Method of producing hydrogen using aluminium

Abstract

Disclosed is a method of producing hydrogen from the reaction of liquid aluminium or a liquid aluminium alloy with water vapour. The method includes the steps of: (a) providing liquid aluminium or liquid aluminium alloy, wherein said liquid has a surface; (b) reacting said liquid with water vapour in order to generate alumina and hydrogen, wherein if the reaction is carried out at a temperature range of 650 to 900 °C and a pressure range of 0.1 to 1 MPa, at least 50% of the hydrogen dissolves in the liquid, and wherein said reaction takes place at the surface and / or in the liquid; (c) extracting hydrogen in the form of gas from the liquid.

Description

[0001] Method of producing hydrogen using aluminium

[0002] Technical Field

[0003] The present invention relates to a method of producing hydrogen and a kit for carrying out the method. In particular, it relates to a method of producing hydrogen from the reaction of liquid aluminium or a liquid aluminium alloy with water vapour.

[0004] Background

[0005] In recent years, the use of hydrogen as a clean fuel for green technologies has garnered significant interest. Current methods of hydrogen production, however, are environmentally problematic, with most hydrogen derived from fossil fuels, through processes that emit significant amount of greenhouse gases. Hydrogen that is produced from the electrolysis of water, although "green" in terms of emissions, is usually heavily reliant on fossil fuels to generate the electricity required in this energy-intensive process. Moreover, electricity represents very high exergy or potential to do work, and can thus be converted into work, i.e. mechanical energy, with ultimate efficiency. The same cannot be said for hydrogen, which, as chemical potential energy and thus a proxy for heat and other derivatives, has much lower exergy or potential to do work, so extra efforts are needed to turn it into exergy. Therefore, using electricity in electrolysis to produce hydrogen is not energy- and cost-efficient.

[0006] At the same time, aluminium is considered an important material for a green future, playing a crucial role in the circular economy due to its infinite recyclability. There is increasing demand for recycled aluminium due to the high carbon footprint and cost of primary aluminium. In providing a method for producing hydrogen as an auxiliary process of aluminium refining or remelting, the environmental problem of hydrogen production as well as reducing the carbon footprint of primary aluminium are addressed using the aluminium-processing infrastructure that will be prevalent in a circular economy of the future.

[0007] The highly exothermic oxidation reaction of solid aluminium with water to produce heat and hydrogen is one that is known and well- documented. Note that this reaction does not require exergy to be initiated or maintained. In practice, powdered aluminium is used to provide a large enough aluminium surface so that the capacity for reaction is worthwhile. However, any further reaction is prevented by the surface oxide that forms, rendering the process inefficient. To overcome this, alkali solutions of water are used, which contaminates a valuable by-product of the reaction, i.e. aluminium hydroxide. Powdered aluminium also carries a significant risk of explosion, raising issues for industrial applicability.

[0008] Attempts to harness the reaction of water with liquid aluminium to produce hydrogen have been made to overcome the limitations of using solid aluminium. In addition, the amount of heat released by the reaction (that can also be utilised) is larger in the case of liquid aluminium, and the reaction product is alumina that can be directly returned into the aluminium refining stage, instead of aluminium hydroxide that requires additional processing stages to convert it into aluminium oxide. The reaction is as follows:

[0009] 2AI (I) + 3H2O (g) = AI2O3 (s) +3H2(g)

[0010] As a result of this exothermic reaction, 1 kg of liquid Al releases

[0011] 17.9 MJ of heat (corresponding to 5 kWh kg-1Al that can be converted back to electricity using a heat engine), 0.11 kg of H2 (corresponding to about 4.4 kWh kg-1Al), and 1.9 kg of alumina (AI2O3) which can be returned into the aluminium production, recycled or sold as pure alumina. See H. Ersoy et al. "Hybrid energy storage and hydrogen supply based on aluminum— a multiservice case for electric mobility and energy storage services". Adv. Mater. Technol. 2022, 7, 2101400.

[0012] DOI: 10.1002 / admt.202101400.

[0013] One example is proposed in US3975913 (ERICKSON), which discloses a method of producing hydrogen by exposing liquid aluminium to a gaseous reactant, preferably water vapour, and bubbling this water vapour through a zone of reaction of at least 6 inches in vertical dimension in liquid aluminium into a gas-collecting vessel. There is no disclosure of hydrogen dissolution in the liquid aluminium.

[0014] W02021004791A1 (SONY SEMICONDUCTOR SOLUTIONS CORP, et al.) discloses a method which involves introducing liquid water droplets into the bottom of a vessel containing molten aluminium.

[0015] These disclosures are surprising and hardly practical as it is well documented that there is an explosion risk associated with instantaneously vaporising water under or inside a volume of molten aluminium, and the subsequent highly exothermic reaction of water vapour with the molten metal equalling an explosion. See for example S.G. Epstein, "The aluminium industry's efforts to prevent molten metal explosions," in Light Metals 2023, Warrendale (PA), TMS, 2023, pp. 679-681. Accordingly, it is possible that in fact the disclosed methods were never tested or verified. CN111056529A (WANG GUANGWU) discloses a hydrogen station for producing hydrogen by reacting water vapour with aluminiummagnesium melt, wherein the water vapour is bubbled through the melt.

