EP3179059A1

EP3179059A1 - Feedwater afterheater

Info

Publication number: EP3179059A1
Application number: EP16397534.5A
Authority: EP
Inventors: Jarmo MANSIKKASALO; Kari Haaga
Original assignee: Valmet Technologies Oy
Current assignee: Valmet Technologies Oy
Priority date: 2015-12-09
Filing date: 2016-12-01
Publication date: 2017-06-14
Anticipated expiration: 2036-12-01
Also published as: EP3179059B1; FI20155929A; FI126904B; ES2692406T3; PT3179059T; PL3179059T3

Abstract

A power plant (900) comprising an arrangement of a steam turbine (410, 420) and a generator (415, 425); a boiler (100) having a drum (250) and feed water piping (210, 220, 310, 312, 320, 322, 330, 332) for feeding water and/or steam into the drum (250). The feed water piping comprises at least a last economizer (210) and an afterheater (310) configured to heat water and/or steam running in the feed water piping (210, 220, 310, 312, 320, 322, 330, 332) by using heating steam. The afterheater (310) is arranged, in the direction of flow of the feed water in the feed water piping (210, 220, 310, 312, 320, 322, 330, 332), upstream from the drum (250) and downstream from the last economizer (210). Moreover, a pipeline (610, 610a, 610b, 610c) is configured to convey heating steam to the afterheater (310) from at least one of (i) a location of the superheater piping (260, 270), the location being arranged, in the direction of flow of steam in the superheater piping, downstream from at least a phase of the steam turbine (410), (ii) the steam turbine (410), and (iii) a steam net configured to receive steam from another boiler.

Description

Technical field

The invention relates to power plants. The invention relates to boilers. The invention relates to production of electricity by burning some fuel. Some embodiments of the invention relate to recovery boilers. Some embodiments of the invention relate to utilization of black liquor.

Background

Power plants are used to produce electricity. Sometimes power plants are further used to produce heat. Oftentimes a power plant comprises a boiler having a furnace. By burning material in the furnace, water can be boiled to produce steam. Steam can be used to produce mechanical energy, which is converted to electricity. Furthermore, some of the heat may be used e.g. for district heating, drying, cleaning, or other purposes, whereby thermal energy (heat of steam/water including latent heat of steam) and/or mechanical energy (pressure) can be used as such.
The effective use of resources, in particular fuel, require that the energy efficiency of the power plant is high. The energy efficiency is related to at least two things. First, how much electricity can be produced per unit of fuel. Second, how low is the temperature of the flue gases coming out of the process. Commonly, the temperature of the flue gases is decreased to a sufficiently low level by applying sufficiently many heat transfer surfaces in the boiler.

Summary

The purpose of the present invention is to increase the efficiency of a power plant, wherein a boiler is used to produce steam and a steam turbine is used to produce mechanical energy by using the steam. A power plant according to an embodiment of the invention is configured to produce more steam than a conventional power plant using the same amount of fuel. In this way, also the amount of electricity per used fuel can be increased.
According to various embodiments, an afterheater is arranged in connection with the boiler. The afterheater is a heat exchanger that is configured to heat feed water by using heating steam. Preferably, the heating steam for the afterheater is superheated. The heating steam may be conveyed to the afterheater from the steam turbine. In the alternative or in addition, heating steam may be conveyed from superheater piping after a phase of a steam turbine. In the alternative or in addition, heating steam may be conveyed from a steam net configured to receive steam from another boiler. The afterheater is arranged, in the direction of feed water flow, after the last economizer and before the drum, i.e. in between the last economizer and the drum and as a part of feed water piping.
The invention is disclosed in more precise terms in the independent claim 1. Preferable embodiments are disclosed in the dependent claims. Methods for operating the power plant are disclosed in the use claims.

Brief description of the drawings

Fig. 1a and1b: show schematic layouts of a power plant according two embodiments of the invention, including the circulations of feed water and steam; and the flow of flue gas, as well as air and fuel feeds,
Fig. 2: shows a principal view of a power plant according an embodiment of the invention,
Fig. 3: shows in detail an afterheater, and
Figs. 4a, 4b, and 5 to: 8 show principal views of a power plants according embodiments of the invention.

