EP3415816B1

EP3415816B1 - A method and a system for extending the load range of a power plant comprising a boiler supplying steam to a steam turbine

Info

Publication number: EP3415816B1
Application number: EP18397514.3A
Authority: EP
Inventors: Juhani Isaksson; Matti Nieminen
Original assignee: Valmet Technologies Oy
Current assignee: Valmet Technologies Oy
Priority date: 2017-05-10
Filing date: 2018-04-24
Publication date: 2020-10-28
Anticipated expiration: 2038-04-24
Also published as: FI20175419A; HUE052435T2; FI128267B; EP3415816A1; ES2848053T3; DK3415816T3; PT3415816T; PL3415816T3

Description

FIELD OF THE DISCLOSED SOLUTION

The disclosed solution relates to the operation of a steam turbine power plant comprising a boiler for steam generation.

BACKGROUND OF THE DISCLOSED SOLUTION

Steam turbine power plants are commonly used for production of electricity. In combined heat and power (CHP) plants, thermal energy not captured by the steam turbine is additionally utilized as useful heat.
In steam turbine power generation, a boiler, burning suitable fuel, produces steam with a mass flow rate, which steam is conveyed to a steam turbine. Steam refers to water in a gaseous state. In the boiler, thermal energy released by the burnt fuel is transferred into the steam.
The steam entering the steam turbine is typically pressurized at 80-180 bar and has a temperature typically in the range of 450-560 °C. Typically, when a power plant is in operation, i.e. electricity is being produced, the pressure of the steam entering the turbine is kept constant while its temperature may change. The steam turbine converts thermal energy in the steam into rotary motion on the turbine output shaft. This shaft then drivers an electric generator via a suitable driveline.
A steam turbine requires steam of sufficiently high quality for viable operation. This typically means that the steam entering the steam turbine must contain a sufficiently high amount of thermal energy to be released in the steam turbine as the steam travels through the turbine. If the incoming steam contains too little thermal energy it is not acceptable for the steam turbine, since such steam may condense prematurely and thereby risk steam turbine damage or failure.
For this reason, the load range of a steam turbine power plant can not be lowered below a threshold at which the boiler burns just enough fuel so that the thermal energy transferred in the boiler into the steam enables satisfying the minimum quality criteria for the steam entering the steam turbine. Below this threshold, i.e. the minimum viable rated load of the power plant, the power plant may not be viably operated.
Therefore, steam turbine power plants have a limitation with respect to their downward load range flexibility, which limits their use in electricity production namely, with low loads i.e. low volume of electricity production.
For example, document GB 682410 A discloses a separate combustion chamber for the purpose of superheating steam in a power plant after the steam has exited a steam generator. However, the teaching is to shut down any auxiliary superheating with low loads and to rely on an internal combustion engine when only low power plant loads are required. Document US 3002347 A discloses a power plant designed for running efficiently at constant high load and with two-staged fuel combustion with a pressurized system requiring separating of combustible gases from solids and liquids between the combustion stages.

SUMMARY OF THE DISCLOSED SOLUTION

The method according to the disclosed solution is defined by what is stated in claims 1 and 2. The system according to the solution is defined by what is stated in claims 7 and 8.
The solution comprises a method of extending the load range for viable operating W_v of a steam turbine power plant comprising a boiler with a superheater for supplying superheated steam to a steam turbine, the steam turbine having a required minimum viable temperature T_v of steam entering the steam turbine, the load range for viable operating W_v of the steam turbine power plant being extended by means of controllably using an auxiliary superheating chamber comprising an auxiliary superheater to further superheat steam.
Such controllable use of the auxiliary superheating chamber may entail that based on determining the temperature T of the steam entering the steam turbine:

(1) when the temperature T of the steam entering the steam turbine is above an alert temperature T_A , which alert temperature T_A is higher than the required minimum viable temperature T_V: fuel is burned in the boiler to generate thermal energy to be transferred into steam in the superheater to generate superheated steam satisfying the required minimum viable temperature T_V of the steam entering the steam turbine; the superheated steam from the superheater is conveyed to the auxiliary superheater of the auxiliary superheating chamber; fuel is burned with a burner in the auxiliary superheating chamber to keep the superheating chamber in a hot standby temperature; and the superheated steam is conveyed from the auxiliary superheater to the steam turbine;
and
(2) when the temperature T of the steam entering the steam turbine is at or below the alert temperature T_A: fuel is burned in the boiler to generate thermal energy to be transferred into steam in the superheater to generate superheated steam; the superheated steam is conveyed from the superheater to the auxiliary superheater of the auxiliary superheating chamber; the superheated steam is further superheated by burning fuel with a burner in the auxiliary superheating chamber to generate thermal energy to be transferred into the superheated steam in the auxiliary superheater such that the required minimum viable temperature T_v of steam entering the steam turbine is satisfied; and the superheated steam is conveyed from the auxiliary superheater to the steam turbine.

Alternatively, such controllable use of the auxiliary superheating chamber may entail that based on determining the temperature T of the steam entering the steam turbine:

(1) when the temperature T of the steam entering the steam turbine is above an alert temperature T_A , which alert temperature T_A is higher than the required minimum viable temperature T_V: fuel is burned in the boiler to generate thermal energy to be transferred into steam in the superheater to generate superheated steam satisfying the required minimum viable temperature T_V of the steam entering the steam turbine; the superheated steam is conveyed from the superheater to the steam turbine by bypassing the auxiliary superheater with a steam diversion pathway; fuel is burned with a burner in the auxiliary superheating chamber to keep the superheating chamber in a hot standby temperature;
and
(2) when the temperature T of the steam entering the steam turbine is at or below the alert temperature T_A: fuel is burned in the boiler to generate thermal energy to be transferred into steam in the superheater to generate superheated steam; the superheated steam is conveyed from the superheater to the auxiliary superheater of the auxiliary superheating chamber; the superheated steam is further superheated by burning fuel with a burner in the auxiliary superheating chamber to generate thermal energy to be transferred into the superheated steam in the auxiliary superheater such that the required minimum viable temperature T_V of steam entering the steam turbine is satisfied; and the superheated steam is conveyed from the auxiliary superheater to the steam turbine.

The solution thereby enables the required minimum viable temperature T_V of steam entering the steam turbine to be satisfied both within the load range for normal operating of the power plant and the load range for low load operating of the power plant.
The solution further comprises a system which may comprise a boiler with a superheater for supplying superheated steam to a steam turbine, the steam turbine having a required minimum viable temperature T_V of steam entering the steam turbine; a controllably usable auxiliary superheating chamber comprising an auxiliary superheater and a burner, the auxiliary superheater installed in a steam forward conveyance pathway between the superheater and the steam turbine; and a control unit adapted to:

determine the temperature T of the steam entering the steam turbine;
when the temperature T of the steam entering the steam turbine is above an alert temperature T_A , which alert temperature T_A is higher than the required minimum viable temperature T_V : control the burning of fuel in the burner such that the superheating chamber is kept in a hot standby temperature; and
when the temperature T of the steam entering the steam turbine at or below the alert temperature T_A: control the burning of fuel in the burner such that thermal energy is transferred into the superheated steam in the auxiliary superheater such that the required minimum viable temperature T_V of steam entering the steam turbine is satisfied.

