Abstract:
A method for retrofitting an existing steam turbine with a steam extraction facility is provided. The stem turbine has a plurality of pressure stages and is integrated into a fossil-fired steam power plant. A steam extraction line is connected to one pressure stage or between two pressure stages of the steam turbine, and a heating steam turbine is connected into the steam extraction line.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the US National Stage of International Application No. PCT/EP2011/071180 filed Nov. 28, 2011 and claims benefit thereof, the entire content of which is hereby incorporated herein by reference. The International Application claims priority to the German application No. 10 2010 062 623.6 DE filed Dec. 8, 2010, the entire contents of which is hereby incorporated herein by reference. 
     
    
     BACKGROUND OF INVENTION 
       [0002]    There is a need to adapt existing fossil-fired power plants to changing requirements. Steam power stations or combined-cycle gas and steam power stations in particular are often subject to demands for adaptation, especially for a retroactive implementation of the capability to extract steam from the steam section of the power station. This additionally extracted steam can be required as process or heating steam for internal processes within the power station process or for supplying other processes outside of the actual power station process. Extracting steam from the steam turbine process reduces the remaining steam volume which is still available for the steam turbine process and which now can make no further contribution to the generation of steam. As a consequence the extraction of steam from the steam turbine process reduces the efficiency of a steam power plant. 
         [0003]    In order to enable a thermodynamically optimized concept to be realized in a steam extraction retrofit that is now to be implemented, the use of an extraction turbine would recommend itself already at the time of construction of the power plant. However, this concept would lead to an increased initial investment since the turbine cannot be optimized simultaneously for operation without extraction and with extraction. Retrofitting a steam turbine plant with steam extraction capability is often technically demanding and complex, as well as cost-intensive in terms of implementation. If a steam extraction capability is realized only as a result of a retrofit, substantial losses in efficiency are likely into the bargain. 
         [0004]    Converting an existing steam turbine plant to provide a retroactive steam extraction capability, in particular for tapping low-pressure steam, can be very complicated and labor-intensive, however. Thus, for example, the dimensions of the power house may not be sufficiently large to accommodate the additional piping for extracting the steam, or the steam turbine or, as the case may be, the power station process is not suitably configured for steam extraction. In steam turbines having a separate casing for the medium- and low-pressure stages it is at least easily possible to tap low-pressure steam at the overflow line. In steam turbines having a medium- and low-pressure stage housed in a single casing, on the other hand, it is often not feasible to carry out retrofits in order to extract the large volume of steam required, for which reason the turbine has to be replaced in this situation. In any event, however, when low-pressure steam is tapped into the low-pressure section from the overflow line, the low-pressure section needs to be adapted to handle the changed swallowing capacity (steam volume flow). 
         [0005]    Extracting steam from other sources within the power station process is often likewise not cost-effective or possible in a suitable manner. Thus, for example, extracting steam from a reheat line of the steam turbine leads to load imbalance in the boiler if no further expensive and complex measures are taken. Extracting higher-value steam for the carbon dioxide separator must also be ruled out unless further measures are undertaken, since this leads to unacceptable energy losses. 
         [0006]    A further problem that arises with the retrofitting of a steam extraction capability is that when the steam extraction is discontinued the steam that is now not required abruptly accumulates to excess. This surplus steam now cannot simply be returned to the steam turbine process, because the latter is configured for operation with steam extraction, in other words for a lower volume of steam. 
       SUMMARY OF INVENTION 
       [0007]    The object of the invention is therefore to disclose a method for retrofitting a steam extraction capability in order to tap steam from the steam process of a fossil-fired power plant, which capability can be realized in a simple and cost-effective manner, and which in addition is thermodynamically favorable, so that the efficiency losses due to the additional steam extraction are minimized. 
         [0008]    The object is achieved according to the invention by the features of the independent claims. Also provided according to the invention is a heating steam turbine which is connected to the overflow line of the steam turbine. 
         [0009]    The invention permits an extraction point to be chosen which lies outside of the turbine. This enables retrofit capability to be integrated without high initial investments. The use of a back-pressure turbine with extraction points permits multistage heating to be realized, which is more beneficial thermodynamically than single-stage heating. Moreover, this retrofit concept permits retroactive thermodynamic optimization, since the extractions are only specified at the time of the retrofitting. 
         [0010]    According to the invention the heating steam extraction is decoupled from the main process through the use of the back-pressure steam turbine. Because the back-pressure steam turbine is not supplied until the time of conversion, no extraction points need to be provided on the main steam turbine. This means that retrofitting is possible even in a power station in which heating steam extraction was not included in the planning at the time of installation. In this case, however, it might be necessary to carry out a modification to the low-pressure turbine. 
         [0011]    Advantageously the steam extraction line is connected to a reheat line. In the event of a deactivation of the steam extraction function the low-pressure steam continues to be tapped from the overflow line. For this reason an auxiliary condenser is connected in parallel with the steam extraction line. The auxiliary condenser is provided so that in the event of failure or intentional deactivation of the steam extraction function the accumulating excess steam will be condensed in the auxiliary condenser. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0012]    Exemplary embodiments of the invention are explained in more detail below with reference to figures, in which: 
