U.S. patent application number 10/613979 was filed with the patent office on 2004-05-20 for process and apparatus for generating hydrogen.
This patent application is currently assigned to SGL Acotec GmbH. Invention is credited to Franz, Marcus, Hartel, Georg, Kunzel, Jurgen.
Application Number | 20040096391 10/613979 |
Document ID | / |
Family ID | 29719467 |
Filed Date | 2004-05-20 |
United States Patent
Application |
20040096391 |
Kind Code |
A1 |
Franz, Marcus ; et
al. |
May 20, 2004 |
Process and apparatus for generating hydrogen
Abstract
A process is described for generating hydrogen through the
oxidation of fuels that contain chemically bound hydrogen, in
particular hydrocarbons, having the following process steps: a)
introducing the fuel (1) as well as the oxidation agent (2) into a
reactor (3) having a porous material (4') that is embodied in such
a way that flame propagation in a direction opposite the direction
of flow is prevented, and b) reacting the fuel with the oxidation
agent in partial oxidation so that hydrogen is obtained in gaseous
form. In addition, an apparatus for generating hydrogen that has a
reactor that contains a porous material (4, 4'), and the reactor
(3) is embodied as a tubular reactor that has a central chamber (5)
to introduce the fuel and the oxidation agent that extends in the
axial direction and is delimited radially toward the outside by a
first wall that has porous material (4), and the first wall is
delimited radially toward the outside by a second wall that
contains the porous material (4'). Also described is an apparatus
to generate hydrogen that has a reactor that contains a porous
material and is characterized by the fact that its porosity in the
direction of flame generation changes so that the pores are larger,
that the porous material is disposed in a first zone and a second
zone, which zones are adjacent to each other, and that a zone that
has a porous material follows the one zone, seen in the direction
of flow.
Inventors: |
Franz, Marcus;
(Schwabmunchen, DE) ; Hartel, Georg; (Freiberg,
DE) ; Kunzel, Jurgen; (Thierhaupten, DE) |
Correspondence
Address: |
DARBY & DARBY P.C.
P. O. BOX 5257
NEW YORK
NY
10150-5257
US
|
Assignee: |
SGL Acotec GmbH
Siershahn
DE
|
Family ID: |
29719467 |
Appl. No.: |
10/613979 |
Filed: |
July 3, 2003 |
Current U.S.
Class: |
423/650 ;
422/600 |
Current CPC
Class: |
C01B 2203/0255 20130101;
B01J 8/0285 20130101; C01B 2203/066 20130101; C01B 2203/142
20130101; B01J 8/025 20130101; H01M 8/0631 20130101; Y02P 20/129
20151101; B01J 8/0411 20130101; Y02E 60/50 20130101; B01J
2219/00265 20130101; B01J 2208/00309 20130101; C01B 3/363 20130101;
C01B 2203/0261 20130101; H01M 8/0618 20130101; C01B 3/386 20130101;
B01J 2208/00141 20130101; B01J 2208/0053 20130101; B01J 2208/00203
20130101; B01J 8/0496 20130101 |
Class at
Publication: |
423/650 ;
422/191 |
International
Class: |
C01B 003/24; B01J
008/04 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 4, 2002 |
DE |
102 30 149.2 |
Claims
What is claimed is:
1. A process for generating hydrogen by means of the oxidation of
fuels that contain chemically bound hydrogen, comprising: a)
introducing a fuel (1) and an oxidation agent (2) into a reactor
(3) having a porous material (4') that is embodied in such a way
that flame propagation in a direction opposite to that of flow is
prevented, b) reacting the fuel (1) with the oxidation agent
through partial oxidization so that hydrogen is obtained in gaseous
form.
2. The process of claim 1, wherein the fuel (1) comprises a
hydrocarbon.
3. The process of claim 1, wherein the oxidation agent (2) is
ambient air.
4. The process of claim 1, wherein the oxidation agent (2) is pure
oxygen or ambient air enriched with oxygen.
5. The process of claim 1, wherein step b) produces heat used to
preheat at least one of the oxidation agent (2) and the fuel
(1).
6. The process of claim 1, wherein step b) is performed at a
pressure of approximately 0.3 to 20 bar absolute.
7. The process of claim 1, wherein the fuel (1) is a hydrocarbon or
an alcohol.
8. The process of claim 1, wherein step b) produces CO, wherein the
CO is reacted in a further step c) to form CO.sub.2.
9. The process of claim 8, wherein steps b) and c) are performed in
different zones of the reactor (3), whereby step b) is performed in
a zone (3b) and step c) is performed in a zone (3c).
10. The process of claim 9, wherein the zone (3c) comprises a
porous material (4").
11. The process of claim 8, wherein step b) produces heat used to
activate the reaction in step c).
12. The process of claim 8, wherein at least one of steam and an
oxygen-containing gas is added prior to step c).
13. The process of claim 8, wherein step c) is performed using a
catalyst.
14. The process of claim 1, wherein step b) is performed using a
catalyst.
15. The process of claim 9, wherein the porous material of at least
one of the zone (3b) and the zone (3c) of the reactor (3) is a
fixed bed, a foam structure, or an open-celled macroporous
structure of ceramic refractory materials.
16. The process of claim 9, wherein the porous material of at least
one of the zone (3b) and the zone (3c) of the reactor (3) comprises
a structured metal.
