Abstract:
In one aspect, an embodiment of the present disclosure provides an Integrated Gasification Combined Cycle (IGCC) apparatus. The apparatus includes a saturator configured to saturate NPG with water vapor, and a heat recovery steam generator (HRSG), a low pressure steam loop through the saturator, wherein the HRSG is configured to heat the low pressure steam loop. The apparatus further includes a compressor and a heat exchanger configured to heat the NPG using waste process heat and extraction air from the compressor, wherein the heated NPG thereby becomes diluent nitrogen.

Description:
BACKGROUND 
       [0001]    The present invention relates generally to Integrated Gasification Combined Cycle (IGCC) power plants, and more specifically to methods and apparatus for using diluent nitrogen with dry syngas in IGCC plants. 
         [0002]    In at least some known IGCC plants and in certain operating conditions, diluent is mixed with syngas to improve the mass flow through a combustor. Nitrogen process gas from an air separator is commonly used as diluent. When additional diluent is needed, the syngas is saturated with water vapor, which acts as an additional diluent. However, the addition of water vapor in the syngas may cause variations in the heating value of the syngas. Such variations can lead to variances in firing temperature, and/or may adversely affect the performance and/or efficiency of the combustor. Moreover, moisture in the syngas adversely lowers the combustor firing temperature and combustor efficiency. As such, the possible adverse affects of the addition of water vapor may outweigh any benefits. 
         [0003]    Thus, methods and apparatus to supply diluent to the combustor are desirable to maintain a constant heating value of the syngas. Maintaining a constant heating facilitates the combustor maintaining a constant firing temperature such that the combustor&#39;s performance and efficiency is also maintained. In addition, methods and apparatus to supply diluent to the combustor so that dry syngas can be burned efficiently are desirable to raise the combustor firing temperature and further enhance the combustor&#39;s performance and efficiency. Also desirable are methods and apparatus to facilitate enhancing the use of waste process heat to increase the efficiency of IGCC plants. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0004]    In one aspect, a method for introducing diluent into a syngas stream in an integrated gasification combined cycle (IGCC) plant having a combustor is provided. The plant also includes a heat recovery steam generator. The method includes producing nitrogen process gas (NPG) and water vapor, saturating the NPG with the water vapor, raising the temperature of the saturated NPG using heat from the heat recovery steam generator (HRSG), waste process heat and compressor extraction air, and supplying essentially dry syngas to the combustor along with the saturated and heated NPG. 
         [0005]    In another aspect, an embodiment of the present disclosure provides a method for using nitrogen saturation and heating to facilitate increasing the efficiency of an IGCC having a combustor. The method includes using low level process heat to pre-heat saturated diluent nitrogen and using low level process heat to heat steam turbine condensate. 
         [0006]    In yet another aspect, an embodiment of the present disclosure provides an Integrated Gasification Combined Cycle (IGCC) apparatus. The apparatus includes a saturator configured to saturate NPG with water vapor, and a heat recovery steam generator (HRSG), a low pressure steam loop through the saturator, wherein the HRSG is configured to heat the low pressure steam loop. The apparatus further includes a compressor and a heat exchanger configured to heat the NPG using waste process heat and extraction air from the compressor, wherein the heated NPG thereby becomes diluent nitrogen. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a block diagram of an exemplary Integrated Gasification Combined Cycle Plant (IGCC) including equipment used to supply saturated diluent and dry syngas to a combustor. 
       
    
    
     DETAILED DESCRIPTION 
       [0008]    As used herein, the term syngas refers to synthesis gas made from partially oxidized hydro-carbonaceous feedstock. Syngas varies in its exact composition based on the feedstock used, but generally includes mostly carbon monoxide, hydrogen, water, carbon dioxide and may also include impurities such as hydrogen sulfide. Syngas is used as fuel in the combustor of at least some Integrated Gasification Combine Cycle (IGCC) plants. 