           [0013]      FIG. 1  is a schematic diagram of a steam turbine arrangement comprising a back-pressure steam turbine according to the invention, 
           [0014]      FIG. 2  is a schematic diagram of a steam turbine arrangement having steam extraction from the overflow line according to the prior art. 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0015]      FIG. 2  shows a steam turbine arrangement having steam extraction from the overflow line according to the prior art. The steam extraction serves in this case to provide a district heating supply using two heating condensers HZ-K. The district heating system is connected to the gas and steam turbine power plant via the overflow line of the steam turbine. There, steam (NAA) is extracted and ducted by way of a steam line from the power house UMC to the district heating building UND. The actual district heating system in the form of 2×50% heating condensers is located in the district heating building UND. Depending on the required district heating capacity, provision of the district heating is effected using a single stage. The two district heating preheaters in combination can thermally transfer 265 MW at the maximum into the district heating system during normal operation. 
         [0016]    Alternatively the district heating system can also be operated with steam from the cold reheat cycle (KZÜ) (emergency operation during steam turbine downtime). Capacity transfer into the district heating grid is thermally limited in this case. 
         [0017]    The district heating return-circuit water that is to be heated is provided at the transfer point at a pressure of approx. 5-22 bar and flows via the two steam-heated district heating preheaters (HzVW1 and HzVW2) back into the district heating flow line to the district heating loads. The district heating flow line and the district heating return line can each be separated from the district heating water grid by means of a motorized butterfly valve. Each HzVW can be shut off individually by means of a manually operated shutoff valve on the input side and by means of a motorized butterfly valve on the output side. They possess a common bypass fitted with a motorized valve. 
         [0018]    The steam for the two HzVWs is tapped during steam turbine operation from the overflow line to the low-pressure (ND) steam turbine (DT) by way of a motorized bleeder valve. Two nonreturn valves in the line prevent backflow to the DT. A steam inspection probe monitors compliance with the maximum permitted pressure in this line. If the set value is exceeded the medium-pressure (MD)·DT quick-action shutoff valve is closed. The DT steam extraction lines are drained via drainage lines fitted with motor-driven shutoff valves to the condenser MAG and preheated. In order to achieve an energetically favorable mode of operation the HzVWs are connected to the system in a staggered manner: For that purpose the bypass of the HzVWs is set to fully open before the district heating steam extraction process is placed into service. The control butterfly valves at the outlet of the HzVWs are closed and the heat extraction begins with the opening of the outlet valve of the HzVW1. After the open position is reached the control butterfly valve in the bypass closes in a controlled manner in order to increase the district heating capacity. As the demand for heat increases the control butterfly valve at the outlet of the HzVW2 is opened in a controlled manner and, as previously in the case of HzVW1, closes the control butterfly valve in the HzVW as the demand for heat increases further until the entire volume flows through the HzVWs. If both HzVWs are in operation with the bypass closed and the requirement for heating capacity continues to increase, the steam pressure in both HzVWs is raised with the aid of the control butterfly valve in the overflow line to the ND turbine and as a result the heat output is increased in a controlled manner. In bypass operation of the steam turbine the steam is tapped from the KZÜ by way of a steam converter station. A steam inspection probe monitors to ensure compliance with the maximum permitted pressure on the low-pressure side. If the set value is exceeded the corresponding converter valve is immediately closed. Any valve leakages that could lead to a further increase in pressure are in each case ducted to the atmosphere via a downstream safety valve. The injection water for cooling the steam of the steam converter station is taken from the condensate system downstream of the condensate pumps. In order to protect against contamination of the injection control fittings the injection water lines are fitted with an upstream dirt strainer. In addition the section of pipeline up to the control valve may be protected by means of a safety valve in certain cases in order to ensure it cannot be damaged due to heating of the enclosed condensate. The steam lines upstream of the steam converter stations are preheated and drained to the drainage system LCM via drainage lines fitted with motor-driven shutoff valves. As the requirement for district heating capacity decreases the HzVWs are powered down in precisely the reverse order to the connection sequence. 
         [0019]    The condensate in the HzVWs drains off geodetically or due to the pressure difference into the main condenser, being ducted in the process through a main condensate preheater in order thereby to operate more energy-efficiently. A control valve in the drain line keeps the fill level in the HzVWs constant within the predefined limits. The two HzVWs remain under pressure on the hot water side when the district heating system is not in operation so that an escape of steam is reliably prevented. Both HzVWs are fitted with a safety valve on the hot water side in order to discharge the expanding heating water in the event of heating and enclosed medium. Valves and fittings that are operated in the vacuum range have a water seal adapter or are implemented with vacuum-tight stems. The impulse lines of the fill level measurements of the HzVWs are kept filled at all times by way of bubbler lines. A safety valve is installed on both HzVWs in order to enable the accumulating heating water to be ducted away in the event of pipeline rupture or leaks. 
         [0020]    The district heating system according to  FIG. 2  has the following tasks:
       ensuring heat input into the district heating grid   regulating the flow line temperature   the mass flow rate is regulated on the power station side       
 
