Abstract:
An electrical generation system including a first gas turbine and a a heat recovery steam generator coupled to the gas turbine and including a low pressure super-heater having a low pressure super-heater output. The electrical generation system also includes a second gas turbine, an output duct coupled to the second gas turbine and a supplemental low pressure super-heater within the output duct and thermally coupled to the low pressure super-heater output.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The subject matter disclosed herein relates to producing electrical power and, in particular, to producing electrical power during peak demand periods by utilizing peaker cycle heat exhaust recovery. 
         [0002]    Baseload (also base load, or baseload demand) is the minimum amount of power that a utility or distribution company must make available to its customers, or the amount of power required to meet minimum demands based on reasonable expectations of customer requirements. Baseload values typically vary from hour to hour in most commercial and industrial areas. The baseload is generated by a so-called “baseload power plant.” 
         [0003]    Peaks or spikes in customer power demand are handled by smaller and more responsive types of power plants called peaker power plants. Of course, a baseload power plant may be co-located with a peaker power plant. The time that a peaker plant operates may be many hours a day or as little as a few hours per year, depending on the condition of the region&#39;s electrical grid. It is expensive to build an efficient power plant, so if a peaker plant is only going to be run for a short or highly variable time, it may not make economic sense to make it as efficient as a base load power plant. In addition, the equipment and fuels used in base load plants are often unsuitable for use in peaker plants because the fluctuating conditions would severely strain the equipment. For these reasons, nuclear, geothermal, waste-to-energy, coal, and biomass plants are rarely, if ever, operated as peaker plants. 
         [0004]    Peaker plants are generally gas turbines that burn natural gas. A few burn petroleum-derived liquids, such as diesel oil and jet fuel, but they are usually more expensive than natural gas, so their use is limited. 
         [0005]    For greater efficiency, a Heat Recovery Steam Generator (HRSG) is added at the exhaust. This is known as a combined cycle plant. Cogeneration uses waste exhaust heat for process or other heating uses. Both of these options are used only in plants that are intended to be operated for longer periods than usual. 
         [0006]    Peak load requirements in the past have been met using different techniques depending on the duration and the maximum power requirements. Some are described below. 
         [0007]    One prior solution is the so-called “duct burning” solution. During the peak load operation, additional fuel is burned in the exhaust stack upstream of a heat recovery steam generator (HRSG) to produce additional heat and hence additional steam flow and concomitant power output in the bottoming cycle. The exhaust gas is oxygen depleted and, thus, the combustion is not highly efficient. Further the non-uniform temperature distribution may lead to HRSG tube life reduction. In addition, the turbines operate slightly in “off-design” conditions to account for more flow during the peak load operation, making the normal combined cycle mode operation less efficient and lower yielding. 
         [0008]    Another solution involves utilizing a simple cycle gas turbine. For applications requiring high peak loads over significantly long periods of time, supplementary simple cycle based gas turbines are used. Start-up time for such turbines must be short, ranging from 7-10 minutes, and it is an important design requirement. Such systems may operate with operating efficiency of about 37% and power output of 175 MW, for example. Such systems, however, may have low peak load efficiency due to un-recovered heat in their exhaust. In addition, these systems may require expensive and less reliable high temperature selective catalytic reduction (SCR) catalysts to reduce peaker cycle NO x  production. Further, exhaust fans for high temperature SCR are very expensive and themselves impose a high auxilary demand power penalty. In cases where an ammonia injection system is used in a peaker system, the external skids for the ammunia injection system is very high. 
       BRIEF DESCRIPTION 
       [0009]    According to one aspect of the invention, an electrical power generation system is provided. The system includes a first gas turbine and a heat recovery steam generator coupled to the gas turbine. The heat recovery steam generator includes a high pressure super-heater having a high pressure super-heater output. The system also includes a second gas turbine and an output duct coupled to the second gas turbine. The system also includes a supplemental high pressure super-heater within the output duct and thermally coupled to the high pressure super-heater output and an attemperator coupled between the high pressure super-heater output and the supplemental high pressure super-heater. 
