Note: Descriptions are shown in the official language in which they were submitted.
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Gas/Solid Phase Reaction
The invention relates to a process and reactor for the quasi-continuous
perform-
ance of a chemical reaction on the surface of a fixed reactant in a gas/solid
phase
reaction. In particular, the invention relates to a thermal process and a
reactor for
the continuous preparation of hydrogen from water vapor on the surface of a
metal
oxide in a gas/solid phase reaction.
On a long-term basis, hydrogen is an important carrier for a sustainable
energy
supply. Today, most of the hydrogen is prepared from fossil sources. However,
the
limited presence of these sources and the indispensable reduction of
greenhouse
gases (mainly CO2) require the exploration of alternative resources or
processes.
Water splitting by means of electrolysis using solar current is possible, but
has the
disadvantage of an enormous influence of the cost of solar current on H2
produc-
tion. The direct utilization of concentrated solar radiation for
thermochemical water
splitting avoids this and has a higher efficiency. Thus, the cost of hydrogen
production can be lowered and production on an industrial scale enabled on a
long-
term basis.
A number of processes are available for the thermal production of hydrogen.
Thus, in DE 44 10 915 Al, hydrogen is produced by the reaction of iron with
carbonic acid with supply of solar-thermal energy. The iron oxide formed is
reduced again using carbon monoxide and is thus available for the process.
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44.
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In DE 42 26 496 Al, hydrogen is produced in a modified continuous iron-water
vapor process, and the iron oxide formed thereby is subsequently supplied to
steel
production again.
JP 03205302 A describes the preparation of highly pure hydrogen using
activated
magnetite as a reactive catalyst.
In 313 2001270701 A, hydrogen is prepared by reacting metallic zinc, magnetite
and water at 600 C.
M. Inoue et al. in Solar Energy (2003 Pergamon Press, Oxford, GB, Vol. 79, 10-
05, pp.
409-421) describes the preparation of hydrogen by means of a water-Zn0-
MnFe204
system. The corresponding ferrite powder of the type Me)(2+Zn1_x2+Fe204 can be
prepared by the method of S. Lorentzou et al. as presented on the conference
Partec
2004 (conference Partec 2004; Aerosol and Particle Technology Laboratory,
Thessaloniki, GR).
According to a press communication by the Deutsches Zentrum fOr Luft- und
Raumfahrt
of October 15, 2004 (Premiere; Wasserstoff Aus Dem Sonnenofen; Deutsches
Zentrum
fur Luft-und Raumfahrt, Press communication; October 15, 2004), hydrogen was
produced for the first time in a solar oven by solar-thermal water splitting.
In the process
described, the hydrogen is produced. discontinuously by splitting the water
vapor over
metal oxide and regenerating the metal oxide.
DE 197 10 986 C2 describes a volumetric radiation receptor for heat recovery
from
concentrated radiation by heating a fluid under pressure without a chemical
reaction occurring in this reactor.
Thus, it is the object of the present invention to provide a process which can
be
performed, in particular, in a reaction chamber system, in which no solid need
be
separated and which proceeds in a quasi-continuous manner at as low tempera-
tures as possible. It is a further object to provide a solar-operated reactor
in which
a product (especially hydrogen) is continuously produced although at least two
process steps (for example, splitting and regeneration) necessarily proceed
sequentially. In particular, it is the object of the present invention to
provide a
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corresponding process for producing hydrogen that can be performed in at least
one single reaction chamber, in particular.
In a first embodiment, this object of the invention is achieved by a process
for the
quasi-continuous performance of a chemical reaction consisting of at least two
sequential reversible steps, characterized in that:
at least two reaction chambers in each of which at least one reactant is
locally
fixed are operated in parallel, wherein
cyclically alternating reaction conditions are provided in the reaction
chambers.
Sequential steps within the meaning of the invention are successive reaction
steps
of a chemical reaction in which the reaction products can be isolated.
Reversible steps within the meaning of the invention are reaction steps in
which
the chemical equilibrium can be adjusted in such a way that alternatively
either the
forward or the backward reaction preferably proceeds.
