Abstract:
An integrated gasification combined cycle system. In one embodiment (FIG.  2 ) a system ( 200 ) includes an ion transport membrane air separation unit ( 210 ) for producing oxygen-enriched gas ( 209 ) and oxygen-depleted air ( 227 ), a gasification system ( 5 ) for generating syngas with the oxygen-enriched gas ( 209 ), a gas combustor ( 234 ) for reacting the syngas ( 224 ), and a subsystem configured to provide a first stream of air to the combustor ( 234 ) at a first pressure and to provide a second stream of air to the air separation unit ( 210 ) at a second pressure greater than the first pressure. The subsystem includes a compressor ( 230 ) having multi-pressure outlets ( 203, 204 ).

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates generally to power generation systems, and more particularly, to integrated gasification combined cycle systems. 
       BACKGROUND OF THE INVENTION 
       [0002]    Integrated Gasification Combined Cycle (IGCC) systems are an economically attractive alternative to Natural Gas Combined Cycle systems (NGCC), as the systems can use more abundant fuel sources such as coal or biomass. IGCC systems gasify the low heating value fuel and produce a mixture comprising hydrogen and carbon monoxide. IGCC systems also have greater potential for efficiency improvement and a decrease in undesirable emissions compared to conventional coal-fired steam power plants. 
         [0003]    IGCC power plants having oxygen-blown gasifiers to generate syngas require a relatively pure stream of oxygen gas. Production of this oxygen supply can be achieved by various means. A well-known technique is the cryogenic air separation method, in which the partial pressure differences between oxygen and other air constituents is exploited at a very low temperature and an elevated pressure to effect phase differences that are used to separate the air components. One disadvantage of using cryogenic systems for oxygen separation is that the compression stage requires significant power consumption. This reduces the plant output and net efficiency. Another air separation technology involves use of an Ion Transport Membrane (ITM) to remove oxygen from a high temperature, pressurized air stream. The resulting ITM system output streams are: (i) an oxygen-enriched gas supply delivered at a high temperature and ambient pressure, and (ii) an oxygen-depleted air supply delivered at a high temperature and a high pressure. A compressor and an air pre-heater are generally employed to provide the high temperature, high pressure air stream, adding significant equipment installation and operational cost for deployment of the ITM technology in IGCC systems. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]    Features of the invention will be best understood when the following detailed description is read in conjunction with the accompanying drawings, wherein: 
           [0005]      FIG. 1  is a schematic representation of a conventional Integrated Gasification Combined Cycle (IGCC) system; 
           [0006]      FIG. 2  is a schematic representation of an IGCC system according to an embodiment of the invention; and 
           [0007]      FIG. 3  is a schematic representation of an IGCC system according to another embodiment of the invention. 
       
    
    
       [0008]    In accord with common practice, the various described device features are not drawn to scale, but are drawn to emphasize specific features relevant to the invention. Like reference characters denote like elements throughout the figures and text. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0009]    IGCC systems employing the ITM air separation technology require two compressed air streams, one for the ITM air separation process and one for combustion of the fuel mixture in a gas combustor. The ITM process requires compressed air, generally in the range of 150-500 psia. The gas combustor of the IGCC gas turbine system requires compressed air, generally in the range of 120 psia-475 psia. It is now recognized as advantageous to generate two compressed air streams from a single compressor. This approach, using, for example, a compressor having multipressure outlets, reduces capital equipment costs as well as the operational costs of IGCC systems. 
         [0010]    In the past, high pressure oxygen-depleted air produced by the ITM has been injected into the fuel mixture entering the combustor of the IGCC gas turbine system. This facilitates temperature control and NO x  emission reduction. The pressure of the oxygen-depleted air relative to the compressed air supplied to the ITM ASU is reduced, due to frictional losses in the ITM system and in return piping. The pressure of the depleted air as it is injected into the fuel flow is lower than the pressure of compressed air that is routed directly from the gas turbine compressor to the combustor. In order to equalize the pressure of the two streams, the higher pressure stream of air from the compressor has been throttled, but this results in a loss of gas turbine efficiency. 