17. The process of claim 1, wherein step (b) includes an
open-celled microporous material (4) having a pore size that has a
Pclet number that is less than the critical Pclet number below
which flame propagation does not take place.
18. The process of claim 1, wherein step (b) includes a material
(4) having fine holes (4a), wherein the flow velocity in the holes
(4a) prevents the flame from propagating in the direction opposite
to flow.
19. An apparatus to produce hydrogen by means of the oxidation of
fuels that contain chemically bound hydrogen, comprising a reactor
(3) that contains a first porous material (4) and a second porous
material (4'), and the reactor (3) comprises a tubular reactor that
has a central chamber (5) for introducing a fuel and an oxidation
agent, said central chamber (5) extending in an axial direction,
wherein the reactor (3) is defined in the radially outward
direction by a first wall that contains the first porous material
(4), and the first wall is delimited radially to the outside by a
second wall that contains the second porous material (4').
20. The apparatus of claim 19, wherein the first porous material
(4) has a pore size that has a Pclet number that is less than a
critical Pclet number below which flame propagation cannot
occur.
21. The apparatus of claim 19, wherein the first porous material
(4) has fine holes (4a), wherein the flow velocity in the holes
(4a) of the first porous material (4), prevents flame propagation
in the direction opposing flow.
22. The apparatus of claim 19, further comprising a third wall of a
porous material (4") that extends radially outward at a given
distance and is parallel to the second wall.
23. The apparatus of claim 22, wherein the porous material (4") of
the third wall comprises catalytically active structures.
24. The apparatus of claim 19, wherein said apparatus is delimited
by an outer wall (6) that extends axially at a given distance from
the third wall.
25. The apparatus of claim 24, wherein the outer wall (6) comprises
a double tube to contain a coolant.
26. The apparatus of claim 24, wherein a membrane (10) is disposed
on a porous substrate (10') between the outer wall (6) and the
adjacent inner wall, said membrane being permeable to hydrogen gas,
but not permeable to the other products that result in the
oxidation.
27. The apparatus of claim 19, wherein a tube (8) is disposed
around the first and second wall, said tube coiled around an outer
side of the first wall in the form of an extended spiral.
28. The apparatus of claim 19, wherein the first porous material
(4) and the second porous material (4') of at least one of the
first wall and the second wall comprises a catalytically active
structure.
29. An apparatus to generate hydrogen by means of the oxidation of
fuels that contain chemically bound hydrogen comprising a reactor
(3) that contains a first porous material (4) and a second porous
material (4'), wherein the porosity of the first and second porous
materials changes in the direction in which a flame develops to
produce larger pores, the first and second porous materials are
disposed in a first and second zone (3a, 3b), wherein said first
and second zones (3a, 3b) are adjacent to each other and seen in
the direction of flow, and a third zone (3c) is provided downstream
from the second zone (3b) and contains a third porous material
(4").
30. The apparatus of claim 29, wherein an intermediate space (12)
into which a pipe extends to introduce at least one of a gas and
steam is provided between the second and third zones (3b, 3c).
31. The apparatus of claim 29, wherein the second and third porous
materials (4', 4") of at least one of the second and third zone
(3b, 3c) comprises catalytically active structures.
32. The apparatus of claim 29, wherein a heat exchanger (14) is
provided to transfer heat produced in the oxidation reaction that
occurs in zone (3b) to at least one of the fuel and oxidation agent
that is being added.
33. The apparatus of claim 29, wherein said apparatus is delimited
by an outer wall (6) that extends axially at a given distance from
the third zone (3c).
34. The apparatus of claim 33, wherein the outer wall (6) comprises
a double tube for containing a coolant.
Description
[0001] The invention relates to a process for generating hydrogen
by means of the oxidation of fuels that contain chemically bound
hydrogen, in particular of hydrocarbons, as well as apparatuses
suitable for performing the process.
[0002] In the prior art, the generation of hydrogen is generally
accomplished through the partial oxidation of hydrocarbons, for
example methane, propane, natural gas, liquid hydrocarbons, or
low-molecular-weight alcohols, etc. In general, the following
equation may be used for the partial oxidation of a hydrocarbon
with oxygen to generate hydrogen:
2C.sub.nH.sub.2n+2+nO.sub.2.fwdarw.2nCO.vertline.2(n+1)H.sub.2(+soot)
(1)
[0003] (See for example: Buchner, Schliebs, Winter, Buchel,
Industrielle Anorganische Chemie [Industrial Inorganic Chemistry],
2nd revised edition, VCH Verlagsgesellschaft mbH, D6940 Weinheim,
1986, pp. 37 ff.) The combustion temperatures that are cited are
generally from 1200.degree. C. to 1500.degree. C. In order to
counteract the formation of soot that is generally observed,
oxidation agents that have a higher oxygen content than air are
used in the prior-art industrial processes. It is not, however,
always possible to avoid the formation of soot as an unwanted
byproduct. A possible reason for this is that the reactants are not
adequately mixed together if the reaction is performed with free
flame formation, so that localized oxygen deficiencies can occur.
In addition, the temperature gradient at the edge of the flame is
relatively high. Thus, complete conversion of the reactants is no
longer possible at the cooler edge of the flame. This problem
occurs with the partial combustion of a hydrocarbon in an even more
pronounced manner because here the reaction mechanism--namely the
suppression of the oxidation of CO to form CO.sub.2--results in
lower temperatures and, thus, a slowing of the reaction. This
mainly occurs at the colder periphery of the flame or combustion
chamber.