A chemical reaction within the meaning of the invention is in principle any
chemical
reaction in which one of the reactants is fixed and in which the energy is
supplied
as heat energy, light energy, nuclear energy or in the form of other
electromag-
netic radiation. Preferably, the process according to the invention is
employed in
the following reaction types listed in an exemplary manner:
Reaction type First step Second step
H2 production MeOx + H20 ¨> H2 + MeOy MeOy ¨) MeOx + 02
Reduction of carbon MeOx +CO2 ¨> Me0y +CO Me0y ¨> MeOx + 1/2 02
dioxide
Cleavage of nitrogen MeOx + NOx ¨> Me0y + 1/2 Me0y ¨> MeOx + 1/2 02
oxides N2
Cleavage of S03/ MeOx + SO3 ¨> Me0y + SO2 Me0y ¨> MeOx + 1/2 02
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production of SO2
Selective oxidation MeOx + 02 ¨> MeOy CmHn + MeOy ¨> MeOx +
Cm H no
Dehydrogenations MeOx + 02 ¨> MeOy CmH,, + MeOy ¨> MeOx +
CmHn-2 +H20
H2 production Me + H20 ¨> H2 + Me0 Me0 ¨> Me + 1/2 02
H2 production MeXy + HX ¨> MeXy-Fi + MeXy+1 ¨> MeXy + 1/2 X2
1/2 H2
In this Table, Me represents a metal atom, X represents a halogen or
pseudohalo-
gen, subscripts n, m, x or y represent positive integers.
In the process according to the invention, throughout the reaction time of a
first
sequential step of a chemical reaction in a first reaction chamber, a second
sequential reaction step other than said first sequential reaction step
proceeds at
at least one time in a second reaction chamber, which is different from the
first.
Thereby, it is achieved that the final product can be provided by the process
at any
time and the reaction chambers are utilized optimally.
Since the different sequential reaction steps can have a different reaction
time, for
optimally utilizing the capacity of the reaction chambers, advantageously:
a) the energy input in the reaction chambers can be selected differently to
adjust
the reaction rates;
b) the mass flow of the reactants can be adjusted; and/or
c) the number of reaction chambers can be adjusted in accordance with the
reaction times in which the reactions proceed in a corresponding time-shifted
mode.
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The latter variant is further illustrated by means of Figure 1. In an
exemplary
manner, it is shown in Figure 1 how a quasi-continuous process according to
the
invention can be operated with three reaction chambers for two reaction steps
in
which the second reaction step takes twice as much time as the first reaction
step.
In the process according to the invention, all reversible reaction steps of
the
chemical reaction are preferably performed sequentially in the same reaction
chambers. Thus, separation or isolation of intermediate products can be
dispensed
with.
Advantageously, in the process according to the invention, radiation-heated
reactors are employed as reaction chambers. Thus, thermal reactions can be
performed with light energy. Any electromagnetic radiation can be employed as
the radiation. According to the invention, photoreactions may also
advantageously
take place when the process is performed. Reactions that are thermal in
principle
may also proceed in a photoassisted manner, in particular, according to the
invention. "Photoassisted" within the meaning of the invention means that the
reaction product is formed with enhancement by a photoreaction.
Said cyclically alternating reaction conditions are preferably provided by
cycling the
temperature of the reaction chambers, for example, by varying the heating
power.
More preferably, the required temperature in the reaction chambers is varied
by
periodically changing the heating power to enable a quasi-continuous product
stream. For example, the different thermal addressing of the reactors enables
simultaneous reactions of water splitting at a lower temperature and
regeneration
at a higher temperature. Thus, the sequence of these different batch processes
enables a quasi-continuous production of hydrogen, for example.
Advantageously, the process is performed in several successive cycles is a
quasi-
continuous reproducible way. For example, one cycle takes a period of time
within
a range of from 0.3 to 1.5 hours, especially from 0.3 to 1 hour. Over a
discontinu-
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ous process, this has mainly economical advantages. However, depending on the
reaction to be performed, the cycles may also be substantially shorter or
longer.
In accordance with the different energy requirements of the reactions
concerned to
be performed sequentially, it is further preferred in this process to set the
cycling
of the temperature of the fixed reactant (for example, the metal oxide) by
varying
the heating power, for example, because the splitting is to take place first
followed
by regeneration.
