US20180298788A1

US20180298788A1 - Combined Gas-and-Steam Power Plant

Info

Publication number: US20180298788A1
Application number: US15/766,178
Authority: US
Inventors: Stefan Becker; Vladimir Danov; Uwe Lenk; Florian Reißner; Erich Schmid; Jochen Schäfer
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 2015-10-07
Filing date: 2016-09-26
Publication date: 2018-10-18
Also published as: EP3359782A1; JP2018534465A; CN108138600A; DE102015219398A1; WO2017060112A1; KR20180059939A

Abstract

The present disclosure relates to power plants. Teachings thereof may be embodied in methods for operating a combined gas-and-steam power plant and/or combined gas-and-steam power plants. For example, some embodiments may include a method for operating a combined gas-and-steam power plant comprising: generating steam with waste gas from a gas turbine; driving a generator for providing electrical current via a turbine device; and using at least part of the heat in the steam to affect an endothermic chemical reaction.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National Stage Application of International Application No. PCT/EP2016/072840 filed Sep. 26, 2016, which designates the United States of America, and claims priority to DE Application No. 10 2015 219 398.5 filed Oct. 7, 2015, the contents of which are hereby incorporated by reference in their entirety.

TECHNICAL FIELD

The present disclosure relates to power plants. Teachings thereof may be embodied in methods for operating a combined gas-and-steam power plant and/or combined gas-and-steam power plants.

BACKGROUND

A gas-and-steam power plant is also referred to as a combined-cycle power plant and typically comprises at least one turbine device, at least one generator, which can be driven by the turbine device, for providing electrical current, and at least one gas turbine. When the generator is driven by the turbine device, the generator can convert mechanical energy into electrical energy or electrical power and provide this electrical energy or the electrical power. The electrical power can then be fed, for example, into a power grid.
The gas turbine here provides waste gas by means of which hot steam is generated. For example, a gaseous fuel such as natural gas is supplied to the gas turbine, wherein the fuel is burnt by means of the gas turbine. In some cases, oxygen or air is supplied to the gas turbine in addition to the fuel such that a fuel/air mixture is formed from the air and the fuel. This fuel/air mixture is burnt, generating waste gas from the gas turbine. For example, water is heated and consequently evaporated by means of the waste gas, generating hot steam. This means that the hot steam is generated by means of the waste gas from the gas turbine in such a way that water is evaporated by means of the hot waste gas from the gas turbine.
The steam is supplied to the turbine device such that the turbine device is driven the steam. As already described, the generator is driven via the turbine device. The combined gas-and-steam power plant is a power plant in which the principles of a gas turbine power plant and a steam power plant are combined. The gas turbine or its waste gas here serves as a heat source for a downstream steam generator by means of which the steam is generated for the turbine device or for driving the turbine device. The turbine device is thus designed as a steam turbine.
Such a combined gas-and-steam power plant needs to be shut down, depending on the power demand, such that the generator does not provide any electrical power and is, for example, not driven and such that no power is fed into the power grid by means of the gas-and-steam power plant. As a result of the shutdown, the combined gas-and-steam power plant can cool down, whereupon a particularly long period of time and a particularly high energy demand are required to switch the combined gas-and-steam power plant on again or power it up. It is therefore usually provided to keep the combined gas-and-steam power plant warm for a period of time during which the combined gas-and-steam power plant is shut down. The combined gas-and-steam power plant is thus kept warm by means of steam. This steam for keeping the plant warm is usually generated by means of a boiler, in particular a gas boiler. A fluid such as water is evaporated by means of the boiler, and a fuel is used for this purpose. The steam generated by means of the boiler is passed at least through a part of the combined gas-and-steam power plant to keep the latter warm or heat it up. The combined gas-and-steam power plant can then, after it is shut down, be started as part of a warm start because the combined gas-and-steam power plant then has a temperature which is already sufficiently high and at which it can be started.
However, the longer the combined gas-and-steam power plant is shut down, the greater the amount of steam required to keep the combined gas-and-steam power plant warm or heat it up because the plant cools down gradually. Usually a very great amount of energy is required to generate or provide steam to keep the shut down combined gas-and-steam power plant warm or heat it up.