Alternatively, the control unit may be adapted to:

determine the temperature T of the steam entering the steam turbine;
when the temperature T of the steam entering the steam turbine is above an alert temperature T_A , which alert temperature T_A is higher than the required minimum viable temperature T_V: control a valve arrangement such that superheated steam is conveyed from the superheater to the steam turbine, the steam thereby bypassing the auxiliary superheater via a steam diversion pathway; and control the burning of fuel in the burner such that the superheating chamber is kept in a hot standby temperature; and
when the temperature T of the steam entering the steam turbine at or below the alert temperature T_A: control the valve arrangement such that superheated steam is conveyed from the superheater to the auxiliary superheater; and control the burning of fuel in the burner such that thermal energy is transferred into the superheated steam in the auxiliary superheater such that the required minimum viable temperature T_V of steam entering the steam turbine is satisfied.

The system may thereby enable the required minimum viable temperature T_V of steam entering the steam turbine to be satisfied both within the load range for normal operating of the power plant and the load range for low load operating of the power plant.
Typically, a steam turbine has specified minimum quality requirements for incoming steam. Steam quality typically refers to (1) steam temperature, (2) steam pressure and (3) proportion of entrained liquid content in the steam. High quality steam is of high enough temperature and pressure and contains little to no entrained liquid. If steam is not of high enough quality, it will condense too early along its way through the steam turbine, whereby condensate water droplets may cause problems. Such problems comprise, for example, formed water droplets hitting turbine blades, typically towards the end of the steam turbine, which creates a risk of turbine damage or failure. The disclosed solution alleviates this problem by enabling supplying, by controllably using an auxiliary superheating chamber comprising an auxiliary superheater, steam of sufficiently high quality also in situations in which the thermal energy production in the boiler is insufficient alone to maintain sufficient steam quality.
Steam of sufficiently high quality requires that a sufficiently large amount of thermal energy is produced in the boiler and then transferred to the steam. Such energy transfer is typically brought about so that water is pre-heated in the boiler to form steam, whereafter steam is further heated in a superheater or superheaters installed in the boiler. Superheater is a device preferably used to convert steam into superheated steam.
Superheated steam is steam at a temperature which is higher than the boiling point of the substance such as water at a particular pressure. Steam in a superheated state contains no entrained liquid. Thus, the temperature of superheated steam may decrease by some amount before entrained liquid begins to form. Therefore, the higher the temperature of superheated steam, the more it may cool, i.e. release energy, before entrained liquid begins to form.
Typically, steam of sufficiently high quality for a steam turbine refers to steam which is superheated to such a temperature that the superheated steam contains high enough energy so that it will not condensate prematurely in the steam turbine as it releases energy while travelling through the steam turbine.
Typically, the pressure of the steam entering the steam turbine is kept constant in a power plant system. This pressure begins to decrease once steam enters the steam turbine, wherein the steam starts to release energy and expand.
Lower boiler load, i.e. lower heat generation in the boiler, results in less thermal energy being transferred to the steam which, in turn, results in lower quality steam. Boiler load may be controlled by adjusting the amount of fuel being fed into the boiler to be burned. As is well known in the industry, the amount of thermal energy generated in the boiler and/or transferred into the steam can be inferred, for example calculated, from the volume of fuel per unit of time being fed into the boiler and the type of fuel being fed. Typically, the boiler comprises an arrangement for automatically adjusting the amount of air and/or other gases required in the fuel burning process as a function of the amount of fuel being fed. Such automatic adjustment of air and/or other gases may be effected, for example, by measuring the level of oxygen present in the combustion gases and adjusting the feed of fuel and/or the air and/or other gases such that an optimal oxygen level in the combustion gases, known as the lambda value, is obtained.
When the load of a boiler is reduced, the volume of steam produced in terms of mass flow rate typically decreases relatively linearly as a function of the load. When the steam mass flow is reduced, the electricity output of the plant, i.e. the load of the power plant, is correspondingly reduced.
At and above its minimum rated load a boiler is capable of transferring sufficient amount of thermal energy to the produced steam mass flow for the steam to be of sufficient quality for the steam turbine. Below the minimum rated load of the boiler, the amount of thermal energy transferred to the produced steam mass flow is no longer sufficient for the steam to be of sufficient quality for the steam turbine, even though the reduced mass flow rate of the steam could still be appropriate for electricity generation purposes at that time.
Thus, the minimum rated load of a boiler is the minimum load with which the boiler is capable of producing steam of sufficient quality for the steam turbine. Below this minimum rated load, the steam is not of acceptable quality for the steam turbine. In practice this usually means that the steam is not sufficiently hot, since the pressure of the steam entering the turbine is typically kept constant when electricity is being produced.
With regard to a large-scale electrical grid, the increasing generation of electricity from renewable sources such as solar and wind power poses challenges. This is because such renewable energy sources are inherently variable in terms of their power output because of, for example, weather conditions and the night-day sunlight cycle. Consequently, in an electrical grid comprising such renewable or otherwise variable power production, other modes of power production in the grid such as steam turbine-based power generation should preferably be capable of flexibly adjusting their power output in order to appropriately complement the variable power sources so that the demand for electricity in the grid is met at all times. As is well known, demand for electricity is variable as well.
An advantage of the disclosed solution is that steam turbine-based power generation may be made more capable of such flexible adjusting of power production. This is because below the minimum rated viable load of the boiler, the auxiliary superheating chamber comprising an auxiliary superheater may be employed to further superheat steam thereby ensuring supplying steam of sufficient quality to the steam turbine.
Without such further superheating, the minimum rated viable load of a boiler defines the minimum rated viable load of a power plant comprising a boiler for steam production. Below this minimum rated viable load of a power plant, the power plant typically must be shut down in order to avoid risking a steam turbine damage. Thus, the minimum rated viable load of a boiler currently in practice defines the range of downward load flexibility of a power plant, from the full rated load of a power plant to its minimum rated viable load. The disclosed solution enables extending the viable load range of a steam turbine power plant downwards below the minimum rated viable load of the boiler.
Conventionally, the minimum rated viable load of a power plant is about 30-40% below its full rated load. Some power plants comprising fluidized bed boilers have a minimum rated viable load of about 60% below the full rated load.
Conventionally, a further hindrance for the load range flexibility of steam turbine power plants is that their shutdown and start-up times may be considerably long, such as up to 12 hours. Therefore, such power plants are not capable of flexibly adjusting to changing electricity demand conditions by means of shutdown-start-up cycles. Such non-productive shutdown and start-up periods are costly as well. The disclosed solution enables reducing the need for such shutdown-start-up cycles by way of enabling a steam turbine power plant to be operated with an extended load range by controllably using an auxiliary superheating chamber comprising an auxiliary superheater to further superheat steam supplied to the steam turbine. Thus, the disclosed solution enables running a steam turbine power generation plants with a wide load range for viable operating, particularly such that the load range for viable operating of the power plant extends below the minimum rated viable load of the power plant as typically determined by the minimum rated viable load of the boiler. Such extended load range for viable operating of the power plant below its minimum rated viable load, as enabled by the disclosed solution, has the benefit of enabling the power plant to better respond to changing power demand conditions.