         [0009]    In the exemplary embodiment, an IGCC plant  21  includes a gasifier  23  and a combustor  25  that is coupled in flow communication with a turbine  29  and a compressor  27 . Compressor  27  is up stream from, and is in flow communication with turbine  29 . Turbine  29  is rotatably coupled to an electrical generator  31  and turbine  29  is also in flow communication with a heat recovery steam generator (HRSG)  33 . HRSG  33  is rotatably coupled to a steam turbine  35  that is also rotatably coupled to an additional electrical generator  37 . In the exemplary embodiment, an air separator  39  is coupled in flow communication with gasifier  23  and combustor  25 . 
         [0010]    During operation, Gasifier  23  partially oxidizes hydro-carbonaceous feedstock to make syngas. Combustor  25  burns the syngas to produce high temperature, high-pressure gas. Compressor  27  compresses ambient air that is then discharged towards turbine  29  along with the high-pressure gas discharged from combustor  25 . As the high-pressure gas expands, it induces rotation of turbine  29 . As turbine  29  rotates, it powers electrical generator  31 . HRSG  33  receives the hot gases discharged from turbine  29  and uses heat contained in such gases to boil water to produce steam. The resulting steam induces rotation of steam turbine  35 , which powers electrical generator  37 . 
         [0011]    In the exemplary embodiment, IGCC plant  21  also includes saturator  41 , heat exchanger  43 , heat exchanger  45  and heat exchanger  47 . In other embodiments, IGCC plant  21  does not include at least one of saturator  41 , heat exchanger  43 , heat exchanger  45 , and/or heat exchanger  47 . 
         [0012]    In operation, air separator  39  receives ambient air and separates the received air into oxygen and nitrogen process gas (NPG) streams. The oxygen stream is channeled to gasifier  23  for use in partially oxidizing hydro-carbonaceous feedstock to make syngas. In the exemplary embodiment, the NPG contains approximately 95% nitrogen by weight and may contain oxygen, argon, and/or other trace atmospheric constituents. Moreover, in the exemplary embodiment, the NPG contains approximately 2% water vapor or less by weight. As the NPG exits air separator  39 , the NPG is at approximately 250° F. (121° C.) and 320 psig (22.5 kg/square cm). Since this NPG is readily available and is non-combustible, in some embodiments, air separator  39  channels the NPG directly to combustor  25 , where the NPG is used as a fuel diluent to facilitate increasing the mass flow through combustor  25 . Increasing the mass flow through combustor  25  facilitates increase in the amount of thrust produced as the gas exits combustor  25 . 
         [0013]    As the NPG exits air separator  39 , the NPG is channeled to saturator  41 . In one embodiment, saturator  41  increases a trayed tower that enables NPG and low pressure steam to mix with little or no pressure losses. In other embodiments, saturator  41  may be other than a trayed tower. In the exemplary embodiment, the low pressure steam circulates from saturator  41  to HRSG  33  and is then returned to saturator  41 . More specifically, in the exemplary embodiment, the steam exits saturator  41  at approximately 210° F. (99° C.) before it is heated in HRSG  33  to approximately 305° F. (152° C.)], prior to being returned to saturator  41  wherein it imparts its heat to the NPG. The saturated NPG that leaves saturator  41  contains approximately 16% water and is at approximately 285° F. (140° C.). In at least one embodiment, before the saturated NPG reaches combustor  25 , the NPG is superheated to facilitate providing water condensation inside combustor  25  and to facilitate minimizing or at least reducing an amount of energy required by combustor  25 . Reducing the amount of energy required by combustor  25  facilitates increasing the overall efficiency of combuster  25 . 
         [0014]    The saturated NPG is heated by heat exchanger  43  and by heat exchanger  45 . After the saturated NPG leaves saturator  41 , the NPG is routed through heat exchanger  43 . In heat exchanger  43 , waste process heat imparts its heat to the saturated NPG stream. In one embodiment, an operator may select the source of the waste process heat. In another embodiment, the source of waste process heat is preselected. Also, in some embodiments, extraction air from compressor  27  discharged at approximately 700° F. (371° C.), imparts its heat to the saturated NPG stream within heat exchanger  43 . As the saturated NPG leaves heat exchanger  43 , the NPG is at approximately 650° F. (343° C.) and 310 psig (22 kg/square cm). The saturated NPG is then channeled through heat exchanger  45 . In heat exchanger  45 , extraction air from compressor  27  at approximately 800° F. (427° C.) and 220 psig (15 kg/square cm) imparts its heat to the saturated NPG stream. 