[0004] The formation of soot can be counteracted by enriching the
combustion air with oxygen or by using pure oxygen as the oxidation
agent. This measure causes the combustion temperature to increase,
and the formation of soot is reduced. However, the use of pure
oxygen or air enriched with oxygen involves additional complexity
from a chemical engineering standpoint and higher costs.
[0005] In general, with various processes to produce hydrogen the
highest possible hydrogen yield is sought. As a result, one of the
requirements is to have the lowest possible fuel/air ratio .lambda.
When methane is used as the fuel, the minimum value for .lambda. is
0.25, based on the above equation (1). But here we encounter the
problem that with a low .lambda. soot is formed.
[0006] Thus, the demands for the least possible soot formation and
the greatest possible hydrogen yield are opposed to each other. One
of the goals in any process used to generate hydrogen is to meet
both of these requirements to the greatest extent possible--in
other words, to keep .lambda. as low as possible without forming
soot.
[0007] EP 0 553 924 describes a partial oxidation process in which
a fixed bed is preheated in a first step. Then a stream of
hydrocarbons is passed through the hot fixed bed, which causes the
soot to be deposited on the fixed bed at the desired low .lambda.
values. Starting at a certain temperature level, largely pure
hydrogen is then passed over the soot that has deposited on the
fixed bed, and as a result the soot is oxidized. One disadvantage
of this process for producing hydrogen is that it discontinuous. In
addition, the process requires pure oxygen as the oxidation
agent.
[0008] The problem of soot formation with the partial oxidation of
hydrocarbons has been studied on numerous occasions in the past. In
particular, the phenomenon of soot formation was studied as a
function of the stoichiometric ratio .PHI. between the actual
amount of fuel and the stoichiometrically ideal amount of fuel or
as a function of its reciprocal value, the fuel/air ratio .lambda.
Examples of such studies include the following scientific
articles:
[0009] Kremer, M. Flamme (1984), "Unterstochiometrische Verbrennung
von Kohlenwasserstoffgasen zur Schutzgaserzeugung"
[Substoichiometric Combustion of Hydrocarbon Gases to Generate
Inert Gas"], GWF-Gas/Erdgas, vol. 125, no. 4, pp. 187-193. In this
article, the soot formation limit when methane or natural gas is
used without air preheating is given as .lambda.=0.6. When the air
is preheated to 400.degree. C., this limit decreases to a value of
.lambda.=0.51.
[0010] H. F. Caleote, D. B. Olson (1982), "Importance of
Temperature on Soot Formation in Premixed Flames," Combustion
Science and Technology, vol. 28, pp. 315-317. In this article, the
increased tendency to form soot with fuels at higher flame
temperatures is described, in addition to the familiar
interrelationships.
[0011] A. Jess (1991). "Synthesegaserzeugung durch katalytische
partielle Oxidation von Methan mit Luft" [Synthesis Gas Generation
Through Catalytic Partial Oxidation of Methane with Air],
dissertation Fridericiana University of Karlsruhe. In this study,
the soot formation limit using a high-turbulence vortex burner was
studied. When methane was used as the hydrocarbon and the air was
not preheated, soot formation limit was .lambda.=0.6. For kinetic
reasons, the theoretical limit is stated as .lambda.=0.46. This
study describes how soot formation increases at adiabatic
temperatures up to 1400.degree. C., while it then begins to
decrease at even higher temperatures.
[0012] H. F. Calcote, D. M. Manos (1983), "Effect of Molecular
Structure on Incipient Soot Formation," Combustion and Flame, vol.
49, pp. 289-304. This article provides an overview of the soot
formation limits with various fuels.
[0013] From the studies cited above, it can be assumed that the
practicable fuel/air ratios .lambda. in a process using partial
oxidation generally have a minimum value below which soot begins to
form. When methane is burned, this minimum fuel/air ratio limit
lies between 0.55 and 0.66 according to the literature. Because of
the deposition of soot in such equipment, partial oxidation
processes cannot be used below this limit.
[0014] The articles cited above show that the lower fuel/air limit
of the formation of soot decreases as the flame temperature
increases with the same fuel (for example with air preheating
and/or combustion at lower nitrogen concentrations=oxygen
enrichment), while when various fuels are used, a fuel-dependent
increase in the flame temperature also causes the fuel/air limit
for soot formation to increase. These dependencies only apply in
cases in which the fuels are already premixed with air at the
beginning of the process. By contrast, in diffusion flames there is
an increased tendency for soot formation upon an increase in the
reaction temperature.
[0015] One possible way to prevent the formation of soot is to use
catalysts. This also allows the hydrogen yield to be increased.
Lower fuel/air ratios than those referred to above can be achieved
when catalysts are used. Processes of this type are referred to,
for example, in the following articles:
[0016] S. Frini, G. Calogero, S. Cavallaro (2000), "Hydrogen
production from methane through catalytic partial oxidation
reactions," Journal of Power Sources, vol. 87, pp. 28-38. C. R. H.
de Smet et al. (2001), "Design of adiabatic fixed bed reactors for
the partial oxidation of methane to synthesis gas. Application to
production of methanol and hydrogen for fuel cells," Chemical
Engineering Science, vol. 56, pp. 4849-4861.
