Abstract:
The present invention relates to a method and a plant for CO 2  enrichment of a CO 2  containing gas, such as an exhaust gas from an industrial plant or a thermal power plant, by means of serially connected centrifuges of cyclones. Enrichment of CO 2  reduces the volume of the exhaust gas, increases the partial pressure of CO 2  therein and makes a less expensive and more effective capture of CO 2  possible. The invention also relates to plants for carrying out the method.

Description:
THE FIELD OF THE INVENTION  
       [0001]    The present invention relates to a method and a plant for CO 2  enrichment of a CO 2  containing gas. More specifically, the present invention relates to a method and a plant for increasing enrichment of CO 2 , or for increasing the partial pressure of CO 2  in a gas stream comprising CO 2  in combination with other gases, such as flue gases from industrial processes and combustion of fossil fuels, to make a subsequent CO 2  capturing more efficient. The invention also relates to a method and a plant for capturing CO 2  from a CO 2  containing gas including said method and plant for CO 2  enrichment. 
       BACKGROUND  
       [0002]    The concentration of CO 2  in the atmosphere has increased by nearly 30% in the last 150 years. The concentration of methane has doubled and the concentration of nitrogen oxides has increased by about 15%. This has increased the atmospheric greenhouse effect, something which has resulted in:
       The mean temperature near the earth&#39;s surface has increased by about 0.5° C. over the last one hundred years, with an accelerating trend in the last ten years.   Over the same period rainfall has increased by about 1%   The sea level has increased by 15 to 20 cm due to melting of glaciers and because water expands when heated up.       
 
         [0006]    Increasing discharges of greenhouse gases is expected to give continued changes in the climate. Temperature can increase by as much as 0.6 to 2.5° C. over the coming 50 years. Within the scientific community, it is generally agreed that increasing use of fossil fuels, with exponentially increasing discharges of CO 2 , has altered the natural CO 2  balance in nature and is therefore the direct reason for this development. 
         [0007]    It is important that action is taken immediately to stabilize the CO 2  content of the atmosphere. This can be achieved if CO 2  generated in a thermal power plant is collected and deposited safely. It is assumed that the collection represents three quarters of the total costs for the control of CO 2  discharges to the atmosphere. 
         [0008]    Thus, an energy efficient, cost efficient, robust and simple method for removal of a substantial part of CO 2  from the discharge gas will be desirable to ease this situation. It will be a great advantage if the method can be realized in the near future without long-term research. 
         [0009]    Discharge gas from thermal power plants typically contains 4 to 10% by volume of CO 2 , where the lowest values are typical for gas turbines, while the highest values are only reached in combustion chambers with cooling, for example, in production of steam. 
         [0010]    There are three opportunities for stabilizing the CO 2  content in the atmosphere. In addition to the capturing of CO 2 , non-polluting energy sources such as biomass can be used, or very efficient power plants can be developed. The capturing of CO 2  is the most cost efficient. Still, relatively little development work is carried out to capture CO 2 , the methods presented up till now are characterized either by low efficiency or by a need for much long-term and expensive development. All methods for capturing CO 2  comprise one or more of the following principles:
       Absorption of CO 2 . The exhaust gas from the combustion is brought into contact with an amine solution, at near atmospheric pressure. Some of the CO 2  is absorbed in the amine solution which is then regenerated by heating. The main problem with this technology is that one operates with a low partial pressure of CO 2 , typically 0.04 bar, in the gas which shall be cleaned. The energy consumption becomes very high (about 3 times higher than if it is cleaned with a CO 2  partial pressure of 1.5 bar). The cleaning plant becomes expensive and the degree of cleaning and size of the power plant are limiting factors. Therefore, the development work is concentrated on increasing the partial pressure of CO 2 . An alternative is that the exhaust gas is cooled down and re-circulated over the gas turbine. The effect of this is very limited due to the properties of the turbine, among other things. Another alternative is that the exhaust gas which is to be cooled down, is compressed, cooled down again, cleaned with, for example, an amine solution, heated up and expanded in a secondary gas turbine which drives the secondary compressor. In this way, the partial pressure of CO 2  is raised, for example to 0.5 bar, and the cleaning becomes more efficient. An essential disadvantage is that the partial pressure of oxygen in the gas also becomes high, for example 1.5 bar, while amines typically degrade quickly at oxygen partial pressures above about 0.2 bar. In addition, costly extra equipment is required. Other combinations of primary and secondary power stations exist.   Air separation. By separating the air that goes into the combustion installation into oxygen and nitrogen, circulating CO 2  can be used as a propellant gas in a power plant. Without nitrogen to dilute the CO 2  formed, the CO 2  in the exhaust gas will have a relatively high partial pressure, approximately up to 1 bar. Excess CO 2  from the combustion can then be separated out relatively simply so that the installation for collection of CO 2  can be simplified. However the total costs for such a system becomes relatively high, as one must have a substantial plant for production of oxygen in addition to the power plant. Production and combustion of pure oxygen represent considerable safety challenges, in addition to great demands on the material. This will also most likely require development of new turbines.   Conversion of the fuel. Hydrocarbon fuels are converted (reformed) to hydrogen and CO 2  in pressurized processing units called reformers. The product from the reformers contains CO 2  with a high partial pressure so that CO 2  can be separated out and deposited or used in another way. Hydrogen is used as fuel. The total plant becomes complicated and expensive, as it comprises a hydrogen-generating plant and a power plant.       
 
