Abstract:
A system ( 10 ) and a method for converting carbonaceous fuel ( 102 ) into a gaseous product ( 42 ). According to one embodiment a fuel slurry ( 118 ) is introduced into a chamber ( 120 ) and heated under sufficient pressure to prevent the carrier component ( 100 ) from boiling so that the carbonaceous component ( 102 ) does not separate from the carrier component ( 100 ). The step of heating the carrier component ( 100 ) may include increasing pressure and temperature to place the carrier component ( 100 ) in a supercritical state while sustaining the carbonaceous component ( 102 ) and carrier component ( 100 ) in a mixed state. In this embodiment a pump ( 136 ) imposes sufficient chamber pressure to prevent boiling of the carrier component ( 100 ) as the mixture is heated to at least 345° C., and a gasifier chamber ( 120 ) is positioned to receive the gaseous mixture ( 118 ) at a lower pressure than the supercritical pressure for creation of syngas ( 42 ).

Description:
FIELD OF THE INVENTION 
     The present invention relates to gasification of carbonaceous fuels and, more particularly, to delivery systems and methods which increase gasifier efficiency. 
     BACKGROUND OF THE INVENTION 
     The efficiency of conventional power plants has been markedly improved with the integration of the combustion turbine and a variety of heat recovery techniques. To improve energy efficiency and enhance the environmental acceptability of fossil fuels, it is advantageous to include a gasification stage in combined cycle power plants. In these systems a carbonaceous fuel such as coal is converted to syngas, a gaseous mixture formed during a high temperature partial oxidation. This combination of features is commonly referred to as an Integrated Gasification Combined Cycle, or IGCC. 
     In an IGCC the syngas is fed to a combustion turbine from which exhaust heat is applied to generate steam for a subsequent power stage, and/or to heat incoming materials associated with the combustion cycle. Components of syngas vary considerably depending on the fuel source and reaction conditions. For coal gasification, typical constituents of syngas include CO 2 , CO, H 2  and CH 4 . Often syngas will include sulfides and nitrous components. The latter are normally removed from the mixture prior to combustion in order to provide an environmentally cleaner exhaust gas from the combustion turbine. 
     The IGCC is coming into greater use in power production because the overall efficiencies are attractive and the technology presents greater opportunities to deploy coal, an abundant resource, in an economical and clean manner. The efficiency advantage of burning gasified coal in power plants stems in part from the combined cycle, wherein hot gases leaving the combustion turbine are used to raise steam temperature in a conventional Rankine cycle. With a typical gasification efficiency of about 80 percent, and a combined cycle efficiency (combustion and steam turbine) of about 58 percent, it is possible to achieve an overall plant efficiency of 46.8 percent. By way of comparison, the overall efficiency of a typical steam turbine power plant is less than 40 percent although newer ultrasupercritical cycle designs may approach efficiencies up to 45 percent. 
     More generally, the cold gas efficiency should be at least 78 percent to render the IGCC commercially attractive. The efficiency of the coal gasification process is dependent in part on the gasification temperature which, in turn, is a function of the reactivity of the coal species. It is desirable to react the coal at as low a temperature as possible, as this will maximize the heating value in the syngas relative to the feedstock. However, due to the equilibrium dynamics of the conversion process, reaction temperatures range from about 1400° C. to about 1500° C. (2550° F. to 2730° F.) for various coal species. As a result, gasification efficiencies above 80 percent have been difficult to achieve in large scale commercial operations. Given these constraints, other means of improving the efficiency of gasification are sought, as even small improvements in plant efficiency have large impacts on the cost and viability of energy production from carbonaceous solid fuel sources. 
     SUMMARY OF THE INVENTION 
     According to one embodiment of the invention, in a process for conversion of carbonaceous fuel to a gaseous product, a fuel slurry is introduced into a high pressure chamber. Initially the slurry may be a mixture of a solid granulated carbonaceous component and a liquid carrier component. The slurry is heated under sufficient pressure to prevent the carrier component from boiling. Accordingly the carbonaceous component does not separate from the carrier component. The mixture is transferred through an orifice or other means of pressure reduction to a lower pressure chamber for combustion without separating the solid component from the gaseous carrier component. In numerous embodiments the carrier component comprises water and the carbonaceous component comprises coal. 
