Integrated solid oxide fuel cell and reformer

Details Integrated solid oxide fuel cell and reformer Integrated solid oxide fuel cell and reformer

2000-06-09

2002-12-10

Kalafut, Stephen (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

With pressure equalizing means for liquid immersion operation

C429S010000, C429S006000, C429S006000

Reexamination Certificate

active

06492050

ABSTRACT:

TECHNICAL FIELD
The present invention relates to devices for the generation of electrical power and in particular to such devices which incorporate solid oxide fuel cells (SOFC).
BACKGROUND ART
A SOFC is an electrochemical device that allows the production of direct current electricity (and/or co-generated heat) by the direct electrochemical combination of a fuel (such as hydrogen, natural gas, coal gas, or other hydrocarbon-based fuels, for example) with an oxidant (such as air). A SOFC consists of an oxygen ion conducting electrolyte (currently based on stabilised zirconia) separating an air electrode (cathode) from a fuel electrode (anode). The fuel is oxidised at the anode and electrons are released to an external circuit, where they are accepted by the cathode. The cathode reaction causes the oxidant gas to be reduced to oxygen ions, which then migrate across the oxygen ion-conducting electrolyte to the anode. The movement of electrons around the external circuit produces an electromotive force (typically 1 volt for a single cell). By the application of a load across the cell, current flows, thus producing a power density, the value of which depends upon the design of the cell and the materials used. The cell is typically run at between 700 and 1000° Celcius. The book “Science and Technology of Ceramic Fuel Cells” authors N. Q. Minh and T. Takahashi (Elsevier, Amsterdam, 1995) describes the principle reactions in SOFC, and the methods by which electricity can be produced.
The most attractive features of the SOFC are its high conversion efficiency (typically 50 to 90% if heat utilisation is included), its low production of emissions, its production of high-grade exhaust heat, and its modularisability (from a few kW of electricity to many MW).
Single SOFC cells are usually stacked, using an interconnect or bi-polar plate (usually based on doped lanthanum chromite, or a high temperature metallic system), to produce a multi-cellular unit. The single cell typically produces 1 volt, however by stacking the single cells either in parallel or series connection, the required voltage can be realised. There are several known structures of the SOFC, and these include the planar, tubular and monolithic designs. It should be stressed, however, that in addition to these three main designs, other designs have been sited, although the basic concept remains the same in all designs, that of an oxygen ion conducting electrolyte separating a fuel gas (at the anode) from an oxidising gas (at the cathode). See, for example, the paper “Ceramic Fuel Cells”, author N. Q. Minh, pages 563-588, Journal of the American Ceramic Society, 76[3] (1993).
For SOFC systems to become fully realised, and thus fully commercial, they must be reliable over long periods of time, and must not be subjected to thermal cracking due to heating or cooling cycles. The systems must also compete financially with conventional technologies, such as gas turbines and diesel generators, and thus they must be relatively inexpensive and easy to assemble. The major drawbacks of the current designs are principally based on sealing of the single cells. The planar design, using flat plate electrolytes with an anode and cathode adhered to either side, appears to be the cheapest system to fabricate, however its major problem still hinges around the issue of sealing the plates without causing excessive stressing of the ceramic plates, or chemical compatibility issues between the sealant and the cells. The tubular design gets around the problem of sealing by using a closed or open-ended tube. In the conventional tubular design, an extruded porous doped-lanthanum manganite support tube is fabricated. The electrolyte (stabilised zirconia) is electrochemically vapour deposited onto the support tube. The anode is then slurry spray-electrochemical vapour deposited onto the electrolyte, and doped-lanthanum chromite is plasma sprayed onto the cell as an interconnect material. The cells are then bundled into multi-cellular units and then packaged as a SOFC system. The air is pumped into the interior of the tubes, while the exterior of the tube is exposed to the fuel gas. The tubes are sealed at one end, so that spent air can flow back through an annular. The spent fuel can also be recycled to allow for heat recovery.
This tubular arrangement has been very successful, However, the design does not allow for rapid heat cycling, as thermal stresses could occur causing the cells to crack. Even with the improvements that have been undertaken, the current limit on this design requires 5 hours for the system to reach 1000° C. operating temperature from ambient. State-of-the-art generators, using the technology described above, are described in U.S. Pat. No. 5,244,752. These are significant improvements on those SOFC systems sited in U.S. Pat. Nos. 4,374,184, 4,395,468, 4,664,986, 4,729,931, and 4,751,152. The design described above is also expensive to fabricate and is not conducive to producing small-scale systems (sub-10kW). The manufacturers of the systems have reduced the manufacturing costs by reducing the cost of the raw materials. For example, 90% of the weight of the cell is in the doped-lanthanum chromite air electrode, and thus by using a cheaper supply of raw materials (high impurity level), the cost can be reduced substantially. As described in a paper “Recent Progress in Tubular Solid Oxide Fuel Cell Technology”, author S. C. Singhal, in Solid Oxide Fuel. Cells Volume V, The Electrochemical Society (New Jersey), pages 37-50 (1997). However, the cost of the system will not allow for small-scale production.
To overcome the problem of high manufacturing cost, the use of extruded thin walled stabilised zirconia tubes is disclosed. See for example Australian Patent 675122. The inner electrode in this design was the fuel electrode, while the outer electrode was the air electrode (usually lanthanum manganite). In the design, tubes were supported within a thermally insulating container from which the exhaust gas can escape through a passageway. In this design, the cost of the tubes has been reduced by using a simple extrusion technique for extruding stabilised zirconia mixed with, for example, polyvinyl butyral and cyclohexane. The design is described as containing an array of the aforementioned tubes, supported within a thermally insulating container from which the combustion products can escape through a passageway. Gas is supplied directly to the top of the tubes. The combustion products escape through the same passageway as the forced air inlet. Although very simple, this design does not allow for the full movement of the cells within the reactor, and it is therefore still liable to stress build up in the cells, which may cause cell damage.
The ability to reform the fuel within the SOFC generator is also not possible in the designs described above. Reforming is a process by which the fuel, in this case usually a hydrocarbon fuel, is combined with water and/or carbon dioxide to produce carbon monoxide and hydrogen. This reformed fuel is then directly used in the SOFC system. In the majority of cases, the fuel is reformed outside the SOFC generator, which requires expensive equipment such as heat exchanges, pumps etc., which also make the whole system a lot more bulky. The reforming reaction, when it takes place outside the generator is highly undesirable as a lot of energy is lost from the system (as heat) and thus causes an overall loss in the efficiency of the system, and an increase in its complexity. This was partially overcome in the U.S. Pat. No. 4,729,9331, where the reforming of a reformable gaseous fuel was performed in the SOFC generator. In this system, the partially spent fuel is divided into two streams; one is combined with the partially spent air stream to form exhaust gas, which is then partially vented. Some of the remaining exhaust gas is then combined with the second spent fuel stream. The combined stream is then mixed with the gaseous reformable fuel. The invention uses the heat balance in the system as a whole to

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