SUMMARY

The teachings of the present disclosure describe methods and combined gas-and-steam power plants which can be heated up or kept warm in a particularly efficient fashion. For example, some embodiments may include a method for operating a combined gas-and-steam power plant (10), in which hot steam is generated by means of waste gas from a gas turbine (12), by means of which at least one generator (10) for providing electrical current is driven via at least one turbine device (22), characterized in that at least part of the heat contained in the steam is used in order to effect an endothermic chemical reaction.
In some embodiments, an exothermic chemical reaction is performed by means of products of the endothermic chemical reaction, in which heat is released by means of which at least part of the combined gas-and-steam power plant (10) is heated up or kept warm.
In some embodiments, steam is generated by means of heat released in the exothermic chemical reaction, by means of which at least part of the combined gas-and-steam power plant (10) is heated up or kept warm.
In some embodiments, at least one heat exchanger (44) is provided, via which at least part of the heat released is transferred to the part of the combined gas-and-steam power plant (10) or to a medium, in particular a fluid or a gas, or a component (40) of the combined gas-and-steam power plant (10).
In some embodiments, at least part of the heat contained in the steam is used during normal operation of the combined gas-and-steam power plant (10) in order to effect the endothermic chemical reaction, wherein during normal operation the generator (30) is driven and electrical power is provided by means of the generator (30).
In some embodiments, after normal operation, operation to keep the power plant warm is performed in which the combined gas-and-steam power plant (10) is heated up or kept warm by means of the released energy, whilst the provision of electrical power, effected by the generator (30), in particular the driving of the generator (30) effected by the turbine device (22), is halted.
In some embodiments, at least one heat exchanger (38) is provided by means of which at least part of the heat contained in the steam is transferred to educts of the endothermic chemical reaction.
As another example, some embodiments may include a combined gas-and-steam power plant (10), with at least one turbine device (22), with at least one generator (30), which can be driven by the turbine device (22), for providing electrical power, and with at least one gas turbine (12) by means of which waste gas for generating hot steam can be provided, by means of which steam the turbine device (22) and, via the latter, the generator (3) can be driven, characterized in that at least one reactor (34) is provided, to which at least part of the heat contained in the steam can be supplied, in order to effect an endothermic chemical reaction in the reactor (34) by means of the supplied heat from the steam.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages, features, and details are explained in the following description of an exemplary embodiment and with the aid of the drawing. The features and combination of features mentioned above in the description, as well as the features and combination of features which are mentioned below in the description of the FIGURE and/or are shown individually in the single FIGURE, can be used not only in the respective given combination but also in other combinations or individually without going beyond the scope of the invention.

In the single FIGURE, the drawing shows a schematic view of a combined gas-and-steam power plant which can be kept warm in a particularly energy-efficient fashion by using an endothermic chemical reaction according to teachings of the present disclosure.