[0016] Energy, vol. 93, 2015, Yang Weijuan et al., "Experimental researches on hydrogen generation by aluminium with adding lithium at high temperature", pp. 451-457 discloses methods of promoting the reactivity of aluminium with water by addition of lithium to the aluminium in molten state. In this method, the incoming water vapour is mixed with the hydrogen that is a product of reaction.

[0017] The present invention seeks to provide an improved method of producing hydrogen and a kit for carrying out the method.

[0018] Summary of Invention

[0019] According to a first aspect of the present invention, there is provided a method of producing hydrogen as claimed in claim 1.

[0020] The method harnesses the dissolution of hydrogen produced from the step (bl) reaction in the liquid, subsequently facilitating extraction of said dissolved hydrogen from the liquid in the form of gas.

[0021] It is well known in the art that hydrogen solubility in liquid aluminium depends on temperature and pressure, see for example: P.N. Anyalebechi. "Hydrogen Solubility in Liquid and Solid Pure Aluminum- Critical Review of Measurement Methodologies and Reported Values", Materials Sciences and Applications, 2022, 13, 158-212, doi.org / 10.4236 / msa.2022.134011. It will be understood, therefore, that the temperature and pressure can be adjusted in order to control the amount of hydrogen dissolved. In a preferred embodiment, in step (bl) at least 50% of the hydrogen generated from the reaction dissolves in the liquid. In other embodiments, the proportion of hydrogen generated from the reaction which dissolves in the liquid is: at least 55%; 60%; 65%; 70%; 75%; 80%; 85%; 90%; and 95%. It is unlikely that greater than 98% dissolution can be achieved due to inevitable losses.

[0022] The proportions in the preferred embodiments above are calculated according to the following formula:

[0023] The method disclosed herein is suitable for incorporating into commercial aluminium processing lines, for example at the refining stage(s) of primary aluminium or remelting operations of recycled aluminium, or it may alternatively be treated as a standalone process. The composition of the liquid can vary depending on the application of the method. When it is incorporated in the primary aluminium production, the liquid is represented by the primary aluminium. When it is incorporated into the remelting or recycling operations, the composition of the liquid represents the composition of the alloys or recycled scrap that is used. When it is used as a standalone process, the composition of liquid can be tuned to increase the solubility of hydrogen and changing the structure of the surface oxide layer by using for example (but not limited to) Li, Sr, Ga, Mg, Ca, Ti, Zr, Ga as alloying additions to aluminium.

[0024] Although the hydrogen may be produced and / or removed in batches, the preferred embodiments disclosed herein are able to provide continuous hydrogen production. Figure 1 provides a concept of using the method (HydrAI) as embedded into the primary aluminium production process.

[0025] In a preferred embodiment, the reaction in step (bl) takes place at the surface and / or in the liquid no further than 10 mm from said surface. As such, there is provided an improved method of producing hydrogen which, by reacting water vapour with aluminium at (or slightly below) a surface of liquid aluminium, safely releases energy from the exothermic reaction of aluminium with water.

[0026] The water provided via the inlet is in the form of water vapour, thus providing a large and controllable partial pressure of reactant at the surface of the liquid such that the reaction proceeds at a faster rate and the solubility of hydrogen in the liquid aluminium is increased. Having water as a vapour assists in containing the reaction to the first surface of the liquid and / or in the liquid no further than 10 mm from said first surface. As such, the alumina produced from the reaction remains relatively undisturbed and can be routinely removed to maximise the available reaction surface. Introducing the vapour in the immediate (within 10 mm) subsurface of the liquid ensures that the byproduct of the reaction, i.e. alumina, does not prevent the reaction from proceeding. Containing the reaction to the surface and / or no further than 10 mm from the surface ensures that thermal energy generated from the exothermic reaction may be released and collected in a controlled and safe manner.

[0027] The hydrogen is preferably extracted in step (cl) at the same or at a faster rate than it is produced from the reaction in step (bl).

[0028] In a preferred embodiment, there is provided a method which additionally includes the steps of (a2) providing apparatus including a first container having an inlet and a second container having an outlet, wherein the first container and second container contain said liquid, and wherein said liquid in the first container has a first surface proximate the inlet and said liquid in the second container has a second surface proximate the outlet; (b2) causing the hydrogen dissolved in said liquid to move to the second container; and (c2) removing said hydrogen gas from the second container via the outlet; wherein in step (bl) said water vapour is supplied to the first container via the inlet; and wherein in step (cl) hydrogen is extracted from liquid in the second container via said second surface.

[0029] Such a method including a first and second container advantageously allows hydrogen, which is dissolved in the first container, to move to the second container from which the dissolved hydrogen can be efficiently and safely extracted.

[0030] Advantageously, the method also provides a concentration of dissolved hydrogen that is greater in the first container than in the second container, so that dissolved hydrogen moves from the first to the second container along a concentration gradient. This concentration gradient means that pressure, temperature or other process conditions do not need to be adjusted, although it is not excluded that they might, for hydrogen to move through to the second container from which it is extracted.

