Abstract:
A method of operating a power generation system is provided. The system includes a first gas turbine engine that uses at least one of a primary fuel and a secondary fuel, and at least one second gas turbine engine that operates using at least one of a primary and secondary fuel. The method includes supplying primary fuel to at least one of the first and second engines from a common fuel source coupled to the first and second engines, and selectively operating the first engine between a first operational position and a second operational position. The first engine using only secondary fuel in the first operational position and operates using only primary fuel in the second operational position. The method includes supplying the second engine with primary fuel from the common fuel source without flaring the primary fuel.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 60/896,177, filed Mar. 21, 2007. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to any multiple gas turbine power plant with a primary and secondary fuel source that requires gas turbine start-up and shutdown on secondary fuel. More specifically, the present invention relates to Integrated Gasification and Combined Cycle (IGCC) power plants, and more particularly, to methods and systems for removing or returning gas turbines from service to vary plant output or facilitate maintenance on a particular unit. 
         [0003]    IGCC plants generally use coal gasification, cooling, cleaning, and saturation equipment, balanced with on-site generation of oxygen and nitrogen to produce a primary plant fuel typically known as syngas. At least some known IGCC plants include a plurality of gas turbines that are powered by syngas received from a common gasification source(s). Accordingly, the common gasification source(s) must respond to any change in fuel demands of the turbines during, for example, a shutdown or start-up of one or more of the gas turbines. Generally, a shutdown of one or more of the turbines may be required during times of maintenance or for a reduction of plant output (i.e, turndown) while maintaining operation of the gasification source(s) at full or reduced rates. 
         [0004]    Many gas turbines consume syngas to generate power. This process requires initial firing and loading to a predetermined point using a secondary or back-up fuel, such as natural gas or distillate, followed by fuel transfer, completely or partially, to syngas operation. To return IGCC plants to increased output by bringing one or more offline gas turbine(s) back online, syngas must be flared until the fuel flow rate and fuel condition meets the requirements for transfer. Generally, such methods of bringing a unit online and taking a unit offline regularly produce undesirable plant emissions. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0005]    The present invention provides a method of transferring a gas turbine from secondary (back-up) fuel, such as natural gas or distillate, to primary fuel, such as syngas, without flaring the primary fuel, while at least one gas turbine is online consuming primary fuel from a common source(s). This sequence of events is commonly called an add-on start of a gas turbine in a multiple gas turbine power plant. This situation typically occurs when a power plant increases plant output after operating at a reduced output (turndown) with at least one gas turbine offline, or after offline maintenance was performed on one or more of the turbines. 
         [0006]    In order to run on primary fuel, a gas turbine must initially operate on a secondary fuel and then transfer to the primary fuel source. For the first unit start-up, flaring primary fuel until primary fuel quality is established for transfer is unavoidable. However, for additional units coming online to be fed from the same primary fuel source, secondary fuel may be used to replace primary fuel in the online unit. This ensures that the required quantity of primary fuel to transfer the unit coming online is available without sacrificing output of the online unit. As a result, rather than flaring primary fuel, back-up fuel is used to maintain output. This sequence may also be reversed to take units offline without flaring. To enable operation of this feature where moisture is present in the primary fuel lines to offline units must circulate the primary fuel continuously. 
         [0007]    In one aspect, a method of operating a power generation system is provided. The system includes a first gas turbine engine that uses at least one of a primary fuel and a secondary fuel, and at least one second gas turbine engine that and that operates using at least one of a primary and secondary fuel. The method includes supplying primary fuel to at least one of the first and second engines from a common fuel source coupled to the first and second engines, and selectively operating the first engine between a first operational position and a second operational position. The first engine using only secondary fuel in the first operational position and operates using only primary fuel in the second operational position. The method includes supplying the second engine with primary fuel from the common fuel source without flaring the primary fuel. 
         [0008]    In a further aspect, a power generation plant is provided. The power generation plant includes a first gas turbine engine configured to operate using at least one of a primary fuel and a secondary fuel, the first gas turbine engine is configured to be selectively operable between a first operational position and a second operational position, wherein the first gas turbine engine operates using only the secondary fuel in the first operational position and operates using only primary fuel in the second operational position. The power generation plant further includes at least one second gas turbine engine configured to operate using at least one of a primary fuel and a secondary fuel. The power generation plant also includes a common fuel source coupled to the first and second gas turbine engines. The fuel source is configured to supply at least one of the first and second gas turbine engines with the primary fuel without flaring the primary fuel. 
