Abstract:
A method and apparatus for optimizing boilers with high flue gas recirculation rate based with laser based diagnostic technology. A tunable diode laser is emitted from a launcher, is altered by the absorption spectra of the gas species that it intersects, and encounters a receiver. The signal is processed, then the information is used to modulate the flowrate of hydrogen blended fuel or oxygen enriched air into the burner.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims the benefit of U.S. Provisional Application No. 60/826,645, filed Sep. 22, 2006, the entire contents of which are incorporated herein by reference. 
     
     BACKGROUND  
       [0002]    The generation of electric power can be a challenging endeavor, often requiring the production of varying amounts of power during different times of day, seasons, or local load fluctuations. In contrast, it is well understood that the optimum efficiencies are typically achieved by operating at steady state or near-steady state conditions. Thus, peak efficiency, and such a varying load, are often not compatible with these widely varying demands. This problem can be addressed by providing various combinations of plants that are either on standby or running. Even if those that are running are not as efficient, if they are providing a smaller portion of the overall electrical load to the system. Gas turbines are well suited for these “peaking” applications because of the ease with which they can be brought on-line. 
         [0003]    Combined cycle power generation plants meet the needs of increased efficiency and flexibility because they blend the best features of peaking and base-load generation by combining a steam turbine system with one or more gas turbines. As previously mentioned, gas turbines have short start up times and respond well to changes in power demands. Gas turbines are, however, relatively inefficient for power generation in simple cycle applications. In contrast, steam turbines are not well-suited for fast start up and for response to varying demand. Combined cycle plants can achieve better efficiencies by utilizing the waste heat from the exhaust of gas turbines to generate steam for the steam turbine, and are among the most efficient means available for producing electricity. 
         [0004]    The overall efficiency of gas turbines is a function of the compressor and turbine efficiencies, ambient air temperature, turbine inlet temperature, overall pressure ratio and the type of cycle used. Certain of these conditions are not controllable by the plant design or operation, but are determined by the equipment design. However, it is possible to control the temperature of the gas entering the combustor. The higher this temperature, the higher the efficiency of the turbine cycle. Thus, it is a one object of the present invention to increase the temperature of the gas entering the combustor. 
         [0005]    A cogeneration plant can be operated with gas turbine on or off. When gas turbine is in operation, high temperature exhaust gas from gas turbine is fed into heat recovery steam generator. The energy contained in the gas turbine exhaust gas is recovered in heat exchangers to produce steam for either processing (i.e. a cogeneration cycle) or for driving a steam turbine (i.e. a combined cycle). When the gas turbine is off, a diverter damper can by-pass exhaust gas to a by-pass stack, which prevents any air from leaking into gas turbine when fresh air fan is running. 
         [0006]    A cogeneration plant may be operated with the gas turbine off, by incorporating an auxiliary fan to provide an air flow through the heat recovery boiler. Such a system is known in the art as operating in Fresh Air mode, or alternately Fresh Air Firing mode. Operating a cogeneration system under such a Fresh Air mode gives the operator the flexibility of being able to separate the production of electricity from the production of the steam. This is particularly interesting in the regions where power market is deregulated and power prices fluctuate over time. In such a region, there is a decoupling of steam demand and desired electrical power output. 
         [0007]    To improve the efficiency under Fresh Air mode for a new cogeneration system, two types of technologies can be used; flue gas recirculation, and a boiler with a combustion chamber. For an existing cogeneration unit, flue gas recirculation will almost always be a far easier and cheaper technology for retrofit. 
         [0008]    Generally, a higher recirculation rate of flue gas can yield less stack loss and improve the efficiency of a cogeneration system, and also reduce emissions. However, with the increase of the recirculation rate of flue gas, oxygen concentration at the upstream of the burner drops. Excessive CO emission and combustion stability appear to be another concern. Thus, maintaining an efficient and stable combustion at the high recirculation rates of flue gas is problematic. 
         [0009]    Extractive sampling or hybrid in-situ ex-situ systems are common technologies within the industry for monitoring cogeneration systems for gas species concentrations. One drawback in either of these cases is that the measurement is done at a single point. Due to the large size of the combustion zone within a heat recovery stream generator, significant non-uniformity of gas species concentration and gas temperature can exist. Also, simply installing more sampling probes can create obstructions to the gas flow. When sampling probes are installed in such a high temperature flame region, maintenance issues can be significant, and must be considered. In addition, due to long sampling lines, the extractive sampling technology with conventional analyzers often has relatively long delay time, which can be up to minutes depending on the system. 
         [0010]    With an increase in recirculation rate of the flue gas, the oxygen concentration upstream of the burner decreases. Excessive CO emissions and combustion instability are other concerns. Hydrogen-blended fuels or oxygen-enriched air can be used to maintain an efficient and stable combustion with little effect on emissions. However, in order to improve the overall performance at the high percentage flue gas recirculation that are of interest, the fuel or oxygen-enriched air inlet flow has to be accurately measured and controlled. Process parameters, such as O 2 , CO, and NO x  concentrations, and gas temperature, are key indicators, and are useful for combustion adjustments to improve the performance. To optimize the combustion, it is desirable to monitor these important parameters and to couple them with a process control strategy. 
         [0011]    Thus, there is a need in the industry for a system that can be retrofitted into existing combined cycle, or cogeneration plants, that will allow real-time, non-intrusive, in-situ measurement and control of gas parameters, such as temperature and composition. 
       SUMMARY  
       [0012]    The present invention is directed toward an optimized high flue gas recirculation rate combustion diagnostic apparatus. The apparatus comprises a combustion zone with gas species to be measured, and burners. The apparatus also comprises a launcher for emitting a tunable diode laser beam at a sequence of wavelengths that correspond to an absorption spectra of said gas species. The apparatus also comprises a receiver for receiving said tunable diode laser beam, and for generating an output signal corresponding to a temperature and/or a degree of absorption encountered. The apparatus comprises a control system for receiving said output signal, wherein said control system thereby regulates the inlet flow of fuel or oxidant to said combustion zone. 
         [0013]    The present invention is also directed toward an optimized high flue gas recirculation rate combustion diagnostic method. This method comprises providing a combustion zone with gas species to be measured, and burners. The method also comprises emitting a tunable diode laser beam from a launcher, wherein said tunable diode laser beam has a sequence of wavelengths that correspond to an absorption spectra of said gas species. The method also comprises receiving said tunable diode laser beam in a receiver, and generating an output signal corresponding to a temperature and/or a degree of absorption encountered. The method also comprises regulating the inlet flow of fuel or oxidant into said combustion zone with a control system for receiving said output signal. 
     