It is advantageous if the absorbed energy of the optical component (preferably
an
attenuator) is utilized for heating fluids. Such fluids may be, inter alia,
reactants,
auxiliary agents or heat transfer media. Being preheated, the fluids do not
require
that much radiated power in the reactor space any more. It is particularly pre-
ferred if the optical component is a tube bundle flowed through by the fluid.
In the process according to the invention, fossil energy, electric energy,
light
energy and/or nuclear energy is preferably employed.
Preferably, the required temperature can be generated by burning fossil energy
carriers and/or utilizing electric energy, because usual processes utilize
these
energy sources.
It is also advantageous to generate the required temperature by nuclear energy
because in nuclear reactions only about one third of the heat produced in the
reactor can be utilized for electric power production. The (residual) heat
produced
can be utilized for generating the required temperature. On an industrial
scale, no
climate-damaging emissions of CO2 are formed thereby.
Advantageously, however, the energy input takes place by light energy,
especially
by concentrated solar radiation, because this energy source is available at
particu-
larly low cost and is suitable for both thermal and photoreactions alike.
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In accordance with one aspect of the present invention there is provided a
process for the
quasi-continuous performance of a chemical reaction consisting of at least two
sequential reversible
steps, characterized in that: at least two reaction chambers in each of which
at least one reactant
selected from ferrites, zinc oxides, manganese oxides, cerium oxides,
lanthanum oxides, lanthanoide
oxides, oxides of general formula Mex2+Zn1_x2+Fe204, wherein Mex2+ is a
divalent metal ion selected from
the group of Mg, Ca, Mn, Fe, Co, Ni, Cu, Zn, Sr, Sn, Ba, C, and Pb, and
mixtures of such oxides, said
reactant is locally fixed on a heat-resistant ceramic support structure in the
reactor, the reaction
chambers are operated in parallel, wherein cyclically alternating reaction
conditions are provided in
the reaction chambers by cycling the temperature of the reaction chambers by
shifting the reaction
chambers relative to the radiation source.
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The generation of the required temperature by means of light energy is advanta-
geous, because conventional energy-producing systems burning fossil energy
carriers are not as resource efficient as the process according to the
invention, and
light energy, such as sunlight, is available worldwide.
Preferably, sunlight can be irradiated into the reaction chamber by means of
optical set-ups in order to generate the required temperature. Such optical
set-ups
have particularly preferred manifestations, such as solar tower systems,
paraboloid
concentrators, sun ovens, elliptical or spherical mirrors or line-focusing
concentra-
tors. By means of solar-thermochemical water splitting, hydrogen can thus be
produced on an industrial scale as a possible energy carrier of the future
without
climate-damaging emissions of carbon dioxide.
The required radiated power is preferably achieved by a group of heliostats,
and
the radiated power required for regeneration is achieved by another group of
heliostats, the focus of the second group being rearranged onto the individual
reaction fields. The heliostat array is separated in such a way that at least
one
group of heliostats covers the base load of the necessary radiated power in
accordance with the reaction step with the lowest energy requirement by being
"regularly" tracked in accordance with the daily course of the sun, and that
at least
one group of heliostats covers additional loads of necessary radiated power
for
reaction steps with an increased energy requirement by guiding the focus of
this
group to another area of the radiation receptor at defined intervals
respectively
after completion of the respective reaction step. By this method, two
different
reaction temperatures can be easily realized.
Advantageously, the reaction chambers are shifted relative to the radiation
source
in order to vary the heating power. A temperature change can be effected uncom-
plicatedly thereby while maintaining the radiated power. Thus, the reaction
chambers can preferably be changeable relative to the optical set-up in order
to
vary the heating power. A temperature change can be effected uncomplicatedly
thereby while maintaining the radiated power.
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For varying the solar-thermal heating power, the use of optical components is
advantageously suitable for reducing the irradiation. More particularly,
optical
attenuators, apertures, deflector mirrors or filters that can be shifted in
space or
are variable in terms of transparency are suitable for this.