[0031] In a preferred embodiment, the average temperature of the liquid, the average pressure exerted on the surface of the liquid, or a combination thereof (or any other physical or chemical means that affects the solubility or concentration of the dissolved hydrogen) differs between the first and second container in order to facilitate movement of the dissolved hydrogen. This is especially preferred in embodiments where, as disclosed above, the hydrogen is extracted in step (cl) at the same rate as produced from the reaction in step (bl).

[0032] In a preferred embodiment, the average pressure exerted on the first surface is greater than the average pressure exerted on the second surface, as the solubility of hydrogen increases with pressure. It is preferred that the average pressure exerted on the first surface is from 0.1 MPa to 0.5 MPa, although it can be from 0.1 MPa to 1 MPa, and the average pressure exerted on the second surface is from 50 Pa to 0.1 MPa.

[0033] The temperature of the liquid aluminium or aluminium alloy depends on the melting operations where this method is embedded in and may be from 650 to 900 °C. In a preferred embodiment, liquid in the first container is of a higher average temperature than liquid in the second container, as the solubility of hydrogen in liquid aluminium increases with temperature. It is preferred that the average temperature in the first container is from 701 °C to 750 °C (most preferably from 720 to 750 °C) and the average temperature in the second container is from 650 °C to 700 °C.

[0034] The method preferably includes ultrasonic cavitation of the liquid in step (cl) in order to extract the hydrogen gas. This may involve placing a vibrating probe in direct contact with, and thus inducing ultrasonic cavitation in, the liquid in the second container. Alternatively, this may involve a contactless electromagnetic induction coil inducing ultrasonic cavitation in the liquid in the second container. In embodiments which include ultrasonic cavitation, the area of the first surface is preferably greater than the area of the second surface, as the smaller volume associated with the smaller area of the second surface increases the efficiency of the ultrasonic cavitation. Alternatively, or additionally, the pressure at the second surface is reduced in step (cl) in order to facilitate extraction of the hydrogen gas. Reducing the pressure at the second surface increases the rate of hydrogen extraction. In some embodiments, a vacuum is created at the second surface. In embodiments which reduce the pressure at the second surface, the area of the first surface is preferably substantially the same as the area of the second surface, as a larger area of the second surface is required to encourage extraction of the hydrogen from the liquid, in the absence of cavitation.

[0035] Preferably, the first and / or second container of the apparatus are flushed with gaseous argon. Doing so replaces the air in the reactor which may oxidise the liquid surface(s) and reduce the efficiency of hydrogen extraction. This is preferably performed before filling the apparatus with the liquid and is most beneficial when carried out in the second container.

[0036] In a preferred embodiment, thermal energy generated from the reaction in step (bl) is recycled. This energy may be used to maintain the temperature of the liquid aluminium or aluminium alloy, heat water to produce water vapour, generate electricity using for example a heat engine, or stored, or exported for other processes.

[0037] In a preferred embodiment, alumina (aluminium oxide) generated from the reaction in step (bl) is recycled into the aluminium refining operations (electrolysis) or exported for other purposes.

[0038] In a preferred embodiment, the first and second container contain liquid aluminium or liquid aluminium alloy from the outset, that is, before the steps of hydrogen production and extraction commence. Alternatively, these containers may be empty at the beginning and then filled with liquid aluminium before the steps of hydrogen production and extraction commence.

[0039] Preferably, the apparatus includes a conduit providing fluid communication between the first and second containers and the hydrogen dissolved in said liquid moves to the second container via the conduit. This conduit design and implementation ensures that the liquid therein is not exposed to the air. The conduit may have a transverse cross-section that decreases towards the second container, particularly in embodiments wherein the second container has a smaller surface area than the first. Alternatively, the conduit may have a uniform transverse cross-section, particularly in embodiments wherein the second container has a surface area that is substantially the same as the surface area of the first container. The conduit may incorporate a filter to collect alumina generated in the first container that sinks into the liquid.

[0040] In a preferred embodiment, alumina produced from the reaction in step bl is removed.

[0041] According to a second aspect of the present invention, there is provided a kit comprising: (a) apparatus including a first container having an inlet and a second container having an outlet; and (b) a device for inducing cavitation by means of providing vibrations in the second container. Preferably, the kit additionally includes a supply of water vapor for the inlet and / or has a one-way valve for the outlet.

[0042] In an alternative aspect of the invention, there is provided a method of producing hydrogen including the steps of: a. providing apparatus including a first container having an inlet and a second container having an outlet, wherein the first container and second container contain liquid aluminium or a liquid aluminium alloy, and wherein said liquid has a first surface proximate the inlet; b. reacting said liquid in the first container with water vapour supplied to the first container via the inlet in order to generate hydrogen which all dissolves in the liquid, wherein said reaction takes place either at said first surface or in the liquid no further than 10 mm from said first surface; c. causing the hydrogen dissolved in said liquid to move to the second container; d. extracting hydrogen in the form of gas from liquid in the second container, wherein liquid in the second container has a second surface proximate the outlet, and wherein the hydrogen gas is extracted via said second surface; and e. removing said hydrogen gas from the second container via the outlet.

