Abstract:
A method of operating a gas turbine power plant and gas turbine power plant are disclosed wherein hydrogen for the combusting process is produced by feeding natural gas mixed with steam through a membrane/partial oxidation reactor and converting the natural gas at least to H 2  and CO. Thereby oxygen is transferred from the compressed air through the membrane of the membrane/partial oxidation reactor and the oxygen is used for the partial oxidation process of the natural gas. The process is followed by converting the syngas in a CO shift reactor and a CO shift reactor to a CO 2  removal equipment to mainly hydrogen.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of the U.S. National Stage designation of co-pending International Patent Application PCT/EP2003/050782 filed Nov. 3, 2003, which claims priority to U.S. provisional application No. 60/424,681 filed Nov. 8, 2002 and European patent application no. 02405995.8 filed Nov. 19, 2002, and the entire contents of these applications are expressly incorporated herein by reference thereto. 

   FIELD OF THE INVENTION 
   The invention relates to a method of operating a gas turbine power plant and a gas turbine power plant. 
   BACKGROUND OF THE INVENTION 
   In the last years different projects were launched with the aim to develop emission free gas turbine based processes using semi-closed cycles with CO 2 /H 2 O mixtures as working fluid. Methods of operating such power plants are known for example from EP-A1-0 939 199 and EP-A1-0 953 748. In these processes the fuel, usually natural gas, reacts with technically pure oxygen generated either in an external air-separation unit or internally in an integrated membrane reactor. One major disadvantage of using air-separation units for these kind of processes is that they consume a great amount of energy, thus penalizing the efficiency and power output of the plant. From the literature it can be found that the energy demand for air-separation units is as high as 0.3 kWh/kg O 2  produced. The energy consumption for separating the oxygen from the air can be decreased very much if oxygen-separating membranes are used. Also this technique has a few disadvantages, namely: metal to ceramic sealing is needed that can withstand temperatures &gt;800° C., the turbine inlet temperature (TIT) and the ceramic sealing temperature are linked, which limits the maximum TIT and thus lowers the performance of the plant and one needs to separate large amounts of air, corresponding to the total O 2  required for full oxidation of fossil fuel powering the gas turbine. 
   SUMMARY OF THE INVENTION 
   The present invention relates to providing a method of operating a gas turbine power plant and a gas turbine power plant which avoid disadvantages of the prior as well as increasing the overall efficiency of the power plant. 
   This present invention is related to making use of so-called partial oxidation (POX) of the natural gas to syngas consisting of CO and H 2 . The oxygen required for this partial oxidation is provided by a ceramic, air separation membrane, thermally integrated into the process. This syngas would then be water gas shifted to produce even more hydrogen and convert the CO to CO 2 , and finally use the produced hydrogen as fuel in a gas turbine. 
   By doing this, one would overcome the temperature limit previously set by the membrane. The membrane reactor unit would be combined to both work as an oxygen transferring membrane and as a reactor for the partial oxidation. One membrane type that can be used to separate the oxygen from the air is a so-called “Mixed Conducting Membrane” (MCM). These materials consist of complex crystalline structures, which incorporate oxygen ion vacancies (5-15%). The transport principle for oxygen transport through the membrane is adsorption on the surface followed by decomposition into ions, which are transported through the membrane by sequentially occupying oxygen ion vacancies. The ion transport is counterbalanced by a flow of electrons in the opposite direction completing the circuit. The driving force is a difference in oxygen partial pressure between the permeate and retentate sides of the membrane. The transport process also requires high temperatures, i.e. &gt;700° C. In an embodiment of the present invention the surfaces of the permeate side of the membrane that contain the syngas are coated with catalytic material to promote the formation of synthesis gas  17   1  and, in particular, hydrogen. Catalyst materials used for autothermal reforming are Rh, Ru, Co, Fe or bimetallic combinations thereof. 
   Optionally, prior to entering the membrane reactor, the air stream from the compressor can be lead to a catalytic burner where the air is heated by means of catalytic combustion. The fuel for the catalyst is either hydrogen or natural gas. Thereby the use of hydrogen is preferred to avoid producing CO 2 . The reason for using a catalytic burner is to increase the average temperature in the membrane/POX reactor thereby increasing the oxygen flux through the membrane. Also, the temperature gradient in the reactor will be lower and thus the thermal stresses for the reactor will decrease. 
   Advantageously the syngas coming from the membrane/POX reactor consisting of hot steam, H 2  and CO can enter a low temperature heat exchanger, where the syngas mixture is cooled down by an incoming stream of the compressed air from the compressor. Another possibility would be to use a medium temperature heat exchanger to raise the temperature of the mixture of steam and natural gas before the mixture enters the membrane/POX reactor. This would flatten out the temperature profile in the membrane/POX reactor and thus lower the temperature gradients in this. 
   After the expansion the hot flue gases of the gas turbine can be utilised in a heat recovery steam generator producing steam for the bottoming steam cycle and producing more power in a steam turbine and electricity in a generator. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the invention are illustrated in the accompanying drawings, in which: 
       FIG. 1  illustrates a gas turbine power plant according to the present invention; and 
       FIG. 2  illustrates the partial oxidation of the membrane/partial oxidation reactor. 
   