         [0015]    The saturated NPG is discharged from heat exchanger  45  at a temperature of approximately 680° F. (360° C.) and a pressure of between about 150 to about 300 pounds per square inch absolute (psia) (10.5-21 kg/square cm). In another embodiment, the saturated NPG is discharged from heat exchanger  45  at a pressure greater than 300 psia (21 kg/square cm). The saturated NPG is then channeled to combustor  25  for use as a diluent. The saturated NPG, essentially dry syngas, oxygen and carbon dioxide are injected into combustor  25 . The use of dry syngas results in an approximately constant heating value of the fuel supplied to combustor  25 . The constant fuel heating value facilitates combustor  37  operating with an approximately constant temperature and with an enhanced efficiency. The dry syngas may also have a constant, but low, moisture level. 
         [0016]    Extraction air from compressor  27 , at approximately 800° F. (427° C.), imparts some of its heat to the saturated NPG within heat exchanger  43 . Extraction air is discharged from heat exchanger  43  at approximately 700° F. (371° C.), wherein the extraction air then imparts more of its heat to the saturated NPG in heat exchanger  45 . Upon discharge from heat exchanger  45 , the extraction air is at approximately 320° F. (160° C.). The extraction air then imparts more of its heat to condensate from steam turbine  35  in heat exchanger  47 . The extraction air is discharged from heat exchanger  47  at approximately 130° F. (54° C.) and is channeled to air separator  39  for reuse. 
         [0017]    In at least one known IGCC system, condensate from a steam turbine is heated in an HRSG. However, in embodiments of the present invention, HRSG  33  heats steam for use in saturator  41 . Condensate from steam turbine  35  is heated with waste process heat. This source of this waste product heat is, in at least one embodiment, selectable by an operator, or in another embodiment, preselected from any other source. 
         [0018]    One distinguishing feature of some embodiments of the present disclosure is that only the diluent nitrogen, and not the syngas, is saturated, and that HRSG  33  is the primary source of heat. 
         [0019]    Condensate from steam turbine  35  is at approximately 100° F. (38° C.) when discharged from steam turbine  35 . The condensate is channeled through heat exchanger  47  where it acquires some of the heat from extraction air discharged from compressor  25 , as well as heat from waste process heat. After the condensate is discharged from heat exchanger  47  it is at approximately 210° F. (99° C.) and it is channeled to HRSG  33  for heating into steam for use by steam turbine  35 . 
         [0020]    The use of low level process heat to increase the temperature of the saturated NPG and the condensate from steam turbine  35  facilitates increasing the heat recovery of IGCC  21  and increasing its efficiency. More specifically, embodiments of the present invention provide methods and apparatus to supply diluent to the combustor maintain a constant heating value of the syngas. Maintaining a constant heating facilitates the combustor maintaining a constant firing temperature such that the combustor&#39;s performance and efficiency is also maintained. In addition, embodiments of the present invention provide methods and apparatus to supply diluent to the combustor so that dry syngas can be burned efficiently to raise the combustor firing temperature and further enhance the combustor&#39;s performance and efficiency. In addition, embodiments of the present invention provide methods and apparatus to facilitate enhancing the use of waste process heat to increase the efficiency of IGCC plants. 
         [0021]    When introducing elements of the present invention or preferred embodiments thereof, the articles “a”, “an”, “the”, and “said” are intended to mean that there are one or more of the elements. The terms “comprising”, “including”, and “having” are intended to be inclusive and allow additional elements other than the listed elements. The phrases “in one embodiment,” “in at least one embodiment,” or “in some embodiments” are not intended to limit the inclusion of any recited features and/or elements to exactly one embodiment. Features and/or elements described as being in any embodiment may be included in any other embodiment, unless such features and/or elements are mutually exclusive. 
         [0022]    The temperatures, pressures, and other data recited herein represent or are indicative of operating conditions of an exemplary embodiment and are not necessarily intended as specifications required for any particular embodiment. 
         [0023]    As various changes could be made in the above constructions and methods without departing from the scope of the invention, it is intended that all matter contained in the above description and shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.