WO2014105720A1

WO2014105720A1 - Apparatus, methods, and systems for recovering heat from a metal casting process

Info

Publication number: WO2014105720A1
Application number: PCT/US2013/077016
Authority: WO
Inventors: Luke ERICKSON; Russell MUREN; Mark A. Wilkinson; Alain L. Bourhis; Francis X. Mcconville; Charles Freeman
Original assignee: Abengoa Solar, Inc.
Priority date: 2012-12-24
Filing date: 2013-12-20
Publication date: 2014-07-03

Abstract

A method for extracting heat from a continuous casting process is provided wherein a heat exchanger is positioned beneath a conveyor belt that is traditionally used in a continuous casting process. In operation, molten metal is deposited on the conveyor belt and the heat exchanger draws heat therefrom to 1) facilitate the molten metal freezing process as opposed to, or in addition to, the use of cooling sprays; and 2) recover the heat energy which is used in a separate heat energy recovery process.

Description

APPARATUS, METHODS, AND SYSTEMS FOR RECOVERING HEAT FROM A

METAL CASTING PROCESS

This application claims the benefit of U.S. Provisional Patent Application Serial No. 61/745,738, filed December 24, 2012, the entirety of which is incorporated by reference herein.

FIELD OF THE INVENTION

Embodiments of the present invention are generally related to apparatus and methods for recovering heat associated with a continuous metal casting process. In one particular embodiment, the metal to be cast is melted at least partially with heat generated by a solar power generation system.

BACKGROUND OF THE INVENTION

Traditional metal casting processes comprise cooling molten metal in a cast or form to yield a finished or semi-finished component. Continuous casting is a process whereby molten metal, such as steel, aluminum, copper, or other similar material, is deposited and cooled on a moving belt. The molten metal is solidified into ingots, billets, or sheets. In the case of sheets, at least one roller may be spaced from the belt that is used to define the thickness, or gauge, of the finished sheet. Before the advent of continuous casting, sheet metal was often formed by repeatedly rolling and forming ingots or billets, which is a time consuming and labor-intensive process. Continuous casting achieves product yield and quality that exceeds that of traditional sheet forming methods. Further, productivity and cost efficiencies are increased over traditional sheet forming processes.

U.S. Patent No. 49,053 to Bessemer describes an alternate process for metal casting wherein molten metal is fed between two spaced rollers that are wetted or otherwise cooled. When the molten metal contacts the cooled rollers, a frozen outer boundary layer is formed that defines the geometry of the finished sheet. The sheet is then fed to a conveyor belt and cut or otherwise formed.

Fig. 1 shows another prior art method of continuous metal casting disclosed in U.S. Patent No. 5,350,009 to Mizoguchi et al. that employs rollers. The twin roll-type sheet continuous casting apparatus comprises a pair of casting rollers 11, 12 that are horizontally disposed in a closely spaced, parallel relation to each other. Cooling water flows through the interior of each of the casting rollers 11, 12. A molten metal reservoir 13 is formed on the upper side of the pair of casting rollers 11, 12 that receives molten metal from a tundish 29 provided above the molten metal reservoir 13. The molten material is passed between the two cool rollers which freeze a film of solid metal on each surface of the molten material to form a metallic sheet. The partially frozen metal sheet is carried by a moving belt by a plurality of water sprays that reduce the sheet's temperature to fully solidify the same. The finishing sheet then further formed or rolled and stored.

Fig. 2 shows another method of fabricating sheet material from molten material by a continuous casting process described in U.S. Patent No. 5,148,855 to Ashok. Here, the partially frozen sheet is carried over a plurality of nozzles that spray cooling fluid onto the back of the conveyor belt to cool and further freeze the sheet. Indeed, most continuous sheet casting systems employ sprayed water cooling. Another example of such a system can be found in US Patent No. 6,561,440 to Hofherr which describes a water-cooling system that maintains the casting equipment at a low temperature.

