U.S. patent application number 10/044526 was filed with the patent office on 2002-08-08 for catalytic burner element inside a fuel cell with structured catalytic coated surfaces.
Invention is credited to Dusterwald, Hans Gerd, Hermann, Ingo.
Application Number | 20020106596 10/044526 |
Document ID | / |
Family ID | 25112088 |
Filed Date | 2002-08-08 |
United States Patent
Application |
20020106596 |
Kind Code |
A1 |
Hermann, Ingo ; et
al. |
August 8, 2002 |
Catalytic burner element inside a fuel cell with structured
catalytic coated surfaces
Abstract
A heat-emitting burner element, especially for use in a reformer
unit of a fuel cell system, consists of two at least essentially
parallel and spaced-apart plates and is characterized by the fact
that the plates form a reaction gap between them, and as a result
of the catalytic combustion of a fuel gas/oxygen mixture there on a
catalytic coating facing the reaction gap provided on at least one
of the plates, generate heat and emit it by radiation, convection
and conduction directly through the coated plates(s) to at least
one neighboring endothermic stage and that at least one of the
plates displays structural elements having the catalytic coating
and also extending into the reaction gap, which structural elements
extend in the flow direction and consist, e.g., of fins or bars. A
device for introducing diluting air transversely to the flow
direction through the reaction gap is preferably provided.
Inventors: |
Hermann, Ingo; (Mainz,
DE) ; Dusterwald, Hans Gerd; (Dusseldorf,
DE) |
Correspondence
Address: |
CARY W. BROOKS
General Motors Corporation
Legal Staff, Mail Code 482-C23-B21
P.O. Box 300
Detroit
MI
48265-3000
US
|
Family ID: |
25112088 |
Appl. No.: |
10/044526 |
Filed: |
January 10, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10044526 |
Jan 10, 2002 |
|
|
|
09778031 |
Feb 6, 2001 |
|
|
|
Current U.S.
Class: |
431/12 ;
431/356 |
Current CPC
Class: |
C01B 3/363 20130101;
H01M 8/04022 20130101; F23D 14/18 20130101; Y02E 60/50 20130101;
H01M 8/0631 20130101 |
Class at
Publication: |
431/12 ;
431/356 |
International
Class: |
F23N 003/00; F23D
014/02 |
Claims
1. A heat-emitting burner element for use with at least one
processing device of a fuel cell system performing an endothermic
process, e.g., with an endothermic stage of a reforming unit where
the burner element consists of at least two plates arranged
essentially parallel to each other and at a distance from each
other, characterized by the fact that the plates form a reaction
gap between themselves and, as a result of the catalytic combustion
of a fuel gas/oxygen mixture there on a catalytic coating provided
on at least one of the plates and facing the reaction gap, generate
heat and emit it by radiation, convection and conduction directly
through the coated plate(s) to at least one neighboring endothermic
stage and that at least one of the plates displays structural
elements also having a catalytic coating and extending into the
reaction gap, which runs in the flow direction, which structural
elements are if necessary in rows arranged transversely to the
direction of flow and offset with respect to each other and
consisting, for example, of fins or bars.
2. A burner element as in claim 1, characterized by the fact that
the element is essentially four-sided in top view, e.g., square,
rectangular or trapezoidal, that the reaction gap displays an inlet
and an outlet on the first and second opposite sides of the
four-sided element so that the fuel gas/oxygen mixture flows in a
flow direction from the inlet on the first side to the outlet on
the second side.
3. A burner element as in claim 1 characterized by the fact that
the plates forming the reaction gap are of wavelike shape, with the
peaks and valleys forming the longitudinal direction of the wave
form extending in the flow direction of the fuel gases.
4. A burner element as in claim 3 characterized by the fact that
the waveform is a rectangular or square wave.
5. A burner element as in claim 2 characterized by the fact that a
device for introducing diluting air transversely to the direction
of flow is provided at least in one and preferably in several
places along at least one of the also oppositely positioned third
and fourth sides of the element.
6. A burner element as in claim 5 characterized by the fact that
the device is designed for introducing diluting air in order to
introduce it perpendicular to the flow direction of the combustion
gases through the reaction gap.
7. A burner element as in claim 5 characterized by the fact that
the catalytic combustion chamber defined by the reaction gap is
subdivided in the flow direction into several structured sections
with the device for introducing diluting air having air openings
which in each case are arranged between the neighboring sections
following one another.
8. A burner element as in claim 7 characterized by the fact that
between two neighboring consecutive sections in each case a
distance is provided in the region of the air openings which is at
least essentially free of structural elements.
9. A burner element as in claim 8 characterized by the fact that
the structured sections display structural elements which bridge
the reaction gap between the plates at least essentially
completely.
10. A burner element as in claim 7 characterized by the fact that
between the two above-mentioned plates on their edge regions,
spacers are provided in which or between which the above noted air
openings are provided.
11. A burner element as in claim 1 characterized by the fact that
the two above-noted plates form on their surfaces facing away from
each other a part of an endothermic stage or a reforming unit.
12. A burner element as in claim 11 characterized by the fact that
the above-noted surfaces of the plates facing away from each other
are also structured and may be coated with a catalyst also.
13. A burner element as in claim 2 characterized by the fact that
the inlet communicates with a feed channel for the fuel/oxygen
mixture arranged in an edge region on the first side of the element
and extending perpendicular to the reaction gap.
14. A burner element as in claim 13 characterized by the fact that
the outlet communicates with an outflow channel arranged on the
second side of the rectangular element and extending perpendicular
to the reaction gap.
15. A burner element as in claim 2 characterized by the fact that
the inlet communicates with several feed-in passages which guide
the fuel/oxygen mixture to different places in the reactor gap
along the first side and thus assure a uniform distribution of the
fuel/oxygen mixture over the width of the reactor gap.
