Patent Publication Number: US-2010107592-A1

Title: System and method for reducing corrosion in a gas turbine system

Description:
BACKGROUND 
     The invention relates generally to power generation systems, and more particularly to a system and method for reducing corrosion in a gas turbine system of a simple or combined cycle with exhaust gas recirculation. 
     It is known that a combined cycle power plant is a high efficiency power generation system. In a combined cycle power plant, steam is generated by using hot exhaust gas discharged from a gas turbine. A steam turbine is driven using the generated steam. One or more generators are driven by the gas turbine and the steam turbine. By using the thermal energy of an exhaust gas from the gas turbine, the combined cycle plant can achieve higher efficiency levels than simple cycle power plants without steam generation. 
     Recirculation of exhaust gas to the inlet of the compressor results in an increase of the concentration of carbon dioxide and potentially reduction of nitrogen oxide emissions at an exhaust manifold of the gas turbine. A problem, however is that combustion by-products containing sulfur are recirculated to the gas turbine. This may cause corrosion on metal surfaces at the intake manifold of the compressor or during the first compression stage or other areas. 
     In a conventional method, reduction in sulfur content of the recirculated exhaust gas is achieved by reducing sulfur content in the exhaust gas under close to atmospheric pressure conditions at a location downstream of the gas turbine, for example, at the exhaust gas recirculation path. This sulfur cleanup strategy, however, requires large equipment and large amounts of energy due to the larger volume flow of gas and lower partial pressure of the sulfur-containing components, respectively, leading to higher capital expenditure and higher operating costs. The sulfur content in the exhaust gas also depends on the exhaust gas recirculation rate. 
     Accordingly, it is desirable to have a system and method that requires smaller equipment and lower energy for reducing the sulfur content in the exhaust gas of a system with exhaust gas recirculation in order to efficiently and economically reduce corrosion risk in a gas turbine. 
     BRIEF DESCRIPTION 
     In accordance with one exemplary embodiment of the present invention, a system includes a gas turbine unit configured to generate a combustion gas by combusting a mixture of compressed air and fuel and generate electric power by expanding the combustion gas. A separator is coupled to the gas turbine unit and disposed upstream of the gas turbine unit. The separator is configured to reduce sulfur content in the fuel before being fed to the gas turbine unit. A portion of an exhaust gas from a gas turbine is recirculated to a gas turbine unit inlet. 
     In accordance with another exemplary embodiment of the present invention, a combined cycle power generation system comprising a heat recovery unit is disclosed. 
     In accordance with another exemplary embodiment of the present invention, a system including a cooler in an exhaust gas recirculation line is disclosed. 
     In accordance with yet another exemplary embodiment of the present invention, a system comprising a carbon dioxide capture unit is disclosed. 
     In accordance with yet another exemplary embodiment of the present invention, a method for reducing corrosion in a gas turbine unit of an electric power generation system is disclosed. 