[0043] A preferred embodiment of the present invention is described below, by way of example only, with reference to and as illustrated in the accompanying drawings, in which:

[0044] Figure 1 is a schematic diagram depicting an integrated process of aluminium and hydrogen production and utilisation.

[0045] Figure 2 is a schematic diagram depicting steps of a preferred method of producing hydrogen by reacting liquid aluminium or liquid aluminium alloy with water vapour in a first container and extracting hydrogen in the form of gas from liquid in a second container via contact (ultrasonic horn) cavitation. This is also a schematic of a first feasibility study.

[0046] Figure 3 is a schematic diagram depicting steps of a second preferred method of producing hydrogen by reacting liquid aluminium or liquid aluminium alloy with water vapour in a first container and extracting hydrogen in the form of gas from liquid in a second container via contactless (electromagnetic coil) cavitation.

[0047] Figure 4 is a schematic diagram depicting steps of a third preferred method of producing hydrogen by reacting liquid aluminium or liquid aluminium alloy with water vapour in a first container and extracting hydrogen in the form of gas from liquid in a second container via vacuum (reduced pressure).

[0048] Figure 5a is a photograph showing the arrangement of a preferred setup of a second feasibility experiment in which liquid aluminium alloy is provided in a first container and reacted with water vapour supplied through a steel tube. The nozzle of the steel tube is submerged in the liquid no further than 10 mm from the surface of the liquid. There is no surface turbulence visible, nor there are any bubbles or melt splashes emerging.

[0049] Figure 5b is a photograph showing the arrangement of an alternative setup of the feasibility study above. In this alternative setup, the nozzle of the steel tube is submerged in the liquid more than 10 mm from the surface of the liquid. There is a large bubble on the surface of the liquid, potentially producing melt splashes.

[0050] Figure 6 is a graph showing the results of a feasibility experiment using the preferred setup in Figure 5a. The graph shows the change in hydrogen concentration in the liquid ("the melt") with time spent reacting with water vapour ("hydrating").

[0051] Figure 7 is a schematic diagram depicting the arrangement of a third feasibility experiment in which liquid aluminium alloy is provided in a container with a lid and reacted with water vapour which is supplied to the cavity volume above the surface of the liquid through a steel tube. A hydrogen probe is submerged in the liquid to measure the hydrogen concentration in the liquid as it is reacted with water vapour (hydrated) and hydrogen is subsequently extracted (degassed using ultrasonic cavitation).

[0052] Figure 8 is a graph showing the results of a feasibility experiment using the setup in Figure 7. The graph shows the change in hydrogen concentration in the liquid ("the melt") with time spent reacting with water vapour ("hydrating stage") and undergoing hydrogen extraction ("degassing stage"). The hydrating stage corresponds to the reaction in the first container in Fig. 2, the degassing stage corresponds to the second container in Fig. 2.

[0053] Specific Description

[0054] Figure 2 shows in schematic form an embodiment of an apparatus (the HydrAI reactor) for producing hydrogen by reacting liquid aluminium with water vapour. There is provided the apparatus including two containers 20,30 and a conduit 40 between them, in which liquid aluminium or liquid aluminium alloy 100 is contained or may eventually be contained.

[0055] A first container has an inlet 60, through which water vapour is supplied. The water vapour pressure is controlled at the inlet 60, such that it is from 0.1 MPa to 0.5 MPa. Optionally, a supply of gaseous argon is also controlled at the inlet 60 to fill the apparatus prior to the supply of liquid aluminium. The first container provides the liquid 100 with a first surface 2 proximate the inlet. The first surface 2 exposes the liquid 100 to the water vapour, such that the following reaction occurs at the first surface 2 or in the liquid 100 no further than 10 mm from said first surface:

[0056] 2AI(i) + 3H2O(g) AhOscs) + 6H(at)

[0057] The hydrogen produced in the reaction dissolves readily in the liquid aluminium, and the alumina forms a removable solid dross 5 on the surface of the liquid.

[0058] It is preferred that the reaction occurs at the surface or no deeper than 10 mm into the liquid aluminium 100 to avert the explosion risk associated with containing the reaction within the volume of liquid aluminium. Furthermore, constraining the reaction to a surface region prevents turbulence and entrainment of alumina into the liquid. This is further discussed and demonstrated in feasibility experiment 2 and Figure 5b. The alumina dross 5 which forms at the reaction surface should be routinely removed to expose fresh liquid to the water vapour and ensure that the reaction is only minimally restricted.

[0059] The hydrogen dissolved in liquid aluminium or aluminium alloy 100 in the first container 20 is caused to move 8 from the first container 20 to a second container 30 via a concentration gradient. This may be controlled by a variety of operating parameters, including hydrogen dissolution rate, hydrogen extraction rate, pressure gradients and temperature gradients, as described below.

