Patent Application
20210340070
This disclosure generally relates to designs, methods, and materials for inhibiting the penetration of molten salts into surfaces. In particular, this disclosure relates to designs, methods, and materials for inhibiting the penetration of molten halide salts, including but not limited to molten chloride salts, through the walls of containment devices at high temperatures. This disclosure also relates to penetration-resistant containment devices made utilizing the designs, methods, and materials of this disclosure. Such devices include, but are not limited to, piping, valves, seals, and thermal energy storage tanks for high-temperature systems, including, but not limited to, high-temperature systems for energy (e.g., electrical power) production, energy storage, waste heat recovery, and chemical processing.
This section introduces aspects that may help facilitate a better understanding of the disclosure. Accordingly, these statements are to be read in this light and are not to be understood as admissions about what is or is not prior art.
Molten salts can possess attractive characteristics for use as heat transfer liquids and as thermal energy storage liquids. Such molten salts include, but are not limited to, molten halides, molten nitrates, molten carbonates, molten sulfates, molten hydroxides, and molten oxides. Nonlimiting examples of molten halides include molten chlorides and molten fluorides.
Molten halide salts can possess particularly attractive characteristics for use as high-temperature heat transfer liquids and as high-temperature thermal energy storage liquids. Such attractive characteristics include chemical stability at high temperatures, modest melting points, low vapor pressures at high temperatures (i.e., high boiling points), modest densities, modest values of viscosity at high temperatures, and high heat capacities per unit volume.
During use as high-temperature heat transfer liquids and high-temperature thermal energy storage liquids, molten salts come into contact with a number of solid materials, including but not limited to solid materials used for containment of the molten salt in the walls of pipes and tanks. In order to allow for prolonged, cost-effective use of molten salts as high-temperature heat transfer liquids and high-temperature thermal storage liquids, the wall of a pipe or tank needs to be resistant to penetration by the molten salts at high temperatures. However, molten salts tend to wet, infiltrate, and penetrate a variety of solid materials, including solid metals, metal alloys, metal-bearing composites, ceramics, and ceramic-bearing composites. Such penetration of the molten salt into the wall of a pipe or tank can degrade desired properties of the wall and, therefore, the pipe or tank, with such properties including, but not limited to, thermal and mechanical properties. Such thermal properties include thermal conductivity. Excessive penetration of the molten salt into the walls of pipes and tanks can also lead to undesired loss of the molten salt during its use as a heat transfer liquid and thermal energy storage liquid.
Earth-abundant, low-cost molten chlorides can be particularly attractive as high-temperature heat transfer liquids and high-temperature thermal energy storage liquids, as a nonlimiting example, at temperatures above 600° C. One such group of molten chlorides are MgCl2—KCl-bearing salts, including binary MgCl2—KCl compositions and ternary MgCl2—KCl—NaCl compositions. Such MgCl2—KCl-bearing salts tend to become contaminated with oxygen-bearing species upon exposure to air at high temperatures, and form MgO or MgO-bearing species upon reaction with the oxygen or water vapor in air at high temperatures. In order to minimize such contamination and formation of MgO or MgO-bearing species, MgCl2—KCl-bearing salts may be sealed from the air environment, and/or contained within an inert or reducing atmosphere, and/or exposed to an agent that can act to remove oxygen-bearing species from the molten salt. However, molten chloride salts including MgCl2—KCl-bearing salts tend to wet, infiltrate, and penetrate solid materials, including solid metals, metal alloys, metal-bearing composites, ceramics, and ceramic-bearing composites. For example, structural steels exhibit appreciable corrosion by molten chlorides, particularly if the molten chlorides are contaminated with oxygen-bearing species, and porous ceramics generally exhibit appreciable molten salt penetration, particularly at temperatures of 550-750° C. or higher. These detrimental behaviors occur in inert atmospheres, including inert argon atmospheres, and in reducing atmospheres, including hydrogen-argon atmospheres. While corrosion and penetration may be addressed by using nickel-based superalloys or high-density (closed porosity) ceramics, large storage tanks and long pipe lengths comprised of these materials would be expensive.