16. A burner element as in claim 15 characterized by the fact that
the outlet communicates with several collecting passages which
collect the exhaust gases from the reactor or the reaction gap at
various places along the second side and feed it to the outflow
channel.
17. A burner element as in claim 15 characterized by the fact that
the feeder passages and the collecting passages are rectangular in
each case and are arranged side by side, that the distance in each
case between the mouth of one of the feeder passages and the inlet
to the collecting passage lying opposite it is always the same.
18. A burner element as in claim 11 characterized by the fact that
the two plates together with the other plate-shaped elements of the
fuel processing system of the reformer unit are stacked into a
stack and the plates or the other plate-shaped elements are welded
together on their four sides to form the stack.
19. A burner element as in claim 7 characterized by the fact that
the combustion chamber is subdivided into three structured sections
and that on at least one of the opposing third and fourth sides two
openings are provided for introduction of air.
20. A heat-emitting burner element for use with at least one
processing device conducting an endothermic process of a fuel cell
system, e.g., with an endothermic stage of a reformer unit, where
the burner element consists of at least two essentially parallel
spaced-apart plates characterized by the fact that the plates form
between themselves a reaction gap and, as a result o the catalytic
combustion of a fuel gas/oxygen mixture there on a catalytic
coating on at least one of the plates facing the reaction gap,
generate heat and emit it by radiation, convection and conduction
directly through the coated plate(s) to at least one neighboring
endothermic stage, that the element is essentially four-sided at
least in top view, e.g., square, rectangular or trapezoidal, that
the reaction gap is divided by at least one separating wall into at
least two slot-like reaction chambers running parallel to each
other and that the one reaction chamber displays on a first side of
the four-sided element an inlet for the one component of the fuel
gas/oxygen mixture while the second reaction chamber on the same
side displays an inlet for another component of the fuel gas/oxygen
mixture, the openings being provided and designed in the separating
wall(s) in order to make an exchange of gases possible in the
reaction chambers or a diffusion equalization, while the gases flow
from the inlet to an outlet on second side lying opposite the first
side.
21. A burner element as in claim 20 characterized by the fact that
the fuel gas is preferably introduced into the first slot-like
reaction chamber and air is preferably introduced into the second
slot-like reaction chamber.
22. A burner element as in claim 21 characterized by the fact that
at least one of the plates displays structural elements also having
a catalytic coating and extending into the reaction gap and which
extend in the direction of flow and consist, for example, of fins
or bars.
23. A burner element as in claim 22 characterized by the fact that
the plates forming the reaction gap are of wavelike design, with
the peaks and valleys forming the wave form in the longitudinal
direction extending in the direction of flow of the fuel gases.
24. A catalyst-coated plate for a heat-emitting burner element
which consists of two at least essentially parallel plates arranged
at a distance from each other characterized by the fact that the
plate in top view is at least essentially four-sided, e.g., square,
rectangular or trapezoidal, that on the first and second opposing
sides of the four-sided element in each case an inlet region and an
outlet region are provided and that the plate displays on the above
noted catalyst-covered surface structural elements extending in the
direction of flow which elements consist, for example of fins or
bars.
25. A plate as in claim 24 characterized by the fact that a device
for introducing diluting air transversely to the direction of flow
is provided at least in one and preferably in several places along
at least one of the opposing third and four sides of the plate.
26. A plate as in claim 24 characterized by the fact that the
structural elements are arranged in at least two groups at a
distance from each other.
27. A plate as in claim 26 characterized by the fact an air intake
site is provided in the spacing region between the groups.
28. A process for controlling the endothermic reforming reactions
in a fuel processing system generating a hydrogen-rich synthetic
gas which contains at least one burner element in which a
fuel/oxygen mixture is introduced into a slot-like reaction
chamber, characterized by the fact that control is accomplished at
least partially by introducing diluting air into the reaction
chamber.
29. A process as in claim 28 characterized by the fact that the
control is accomplished by varying the quantity of diluting air fed
in.
30. A process for controlling the endothermic reformation reaction
in a fuel processing system generating a hydrogen-rich synthetic
gas which contains at least one burner element in which the
components of the fuel-oxygen mixture are introduced into slot-like
reaction chambers separated by a perforated separating wall and
adjacent to one another, characterized by the fact that the control
is accomplished by varying the total quantity of the constituents
supplied.
Description
RELATED APPLICATION
[0001] This is a continuation-in-part of co-pending U.S. patent
application Ser. No. 09/778,031 filed Feb. 6, 2001, and assigned to
the assignee of the present invention.
TECHNICAL FIELD
[0002] The present invention concerns a heat-emitting burner
element for use with at least one processing device of a fuel cell
system carrying out an endothermic process, e.g., with an
endothermic stage of a reforming unit, said burner element
consisting of at least two at least essentially parallel plates
arranged at a distance from each other and a process for
controlling the endothermic reforming reaction in a fuel-processing
system generating a hydrogen-rich synthetic gas which contains at
least one such burner element.
BACKGROUND OF THE INVENTION
[0003] Fuel cell systems require the energy carrier hydrogen for
the generation of current. This hydrogen is frequently generated by
an endothermic conversion process from liquid energy carriers such
as methanol with introduced water, ethanol, methane and higher
hydrocarbons such as gasoline, naphtha, DME, natural gas, kerosene
and synthetic fuels, e.g., diesel oil. The necessary process heat
is supplied by exothermic reactions which are coupled into the
process mode. The combination of a heat-generating and
hydrogen-producing unit is ordinarily called a "fuel processor",
i.e. a fuel preparation system.