BRIEF DESRCIPTON OF THE FIGURES

Figure 1: schematically illustrates the steam production system of a steam turbine power plant according to an embodiment of the invention, the system comprising an auxiliary superheating chamber for further superheating steam as needed.
Figure 2: schematically illustrates the steam production system of Figure 1, wherein the fuel supplied to the auxiliary superheating chamber and optionally supplied to the boiler is gaseous and produced in a gasifier.
Figure 3: schematically illustrates the steam production system of Figure 1, wherein steam may be selectably conveyed and not conveyed through the auxiliary superheating chamber.
Figure 4a: schematically illustrates the idealized relationships of the mass flow (ṁ) of steam and the temperature (T) of steam entering a steam turbine to the load (W) of a steam turbine power plant in a conventional steam production system.
Figures 4b-c: schematically illustrate the idealized relationships of the mass flow (m) of steam and the temperature (T) of steam entering a steam turbine to the load (W) of a steam turbine power plant in a steam production system according to the disclosed solution.
Figure 5: schematically illustrates the steam production system of a steam turbine power plant according to another embodiment of the invention, the system comprising an auxiliary superheating chamber for further superheating steam as needed.
Figure 6a: schematically illustrates the idealized relationship between the load (W) and the heat consumption of a power plant.
Figure 6b: schematically illustrates the idealized relationship between the fuel consumption and the heat consumption of a power plant.

The figures are not in scale or suggestive of the physical layout or the dimensions of system components.

DETAILED DESCRIPTION OF THE INVENTION

In the text, reference is made to the figures with the following numerals and denotations:

W: Load of a power plant at a point of time
W_F: Full rated load of a power plant
W_MV: Minimum rated viable load of a power plant
W_MVA: Minimum viable load of a power plant
W_N: Load range for normal operating of a power plant
W_L: Load range for low load operating of a power plant
W_U: Load range for unviable operating of a power plant
W_V: Load range for viable operating of a power plant
T: Temperature of steam entering a turbine
T_F: Temperature of steam entering a turbine with full power plant load
T_V: Minimum viable temperature of steam entering a steam turbine
ṁ: Mass flow rate of steam
ṁ _F: Mass flow rate of steam with full boiler load
1: Auxiliary superheating chamber
2: Auxiliary superheater
3: Steam turbine
4: Gasifier
5: Cooler
6: Fuel source
7: Filter
8: Heat exchanger
9: Generator
10: Boiler
11: Superheater
13: Electricity-consuming process
14: Condenser
15: Heat exchanger
20: Pump
21: Fuel source
22: Heat-consuming process
23: Control unit
24: Burner
25: Heat exchanger
26: Valve arrangement
30 to 58: Line
70: Driveline
71: Duct
72: Burner

