Patent Application
20170284656
The present disclosure relates to the boiler arts, with illustrative embodiments including sub-critical boilers, sub-critical natural circulation boilers, coal-fired boilers, sub-critical coal-fired boilers, sub-critical natural circulation coal-fired boilers, and to methods of manufacturing and operating the same.
Small coal-fired boilers find application in diverse settings, such as where power requirements are relatively low (e.g. rural areas, underdeveloped regions), where coal is readily available, and so forth. Typical small coal-fired boilers for electric power generation employ a sub-critical natural circulation design. An example of such a boiler design is the Babcock & Wilcox Carolina-Type Radiant Boiler design. This design employs a furnace with membraned water-cooled furnace walls that feed one or more steam drums. Water passing through the furnace walls absorb heat energy, in effect cooling the tubes/pipes directly exposed to the combustion heat. The steam drum(s) feeds one or more primary superheaters located inside a convection pass, and one or more secondary pendant superheaters located inside the upper portion of the furnace. This superheated steam is used to run a high-pressure turbine. The steam exiting the high-pressure turbine is then sent through reheaters to increase the temperature again, so that the steam can then be used to run a low-pressure turbine.
Water-cooled pipes or tubes are designed to carry wet steam (i.e. a steam/water mixture, or equivalently, steam quality less than 100%). For a given operating pressure, the temperature of wet steam is thermodynamically limited to the boiling temperature of liquid water at the given operating pressure. In practice, water-cooled pipes are designed for an operating temperature of about 650° F.−670° F., corresponding to an operating pressure of about 2200-2600 psig. In a sub-critical boiler, water-cooled pipes feed wet steam into the steam drum.
By contrast, steam-cooled pipes or tubes are designed to carry superheated steam having a steam quality of 100% (i.e., no liquid component). The temperature of superheated steam is not thermodynamically limited for a given pressure, and in Carolina-Type designs the steam-cooled superheaters generally carry superheated steam at temperatures of about 1000° F.−1050° F.
Because of the differences in temperature, water-cooled pipes can be made of lower cost carbon steel, whereas steam-cooled pipes are made of more costly steel compositions. A design such as the Carolina-Type Radiant Boiler advantageously leverages these factors by designing the entire furnace to be water-cooled, so that the membraned walls can use lower cost carbon steel pipes and connecting membranes. The higher alloy superheater components are located within the furnace and convection pass (i.e inside the walls of the boiler), and are not membraned. In such designs, the membraned water-cooled walls are generally cooler than the flue gas to which the steam-cooled superheaters are exposed, due to more efficient heat transfer to the steam/water mixture carried by the water-cooled pipes.
In certain applications, it is desirable to obtain steam at high temperatures after superheating and after reheating, e.g. about 1050° F. after both cycles. This can be difficult in small designs, and further designs and methods are needed to obtain such high temperatures.
The present disclosure thus relates to small high pressure sub-critical boilers that can have natural circulation and achieve high superheater temperatures of about 1050° F. Generally, the lower furnace of such boilers uses water-cooled tubes/pipes, and the upper furnace uses steam-cooled tubes/pipes. Put another way, the upper furnace of the boiler is made of superheater tubes/pipes. The upper furnace is connected to a convection pass, and they share a common steam-cooled membrane wall.
Disclosed in various embodiments herein is a boiler, comprising: an upper furnace having steam-cooled membrane walls, the steam-cooled membrane walls comprising pipes sealed by membrane disposed between and connected to the pipes; and a convection pass downstream of the upper furnace, the convection pass having steam-cooled membrane walls and extending downwards; wherein the upper furnace and the convection pass share a common steam-cooled membrane wall, the common steam-cooled membrane wall being both a wall of the upper furnace and a wall of the convection pass.
The common steam-cooled membrane wall may be a rear wall of the upper furnace and a front wall of the convection pass.
In particular embodiments, the upper furnace includes a side wall, the convection pass includes a side wall having a lower edge, and a seal is present at a junction of the common steam-cooled membrane wall, the upper furnace side wall, and the lower edge of the convection pass side wall.
