Patent Application
20070157516
The present invention relates to hydrocarbon reformers for producing fuel for fuel cells; more particularly, to such a reformer comprising a plurality of sequential reforming stages; and most particularly, to a staged reformer system wherein reforming is controlled and limited in a first stage to prevent thermal degradation of the reformer.
Partial catalytic oxidizing (CPOX) reformers are well known in the art as devices for converting hydrocarbons to reformate containing hydrogen (H2) and carbon monoxide (CO) as fuel for fuel cell systems, and especially for solid oxide fuel cell (SOFC) systems.
Prior art CPOx reformers typically comprise a catalyst bed formed of a durable inert substrate coated with an active catalytic wash coat. The substrate is typically porous, presenting a large surface area for catalysis.
A serious problem for prior art catalyst beds is that intense exothermic catalysis occurs at the leading edge of the bed where the concentration of reactants is highest and the dispersal of heat is lowest, causing rapid exothermic heat release and buildup which results in unacceptably elevated substrate, washcoat, and catalyst temperatures.
During sustained use of the reformer, the catalyst bed is progressively eroded thermally along the leading edge, resulting in a progressively smaller bed and eventual failure of the reformer.
What is needed in the art is a hydrocarbon reformer wherein temperature of the catalyst bed is controlled inherently by the arrangement of the bed, whereby thermal degradation of the catalyst bed is prevented.
It is a principal object of the present invention to prevent failure of a hydrocarbon reformer by thermal degradation of the leading edge of the catalyst bed.
Briefly described, a hydrocarbon reformer in accordance with the invention comprises a plurality of sequential reforming stages, preferably two, separated by non-reforming mixing spaces. The first reforming stage is arranged such that only a predetermined portion of the substrate surface available to hydrocarbon fuel and air in the first stage is provided with catalyst. The first stage catalyst bed is an egg-crate structure divided into fully active, partially active, and inactive flow cells. The cells are arranged such that fully active cells are adjacent to partially active and inactive cells, allowing fuel and air flowing through the partially active and inactive cells to cool the catalyst bed, thereby preventing thermal excess in the first stage and consequent bed erosion. Fuel and oxygen thus heated in the first stage are mixed at the end of the first stage with the reformate formed, and the mixture is provided to the next stage, which is preferably fully catalytic. Because the fuel and oxygen in the second stage are diluted with reformate from the first stage, insufficient heat is generated in the second stage to cause thermal degradation thereof.
The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:
The distinctions and benefits of the present invention may be better appreciated by first considering the elements and limitations of a prior art catalytic reformer.
Referring to
In operation, a mixture 22 of hydrocarbon and oxygen, typically in the form of air, is introduced into reformer 10 through inlet 14, and is passed through catalyst bed 18 wherein the hydrocarbon fuel and air are converted to a reformate 24 comprising a mixture of molecular hydrogen and carbon monoxide.
As noted above, a shortcoming of a prior art reformer such as reformer 10 is that the leading edge 26 of catalyst bed 18 becomes overheated and suffers thermal erosion over time of use, resulting in a recession of bed edge 26 to a new bed edge 26a which continues to recede with continued use of the reformer.
Referring to
In operation, a mixture 22 of hydrocarbon and oxygen, typically in the form of air, is introduced into reformer 110 through inlet 114, and is passed through first catalyst bed 118a wherein a first portion of the hydrocarbon fuel and air are converted to molecular hydrogen and carbon monoxide, resulting in an intermediate mixture 124a of hydrogen, carbon monoxide, fuel, and air in intermediate chamber 119. Mixture 124a is homogenized by turbulence and passed through second catalyst bed 118b which is fully catalytic wherein the remaining fuel is reformed, yielding reformate 24. Referring now to
Referring to
It is obvious that when both sides 136, 138 of all plates 128′, 128″ are coated with catalyst composition prior to assembly of the plates into structure 126, all four walls of each flow cell 132 are catalytically active. In such an arrangement, catalysis proceeds in each cell, and no mechanism or path exists to remove the exothermic heat of catalysis other than the materials passing through the cells and being catalyzed. As has been recited for prior art catalyst beds, the result is that the bed will overheat and become thermally eroded.
What is needed is a means to remove some of the heat being generated, which can be achieved by rendering some of the flow cells non-catalytic or only partially catalytic. Fortunately, this is readily accomplished in accordance with the invention as follows.
Each plate 128 is not fully coated to a catalyst composition on each side, thus lo reducing the active catalytic area of each plate. Preferably, each plate 128 is fully coated on only side 136 or 138, which automatically reduces by one-half the total catalytic area of structure 126. If plates thus coated are assembled in random side-to-side orientation to form structure 126, the resulting flow cells will be a mixture of zero-, one-, two-, three-, and four-sided catalytically coated cells. Cells having little or no catalytic capability provide little or no reforming and thus contribute little or no heat to the overall heating. Rather, fuel/air mixture flowing through such cells abstracts heat from the structure being generated in those cells having greater reforming capability.
Thus, in first stage 118a, temperature is inherently controlled by decreased catalysis and increased flow of cooling medium.
Referring to
Referring to
Note that the catalytic capacity of the structure shown in
Note also that the fully catalytic 4 cells are non-contiguous, being separated by half-catalytic 2 cells on all four sides and by non-catalytic 0 cells on all four diagonals.
Other selective coatings and orientations of plates 128 as may become obvious to those of skill in the art are fully comprehended by the invention. The invention thus provides a nearly infinite number of arrangements available to tailor the competing catalytic and cooling characteristics of an egg-crate catalyst bed to provide maximum catalytic capacity without risk of overheating the leading edge of the structure and resultant thermal erosion thereof.
The number of plates 128′, 128″, and the corresponding number of flow cells 132, can vary depending upon application needs from as little as a few dozen to thousands. Each flow cell and the whole grid structure can be designed for low pressure loss which enhances overall reformer system efficiency. For example, as many as 100 plates might be used in each direction to provide upwards of 10,000 flow cells in a single module.
The invention provides a modular approach toward reformer power requirements.
Rather than increasing the number of plates and cells to form a very large single catalyst bed, smaller structures 126 may be joined together in parallel in a large frame to form a single large bed as may be desired.
Referring again to
While the invention has been described by reference to various specific embodiments, it should be understood that numerous changes may be made within the spirit and scope of the inventive concepts described. Accordingly, it is intended that the invention not be limited to the described embodiments, but will have full scope defined by the language of the following claims.