Because melting metal requires a great amount of heat energy, it is desirous to capture some of the heat when the molten metal is later cast. The recovered heat could be used to heat raw material or for electricity generation. One method of heat recovery is disclosed in U.S. Published Patent Application No. 2012/0118526 to Sudau et al, which describes a system for collecting thermal energy released or expelled by a cooling metal object subsequent to casting. Sudau describes isolating the still hot metal and transferring heat to air that is passed by the hot metal. The high-temperature air is then used in a heat power cycle. Sudau does not disclose, however, capturing the heat energy that is lost during the casting process, i.e., the initial metal freezing process. More specifically, when material such as aluminum, is cast, the latent heat of freezing, i.e., the heat expelled, is typically 40-60% of the total energy stored by the hot metal. Additionally, the energy associated with the expelled heat is very high grade because it is recovered at high temperature. Thus, to maximize heat recovery, it is very desirable to capture the heat associated with the initial cooling of the sheet metal.

Thus it is a long felt need to provide apparatus, systems, and methods that allow for the capture and reuse of heat associated with initial cooling of a molten material. The following disclosure describes apparatus and improved methods of capturing heat that is usually lost during a continuous casting process.

SUMMARY OF THE INVENTION

It is one aspect of embodiments of the present invention to provide a method of obtaining heat from a molten metal as it is cast. Traditionally, casting processes comprise transferring molten metal from a tundish, i.e. a reservoir, onto a moving belt. The moving metal is cooled by series of air or water jets, which solidifies the molten metal into a sheet. The sheet is then rolled, stamped, cut, or otherwise formed. One embodiment of the present invention does not use water sprays and, alternatively, cools the metal by drawing heat from the molten material deposited on the belt with a heat exchanger. A continuous casting process utilizing the contemplated heat exchanger may employ twin roller casters, belt casters, or direct chill casters, or other sheet-forming tools and methods commonly used in the art. The heat transfer fluid used in the heat exchanger can be one or more of water/steam, air, oil, or carbon dioxide. The heat transfer fluid may be carried via pipes, grooves, or other known methods commonly employed in heat exchanging devices.

Another embodiment of the present invention uses a roller that includes an integral heat transfer element. More specifically, the roller is placed in direct contact with a reservoir of molten metal such that when rolled, molten material is gathered by the roller. The molten metal will transfer heat to the heat transfer element positioned within the roller and cooler metal will be directed to conveyor or sheet rolling device. One of skill in the art will appreciate that the roller of this design could be associated with a conveyor belt as described above.

In yet another embodiment of the present invention, molten metal is extruded directly into a fluid. The molten material flash freezes when it contacts the fluid, wherein the heated fluid can be used in a heat transfer process.

It is yet another aspect the present invention to recover heat from a casting process that can be used to generate electricity. More specifically, the heat obtained from the casting process using one or more of the contemplated methods described herein may be used in a thermal power cycle. For example as one skill that will appreciate, the captured heat may be used to produce steam that drives an electricity-producing turbine generator. If the molten material is aluminum or a copper-based alloy, recovered heat could be used in a steam Rankine or supercritical carbon dioxide power cycle. If the molten material is steel, the recovered heat may rise to a temperature level suitable for use in an air Brayton cycle. One of skill in the art will appreciate that metal alloys heat differently and the proper method of extracting heat from the molten material during casting will depend on the heat transfer fluid being used, the method of casting, the amount of material used, the material's heat coefficient, etc.