         [0009]    In another aspect, a fuel control system is provided. The fuel control system controls a flow of fuel to a first gas turbine engine that operates using at least one of a primary fuel and a secondary fuel, and controls a flow of fuel to at least one second gas turbine engine to ensure the second gas turbine engine operates using at least one of a primary fuel and a secondary fuel. The fuel control system is configured to supply at least one of the first and second gas turbine engines with primary fuel from a common fuel source coupled to the first and second gas turbine engines, selectively position the first gas turbine engine between a first operational position and a second operational position, wherein the first gas turbine engine operates using the secondary fuel in the first operational position and operates using the primary fuel in the second operational position, and supply the second gas turbine engine with the primary fuel from the common fuel source without flaring the primary fuel. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a schematic view of an exemplary integrated gasification combined cycle power system. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0011]    The present invention facilitates completing a transfer associated with the start up, or shut down, of a combustion turbine without flaring or changing a flow rate of primary fuel. In the exemplary embodiment, this invention relates to multiple train integrated systems where a primary fuel source is generated and the rate of change of fuel gas production may be dissimilar to a gas turbine transfer rate. Further, in the exemplary embodiment, this invention utilizes peer-to-peer communication between operating unit controllers to establish the initial operating conditions. 
         [0012]      FIG. 1  is a schematic view of an exemplary integrated gasification combined cycle power system  100 . In the exemplary embodiment, system  100  includes a primary source of syngas  110  in flow communication with a first turbine  120  and a second turbine  130 . In the exemplary embodiment, first turbine  120  includes a secondary manifold  124  and a primary manifold  128 . Second turbine  130  also includes a primary manifold  134  and a secondary manifold  138 . In this exemplary embodiment, each primary manifold continually has primary (i.e., syngas) fuel flow, secondary (i.e, natural gas) fuel flow, or some combination thereof during operation, and the secondary manifold has only primary fuel. Specifically, in the exemplary embodiment, each secondary manifold  124  and  138  is coupled in flow communication with the primary source of syngas  110  via respective piping  140  and  150 . Further, in the exemplary embodiment, each primary manifold  128  and  134  is coupled in flow communication with a secondary fuel source  160  via piping  170  and  180 . As will be appreciated by one of ordinary skill in the art, system  100  can include any number of turbines fluidly coupled to the primary source of syngas  110 . 
         [0013]    In the exemplary embodiment, system  100  also includes a flare  200  and a first scrubber  210  that are each in flow communication between first turbine  120  and the primary source of syngas  110 . Further, in the exemplary embodiment, a second flare  220  and a second scrubber  230  are each in flow communication between second turbine  130  and the primary source of syngas  110 . 
         [0014]    In the exemplary embodiment, each turbine  120  and  130  receives secondary fuel from secondary fuel source  160  to facilitate a start-up of turbines  120  and  130 . Each turbine  120  and  130  receives primary fuel from scrubbers  210  and  230 , respectively. During start conditions in some known systems  100 , the primary fuel is ignited by flares  200  and  220  prior to reaching turbines  120  and  130 , respectively. 
         [0015]    In the exemplary embodiment, a method for shutting down or transferring one of first turbine  120  or second turbine  130  without having to flare the primary fuel is provided. In the exemplary embodiment, the method is utilized in an overnight turndown of system  100  to facilitate maximizing the efficiency of system  100 , while minimizing flaring. As such, emissions from system  100  are facilitated to be reduced. 
         [0016]    The objective of maximizing the efficiency of exemplary system  100  may be best achieved by shutting down either turbine  120  or turbine  130  each night. While shutting down a turbine  120  or  130  can be achieved in multiple ways, the exemplary method maximizes utilization of gasifier output and also minimizes flaring. Specifically, system  100  could be shifted from a two gas turbine co-fire mode or two gas turbines on primary fuel to a single gas turbine primary fuel mode using a fuel-trade concept. Fuel-trading enables a fuel transfer to secondary fuel, such as natural gas, on second turbine  130 , while utilizing the excess syngas with first turbine  120 . 
         [0017]    The concept of fuel-trading has been developed to address the relative difference in rate of change that may exist between fuel production and consumption systems. Following fuel transfer, the gasifiers produce excess primary fuel, which may be flared or routed to an alternative location until the production rates match the consumption requirements. With fuel-trading, the excess primary fuel from second turbine  130  is routed to first turbine  120 . This can be done in at least two modes including a co-fired mode and a primary fuel mode only. 