DETAILED DESCRIPTION

A combined gas-and-steam power plant can be kept warm or heated up in a particularly efficient fashion, according to the teachings herein in that at least part of the heat contained in the steam generated by means of the gas turbine is used in order to effect an endothermic chemical reaction, e.g. a chemical reaction in which heat is absorbed. For the sake of simplicity and to make the text easier to read, the combined gas-and-steam power plant is referred to below as a COGAS power plant or simply as a power plant. By using the heat contained in the steam to affect the endothermic chemical reaction, it is possible to store at least part of the heat which is used to effect the endothermic chemical reaction in products of the endothermic chemical reaction, wherein this stored heat can be used, for example, at a later point in time in order to heat up or keep warm a part or a subsection of the power plant.
By using an endothermic chemical reaction, heat can thus be stored effectively and efficiently with or without the conversion of heat such that the power plant can, for example, be kept warm whilst the latter is shut down. In particular, different components can be heated up or kept warm even after the power plant has been shut down for a relatively long period of time in order thereby to affect a warm start of the power plant following the shutdown of the power plant. As a result, the power plant can be activated and started up in a particularly quick and energy-efficient way.
In some embodiments, the power plant can be kept warm or heated up by using the endothermic chemical reaction much more efficiently than if steam for keeping the power plan warm were to be generated by means of a boiler, such as a gas boiler. Such a gas boiler is a gas burner by means of which steam for keeping the power plant warm or heating it up is generated using fuel, in particular gaseous fuel.
In some embodiments, at least part of the energy contained in the steam generated anyway by means of the waste gas from the gas turbine is diverted and used to effect the endothermic reaction in order to store energy or heat in the products of the endothermic reaction, which can then be used later, when the power plant is shut down, to keep the power plant warm. A thermochemical heat store, in which for example the endothermic reaction takes place, is thus used. The heat stored in the products of the endothermic reaction can be stored in the heat store and used effectively and efficiently for later purposes.
In some embodiments, an exothermic chemical reaction is performed by means of products of the endothermic chemical reaction, in which heat is released by means of which at least part of the combined gas-and-steam power plant is heated up or kept warm. The endothermic reaction is performed, for example, in an endothermic reactor. The exothermic reaction is performed, for example, in an exothermic reactor by means of which the heat released in the exothermic reaction is used at least indirectly to keep the power plant warm whilst it is shut down.
In some embodiments, steam is generated by means of heat released in the exothermic chemical reaction, by means of which at least part of the combined gas-and-steam power plant is heated up or kept warm. In some embodiments, at least one heat exchanger is provided, via which at least part of the heat released in the exothermic reaction is transferred to the part of the combined gas-and-steam power plant or to a medium, in particular a fluid or a gas, or a component of the combined gas-and-steam power plant.
The medium is used, for example, to keep the power plant warm. The medium is, for example, water which can be evaporated by means of the heat released. The medium can contact, for example, educts and/or products of the exothermic reaction, in particular can flow onto or around them, to heat the medium and consequently, for example, to evaporate it. The component can be a heat exchanger via which the released heat is transferred, for example, to the medium, to heat the medium, in particular to evaporate it. Because of the use of the heat exchanger, spatial separation of the medium from the educts and/or products of the exothermic reaction is possible such that the medium does not contact directly the products and/or educts of the exothermic reaction.
In some embodiments, at least part of the heat contained in the steam generated by means of the waste gas from the gas turbine is used during normal operation of the combined gas-and-steam power plant in order to effect the endothermic chemical reaction, wherein during normal operation the generator is driven and electrical power is provided by means of the generator. In some embodiments, after normal operation, the combined gas-and-steam power plant is heated up or kept warm by means of the released energy, whilst the provision of electrical power, effected by the generator, in particular the driving of the generator effected by the turbine device, is halted. During operation to keep the power plant warm, the latter is thus shut down.
In some embodiments, at least one heat exchanger is provided by means of which at least part of the heat contained in the steam generated by means of the waste gas from the gas turbine is transferred to educts of the endothermic chemical reaction. The diverted steam can, for example, contact directly or flow onto or around the educts of the endothermic reaction to affect the transfer of heat from the steam to the educts of the endothermic reaction. By using a heat exchanger, a spatial separation of the generated steam from the educts of the endothermic reaction can be achieved such that the generated steam does not directly contact the educts of the endothermic reaction.
In some embodiments, there is at least one reactor to which at least part of heat contained in the steam generated by means of the gas turbine can be supplied, to affect an endothermic chemical reaction in the reactor by means of the supplied heat from the steam. Embodiments of the method described herein should be seen as embodiments of the combined gas-and-steam power plant, and vice versa.
The single FIGURE shows, in a schematic view, a combined gas-and-steam power plant 10 referred to as a COGAS power plant or a power plant. The power plant 10 comprises at least one gas turbine 12 to which fuel is supplied, for example as part of a method for operating the power plant. This supply of fuel to the gas turbine 12 is illustrated in the FIGURE by a direction arrow 14. The fuel may be a gaseous fuel such as, for example, natural gas. Air is moreover supplied to the gas turbine 12, as illustrated in the FIGURE by a direction arrow 16. The fuel is burnt by means of the gas turbine 12, which results in waste gas from the gas turbine 12. The gas turbine 12 thus provides the waste gas, as illustrated in the FIGURE by a direction arrow 18. A mixture of the fuel and air is formed, for example, in the gas turbine 12, this mixture being burnt. The waste gas from the gas turbine 12 results therefrom.
