Fuel tanks can contain potentially combustible combinations of fuel vapors, oxygen, and ignition sources. To prevent combustion, inert gas, such as nitrogen-enriched air (NEA) or oxygen-depleted air (ODA), is introduced into the ullage of a fuel tank, in order to keep the oxygen concentration in the ullage below 12%. A variety of membrane-based technologies have conventionally been used to inert fuel tank air. Similarly, fire suppression systems, such as fire suppression systems deployed in aircraft cargo holds, can function with inert gas.
In one embodiment, a gas inerting system includes a solid oxide electrochemical gas separator system, a dilution air source configured to selectively add dilution air to the incoming process air or the oxygen-enriched air, a controller configured to control the dilution air source, and an outlet configured to direct the oxygen-depleted air to a location requiring inerting. The solid oxide electrochemical gas separator system includes a cathode configured to receive incoming process air and produce oxygen-depleted air, and an anode configured to evolve oxygen.
In another embodiment, a gas inerting method includes separating incoming process air into oxygen-enriched air and oxygen-depleted air in a solid oxide electrochemical gas separator system, selectively temperature controlling the solid oxide electrochemical gas separator system with dilution air, selectively diluting the oxygen-enriched air with dilution air, and inerting a space with the oxygen-depleted air.
The present disclosure describes a system and method to generate inert gas for use in combustion prevention and fire suppression. In particular, the system can be applied to fuel tank inerting or to fire suppression for aircraft cargo areas, dry bays, and other areas that require fire protection. The system uses solid oxide electrochemical gas separators (SOEGS) cells configured to transport oxygen out of incoming process air, resulting in inert oxygen-depleted air. The use of SOEGS cells is beneficial for purposes of energy efficiency and lower system weight. In addition, the replacement of ozone-depleting organic halides such as Halon that are used as fire extinguishing agents on aircraft with an inert gas generation system is more environmentally benign.
Ceramic solid oxide fuel cells have been leveraged in a variety of systems. Generally, past uses configure the system as a fuel cell for producing electrical current. In this configuration, both fuel and air are fed into the cells, resulting in a voltage difference across the cell that can be used to generate an electric current. In this traditional configuration, the cathode of the fuel cell is positive, while the anode of the fuel cell is negative. In similar configurations, solid oxide systems have been used to accomplish electrolysis of water or carbon dioxide, splitting the water or carbon dioxide into separated components. However, solid oxide technology has scarcely been used in a “gas separator” configuration.
Rarely have ceramic solid oxide cells been used as solid oxide electrochemical gas separators (SOEGS). In a gas separator configuration, the polarity of the cell changes sign in comparison to a fuel cell according to convention. The cathode is negative, and the anode is positive (higher potential). Nonetheless, the anode is the site of oxidation and the cathode is the site of reduction reactions. When a solid oxide cell is used in such a configuration, instead of generating a current, the SOEGS generates oxygen-depleted air. In the SOEGS configuration, an applied DC voltage induces a current that causes incoming oxygen to reduce in the cathode and be transported through the oxygen-conducting electrolyte to the anode.
The use of SOEGS has several benefits. First, the use of an SOEGS is more energy efficient in operation than the use of other types of electrochemical gas separators, such as those containing a proton exchange membrane. Second, the use of SOEGS has the potential to decrease the weight of the inert gas and fire suppression systems. Finally, the proposed SOEGS gas separation system exhaust comes out dry with no need to remove humidity from the system, as compared to proton exchange membrane gas separator systems.
Cathode 4 and anode 6 are generally made of ceramic material such as lanthanum strontium manganite, lanthanum strontium cobaltite, and lanthanum strontium cobalt ferrite; or composite material such as noble metal supported on a ceramic substrate. Electrolyte 8 is an oxygen ion conductor, such as yttrium-stabilized zirconia or ceria doped with rare earth metals. Electrolyte 8 can be a thin film between anode 6 and cathode 4, while anode 6 and cathode 4 may consist of porous ceramic materials that can support the electrolyte. When SOEGS cell 2 is running, a bias voltage 10 of about 1 V per SOEGS cell is applied across SOEGS cell 2 from anode 6 to cathode 4. Incoming heated process air (HPA) is heated outside the SOEGS (see
While SOEGS cell 2 is running, anode process air is flowed through anode 6 to reject waste heat from SOEGS cell 2 and to dilute the evolved oxygen. The difference in temperature between the sides of the SOEGS should be no more than approximately 200 degrees Celsius to prevent mechanical failure due to thermally induced stresses. Temperature control air exits anode 6 along with oxygen that is evolved at anode 6; this flow stream contains oxygen previously removed from the incoming heated process air in cathode 4.