It is yet a further aspect of embodiments of the present invention to provide a metal casting system that can be used in conjunction with the solar power generation system. Common concentrated solar power generation systems are comprised of a tower with a solar energy receiver. A plurality of reflectors are positioned about the tower that concentrate solar energy onto the receiver which heats salt or other heat transfer medium. Once the heat transfer medium is melted, it is pumped to a heat exchanger that draws the heat energy from the media. In one embodiment of the present invention, the collected solar energy is used to melt the metallic material that is later recast to obtain the collected solar energy using the apparatus and methods described herein. The metallic material being melted can be the cast metal formed by the methods contemplated herein, raw metallic material, etc. A similar system can be used with solar power systems that employ one or more parabolic mirrors that collect solar energy as described by PCT Patent Application Serial No. PCT/US1259210, filed October 8, 2012, entitled "HEAT TRANSFER FLUID HEATING SYSTEM AND METHOD FOR A PARABOLIC THROUGH SOLAR CONCENTRATOR," the entire disclosure of which is incorporated by reference herein.

One advantage of embodiments of the present invention is that the heat associated with casting that was previously lost is now captured at a usefully high temperature. The quality of heat energy exceeds that captured by the prior art methods that rely on capturing energy of a formed material from steam generated by water sprays contacting the molten metal, meaning it can later be used to perform useful work.

Thus, it is one aspect of the present invention to provide a metal casting system, comprising: a drive pulley spaced from an adjustable pulley; a conveyor belt extending between the drive pulley and the adjustable pulley; a heat exchanger positioned between the drive pulley and the adjustable pulley and in contact with an upper portion of the conveyor belt; and wherein molten metal is deposited on the conveyor belt adjacent to the heat exchanger and the heat exchanger draws heat energy from the molten metal to freeze the same into sheet.

It is yet another aspect of embodiments of the present invention to provide a metal casting system, comprising: a drive pulley spaced from an adjustable pulley; a conveyor belt extending between the drive pulley and the adjustable pulley; a heat exchanger positioned between the drive pulley and the adjustable pulley and in contact with an upper portion of the conveyor belt; a fluid flowing through the heat exchanger extracting heat from the heat exchanger; and wherein molten metal is deposited on the conveyor belt adjacent to the heat exchanger and the heat exchanger draws heat energy from the molten metal to freeze the same into sheet.

It is still yet another aspect of embodiments of the present invention to provide a method of generating energy from a metal casting process, comprising: providing a conveyor belt supported and driven by a drive pulley and an adjustable pulley; providing a heat exchanger associated with the conveyor belt; depositing molten metal on the conveyor belt; moving the conveyor belt to place the molten metal in close proximity to the heat exchanger; and transferring heat energy from the molten material to the heat exchanger.

The Summary of the Invention is neither intended nor should it be construed as being representative of the full extent and scope of the present invention. Moreover, references made herein to "the present invention" or aspects thereof should be understood to mean certain embodiments of the present invention and should not necessarily be construed as limiting all embodiments to a particular description. The present invention is set forth in various levels of detail in the Summary of the Invention as well as in the attached drawings and the Detailed Description of the Invention and no limitation as to the scope of the present invention is intended by either the inclusion or non-inclusion of elements, components, etc. in this Summary of the Invention. Additional aspects of the present invention will become more readily apparent from the Detail Description, particularly when taken together with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention and together with the general description of the invention given above and the detailed description of the drawings given below, serve to explain the principles of these inventions.

Fig. 1 is a schematic of a continuous casting process of the prior art;

Fig. 2 is a schematic of a continuous casting process of the prior art;

Fig. 3 is a schematic of a continuous casting process of one embodiment of the present invention;

Fig. 4 is a perspective view showing a heat transfer element employed by of one embodiment of the present invention;

Fig. 5A is a representation of a layer of a heat exchanger employed by one embodiment of the present invention; Fig. 5B is a representation of a layer of a heat exchanger employed by one embodiment of the present invention;

Fig. 5C is a representation of a layer of a heat exchanger employed by one embodiment of the present invention;

Fig. 6 is a schematic of a roller with an integral heat exchanger employed by another embodiment of the present invention;

Fig. 7 is a cross-sectional view of the roller in Fig. 6; and

Fig. 8 is a schematic of another embodiment of the present invention.

