EP2698507A1

EP2698507A1 - System and method for temperature control of reheated steam

Info

Publication number: EP2698507A1
Application number: EP12180786.1A
Authority: EP
Inventors: Christian Dicker; Julia Kirchner
Original assignee: Alstom Technology AG
Current assignee: General Electric Technology GmbH
Priority date: 2012-08-17
Filing date: 2012-08-17
Publication date: 2014-02-19
Anticipated expiration: 2032-08-17
Also published as: EP2698507B1; WO2014026995A2; WO2014026995A3

Abstract

A temperature control system, particularly for low load conditions, is described including a by-pass line (132) with a by-pass control valve (133) branching off a live steam circuit (13) to a first steam turbine (14) in a sequence of at least two steam turbines (14, 18), with a steam exit line of the first turbine merging with the by-pass line into the inlet of a reheater (16) heated by a steam generator (10) and connected to a second steam turbine (18) of the sequence of at least two steam turbines, and having a steam mixing chamber (151) at or after the point of merging and before the inlet into the reheater. The desired temperature is set by controlling the mass flow through the by-pass line and/or the first turbine.

Description

This invention relates generally to a method and system for controlling the hot reheater outlet steam temperature, particularly at low turbine load.

Background

As described for example in the United States patent no. 5,605,118 , a modern steam generator can include a complex configuration of various thermal and hydraulic units for preheating and evaporating water, and for superheating steam. The surfaces of such units are arranged to facilitate: a complete and efficient fuel combustion while minimizing emissions of particulate and gaseous pollutants; steam generation at a desired pressure, temperature and flow rate; and to maximize recovery of the heat produced upon combustion of the fuel.
As part of the modern steam generator, reheaters and superheaters are specially designed tube bundles capable of increasing the temperature of saturated steam. Additionally, reheaters and superheaters are designed to obtain specific steam outlet temperatures, while keeping metal temperatures from becoming too elevated, and limiting steam flow pressure losses. Essentially, a reheater or a superheater is a single-phase heat exchanger comprising tubes through which steam flows, and across which the combustion or flue gas passes on the outside of the tubes. Reheater and superheater tube bundles are often made of a steel alloy capable of withstanding high operating temperatures.
The reheater typically provides the steam for a second turbine following the first turbine, which is typically fed directly from the feed water cycle going through the steam generator. Referring to the respective state of expansion, the first turbine is typically known as high-pressure or HP turbine and the second turbine or turbine group as intermediate pressure or IP turbine or turbine group. The reheater receives its feed stream from the steam which exits the HP turbine.
It can be important for the heat rate and cycle efficiency of a steam generator, such as a carbonaceous fuel boiler-turbine power plant, to regulate and control reheater steam temperature within narrow limits. To maintain a constant temperature can be particularly challenging when a power plant operates at a low load, for example during start-up, as the pressure of the reheat section is also very low. Depending of the type of steam generator of boiler, the reheater outlet temperature (RHO) required at main continuous rated (MCR) conditions may not be achieved at certain load levels. As a consequence the IP turbine will not receive steam heated to the optimal operational temperature.
A known method to control reheat steam temperature is by water spray, also called direct contact attemperation or de-superheating, by which a cooling water spray is added to the fluid entering the reheater. This method can have a negative effect on cycle efficiency. Another method is to use excess air supplied to the boiler for reheater steam temperature control. This method can have a negative effect on boiler efficiency. Other solutions include drawing off steam from the super heater and/or reheater, leaving however the problem of finding a disposal path for the extracted steam. The methods described above can only be applied if the steam is too hot.
In view of the prior art it is seen as an object of the present invention to provide more efficient means and methods for controlling the temperature of the reheater, particularly at low (i.e sub-operational) loads or pressure in the steam path.