Specifically, when heated process air enters cathode 4, the oxygen in heated process air reacts with electrons (e−) from applied bias voltage 10 in the following reaction:
O2+4e−→2O═
The resulting oxygen ions are transported across electrolyte 8 where they recombine into oxygen molecules and electrons in the following reaction:
2O═→O2+4e−
Thus, air leaving anode 6 contains additional oxygen molecules and is oxygen-enriched air.
A plurality of SOEGS cells 2 arranged in SOEGS stacks 14A, 14B can be used to produce enough oxygen-depleted air for use in aircraft systems through the chemical reactions described in reference to
In SOEGS system 26A, process air (PA) enters system 26A through inlet 54 and continues to cathode heat recovery heat exchanger 28. Process air can be bleed air, compressed air, cabin air, ram air or fan air. Incoming process air should be purified (not pictured) to remove impurities prior to entering the system, and may have to be mechanically compressed (not pictured) if it is ram air or fan air. Incoming process air contains higher than 12% oxygen upon entering the system, and must be temperature-conditioned before being reduced in SOEGS stack 14.
Thus, heat exchangers 28, 34, flow control valves 40, 42, 43, and burner 44 are in SOEGS system 26 to temperature control incoming process air to a range of at least 500 degrees Celsius and no more than 1000 degrees Celsius. Ideally, process air is heated to a temperature of approximately 650-850 degrees Celsius. If process air is too cold, then the kinetics of the reaction in the solid oxide electrochemical gas separator cells will be adversely affected. If process air is too hot, then the longevity and consistency of the solid oxide electrochemical gas separator cells may be compromised due to the microstructural aging of the ceramic materials. Temperatures outside the ideal range may cause downgraded performance of the solid oxide electrochemical gas separator system.
Process air first enters cathode heat recovery heat exchanger 28. Cathode heat recovery heat exchanger 28 has two sides: cold side 30 and hot side 32. Process air enters cathode heat recovery heat exchanger 28 in cold side 30, where process air is heated from the hot inert product gas.
Heated process air (HPA) is then either routed to anode heat recovery heat exchanger 34 for further heating, or through bypass valve 40. Anode heat recovery heat exchanger 34 consists of two sides: cold side 36 and hot side 38. If heated process air enters anode heat recovery heat exchanger, then heated process air goes in anode heat recovery heat exchanger 34 cold side 36 where heated process air is further temperature-conditioned before flowing to SOEGS stack 14. Subsequently, heated process air exits anode heat recovery heat exchanger cold side 36 and is routed to solid oxide electrochemical gas separator (SOEGS) stacks 14. If some heated process air is routed through bypass valve 40, then it can mix with heated process air from anode heat recovery heat exchanger 34 cold side 36 in order to temperature-control gases to SOEGS stack 14.
Flow of heated process air into SOEGS stack 14 can be regulated by flow control valves 42, 43, allowing for both temperature control of SOEGS 14 and safety controls. Flow control valve 43 controls flow of process air into cathode 4. Flow control valve 42 can optionally regulate and shut off flow of heated process air into anode 6. For example, if flow control valve 42 is open and heated process air is flowed into anode 6, the heated air can warm up SOEGS 14 and allow quicker startup of reactions within SOESG 14 by promoting the kinetics of those reactions. Less activation energy is required for the reactions in SOEGS 14 when the stack is at higher temperatures. At low oxygen removal rates, heating air may be required to maintain a desirable operating temperature.
Alternatively, when SOEGS 14 is operating, cooling anode process air may be necessary in anode 6 to remove the power due to internal resistance losses resulting from irreversible processes. This allows for increasing flexibility and tailoring of SOEGS system 26A. Optionally, system 26A can include a temperature sensor proximate to SOEGS stack 14 in communication with controller 56 (discussed in detail below) so that the flow of cooling air or heated air through SOEGS stack 14 can be controlled based on current temperatures.