[0060] Hydrogen is extracted from the liquid aluminium or aluminium alloy 100 in the second container 30 in an extraction step. The second container 30 may be provided with liquid aluminium 100 from the outset, or this may be later introduced from the first container 20 or an alternative source. The second container 30 includes an outlet 70, through which the extracted hydrogen 50 is removed as a gas. The liquid 100 has a second surface 30 proximate the outlet 70. Optionally, the second container 30 also includes a layer of argon gas 54 to prevent oxidation at the second surface 3 and separating extracted hydrogen 50 from the liquid aluminium 3 (argon is much heavier than hydrogen). Argon gas may be introduced via the first container 20 to flash the entire apparatus and displace air. Argon gas will then remain in the second container 30 above the second surface 30 as the reaction commences, as described above. The extraction step may be carried out using a variety of methods, as described below.

[0061] Dissolved hydrogen which is extracted from the second container 30 is removed from said second container as a gas, via the outlet 70. The outlet 70 includes a one-way exit valve for removing the hydrogen and for controlling the hydrostatic pressure at the second surface 3. The hydrogen may be removed via the outlet in a continuous stream, or in batches.

[0062] Figure 2 also shows a conduit 40 between the first and second containers 20,30. The conduit 40 serves to provide fluid communication between the first and second containers. Hydrogen dissolved in the liquid in the first container 20 is transported 8 through the liquid 100 to the second container 30 via the conduit 40. The conduit 40 may be cylindrical or rectangular in cross-section or take any other hollow form which allows transport of dissolved hydrogen from a first container to a second container. Given the temperatures associated with the method and with liquid aluminium or liquid aluminium alloy, the apparatus is formed from material which can withstand high temperatures and be non-reactive with liquid aluminium, for example refractory ceramic.

[0063] Liquid aluminium has a vast capability to dissolve hydrogen generated from the reaction of water vapour with liquid aluminium. The method disclosed herein achieves a large partial pressure of hydrogen at the interface between water vapour and liquid aluminium. At this interface, the potential concentration of dissolved hydrogen in liquid aluminium at 727 °C and atmospheric pressure conditions is found to be, in theory, as high as 0.3xl06cm3 / 100 g or 27 g H2 / IOO g Al. Although these values are not practically achieved due to the barrier oxide layer and dynamic equilibrium at the interface, values from 4 to 44.5 cm3 / 100 g have been reported. As the equilibrium solubility of H2 in liquid Al at 727 °C is about 1.1 cm3 / 100 g, the liquid will be supersaturated with hydrogen. It is this solubility of hydrogen in liquid aluminium or liquid aluminium alloy 100 which enables the movement of hydrogen in the apparatus. This solubility is also what necessitates an engineered extraction step; it is otherwise energetically unfavourable for hydrogen to escape the liquid.

[0064] Various methods may be used to extract hydrogen from the liquid aluminium or liquid aluminium alloy 100. Ultrasonic cavitation is the chosen method in preferred embodiments. The ultrasonic cavitation process is a method of degassing liquid metal that does not involve any other gases or reagents. It is a well-studied (for the typical, rather low levels of hydrogen in aluminium, i.e., less than 1 cm3 / 100 g) and energy-efficient process. The process depends on the formation of cavitation bubbles 55 in the liquid subjected to high frequency (e.g. 17- 25 kHz), high-amplitude (e.g. 5-40 pm) oscillations. For ultrasonic cavitation in the preferred embodiments disclosed herein, the acoustic pressure in the liquid should exceed the cavitation threshold, i.e., 0.2 MPa at the range of acoustic frequencies 16 to 40 kHz. It is preferred that the acoustic pressure in the liquid is from 0.4 to 0.8 MPa. The acoustic pressure can be measured and controlled by a cavitometer inserted in the liquid in the second, or outlet, chamber, which provides feedback to the ultrasonic device 51 / 52. The ultrasonic frequency range is from 16 to 40 kHz (preferred 17-20 kHz).

[0065] Figure 2 depicts an embodiment of contact cavitation as a preferred method of extracting hydrogen from the liquid 100. In this method, an ultrasonic probe 51 is inserted in the liquid 100 and induces vibrations in the liquid, causing cavitation of dissolved hydrogen in the second container 30. The probe 51 is made from a material that does not react with liquid aluminium under cavitation conditions, for example, a Nb-based alloy or a sialon-based ceramic, and a provision is made to make the manifold (where the probe in introduced in the second container) airtight.

[0066] Figure 3 depicts an embodiment of contactless cavitation as a preferred method of extracting hydrogen from the liquid 100. In contactless cavitation, an electromagnetic induction coil 52 surrounds the second container 30. The induced Lorentz force, tuned to achieve acoustic resonance, allows upscaling of the process to larger volumes of liquid in the second container 30. The driving AC current frequency in the induction coil 52 is adjustable in the range 10-20 kHz, resulting in forced ultrasonic vibrations in the liquid 100, in the range 20-40 kHz. The driving current magnitude ranges from 0.5 to 2 kA, depending on the liquid volume in the second container 30, taking advantage of acoustic resonance to improve efficiency. There may be one or more electromagnetic induction coils 52. Where the cavitation threshold in the liquid in the second container 30 is expected to be low, as is the case in the preferred embodiments, only one electromagnetic induction coil 52 may be implemented, as stable cavitation maybe sufficient to allow hydrogen bubbles 55 to grow and float. Where the hydrogen cavitation threshold is expected to be relatively high, there may be a need for transient cavitation for hydrogen bubbles 55 to grow and float requiring a higher driving current or additional coils.