[0004] The present invention is based on a layered structure of a
fuel processor system in which flat structures of different
functionality are stacked one above the other in space and coupled
to each other in accordance with their technical tasks. In such a
fuel processor system the zones, for example, catalytic combustion,
reforming and water-gas shift alternate with each other. A strongly
endothermic reaction stage such as reformation must necessarily be
surrounded on both sides by heat-supplying combustion stages.
[0005] A fuel processor of the type described initially is
disclosed in the European preliminary published application EP 0
861 802 A2.
[0006] In the known device, a reforming unit is present between
catalytically acting burner elements. In all layers of the known
fuel processor with catalysts, said catalysts are present in the
form of pellets and are fixed in loose layers on the corresponding
stages. This system has a number of disadvantages: (1) the fixation
of the loose pellets in space in order to assure functionality; (2)
mechanical abrasion of the pellets and loss of catalytic activity
as a result of vibration and the mobile use of the fuel processor;
and (3) heat and mass transfer inhibition of catalytic reactions in
the loose bed.
[0007] The reformation reaction is extremely influenced by the heat
balance. For high yields, a homogeneous temperature distribution in
the reaction layer is necessary. This does not exist in the case of
pellets in the loose bed since a certain empty space volume is
always present. This results in a lower yield per reaction volume
and therefore per catalyst unit mass, entailing a higher catalyst
quantity for complete conversion. The consequences are a larger
structural volume and weight as well as high costs.
[0008] The catalytic combustion reaction on pellets is also limited
by heat and mass transfer. In particular, the produced heat must be
transferred efficiently to the neighboring zones, and this is also
difficult in the case of loose layers; it has been attempted to
alleviate this shortcoming by constructive measures such as
heat-transferring fins.
[0009] The regulation of the heat balance, especially in the case
of dynamic operation of such a fuel processor, is also an
unresolved problem. The heat must be available wherever it is
required by the endothermic reaction steps. A heat deficit or a
heat surplus interfere with the reaction and lead to functioning
failures or damage to the equipment.
SUMMARY OF THE INVENTION
[0010] The purpose of the invention is to avoid the above-described
disadvantages and to devise a burner element which, in a compact
design, achieves a high heat output per reaction volume unit with
efficient transfer of heat to the neighboring heat absorbing
elements of the fuel processor as well as efficient heat and mass
transfer, in which case the burner element should be designed in
such a way that an efficient control of the catalytic combustion
reaction can be achieved. Another objective of the invention is to
devise a process for controlling the exothermic combustion process
in a burner element and therefore also to control the endothermic
reformation reactions in a reforming unit adjacent to the burner
element.
[0011] A first solution o these problems according to the invention
with a burner element of the type described initially is
characterized by the fact that the plates form a reaction gap
between themselves and generate heat as a result of the catalytic
combustion of a fuel gas/oxygen mixture there on a catalytic
coating provided on at least one of the plates and facing toward
the reaction gap and transfer it by radiation, convection and
conduction directly through the coated plate(s) to at least one
neighboring exothermic stage and that at least one of the plates
extending into the reaction gap also displays structural elements
displaying catalytic coatings which run in the flow direction,
which structural elements are possibly in rows which are arranged
transversely to the direction of flow and may be offset from each
other and consist, for example, of fins or bars.
[0012] Through the use of plates displaying catalyst-coated
structural elements instead of catalysts in pellet form, it is
possible to achieve a very large ratio of surface to volume and
smaller reaction volumes, thus realizing a very efficient heat and
mass transfer. Because the catalyst is applied directly to the
plates of the burner element, heat generated in the burner element
is passed on directly to the side of the plates facing the reaction
gap and directly through the plate to the neighboring
heat-absorbing elements. Therefore the heat transfer from the
catalyst to the plate of the burner element is much more
efficiently configured.
[0013] The structural elements of the plates according to the
invention can also be realized by designing the plates forming the
reaction gap to be wavelike or corrugated, in which case the
longitudinal direction of the peaks and valleys forming the waves
runs in the flow direction of the combustible gases.
[0014] The use of corrugated plates in a reactor for catalytic
treatment of gaseous fluids is known from DE-OS 42 14 579 A1. Here,
however, the waves of the plates run perpendicular to the flow
direction so that, on the one hand, the arrangement is not of
space-saving design, and on the other, the flow resistance is
increased. In addition, the known arrangement is not used for a
fuel processor but rather for the purification of exhaust air. For
this purpose the countercurrent principle is used there with which
the same medium flows in neighboring flow channels in opposite
directions so that the reaction heat of one stream leads to a
heating of the opposing stream.
[0015] Because the structural elements displaying the catalytic
coating extending into the reaction gap according to the invention
also run in the flow direction of the combustible gas through the
burner element, the surface supporting the catalyst material can be
enlarged without generating an unacceptable resistance for the fuel
gas/oxygen mixture flowing through the reaction gap.
[0016] Due to the fact that the fuel gas/oxygen mixture flows from
the inlet on one side of a four-sided element to the outlet on the
opposite side of the element, the possibility exists of introducing
diluting air into the mixture on one or both of the two remaining
sides, e.g. by providing a device for introducing diluting air
transversely to the direction of flow in at least one and
preferably in several places along at least one of the also
opposite third and fourth sides of the element so that control of
the catalytic reaction is made possible.
[0017] Such a control of the catalytic reaction is desirable
according to the invention in order to match the heat consumption
and production to each other. Too low temperatures on the reforming
side inhibit the reaction, while too high temperatures excessively
accelerate the reforming reaction, disturb the uniform course of
the coupled reactions and may locally lead to strong thermal
imbalances. This may lead to intensified catalyst aging. The
catalytic oxidation reaction therefore is preferably controlled by
the introduction of air perpendicular to the flow direction of the
fuel gas.
