Abstract:
A power generation plant and a method of generating electric energy from recovered heat during an industrial process that uses steam as a means of transferring energy. The method comprises: a) generating a first saturated steam in a first heat exchanger heated by a first source of recovered heat; b) feeding the first saturated steam into a first steam turbine generator, where the first steam turbine generator outputs exhaust steam; c) removing moisture from the exhaust steam with a moisture separator; d) superheating the moisture reduced exhaust steam from step c) in a main heat exchanger with a heat source; and e) feeding the superheated exhaust steam into a second steam turbine generator. The power generation plant comprises a first source of saturated steam, a first steam turbine generator, a moisture separator, a second source of saturated steam, a heat exchanger and a second steam turbine generator.

Description:
RELATED APPLICATIONS 
     This application claims the benefit of 35 USC 119 based on the priority of U.S. Provisional Patent Application 61/221,593, filed Jun. 30, 2009, the entirety of the contents of which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention generally relates to the field of energy recovery in plants and processes such as those used for coal-to-liquid production. 
     BACKGROUND OF THE INVENTION 
     In its International Energy Outlook for 2008, the Energy Information Administration forecast that global energy consumption will grow by 50% between 2005 and 2030. Total global transportation fuel production is expected to reach 93 millions barrels of oil per day (MMbbl/d) by 2020. It is expected that coal-to-liquid (CTL) will become increasingly necessary to compensate for the increase in oil consumption. World production of unconventional resources (coal-to-liquid, gas-to-liquid, etc) totaled only 2.5 MMbbl/d in 2005, yet is expected to increase to at least 9.7 MMbbl/d by 2030, accounting for 9% of the total world liquid supply on an oil-equivalent basis. 
     Promising prospects are also predicted for other industrial gasification processes including turning vast world reserves of coal, oil sands, waste coal and petcoke into an array of higher value products such as electrical power, liquid fuels, SNG, fertilizers and other chemical feed stocks. High oil and gas prices make this a virtually irresistible option. 
     Because many of these industrial processes use large quantities of energy and are therefore costly, efforts are made to recover as much as possible of wasted energy, which is often in the form of heat. Hence, it is known to use power generation plants to recover waste energy during these industrial processes. 
     For example, processes with entrained flow gasifiers or other exothermal reactors are often characterized by the generation of large quantities of waste heat, which may be at least partly transferred to saturated steam at various pressures according to the location in the process. In a typical CTL plant using gasifiers and producing 40,000 bbl/d, a waste heat boiler and a Fischer-Tropsch (F-T) reactor, each produce approximately 1,000 tonne/h of saturated steam that can be used for power generation. 
     In a conventional power generation plant, each source of saturated steam is superheated in its own fired superheater before being fed to conventional steam turbines. The fuel used to fire the superheaters is usually a fuel gas purged from a synthesis loop of the industrial process. However, the fired superheaters are expensive pieces of equipment, their efficiency is poor, and they are sources of air polluting emissions. 
     Considering the quantity of waste energy involved in CTL and various other industrial processes, it is clear that improving the efficiency of the power generation process, or of one of its steps, can lead to significant additional energy recovery. Moreover, in view of ever increasing environmental considerations, improvements in the efficiency of the power generation process and plant are clearly advantageous. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a method of generating electric energy from waste heat during an industrial process that overcomes or mitigates one or more disadvantages of known methods, or at least provides a useful alternative. 
     It is another object of the present invention to provide a power generation plant for an industrial process plant producing saturated steam from wasted heat that overcomes or mitigates one or more disadvantages of known power generation plants for industrial process plants producing saturated steam from wasted heat. 
     The invention provides the advantages of providing a more efficient power generation plant that uses a more efficient method of generating electric energy that yields an industrial plant installation that can be cheaper than comparable known industrial plants. Moreover, the improved industrial plant recovers more wasted energy and has lower air emissions. Because the industrial plant is less expensive to manufacture and to build, and because wasted energy is recovered more efficiently during the industrial process, the industrial plant economics can be improved. 
     In accordance with one embodiment of the present invention, there is provided a method of generating electric energy from recovered heat during an industrial process such as a CTL process, that uses steam as a means of transferring energy. The method comprises the following steps: a) generating a first saturated steam in a first heat exchanger heated by a first source of recovered heat; b) feeding the first saturated steam into a first steam turbine generator, where the first steam turbine generator outputs exhaust steam; c) removing moisture from the exhaust steam with a moisture separator; d) superheating the moisture reduced exhaust steam from step c in a main heat exchanger with a heat source; and e) feeding the superheated exhaust steam into a second steam turbine generator. 
