Abstract:
A diluent nitrogen compressor inlet cooling system comprises a bottoming cycle heat source; a vapor absorption chiller powered by the bottoming cycle heat source, the vapor absorption chiller being configured to cool diluent nitrogen; and a diluent nitrogen compressor that receives the cooled diluent nitrogen from the vapor absorption chiller. A method for cooling an inlet of a diluent nitrogen compressor comprises powering a vapor absorption chiller using a bottoming cycle heat source from the integrated gasification combined cycle system; indirectly cooling diluent nitrogen by the vapor absorption chiller; and sending the cooled diluent nitrogen to a diluent nitrogen compressor inlet.

Description:
BACKGROUND OF THE INVENTION 
     The subject matter disclosed herein relates to an improved integrated gasification combined cycle (IGCC) power plant. 
     An integrated gasification combined cycle (IGCC) power generating plant performs a two-stage combustion with cleanup between stages. The first stage includes a gasifier for partial oxidation of a fossil fuel, such as coal or heavy fuel oil, and the second stage utilizes a gas turbine combustor for burning the fuel gas produced by the gasifer. Performance of the gas turbine combustor is enhanced by the addition of compressed diluent nitrogen. Diluent nitrogen from an air separation unit (ASU) in the IGCC is compressed in stages by a diluent nitrogen compressor (DGAN) and inter-cooled between stages by a cooling tower water source. The compressed nitrogen is then supplied to the gas turbine combustor. The DGAN compressor consumes power as an auxiliary load. The DGAN compressor may consume a large amount of power, lowering the overall efficiency of the IGCC power plant. 
     Accordingly, there remains a need in the art for a reduction in the power load consumed by a DGAN compressor operated in conjunction with an IGCC power plant. 
     BRIEF DESCRIPTION OF THE INVENTION 
     According to one aspect of the invention, a diluent nitrogen compressor inlet cooling system comprises a bottoming cycle heat source; a vapor absorption chiller powered by the bottoming cycle heat source, the vapor absorption chiller being configured to cool diluent nitrogen; and a diluent nitrogen compressor that receives the cooled diluent nitrogen from the vapor absorption chiller. 
     According to another aspect of the invention, a method for cooling an inlet of a diluent nitrogen compressor comprises powering a vapor absorption chiller using a bottoming cycle heat source from the integrated gasification combined cycle system; indirectly cooling diluent nitrogen by the vapor absorption chiller; and sending the cooled diluent nitrogen to a diluent nitrogen compressor inlet. 
     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 
       The subject matter that 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 objects, features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
         FIG. 1  is an embodiment of a vapor absorption chiller (VAC). 
         FIG. 2  is an embodiment of a DGAN compressor inlet in conjunction with a VAC. 
         FIG. 3  is an embodiment of a DGAN compressor inlet in conjunction with a VAC. 
         FIG. 4  is an embodiment of a method of cooling nitrogen supplied to a DGAN compressor inlet. 
     
    
    