This may be advantageously achieved, inter alia, by varying the focal position
due
to a change in the orientation of mirrors or mirror arrays, so-called
heliostat
arrays. This can be realized substantially more easily than the shifting of
the
reactor, which is mostly very heavy.
Preferably, a temperature within a range of from 500 C to 1000 C, especially
up
to 900 C, is set in a first reaction chamber, and a temperature within a
range of
from 1000 C to 1400 C is set in a second reaction chamber, in order to
perform,
for example, the hydrogen production at a particularly low temperature in a
first
reaction chamber and simultaneously the regeneration at a higher temperature
in
a second reaction chamber. However, depending on the reaction to be performed,
the temperatures employed may also deviate substantially from these values.
The fixed reactant in the two reaction chambers is advantageously selected
from
the group of metal hydrides, dyes, chemical compounds having redox properties,
and complexing agents. Chemical compounds having redox properties within the
meaning of the invention are those compounds that can be reversibly oxidized
and
reduced. Advantageously, these chemical compounds having redox properties are
selected from the group of metal oxides, mixed metal oxides and/or doped metal
oxides. Of these reactants, metal oxides have proven particularly advantageous
because they are most versatile to employ and can be particularly easily
fixed, for
example, in contrast to metal hydrides.
More preferably, a multivalent metal oxide is employed as a fixed reactant
because
it can be fixed and regenerated particularly easily. "Multivalent" within the
meaning
of the invention means a metal oxide having several coexisting oxidation
states,
especially if the metal is in an oxidation state of more than +1, especially
more
than +2.
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Preferably, the metal oxides include ferrites and/or zinc oxides and/or
manganese
oxides and/or cerium oxides and/or lanthanum oxides and/or other lanthanoide
oxides and/or oxides of general formula Mex2+Zn1_x2+Fe204, wherein Mex2+ is a
divalent metal ion selected from the group of Mg, Ca, Mn, Fe, Co, Ni, Cu, Zn,
Sr,
Sn, Ba, Cd or Pb, and/or mixtures of such oxides, or oxides with general
formula
Me'xMeu1_xFe0, wherein Me' and Me" are metal ions selected from the group of
Mg, Ca, Mn, Fe, Co, Ni, Cu, Zn, Sr, Sn, Ba, Cd, Pb or lanthanoides, because
these can be employed particularly efficiently in hydrogen splitting, wherein
x is
a number in a range of from 1 to 5, especially from 2 to 3.
Advantageously, the chemical compound having redox properties is employed as
a coating of a heat-resistant support structure, more preferably a ceramic
one.
Due to the use of a support structure, the chemical compound having redox
properties need be in the reaction chambers only in a thin layer.
Preferably, a support structure having a conical, hemispherical or paraboloid
shape is employed because scattered radiation from the radiation source can be
optimally utilized in the reaction chamber thereby.
In addition to the fixed reactants, mobile reactants are also employed.
In the process according to the invention, at least one of the mobile
reactants is
advantageously selected from the group of water, alcohols, carbon dioxide,
hydrogen sulfide, nitrogen oxides, hydrocarbons, halo- or pseudohalohydrocar-
bons, ammonia and sulfur oxides. Of these mobile reactants, water has proven
to be particularly advantageous because it is readily available and is an
easily
handled reactant, above all in the gas phase.
Further, at least one, more preferably all mobile reactants in the process
according to the invention are advantageously gaseous. In this way, the
reactant
or reactants can be transferred to the reaction chambers particularly easily.
In
addition, preferably at least one and more preferably all mobile reaction prod-
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ucts are gaseous, because it is equally easy then to extract them from the
reaction chambers.
Therefore, the object of the invention is advantageously achieved by a process
for
producing hydrogen from water vapor on a surface of at least one chemical
compound having redox properties, wherein:
in the first step, water vapor is split by associating oxygen to the excited
chemical
compound having redox properties to release hydrogen; and
in the second step, the chemical compound having redox properties is
regenerated
at a temperature which is higher than that of the first step to release bound
oxygen.
Thus, the invention can relate to a process of splitting water vapor thermally
in a
multi-step process by using concentrated radiation and thus producing solar
hydrogen.