In the text and in the figures, the notion of a "line" is used to refer to any suitable conveyance passageway without any definite characterization of the physical properties of the passageway. It is to be appreciated that a person skilled in the art is capable of determining the physical properties of a passageway according to the properties and the volume of the material to be conveyed as well as other such pertinent conveyance parameters and requirements.
In the text, unless otherwise specified, the notion of a "power plant" is used to refer to a steam turbine power plant wherein a steam turbine 3 may be used to convert thermal energy of steam into mechanical work, which mechanical work may be converted into electricity by a generator 9.
Figure 1 schematically illustrates, according to an embodiment of the invention, a system for producing steam to be used in a steam turbine 3 according to the solution. Steam enters the steam turbine 3 with a mass flow rate ṁ, defined as the mass of steam entering the steam turbine 3 per unit of time. The steam turbine may have minimum requirements for the properties of the steam entering the steam turbine for viable operating of the steam turbine 3. Such minimum requirements may comprise, for example, a minimum viable temperature T_V for the steam entering the steam turbine 3 such that if the temperature T of the steam entering the steam turbine is below the minimum viable temperature T_V , the steam turbine may not be operated viably. There may be additional such minimum requirements as well, such as a minimum viable pressure for the steam entering the steam turbine 3 such that if the pressure of the steam entering the steam turbine 3 is below the minimum viable pressure, the steam turbine 3 may not be operated viably.
Determining steam properties may be performed by installing a flowmeter and/or a pressure sensor and/or a temperature sensor in a steam conveyance line, and relaying the signal(s) from this/these instruments to an apparatus such as a dedicated steam flow computer and/or a control unit 23 for processing and/or storage. It is to be appreciated that determining steam properties is well known in the industry and appropriate equipment for this purpose commercially available. Steam properties, once determined for example through measurement, may be used as control input data by the control unit 23.
Referring to Figure 1 , steam properties may be determined by way of measurement at least in a line 31 at its terminus at the steam turbine 3. In addition, as will be explained below, steam properties may be determined otherwise as well, for example based on measurements in other loci in the system.
Determining of the properties of steam, including its temperature, as it enters into the steam turbine 3 may be indirect. This means that steam properties are measured further upstream or downstream, for example in the steam conveyance pathway which terminates at the steam turbine 3, and that the measurement results are converted into values for the properties of the steam to be determined, for example as it enters into the steam turbine 3, by using known conversion factors. Such known conversion factors may be obtained through, for example, comparative measurements at loci of interest, of they may be derived from calculations based on the physical properties of the system. The notion of determining steam properties, as it used in this text, includes also indirect determining as just described.
In a power plant, the pressure of the steam may be kept constant, at least when the power plant produces electricity, in which case the minimum requirements for the properties of the steam entering the steam turbine 3 may in practical terms be captured by the minimum viable temperature T_V for the steam entering the steam turbine 3.
The steam turbine 3 may be adapted to drive, via a driveline 70, an electric generator 9 which may supply electricity to an electricity-consuming process via a line 44. The electricity-consuming process may be a specific and/or localized process such as in a manufacturing facility, or the electricity-consuming process may be aggregate electricity consumption in an electrical grid such as a district, a regional or a national electrical grid.
In the system, superheated steam is produced by superheating steam in a superheater 11 installed in a boiler 10. As is well known, apparatuses upstream from the superheater 11, such as a heat exchanger 25 or a plurality of such heat exchangers may be employed to vaporize the water circulating in the system such that the water is already steam when it enters the superheater 11. In addition, upstream from the superheater 11 may be additional apparatuses such as a heat exchanger or heat exchangers 15 for pre-heating the circulating water before vaporization.
The superheater 11 may be a single superheater device. Alternatively, the superheater 11 may be an aggregate of a plurality of individual superheater devices. Superheating the steam is brought about by transferring to the steam thermal energy resulting from burning fuel in in the boiler 10 with a burner (not depicted). The same applies to the heat exchanger or heat exchangers 25.
The boiler 10 may be of a known type, such as of the fluidized bed type or of the pulverized coal-fired type. Fuel may be supplied from a fuel source 21 via a line 49 to the boiler to be burned. Air or other suitable gas or gas mixture required for burning the fuel may be conveyed to the boiler 10 via a line 50 or multiple such lines. Non-gaseous combustion residues such as ash resulting from burning the fuel may be expelled from the boiler 10 via a line 51. Combustion gases resulting from burning the fuel may be expelled from the boiler 10 via a duct 71.
Before combustion gases are expelled from the boiler, thermal energy may be captured from the combustion gases with means in addition to the superheater 11. According to the embodiment depicted in Figure 1 , such additional means of thermal recovery may comprise a heat exchanger or heat exchangers 15. The varieties and using of heat exchangers are well known in the industry, and such knowledge readily applies to the heat exchanger or heat exchangers 15.
The power plant may be run with a load W, which refers to the amount of electricity generated by the generator 9 per unit of time, as driven by the steam turbine 3. As is well known in the industry, the amount of electricity generated by the generator 9 per unit of time, i.e. the power output of the generator 9, may be measured based on the voltage and current of the electricity output. Alternatively or in addition, the load W of the power plant may be inferred from the heat consumption of the power plant, since it is well known that the load W of a power plant is highly correlated with its heat consumption. The heat consumption, in turn, can be inferred from the amount of fuel burned in a unit of time for the purposes of heat generation in the power plant. The precise relationships between the amount of fuel burned in a unit of time and heat consumption, and between heat consumption and load W is typically plant-specific due to, for example, plant-specific energy losses. Figures 6 a and 6 b illustrate the idealized relationships between these measures. Such measurements are a part of normal power plant instrumentation.
The volume of fuel being burned in a power plant for the purposes of producing and heating steam and the load W of the power plant have a correlation which is characteristic for each power plant and known by the operators and/or programmed into the control apparatuses of the power plant. Thus, the load W of the power plant may be controlled by adjusting the fuel being burned for the purposes of producing and heating steam.
The power plant may be run with different loads W, as illustrated in Figures 4 a to 4 d.
Figure 4 a represents the operation of a conventional system for producing steam to be used in a steam turbine 3. Figures 4 b to 4 d represent the operation a system for producing steam to be used in a steam turbine 3 according to the disclosed solution comprising a controllably utilizable auxiliary superheating chamber comprising an auxiliary superheater.
As illustrated in Figure 4 a, a power plant may have a full rated load W_F . This refers to the load W of the power plant which may be obtained with a full rated load of the boiler 10, i.e. when a maximum amount of thermal energy is being transferred in the boiler 10 into the steam fed into the steam turbine 3. At this full rated load W_F of the power plant, the temperature T of the superheated steam entering into the steam turbine 3 is at its maximum, i.e. at the temperature T_F of the steam with full power plant load.