     In accordance with another embodiment of the present invention, there is provided a power generation plant for an industrial process plant such as a CTL plant, that produces saturated steam from recovered heat. The power generation plant comprises a first source of saturated steam, a first steam turbine generator, a moisture separator, a second source of saturated steam, a heat exchanger and a second steam turbine generator. The first steam turbine generator has a first inlet and a first outlet. The first inlet is connected to the first source of saturated steam. The moisture separator is connected to the first outlet of the first steam turbine generator. The moisture separator is adapted to remove moisture from an exhaust steam coming from the first outlet and thereby produce dried exhaust steam. The heat exchanger is connected to the moisture separator and to the second source of saturated steam. The heat exchanger is adapted to receive and superheat the dried exhaust steam with the saturated steam from the second source. The heat exchanger has a heat exchanger outlet to exhaust the superheated dried exhaust steam. The second steam turbine generator connects to the heat exchanger outlet so as to receive the superheated dried exhaust steam. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       These and other features of the present invention will become more apparent from the following description in which reference is made to the appended drawings wherein: 
         FIG. 1  is a schematic diagram of a coal-to-liquid plant in accordance with an embodiment of the present invention; and 
         FIG. 2  is a schematic diagram of a detailed view of a power generation plant used in the coal-to-liquid plant of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Power generation plants are often used in industrial processes to recover waste energy and transform it into usable energy such as electricity. Typical known power generation plants are used to recover by-product heat from the industrial process by using the heat to transform water into steam for driving generators. These power generation plants typically use superheaters to heat saturated steam before using the steam to power steam turbines which in turn generate electricity. Superheaters are used to remove excess humidity in the superheated steam, which would otherwise be detrimental to the steam turbines by eroding the turbine&#39;s blades and by lowering its expansion efficiency. Purge gas by-produced by the industrial process is used to fire the superheaters and also in a combined cycle unit. 
     In the power generation plant of the present invention, the superheaters are replaced by a moisture separator and reheater. Moreover, purge gas is used solely in the combined cycle unit. By doing so, more energy can be produced from the same amount of purge gas since the combined cycle has a better efficiency than the Rankine cycle, of the superheaters used in typical known power generation plants. 
     Although the power generation plant and associated method of producing electric energy of the present invention could be used in different types of industrial plants, the present embodiment will be described with respect to a coal-to-liquid (CTL) gasification plant. 
       FIG. 1  depicts a diagram of a CTL plant  10  in accordance with an embodiment of the present invention. The CTL plant consists of the following plant areas: coal preparation  12  (drying, pulverizing and feeding), air separation unit (ASU)  14 , gasification  16 , waste heat boiler  18  for heat recuperation, syngas cleaning and purification  20 , Fischer-Tropsch (F-T) gas-to-liquid synthesis reactor  22 , refining  24 , and power generation plant  26 .  FIG. 2  shows the details of the power generation plant  26 . 
     In some examples, the power generation plant  26  is composed of a first steam turbine generator  28  and a second turbine generator  29 , connected by a moisture separator and reheater (MSR)  30 , and a combined cycle unit  32 . A source of feed water  34 , in which condensate from condensers is heated and de-aerated, supplies feed water  36 . A first portion of the feed water  36  enters the waste heat boiler  18  and gets evaporated into waste heat boiler saturated steam  38 . A first portion of the waste heat boiler saturated steam  38  enters the first steam turbine generator  28 , which produces electricity from the energy contained in the pressurized waste heat boiler saturated steam  38 . 
     Waste heat boiler saturated steam  38  can be at a pressure of 65 bars, a temperature of approximately 280° C. and at approximately 0% moisture. Optionally, a moisture separator  19  may be used to remove moisture from the waste heat boiler saturated steam  38 . Waste heat boiler saturated steam  38  enters the first steam turbine generator  28 , which is of a wet steam type, through its inlet and expands inside the turbine, thereby producing work. Waste heat boiler saturated steam  38  exits the first steam turbine generator in the form of exhaust steam  40 . The exhaust steam  40  can be at a pressure of 18.5 bars, a temperature of approximately 208° C. and at approximately 12% moisture. The first steam turbine generator  28  may be equipped with its own moisture separator. 
     In turn, the exhaust steam  40  enters the MSR  30 . Some examples of the MSR  30  are comprised of two portions: a first portion is a moisture separator  42  and a second portion is a heat exchanger  44 . The moisture separator  42  may comprise baffles so that the exhaust steam has to travel around the baffles, which drains some of the humidity in the exhaust steam  40 . The moisture separator  42  can reduce the moisture content of the exhaust steam  40  to approximately 5% at the moisture separator outlet. 
     Before the dried exhaust steam  40  enters the heat exchanger  44 , saturated steam  46  from the F-T reactor  22  is mixed with the dried exhaust steam  40 . This mixed exhaust steam  48  then enters the heat exchanger  44  to be reheated. 
