Abstract:
The invention relates to a steam generation plant, comprising a steam generator ( 1 ) with a combustion chamber ( 8 ), an evaporator, a superheater ( 9 ), an intermediate superheater ( 12 ), a condenser ( 14 ), a feed water preheater ( 16, 19, 19 ′) regeneratively heated by steam, a steam turbine set ( 2 ) with a high-pressure section ( 4 ), a medium pressure section ( 5 ) and a low-pressure section ( 6 ), a flue gas line ( 22 ), connected to the combustion chamber ( 8 ), an air supply line ( 21 ), for the supply of combustion air to the burner in the combustion chamber ( 8 ) and an air preheater ( 3 ) with flue gas and combustion air passing therethrough. An air line ( 23 ) branches off from the air supply line ( 21 ) downstream of the air preheater ( 3 ) in said steam generation plant and supplies an air-fractionation unit ( 25 ). Air coolers ( 34, 35 ) are arranged in the air line ( 23 ) through which the condensate or feed water from the condensate/feed water circuit from the steam generator ( 1 ) flows. The oxygen output from the air fractionation unit ( 25 ) is connected to the burner of the combustion chamber ( 8 ) by means of an oxygen line ( 26 ).

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a U.S. national stage of International Application No. PCT/EP2006/005334, filed on 3 Jun. 2006. Priority is claimed on German Application No. 10 2005 026 534.0, filed on 8 Jun. 2005. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention pertains to a steam generating plant of the type having a steam generator including a combustor; a set of steam turbines; a condensate circuit connected to the steam generator; a flue gas line connected to the combustor; an air feed line for supplying combustion air to the combustor; and an air pre-heater wherein the combustion air is pre-heated by the flue gas. 
     2. Description of the Related Art 
     By burning fossil fuels, these types of steam generating plants generate the CO 2  which is held responsible for the destruction of the ozone layer of the atmosphere. Industry and various universities are therefore conducting joint research projects to develop ways of separating CO 2  from the flue gas produced in the power generation industry. 
     These joint development projects include the conversion of CO with H 2 O to CO 2  and H 2 , followed by separation of the CO 2  in an integrated gasification combined cycle (IGCC process), and the combustion of fossil fuel with pure oxygen and subsequent separation of the CO 2  (oxy-fuel process). According to current estimates, it will take some time, e.g., about 10-20 years, before it will be possible to realize the construction of new steam generating plants based on the oxy-fuel process, and even then it will be associated with high investment costs. 
     It could be much more favorable to retrofit existing, conventionally fired power plants, because the investments would be much lower. Because of the CO 2  separations associated with combustion with pure oxygen, only power plant blocks of the higher power output ranges of 100-300 MW can be considered for an oxy-fuel retrofit. This is true not only because of the cost but also because of the size of the compressor units which must be used. 
     SUMMARY OF THE INVENTION 
     The invention is therefore based on the task of retrofitting a steam generating plant of the general type in question fired with pure oxygen (oxy-fuel process) in such a way that the steam generating plant can be operated according to either the oxy-fuel process or the conventional process. 
     For a steam generating plant of the general type in question and for a process for operating or retrofitting a plant of this type, an air line is connected to the air feed line at a connection downstream of the air pre-heater, air coolers cooled by condensate from the condensate circuit are installed in the air line downstream of the connection, an air separation plant is connected to the air line downstream of the air coolers, and an oxygen line is installed between the separation plant and the combustor. 
     As a result of the inventive retrofitting to the oxy-fuel process, the existing regenerative feed water preheating and the existing air preheating on the flue gas side of the steam generator are integrated into the inventive steam generating plant, and use is made of intermediate superheating. The circuit of the steam generating plant on the combustion air side is selected in such a way that operation with air as the sole oxygen source remains possible without restriction. A conventional steam generating plant can thus be retrofitted to an oxy-fuel plant without any impairment to the ability of the conventional plant to operate with fresh air. Scaling to larger units is possible. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a circuit diagram of a steam generating plant retrofitted to operate with oxygen (oxy-fuel mode); and 
         FIGS. 2-4  show additional embodiments of the steam generating plant according to  FIG. 1 . 
     
    
    
     The steam generating plant comprises a steam generator  1  with a water-steam circuit, a set  2  of steam turbines, an air feed, a flue gas discharge line, and a regenerative air preheater (combustion air preheater)  3 , heated by flue gas. To this extent, the steam generating plant is conventional in design. It is explained briefly in the following only to the extent that this is necessary to understand the invention. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The set  2  of steam turbines consists of a high-pressure turbine  4 , an intermediate-pressure turbine  5 , and a low-pressure turbine  6 , which are mounted on a common shaft and which drive a generator  7  to generate electrical energy. 
