Abstract:
A method for separation of CO 2  from the combustion gas of a gas turbine comprising the steps of: withdrawing the combustion gas at an intermediate stage of the turbine, introducing the withdrawn combustion gas into a burner together with compressed air and additional carbonaceous fuel to cause a secondary combustion therein, cooling the combustion gas from the burner, introducing the cooled combustion gas into a CO 2  capturing unit, to separate the combustion gas into a CO 2  rich gas, that is withdrawn for deposition, and a CO 2  lean gas, and reheating and reintroducing the CO 2  lean gas into the turbine at an intermediate level and further expand the gas before it is released into the atmosphere, is described. A power generation plant utilizing the method is also described.

Description:
THE FIELD OF THE INVENTION 
       [0001]    The present invention relates to a method and a plant for capturing CO 2  that may be implemented on an existing gas turbine power plant. The invention also relates to a gas turbine power plant including the inventive CO 2  capturing, or CO 2  abatement, plant. 
       BACKGROUND 
       [0002]    The last years, or the last decade, the increasing concentration of CO 2  in the atmosphere due to increased combustion of fossil fuel, has caused great concern. The increased greenhouse effect caused by the increasing concentration of CO 2  is expected to cause a substantial temperature increase at the planet earth and an enormous environmental impact in the next few decades. 
         [0003]    Actions therefore have to be taken to stabilize the CO 2  concentration in the atmosphere. A substantial part of the man made CO 2  emission is a result of power generation from oil, gas or coal. Accordingly, substantial efforts have been made to develop thermal power plants including capturing of CO 2 . Captured CO 2  may be safely deposited into geological formations such as e.g. depleted oil or gas wells, or may be used as pressure support for increasing the production of oil or gas. 
         [0004]    The suggestions for CO 2  capture mainly follow three lines of development:
       Post-combustion or “end of pipe” absorption of CO 2  from the exhaust gas from a thermal power plant,   Pre-combustion conversion of fuel where fossil fuel is converted mainly to hydrogen and CO 2  in reformers. The product from the reformers contains CO 2  at a high partial pressure and this CO2 is therefore relatively easy to separate from the hydrogen that is to be used as fuel, and   Oxy-fuel systems where oxygen obtained by air separation is used together with CO2, replacing the normal air supply. This eliminates N2 from the system and increases the partial pressure of CO2, facilitating the separation of CO2.       
 
