Abstract:
A method of using an apparatus for producing electric power or mechanical drive in which fuel gas produced in a gasifier using air as an oxidant is supplied to the gasifier by extraction from the exit of the air compressor of a gas turbine.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     Not applicable. 
     FEDERALLY SPONSORED RESEARCH 
     Not applicable. 
     REFERENCE TO MICROFICHE APPENDIX 
     Not applicable. 
     FIELD OF THE INVENTION 
     The invention relates to the operation of an Integrated Gasification Combined Cycle Power Plant (IGCC). 
     BACKGROUND OF THE INVENTION 
     The gasification of coal by reaction with steam and air has been practiced for more than a century. For most of those years it was produced in relatively simple equipment at a pressure slightly higher than atmospheric pressure. Gas produced through coal gasification has been used to provide fuel gas for lighting and heating. In such simple gasification processes, by-products of coke, tars and light oils were produced in sufficient quantities that their values influenced the selling price of the gas. In recent years many of the outlets for the by-products have been replaced by electricity, natural gas and petroleum distillates. In order to compete with electricity, natural gas, and petroleum distillates, more sophisticated equipment for the gasification of coal has been designed and developed. Such newer sophisticated coal gasification processes operate at high pressures and use, as reactants, steam and pure or nearly pure oxygen. Such equipment is costly and complex to operate with the result that such operating units must be large to achieve good economics. 
     At the present time power plants for the production of electricity are large and the electricity is distributed through nation-wide networks. There is, however, still a need for plants of all sizes to supply local demands for power and cogeneration. Irrespective of the size of the plant, plant emissions must meet environmental requirements set by governmental authorities. These factors have led to difficulties in the use of coal as the primary fuel for power generation or co-generation plants. 
     The design of power plants has also changed with the development of the gas turbine as a highly efficient power unit, especially when combined with a heat recovery steam generator and steam turbine. Gas turbines, however, operate solely on gaseous or liquid fuels. Clean fuel gas produced by large coal gasifiers, in conjunction with clean up equipment, may be used in generating power with gas turbines. Large gasifiers which can convert from 1,200 to 2,500 tons of coal per day to gas and provide fuels to gas turbine combined cycle plants of 250 to 500 megawatt capacity are known. 
     Although there is a demand for gas turbine based power plants in that size range, gas produced from coal has not generally been used to produce electrical power because the complexity of the equipment and operating costs of oxygen based systems make the use of coal economically impractical. However, for many decades small and medium sized gasifiers have been used to provide fuel gas, at low pressure, to such plants as brick works, glass plants and lime kilns. Such small and medium size gasifiers use air as a reactant and so the quality of the gas produced is lower than that of the large plants that use oxygen, particularly as regards gas heating value and composition. Because the operating pressure is low the normal method for producing the air necessary for the gasification is through the installation of air blowers. The use of air blowers is very inefficient with the result that the gasification process has a low efficiency. In cases where the quantity of air is large and the pressure higher, axial flow compressors, which have high efficiencies, can be used. Such is the case with the compressors used in gas turbine engines. 
     The gas turbine consists of a highly efficient air compressor that supplies air to a combustion chamber in which fuel is burned. The hot gas produced flows into an expander where it produces work. This expander is connected to the compressor and also to a mechanical drive commonly to an electricity generator. It is usual to supply the fuel in the form of natural gas or fuel oil, which have heating values on the order of 21,000 British Thermal Units per pound and the fuel supplied amounts to slightly less than 2% of the air supplied. The heating value of coal gas is about 3,000 British Thermal Units per pound and to achieve the same gas temperature in the gas turbine the amount of fuel added must be at least 14% of the airflow. This leads to a substantial increase in gas flow through the expander and a consequent increase in the power produced. This higher power, in some cases, is higher than the value that can be transmitted to the electricity generator without exceeding the mechanical limits of the drive shaft. The normal method of preventing this condition is to reduce the airflow to the compressor and hence the flow through the turbine. 
     SUMMARY OF THE INVENTION 
     Embodiments of the invention provide for the improvement of the operating efficiency of low pressure gasifiers by linking the air supply to the compressors of the gas turbine being used in an Integrated Gasification Combined Cycle Power Plant (IGCC) by extracting air from the outlet of the gas turbine compressor and passing it through an expander to reduce the pressure to that required by the gasifier and at the same time producing power. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic representation of a gas turbine and gasifier system. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the invention proposes a more efficient way to operate a gas turbine by allowing the normal amount of air to enter the compressor by extracting an amount of air from the exit of the compressor equal to the air quantity requirement of the gasifier or gasifiers. In embodiments of the inventive process, air in an amount equal to that needed by the gasifiers is withdrawn from the gas turbine air compressor, passed through an expander and fed into the gasifier, thus eliminating the need for air blowers and saving the power that such air blowers would have consumed. 