         [0014]    A common feature of the alternative methods for capture of CO 2  from a power plant is that they strive for a high partial pressure of CO 2  in the processing units where the cleaning is carried out. In addition, alternative methods are characterized by long-term, expensive and risky developments, with a typical time frame of 15 years research and a further 5 to 10 years or more before operating experience is attained. Expected electrical efficiency for large turbines under test conditions and full load, are up to 56 to 58%, whereas the efficiency under normal operating conditions with part load and degeneration of turbines, is about 52 to 54%, for a plant without cleaning. Expected efficiency for a plant including CO 2  capturing is expected to be below 40%. 
         [0015]    An extended time frame is enviromnentally very undesirable. In a United Nations Economic Commission for Europe (UNECE) conference in the autumn of 2002, “an urgent need to address the continuing exponential rise in global CO 2  emissions” was emphasised and words such as “as soon as possible” and “need to go far beyond Kyoto protocol targets” were used. 
         [0016]    Capturing of CO 2  from a gas by means of absorption seems to be the most promising solution at least for providing an effective capturing of CO 2  at a short to medium time horizon. Accordingly, substantial efforts have been made to increase the efficiency, and reduce the cost for CO 2  capturing. 
         [0017]    Important issues in capturing CO 2  by absorption are the large volume of gas that is to be treated and the relative low partial pressure of CO 2  normally present in combustion gases. The exhaust gas from a gas fired power plant of 400 MW constitutes about 1000 m 3 /sec at atmospheric pressure, and the CO 2  partial pressure is about 0.004. Due to the low partial pressure of CO 2  only high capacity absorbents, such as amines, may be used. The amine absorbents will be degraded in the presence of oxygen, that are present in the exhaust gas at a partial pressure of typically about 0.14, to form toxic waste. Additionally, amines will be released to the atmosphere. 
         [0018]    WO 2005/045316, which is included as reference in its entirety, to the present inventors, relates to a method for separation of CO 2  from the combustion gas from a thermal power plant fired with fossil fuel, and a plant for performing the method, the method comprising the steps of: 
         [0019]    a) cooling and mixing the combustion gas from the thermal power plant with air; 
         [0020]    b) compressing the combustion gas—air mixture; 
         [0021]    c) reheating the compressed gas from step b) by using it as an oxygen containing gas for combustion of natural gas in a pressurized combustion chamber to form an exhaust gas; 
         [0022]    d) regulating the supply of natural gas and oxygen containing gas in the combustion chamber so that the exhaust gas contains less than 6% rest oxygen; 
         [0023]    e) keeping the temperature in the exhaust gas between 700 and 900° C. by generation of steam in tubular coils in the combustion chamber; 
         [0024]    f) cooling the the exhaust gas and bringing it in contact with an absorbent absorbing CO 2  from the exhaust gas to form a low CO 2  stream and an absorbent with absorbed CO 2 ; 
         [0025]    g) heating the low CO 2  stream by means of heat exchanges against the hot exhaust gas leaving the combustion chamber; and 
         [0026]    h) expanding the heated low CO 2  stream in turbines. 
         [0027]    The total volume of the incoming combustion gas from the thermal power plant, and the relative low content of CO 2  therein, add both investment cost and energy cost during operation of the plant. 
         [0028]    The use of centrifugal forces for separation of gases having different molecular weight, is known. WO 2005/049175 describes the use of cyclone separators for separation gaseous mixtures comprising at least two gases having different molecular weight. U.S. Pat. No. 6,363,923 relates to a use of a centrifuge for oxygen enrichment for air for an internal combustion engine. U.S. Pat. No. 6,716,269 relates to a centrifuge and a cascade of centrifuges for separation of gases, more specifically, the publication relates to separation CO 2  and other heavy gases, such as H 2 S, from methane in produced natural gas. The separated heavy gases may be further treated and/or deposited. Additionally, centrifuges have been used for decades for separation of Uranium 235 and Uranium 238. 
         [0029]    An object of the present invention is to improve CO 2  capturing from gases comprising CO 2 . Other objects will be clearer after reviewing the present description and claims. 
       SUMMARY OF THE INVENTION  
       [0030]    According to a first aspect, the present invention relates to a method for enrichment of CO 2  from a gas mixture comprising at least CO 2  and nitrogen, comprising the steps of:
       a) introducing the gas mixture into a centrifugal separation device separating the gas mixture based on the molecular weight of the gases therein in a centrifugal field created by rotation of the gas mixture about an axis of rotation in a centrifuge or cyclone,   b) withdrawing a low CO 2  stream close to the axis of rotation and a CO 2  enriched stream is withdrawn close to the periphery of rotation,   c) releasing the low CO 2  stream into the surroundings,   d) introducing the stream enriched in CO 2  from one centrifuge or cyclone into a next centrifuge or cyclone, and   e) withdrawing a low CO 2  stream, that is released to the surroundings, and a CO 2  enriched stream from this next centrifuge or cyclone.       
 