The boiler may further comprise a lower furnace below the upper furnace, the lower furnace having water-cooled membrane walls comprising pipes sealed by membrane disposed between and connected to the pipes. The boiler can also further comprise a steam separator configured to deliver dry steam to a steam cooled circuit including the steam cooled membrane walls of the upper furnace and configured to receive wet steam from a water-cooled circuit including the water-cooled membrane walls of the lower furnace.
The boiler may further comprise one or more superheaters disposed in the upper furnace or the convection pass, the one or more superheaters comprising pipes without membrane.
The common steam-cooled membrane wall may comprise stainless steel pipes sealed by membrane disposed between and connected to the stainless steel pipes.
The boiler may further comprise a steam separator; and a steam circuit connected to a dry steam outlet of the steam separator; wherein the steam circuit includes the common steam-cooled membrane wall shared by the upper furnace and the convection pass.
The boiler can also further comprise burners arranged to combust fuel to generate heated flue gas that flows upward through the upper furnace and then downward through the convection pass; wherein the common steam-cooled membrane wall is heated by the heated flue gas that flows upward through the upper furnace and by the heated flue gas that flows downward through the convection pass.
Also disclosed in various embodiments is a boiler, comprising: a furnace having steam-cooled membrane walls forming an upper portion of the furnace, the steam-cooled membrane walls comprising pipes sealed by membrane disposed between and connected to the pipes; and a convection pass having steam-cooled membrane walls forming at least a portion of the convection pass, and being fluidly connected to the upper portion of the furnace; wherein the furnace and the convection pass share a common steam-cooled membrane wall comprising a single layer of pipes sealed by a single layer of membrane disposed between and connected to the single layer of pipes.
In some embodiments, the furnace includes: an upper furnace having the steam-cooled membrane walls and the common stream-cooled membrane wall; and a lower furnace having water-cooled membrane walls, the water-cooled membrane walls comprising pipes sealed by membrane disposed between and connected to the pipes.
The boiler may further comprise a steam separator having a dry steam outlet connected to a steam cooled circuit including the steam cooled membrane walls of the upper furnace and a saturated steam inlet connected with a water-cooled circuit including the water-cooled membrane walls of the lower furnace.
The steam cooled circuit can further include one or more superheaters disposed in the upper furnace or the convection pass and comprising pipes without membrane.
The common steam-cooled membrane wall may comprise stainless steel pipes and stainless steel membrane.
The boiler can further comprise burners arranged to combust fuel to generate heated flue gas that flows upward through the upper furnace and then downward through the convection pass; wherein the common steam-cooled membrane wall is heated by the heated flue gas that flows upward through the upper furnace and by the heated flue gas that flows downward through the convection pass.
Also disclosed are methods of operating a boiler, comprising: combusting fuel to generate heated flue gas that flows sequentially upwards through an upper furnace and downwards through a convection pass; and sending superheated steam through a common steam-cooled membrane wall to capture heat energy from the heated flue gas and further superheat the superheated steam; wherein the common steam-cooled membrane wall is heated both by the heated flue gas flowing upward through the upper furnace and by the heated flue gas flowing downward through the convection pass.
The superheated steam sent through the common steam-cooled membrane wall can be generated from water that is heated in a lower furnace to obtain wet steam that is subsequently separated in a steam separator to obtain water and dry steam. The dry steam may pass through at least one superheater prior to being sent to the common steam-cooled membrane wall.
The common steam-cooled membrane wall can comprise a single layer of pipes separated by membrane therebetween. In particular embodiments, the upper furnace includes a side wall, the convection pass includes a side wall having a lower edge, and a seal is present at a junction of the common steam-cooled membrane wall, the upper furnace side wall, and the lower edge of the convection pass side wall.
These and other non-limiting aspects and/or objects of the disclosure are more particularly described below.
The following is a brief description of the drawings, which are presented for the purposes of illustrating embodiments disclosed herein and not for the purposes of limiting the same.
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.
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.
The modifier “about” used in connection with a quantity is inclusive of the stated value and has the meaning dictated by the context (for example, it includes at least the degree of error associated with the measurement of the particular quantity). When used with a specific value, it should also be considered as disclosing that value. For example, the term “about 2” also discloses the value “2” and the range “from about 2 to about 4” also discloses the range “from 2 to 4.”