[0017] However, other disadvantages are encountered when catalysts
are used, such as "catalyst poisoning," the sensitivity of
catalysts to excessive temperatures, contaminants such as sulfur
and its compounds, and very low soot concentrations. In addition,
reactors using catalysts require special start-up procedures.
[0018] In the article "Hydrogen Production by Methane-Rich
Combustion in a Ceramic Burner," Journal of Chemical Engineering of
Japan, vol. 35, no. 1, pp. 46-56, a process utilizing partial
oxidation in which the reactor is configured as a radiant surface
burner is described. The radiant surface burner consists of a fine
porous structure. No catalyst is used. In the reactor that is
described, most of the combustion reactions occur outside the
porous body, as is typical of radiant surface burners. In this
process, the soot formation limit when methane is burned is
.lambda.=0.66. Experiments were also performed with a lower
fuel/air ratio (.lambda.=0.5) in order to increase the hydrogen
yield. However, soot was formed at this level. The fact that the
soot formation limit involves a higher fuel/air ratio compared with
reactors in which there is free flame formation is because in
radiant surface burners the reaction zone experiences cooling due
to the radiation of heat from the burner surface.
[0019] The object of the invention is to provide a process for
generating hydrogen through the oxidation of fuels containing
chemically bound hydrogen in which the formation of soot is
avoided, even at low fuel/air ratios, and at the same time a high
hydrogen yield is obtained. Another object of the present invention
is to characterize a reactor suitable for the process.
[0020] The object of the invention is accomplished by a process of
claim 1, and by a reactor of claims 19 or 29, respectively.
[0021] The process of the invention to produce hydrogen by means of
the oxidation of fuels that contain chemically bound hydrogen, in
particular hydrocarbons, has the following process steps: a)
feeding the fuel that contains chemically bound hydrogen, as well
as the oxidation agent, into a reactor having a porous material
that is designed in such a way that flame propagation in the
opposite direction from the flow of the fuel is prevented; b)
reacting the fuel with the oxidation agent in partial oxidation, so
that the hydrogen is obtained in gaseous form.
[0022] In the present invention, "partial oxidation" of
hydrocarbon-containing fuels means any reaction in which the
fuel/air ratio is less than 1.
[0023] There are various ways to stabilize combustion reactions in
porous materials, and they basically can be divided into two
different categories:
[0024] The first option is to stabilize the combustion by means of
fluid technology in such a way that the flow velocity at a given
location in the burner is greater than the flame propagation
velocity in order to prevent the undesirable propagation of flames
in the direction opposite to that of the flow of fuel (U.S. Pat.
No. 755,376, Ser. No. 70,230, U.S. Pat. No. 755,377, Ser. No.
70,230, both C. E. Lucke; both 1901: German patent 1,303,596,
1966). This can be accomplished, for example, by an abrupt widening
of cross section, upstream from which the flow velocity is greater
than the flame propagation velocity, and downstream from which the
flow velocity is smaller. The same principle also works with a
gradual increase in cross section, with the difference being that
the flame stabilizes at different locations, depending on the flow
velocity (U.S. Pat. No. 1,015,261 to W. A. Bone, Ser. No. 595,438,
issued Jan. 16, 1912). Another way to utilize the difference
between the flame propagation velocity and the flow velocity is the
variable adjustment of different flows of gas and air, for example
using temperature measurements in porous materials (U.S. Pat. No.
5,165,884, dated 1992; EP 524736 A2, dated 1993).
[0025] The second way to stabilize combustion reactions in porous
media is to sufficiently cool the gas mixture just upstream from
the desired combustion site. This can be accomplished, for example,
by means of active cooling measures such as water-cooled pipes
(U.S. Pat. No. 1,225,381, Ser. No. 11,365) or by cooling surfaces
(Schwank, G. Gasheizung und Strahlung, vol. 90, no. 8, 1949, pp.
169-174). A specific and advantageous way to stabilize combustion
reactions in porous media based on thermal effects is that for the
propagation of the flame in a porous structure there must be a
certain minimum ratio between the heat produced by the combustion
and the heat removed by the surrounding medium. This ratio can be
stated by a modified Pclet number Pe, which is not formed with the
flow velocity, but rather by the laminar flame velocity of the
fuel/air mixture S.sub.1 1 P e - S 1 d P , eff p f c p , f f - S 1
d P , eff a f ( 2 )
[0026] In addition, an effective pore size d.sub.P,eff, as well as
the thermal conductance of the gas mixture a.sub..rho. as a
material property are used in Equation (2). There exists a critical
Pclet number, which for hydrocarbons generally has a value of about
65, above which flame propagation is possible in porous material
and below which, however, the flame is extinguished. In the process
of the invention, as a result of the increase in the pore size in
the direction of flow there initially is a zone (having a small
pore size) in which the Pclet number is sub-critical and in which
the flames are extinguished. As the pore size increases, a zone
having a supercritical Pclet number is produced, in which the
flames can propagate. By means of this method of stabilization, a
self-stabilizing flame wave can be generated in the process of the
invention. This option for achieving combustion stabilization in a
porous burner is described, for example, in EP 0657011 B1, and it
often appears to be advantageous, since stabilization is possible
within a broad range, independent of the flow velocity, which
thereby permits a broad range of throughputs.