         [0036]    The large gas volume and low concentration of CO 2  are two main obstacles in the capture of CO 2  from industrial plants and thermal power plants. Enrichment of CO 2  from a CO 2  containing gas before capturing the CO 2  makes it possible to achieve a more efficient and less expensive capture of CO 2  from the exhaust gas. 
         [0037]    Steps d) and e) may repeated one or more times to allow for a higher degree of enrichment of CO 2  to reduce the volume of the gas and to increase the partial pressure, or concentration of CO 2 , in the gas to be treated during the CO 2  capturing process, to increase the efficiency of the capturing process even more. 
         [0038]    According to a second aspect, the present invention relates to method for separation of CO 2  from a gas mixture comprising at least CO 2  and nitrogen, comprising the steps of:
       a) introducing the gas mixture into a centrifugal separation device separating the gas mixture based on the molecular weight of the gases therein in a centrifugal field created by rotation of the gas mixture about an axis of rotation in a centrifuge or cyclone,   b) withdrawing a low CO 2  stream close to the axis of rotation and a CO 2  enriched stream is withdrawn close to the periphery of rotation,   c) releasing the low CO 2  stream into the surroundings,   d) introducing the stream enriched in CO 2  from one centrifuge or cyclone a next centrifuge or cyclone,   e) withdrawing a CO 2  low stream, that is released into the surroundings, and   f) withdrawing a CO 2  enriched stream,   g) introducing the CO 2  enriched stream into a absorption device for separation into a CO 2  lean gas that is released into the surroundings, and CO 2 .       
 
         [0046]    According to this third aspect the method according to the first aspect is included in a method for capturing CO 2 , and this method includes all the advantages of this overall method. 
         [0047]    According to third aspect the invention relates to a method for separation of CO 2  from a gas mixture comprising at least CO 2  and nitrogen, comprising the steps of:
       a) introducing the gas mixture into a centrifugal separation device separating the gas mixture based on the molecular weight of the gases therein in a centrifugal field created by rotation of the gas mixture about an axis of rotation in a centrifuge or cyclone,   b) withdrawing a low CO 2  stream close to the axis of rotation and a CO 2  enriched stream is withdrawn close to the periphery of rotation,   c) releasing the low CO 2  stream into the surroundings,   d) introducing the stream enriched in CO 2  from one centrifuge or cyclone a next centrifuge or cyclone,   e) withdrawing a CO 2  low stream, that is released into the surroundings, and   f) withdrawing a CO 2  enriched stream,   g) cooling and mixing the CO 2  enriched stream with air and/or oxygen;   h) compressing the combustion gas—air/oxygen mixture;   i) reheating the compressed gas from step h) by using it as an oxygen containing gas for combustion of natural gas in a pressurized combustion chamber to form an exhaust gas;   j) regulating the supply of natural gas and oxygen containing gas in the combustion chamber so that the exhaust gas contains less than 6% rest oxygen;   k) keeping the temperature in the exhaust gas below 400° by generation of steam in tubular coils in the combustion chamber;
           f) cooling the exhaust gas and separating the exhaust gas into a CO 2  depleted stream and CO 2 .   
               
 
         [0060]    According to this third aspect, the method of the first aspect is included in another method of capturing of CO 2  than the one mentioned in the second aspect. 
         [0061]    According to the fourth aspect, the present invention relates to a plant for enrichment of CO 2  from a gas comprising nitrogen and CO 2 , the plant comprising two or more centrifuges or cyclones separating the gas based on the molecular weight thereof in a centrifugal field, means to withdraw and release into the surroundings, a low molecular weight, or low CO 2  stream close to the axis of rotation of the axis of rotation of the centrifugal field, and means to withdraw a CO 2  enriched stream from the periphery of the centrifugal field, and means to transfer the CO 2  enriched stream from one centrifuge or cyclone to a next centrifuge or cyclone in a serially connected row of two or more centrifuges or cyclones, and means to withdraw the CO 2  enriched gas stream from the last centrifuge or cyclone in the row of serially connected centrifuges or cyclones and transfer the gas to means for further treatment. 
         [0062]    According to one embodiment, the plant comprises two or more rows or serially connected centrifuges or cyclones arranged in parallel. Arranging the centrifuges or cyclones in parallel makes it possible to increase the capacity of the enrichment plant. 
         [0063]    According to a fifth embodiment, the invention relates to a plant for capturing, or separating, CO 2  from a CO 2  containing gas, the plant comprising a plant as described for the forth embodiment, for enrichment of CO 2  and absorption means to separate the CO 2  enriched gas into a CO 2  depleted stream that is released to the surroundings, and CO 2 . 
         [0064]    According to a sixth aspect, the invention relates to a plant for capturing, or separating, CO 2  from a CO 2  containing gas, the plant comprising a plant according to the forth embodiment, for enrichment of CO 2 , means for cooling and mixing the CO 2  enriched stream with air and/or oxygen, means for compressing the combustion gas—air/oxygen mixture; a combustion chamber for reheating the compressed gas by using it as an oxygen containing gas for combustion of natural gas in a pressurized combustion chamber to form an exhaust gas; means for regulating the supply of natural gas and oxygen containing gas in the combustion chamber so that the exhaust gas contains less than 6% rest oxygen; tubular coils arranged in the combustion chamber for keeping the temperature in the exhaust gas below 400° by generation of steam in the tubular coils; means for cooling the exhaust gas; and absorption means for separating the exhaust gas into a CO 2  depleted stream and CO 2 . 
     