[0067] A combination of contactless and contact cavitation may be used, either simultaneously or in an alternating manner.

[0068] In embodiments where at least one ultrasonic cavitation method is employed, the volume of the second container 30 and, by association, the second surface 3, is preferably smaller than the first container 20 and the first surface 2. This is provided to maximise the effectiveness of the ultrasonic cavitation method. The method would still work, albeit less efficiently, if the second surface 3 is substantially the same or greater than the first surface 2.

[0069] It is also conceived that the extraction may take place in the absence of ultrasonic cavitation, and instead by other methods such as reducing the pressure at the second surface 3, especially by inducing a vacuum as depicted in Figure 4. This can be achieved by adding another one-way valve connected to a vacuum pump 53 near to or at the outlet 70 to reduce the hydrostatic pressure at the second surface 3. It is advantageous that the pressure at the second surface 3 is from 50 Pa to 100 Pa. Accompanying this reduced pressure at the second surface 3 with an increase in water vapour pressure at the first surface 2 (0.1 to 0.5 MPa, preferred range 0.1 to 0.2 MPa) helps to saturate the liquid in the first container 20 with hydrogen and further increase the hydrogen concentration gradient.

[0070] This pressure gradient-driven extraction method may be used in combination with ultrasonic cavitation or other methods of extraction. Where an ultrasonic cavitation step is employed, the hydrostatic pressure at the outlet 70 is preferably from 0.05 MPa to 0.1 MPa, a larger pressure than that which is required in the absence of ultrasonic cavitation.

[0071] Where a reduced pressure is applied to the second surface 3 to extract hydrogen from the liquid 100, the second surface 3 is preferably provided with an area that is substantially the same as the first surface 2, such that there is a large enough volume of liquid exposed for hydrogen extraction, in the absence of induced cavitation. The method would still work, albeit less efficiently, if the second surface 3 is smaller than the first surface 2. Any of the hydrogen extraction methods may be used in isolation or in combination with one another, as appropriate. Alternative methods of hydrogen extraction are also not excluded from the scope of the claims, either on their own or in combination with the methods described.

[0072] In order to assist in moving the dissolved hydrogen 8 from the first container 20 to the second container 30, the preferred embodiments employ a temperature gradient, in combination with any other method or methods of creating or facilitating the hydrogen concentration gradient. A higher temperature in the first container 20 will increase the solubility and therefore dissolution of hydrogen at the first surface 2 of liquid aluminium or liquid aluminium alloy 100, while a lower temperature in the second container 30 will decrease the solubility of hydrogen in the liquid 100 and facilitate its extraction. The average temperature of liquid in the first container 20 should be from 650 °C to 900 °C. The actual temperature may depend on the temperature of the source of liquid aluminium or liquid aluminium alloy. A temperature from 720 °C to 750 °C at the inlet 60 with 650 °C to 700 °C at the outlet 70 is optimal.

[0073] As previously mentioned, the apparatus may be flushed with argon gas 54 in the first container 20 and / or the second container 30. This ideally occurs before introducing liquid aluminium or liquid aluminium alloy 100 to the apparatus. Where there is a reduced pressure at the outlet 70, the residual argon is first evacuated before extracted hydrogen may be removed from the second container 30 through a one-way valve. Where there is initially atmospheric pressure in the second container 30, the argon 54 is not evacuated, and instead (being much heavier than hydrogen) remains at the second surface 3, effectively cushioning the liquid 100 and protecting it from oxidation. It also acts as a separator between the second surface 3 and the extracted hydrogen 50, so hydrogen will accumulate above the argon layer 54 and can be subsequently transported out of the second container 30 through a pressure-controlled one-way valve.

[0074] It will be appreciated that the reaction at the first surface 2 is exothermic. The thermal energy generated at the first surface 2 may be recycled, either to maintain the temperature of the liquid 100 in the apparatus, to generate water vapour (steam), to generate electricity, or to be stored. The alumina dross 5 produced may be recycled back to the primary aluminium production or exported elsewhere. See Figure 1 for a concept.

[0075] It will be appreciated that aluminium / aluminium alloy will be consumed in reaction and will need to be replaced in order to allow a continuous process. It is envisaged that it is replaced via a separate (from the water vapour) inlet in the first chamber 20.

[0076] Examples

[0077] Feasibility Experiment 1

[0078] In a first feasibility experiment, a setup similar to the embodiment in Figure 2 is constructed from refractory ceramic. The inlet and outlet have diameters of 100 mm and are 150 mm in height; the closed trough connecting these two chambers has a cross section of 100 mm and is 200 mm long. The inlet and outlet chambers are opened during the experiment, but a cushion of Ar is applied to the surface of the liquid at the outlet. The setup is filled with 9 kg of molten commercial purity aluminium at a temperature of 750 °C. Water vapour at a temperature of 150 °C is applied to the liquid surface at the inlet chamber. The ultrasonic cavitation processing is administered in the outlet chamber by a Nb-alloy horn (20 mm in diameter) driven by a 17.5 kHz magnetostrictive transducer (power 3.5 kW, vibration amplitude ±15 pm) inducing 0.8 MPa of acoustic pressure in the liquid as measured by an NPL-calibrated high-temperature cavitometer. The hydrogen levels in the inlet and outlet chambers are controlled by (a) ALSPEC-H direct measurement method and (b) reduced pressure test as an indirect method. The difference between these measurements in the inlet and outlet locations is taken as the amount of hydrogen produced. The alumina dross formed as a result of the reaction at the liquid surface in the inlet chamber is collected and weighed. The experiment continues for 15 min.

