Abstract:
Disclosed are an apparatus and a method for recovering energy after carbon dioxide capture. The apparatus includes an energy recovery unit at a discharge part of a carbon dioxide capturing apparatus through which captured carbon dioxide is discharged. The energy recovery unit reduces a discharge pressure of the carbon dioxide to a pressure level suitable for a fixation or conversion treatment, and simultaneously generates and recovers energy generated during the pressure reduction.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims under 35 U.S.C. §119(a) the benefit of Korean Patent Application No. 10-2012-0024182 filed Mar. 9, 2012, the entire contents of which are incorporated herein by reference. 
       BACKGROUND 
       [0002]    (a) Technical Field 
         [0003]    The present invention relates to an apparatus and method for recovering energy after carbon dioxide capture. More particularly, it relates to an apparatus and method for recovering energy after carbon dioxide capture, which can recover energy from a discharge pressure of captured carbon dioxide when the captured carbon dioxide is treated by a method such as fixation or conversion. 
         [0004]    (b) Background Art 
         [0005]    Generally, methods for capturing carbon dioxide include an absorption method, an adsorption method, and a separation membrane method. Absorption methods can treat a large amount of exhaust gas compared to adsorption methods and separation membrane methods, and provides a high removal efficiency even in cases when the CO 2  concentration condition is about 7% to about 30%. Further, adsorption methods have a high economical efficiency and are easy to apply. 
         [0006]    The carbon dioxide, once captured, can then be stored, or can be treated by fixation and conversion methods. Among these, the method of storing CO 2  in the ground or deep sea is easy even with a large amount of CO 2 , in contrast to the other treatment methods. Such storage methods are currently available are have been commercialized. However, the cost for storing CO 2  in the ground or deep sea is high, and the stored CO 2  cannot be fundamentally removed, making additional profit-making difficult. 
         [0007]    The methods of converting captured CO 2  into other chemical substances using CO 2  as a carbon source, and fixing CO 2  using plants and seaweeds are both being studied. If such methods reach a commercialization stage, CO 2  can be fundamentally removed, and useful products produced thereby can allow for additional profit-making. Accordingly, these methods are being evaluated as more economical and preferable technologies. 
         [0008]    Among the absorption methods for capturing carbon dioxide, a chemical absorption method is currently being most widely developed. In a chemical absorption method, CO 2  is selectively separated from exhaust gas by a chemical reaction. With chemical absorption, the amount of absorption is not significantly affected by the CO 2  partial pressure. Accordingly, there is an advantage in that the CO 2  removal efficiency is high even when the CO 2  partial pressure is low. However, the chemical absorption method is limited because high energy consumption is required in a subsequent recovery process in which CO 2  is separated from an absorbent. For example, it is known that the energy cost for recovery accounts for about 60% or more of the total CO 2  recovery cost of a CO 2  capturing apparatus. In particular, it is known that the energy cost for separating CO 2  from absorbent in a recovery tower accounts for about 80% of the energy cost for CO 2  recovery, and the energy cost for maintaining process equipment such as a pump accounts for about 20% of the energy cost for CO 2  recovery. 
         [0009]    Accordingly, an improved absorption technology is needed for capturing carbon dioxide wherein energy consumed in absorbent recovery is reduced, thereby reducing the cost for collecting carbon dioxide. 
         [0010]    Hereinafter, a typical carbon dioxide capturing processing will be described in brief. 
         [0011]    As shown in  FIG. 2 , an exhaust gas containing CO 2  is supplied to an absorption tower  10  that has a wide surface area for smooth gas-liquid contact and which is filled with filling substances. 
         [0000]    In this case, a liquid absorbent is supplied from an absorbent storage tank  12  to an upper part of the absorption tower  10 , and an exhaust gas is supplied to a lower part of the absorption tower  10 . the exhaust gas contacts the liquid absorbent (absorption solution) at an atmospheric pressure in the upper end of the absorption tower  10 , allowing CO 2  in the exhaust gas to be absorbed into the absorption solution, generally within a temperature range of 40° C. to 70° C. 
         [0012]    The absorbent that absorbs CO 2  is discharged from the absorption tower  10  and is supplied to a recovery tower  14  where it undergoes a recovery process in which the absorbent is heated to a temperature of 100° C. to 160° C. Thereafter, the absorbent is discharged from the lower part of the recovery tower  14  (“used CO 2  absorbent”) and it is resupplied to the absorption tower  10  through an absorbent supplying line  22 . 
         [0013]    Absorbent that is resupplied to the absorption tower  10  is heated by passing through a heat exchanger  16 . As shown, absorbent newly supplied to the recovery tower  14  from the absorbent storage tank  12  can be preheated by heat exchange with the heated absorbent that is resupplied from the lower part of the recovery tower  14 . This combined heated absorbent is then supplied to the upper part of the recovery tower  14 . 