With the process according to the invention, water vapor can be thermally
split by
concentrated sunlight to produce hydrogen. This is the basis for developing
the
process according to the invention with which hydrogen can be produced by a
solar-thermal process. In contrast to direct thermal water splitting, which
takes
place only at a few thousand degrees centigrade, hydrogen is produced here
from
water vapor in a two-step cycle process, preferably at temperatures within a
range
of from 800 C to 1200 C. What is recirculated, for example, is a metal oxide
system that can cleave oxygen from water molecules and reversibly bind it into
its
crystal structure.
Reaction 1: splitting Me rod + H20 ¨> Me00x + H2
Reaction 2: Regeneration MeOox MeOred + 02
Preferably, metal oxides (Me0) with different doping are employed and are
cyclically oxidized and reduced. In the first step, the hot water vapor
flowing past
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the metal oxide is split by binding the oxygen to the excited metal oxide
lattice at
temperatures preferably within a range of from 500 to 1000 C, especially from
550 to 850 C, to release hydrogen. In the second step, the oxygen previously
incorporated into the lattice is released again at temperatures preferably
within a
range of from 1000 to 1400 C, especially from 1050 to 1350 C, and the metal
oxide is regenerated or reduced again to the high-energy state. These tempera-
tures preferably apply to ferrites or iron mixed oxides. More preferably, the
reaction temperature may advantageously be within a range of from 600 C to
800 C, and the regeneration temperature may be within a range of from 900 C
to 1200 C. Thus, all in all, water is split into its elements by means of the
metal
oxide. The metal oxides employed are advantageously mixed oxides, more
preferably zinc-doted ferrites.
One important innovation of the process is the advantageous combination of a
ceramic support and absorber structure that can be heated at high temperatures
with concentrated solar radiation, with a redox system that is capable of
reversibly
splitting water, for example. Preferably, porous honeycomb structures
functioning
as radiation absorbers are coated with ferrites. This includes advantages over
comparable processes since the complete process can be performed in a single
converter here. Thus, solids need not be circulated, and because the oxygen
binds
to the metal oxide, the product separation is reduced to one gas separation.
In
addition, this system enables the water splitting process to be performed at
clearly
lower temperatures that can be mastered in terms of material technology.
Preferably, the metal oxide is recycled, so that only water is consumed. All
these
technical advantages also offer economical advantages over other processes for
hydrogen production.
The ceramic structure coated with metal oxide advantageously forms the core of
a
receiver reactor. By being coupled to a concentrating solar plant (preferably
a solar
tower), the structure is brought to the necessary temperature by the incident
concentrated solar radiation. The reactions take place on the surface of the
coated
ceramics. The reactor is preferably integrated in a small plant for checking
and
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optimizing the operational behavior during water splitting or regeneration.
This
plant preferably comprises fittings and mass flow controllers for supplying
the
necessary gases, a water vapor dosing system, measuring systems for pressure
and temperature, product gas treatment, and data acquisition and control. The
analysis of the concentrations of produced hydrogen or released oxygen is
preferably effected by a mass spectrometer.
For an efficient utilization of the reactor, it is preferably required that a
continuous
operation for producing the product hydrogen can take place. Since two
reactions
with different conditions are to be performed, a cyclic change of the reaction
conditions or gases and of the required energy (temperature) must be effected.
Preferably, the water vapor is split at a temperature within a range of from
500 C
to 1000 C, especially up to 900 C, even more preferably from 550 to 850 C,
and
the metal oxide is regenerated at a temperature within a range of from 1000 C
to
1400 C, especially from 1050 to 1350 C. To date, it has been necessary to
employ temperatures of a few thousand degrees, but at least 2000 C, for one-
step thermal water splitting. The lower temperature range is more easily
handled
in terms of materials and process technology and significantly reduces the
cost of
the process.
Therefore, the object of the invention is preferably achieved by a process for
the
quasi-continuous production of hydrogen from water vapor on a surface of a
metal
oxide followed by regeneration of the surface.