    
     
       SHORT DESCRIPTION OF THE FIGURES  
         [0065]      FIG. 1  is a sectional view of a centrifuge, 
           [0066]      FIG. 2  is a cross section A-A in  FIG. 1 , 
           [0067]      FIG. 3  is a sectional view of a series of centrifuges, 
           [0068]      FIG. 4  is an exemplary plant including the present enrichment plant, and 
           [0069]      FIG. 5  is an alternative exemplary plant including the present enrichment plant. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0070]    Gases having different molecular weights will be separated, at least partly, by a strong gravitational field caused by e.g. a centrifuge or a cyclone separator. A typical gas to be treated according to the present invention is combustion gas from a thermal power plant, or a CO 2  containing industrial waste gas. Combustion gas mainly comprises N 2 , O 2 , and CO 2  having molecular weights of about 28, 32 and 44, respectively. In a strong gravitational field in a centrifuge or a cyclone, the heavier molecules, i.e. CO 2 , will migrate towards the periphery of the centrifugal field whereas the lighter molecules, N 2  and O 2  will migrate towards the longitudinal axis of the centrifuge or cyclone. Minor amounts of water remaining in the combustion gas after cooling and condensation, having a molecular weight of 16, will migrate towards the axis of rotation. 
         [0071]    Accordingly, by exposing the combustion gas for a centrifugal separation step, N 2  and O 2  being the lighter molecules in the combustion will be enriched along the longitudinal axis of the centrifugal field and a CO 2  depleted gas may be withdrawn from an outlet close to the axis of rotation. CO 2  will be enriched towards the periphery of the centrifugal, and a CO 2  enriched gas may be withdrawn from the periphery of the centrifuge or cyclone. The CO 2  depleted gas may be released into the air, whereas the CO 2  enriched gas, being substantially reduced in volume, preferably is further processed. 
         [0072]    The present invention will now be illustrated based on centrifuges but the principle applies correspondingly for cyclones. 
         [0073]      FIG. 1  is a longitudinal section of a centrifuge  1  for CO 2  enrichment, whereas  FIG. 2  is a cross section along A-A in  FIG. 1 . The centrifuge comprises three main parts, an inlet static body  2 , a rotary part  3  and an outlet static body  4 . The inlet and outlet static bodies  2 ,  4  are fastened to supports  6 ,  6 ′. The rotary part  3  is rotary connected to both the inlet and outlet static bodies  2 ,  4 . Labyrinth packings  8 ,  9  are provided in the rotary connections between the rotary body and the static bodies to make the rotary connections substantially gas tight. 
         [0074]    The inlet and outlet static bodies  2 ,  4  are preferably additionally connected by means of an outer shell  5  surrounding the rotary part  5 . An annular space  10  is created between the rotary part  3  and the shell  5 . The annular space  10  is closed towards the inlet and outlet parts  2 ,  4 . The annular space  10  is preferably evacuated to reduce the friction towards the rotating part  3 , by withdrawing gas from the annular space through a vacuum pipe  11 . The gas withdrawn from the annular space is gas that has leaked through the labyrinth packings at the periphery of the centrifuge, and is thus is CO 2  rich and is combined with the CO 2  rich gas for further treatment. 
         [0075]    The rotary part  3  comprises an outer tubular body  12  that is connected to an axial shaft  13  by means of rods  14 ,  14 ″,  14 ′″. The shaft  13  is rotated by means of a motor  15  that is connected to the axial shaft  13  via a dampening connection  16 . The shaft  13  is connected to the inlet and outlet static parts, respectively, by means of bearings  17 ,  17 ′. 
         [0076]    According to the embodiment illustrated in  FIG. 1 , the diameter of the shaft  13  is gradually increasing from the inlet end of the centrifuge, to have a diameter of about 50% of the diameter of the outer tubular body at a distance of about 25% from the inlet end of the centrifuge. The diameter of the shaft is thereafter gradually reduced to the same diameter as in the inlet end at a distance of about 50% from the inlet end of the centrifuge. The gas entering the centrifuge is accelerated to the rotating speed of the centrifuge in the lower part thereof. Due to the increasing diameter of the shaft, all the gas will be forced outwards from the axis of the centrifuge by the shaft. When the diameter of the shaft again is reduced, the heavier components of the exhaust gas, mainly CO 2 , will remain close to the outer tubular body, whereas the lighter components, i.e. nitrogen and oxygen, will flotate in the gravitational field of the centrifuge and thus migrate towards the center to cause a separation of the gases. 
         [0077]    The inlet static part  2 , comprises a substantially cylindrical inlet chamber  18  and an inlet pipe  19  introducing the gas to be separated substantially tangential into the inlet chamber  18  to cause the gas to rotate therein. The gas in the inlet chamber  18  is then introduced into the rotating part wherein the gas is further accelerated due to the rotation of the rotating part. Longitudinal frames  20 ,  20 ′ are preferably provided at the inside of the tubular body  12  and on the shaft  38 , to ensure that the gas in the rotating part is rotated by the rotation of the rotating part. 
         [0078]    After partly separation of constituents of the gas in the rotating part, the gas is introduced into a substantially cylindrical outlet chamber  21  in the outlet part  4 . A light gas outlet pipe  22  is provided substantially radial in the outlet chamber to withdraw the gas in a zone being closest to the axis of rotation of the rotating part and the rotating gas. A heavy gas outlet pipe  23  is provided substantially tangential to the outlet chamber to avoid disturbing the gas flow in the outlet chamber. 
         [0079]    The tubular part  12  is preferably substantially conical, having its smallest diameter towards the inlet part  2 , and its greatest diameter towards the outlet part. The conical shape will result in a gradually increased angular velocity of the flue gas with distance from the inlet end of the rotary part, thus reducing the risk of cavitations and reduce the power demand. The diameter of the light gas outlet pipe  22  is adjusted to be able to withdraw a controlled amount of the total gas volume. The diameter of the light gas outlet pipe and thus the part of the total gas that is withdrawn, depends on the design criteria, for the centrifuge, such as total gas volume, axial flow velocity, the composition of the introduced gas, etc. The opening area of the light gas outlet may thus be from about 10% to about 80%, such as e.g. from about 15 to about 70%, of the total area at the outlet chamber  21 . 
         [0080]    Two or more centrifuges are preferably serially connected, so that the gas leaving the first centrifuge through the heavy gas outlet pipe  23  is introduced into the inlet pipe of the next centrifuge. The gas withdrawn through the light gas outlet pipe, may for a combustion gas, be released into the atmosphere. When two or more centrifuges are serially connected, a portion of the gas is reduced for each centrifuge. The reduced volume of gas from one centrifuge to the next will result in a reduced axial velocity and an increased separation retention time. Accordingly, the separation is more efficient in the last centrifuge than in the previous ones for identical centrifuges. 
         [0081]      FIG. 3  illustrates four serially connected centrifuges  1 ,  1 ′,  1 ″ and  1 ′″ for CO 2  enrichment. Gas containing CO 2 , such as combustion gas from a thermal power plant, is introduced through a flue gas manifold  30  that is connected to the inlet pipe  19  of the first centrifuge  1 . CO 2  enriched gas withdrawn from the first centrifuge  1  is introduced into the second centrifuge  1 ′ through a connection line  31 , whereas the light gas is withdrawn into a light gas withdrawal line  32 . 
