Abstract:
A cryogenic air separation unit is used to separate carbon dioxide from a flue gas stream. The temperature and pressure of the carbon dioxide are controlled so that the separated carbon dioxide coming from the cryogenic air separation unit is in a liquid phase. The liquid phase carbon dioxide is converted to carbon monoxide by safely reacting the carbon dioxide with carbon at high temperature in a plasma arc reactor. The carbon monoxide produced by this reaction has sufficient energy potential to be used as a fuel.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of provisional patent application Ser. No. 60/786,280 filed Mar. 28, 2006. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates to a process for removing carbon dioxide (CO 2 ) from a combustion gas stream by converting the CO 2  to a liquid form and then reacting the CO 2  with carbon to form carbon monoxide (CO) and using the CO to generate the energy required to heat a reaction high enough to have it proceed satisfactorily. 
       BACKGROUND OF THE INVENTION 
       [0003]    A significant and substantial portion of the energy generation needs of the United States depends on the combustion of carbon-containing fuels. Any such combustion will inevitably produce CO 2 . The effects of the production of CO 2  have been the subject of much research and there is recognition in the scientific field that man-made production of CO 2  has resulted in potentially damaging climate change. The problem with man-made CO 2  production has led to the Kyoto protocol and other measures to limit the production of CO 2 . Although the United States is not a member of the Kyoto protocol, delegates representing the United States have outlined certain technological solutions which the United States is proposing to control CO 2  emissions such as burying CO 2  deep underground. The delegates did not publicly propose any technology that would recover all or any portion of the CO 2  generated by power plants or other operations of the like. 
         [0004]    One proposed attempt to lower the concentration of CO 2  is to bury the CO 2  by injecting it deep in the ground. The cost of this proposal is high and does not produce any secondary beneficial use of the CO 2 . There is a need for a process which decreases the amount of CO 2  in as energy efficient manner as possible. 
       SUMMARY OF THE INVENTION 
       [0005]    The present invention is a method for the removal of CO 2  from flue gas by means of a cryogenic air separator and the subsequent reaction of CO 2  with carbon to create CO which has a high calorific value. It has been reported that a 1000 megawatt electric utility plant creates 6 million tons of CO 2  per year. According to eia data (Number of Plants at U.S. Electric Utilities by Census Division and State, 2000), there are 2,776 electric utility plants in the United States. The average capacity of these electric utility plants is estimated to be 350 megawatts. Thus, the total megawatt output of all of these plants would have a combined CO 2  output of 5.829×10 9  tons of CO 2  per year, which is equivalent to 5.205×10 11  moles of CO 2 . The present process allows for further utilization of this CO 2  after it is converted to CO. 
         [0006]    The preferred process uses a cryogenic air separation unit to separate the CO 2  from a flue gas stream. The temperature and pressure of the CO 2  are controlled according to the carbon-oxygen phase diagram of  FIG. 2  so that the separated CO 2  coming from the cryogenic air separation unit is in a liquid phase. The liquid phase CO 2  is converted to carbon monoxide by safely reacting the CO 2  with carbon at high temperature in a plasma arc reactor. The CO produced by this reaction has sufficient energy potential to be used as a fuel. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a series of graphs showing the equilibrium constant as a function of temperature for various reactions involving carbon. 
           [0008]      FIG. 2  is a phase diagram of the carbon oxygen system. 
           [0009]      FIG. 3  is a graph showing the conversion of CO 2  to CO at a range of temperatures. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0010]    The present invention uses the process steps of separating CO 2  from a flue gas stream and converting at least a fraction of that CO 2  to CO. The CO 2  is preferably separated from the flue gas stream though a cryogenic air separation process, in which the CO 2  is converted to its liquid phase. In order to ensure that the CO 2  is in liquid phase, it is preferred that the pressure be maintained at approximately 7.0 atmospheres and the temperature maintained at approximately 78° C. The temperature and pressure needed to contain the CO 2  as a liquid can be estimated from the phase diagram of the carbon oxygen system as shown in  FIG. 2 . 
         [0011]    Cryogenic air separation units are well known. Universal Industrial Gases, Inc. has installed such plants to recover the CO 2  from high-purity or low-purity feed streams generated by various sources such as ammonia, ethanol or hydrogen plants. In addition, cryogenic air separation units have also been used in the steel industry which are capable of removing 40.8 tons of O 2  in one hour. 