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. The terms “upstream” and “downstream” are relative to the direction in which a fluid flows through various components, i.e. the fluid flows through an upstream component prior to flowing through the downstream component. It should be noted that in a loop, a first component can be described as being both upstream of and downstream of a second component. Similarly, the terms “upper” and “lower” are relative to each other in location, i.e. an upper component is located at a higher elevation than a lower component.
The terms “horizontal” and “vertical” are used to indicate direction relative to an absolute reference, i.e. ground level. However, these terms should not be construed to require structures to be absolutely parallel or absolutely perpendicular to each other. For example, a first vertical structure and a second vertical structure are not necessarily parallel to each other.
The terms “top” and “bottom” or the terms “roof” and “floor” are used to refer to locations/surfaces where the top/roof is always higher than the bottom/floor relative to an absolute reference, i.e. the surface of the earth. The terms “upwards” and “downwards” are also relative to an absolute reference; an upwards flow is always against the gravity of the earth.
The term “plane” is used herein to refer generally to a common level, and should be construed as referring to a volume, not as a flat surface.
A fluid at a temperature that is above its saturation temperature at a given pressure is considered to be “superheated.” The temperature of a superheated fluid can be lowered (i.e. transfer energy) without changing the phase of the fluid. As used herein, the term “wet steam” refers to a saturated steam/water mixture (i.e., steam with less than 100% quality where quality is percent steam content by mass). As used herein, the term “dry steam” refers to steam having a quality equal to or about 100% (i.e., no liquid water is present).
The terms “pipes” and “tubes” are used interchangeably herein to refer to a hollow cylindrical shape, as is commonly understood.
The term “natural circulation”, as used herein, refers to the circulation of water through the boiler due to differences in density as the water is heated. Water circulation can occur without the need for a mechanical pump.
To the extent that explanations of certain terminology or principles of the boiler and/or steam generator arts may be necessary to understand the present disclosure, the reader is referred to Steam/its generation and use, 42nd Edition, edited by G. L. Tomei, Copyright 2015, The Babcock & Wilcox Company, ISBN 978-0-9634570-2-8, the text of which is hereby incorporated by reference as though fully set forth herein.
Higher wet steam pressures are not desired for many small boiler applications due to cost and safety issues (e.g. higher minimum pipe wall and membrane thicknesses). In a small boiler using only water-cooled furnace walls, it is difficult or impossible to achieve superheater and reheater outlet temperatures both at 1050° F. for 150-300 MW net power generation, because it is not possible to provide sufficient superheater/reheater surface area for heat transfer to the dry steam to obtain such high temperatures.
One possible alternative is to employ a drum less once-through boiler design, such as one of the Babcock & Wilcox Universal Pressure boiler designs. However, these designs employ once-through steam generation. In a once-through design, the transition point from wet steam to superheated steam depends on operating conditions, rather than being defined using a steam separator (e.g. a steam drum). As a result, more expensive piping is typically used for all piping/tubing in such once-through designs for safety. This results in increased capital costs.
In the sub-critical boiler designs of the present disclosure, the furnace is divided into two sections: a lower furnace using water-cooled membrane walls that feeds into the steam separator, and an upper furnace using steam-cooled membrane walls that is fed (directly or indirectly) by the dry steam outlet of the steam separator. This approach advantageously enables lower cost carbon steel to be used for the lower furnace walls, with more expensive piping being used only in the upper steam-cooled furnace (including the convection path walls in some embodiments). Cost is lowered by retaining the steam separator. A higher steam output temperature is attainable because the use of a steam-cooled upper furnace and convection pass walls provides additional surface area for heat transfer from combustion/flue gases to the dry steam within the steam-cooled walls, resulting in superheated steam of desired temperatures. Again, such high superheated steam temperatures cannot be obtained with conventional water-cooled walls in the upper furnace.