     The heat exchanger  44  of the MSR  30  may receive saturated steam from multiple sources: for example a first source of saturated steam is the waste heat boiler  18 . As mentioned, a portion of the waste heat boiler saturated steam  38  provided by the waste heat boiler powers the first steam turbine  28 . A second portion of the waste heat boiler saturated steam  38  is directed to the heat exchanger  44  to reheat the mixed exhaust steam  48 . The heat exchanger  44  may also receive saturated steam  50  from a heat recovery steam generator (HRSG)  52  within the MSR  30 . The HRSG saturated steam  50  enters the heat exchanger  44  at a temperature of approximately 314° C. and a pressure of approximately 104 bars. Both the HRSG saturated steam  50  and the second portion of saturated steam  38  from the waste heat boiler reheat the mixed exhaust steam  48  to a superheated state at a temperature of approximately 304° C. and a pressure of approximately 17 bars. 
     The MSR  30  may have different stages. For example, a first stage may be the moisture separator  42 , a second stage may be a first portion of the heat exchanger  44  where the second portion of the waste heat boiler saturated steam  38  reheats the mixed exhaust steam  48 , while a third stage may be a second portion of the heat exchanger  44  where the HRSG saturated steam  50  further reheats the mixed exhaust steam  48 . At any one or more, and optionally at all of the stages of the MSR  30 , drain water  54  is collected. This drain water  54  comes either from moisture separated from the exhaust steam  40  or from condensed water from the second portion of waste heat boiler saturated steam  38  and the HRSG saturated steam  50  that entered the heat exchanger  44  to reheat the mixed exhaust steam  48 . The drain water  54  can be recycled in the power plant  26 . 
     The superheated mixed exhaust steam  48  then enters the second steam turbine generator  29  where it expands to produce further work. This allows the second steam turbine generator  29  to produce electricity. 
     The combined cycle unit  32  used in the power plant  26  has a similar disposition to that of known power plants, except that first, it receives more purge gas  56 , which is a by-product of the industrial process used in the CTL plant  10  and secondly, as mentioned, a portion of the saturated steam  50  from the HRSG  52  is directed to the MSR  30 . 
     Preferably all, or substantially all of the by-produced purge gas  56  is directed to the combined cycle unit  32 . Inside the combined cycle unit  32 , a first portion of the purge gas  56  is used to power a gas turbine generator  58 , which produces electricity. 
     The gas turbine generator  58  produces hot exhaust gases  60 . The heat of these hot exhaust gases  60  can also be recovered in the HRSG  52  and used to heat, evaporate and superheat a second portion of feed water  36  that enters the HRSG  52 . The hot exhaust gases  60 , having transferred a portion of their heat, can exit the HRSG  52  to the atmosphere as cooled flue gases  62 . 
     A second portion of the purge gas  56  is fired in a duct burner within the HRSG  52  to transfer additional heat to the feed water  36  and produce superheated saturated steam  50 . 
     Not all of the saturated steam  50   a  is directed to the MSR  30 . A second portion of the saturated steam  50  is superheated and fed to a third steam turbine generator  64  where it expands and produces work, thereby allowing the third steam turbine generator  64  to generate electricity. 
     Advantageously, the power plant  26  of the present invention is more efficient, that is, it generates more electricity for a given amount of energy used than known power plants. This is because all purge gas by-produced by the industrial process is used in a combined cycle, rather than partly in a combined cycle and partly in a Rankine cycle as in known power plant processes. The efficiency of the combined cycle is approximately 50% while the efficiency of the Rankine cycle is only approximately 30%. 
     The power plant configuration of the present invention can be applied to single saturated steam source or to multiple saturated steam sources depending on the process. With different sources and parameters of saturated steam, different power generation configurations can be developed by using the moisture separation power cycle concept. 
     The power plant configuration of the present invention applies to various industrial processes, and especially gasification processes, where saturated steam is produced, such as CTL, ammonia, methanol, hydrogen, and SNG production. 
     The power plant configuration of the present invention may also be applied to industrial processes such as iron and steelmaking, bauxite and alumina production, where common headers of steam with different pressure exist. 
     The CTL gasification process of the embodiment of the invention shown in  FIGS. 1 and 2  and a conventional CTL gasification process were both simulated on computer using Aspen Plus™ and GTPro/GTMaster™ software applications for comparison as Case A and Case B. Key streams parameters for Case A and Case B are shown in Tables A and B, respectively. (The feed water system is simplified to reflect a mass balance of steam generation of a power plant.) A CTL gasification plant with capacity of 40,000 bbl/d is simulated, in which dry-fed entrained flow gasifiers and cobalt catalyst F-T synthesis reactors are used. 
     Case A: Conventional Power Generation 
     According to the total quantity of purge gas available, which is the same for both Case A and Case B, a GE 7EA gas turbine is selected for Case A. This gas turbine operates at 94% load due to the limitation from low Btu gas application. There is no supplementary firing in the HRSG. A condensing steam turbine with steam induction from a HRSG is used. 
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE A 
               