         [0014]    During the recovery process in which absorbent is heated to a temperature of 100° C. to 160° C. in the recovery tower  14 , evaporated absorbent and CO 2  is discharged from the upper part of the recovery tower  14 . Absorbent with CO 2  is discharged from the lower part of the recovery tower  14  and is heated to a temperature range of 100° C. to 160° C. by a heater  18 , such as a boiler, to separate CO 2  from the absorbent. 
         [0015]    CO 2  separated in the recovery tower  14  is discharged through a condenser to locations for storage, fixation, and conversion, and the evaporated absorbent is condensed in the condenser  20  then fed back to the recovery tower  14 . 
         [0016]    The CO 2  separated in the recovery tower  14  is a high concentration of gaseous CO 2  (90% to 100%), and is discharged from the recovery tower  14  at a pressure range of 1.9 atm to 6 atm to be finally treated by a storage, fixation, or conversion method. 
         [0017]    In order to store captured CO 2  in the ground or deep sea, the pressure of a high concentration of CO 2  discharged from the upper end of the recovery tower  14  must be increased to a high pressure of about 70 atm to about 100 atm. For this pressure increase, additional energy is required. 
         [0018]    On the other hand, when captured CO 2  is directly treated by fixation or conversion instead of storage, the captured CO 2  can be treated by a pressure of just 1.2 atm or less and, thus, a process of increasing the pressure of CO 2  with a compressor is unnecessary. 
         [0019]    The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art. 
       SUMMARY OF THE DISCLOSURE 
       [0020]    The present invention provides an apparatus and method for recovering energy after carbon dioxide capture, which reduces a discharge pressure of captured CO 2  to a pressure necessary for fixation or conversion. The apparatus and method further simultaneously generates energy (e.g. in a generator connected to a turbine), wherein the energy is generated by fixation or conversion of the captured CO 2  instead of storing the captured CO 2  in the ground or deep sea. This generated energy can be supplied to process operating units of the present apparatus and, thus, can be used by any of the operating units to capture CO 2 . 
         [0021]    In one aspect, the present invention provides an apparatus for recovering energy after carbon dioxide capture, including an energy recovery unit at a carbon dioxide discharge part of a carbon dioxide capturing apparatus, wherein the energy recovery unit reduces a discharge pressure of the carbon dioxide to a pressure level suitable for a fixation or conversion treatment. According to various embodiments, energy generated during the pressure reduction can be simultaneously recovered by the energy recovery unit. 
         [0022]    In an exemplary embodiment, the energy recovery unit may be in connection with one or more process operating units of the carbon dioxide capturing apparatus to supply the recovered electrical energy to the desired process operating units. 
         [0023]    In another exemplary embodiment, the energy recovery unit may include: a turbine disposed at an outlet of a condenser, wherein the outlet is a discharge part of the carbon dioxide capturing apparatus; and a generator connected to the turbine. 
         [0024]    In another aspect, the present invention provides a method for recovering energy after carbon dioxide capture, including: capturing, by a carbon dioxide capturing apparatus, carbon dioxide from an exhaust gas; discharging, by the carbon dioxide capturing apparatus, the captured carbon dioxide; reducing a discharge pressure of the discharged carbon dioxide to a pressure level suitable for a fixation or conversion treatment; and recovering energy generated during the pressure reduction. 
         [0025]    In an exemplary embodiment, the method may further include supplying the recovered energy to one or more desired process operating units of the carbon dioxide capturing apparatus to utilize the recovered energy. 
         [0026]    In another exemplary embodiment, the energy recovery may include: rotating a turbine using the discharge pressure of the carbon dioxide captured by the carbon dioxide capturing apparatus; continually reducing a final discharge pressure of the carbon dioxide that has passed the turbine to the pressure level suitable for the fixation or conversion treatment; and delivering a rotary force of the turbine to a generator connected to the turbine to enable generation of energy by the generator. 
         [0027]    In still another exemplary embodiment, when the discharge pressure of the carbon dioxide captured by the carbon dioxide capturing apparatus ranges from about 1.8 atm to about 6 atm, the final discharge pressure of the carbon dioxide that has passed the turbine may be reduced to a pressure of less than about 1.8 atm, less than about 1.6 atm, less than about 1.4 atm, or a pressure of about 1.2 atm which is suitable for the fixation or conversion treatment. 
         [0028]    Other aspects and exemplary embodiments of the invention are discussed infra. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0029]    The above and other features of the present invention will now be described in detail with reference to certain exemplary embodiments thereof illustrated the accompanying drawings which are given hereinbelow by way of illustration only, and thus are not limitative of the present invention, and wherein: 
           [0030]      FIG. 1  is a diagram illustrating an apparatus for recovering energy after carbon dioxide capture according to an embodiment of the present invention; and 
           [0031]      FIG. 2  is a diagram illustrating a typical carbon dioxide capturing apparatus. 
       