Decreasing the load W of the power plant from the full rated load W_F means that less thermal energy is being transferred in the boiler 10 into the stem fed into the steam turbine 3. Consequently, the temperature T of the superheated steam entering into the steam turbine 3 decreases from the temperature T_F obtainable at with the full rated power plant load W_F . Typically, the pressure of the superheated steam is kept constant, but the mass flow rate ṁ of the superheated steam entering the steam turbine 3 typically decreases as a function of the energy being transferred in the boiler 10 into the steam, as illustrated in Figure 4 a.
Still referring to Figure 4 a, when further decreasing the load W of the power plant as just described, the temperature T of the superheated steam entering into the steam turbine 3 further decreases and eventually reaches a minimum viable temperature T_V , which refers to the lowest acceptable temperature for the superheated steam entering the steam turbine 3. This load W of the power plant, at which the temperature T of the steam entering the steam turbine 3 is at the minimum viable temperature T_V is referred to as the minimum rated viable load W_MV of the power plant. Conventionally, the load range between for viable operating W_V of the power plant is, inclusively, the load range between the minimum rated viable load W_MV and the full rated load W_F .
The load range for unviable operating W_U of the power plant is the load range with loads below the range for viable operating W_V .
Thus, in the range for viable operating W_V a power plant, the temperature T of the steam entering the steam turbine 3 is at or above the minimum viable temperature T_V . In the range for unviable operating W_U of a power plant, the temperature T of the steam entering the steam turbine 3 is below the minimum viable temperature T_V.
According to the solution, as illustrated in Figure 1 , the system comprises an auxiliary superheating chamber 1 which comprises an auxiliary superheater 2. The auxiliary superheater 2 may be used to further superheat the steam originating from the superheater 11 of the boiler 10. The auxiliary superheater 2 may be a single superheater device. Alternatively, the auxiliary superheater 2 may be an aggregate of a plurality of individual superheater devices.
The auxiliary superheating chamber 1 may include a burner 24 which burns fuel thereby releasing thermal energy. Fuel may be burned in the burner 24 in such an amount that the released thermal energy is transferred to steam flowing through the auxiliary superheater 2 of the auxiliary superheating chamber 1 to the extent that the steam is further superheated in the auxiliary superheater 2.
The auxiliary superheating chamber 1 may additionally be connected to a line 54 constituting an inlet supplying incoming air or other suitable gas or gas mixture for the fuel burning process.
According to the solution and Figure 1 , the auxiliary superheating chamber 1 comprising the auxiliary superheater 2 is installed in a steam forward conveyance pathway comprising lines 30, 31. The steam forward conveyance pathway thus runs from the superheater 11 to the steam turbine 3.
A back conveyance pathway begins with a line 34 at the steam turbine 3 and terminates with a line 55 at the superheater 11. In the back conveyance pathway, water circulating in the system may at a given point be in a gaseous state, i.e. steam, and/or in a liquid state, depending on its temperature and/or its pressure at that given point.
According to the embodiment depicted in Figure 1 , steam may be continuously conveyed through the auxiliary superheater 2 en route from the superheater 11 to the steam turbine 3.
According to another embodiment depicted in Figure 3 , steam may be selectably conveyed either through the auxiliary superheater 2 en route from the superheater 11 to the steam turbine 3 or by bypassing the auxiliary superheater 2 via a line 58 constituting a steam diversion pathway. Such selectable conveyance may be effected by a valve arrangement 26. The valve arrangement 26 may be controlled by the control unit 23. The control unit 23 may effect the selected conveyance, i.e. control the valve arrangement 26, by using steam quality measurements as input data. Such input data may comprise, for example, at least the temperature T of the steam entering the steam turbine 3, for example as measured at the terminus of the line 31 at the steam turbine 3, whereby steam may be selectably conveyed via the auxiliary superheater 2 when the temperature T of the steam entering the steam turbine is near, at or below the minimum viable temperature T_V .
According to the embodiment depicted in Figure 1 , fuel may be burned continuously in the auxiliary superheating chamber 1 when the power plant is producing electricity or is in a hot standby mode. Hot standby means that while electricity is not being produced in the power plant, the boiler 10 and the superheating chamber 1 and the steam turbine 3 are kept at an elevated temperature, i.e. above ambient temperature, in order to reduce the time required to heat the said system components to a production temperature.
The volume of fuel being burned in the auxiliary superheating chamber 1 may vary. A low amount of fuel being burned in the auxiliary superheating chamber 1 may be such that the auxiliary superheating chamber 1 may be kept at a hot standby temperature. This hot standby temperature of the auxiliary superheating chamber 1 may be defined such that at such a temperature the superheated steam travelling through the auxiliary superheater 2 is not further superheated in the auxiliary superheater 2. For example, such a hot standby temperature may be equal to or less than the temperature T of the superheated steam entering the auxiliary superheater 2. The hot standby temperature is higher than ambient temperature.
It is to be appreciated that the notion of temperature and its measurement with respect to both the boiler 10 and correspondingly the auxiliary superheating chamber 1 is well known in the industry including established methodology and equipment for this purpose.
According to the embodiment depicted in Figure 1 , the boiler 10 produces steam with a mass flow ṁ and a temperature T. The auxiliary superheater 2 may be used to further superheat this steam as needed. In other words, the auxiliary superheater 2 may be used, as needed, to increase the temperature T of the steam originating from the boiler 10 before the steam enters the steam turbine 3. Thus, the auxiliary superheater may be controllably used to superheat steam.
Such controllable use refers to controlling the amount of fuel being burned in the burner 24 in the superheating chamber 1, thereby controlling the amount of thermal energy released and transferred into the steam flowing through the auxiliary superheater 2. Furthermore, in embodiments comprising a steam diversion pathway 58, as is depicted in Figure 3 , such controllable use additionally refers to the selective conveyance and non-conveyance of steam through the auxiliary superheater 2, for example as effected by a valve arrangement 26.
Referring to Figure 4 b, when the power plant operates in the load range W_N for normal operating, i.e. between its full rated load W_F and minimum rated viable load W_MV , inclusively, the thermal energy transferred into the steam in the boiler 10 is sufficient to maintain the temperature T of the steam entering the steam turbine 3 at or above the minimum viable temperature T_V . In such a case, the auxiliary superheating chamber 1 may be kept at a standby temperature as explained above.
Still referring to Figure 4 b, when the power plant operates in the load range below its minimum viable load W_MV , the thermal energy transferred into the steam in the boiler 10 may not be sufficient alone to maintain the temperature T of the steam entering the steam turbine 3 at or above the minimum viable temperature T_V . In such a case, the auxiliary superheating chamber 1 may be employed to further superheat the steam conveyed through the auxiliary superheater 2. Such further superheating may be brought about by increasing the volume of fuel being fed to the burner 24. Hence, the more the load W of the power plant is below its minimum rated viable load W_MV , i.e. the minimum viable load without further superheating of steam between the boiler 10 and the steam turbine 3, the higher the load of the auxiliary superheating chamber 1 advantageously may be set.
The load of the auxiliary superheating chamber 1 and/or the boiler 10 may controlled by the control unit 23. The control unit 23 may effect such load control by using steam quality measurements as input data and/or controlling the volume of fuel fed to the burner 24 and/or the boiler 10. The said input data may comprise, for example, at least the temperature T of the steam entering the steam turbine 3, for example as measured at the terminus of the line 31 at the steam turbine 3.