         [0008]    Much effort has been done on post-combustion absorption of CO 2 , both due to the fact that this technology is most developed and that the degree of integration with the power plant is small. Post combustion systems may be implemented on existing plants. 
         [0009]    The very low partial pressure of CO 2  in the exhaust gas from a thermal power plant is, however, a major problem in making the CO 2  capture economically acceptable. Absorbers become much larger than can be guaranteed with current technology, impeding the absorbent distribution in the absorber column and thus reducing the absorption efficiency. Additionally, the residual O 2  concentration in the exhaust gas of most fossil fuel based thermal power plants is relatively high. This causes degradation problems for the required organic absorbent in the CO 2  capturing system and, depending on the absorbent selectivity, potential contamination of the CO 2  product. 
         [0010]    Pre-combustion conversion of fossil fuel to hydrogen is attractive because the reforming products are pressurized with high concentration of CO2. The CO2 is therefore much easier to capture than in post combustion systems. Conventional pressurized absorption columns may be employed. Disadvantages with the process include very complex processes for coal gasification, and the need to develop gas turbines for hydrogen fuel. 
         [0011]    Similar to the pre-combustion conversion of fossil fuel, oxy-fuel systems produce relatively high partial pressures of CO2. The CO2 therefore becomes much easier to capture than for post combustion systems. Disadvantages with the system include the need for very large and expensive air separation units, high energy requirement for oxygen production, and the new technologies required to use CO2 instead of nitrogen to cool flame temperatures. New gas turbines are also required when CO2 replaces nitrogen in the motive fluid. Risks of leaks and fire involving pure oxygen from the oxygen production unit will require large spatial separation between the oxygen plant and the power plant. This requirement increases the total area needed and will in particular increase the cost of offshore applications. Furthermore, the produced CO2 will contain unburned oxygen. This oxygen must be separated from the CO2 prior to for example injection in oil fields for enhanced oil recovery. 
         [0012]    WO 2004/001301, which is included as reference in the present application, relates to a low CO 2  emission thermal power plant. CO 2  is absorbed from the combustion gas from a combustion chamber in an absorber, wherein a liquid absorbent flows countercurrent to the combustion gas. This enriches the absorbent in CO2. The rich absorbent is regenerated by heating and stripping in a regeneration column by countercurrent flow to steam generated in a reboiler connected to the lower part of the regeneration column, to produce a stream of CO 2  that is exported from the plant for deposition, and regenerated absorbent that is returned to the absorber. The partial pressure of CO 2  is increased and the volume flow of flue gas to be purified is decreased, relative to the power produced, by substantially complete combustion of both oxygen and fuel in a pressurized combustion chamber. This improves the capture of CO 2 , which occurs at high pressure. WO 2004/001301 is, however, suitable either for thermal power plants having a pressurised combustion chamber for production of steam, or for new plants. 
         [0013]    WO 2005/045316 relates to a purification works for a thermal power plant, where the combustion gas from an existing thermal power plant is used as all, or a substantial part of, the oxygen containing gas that is introduced into a plant built at the basic principle of WO 2004/001301, to capture the CO 2  from both plants and increase the total production of electrical power, at the same time. A highly efficient gas turbine is used as a primary power plant. Air is first compressed in a primary power plant compressor, then heated and expanded to atmospheric pressure. A secondary power plant provides additional power and carries out CO2 capture under pressure. The main shortcoming of this technology is the need to re-compress the gas from the primary unit. Such re-compression requires significant work and causes loss of thermal efficiency. 
         [0014]    There is, however, need for a system which as far as possible uses the advantages of the highly efficient, high temperature gas turbines in combination with pressurized CO2 capture, without the need to re-compress gas from atmospheric pressure. 
         [0015]    An objective is therefore to provide an improved method and plant for capturing CO 2  from a gas turbine. It is also an objective to provide solution that is suitable for post-installation for an existing gas turbine or combined cycle power plant. 
       SUMMARY OF THE INVENTION 
       [0016]    According to a first embodiment, the present invention relates a method for separation of CO 2  from the combustion gas of a gas turbine where carbonaceous fuel and compressed oxygen containing gas are combusted, and the combustion gas is expanded over a turbine to produce electrical power in a generator before the expanded combustion gas is released into the atmosphere, the method additionally comprises the steps of:
       a) withdrawing the combustion gas at an intermediate stage of the turbine,   b) introducing the withdrawn combustion gas into a burner together with compressed air and additional carbonaceous fuel to cause a secondary combustion therein,   c) cooling the combustion gas from the burner,   d) introducing the cooled combustion gas into a CO 2  capturing unit, to separate the combustion gas into a CO 2  rich gas, that is withdrawn for deposition, and a CO 2  lean gas, and   e) reheating and reintroducing the CO 2  lean gas into the turbine at an intermediate level and further expand the gas before it is released into the atmosphere.       
 