    
    
       [0032]    Reference numerals set forth in the Drawings includes reference to the following elements as further discussed below:
         10 : absorption tower     12 : absorbent storage tank     14 : recovery tower     16 : heat exchanger     18 : heater     20 : condenser     22 : absorbent supplying line     30 : energy recovery unit     32 : turbine     34 : generator       
 
         [0043]    It should be understood that the appended drawings are not necessarily to scale, presenting a somewhat simplified representation of various preferred features illustrative of the basic principles of the invention. The specific design features of the present invention as disclosed herein, including, for example, specific dimensions, orientations, locations, and shapes will be determined in part by the particular intended application and use environment. 
         [0044]    In the figures, reference numbers refer to the same or equivalent parts of the present invention throughout the several figures of the drawing. 
       DETAILED DESCRIPTION 
       [0045]    Hereinafter reference will now be made in detail to various embodiments of the present invention, examples of which are illustrated in the accompanying drawings and described below. While the invention will be described in conjunction with exemplary embodiments, it will be understood that present description is not intended to limit the invention to those exemplary embodiments. On the contrary, the invention is intended to cover not only the exemplary embodiments, but also various alternatives, modifications, equivalents and other embodiments, which may be included within the spirit and scope of the invention as defined by the appended claims. 
         [0046]    It is understood that the term “vehicle” or “vehicular” or other similar term as used herein is inclusive of motor vehicles in general such as passenger automobiles including sports utility vehicles (SUV), buses, trucks, various commercial vehicles, watercraft including a variety of boats and ships, aircraft, and the like, and includes hybrid vehicles, electric vehicles, plug-in hybrid electric vehicles, hydrogen-powered vehicles and other alternative fuel vehicles (e.g., fuels derived from resources other than petroleum). As referred to herein, a hybrid vehicle is a vehicle that has two or more sources of power, for example both gasoline-powered and electric-powered vehicles. 
         [0047]    The above and other features of the invention are discussed infra. 
         [0048]    Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. 
         [0049]    The present invention provides an apparatus and method that improves the economical efficiency of CO 2  capture by reducing the CO 2  absorption cost, particularly by reducing energy costs. 
         [0050]    According to the present invention, a CO 2  capturing apparatus is designed to discharge captured CO 2  for conversion or fixation after reducing the discharge pressure of the CO 2  to a pressure level suitable for fixation or conversion treatment, wherein energy is recovered during the pressure reduction, and the recovered energy is supplied to one or more of process units of the CO 2  capturing apparatus. 
         [0051]    According to an exemplary embodiment, as shown in  FIG. 1 , an energy recovery unit  30  may be disposed around a location where CO 2  captured by the CO 2  capturing apparatus is discharged. For example, the energy recovery unit  30  may be disposed at the side of an outlet of a condenser  20  connected to a recovery tower  14  of the CO2 capturing apparatus. 
         [0052]    When CO 2  captured in the CO 2  capturing apparatus is discharged from the outlet of the condenser  20 , the energy recovery unit  30  may be configured and arranged to reduce the discharge pressure of CO 2  to a pressure level suitable for fixation or conversion treatment, and to recover energy generated during the pressure reduction. 
         [0053]    More specifically, the energy recovery unit  30  according to an embodiment of the present invention may include a turbine  32  disposed at an outlet of the condenser  20  that is a discharge part of the CO 2  capturing apparatus. A generator  34  can be provided in connection with the turbine  32 , for example, by concentrically connecting the generator  34  to the turbine  32  or by other suitable arrangements. 
         [0054]    The generator  34  of the energy recovery unit  30  may be connected to one or more process operating units (e.g., pump and blower disposed in each capture process, which are typically driven by electrical energy) of the CO 2  capturing apparatus so as to supply generated electrical energy to the respective process operating units. 
         [0055]    Hereinafter, a method of recovering energy after CO 2  capture according to an embodiment of the present invention will be described as follows. 
         [0056]    As shown in  FIG. 1 , exhaust gas containing CO 2  may be supplied into an absorption tower  10 , and absorbent, typically liquid absorbent, may be supplied from an absorbent storage tank  12  to an upper part of the absorption tower  10 . 
         [0057]    The exhaust gas supplied into the absorption tower  10  may contact liquid absorbent (absorption solution), typically at an atmospheric pressure, in the absorption tower  10  (e.g. in the upper part of the absorption tower  10 ), and CO 2  within the exhaust gas may be absorbed by the absorbent. 