Still referring to Figure 4 b, there may be an alert temperature T_A for the temperature T of the steam entering the steam turbine 3. Advantageously, the alert temperature T_A is higher than the minimum viable temperature T_V and lower than the temperature of steam entering the steam turbine 3 with full power plant load T_F . The primary purpose of the alert temperature T_A is to serve as a control signal for the controllable use of the auxiliary superheating chamber 1 such that lowering the load of the boiler 10 does not result in the temperature T of the steam entering into the steam turbine 3 dropping below the minimum viable temperature T_V until the load W of the power plant reaches the minimum viable load W_MVA with the auxiliary superheating chamber 1 in use.
That the alert temperature T_A serving as a control signal means that when the temperature T of the steam entering the steam turbine 3 reaches the alert temperature T_A , this may be used as a trigger for control actions. Such control actions may be effected, for example, by the control unit 23.
According to what is depicted in Figure 4 b, the alert temperature T_A may be fixed to a certain temperature, for example as being a certain number of temperature degrees above the minimum viable temperature T_V .
Alternatively, according to what is depicted in Figure 4 d, the alert temperature T _A may be variable, for example as a function of the load W of the power plant, or depending on the rate with which the temperature T of the steam entering the steam turbine 3 is changing. Thus, the value or the value function of the alert temperature T_A may be specified according to the properties of the power plant system and its behavior in different situations such that the auxiliary superheating chamber 1 may be appropriately employed to further superheat steam so that lowering the load of the boiler 10 does not result in the temperature T of the steam entering into the steam turbine 3 dropping below the minimum viable temperature T _V until the load W of the power plant reaches the minimum viable load W_MVA with the auxiliary superheating chamber 1 in use.
Even if the alert temperature T_A is specified to be a fixed temperature, the controllable use of the auxiliary superheating chamber 1 does not necessarily have to be such that it strictly maintains the steam at a fixed temperature at or near the alert temperature T_A . In other words, the employment of the auxiliary superheating chamber 1 may be such that the temperature T of the steam entering the steam turbine 3 may fluctuate above the minimum viable temperature T_V, as illustrated in Figure 4 c .
Using the alert temperature T_A as a control signal as described above, and in accordance with Figures 4 b to 4 d, the range of load W of the power plant in which the auxiliary superheating chamber 1 is used to further superheat steam is referred to as the load range for low load operating W_L of the power plant. Thus, with the auxiliary superheating chamber 1, the load range for viable operating W_V of the power plant may be extended below the minimum rated viable load W_MV (W_MV being unviable without the auxiliary superheating chamber 1 used to further superheat the steam originating from the superheater 11) such that the load range for viable operating W_V comprises both the load range for normal operating W_N and the load range for low load operating W_L , as illustrated in Figures 4 b to 4 d.
Thus, decrease in the load of the boiler 1, as effected by decrease in the fuel feed into and to be burned in the boiler 10, may result in decrease in the mass flow ṁ and the temperature T in the steam originating from the boiler 10. As a result of the decrease in the mass flow ṁ of the steam, the load W of the power plant decreases. And, when the decreased load of the boiler 1 no longer is sufficient to produce steam at or above the alert temperature T_A as the steam enters the steam turbine 3, the load of the superheating chamber 1 may be increased, by increasing the fuel feed to the burner 24, to compensate the reduced load of the boiler 1, and thusly maintain the reduced mass flow rate ṁ of the steam at or above the minimum viable temperature T_V when the steam enters the steam turbine 3.
In accordance with Figures 4 b to 4 d, as the load W of the power plant still decreases, eventually the capacity of the superheating chamber 1 is no longer sufficient to transfer a sufficient amount of energy to the steam so that the minimum viable temperature T_V of the steam entering the steam turbine could be satisfied. This low load W is referred as the minimum viable load W_MVA of the power plant (with the auxiliary superheating chamber 1 in operation). The range of load W of the power plant below this minimum viable load M_MVA is referred to as the load range for unviable operating W_U of a power plant comprising an auxiliary superheating chamber 1.
Referring to Figures 4 b to 4 d, the load range for viable operating W_V of the power plant may be extended downwards below the minimum rated viable load W_MV (without the auxiliary superheater 2, as illustrated in Figure 4 a) by installing and controllably using the auxiliary superheater 2 between the boiler 10 and the steam turbine 3.
According to the embodiment depicted in Figure 3 , such controllable use of the auxiliary superheater may additionally mean selectably conveying or not conveying steam through the auxiliary superheater 2 with the valve arrangement 26 as explained above.
Thus, referring to Figures 4 b to 4 d, according to the solution, the minimum viable temperature T_V of the steam entering into the steam turbine 3 may be satisfied according to the solution both within the load range W_N for normal operating of a power plant as well as within the load range W_L for low load operating of a power plant.
According to the embodiment illustrated in Figure 1 , fuel may be supplied to the auxiliary superheating chamber 2 from a fuel source 6 via a line 42. Such fuel may be the same as supplied to the boiler 10. Alternatively, the fuel may be different from the fuel which is supplied to the boiler 10. In the case of the boiler 10 and the auxiliary superheating chamber 1 consuming the same fuel, the fuel sources 6, 21 may be, but need not be, combined into a common fuel source.
According to the embodiment illustrated in Figure 2 , the fuel supplied to and burned in the auxiliary superheating chamber 1 may be product gas generated by gasification with a gasifier 4. In such a case, the fuel supply 6 may contain initial fuel to be gasified into final fuel burned in the auxiliary superheating chamber 1. Such initial fuels to be gasified may comprise, for example, biomass and/or waste. As illustrated in Figure 2 , initial fuel may be conveyed from the fuel source 6 via a line 39 to the gasifier 4. The final fuel may be conveyed from the gasifier 4 to the auxiliary superheating chamber 1 via a line or lines 40, 41, 42.
The gasifier 4 may be of a known type, such as of the fluidizing bed type. For gasification, air or other suitable gas or gas mixture may be supplied to the gasifier 4 via a line 38 or multiple such lines. Gasification residues may be expelled from the gasifier via a line 36.
In the embodiment in which the auxiliary superheating chamber 1 uses as fuel product gas generated with the gasifier 4, the product gas conveyance pathway comprising the lines 40, 41, 42 may further comprise a cooler 5 for cooling the product gas and/or a filter 7 for filtering out undesirable substances from the product gas before the product gas is supplied to the auxiliary superheating chamber 1. The varieties and using of coolers and filters are well known in the industry, and such knowledge readily applies to the cooler 5 and the filter 7.
If the cooler 5 is so used, as illustrated in Figure 2 , heat energy may be conveyed from the cooler 5 to a heat exchanger 8 installed in the back conveyance pathway between lines 32 and 48, wherein the heat energy may be released into the water in the back conveyance pathway en route to be vaporized. Such heat conveyance from the cooler 5 to the heat exchanger 8 may be brought about by circulating an appropriate heat transfer medium in the lines 52 and 53 between the cooler 5 and the heat exchanger 8. Thus, the heat exchanger 8 may be used to pre-heat the water to be vaporized.
In the embodiment in which the auxiliary superheating chamber 1 uses as fuel product gas generated with the gasifier 4, product gas may be additionally conveyed via line 47 to the boiler 10 to be used as fuel. Such fuel use may be advantageously used, for example, as fuel being burned during hot standby, and/or as supplementary fuel, and/or as main fuel and/or as the only fuel for the boiler 10. During hot standby, the fuel conveyed via line 47 to the boiler may be burned with a burner 72 installed in the boiler 10. This burner 72 may be dedicated to burning fuel to maintain the boiler 10 in hot standby. Alternatively, the burner 72 may have multiple functionalities, such as burning fuel conveyed via lines 49 and/or 47 during normal operations when electricity is being produced in the power plant and/or burning fuel conveyed via lines 49 and/or 47 during hot standby.
