Durable catalytic burner system

Details Durable catalytic burner system Durable catalytic burner system

2001-04-20

2002-12-24

Bennett, Henry (Department: 3743)

Combustion

Porous, capillary, particulate or sievelike flame holder,...

Means supplying fuel for passage through the flame holding...

C431S170000, C431S007000, C431S268000

Reexamination Certificate

active

06497571

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The subject durable catalytic burner system is generally directed to a system for combustively oxidizing an inflowing mixture of fuel and air (referred to herein simply as a fuel stream) to generate heat. More specifically, the subject durable catalytic burner system incorporates a catalytic bed assembly which, at its steady state, operates in a flameless catalytic mode to effect heat-releasing oxidation reactions of the fuel stream. The subject durable catalytic burner system incorporates a combination of mechanical features which collectively yield thermodynamic efficiencies that afford the catalytic bed assembly a longer operational life.
There is a need in numerous applications for burner systems that are operable for extended periods of continuous use. In electric power generator systems located at remote, unmanned stations, for example, burner systems are employed which reactively consume an inflowing stream of fuel to generate heat that, then, is thermoelectrically converted to electric power. Typically, the burner system is mounted directly to a thermoelectric conversion unit for this purpose. The heat generated by the burner system's operation is transferred through its heat exchanger portion to the thermoelectric conversion unit for appropriate transduction.
Catalytic burner systems are often employed in these and other such applications for the thermodynamic advantages inherent to their steady state operation. A conventional catalytic burner system
1
known and typically used in the prior art is shown in
FIGS. 5 and 6
. System
1
includes a housing
10
in which a burner chamber
12
is formed. In the walls surrounding this chamber
12
are an upstream opening
14
and a downstream opening
16
. A fuel supply stream is introduced to and exhausted from chamber
12
through these openings
14
,
16
as indicated by the directional arrows
15
. Housing
10
is formed of a metallic or other suitable material such that the walls and floor defining burner chamber
12
serve collectively as a heat exchanger that effectively transfers the heat generated within chamber
12
to a thermoelectric conversion or other such unit mounted therebeneath.
Disposed within chamber
12
is a catalytic bed assembly
20
that, upon sufficient initial heating, catalyzes oxidation of the fuel/fair mixture constituting the introduced stream of fuel to sustain a level of generated heat. The assembly—which is supported in part by a plurality of heat conductive posts
18
projecting upward from the floor of burner chamber
12
—includes upstream and downstream mesh retaining members
22
,
24
. Mesh retaining members
22
,
24
serve as fuel-pervious retaining wall structures between which a bed of catalytic bead members
26
are retained.
While not shown, a cover is typically installed directly over chamber
12
. Such cover is coupled to housing
10
, so as to fit tightly against catalytic bed assembly
20
and thereby prevent the incoming fuel stream from bypassing that catalytic bed assembly
20
.
Briefly, operation of system
1
occurs as follows. As the fuel stream traverses catalytic bed assembly
20
, it is initially ignited within that chamber
12
, downstream of catalytic bed assembly
20
. As the resulting flame burns within chamber
12
, the individual catalytic bead members
26
are gradually heated until enough of them attain a sufficient temperature to catalyze a flameless oxidation reaction o f at least a portion of the fuel stream. Enough of the fuel in the stream is eventually consumed in this manner that an insufficient concentration of fuel remains to sustain the flame combustion. The initially ignited flame thus extinguishes, and flameless catalytic combustion prevails, whereby the catalytic bead members
26
are maintained in their sufficiently heated state by heat released from the ongoing catalyzed oxidation reactions. The region of most intense oxidation—thus, of most intense heat production—then propagates upstream through catalytic bed assembly
20
until the upstream-most layer of catalytic bead members
26
come to oxidize much of the fuel in the passing fuel stream.
While adequate for basic operation, such prior art catalytic burner systems are encumbered by a number of shortcomings. First, its mechanical features permit the premature degradation of catalytic bead members
26
, permitting in turn the premature degradation of the burner system's thermodynamic efficiency. Each type of catalyst composition (typically, coated onto the surface of a ceramic or other suitable substrate to form catalytic bead members
26
) that may be employed for catalytic bed
20
is characterized by a range of temperatures at which it serves its catalyzing function in stable manner. At temperatures above this range, a given catalyst composition becomes unstable and sustains a measurable damage if maintained at the excessive temperatures. Even within its range of temperatures, a catalyst composition's ultimate durability is closely correlated with the temperatures at which it is maintained during burner operation. Generally, the lower the temperature at which a catalyst composition is maintained during operation, the longer its useful life. Conversely, the higher the temperature at which a catalyst composition is maintained during operation, the quicker it degrades. Particularly in applications requiring extended periods of burner operation, therefore, it becomes important to minimize the catalyst composition's operating temperature within the permissible range. Adequate measures to so minimize the catalyst composition's temperature are not provided in catalytic burner systems heretofore known utilized as sources of heat.
The sectional or transverse area of the catalyst bed's upstream side, or ‘face’ is found to be an important factor in this context. An increase in the transverse area yields a corresponding increase in the spatial distribution of the total heat produced by the catalyzed reaction. Increasing the transverse area consequently affords a lower operating temperature for each individual catalyst bead member within a catalytic bed. Moreover, as it is the upstream-most transverse layer of catalytic bead members
26
that first reacts with the stream of fuel impinging thereon, the transverse area at the upstream face of the catalytic bed proves to be of particular importance.
When subjected to substantial periods of normal use, many of the beads
26
forming the bed's upstream-most portions in the prior art burner system
1
are visibly degraded, having lost a substantial proportion of their catalytic capacity. A disproportionately greater degradation is typically revealed in catalytic bead members
26
at the upstream-most regions of the catalytic bed than at the downstream-most regions. In long term operation, the catalytic bed's upstream-most layer degrades in catalytic performance until it becomes inactive, causing the next layer of bead members
26
to become the most active. This continues, in turn, for successive layers of bead members
26
, such that the catalytic bed is progressively destroyed from its upstream-most to its downstream-most portions, until its catalytic performance is diminished beyond acceptable levels.
The problem is aggravated where a concentration of flow occurs at the catalytic bed's upstream-most portions. Variations in flow resistance in the bed cause the flow to be more concentrated along certain stream paths through the catalytic bed. Directional arrows
17
and
19
illustrate examples of stream paths potentially of differing flow concentration.
Another shortcoming found in the prior art catalytic burner system
1
is that of inefficient catalytic bed heating during the initial phases of burner operation. The initially ignited flame bears against the downstream face of the catalytic bed to provide the required bed heating. Without measures to intensify the heat of the flame, it is not uncommon in many applications for insufficient heating of the cat

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