In some embodiments, further improvement is attained in such a design by reducing the cross-section of the upper steam-cooled furnace compared with the lower water-cooled furnace. This increases flue gas flow velocity in the upper steam-cooled furnace compared with the lower water-cooled furnace, which provides more efficient heat transfer in the high temperature gas path, and also reduces the amount of materials and manufacturing cost.
In Carolina-Type Radiant Boiler designs, the convection pass is spaced apart from the furnace by a horizontal convection pass whose horizontal length creates a spacing between the furnace and the convection pass. As a result, in the Carolina-Type Radiant Boiler design, the furnace includes a rear wall and the convection pass includes a front wall. In a folded design, these two walls are combined into one wall. In the present disclosure, a common membraned steam-cooled wall is used to separate the upper furnace up-pass and the adjacent convection pass. This eliminates the open pass between the furnace and the convection pass, providing improved compactness for the boiler and reduces the amount of materials and manufacturing cost.
These benefits set forth above are attained by replacing the conventional water-cooled furnace with a two-part design in which the upper furnace is steam-cooled. However, such a modification has certain potential disadvantages. Overall material cost is increased due to the higher-cost alloys used in the upper furnace, but this can be mitigated by approaches disclosed herein (e.g., reduced upper furnace cross-section, employing a common steam-cooled wall between the furnace and the convection pass). Another potential disadvantage is structural complications for the preferred top-supported arrangement. This potentially arises because the lower furnace pipes are preferably carbon steel to reduce cost, while the upper furnace pipes are higher cost alloys for compatibility with steam cooling. Such a difficulty is also encountered in some once-through super-critical furnaces that employ carbon steel pipes in a lower furnace section to reduce cost. An example of such a design is the Babcock & Wilcox Spiral Wound Universal Pressure (SWUP) Boiler. In the SWUP once-through super-critical boiler design, the lower furnace is top-supported via dedicated lower furnace support components that connect with the upper boiler support via an array of vertical tie rods and/or paddle connections. The resulting assembly is complicated as the lower furnace must be secured by installing its supports, followed by performing the pipe welding.
Such an approach employing dedicated lower furnace supports can also be employed in sub-critical boiler designs with an upper steam-cooled furnace and lower water-cooled furnace. However, as disclosed herein, such dedicated prior art supports and complex pipe welding operations associated therewith are eliminated, and in their place a transition section with integral transition piping is employed. The transition section contains both water pipes and steam pipes, and provides a location where these pipes can be run to headers. In the transition section, at least some transition pipes are designed to be vertically oriented pipes, and lower furnace support is achieved by tensile support via welds to these vertically oriented transition pipes. The transition section is designed to support twice the load of the lower furnace. The transition section can be made of a cast stainless steel material that is compatible with steam-cooling—it is therefore overdesigned for the water-cooled transition pipes, but the ability to maintain top support for the lower furnace outweighs the additional cost entailed by overdesigning these relatively short water-cooled transition pipes. The transition section also acts as a pressure seal between the lower furnace and the upper furnace.
Some illustrative embodiments of such sub-critical boilers are diagrammatically shown and described below. These are merely illustrative examples, and a given embodiment may include one, two, more, or all disclosed novel features described herein.
With reference to
Combustion/flue gas 22 are diagrammatically indicated by arrows, and these gases flow through the boiler and heat the water/steam in the various walls of the boiler. More specifically, combustion air is blown into the lower furnace 12 through an air inlet 24, where it is mixed with a combustible fuel such as coal, oil, or natural gas. In some preferred embodiments, the fuel is coal, which is pulverized by a pulverizer (not shown). A plurality of burners 26 combusts the fuel/air mixture, resulting in flue gas. The flue gas rises by natural convection through the up-pass formed by the lower furnace 12, the transition section 16, and the upper furnace 14, then flows horizontally through the convection pass, which includes the convection pass 18 and finally exits through a flue gas outlet 28 for further downstream processing. Preferably, a hopper 33 is provided to capture ash or other contaminants in the exiting flue gas.