         [0082]    The second centrifuge  1 ′ is correspondingly connected to the third centrifuge and a light gas withdrawal line  32 ′, and so on. The light gas withdrawn from a series of centrifuges through lines  32 ,  32 ′ is collected in a light gas manifold  33  and may be released into the atmosphere. The heavy gas from the last centrifuge in a series,  1 ′″, is withdrawn trough a manifold for CO 2  enriched gas  34 . 
         [0083]    A vacuum pipe  11 ′ connects the vacuum pipes  11  from each of the centrifuge units with a vacuum pump  35 . The gas withdrawn through lines  11  is rich in CO 2  as the gas leaking into the annular space  10  comprises the heavy gas closest to the tubular body  12 . The gas from the vacuum pump is therefore preferably connected to the manifold for CO 2  enriched gas  34 , and further treated. 
         [0084]    To increase the capacity of the separation, two or more serially connected centrifuges as illustrated above may be arranged parallel. 
         [0085]      FIG. 4  illustrates a CO 2  enrichment plant integrated into a plant as described in the above identified WO 2005/045316, which is included as reference in its entirety, in which added features is introduced to reduce costs both for investments and during operation of the plant. 
         [0086]    Fuel and oxygen containing gas, such as air, are introduced into a thermal power plant  50  through a fuel line  51  and an air line  52 , respectively. The power plant may be any traditional thermal power plant for combustion of carbonaceous fuel to generate electrical power and/or heat that is exported from the plant in line(s)  53 . The power plant  50  may be fired with fossil fuels, such as natural gas or coal, or any other carbonaceous fuel, or a combination of different carbonaceous fuels. Alternatively, the thermal power plant may be substituted by an industrial process emitting CO 2 . 
         [0087]    Combustion gas from the power plant  50 , leaves through a combustion gas line  54  and is introduced into a combustion gas blower  55  to compensate for pressure drop in the system. The gas leaving the combustion gas blower is introduced into a combustion gas separator unit  56 . In the combustion gas separator unit  56 , the combustion gas is separated by means of centrifugal fields, such as by using centrifuges as described above in more detail above with reference to  FIGS. 1 ,  2  and  3 , into a CO 2  depleted fraction which is released into the atmosphere through a line  57 , and a CO 2  enriched fraction that is withdrawn through a line  58 . 
         [0088]    The gas leaving the combustion gas separation unit through line  58  is introduced into a cooler in which the combustion gas is cooled against a cooling medium that is introduced through a line  60 . The cooled and CO 2  enriched combustion gas is withdrawn from the cooler  59  in a line  61  and is introduced into a mixer  62 , in which the CO 2  enriched combustion gas is mixed with an oxygen containing gas, such as air through a line  63 , oxygen or oxygen enriched air through line  67 . Two different options for introduction of the oxygen containing gas are illustrated in  FIG. 4 . According to the first option, air is introduced into the mixer through a line  63 . A blower  64  may be provided in the line to provide the necessary pressure of the air. According to the second option, air is introduced into an air separation unit  65  through a line  66 . The air separation unit  65  may be any kind of air separation unit known in the art that can produce oxygen or oxygen enriched air. The oxygen enriched air or oxygen is withdrawn from the air separation unit through a line  67  and introduced into the mixer  62 , whereas oxygen depleted air is released into the atmosphere from the air separation unit through a line  68 . The gas in line  67  is preferably oxygen enriched air having a concentration of oxygen above 50%. The oxygen requirement is dependent on the oxygen content in the exhaust gas from the thermal power plant  50 . The exhaust gas composition is dependent on the load on the gas turbine and supplementary gas firing in the exhaust gas. A convenient, and cost effective air separation unit for this purpose is a membrane based separation unit. The two options for addition of oxygen may be combined, i.e. both air and oxygen may be introduced through lines  63  and  67 , respectively. 
         [0089]    The gas from the mixer  62  is withdrawn through a line  69  and introduced into combined plant for thermal power production and CO 2  capturing  70 . The combined plant is preferably a plant according to WO 00/57990 or WO 2004/001301, both to the same applicants, or an alternative embodiment thereof described in further detail below. 
         [0090]    The gas entering the combined plant  70  through line  69  is used as an oxygen containing gas for combustion under elevated pressure of natural gas entering the plant  70  through a line  71 . Electrical power, and optionally heat, is exported from the plant  70  in a line  72 . The exhaust gas from the combustion in the combined plant  70  is separated to a CO 2  stream leaving the plant through line  73 , and a CO 2  depleted stream that is released into the atmosphere through a line  74 . 
         [0091]      FIG. 6  illustrates an alternative embodiment of the combined plant for thermal power production and CO 2  capturing than the ones described in the above mentioned WO applications of the present applicants. An exhaust gas comprising CO 2 , N 2  and O 2 , preferably a gas that has been enriched in CO 2  by means of the present gas separation unit, is introduced through a line  34  into a heat exchanger  101 , in which the exhaust gas is cooled by heat exchanging against CO 2  depleted gas in a line  99 . The gas cooled in the heat exchanger  101  may be even further cooled in a cooler  102  before it is mixed with air or oxygen in a mixer  62 . The introduction of air of oxygen is in the figure illustrated by a line  63  and a blower  64  for introduction of air. Oxygen from a air separation unit  65  may, however, be introduced as described through a line  67 . The air separation unit and line  67  is omitted in  FIG. 5 . 
         [0092]    After mixing of air and the cooled gas from line  34 , the combined gas flow is entered into a compressing unit  103 , comprising one or more compressors and optional intercooler(s). The compressing unit is operated by a steam turbine  120 . An electrical motor  104  may be provided for starting up the compressing unit and turbine after a stop. 
         [0093]    The compressed gas mixture leaving the compressor unit  103  is carried in a line  105  and introduced into a combustion chamber  106 , where natural gas, introduced from line  71 , is combusted at an elevated pressure using the compressed gas mixture as an oxygen containing gas. The pressure in the combustion chamber is preferably between 5 and 20 bar, such as from about 8 bar to about 16 bar, such as e.g. about 10 bar. As indicated in the figure, the compressed gas may be introduced into a mantle  106 ′ surrounding at least the combustion chamber, to cool the outer walls of the combustion chamber and to heat the compressed gas before it is introduced into the combustion chamber. 
         [0094]    In the combustion chamber  106  steam is generated in a closed tubular system connecting a water inlet line  107  and a steam line  108 . It is preferred that the temperature in the combustion is reduced by generation of steam so that flue gas leaving the combustion chamber through a flue gas line  115 , has a temperature below about 400° C., e.g. about 350° C. By reducing the temperature of the flue gas to about 350° C., the requirement for high cost, high grade steel for the succeeding equipment is omitted. An exemplary combustion chamber for use in the present plant is described with reference to FIG. 3 in WO2004001301. 