             
             
               
                   
               
               
                 Computer Simulated Stream Parameters for Conventional Power 
               
               
                 Generation 
               
             
          
           
               
                   
                   
                 Pres- 
                 Temper- 
                 Flow 
               
               
                   
                   
                 sure 
                 ature 
                 Rate 
               
               
                   
                 Stream Name 
                 bar 
                 ° C. 
                 tonne/h 
               
               
                   
                   
               
             
          
           
               
                 Known 
                 Feed Water 
                   
                   
                 2,285 
               
               
                 CTL 
                 Feed Water to Waste Heat Boiler 
                   
                   
                 1,108 
               
               
                 Plant 
                 Steam from Waste Heat Boiler 
                 35.0 
                 242.6 
                 1,086 
               
               
                   
                 Fuel Gas to Fired Superheater (1) 
                 3.0 
                 34.0 
                 29 
               
               
                   
                 Steam Turbine (1) Inlet Steam 
                 32.2 
                 420.0 
                 1,086 
               
               
                   
                 Feed Water to F-T Reactor 
                   
                   
                 1,018 
               
               
                   
                 Steam from F-T Reactor 
                 18.0 
                 207.2 
                 997 
               
               
                   
                 Fuel Gas to Fired Superheater (2) 
                 3.0 
                 34.0 
                 20 
               
               
                   
                 Steam Turbine (2) Inlet Steam 
                 16.6 
                 350.0 
                 997 
               
               
                   
                 Fuel Gas to Gas Turbine 
                 3.0 
                 34.0 
                 51 
               
               
                   
                 Gas Turbine Exhaust 
                 1.02 
                 531.3 
                 1,121 
               
               
                   
                 Flue Gas to Stack 
                 1.00 
                 113.1 
                 1,121 
               
               
                   
                 Steam Turbine (3) Inlet Steam 
                 69.0 
                 510.0 
                 129 
               
               
                   
                 Feed Water to HRSG 
                   
                   
                 160 
               
               
                   
               
             
          
         
       
     
     For the conventional known power generation plant configuration, the results indicate: 
     Total heat input, HHV: 571.4 MWth 
     Total Power output: 643.6 MWe 
     It is noted that the heat for producing saturated steam in the waste heat boiler of gasification island and in the F-T reactor is not included in the total heat input. 
     Case B: Power Generation in Accordance with the Present Invention 
     The same 40,000 bbl/d CTL gasification plant is simulated based on a configuration for power generation in accordance with the present invention. Because the method of the present invention provides more purge gas to the gas turbine than with the known method, a GE 7FA gas turbine is selected. This gas turbine also operates at 94% load. In order to handle the total quantity of purge gas, a small portion of it is used as supplementary firing in the HRSG. 
     