[0079] During the entire experiment the surface of the liquid in the first container is quiet without any turbulence (no entrainment of alumina into the liquid) and the dross formation develops gradually without forming a thick continuous layer, therefore not restricting the water- aluminium reaction.

[0080] Once a steady state is reached, the amount of hydrogen dissolved in the liquid in the inlet chamber is measured to be 1 cm3 / 100 g, while the amount of hydrogen remaining in the liquid in the outlet chamber is below 0.1 cm3 / 100 g. These are instantaneous values, and it takes about 60 seconds to reach this level in such a volume. Therefore, the total amount of hydrogen produced in 15 min using this setup is up to approximately 1215 cm3or 81 g.

[0081] In a second feasibility experiment, a setup is arranged which includes a first container 20 constructed from refractory ceramic, a steel tube 61 connected to a steam generator, and a hydrogen probe (not shown). The container is filled with 3 kg of molten recycled A356 aluminium alloy 100 at a temperature varying from 710 to 730 °C. Water vapour is supplied through the tube nozzle 62. In a preferred setup shown in Figure 5a, the tube nozzle 62 is submerged less than 10 mm from the surface of the liquid 1. In an alternative setup shown in Figure 5b, it is submerged more than 10 mm from the surface of the liquid.

[0082] Using the alternative setup in Figure 5b, submerging the tube nozzle 62 in the liquid 100 more than 10 mm from the surface 1, produces a large bubble at the surface 6 containing mostly unreacted water vapour. This is unsafe as bursting this bubble produces melt splashes and can result in an explosion. Using the preferred setup in Figure 5a, submerging the tube nozzle 62 in the liquid 100 no further than 10 mm from the surface 1, does not produce such bubbles, suggesting that the water vapour reacted with the liquid.

[0083] Figure 6 is a graph showing the results of a feasibility experiment using the setup in Figure 5a. The graph shows the change in hydrogen concentration in the liquid ("the melt") with time spent reacting with water vapour ("hydrating"). It shows that the amount of dissolved hydrogen in a liquid aluminium alloy (A356) increased significantly from 0.14 cm3 / 100 g to 0.5 cm3 / 100 after just 5 minutes of reacting the liquid with water vapour applied in the liquid no further than 10 mm from the surface. The dissolved hydrogen stayed in the liquid after the end of hydrating and decreased insignificantly to 0.47 cm3 / 100 g after 30 min, demonstrating that an extra step (cl) is required for extracting hydrogen as no "bubbling out" happens contrary to the claims in reported methods. The reaction of water vapour with liquid aluminium also produced 136 g of alumina dross 5 (aluminium oxide), corresponding to approximately 71.5 g reacted aluminium and 7.8 g of generated H2. From the reaction mass balance, 136 g of alumina should correspond to 8 g H2, which means that in our experiment 97.5% hydrogen dissolved in liquid aluminium.

[0084] In a third feasibility experiment, a setup is arranged according to Figure 7 which includes a combined dissolution / extraction container 15 constructed from refractory ceramic with a lid 25, a steel tube 61 connected to a steam generator, and an Alspek-H hydrogen analyser (hydrogen probe) 80. The container is filled with 2.5 kg of molten A356 aluminium alloy 100 at a temperature of 740 °C. Water vapour is supplied to the cavity volume 26 above the surface of the liquid through the tube nozzle 62 in the cavity volume 26. The lid 25 is closed to create some overpressure of the water vapour in the cavity volume 26. The hydrogen probe 80 is submerged in the liquid 100 to measure hydrogen concentration simultaneously. The results of the experiment are shown in the graph of Figure 8.

[0085] In this setting, this closed volume of the combined dissolution / extraction container 15 plays the role of the first container 20 (Figures 2, 3 and 4) where the reaction takes place and hydrogen is dissolved in the liquid 100 (hydrating stage). As seen in Figure 8, upon hydration, the hydrogen concentration steadily increased 4-fold from 0.2 to 0.8 cm3 / 100 g.

[0086] The liquid 100 was then subjected to hydrogen extraction (degassing stage) by ultrasonic cavitation using a Nb-alloy horn producing vibration at 17.5 kHz and 20-pm amplitude. In this case the container 15 plays role of the second container 30 (Figure 2). As seen in Figure 8, upon ultrasonic degassing, the hydrogen concentration in the liquid aluminium decreased back to 0.2 cm3 / 100 g, demonstrating successful extraction of the dissolved hydrogen.

[0087] The disclosures in UK patent application no. 2404147.7, from which this application claims priority, and in the abstract accompanying this application are incorporated herein by reference.