To assist in the understanding of one embodiment of the present invention the following list of components and associated numbering found in the drawings is provided herein:

# Component

11 Casting roller

12 Casting roller

13 Molten metal reservoir

14 Nozzle

15 Supply reel

16 Support sheet

17 Coiler

18 Support rolls

19 Sheet

21 Control device

25 Take-up reel

26 Injection nozzles

29 Tundish

110 Molten metal

114 Vessel

116 Outlet orifice

118 Plunger

120 Manifold

124 Rollers

126 Water jets

130 Upper run # Component

132 Tundish

134 Stream

136 Bottom surface

138 Side edges

140 Discharge end

142 Metal pool

144 Slab

146 Roller system

200 Casting system

204 Heat exchanger

208 Conveyor belt

212 Molten material

216 Tundish

220 Drive pulley

224 Adjustable pulley

228 Guard heater

232 Sheet

236 Sheet roller

240 Outlet line

244 Inlet line

248 Flow channel

252 Aft end

256 Forward end

300 Casting device

304 Roller

308 Tundish

312 Heat exchanger

316 Molten metal

320 Annular channel

324 Inlet channel

328 Outlet channel

332 Metal roller # Component

334 End plate

336 Pressure seal

340 End seal

404 Clamshell cooler

408 Tundish

412 Molten metal

416 Cooling passages

420 Fluid inlet

424 Fluid outlet

428 Spring-loaded clamp

432 Cutter

It should be understood that the drawings are not necessarily to scale. In certain instances, details that are not necessary for an understanding of the invention or that render other details difficult to perceive may have been omitted. It should be understood, of course, that the invention is not necessarily limited to the particular embodiments illustrated herein.

DETAILED DESCRIPTION

Fig. 3 is a schematic showing the casting system 200 of one embodiment of the present invention that utilizes a heat exchanger 204 that is associated with a conveyor belt 208 that carries molten material 212. The molten material 212 is transferred from a tundish 216 onto the conveyor belt 208. The conveyor belt 208 is supported and moved by at least one drive pulley 220 and at least one adjustable pulley 224, which can also function as a drive pulley. Rotation of the drive pulley 220 will pull the molten metal 212 away from the tundish 216 and across the heat exchanger 204. One of skill in the art will appreciate that the conveyor belt speed may vary heat transfer rate or effectiveness. Although the conveyor belt 208 is shown supported by a drive pulley 220 and the adjustable pulley 224, some embodiments of the present invention employ additional rollers and/or at least one heat exchanger that contact the top surface of the molten metal 212.

The system 200 may be surrounded by insulation, such as fiber board, graphite wool, or other known insulating materials. Further, the heat exchanger 204 and adjacent conveyor belt 208 may be surrounded by a guard heater 228 that helps reduce molten metal 212 temperature losses and metal corrosion. If the desired product is a sheet, after the frozen or partially frozen metal sheet 232 exits the guard heater 228 it is directed to a hot roll process to reduce the thickness and then to a sheet roller 236 (see Fig. 4). Because the sheet may remain somewhat hot for some time, it can be directed to another area where heat is drawn off the cooling sheet roll. If the cast metal is being used as a heat transfer material as part of a concentrated solar power plant, the sheet may be shredded for ease of handling.

Figs. 4 and 5 show the heat exchanger 204 that is used by embodiments of the present invention. The heat exchanger 204 is positioned under the metallic conveyor belt 208 and includes an inlet line 240 that carries cool heat exchange medium and an outlet line 244. The heat exchanger 204 may include a printed circuit made of Haynes 230 with etched flow channels 248 which is bonded into a monolithic structure. Further, to reduce the thermal contact resistance, a high conductance, low friction material such as graphite can be positioned between the belt 208 and the heat exchanger 204. One of skill in the art will appreciate that as opposed etching, other methods of forming a flow channel within the heat exchanger 204 may be employed without departing from the scope of the invention. The fluid running through the heat exchanger is, in one embodiment of the present invention, supercritical carbon dioxide that is supplied at the aft end 252 of the heat exchanger and which flows towards the forward end 256 of the heat exchanger. Other embodiments of the present invention utilize carbon dioxide, nitrogen, helium, argon, water vapor, supercritical nitrogen, supercritical helium, supercritical argon, or supercritical water vapor. If needed, extraction of carbon dioxide can be made through the walls of the heat exchanger. Ideally, the temperature difference between the carbon dioxide in the outlet line 244 and the molten metal is minimized such that the maximum amount of heat is extracted from the molten metal, which means that the majority of the latent heat of freezing is captured. Fig. 5 shows different methods of circulating fluid through the heat exchanger 204. One of skill in the art will appreciate that the flow channels 248 may have other flow patterns without departing from the scope of the invention.