         [0010]    According to one aspect of the invention, an electrical power generation system is provided. The system of this aspect includes a combined cycle and a peaker cycle. The combined cycle includes a gas turbine and a heat recovery steam generator coupled to the gas turbine. The heat recovery steam generator includes an intermediate pressure super-heater having an intermediate pressure super-heater output and a low pressure super-heater having a low pressure super-heater output. The peaker cycle includes a peaker gas turbine and an output duct coupled to the peaker gas turbine. The peaker cycle also includes a supplemental intermediate pressure super-heater within the output duct and thermally coupled to the high pressure super-heater output and a supplemental low pressure super-heater within the output duct and thermally coupled to the low pressure super-heater output. 
         [0011]    According to yet another aspect of the invention, a method for operating a system including a combined cycle and a peaker cycle, the combined cycle including a gas turbine and a heat recovery steam generator coupled to the gas turbine and a peaker cycle including a peaker gas turbine and an output duct. The method includes superheating a high pressure output product in the heat recovery steam generator; superheating the output product in a supplemental high pressure super-heater within the output duct after the output product has be superheated in the heat recovery steam generator and before the output product is provided a another turbine; and mixing the output product with water in an attemperator before the output product is superheated in the supplemental high pressure super-heater. 
         [0012]    These and other advantages and features will become more apparent from the following description taken in conjunction with the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
           [0014]      FIG. 1  is system diagram of an electrical power generation system; 
           [0015]      FIG. 2  is system diagram for the system shown in  FIG. 1  including an attemperator; 
           [0016]      FIG. 3  is system diagram of an electrical power generation system according to another embodiment of the present invention; and 
           [0017]      FIG. 4  is system diagram of an electrical power generation system according to another embodiment of the present invention. 
       
    
    
       [0018]    The detailed description explains embodiments of the invention, together with advantages and features, by way of example with reference to the drawings. 
       DETAILED DESCRIPTION 
       [0019]      FIG. 1  shows an example of an electrical power generation system  100  according to one embodiment of the present invention. The system  100  includes combined cycle  102  and a peaker cycle  104 . 
         [0020]    The combined cycle  102  may include a compressor  106  that includes an air intake  107 . The compressor  106  is coupled to a combustor  108  that combusts a gas or fuel oil in a stream of compressed air. The compressor  108  is coupled to a gas turbine  110 . The gas turbine  110  extracts energy from a flow of hot gas produced by combustion of gas or fuel. In one embodiment, the extracted engery is converted to electricity. 
         [0021]    The output  112  of the gas turbine  110  is an exhaust gas that may be used in other cycles of the combined system  100 . The exhaust gas may be used, for example, to heat steam for use in a steam turbine (not shown). The combined cycle, thus, includes an HSRG  114 . The HSRG  114  may include a high pressure super-heater  116 , an intermediate pressure super-heater  118  and a low pressure super-heater  120 . The HSRG  114  may include only the low pressure super-heater  120  or any combination of the low pressure super-heater  120  and another super-heater. 
         [0022]    The exhaust gas may be at a temperature of approximately 1150° F. Ultimately, the exhaust gas is processed in an exhaust duct  120  which includes a low temperature SCR  124  to treat the exhaust gas before it is released through the stack  126 . 
         [0023]    As described above, the system  100  also includes a peaker cycle  104 . In the prior art, such systems, however, had low peak load efficiency. One cause for this low efficiency may be due to un-recovered heat from exhaust gas in the peaker cycle. In addition, these systems required the utilization of less reliable and expensive high temperature SCR catalysts and an additional external high cost cooling fan to cool the exhaust gas to high temperature SCR levels. Further, such systems typically required an external skid for an ammonia injection system to effectively operate a high temperature SCR. 