Detailed description

In this description, pressure is given in units of bar(a), meaning absolute pressure in bars, unless a pressure difference is indicated. In the description the terms upstream and downstream, as well as after, before and between refer to directions of some flow of some medium in the power plant when the power plant is used.
In this description, the expression a thing for a purpose is to be understood to disclose (a) what has been explicitly disclosed, (b) an embodiment, wherein the thing is suitable for the purpose and (c) an embodiment, wherein the thing is configured to perform the purpose.
The embodiments of the invention are discussed in such a context, wherein the power plant comprises a recovery boiler i.e. a soda recovery boiler. A recovery boiler is the part of a pulping process where black liquor is burned to recover inorganic chemicals for reuse in the pulping process. In addition, heat is generated, and the heat is typically used also to produce electricity. Apparently, embodiments of the invention can be utilized also with other types of boilers having a furnace configured to burn or oxidize some fuel.
Because a recovery boiler is part of the Kraft process of pulping, the heat produced by the boiler usually cannot be utilized for district heating, because pulping plants are physically located so far away from inhabitants. As the invention aims mainly in increasing the amount of electricity produced from the same amount of fuel, the embodiments are particularly suitable for recovery boilers. However, embodiments may be used also in connection with other types of boilers.
In the embodiments, a steam turbine is used to produce mechanical energy, in particular rotational energy, using the energy (heat and pressure) of steam. In the art, different types of steam turbines are known, including condensing turbines, back pressure turbines, and reheat turbines.
Condensing turbines are most commonly found in electrical power plants. These turbines receive steam from a boiler and exhaust it to a condenser. The exhausted steam is at a pressure well below atmospheric, and is in a partially condensed state, typically of a steam quality near 90%. The term steam quality refers to the proportional amount (mass fraction) of steam to the total amount of saturated steam/water mixture.
Non-condensing, i.e. back pressure, turbines are most commonly used for process steam applications. The exhaust pressure is controlled by a regulating valve to suit the needs of the process steam pressure. These are commonly found at refineries, district heating units, pulp and paper plants, and desalination facilities where large amounts of low pressure process steam are needed. The pressure of the steam coming out of the steam turbine is typically at least 5 bar(a).
Reheat turbines are most commonly used in electrical power plants. In a reheat turbine, steam flow exits from a high pressure section of the turbine and is returned to the boiler where additional superheat is added. The steam then goes back into an intermediate pressure section of the turbine and continues its expansion. Using reheat in a cycle increases the work output from the turbine and also the expansion reaches conclusion before the steam condenses, thereby minimizing the erosion of the blades in last rows. In most of the cases, maximum number of reheats employed in a cycle is two as the cost of super-heating the steam negates the increase in the work output from turbine.
The power plant of the embodiments of the invention may comprise a back pressure steam turbine or a condensing turbine. However, it has been noticed that a power plant having a back pressure steam turbine to power an electricity generator benefits more from an afterheater than a condensing turbine. This is due to the reduced amount of steam for the turbine after letting out some heating steam for an afterheater. Thus, more preferably, the power plant comprises a back pressure steam turbine.
The back pressure steam turbine is configured to let out only such steam that has a pressure of at least 5 bar(a). The back pressure steam turbine may also be configured to let out medium or high pressure steam, such as heating steam to be used for heating feed water in an afterheater, and optionally also an interheater and/or a preheater. Referring to Fig. 1a, in case a reheat steam turbine 410 is used, the reheat steam turbine 410 is, by definition, a back pressure steam turbine, since steam is let out and conveyed to a subsequent superheater 270 at a reasonably high pressure. When a steam turbine 410 comprises at least two outlets for steam in two different pressures, the steam turbine is considered to comprise at least two phases. Steam may be conveyed to the afterheater 310 e.g. from such an outlet (see the pipelines 610, 610a), or after having been conveyed within superheater piping, from such a location of the superheater piping 260, 270 that is arranged, in the direction of flow of steam in the superheater piping and along the superheater piping, downstream from at least a phase of the steam turbine (see the pipelines 610, 610b).
Referring to Fig. 1a, a power plant 900 comprises a steam turbine 410, such as a back pressure steam turbine 410, for producing rotational mechanical energy, such a torque acting on a rotating shaft, using steam. The steam turbine 410 is connected to a generator 415 configured to generate electricity from the mechanical energy. As indicated in Fig. 1a, the power plant 900 may comprise also another steam turbine 420 and another generator 425. As indicated above, reheating can be applied even if only one steam turbine is used. From the last steam turbine 420, some low pressure steam is conveyed for use in e.g. drying or cleaning, and some steam, optionally after said use, may be condensed and circulated back to the feed water tank 200. Feed water is fed to a feed water piping, including economizers 220, 210, from a feed water tank 200. Feed water piping and feed water flow is indicated in the figure by the lines, and the direction of flow of water is indicated by the arrows. Also superheater piping and steam flow is indicated in the figure by the lines, and the direction of flow of steam is indicated by the arrows.
The power plant 900 further comprises a boiler 100, such as a recovery boiler. The walls of the boiler 100 limit a furnace 105 and a flue gas duct 110 for leading flue gases out of the furnace 105. As known, heat is recovered from the flue gases by heat exchangers, such as superheaters (260, 270), economizers (210, 220), and a boiling circulation 290. The boiling circulation 290 may comprise e.g. heat transfer pipes embedded in the walls of the boiler 100. In addition or alternatively, the boiling circulation 290 may comprise a water screen (not shown) arranged in the furnace. In addition or alternatively, the boiling circulation 290 may comprise a boiler bank arranged in the furnace. Typically, the boiling circulation 290 comprises downcomers for conveying water to other parts of the boiling circulation 290.
Water to be heated and boiled is commonly referred to as feed water. Feed water is fed to the boiler 100 from a feed water tank 200. Feed water runs through the feed water piping including the economizers 210, 220 (or an economizer 210) to a drum 250. The water of the drum is circulated through the boiling circulation 290, whereby the water boils, and the steam such produced is conveyed back to the drum 250. As indicated above, in use, the drum 250 contains both liquid water and steam. From the drum, steam is conveyed through a superheater 260 to a first steam turbine 410, after which the steam is reheated in the second superheater 270 and conveyed to a second steam turbine 420. As indicated in Fig. 5, it is also possibly to have only one steam turbine 410. As indicated in Fig. 5, it is also possibly to have only one superheater 260. As indicated in Fig. 5, it is also possibly to have only one economizer 210.
An economizer (210, 220, 230) is a heat exchanger configured to heat feed water by exchanging heat with flue gas. In particular, in the economizer, the feed water is in liquid form. As known, heat transfer from a hot heat transfer surface to a liquid (e.g. feed water) is much more efficient, e.g. in terms of thermal resistance, than to a gas (e.g. steam). This results in that, in use of the boiler 100, the materials of the economizer, in particular the heat transfer surfaces thereof, have a much lower temperature than if steam would be heated therein. Correspondingly, the materials of the economizers are designed to withstand only a relatively low temperature (relative to e.g. superheaters), because such materials are cheaper than e.g. the materials suitable for the superheaters. Therefore, in use, the boiler 100 is driven so that the feed water does not boil in the economizers 210, 220, 230, in particular the last economizer 210. Boiling would reduce the heat transfer from the economizer to the heat transfer medium in the economizers, and would correspondingly increase the temperature of the economizer above the design limit. This could lead to breaking of the economizer, which would cause explosion, since the pressure of the feed water running in the economizer is high.
Through the economizers 210, 220 (and optionally also 230 and 240, see Figs. 6 and 8), the feed water is fed to the drum 250. The feed water is fed from a feed water tank 200. Typically the feed water is fed using at least a feed water pump 202. The pressure in the feed water tank 200 may be e.g. from 4 bar(a) to 6 bar(a). The feed water pump 202 is configured to increase the pressure of feed water to such a level that water can be fed to the drum 250. Typically a sufficient level is more than the pressure in the drum 250. The feed water is conveyed from the tank 200 to the drum 250 via feed water piping including the economizers 210, 220, 230, 240 and the various heaters 310, 320, 330, 322, 332. Thus, the term "feed water piping" is considered to include only pipelines that are, in the direction of flow of feed water, in between the feed water tank 200 and the drum 250. Thus, in use, feed water is configured to flow from the feed water tank 200 to the drum 250 through the feed water piping. Conversely, such parts (if any) of piping though which steam and/or condensate is, in use, configured to flow from the drum 250 to the feed water tank 200 do not form part of the feed water piping.
As indicated above, in the drum 250, the temperature and pressure correspond to a saturation temperature and pressure of water, because water is boiled in the boiling circuit 290. Examples of pressures in the drum 250 include 90 bar(a), 100 bar(a), 110 bar(a), 120 bar(a), 130 bar(a), 140 bar(a), and 150 bar(a). However, the pressure may be significantly less, such as 40 bar(a), depending on design. Moreover, as indicated above, in the economizers 210, 220, the temperature must be somewhat lower than a corresponding saturation temperature to avoid boiling. Thus, the boiling circulation 290 not only boils water, but also heats water up to boiling point. It has been noticed that the amount of electricity available at the generator 415 can be increased by increasing the temperature of the feed water entering the drum 250. This may be at least partly because of the increase in the amount of steam produced. However, for safety reasons, there must be a reasonable temperature difference between the temperature of the feed water in the last economizer 210 and the temperature of the water/steam in the drum 250. The problem of boiling feed water already in the economizers is easily encountered in situations, where an interheater (320, 322) and/or a preheater (330, 332) is used (see e.g. Figs. 1 a and 2).
As for the term "last economizer", the last economizers 210 refers to a heat exchanger

of which at least a part, such as a heat transfer surface 212, 214, is arranged in the flue gas duct 110,
that is configured to heat feed water of the feed water piping by exchanging heat with flue gases,
that is arranged, in the direction of flow of feed water (the flow being inside the heat transfer surface, e.g. piping), upstream from the drum 250 and
that is arranged at such a location that no other economizer (e.g. 220, 230) is arranged in the direction of flow of feed water in between the last economizer 210 and the drum 250 in the feed water piping.

As for the term "other economizer" (220, 230) in the last point, the other economizer 220 refers to a heat exchanger

of which at least a part, such as a heat transfer surface 222, is arranged in the flue gas duct 110 and
that is configured to heat feed water of the feed water piping by exchanging heat with flue gases.