     The steam generator  1  illustrated here is designed as a forced-flow steam generator. The following description can also be applied to a drum boiler. The steam generator  1  has a combustor  8 , which is fired with gaseous fuel. In principle, the generator could also be coal-fired in conjunction with a flue gas purification system. A superheater  9  is provided after the evaporative heating surfaces of the combustor  8 . A high-pressure steam line  10  connected to the superheater  9  leads to the high-pressure turbine  4  of the steam turbine set  2 . The exhaust steam outlet of the high-pressure turbine  4  is connected to an intermediate superheater  12  of the steam generator  1  by a connecting line  11 . The outlet side of the intermediate superheater  12  is connected to the intermediate-pressure turbine  5  of the steam turbine set  2  by an intermediate steam line  13 . The exhaust steam side of the intermediate-pressure turbine  5  is connected to the low-pressure turbine  6 . 
     The exhaust steam outlet of the low-pressure turbine  6  is connected to a condenser  14 . A condensate line  15 , in which a condensate pump  15 ′ is installed, is connected to the condenser  14 . Several low-pressure feed water preheaters  16 , a thermal feed water degasser  17 , a high-pressure feed water pump  18 , and several high-pressure feed water preheaters  19 ,  19 ′ are installed in series in the condensate line  15 . The last high-pressure feed water preheater  19 ′ is connected to an additional, flue gas-heated feed water preheater or to the evaporator of the steam generator  1 . The feed water preheaters  16 ,  19  are heated by steam tapped from the high-pressure turbine  4 , from the intermediate-pressure turbine  5 , and from the low-pressure turbine  6  of the steam turbine set  2 . 
     A blower  20  is installed in an air feed line  21 , which is connected to the inlet-side air section of the regenerative air preheater  3 . On the downstream side of the air preheater  3 , this air feed line leads to the firing unit of the combustor  8  to supply it with combustion air. A flue gas line  22  is connected to the flue gas outlet of the steam generator  1 ; this flue gas line proceeds to the inlet-side gas section of the regenerative air preheater  3 . Because of the constraints of the drawing, the flue gas line  22  is interrupted at points “b”. Following the air preheater  3 , the flue gas line  22  proceeds to the stack  36 . 
     The description given so far pertains to a conventional steam generating plant. How the steam generating plant can be retrofitted to create an oxy-fuel plant will be described next. This retrofitting can be undertaken afterwards on an already existing plant or can be done from the very start in a new steam generating plant. 
     An air line  23  is branched off from the air feed line  21  at a point downstream of the air preheater  3  and extended to an air compressor  24 . The outlet of the air compressor  24  is connected to an air separation plant  25 . The oxygen outlet of the air separation plant  25  is connected to a gas mixer  27  by an oxygen line  26 ; the gas mixer is installed in the air feed line  21  extending between the air preheater  3  and the firing unit of the combustor  8  of the steam generator  1 . 
     The air compressor  24  is driven by a steam drive turbine  28 . In the embodiment according to  FIG. 1 , the steam drive turbine  28  is operated with steam taken by way of a steam line  29  from the intermediate steam line  13  located between the intermediate superheater  12  of the steam generator  1  and the intermediate-pressure turbine  5  of the steam turbine set  2 . Because of the constraints of the drawing, the steam line  29  is interrupted at the points “a”. Upstream of the entrance to the steam drive turbine  28 , a control valve  30  is installed in the steam line  29 . The exhaust steam from the steam drive turbine  28  is sent to a condenser  31 , which is connected to the condensate line  15  of the steam generator  1  by a condensate line  32 , in which a pump  33  is installed. The connection is established at a point upstream of the first low-pressure feed water preheater of the low-pressure feed water preheater group  16 . 
     If the way in which the main condenser  14  of the steam generator  1  is designed allows the possibility, the exhaust steam from the steam drive turbine  28  can be sent to this main condenser  14 . As a result, the condenser  31  of the drive-steam turbine  28  and the associated condensate pump  33  can be eliminated. 
       FIG. 2  shows a steam drive turbine  28 ′, which is operated not with intermediate steam but rather with tapped steam. The tapped steam is taken from a suitable tapping stage  47  of the steam turbine set  2  and sent to the steam drive turbine  28 ′ through a steam line  29 ′. 
