terminology or principles of the steam generating arts may be necessary to understand the present disclosure, the reader is referred to Steam/its generation and use, 41st Edition, Kitto and Stultz, Eds., Copyright 2005, The Babcock & Wilcox Company, the text of which is hereby incorporated by reference as though fully set forth herein.

(11) Referring now to the drawings, FIG. 4 is a schematic diagram of a preferred boiler economizer arrangement 1 embodying the invention.

(12) The economizer arrangement 1 will typically be part of a larger arrangement for capturing heat energy from a flowing gas and transferring it to another flowing substance for use in power generation. This may be capturing furnace combustion heat from hot flue gas. Preferably, the economizer arrangement 1 is located in the path of moving heated flue gas flow 4 downstream from other heat absorbing equipment, such as superheaters, which will have partially cooled the flue gas flow 4 by the time it reaches the economizer. However, the present invention is not limited to economizer arrangements which are physically part of the boiler and furnace combustion equipment, and alternatively may be a separately located arrangement of an economizer at the plant.

(13) The heated flue gas is conveyed from the heat source down a path which may include the first economizer side and second economizer side walls 6, 8, respectively. As used herein, the term economizer side wall refers to enclosure walls which convey the flue gas and which surround the economizer arrangement 1. These enclosure walls are typically casing, but may be comprised of heating surface, conveying water, steam, or mixtures thereof. The path of the flue gas flow 4 may be generically referred to as a cavity 2 for conveying heated flue gas. Cavity 2 may also be referred to as an enclosure which conveys the heated flue gas. Preferably, the cavity 2 is defined by a first economizer side wall 6 and a second economizer side wall 8, with the first and second economizer side walls being opposite each other. The flue gas path may be a single continuous cavity, or it may split or branch as needed. The cavity 2 has an upstream direction 10 where heated flue gas comes from, often being the direction where combustion or other heat-generating reaction takes place. The cavity also has a downstream direction 12 that eventually leads to an opening to atmosphere. The cavity 2 will often be rectangular in cross section but is not limited to any particular shape.

(14) An economizer bank 20 stretches substantially from a first economizer side wall 6 to a second economizer side wall 8. Preferably the economizer bank takes up most or all of a cross-section of the cavity 2 so that a maximum portion of the passing flue gas flow 4 is forced to contact the bank for maximum heat transfer. The economizer bank includes at least two banks, typically including a cold pass bank economizer 22 where feedwater transits first, and a hot pass bank economizer 24 where the feedwater transits later. Preferably, the cold pass bank economizer 22 and the hot pass bank economizer 24 are positioned in a parallel arrangement relative to the flue gas flow 4 to collectively span substantially across the width of the cavity 2 as shown, for example, in FIGS. 4-5. Similar arrangements using more than two banks are possible. Different shapes and arrangements can be used without departing from the general concept of filling a single cross-section of the cavity with more than one separate pass bank for heat transfer. The pass banks may be of equal size, or of different sizes.

(15) In a preferred embodiment each cold pass bank economizer 22 and hot pass bank economizer 24 includes at least one collection header 26 and a plurality of mini-headers 28 connected to each collection header 26. There may be one hot pass split collection header 26 for the hot pass bank economizer 24 and one cold pass split collection header 26 for the cold pass bank economizer 22. As shown in FIG. 5, the cold pass split collection header 26 extends from end 50 to end 51. The hot pass split collection header 26 extends from end 52 to end 53. Collection header split 46 is defined at a location between end 51 and end 52. Each mini-header may in turn be connected to a number of pipes or tubes 29. See, generally, FIG. 5 in light of FIG. 2. Many other economizer designs may be used with the arrangement, however, to maximize the surface area available for heat transfer from the flue gas flow 4 to the feedwater 32. The general principle is that feedwater enters each economizer bank through preferably one opening, then spreads out through a network of (typically branching, winding, and/or having heat-conducting protrusions) pipes and tubes to increase surface area and residence time in the heated zone, and then consolidates back down to preferably another single opening which routes warmed feedwater out of the economizer bank.

(16) One aspect of the invention is that a water coil air heater 30 (WCAH) is positioned in the flow path for the feedwater 32 upstream of at least one hot pass bank economizer 24 and downstream of at least one cold pass bank economizer 22. The WCAH 30 will typically need to be positioned outside of the cavity 2 containing the flow of heated flue gas flow 4, preferably in a stream of cooler air which may be routed into the a furnace. This is so that some heat will be transferred back out of the newly-warmed feedwater 32, via the WCAH 30, and into the stream of cooler air. After the feedwater is cooled in the WCAH 30, it proceeds to another pass bank economizer 24 to be heated again by the flue gas flow 4. Various embodiments of this general concept, such as alternating three or more pass banks with two or more WCAHs, are possible. The WCAH can take a number of forms, and the arrangement is not limited to a particular type of WCAH.

