Note: Descriptions are shown in the official language in which they were submitted.
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METHOD FOR PRODUCING FUEL FROM CAPTURED CARBON
DIOXIDE
FIELD OF THE INVENTION
The present invention relates to a method for capturing carbon dioxide from
a gaseous mixture containing carbon dioxide, e.g., from the atmosphere, and
subsequently using this carbon dioxide for the production of fuel.
BACKGROUND OF THE INVENTION
Greenhouse gases include carbon dioxide, methane, nitrous oxide and water
vapor. While greenhouse gases occur naturally in the atmosphere, human
activities
also produce greenhouse gas emissions and are responsible for creating new
ones.
Carbon dioxide (C02) is the most common greenhouse gas released by human
activities, resulting from the extensive use of fossil fuel (coal, petroleum,
natural
gas). One of the main challenges modem civilization is facing is the increase
of
carbon dioxide in the atmosphere, affecting the greenhouse effect and global
warming. Another problem arises from the extensive use of fossil fuel thus
diminishing the global fuel reserves.
Renewable energy sources, that capture their energy from existing flows of
energy, from on-going natural processes, such as sunshine, wind, flowing
water,
biological processes and geothermal heat flows, can be used for generating
electricity, and there is a growing demand for methods of producing fuel using
electricity.
Numerous attempts for extracting CO2 directly from car exhausts or power
plants have been made, most of them involving reactions of exhausted gases
with
organic amine compounds or strong bases like calcium hydroxide or sodium
hydroxide. In processes using organic amines, a solution of amine and water is
contacted with the gas, whereby the amine and the CO2 undergo a chemical
reaction
forming a rich amine that is soluble in the water. The rich amine solution is
pumped
to a desorber where it is heated, reversing the reaction and releasing pure
CO2 gas.
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The disadvantage of this method is the fact that organic amine bases are
expensive
and unstable.
Carbon dioxide and mixtures containing it have been proposed for
production of combustible fuels. For example, US Patent No. 4,140,602
discloses a
chemical method for combustible fuel production by converting carbon dioxide
in
the atmosphere to a carbonate such as an alkali carbonate, following which the
recovered carbonate is combined with hydrogen gas to produce combustible fuels
e.g. methane and methanol. The method includes the additional step of reacting
the
alkali carbonate with calcium hydroxide to form calcium carbonate. The
disadvantages of this method resides in the use of the strong base compound
Ca(OH)2, forming CaCO3, that requires considerable amount of energy for the
thermal release of COZ.
SUMMARY OF THE INVENTION
The present invention relates to a method for producing combustible fuels
from a gaseous mixture containing carbon dioxide, which comprises:
(i) capturing CO2 from said gaseous mixture by means of K2C03, thus
forming KHCO3;
(ii) releasing the COZ from said KHCO3 ; and
(iii) subsequently producing fuel from the released CO2 by reaction with
hydrogen.
DETAILED DESCRIPTION OF THE INVENTION
The method of the present invention enables the production of combustible
fuels, using as preferred starting material the highly available atmospheric
carbon
dioxide, and retuniulg the CO2 produced by fuel combustion to the atmosphere,
thus
maintaining the equilibrium of the CO2 in the atmosphere. The method is based
on
well known in the art reactions such as thermal catalytic and electrochemical
reactions, utilizing the reversibility of these reactions and carrying out the
reverse
reaction by modifying the operating pressure and/or the electrical voltage
supplied
to the process.
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The reaction between the CO2 and K2C03 in step (i) may be performed by
bubbling air in water through an aqueous solution of K2C03 or by spraying
droplets
of K2C03 in aqueous solution into a stream of air. In both methods, the
atmospheric
COa reacts with the K2C03 to form KHCO3 according to the following reaction:
K2C03 + H20 + CO2 4 2KHCO3
In the next step, CO2 is released from the KHCO3.
In one embodiment of the invention, the CO2 is released by heating the
KHCO3 to a temperature sufficient to liberate the CO2, according to the
following
reaction, thus recycling the K2CO3:
2KHCO3 + Heat 4 K2C03 + H20 + CO2
In another embodiment, the CO2 is released from the KHCO3 obtained by an
electrochemical process, according to the following reaction:
HC03- - e 4 -OH + CO2
4(-OH) 4 2H20 + 02
The CO2 obtained in step (ii) is then reacted with hydrogen to produce
combustible fuels, such as methane and methanol.