     The detailed description explains embodiments of the invention, together with advantages and features, by way of example with reference to the drawings. 
     DETAILED DESCRIPTION OF THE INVENTION 
     DGAN power consumption is directly proportional to DGAN compressor inlet temperature. For a given mass flow, the power consumption of the DGAN compressor is higher at a higher temperature; the density of nitrogen decreases as temperature rises, requiring more compression. Cooling the inlet nitrogen temperature results in denser nitrogen, requiring less compression, reducing DGAN compressor power consumption. For example, power consumption of a DGAN compressor power may be reduced by about 2 MW by a reduction of the inlet temperature of about 40° F. 
     Low grade or waste heat from the bottoming cycle of the IGCC power plant may be used to run a vapor absorption chiller (VAC) system. The VAC may generate a chilling media to cool the nitrogen before it reaches the inlet of the DGAN compressor. Cooling reduces the volumetric flow through the compressor, resulting in reduction of compressor work. Thus the plant auxiliary load is reduced, resulting in an output gain of approximately 1 MW to 1.8 MW and a net efficiency gain of approximately 0.08%-0.12% for embodiments of an IGCC power plant. 
       FIG. 1  shows an embodiment of a vapor absorption chiller (VAC)  100 . VAC  100  comprises four sections: absorber  101 , generator  102 , condenser  103 , and evaporator  104 . Evaporator  104  is kept at a low pressure, or vacuum. The vacuum in evaporator  104  causes a refrigerant, such as ammonia (NH 3 ), to boil at very low temperature; the boiling refrigerant absorbs heat from a cooling media that is circulated to and from the evaporator  104  via pipes  109  and  110 . The heat transfer between the cooling media and the refrigerant converts the refrigerant into a vapor. The refrigerant vapor is sent to absorber  101 . In absorber  101 , the refrigerant vapor is absorbed into water. The refrigerant-enriched water is pumped from absorber  101  to generator  102  via pipe  108 . Heat from a bottoming cycle heat source in the form of hot water or steam is received by generator  102  via pipe  115 , and water or steam at a reduced temperature is output from the generator  102  via pipe  116 . The heat from the hot water or steam in pipe  115  is transferred to the refrigerant-enriched water from pipe  108  in generator  102 . The heat from the hot water or steam from pipe  115  boils the refrigerant off from the refrigerant-enriched water received from pipe  108 , resulting in refrigerant vapor and hot water. The hot water is sent to the absorber via pipe  107 . Excess heat is removed from the hot water in absorber  101  by cooling water flow from a cooling tower, circulated via pipes  111  and  112 . The refrigerant vapor from the generator  102  is sent to condenser  103  via pipe  105 , where it is converted to a liquid by exchanging heat with cool water from a cooling tower, circulated via pipes  113  and  114 . The liquid refrigerant is then sent to back the vacuum in the evaporator  104  via pipe  106 , where it absorbs heat from the cooling media circulating in pipes  109  and  110 . The cooling media circulates via pipes  109  and  110  between evaporator  104  and a nitrogen chiller, which is discussed below with regards to  FIGS. 2 and 3 . 
     The heat source that powers the VAC  100  may be selected so as not to affect the overall performance of the IGCC power plant. Any bottoming cycle heat source in the IGCC power plant that has sufficient flow, pressure, and temperature for proper operation of the VAC  100  may be selected. Three examples of bottoming cycle heat sources in the IGCC power plant that may be used to power VAC  100  include: stack flue gas heat, steam from a steam seal regulator (SSR), or evaporator blow down flow. 
     Stack flue gas may be used to heat water. In some embodiments, a low pressure economizer (LPE) may heat the water using the stack flue gas. The heated water may be used to power VAC  100 . Water heated using stack flue gas may reach a temperature of about 160° F. and a pressure of about 14.7 psi. 
     Steam comes from an SSR outlet at about 600° F. and about 20 psi pressure. This steam may also be used to power VAC  100  in some embodiments. The VAC  100  may require a minimum of about 21 psi pressure to operate, so the steam pressure may be increased to about 25 psi by using a steam compressor. The steam compressor will increase the steam temperature to about 665° F. The steam is condensed in the VAC and is discharged to a gland seal condenser (GSC) as water at a temperature of about 240° F. The water may be used in the GSC for pre-heating condensate, or may by-pass the GSC if pre-heating is not necessary for the particular IGCC power plant. 
     Water comes from the evaporator blow down flow at about 200° F. This water may also be harnessed to run VAC  100  in some embodiments. 
       FIG. 2  illustrates an embodiment  200  of a DGAN compressor inlet in conjunction with a VAC that is powered by water heated using stack flue gas. Stack flue gas is input to low pressure economizer (LPE)  212  via pipe  213 , and flue gas is output to the stack by pipe  214 . LPE  212  heats water using the stack flue gas and sends the hot water via pipe  204  to VAC  201 . The VAC  201  uses the heated water received through pipe  204  to power the generator, which powers the absorber, evaporator, and condenser, as discussed above with regards to  FIG. 1 . Water or steam at reduced temperature is output from VAC  201  via pipe  205 . The cooling media from the VAC  201  travels to and from the nitrogen cooler  202  via pipes  206  and  207 . Nitrogen cooler  202  receives diluent nitrogen through pipe  208 , chills the diluent nitrogen using the cooling media circulating via pipes  206  and  207 , and outputs cooled diluent nitrogen to DGAN compressor inlet  203  via pipe  209 . 
       FIG. 3  illustrates an embodiment  300  of a DGAN compressor inlet in conjunction with a VAC  301  that is powered by steam or hot water. VAC  301  receives steam or hot water through pipe  304 . Pipe  304  may carry steam from, for example, an SSR outlet or hot water from an evaporator blow down flow. The steam or hot water from pipe  304  is used to power the generator, which powers the absorber, evaporator, and condenser, as discussed above with regards to  FIG. 1 . Water or steam at reduced temperature is output from VAC  301  via pipe  305 ; in embodiments that receive steam from the SSR, pipe  305  may connect to a gland seal condenser (GSC). A cooling media generated by VAC  301  travels to and from diluent nitrogen cooler  302  via pipes  306  and  307 . Nitrogen cooler  302  receives nitrogen from pipe  308 , cools the diluent nitrogen with the cooling media circulating via pipes  306  and  307 , and outputs the cooled diluent nitrogen to DGAN compressor inlet  303  via pipe  309 . 
       FIG. 4  illustrates an embodiment of a method  400  of cooling nitrogen supplied to a DGAN compressor inlet. In block  401 , heat from a bottoming cycle heat source is used to power a VAC. In block  402 , the VAC cools diluent nitrogen. In block  403 , the cooled diluent nitrogen is sent to the DGAN compressor inlet. 
     For an example IGCC power plant, specifically, a General Electric Multi-Shaft STAG 207FB IGCC, with 2 GTs and 2 gasifiers burning Illinois Basin coal at ISO day, the initial DGAN compressor inlet temperature is about 80° F. The DGAN compressor inlet temperature may be brought down to about 64° F. by a VAC using SSR steam as a heat source, improving the power consumption of the DGAN compressor by about 1 MW, and giving an efficiency gain of about 0.08%. The DGAN compressor inlet temperature may be reduced to about 44° F. by a VAC using stack flue gas as a heat source, improving the DGAN compressor power consumption by about 1.8 MW, and giving an efficiency gain of about 0.12%. The initial investment required to install the VAC system is low compared to the savings due to efficiency improvements over the IGCC power plant&#39;s life cycle. 
     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.