Summary

According to an aspect of the present invention, there is provided a temperature control system including a by-pass line with a by-pass control valve branching off a live steam circuit to a first turbine in a sequence of at least two turbines, with a steam exit line of the first turbine merging with the by-pass line into the inlet of a reheater heated by a steam generator and connected to a second turbine of the sequence of at least two turbines, and having a steam mixing chamber at or after the point of merging and before the inlet into the reheater.
The steam mixing chamber has preferable a first inlet connected to the by-pass line and a second inlet connected to the steam exit line of the first turbine. The steam mixing chamber includes preferably a static steam mixer.
The line to the by-pass can be preferably connected to the main/live steam line at a location between the steam generator and the main steam inlet of the first turbine or at a location within the steam generator downstream the primary superheater, e.g. as intermediate boiler extraction.
According to another aspect of the invention, there is provided a method for controlling the outlet temperature of a reheater positioned in the steam path between a first turbine and a second turbine in a sequence of at least two turbines, wherein at low unit loads the steam temperature at the inlet to the reheater is raised at a pressure lower than the main continuous rated operation pressure by extracting steam from a live steam line connecting the steam generator with the first turbine and mixing the extracted steam with the steam from the first turbine such that the inlet steam temperature to the reheater is raised to a temperature sufficient to maintain the outlet temperature at the desired level, e.g. close to the MCR temperature or closer to the HP turbine/live steam temperature. Hence it is a preferred object of the present invention to increase the reheater outlet temperature.
The above and further aspects of the invention will be apparent from the following detailed description and drawings as listed below.

Brief Description of the Drawings

Exemplary embodiments of the invention will now be described, with reference to the accompanying drawings, in which:

FIG. 1 is a schematic diagram of an exemplary embodiment in accordance with the invention ; and
FIG. 2 is a schematic diagram of another exemplary embodiment of the invention.

Detailed Description

Aspects and details of examples of the present invention are described in further details in the following description. Exemplary embodiments of the present invention are described with references to the drawings, wherein like reference numerals are used to refer to like elements throughout. In the following description, for purposes of explanation, numerous specific details are set forth to provide a thorough understanding of the invention. However, the present invention may be practiced without these specific details, and is not limited to the exemplary embodiments disclosed herein
Fig. 1 shows a schematic diagram of a section of a steam power plant designed to provide power to a public power grid. The steam is generated in a boiler 10. The boiler 10 receives water from condenser (not shown) or a source external to the plant. The water passes through a feed water preheater 11 as shown before entering the boiler. The boiler 10 can be fired directly by fossil fuels such as coal or gas or by a non-convention heat source in form of a secondary heat exchange cycle or as is otherwise known in the industry.
The live steam is generated within a cascade of heat exchangers 12 before exiting the boiler into the main/live steam pipework 13. The feed pipe 13 directs the steam into the inlet of a first turbine 14, which is in this example a high-pressure (HP) turbine within several turbine stages of. The main steam control valve 131 controls the amount of steam entering the HP turbine 14. At the exit of the HP turbine the partially expanded steam is returned in the cold reheat (CRH) line 15 to the inlet of a reheater section 16 of the boiler. Within the reheater 16 the partially expanded steam is reheated using the heat generated in the boiler 10.
The reheat outlet (RHO) is connected through the hot reheater (HRH) line 17 with the inlet of a second turbine 18, which is in this example an intermediate-pressure (IP) turbine within several turbine stages. The turbines share a single rotor 19 which drives a generator (not shown). Further turbine stages may include additional IP turbines or one or more low pressure (LP) turbines with or without additional reheating circuits. As will be however evident from the following description of examples, the principles of the present invention can be applied to any steam power plant with a reheating system and at least two turbines connected through a reheating system.
The system of FIG. 1 further includes a by-pass line 132 connecting the main steam line 13 with the CRH line 15. The amount of live steam taken from the main line is controlled by a by-pass valve 133. A spray cooler 134 is installed at the outlet of the HP by-pass valve to control the temperature T2 in the HP by-pass exhaust line. The by-pass line directs steam into a mixing chamber 151 within the CRH line 15. The second inlet to the mixing chamber 151 is connected to the exit of the HP turbine 14. From the mixing chamber 151 the CRH line 15 connects to the inlet of the reheater 16.
The mixing chamber 151 of this example is a static mixer with fixed elements made of steel or steel alloy suitable for the steam temperature required for this application. Such mixers are commercially available for example as STATIFLO™ series 800/850.
During operation in a conventional power plant with a by-pass the HP bypass system has the following standard functions according to the standard ANSI/ISA-77.13.01-1999:

a) Control the pressure of the steam bypassing the HP turbine
b) Control the pressure of the main steam from the boiler
c) Control the flow and temperature of steam through the cold reheat line to cool the boiler reheater tubing
d) Control the flow of steam through the main steam line to cool the boiler final superheater in case of sliding pressure operation
e) Prevent lifting of main steam and hot reheat safety valves during transient operations
f) act as safety valve for the superheater section of the boiler (TRD 421, DIN EN ISO 4126).

To cool the boiler reheater tubing the by-pass line 132 typically includes a de-superheater 134, which can be a spray cooling chamber located within or after the by-pass valve 133.
In contrast to known applications for reheat temperature control, it is the aim of the present example to raise the inlet temperature T3 of the steam to the reheater section 16, which will, assuming a constant or similar heat input into the boiler, result in an increased reheater outlet (RHO) steam temperature T4. The HP bypass valve 133 is operated in a way to control the RHO temperature T4 by means of by-passing live steam around the HP turbine 14 directly into the cold reheat (CRH) line 15. In the HP by-pass valve 133 the pressure of the live steam is reduced to the pressure of the cold reheat (CRH) steam. This leads to a temperature reduction of the life steam by throttling (Joule-Thompson-Effect). Depending on the live steam pressure, the temperature reduction can be in the range of for example 40 K. If, for example, reducing the live steam pressure and temperature by throttling from 100 bar overpressure and 540°C to 15 bar overpressure, the resulting steam temperature T2 after the bypass valve 132 is 500°C. If the maximum (= design) temperature of the CRH line is less than this temperature, the piping downstream of the HP bypass valve and even downstream of the steam mixer 151 must be either exchanged for a high-temperature-resistant material before applying this example of the invention in plants already equipped with an HP turbine bypass. Another solution is to cool the steam further by spraywater injection in the spray cooler 134.
Depending on the design and operational parameters of the bypass valve 133 the steam downstream the HP by-pass valve 133 has a temperature T2 higher than the temperature T1 of CRH steam coming from the HP turbine exhaust. By mixing the steam from the by-pass line 132 with the HP exhaust steam in the CRH line 15 the steam temperature T3 at the inlet of the reheater 16 is increased. In turn, the RHO steam temperature T4 can be maintained at the operational level for the IP turbine 18, even at low load.
The above basic steps to increase the temperature can be refined to match any existing design parameters of the power plant. In case the plant allows for example for higher temperature in the by-pass line, the reduction of pressure in the HP by-pass valve 132 can be adjusted to maximum the pressure of the cold reheat (CRH) steam design values.
If it is not feasible or desired by the power plant to use high-temperature-resistant material in the piping downstream of the by-pass valve 132, the temperature T2 of the by-pass steam can be reduced by suitable cooling such as spray water cooling in the de-superheater 134. In this case, the mixing can take place in the standard CRH piping 15. However the by-pass mass flow required to achieve a high CRH inlet temperature T3 may be larger than in the first example above. But the loss of efficiency can be offset by the reduced costs in implementing this variant of the invention. This kind of operation reduces the mass flow through the HP turbine more than the solution without using spraywater. As a large amount of spray water is required, the mass flow to the second turbine is much greater than when not using spray water.
It is further possible to extract the steam for the by-pass line 152 from a location within the boiler 10. An example of this variant of the invention is shown in FIG. 2. In FIG. 2, the by-pass line 132 extracts steam from an intermediate point in the superheater section 12, which is located after the steam/water separator 101. By extracting the steam between the steam/water separator 101 and the boiler outlet of the main steam line 13, the live steam temperature T5 may be increased. The temperature T2 after the throttling will however be lower as the steam temperature T6, this is again due to the Joule-Thompson rule.
In the above examples, the hot reheater outlet temperature (HRH temperature) T4 is controlled by adjusting the cold reheater inlet temperature (CRH temperature) T3. The CRH temperature T3 is set such that a desired HRH temperature can be obtained with the heat input from the boiler 10 into the reheater 16. This heat input can be calculated from the boiler and boiler fuel feed parameters. The CRH temperature after mixing the mass flow from the turbine 14 with the mass flow from the by-pass 132 can be adjusted by enthalpy balances over the HP 14 turbine and the HP by-pass valve 133.
The enthalpy balance over the HP turbine yields temperature T1 at the exit of the HP turbine 14. Similarly, an enthalpy balance over the HP by-pass 132 delivers the temperature T2 downstream the HP bypass valve. The mixing temperature T3 can then be calculated. If required, the HP by-pass valve exhaust temperature T2 can be adjusted by spraying water in the de-superheater 134.
The pressure drop over the HP by-pass valve 133 is controlled in a similar way as the temperature. The pressure in the HP bypass exhaust line 132 must be slightly higher than the CRH pressure, to be sure that the HP by-pass exhaust steam can enter the CRH line 15.
The present invention has been described above purely by way of example, and modifications can be made within the scope of the invention, particularly as relating to the way of measuring and calculating temperatures and enthalpy balances. The invention also may comprise any individual features described or implicit herein or shown or implicit in the drawings or any combination of any such features or any generalization of any such features or combination, which extends to equivalents thereof. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments.
Each feature disclosed in the specification, including the drawings, may be replaced by alternative features serving the same, equivalent or similar purposes, unless expressly stated otherwise.
Unless explicitly stated herein, any discussion of the prior art throughout the specification is not an admission that such prior art is widely known or forms part of the common general knowledge in the field.