         [0058]    The absorbent that absorbs CO 2  (“used CO 2  absorbent”) is discharged from the absorption tower  10 , and is supplied to a recovery tower  14  where it may then undergo a recovery process. In particular, in the recovery process the absorbent is heated to a suitable temperature (such as a temperature of about 100° C. to about 160° C.) in the recovery tower  14 . 
         [0059]    The absorbent recovered in the recovery process is discharged from the lower part of the recover tower  14 , and may then be resupplied to the absorption tower  10  via an absorbent supplying line  22  which connects the absorbent storage tank  12  and the absorption tower  10 . 
         [0060]    As shown, the resupplied absorbent passes through a heat exchanger  16 , and thereafter combines with CO 2  absorbent newly supplied from the absorption tower  10 . As such, the newly supplied CO 2  absorbent may be preheated by heat exchange with the heated resupplied absorbent, and the combined absorbent (newly supplied absorbent and resupplied absorbent) may then be supplied to the upper part of the recovery tower  14 . 
         [0061]    During the recovery process in which absorbent is heated to a suitable temperature, such as a temperature of about 100° C. to about 160° C., evaporated absorbent with CO 2  may be discharged from the upper part of the recovery tower  14 . Further, liquid absorbent with CO 2  may be discharged from the lower part of the recovery tower  14 , may pass through a heater  18  (e.g. a boiler or the like) where it is heated to a suitable temperature range, such as a temperature of about 100° C. to about 160° C., so as to separate CO 2  from the absorbent. 
         [0062]    CO 2  separated in the recovery tower  14 , i.e., CO 2  with evaporated absorbent, may be discharged to a condenser  20 . From the condenser, condensed absorbent may be resupplied to the recovery tower  14 , while separated CO 2  may be discharged to a location for fixation or conversion treatment. 
         [0063]    When separated CO 2  is discharged from the condenser  20  to the location for the fixation or conversion treatment, the pressure of CO 2  may range from about 1.8 atm to about 6 atm. A suitable discharge pressure of CO 2  necessary for the fixation or conversion treatment may be less than this discharge pressure, and, for example, may be less than 1.8 atm, less than 1.6 atm, less than 1.4 atm, and in some embodiments, may be about 1.2 atm. 
         [0064]    As shown in the embodiment of  FIG. 1 , separated CO 2  discharged from the condenser  20  at a pressure of about 1.8 atm to about 6 atm is passes through the turbine  32  of the energy recovery apparatus  30 . As the separated CO 2  passes through the turbine, the turbine  32  may be rotated, and the rotary force of the turbine  32  may be delivered to the generator  34 . 
         [0065]    While the separated CO 2  discharged from the condenser  20  passes through the turbine  32 , the pressure of CO 2  may be reduced to a suitable pressure level for the fixation or conversion treatment. In particular, according to an exemplary embodiment, separated CO 2  is discharged from the condenser  20  at a pressure of about 1.8 atm to about 6 atm, and passes through the turbine  32  where the pressure of the CO 2  is constantly or continuously reduced as needed to a suitable pressure level for fixation or conversion treatment. 
         [0066]    For example, the final discharge pressure of CO 2  that has passed through the turbine  32  may be reduced to a pressure of about 1.2 atm, which is a suitable pressure for the subsequent fixation or conversion treatment. 
         [0067]    As the CO 2  passes through the turbine and is reduced in pressure, the rotary force of the turbine  32  may be delivered to the generator  34 , enabling the generation of energy by the generator  34 . Electrical energy generated in the generator  34  may be supplied to and consumed in one or more of the process operating units (e.g., pump and blower disposed in each capture process and driven by electrical energy) of the CO 2  capturing apparatus. 
         [0068]    As a result, the amount of energy that must be supplied (i.e. external energy) to operate the CO 2  capturing apparatus can be significantly reduced, and costs can be saved by utilizing electrical energy generated in the generator  34  of the energy recovery unit  30  as energy for powering one or more of the process operating units of the CO 2  capturing apparatus. 
         [0069]    As a test example of the present invention, a test of energy recovery was performed using a process simulation program in which the amount of CO 2  capture (removal) was about 1000 ton/day. The CO 2  absorption process conditions are shown in Table 1 below. 
         [0000]    
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Gas-liquid flow ratio 
                 125 
                 150 
                 175 
                 200 
                 225 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Flow rate of exhaust gas, 
                 2,125.6 
                 2,125.6 
                 2,125.6 
                 2,125.6 
                 2,125.6 
               