         [0095]    The steam leaving the combustion chamber through the steam line  108 , is expanded over one or more steam turbine(s)  109 . Electrical power is generated from the steam turbine(s)  109  in an electrical generator  110 . 
         [0096]    Low temperature steam is released from the turbine(s)  109  in a line  112  and is cooled and finally condensed in one or more heat exchanger(s)  113  and cooler(s)  114 . The condensed water may again be partly reheated in an economizer  121 , before the water again is reintroduced into the combustion chamber through line  107 . 
         [0097]    A part stream of the steam in steam turbine  109  may be withdrawn through a line  122  to be introduced into the turbine  120  to operate the compressor unit  103 . Partly expanded steam from the turbine  120  may then again be reintroduced into the turbine  109  through a line  123   
         [0098]    The flue gas that is withdrawn through line  115  may, if required, be introduced into a selective catalytic reduction (SRC) unit  116  in which an aqueous solution of a reductant, such as ammonia or urea, is introduced through a line  117  in a way known by the skilled man in the art. The temperature of the flue gas is reduced by evaporation of the water including the reductant in the SCR unit  116 . The temperature of the flue gas is further reduced in the above mentioned economizer  121  downstream to the SCR unit, against condenced water from the heat exchanger  113  and cooler  114 . The flue gas leaving the economizer has a temperature of about 170° C., and is withdrawn through a line  124 . 
         [0099]    The flue gas in line  124  is split in a line  125  which is further cooled in a heat exchanger  127 , and a line  126  which is further cooled in a cooler  128 . Both streams are cooled to a temperature of about 90° C., before they are introduced into a condenser  129  before being introduced into a CO 2  capturing plant  130 . 
         [0100]    The CO 2  capturing plant  130  is of the adsorption/resorption type as described in the above mentioned WO publications from the applicant. The preferred absorbent is an aqueous carbonate solution. 
         [0101]    CO 2  that is captured in the plant is withdrawn through a line  131  for export from the plant, whereas the not captured gas, being low in CO 2  is withdrawn through a line  132 . 
         [0102]    The gas in line  132  is heated in the heat exchanger  127  against the gas in line  125 , and is expanded over a turbine  133  to produce electricity in a generator  134 . The low temperature gas is withdrawn from the turbine  133  in a line  99  and may be used to cool the incoming gas in line  34  in the heat exchanger  101  before the gas is released into the atmosphere through the line  74 . 
         [0103]    An important feature with the plant described with reference to  FIG. 6  is that the flue gas in the combustion chamber is cooled to below 400° C. before leaving the combustion chamber. Accordingly, the gas turbine that is described as essential parts in above mentioned WO publications of the present applicants, may be omitted without loosing efficiency. This makes it practically possible to scale the plant more easily for different purposes. 
         [0104]    The efficiency of absorption in the CO 2  capturing plant is assumed to be proportional to the partial pressure of CO 2  up to the maximal absorption capacity of the plant. The CO 2  capturing plant is dimensioned to the CO 2  load from the combusted carbonaceous fuel. By reducing the total gas load and increasing the concentration, or partial pressure, of CO 2  the efficiency of the capturing becomes higher than it would be for the untreated exhaust gas at atmospheric pressure. This makes it possible to use carbonates as absorbents in stead of the more efficient amines. Carbonates are relatively inexpensive, does not give raise to toxic waste or unpleasant smell from the plant. Additionally, the carbonates are not, as the amines, prone to deactivation by oxygen and other constituents of the exhaust gas. 
       Examples 
       [0105]    The present invention will now be described in further detail with reference to plants in which the present invention may form a part to increase the CO 2  capturing or to reduce the cost thereof. 
       Example 1  
       [0106]      FIG. 5  illustrates a combined heat and power plant  80  connected to a CO 2  enrichment plant  81 . The combined power and heat plant  80  is illustrated by a gas turbine  82 , heat exchange means  83  and a flue gas manifold  30 . A plurality of centrifuges  1 ,  1 ′,  1 ″,  1 ′″ are connected in centrifuge trains in which four centrifuges are connected in series. Several compressor trains are parallel arranged between the flue gas manifold and a light gas manifold  33  and a manifold for CO 2  enriched gas  34  as described with reference to  FIG. 3 . 
         [0107]    The plant according to this example comprises two units according to  FIG. 5 , in a plant as illustrated in  FIG. 4 . Each unit according to  FIG. 5 , comprises a 130 MW gas turbine, resulting in a exhaust gas flow of 400 m 3 /sec, giving a total effect of 260 MW and a total exhaust gas volume of 800 m 3 /sec. 
         [0108]    Each unit has its own CO 2  enrichment plant  81  comprising 5 parallel centrifuge trains, each with four centrifuges in series. The centrifuges are preferably identical with the exception of the diameter of the light gas outlet pipe  22 . 
         [0109]    The rotary part of each centrifuge is a conical tubular construction having a mean diameter of 3.30 m, a maximum diameter of 3.60 m and an effective height of 20 m. The rotary part is rotated at a speed of 1800 rpm to give a maximum radial force of 6513 G. 
         [0110]    Due to the large radial G-force, the tubular body must be built of a material with light weight and high strength. An example of a material that may be used for the tubular body  12  is a titan alloy of the composition TiAl5Cr2Mo2. 
         [0111]    The partial pressure of gaseous components in a centrifuge, assuming a solid body rotation, is given from the Maxwell-Bolzmann Distribution Law: 
         [0000]      α=exp(( M   2   −M   1 )(Ω r ) 2 /2 RT ), where   Eq. 1 
         [0112]    M 2  and M 1  are the molecular weight of CO 2  and N 2 , i.e. 44 and 28, respectively. 
         [0113]    Ω=angular velocity 
         [0114]    r=radius from center of rotation 
         [0115]    R=gas constant 
         [0116]    T=temperature (° K) 
         [0117]    This results in the following equation for exhaust gas separation: 
         [0000]      α=exp((44−28)(188.4×1.80) 2 /2×8314.3×293=1.46 
         [0118]    Using a centrifuge where the diameter of the shaft gradually increases the first 25% of the length of the centrifuge, i.e. the first 5 meters, and then decreases again, the effective axial separation length of the centrifuge is 15 meters. The flow delivered to the first centrifuge in the centrifuge train is 400 m 3 /sec/5=80 m 3 /sec. The average cross section area in the effective part of the centrifuge is 9.14 m 2 , and the axial exhaust gas velocity is 8.75 m/sec, giving an exhaust gas retention time in the effective part of the centrifuge of 1.71 m/sec. 
         [0119]    Based on the gas concentration ratio and the retention time as calculated above, it is expected that a separation ratio of 20% can be achieved for N 2  and O 2 , together with 2% of the CO 2  for the first of the serially connected centrifuges. For the succeeding serially connected centrifuges the retention time will increase due to reduced mass flow. 
         [0120]    Table 1 is an overview over critical parameters in one train of centrifuges. 
         [0000]    
       