     According to  FIG. 3 , a steam drive turbine  28 ″ can also be used, which is operated with steam from an external steam source  48  via a steam line  29 ″. This external steam source can be a directly fired steam generator. 
     In place of a steam drive turbine  28 ,  28 ′,  28 ″, it is also possible to use an electric motor  49 , as shown in  FIG. 4 , to drive the air compressor  24 . 
     Two air coolers  34 ,  35  are installed in the air line  23  between the air preheater  3  and the air compressor  24 . The air coolers  34 ,  35 , like the steam drive turbine  28  which drives the air compressor  24 , are integrated into the water-steam circuit of the steam generator  1 . High-pressure feed water flows through the air cooler  34  installed downstream of the air preheater  3 ; this feed water is taken from the condensate line  15  upstream of the high-pressure feed water preheater  19  and is returned to the condensate line  15  upstream of this high-pressure feed water preheater  19 . If the temperature of the steam in the intermediate superheater  12  of the steam generator  1  is controlled by internal recirculation of flue gas, then the last high-pressure feed water preheater  19 ′ can also be connected to the air cooler  34 . Low-pressure feed water flows through the air cooler  35  installed downstream of the air cooler  34 ; this feed water is taken from the condensate line  15  upstream of the low-pressure feed water preheater group  16  and is returned to the condensate line  15  downstream of the low-pressure feed water preheater group  16 . 
     A recirculation blower  37  is installed in the flue gas line  22 , downstream from the regenerative air preheater  3  and from a branch leading to the stack  36 . Downstream from the recirculation blower  37 , the flue gas line  22  branches into two flue gas secondary lines  38 ,  39 . The first flue gas secondary line  38  leads to the gas mixer  27 . 
     The second flue gas secondary line  39  leads to a CO 2  compressor  40 . The CO 2  compressor  40  is driven by an expander  42  and a motor/generator  41 . The CO 2  compressor  40  and the expander  42  are mounted on the same shaft as the motor/generator  41 . 
     As illustrated by way of example in  FIGS. 3 and 4 , the motor/generator  41  can be eliminated. Instead, the air compressor  24 , the expander  42 , and the CO 2  compressor  40  are mounted together with either the steam drive turbine  28 ″ or the electric motor  49  as drive means on a single-shaft power train  50 . It should be emphasized that the drive train shown in  FIGS. 3 and 4  can also be used in a steam generating plant according to  FIGS. 1 and 2 , just as it is also possible for the drive train according to  FIGS. 1 and 2  to be used in a steam generating plant according to  FIGS. 3 and 4 . 
     Heat exchangers  43  for cooling the flue gas below the water dew point are installed in the second flue gas secondary line  39  before it arrives at the CO 2  compressor  40 , as a result of which water is separated from the flue gas. The heat exchangers  43  are connected to the expander  42  by a connecting line  44 ′, thus forming a Rankine cycle  44 , in which a coolant with a low boiling point, e.g., NH 3 , is used as the working medium. A pump  45  connected to the outlet of the expander  42  circulates the working medium through the heat exchangers  43  and the expander  42 . 
     As shown in the drawing and described above, the air line  23 , which leads via the air compressor  24  to the air separation plant  25  and includes the air coolers  34 ,  35 , and the oxygen line  26 , which leaves the air separation plant  25 , are connected in parallel to the air feed line  21  leading to the combustor  8 . Shutoff/control valves  46  in the air feed line  21 , in the air line  23 , in the oxygen line  26 , in the first flue gas secondary line  38 , and in the second flue gas secondary line  39  make it possible to shut off the line in question and to control the medium flowing through it. 
     The previously described steam generating plant is operated as follows. The air required for the oxy-fuel process, that is, the air required to operate the plant with oxygen, is cooled to the lowest possible temperature in the coolers  34 ,  35  downstream of the regenerative air preheater  3  by the use of steam turbine condensate and then compressed in the air compressor  24  to the pressure required for the air separation plant  25 . 
     The air compressor  24  is driven by the steam drive turbine  28 ,  28 ′, which is fed with intermediate steam from the intermediate superheater  12  or with tapped steam from the tapping stage  47  of the intermediate-pressure turbine  5  of the steam turbine set  2 . The power loss of the steam turbine set  2  is small, because the removal of the intermediate or tapped steam is partially compensated quantitatively by shifting the heat of the combustion air into the condensate circuit of the steam generator  1 . This is done by closing or only partially opening the tapping points of the steam lines on the medium-pressure and the low-pressure side. The condensate accumulating from the steam drive turbine  28 ,  28 ′ is added to the condensate circuit of the steam generator  1 . As a result, there is no need for an additional degasser or an additional steam condensate system. Because the heat of the combustion air is shifted from the air preheater  3  into the condensate feed water circuit of the steam generator  1 , the power loss caused by the removal of intermediate steam or tapped steam to drive the steam drive turbine  28 ,  28 ′ is almost completely compensated. 