         [0024]    The peaker cycle  104  includes a peaker gas turbine  130 . The peaker gas turbine  130  is coupled to a peaker compressor  132  by a peaker combustor  134 . The output temperature of the peaker exhaust gas  140  is about 1150° F. and is passed through a peaker exhaust duct  142 . The peaker exhaust duct  142  includes a low pressure supplementary super-heater  136  and a peaker low-temperature SCR  138 . Of course, the peaker HSRG  142  may be coupled to the stack  126 . 
         [0025]    An output of the low pressure super-heater  114  is coupled to an input of the supplementary low presssure super-heater  136 . An input of the low pressure super-heater may be coupled to a low pressure condenser (not shown). 
         [0026]    The temperature of the output product (typically steam) of the low pressure super-heater  114  is typically about 600° F. As the exhaust gas from the peaker gas turbine  130  passes through the peaker exhaust duct  142  it heats the output product to about 1050° F. while the gas itself cools to about 650° F. The cooling of the peaker gas turbine&#39;s  130  exhaust gas to this temperature allows for normal, rather than high temperature, SCR to be performed thereon. In addition, because output product has been heated in the supplementary low pressure super-heater  136 , the waste heat from the peaker cycle  104  has been recovered and, thus, the net efficiency of the combined cycle and peaker cycle is improved. 
         [0027]    The output of the supplemental low pressure super-heater  136  is provided to a low pressure turbine inlet nozzle. In one optional embodiment, and as indicated by the dashed arrow labeled  144 , the output of the supplemental low pressure super-heater  136  may be diverted to different stages of the low pressure turbine as taught, for example, in U.S. Pat. No. 6,442,924. 
         [0028]      FIG. 2  shows an example of an electrical power generation system  200  similar to that shown in  FIG. 1  with an optional low pressure water flow  202 . The optional low pressure water flow  202  includes a water pump  204  and a low pressure steam attemperator  206 . The optional additional water flow  202  may serve to reduce the temperature of the output product before introduction into a low pressure turbine. For example, the temperature of the output product entering the low pressure steam attemperator  206  may be about 1050° F. and leave at a temperature of about 700-800 ° F. 
         [0029]    It will be understood from the above description describes a system that allows for the recovery of energy from the peaker cycle. In particular, exhaust gas from the peaker turbine is used to superheat low pressure steam for later use by a combined cycle system and, thereby, increases the efficiency of the combined cycle system. 
         [0030]    Typically, intermediate and high pressure steam temperatures for a typical combined cycle plant are about 1050° F. This makes any additional super heating of these steams using peaker exhaust heat impossible. In one embodiment of the present invention, high pressure steam from the combined cycle is attemperated from about 1050° F. down to about 650° F. before allowing it to pass through the supplementary high pressure super-heater section in the peaker cycle exhaust path. This enables the peaker cycle exhaust heat (temperature about 1150° F.) recovery using high-pressure steam that has high heat-work conversion efficiency. In addition to that, optional LP steam circuit from the combined cycle is also used to reduce the exhaust heat to lowest possible temperature. Overall, this embodiment may enable additional power output gain of 50 MW during peaker cycle operation, compared to 35 MW gain from previous designs. Further, the temperature levels upstream of the catalyst can be maintained as low as required for a combined cycle catalyst to ensure efficient operation of the catalyst. 
         [0031]    In addition, because the energy has been recovered from the exhaust gas it is at a reduced temperature and, therefore, may be treated with normal, rather than high temperature, SCR catalyst. The above description has described only superheating the low pressure steam. Of course, and described below, high and intermediate pressure steam may also be superheated according to embodiments of the present invention. Of course, only high pressure steam, intermediate pressure steam or a combination of both may be superheated by the peaker exhaust gas according to different embodiments of the present invention. That is, embodiments of the present invention are directed to superheating one or more of low, intermediate and high pressure steam with peaker exhaust gas. 