     Referring to  FIG. 1 , coal ( 1 ) is passed into a gasifier ( 2 ) where it is turned into gas. The gas leaves the gasifier at exits ( 3 ) and ( 4 ). The gas is passed into a scrubber tower ( 5 ) that removes sulfur from the gas. Some of the gas enters a compressor ( 6 ), from which it exits at a high enough pressure ( 7 ) to that needed for entry to the combustion chamber ( 8 ) of the gas turbine. Air from the gas turbine compressor ( 9 ) is split into two streams ( 10 ) and ( 11 ) of which stream ( 11 ) passes to an expander ( 12 ) and thence to the bottom of the gasifier ( 13 ). Meanwhile stream ( 10 ) passes to the combustion chamber ( 8 ) to provide the oxygen needed for the fuel to burn. The hot gas from the combustion chamber ( 8 ) passes into the gas turbine ( 14 ), which creates the power needed to drive the compressor ( 9 ) and the alternator ( 15 ). The exhaust gas from the gas turbine ( 16 ) passes to a heat recovery steam generator ( 18 ) where it is cooled by heat exchange with water ( 19 ) to provide high pressure steam ( 20 ) to drive a steam turbine ( 21 ) and electricity generator ( 22 ). Steam exiting the steam turbine ( 21 ) is cooled in a condenser ( 23 ) by cold water ( 24 ) supplied from a cooling tower ( 25 ) or by air in an air-cooled condenser. In some cases more steam, and hence more power, can be produced by burning some of the coal gas ( 17 ) in the entry duct of the heat recovery steam generator. 
     This embodiment of the invention relates to the gasification of coal using air and steam as the reactants to produce a product gas, which has properties suitable for use as a fuel for gas turbines. As shown in  FIG. 1 , air is introduced into the bottom of the gasifier to produce gas, which rises through the gasifier and leaves the ash in the coal to be removed through the bottom of the gasifier. In its passage through the gasifier the coal is converted to an acceptable fuel gas, and such reaction should occur at a pressure of from 20 to 90 pounds per square inch absolute. In some embodiments, the gasifier converts biomass into fuel gas. In other embodiments, the gasifier converts waste such as old tires, sewage sludge or corn stalks into fuel gas. In still other embodiments, the gasifier converts a mixture of coal and other materials into fuel gas. In alternate embodiments, the gasifier is a two-stage gasifier. 
     The gas produced is comprised of a mixture of carbon monoxide, carbon dioxide, hydrogen, nitrogen, methane, ethane, ethylene, hydrogen sulphide and a trace of carbonyl sulphide and carbon disulphide. The sulfur compounds are removed in a clean-up plant and the gas is then ready to be used and is environmentally suitable for any purpose and particularly as fuel for gas turbines. 
     As an example coal, with a composition of carbon 52.58%, Hydrogen 3.65%, Nitrogen 0.72%, Oxygen 13.10%, Sulfur 0.23%, Water 25.00%, Ash 4.72%, can be gasified under a pressure of 30 pounds per square inch absolute to provide gas for a gas turbine. The approximate quantity of gas produced from one gasifier would be 588,000 standard cubic feet per hour and would have a heating value of 3,118 British thermal units per pound. The composition of the gas would be approximately 28.39% Carbon Monoxide, 18.26% Hydrogen, 41.61% Nitrogen, 5.86% Carbon Dioxide, 5.08% Methane, 0.10% Ethylene, 0.13% Ethane and less than 0.01% sulfur. 
     The output of thirteen gasifiers producing this quality of fuel would be sufficient for a single SGT6-3000E or equivalent gas turbine to produce, at sea level and 59° F., 144 Mw of electricity. This quantity, however, exceeds the maximum, of 135 Mw, that can be transmitted to the electricity generator. Conventionally, this would be achieved by throttling the flow of air to the compressor of the gas turbine by about 10%. An alternative way to reduce the flow to the combustion chamber is to extract air from the outlet of the compressor and use it for some other purpose. 
     Gasifiers typically operate at a pressure of from about 30 to about 90 psia and the air must be compressed to that level. This may be achieved by the use of centrifugal blowers. It has been discovered that by extracting air from the gas turbine and reducing its pressure to that of the gasifier, by passing it through an expander, a considerable saving in the power can be gained compared with using blowers. The effect on performance and efficiency of throttling the air or extracting the air is presented in the table below. 
     In another example coal, with a composition of Carbon 49.21%, Hydrogen 3.51%, Nitrogen 0.71%, Oxygen 11.42%, Sulfur 0.35%, Water 29.90%, Ash 4.90%, can be gasified under a pressure of 30 pounds per square inch absolute to provide gas for a gas turbine. The approximate quantity of gas produced from one gasifier would be 575,000 standard cubic feet per hour and would have a heating value of 3,132 British thermal units per pound. The composition of the gas would be approximately 30.60% Carbon Monoxide, 16.62% Hydrogen, 42.64% Nitrogen, 4.53% Carbon Dioxide, 5.54% Methane, 0.12% Ethylene, 0.19% Ethane and less than 0.01% sulfur. 
     The output of a Frame  7 FA turbine operating on this quality of fuel and at a temperature of 59° F. has been compared for cases with extraction and without extraction at altitudes of sea level and 5000 feet. The results are given in the table below. 
     