[0027] The use of an open-celled macroporous material having a
changing pore size that has a critical Pclet number on an internal
boundary surface permits extremely stable process management, even
at comparatively low fuel/air ratios, without producing soot. In
this way, the hydrogen concentration that is produced as well as
the hydrogen yield can be increased. This results in a decrease in
the combustion temperature, and therefore also the reactor
temperature, so that, for example, metallic materials in the form
of open-celled macroporous material can be used. It was found that
the fuel/air ratio below which soot begins to form can be lowered
to values of about .lambda.=0.45, and even lower, without the use
of catalysts. Extrapolation of the values that were determined
allows one to expect that if the process described above is
optimized systematically, values of .lambda.=0.4 can even be
achieved without catalyst. These astonishingly low values are
apparently due to the heat transport mechanisms prevailing inside
the porous material. By using open-celled microporous material
having the stated characteristics, the homogeneity of the
temperature matrix within the combustion zone is increased. This
permits a mode of operation that is much closer to the soot limit
than in other reactors that are subject to larger temperature and
concentration variations. In addition, the oxidative reaction of
soot precursors increases faster at higher temperatures than their
rates of formation. This is the general reason why the lower
fuel/air ratio limit for the formation of soot decreases as the
flame temperature increases with the same fuel. However, if soot
nuclei are present in an open-flame reactor used for partial
oxidation, their temperature decreases abruptly due to free
radiation, which contributes to the formation of soot. In a
stabilized porous reactor such as that used in the process of the
invention, on the other hand, all soot precursors are in a thermal
equilibrium with the porous structure, which is why soot nuclei
cannot cool down as the result of free heat radiation.
[0028] The open-celled microporous material used in the process of
the invention can have a continuous transition from small-diameter
pores to larger diameter pores in the interior of the reactor, so
that then the flame propagation begins at a characteristic pore
diameter having the critical Pclet number. When different fuel/air
mixtures are used, the critical Pclet number can also vary. With a
continuous increase in the pore size of the porous material, this
would have the disadvantage that the flame could move under
different conditions. In order to establish a defined position for
flame development, it is therefore advantageous that two zones of
different pore size located adjacent to one another be present, and
that the first zone located downstream from the inlet have a Pclet
number for flame development that is smaller than the critical
Pclet number and the second zone further downstream from the inlet
have a Pclet number that is larger than the critical Pclet number.
As a result of this measure, the generation of the flame is
restricted to the surface or to the area between the two zones and
is essentially independent of operating parameters that could
result in a variation of the critical Pclet number. The measure to
define the site of flame creation referred to above therefore
further increases stability. See EP 0 657 011 for further details
regarding the configuration of the zones in the reactor.
[0029] The process of the invention allows ambient air to be used
as the oxidation agent. Enrichment with oxygen is not necessary. In
general, a hydrogen yield relative to the hydrogen bound in the
fuels of at least 70% can be achieved without generating soot. The
use of air has the advantage of offering particularly economical
process management, and it has the further advantage of producing
low adiabatic combustion temperatures. The process of the invention
even allows the reaction to be maintained at a temperature level
near the minimum ignition temperature for the reaction mixture. Of
course, it is also possible to use oxygen or air enriched with
oxygen as the oxidation agent.
[0030] In a preferred embodiment of the process of the invention,
the heat that is given off resulting from step b) is used to
preheat the oxidation agent and/or the hydrogen-containing fuel.
The preheating can be accomplished, for example, by means of a heat
exchanger embedded in the hot reaction zone, or placed in the hot
gas stream, through which the oxidation agent and/or the
hydrogen-containing fuel is directed. Another way to accomplish
preheating is to design the first zone, which serves as a flame
barrier, in such a way with regard to its heat transfer
characteristics that a sufficient part of the heat of reaction is
removed directly out of the reaction zone in the direction opposite
the direction of flow. Using this heat, the educt mixture flowing
through the flame barrier can be heated within the flame barrier
before the reaction zone is reached. A combination of both
preheating techniques is also possible. In this way, the reaction
temperature can be controlled in such a way that the hydrogen yield
is optimized. This is because the chemical equilibrium is shifted
into the range having a desired high hydrogen yield solely based on
the temperature increase, which is why the chemical composition of
the fuel and/or the oxidation agent does not need to be
changed.
[0031] Since the porous material itself offers resistance to the
gas flow, the process of the invention can essentially be used in a
wide range of pressures. In particular, in step b) pressures from
about 0.3 bar to 20 bar absolute are possible--in other words in a
range from vacuum to high pressure.
[0032] In addition to the usual hydrogen-containing fuels such as
methane, propane, or natural gas, or liquid hydrocarbons, alcohols
can also be used in the process of the invention, in particular
low-molecular-weight alcohols.
[0033] In an additional preferred embodiment of the process of the
invention in step b) the CO that results is reacted with steam to
form CO.sub.2 with water vapor in a further step c). This generates
additional hydrogen and prevents the formation of toxic CO.
[0034] Steps b) and c) are preferably performed in different
reactor zones. This means that steps b) and c) are separate from
each other, both spatially as well as in their chronological order.
The process of the invention can be performed continuously, so that
it is not necessary to interrupt the course of the project.
[0035] Based on the already described properties of the porous
material with respect to heat transfer or to the generation of a
homogeneous temperature matrix, it is preferred that the zone used
to perform step c) also contain porous material.