         [0024]    Example process parameters:
       return line temperature: 60-75° C.   flow line temperature: 90-110° C.   heating water mass flow rate: max. 1400 kg/s   district heating capacity: approx. 20-265 MW.       
 
         [0029]    The district heating system consists of the following main components:
       two 50% district heating preheaters   heating condensate system without heating condensate pumps   steam provisioning via steam turbine extraction (NM)   steam provisioning system from cold ZÜ/KZÜ(LBC) incl. condensate injection cooling (LCE).       
 
         [0034]      FIG. 1  shows a steam turbine arrangement comprising a back-pressure steam turbine according to the invention. 
         [0035]    The district heating system is connected to the gas and steam turbine plant exactly as in  FIG. 2 . Steam (NM) is extracted from the overflow line of the steam turbine (DT) and ducted by way of a steam line from the power house UMC to the district heating building UND. Located there is a heating steam turbine including all ancillary equipment necessary for operation, such as e.g. lubricating oil system, evacuation system and drainage facilities. The steam from the NM system is ducted either to the steam turbine only or additionally to a third heating condenser (HzVW3). The heating power output of the district heating system is realized in up to three stages depending on the district heating capacity required. Accordingly, two or even three heating condensers are operated on the steam side as a function of demand. A heating condenser (HzVW1 and HzVW2) is located under each steam turbine outflow. Operating in combination at maximum steam turbine load, these heating condensers can transfer, for example, 120 MW equivalent thermal energy from the NM steam system into the district heating grid. If an increased steam output of more than 120 MW equivalent thermal energy is to be extracted, steam is injected into the heating condenser 3 (HzVW3) in addition. The latter is supplied directly with steam from the NM system. Alternatively the district heating system can also be operated with steam from the cold reheat cycle (KA) (emergency operation during steam turbine downtime). Capacity transfer into the district heating grid is thermally limited to, for example, 220 MW in this case. In the event of downtime/failure of the heating steam turbine the entire district heating output can be transferred into the district heating grid by way of the HzVW3. In this case the steam supply to the heating steam turbine is interlocked and the steam is supplied exclusively to the HzVW3. 
         [0036]    The district heating system according to  FIG. 1  has the following tasks:
       ensuring heat input into the district heating grid   regulating the flow line temperature   the mass flow rate is regulated on the power station side       
 
         [0040]    Example process parameters:
       return line temperature: 60-75° C.   flow line temperature: 90-110° C.   heating water mass flow rate: max. 1400 kg/s   district heating capacity: approx. 20-265 MW.       
 
         [0045]    The district heating system consists of the following main components:
       double-flow heating steam turbine with a max. terminal output power of, for example, approx. 14 MW   3× district heating preheaters   heating condensate system including heating condensate pumps steam provisioning via steam turbine extraction (NM) steam provisioning systems from cold ZÜ (KZÜ) LBC incl. condensate injection cooling (LCE).       
 
         [0049]    The district heating system can be housed in a separate building UND. A larger district heating building may be necessary on account of the increased space requirement for the heating steam turbine incl. ancillary equipment.