    
    
     
       DRAWINGS 
       These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
         FIG. 1  is a diagrammatical representation of a system e.g. a power generation system having a separator disposed upstream of a combustor in accordance with an exemplary embodiment of the present invention; 
         FIG. 2  is a diagrammatical representation of a combined cycle power generation system having a heat recovery unit, an exhaust gas recirculation cooler, a carbon dioxide capture unit, and a separator disposed upstream of a combustor in accordance with an exemplary embodiment of the present invention; 
         FIG. 3  is a diagrammatical representation of a system e.g. a power generation system having a carbon dioxide capture unit, and a separator disposed upstream of a combustor in accordance with an exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     As discussed in detail below, embodiments of the present invention discloses a system and method for reducing corrosion in a gas turbine of an electric power generation system. The electric power generation system includes a gas turbine unit configured to generate a combustion gas by combusting a mixture of compressed air and fuel and generate electric power by expanding the combustion gas. A separator is coupled to the gas turbine unit and disposed upstream of the gas turbine unit. The separator is configured to reduce sulfur content in the fuel before being fed to the gas turbine unit. A heat exchanger is optionally coupled to the gas turbine unit and configured to transfer an exhaust gas from the gas turbine unit in heat exchange relationship with a cooling medium so as to generate a cooled exhaust gas. A portion of the exhaust gas from the gas turbine unit or the heat exchanger is recirculated to an inlet of the gas turbine unit. In one embodiment, the electric power generation system is a combined cycle power generation system. In another embodiment, the system includes an exhaust gas recirculation cooler in an exhaust gas recirculation duct of the system. In yet another embodiment, the system includes a carbon dioxide capture unit configured to remove carbon dioxide from the exhaust gas. In yet another embodiment, a method for reducing corrosion in a gas turbine unit of an electric power generation system is disclosed. In accordance with the embodiments of the present invention, sulfur content is reduced in a fuel stream in a separator upstream of a gas turbine unit. Hence smaller equipment and lower amounts of energy are required for reducing sulfur content in the fuel stream, when compared to a system separating sulfur-containing components from the exhaust gas stream. Also, the separation of sulfur content from the fuel stream does not depend on an exhaust gas recirculation rate in the power generation system. 
     Referring to  FIG. 1 , an exemplary electric power generation system  10  configured to generate electric power is illustrated. The system  10  includes a gas turbine unit  12  including a compressor  14 , a combustor  16 , and a turbine  18 . The compressor  14  is configured to receive ambient air at atmospheric pressure and to compress the air to a higher pressure. The compressed air is mixed with a gaseous or liquid fuel, for example natural gas, and combusted in the combustor  16 . Combustion exhaust gas from the combustor  16  is expanded via the gas turbine  18 . A generator  20  coupled to the turbine  18  transforms the mechanical energy into electrical power. It should be noted herein that in some embodiments, the turbine  18  is coupled to a mechanical drive unit. The gas turbine  18  typically drives the compressor  14  via a shaft  22 . 
     The exhaust gas from the gas turbine  18  is optionally passed in heat exchange relationship with a cooling medium, for example feed-water, through a heat exchanger  24 . The cooling medium is fed to the heat exchanger  24  through a feed line  26 . As a result cooling medium is heated to generate steam. In the illustrated embodiment, one portion of the cooled exhaust gas is vented from the heat exchanger  24  to the atmosphere via a carbon dioxide capture unit  28  and a stack (not shown). Conventional power plants generate pollutants that may be toxic, for example, certain metals and polyaromatic hydrocarbons; sulfur oxides (SOx) such as sulfur dioxide (SO2), and nitrogen oxides (NOx); nitrogen dioxide and reactive organic gases; particulate matter; and greenhouse gases, notably carbon dioxide. The use of natural gas (NG) as a fuel instead of petroleum or coal reduces many emissions and solids wastes; however, burning natural gas in air still produces quantities of nitrogen dioxide, reactive organic gases, and carbon dioxide. Carbon dioxide can be “captured” or removed from the flue gas using methods including chemical or physical absorption, cryogenic fractionation, membrane separation, or the like. Another portion of the cooled exhaust gas is recirculated to the gas turbine unit  12 . Flue gas recycling is used for example, to increase the carbon dioxide concentration in the exhaust gas stream and to reduce nitrogen oxide emissions. The other portion of the exhaust gas from the heat exchanger  24  is returned to an air inlet of the compressor  14  via a recirculation duct  30 . In other words, the recirculated exhaust gas is mixed with the inlet air of the compressor  14 . In the illustrated embodiment, the duct  30  is provided with an exhaust gas recirculation cooler  31  configured to further cool the recirculated portion of the exhaust gas from the heat exchanger  24 . 