     By way of example, the step of heating the carrier component may include increasing pressure and temperature to place the carrier component in a supercritical state while sustaining the carbonaceous component and the carrier component in a mixed state. The desired pressure may be attained by pumping a sufficient quantity of the slurry into the high pressure chamber to reach a supercritical pressure while elevating the temperature of the mixture and before transferring the mixture through the orifice. 
     A system is also provided for converting a slurry comprising solid, granulated carbonaceous material such as coal or coke into a syngas. One embodiment of the system includes a high pressure chamber for receiving a mixture of the solid carbonaceous material and a liquid carrier material. A pump imposes sufficient chamber pressure to prevent boiling of the carrier material as the mixture is heated to at least 345° C. (650° F.). A heat source effects heating of the mixture to at least 345° C. (650° F) in the chamber. A gasifier chamber is positioned to receive the gaseous mixture at a lower pressure than the supercritical pressure for creation of syngas. The combination of the pump and the heat source impart sufficient pressure and thermal energy to place the liquid carrier material above its critical point. 
     A power system according to the invention includes a combustion turbine, a gasifier coupled to provide syngas to the turbine, and a high pressure chamber for receiving a fuel slurry. The slurry may be a mixture of a solid granulated carbonaceous component and a carrier component. A pump imposes sufficient chamber pressure to prevent boiling of the carrier component as a heat source effects heating of the mixture to at least 345° C. (650° F.) in the chamber. An expansion valve is positioned to pass the mixture while under a relatively high pressure into a lower pressure region within the gasifier. 
     In the disclosed embodiments the pump may be configured to pressurize a slurry to at least 218bara (3161 psia) and the heat source elevates the slurry from ambient conditions to a temperature of at least 345° C. (650° F.) without allowing the carrier component, e.g., water, to boil. The slurry may be pressurized to a supercritical level prior to heating the slurry or pressurization to the supercritical level may in conjunction with heating of the slurry to at least 345° C. (650° F.). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects of the invention will become more apparent in light of the following detailed description when read in conjunction with the accompanying drawings, wherein: 
         FIG. 1  illustrates components of a power plant incorporating the invention; 
         FIG. 2  illustrates a gasification system in the power plant of  FIG. 1 ; 
         FIG. 3  illustrates a conventional heating process; and 
         FIG. 4  illustrates a heating process according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to  FIG. 1  there is shown an exemplary IGCC power plant  10  incorporating principles of the invention. While the invention is described with reference to an IGCC plant, the concepts are directly transferable to the many applications and other systems in which carbonaceous solid fuels are gasified, including production of gases which are transported for combustion at a remote location. 
     The plant  10  includes numerous well-known components, including a gas turbine section  1 , a heat recovery steam generator  2  (“HRSG”), a steam turbine  3 , and a condenser  4 . The gas turbine section  1  includes a compressor  6 , a turbine  7  having a rotor shaft  8  connected to the compressor  6  and to an electrical generator  24 , and a combustor  9 . As more fully described herein, the combustor receives fuel from a gasification system  12  constructed according to the invention. 
     The HRSG  2  includes a superheater  13 , an evaporator  14 , a steam drum  18 , and an economizer  16 . The steam turbine  3  includes a rotor  38  mounted for rotation within a casing  33  so as to form a flow path wherein steam travels across a plurality of the rotating blades  34  and stationary vanes  37  to transfer power. 
     In operation, the compressor  6  inducts ambient air  40  and compresses it, thereby producing compressed air  41 . The compressed air  41  will typically be heated in excess of 260° C. (500° F.), at a pressure on the order of 700bara (100 psia) when the gas turbine rotor shaft  8  is at a steady state operating speed, e.g., 3600 RPM. 
     A portion (not shown) of the compressed air  41  produced by the compressor  6  is typically directed to the turbine  7  for cooling therein. During steady state operation of the power plant, the remainder  43  of the compressed air  41  is directed to the combustor  9 , along with a syngas fuel  42  received from the gasification system  12 , according to the invention. The compressed air  43  may be oxygen enriched. During start-up, a portion  56  of the compressed air  41  produced by the compressor  6  may be used for warming the steam turbine  3 . 
