Abstract:
The present disclosure relates to a system for carbon dioxide seperation and capture. The system includes a porous metal membrane comprising Ni, Ag, or combinations thereof and having molten carbonate within the pores. A CO 2  containing flue gas input stream is separated from a reactant gas input stream by the membrane. The CO 2  is removed from the flue gas input stream as it contacts the membrane resulting in a CO 2  free flue gas output stream and a CO 2  containing reactant gas output stream.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application is based on and claims priority to U.S. Provisional Application 61/335,753 having a filing date of Jan. 12, 2010, which is incorporated by reference herein. 
    
    
     BACKGROUND 
     Fossil fuels will be the primary energy resources to power society for at least the next fifty years. The use of fossil fuels generates CO 2 . Unfortunately, CO 2  is a greenhouse gas, the emissions of which to the atmosphere cause global warming, an alarming threat to humankind. An increased awareness of this problem is recognized in the changes of energy policy at both national and international levels. For the scientific and engineering communities, research activities in searching for means of effectively reducing CO 2  emissions has also increased considerably in recent years. This trend will continue for a sustained period of time in near future. 
     The technical approach to reduce CO 2  emissions is simply to avoid the emissions to the atmosphere by capturing and storing it at either pre-combustion or post-combustion stage. At either stage, CO 2  has to be separated from either a reducing fuel stream (pre-combustion) or an oxidizing flue gas stream (post-combustion) into a highly concentrated form, from which CO 2  can then be further compressed into a liquid form and stored via geologic sequestration. The technologies available for the CO 2  separation include primarily mechanical scrubbing and poly-amine solvent-based physical adsorptions. The former is mainly used in separating CO 2  from CH 4  (or natural gas) and the latter is widely employed in CO 2  separation in flue and fuel gas streams of power plants. Unfortunately, both methods are energy intensive and cost prohibitive for large-scale commercial applications. For example, it is estimated that the cost will be at $50-$60 per metric ton of CO 2  captured, which translates to $100 million per year for a refinery to invest the CO 2  capture technology. Therefore, developing a cost-effective and separation-efficient CO 2  capture technology is both scientifically interesting and industrially demanding. 
     As such, a need exists for a CO 2  capture technology that addresses the shortcomings of conventional approaches. Methods of utilizing such technology would also be desirable. 
     SUMMARY 
     The present disclosure relates to a system for carbon dioxide separation and capture. The system includes a porous metal membrane comprising nickel (Ni), silver (Ag), or combinations thereof and having molten carbonate within the pores. A CO 2  containing flue gas input stream is separated from a reactant gas input stream by the membrane. The CO 2  is removed from the flue gas input stream as it contacts the membrane resulting in a CO 2  free flue gas output stream and a CO 2  containing reactant gas output stream. 
     In certain embodiments of the present disclosure, a method of carbon dioxide separation and capture is provided. The method includes contacting a CO 2  containing flue gas input stream with a porous metal membrane comprising nickel (Ni), silver (Ag), or combinations thereof and having molten carbonate within the pores. The membrane separates the CO 2  containing flue gas input stream from a reactant gas input stream. The CO 2  is removed from the flue gas input stream as it contacts the membrane resulting in a CO 2  free flue gas output stream and a CO 2  containing reactant gas output stream. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       A full and enabling disclosure of the present subject matter, including the best mode thereof, to one of ordinary skill in the art, is set forth more particularly in the remainder of the specification, including reference to the accompanying figures in which: 
         FIG. 1  illustrates a conceptual cross-sectional schematic of the mixed carbonate ion and electron conducting membrane in accordance with certain embodiments of the present disclosure. 
         FIG. 2  illustrates a schematic of the CO 2  separation reactor based on mixed conducting membrane in accordance with certain embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to embodiments of the disclosed subject matter, one or more examples of which are set forth below. Each example is provided by way of explanation of the subject matter, not limitation of the subject matter. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present disclosure without departing from the scope or spirit of the subject matter. For instance, features illustrated or described as part of one embodiment, can be used on another embodiment to yield a still further embodiment. 
     The present disclosure describes a novel mixed carbonate ion and electron conducting membrane that is capable of separating CO 2  from a flue gas stream. Different from the current poly-amine solvent-based physical-adsorption-in-principle technology, the proposed composite mixed conducting CO 2  separation membrane is based on electrochemical principles, and therefore has a sole selectivity to CO 2  but no limitation to the separation efficiency. The chemical reactors that are built on the membrane and assisted by the reactant gases are also presented. 
     The disclosed mixed carbonate ion and electron conductor is comprised of two phases. One phase is the porous substrate made of metal such as pure Ni, pure Ag, or combinations thereof. Another phase is the molten carbonates that are held within pores of the porous metal substrate by the capillary force.  FIG. 1  shows the conceptual cross-sectional schematic of the composite mixed conductor in a tubular geometry. The functionality of the porous metal substrate is multiple. First, it provides the pathway for electrons to react at the gas-solid-liquid surfaces. Second, it also provides the physical pores for holding the molten electrolyte by the capillary force and mechanical structure for the membrane. During separation, the carbonate ions in molten carbonate electrolytes migrate from high end of chemical potentials of CO 2  and O 2  in the flue gas stream to the low end in the reactant gas. The electrode reactions at the two reactive surfaces are represented by:
 