               
                 m3/min 
               
               
                 CO 2  concentration, 
                 20 
                 20 
                 20 
                 20 
                 20 
               
               
                 mol % 
               
               
                 Flow rate of absorbent, 
                 17.0 
                 14.2 
                 12.1 
                 10.7 
                 9.4 
               
               
                 m3/min 
               
               
                 MEA concentration in 
                 35 
                 35 
                 35 
                 35 
                 35 
               
               
                 absorption tower, wt % 
               
               
                   
               
             
          
         
       
     
         [0070]    The consumed energy kW of a reboiler (e.g., heater  18  connected to the lower part of the recovery tower  14 ) for each gas-liquid flow ratio and the flow rate of CO 2  gas discharged from the condenser are shown in Table 2 below. 
         [0000]    
       
         
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
             
             
               
                   
                   
               
               
                   
                 Gas-Liquid Flow Ratio 
               
             
          
           
               
                   
                 125 
                 150 
                 175 
                 200 
                 225 
               
               
                   
                   
               
             
          
           
               
                 Amount of 
                 1,000 
                 1,000 
                 1,000 
                 1,000 
                 1,000 
               
               
                 CO 2   
               
               
                 Removal, 
               
               
                 ton/day 
               
               
                 Energy Used 
                 697.2 
                 577.2 
                 489.8 
                 425.0 
                 376.0 
               
               
                 in Absorbent 
               
               
                 Pump, kW 
               
               
                 Flow Rate of 
                 77.95 
                 79.50 
                 80.88 
                 81.97 
                 83.50 
               