An economizer typically comprises multiple heat transfer pipes. The heat transfer pipes typically comprise at least straight parts, as shown in Figs. 1a and 1b. The straight parts are typically oriented parallel to each other. To transfer heat from the flue gases to the feed water through the heat transfer pipes, flue gases are, in use, configured to flow in between the heat transfer pipes.
Fig. 1 a shows an embodiment with two economizers 210, 220 arranged in a flue gas duct 110. Within the flue gas duct, the flue gases, in use, flow in between the heat transfer surfaces of the last economizer 210 downwards. Moreover, within the flue gas duct, the flue gases, in use, flow in between the heat transfer surfaces of the other economizer 220 downwards. Moreover, a flue gas passage 110a is arranged horizontally in between the economizers 210, 220 to convey flue gases from the bottom of the last economizer 210 to the top of the other economizer 220. Flue gases may exit the flue gas duct 110 from below, or they may continue flowing in the duct to further components, such as a cleaning device 510 and/or a flue gas cooler 520.
Referring to Fig. 1a, the last economizer 210 may be arranged in a longitudinal direction relative to the average direction of flow of flue gases between the heat transfer pipes of the last economizer 210. Quite generally, the direction of flue gas flow is vertical. Thus, the last economizer 210 may comprise heat transfer pipes, of which straight parts are oriented in a substantially vertical direction. This orientation is beneficial, in particular for the last economizer 210, because this orientation reduces the accumulation of foul onto the heat transfer surfaces of the last economizer 210. A substantially vertical direction may form an angle of at most 30 or at most 10 degrees with the vertical direction. In this embodiment, the last economizer 210 comprises heat transfer pipes having straight parts that are oriented parallel to each other. Moreover, the longitudinal direction of the straight parts forms an angle at most 30 degrees or at most 10 degrees with the average direction of flow of flue gases between said heat transfer pipes of the last economized 210.
Referring to Fig. 1b, the last economizer 210 may be arranged in a cross direction relative to the average direction of flow of flue gases between the heat transfer pipes of the last economizer. Quite generally, the direction of flue gas flow is vertical. Thus, the last economizer 210 may comprise heat transfer pipes, of which straight parts are oriented in a substantially horizontal direction. A substantially horizontal direction may form an angle of at most 30 degrees or at most 10 degrees with a horizontal direction. This orientation may be beneficial in other economizers, in particular the post economizer 240 (see Figs. 7 and 8), since such orientation may increase the heat transfer coefficient between the flue gas and the heat transfer pipes. Moreover, in the post economizer 240, the flue gases may have been cleaned e.g. with an cleaner 510, such as an electrostatic precipitator 510. In the embodiment of Fig. 1b, the last economizer 210 comprises heat transfer pipes having straight parts and curved parts in such a way that the straight parts are oriented parallel to each other. Moreover, the longitudinal direction of the straight parts forms an angle of at least 60 degrees or at least 80 degrees with the average direction of flow of flue gases between said heat transfer pipes of the last economized 210.
As indicated above, the last economizer 210 comprises a heat transfer surface 212, i.e. a last heat transfer surface 212 of the last economizer 210, in the flue gas duct 110, the last heat transfer surface 212 being, in the direction of flow of feed water, the last such heat transfer surface that is configured to heat feed water of the feed water piping by exchanging heat with flue gases.
As indicated in the figures, the last economizer 210 is typically also, in the direction of flue gas flow, the first economizer. That is, the term "last economizer", may further refer to a heat exchanger comprising a heat transfer surface 214, i.e. a first heat transfer surface 214 of the last economizer 210, in the flue gas duct 110, the first heat transfer surface 214 being, in the direction of flow of flue gases (the flow being outside of the heat transfer surface, e.g. piping) the first such heat transfer surface that is configured to heat feed water of the feed water piping by exchanging heat with flue gases. In other words, the last economizer 210 may refer to an economizer,

that is the only economizer or
of which a heat transfer surface is arranged in the flue gas duct 110, in the direction of flow of flue gases (the flow being outside of the heat transfer surface, e.g. piping), upstream from all other economizers.