Advantageously, fuel is continuously consumed in the burner 24 of the auxiliary superheating chamber 1 and/or the boiler 10 so that the gasifier 4 may be kept in continuous operation. This has the benefit of avoiding start-up (i.e. heating) and shut-down periods for the gasifier 4. Such start-up and shut-down periods may be several hours in duration, which imposes disadvantageous restrictions on the flexibility in terms of the load variability of the power plant.
The combustion gases resulting from burning fuel in the auxiliary superheating chamber 1 may be conveyed via a gas conveyance passage into the boiler 10 in which thermal energy may be recovered from the combustion gases. The combustion gases may be conveyed from the auxiliary superheating chamber 1 into the boiler 10 via a line 43 as the gas conveyance passage as illustrated in Figure 1 . If the auxiliary superheating chamber 1 is installed to the outer wall of the boiler 10 the combustion gases may be conveyed from the auxiliary superheating chamber 1 to the boiler 10 via an opening between the auxiliary superheating chamber 1 and the boiler 10. Thermal recovery of the said combustion gases in the boiler 10 may be effected, for example, by the superheater 11 and/or the heat exchanger 15 and/or any other means of recovering thermal energy that the boiler 1 comprises.
According to another embodiment, as illustrated in Figure 5 , the combustion gases resulting from burning fuel in the auxiliary superheating chamber 1 may be vented out from the auxiliary superheating chamber via a line 37 such that the said combustion gases are not conveyed into the boiler 10. In such a case, the said combustion gases may be conveyed via a line 37 to elsewhere in the system (not shown), or alternatively to another system or process (not shown), or alternatively to the atmosphere (not shown). In the case that the said combustion gases are not conveyed into the boiler 10, thermal energy may be captured from these gases with a suitable heat exchanger arrangement, for example in a manner similar to how the heat exchanger 15 may be used to capture thermal energy from the combustion gases expelled from the boiler 10.
Referring back to Figure 1 , after the steam conveyed to the steam turbine 3 has been utilized in the steam turbine 3, it (i.e. water either in a gaseous or in a liquid state depending on its temperature and pressure) may be conveyed back to the boiler 10 via a line or lines 34, 35, 33, 32, 48, 55 which constitute the back conveyance pathway terminating at the superheater 11. As illustrated in Figure 1 , the back conveyance pathway may additionally comprise a condenser 14 for recovering heat from the steam and transferring it to a heat-consuming process 22. The condenser 14 may comprise a single condenser device, or it may comprise a plurality of individual condenser devices. The varieties and using of condensers are well known in the industry, and such knowledge readily applies to the condenser 14. The condenser 14 may be connected to a heat-consuming process 22 with lines 45, 46, in which a heat transfer medium, such as water, may circulate between the condenser 14 and the heat-consuming process 22.
In addition, or alternatively, the back conveyance pathway may comprise a pump or pumps 20 for effecting the circulation of the circulating substance between the boiler 10 and the steam turbine 3.
In addition, or alternatively, the back conveyance pathway may comprise a heat exchanger 25 in the boiler 10 for pre-heating or preferably vaporizing the water before it enters the superheater 11. The heat exchanger 25 may comprise a single heat exchanger device, or it may comprise a plurality of individual heat exchanger devices. The properties and use of such heat exchangers is well known in the industry and this knowledge readily applies to the heat exchanger 25. According to an example the heat exchanger 25 may comprise a plurality of tubes (not specifically illustrated) integrated into the side walls of the boiler 10.
Additional pre-heating of water in the back conveyance pathway may be effected by a heat exchanger 15 installed in the boiler 10, such as in the duct 71 for expelling the combustion gases from the boiler 10 as illustrated in Figure 1 . The heat exchanger 15 installed in the boiler 10 may comprise a single heat exchanger device, or it may comprise a plurality of individual heat exchanger devices. If installed in the duct 71 as illustrated in Figure 1 , the heat exchanger 15 may capture thermal energy from the combustion gases of the boiler 10 and/or the auxiliary superheating chamber 2 before the combustion gases are expelled from the boiler 10 via the duct 71 of the boiler.
Advantageously, and referring to Figures 4 b to 4 d, the capacity of the auxiliary superheating chamber 1, i.e. the amount of thermal energy it can generate in a unit of time, is selected such that when the power plant is run with a load W below its minimum rated viable load W_MV , the auxiliary superheating chamber 1 is capable of further superheating the steam originating from the superheater 11 such that the minimum viable temperature T_V of the steam entering the steam turbine 3 is satisfied. That is, when the thermal energy transferred to steam in the superheater 11 is not sufficiently high for the steam fulfil the minimum viable temperature T_V of steam entering the turbine 3 due to the boiler 10 being run at a low load, the auxiliary superheating chamber 1 advantageously is capable of transferring to the steam the thermal energy required after the steam has been superheated at the superheater 11 so that the temperature T of the steam entering the steam turbine 3 is at least the minimum viable temperature T_V such that the load range W_L for the low load operating of the power plant extends substantially below its minimum rated viable load W_MV .
In this manner, and according to the disclosed solution, the auxiliary superheating chamber 1 comprising the auxiliary superheater 2 may be used to extend the load range for viable operating of a power plant W_V such that the load range for viable operating W_V of the power plant comprises both the load range for normal operating W_N and the load range for low load operating W_L , wherein the auxiliary superheating chamber 1 may be used to further superheat steam in the load range for low load operating W_L . According to the disclosed solution, the auxiliary superheating chamber 1 may be used within the load range for normal operating of the power plant W_N as well for example so as to use the auxiliary superheater 2 to further superheat the steam originating from the superheater 11 already before the minimum viable temperature T_V of the steam entering the steam turbine 3 is reached, as illustrated in Figures 4 b to 4 d.
Referring to Figures 4 b to 4 d, in practice there may a minimum viable load W_MVA of a power plant with an auxiliary superheating chamber 1 in operation such that with a load of the power plant W below the minimum viable load W_MVA the thermal energy generation capacity of the auxiliary superheating chamber 1 is not sufficient to satisfy the minimum viable temperature T_V of steam entering the turbine 3. Consequently, the load range for viable operating W_V of the power plant is, inclusively, the load range between its minimum viable load W_MVA and its full rated load W_F .
The auxiliary superheating chamber 1 may additionally be utilized as a means to maintain the system in a hot standby mode. Standby means that no electricity is being produced in the power plant, in which case the boiler 10 may be run down for example by way of no or very little fuel being burned in the boiler 10, or kept in a hot standby mode by way of burning fuel in the boiler in an amount sufficient to keep the boiler 10 at an elevated temperature. Hot standby for the system means that the circulating water, in gaseous and/or liquid state, is maintained in such a high temperature that the system can be started up, i.e. electricity production with the generator 9 be started, with a start-up time substantially shorter than would be the case of the system was shut down without maintaining high temperature in the circulation of the circulating substance.
During hot standby, the thermal energy release to the circulating water required for maintaining the system in a hot standby temperature may be brought about by the superheating chamber 1 in which case no fuel or very little fuel may be burned in the boiler 10. Alternatively, during hot standby, the thermal energy release to the circulating water required for maintaining the system in a hot standby temperature may be brought about by both the superheating chamber 1 and the boiler 10.