At CO 2 -rich side with flue gas: CO 2 (h)+½O 2 (h)+2 e   − =CO 3   2−   (1)
 
At CO 2 -lean side with reactant gas A: A(l)+CO 3   2− =AO(l)+CO 2 (l)+2 e   −   (2)
 
Overall reaction: CO 2 (h)+½O 2 (h)+A(l)=AO(l)+CO 2 (l)  (3)
 
     Reaction (3) depicts the completion of the separation of CO 2  from one side (high end) to another side (low end). 
     Selection of the reactant gas A is critical to the design of chemical reactors and successful separation of CO 2 . It is apparent that the formed mixture of AO and CO 2  in the effluent has to be easily separable to allow a true separation of CO 2 . One candidate reactant gas is H 2 , with which a mixture of H 2 O and CO 2  can be formed in the effluent as a result of the reaction (2). A simple physical condensation of H 2 O at the downstream of the reactor can then be applied to obtain a pure stream of CO 2 . Another candidate is CO, with which only CO 2  is the product after reacting with the membrane. No secondary separation is needed in this case. In practice, the mixture of H 2  and CO is an easier and more common form to obtain by traditional industrial processes such as coal gasification or methane reforming. Therefore, in  FIG. 2  both kinds of reactant gases are used to illustrate the principle of the CO 2  separation chemical reactor built upon the proposed mixed conducting membrane. As shown, a component labeled Coal Gasification Reactor or Steam Methane Reactor is added to regenerate CO and H 2  that are needed for the subsequent CO 2  separation reactions by allowing a fraction of produced H 2 O and CO 2  to be recycled. The electrode and overall chemical reactions with H 2  and CO as the reactant gases are represented by
 
At flue gas side: CO 2 (h)+½O 2 (h)+2 e   − =CO 3   2−   (4)
 
At reactant gas side: H 2 (l)+CO 3   2− =H 2 O(l)+CO 2 (l)+2 e   −   (5)
 
CO(l)+CO 3   2− =2CO 2 (l)+2 e   −   (6)
 
Overall reaction: H 2 (l)+CO 2 (h)+½O 2 (h)=H 2 O(l)+CO 2 (l)  (7)
 
CO(l)+CO 2 (h)+½O 2 (h)=2CO 2 (l)  (8)
 
     At the end, the chemical reactor produces two product lines: one is the CO 2 -free stream containing only N 2  and H 2 O; another is the pure stream of CO 2  after physical condensation of H 2 O. 
     In the interests of brevity and conciseness, any ranges of values set forth in this specification are to be construed as written description support for claims reciting any sub-ranges having endpoints which are whole number values within the specified range in question. By way of a hypothetical illustrative example, a disclosure in this specification of a range of 1-5 shall be considered to support claims to any of the following sub-ranges: 1-4; 1-3; 1-2; 2-5; 2-4; 2-3; 3-5; 3-4; and 4-5. 
     These and other modifications and variations to the present disclosure can be practiced by those of ordinary skill in the art, without departing from the spirit and scope of the present disclosure, which is more particularly set forth in the appended claims. In addition, it should be understood that aspects of the various embodiments can be interchanged both in whole or in part. Furthermore, those of ordinary skill in the art will appreciate that the foregoing description is by way of example only, and is not intended to limit the disclosure so further described in such appended claims. The metals Ni and Ag are used herein to illustrate the porous metal membrane of the present invention but are not intended to limit the scope of the invention.