[0018] The quantity of air is controlled by the pressure loss of
the inlet openings over the running length. The dilution with air
reduces the rate of the catalytic reaction, less heat is liberated
and the heat can be managed selectively. Air can therefore be
metered in a controlled way over the entire length or width of the
catalytic combustion zone.
[0019] It is especially favorable to provide the sides of the
plates of the burner element facing away from the reaction gap also
with a catalyst material which is necessary for the work of
reforming. This arrangement especially utilizes the basic idea of
the invention, i.e. to couple the heat source and the heat sink
directly in space in order to generate the heat where it is
consumed. The plates of the burner element are therefor utilized to
generate a direct coupling between reforming and catalytic
combustion. Each plate functions as a separating layer on one side
of which the oxidation catalyst of the burner element and on the
other side the reforming catalyst are present.
[0020] The heat transfer takes places by radiation, convection and
conduction directly through the separating layer. This separating
layer may be either planar or structured. The basic idea of this
compact, efficient "sandwich" concept is the switchover from pellet
catalyst to coated surfaces.
[0021] As a result of the design of the burner element according to
the invention, it is possible to provide an alternating sequence of
burner elements and reforming units which follow one another
directly so that an extremely compact and highly efficient
structure is achieved. Since the heat conduction is efficient,
uniform temperatures can be achieved in the entire structure so
that the reactions always take place under precisely defined
temperature conditions and premature failure of the fuel processor
due to local overheating can essentially be avoided.
[0022] In process terms, the present invention therefore envisages
a process for controlling the endothermic reformation reaction in a
fuel processing system generating a hydrogen-rich synthetic gas
containing at least one burner element in which a fuel/oxygen
mixture is introduced into a slot-like reaction chamber, the
process being characterized by the fact that it is controlled at
least partially by controlling the quantity of diluting air
introduced into the burner element.
[0023] A second solution to the problems posed above according to
the invention is for the plates to form a reaction gap between them
and as a result to the catalytic combustion of a fuel gas/oxygen
mixture there, heat is generated on a catalytic coating facing the
reaction gap provided on at least one of the plates and the heat is
transferred by radiation, convection and conduction directly
through the coated plate(s) to at least one neighboring endothermic
stage, that the element in top view is at lest essentially
four-sided, e.g. square, rectangular or trapezoidal, that the
reaction gap is divided at least by one separating wall into at
lest two parallel running slot-like reaction chambers and that the
one reaction chamber on a first side of the four-sided element
displays an inlet for the one component of the fuel gas/oxygen
mixture, while the second reaction chamber on the same side
displays an inlet for another component of the fuel gas/oxygen
mixture, openings being provided in the or in each separating wall
and designed in order to make an exchange of gases possible in each
of the reaction chambers or to promote a diffusion equalization
while it flows from the inlets to the outlet on a second side
opposite the first side.
[0024] In this burner element the combustible gas is preferably
introduced into the first slot-like reaction chamber and air is
preferably introduced into the second slot-like reaction chamber.
Here also the plates extending into the reaction gap may also
display structural elements having catalytic coatings which extend
in the direction of flow and consist, e.g., of fins or bars. The
structural elements may also be designed by making the plates
forming the reaction gap waved, in which case the longitudinal
direction of the peaks and valleys forming the waves extends in the
flow direction of the combustible gases.
[0025] Because the two components of the fuel gas/oxygen mixture
flow over both sides of the separating wall and this flow is
disturbed because of the openings present in the separating wall
and the structural elements, diffusion processes occur from both
sides of the separating wall so that the two above-mentioned
components o the fuel gas/oxygen mixture are mixed on both sides of
the separating wall and react chemically with each other there with
the aid of the catalytic coatings and generated heat. Since this
diffusion equalization or mixing of the two components takes place
over the entire length of the separating wall, the chemical
reaction also takes place over the entire length of the reaction
chambers and transversely to the separating wall so that a uniform
production of heat results and the burner element does not suffer
the disadvantage that too much heat is generated at one place,
while little heat arises in other regions. This means that the
design according to the invention leads to an essentially more
uniform temperature distribution over the entire area of the burner
element, and this is beneficial for the endothermic process which
is conducted on the outside of the one or both plates and utilizes
the heat emitted by the burner element.
[0026] Because the separating wall is realized as an extremely thin
part, preferably of perforated sheet metal, it occupies little
space so that a very compact design of the burner element is
attained.
[0027] At this point it should be mentioned that a mixed
composition deviating from the stoichiometrically ideal composition
during the catalytic transformation of a fuel gas/oxygen mixture
into heat does not lead to the formation of soot or to other
undesired deposits, because only as much of each component is
reacted as can react chemically with the other and the unconsumed
constituents are removed with the exhaust gases on the outlet side
and may if necessary be fed to another burner element or used for
other purposes. Since the separating wall assures an increasing
mixing of the two components over the entire length and width of
the burner element, on the one hand, the desired uniform
temperature distribution is created. This property is responsible
on the other hand for the fact that by controlling the total
quantity of fuel gas/oxygen mixture fed in, a power-related
adaptation of the heat generated in the burner element and of the
heat made available to the neighboring endothermic processing steps
is achieved so that the process can be governed as a whole, and for
example, in this way an adaptation to different load cycles can be
achieved.
[0028] The invention also concerns a catalyst-coated plate for a
heat-emitting burner element which consists of two plates arranged
essentially parallel to each other and at a distance apart with the
special characteristics that the plate in top view is essentially
four-sided, for example, square, rectangular or trapezoidal, that
on the first and second opposite sides of the four-sided element in
each case an inlet zone and an outlet zone are provided and that
the plate displays on the above mentioned surface covered with
catalyst structural elements extending in the flow direction
consisting, for example, of fins or bars.