As for the term "other economizers", see above. As is evident, if the power plant comprises only one economizer, the only economizer is also the first economizer in the flue gas duct, in the direction of flue gas flow. As for the last and first heat transfer surfaces 212, 214 of the last economizer 210, they may be the same surface.
The last economizer 210 comprising the last heat transfer surface 212 ensures that the afterheater works in a proper way. The last economizer 210 comprising the first heat transfer surface 214 ensures that all the economizers 210, 220, 230 recover heat from the flue gases efficiently.
Referring to Figs. 1a, 1b, 2, and 3, to increase the temperature of the feed water entering the drum 250 (for reasons, see above), a feed water afterheater 310 (i.e. an afterheater 310 for short) is arranged, in the direction of feed water flow (and along the feed water piping), after (i.e. downstream from) the last economizer 210 and before (i.e. upstream from) the drum 250.
Referring to Fig. 3, the afterheater 310 is a heat exchanger that is configured to heat feed water by utilizing heating steam. Preferably, the afterheater 310 is configured to use superheated heating steam. Preferably, the afterheater 310 is configured to use heating steam, of which temperature is at least 350 °C. Thus, in an embodiment, the temperature of the steam in at least some location within the pipeline 610, 610a, 610b, 610c, such as at the outlet 412, is, in use, at least 350 °C.
The afterheater 310 comprises heat exchanger piping 312, in which feed water is configured to run. The afterheater 310 further comprises a covering 314 such that the heating steam is configured to run in between the covering and the heat exchanger piping. In the alternative, heating steam could run through pipes 312, and feedwater could run in between the covering 314 and the pipes 312. The materials for the parts of the afterheater 310 are selected in such a way that they withstand the design temperature and pressure in use. In particular, in such an embodiment, where the heating steam is configured to run in between the covering and the heat exchanger piping (see above), the material of the covering 314 is selected in such a way that the covering 314 is configured to withstand steam having the pressure of at least 40 bar(a) and the temperature of at least 250 °C. However, in an alternative design, stronger materials may be needed. Thus, in an embodiment the covering 314 is configured to withstand steam having the pressure of at least 60 bar(a) and the temperature of at least 500 °C (i.e. materials are selected and structure is designed accordingly).
Referring to Fig. 1a, a pipeline 610 is configured to convey heating steam into the afterheater 310. The power plant 900 may comprise a pipeline 610a, 610 configured to convey heating steam from the steam turbine 410 into the afterheater 310. In addition of alternatively, the power plant 900 may comprise a pipeline 610b, 610 configured to convey heating steam from superheating piping 260, 270, from a location downstream from at least a phase of a steam turbine (410, 420) in the direction of flow of steam, into the afterheater 310. In addition of alternatively, the power plant 900 may comprise a pipeline 610c, 610 configured to convey heating steam from a steam net into the afterheater 310. The steam net is configured to receive steam from another boiler, such as from (a) another boiler of the same mill or another mill and/or (b) another steam turbine of the same mill or another mill. Also the other boiler may comprise a furnace.
Referring to Fig. 1a, the power plant 900 may comprise more than one steam turbine in case a reheat turbine is used. In such a case, the pipeline 610 may be configured to convey heating steam from any one of the steam turbines 410, 420, or from superheater piping thereafter. Preferably, as indicated in Fig. 1, the pipeline 610 is configured to convey heating steam from such a steam turbine 410 (or from a location within superheater piping after a phase of that turbine), that is in the direction of flow of steam the first steam turbine after the drum 250, to the afterheater 310. In an embodiment, the pipeline 610 is suitable for conveying superheated steam, or so configured. Another pipeline 612 is configured to convey heating steam and/or a condensate thereof out of the afterheater 310. The pipeline 612 may be configured to convey the heating steam and/or a condensate thereof to an interheater 320, a preheater 330, a steam turbine (410, 420), the superheater piping, the feed water tank 200, or to some other use, such as cleaning of heat exchange surfaces. However, the pressure in the pipeline 612 may be controlled e.g. with the valve 613 (see Fig. 3). Since the pressure upstream from the valve 613 may be higher than downstream from the valve 613, condensed heating steam may be let out from the afterheater 310 through the pipeline 612. Moreover, as the pressure is lowered at the valve 613, the condensed steam may boil to steam with higher quality. Thus, even if (high pressure) water is let out the afterheater 310, it may be used as (low pressure) steam subsequently. The terms high and low pressure herein do not refer to any absolute pressure, but only relative to each other.
Referring to Fig. 8, in an embodiment, the heating steam and/or condensate thereof is conveyed into a flash tank 350. The flash tank 350 may be used e.g. when the heating steam and/or condensate thereof is used for purposes other than heating feedwater and/or combustion air. Cleaning, such as soot blowing, is an example of such use. However, in view of production of electricity, it may be beneficial to re-use the heating steam to produce electricity, since the steam still has some thermomechanical energy (heat and pressure). Thus, an embodiment comprises a pipeline 351 configured to convey (i) the heating steam and/or condensate thereof to a superheater 270, (ii) the heating steam to another steam turbine 420, and/or (iii) the heating steam to the same steam turbine 410 from which the heating steam is taken from. For the last option (iii) to be functional, the steam turbine 410 should have an inlet for lower pressure steam. However, because the steam in the pipeline 351 may have a pressure significantly higher than the low-end pressure of the steam turbine 410, this arrangement may also be technically feasible.
As for the terms "interheater" and "preheater" a preheater 330 is arranged, in the direction of flow of the feed water in the feed water piping, upstream from all such economizers (240 in Fig. 7) that is/are arranged, in the direction of flow the flue gas, upstream a from flue gas cleaning device 510. As indicated in Fig. 4a, when the power plant is free from such economizers (240, 220 in Fig. 8; or 240 in Fig. 7) that is/are arranged, in the direction of flow the flue gas, upstream from a flue gas cleaning device 510, the preheater 330 is arranged, in the direction of flow of the feed water in the feed water piping, upstream from all the economizers (210, 220, 230). In case the power plant is free from the cleaning device 510,
the preheater 330 may be arranged, in the direction of flow of the feed water in the feed water piping, upstream from all the economizers (see Fig. 5, even if that embodiment does not comprise a preheater). Moreover, when the boiler comprises two or more post economizers (220, 240, Fig. 8) on the clean side of the flue gas duct 110 (the clean side referring to points downstream from the cleaning device 510), a steam-operated feed water heater in between such economizers may be referred to as an interheater 320 (Fig. 8).
As the heating steam already has a reasonably high pressure and temperature (see below), the feed water may even boil in the afterheater 310 without a risk of explosion. Even if the feed water would boil in the afterheater 310, the pressure difference in between feed water and the heating steam would be so low, that the piping of the afterheater 310 would tolerate the pressure difference. In addition, since heat transfer from heating steam to a heat exchanger piping is much more efficient than from flue gases to a heat exchanger piping, the temperature of the heat exchanger piping in the afterheater 310 will remain at reasonable level even if the feed water boils in the afterheater. However, as will be discussed, typically the feed water is not boiled in the afterheater 310.
As for the pressures of the boiler in use and referring to Fig. 3, in the drum 250 the water has a first pressure p1 and a first temperature T1. These correspond to saturation pressure and temperature (p1, T1) of water, such as (80 bar(a), 296 °C), (90 bar(a), 304 °C), (100 bar(a), 312 °C), (110 bar(a), 319 °C), (120 bar(a), 325 °C), (130 bar(a), 331 °C), (140 bar(a), 337 °C), or (150 bar(a), 343 °C); or other suitable saturated pressure and temperature.
In the superheaters 260, 270 the temperature increases because of the heat exchanges. However, the pressure decreases, because the steam is driven by its own pressure, whereby there typically is a 6 - 20 bar pressure drop, between the pressure after the superheater(s) and the pressure of the drum. Thus, p1-p2 may be e.g. from 6 bar to 20 bar.
As for suitable pressures for the heating steam, the pressure p3 of the heating steam at the first outlet 412 (see Fig. 3), should be reasonably high, e.g. at least 40 bar(a). In addition, from the point of view of electricity production, it is feasible to use the steam with highest pressure to produce electricity. Thus, preferably, the pressure p3 of the heating steam at the first outlet 412 (see Fig. 3) is preferably at most a maximum value, wherein the maximum value is 10 bar less than the pressure p2 of the steam entering the steam turbine 410. Thus, p3 may be in the range from 40 bar(a) to p2 - 10 bar. In addition or alternatively, p3 may be in the range from 40 bar(a) to 90 bar(a). The pressure p3 of the heating steam may be e.g. from 40 % to 90 % of the pressure p2 of the steam entering the steam turbine 410 from which the heating steam is let out. Preferably, the pressure p3 of the heating steam may be from 50 % to 80 % or from 60 % to 80 % or most preferably from 60 % to 70 % of the pressure p2 of the superheated steam. Operating the power plant 900 in such a way that these values are achieved has been found to increase the production of electricity particularly well.