It can be seen from the direction arrow 18 that the waste gas is supplied to a steam generator 20 of the power plant 10. The steam generator 20 is also referred to as a boiler or evaporator. A fluid, e.g. water, is supplied to the steam generator 20. Heat is thus transferred from the waste gas of the gas turbine 12 to the water, as a result of which the water is heated up and evaporates. Steam is generated from the water as a result. This means that steam is generated from the water (fluid) supplied to the steam generator 20 by means of the waste gas from the gas turbine 12 and by means of the steam generator 20. Following this transfer of heat from the waste gas to the water, the waste gas is cooled such that it is discharged from the steam generator 20, for example at a first temperature T1. The first temperature T1 is, for example, at least predominantly 90° C. (degrees Celsius).
The power plant 10 comprises a turbine device 22 comprising a first turbine 24 and a second turbine 26. The turbine 24 may be, for example, a high-pressure turbine. Turbine 26 may be a medium-pressure and/or low-pressure turbine. The steam generated by means of the waste gas from the gas turbine 12 and with the aid of the steam generator 20 is supplied to the turbine device 22 such that the turbine device 22 is driven by the hot steam. Energy contained in the hot steam is converted into mechanical energy by means of the turbine device 22, wherein the mechanical energy is provided via a shaft 28. The turbine device 22 comprises, for example, turbine wheels which are not shown in detail in the FIGURE and to which the steam is supplied. The turbine wheels are driven by means of the steam as a result. The turbine wheels are, for example, connected non-rotationally to the shaft 28 such that the shaft 28 is driven by the turbine wheels when the turbine wheels are driven by means of the steam.
The power plant 10 comprises at least one generator 30 driven via the shaft 28 of the turbine device 22. The mechanical energy provided via the shaft 28 is thus supplied to the generator 30, wherein at least part of the mechanical energy supplied is converted by means of the generator 30 into electrical energy or electrical power. The generator 30 can provide this electrical power which can be fed, for example, into a power grid.
The steam is discharged from the turbine device 22 and supplied to a heat exchanger 32 which functions or is designed as a condenser. The steam is cooled by means of the heat exchanger 32, the steam condensing as a result. The steam consequently becomes the abovementioned water again, which can be resupplied to the steam generator 20.
In some embodiments, to cool the steam by means of the heat exchanger 32, for example a cooling medium, e.g., a cooling fluid, is supplied to the heat exchanger 32. Heat can thus be transferred from the steam to the cooling fluid, as a result of which the steam is cooled and then condenses.
The power plant 10 may have multiple lines (not shown in detail in the FIGURE) through which respective flows of the steam generated by means of the waste gas from the gas turbine 12 flow. These flows can have different temperatures. Different temperatures T2, T3, and T4 of the steam generated by means of the waste gas from the gas turbine 12 are shown in the FIGURE, the temperature T2 being, for example, 595° C., the temperature T3 being 360° C., and the temperature T4 being 240° C. The water leaves the condenser, for example, at a temperature T5 which is, for example, 40° C.
Depending on the amount of electrical power to be provided by the power grid, the power plant is activated and/or started up, and deactivated and/or shut down. The power plant is, for example, shut down when there is only a low power demand. If the power demand increases, the power plant is started up again after being shut down. This start-up, which takes place following a shutdown of the power plant, preferably takes the form of a warm start to switch the power plant on quickly and in an energy-efficient fashion. To perform a warm start, in particular to perform a particularly energy-efficient warm start, the power plant is kept warm or heated up after the shutdown and for a period of time in which the power plant is shut down, in order to prevent excessive cooling-down or cooling-off of the power plant. In some embodiments, to keep the power plant warm or heat it up in a particularly energy-efficient fashion, at least part of the heat contained in the steam generated by means of the gas turbine 12 is used to affect an endothermic chemical reaction. For this purpose, the power plant comprises at least one reactor 34 in which the endothermic chemical reaction can take place.
For example, the steam generated by means of the waste gas from the gas turbine 12 or one of the abovementioned flows or at least part of one of the abovementioned flows of the steam generated by means of the waste gas from the gas turbine 12 is diverted from at least one of the abovementioned lines. This diverted steam generated by means of the waste gas from the gas turbine 12 can, for example, flow through a line 36 of the power plant and is supplied to the reactor 34 by means of the line 36. At least part of the heat contained in the steam flowing in the line 36 is used to affect the endothermic chemical reaction in the reactor 34.
For this purpose, the heat of the steam flowing through the line 36 is supplied to educts of the endothermic chemical reaction. The endothermic chemical reaction is a reaction which absorbs energy or heat. This heat thus comes from the steam flowing through the line 36. In some embodiments, the steam supplied to the reactor 34 flows through a heat exchanger 38 by means of which at least part of the heat contained in the steam flowing through the heat exchanger 38 is transferred to the educts of the endothermic chemical reaction. Owing to the endothermic chemical reaction, products of the endothermic chemical reaction result from the educts. At least part of the heat which is used to affect the endothermic chemical reaction is stored in these products.
The endothermic chemical reaction is, for example, a forward reaction of a chemical equilibrium reaction. This chemical equilibrium reaction comprises, for example, a backward reaction which is an exothermic chemical reaction. As part of the backward reaction, the products of the endothermic chemical reaction (forward reaction) are educts of the backward reaction, wherein the educts of the forward reaction are products of the backward reaction.
As part of the backward reaction (exothermic chemical reaction), heat is released. This released heat can be used at least in part to keep the power plant warm, e.g. when the power plant is shut down. This means that an exothermic chemical reaction (backward reaction) is performed by means of the products of the endothermic chemical reaction, in which heat is released. At least part of the power plant is heated up or kept warm by means of the released heat. A thermochemical heat store is thus created in which heat is stored during the operation of the power plant. This stored heat is used, whilst the power plant is shut down, to heat up the power plant or keep it warm. This means that the thermochemical heat store is charged during the operation of the power plant and is discharged whilst the power plant is shut down.