[0029] Especially preferred variants of the burner element of the
invention and of the plate in the process for controlling the
endothermic reformation reactions in a fuel-processing system can
be derived from the claims and the further description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] The invention will now be explained in detail with reference
to examples and the drawings in which:
[0031] FIG. 1 shows the layered structure of a fuel-processing
system according to the invention with different layers of
alternating functionality, in which the structure shown can
continue, for example, according to the broken arrow I;
[0032] FIG. 2 is a top view of a planar burner element
corresponding to the invention with gas and air feed lines;
[0033] FIG. 3 is a cross section corresponding to sectional plane
III-III in FIG. 2;
[0034] FIG. 4 is a possible configuration of the one plate of the
burner element in FIGS. 2 and 3 in the region of the boundary
surface with the neighboring reforming unit;
[0035] FIG. 5 shows a schematic representation in order to explain
possible structuring of the sides of the burner element according
to the invention facing the reaction gap;
[0036] FIG. 6 is a top view corresponding to FIG. 2 of a burner
element divided into three sections with a diluting air supply;
[0037] FIG. 7 is a cross section to the burner element in FIG. 6
corresponding to sectional plane VII-VII;
[0038] FIGS. 8A and 8B shows diagrams explaining the temperature
curve in a burner element without the side feed of diluting air and
with such a side feed as shown in FIG. 7;
[0039] FIG. 9 is a perspective schematic representation of a
segment of a burner element according to the invention;
[0040] FIG. 10 is a top view of the plate for a burner element
according to the invention;
[0041] FIG. 11 is an enlarged representation of the structural
elements with the plate in FIG. 10 corresponding to the region XI
shown there; and
[0042] FIG. 12 is an alternative design of a burner element
according to the invention with a sectional representation similar
to that in FIGS. 3 and 7.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0043] FIG. 1 shows in purely schematic form the alternating layers
of a fuel processing system 10 in order to represent the possible
application of the catalytic burner element 12 according to the
invention.
[0044] For example, as disclosed in European preliminary published
application EP 0 920 064 A1, a fuel processing device for fuel
cells has the purpose of transforming a fuel consisting of a
hydrocarbon, usually in the form of CH.sub.3OH, into a
hydrogen-rich synthetic gas which is supplied to the actual fuel
cell arrangement for generating current. For this purpose, methanol
together with water, is fed into the fuel processing system 10 and
preheated by heat exchange with the reform gases or exhaust gases
of the system. Then the methanol/water mixture is evaporated in an
evaporation stage, here denoted by 14A. The heat necessary for this
evaporation is generated by a first burner element 12A according to
the invention which is adjacent to one side of the evaporation
device 14A. On the side of the burner element 12A facing away from
the evaporation device 14A is a so-called superheating device 16A
which has the purpose of heating the fuel/oxygen mixture (oxygen
usually fed in as air) already transformed into vapor in the
evaporation unit 14A to circa 300.degree. C. The corresponding
superheating unit 16A receives heat not only from the first burner
element 12A shown in FIG. 1 at the top but also from a second
burner element 12B which is arranged below the superheating unit
16A in FIG. 1.
[0045] In the schematic representation in FIG. 1, two additional
burner elements 12C and 12D are shown, a reforming unit 18 being
arranged between the two central burner elements 12C and 12B in
FIG. 1 which transforms the methanol/water mixture heated in the
superheating unit 16A into a hydrogen-rich synthetic gas which
consists predominantly of H.sub.2 and CO.sub.2 but also contains
N.sub.2, CO and water. The reforming unit 18 thus receives heat
from both sides, from the burner elements 12B and 12C. Below the
burner elements 12C is another superheating unit 16B which is
positioned between the burner element 12C according to the
invention and the other burner element 12D according to the
invention. Below the bottom burner element 12D in FIG. 1 again is
another evaporation unit. The incoming methanol/water mixture is
accordingly fed to both evaporation units 14A and 14B by
corresponding feed lines which are formed in the stacked units in
FIG. 1. The mixture preheated in the evaporation units 14A and 14B
is accordingly supplied also to the two superheating units 16A and
16B whose outgoing streams are fed to the reforming unit 18.
[0046] FIG. 1 shows no gas and liquid feed or removal lines, but
such lines are realized at least partially by corresponding passage
ducts inside the fuel processing system 10 in FIG. 1 which is
composed of planar elements. Such passage ducts are well know,
e.g., from the above mentioned document EP-A 0 861 802.
[0047] The arrow I indicates that the basic structure shown in FIG.
1 can be repeated, which is usually the case. The possibility of
repeating the structural units has the special advantage that a
modular structure is achieved which can be adapted by the
corresponding choice of the total number of units present to any
power requirement which may arise. Therefore, the units shown
schematically in FIG. 1 can be produced economically.
[0048] At this point it should be emphasized that the sequence of
units shown in FIG. 1 is not compulsory. Other sequences are also
possible such as the sequences shown in EP-A 0 861 802. The
possibility also exists of supplying the methanol and water
separately to the fuel processing device and treating them
selectively in each case before they are supplied to the reforming
unit(s). Otherwise the stack 10 shown in FIG. 1 is not absolutely
complete. Other units may be provided such as so-called hydrogen
shift units and units for transforming carbon monoxide into carbon
dioxide.
[0049] The central point of the present invention, however, is not
the overall design of the fuel processing system, but rather the
design of the burner elements 12A-D which can be utilized in such a
fuel processing device.
[0050] Within the scope of the present description, several
examples will now be discussed for the burner element 12 according
to the invention to carry out the catalytic combustion inside such
a compact processing system with a flat catalyst coating.
[0051] The burner element 12 in FIGS. 2 and 3 consists of two flat
metal plates 20, 22 lying one above the other, e.g., made of
stainless steel, which form a reaction gap 24 between them. Both
surfaces of the plates 20, 22 facing toward the reaction gap 24 are
coated with a defined quantity of an oxidation catalyst such as
platinum or palladium. The known catalyst coating processes are
optimized to such an extent that a defined film thickness can be
maintained.