As for the first outlet 412, the pressure p3 may be e.g. in the aforementioned range. Saturation temperatures for these pressures would be 252 °C and 304 °C, respectively. However, the heating steam, that is let out from the first outlet 412 is preferably superheated by at least 50 °C, such as from 150 °C to 200 °C. Preferably, at least some of the heating steam is condensed in the afterheater 310. Moreover, because of the technical function of the afterheater 310, in use thereof, the process parameters a selected such that the temperature of the heating steam in the afterheater 310 (e.g. in between the core 314 and the piping 312) exceeds the temperature of the feed water in the afterheater 310 (e.g. in the piping 312). The temperature difference between the drum 250 and the feed water in the afterheater may significantly depend on the other components of the power plant 900 (e.g. the presence of an interheater 320 and/or a preheater 330 and/or the number of economizers 210, 22, 230, 240). In an embodiment, the temperature T3 of the heating steam, before the afterheater 310, may, in use, be e.g. from 350 °C to 500 °C.
As indicated above, in an embodiment, the power plant is used such that heating steam is superheated at a location upstream from the afterheater 310 in the direction of flow of the heating steam. As evident, the heating steam is also superheated at a location downstream from at least some superheater. Thus, in a use, such heating steam is used that is superheated or has been superheated at a location in between the afterheater 310 and a superheater 260,270.
To control the temperature T0 of the feed water coming out of the afterheater 310, the power plant may comprise a first valve 611 configured to control the flow of the heating steam from the steam turbine 410 to the afterheater 310. Alternatively or in addition, to control the temperature T0 of the feed water coming out of the afterheater 310, the power plant may comprise a valve 613 configured to control the flow of the condensate thereof coming out of the afterheater 310. By controlling the flow out, the valve 613 is configured to control the condensate level in the afterheater 310. As indicated above, at least some of the heating steam is preferably condensed in the afterheater 310. Thus, the afterheater 310 comprises some condensed heating steam (i.e. water) and some heating steam. Therefore, in the afterheater 310, in use, there is a surface of the water, the surface being horizontal. The aforementioned condensate level may refer to vertical location of the surface of water in the afterheater 310. As is evident, the condensate level is thus also indicative of the amount of water (i.e. condensed heating steam) in the afterheater 310. The condensate level affects the heat transfer rate from the heating steam to the feed water and therefore affects the temperature T0. The condensate level, on the other hand, may be controlled with one (or both) of the valves 611,613.
The power plant may comprise a temperature sensor 652 configured to sense the temperature T0 of the feed water coming into drum 250. In case a sweet water condenser 390 is not used, this may correspond to the temperature of the feed water coming out of the afterheater 310. The power plant may comprise a control unit 600 configured to control at least one of (a) the opening of the first valve 611 and (b) the opening of the second valve 613, using a signal indicative of the temperature T0 of the feed water coming into the drum 250. Such a signal may be received e.g. from the sensor 652. The power plant may comprise a control unit 600 configured to control at least one of (a) the opening of the first valve 611 and (b) the opening of the second valve 613, using a signal indicative of the temperature T0 of the feed water coming into the drum 250 in such a way that the temperature T0 of the feed water coming out of the afterheater 310 at least 1 °C colder than the water in the drum 250. This ensures that the last economizer 210 will not boil the feed water. In addition, it may be preferable, from the point of view of electricity production capacity, that there is a reasonable temperature difference in between the water in drum and the feedwater in the afterheater.
Preferably, the control unit 600 is configured to control at least one of (a) the opening of the first valve 611 and (b) the opening of the valve 613 such that the temperature difference T1-T0 between the saturation temperature T1 in the drum 250 and the temperature T0 (see above) is in a target range. The target range may be e.g. from 1 °C to 60°C, such as from 5 °C to 20°C. The saturation temperature T1 may be measured with another sensor. In the alternative, the saturation temperature T1 may be determined by measuring the saturation pressure p1 in the drum 250 and calculating the corresponding saturation temperature T1. Still further, the saturation temperature and/or pressure may be designed to have, in use, a value. At least one of the valves 611, 613 may be controlled using the designed temperature value T1, designed temperature difference T1-T0, and measured temperature T0. In practice it may suffice to control only one of the valves 611, 613.
An embodiment of the power plant further comprises another temperature sensor 615 configured to sense the temperature TA of the feed water entering the afterheater 310 (e.g. coming out of the last economizer 210). The control unit 600 of the power plant 900 may be configured to control at least one of (a) the opening of the first valve 611 and (b) the opening of the second valve 613, using a first signal indicative of the temperature T0 of the feed water coming into the drum 250 and a second signal indicative of the temperature TA of the feed water entering the afterheater 310. The control unit of the power plant 900 may be configured to control the openings of the valves to achieve a proper temperature difference T1-T0 as discussed above.
Referring still to Fig. 3, the steam turbine 410 may comprise a second outlet 414 for letting out steam for other purposes, such as soot blowing. The pressure p3 of the steam for the afterheater 310 is, in general, higher than the pressure of the steam at the second outlet 414. Moreover, the temperature T3 of the steam for the afterheater 310 is, in general, higher than the temperature of the steam at the second outlet 414. The pressure of steam at the second outlet 414 may be e.g. from 30 bar(a) to 50 bar(a), such as 40 bar(a).
Referring to Figs. 1 a and 2, in an embodiment feed water piping comprises also another economizer 220. Feed water is configured to flow into the last economizer 210 via the other economizer 220. In between the economizers 210, 220 (in the direction of feed water flow), an interheater 320 is arranged. The interheater is a heat exchanger configured to heat feed water using heat of some steam. At least part of the steam for the interheater 320 may be received from the afterheater 310 (as e.g. in the figures 1 a and 2). In addition or alternatively, some steam may be conveyed to the interheater 320 from elsewhere, such as the steam turbine 410 and/or superheater piping. Correspondingly a power plant 900 may comprise a pipeline 321 suitable for the purpose (see Fig. 2) and configured to covey steam in the manner described. In the alternative, all steam for the interheater 320 may be conveyed from elsewhere than the afterheater 310, e.g. from the steam turbine 410, such as from the second outlet 414 thereof (see Fig. 3).
Referring to Figs. 1a and 2, in an embodiment, a preheater 330 is arranged before (i.e. upstream in the direction of the feed water flow from) the economizers 210, 220. The preheater 330 is arranged after (i.e. downstream in the direction of the feed water flow from) the feed water tank 200. The preheater 330 is a heat exchanger configured to heat feed water using heat of some steam. At least some of the steam for the preheater 330 may be received from the interheater 320. In the figures, for clarity, other steam sources for the preheater 330 are not indicated. As discussed in the context of the interheater 320, at least some of the steam for the preheater 330 may be received from the afterheater 310. This is the case in particular, when the power plant does not comprise an interheater 320 (see Fig. 5, even if Fig. 5 does not show a preheater 330). In the alternative, steam for the preheater 330 may be conveyed e.g. from the steam turbine 410, such as from the second outlet 414 thereof.
A power plant 900 may comprise an afterheater 310 without comprising an interheater 320 or a preheater 330, as indicated in Fig. 5.
Referring to Fig. 6, a power plant may comprise a further economizer 230. Such a power plant may comprise a further interheater 322. Such a power plant may comprise a further preheater 332. Such a power plant comprises only one afterheater 310 or more than one afterheaters 310, wherein the afterheater 310 (or all afterheaters) is/are arranged after (in the direction of feed water flow) the last economizer 210. Such a power plant may comprise only one preheater or more than one preheaters, wherein the preheater (or all preheaters) are arranged before (in the direction of feed water flow) the first economizer, or before (in the direction of feed water flow) the first economizer that is on the dirty side of a flue gas cleaning device 510; the dirty side referring to locations within the flue gas duct upstream from the cleaning device 510 in the direction of flow of flue gases. Such a power plant may comprise at least two economizers and at least one interheater, wherein the interheater (or all interheaters) are arranged in between (in the direction of feed water flow) the first economizer (such as a post economizer 240) and the last economizer 210. The power plant may comprise e.g. one interheater in between each pair of subsequent economizers. The pair of subsequent economizers refers to two such economizers that no other economizer is arranged in between them in the direction of flow of feed water.