[0036] As is known, a minimum temperature must be present in order
to react CO into CO.sub.2. In a preferred embodiment of the process
of the invention, this temperature at least partially comes from
the heat energy that is formed in step b) and that is contained in
the gases. Heat can be removed from porous structures particularly
well. For example, it is possible to connect the porous materials
in the zones in steps b) and c) to each other by means of a
conventional heat exchanger. Of course, the heat produced in step
b) can also be used in some other way.
[0037] In order to react the CO to CO.sub.2, steam can be
introduced prior to step c). The steam is preferably introduced
between the zone in which step b) is performed and the zone in
which step c) is performed. The introduction of steam (steam
reforming) has the further advantage of increasing the hydrogen
yield further.
[0038] A further improvement in efficiency can be achieved by using
a catalyst when performing step b) and/or step c). Thus, for
example, a catalytic coating on the porous material is conceivable.
If the process of the invention is performed with a catalytically
coated combustion zone, in this case the stabilization of
combustion is significantly better than when the process is
performed with a conventional catalytic reactor. The use of a
catalyst in step c) also allows the selectivity of the oxidation
reaction to be controlled better.
[0039] In one embodiment, the open-celled macroporous material of
at least one of the zones of the reactor can contain a fixed bed of
ceramic refractory materials. Conceivable porous materials used in
this regard are, for example, ceramic packing bodies in the form of
rings, balls, open-celled foams, or also in the form of other
regular ceramic structures.
[0040] In one alternative, the porous material of at least one of
the zones of the reactor can also contain a structured metal. Metal
has the advantage of having particularly high thermal conductivity,
so that a homogeneous temperature matrix is quickly established.
Conceivable metal structures include, for example, lattices,
screens, mixer structures, or open-celled metal foams.
[0041] The process of the invention can be used for numerous
applications, i.e., chemical engineering. For example, the hydrogen
can be generated for various large-scale chemical processes, such
as increasing the mass flow of H.sub.2 in HCl synthesis plants, as
well as H.sub.2 generation in compact plants in order to meet
peak-load requirements.
[0042] A further area for the use of the process of the invention
that is becoming increasingly important is in the preparation of
fuel for fuel cells (FCs). Fuel cells are typically used in small
and medium sizes in automotive vehicles as well as for the
decentralized generation of electrical power and heat, and they are
currently in the advanced stage of development. The process of the
invention can be used advantageously in this application in
particular. In order to generate the hydrogen required by the fuel
cell, a so-called reformer is typically used. Depending on the fuel
cell and reformer type, a "starting burner" is needed to bring the
fuel cell temperature and the fuel and reformer equipment to the
necessary starting temperature. When the process of the invention
is used as the reformer, such a starting burner is not needed for
the fuel cell. All that is needed is to modify the process when the
system is started so that superstoichiometric conditions prevail
for the oxidation. After the system start, the system can be
shifted to substoichiometric operation, so that the entire reformer
process or a part of it is carried out by means of the process of
the invention.
[0043] When the same porous structures as those in the process of
the invention are used, combustion of the combustible residue gases
from the fuel cell and the reformer can burn in a safe and stable
manner.
[0044] An especially suitable device for generating hydrogen by
means of the oxidation of fuels that contain chemically bound
hydrogen has a reactor that contains a porous material, this
reactor being designed as a tubular reactor having a central
chamber for introducing the fuel and the oxidation agent that
extends in the axial direction and is radially delimited on the
exterior by a first wall consisting of porous material, whereby the
first wall is delimited radially on the exterior by a second wall
that contains an additional porous material.
[0045] As already noted in conjunction with the process of the
invention, a stabilization of the reactor of the invention can be
accomplished if the material has fine holes, so that the rate of
flow in the material or in the holes prevents the flame from
traveling in the direction opposite to flow.
[0046] In one alternative, the pore size of the macroporous
open-celled material can change to larger-sized pores in the
direction of flame development. In this case, a boundary surface
within the porous material can have a critical Pclet number above
which flame generation occurs and below which it is suppressed.
[0047] In the reactor of the invention, the two porous materials on
whose boundary surface the critical Pclet number is established are
adjacent to each other and are disposed concentrically about the
central longitudinal axis of the tubular reactor. To perform the
reaction, the oxidation agent as well as the fuel are fed into the
central chamber, and then they flow through the two porous walls
radially toward the outside. Within the first wall, the Pclet
number is smaller than the critical value, which is why flame
propagation is not possible. Within the second porous material of
the second wall, the Pclet number is larger than the critical
value, so that combustion can occur.
[0048] The reaction products can be brought together in an open
space that is delimited radially on the inside by the second wall
and is delimited radially on the outside by an outer wall. The
chamber thus extends essentially concentric to the walls and to the
inner hollow chamber. From this chamber, the reaction product can
be transported in an axial or radial direction out of the
reactor.
[0049] In a preferred embodiment, the outer wall is embodied as a
double tube in order to accept a coolant. In this way, the
resulting gases can be cooled before they leave the reactor. The
double tube can be advantageously equipped with ribs on the reactor
side, so that the heat from the reaction mixture (not shown) can be
removed better.
[0050] In order to subsequently react the CO that results in the
process of the invention, the apparatus of the invention preferably
has an additional wall made of a porous material that extends at a
given distance radially toward the exterior and essentially
parallel to the second wall. In other words, a concentric chamber
is formed between the second and third wall. Pipes to supply
oxygen-containing gases and/or steam, for example, can lead into
this chamber. The gases or steam that are fed flow radially toward
the exterior through the third wall together with the product gases
from the partial oxidation reaction.
