Patent Application
20240253008
The present invention relates to an electrical heater suitable for use in industrial chemical processes and plants. A process for manufacturing the electrical heater, and various uses of the heater are also provided.
Process gases for various chemical processes should be free from contaminants which might negatively affect the processes or systems in which the process gases are used. In particular, certain contaminant gases may damage electrochemical or catalytic components of a reactor.
For instance, the presence of oxygen contaminant in a process gas for a solid oxide electrolyser cell (SOEC) can enter the stack and cause damage. In particular, oxygen may oxidize the (Ni-based) material in the SOEC cell, thereby losing effectivity (performance) It is known that contaminant gases can be removed from a process gas by means of dedicated units; e.g. a partial oxidation (PROX) reactor or a bed of reduced metal catalyst (Ni or Cu) reactor—or combinations thereof—can be used to remove oxygen from a gas stream. However, such reactors are usually specific for a particular contaminant gas, and cannot easily be used for different contaminant gases or for large volumes of gas. Also, the introduction of additional units in a process gas stream can cause a significant pressure drop in the process gas stream.
WO97/02092 describes a ceramic-coated metal net, and its use in catalytically cleaning a gas stream. WO03/025490 discloses a device for performing catalytic reactions, which is in the form of a plate heat exchanger.
It is therefore an object of the invention to provide systems and processes which can remove contaminant gases from process gases. In particular, it would be useful to provide such systems and processes while minimising equipment use, and without introducing a significant pressure drop in the process gas.
It has been found by the present inventors that a catalytic coating on an electrical heater can remove contaminant gases from a gas stream, and heat generated by the catalytic reaction can reduce, or completely remove, the need for external electrical power to the heater.
So, in a first aspect the present invention relates to an electrical heater, said electrical heater comprising a ceramic monolith and a metal resistance wire supported on said ceramic monolith, wherein at least a portion of said metal resistance wire, and optionally at least a portion of said ceramic monolith, is coated with a ceramic coating, and wherein at least a portion of said ceramic coating is impregnated with a catalytic metal selected from the group consisting of Cu, Mn, Cr, Ni, Pt, Ir, Pd, Rh, Ru and combinations thereof.
A process for manufacturing the electrical heater is also provided, said process comprising the steps of:
A process is provided for simultaneously heating a process gas and removing a contaminant gas from said process gas, said process comprising the steps of
The present technology also provides a solid oxide electrolyser system, a fuel cell system and a syngas system each comprising an electrical heater as described herein.
Additional aspects are set out in the dependent claims, the figures and the following description text.
The technology is described with reference to the enclosed schematic figures, in which:
Unless otherwise specified, any given percentages for gas content are % by volume.
The term “synthesis gas” is used interchangeably with the term “syngas” and is meant to denote a gas comprising hydrogen, carbon monoxide and also carbon dioxide and small amounts of other gasses, such as argon, nitrogen, methane, etc.
As noted above, and as illustrated in the Figures, an electrical heater is provided, which is suitable for heating a gas stream. The electrical heater is mounted in a passage, and a gas stream flows around and/or through the electrical heater, thus raising the temperature of the gas stream downstream the electrical heater.
The electrical heater comprises a ceramic monolith. The ceramic monolith is suitably molded or extruded into the required form.
The ceramic monolith may be formed of one or more ceramic materials selected from steatite, cordierite, alumina, silica, or aluminates (e.g. XAl2O4 where X is Mg or Ca).
As illustrated in
In the specific case illustrated, the ceramic monolith has a substantially cylindrical form. However, other forms, e.g. cuboid are possible.
A metal resistance wire is supported on the ceramic monolith. Typically, the metal resistance wire is wound around the monolith, and may also be located within open passages of the monolith, where present. Therefore, the resistance wire suitably extends within at least one passage of the ceramic monolith (e.g. from first to second end of the monolith).