Referring to Figs. 1a and 6, heat from flue gas may be recovered to combustion air by an air preheater 124. In a simple form, the air preheater 124 may be a heat exchanger arranged in the flue gas duct 110. In addition or alternatively, referring to Fig. 2, heat from flue gas may be recovered to a separate water circulation 520, such as a flue gas cooler. Referring to Figs. 7 and 8, in addition or alternatively, feed water can be heated in a heat exchanger 240, such as a post economizer 240, by using the heat of flue gases. As indicated in Fig. 8, preferably heat is recovered from flue gases both to feed water and by using a separate water circulation 520. The separate water circulation 520 may be configured to heat combustion air as in Fig. 8 and/or feed water entering the feed water tank as indicated in Fig. 4.
Referring to Figs. 7 and 8, an embodiment comprises a post economizer 240 configured exchange heat between flue gases and feed water. The embodiment of Fig. 8 further comprises another post economizer 220. A part of the post economizer 240 is arranged in the flue gas duct 110. Moreover, the post economizer 240 is arranged downstream from the feed water tank 200 in the direction of flow of the feed water. Still further, the term "post economizer" refers to an economizer that is arranged on a clean side of a flue gas cleaning device 510. The clean side, on the other hand, refers to locations of the flue gas duct that are, in the direction of flow of flue gases, downstream from the flue gas cleaning device 510.
Referring to Fig. 8, an embodiment further comprises a separate water circulation 520. A part of the separate water circulation 520 may be arranged in the flue gas duct 110, the part being also arranged downstream from the post economizer 240, in the direction of flow a flue gases. Moreover, heat of the steam and/or condensate from the afterheater 310 or interheater 320 or preheater 310 may be utilized to heat combustion air in an air preheater 122 (see Fig. 1 a) or feed water in a water preheater before the feed water tank 200.
As an alternative to the post economizer 240, the boiler may comprise a combustion air pre-heater 124 (see Figs. 6 and 1a) configured to heat combustion air using heat of flue gases, preferably clean flue gases. Typically, even if technically possible, it is not economically feasible to have both a post economizer 240 and such a combustion air pre-heater 124.
The flue gas cooler 520 can be formed as a separate water circulation and may be configured to heat [a] feed water, e.g. before the feed water tank 200 (see Fig. 4) or [b] combustion air (see Fig. 2), or [c] both feed water and combustion air, e.g. successively. Also other possibilities will follow. It has been observed that the flue gas cooler 520 effectively cools the flue gases, thereby further reducing the amount of heat let out from the power plant. The flue gas cooler 520 has a separate cooling water circulation, i.e. separate from the feed water circulation, and it is arranged as a last stage of economizers. As the cooling water circulation is separate, no water of the cooling water circulation is conveyed to the feed water piping (210, 220, 310, 312, 320, 322, 330, 332). The flue gas cooler may be arranged further cool flue gases by a heat exchanger 522 configured to transfer heat of the water of the cooling water circulation for utilization.
As for utilizing the heat of the water circulation of the flue gas cooler there are some options that are not mutually exclusive. First, the heat exchanger 522 may be arranged to heat combustion air, as indicated in Fig. 8. Second, the heat exchanger 522 may be arranged to heat feed water at a point upstream from the feed water tank, in the direction of flow of feed water, as indicated in Fig. 4b. Third, the heat exchanger 522 may be arranged to heat feed water at a point downstream from the feed water tank, e.g. in between the feed water tank and an economizer (240, 230), in the direction of flow of feed water (not shown). And fourth, the heat exchanger 522 may be used for purposes of drying, e.g. drying biomass to be burned. The heat exchanger 522 may be configured to heat the biomass and/or drying air directly or via another circulation of some heat transfer medium.
Referring to Figs. 1a, 2, 4a, 7, and 8, a cleaning device 510 may be used to clean the flue gases. The cleaning device 510 may comprise an electrostatic precipitator. In addition or alternatively, the cleaning device 510 may comprise a filter, such as a bag filter. In the following, the cleaning device 510 is an electrostatic precipitator. The electrostatic precipitator 510 may be arranged, in the direction of flue gas flow, downstream from the last economizer 210. Referring to Fig. 4a, the electrostatic precipitator 510 may be arranged, in the direction of flue gas flow, downstream from all such economizers, through which feed water is configured to flow. Referring to Figs. 7 and 8, the electrostatic precipitator 510 may be arranged, in the direction of flue gas flow, downstream from the last economizer 210 and upstream from another economizer 240. The flue gas cooler 520 may be arranged further downstream from the electrostatic precipitator 510, in the direction of flue gas flow.
Other components, such as a flue gas scrubber and/or an SCR unit (selective catalytic reduction unit) may be used to clean the flue gas, as known in the art. The power plant may comprise a scrubber, of which a part is arranged, in the direction of flue gas flow, in the flue gas duct downstream from the last economizer 210. The power plant may comprise an SCR unit, of which a part is arranged in the flue gas duct downstream from the last economizer 210.
Referring to Fig. 4a an embodiment of the power plant 900 comprises subsequent superheaters 260, 270, but only one steam turbine 410. The power plant comprises an afterheater 310, an interheater 320, and a preheater 330; as well as a first economizer 220 and a last economizer 210, as discussed above.
To control the temperature T2 (compare to Fig. 3) of the steam after the last superheater 270, the power plant 900 of Figs. 4a and 4b comprises an attemperator arrangement 385 (i.e. attemperator). The attemperator 385 of Fig. 4a comprises a sweet water condenser 390 configured to condense some of the steam from the drum 250 to sweet water. The attemperator 385 comprises nozzles 395 for spraying sweet water to the steam in between the superheaters 260 and 270. Such an attemperator may be referred to as a Dolezal or a Dolezal attemperator. Even if the power plant 900 does not comprise a sweet water condenser 385, the power plant 900 may comprise nozzles 395 for spraying sweet water (such as feed water) to the steam in between the superheaters 260 and 270. Moreover, the power plant may comprise nozzles 395 configured to spray such water that has been condensed from steam in a sweet water condenser 390 to the steam in between the superheaters 260 and 270, and other nozzles 395 configured to spray other water to the steam in between the superheaters 260 and 270.
The afterheater 310 is particularly suitable for power plants 900 having an attemperator arrangement 385 with a sweet water condenser 390. This is due to the fact that in case an attemperator arrangement 385 with the sweet water condenser 390 is used, the feed water temperature before the drum should be reasonable low to ensure proper functioning of the sweet water condenser 390. However, an attemperator arrangement 385 does not necessarily comprise a sweet water condenser 390. It suffices to spray water from another source into the superheater piping. Typically, in an attemperator, water is sprayed to a location before a superheater. Preferably water is sprayed in between two superheaters.
As indicated in the Figure 4a, in an embodiment, the afterheater 310 may be placed, in the direction of feed water flow, before (i.e. upstream from) the sweet water condenser 390. This has the effect that the afterheater 310 operates effectively, as the feed water is colder before the sweet water condenser 390 than after the sweet water condenser 390. This arrangement is therefore beneficial from the point of view of the amount of energy produced.
However, from the point of view of investment costs, as indicated in Fig. 4b, the sweet water condenser 390 and the afterheater 310 may be arranged also in the reverse order with respect to the direction of low of feed water. In that embodiment, the afterheater 310 is located, in the direction of feed water flow, after the sweet water condenser 390. In such a case, a smaller sweet water condenser 390 suffices, as the feed water therein is cold. This reduces the investment costs related to the sweet water condenser 390. Therefore, depending on case, either of these arrangement may be preferred.
Fig. 5 shows a simple embodiment of the invention. Therein a steam turbine 410 is configured to operate a generator 415 by using steam, which is superheated in a superheater 260. Saturated steam is fed to the superheater 260 from a drum 250. The power plant also comprises an economizer 210, which, being the only economizer, is a last economizer 210 before the drum 250 in the direction of flow of feed water. In the flue gas duct 110, the last economizer 210 is arranged, in the direction of flow of flue gases, downstream from the superheater. The power plant comprises an afterheater 310 configured to heat water and/or steam running in the feed water piping by using heating steam. The afterheater 310 is arranged, in the direction of flow of the feed water in the feed water piping and along feed water piping upstream from the drum 250. The afterheater 310 is arranged, in the direction of flow of the feed water in the feed water piping and along feed water piping downstream from the last economizer 210. The power plant further comprises a pipeline 610 configured to convey heating steam from the steam turbine 410 to the afterheater 310.
In addition to the aforementioned embodiments, the claims as filed and the following examples disclose further embodiments:

Example 1. The power plant (900) according to any of the claims 8 to 10, comprising
1. [A]
  - a further economizer (230)
    - o configured to heat water of the feed water piping (210, 220, 230, 240, 310, 312, 320, 322, 330, 332),
    - o comprising a heat transfer surface arranged in the flue gas duct (110), and
    - o arranged upstream from the other economizer (220) in the direction of flow of the feed water in the feed water piping (210, 220, 230, 240, 310, 312, 320, 322, 330, 332) and
2. [B]
  - a further interheater (322) in between the other economizer (220) and the further economizer (230) in the direction of flow of the feed water in the feed water piping (210, 220, 230, 240, 310, 312, 320, 322, 330, 332) and/or
  - a further preheater (332) upstream from the further economizer (230) in the direction of flow of the feed water in the feed water piping (210, 220, 230, 240, 310, 312, 320, 322, 330, 332).
Example 2. The power plant (900) according the Example 1 or according to any of the claims 1 to 10 further comprising an attemperator arrangement (385) comprising
- nozzles (395) for spraying water to the superheater piping (260, 270); optionally the attemperator arrangement (385) further comprises
- a sweet water condenser (390) configured to condense some steam to water,
- a pipeline (690) for conveying steam from the drum (250) to the sweet water condenser (390),
- a pipeline (692) for conveying water from the sweet water condenser (390) to the nozzles (395) for spraying water to the superheater piping (260, 270),
- a pipeline (694) for conveying feed water to the sweet water condenser, and
- a pipeline (696) for conveying feed water from the sweet water condenser (390) to the drum (250), optionally via the afterheater (310), whereby
- the sweet water condenser (390) is configured to condense steam using the feed water;
preferably,
- the afterheater (310) is arranged, in the direction of flow of the feed water in the feed water piping (210, 220, 230, 240, 310, 312, 320, 322, 330, 332), upstream from the sweet water condenser (390), whereby
- the pipeline (694) is configured to convey feed water from the afterheater (310) to the sweet water condenser (390) OR
- the afterheater (310) is arranged, in the direction of flow of the feed water in the feed water piping (210, 220, 230, 240, 310, 312, 320, 322, 330, 332), downstream from the sweet water condenser (390), and
- the pipeline (696) is configured to convey feed water from the sweet water condenser (390) to the drum via the afterheater (310).
Example 3. The power plant (900) according to Example 1 or 2 or any of the claims 1 to 12, comprising
- a flue gas cleaning device (510) in the flue gas duct (110), wherein
- the flue gas cleaning device (510) is arranged, in the direction of the flow of flue gases in the flue gas duct (110), downstream from last economizer (210);
preferably
- the flue gas cleaning device (510) is arranged, in the direction of the flow of flue gases in the flue gas duct (110), upstream from the flue gas cooler (520) of claim 11 and/or
- the flue gas cleaning device (510) is arranged, in the direction of the flow of flue gases in the flue gas duct (110), upstream from the post economizer (240) of claim 12.
Example 4. The power plant (900) according to any of the Examples 1 to 3 or any of the claims 1 to 12, comprising
1. [A]
  - a first valve (611) configured to control the flow of the heating steam from the steam turbine (410) to the afterheater (310) and/or
  - a second valve (613) configured to control the condensate level of the afterheater by controlling the flow of the condensate coming out of the afterheater (310) and
2. [B]
  - a control unit (600) configured to control the first valve (611) and/or the second valve (613).
Example 5. The power plant (900) according to any of the Examples 1 to 4 or any of the claims 1 to 12, comprising
- a pipeline configured to convey the heating steam and/or a condensate made by condensing the heating steam, from the afterheater (310), optionally via a vessel (350), to at least one of
  - the steam turbine (410),
  - another steam turbine (420), and
  - the superheater piping (260, 270) to be reheated.
Example 6. Use of the power plant (900) of any of the Examples 1 to 5, comprising
- conveying heating steam from an outlet (412) of the steam turbine (410) to the afterheater (310) in such a way that
- the pressure (p3) of the heating steam at the outlet (412) is at least 10 bar less than the pressure (p2) of superheated steam in a superheater (260, 270);
optionally,
- the pressure (p3) of the heating steam at the outlet (412) is at least 40 bar(a) and/or
- the temperature (T3) of the heating steam at the outlet (412) is from 350 °C to 600 °C.
Example 7. Use of the power plant (900) of any of the Examples 1 to 5, comprising
- guiding superheated steam to the steam turbine (410), the superheated steam having a second pressure (p2),
- letting out heating steam from an outlet (412) of the steam turbine (410), the heating steam having a third pressure (p3) at the outlet (412), wherein
- the ratio of the third pressure and the second pressure (p3/p2) is from 40 % to 90 %; preferably from 50 % to 80 %; more preferably from 60 % to 80 %;
and most more preferably from 60 % to 70 %.
Example 8. Use of the power plant (900) of any of the Examples 1 to 5, wherein
- the heating steam is superheated at a location upstream from the afterheater (310) in the direction of flow of the heating steam.