     
       BRIEF DESCRIPTION OF DRAWINGS  
         [0014]    For a further understanding of the nature and objects for the present invention, reference should be made to the following detailed description, taken in conjunction with the accompanying drawings, in which like elements are given the same or analogous reference numbers and wherein: 
           [0015]      FIG. 1  is a schematic illustration of one embodiment in accordance with the present invention; 
           [0016]      FIG. 2  is a schematic illustration of another embodiment in accordance with the present invention; and 
           [0017]      FIG. 3  is a schematic illustration of a the duct burner as positioned in the transition duct between the gas turbine and the heat recovery stream generator, in accordance with one embodiment the present invention. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0018]    A cogeneration unit with flue gas recirculation can be continuously operated under Fresh Air mode for a long period with a competitive efficiency that is roughly comparable to the typical value for a conventional boiler. While keeping the total flow of flue gas that exits the stack virtually constant, an increase in flue gas recirculation rate yields less stack loss and reduces emissions. However, with the increase of the flue gas recirculation rate, the oxygen content to the inlet of duct burners can easily decrease to a level that causes excessive CO emission and result in combustion instability. To overcome this difficulty, a practical solution is to add a more reactive fuel, such as hydrogen, to the main fuel (typically natural gas). A system that utilizes a fuel blended with hydrogen in the combustion system, can be used to maintain an efficient and stable combustion in heat recovery steam generator. Such a system will also reduce emissions at these high flue gas recirculation rates. In addition, the use of hydrogen as part of the fuel blend, the greenhouse gas (CO 2 ) production is reduced. Therefore, this solution can serve as a transition strategy to a carbon free energy system at some point in the future. 
         [0019]    A hydrogen fuel blend combustion system is proposed to solve the problem of combustion instability and to improve the efficiency of a cogeneration system at a high flue gas recirculation rate. In this solution, the fresh air is mixed with a part of the total flue gas and then this mixture is recycled back to the inlet duct of Heat Recovery Steam Generator. The more reactive fuel, such as hydrogen (or hydrogen/CO), is blended with the primary fuel, or fuels, and then the blended fuel is injected into combustion chamber. In one embodiment, a stable and efficient combustion can be maintained when a large portion of flue gas is recycled, and the emissions (NOx and CO) are also reduced to the required regulation levels at the same time. 
         [0020]    To improve the overall performance, it is critical to accurately control the fuel or oxygen-enriched air inlet flow. The important processing parameters, such as O 2 , CO, and NO x  concentrations, and gas temperature, are linked to the performance. To solve this issue, a laser-based diagnostic technology, such as Tunable Diode Laser (TDL) sensors, can provide the non-intrusive in-situ fast response measurements of important gas species concentrations and gas temperature. The sensors are coupled with feedback control to accurately and timely adjust gas and fuel inlet flows, which can improve the overall performance of a cogeneration system at a high percentage flue gas recirculation. 
         [0021]    A laser-based diagnostic technology, such as Tunable Diode Laser (TDL) sensor, can provide the non-intrusive in-situ fast response measurements of important gas species concentrations and gas temperature at different cross sections in the combustion chamber. The sensors are then coupled with feedback control to accurately and timely adjust the inlet flow rates of the hydrogen-blended fuel or oxygen-enriched air. The ability of monitoring key process parameters coupled with the feedback control of inlet flows plays an important role for maintaining an efficient and stable combustion with limited emissions at a high percentage flue gas recirculation in a cogeneration system. 
         [0022]    Turning to  FIG. 1 , schematic diagram  100 , which represents a cogeneration system utilizing a hydrogen blended combustion system is shown. 