For the quasi-continuous production of hydrogen from water vapor on a surface
of
a metal oxide followed by regeneration of the surface, it is advantageous if a
quasi-continuous synthesis is performed in at least two reaction chambers,
whereby water vapor can be converted to hydrogen and simultaneously another
reaction chamber can be regenerated in order to convert water vapor to
hydrogen
again immediately afterwards. This quasi-continuous process can substantially
simplify the thermal production process of hydrogen.
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Thus, in the process according to the invention, advantageously the hydrogen
synthesis by water splitting can take place in one reactor while the
regeneration of
the metal oxide takes place in another. In the subsequent cycle, the
regenerated
reaction chamber can then take up new reactants again. Thus, the thermal
production of hydrogen can be effected continuously and simply as compared to
the prior art.
In another embodiment, the object of the invention is achieved by a thermal
process for the preparation of hydrogen from water vapor on a surface of a
metal
oxide in a gas/solid phase reaction, wherein in a reaction chamber, in a first
step,
water vapor is split by associating oxygen to the excited metal oxide to
release
hydrogen, and in a second step, the metal oxide is regenerated at a
temperature
which is higher than that of the first step to release bound oxygen, so that
the
metal oxide is available for further reactions.
Thus, the invention relates to a process for thermally splitting water vapor
in a
multistep process by utilizing concentrated radiation and thus to produce
solar
hydrogen.
In a further embodiment, the object of the invention is achieved by a
photoreactor
for performing the process according to the invention, characterized by having
two
reaction chambers.
In a still further embodiment, the object of the invention is achieved by a
radia-
tion-heated reactor for performing the process according to the invention,
charac-
terized by having two reaction chambers.
This reactor is preferably a reactor for the thermal preparation of hydrogen
from
water vapor on a surface in a gas/solid phase reaction comprising at least one
connected tube that enables a gas stream of educt gases to flow into a
reaction
chamber and of product gases to flow out, and a heat source, metal oxide being
provided as a reactant in one reaction chamber.
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Preferably, the metal oxide is coated on a heat-resistant ceramic support
structure
in the reactor. This fixation has the advantage that the metal oxide is always
available and thus can be exposed to the heat source optimally in the reactor.
Due
to the fixation of the metal oxide on the support structure, the metal oxide
need
not be recovered tediously by separation processes. Advantageously, the heat
necessary for the reactions can also be supplied out of the support structure.
More preferably, the ceramic support structure consists of a porous honeycomb
structure, because porous ceramic honeycomb structures have proven
particularly
heat-resistant. Pores within the meaning of this invention are the interstices
formed by the honeycomb structure. This does not exclude that the material as
such advantageously has itself a porosity within a range of from 10 to 60%.
The
porosity is obtained from the weight ratio of the actual weight to the weight
when
the theoretical maximum density is assumed.
Advantageously, the ceramic support structure has a conical, hemispherical or
paraboloid shape in order to capture the radiation optimally onto the metal
oxide. In contrast to known shapes (for example, cylindrical), peripheral
radiation can also be captured more readily thereby.
The reaction chamber is advantageously provided with a transparent window
because the light source can be arranged outside the actual reactor in this
way.
Advantageously, tubes that attenuate the energy flow run between the reaction
chamber and energy source, because this enables a better control of the
reaction.
Preferably, the tubes contain a fluid because the heat exchange can be
adjusted
individually thereby.
The reactor is advantageously provided with a multiport valve to enable the
supply of the gaseous educts.
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Preferably, the multiport valve has such a design that the gaseous products
can
be removed separately.
Advantageously, the reactor has a modular structure consisting of at least two
reaction chambers, because the quasi-continuous process described above can
be implemented particularly easily thereby.
Preferably, the two reaction chambers are alternately provided with water
vapor or
inert gas, especially nitrogen, the switching being effected in such a way
that a
hydrogen production constant in time is provided.
Preferably, the metal oxides are ferrites and/or zinc oxides and/or manganese
oxides and/or lanthanum oxides and/or oxides of general formula
Mex2+Zn1_x2+Fe204, wherein Mex2+ is a divalent metal ion selected from the
group of
Mg, Ca, Mn, Fe, Co, Ni, Cu, Zn, Sr, Sn, Ba, Cd, Pb or lanthanoides, and/or
mixtures of such oxides, because these can be employed particularly
efficiently
in water splitting, wherein x is a number within a range of from 1 to 5,
especially
from 2 to 3.