     In the combustor  9 , the fuel  42  is introduced into the compressed air  43  via a nozzle (not shown). The fuel  42  burns in the compressed air, thereby producing a hot, compressed gas  44 . The hot gas  44  is then directed to the turbine  7 . In the turbine  7 , the hot gas  44  is expanded, thereby producing power in the rotor shaft  8  that drives both the compressor portion of the rotor and the electrical generator  24 . The expanded gas  46  is then exhausted from the turbine  7 . As a result of having been expanded in the turbine  7 , the temperature of the expanded gas  46  exhausting from the turbine  7  is considerably less than the temperature of the hot gas  44  entering the turbine. Nevertheless, in a modern gas turbine operating at full load, the temperature of the expanded gas  46  is still relatively hot, typically in the range of 450°-620° C. (850°-1150° F.). 
     From the turbine  7 , the expanded gas  46  is directed to the HRSG  2  and through ductwork so that it flows successively over the superheater  13 , the evaporator  14  and the economizer  16 . A portion  47  of the expanded gas may also be directed to a high temperature heat exchanger  144  in the gasification system  12 . See also  FIG. 2 . After flowing through the HRSG  2 , the cooled, expanded gas  48  is then discharged to atmosphere via a stack  19 . As is conventional, the superheater  13 , the evaporator  14  and the economizer  16  may have heat transfer surfaces formed of finned tubes. The expanded gas  46  flows over these finned tubes while feed water or steam flows within the tubes. In the HRSG  2 , the expanded gas  46  transfers a considerable portion of its heat to the feedwater/steam, thereby cooling the gas and transforming the feedwater into steam. 
     In addition to the expanded gas  46  discharged by the gas turbine  1 , the HRSG  2  receives a flow of feed water  50  from the condenser  4  that has been pressurized by pump  20 . As is conventional, the feed water first flows through the heat transfer tubes of the economizer  16 , where its temperature is raised to near the saturation temperature. The heated feedwater from the economizer  16  is then directed to the steam drum  18 . From the steam drum  18 , the water is circulated through the heat transfer tubes of the evaporator  14  which converts the feedwater into saturated steam  52  which is then directed to the superheater  13 , wherein its temperature is raised into a superheated region and then provided to a steam chest  22  that distributes the steam to the inlet of the steam turbine  3 . 
     In the steam turbine  3 , the steam  54  flows through the casing  33  and over the rows of rotating blades  34  and stationary vanes  37 , only a few of which are shown in  FIG. 1 . In so doing, the steam  54  expands and generates shaft power that drives the rotor  38  which, in turn, drives a second electrical generator  26 . Alternately, the steam turbine rotor  38  could be integrally formed along the gas turbine rotor shaft  8  to drive a single electrical generator. The expanded steam  58  exhausted from the steam turbine  3  is directed to the condenser  4  and eventually returned to the HRSG  2 . A portion  59  of the expanded steam  58  may be diverted to a low temperature heat exchanger  142  in the gasification system  12 . 
     With reference to  FIG. 2 , a carrier component  100  and a finely granulated (e.g., less than 10 mm in size) solid carbonaceous fuel component  102 , such as coal, combine to form a slurry  118  which is injected into a pressurized gasifier chamber  120 . The carrier component may be water, but water-based mixtures and other liquids may form the carrier component. An oxygen supply  121  is separately injected to react with the slurry  118  and produce an intermediate gas product  122  at a reaction temperature on the order of 1500° C. (2730° F.). Slag  124  is removed from a lower portion of the chamber  120 . The gas product is passed through a cooler  128  prior to removal of char in an extraction stage  130 , followed by removal of sulfur and corrosive constituents in a cleaner stage  132 . The resulting syngas  42  may, for example, predominantly consist of carbon monoxide, hydrogen, carbon dioxide and steam. 
     A feature of the invention is the provision of the slurry  118  to the gasifier chamber  120  under supercritical or near-supercritical conditions. The ratio of heating value of the product gas to the heating value of the coal feedstock is a function of the reaction temperature in the gasifier chamber  120 . Consequently, the cold gas efficiency of the product gas is a direct function of the reaction temperature. 