         
               
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Centrifuge No. 
                   
               
             
          
           
               
                   
                 I 
                 II 
                 III 
                 IV 
                 Total 
               
               
                   
                   
               
             
          
           
               
                 Flue gas to centrifuge 
                 80 
                 62 
                 44 
                 26 
                   
               
               
                 inlet 
               
               
                 Nitrogen rich gas to 
                 18 
                 18 
                 18 
                 18 
                 72 
               
               
                 atmosphere (m 3 /s) 
               
               
                 CO 2  enriched gas 
                 62 
                 44 
                 26 
                 8 
               
               
                 out of centrifuge 
               
               
                 (m 3 /s) 
               
               
                 Axial flow velocity in 
                 8.6 
                 6.7 
                 4.7 
                 2.8 
               
               
                 centrifuge (m/s) 
               
               
                 Retention time (s) 
                 1.7 
                 2.2 
                 3.2 
                 5.4 
                 12.5 
               
               
                 Separation ratio 
                 0.22 
                 0.29 
                 0.41 
                 0.69 
               
               
                 Nitrogen rich gas 
                 2.04 
                 2.69 
                 3.81 
                 6.41 
               
               
                 exit area (22) (m 2 )* 
               
               
                 Power requirement 
                 440 
                 341 
                 242 
                 143 
                 1166 
               
               
                 (motor) (KW) 
               
               
                 Temperature 
                 4.5 
                 3.5 
                 2.5 
                 1.5 
                 12 
               
               
                 increase (° C.) 
               