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE B 
               
             
             
               
                   
               
               
                 Computer Simulated Stream Parameters for Power Generation in 
               
               
                 Accordance with the Present Invention 
               
             
          
           
               
                   
                 Stream 
                   
                   
                   
                   
               
               
                   
                 No. in 
               
               
                   
                 FIGS. 
                   
                 Pressure 
                 Temperature 
                 Flow Rate 
               
               
                   
                 1 &amp; 2 
                 Stream Name 
                 bar 
                 ° C. 
                 tonne/h 
               
               
                   
                   
               
             
          
           
               
                 CTL Plant 
                 36 
                 Feed Water 
                   
                   
                 2,451 
               
               
                 using 
                   
                 Feed Water to Waste Heat 
                   
                   
                 1,124 
               
               
                 Power 
                   
                 Boiler 
               
               
                 Generation 
                 38 
                 Steam from Waste Heat Boiler 
                 70.0 
                 285.9 
                 1,101 
               
               
                 of the 
                   
                 Steam Turbine (1) Inlet Steam 
                 65.0 
                 280.9 
                 802 
               
               
                 Present 
                 40 
                 Steam Turbine (1) Exhaust 
                 18.5 
                 208.4 
                 802 
               
               
                 Invention 
                   
                 Steam 
               
               
                   
                   
                 Steam to Stage-1 Reheater 
                 65.0 
                 280.9 
                 299 
               
               
                   
                   
                 Steam to Stage-2 Reheater 
                 104.0 
                 313.9 
                 29 
               
               
                   
                   
                 Drain from Moisture Separator 
                 18.0 
                 207.2 
                 58 
               
               
                   
                   
                 Drain from Stage-1 Reheater 
                 65.0 
                 280.9 
                 299 
               
               
                   
                   
                 Drain from Stage-2 Reheater 
                 104.0 
                 313.9 
                 29 
               
               
                   
                   
                 Feed Water to F-T Reactor 
                   
                   
                 1,018 
               
               
                   
                 46 
                 Steam from F-T Reactor 
                 18.0 
                 207.2 
                 997 
               
               
                   
                 48 
                 Steam Turbine (2) Inlet Steam 
                 16.6 
                 303.9 
                 1,741 
               
               
                   
                 56 
                 Fuel Gas to Gas Turbine 
                 3.0 
                 34.0 
                 99 
               
               
                   
                 60 
                 Gas Turbine Exhaust 
                 1.03 
                 604.1 
                 1,672 
               
               
                   
                 62 
                 Flue Gas to Stack 
                 1.00 
                 82.9 
                 1,679 
               
               
                   
                   
                 Steam Turbine (3) Inlet Steam 
                 101.9 
                 549.1 
                 224 
               
               
                   
                   
                 Feed Water to HRSG 
                   
                   
                 310 
               
               
                   
               
             
          
         
       
     
     For the power generation plant of the present invention, the results show: 
     Total heat input, HHV: 571.4 MWth 
     Total Power output: 693.8 MWe 
     By comparing the results of Case A and Case B, it can be seen that, for the same quantity of heat and of purge gas provided by the industrial process, the output of the power generation plant of the present invention is 50.2 MWe, or 7.8%, greater. 
     The increase of power by 50.2 MW benefits not only from the shift of fuel gas utilization from the Rankine cycle to the combined cycle, which accounts for 35.0 MW, but also from using a more advanced gas turbine (7FA vs. 7EA), which accounts for 15.2 MW. 
     The annual economic benefit of this additional power generation can reach 28 M$ when an operational availability factor of 92% and electricity rate of 0.07 $/kWh are selected. Moreover, by removing two fired superheaters used in the known power generation plant, the capital cost of the CTL gasification plant may be reduced. Also, it is noted that the specific emissions of green house gases (GHG) per giga joule (GJ) of energy produced may also be reduced. By using a power generation plant configuration in accordance with the present invention, the overall economics of a CTL plant or other industrial plant, can be improved. 
     The present invention has been described with regard to preferred embodiments. The description as much as the drawings were intended to help the understanding of the invention, rather than to limit its scope. It will be apparent to one skilled in the art that various modifications may be made to the invention without departing from the scope of the invention as described herein, and such modifications are intended to be covered by the present description. The invention is defined by the claims that follow.