LIST OF REFERENCE SIGNS AND NUMERALS

boiler 10
feed water preheater 11
superheater section 12
main/live steam line/pipe 13
HP turbine valve 131
by-pass line 132
by-pass valve 133
de-superheater 134
first (HP) turbine 14
cold reheat (CRH) line 15
mixing chamber 151
reheater 16
Hot Reheat (HRH) line 17
second (IP) turbine 18
rotor 19
cold reheat (CRH) steam temperature T1
by-pass steam temperature T2
reheater inlet temperature T3
hot/outlet reheat (HRH) steam temperature T4
main/live steam temperature T5
steam temperature at the extraction point T6

Claims

A temperature control system including a by-pass line with a by-pass control valve branching off a live steam circuit to a first steam turbine in a sequence of at least two steam turbines, with a steam exit line of the first turbine merging with the by-pass line into the inlet of a reheater heated by a steam generator and connected to a second steam turbine of the sequence of at least two steam turbines, and having a steam mixing chamber at or after the point of merging and before the inlet into the reheater.
The control system of claim 1 wherein the steam mixing chamber has a first inlet connected to the by-pass line and a second inlet connected to the steam exit line of the first steam turbine.
The control system of claim 1 wherein the steam mixing chamber includes a static mixer.
The control system of claim 1 wherein the by-pass line is connected to the main live steam line at a location between the steam generator and the main steam inlet of the first turbine.
The control system of claim 1 wherein the by-pass line is connected to the main/live steam line at a location within the steam generator.
A method for controlling the outlet temperature of a reheater positioned in the steam path between a first turbine and a second turbine in a sequence of at least two steam turbines, wherein the steam temperature at the inlet to the reheater is raised by extracting steam from a live steam line connecting the steam generator with the first turbine and mixing the extracted (and expanded) steam with the steam from the outlet of the first turbine such that the inlet steam temperature to the reheater is raised to a temperature sufficient to maintain the outlet temperature at the desired level.
The method of claim 6 wherein under low load conditions the desired level the outlet temperature is maintained equal or close to the outlet temperature at normal operating conditions.
The method of claim 6 wherein the steam mass flow through the by-pass line and/or through the first turbine are controlled in response to the inlet steam temperature for the reheater.
The method of claim 6 wherein the steam mass flow through the by-pass line and/or through the first turbine are controlled in response to the outlet steam temperature for the reheater.