Additionally, when SOEGS stack 14 is running, anode 6 evolves oxygen as described in reference to
Flow of heated process air into SOEGS stack 14 is regulated by flow control valves 42 and 43. Heated process air is flowed through SOEGS stack 14 cathode side 4, while anode process air is flowed through SOEGS stacks anode side 6. Temperature control air is used to maintain a desired temperature inside SOEGS stack 14 and to dilute jot oxygen evolved at the anode. In various modes of operation, APA can be cooling air to reduce heat in SOEGS 14, or hot air to jump start kinetics inside SOEGS 14 at start up. Power source 10 represents a bias voltage that produces a DC current, resulting in oxygen molecules in heated process air reducing in normal operation, and subsequent oxygen ions moving from heated process air in cathode 4 through electrolyte 8 to anode 6. The chemical reactions which occur in cathode 4 and anode 6 are described in detail with reference to
Air leaving cathode 4 is oxygen-depleted air (ODA). Air leaving anode 6 is oxygen-enriched air (OEA). Oxygen-depleted air is inert air with depressed oxygen content, e.g. below 12% for fuel tank inerting of commercial aircraft fuel tanks or below 15% for fire suppression purposes. Oxygen-depleted air is routed back to cathode heat recovery heat exchanger 28, where oxygen-depleted air passes through cathode heat recovery heat exchanger 28 hot side 32 and is cooled to a temperature safe for use in inerting applications. Failure to cool oxygen-depleted air may result in damage to other materials, structures, and equipment when used for inerting applications. Preferably, oxygen-depleted air is cooled to ambient temperature, however, cooling to a temperature below 80 degrees Celsius for safe use with tank structural materials is acceptable. Oxygen-depleted air is then routed out of SOEGS system 26 through outlet to a second location, where ODA will be used to inert fuel tanks or in cargo hold fire suppression (not pictured).
If ODA is being used for cargo hold fire suppression purposes, the presence of water vapor in combustion gases exiting burner 44 may be acceptable. In this case, combustion gases exiting burner 44 may be combined with oxygen-depleted air downstream of heat exchanger 28 cold side 32 in order to maximize the temperature difference between hot side 30 and cold side 32 in heat exchanger 28.
Oxygen-enriched air exiting SOEGS stack 14 reacts with fuel in burner 44 to create combustion gases that contain water vapor. Depending on the stoichiometry, the exiting combustion gases may be sufficiently depleted of oxygen to consider as inert gas, however, for fuel tank inerting, the water vapor is difficult to remove to the required extent (sub-freezing dew point) and so it may be discarded through an outlet (e.g., overboard). Burner 44 is fed by fuel from a fuel tank (not pictured). Burner 44 heats oxygen-enriched air (OEA) to a range of 500-2000 degrees Celsius. The heated gas may then be routed back through anode heat recovery heat exchanger 34 hot side 38, where oxygen-enriched air is cooled by transferring its heat to process air which is heated to the range of 650-850 degrees Celsius. Heated process air is then routed out of anode heat recovery heat exchanger 34 towards SOEGS stacks 14. Simultaneously, the combustion gases exiting anode heat recovery heat exchanger 34 hot side 38 leaves the system through an outlet.
Controller 56 allows for manipulation of components in system 26A. Controller 56 is operatively coupled (e.g., electrically and/or communicatively) to components as depicted in
Controller 12 can include one or more processors and computer-readable memory encoded with instructions that, when executed by the one or more processors, cause controller device 12 to operate in accordance with techniques described herein. Examples of the one or more processors include any one or more of a microprocessor, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field-programmable gate array (FPGA), or other equivalent discrete or integrated logic circuitry. Computer-readable memory of controller device 12 can be configured to store information within controller device 12 during operation. The computer-readable memory can be described, in some examples, as computer-readable storage media. In some examples, a computer-readable storage medium can include a non-transitory medium. The term “non-transitory” can indicate that the storage medium is not embodied in a carrier wave or a propagated signal. In certain examples, a non-transitory storage medium can store data that can, over time, change (e.g., in RAM or cache). Computer-readable memory of controller device 12 can include volatile and non-volatile memories. Examples of volatile memories can include random access memories (RAM), dynamic random access memories (DRAM), static random access memories (SRAM), and other forms of volatile memories. Examples of non-volatile memories can include magnetic hard discs, optical discs, floppy discs, flash memories, or forms of electrically programmable memories (EPROM) or electrically erasable and programmable (EEPROM) memories. Controller 56 can be a stand-alone device dedicated to the operation of the catalytic oxidation unit, or it can be integrated with another controller.
Similarly to systems 26A-26D, SOEGS system 26E in
In system 26E, dilution air source 55 is located on oxygen enriched air (OEA) line downstream of anode 6 of SOEGS stack 14 as depicted in
Without incoming anode process air or cooling air into anode 6, pure oxygen is evolved which exits SOEGS stack 14 in the OEA line. Hot, pure oxygen is extremely reactive and potentially dangerous. Thus, diluting the oxygen exiting anode 6 immediately downstream of SOEGS stack 14 provides a safety measure to ensure highly oxidizing pure oxygen does not pose a danger. Adding dilution air downstream of SOEGS stack 14 reduces concerns of thermal stress on SOEGS stack 14 that would occur with adding upstream cooling air to anode 6 due to temperature differentials within SOEGS stack 14. Cool air that is not preheated can be used as the dilution air. Optionally, because anode 6 is not preheated, SOEGS stack 14 can be outfitted with resistance elements for heating, for example during system start-up. During regular operation, SOEGS stack 14 produces waste heat that must be removed in order to avoid overheating ceramic constituents so means to reject heat is envisioned such as heat exchangers integrated into the stack. In this embodiment (not shown), cooling air can be heated process air.