         [0022]    Withdrawing partly expanded gas from an intermediate level of the turbine and introduction of the partly expanded, gas that are still at an elevated pressure, into the burner for a second combustion, allows for a combustion and succeeding CO 2  capture at an elevated pressure. Additionally, the heat and pressure energy of the gas that is withdrawn is at least partly conserved and is used by reheating of the CO 2  lean gas and expanding the same over the turbine, 
         [0023]    According to one embodiment, the method additionally comprises the step of cooling the withdrawn combustion gas of step a) before the gas is introduced into the burner in step b). Cooling of the gas before it is introduced into to burner reduces the temperature of the flare in the burner, as the flare otherwise may become too hot and produce high levels of NOx. Additionally high temperatures may result in problems related to the materials of the components of the plant. 
         [0024]    The secondary combustion in the burner adds mass to the total gas flow to substitute the mass of CO 2  that is removed from the total mass of gas. Performing this combustion and the CO 2  capture downstream of the burner reduces the oxygen level in the gas and increases the CO 2  level, which both are important for the efficiency of the capturing process. Re-heating of the CO 2  depleted gas and expanding the gas over the turbine increases the energy efficiency of the plant considerably. 
         [0025]    According to a second aspect, the present invention relates to a plant for generation of power comprising a gas turbine, and a generator operated by the gas turbine, wherein the plant additionally comprises a gas side draw unit for withdrawal of partly expanded gas from an intermediate stage of the turbine, a burner for a secondary combustion of fuel, using the partly expanded gas and additional air as sources for oxygen, one or more heat exchanger(s) for cooling the combustion gas from the secondary combustion, CO 2  separation unit for separation of the cooled combustion gas into a CO 2  rich gas that is treated further and exported from the plant, and a CO 2  lean gas, one or more heat exchanger(s) for reheating the CO 2  lean gas, and gas a gas return line and a turbine inlet unit for introduction of the heated CO 2  lean gas at an intermediate level of the turbine for further expansion. 
     
    
     