In some embodiments, steam is generated by means of the heat released in the exothermic chemical reaction. For this purpose, a steam generator 40 which differs from the steam generator 20 and is provided in addition is, for example, provided, to which at least part of the heat released in the backward reaction is supplied. A fluid, in particular water, is heated and consequently evaporated by means of the heat supplied to the steam generator 40, as a result of which steam is generated by means of the steam generator 40. The water is thus supplied to the steam generator 40. The steam generated by means of the steam generator 40 with the aid of the released heat is discharged from the steam generator 40, as illustrated in the FIGURE by a direction arrow 42. The steam discharged from the steam generator 40 is, for example, supplied to at least part of the power plant to keep warm or heat up at least this part of the power plant by means of the steam which was generated with the aid of the heat released in the backward reaction.
The power plant 10 may comprise a further heat exchanger 44 via which at least part of the heat released in the exothermic chemical reaction is transferred to that part of the power plant which is to be kept warm or to a medium for keeping the power plant warm or alternatively to a component of the power plant. The medium is, for example, the water which is supplied to the steam generator 40.
In some embodiments, the heat released in the exothermic chemical reaction is supplied to the steam generator 40 via the heat exchanger 44. This is illustrated in the FIGURE by a direction arrow 46. Steam can be generated subsequently in the described fashion by means of the steam generator 40 with the aid of the heat released as part of the exothermic chemical reaction, by means of which steam at least part of the power plant can finally be kept warm. The heat exchanger 44 is, for example, arranged in the steam generator 40 such that the heat released in the backward reaction can be supplied to the water supplied to the steam generator 40, as a result of which the water supplied to the steam generator 40 is evaporated.
In some embodiments, the systems and methods keep warm or heat up components of the combined gas-and-steam power plant 10 whilst the power plant 10 is shut down. As a result, when it is desired to switch it off, the power plant 10 has a sufficiently high temperature to be able to reactivate or switch on the power plant by means of a warm start.
The teachings herein use, during the normal operation of the power plant 10, heat sources which are available anyway, such as for example the steam generated by means of the waste gas of the gas turbine 12, to obtain heat therefrom which can be stored with the aid of the endothermic chemical reaction. In some embodiments, at least part of the heat contained in the steam generated by means of the gas turbine 12 is used during normal operation of the power plant 10 to affect the endothermic chemical reaction, wherein the generator 30 is driven during normal operation such that electrical power is provided by means of the generator 30 during normal operation.
Normal operation is ended by switching off the power plant 10. In order to prevent excessive cooling of the power plant 10, after normal operation and following the shutdown of the power plant 10, operation to keep the power plant 10 warm is performed in which the power plant 10 is heated up or kept warm by means of the energy released, whilst the provision of electrical power, effected by a generator 30, is halted, in particular during the driving of the generator 30 effected by the turbine device 22.
The diverted steam which is generated by means of the waste gas from the gas turbine 12, the heat of which is used to affect the forward reaction and is stored in particular in the products of the forward reaction, has for example a mass flow of 14.4 kg/s and a temperature of 152° C. By transferring the heat from the steam to the educts of the forward reaction via the heat exchanger 38, the steam is cooled, for example, from 152° C. to 105° C. The steam can moreover have, for example, a pressure of 5 bar.
In some embodiments, the steam generated by means of the waste gas from the gas turbine 12 has a pressure of 38 bar and a mass flow of less than 21.4 kg/s. Following the transfer of heat from the steam flowing through the line 16 or the heat exchanger 38 to the educts of the forward reaction via the heat exchanger 38, the steam is cooled, for example, from 334° C. to approximately 247° C. This means that the forward reaction takes place at, for example, 152° C. or 334° C. The backward reaction takes place, for example, at 250° C. such that, for example, the steam which is generated by means of the heat released in the backward reaction has a temperature of 250° C.
Some embodiments provide steam by means of the heat released in the exothermic chemical reaction. The steam for keeping the power plant warm can thus have a temperature of 250° C. and thus be heated, for example, in the manner described from a temperature to 250° C. or from 250° C. to an even higher temperature. The steam for keeping the power plant warm has, for example, a mass flow of 1.4 kg/s to 5 kg/s and a pressure of 5 bar. As part of keeping the power plant warm, heat is transferred from the steam generated by means of the heat released in the backward reaction to the power plant or to components of the power plant such that the steam cools down, for example, from 250° C. to a comparably lower temperature, in particular 150° C.
In some embodiments, the steam by means of which at least part of the power plant is kept warm or heated up has a temperature of 250° C. The part of the power plant which is to be kept warm is supplied to the steam for keeping the power plant warm such that heat can be transferred from the steam for keeping the power plant warm to the power plant or to the part of the power plant which is to be kept warm. As a result, the steam for keeping the part of the power plant warm is cooled such that the temperature of the steam falls, for example, from 250° C. to 150° C. The steam then has a very high temperature such that heat which is contained in the steam for keeping at least the part of the power plant warm may be used for other purposes.
Owing to the use of the heat exchanger 38, a spatial separation of the educts of the forward reaction from the diverted steam can be achieved such that the diverted steam does not directly contact the educts of the forward reaction. In some embodiments, the diverted steam directly contacts and thus flows onto or around the educts of the forward reaction. There is then, for example, no need for the heat exchanger 38. This can also be replicated in backward reaction: owing to the use of the heat exchanger 44, a spatial separation of the educts and/or products of the backward reaction from the medium which us heated by means of the released steam and is used to keep the power plant warm can be achieved such that the medium does not directly contact the educts and/or products of the backward reaction. In some embodiments, the medium directly contacts and thus flows onto or around the educts and/or products of the backward reaction. There is then, for example, no need for the heat exchanger 44.
It can be seen as a whole from the FIGURE that heat which is already available can be stored efficiently and effectively by using the endothermic reaction. The stored steam can be used effectively and efficiently, in particular at a later point in time, in order to heat up or keep warm a part of the power plant.