Claims

A power plant (900) comprising
- an arrangement of a steam turbine (410, 420) and a generator (415, 425) for producing electric energy using steam,

- a boiler (100) having
• a furnace (105),

• a flue gas duct (110) for leading flue gases from the furnace (105),

• a drum (250),

• superheater piping (260, 270) for superheating steam and conveying steam from the drum (250) to the steam turbine (410),

• feed water piping (210, 220, 230, 240, 310, 312, 320, 322, 330, 332) for feeding water and/or steam into the drum (250),

• at least a last economizer (210) that is in the direction of flow of the feed water in the feed water piping (210, 220, 230, 240, 310, 312, 320, 322, 330, 332) the only or the last economizer (210), the last economizer (210) forming a part of the feed water piping,

- an afterheater (310)
• configured to heat water and/or steam running in the feed water piping (210, 220, 230, 240, 310, 312, 320, 322, 330, 332) by using heating steam and

• arranged, in the direction of flow of the feed water in the feed water piping (210, 220, 230, 240, 310, 312, 320, 322, 330, 332) and along the feed water piping (210, 220, 230, 240, 310, 312, 320, 322, 330, 332), upstream from the drum (250) and downstream from the last economizer (210), and

- a pipeline (610, 610a, 610b, 610c) for conveying the heating steam to the afterheater (310), wherein

- the pipeline (610, 610a, 610b, 610c) is configured to convey the heating steam to the afterheater (310) from at least one of
• a location of the superheater piping (260, 270), the location being arranged, in the direction of flow of steam in the superheater piping, downstream from at least a phase of the steam turbine (410),

• the steam turbine (410), and

• a steam net configured to receive steam from another boiler.
The power plant (900) according to claim 1, wherein
- the last economizer (210) is configured to heat water of the feed water piping (210, 220, 230, 240, 310, 312, 320, 322, 330, 332),

- at least a part of the last economizer (210) is arranged in the flue gas duct (110), and

- the last economizer (210) is, in the direction of flow of flue gas in the flue gas duct, the only or the first economizer (210);
optionally,
- the last economizer (210) comprises heat transfer pipes comprising straight parts in such a way that the straight parts are oriented parallel to each other, wherein
• the longitudinal direction of the straight parts forms an angle at most 45 degrees, preferably at most 30 degrees, with the direction of flow of flue gases between said heat transfer pipes or

• the longitudinal direction of the straight parts forms an angle of more than 45 degrees, preferably at least 60 degrees, with the direction of flow of flue gases between said heat transfer pipes.
The power plant (900) according to claim 1 or 2, wherein
- the boiler (100) comprises nozzles (106) suitable for feeding black liquor into the furnace (105), whereby the boiler (100) is a recovery boiler.
The power plant (900) according to any of the claims 1 to 3, wherein
- the material of the pipeline (610) for conveying the heating steam is selected in such a way that the pipeline (610) for conveying the heating steam is configured to withstand steam having the pressure of at least 40 bar(a) and the temperature of at least 250 °C;
optionally,
- the pipeline (610) is configured to withstand steam having the pressure of at least 60 bar(a) and the temperature of at least 500 °C.
The power plant (900) according to any of the claims 1 to 4, wherein
- the afterheater (310) comprises
• heat exchanger piping (312), in which feed water is configured to run and

• a covering (314) such that the heating steam is configured to run in between the covering (314) and the heat exchanger piping (314), wherein

- the material of the covering (314) is selected in such a way that the covering (314) is configured to withstand steam having the pressure of at least 40 bar(a) and the temperature of at least 250 °C;
optionally,
- the covering (314) is configured to withstand steam having the pressure of at least 60 bar(a) and the temperature of at least 500 °C.
The power plant (900) according to any of the claims 1 to 5, comprising
- a back pressure steam turbine (410, 420) or a condensing turbine (410, 420) in connection with the generator (415, 425);
preferably,
- the power plant (900) comprises a back pressure steam turbine (410, 420) in connection with the generator (415, 425).
The power plant (900) according to any of the claims 1 to 6, comprising
- a preheater (320, 330, 322, 332) configured to heat water and/or steam running in the feed water piping (210, 220, 230, 240, 310, 312, 320, 322, 330, 332) by using some steam, wherein

- the preheater (320, 330, 322, 332) is arranged, in the direction of flow of the feed water in the feed water piping (210, 220, 310, 312, 320, 322, 330, 332), upstream from the last economizer (210).
The power plant (900) according to any of the claims 1 to 6, comprising
- another economizer (220, 230), wherein the other economizer (220, 230)
• is configured to heat water of the feed water piping (210, 220, 230, 240, 310, 312, 320, 322, 330, 332),

• comprises a heat transfer surface arranged in the flue gas duct (110), and

• is arranged upstream from the last economizer (210) in the direction of flow of the feed water in the feed water piping (210, 220, 230, 240, 310, 312, 320, 322, 330, 332).
The power plant (900) according to claim 8, comprising
- an interheater (320, 322) configured to heat water and/or steam running in the feed water piping (210, 220, 230, 240, 310, 312, 320, 322, 330, 332) by using some steam, wherein

- the interheater (320, 322) is arranged, in the direction of flow of the feed water in the feed water piping (210, 220, 230, 240, 310, 312, 320, 322, 330, 332), upstream from the last economizer (210) and downstream from the other economizer (220, 230).
The power plant (900) according to claim 8 or 9, comprising
- a preheater (330, 332) configured to heat water and/or steam running in the feed water piping (210, 220, 230, 240, 310, 312, 320, 322, 330, 332) by using some steam, wherein

- the preheater (330, 332) is arranged, in the direction of flow of the feed water in the feed water piping (210, 220, 230, 240, 310, 312, 320, 322, 330, 332), upstream from the other economizer (220, 230);
optionally,
- the preheater (330, 332) is arranged, in the direction of flow of the feed water in the feed water piping (210, 220, 230, 240, 310, 312, 320, 322, 330, 332), upstream from all the economizers (210, 220, 230) or

- the preheater (330, 332) is arranged, in the direction of flow of the feed water in the feed water piping (210, 220, 230, 240, 310, 312, 320, 322, 330, 332), upstream from all such economizers (240) that is/are arranged, in the direction of flow the flue gas, upstream from a flue gas cleaning device (510).
The power plant (900) according to any of the claims 1 to 10, comprising
- a flue gas cooler (520) arranged in the flue gas duct (110),

- a cooling water circulation configured to circulate cooling water through the flue gas cooler (520) in such a way that water of the cooling water circulation is not conveyed to the feed water piping (210, 220, 230, 240, 310, 312, 320, 322, 330, 332), and

- a heat exchanger (522) configured to transfer heat of the water of the cooling water circulation for utilization.
The power plant (900) according to any of the claims 1 to 11, comprising
- a post economizer (240) configured exchange heat between flue gases and feed water, wherein

- a part of the post economizer (240) is arranged in the flue gas duct (110) and

- the post economizer (240) is arranged downstream from the feed water tank (200) in the direction of flow of the feed water and upstream from the last economizer (210) in the direction of flow of the feed water;
optionally
- the post economizer (240) is arranged, in the direction of flow of flue gases, downstream from a flue gas cleaning device 510.
Use of the power plant (900) of any of the claims 1 to 12, comprising
- conveying heating steam from an outlet (412) of the steam turbine (410) to the afterheater (310) in such a way that

- the pressure (p3) of the heating steam at the outlet (412) is at least 10 bar less than the pressure (p2) of superheated steam in a superheater (260, 270);
optionally,
- the pressure (p3) of the heating steam at the outlet (412) is at least 40 bar(a) and/or

- the temperature (T3) of the heating steam at the outlet (412) is from 350 °C to 600 °C.
Use of the power plant (900) of any of the claims 1 to 12, comprising
- guiding superheated steam to the steam turbine (410), the superheated steam having a second pressure (p2),

- letting out heating steam from an outlet (412) of the steam turbine (410), the heating steam having a third pressure (p3) at the outlet (412), wherein

- the ratio of the third pressure and the second pressure (p3/p2) is from 40 % to 90 %; preferably from 50 % to 80 %; more preferably from 60 % to 80 %; and most more preferably from 60 % to 70 %.
Use of the power plant (900) of any of the claims 1 to 12, wherein
- the heating steam is superheated at a location upstream from the afterheater (310) in the direction of flow of the heating steam.