Abstract:
A method and apparatus for preparing pulverized coal used in the production of synthesis gas combusted in gas turbines used in integrated gasification combined cycle power plants uses gas turbine exhaust gas to dry and convey pulverized coal from a pulverizer to a coal/gas separation device.

Description:
FIELD AND BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates generally to pulverized coal preparation and, in particular, to a method and apparatus for preparing pulverized coal used in the production of synthesis gas combusted in gas turbines used in integrated gasification combined cycle power plants. 
         [0002]    Integrated gasification combined cycle (IGCC) power plants produce synthesis gas fuel in gasifiers via the partial combustion of coal. The synthesis gas is used as fuel for gas turbines of these IGCC power plants. 
         [0003]    The coal used to produce the synthesis gas is ground or pulverized to a desired particle size. The pulverized coal is heated by a drying stream to remove a portion of the moisture in the coal. The pulverized coal, after being separated from the drying stream, is then transported to a temporary storage location from where it is then conveyed to the gasifier to produce the synthesis gas; systems of this type are known as indirect or bin systems. Moisture is removed so that the pulverized coal does not agglomerate when stored in the bin. However, in removing the moisture, the drying stream itself retains moisture removed from the coal. 
         [0004]    One current system uses a closed loop coal drying/grinding/transport gas of nitrogen with very low oxygen concentration, thus reducing the risk of coal combustion. Since the nitrogen is drying the coal, the moisture must be removed from the nitrogen by cooling it below the dew point, and removing the moisture, and then reheating it for recirculation to the pulverizer to dry and transport more coal. This system requires a cooler, a heater, associated pumps, valves and piping and continuous use of steam and cooling water. Since the nitrogen would have to be purchased, the coal preparation and drying system would be operated as a closed system in order to minimize nitrogen losses from the system, and one or more heat exchangers would be required to remove moisture from the recirculating nitrogen drying stream. 
       SUMMARY OF THE INVENTION 
       [0005]    It is an object of the present invention to provide an improved method and apparatus for preparing coal for use in producing synthesis gas fuel used by gas turbines of power plants to produce electricity. 
         [0006]    It is another object of the present invention to provide an improved method and apparatus for preparing coal for use in producing synthesis gas fuel which does not require a closed loop system. 
         [0007]    It is a further object of the present invention to provide an improved method and apparatus for preparing coal for use in producing synthesis gas fuel which does not require heat exchangers to remove moisture from the drying stream. 
         [0008]    Accordingly, one aspect of the present invention is drawn to an improved method and apparatus for preparing coal for use in producing synthesis gas which uses gas turbine exhaust gas to dry and convey pulverized coal from a pulverizer to a coal/gas separation device. 
         [0009]    The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which a preferred embodiment of the invention is illustrated. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0010]    The sole FIGURE is a schematic drawing of the apparatus utilized in a preferred embodiment of the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0011]    The FIGURE is a schematic illustration of a portion of an IGCC power plant, generally designated  10 , which employs the principles of the present invention to prepare pulverized coal used in the production of synthesis gas to generate power. Synthesis gas  12  from a gasifier  90  and air  14  are combusted in gas turbine  16  to produce energy for the IGCC power plant  10 . The gas turbine is coupled to an electric generator (not shown) which produces a portion of the electricity produced by IGCC plant  10 . Hot turbine exhaust gas  18  is emitted from gas turbine  16  as a result of the combustion of the synthesis gas  12  and air  14 . The hot turbine exhaust gas  18  is then conveyed to a NO x  reduction device, such as a selective catalytic reduction (SCR) device  20 , of known design and operation. 
         [0012]    A portion  22  of the hot turbine exhaust gas  18  is then conveyed through a waste heat recovery boiler  40  of known design, while a second portion  24  of the hot turbine exhaust gas is directed to a coal pulverizer  60 , as described below. The purpose of waste heat recovery boiler  40  is to generate steam to power a steam turbine and electric generator (not shown) for additional electric power generation by the IGCC plant  10 . The temperature of the hot turbine exhaust gas flowing through the boiler  40  is reduced due to the heat transfer taking place within the boiler  40  and which produces the steam for the steam turbine (not shown). Cooled turbine exhaust gas exits from the boiler  40 . Another portion  26  of the cooled turbine exhaust gas exiting from the boiler  40  is also directed to the coal pulverizer  60 , as described below. The balance  28  of the cooled turbine exhaust gas exiting from the boiler  40  is then conveyed to an environmental clean up unit (schematically indicated at  110 ) of known design and thereafter to a boiler stack  120 . The environmental clean up unit would actually be positioned so as to receive and treat the flow stream  28  and the gas exiting via line  7  from a coal/gas separation device  70  described below. The environmental clean up unit  110  may, for example, comprise wet or dry scrubbers of known design used to remove sulfur oxides from the turbine exhaust gas. 