         [0018]    If both turbines  120  and  130  are in co-fired mode, the second turbine  130  is transferred from co-fired operation to primary fuel only. The second turbine  130  is then reduced to a primary fuel load that matches the heat rate required to move first turbine  120  from a co-fired mode to a syngas only mode. Second turbine  130  is then transferred to secondary fuel, and, at the same time, primary fuel from turbine  130  is routed to the first turbine  120 . This may result in turbine  120  transferring to a primary fuel only operation, or the turbine may remain in co-fire mode. This method facilitates absorbing all of the available primary fuel that was being used by second turbine  130 , such that second turbine  130  can be shut down. In the exemplary embodiment, a restart of second turbine  130  occurs in the reverse order of the method described herein. 
         [0019]    If both turbines are in a primary fuel only mode, the primary fuel rate to the first turbine  120  must first be reduced to absorb the excess primary fuel that will come from second turbine  130  during the fuel-trade. This can be done in a controlled rate that avoids flaring by either unloading or co-firing on gas turbine  120 . The second turbine  130  is then transferred from primary-only operation to secondary-only operation and the excess primary fuel is routed to the first turbine  120 . 
         [0020]    In the exemplary embodiment, the method also includes determining and setting a transfer rate to facilitate initializing a peer-to-peer transfer at equal fuel change rates, thereby transferring the minimum required amount of fuel into/out of the combustion turbine going on or offline. 
         [0021]    In one embodiment, the resultant conditions following a transfer consist of a reduced number of units operating on primary fuel. In another embodiment, the resultant conditions following a transfer consist of multiple units operating at a load point above the minimum specified operational point on primary fuel such that they may be loaded on primary fuel to desired output. As will be appreciated by one of ordinary skill in the art, the above-described method is not limited to the steps disclosed herein. 
         [0022]    One concern with shutting down a gas turbine train is the subsequent cooling of the syngas piping. Cooling of the syngas piping can result in moisture condensation due to the moisturized state of the gas. While the line could be thoroughly purged with nitrogen, such purging may result in flaring to re-inventory and a need to reheat the line the next morning. Accordingly, to prevent such problems, in the exemplary embodiment, the line is continually heated using syngas from second turbine  130 . 
         [0023]    Specifically, in the exemplary embodiment, the piping  140  of first turbine  120  is kept warm by creating a pressure drop that induces a syngas flow from the piping  150  of second turbine  130  through crossover line  190 . In an exemplary embodiment, the pressure drop is created by modulating a valve (not shown) that feeds second turbine  130  while leaving the line to first turbine  120  and crossover line  190  open. The induced flow heats piping  140  until first turbine  120  is restarted. 
         [0024]    The above-described methods facilitate minimizing primary fuel emissions in transitioning for turbine maintenance or turndown. Accordingly, the methods will provide advantageous operational flexibility for power plant operations in avoiding potentially reportable events. 
         [0025]    At least some known systems are configured to support a single gas turbine start or shutdown for the purposes of maintenance or turndown while maintaining operation of sources of primary fuel gas. The above-described systems and method facilitate a peer-to-peer controlled transfer of primary fuel, syngas, from an operating combustion turbine to a peer unit undergoing a transfer into or out of operation on syngas fuel in support of maintenance or turndown activities. In general, the methods of preparing and executing the fuel-trade event includes, but is not limited to, achieving an initial load point through specified turbine cycle conditions for the unit operating on primary fuel sufficient to enable the fuel-trade, and establishing a fuel split using secondary fuel for the peer unit in preparation for transfer from syngas to back-up fuel or vice versa. 
         [0026]    As used herein, an element or step recited in the singular and proceeded with the word “a” or “an” should be understood as not excluding plural said elements or steps, unless such exclusion is explicitly recited. Furthermore, references to “one embodiment” of the present invention are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features. 
         [0027]    The above-described methods and system enable transitioning for turbine maintenance or turndown while minimizing primary fuel emissions. Accordingly, the above-described methods and system provide advantageous operational flexibility for power plant operations in avoiding potentially reportable events. 
         [0028]    Exemplary embodiments of systems and methods for start-up and/or turndown of a gas turbine associated with an IGCC plant are described above in detail. The systems and methods illustrated are not limited to the specific embodiments described herein, but rather, components of the system may be utilized independently and separately from other components described herein. Further, steps described in the method may be utilized independently and separately from other steps described herein. 
         [0029]    While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.