In one embodiment of the present invention, eutectic aluminum-silicon alloy is used as the molten metal that is initially about 600°C, and which freezes at about 577°C. The material is cooled to about 300°C before the sheet is rolled. The temperature of the supercritical carbon dioxide entering the heat exchanger is about 300°C and is heated and maintained to about 560 to 570°C by controlling its mass flow rate, which can be reduced by extracting carbon dioxide at, for example, the location where the molten material begins to freeze. In one embodiment, the heat exchanger is about 8-32 feet (about 2.44 - 9.75 m) long by about 5 feet (about 1.52 m) wide and can capture enough heat that is used to generate about 250 kW - 1 MW of power.

Small-scale tests have been conducted to validate the apparatus and systems described herein. The average heat transfer coefficient is about 550-700 W/m²/K when using eutectic aluminum-silicon alloy. The higher that the coefficient is for a particular material, the more rapidly that heat will be transferred through that material. Thus, as the contemplated design is scaled up, the heat transfer coefficient will be suitable for cost- effective use in many desired applications.

Figs. 6 and 7 show another casting device 300 of embodiments of the present invention that uses a roller 304 that draws molten metal 316 from a tundish 308 positioned adjacent to the roller 304. The roller 304 surrounds an internally-positioned and stationary heat exchanger 312. The heat exchanger 312 maintains its position while the roller 304 rotates and draws molten metal 316 from the tundish 308. The heat exchanger 302 includes an outer annulus or groove 320 that carries heat exchanging medium that is pumped from one or more inlet channels 324 to one or more outlet channels 328. As the roller 304 rotates, molten metal 316 is pulled from the tundish 308, the molten material rides on the roller, it is cooled by the transfer heat to the heat exchange medium in the annular channel 320. When the molten material has traveled, this example about 180° about the center of the roller, it is at least partially frozen and can be transferred to a metal roller 332. The hot heat transfer medium is then transferred to a location where the heat energy is drawn therefrom.

The roller 304 of one embodiment of the present invention is about 2 meters long and has a diameter of about 25 centimeters. The roller 304 may be made of steel, carbide ceramic, or any other suitable material. The heat exchanger 312 is interconnected to end plates 334 of the roller 304 by rotating pressure seals 336 that allow the roller to be rotated while the heat exchanger 312 remain stationary. End seals 340 may also be provided to prevent leaks of heat exchange medium.

Fig. 8 shows yet another embodiment of the present invention that employs a clamshell cooler 404. Here, molten metal 412 is drawn into the clamshell cooler 404, which includes a plurality of cooling passages 416 integrated into its wall thickness. The cooling passages 416 include a fluid inlet 420 and a fluid outlet 424. Further, the clamshell cooler includes a plurality of spring-loaded clamps 428 that allow the volume of the clamshell cooler 404 to be selectively altered.

In operation, molten metal 412 is drawn from a tundish and into the clamshell cooler 404. As the metal within the clamshell cooler 404 cools, the spring-loaded clamps 424 bias the top or bottom portion of the clamshell cooler to accommodate the shrinking metal. The heat exchange medium flowing through the cooling passages 416 capturing expelled heat which can be used in another process as described above. The solidified metal is taken from the clamshell cooler 404 and can be cut by a cutter 432 or otherwise modified.