The sub-critical boiler 10 is top-supported to the building structure via suitable upper anchor points 30. These are diagrammatically indicated in
It is desirable to capture the heat energy present in the combustion/flue gas 22 for tasks such as driving an electrical power generation turbine (for example). To do so, the sub-critical boiler 10 includes cooling surfaces comprising pipes or tubes through which wet steam flows (these pipes or tubes are referred to herein as water-cooled) or through which superheated steam flows (these pipes or tubes are referred to herein as steam-cooled). More particularly, with reference to Inset A of
The upper furnace 14 and convection pass 18 are analogously made of a steam-cooled membrane wall 46 comprising steam-cooled tubes 42 with membrane 44 disposed between and welded or otherwise connected to the tubes 42 (see Inset B of
The tubes 32 of the water-cooled membrane wall generally have a greater diameter than the tubes 42 of the steam-cooled membrane wall. In particular, embodiments, the inner diameter of the water-cooled tubes is at least 0.5 inches greater than the inner diameter of the steam-cooled tubes. The tubes of the water-cooled membrane wall have an inner diameter of about 1.5 inches to about 2.0 inches, while the tubes of the steam-cooled membrane wall have an inner diameter of about 1.0 inches to about 2.5 inches. The tubes of the water-cooled membrane wall have an outer diameter of about 2.0 inches to about 2.5 inches, while the tubes of the steam-cooled membrane wall have an outer diameter of about 1.3 inches to about 2.3 inches. The tubes themselves may have a thickness of about 0.2 inches to about 0.5 inches.
In a typical steam flow circuit for the sub-critical boiler 10, water is inputted to the lower ends of the water-cooled tubes 32 via a lower inlet header 50. As the water travels upwards through these water-cooled tubes 32, the water cools the tubes exposed to high-temperature flue gas in the lower furnace 12 and absorbs energy from the flue gas to become a steam-water mixture (i.e. wet steam) at subcritical pressure.
The wet steam exits the upper ends of the water-cooled tubes 32 and flows via a wet steam outlet header 52 into an inlet 53 of a steam separator 54. The wet steam outlet header 52 is preferably welded to water-cooled transition pipes within the transition section 16. Preferably, the wet steam outlet header 52 facilitates venting of the tubes 32 as appropriate during start up, shut down, or maintenance, etc. Any type of steam separator may be used, e.g. employing cyclonic separation or so forth. In particular embodiments, a vertical steam separator is used, such as that described in U.S. Pat. No. 6,336,429. A dry steam outlet 56 of the steam separator 54 at an upper end of the steam separator outputs substantially dry steam (i.e., steam with 100% quality). A drain or water outlet 58 near the lower end of the steam separator 54 collects water extracted from the wet steam for recycle back to the lower inlet header 50 feeding the lower furnace 12.
The steam output from the dry steam outlet 56 flows to the convection pass 18 and then to the upper furnace 14. To provide additional surface for heat transfer, one or more primary superheaters 60, re-heaters 62, and/or secondary superheaters 64 may be provided in the interior volume of the boiler, within the upper furnace 14 and the convection pass 18. As illustrated here, one or more superheaters 60 disposed in the convection pass 18; one or more re-heaters (or re-heating superheaters) 62 are disposed in the convection pass 18 and/or in the upper furnace 14; and one or more secondary superheaters 64 are disposed in the upper furnace 14. Again, the steam-cooled furnace walls 46 of the upper furnace 14 act as superheater surfaces as well. A more detailed illustrative steam circuit is described in later drawings. It is to be understood that the illustrative steam circuit is merely an example, and other steam circuit configurations are contemplated, e.g. various superheater components may be omitted, and/or located elsewhere, etc.