To provide good gas-to-surface contact area, the resistance wire can be arranged in spirals from first to second end of the monolith, as illustrated in
Suitably, the electrical heater comprises first and second electrical terminals, through which electrical current can be supplied to the heater. The resistance wire extends between the first and second electrical terminals, and is arranged to receive an electrical current applied between the first and the second electrical terminals and convert it into heat.
For ease of construction, the first and second electrical terminals may be arranged at the same end of the ceramic monolith.
The resistance wire suitably extends within multiple passages of the ceramic monolith, so as to increase the heating capabilities of the monolith. Preferably, at least a first portion of the resistance wire extends from the first end to the second end of the monolith within a first passage of the monolith, and at least a second portion of the resistance wire extends from the first end to the second end of the monolith within a second passage of the monolith. In other words, the resistance wire is threaded back and forth along open passages in the monolith. Most preferably, the resistance wire extends from the first electrical terminal, along alternating passages of the ceramic monolith to the second electrical terminal. Preferably, the resistance wire is threaded through every open passage in the monolith.
For optimal heating, the resistance wire must be stable in air when hot. The metal resistance wire suitably comprises one or more metals selected from the group consisting of iron, chromium, nickel or aluminium, preferably one or more metals selected from iron and aluminium. It is particularly of interest that the metal resistance wire is able to form aluminium oxide on the surface thereof, as this improves thermal contact with an overlying ceramic coating. Therefore, the metal resistance wire may comprise or consist of an aluminium (Al) alloy, such as an iron chromium aluminium (FeCrAl) alloy, suitably consisting of iron, chromium (20-30%) and aluminium (4-7.5%).
Kanthal is the trademark for a family of iron-chromium-aluminium (FeCrAl) alloys used in a wide range of resistance and high-temperature applications. The alloys are known for their ability to withstand high temperatures and having intermediate electric resistance. As such, it kanthal alloys are frequently used in heating elements.
Kanthal FeCrAl alloy forms a protective coating of aluminium oxide (alumina). Aluminium oxide has high thermal conductivity but is an electrical insulator, so special techniques may be required to make good electrical connections.
Ordinary Kanthal FeCrAl alloy has a melting point of 1,425° C. Special grades can be used to provide melting points as high as 1,500° C. Depending on specific composition the resistivity is about 1.4 μΩ·m and temperature coefficient is +49 ppm/K (+49×10−6 K−1).
At least a portion of the metal resistance wire, and optionally at least a portion of said ceramic monolith, is coated with a ceramic coating. Suitably, both the metal resistance wire and the ceramic monolith are coated with a ceramic coating, so as to provide a more uniform heat distribution.
Coating of the ceramic coating is suitably performed in one or more steps by slurry coating, or any other suitable known method for coating ceramic materials in a coating. The ceramic coating is porous, so that catalytic metal can be incorporated.
The ceramic coating may comprise or consist of a ceramic selected from the group consisting of alumina (Al2O3), an aluminate (XAl2O4 in which X is Mg, Ca, Ba, or mixtures thereof), silica, alumina-silicate (zeolite), zirconia-alumina and combinations thereof. Most preferred materials for the ceramic coating are zirconia-alumina, alumina, alumina titanate, zeolite, magnesium aluminate or combinations thereof—
At least a portion of- and preferably the entirety of—the ceramic coating is impregnated with a catalytic metal selected from the group consisting of Cu, Mn, Cr, Ni, Pt, Ir, Pd, Rh, Ru and combinations thereof. Preferably, the catalytic metal is selected from Pt, Ir, Pd, Rh or Ru In particular, selection of Pd as the catalytic metal has advantages, as it allows the content of the metal to be low (as low as 1 wt % considering coat+metal).
The addition of catalytic activity to the electrical heater in this manner provides low temperature activity to e.g. convert all oxygen in a gas stream to water. In particular, the catalytic metal is selected such that it can convert oxygen by reacting with hydrogen, forming water (2H2+O2=>2H2O).
The present technology allows the formation of a ceramic coating that both will attach to the monolith surface as well as to the surface of the metal wire, and which then can be impregnated with the active metal for the catalysis.