(17) The economizer arrangement 1 preferably includes at least one feedwater inlet 40 for receiving water into the economizer arrangement. The feedwater inlet 40 may lead to an economizer pass bank. The arrangement also preferably includes at least one heated water outlet 42 for water flow exiting the economizer arrangement 1.

(18) Preferably, the economizer arrangement includes at least one valve 44 for controlling a flow of water between the cold bank economizer 22 and the water coil air heater 30. Valves 44 might be adapted for biasing feedwater flow between economizer banks (22, 24), and for either routing water into a WCAH 30 or bypassing a WCAH 30.

(19) In the illustrative embodiments, feedwater 32 enters the economizer arrangement 1 at the feedwater inlet 40. The feedwater proceeds through the cold bank economizer 22 where it flows through a branching series of header(s), mini-headers, and tubes which have a large collective surface area. Heat is transferred from the flowing flue gas flow 4 to the feedwater 32 through the surfaces of the cold bank economizer 22. The feedwater converges again, typically in a header, and leaves the cold bank economizer. The feedwater then proceeds via a pipe out of the second economizer side wall 8 of the cavity 2, through an open valve 44, and into a WCAH 30. In the WCAH 30 the feedwater sheds some heat energy into a passing stream of air 34. The cooled feedwater then flows out of the WCAH 30, back into the cavity 2 and into the hot bank economizer 24. The feedwater is heated again by the hot gas flow 4 through the branching flow paths of the hot bank economizer 24 similar to the cold bank economizer 22. The reheated water then proceeds out of the enclosure via an outlet 42 and eventually to a drum (in recirculating boilers) or furnace surface (once-through boilers).

(20) TABLE-US-00001 TABLE 1 Prior Art Long Flow (mini-header) Economizer vs. Side-by-Side Long Flow (mini-header) Economizer Prior Art Parallel Economizer, Economizer (50:50 hot:cold split) Economizer Height ft 42 42 100 Subcooling Lvg Econ F. 27 101 40 Gas temperature F. 588 576 430 leaving

(21) Table 1 illustrates that a multiple gas path, parallel (with an intermediate WCAH) economizer (with hot and cold pass banks in parallel relative to the gas flow) provides an additional 70+ degrees of subcooling over a similar sized conventional economizer arrangement (with two 42 ft economizer columnshot and cold pass banks in series relative to the gas flow). With this additional subcooling, the economizer heating surface can be increased while maintaining steaming economizer design margins. Table 1 shows that a 100 ft tall economizer bank (far right column) can achieve low economizer exit gas temperatures (EEGT) while still maintaining 40 F subcooling. Thus, the current arrangement both improves economizer performance and lowers costs.

(22) The arrangement is particularly useful for retrofitting older installations where space is fixed and limited, but where the efficiency advantages of a WCAH are desired.

(23) For example, the arrangement could be applied successfully in process recovery (PR) boilers undergoing low odor conversions. Environmental regulations are driving low odor conversions in the existing direct contact evaporator recovery boiler fleet. A recovery boiler is used in the Kraft process of wood pulping where chemicals for white liquor are recovered and reformed from black liquor, which contains lignin from previously processed wood. The black liquor is burned, generating heat, which is usually used in the pulping process or in making electricity, much as in a conventional steam power plant. When a low odor conversion of a pulping facility is completed, the direct contact evaporators are replaced with multiple effect evaporators. As a result of this change, the flue gas temperature leaving the unit no longer needs to be 600+ degrees F. Typically, to re-gain efficiency on low odor conversions, gas temperature is reduced by the addition of economizer surface. The multi-gas path arrangement with an intermediate WCAH of the present arrangement increases efficiency over that which is possible with traditional single or multiple bank longflow economizer arrangements.

(24) Additionally, the multi-gas path economizer arrangement could be applied to other types of boilers, including but not limited to waste-to-energy applications and biomass combustion technologies.

(25) The multi-gas path parallel economizer banks design brings a number of advantages. The arrangement achieves higher heat absorption rates within a single long flow bank than were previously possible. It was previously necessary to add a second full flow bank in series (with respect to gas flow as in FIG. 1) in order use a WCAH and thereby to more efficiently cool flue gas. The arrangement includes the flexibility to define shapes and relative sizes of the cold and hot pass heating surfaces. The location of a collection header split 46 can be tailored to maximize unit performance (see FIG. 5).

(26) The integration of economizers to a WCAH 30 allows the biasing of water between the components, including by using valves 44. The arrangement has the capability to control gas temperature leaving the economizer, water temperature leaving the economizer, and/or air temperature leaving the water coil air heater.

(27) The arrangement could also be implemented, for example, using a horizontal flow continuous tube economizer instead of long flow-mini header type economizer banks. A continuous tube economizer could be split with intermediate headers which leave a cavity 2, bring feedwater to a WCAH 30, and then return cooled feedwater to the continuous tube economizer.

(28) The present disclosure has been described with reference to exemplary embodiments. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the present disclosure be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.