         [0032]      FIG. 3  shows a system  300  according to another embodiment of the present invention. The system includes a peaker cycle  302 . The remainder of the system  300  not included in the peaker cycle  302  shall be referred to herein as a combined cycle. 
         [0033]    The peaker cycle  302  includes a compressor  303 , a combustor  304  and a gas turbine  306 . As discussed above, the gas turbine  306  extracts energy from a flow of hot gas produced by combustion of gas or fuel. In one embodiment, the extracted engery is converted to electricity. The peaker cycle  302 , also includes an output duct  308 . The output duct  308  processes exhaust gas from the gas turbine  306  before it is expelled to the environment. The processing of the exhaust may be accomplished by SCR  314 . In one embodiment, the SCR  314  utilizes normal, rather than high, temperature catalysts. 
         [0034]    The output duct  308  also includes a supplemental low pressure super-heater  310  and a supplemental high pressure super-heater  312 . The supplemental super-heaters  310  and  312  are, respectively, connected to the output of a low pressure super-heater  316  and a high pressure super-heater contained in the combined cycle. 
         [0035]    The combined cycle may include a compressor  320  that includes an air intake  321 . The compressor  320  is coupled to a combustor  322  that combusts a gas or fuel oil in a stream of compressed air. The compressor  322  is coupled to a gas turbine  324 . The gas turbine  324  extracts energy from a flow of hot gas produced by combustion of gas or fuel. In one embodiment, the extracted energy is converted to electricity. 
         [0036]    The output of the gas turbine  324  is an exhaust gas that may be used in other cycles of the combined system. The exhaust gas may be used, for example, to heat steam for use in a high pressure steam turbine  326  and a low pressure steam turbine  328 . To that end, the combined cycle includes an HSRG  330 . The HSRG  330  may include a low pressure super-heater  316  and high pressure super-heater  318 . 
         [0037]    Steam passing through the high pressure super-heater  318  exits at temperature of about 1050° F. The steam is mixed with water by the high pressure pre-attemperator  332 . In one embodiment, the steam leaves the high pressure pre-attemperator  332  at a temperature of about 650° F. The high pressure pre-attemperator  332  receives the steam from an output of the high pressure super-heater  318  and water from a first pump  336 . 
         [0038]    Steam passing through the low pressure super-heater  3   16  exits at temperature of about 550° F. The steam is mixed with water by the low pressure pre-attemperator  334 . In one embodiment, the steam leaves the low pressure pre-attemperator  334  at a temperature of about 350° F. The low pressure pre-attemperator  334  receives the steam from an output of the low pressure super-heater  316  and water from a second pump  338 . The super-heaters  316  and  318  receive water from a third pump  339 . 
         [0039]    The output of both the high pressure pre-attemperator  332  and the low pressure pre-attemperator  334  are then superheated in the output duct  308  of the peaker cycle  302 . In particular, the output of the high pressure pre-attemperator  332  is connected to an input of the supplementary high pressure super-heater  3   12 . In one embodiment, the output of the high pressure pre-attemperator  332  is superheated to a temperature of about 1050° F. by the supplementary high pressure super-heater  312 . The output of the supplementary high pressure super-heater  312  is coupled to an input of a high/intermediate pressure steam turbine  340 . 
         [0040]    The output of the low pressure pre-attemperator  334  is connected to an input of the supplementary low pressure super-heater  310 . In one embodiment, the output of the low pressure pre-attemperator  334  is superheated to a temperature of about 600° F. by the supplementary low pressure super-heater  31   0 . The output of the supplementary low pressure super-heater  310  is coupled to an input of a low pressure steam turbine  342 . Remaining steam from the high/intermediate pressure steam turbine  340  and the low pressure steam turbine  342  is condensed in the condenser  344  and the water produced therein is provided to the pumps  336 ,  338  and  339 . 
