A METHOD FOR HEATING A HEAT EXCHANGE MEDIUM IN A FLUIDIZED BED BOILER, A FLUIDIZED BED BOILER, AND A LOOPSEAL HEAT EXCHANGER

A method for heating a heat exchange medium in a fluidized bed boiler (100), the method comprising burning first fuel (165) in a first furnace (162) of the fluidized bed boiler (100) to produce first flue gas (163); recovering heat from the first flue gas (163) to a heat exchange medium using a first heat exchanger (310); conveying the heat exchange medium from the first heat exchanger (310) to a second heat exchanger (320), of which at least a part is arranged in contact with a fluidized bed of the fluidized bed boiler (100); burning second fuel (175) in a second furnace (172) of the fluidized bed boiler (100) to produce second flue gas (173); conveying the heat exchange medium from the second heat exchanger (320) to a third heat exchanger (330); and recovering heat from the second flue gas (173) to the heat exchange medium using the third heat exchanger (330). A fluidized bed boiler (100) for performing the method. A loopseal heat exchanger (400) that is, when installed in a loopseal of a circulating fluidized bed boiler, configured to burn second fuel (175) in a second furnace (172) of the loopseal heat exchanger (400) to produce second flue gas (173); convey the heat exchange medium from the second heat exchanger (320) to a third heat exchanger (330); and recover heat from the second flue gas (173) to the heat exchange medium using the third heat exchanger (330).

TECHNICAL FIELD

The invention relates to fluidized bed boilers. The invention relates to fluidized bed boilers of the circulating bed type. The invention relates to loopseal heat exchangers for circulating fluidized bed boilers. The invention relates to production of steam by boiling water. The invention relates to production of sufficiently hot steam to be used in steam turbines for power generation. The invention relates to methods for reducing corrosion of heat transfer surfaces. The invention relates to burning low-quality fuel to produce steam. The invention relates to operating fluidized bed boilers at low loads.

BACKGROUND

In order to efficiently produce mechanical energy from heat, e.g. for purposes of generating electricity, superheated steam is needed. Saturated steam can be produced by boiling water, and steam can be further heated (i.e. superheated) in a superheater (i.e. a first heat exchanger) recovering heat from flue gases. However, when low-quality fuel is burned, the flue gases contain a lot of alkali and/or halogen, which corrode the heat transfer surfaces at certain temperatures, for example when the gaseous alkali halides condense on the heat transfer surfaces, which are cooler than the flue gas. Thus, this poses on upper limit for the temperature of the superheated steam. For reasons of efficiency, hotter steam is required.

In fluidized bed boilers, this limit can be exceeded by applying downstream from the first heat exchanger another superheater (i.e. a second heat exchanger), which is arranged inside a fluidized bed of solid particulate bed material. Within the fluidized bed, the content of corrosive components of the flue gas is much less, and the heat transfer from the fluidized bed to the second heat exchanger is much better. Thus, there are less compounds to condense on heat transfer surfaces and also a higher surface temperature on the heat transfer surfaces, whereby the corrosion is reduced.

When designed in such a way, under normal operating conditions, high temperature steam can be produced by heating heat transfer medium subsequently in the first and second heat exchangers.

However, when the load of the fluidized boiler decreases, less fuel is being burnt. As a result, the temperature of the bed material and/or an amount of circulated bed material may become so low that the second heat exchanger cannot sufficiently superheat the steam for purposes of the steam turbine. In such a case, the steam turbine needs to be shut down to prevent turbine failure. Thus, no electricity can be produced, even if that would be a purpose of the power plant comprising the fluidized bed boiler. Operating the boiler with excess fuel at low load would also considerably decrease efficiency, if even possible.

SUMMARY

It has been found that the steam coming from the second superheater, i.e. from a second heat exchanger, can be further heated in a third heat exchanger. The heat required for the third heat exchanger can be supplied by burning second fuel. Preferably, the second fuel is of high quality in order not to arrive at the corrosion problems detailed in the background for the first heat exchanger. The method is disclosed in more specific terms in claim1. The fluidized bed boiler is disclosed in more specific terms in claim11. The fluidized bed boiler can be a part of a power plant, as disclosed in claim15. The second fuel can be burnt in a loopseal heat exchanger. The loopseal heat exchanger is disclosed in more specific terms in claim16. The loopseal heat exchanger is suitable for use in a circulating fluidized bed boiler. The dependent claims disclose some preferable embodiments in specific terms. The description and figures disclose these and other embodiments.