In one embodiment, in which heat source producing very high temperatures
is available, the reaction of COZ and hydrogen is conducted as a thermal
catalytic
reaction. One possible thermal catalytic reaction is a reverse operation of
methane
reforming. In steam methane reforming, methane is brought into contact with
(excess) steam at high temperature and pressure, typically 800-1000 C and 30-
40
bar, over a catalyst, to produce a mixture of H2, CO and CO2. In the industry,
the
process is usually carried out in fixed bed or fluidized bed membrane
reactors, using
a Ni as the preferred catalyst, because of its low cost, or a noble metal
catalyst such
as Ru, Rli, Pd, Ir or Pt. The reverse methane reforming according to the
invention is
carried in the same type of reactors and using the same catalysts as in steam
methane reforming, but using pressures varying according to the
characteristics of
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the specific process, said pressure being always higher than the pressure used
for
the methane reforming.
In another embodiment, the reaction of CO2 and hydrogen according to the
invention is an electrochemical process, such as a reverse operation of a fuel
cell.
A fuel cell is an electrochemical energy conversion device that converts the
chemical energy of a fuel, e.g. hydrogen, and an oxidant, e.g. oxygen, to
electrical
energy and heat, without combustion. The device is similar to a battery but,
unlike a
battery, the fuel cell is designed for continuous replenishment of the
reactants
consumed, i.e., the fuel and the oxidant are typically stored outside of the
fuel cell
and transferred into the fuel cell as the reactants are consumed. In a typical
fuel cell,
the fuel is consumed at the anode and the oxidizer is consumed at the cathode.
There are several types of fuel cells, each using a different chemistry. Fuel
cells are
usually classified by the type of electrolyte they use, and include phosphoric
acid-
based, proton exchange membrane, solid polymer, molten carbonate, solid oxide,
alkaline, direct methanol, regenerative, zinc-air and protonic ceramic fuel
cells.
In a fuel cell, if a hydrocarbon, such as methane, is the fuel, said
hydrocarbon is reacted with oxygen obtained by electrolysis of water within
the
cell, thus forming CO2 and hydrogen and generating electricity.
According to the present invention, a reverse operation of a fuel cell is
carried out whereby electricity is supplied to a fuel cell containing C02,
that reacts
with hydrogen formed in situ by electrolysis of water, thus producing the
desired
hydrocarbon, e.g. methane fuel. The electrical voltage supplied to the process
is
determined based on the characteristics of the specific process performed but
it is
always higher than the electrical voltage generated in the opposite process,
namely,
the regular operation of the fuel cell.
In one preferred embodiment, the electrochemical process corresponds to an
inverted direct methanol fuel cell (DMFC) and the fuel obtained is methanol.
DMFCs are low-temperature fuel cells operating at temperatures of 30-130 C
and using liquid methanol as the electrolyte, according to the reaction:
CH3OH + 3/202 4 CO2 + 2H2O
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The central component of DMFCs is the membrane electrode assembly,
composed of membrane, catalyst and diffusion layers. The membrane may be a
polymer with acid groups that are capable of splitting off protons and has
them
migrate through the membrane. The diffusion layer passes the fuels to the
catalyst
layer and removes the combustion products. In the catalyst layers, the
electrochemical reaction takes place, in which chemical energy is converted
into
electric energy. The catalyst is provided with additives to apply it as a
paste on a
substrate, and it is usually based on a noble metal, such as platinum and
platinum/ruthenium.
].0 According to the present invention, the catalysts used for the reverse
operation of the DMFC are the same used in the regular operation mode of the
methanol fuel cell, and other parameters such as temperature and electrical
voltage
supplied to the process are determined based on the characteristics of the
specific
process performed.
In another preferred embodiment, the electrochemical process corresponds to
an inverted molten carbonate fuel cell (MCFC) and the fuel obtained is a
hydrocarbon, such as methane.
MCFCs are high-temperature fuel cell operating at temperatures of 600-
650 C, and thus can achieve higher fuel-to-electricity and overall energy use
efficiencies than low temperature fuel cells. The electrolyte used in MCFCs is
an
alkali carbonate such as Na2CO3, K2C03, Li2CO3 or combinations thereof, that
may
be retained in a ceramic matrix, e.g. of LiAlO2. In the fuel cell, the alkali
carbonates
melt into a highly conductive molten salt with carbonate ions providing ionic
conduction through the electrolyte matrix. Nickel and nickel oxide are
adequate to
promote reaction on the anode and cathode, respectively, and expensive
catalysts
(noble metals) are not required.
The fuel consumed in MCFCs is usually a natural gas, mainly methane, and
in this case methane and steam are converted into a hydrogen-rich gas inside
the
fuel cell stack (a process called "internal reforming"). The overall reaction
performed within the cell is:
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CH4+02 4 C02 +2H2
According to the present invention, the operating conditions for the reverse
operation of the MCFC (temperature and pressure) are similar to these in the
regular
operation mode of this cell. The exact conditions, as well as the voltage
supplied to
the process, are determined based on the characteristics of the specific
process
performed.
The methane or methanol obtained by the method of the invention may later
be converted into longer hydrocarbons, using known chemical reactions.
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