Management of anode gas flow within SOEGS system 26E allows for safety measures, thermal regulation, and tailoring of inert gas concentrations. Dilution of hot oxygen with cooling air prevents potentially harmful, deleterious reactions. Temperature regulation of anode 6 with cooling air or heated process air allows for reduced electric input at higher temperatures due to a lower of activation energy which reduces operating costs. Overall, managing anode gas flow allows for greater flexibility in inert gas production.
In all of the preceding embodiments, the inert product gas may not be regulated to the desired temperature. For example, for fuel tank inerting, a maximum inert gas temperature of 80 degrees Celsius is desired to avoid structural damage to fuel tank components. Additional temperature regulation may be required beyond the heat exchangers and is envisioned within the scope of this invention.
The following are non-exclusive descriptions of possible embodiments of the present invention.
In one embodiment, a gas inerting system includes a solid oxide electrochemical gas separator system, a dilution air source configured to selectively add dilution air to the incoming process air or the oxygen-enriched air, a controller configured to control the dilution air source, and an outlet configured to direct the oxygen-depleted air to a location requiring inerting. The solid oxide electrochemical gas separator system includes a cathode configured to receive incoming process air and produce oxygen-depleted air, and an anode configured to evolve oxygen.
The gas inerting system of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations and/or additional components:
The incoming process air is selected from the group consisting of engine bleed air, compressed air, ram air, cabin air, and fan air.
The dilution air source is located upstream of the anode.
The dilution air source is located downstream of the anode.
The gas inerting system includes an oxygen sensor downstream of the dilution air source configured to detect oxygen concentration in the oxygen enriched air and communicate with the controller.
The gas inerting system includes a temperature sensor configured to detect temperature of the solid oxide electrochemical gas separator system and communicate with the controller.
The gas inerting system includes further comprising a burner configured to receive the oxygen-enriched air from the electrochemical gas separator system and combust the oxygen-enriched air to heat the electrochemical gas separator system.
The gas inerting system includes a heater configured to heat the incoming process air upstream of the electrochemical gas separator system.
The gas inerting system includes a first heat exchanger configured to receive and temperature control the oxygen-depleted air from the electrochemical gas separator system and the incoming process air.
The gas inerting system includes a second heat exchanger configured to receive and temperature control the oxygen-enriched air from the electrochemical gas separator system and the incoming process air.
The solid oxide electrochemical gas separator system is configured to produce oxygen-depleted air with varying oxygen concentrations.
The oxygen-depleted air contains less than 15% oxygen.
The oxygen-depleted air contains less than 12% oxygen.
In another embodiment, a gas inerting method includes separating incoming process air into oxygen-enriched air and oxygen-depleted air in an electrochemical gas separator system, selectively temperature controlling the electrochemical gas separator system with dilution air, selectively diluting the oxygen-enriched air with dilution air, and inerting a space with the oxygen-depleted air.
The method of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations and/or additional components:
Selectively diluting the oxygen-enriched air comprises manipulating oxygen content of the oxygen-enriched air.
The method includes simultaneously selectively temperature controlling the solid oxide electrochemical gas separator system and selectively diluting the oxygen-enriched air.
The incoming process air is selected from the group consisting of engine bleed air, compressed air, ram air, cabin air, and fan air.
The method includes heating the incoming process air with a heater.
The method includes combusting the oxygen-enriched air in a burner and heating the electrochemical gas separator system.
The method includes temperature controlling the incoming process air in a cathode recovery heat exchanger by routing the oxygen-depleted air to the cathode recovery heat exchanger.
The method includes temperature controlling the incoming process air in an anode recovery heat exchanger by routing the oxygen-enriched air to the cathode recovery heat exchanger.
While the invention has been described with reference to an exemplary embodiment(s), it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment(s) disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.
This application is a continuation in part of U.S. application Ser. No. 14/969,398 filed Dec. 15, 2015 for “SOLID OXIDE ELECTROCHEMICAL GAS SEPARATOR INERTING SYSTEM” by D. Tew, S. Tongue and J. Rheaume.
Number | Date | Country | |
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Parent | 14969398 | Dec 2015 | US |
Child | 15866184 | US |