         [0013]    While the FIGURE illustrates a series arrangement of gas turbine  16 , SCR  20  and waste heat recovery boiler  40 , it will be appreciated by those skilled in the art that the SCR is probably integrated with the waste heat recovery boiler  40 . Since the preferred inlet gas temperature entering the SCR  20  is about 700 F, it is likely that the designer would actually place some high temperature sections of the waste heat recovery boiler  40  in between the gas turbine  16  and the SCR  20 , and then locate some low temperature sections of the waste heat recovery boiler  40  downstream of the SCR  20 . In the present invention, it is preferred that the hot turbine exhaust gas  24 , from which some of the NO x  has been removed, be provided at a gas temperature of about 650 F, with the cooled turbine exhaust gas  26  exiting from the waste heat recovery boiler  40  at a gas temperature of about 300 F or less. This provides sufficient heat and cooling capability in the turbine exhaust gas for use as a drying agent in the coal pulverizer  60  as described below. 
         [0014]    The hot  24  and cooled  26  turbine exhaust gas are conveyed to the coal pulverizer  60  to dry and convey pulverized coal from the pulverizer to a coal/gas separation device  70  in a controlled fashion, as described below. The turbine exhaust gas has a low oxygen concentration, reducing the risk of coal combustion. 
         [0015]    Raw coal  30  is supplied to one or more coal pulverizers  60  and ground to a desired fineness to produce a stream of pulverized coal. The hot turbine exhaust gas  24  is mixed with the cooled turbine exhaust gas  26  at  32 , and blown into the pulverizer  60  by hot pulverizer fan  50 . The relative amounts of hot turbine gas  24  and cooled turbine gas  26  provided to the coal pulverizer  60  are regulated by motor operated gas dampers  34 ,  36  under the control of a controller (not shown) which operates to achieve a desired pulverized coal mill outlet temperature as measured by temperature sensor  38 . The turbine exhaust gas dries the pulverized coal and transports it from the pulverizer  60 . The dried pulverized coal exits from the pulverizer  60  and is conveyed to a coal/gas separation device  70 , where the pulverized coal is separated from the gas. The mixed turbine exhaust gas which was conveyed to the coal pulverizer  34  removes the moisture from the pulverized coal. A direct fired-air heater (not shown) may be utilized to supply heat to the coal pulverizer exhaust gas input line if hot turbine exhaust gas is not available. 
         [0016]    The coal/gas separation device  70  is advantageously a fabric filter, a cyclonic separation device, an electrostatic precipitator, or a combination thereof, and separates the pulverized coal from the turbine exhaust gas conveying stream. If device  70  is a fabric filter, it may be a single module, intermediate pressure, pulse-jet collector with automated cleaning of the filter bags in an on-line cleaning mode. The device  70  may include a pre-cleaner inlet to reduce the grain loading to the bags. Such a design provides for both separation of particles from the gas stream and reduces the gas stream velocity by introducing a cross-flow gas pattern into the side of the device  70 . A compressed nitrogen system  100  may be provided for device  70  to provide nitrogen pulse jets to dislodge accumulated pulverized coal from the bags of the fabric filter. The nitrogen pulse jets also provide a nitrogen blanket over the pulverized coal to reduce the chance of combustion. Infiltration of air into the device  70  is also reduced by the positive pressure established by the hot pulverizer fan  50 . 
         [0017]    The pulverized coal separated from the exhaust gas in device  70  is collected in hoppers and then conveyed to pulverized coal storage bins  80  for eventual transport to the gasifier  90  where the synthesis gas is produced. The relatively clean exhaust gas is then conveyed from the coal/gas separator  70  to the environmental clean up unit  110  for final clean-up prior to discharge to the atmosphere. 
         [0018]    The present invention may be employed in the construction of new IGCC power plants or in the repair, modification or retrofit of existing IGCC power plants. It is also understood that, depending upon the specific applications, certain features of the invention may be employed without other features of the invention. Thus, while a specific embodiment of the invention has been shown and described in detail to illustrate the application of the principles of the invention, it will be understood that the invention may be embodied otherwise without departing from such principles.