       SHORT DESCRIPTION OF THE FIGURES 
         [0026]      FIG. 1  is a principle sketch of a combined cycle gas powered power plant according to the state of the art, 
           [0027]      FIG. 2  is a principle sketch of an embodiment of the present invention, 
           [0028]      FIG. 3  is a graph illustrating the net power output from a power plant according to the invention as a function of gas turbine load relative to total duty, 
           [0029]      FIG. 4  is a graph illustrating the net electric efficiency from a power plant according to the invention as a function of gas turbine load relative to total duty, 
           [0030]      FIG. 5  is a graph illustrating the residual oxygen in the exhaust gas to be purified in a power plant according to the invention as a function of gas turbine load relative to total duty, 
           [0031]      FIG. 6  is a graph illustrating partial pressure of CO 2  in the exhaust gas to be purified in a power plant according to the invention as a function of gas turbine load relative to total duty, and 
           [0032]      FIG. 7  is a graph illustrating the actual volume of exhaust gas to be purified in a power plant according to the invention as a function of gas turbine load relative to total duty. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0033]      FIG. 1  illustrates a combined cycle gas turbine power plant  1  according to prior art. The prior art plant will be discussed as the present invention relates to a method and modification for capturing CO 2  from a power plant based on a combined cycle power plant. The term “gas turbine” is in the present invention used for a unit comprising a compressor  2 , a combustion chamber  8  and turbine  4  mechanically connected to the compressor, most preferably connected on a common shaft  11 . A “turbine” is used in the meaning of an expansion unit for converting of the energy of high temperature gas to rotational energy. 
         [0034]    The terms “carbonaceous fuel” or “fuel” are in the present invention used for fuel suitable for a gas turbine such as natural gas, fluid hydrocarbons and oxygenated hydrocarbons such as methanol, ethanol etc., that will be in gas phase in the combustion chamber of a gas turbine, or gasified fuels such as gasified coal, gasified coke, gasified organic materials etc. 
         [0035]    Air is introduced into the compressor  2  through an air inlet line  3 . The compressed air from the compressor  2  is introduced into a combustion chamber  8  via a compressed air line  7 . Fuel, such as e.g. natural gas, is introduced into the combustion chamber through a gas line  9 . Combustion gas from the combustion chamber is led through a combustion gas line  10  and is expanded over a turbine  4  before the expanded gas is released through an exhaust gas line  12 . 
         [0036]    As indicated in the figure, the compressor  2 , turbine  4  and a generator  5  for production of electric power, are arranged on a common shaft  11 . 
         [0037]    The exhaust gas in the exhaust gas line  12  is still hot, typically from 500 to 600° C., and is cooled by means of one or more heat exchanger(s)  13  to produce steam and cooled exhaust gas that is released into the surroundings through an exhaust outlet  12 ′. 
         [0038]    The steam produced in the heat exchanger(s)  13  is led in a steam line  14  into a steam turbine  15  where the steam is expanded. A generator  16  is connected to the steam turbine for production of electrical power. 
         [0039]    The expanded steam is led in an expanded steam line  17 , cooled on a cooler  18 , suitably against water, to condense the steam. The condensate is pumped by means of a pump  19  trough a water line  20  and is reintroduced into the heat exchanger(s)  13 . 
         [0040]    Typically, about 75 to 80% of the electrical power from a combined cycle power plant is generated in the generator  5  operated by the turbine  4  and the rest in generator  16 , operated by the steam turbine  15 . 
         [0041]      FIG. 2  illustrates a plant according to the present invention, comprising a modified combined cycle gas turbine part A and a CO 2  abatement part B. 
         [0042]    Parts corresponding to parts described with reference to  FIG. 1  are referred to using the same reference numbers as used for  FIG. 1 . 
         [0043]    The turbine  4  normally comprises a high pressure turbine  4 ′ and a low pressure turbine  4 ″. According to the present invention, partly expanded combustion gas is withdrawn from the turbine at an intermediate level of expansion, suitably between the high pressure  4 ′ and low pressure  4 ″ turbines, into a gas withdrawal line  20 . A gas side draw unit  21  is preferably inserted at the shaft  11 , after the high pressure turbine to facilitate the withdrawal of the partly expanded gas. The pressure at the point of gas withdrawal is for example in the range from 6 to 16 bara, such as 10 to 14 bara. 
         [0044]    The partly expanded gas in line  20  is combined with pressurized and heated air and introduced into a cooling chamber, where the combined gas is cooled by heating steam and/or generating of steam in a heating tube  22  in a primary cooling chamber  23 . The gas mixture entering the cooling chamber has a temperature of about 1000° C. and is cooled therein to a temperature of about 400 to 500° C. The combined and cooled gas in cooling chamber  23  is then introduced to a secondary cooling chamber  24  through a burner  25  where the combined gas is mixed with fuel gas that is introduced through a secondary fuel line  26 . Air enters through an air supply line  52  and is compressed in a compressor  53  operated by means of an electric motor  54 . 