         [0023]    For a cogeneration unit that is operating in Gas Turbine mode (i.e. with the gas turbine on), primary gas turbine fuel  101  is injected into gas turbine  102  and the high temperature exhaust gas  103  exits the gas turbine  102 . When operating in the Gas Turbine mode, the damper  104  remains open, and all the exhaust gas  103  is directed toward the heat recovery steam generator  111 , where this heat content is exploited to produce steam. The damper  104  and by-pass stack  105  will be used during the switching period between Gas Turbine mode and Fresh Air mode (i.e. when the gas turbine is off). When operating in Fresh Air mode, fresh air  115  is fed into the heat recovery steam generator  111  and the damper  104  can prevent air from leaking into the gas turbine ducting. 
         [0024]    Hydrogen fuel  108  may be blended with primary HRSG fuel  109 , and then burned in duct burner  110 . As the exhaust gas  112  exits the heat recovery steam generator  111 , instead of being completely exhausted into main stack  114 , a portion of the flue gas  113  is recycled back to displace an equivalent volume of fresh air  115 . 
         [0025]    The hydrogen (or hydrogen/CO) blended fuel  118  may go through a fan  119  to increase the pressure for better injection and mixing. After being burned in duct burner  110 , the heated gas stream enters heat recovery steam generator  111 , where it mixes with gas turbine exhaust gas  103 . The velocity with which blended fuels  118  are introduced into the duct burner  110  depends on the structure of duct burner, the size and the geometry of the combustion zone of the heat recovery steam generator, the velocity and the temperature of combustion gases, and the structure of heat recovery steam generator. 
         [0026]    For a cogeneration unit that is operating in Fresh Air mode (i.e. with the gas turbine off), the damper  104  will be closed. When operating in Fresh Air mode, fresh air  115  is fed into the heat recovery steam generator  111  and the damper  104  can prevent air from leaking into the gas turbine ducting. Fresh air  115 , and the recirculated flue gas to be discussed below, may go through a fan  117  to increase the pressure as needed. 
         [0027]    Hydrogen fuel  108  may be blended with primary HRSG fuel  109 , and then burned in duct burner  110 . As the exhaust gas  112  exits the heat recovery steam generator  111 , instead of being completely exhausted into main stack  114 , a portion of the flue gas  113  is recycled back to displace an equivalent volume of fresh air  115 . 
         [0028]    The hydrogen (or hydrogen/CO) blended fuel  118  may go through a fan  119  to increase the pressure for better injection and mixing. After being burned in duct burner  110 , the heated gas stream enters heat recovery steam generator  111 , where it mixes with gas turbine exhaust gas  103 . The velocity with which blended fuels  118  are introduced into the duct burner  110  depends on the structure of duct burner, the size and the geometry of the combustion zone of the heat recovery steam generator, the velocity and the temperature of combustion gases, and the structure of heat recovery steam generator. 
         [0029]    Shown in  FIG. 2  is a schematic diagram of a cogeneration system utilizing one embodiment of the laser-based diagnostic technology for optimizing combustion at a high flue gas recirculation rate. 
         [0030]    The laser launcher  202  and laser receiver  204  are mounted at the downstream of duct burners  110  on opposite side walls of combustion zone of the heat recovery steam generator  111 . To have improved control of the combustion process, one preferred location of monitoring gas species concentrations is close to the end of the flame  201 . At the downstream of flame  201 , the combustion gases start to mix with the surrounding extra air/flue gas to achieve a uniform distribution of gas species concentrations and temperature as the combustion gases exit the combustion zone of the heat recovery steam generator  111  and enter the heat transfer sections of the heat recovery steam generator. 