     In the past, slurry has only been heated to a limited degree prior to the gasification reaction, e.g., to about 177° C. (350° F.). Heating the slurry to a significantly higher temperature has been avoided because this is commonly regarded as problematic. That is, heating to higher temperatures is known to result in a separation of solid granulated fuel from the water. If the slurry carrier is allowed to enter the vapor phase it will separate from the solid components, creating non-uniform and unacceptable slurry flow characteristics. Recognizing this constraint, prior to injection of the slurry into the gasification chamber, it is conventional to retain the mixture of solid granulated carbonaceous component (e.g., coal) and carrier component (e.g., water) in a liquid slurry form, pressurized slightly higher than the pressure in the gasification chamber. Only after the liquid slurry enters the relatively hot environment of the gasification chamber (at a somewhat lower pressure), has the liquid carrier component of the slurry entered the vapor phase. 
     An example of this conventional heating process is shown in  FIG. 3 , in which the temperature increase of a liquid, non-boiling carrier component is plotted with respect to the entropy. Specifically, prior to injection into a gasification chamber, the water-based slurry  118 , initially at room temperature and atmospheric conditions, is preheated up to 177° C. (350° F.) under sufficient pressure, i.e., at least 14bara (205 psia) to prevent the water therein from boiling. Once the slurry reaches the desired temperature it enters the gasification chamber where the temperature reaches 1500° C. (2730° F.) during syngas production. 
     In the gasification system  12  a pump  136  delivers the slurry  118  into a pressurized heating flow path  138 . The pump  136  subjects the slurry to a large pressure transition, e.g., from atmospheric pressure to above 218 bara (3160 psia). Generally, with water being the primary constituent of the carrier material, the flow path  138  may be pressurized in the range of 207bara to 552bara (3000 to 8000 psia) or higher. In the flow path  138  the liquid slurry is passed through one or more heating stages. As illustrated in  FIG. 2 , the flowing slurry may initially pass through a low-temperature heat exchanger  142  which may transfer heat to the slurry  118  from a portion  59  of the expanded steam  58  to elevate the slurry temperature. A higher temperature elevation is then effected by directing heat from the portion  47  of the turbine exhaust gas  46  into a high temperature heat exchanger  144  to bring the carrier temperature above 375° C. (750° F.). Heat transferred to the cooler  128  may also be applied along the heating flow path  138 . 
     An exemplary heating process for gasifying the slurry  188  is further illustrated in the entropy-temperature diagram of  FIG. 4 . The characteristic liquid-vapor region of the slurry water, within the confines of the curve  150 , is shown in order to compare state conditions according to the invention with conventional heating processes. The curve  150  includes a maximum corresponding to the characteristic critical point  154  of water. Above the critical point, the carrier component only exists in the gaseous state, referred to as a supercritical gaseous state. An example of the inventive process by which the slurry temperature is elevated is illustrated by the curve  156 . With the slurry initially at ambient atmospheric conditions, the slurry is pressurized to 276bara (4000 psia) and is then heated to approximately 427° C. (800° F.). 
     Temperature elevation under this pressure condition assures that the water does not simultaneously exist as both a vapor and a liquid at any given temperature, i.e., the state conditions remain outside the curve  150 . Under these conditions the water can remain in a continuous fluid state without a separation of vapor from liquid. Such separation would result in segregation of the solid coal fuel component from a vapor component. In the example of  FIG. 4  the slurry water exceeds the critical point  154 . The 427° C. (800° F.) slurry is then delivered to the gasifier chamber  120  through a pressure reduction element  160  such as a contollable expansion valve or an orifice, to resume a somewhat lower pressure. 
     By elevating slurry temperature while the slurry is at a supercritical pressure, i.e., in the high pressure flow path  138 , the slurry water remains under state conditions external to the curve  150 . Thus the heated slurry does not separate and also carries greater thermal energy as it flows into the gasifier chamber  120 . With this higher internal energy it becomes possible to achieve higher gasifier efficiency. 
     According to the invention, it is desirable to heat the slurry  118  to a temperature in excess of 374° C. (705° F.) prior to introduction to the gasification chamber and thus increase the efficiency of the subsequent gasification process. The novel method of heating the slurry overcomes impediments associated with conventional gasification systems. The concepts disclosed can be applied to improve overall efficiency in power systems and energy conversion processes. In particular, the invention renders coal gasification more commercially attractive. 
     The invention has been illustrated with reference to an example embodiment but may be applied in a variety of other ways. Many equivalents, alternatives and modifications will be apparent without departing from the invention. Accordingly the scope of the invention is only limited by the claims which follow.