[0051] In order to allow the process of the invention to proceed
catalytically, the porous material of the second wall can contain
catalytically active structures. For example, the porous material
may have a catalytic coating. Similarly, it is also possible to
configure the porous material of the third wall with a
catalytically active structure that is able to use steam to react
CO to form H.sub.2 and CO.sub.2.
[0052] In order to be able to use catalysts that are also effective
at temperatures below the reaction temperature, an intermediate
cooling system is provided, preferably between the first and second
wall. This cooling system can be configured as a tube wound around
the exterior of the second wall in the form of an extended spiral.
Of course, other configurations of a cooling tube are conceivable
in the space between the second and third wall, in order to adjust
the temperatures to advantageous values.
[0053] The hydrogen gas that is generated is frequently required in
pure form. In this case, a membrane that is only permeable to
hydrogen gas is disposed in the apparatus of the invention between
the outer wall and the adjacent inner wall. This membrane, however,
is not permeable to the other products resulting in the reaction.
Suitable membranes are, for example, metal membranes of the
platinum group, preferably of palladium. In one alternative,
however, it is also possible to clean the resulting hydrogen gas
with the help of conventional processes (for example, after
separation of CO/CO.sub.2 by means of low-temperature
rectification).
[0054] A further apparatus of the invention for generating hydrogen
through the oxidation of fuels that contain chemically bound
hydrogen has a reactor that contains a porous mass whose pore size
changes so that the pores become larger in the direction of flame
development, whereby a boundary surface within the porous material
has a critical Pclet number above which flame development occurs
and below which it is suppressed, whereby the porous material is
arranged into a first zone and a second zone, and whereby, seen in
the direction of flow, a third zone is provided downstream and also
contains a porous material. The individual zones may, unlike the
reactor described above, be disposed linearly in series, so that in
this case the direction of flow proceeds in an axial direction
through the reactor. In this type of reactor as well, the actual
oxidation reaction occurs within the second zone, and the first
zone, which contains the porous material having the subcritical
Pclet number, serves as a flame barrier. In the third zone, the
resulting CO is reformed to CO.sub.2, and is spatially separate
from the main reactor. For this reason, the entire reaction process
can be run simultaneously, so that it is not necessary to interrupt
the reaction process flow. However, because the porous flow from
the third zone is located within the reaction mixture in the
reactor, however, a very compact design is possible.
[0055] An intermediate space is preferably disposed between the
second and third zone, and into this space a pipe for introducing
gas and/or steam is disposed. Here, the reaction of CO to form
CO.sub.2 is supported by the introduction of oxygen-containing gas
and/or steam, as already described in conjunction with the process
of the invention.
[0056] If use of a catalyst is desired, the porous material of the
second and/or third zone can also contain catalytically active
structures in this case.
[0057] In order to remove the heat of reaction that results in the
oxidation reaction that occurs in the second zone, the apparatus of
the invention preferably has a heat exchanger pipe. The heat
exchanger pipe can be used, for example, to transfer the resulting
heat to the incoming fuel and/or the incoming oxidation agents. In
addition, the pipe can be used to heat the third zone within which
the oxidation of CO to CO.sub.2 (with H.sub.2O steam) occurs, thus
providing the necessary activation energy for this reaction.
[0058] In general it must be noted that the porous structure in the
reaction chamber plays a key role for the transport of heat in the
reactors of the invention. In this regard, materials that have a
high radiation intensity already at low temperatures--in other
words, that increase the transfer of heat--have been found to be
advantageous for the porous structure. Metals in spherical form,
knitted fabrics or orifice diaphragms and other porous bodies,
open-celled foams (in particular in small pieces of equipment), and
fixed beds or stratified beds can be considered, such as are
generally used in thermal processing technology. Ceramic materials
that are based on Mg, Ca, Al, Si, or Zr oxide and compounds thereof
have been found to be particularly suitable. Nitrides and carbides
can also be used in this conjunction. The possible configurations
of the packing materials are extremely varied, however preferably
Hiflow rings, saddle packing, balls, and cylindrical packing are
used.
[0059] The apparatuses of the invention can be used as combined
starting burners and reform burners to prepare the fuel for fuel
cells. Detailed design options for these apparatuses have already
been described in combination with the process of the
invention.
[0060] We shall now explain the invention in greater detail below
based on examples of embodiments and with reference to the attached
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0061] FIG. 1A first embodiment of an apparatus of the invention
for generating hydrogen through the oxidation of
hydrocarbon-containing fuels shown in a sectional view;
[0062] FIG. 2a A modified embodiment of the apparatus of FIG. 1,
shown in a sectional view;
[0063] FIG. 2b A modified embodiment that shows a different way to
stabilize the flame propagation;
[0064] FIG. 3 An additional modified embodiment of the apparatus of
FIG. 1 in a sectional view;
[0065] FIG. 4A further apparatus of the invention for generating
hydrogen through the oxidation of hydrocarbon-containing fuels.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0066] FIGS. 1 to 3 each show an example of an apparatus of the
invention, configured as reactor 3 in order to generate hydrogen by
means of the oxidation of fuels that contain chemically bound
hydrogen. Reactor 3 has the shape of an extended pipe, into whose
interior a first wall extends in the axial direction from a first
porous material 4 and, adjacent to this first wall in the radially
exterior direction, a second wall made of a second porous material
4' extends. The two walls of porous materials 4 and 4'
concentrically surround a central chamber 5 that also extends
axially and into which a connection fitting 5a is directed to
introduce a fuel 1 as well as an oxidation agent 2. The directions
in which the gases flow are indicated in the figures by arrows. The
reactor 3 is defined on the exterior by an outer wall 6, which here
is embodied as a double hallow tube through which, for example, a
coolant can be passed.