         [0011]    By way of example, to avoid this loss in efficiency, a compressor having multipressure outlets may be integrated with a gas turbine system and a gasification system. Two compressed air streams can be generated, each at a different pressure. The integrated system eliminates the need for throttling of the gas turbine compressor outlet stream that is routed to the combustor. The lower pressure air stream output from the compressor is mixed with the oxygen-depleted air from the ITM prior to introduction to the combustor. 
         [0012]    A conventional IGCC system  100 , shown in  FIG. 1 , comprises a gasification system  1 , a gas turbine system  2 , a heat recovery steam generator (HRSG)  3 , and a steam turbine system  4 . The gasification system  1  includes a carbonaceous fuel  106 , such as a coal slurry, a gasifier  108  and an ITM Air Separation Unit (ASU)  110 . The gas turbine system  2  includes an air-compressor  130 , a throttle valve  132 , a combustor  134 , a gas turbine  136 , and an electrical generator  140 . The HRSG  3  comprises an economizer  172 , a steam drum  174 , an evaporator  176 , and a super heater  178 . The steam turbine system  4  includes a steam turbine  160 , an electrical generator  162 , a condenser  164  and a feed-water pump  166 . 
         [0013]    In the gasification system  1 , oxygen  109 , e.g., in an oxygen-enriched gas supply, is provided to the gasifier  108  from the ITM ASU  110 . The ASU  110  produces oxygen-enriched gas  109  while operating in a temperature range of about 1300-1700° F. and with an oxygen partial pressure differential across an ion transport membrane of 160 to 285 psia. The compressor  130  in the gas turbine system  2  develops a stream of high pressure air  131  from ambient air  129 . A portion  111  of the high pressure air  131  is delivered to the pre-heater  112 , where the membrane operating temperature is achieved by heat exchange to extract sensible heat from one or more sources, including the hot gas  137  exiting the gas turbine  136 . In the gasifier  108 , the carbonaceous fuel  106  undergoes partial oxidation with the oxygen-enriched gas  109  to generate syngas  117 , which primarily comprises carbon monoxide and hydrogen, in a highly exothermic reaction, generally in the temperature range of about 2000° F.-2800° F. To meet air quality requirements, impurities such as sulfides, nitrous components, and dust particles are removed in the gas clean-up unit  120 . The syngas cooler  118  reduces the syngas temperature before introduction to the gas clean-up unit  120 . The cooler  118  may, as illustrated, use a portion  191  of feed-water  167  from the steam turbine system  4  to recover the syngas heat. The steam  192  produced from the feed-water  191  by the syngas cooler  118  can be sent to the steam chest  152 . The cleaned syngas  124  is mixed with steam  126  from the steam chest  152  to regulate the combustion process temperature, the internal combustor temperature profile, and the combustor exit temperature by varying the steam flow rate. The mixture  128  of steam  126  and syngas  124  flows through the fuel manifold  123  and into the combustor  134  of the gas turbine system  2 . 
         [0014]    In the gas turbine system, the compressed air  131  produced by the compressor  130  is mixed with oxygen-depleted air  127  from the ITM ASU  110 , forming a high pressure air mixture  133  directed to the combustor  134 . Mixing with oxygen-depleted air  127  helps to control the flame temperature and reduce the formation of NO x  in the combustor  134 . Due to frictional losses in the piping  107  and in the ITM ASU  110 , the pressure of the oxygen-depleted air  127  can be lower than the pressure of the air stream  131  coming directly from the compressor  130 . In order to prevent back-flow of oxygen-depleted air  127 , the stream of pressurized air  131  exiting the compressor  130  is throttled by a valve  132 , leading to a significant loss of gas turbine efficiency. The fuel mixture  128  entering the gas combustor  134  reacts with the high pressure air mixture  133  to produce a hot, pressurized gas  135  which powers gas turbine  136  and turns the rotor shaft  138  to drive both the compressor  130  and the electrical generator  140 . As a result of having been expanded in the turbine  136 , the temperature of the exhaust gas  135  from the turbine  136  is considerably lower than the temperature of the hot gas  135  entering the turbine  136 . The exhaust gas  135 , typically in the range of 850° F.-1100° F., is directed from the turbine  136  to the air pre-heater  112  of the gasification system  1  for transfer of sensible heat to the compressed air  111  supplied from the compressor  130 . The cooled gas  135  exiting the pre-heater  112 , still relatively hot (typically in the range of 750° F.-1000° F.), is sent to the HRSG  3  for further recovery of heat. 