     As discussed previously, recirculation of exhaust gas to the inlet of the compressor  14  facilitates to increase concentration of carbon dioxide and potentially reduced nitrogen oxide emissions at an exhaust manifold of the gas turbine. A problem, however is that combustion by-products containing sulfur are recirculated to the gas turbine. This may cause corrosion on metal surfaces at the intake manifold of the compressor  14  or during the first compression stage or in other areas. In the illustrated embodiment, the fuel is fed from a fuel source (not shown) to the combustor  16  via a separator  32  configured to separate sulfur content in the fuel so as to reduce sulfur content in the fuel before being fed to the combustor  16 . The separator  32  is disposed upstream of the combustor  16 . The sulfur content in the fuel may include but is not limited to hydrogen sulfide, sulfur dioxide, mercaptan, or the like. The separator  32  may separate sulfur content from the fuel using techniques including but not limited to membrane separation, absorption into a solvent, liquid phase oxidation, adsorption with a sorbent, or combinations thereof. In the illustrated embodiment, the separator is configured to reduce the sulfur content to about 100 to 530 parts per billion by weight at a pressure in the range of about 10 to about 100 bars before being fed to the gas turbine unit  12 . In the illustrated embodiment, the sulfur content in the exhaust gas generated from the gas turbine unit  12  is about 30 parts per billion by weight when sulfur content in the fuel is reduced upstream of the combustor  16 . In one exemplary embodiment, the separator  32  reduces the amount of sulfur dioxide in the fuel from about 4043 parts per billion by weight to about 530 parts per billion by weight. If 40% of the exhaust gas is recirculated from the gas turbine  18  is recirculated to the inlet of the compressor  14 , then the amount of sulfur dioxide in the exhaust gas recirculated to the compressor  14  may be around 24 parts per billion by weight, resulting in an inlet gas to the gas turbine compressor  14  with about 10 parts per billion weight. 
     If reduction in sulfur content of the recirculated exhaust gas is achieved by reducing sulfur content in the exhaust gas under atmospheric pressure conditions at a location downstream of the gas turbine, for example, exhaust gas recirculation path, this requires large equipment and large amount of energy for reducing sulfur content to lower levels at atmospheric conditions, leading to higher capital expenditure and higher operating costs. On the contrary, if the sulfur content is removed from a fuel stream in the separator  32  disposed upstream of the gas turbine operating with exhaust gas recirculation (EGR), less expensive clean up systems and less energy are required. Sulfur content will be most concentrated at highest pressure upstream of the combustor  16 . In the illustrated embodiment, the separator  32  is smaller and provides large driving forces for separating sulfur content from the fuel. This leads to lower capital expenditure and lower operating costs. 
     Referring to  FIG. 2 , an exemplary electric power generation system  10  configured to generate electric power is illustrated. In the illustrated embodiment, the system  10  includes a combined cycle power generation system. The system  10  includes a gas turbine unit  12  including the compressor  14 , the combustor  16 , and the turbine  18 . Combustion exhaust gas from the combustor  16  is expanded via the gas turbine  18 . The generator  20  coupled to the turbine  18  transforms the mechanical energy into electrical power. 
     The exhaust gas from the gas turbine  18  is passed in heat exchange relationship with a cooling medium e.g. feed-water through the heat exchanger  24  of a heat recovery unit  34  configured to produce additional power and increase plant efficiency. In the illustrated embodiment, the heat exchanger  24  is a heat recovery steam boiler. As a result feed-water is heated to generate steam. One portion of cooled exhaust gas from the heat exchanger  24  is returned to an air inlet of the compressor  14  via the recirculation duct  30 . Similar to the previous embodiment, the duct  30  may be provided with the exhaust gas recirculation cooler  31  configured to further cool the recirculated portion of the exhaust gas from the heat exchanger  24 . In the illustrated embodiment, the remaining portion of the cooled exhaust gas is vented from the heat exchanger  24  to the atmosphere via the carbon dioxide capture unit  28  and a stack (not shown). 