A nonlimiting example of a system that requires containment materials that are resistant to excessive penetration by molten salts at high temperatures is a concentrated solar power (CSP) plant. Molten chloride salts, including but not limited to MgCl2—KCl—NaCl salts, can be attractive for use as low-cost, readily available (sea-water-derived), high-temperature fluids for heat transfer (e.g., to transfer solar heat from a receiver to a working fluid through a heat exchanger, or to transfer solar heat to a thermal energy storage tank). However, excessive molten salt penetration into the wall of a pipe or tank would result in an undesired loss of the molten salts and would result in degradation of the thermal and mechanical properties of the containment materials. For example, the thermal insulation capability of a thermal energy storage tank can be significantly degraded by the excessive penetration of a molten salt into a wall of the tank (since air is a much better thermal insulator than the molten salt). Hence, the effective use of such molten salts as heat transfer media and as thermal energy storage media requires containment devices (including but not limited to pipes and thermal energy storage tanks) with molten-salt-facing walls that are: i) resistant to excessive penetration by the molten salts at high temperatures, and ii) resistant to recession or removal, such as by abrasion during installation of the containment material, or during use of the containment material, such as by erosion during use of the containment material (i.e., by erosion from contact with the flowing molten salt) or such as by thermal stresses that may be encountered during heating or cooling of the containment material. Such penetration-resistant walls of molten-salt-containment devices also need to be fabricated from inexpensive materials and need to be formed into desired shapes by cost-effective processes.
Existing methods to reduce molten salt infiltration and penetration into containment materials include: i) the use of dense (non-porous) coatings on molten-salt-facing surfaces of containment materials that prevent the penetration of a molten salt into the interiors of the containment materials, and ii) the use of non-dense coatings on molten-salt-facing surfaces of containment materials that are not wetted by a molten salt and that thereby inhibit infiltration of the molten salt into the interior of the containment materials. As a nonlimiting example, the application of a high-density graphite coating on a molten-salt-facing surface of a containment material may be considered as a means of inhibiting penetration of a molten salt (including but not limited to a molten chloride salt) into an underlying containment material. However, such coatings, or sections of such coatings, are prone to unintentional removal from a molten-salt-facing surface of a containment material during installation of the containment material, such as by abrasion during installation of the containment material, or during use of the containment material, such as by erosion during use of the containment material (i.e., by erosion from contact with the flowing molten salt), or by thermal stresses that may be encountered during heating or cooling of the coated containment material. The loss of continuity of such coatings on a molten-salt-facing surface would then provide a pathway for the infiltration and penetration of the molten salt into the containment material. Such coatings would require regular inspections to determine whether the coatings have been retained as a continuous layer within a given time, and whether the coating needs to be repaired if the coating has been partially removed. The requirement for regular inspections and regular repairs can result in undesired downtime and additional undesired costs.
Graphite plates or blocks or tubes (or other shapes) of high relative density (i.e., of low relative porosity) may also be considered as liner materials for inhibiting the penetration of a molten salt (including but not limited to a molten chloride salt) into surrounding materials. However, even such high-density graphite is prone to removal during use, such as by erosion from contact with a flowing molten salt. Graphite plates or blocks or tubes (or other shapes) of high relative density (i.e., of low relative porosity) are also expensive, owing to the high temperatures required to fire their graphite materials in order to obtain a high relative density (and low relative porosity).
Thus, there is an unmet need for cost-effective designs, materials, and fabrication methods for the containment of molten salts that would avoid excessive penetration of molten salts at high temperatures into the walls of the containment devices and provide containment devices with walls that are resistant to recession or removal, such as by abrasion during installation of the containment material, or during use of the containment material, such as by erosion during use of the containment material (i.e., by erosion from contact with the flowing molten salt) or such as by thermal stresses that may be encountered during heating or cooling of the containment material.
The present invention provides materials systems that are resistant to penetration of molten salts and may be present within a molten-salt-facing wall of a device for containing a molten salt bath at an elevated temperature and provides methods for producing such materials systems and molten-salt-facing walls and devices formed by such materials systems.
According to one aspect of the invention, a materials system is provided that is resistant to penetration of a molten salt. The materials system is present within a molten-salt-facing wall of a device for containing a molten salt bath at an elevated temperature. The materials system includes a first layer that comprises a first material and defines an outer surface of the materials system for direct contact with the molten salt bath. The first layer possesses resistance to erosion in the molten salt at the elevated temperature, possesses resistance to corrosion in the molten salt at the elevated temperature, and is penetrable by the molten salt at the elevated temperature. The materials system further includes a second layer that comprises a second material and is located adjacent to the first layer, such that the first layer is located between the second layer and the outer surface of the materials system. The second layer exhibits little or no wetting by the molten salt so that at least a portion of a thickness of the second layer is not penetrable by the molten salt. The materials system further includes a third layer that comprises a third material and is located adjacent to the second layer. The third layer is porous and exhibits a low thermal conductivity at the elevated temperature.