         [0015]    The HRSG  3  receives feed-water  167  sent from the steam turbine system  4  by the feed-water pump  166 . The feed-water  167  is heated with heat transferred from the relatively hot gas  135  exiting the gas turbine system  2 . The feed-water  167  first flows through the heat transfer tubes of the economizer  172 , where its temperature is raised to near the boiling point and is then directed to the steam drum  174  from which the water is circulated through the heat transfer tubes of the evaporator  176  where the heated feed-water  167  is converted into saturated steam  177 . The steam temperature is further elevated as it flows through the superheater  178  before entering the steam chest  152 . After flowing through the HRSG  3 , the cooled, expanded gas  135  is then discharged to atmosphere via a stack  156 . 
         [0016]    In the steam turbine system  4 , steam  192  from the syngas cooler  118  of the gasification system  1  and steam  177  from the HRSG  3  are merged in the steam chest  152 . Steam  153  flows from the steam chest  152  to the steam turbine  160  and steam  126  flows from the steam chest  152  to the fuel supply line  125  for entry to the fuel manifold  123  with the cleaned syngas  124  as the fuel mixture  128 . Within the steam turbine  160 , the steam  153  expands, turning the rotor shaft  161  to drive the electrical generator  162 . In other designs, the steam turbine  160  may be coupled to the shaft  138  and generator  140  of the gas turbine system  2 . After passing through the turbine  160  the cooled, expanded steam  163  enters the condenser  164  for recycling as feed-water  167 . Fresh water  165  is supplied to the condenser  164  to compensate for water loss in the system  100 . 
         [0017]    In the embodiment of  FIG. 2 , an IGCC system  200  generates compressed air  211  for an ITM process and compressed air  231  for combustion from a compressor  230  having multipressure outlets  203  and  204 . The system  200  comprises a HRSG  3 , and a steam turbine system  4  as described with respect to  FIG. 1 , a gasification system  5 , and a gas turbine system  6 . 
         [0018]    The gasification system  5  includes a gasifier  208  which receives a fuel source  206  and an oxygen-enriched gas supply  209  from an ITM ASU  210 . Syngas  217  produced in the gasifier  208  is sent to a syngas cooler  218  to reduce the syngas temperature prior to clean-up of impurities, e.g., sulfur, nitrous oxide, and dust particles in a gas clean-up unit  220 . The cleaned syngas  224  is mixed with steam  126  from the steam chest  152  of the steam turbine system  4  to form a fuel mixture  228  which flows through the manifold  223  and passes through multiple ports  239  thereof, into the gas combustor  234  of the gas turbine system  2 . A supply of compressed air  211  delivered to an ASU  210  first passes through a syngas cooler  218  where it receives sufficient heat from hot syngas  217  to elevate the temperature as required for ITM oxygen separation. After being separated from the air  213  in the ASU  210 , oxygen-enriched gas  209  is delivered to a gasifier  208  and oxygen-depleted air  227  is delivered to an air chest  214  in the gas turbine system  6 . Although not shown, the syngas cooler  218  may include an additional heat exchanger to generate steam from a portion of the feed-water  167 . 
