Abstract:
The invention provides 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 K2CO3, thus forming KHCO3; (ii) releasing the CO2 from said KHCO3; and (iii) subsequently producing fuel from the released CO2 by reaction with hydrogen.

Description:
FIELD OF THE INVENTION  
       [0001]     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  
       [0002]     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 (CO 2 ) 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 modern 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.  
         [0003]     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.  
         [0004]     Numerous attempts for extracting CO 2  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 CO 2  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 CO 2  gas. The disadvantage of this method is the fact that organic amine bases are expensive and unstable.  
         [0005]     Carbon dioxide and mixtures containing it have been proposed for production of combustible fuels. For example, U.S. Pat. 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 CaCO 3 , that requires considerable amount of energy for the thermal release of CO 2 .  
       SUMMARY OF THE INVENTION  
       [0006]     The present invention relates to a method for producing combustible fuels from a gaseous mixture containing carbon dioxide, which comprises: 
        (i) capturing CO 2  from said gaseous mixture by means of K 2 CO 3 , thus forming KHCO 3 ;     (ii) releasing the CO 2  from said KHCO 3 ; and     (iii) subsequently producing fuel from the released CO 2  by reaction with hydrogen.       
 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0010]     The method of the present invention enables the production of combustible fuels, using as preferred starting material the highly available atmospheric carbon dioxide, and returning the CO 2  produced by fuel combustion to the atmosphere, thus maintaining the equilibrium of the CO 2  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.  
         [0011]     The reaction between the CO 2  and K 2 CO 3  in step (i) may be performed by bubbling air in water through an aqueous solution of K 2 CO 3  or by spraying droplets of K 2 CO 3  in aqueous solution into a stream of air. In both methods, the atmospheric CO 2  reacts with the K 2 CO 3  to form KHCO 3  according to the following reaction: 
 
K 2 CO 3 +H 2 O+CO 2 →2KHCO 3  
 
         [0012]     In the next step, CO 2  is released from the KHCO 3 .  
         [0013]     In one embodiment of the invention, the CO 2  is released by heating the KHCO 3  to a temperature sufficient to liberate the CO 2 , according to the following reaction, thus recycling the K 2 CO 3 : 
 
2KHCO 3 +Heat→K 2 CO 3 +H 2 O+CO 2  
 
         [0014]     In another embodiment, the CO 2  is released from the KHCO 3  obtained by an electrochemical process, according to the following reaction: 
 
HCO 3   −   −e →.OH+CO 2  
 
4(.OH)→2H 2 O+O 2  
 
         [0015]     The CO 2  obtained in step (ii) is then reacted with hydrogen to produce combustible fuels, such as methane and methanol.  
         [0016]     In one embodiment, in which heat source producing very high temperatures is available, the reaction of CO 2  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 H 2 , CO and CO 2 . 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, Rh, 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 the specific process, said pressure being always higher than the pressure used for the methane reforming.  
         [0017]     In another embodiment, the reaction of CO 2  and hydrogen according to the invention is an electrochemical process, such as a reverse operation of a fuel cell.  
         [0018]     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.  
         [0019]     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 CO 2  and hydrogen and generating electricity.  
         [0020]     According to the present invention, a reverse operation of a fuel cell is carried out whereby electricity is supplied to a fuel cell containing CO 2 , 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.  
         [0021]     In one preferred embodiment, the electrochemical process corresponds to an inverted direct methanol fuel cell (DMFC) and the fuel obtained is methanol.  
         [0022]     DMFCs are low-temperature fuel cells operating at temperatures of 30-130° C. and using liquid methanol as the electrolyte, according to the reaction: 
 
CH 3 OH+ 3/2O 2 →CO 2 +2H 2 O 
 
         [0023]     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.  
         [0024]     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.  
         [0025]     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.  
         [0026]     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 Na 2 CO 3 , K 2 CO 3 , Li 2 CO 3  or combinations thereof, that may be retained in a ceramic matrix, e.g. of LiAlO 2 . 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.  
         [0027]     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: 
 
CH 4 +O 2 →CO 2 +2H 2  
 
         [0028]     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.  
         [0029]     The methane or methanol obtained by the method of the invention may later be converted into longer hydrocarbons, using known chemical reactions.