[0088] All optional and preferred features and modifications of the described embodiments and dependent claims are usable in all aspects of the invention taught herein. Furthermore, the individual features of the dependent claims, as well as all optional and preferred features and modifications of the described embodiments are combinable and interchangeable with one another.

Claims

CLAIMS1. A method of producing hydrogen including the steps of: al. providing liquid aluminium or liquid aluminium alloy, wherein said liquid has a surface; bl. reacting said liquid with water vapour in order to generate alumina and hydrogen, wherein if the reaction is carried out at a temperature range of 650 to 900 °C and a pressure range of 0.1 to 1 MPa, at least 50%% of the hydrogen dissolves in the liquid, and wherein said reaction takes place at the surface and / or in the liquid; cl. extracting hydrogen in the form of gas from the liquid.

2. A method as claimed in claim 1, wherein in step bl at least 95% of the hydrogen dissolves in the liquid.

3. A method as claimed in claims 1 or 2, wherein the reaction in step bl takes place at the surface and / or in the liquid no further than 10 mm from said surface.

4. A method as claimed in any of claims 1 to 3, additionally including the steps of: a2. providing apparatus including a first container having an inlet and a second container having an outlet, wherein the first container and second container contain said liquid, and wherein said liquid in the first container has a first surface proximate the inlet and said liquid in the second container has a second surface proximate the outlet;b2. causing the hydrogen dissolved in said liquid to move to the second container; and c2. removing said hydrogen gas from the second container via the outlet; wherein in step (bl) said water vapour is supplied to the first container via the inlet; and wherein in step (cl) hydrogen is extracted from liquid in the second container via said second surface.

5. A method as claimed in claim 4, wherein the area of the first surface is substantially the same or greater than the area of the second surface.

6. A method as claimed in claim 4, wherein the area of the first surface is substantially the same or smaller than the area of the second surface.

7. A method as claimed in any of claims 4 to 6, wherein the concentration of dissolved hydrogen is greater in the first container than in the second container, so that dissolved hydrogen moves along a concentration gradient from the first to the second container.

8. A method as claimed in any of claims 4 to 7, wherein the average temperature of the liquid, the average pressure exerted on the surface of the liquid, or a combination thereof differs between the first and second container in order to facilitate movement of the dissolved hyd rogen.

9. A method as claimed in claim 8, wherein the average pressure exerted on the first surface is greater than the average pressure exerted on the second surface.

10. A method as claimed in claim 9, wherein the average pressure exerted on the first surface is from 0.1 MPa to 0.5 MPa and the average pressure exerted on the second surface is from 50 Pa to 0.1 MPa.

11. A method as claimed in claim 9 or 10, wherein liquid in the first container is of a higher average temperature than liquid in the second container.

12. A method as claimed in claim 11, wherein the average temperature in the first container is from 701°C to 750°C and the average temperature in the second container is from 650°C to 700°C.

13. A method as claimed in any preceding claim, wherein step (cl) includes ultrasonic cavitation of the liquid in order to extract the hydrogen gas.

14. A method as claimed in claim 13, wherein a vibrating probe is situated in direct contact with, and induces ultrasonic cavitation in, the liquid in the second container.

15. A method as claimed in claim 13, wherein a contactless electromagnetic induction coil induces ultrasonic cavitation in the liquid in the second container.

16. A method as claimed in any of claims 4 to 15, wherein step (cl) includes reducing the pressure at the second surface in order to facilitate extraction of the hydrogen gas.

17. A method as claimed in claim 16, wherein a vacuum is created at the second surface.

18. A method as claimed in any of claims 4 to 17, wherein the first and / or second container of the apparatus is flushed with gaseous argon before the container(s) are provided with said liquid.

19. A method as claimed in any of claims 4 to 18 wherein said liquid is supplied to the first container to replace liquid which reacts in step bl.

20. A method as claimed in any of claims 4 to 19, wherein the apparatus includes a conduit providing fluid communication between the first and second containers and wherein the hydrogen dissolved in said liquid is transported to the second container via the conduit.

21. A method as claimed in claim 20, wherein the conduit has a transverse cross-section that decreases towards the second container.

22. A method as claimed in claim 20, wherein the conduit has a uniform transverse cross-section.

23. A method as claimed in any preceding claim, wherein alumina produced from the reaction in step (bl) is removed.

24. A method as claimed in any preceding claim, wherein the hydrogen is extracted in step (cl) at the same or a faster rate than it is produced from the reaction in step (bl).

25. A method as claimed in any preceding claim, wherein the hydrogen gas is removed continuously or in batches.

26. A method as claimed in any preceding claim, wherein thermal energy generated from the reaction in step (bl) is recycled.

27. A method as claimed in any preceding claim, wherein not all of said liquid is reacted.

28. A kit comprising:(a) apparatus including a first container having an inlet and a second container having an outlet; and(b) a device for inducing cavitation by means of providing vibrations in the second container.

29. A kit as claimed in claim 28, additionally including a supply of water vapour for the inlet.

30. A kit as claimed in claim 28 or 29, additionally including a one-way gas valve for the outlet.

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