[0052] According to FIGS. 2 and 3, the fuel gas/oxygen mixture
flows into the reaction gap at the inlet 26 to a first side 28.
Regarding this it may be said that the representations shown in
FIGS. 2 and 3 are very schematic in this respect. In its practical
version the fuel gas/air mixture is fed through a channel in the
reaction gap which stands perpendicular to the plane in FIG. 2 and
which is provided in the edge region of the first side 28, as will
be explained in more detail below. The flow in the reaction gap is
axial (in the direction of arrow 30). The completely reacted
exhaust gases consisting of H.sub.2O, CO.sub.2, and N.sub.2, emerge
at the outlet 32 on the second side 34 of the burner element 12
lying opposite the inlet side 28. Here also FIGS. 2 and 3 are to be
understood schematically. In a specific variant, the exhaust gases
from the burner element are carried away by channels formed inside
the stack.
[0053] The heterogeneous catalyzed combustion reactions of the fuel
gas/air mixture take place on the surface of the catalyst. The heat
supplied to the neighboring zones is absorbed by the endothermic
processes, which the evaporation units 14A, 14B, the superheating
units 16A and 16B and the reforming unit 18 of FIG. 1 represent, by
convection, conduction and radiation.
[0054] Control of the catalytic reaction is absolutely necessary in
order to be able to match the local heat consumption and production
to each other. Too low temperatures on the reforming side inhibit
the reaction, while too high temperatures excessively accelerate
the reforming reactions, interfere with the uniform course of the
coupled reactions and locally may lead to strong thermal
imbalances. This can lead to intensified catalyst aging.
[0055] The catalytic oxidation reaction is controlled in the
variants described above by introducing air perpendicular to the
flow direction of the fuel gas as represented by the arrows 36 in
FIGS. 2 and 3. The quantity of air is controlled by the pressure
loss of the inlet opening over the length of the reaction gap. The
dilution with air reduces the rate of the catalytic reaction; less
heat is released and the heat can be selectively managed. By
injecting air on the opposite third and fourth sides of 38, 40 of
the burner element, one succeeds in metering air over the entire
length and width of the catalytic combustion zone, i.e. the
reaction gap, in a controlled way. This is explained in more detail
below in connection with FIGS. 8A and 8B.
[0056] The catalytic combustion zone can exhibit different
geometries. One of the possibilities is shown in FIG. 4. The
reference number 20 here indicates the upper plate (corresponding
to FIG. 3) of the catalytic combustion element 12C of FIG. 1 which
forms the boundary surface with the reforming unit 18. Here the
plate 20 is designed in a wavelike shape (here with a square
waveform, which is also not absolutely necessary). The plate is
provided on the bottom side with an oxidation catalyst 19 and on
the topside with a reforming catalyst 25. Between the two catalysts
19 and 25, only an extremely thin walled separating layer 42 exists
(the plate itself) which is supposed to prevent the passage of
gases between the burner element and the reforming unit. This means
that the plate 20 is a component both of the burner element 12C and
also a component of the reforming unit 18. This has the special
advantage that the heat transfer by radiation, conduction and
convection takes place directly through the separating layer 42
provided between the oxidation catalyst 19 and the reforming
catalyst 25. The circles with crosses in the center represent the
arrows 30 shown in FIG. 2 and indicate the direction of flow of the
fuel gas/air mixture in the burner element 12C, i.e. perpendicular
to the plane of the drawing in FIG. 4, into the drawing. In other
words, the square peaks and valleys or grooves 44 formed by the
wave shape of the plate 20 are aligned in the direction of flow.
Here also diluting air can be introduced in the direction of the
arrow 36 from both sides.
[0057] FIG. 5 shows the bottom plate 22 of the burner element 12C
in FIG. 3 with examples of possible structuring arranged on the
topside of the plate, i.e. inside the reaction gap 24. On the left
side in FIG. 5, as an example, fm segments 46 are shown which are
aligned in the direction of flow 30 and in this example stand
perpendicular to the plate 22. On the right side of FIG. 5, channel
segments 48 are shown which are also arranged parallel to the flow
direction 30.
[0058] Both the fins 46 and the channels 48 are covered with an
oxidation catalyst 19. In this example of embodiment, the
corresponding structural features are also provided on the bottom
side of the (here not shown) upper plate 20. However, this is not
shown for the sake of clarity, since they would only represent an
inverted arrangement with respect to FIG. 5. Such a structuring,
i.e. on the bottom side of the not-shown upper plate 20, however,
is not absolutely necessary, because the bars 46, for example, can
bridge the entire reaction gap so that the bottom side of the upper
plate can be of planar design. Finally it is also possible to
provide different structuring on the bottom side of the plate 22
and on the top side of the (not shown) upper plate 20, e.g.
whenever for any reason the heat emission on both sides of the
burner element is to be different. The structuring of the bottom
side of the plate 22 and the top side of the plate 20, however, is
also not absolutely necessary as will be explained in more detail
below with reference to FIG. 9.
[0059] The catalytic oxidation reaction can take place on such
structured surfaces. The structuring, due to its large ratio of
surface to volume and the favorable flow mechanics of the
geometrical arrangement with flow channels formed in the direction
of flow causes a distinct increase in the heat transfer. From this
a high efficiency results for heat transfer and therefore a greater
catalyst utilization.
[0060] Here also a control of the catalytic combustion reaction
must be assured. If the structural height of the structuring
elements is smaller than the gap height, i.e. the height of the
reaction gap, a side air injection must also be performed. If the
structuring elements, conversely, fill the entire gap height which
is possible according to the invention--the cross exchange could be
prevented.