Advantageously, a concentrating solar-thermal system, such as a solar tower
system, a paraboloid concentrator, a sun oven, an elliptical or spherical
mirror or a
line-focusing concentrator, is employed as the energy source.
Preferably, the required radiated power is achieved by a group of heliostats,
and
the radiated power required for regeneration is achieved by another group of
heliostats, the focus of the second group being rearranged onto the individual
reaction fields.
TM
In the following, an Example of the invention, which is the Konti -reactor, is
further
illustrated by means of Figures. This Example is not to be understood as
limiting
the scope of protection of the invention.
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The process according to the invention for solar-thermochemical water
splitting on
the basis of metal Ades for continuous hydrogen production can be performed
continuously by means of the design of an appropriate receiver reactor as de-
scribed herein.
In the Figures:
Figure 1 shows a schematic representation of the time course of different
reactions
in different reaction chambers in the quasi-continuous process according to
the
invention.
Figure 2 shows a perspective schematic representation (vertical-horizontal
section)
of the Konti reactor according to the invention.
Figure 3 shows a horizontal section through the reactor.
Figure 4 shows a representation of the heat-resistant four-way valve in the
reactor.
Figure 2 shows the receiver reactor, the concentrated solar radiation being
incident
from the right-hand side onto the aperture with quartz windows (1). The power
of
the incident light can be adjusted by an aperture. The receiver reactor is
based on
the connection as described above of the metal oxide redox system with a
support
and absorber structure which consists of a ceramic monolith having a honeycomb
structure (2). The monolith is coated with the metal oxide and built into a
cylindri-
cal housing (3). In a directly absorbing receiver, the honeycomb structure
enables
high temperatures to be generated with low back radiation losses. The reactor
consists of a modular two-component system of permanently installed honeycomb-
like absorbers. Two neighboring, but separated reaction chambers form a mini-
mum arrangement of modules for the continuous production of hydrogen. The
square aperture (1) allows the formation of large and flexible receiver areas
by
serial connection of individual modules. A double tube is provided for
preheating
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the supplied gases nitrogen and water vapor by recovering the heat from the
product gas (4).
The operation of the Konti reactor is based on the simultaneous use of the two
modules. While water is split in one of the reaction chambers, regeneration
takes
place in the other. After the reactions are completed, the regenerated module
is
switched for splitting and vice versa by swapping the gas supply. A
precondition of
this continuous operation and the hydrogen production is the separate supply
of
nitrogen gas, which is employed as a carrier gas or scavenging gas, and water
vapor (6). In addition, separate lines for the products of the splitting on
the one
hand and for the oxygen-containing scavenging gas for the regeneration on the
other hand are necessary (7). This is enabled by four-way valves (5), which
are respectively switched over after a reaction step is completed. One of
these
valves (5) must withstand high temperatures of up to 600 C. Figure 4 shows
the
positions of this valve.
The two steps of the process are performed in the same reactor at different
temperature levels with a different heat demand. The regeneration is
endothermal
and advantageously proceeds within a temperature range of from 1100 to
1200 C. The splitting of the water vapor is slightly exothermal and takes
place at
800 C. Therefore, part of the modules (regeneration) requires a higher solar
flux
density (intensity) as compared to the second part, i.e., that for the
splitting of
water, which demands only a little energy for the compensation of heat losses.
Thus, cycling of the irradiation intensity is required when the cycle is
switched over
from regeneration to splitting or vice versa. For this purpose, a change of
mirror
focusing between two equal foci by suitably adjusting the concentrating
mirrors of
the solar plant is provided. The periodic change of the irradiation intensity
is
achieved by optical components that can be changed in time, for example,
optical
lattices as attenuators, deflector mirrors or semitransparent mirrors. Such a
component is moveable and is positioned in front of one of the two apertures.
When the supplied gas is changed, the position of the component can be
switched
over accordingly. It is also possible, though with higher technical
expenditure, to
CA 02608085 2007-10-15
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change the receiver position in time between locations with different
irradiation
intensities.