               
                 Concentration 
                 1.02 
                 1.03 
                 1.04 
                 1.06 
                 1.158 
               
               
                 ratio O 2 /N 2 ** 
               
               
                   
               
               
                 *Pipe segment arranged upstreams line 22 
               
               
                 **Eq. 1 
               
             
          
         
       
     
         [0121]    The composition of the exhaust gas entering the first centrifuge in the centrifuge train (centrifuge I); 80 m 3 /sec, is as follows:
       O 2  8.80 m 3 /sec, 9.68 kg/sec   CO 2  2.84 m 3 /sec, 4.35 kg/sec   N 2  68.36 m 3 /sec, 66.30 kg/sec       
 
         [0125]    Ratio O 2 /N 2 =8.80/68.36=0.129 
         [0126]    The composition out of the last centrifuge in the centrifuge train (centrifuge IV), 8 m3/sec, delivered to line  34 , is as follows:
       CO 2  2.67 m 3 /sec   N 2 +O 2 ; 8.00−2.67 m 3 /sec=5.33 m 3 /sec       
 
         [0129]    Ratio O 2 /N 2  adjusted for concentration ratio; 
         [0130]    Ratio O 2 /N 2 =0.129×1.158 kg/sec=0.149
       O 2 ; 0.79 m 3 /sec-0.87 kg/sec   Total flow of separated exhaust gas; 80.0 m 3 /sec   O 2  content in the separated exhaust gas; 7.9 m 3 /sec   Combustion flow in line  69  required for the combined plant for thermal power production and CO 2  capturing 70 of 100 MW; 96 m 3 /sec   O 2  required in line  69  for introduction into plant  70 ; 20.2 m 3 /sec   Additional O 2  required; 20.2−7.9 m 3 /sec=12.3 m 3 /sec, to be added as:
           Additional air in line  63 : 6.2 m 3 /sec—O 2 ; 1.3 m 3 /sec   Additional O 2  in in  67 ; 11.0 m 3 /sec, 12.1 kg/sec.   
               
 
         [0139]    The amount of O 2  required for combustion in the plant  70  is governed by the load on the power plant  50 , and the supplementary gas firing in the plant  70 . The supplementary firing is not energy efficient, and could be replaced by low energy heat delivered from the plant  70 . 
         [0140]    The combustion and CO 2  capturing in the plant  70  is operated under elevated pressure as described in further detail in WO 2005/045316, such as e.g. at 11 bar. 
         [0141]    The CO 2  partial pressure in line  69 , i.e. at substantially atmospheric pressure, is 26.7/96.0=0.278 
         [0142]    The partial pressure of CO 2  in the gas that is introduced into the CO 2  absorption column is 3.06 bar. 
         [0143]    The partial pressure ratio for CO 2  in the exhaust gas introduced into the absorption column absorption to the CO 2  in the exhaust leaving the thermal power plant  50  is 3.06/0.04=76.5. 
         [0144]    The CO 2  enriched gas in line  34 ,  58 , may then be introduced as an oxygen containing gas into a combined CO 2  capturing plant and power plant  70  as described in WO 
       Example 2  
       [0145]      FIG. 7  illustrates an exemplary plant for treatment of flue gas from an aluminum factory  150 . Flue gas from the aluminum factory  150  is introduced as a fuel into a ground burner  151  through a line  152 . The flue gas contains CO that is used as fuel in the ground burner and is burned at a temperature above 700° C. Air is introduced through an air line  153  into a cooling mantle  154  partly surrounding the ground burner  151 , an exhaust gas line  155  for transferring exhaust gas from the ground burner to a economizer  156 , and the economizer to cool the economizer, the transfer line and a part of the ground burner. The heated air in the mantle is then introduced into the ground burner as oxygen containing gas for the combustion therein. 
         [0146]    A gas burner  157 , fed by natural gas from gas line  158  and air from air line  159 , is provided in the ground burner  151 , both to ignite the ground burner to start the combustion of CO, and to withheld a temperature high enough to ascertain combustion of CO. 
         [0147]    The flue gas leaving the economizer  156  through a line  160 , is cooled by means of heat exchangers/coolers  161  before it is introduced into a CO 2  enrichment plant  56 , comprising centrifuges as illustrated above. In the CO 2  enrichment plant, the low weight gas, that is low in CO 2 , is released to the atmosphere through line  57 , whereas the CO 2  enriched gas is withdrawn through line  58 . The gas from line  58  is mixed with air from line  63 , or with oxygen or oxygen enriched air from an air separation unit  65 , in a mixer  62 . After the mixing with air the flue gas is introduced into a combined plant  70  as describe above. A fluorine abatement unit  162  is preferably provided to remove fluorine from the flue gas before the CO 2  enrichment unit. 
         [0148]    In an exemplary aluminum factory  150 , a total flue gas flow of 555 m 3 /s is produced, the flue gas comprising about 4% CO and 8.6 kg/sec CO 2 . Traditionally, the flue gas is released into the atmosphere after removal of fluorine. The CO in the flue gas that represents a potential useable energy potential of about 150 MW, is released into the air where it is further reacted with oxygen to form CO 2 . 
         [0149]    The combustion of CO will generate about 34.0 kg/sec of CO 2  and will together with the CO 2  already in the flue gas, give a total of 42.6 kg/sec or 1.343 Mt/y. It is expected that the price of quota for release of CO 2  will be about NOK 100/t. Having CO 2  quota at this price combined with the price of electrical energy and heat energy from the plant  70 , will give a net income in the order of about NOK 270 Mill per year at an estimated investment of 3.0 billion NOK.