[0061] In the case of structuring elements which fill the entire
height of the reaction gap, in order nevertheless to achieve a
cross flow of diluting air and therefore the desired cross
exchange, according to FIGS. 6 and 7, the catalytic combustion zone
can be subdivided into several structured sections (section 1,
section 2, section 3) which are arranged in each case at a distance
from each other, and the diluting air then as before can be
injected form the four sides between these partial segments, i.e.
in FIGS. 6 and 7 between section 1 and section 2 and between
section 2 and section 3 through the corresponding inlet openings
48.
[0062] As FIG. 8A shows, the catalytic combustion takes place
without the introduction of diluting air from the side so that the
temperature increases up to a maximum Tmax which is achieved at a
site along the reaction gap which lies at about 25% of the total
length of the reaction gap, and after this point the temperature
gradually decreases to the outlet 32.
[0063] In the arrangement with the side injection of air in two
places as shown in FIG. 7 at 48, the temperature in the fuel cell
reaches three peaks Tmax which turn out to be somewhat smaller than
the peak Tmax shown in FIG. 8A, while the temperature along the
reaction gap decreases between neighboring maximum Tmax values by
an amount which is clearly smaller than the temperature drop in
FIG. 8A. This means that--for the same quantity of the fuel
gas/oxygen mixture--a more uniform temperature distribution is
achieved over the entire length of the reaction gap, which on the
whole is more advantageous for conducting the process than the
temperature curve shown in FIG. 8A.
[0064] FIG. 9 shows in schematic form how a burner element
according to the invention can be constructed from several
plate-shaped elements. For this version the same reference numbers
are used as before but increased by the base number 100. The
description provided for the structural part of the corresponding
reference numbers is also valid here for the elements with the
corresponding reference numbers.
[0065] With reference to FIG. 9, one sees a schematically
representative segment from a burner element 112 according to the
invention, which consists of three platelike parts, i.e. the upper
plate 120, the lower plate 122 and between them a plate-shaped
spacer or frame 121. For purposes of representation, the three
plates 120, 121, and 122 are pulled apart somewhat so that the
internal structure and the structure of the burner element 112 can
be understood more easily. It should be emphasized here that this
drawing is schematic to the extent that the width of the reaction
gap 124, i.e. in the horizontal direction in FIG. 9, is shown
substantially shortened. This is also true for the length of the
reaction gap 124 of which only a segment is shown in FIG. 9, this
length extending in the direction of the arrow 125.
[0066] Since only a segment of the burner element is shown in FIG.
9, the front side 127 and the back side 129 cannot be equated with
the first side 28 and the second side 34 of FIG. 2, although the
front side 127 can be considered as positioned adjacent to the
inlet and the side 129 as adjacent to the outlet.
[0067] Below the burner element 112 is another plate 131 which
belongs to an endothermic process stage of the reforming unit which
is supposed to be supplied by the burner element 112 with heat.
This plate 131 is shown at a vertical distance away from the plate
122. In a practical variant all plates 120, 121, 122 and 131 lie
directly one on the other and are welded together on the outer
surfaces so that a sealed-off structure results.
[0068] FIG. 9 shows on the lower plate 122 in the center of a
depressed region 133, upright bars 146 arranged at regular
intervals and in a regular pattern.
[0069] The upper plate 120 is also provided with bars 146A arranged
in a mirror image whose bottom side in this example is at a
distance from the top side of the corresponding bar 146 of the
lower plate 122, said distance being determined in this example by
the height of the frame plate 121. The bars 146A of the upper plate
120 are arranged according to the arrangement in the bottom plate
122 in a recess 133A of the upper plate 120. Bars 146 and 146A are
arranged in rows across the direction of arrow 125 and the rows are
in each case offset by half a division with respect to each
other.
[0070] The example of FIG. 9 shows that the topside of the upper
plate 120 also displays structuring elements, here denoted by the
reference numbers 135 and 137. The structural elements 135 and 137
are arranged in a recess 139 in the top side of the outer plate 120
so that their top sides in each case are arranged flush with the
top side of the plate 120. The bars 135 in this example correspond
in size and shape to the bars 146 of the lower plate 122, the bars
137 here are also arranged in rows which, as an example, are square
when viewed from the top, and which are arranged offset with
respect to each other in the transverse direction, i.e.
corresponding to the arrow 141 in FIG. 9.
[0071] The structural elements 135 and 137 in the recess 139 belong
to the endothermic reaction gap of a processing stage of the fuel
processing system which are also supposed to be supplied with heat
from the burner element 112 and are also coated with a
corresponding catalyst.
[0072] While the upper side of the upper plate 120 in FIG. 9 is
provided with structural elements, this is not absolutely
necessary; the top side of plate 120 can also be of planar design,
like the bottom side of the lower plate 122 of the burner element
112. The reaction gap 143 formed between the lower plate 122 and
the plate 131 which is designed for carrying out endothermic
reactions and receives heat from the burner element 112 for this
purpose is therefore defined by the structural elements 145 of the
lower plate 131.
[0073] Below the plate 131 again structural elements 147 are shown
which belong to another reaction gap 149, this reaction gap 149 in
turn involving the reaction gap of an (additional) burner element
such as 112, i.e. the recess 149 of the bottom plate 131 in this
example corresponds to the recess 133A of the upper plate 120.
[0074] The reference number 151 in FIG. 9 indicates a feed channel
for diluting air, the channels 151 extending in the longitudinal
direction of the reaction gap i.e. corresponding to the arrow 125
and open at suitable places 148 (of which only the place on the
left side in FIG. 9 is shown) into the reaction gap 124 of the
burner element 112 in order to supply diluting air into this
reaction gap 124. The possibility of designing the air feed
channels 151 and the cross channels forming the opening 148 in one
side of the plate shaped spacing frame 121 is very advantageous in
practice, because as a result of the small dimensions it would
scarcely be possible to create these feeder channels by
corresponding borings.