       
         
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Frame 7FA 
               
             
          
           
               
                 Turbine 
                 SGT6-3000E 
                 Extraction 
                 No Extraction 
                 Extraction 
                 No Extraction 
               
             
          
           
               
                 KW 
                 Throttling 
                 Extraction 
                 Sea Level 
                 Sea Level 
                 5,000 ft. 
                 5,000 ft. 
               
               
                   
               
             
          
           
               
                 Gas Turbine 
                 131,631 
                 131630 
                 187,552 
                 210.624 
                 156,532 
                 175,520 
               
               
                 Steam Turbine 
                 80,5443 
                 80,544 
                 124,638 
                 130,798 
                 100,797 
                 111,516 
               
               
                 Expander 
                 0 
                 7,847 
                 11,028 
                 0 
                 9,076 
                 0 
               
               
                 Parasitic Loss 
                 6,365 
                 6,365 
                 9,366 
                 10,243 
                 7,720 
                 8,611 
               
               
                 Compressor 
                 19,977 
                 19,977 
                 28,316 
                 31,998 
                 23,635 
                 26,665 
               
               
                 Blower 
                 6585 
                 0 
                 0 
                 9,162 
                 0 
                 8,847 
               
               
                 Gas Clean Up 
                 777 
                 777 
                 1,017 
                 1,017 
                 837 
                 837 
               
               
                 Solids Handling 
                 1,165 
                 1,166 
                 1,526 
                 1,705 
                 1,256 
                 1,435 
               
               
                 Net Power 
                 179,248 
                 185,055 
                 282,993 
                 287,297 
                 232,959 
                 240,540 
               
               
                 Heat Rate Btu/Kwh 
                 9,872 
                 9,562 
                 8,124 
                 8,945 
                 8,121 
                 8,991 
               
               
                 Dry, Ash Free Coal 
                 1,565 
                 1,565 
                 2,048 
                 2,290 
                 1,686 
                 1928 
               
               
                 Used ton/day 
               
               
                   
               
             
          
         
       
     
     As can be seen from the table the use of extraction leads to up to 10% reduction in the fuel required and a reduction in the number of gasifiers needed per kilowatt generated. 
     Certain embodiments of this invention are not limited to any particular individual features disclosed, but include combinations of features distinguished from the prior art in their structures and functions. Features of the invention have been described so that the detailed descriptions that follow may be better understood, and in order that the contributions of this invention to the arts may be better appreciated. These may be included in the subject matter of the claims to this invention. Those skilled in the art who have the benefit of this invention, its teachings, and suggestions will appreciate that the conceptions of this disclosure may be used as a creative basis for designing other structures, methods and systems for carrying out and practicing the present invention. This invention is to be read to include any legally equivalent devices or methods, which do not depart from the spirit and scope of the present invention. 
     In conclusion, therefore, it is seen that the present invention and the embodiment(s) disclosed herein are well adapted to carry out the objectives and obtain the ends set forth. Certain changes can be made in the subject matter without departing from the spirit and the scope of this invention. It is realized that changes are possible within the scope of this invention and it is further intended that each element or step recited is to be understood as referring to all equivalent elements or steps. The description is intended to cover the invention as broadly as legally possible in whatever forms it may be utilized.