Other aspects of the invention include methods of producing the materials system as described above, as well as devices that comprise the materials system and molten-salt-facing walls comprising the materials system.
Aspects and advantages of this invention will be appreciated from the following detailed description.
Some of the drawings shown herein may have been created from scaled drawings or from photographs that are scalable. It is understood that dimensions or relative scaling within a drawing are by way of example, and not to be construed as limiting. Further, in this disclosure, the drawings are shown for illustrative purposes and not to scale, and those skilled in the art can readily recognize the relative dimensions of different drawings depending on how the principles of the disclosure are used in practical applications.
For the purposes of promoting an understanding of the principles of the disclosure, reference will now be made to the embodiments illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the disclosure is thereby intended, such alterations and further modifications in the principles of the disclosure, and such further applications of the principles of the disclosure as illustrated therein being contemplated as would normally occur to one skilled in the art to which the disclosure relates.
The present invention provides materials systems suitable for use as walls of molten-salt-containment devices to inhibit excessive penetration of molten salts at high temperatures into the walls of the molten-salt-containment devices, and thereby to provide the walls of molten-salt-containment devices with desired thermal, chemical, and mechanical properties. The present invention also provides methods of producing the materials systems, as well as molten-salt-facing walls and devices that make use of the materials systems. The materials systems generally utilize a multilayer wall comprising one or more layers of a rigid, chemically robust, porous ceramic compound that provides resistance to erosion by molten chloride salts, alternating with one or more porous layers comprising a carbon particulate material that provides resistance to molten chloride penetration. Material systems comprising layers of such materials may be fabricated by relatively low-cost casting processes. High temperatures of interest will depend on the particular application. For applications involving the use of high-temperature heat transfer liquids and high-temperature thermal energy storage liquids, high temperatures of interest are typically above 600° C. and often above 750° C. However, it is foreseeable that other applications may benefit from the materials systems described below, and that operating temperatures for such applications may be above 25° C., for example, above 100° C., above 300° C., above 500° C., above 700° C., above 750° C., or above 1000° C.
In studies leading to this disclosure, a molten MgCl2—KCl—NaCl salt was determined to infiltrate porous cast CaO—Al2O3-based ceramics in an argon atmosphere at 750° C. Photographs of porous CaO—Al2O3-based ceramic crucibles, before and after exposure to a molten MgCl2—KCl—NaCl salt, are shown in
In further studies leading to this disclosure, additional ceramic crucibles and solid chloride salt compositions were prepared and investigated. In these studies, MgCl2 (anhydrous, 99.0% purity), KCl (99.9% purity), and NaCl (99.0% purity) were used to prepare MgCl2—KCl—NaCl salt mixtures. A calcium hexa-aluminate (CA6) based castable composition (BLG, 5.9:1 Al2O3:CaO molar ratio, Westmoreland Advanced Materials, Inc., Charleroi, Pa., USA) was used to generate rigid, porous ceramic crucibles. Synthetic graphite particles (K106 grade, ≥99.0% purity, ≥89% with sizes between 149 μm (100 mesh) and 840 μm (20 mesh) (Carbon Graphite Materials, Inc., Brocton, N.Y., USA) were used to generate packed particulate-based layers.
The MgCl2—KCl—NaCl salt mixtures were prepared to contain 40 mol % MgCl2, 40 mol % KCl, and 20 mol % NaCl was prepared by mixing and melting the pure chloride salt components. The ceramic crucibles formed of calcium hexa-aluminate were prepared by casting to produce crucibles of two different sizes, with smaller crucibles sized to fit within cavities of the larger crucibles such that a gap is present between and completely separates the two crucibles. The cast crucibles were cured for 20 h at 35° C. in a water-vapor-saturated atmosphere within a sealed oven, then heated for an additional 24 h at 110° C. in air. The cast cured crucibles were then heated at 100° C./h to a peak temperature of 750° C. in industrial grade Ar and held at this temperature for 24 h, yielding porous calcium hexa-aluminate-based crucibles.