In the figures, Sx, Sy, and Sz depict three mutually perpendicular directions, directed according to the right hand rule, i.e. the vector product (i.e. cross product) of Sx and Sy, in this order, equals Sz. In use, the direction Sz is upward vertical, i.e. reverse to the gravitational force.

DETAILED DESCRIPTION

FIGS.1aand1bshow embodiments of fluidized bed boilers100. The fluidized bed boiler ofFIG.1ais a circulating fluidized bed boiler100. The fluidized bed boiler ofFIG.1bis a bubbling fluidized bed boiler100.

Referring toFIGS.1aand1b, the fluidized bed boiler100comprises a first furnace162for burning first fuel165to produce first flue gas163. The first fuel may be of low-quality. Typically, the first fuel165comprises solid material, such as biomass and/or residue derived fuel. In order to recover heat from the first flue gas163, the fluidized bed boiler100comprises a first heat exchanger310for recovering heat from the first flue gas163to a heat exchange medium. The first heat exchanger310may be a superheater, i.e. a heat exchanger configured to receive and heat steam. The heat exchange medium comprises at least one of water and steam. For example, saturated steam comprises H2O both in gaseous form and in liquid form, i.e. steam and water. However, superheated steam is free from liquid water. As detailed in background, a purpose of a fluidized bed boiler is to produce superheated steam from water.

For reasons indicated in background, the steam coming from the first heat exchanger310needs to be further heated. Thus, the fluidized bed boiler100comprises a second heat exchanger320and a first pipeline312for conveying the heat exchange medium from the first heat exchanger310to the second heat exchanger320. Preferably, the first pipeline312does not comprise a heat exchanger configured to heat or cool the heat exchange medium in between the first and second heat exchanger310,320. In this way, preferably, in the direction of flow of steam within the steam circulation of the fluidized bed boiler100, the first heat exchanger310is such a last heat exchanger that no other heat exchanger that is arranged to be in contact with the first flue gas163is arranged downstream from the first heat exchanger310. Thus, in such a case, the first heat exchanger310is also such a last heat exchanger that no other heat exchanger in which in use the steam is at least as hot as in the first heat exchanger310is arranged to be in contact with only the first flue gas163(seeFIGS.1aand1b). Naturally, downstream from the steam turbine152the steam may condense and be recirculated to an economizer122, which is in contact with the first flue gas163. Moreover, the second heat exchanger320is not arranged to be in contact with only the first flue gas, as it is in contact with bed material. In addition, a third heat exchanger330is preferably not in contact with the first flue gas163at all, as detailed below.

The second heat exchanger320is arranged in such a location that in use, a fluidized bed of bed material is configured to contact the second heat exchanger320. The bed material comprises solid and heat resistant particulate material. Thus, the bed material that contacts the second heat exchanger320can be fluidized by blowing a sufficient amount of fluidizing gas therethrough. The bed material is heat resistant so that it does not burn in the first furnace162. Benefits of having the second heat exchanger in contact with a fluidized bed have been discussed in the background.

For example, the second heat exchanger320may be arranged in a first chamber (412,162) of the fluidized bed boiler100or in a wall of the first chamber (412,162), wherein in the first chamber (412,162), a fluidized bed is configured to be formed in use. The first furnace162can be considered to be a chamber in this sense.

Referring toFIG.1a, in a circulating fluidized bed boiler, the first chamber412may be a chamber of a loopseal heat exchanger400. Heat exchange surfaces, such as heat exchange pipes, of the second heat exchanger320may be arranged within the first chamber412. In addition or alternatively, heat exchange surfaces, such as heat exchange pipes, of the second heat exchanger320may be arranged in walls of the first chamber412.

Referring toFIG.1b, in a bubbling fluidized bed boiler, the first furnace162may serve as the first chamber. Heat exchange surfaces, such as heat exchange pipes, of the second heat exchanger320may be arranged within such a lower part of the first furnace162, that the bubbling fluidized bed is configured to be formed in the lower part of the first furnace162.