     In the illustrated embodiment, the steam from the heat exchanger  24  is fed to a steam turbine  36  via a pipe  38 . The pipe  38  may be provided with a valve  40  configured to control the flow of steam through the pipe  38 . The steam from the heat exchanger  24  is expanded via the steam turbine  36 . A generator  42  coupled to the turbine  36  transforms the mechanical energy into electrical power. The steam exiting the steam turbine  36  passes through a condenser  44  where steam is transformed into water. The condenser  44  is cooled using water from a cooling unit, for example, a cooling tower  46  dissipating the steam&#39;s latent heat into the atmosphere. The condenser  44  may also be cooled using ambient air or any other cooling medium. The water from the condenser  44  is then passed through a feed pump  48 , a deaerator  50 , and another feed pump  52  to the heat exchanger  24 . The pump  52  feeds the water to the heat exchanger  24  via the feedline  26 . It should be noted herein even though the heat recovery unit  34  includes a steam turbine unit; other types of heat recovery system are also envisaged. 
     Similar to the previously discussed embodiment, the fuel is fed from the fuel source (not shown) to the combustor  16  via the separator  32  configured to separate sulfur content in the fuel so as to reduce sulfur content in the fuel before being fed to the combustor  16 . The separator  32  is disposed upstream of the combustor  16 . The sulfur content in the fuel may include but is not limited to hydrogen sulfide, sulfur dioxide, mercaptan, or the like. The separator  32  may separate sulfur content from the fuel using techniques including but not limited to membrane separation, absorption into a solvent, liquid phase oxidation, adsorption with a sorbent, or combinations thereof. It should be noted herein that sulfur content may be removed from the fuel upstream of the combustor  16  by any separation process, for example, physical adsorption, chemical adsorption, using membranes, absorption, using differences in boiling point, or the like. 
     In certain embodiments, the power generation system may include a simple cycle gas turbine provided with exhaust gas recirculation. In such embodiments, the exhaust gas stream from the gas turbine  18  is not cooled via the heat exchanger  24  before being fed to the inlet of the compressor  14 . In other words, the exhaust gas stream from the gas turbine  18  is recirculated to the inlet of the compressor  14  via the duct  30 . It should be noted herein that in some embodiments, the cooling of recirculated portion of the exhaust gas in the recirculation duct  30  is also optional. 
     In certain embodiments, one portion of the exhaust gas from the gas turbine  18  is passed in heat exchange relationship with a cooling medium through the heat exchanger  24  and the remaining portion of the exhaust gas from the gas turbine  18  is returned to an air inlet of the compressor  14  via the recirculation duct  30 . 
     Referring to  FIG. 3 , an exemplary electric power generation system  10  configured to generate electric power is illustrated. The system  10  includes a gas turbine unit  12  including the compressor  14 , the combustor  16 , and the turbine  18 . The exhaust gas from the gas turbine  18  is passed in heat exchange relationship with a cooling medium e.g. feed-water through the heat exchanger  24 . As a result feed-water is heated to generate steam. In the illustrated embodiment, the cooled exhaust gas is vented from the heat exchanger  24  to the atmosphere via the carbon dioxide capture unit  28  and a stack (not shown). 
     Similar to the previously discussed embodiments, the fuel is fed from the fuel source (not shown) to the combustor  16  via the separator  32  configured to separate sulfur content in the fuel so as to reduce sulfur content in the fuel before being fed to the combustor  16 . The separator  32  is disposed upstream of the combustor  16 . It should be noted herein that in the illustrated embodiment, the system  10  does not include exhaust gas recirculation. The exhaust gas from the gas turbine  18  includes low sulfur content suitable for passing through the carbon dioxide capture unit  28 . In the illustrated embodiment without exhaust gas recirculation, use of a separator prior to the combustor that removes sulphur content could be beneficial over conventional flue gas desulphurization methods which are used downstream of the power plant combustor. 
     While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.