         [0045]    The compressed air is supplied through lines  55  and  55   a  and used to protect pipes  20  and  39  and to cool the pressure container  50 , before flowing to the secondary burner for firing purposes. Some of the air is supplied through line  55   b  and routed directly to combustor  25 . The total amount of air from compressor  53  is adjusted relative to the captured CO2 withdrawn in line  34 , so that the volume flow of gas to the gas turbine through line  39  is the same as, or very close to, the volume flow of gas withdrawn from the gas turbine through line  20 . The fuel introduced into the secondary burner is adjusted so that the combustion in the secondary combustion chamber  24  is substantially complete, both with regard to oxygen and fuel. 
         [0046]    The combustion gases in the secondary cooling chamber are cooled by heating gas in a gas heating tube  27  and by superheating of steam from the heating tube  22  in a superheater tube  28 . Heating tube  22  is connected to superheating tube  28  though a line  14   a . The superheated steam in the superheating tube  28  is withdrawn through a line  14   b  and introduced into steam turbine  15  to produce electrical energy, condensed and returned to the heat exchanger  13  as described above with reference to  FIG. 1 . 
         [0047]    Exhaust gas from the secondary cooling chamber  24  is withdrawn through an exhaust line  29  and is cooled in a heat exchange assembly  30 . Preferably a SCR (Selective Catalytic Reduction unit) or SNCR (Selective Non-Catalytic Reduction unit)  31  is provided in the heat exchange assembly  30  to remove NO x  from the exhaust gas. 
         [0048]    The cooled gas from the heat exchange assembly  30  is withdrawn through a line  32  and introduced into a CO 2  separation unit  33 . The CO 2  separation unit  33  is a standard unit according to the state of the art, e.g. a separation unit as described in WO 00/57990, where CO 2  in the CO 2  containing gas is absorbed by countercurrent flow to a liquid absorbent in an absorber to produce a CO 2  lean stream that is withdrawn through a line  35 . The CO 2  loaded absorbent is thereafter regenerated to produce a stream of CO 2  that is dried and compressed and is withdrawn through line  34  for export from the plant, and regenerated absorbent that is returned to the absorber. The absorbent may be any conventionally used absorbent, such as aqueous solutions of amines, amino acids, carbonates etc. The CO 2  capture unit may also include gas scrubbing and a direct contact gas cooler upstream of the CO 2  capture unit. 
         [0049]    A pressurized mantle  41  is preferably covering the high pressure and high temperature lines  20 ,  39 . The mantle surrounding lines  20  and  39  is pressurized using air from a branch line  55   a  dividing from the compressed air line  55 . The mantle reduces the pressure difference across the hot inner pipe wall, thus reducing the wall thickness and possibilities for cracks during temperature transients. Heated air from inside the mantle  41  is led from the mantle  41  to the mantle  50  through a line  42 . 
         [0050]    If necessary, additional air for the combustion in the secondary combustion chamber may be introduced through a second branch line  55   b  dividing from the compressed air line  55 , to deliver additional air to the burner  25 . This additional air has higher oxygen content than the air in line  20 , and will stabilize the flame in one or more of the burners  25 . 
         [0051]    The CO 2  lean stream in line  35  is compressed in one or more compressor(s)  36  operated by motor(s)  37 , and is thereafter heated in the heat exchange assembly  30  towards the warm gas that is introduced through line  29 . The heated CO 2  lean stream leaves the heat exchange assembly through a line  38  leading to the gas heating tube  27 , where the gas is heated by the combustion gases from burner  25 . The CO 2  lean gas leaves the gas heating tube  27  and is introduced into a gas return line  39  that is connected to a turbine inlet device  40  that is arranged on the shaft  11 . The gas introduced to the inlet device  40  is then expanded over the low pressure turbine  4 ″ and released into the exhaust gas line  12  as described with reference to  FIG. 1 . 
         [0052]    Closing valves  45 ,  46  on lines  20 ,  39  respectively, and a shortcut line  43  with a shortcut valve  44 , are preferably provided to close line  20  and  39  and to allow the flow from the gas side draw unit  21  to flow directly into the gas inlet device  40  if necessary. 
         [0053]    To balance the turbine correctly, the pressure, temperature and flow of the gas leaving the high pressure turbine  4 ′ through line  20  should substantially be the same as the pressure, temperature and flow of the gas entering the low pressure turbine  4 ″. The combustion in the secondary cooling chamber  24  adds temperature to the total gas, and especially to the CO 2  lean stream in line  38 , and adds mass to the total gas to at least party compensate for the mass loss due to the removal of CO 2 . Additionally, heat is added to the steam cycle making it possible to increase the power production from the plant compared with the exemplary combined cycle plant according to  FIG. 1 . 
         [0054]    Table 1 illustrates typical temperatures, mass flow and pressure, in addition to produced or consumed power for a typical combined cycle plant producing about 500 MW electrical power according to  FIG. 1 . 
         [0000]    
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Temp. 
                 Mass flow, 
                 Pressure, 
                 Produced/consumed 
               