               
                 CO 2   
               
               
                 discharged 
               
               
                 from 
               
               
                 Condenser, 
               
               
                 m3/min 
               
               
                 Pressure of 
                 4.41 
                 4.32 
                 4.25 
                 4.20 
                 4.12 
               
               
                 CO 2   
               
               
                 discharged 
               
               
                 from 
               
               
                 Condenser, 
               
               
                 atm 
               
               
                   
               
             
          
         
       
     
         [0071]    The simulation results of energy generated through the turbine for each gas-liquid flow ratio according to the above test conditions are shown in Table 3 below. 
         [0000]    
       
         
               
               
             
               
               
               
             
               
               
             
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 3 
               
             
             
               
                   
                   
               
               
                   
                 Division 
               
             
          
           
               
                   
                 Wet radical flow 
                 Dry radical flow 
               
             
          
           
               
                   
                 Gas-Liquid Flow Ratio 
               
             
          
           
               
                   
                 125 
                 150 
                 175 
                 200 
                 225 
                 125 
                 150 
                 175 
                 200 
                 225 
               
               
                   
                   
               
             
          
           
               
                 Pressure of 
                 4.41 
                 4.32 
                 4.25 
                 4.20 
                 4.12 
                 4.41 
                 4.32 
                 4.25 
                 4.20 
                 4.12 
               
               
                 CO 2  injected 
               
               
                 into turbine, 
               
               
                 ata 
               
               
                 Pressure of 
                 1.20 
                 1.20 
                 1.20 
                 1.20 
                 1.20 
                 1.20 
                 1.20 
                 1.20 
                 1.20 
                 1.20 
               
               
                 CO 2   
               
               
                 discharged 
               
               
                 from turbine, 
               
               
                 atm 
               
               
                 Turbine 
                 0.75 
                 0.75 
                 0.75 
                 0.75 
                 0.75 
                 0.85 
                 0.85 
                 0.85 
                 0.85 
                 0.85 
               
               
                 efficiency 
               
               
                 Generated 
                 177.3 
                 176.1 
                 175.1 
                 174.2 
                 173.0 
                 223.9 
                 222.4 
                 221.0 
                 220.0 
                 218.5 
               
               
                 energy, kW 
               
               
                 Reduction rate 
                 25.4 
                 30.5 
                 35.7 
                 41.0 
                 46.0 
                 32.1 
                 38.5 
                 45.1 
                 51.8 
                 58.1 
               
               
                 of energy used 
               
               
                 in absorbent 
               
               
                 pump, % 
               
               
                   
               
             
          
         
       
     
         [0072]    As shown in Table 3, the pressure of CO 2  inputted into the turbine ranged from about 4.12 atm to about 4.41 atm regardless of a wet or dry flow, and the pressure of CO 2  discharged into a fixation or conversion treatment unit through the turbine is constantly reduced to about 1.20 atm. Also, as energy generated by the generator increased according to the turbine efficiency, energy used in the absorbent pump of the CO 2  capturing apparatus was reduced. 
         [0073]    According to the embodiments of the present invention, when CO 2  captured by the CO 2  capturing apparatus are processed by fixation or conversion treatment instead of a method of storing CO 2  in the ground or deep sea, the discharge pressure of CO 2  captured by the CO 2  capturing apparatus can be reduced to a pressure necessary for the fixation or conversion treatment. An energy recovery unit can be provided to generate energy from the pressure reduction, particularly wherein a turbine is positioned through which captured CO 2  passes such that the rotary force of the turbine can be delivered to a generator to obtain an energy recovery effect in which electrical energy is produced. 
         [0074]    Also, since electrical energy produced in the generator can be utilized as energy for driving various process operating units (e.g., pump and blower) of the CO 2  capturing apparatus, energy (i.e., externally supplied energy) necessary for operating the apparatus to capture CO 2  can be significantly saved. 
         [0075]    The invention has been described in detail with reference to exemplary embodiments thereof. However, it will be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the appended claims and their equivalents.