   The drawings show only the parts important for the invention. Same elements will be numbered in the same way in different drawings. 
   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  shows a syngas based low emission power plant according to the present invention. Air  1  is fed through a compressor  2  before the compressed air  3  is fed at least through a membrane/partial oxidation (POX) reactor  4 . After the membrane/POX reactor  4  the air is burned in a combustion chamber  5  together with hydrogen  6 . The flue gases are then expanded in a turbine  7 , which is driving the compressor  2  and producing electricity in a generator  8 . After the expansion the hot flue gases  9  are utilised in a heat recovery steam generator  10  producing steam for the bottoming steam cycle  11  and producing more power in a steam turbine  12  and electricity in a generator  13 . 
   As can be seen from  FIG. 1 , natural gas  14  is being mixed with superheated intermediate pressure steam  15  and is then lead to the membrane/POX reactor  4 . One possibility here would be to use a medium temperature beat exchanger  16  to raise the temperature of the mixture of steam  15  and natural gas  14 . This would flatten out the temperature profile in the membrane/POX reactor  4  and thus lower the temperature gradients in this. Since the temperature involved is not too high (&lt;900° C.), it might be possible to use a metal heat exchanger. 
   As seen in  FIG. 2 , in the membrane/POX reactor  4 , oxygen is transferred through a membrane  18  from a first side to a second side and is partially oxidised (as well as reformed with steam) on the membrane  18  surface with the natural gas  14  by the following reactions:
 