               
                 Part No. 
                 ° C. 
                 kg/s 
                 bara 
                 electric power, MW 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                  3 
                 20 
                 869 
                 1 
                   
               
               
                  5 
                   
                   
                   
                 390 
               
               
                  9 
                   
                 18.3 
               
               
                 10 
                 1343 
                 887.3 
                 30 
               
               
                 12 
                 520 
                 887.3 
                 1.04 
               
               
                     12′ 
                 88 
                 887.2 
                 1 
               
               
                 14 
                 500 
                   
                 165 
               
               
                 16 
                   
                   
                   
                 110 
               
               
                   
               
             
          
         
       
     
         [0055]    Table 2 illustrates typical temperatures, mass flow and pressure, in addition to produced or consumed power for a typical plant with CO 2  capture according to the present invention, based on the combined cycle plant illustrated in table 1. 
         [0000]    
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                   
                 Temp. 
                 Mass flow, 
                 Pressure, 
                 Produced/consumed 
               
               
                 Part No. 
                 ° C. 
                 kg/s 
                 bara 
                 electric power, MW 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                  3 
                 20 
                 869 
                 1 
                   
               
               
                  5 
                   
                   
                   
                 390 
               
               
                  9 
                   
                 18.3 
               
               
                 10 
                 1343 
                 887.3 
                 30 
               
               
                 12 
                 520 
                 834.3 
                 1.04 
               
               
                     12′ 
                 88 
                 834.3 
                 1 
               
               
                  14b 
                 565 
                   
                 165 
               
               
                 16 
                   
                   
                   
                 828 
               
               
                 20 
                 1013 
                 887.3 
                 10 
               
               
                 26 
                   
                 31 
               
               
                 52 
                   
                 51 
               
               
                 34 
                   
                 135 
               
               
                 39 
                 13 
                 834.3 
                 10 
               
               
                 37 
                   
                   
                   
                 −13 
               
               
                 54 
                   
                   
                   
                 −17 
               
               
                 33 
                   
                   
                   
                 −84 
               
               
                   
               
             
          
         
       
     
         [0056]      FIGS. 3 to 7  illustrates a plant according to the present invention as described with reference to  FIG. 2  (filled circles connected with a solid line) and a comparative example according to  FIG. 1  is done with a 78% gas turbine load and a 22% steam turbine load for a standard combined cycle plant (solid square) 
         [0057]      FIG. 3  illustrates the net electric power from a plant according to  FIG. 2 , as a function of gas turbine load, included CO 2  capture and compression. The figure illustrates that the net electrical power output is reduced as the relative load on the gas turbine increases. The difference between the solid line for the present system including CO 2  capture and the comparative example is the electric output cost for the CO 2  capture. The production of electrical power from the gas turbine is constant, whereas the production from the steam turbine increases. The increased power production improves the lifetime production and economy of the plant. 
         [0058]      FIG. 4  illustrates the net electrical efficiency as a function of the relative loads of the gas turbine and the steam turbine, including CO 2  capture for the plant according to the present invention. The difference between the solid line representing the present invention and the comparative example is the cost for CO 2  capture. The curve also illustrates that net electrical efficiency is reduced as the relative load of the gas turbine is reduced, as the steam turbine part of the process is less efficient than the gas turbine part. 
         [0059]      FIG. 5  illustrates the effect of the relative load on gas turbine and steam turbine on the residual oxygen content in the exhaust gas, or the gas to be treated by CO 2  capture. The curve clearly illustrates that the oxygen concentration is reduced with increasing steam turbine load. A low O 2  concentration is advantageous for the quality of the captured CO 2 . Oxygen present in the gas to be purified will be partly captured and will contaminate the CO 2 . CO 2  having a too high concentration of oxygen has to be further purified before deposition, a process that will add cost to the process. 
         [0060]      FIG. 6  illustrates the partial pressure of CO 2  at the point of capture (i.e. in the exhaust gas in line  32  for the present invention, and line  12 ′ for the comparative example). The higher the steam turbine load is, the higher is partial pressure of CO 2 . The difference between CO 2  partial pressure in the present plant and the comparative example, at a low steam turbine load, is due to the higher total pressure (of about 10 bara) in the combustion gas in line  32 , compared with 1 bara in line  12 ′. A higher partial pressure improves the CO 2  capture and enables the use of large scale commercial capture units, in addition to allowing the use of low energy absorbents, such as e.g. carbonates. 
         [0061]      FIG. 7  illustrates the total volume of exhaust gas to be purified in a plant according to the present invention and the comparative example. The difference in total volume is a result of a difference in pressure (1 bara versus  10  bara). A smaller volume means that the process equipment may be less space consuming, and makes it possible to make more compact equipment and thereby reduce the capture equipment cost.