         [0012]    The conversion of the separated CO 2  to CO is accomplished by introducing carbon to the liquid CO 2  in the presence of heat to enable the following reaction to proceed: 
         [0000]      CO 2 +C=2CO  (1) 
       As shown in FIG. 3, nearly all of the CO 2  is converted to CO at temperatures in excess of 1200° C. 
       [0013]    The carbon used in this reaction can be of various forms including graphite. The liquid CO 2  and carbon are passed into a plasma furnace where the reaction will take place to form CO. The CO generated from this reaction can be used to supplant other forms of energy such as coal, hydrogen, or natural gas. Other forms of carbon injection may be utilized, but the results of other methods of adding carbon to the gas stream may not result in the same extent of creation of CO as obtained by mixing the graphite with the liquid CO 2 . 
         [0014]    Preferably, the carbon that is used in the process is graphite, which is the purest form of carbon that can be found. If the goal of the process is to remove all of the deleterious materials such as arsenic, sulfur, mercury and the like, then graphite powder should be used to prevent all such deleterious materials commonly found in coal and the like from ever entering the flue gas. If it is not required to have flue gas of such purity then other forms of carbon such as charcoal and the like may be utilized. The form of carbon used can be mixed with the liquid CO 2  and can go through the plasma arc furnace together to form CO. 
         [0015]    The CO produced by the reaction of CO 2  and carbon provides a source of potential energy. It is possible to sequester the CO 2  to a tank that could contain substantially enough liquid CO 2  that when reacted with carbon would create enough CO that its combustion would provide enough energy to maintain the generation of heat at substantially the same level so that the reaction would function with either carbon or such hydrocarbon products as a fuel. 
         [0016]    The CO 2  can be transferred from the storage pressure vessel to a pressure vessel immediately adjacent to burners such as are used on ships or on the boilers of electric generation plants. Any pressure vessel situated adjacent to the burners should be double walled and water filled so that the heat generated by the action of plasma arc system would be absorbed by the water flowing through the walls of the pressure vessel. This absorbed heat can be utilized in the boilers or other methods of utilizing heat to produce steam or other requirements for heat in other applications. 
         [0017]    When the heated gases are finally admitted to the burners of the boiler or other requirement for heating they contain 32.9 kcal of energy resulting from the transformation of CO 2  to CO. When the 2 moles of CO are mixed with oxygen, the combustion results in the formation of an additional 164.367 kcals. Therefore, the sequestration of the CO 2  prior to its transformation to CO results in an additional amount of heat that when combined with the energy created by combustion of the CO amounts to a substantial increase in the heating value of the gas. In addition, the safety of the operation would be advanced in that large quantities of CO would not have to be either sequestered or transported to the burners where they would be utilized. 
         [0018]    A thermal plasma heating system always contains some mechanism of inducing the flow of electricity through an ionized working gas. The current flow heats the gas to a very high temperature through the mechanism of resistive or Joule heating. Through electronic, atomic, and molecular collisions the gas is maintained in an ionized state and the plasma becomes self sustaining. Typical thermal plasma temperatures are in the range of 10,000° K to 30,000° K and result in heat transfers that are difficult to match by alternative processing techniques. The liquid CO 2  and carbon are passed through the plasma arc with the formation of CO according to Equation (1). 
         [0019]    Specific methods and compositions described herein are representative of preferred embodiments and are exemplary and not intended as limitations on the scope of the invention. Other objects, aspects, and embodiments will occur to those skilled in the art upon consideration of this specification, and are encompassed within the spirit of the invention as defined by the scope of the claims. Where examples are given, the description shall be construed to include but not to be limited to only those examples. It will be readily apparent to one skilled in the art that varying substitutions and modifications may be made to the invention disclosed herein without departing from the scope and spirit of the invention, and from the description of the inventions, including those illustratively set forth herein, it is manifest that various modifications and equivalents can be used to implement the concepts of the present invention without departing from its scope. A person of ordinary skill in the art will recognize that changes can be made in form and detail without departing from the spirit and the scope of the invention. The described embodiments are to be considered in all respects as illustrative and not restrictive. Thus, for example, additional embodiments are within the scope of the invention and within the following claims.