A multilayer materials system was then formed for containment of the MgCl2—KCl—NaCl salt mixture. The material system included one each of the smaller and larger calcium hexa-aluminate-based ceramic crucibles, with the smaller crucible placed within the cavity of the larger crucible and a packed graphite particulate layer provided within the gap between the two crucibles. Prior to testing, an amount of the MgCl2—KCl—NaCl salt mixture was premelted and placed within the cavity of the smaller crucible, which was then heated in flowing industrial-grade (99.99% purity) Ar at 100° C./h to 750° C. and held at the latter temperature for 2 h. After cooling to room temperature, weight change measurements indicated that the smaller crucible contained 18.3 grams of solidified salt. A layer of graphite powder of approximately 9 mm thickness was then deposited within the cavity of the larger crucible and vibrated before placing the salt-bearing smaller crucible (from the previous step) within the cavity of the larger crucible and on the layer of graphite powder. The remaining gap between the walls of the smaller (inner) and larger (outer) crucibles was then filled with graphite powder and the whole assembly was vibrated. Weight measurements indicated that a total of 83.9 grams of graphite powder was present within the gaps between the inner and outer crucibles.
A molten salt penetration experiment was then conducted. For this experiment, an additional amount (21.9 grams) of the MgCl2—KCl—NaCl salt mixture was placed inside the cavity of the inner crucible and the entire assembly (the inner and outer crucibles and the graphite powder therebetween) was heated in flowing industrial-grade Ar at 100° C./h to 750° C. and held at the latter temperature for 24 h. After cooling to room temperature, weight measurements of the entire assembly indicated that the remaining salt content contained within the assembly was 37.8 grams (i.e., 2.4 grams of the salt had been lost by evaporation). The graphite powder separating the inner and outer crucibles remained loose and was easily removed from the assembly, with no appreciable increase in apparent agglomeration due to salt penetration into the graphite powder. The graphite powder weighed 80.9 grams, which was 3.0 grams (3.6%) less than the weight of the graphite powder that had been placed in the assembly prior to the experiment. This weight loss was attributed to the graphite powder remaining attached to the inner surfaces of the outer crucible and the outer surfaces of the inner crucible. The outer crucible weighed 0.5 grams more than what the outer crucible weighed prior to the experiment, and the inner crucible weighed 2.4 grams more than what the inner crucible previously weighed (after considering the weight of the salt remaining in the inner crucible after the 24 h test at 750° C.). Hence, the combined weight of the graphite remaining attached to the crucibles (2.9 g) and the graphite removed from the assembly (80.9 g) was close to the weight of the graphite powder originally placed in the assembly (83.8 g vs. 83.9 g, respectively). Hence, it was concluded that no appreciable weight gain of the graphite powder occurred due to penetration of the molten salt through the inner crucible and into the graphite layer.
X-ray diffraction analysis was used to determine whether any molten salt had penetrated into the graphite powder. Three graphite powder batches extracted from the assembly after the experiment did not exhibit any detectable diffraction peaks from the salt. The absence of any detectable salt in the extracted graphite powder demonstrated that the experimental materials system was effective for preventing molten salt penetration into the intermediate graphite powder layer as well as the porous outer crucible, such that the performance of these layers of the materials system would not have been adversely affected by the molten salt.
On the basis of the experiments, it is believed that similar or equivalent results can be obtained with other materials used for the crucibles and intermediate layer separating them. Another group of molten chlorides is CaCl2)—NaCl-bearing salts, including binary CaCl2)—NaCl compositions, ternary CaCl2)—NaCl—BaCl2, and quaternary CaCl2)—NaCl—BaCl2—KCl compositions. Such CaCl2)—KCl-bearing salts tend to wet, infiltrate, and penetrate solid materials, including solid metals, metal alloys, metal-bearing composites, ceramics, and ceramic-bearing composites in air, in inert atmospheres, including inert argon atmospheres, and in reducing atmospheres, including hydrogen-argon atmospheres. However, it was determined that molten CaCl2)—NaCl-bearing salts do not wet, and do not penetrate into, graphite crucibles at 750° C.
Molten salts of particular interest include, but are not limited to, any one of the following materials or any combinations of the following materials: halide-bearing liquids, nitrate-bearing liquids, carbonate-bearing liquids, sulfate-bearing liquids, hydroxide-bearing liquids, and oxide-bearing liquids. Such molten halides include, but are not limited to, chloride-bearing liquids and fluoride-bearing liquids.