CH 4 +0.5O 2             2H 2 +CO+35.67 kJ/mol
 
CH 4 +H 2 O         CO+3H 2 −205 kJ/mol
 
CO+H 2 O         CO 2 +H 2 +41.15 kJ/mol

   In sum, the three reactions combine to produce a mixture of H 2 , CO and CO 2 ; the overall heat balance and product mixture is dictated by the amount of oxygen (and endothermic reactions) that is present. The design of the membrane/POX reactor  4  is such that the overall process is autothermal, and the membrane temperature is of ca. 800° C. The membrane/POX reactor  4  would be combined to both work as an oxygen transferring membrane and as well as doing the partial oxidation. One membrane type that can be used to separate the oxygen from the air is a so-called “Mixed Conducting Membrane” (MCM). These materials consist of complex crystalline structures, which incorporate oxygen ion vacancies (5-15%). The transport principle for oxygen transport through the membrane  18  is adsorption on the surface followed by decomposition into ions, which are transported through the membrane by sequentially occupying oxygen ion vacancies. The ion transport is counterbalanced by a flow of electrons in the opposite direction. The driving force is a difference in oxygen partial pressure between the permeate and retentate sides of the membrane  18 . The transport process also requires high temperatures, i.e. &gt;700° C. In an embodiment of the present invention the surfaces of the permeate side of the membrane  18  (that containing the syngas  17   1 ) is coated with catalytic material to promote the formation of synthesis gas  17   1  and, in particular, hydrogen. Catalyst materials used for autothermal reforming are Rh, Ru, Co, Fe or bimetallic combinations thereof (e.g. Co/Fe). 
   The syngas  17   1 , now consisting of hot steam, H 2  and CO enters a low temperature heat exchanger  19 , where the syngas  17   1  mixture is cooled down by an incoming stream of the compressed air  3  from the compressor  2 . Optionally, the air stream from the low temperature heat exchanger  19  can then be lead to a catalytic burner  20  where the air is heated by means of catalytic combustion. The fuel for the catalytic burner  20  is either hydrogen  21  or natural gas  14 . Use of hydrogen  21  is preferred to avoid producing CO 2 . The reason for using a catalytic burner  20  is to increase the average temperature in the membrane/POX reactor  4 , increasing the oxygen flux through the membrane  18 . Also, the temperature gradient in the reactor  4  will be lower and thus the thermal stresses for the reactor  4  will decrease. This catalytic burner  20  can also be used to help control process conditions within the MCM reactor during start up or to address instabilities within the membrane/POX reactor  4  associated with the autothermal reforming and potential catalyst deactivation. The temperature of the MCM reactor will be very sensitive to the amount of O 2  present and there could be some strange transients during start up. A quick reacting catalytic burner  4  running on H 2  could help for process control. 
   After the syngas  17   1  has been cooled down in the low temperature heat exchanger  19 , the syngas  17   1  is then further cooled down in a CO shift reactor  22 , lowering the temperature further to about 200-300° C. Depending on the chosen cooling temperature, water will condense out or not. Since a low temperature favors the CO shift reaction it might be wise to keep the temperature low. This will also lower the water consumption for the cycle since the condensed water  23  can be re-injected in the bottoming steam cycle  11 . The medium used for the cooling is boiler feed water  24   1 ,  24   2  from a bottoming steam and water cycle  11 . During the cooling of the syngas  17   1 , in the CO shift reactor  22 , the syngas  17   1  undergoes the following reaction:
 
CO+H 2 O           H 2 +CO 2 +41.15 kJ/mol

   The CO shift reactor  22  is in other words used to convert CO and water to CO 2  and more hydrogen. Also this reaction is mildly exothermic, leading to some of the water which was condensed out during the cooling (or all water if the cooling temperature is high) being evaporated again, taking heat from the exothermic process described above. After the CO shift reactor  22  the syngas  17   2  consists ideally of H 2 , CO 2  and H 2 O. This syngas  17   2  is then lead to some kind of CO 2  absorption equipment  25 , based on either chemical or physical absorption. The CO 2  removal rate in this kind of equipment is around 90%. Low pressure steam  26  needed for the CO 2  removal is extracted from the steam turbine  12 , and the condensed water  27  is lead back to the feed water tank of the steam cycle  11 . The removed CO 2    28  is further compressed by means of inter-cooling in a compressor  29 , producing liquid CO 2    30  that might be deposited or used in for instance enhanced oil recovery. 
   After removing most of the CO 2 , the syngas  17   3  mainly consisting of H 2 , H 2 O and some remaining CO 2  is lead to a combustion chamber  5 , to be burned together with air from the first side of the membrane/POX reactor  4 . The water in the syngas  17   3  helps control the combustion temperature and thus lowers NO x  formation. A part of the resulting syngas  17   3  comprising hydrogen  6  from the CO 2  removal equipment  25  can as well be burned in the catalytic burner  20 . 
   LIST OF DESIGNATIONS 
   
       
         1  Air 
         2  Compressor 
         3  Compressed air 
         4  Membrane/partial oxidation (POX) reactor 
         5  Combustion chamber 
         6  Hydrogen 
         7  Gas turbine 
         8  Generator 
         9  Hot flue gases 
         10  Heat recovery steam generator 
         11  Bottoming steam cycle 
         12  Steam Turbine 
         13  Generator 
         14  Natural gas 
         15  Superheated steam 
         16  Medium temperature heat exchanger 
         17   1 ,  17   2 ,  17   3  Syngas 
         18  Membrane 
         19  Low temperature heat exchanger. 
         20  Catalytic burner 
         21  Hydrogen 
         22  CO shift reactor 
         23  Condensed water 
         24  Boiler feed water 
         25  CO 2  absorption equipment 
         26  Low pressure steam 
         27  Condensed water 
         28  CO 2    
         29  compressor 
         30  liquid CO 2