Claims

A method of extending the load range for viable operating W_V of a steam turbine power plant comprising a boiler (10) with a superheater (11) for supplying superheated steam to a steam turbine (3), the steam turbine having a required minimum viable temperature T_V of steam entering the steam turbine, the load range for viable operating W_V of the steam turbine power plant being extended by means of controllably using an auxiliary superheating chamber (1) comprising an auxiliary superheater (2) to further superheat steam; the method comprising:
- determining the temperature T of the steam entering the steam turbine;

- when the temperature T of the steam entering the steam turbine is above an alert temperature T_A , which alert temperature T_A is higher than the required minimum viable temperature T_V ;
• burning fuel in the boiler to generate thermal energy to be transferred into steam in the superheater (11) to generate superheated steam satisfying the required minimum viable temperature T_V of the steam entering the steam turbine;

• conveying the superheated steam from the superheater (11) to the auxiliary superheater (2) of the auxiliary superheating chamber (1);

• burning fuel with a burner (24) in the auxiliary superheating chamber (1) to keep the auxiliary superheating chamber in a hot standby temperature, which temperature is higher than ambient temperature and not higher than the temperature of the superheated steam entering the auxiliary superheater (2);

• conveying the superheated steam from the auxiliary superheater (2) to the steam turbine; and

- when the temperature T of the steam entering the steam turbine is at or below the alert temperature T_A ,
• burning fuel in the boiler to generate thermal energy to be transferred into steam in the superheater (11) to generate superheated steam;

• conveying the superheated steam from the superheater to the auxiliary superheater (2) of the auxiliary superheating chamber (1);

• further superheating the superheated steam by burning fuel with a burner (24) in the auxiliary superheating chamber (1) to generate thermal energy to be transferred into the superheated steam in the auxiliary superheater (2) such that the required minimum viable temperature T_V of steam entering the steam turbine is satisfied;

• conveying the superheated steam from the auxiliary superheater (2) to the steam turbine.
A method of extending the load range for viable operating W_V of a steam turbine power plant comprising a boiler (10) with a superheater (11) for supplying superheated steam to a steam turbine (3), the steam turbine having a required minimum viable temperature T_V of steam entering the steam turbine, the load range for viable operating W_V of the steam turbine power plant being extended by means of controllably using an auxiliary superheating chamber (1) comprising an auxiliary superheater (2) to further superheat steam; the method comprising:
- determining the temperature T of the steam entering the steam turbine;

- when the temperature T of the steam entering the steam turbine is above an alert temperature T_A , which alert temperature T_A is higher than the required minimum viable temperature T_V ;
• burning fuel in the boiler to generate thermal energy to be transferred into steam in the superheater (11) to generate superheated steam satisfying the required minimum viable temperature T_V of the steam entering the steam turbine;

• conveying the superheated steam from the superheater (11) to the steam turbine by bypassing the auxiliary superheater (2) with a steam diversion pathway (58);

• burning fuel with a burner (24) in the auxiliary superheating chamber (1) to keep the auxiliary superheating chamber in a hot standby temperature, which temperature is higher than ambient temperature and not higher than the temperature of the superheated steam entering the auxiliary superheater (2);

- when the temperature T of the steam entering the steam turbine at or below the alert temperature T_A ,
• burning fuel in the boiler to generate thermal energy to be transferred into steam in the superheater (11) to generate superheated steam;

• conveying the superheated steam from the superheater (11) to the auxiliary superheater (2) of the auxiliary superheating chamber (1);

• further superheating the superheated steam by burning fuel with a burner (24) in the auxiliary superheating chamber (1) to generate thermal energy to be transferred into the superheated steam in the auxiliary superheater (2) such that the required minimum viable temperature T_V of steam entering the steam turbine is satisfied;

• conveying the superheated steam from the auxiliary superheater (2) to the steam turbine.
The method according to claim 1 or 2, the method further comprising conveying combustion gases from the auxiliary superheating chamber to the boiler for thermal energy to be recovered from the said combustion gases.
The method according to claim 1 or 2, the method further comprising conveying combustion gases via a line (37) from the auxiliary superheating chamber elsewhere than into the boiler, for example in the atmosphere.
The method according to any of the preceding claims, the method further comprising generating product gas by gasification with a gasifier (4) and supplying the product gas to the burner (24) to be used as the fuel being burned.
The method according to claim 5, the method further comprising conveying at least some of the product gas generated in the gasifier to the boiler to be used as fuel being burned in the boiler with a burner (72) during hot standby, and/or as supplementary fuel, and/or as main fuel and/or only fuel for the boiler.
A system comprising
- a boiler (10) with a superheater (11) for supplying superheated steam to a steam turbine (3), the steam turbine having a required minimum viable temperature T_V of steam entering the steam turbine;

- a controllably usable auxiliary superheating chamber (1) comprising an auxiliary superheater (2) and a burner (24), the auxiliary superheater installed in a steam forward conveyance pathway (30, 31) between the superheater (11) and the steam turbine;

- a control unit (23) adapted to:
• determine the temperature T of the steam entering the steam turbine;

• when the temperature T of the steam entering the steam turbine is above an alert temperature T_A , which alert temperature T_A is higher than the required minimum viable temperature T_V ,
▪ control the burning of fuel in the burner such that the auxiliary superheating chamber (1) is kept in a hot standby temperature, which temperature is higher than ambient temperature and not higher than the temperature of the superheated steam entering the auxiliary superheater (2);

• when the temperature T of the steam entering the steam turbine at or below the alert temperature T_A ,
▪ control the burning of fuel in the burner (24) such that thermal energy is transferred into the superheated steam in the auxiliary superheater (2) such that the required minimum viable temperature T_V of steam entering the steam turbine is satisfied.
A system comprising:
- a boiler (10) with a superheater (11) for supplying superheated steam to a steam turbine (3), the steam turbine having a required minimum viable temperature T_V of steam entering the steam turbine;

- a controllably usable auxiliary superheating chamber (1) comprising an auxiliary superheater (2) and a burner (24), the auxiliary superheater installed in a steam forward conveyance pathway (30, 31) between the superheater (11) and the steam turbine;

- a control unit (23) adapted to:
• determine the temperature T of the steam entering the steam turbine;

• when the temperature T of the steam entering the steam turbine is above an alert temperature T_A , which alert temperature T_A is higher than the required minimum viable temperature T_V ,
▪ control a valve arrangement (26) such that superheated steam is conveyed from the superheater (11) to the steam turbine, the steam thereby bypassing the auxiliary superheater (2) via a steam diversion pathway (58);

▪ control the burning of fuel in the burner (24) such that the auxiliary superheating chamber (1) is kept in a hot standby temperature, which temperature is higher than ambient temperature and not higher than the temperature of the superheated steam entering the auxiliary superheater (2);

• when the temperature T of the steam entering the steam turbine at or below the alert temperature T_A ,
▪ control the valve arrangement (26) such that superheated steam is conveyed from the superheater (11) to the auxiliary superheater (2);

▪ control the burning of fuel in the burner such that thermal energy is transferred into the superheated steam in the auxiliary superheater (2) such that the required minimum viable temperature T_V of steam entering the steam turbine is satisfied.
The system according to claim 7 or 8, the system further comprising gas conveyance passage (43) adapted to convey combustion gases from the auxiliary superheating chamber (1) to the boiler.
The system according to claim 7 or 8, the system further comprising a line (37) adapted to convey combustion gases from the auxiliary superheating chamber (1) to elsewhere than the boiler, for example in the atmosphere.
The system according to any of the claims 7 to 10, the system further comprising:
- a gasifier (4) adapted to generate product gas; and

- lines (40, 41, 42) adapted to convey the product gas from the gasifier to the burner (24).
The system according to claim 11, the system further comprising a line (47) adapted to convey product gas from the gasifier to the boiler.