         [0019]    Still referring to  FIG. 2 , the compressor  230  in the gas turbine system  6  receives ambient air  229  to generate the source of high pressure air  211  exiting the first outlet  203 , generally in the range of 200-300 psia, and a source of low pressure air  231  exiting the second outlet  204 , having substantially same pressure as the oxygen-depleted air  227 , e.g., generally in the range of 160-285 psia. The high pressure stream of air  211  is extracted from a high pressure port  203  of the compressor  230  and is delivered to the syngas cooler  218  for heat exchange prior to entering the ITM ASU  210  for oxygen separation. The low pressure air stream  231  is routed to the air chest  214  where it is mixed with the oxygen-depleted air  227  traveling from the ASU  210  through a line  207 , generating an air mixture  233 . The air mixture  233  is delivered to the combustor  234  to react with the fuel mixture  228  to produce a hot, pressurized gas  235  which powers the gas turbine  236 , turning the rotor shaft  238  to drive both the compressor  230  and the electrical generator  240 . As a result of having been expanded in the turbine  236 , the temperature of the exhaust gas  237  exiting from the turbine  236  is considerably lower than the temperature of the hot gas  235  entering the turbine  236 . The exhaust gas  237 , typically in the range of 850° F.-1100° F., is directed to the HRSG  3  for recovery of heat. After flowing through the HRSG  3 , the cooled, expanded gas is discharged to the atmosphere via a stack  156 . 
         [0020]    In the embodiment of  FIG. 3 , an IGCC system  300  generates a supply of low pressure compressed air  331  for combustion from a supply of high pressure compressed air  311  with an air turbine  341 . The system  300  comprises a HRSG  3 , a steam turbine system  4 , a gasification system  5 , each as described with respect to  FIG. 1  and  FIG. 2 , and a gas turbine system  7 . 
         [0021]    In the gas turbine system  7 , a compressor  330  generates a supply of high pressure air  311 , generally at 200-300 psia. The high pressure air  311  passes through the syngas cooler  218  in the gasification system  5  where it receives sufficient heat from hot syngas  217  to elevate the temperature as required for ITM oxygen separation in the ASU  210 . After being separated from the air  213  in the ASU  210 , oxygen-enriched gas  209  is delivered to the gasifier  208  in the gasification system  5  and oxygen-depleted air  227  is delivered to an air chest  314  in the gas turbine system  7 . Although not shown, the syngas cooler  218  may include an additional heat exchanger to generate steam from a portion of the feed-water  167 . 
         [0022]    A portion  312  of the air  311  is delivered to the air turbine  341  to produce a stream of lower pressure air required for combustion in a combustor  334 . The high pressure air  312  expands in the air turbine  341  and turns a rotor shaft  338  coupled to drive both the compressor  330  and the electrical generator  340 . The air turbine  341 , in other designs, may be coupled to a separate rotor shaft and a separate generator. The air  331  exiting the air turbine  341 , generally at 160-285 psia, is routed to an air chest  314 . 
         [0023]    Pressurized, oxygen-depleted air  227  from the ITM ASU  210  of the gasification system  5  mixes with the air  331  in the air chest  314 , providing an air mixture  333 . The air mixture  333  is delivered to the combustor  334  to react with the fuel mixture  228  to produce a hot, pressurized gas  335  which powers a gas turbine  336 , turning the rotor shaft  338  to drive both the compressor  330  and the electrical generator  340 . As a result of having been expanded in the turbine  336 , the temperature of the exhaust gas  337  exiting the turbine  336  is considerably lower than the temperature of the gas  335  entering the turbine  336 . The exhaust gas  337  exiting from the turbine  336 , typically in the range of 850° F.-1100° F., is directed to the HRSG  3  for recovery of heat. After flowing through the HRSG  3 , the cooled, expanded gas is then discharged to the atmosphere via a stack  156 . 
         [0024]    While the preferred embodiments of the invention have been illustrated and described, it will be clear that the invention is not so limited. Numerous modifications, changes, variations, substitutions and equivalents will occur to those skilled in the art without departing from the spirit and scope of the present invention as described in the claims.