         [0031]    The laser beam  205  is transported to the launcher module  202  by fiber optic cable  207 . The laser beams  203  are multiplexed to monitor gas species concentrations and gas temperature for different rows of duct burner  110 . This may require different sets of launcher  202  and receiver  204  modules mounted on the opposite side of the combustion zone of the heat recovery steam generator  111 . However, only one signal laser and acquisition system  205  is required. The beams  203  are launched across the combustion zone of the heat recovery steam generator  111  and collected in the receiver module  204  at the opposite side. The resulting measurement provides a path averaged concentration and/or temperature of the gas volume that the laser beam  203  intercepts. The signal is collected in the data acquisition unit  205  where the resulting measured laser beam attenuation can be related to the concentration of a resonant absorption transition. The measured process parameters are sent to a control system  206  to perform the control of the inlet flow of the hydrogen-blended fuel  108 , 109  or oxygen-enriched air  119 . As used within this application, oxygen-enriched air contains greater than atmospheric concentrations of oxygen. 
         [0032]    The monitored gas species include CO, H 2 O, and O 2 . By monitoring O 2  and CO, the NO can also be indirectly monitored to some extent. The combustion gas temperature is also measured. When multiple water lines are used, the non-uniformity of gas temperature along the laser path  203  can also be evaluated by comparing the temperatures calculated from each water line. If the gas temperature is uniform along the laser path  203  in the combustion zone of heat recovery steam generator  111 , the calculated temperatures from all water lines will be equal. 
         [0033]    For the lasers generating multiple water lines, two options are available. One option is to use different lasers and another option is to use one sweeping laser. 
         [0034]    A cross section  300  of duct burner  110 , as it is positioned within the transition duct between gas turbine  102  and heat recovery steam generator  111  is shown in  FIG. 3 . A portion of cross section  300  is occupied by the rows of duct burner  301 . The mixture of air and flue gas  107  passes through the remaining portion of the cross section  303 . If hydrogen-blended fuel is used, the fuel may be injected into the combustion zone of heat recovery steam generator  111  through the fuel nozzles  302 . In another embodiment, the oxygen-enriched air is injected through separate nozzles  304 . The optimal air/fuel ratio, velocity ratio and the turbulent intensities (of the blended fuels  118  and/or the mixture of air/flue gas  107 ) are dependent on the configuration of the cogeneration system. 
         [0035]    As shown in Table 1, with the increase of the percentage of the recirculated flue gas, the thermal efficiency of the heat recovery steam generator increases. Simultaneously, the oxygen content to the burner decreases with an increase of flue gas recirculation rate. When a hydrogen-blended combustion system is used, a stable and efficient combustion can be maintained even if the oxygen content of the mixed gas of air/flue gas  107  at the upstream of the burners drops to a level that is not acceptable for a stable combustion. The last three rows of Table 1, generally represent cases in which a hydrogen-blended combustion system may be needed. The percentages of the blended hydrogen fuel depend on the oxygen content of the mixed gas of air/flue gas  107  at the upstream of the burners and several other factors (such as the structure of duct burner, the size and the geometries of the combustion chamber, the velocity and the temperature of combustion gases). It is anticipated that a hydrogen fuel ratio of up to 20% is desirable for this application. 
         [0000]    
       
         
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 The effects of different recirculation rates 
               
               
                 of flue gas on a cogeneration system 
               
             
          
           
               
                 Recirculation rate 
                 Thermal efficiency 
                 O 2  to burner 
                 O 2  in exhaust gas 
               
               
                   
               
               
                  0% 
                   83% 
                  20.7% 
                 13.5% 
               
               
                 20% 
                 85.8% 
                  18.9% 
                 11.9% 
               
               
                 30% 
                 87.2% 
                 17.45% 
                 10.6% 
               
               
                 35% 
                 88.0% 
                 16.73% 
                 9.95% 
               
               
                 40% 
                 88.8% 
                   16% 
                  9.3% 
               
               
                 45% 
                 89.6% 
                  14.6% 
                 7.98%