[0075] In order to give an idea of the orders of magnitude of the
thickness of the plates, the depths of the reaction gaps and the
dimensions of the bars as well as their mutual spacing values are
entered in FIG. 9 which are to be understood as data in
millimeters.
[0076] It should be emphasized that FIG. 9 is given only as an
example; the exact design of the plates and the structural elements
can be selected differently depending on the task. It should be
emphasized that the surfaces of all recesses and structural
elements are provided with a corresponding catalyst coating which
is adapted to the purpose in question.
[0077] In the examples of embodiment shown in FIGS. 10 and 11,
again the same reference numbers are used as for the previous
examples, but increased by the base number 200. Here also it is
true that the previous description of structural parts with
corresponding reference numbers is valid unless otherwise
stated.
[0078] In the representation of plate 222 in FIGS. 10 and 11, the
specific dimensions are also reported in millimeters, i.e. these
two drawings are drawn true to scale.
[0079] FIGS. 10 and 11 show a top view of a single plate 222 of a
burner element according to the invention which here is provided
according to the invention with bars, but not with side air feed
openings although this would be possible as an option, e.g., either
by using the plate in 222 in FIG. 10 with a plate-shaped spacing
frame similar to the plate-shaped spacing frame 121 in FIG. 9 by
providing corresponding air channels in the edge regions 271
appearing as white regions on the plate shaped elements 222 of FIG.
10. The plate 222 in FIG. 10 is essentially rectangular with first
and second opposing sides 228 and 234 respectively and third and
fourth opposing sides 238, 240. On the first side 228, an
approximately semicircular projection 229 is shown with a boring
231 perpendicular to the plane of the plate 22 which is to be used
as a feed channel for a fuel gas/oxygen mixture which is to be
passed through the reaction gap 224 formed by the plate 222.
[0080] On the second side 234 of the plate 222 is a projection 233
also of semicircular shape, which also displays a vertically
arranged boring 235 which in this example forms an exhaust gas
channel for the exhaust gases formed in the reaction gap 224.
[0081] Adjacent to the feeder channel 231, several metering
passages 237 of rectangular cross section in top view are arranged
which are separated form each other by corresponding bars 239, also
appearing rectangular in top view, which have the function of
distributing the fuel gas/oxygen mixture supplied through the
feeder channel 231 to different places over the width of the
reaction gap 224, i.e. corresponding to the arrow 241, so that a
uniform flow occurs along the reaction gap corresponding to the
direction of arrow 225 over the entire width of the reaction
gap.
[0082] In a corresponding manner, on the outlet side 234 of the
plate 222, collecting passages 260, also appearing rectangular in
top view, are arranged which are formed between bars 262, also
appearing rectangular in top view, which have the function of
collecting the exhaust gases at the end of the reaction gap 242 and
carrying them to the discharge channel 235.
[0083] The inlet passages 237 and the collecting passages 260 are
arranged in such a way that the distance between the mouth of an
inlet passage 237 and the inlet of the opposing collecting passage
in each case is always the same.
[0084] FIG. 11 shows a 10-fold enlarged representation of the
arrangement of the bars 246 in the reaction gap 224 of the plate of
FIG. 10. One will note that the bars 246 are arranged in rows which
are arranged in the width direction 241 of the plate 222 and that
the bars in neighboring rows in each case are offset by half a
division with respect to each other.
[0085] Another possibility is shown in FIG. 12. It consists of
dividing the catalytic combustion zone in two, i.e. the reaction
gap shown in FIG. 12. Here also the same reference numbers as
before are used, but increased by the base number 300. The division
plane 350 here lies between the structured surfaces of the two
plates 320, 322 of the burner element 312 and is realized by a
separating layer or separating wall 350 with defined openings and
defined opening cross sections. Here a separate supply of fuel gas
and air is provided, such that fuel gas flows according to arrow
352 in this example into the upper slot like reaction chamber 354
of the burner element 312, and air flows according to arrow 356
into the lower slot like reaction chamber 358 of the burner element
312. Through the openings of the separating layer, a diffusion
balancing controlled by pressure losses takes place because the gas
flows from the top to the bottom and conversely from the bottom to
the top. This mixing-inducing flow arises, because the directed
flow above the openings in the separating wall 350 causes
turbulence at the openings which assures the desired flows of fuel
gas and air into the other chamber in each case. As a result the
heterogeneously catalyzed combustion reaction takes place in both
of the slot like reaction chambers 354 and 358 of the reaction gap.
This type of control is efficiently achieved only if, as provided
by the invention, coated catalyst surfaces are used. Otherwise the
pellets would plug up the opening cross sections and thus prevent
the diffusion equalization. As a result of this variant, a uniform
temperature distribution is achieved along the reaction gap and in
the transverse direction of the reaction gap.
[0086] By means of an appropriate coating technology, structured
areas can be defined and coated homogeneously with catalyst. Based
on such layers, a fuel processor can be constructed for the
generation of fuel gas which is especially compact and operates
efficiently.
[0087] Within the scope of the present invention, the layer of
catalytic combustion merits particular attention. The concept
described above offers the following advantages:
[0088] efficient heat balancing due to the high surface of the
structured layers by radiation, convection and conduction,
[0089] avoidance of mass transfer inhibition by switching from
catalyst pellets to applied catalyst layers,
[0090] greater catalyst utilization and therefore lower catalyst
mass
[0091] small structural volume and weight, and
[0092] control of the endothermic reformation reaction by a
controlled air supply to the zone of catalytic combustion.
* * * * *