In a particular embodiment, the ceramic monolith comprises steatite, the metal resistance wire is an iron chromium aluminium (FeCrAl) alloy, the ceramic coating is a zirconia/alumna coat and the catalytic metal is Pd.
Heat can be generated in the electrical heater by passing electricity along the metal resistance wire and/or through catalytic reactions which are promoted by the catalytic metal.
A process for manufacturing the electrical heater is also provided. The process comprises the steps of
Details of the process are as follows:
The electrical heater described herein can be used in various systems, to provide heat as well as to simultaneously remove one or more contaminants from a gas stream.
The term “contaminant gas” is used to define a gaseous component in the process gas which has the potential to inhibit catalytic sites in the catalytic metal in question. For instance, sulfur, or sulfur-containing organic compounds.
In contrast thereto, the term “inert gas” refers to a gaseous component in the process gas which are not involved in the reactions in question, and which do not inhibit catalytic sites in the catalytic metal in question. This includes for instance gases such as argon or nitrogen.
One particularly interesting system is a solid oxide electrolyser system, comprising an electrolyser unit, preferably comprising one or more solid oxide electrolyser cells (SOEC). A supply of process gas is provided to the electrolyser unit. An electrical heater as described above is arranged to heat the process gas prior to it being fed to the electrolyser unit.
Typically, such a system will comprise one electrical heater, and several SOEC cells, so the gas goes through the heater/catalyst and is then distributed to several cells all incorporated in the same stack.
The use of an electrical heater with catalytic activity simplifies the layout of the electrolyser unit, while combining the functions of heating and a catalytic removal of oxygen. This—in turn—allows minimization of equipment and maintains a low(er) pressure drop.
It is beneficial that oxygen is converted prior to the electrolyser unit. The catalyzed heater can both be used to convert oxygen in a fuel rich atmosphere or to convert hydrogen (fuel) in a fuel lean (oxidizing) atmosphere generated (additional) thermal energy. If the thermal energy is sufficient for the heat requirement the heater can be operated without providing electrical power (i.e. electrical current) to the heater.
Another system of interest is a fuel cell system, comprising a fuel cell and a supply of process gas to said fuel cell, wherein an electrical heater as described herein is arranged to heat the process gas prior to it being fed to the fuel cell. The process gas here is typically a mixture of methane, steam, hydrogen and possibly carbon monoxide, carbon dioxide and inerts (such as nitrogen).
A further system of interest is a syngas system, comprising a syngas unit and a supply of process gas to said syngas unit, wherein an electrical heater as described herein is arranged to heat the process gas prior to it being fed to the syngas unit. The process gas here is typically a mixture of methane, steam, hydrogen and possibly carbon monoxide, carbon dioxide and inerts (such as nitrogen).
Furthermore, a process for simultaneously heating a process gas and removing a contaminant gas from said process gas is provided. The process comprising the general steps of:
All particulars of the electrical heater described above are relevant—mutatis mutandis—to the various processes and systems described herein.
A SOEC stack producing hydrogen by electrolysis of water uses electricity to generate the reaction, the water feed is evaporated and mixed with a small flow of hydrogen. The hydrogen has the ability to ensure the material of the electrolysis cell remains in a reduced state. A concern is that the content of oxygen dissolved in the water is a potential damaging agent for the material in the electrolysis cell. The mixed feed of steam and hydrogen is preheated in an electrical heater, however it is not certain that the oxygen present will react with hydrogen although there is a surplus of hydrogen in the flow. To ensure the reaction of oxygen with hydrogen to produce water upstream the electrolysis cell, the electrical heater is coated to contain a catalyst enabling the reaction between oxygen and hydrogen on the catalytic surface. This arrangement avoids a separate catalytic element that will introduce additional pressure drop to the system.
The present invention has been described with reference to a number of aspects and embodiments. These aspects and embodiments may be combined at will by the person skilled in the art while remaining within the scope of the patent claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 21176587.0 | May 2021 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2022/064264 | 5/25/2022 | WO |