Molten-salt-containment devices of particular interest include, but are not limited to, pipes, valves, seals, and thermal energy storage tanks.
The present invention provides high-temperature systems utilizing the methods, materials, and designs of the walls of molten-salt-containment devices, with such methods, materials, and designs providing resistance to excessive penetration of molten salts at high temperatures into the walls of the molten-salt-containment devices, and thereby providing the walls of the molten-salt-containment devices with desired thermal, chemical, and mechanical properties. Examples of such high-temperature systems include, but are not limited to, systems for energy (e.g., electrical power) production, energy storage, waste heat recovery, and chemical processing. Examples of electrical power production systems in the context of this invention include, but are not limited to, systems for fossil fuel-based electricity generation, solar energy-based electricity-generation, hydrothermal energy-based electricity generation, and nuclear energy-based electricity generation. Solar energy-derived power production systems in the context of this invention include but not limited to concentrating solar power production systems.
A nonlimiting embodiment of a wall of a molten-salt-containment device of the present invention is schematically represented in
By providing a layer A that is rigid and resistant to erosion by the flowing molten salt, layer B (e.g., corresponding to the intermediate layer of the experiments) is able to contain a material that is not erosion resistant (since layer B is not exposed to the flowing molten salt). Layer B may contain a lower-cost material, or a lower-cost mixture of materials. Layer B may contain materials with forms selected from a list including, but not limited to, solid particles and hollow particles. Such solid and hollow particles may possess shapes including, but not limited to, spheres, fibers, flakes, platelets, and irregular shapes.
By providing a layer A containing one or more materials that is/are wetted by the molten salt at elevated temperatures, and a layer B that contains one or more materials that exhibit little or no wetting by the molten salt, there is a thermodynamic driving force to retain the molten salt in layer A (since the salt prefers to wet the material or materials in layer A).
By providing a layer C (e.g., corresponding to the outer crucible of the experiments) containing a highly porous material that exhibits, or contains a highly porous mixture of materials that exhibit, low thermal conductivity, the wall of the containment device can be thermally insulating, that is, a low rate of heat loss can be achieved through the wall of such a containment device, so that the heat can be retained in the flowing or stagnant molten salt bath.
Another nonlimiting embodiment of a wall of a molten-salt-containment device of the present invention is schematically represented in
The following description provides specific examples of the above general concepts of this disclosure with reference to several solid materials, alloys, ceramics, and liquids. These are merely exemplary embodiments and are not intended to limit the scope of this disclosure.
The layers A in
The layers B in
Suitable non-wetted solids or non-wetted mixtures of solids for use in layers B in
Any of the non-wetted solids and non-wetted mixtures of solids described above for layers B in
The layer C in
One example of the general design concept of the wall of a molten-salt-containment device of the present invention shown in
One example of the general design concept of the wall of a molten-salt-containment device of the present invention shown in
The present disclosure is believed to describe cost-effective designs, materials systems, and fabrication methods for the containment of molten salts, and that such materials systems are capable of inhibiting or preventing penetration of molten salts at high temperatures into the walls of containment devices. Furthermore, the materials systems are believed to be resistant to recession or removal, such as by abrasion during installation of the containment material, or during use of the containment material, such as by erosion during use of the containment material (i.e., by erosion from contact with the flowing molten salt) or such as by thermal stresses that may be encountered during heating or cooling of the containment material. Based on experimental results, it is believed that the first layers (layer A) of the materials systems may be capable of exhibiting recession rates of 100 microns per year or less, possibly as low as 10 microns per year.
While the present disclosure has been described with reference to certain embodiments, it will be apparent to those of ordinary skill in the art that other embodiments and implementations are possible that are within the scope of the present disclosure without departing from the spirit and scope of the present disclosure. For example, the materials comprising layers A, B, and C in the design of
This application claims the benefit of U.S. Provisional Application No. 63/003,746 filed Apr. 1, 2020, the contents of which are incorporated herein by reference.
This invention was made with government support under Contract No. DE-EE0008375 awarded by the U.S. Department of Energy. The government has certain rights in the invention.
| Number | Date | Country | |
|---|---|---|---|
| 63003746 | Apr 2020 | US |