[0067] In the simple embodiment shown in FIG. 1, only the first
wall of porous material 4 and the second wall of porous material 4'
are disposed inside the reactor 3. The first porous material is
embodied in such a way that a Pclet number that lies below the
critical Pclet number is established, so that no flame can develop
within this wall. On the other hand, porous material 4' in the
second wall has a Pclet number that is larger than the critical
Pclet number, so that stable combustion can occur within this wall.
The resulting product gases are discharged to the outside in the
axial direction in the simple embodiment shown in FIG. 1 by means
of a central manifold 7.
[0068] FIG. 2a shows a modified embodiment of the reactor of FIG. 1
which is also equipped with a tube 8 that is wound in the form of
an extended spiral about the two inner walls of porous material 4,
4'. The tube 8 is used for the intermediate cooling of the reaction
products that leave the second porous wall and that may also be
necessary if a catalyst is used for the subsequent reaction of CO
by means of steam. In addition, one can see in this figure a third
wall of a porous material 4" that is disposed concentrically and at
a given distance around the second wall. The further oxidation of
CO to CO.sub.2 takes place within this wall. In order to be able to
supply additional reactants such as steam or oxygen-rich air for
the reactions that take place in the third wall, additional small
pipes are inserted in the space between the second and third wall,
these pipes have holes 9a through which additional components can
be mixed into the radially flowing gases. In the embodiment shown
in FIG. 2a, the outer wall 6 is embodied as a simple tube.
[0069] An alternative stabilization mechanism is shown in FIG. 2b.
In this figure, identical reference numbers are assigned to the
elements, which are embodied in a manner analogous to that in FIGS.
1 and 2. This embodiment differs from that of FIG. 2a in that small
holes 4a are present in the material 4 through which the fuel/gas
can pass. As a result of the flow moving radially to the outside
through the holes 4a, the flame cannot advance against the
direction of flow.
[0070] The embodiment of FIG. 3 is further modified with respect to
that of FIG. 2a in that between the outer wall 6 and the third wall
of the porous material 4" a membrane 10, for example one made of
palladium, is placed on a porous substrate 10' through which only
the hydrogen that is generated can pass. In this way the hydrogen
is separated from the other gases, and it flows between the
membrane 10 and the outer wall 6 in the axial direction until it
reaches a deflector 11 positioned here in the radial direction, by
means of which it is discharged to the outside, while the other
gases flow between the membrane and the third porous wall in the
axial direction to the manifold 7 and from there to the
outside.
[0071] Another type of reactor 3 is represented schematically in
FIG. 4. In its interior, reactor 3 has a first zone 3a filled with
the first porous material 4. Based on the already explained pore
structure of the material 4, this zone functions as a flame
barrier. Adjacent thereto in the direction of flow a second zone 3b
is disposed, which is filled with porous material 4' whose Pclet
number is greater than the critical Pclet number. Next to zone 3b
there is located an intermediate space 12 into which steam can be
fed, for example, as represented schematically in the drawing by an
arrow. Zone 3c is adjacent to the intermediate space 12 and is
filled with porous material 4". The oxidation of CO to CO.sub.2 as
well as the further generation of H.sub.2 when steam is fed in
occur in this zone. In the lower area of reactor 3, seen upstream
from the flame barrier, there is located a premix chamber 13
through which the oxidation agent 2 as well as the fuel 1 enter the
reactor 3. As can be seen in the figure, a heat exchange pipe 14
can be present in order to remove the heat arising from the partial
oxidation. The heat given off by the reaction can be used in
particular to preheat the oxidation agent, as is indicated in the
figure by the thin arrows. In an alternative embodiment, which is
not shown here, the heat exchanger pipe 14 can be routed in such a
way that the heat given off by the reaction is carried to the
downstream zone 3c in order to provide the activation energy for
the reactions that occur there at the corresponding temperature
level. Suitable grates 15 are disposed inside the reactor 3 to
support the porous materials 4, 4', 4".
[0072] Needless to say, in this apparatus of the invention it is
also possible to separate the hydrogen gas by means of a membrane.
Appropriate measures can easily be taken by a person of average
skill in the art.
[0073] In performing the process of the invention in a reactor such
as that shown in FIG. 4, methane and air were combusted at a
fuel/air ratio of 0.5. The CO/CO.sub.2 ratio measured at the gas
outlet of the reactor was 2, corresponding to a hydrogen yield,
based on the hydrocarbon fed into the reactor, of approximately
55%. By preheating the air to approximately 180.degree. C. at
.lambda.=0.4, a soot-free product gas having a hydrogen yield of
70% was produced. The residence time within the reaction zone was
only about 0.1 seconds.
[0074] By contrast, prior-art processes such as that described in
U.S. Pat. No. 4,699,631, require a substantially higher residence
time, which can be up to 10 seconds long. Thus, the apparatus
described here and the corresponding process achieve a
significantly higher throughput with the same reactor dimensions.
One of the reasons for this is the heat transfer properties that
occur in a porous structure compared to those with a free-burning
flame.
* * * * *