Unlike the membrane walls 46 of the steam-cooled upper furnace 14 and the convection pass 18, the superheaters 60, 62, 64 located within the boiler are formed from loose pipes/tubes 72 without membranes joining the tubes together (see Inset C of
As seen in
The illustrative sub-critical boiler 10 employs certain features that enhance compactness and efficiency. One feature is a reduced cross-sectional area for the combustion/flue gas flow 22 through the upper furnace 14 compared with the lower furnace 12. The referenced cross-sectional area is the horizontal cross-section in the illustrative design in which the flue gas 22 flows vertically upward. In the illustrative boiler 10, the reduction in cross-sectional area of the upper furnace 14 relative to the lower furnace 12 is obtained via a “arch” surface 76, which is slanted as the upper furnace continues upward from the transition section 16, to reduce turbulence at the transition to higher flow velocity. This has at least two benefits. First, the reduced cross-sectional area of the upper furnace 14 reduces the amount of material (e.g. total surface area of membrane wall 46) which reduces capital cost. Second, the higher velocity of the flue gas flow 22 due to the reduced cross-sectional area increases the efficiency of heat transfer to the steam-cooled pipes 42, 72. The transition section 16 is located below the arch surface 76. The arch 76 is part of the upper furnace, and is also a steam-cooled membrane wall.
Another feature that enhances the compactness and efficiency of this boiler design is the use of a common steam-cooled membrane wall 80. The common steam-cooled membrane wall 80 is both a “rear” wall of the upper furnace 14 and a “front” wall of the convection pass 18. The upper furnace 14 and the convection pass 18 thus share the common steam-cooled membrane wall 80, which comprises a single layer of pipes sealed by a single layer of membrane disposed between and connected to the single layer of pipes. The use of the common steam-cooled membrane wall 80 has numerous advantages. The usual open pass between the furnace and the convection pass is eliminated, providing a more compact design and reducing capital costs due to lower surface area. The common steam-cooled membrane wall 80 is advantageously heated both by flue gas flowing upward through the upper furnace 14 and by flue gas flowing downward through the convection pass 18.
One issue with employing the common steam-cooled membrane wall 80 is the large temperature variation between the flue gas temperature in the upper furnace 14, on the one hand, and the flue gas temperature in the convection pass 18 on the other hand. This differential between the two flue gas temperatures will be felt by the common steam-cooled membrane wall 80. Using transient modeling and finite element analysis to determine the resulting stress in the walls of the boiler, it was found that maximum thermal differential stress occurs during start-up, and more particularly occurs in a small area about the bottom of the common steam-cooled membrane wall 80 adjacent walls 114, 124 on one side and walls 116, 126 on the other. Intuitively, this can be understood since this bottommost part of the common steam-cooled membrane wall 80 is where there is the greatest temperature differential between the upward flue gas flow in the upper furnace 14 and the downward flue gas flow in the convection pass 18. This stress can cause boiler bowing/tearing, and is accommodated by providing seals at the junction of the bottom of the common steam-cooled membrane wall 80, the furnace side walls 114, 116, and the convection pass side walls 124, 126, as analysis showed that the overstressed area does not extend significantly up the common steam-cooled membrane wall 80. These seals are illustrated in
Referring now to
This arrangement is also seen from the side in
It should be noted that the various improvements disclosed herein can be used to advantage individually or in various combinations, and/or in various types of boilers.
With reference now to
As shown in
Four reheaters RSH1, RSH2, RSH3, and RSH4 are also employed. Three of these (RSH1, RSH2, and RSH3) are disposed in the front convection pass 17, while the fourth reheater RSH4 is disposed near the top of the upper furnace 14. Cross-over piping labeled RSH XOVER conveys steam from RSH3 in the convection pass 18 to RSH4 in the upper furnace 14. Steam flow is from a lower inlet header RSH IN, through the reheaters in sequential order, to an outlet header RSH OUT shown to the left of the upper furnace 14 in
Four secondary superheaters SSH1, SSH2, SSH3, and SSH4 are disposed in the upper furnace 14 below the fourth re-heater RSH4. Superheated steam flows from the PSH OUT header to the SSH IN header shown to the left of the upper furnace 14, then upwards successively through SSH1, SSH2, SSH3, and SSH4 and to the SSH OUT header again shown to the left of the upper furnace 14 above the SSH IN header.
The steam-cooled circuit further includes superheater stringers denoted SH STRINGER in
Referring now to
The two dry superheated steam streams from the upper baffle wall and the lower baffle wall are then combined and flow into the primary superheaters 60 (i.e. PSH1, PSH2, PSH3, PSH4 in
Referring back to
The steam-cooled circuit of
The present disclosure as 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.