         [0041]      FIG. 4  shows a system  400  according to another embodiment of the present invention. The system includes a peaker cycle  402 . The remainder of the system  400  not included in the peaker cycle  402  shall be referred to herein as a combined cycle. 
         [0042]    The peaker cycle  402  includes a compressor  403 , a combustor  404  and a gas turbine  406 . As discussed above, the gas turbine  406  extracts energy from a flow of hot gas produced by combustion of gas or fuel. In one embodiment, the extracted engery is converted to electricity. The peaker cycle  402  also includes an output duct  408 . The output duct  408  processes exhaust gas from the gas turbine  406  before it is expelled to the environment. The processing of the exhaust may be accomplished by SCR  414 . In one embodiment, the SCR  414  utilizes normal, rather than high, temperature catalysts. 
         [0043]    The output duct  408  also includes a supplemental low pressure super-heater  410  and a supplemental intermediate pressure super-heater  412 . The supplemental super-heaters  410  and  412  are, respectively, connected to the output of a low pressure super-heater  416  and a intermediate pressure super-heater  418  contained in the combined cycle. 
         [0044]    The combined cycle may include a compressor  420  that includes an air intake  421 . The compressor  420  is coupled to a combustor  422  that combusts a gas or fuel oil in a stream of compressed air. The compressor  422  is coupled to a gas turbine  424 . The gas turbine  424  extracts energy from a flow of hot gas produced by combustion of gas or fuel. In one embodiment, the extracted engery is converted to electricity. 
         [0045]    The output of the gas turbine  424  is an exhaust gas that may be used in other cycles of the combined system. The exhaust gas may be used, for example, to heat steam for use in an intermediate pressure steam turbine  426  and a low pressure steam turbine  428 . To that end, the combined cycle includes an HSRG  430 . The HSRG  430  may includes a low pressure super-heater  416  and an intermediate pressure super-heater  418 . 
         [0046]    Steam passing through the intermediate pressure super-heater  418  exits at temperature of about 1050° F. The steam is mixed with water by the intermediate pressure pre-attemperator  432 . In one embodiment, the steam leaves the intermediate pressure pre-attemperator  432  at a temperature of about 550° F. The intermediate pressure pre-attemperator  432  receives the steam from an output of the intermediate pressure super-heater  418  and water from a first pump  436 . 
         [0047]    Steam passing through the low pressure super-heater  416  exits at temperature of about 600° F. The steam is mixed with water by the low pressure pre-attemperator  434 . In one embodiment, the steam leaves the low pressure pre-attemperator  434  at a temperature of about 350° F. The low pressure pre-attemperator  434  receives the steam from an output of the low pressure super-heater  416  and water from a second pump  438 . The super-heaters  416  and  418  receive water from a third pump  439 . 
         [0048]    The output of both the intermediate pressure pre-attemperator  432  and the low pressure pre-attemperator  434  are then superheated in the output duct  408  of the peaker cycle  402 . In particular, the output of the intermediate pressure pre-attemperator  432  is connected to an input of the supplementary intermediate pressure super-heater  412 . In one embodiment, the output of the intermediate pressure pre-attemperator  432  is superheated to a temperature of about 1050° F. by the supplementary intermediate pressure super-heater  412 . The output of the supplementary intermediate pressure super-heater  412  is coupled to an input of a intermediate pressure steam turbine  440 . 
         [0049]    The output of the low pressure pre-attemperator  434  is connected to an input of the supplementary low pressure super-heater  410 . In one embodiment, the output of the low pressure pre-attemperator  434  is superheated to a temperature of about  600  OF by the supplementary low pressure super-heater  410 . The output of the supplementary low pressure super-heater  410  is coupled to an input of a low pressure steam turbine  442 . Remaining steam from the intermediate pressure steam turbine  440  and the low pressure steam turbine  442  is condensed in the condensor  444  and the water produced therein is provided to the pumps  436 ,  438  and  439 . 
         [0050]    While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.