While various embodiments of the present invention have been described in detail, it is apparent that modifications and alterations of those embodiments will occur to those skilled in the art. However, it is to be expressly understood that such modifications and alterations are within the scope and spirit of the present invention, as set forth in the following claims. Further, the invention(s) described herein is capable of other embodiments and of being practiced or of being carried out in various ways. In addition, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of "including," "comprising," or "having" and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items.

Claims

What is claimed is:

1. A metal casting system, comprising:

a drive pulley spaced from an adjustable pulley;

a conveyor belt extending between said drive pulley and said adjustable pulley; a heat exchanger positioned between said drive pulley and said adjustable pulley and in contact with an upper portion of said conveyor belt;

a fluid flowing through the heat exchanger extracting heat from the heat exchanger; and

wherein molten metal is deposited on said conveyor belt adjacent to said heat exchanger and said heat exchanger draws heat energy from said molten metal to freeze the same into sheet.

2. The system of claim 1, wherein said heat exchanger is surrounded by a guard heater.

3. The system of claim 1, wherein said heat exchanger includes at least one inlet line and at least one outlet line wherein said heat exchanger accommodates cooling fluid that exits the heat exchanger at about the same temperature as the molten material.

4. The system of claim 1, further comprising a metal roll positioned adjacent to said adjustable pulley that receives frozen sheet metal.

5. The system of claim 1, wherein said heat exchanger is in contact with said conveyor belt.

6. The system of claim 1, wherein said heat exchanger is in contact with said conveyor belt with a high conductance, low friction material positioned therebetween.

7. The system of claim 1, wherein said conveyor belt is coated.

8. The system of claim 1, wherein at least one of said drive pulley and said adjustable pulley includes an integral heat exchanger.

9. The system of claim 1, wherein said heat exchanger utilizes gas that enters said heat exchanger at about 309°C and exits said heat exchanger at about 560 to 570°C.

10. The system of claim 9, wherein said gas is at least one of carbon dioxide, nitrogen, helium, argon, water vapor, supercritical carbon dioxide, supercritical nitrogen, supercritical helium, supercritical argon, and supercritical water vapor.

11. The system of claim 9, wherein the temperature of said gas is controlled by altering the mass flow thereof through said heat exchanger.

12. A method of generating energy from a metal casting process, comprising: providing a conveyor belt supported and driven by a drive pulley and an adjustable pulley;

providing a heat exchanger associated with said conveyor belt;

depositing molten metal on said conveyor belt;

moving said conveyor belt to place said molten metal in close proximity to said heat exchanger; and

transferring heat energy from said molten material to said heat exchanger.

13. The method of claim 12, further comprising:

producing steam with said heat energy;

directing said steam to an electically-producing turbine generator.

14. The method of claim 12, wherein said heat exchanger employs at least one of an aluminum or a copper based alloy as a heat transfer medium, and the heat energy is used in a steam Rankine power cycle.

15. The method of claim 12, wherein said heat exchanger employs at least one of an aluminum based alloy, a copper based alloy, a zinc based alloy, and an iron based alloy as a heat transfer medium, and the heat energy is used in a supercritical carbon dioxide power cycle.

16. The method of claim 12, wherein said heat exchanger employs steel as a heat transfer medium, and the heat energy is used in an air Brayton cycle.

17. The method of claim 12, further comprising concentrating solar energy onto solid metal to transform said solid metal into said molten metal.

18. The method of claim 12, wherein said at least one of said drive pulley and said adjustable pulley includes a second heat exchanger and further comprising drawing heat from said molten metal with said second heat exchanger.

19. The method of claim 12, wherein said heat exchanger utilizes supercritical carbon dioxide as a heat transfer medium that enters said heat exchanger at about 309°C and exits said heat exchanger at about 560 to 570°C.

20. The method of claim 19, wherein the temperature of said supercritical carbon dioxide is controlled by altering the mass flow thereof through said heat exchanger.