Claims

What is claimed is:

1. A method for operating a combined gas-and-steam power plant, the method comprising:

generating steam with waste gas from a gas turbine;

driving a generator for providing electrical current via a turbine device; and

using at least part of the heat in the steam to affect an endothermic chemical reaction.

2. The method as claimed in claim 1, further comprising performing an exothermic chemical reaction fusing products of the endothermic chemical reaction;

wherein heat is released by the exothermic chemical reaction to heat at least part of the combined gas-and-steam power plant.

3. The method as claimed in claim 2, further comprising generating steam with heat released in the exothermic chemical reaction to heat at least part of the combined gas-and-steam power plant.

4. The method as claimed in claim 2, wherein a heat exchanger transfers at least part of the heat released to the at least part of the combined gas-and-steam power plant or to a medium for heat storage.

5. The method as claimed in claim 1, wherein at least part of the heat in the steam is used during normal operation of the combined gas-and-steam power plant to effect the endothermic chemical reaction, wherein normal operation includes driving the generator and providing electrical power with the generator.

6. The method as claimed in claim 5, further comprising, after normal operation, supplying heat to the combined gas-and-steam power plant by means of the released energy, whilst the provision of electrical power effected by the generator is halted.

7. The method as claimed in claim 1, further comprising transferring heat from the steam through a heat exchanger to educts of the endothermic chemical reaction.

8. A combined gas-and-steam power plant comprising:

a turbine device;

a generator driven by the turbine device to provide electrical power; and

a gas turbine exhausting waste gas for generating steam for driving the turbine device;

a reactor supplied with heat from the steam to affect an endothermic chemical reaction in the reactor.