     If the absorption capacity of the steam turbine set  2  is sufficient and if the generator  7  still has additional reserves, it would be possible to disconnect the drive of the air compressor  24  from the intermediate steam rail of the intermediate-pressure turbine  5  of the steam turbine set  2 , this rail consisting of the intermediate steam line  13  and the tapping stage  47 . Either an electric motor or a pure steam turbine process with a direct-fired steam generator could be used a drive source. The advantage of such conceptions lies in the latter case both in the freedom of choice with respect to the steam parameters and in the improved dynamics of the process of switching the steam generating plant over to operation with pure air in the event that the additional turbomachines used for the oxy-fuel process have to be tripped. To increase the efficiency of the drive process, the intermediate heat and the recooler heat of the air compressor  24  could also be integrated beneficially into the concept of the retrofitted plant. 
     The air for the air separation plant  25  is compressed by the air compressor  24  to the pressure necessary for the air separation plant  25 . It can be effective to combine axial and radial compressors with intermediate coolers and recoolers in steam generating plants with higher power outputs. In principle, the drive power can also be supplied exclusively by electric motors. 
     The startup of the steam generating plant occurs with the blower  20  at 100% load, where approximately 60% of the air, which represents the minimum load of the air separation plant  25 , is sent to the air separation plant  25 , and approximately 40% of the air, which represents the minimum load of the forced-flow steam generator or of a natural-convection boiler, is sent to the steam generator  1 . These values can be varied as appropriate, depending on the process. The steam generator  1  operates in partial-load, fresh-air mode until the air separation plant  25  is producing O 2  of the desired quality. Then the plant is switched over from partial-load air mode to the corresponding partial-load oxygen mode of the oxy-fuel process. The loads are increased further under consideration of the allowable values of the air separation plant  25 . A plant operating in oxygen mode is shut down in the opposite direction by switching back to air mode first. 
     Because no nitrogen is present during combustion with oxygen, the mass flow rate of the flue gas in the flue gas line of the steam generator  1  is correspondingly lower than that which occurs during fresh-air mode, and the firing temperatures are also significantly higher. This increase in the firing temperatures would lead to considerable thermal loads on the pipes in the combustor  8  of the steam generator  1 , but, by spraying a high predetermined return flow of flue gas into the firing system of the steam generator  1  via the gas mixer  27 , both the mass flow rate and the combustion temperatures are adjusted to values similar to those of fresh-air mode. Through the mixing of oxygen and recirculated flue gas in the gas mixer  27 , O 2  contents similar to those of fresh-air mode are achieved. For thermodynamic reasons, the recirculated flue gas is discharged downstream of the air preheater  3 . 
     As previously mentioned, all of the plant components belonging to the oxy-fuel process are connected in parallel to the steam generating plant. In addition, shut-off/control valves  46  are installed in the air feed line  21 , in the air line  23 , in the oxygen line  26 , in the first flue gas secondary line  38 , and in the second flue gas secondary line  39 . In this way, the oxy-fuel process is integrated into the steam generating plant  1  in such a way that it is also possible at any time to operate in pure fresh-air mode without supplying oxygen. For this purpose, the appropriate shutoff/control valves  46  are to be closed. The steam generating plant  1  can also be operated in pure fresh-air mode in the event that one of the turbomachines such as the air compressor  24 , the expander  42 , or the CO 2  compressor  40  breaks down or if the machines are shut off. After completion of the installation of the parallel-connected plant components belonging to the oxy-fuel process, a short-circuit is created during the time that the steam generating plant is being inspected. 
     To remove the water, the remaining flue gas, consisting primarily of CO 2 , is cooled to a temperature far below the water dew point of the flue gas by way of the Rankine cycle  44  based on NH 3 . As a result of the release of the heat of evaporation of the steam component and the latent heat of the flue gas, it is possible to recover additional electrical energy via the expander  42 . 
     The expander  42  drives the CO 2  compressor via the motor/generator  41 ; the compressor produces the predetermined final CO 2  pressure required for the purpose in question. A compression to 200 bars can be achieved for an EOR process (Enhanced Oil-Recovering process). Depending on the required drive power of the compressor  40 , operation will proceed in either motor or generator mode.