Referring toFIGS.1aand1b, in order to further heat the heat exchange medium, the fluidized bed boiler100comprises a second furnace172for burning second fuel175to produce second flue gas173, and a third heat exchanger330for recovering heat from the second flue gas173to the heat exchange medium received from the second heat exchanger320.FIG.2ashows in more detail the second furnace172, the second fuel175, the second flue gas173, the third heat exchanger330arranged in a second chamber422, and a passage178for conveying the second flue gas173to the second chamber422. As indicated inFIG.2a, a burner176is arranged in the second furnace172. The burner176is configured to burn the second fuel175. This applies to at least one of the fluidized bed boiler100and a loopseal heat exchanger400for a circulating fluidized bed boiler. In this way the fluidized bed boiler100or a loopseal heat exchanger400comprises a burner176arranged in the second furnace172. The burner176may be configured to burn gaseous or liquid second fuel175. As is evident a burner176may be arranged to the second furnace172of the other Figures, too, even if not explicitly shown.

Referring toFIG.1a, the second and third heat exchangers320,330may be arranged close to each other. Thus, the heat exchange medium may flow directly from the second heat exchanger320to the third heat exchanger330, e.g. through a short pipe, which may be considered to be a part of one of the heat exchangers320,330, or a part of a second pipeline322.

Referring toFIG.1b, the fluidized bed boiler100may comprise a second pipeline322for conveying the heat exchange medium from the second heat exchanger320to the third heat exchanger330. Preferably, the second pipeline322does not comprise a heat exchanger configured to heat or cool the heat exchange medium in between the second and third heat exchangers320,330.

Referring toFIGS.2aand2b, the fluidized bed boiler100and/or a loopseal heat exchanger400for a circulating fluidized bed boiler may be operated in two modes. Referring toFIG.2a, in a first mode (i.e. at a first period of time), the second furnace172is used to burn the second fuel175and in this way to further heat the heat exchange medium through second flue gas173. In the first mode, the second heat exchanger320needs not heat the heat exchange medium. Referring toFIG.2b, in a second mode (i.e. at a second period of time), the second furnace172is used not used (i.e. second fuel175is not burnt), or a much smaller amount of the second fuel175is burnt. Thus, effectively, the third heat exchanger330is not used.

More specifically, referring toFIGS.1aand1b, in the first mode and in the second mode, a method for heating a heat exchange medium in a fluidized bed boiler100comprises burning first fuel165in a first furnace162of the fluidized bed boiler100to produce first flue gas163and recovering heat from the first flue gas163to a heat exchange medium using a first heat exchanger310. From the first heat exchanger310, the heat exchange medium is conveyed to the second heat exchanger320. As detailed above, at least a part of the second heat exchanger320is arranged in contact with a fluidized bed of the fluidized bed boiler100. More preferably, all heat transfer surfaces of the second heat exchanger320are arranged in contact with a fluidized bed of the fluidized bed boiler100. The fluidized bed may be arranged in a loopseal heat exchanger400(FIG.1a) or in the first furnace162(FIG.1b). The method further comprises conveying the heat exchange medium through the second heat exchanger320and from the second heat exchanger320to the third heat exchanger330.

Referring toFIG.2a, in the first mode, i.e. at the first period of time, the method comprises burning second fuel175in a second furnace172of the fluidized bed boiler100to produce second flue gas173and recovering heat from the second flue gas173to the heat exchange medium using the third heat exchanger330. The first mode may correspond to an operating condition, wherein a load of the fluidized boiler100is low. Thus, in an embodiment, during the first period of time a load of the fluidized boiler100is less than a threshold. Moreover, because the fluidized bed, in which at least a part of the second heat exchanger320is arranged, needs not be hot at the first period of time, the heat exchange medium needs not be heated in the second heat exchanger320. However, the heat exchange medium is heated in the third heat exchanger330. Moreover, typically the fluidized bed is somewhat hotter than the heat exchange medium running through the second heat exchanger320. Thus, preferably the method comprises also during the first period of time recovering heat from a fluidized bed of the fluidized bed boiler100to the heat exchange medium using the second heat exchanger320, of which at least a part is arranged in contact with the fluidized bed of the fluidized bed boiler100.

As for arranging at least a part of the second heat exchanger320in contact with a fluidized bed of the fluidized bed boiler100, reference is made toFIGS.1aand1b. In case of a circulating fluidized bed boiler100(FIG.1a), a circulating fluidized bed is arranged in the first furnace162. Heating the heat exchange medium in a circulating fluidized bed boiler comprises circulating bed material from the first furnace162to a cyclone132, from the cyclone132to a loopseal140, and from the loopseal140to the first furnace162, e.g. via a return channel136. However, the loopseal140may be arranged in contact with a wall of the first furnace162, whereby the return channel136may be short, e.g. an aperture in a wall. In use, another fluidized bed is arranged in the loopseal140of the circulating fluidized bed boiler100to facilitate circulation of the bed material through the loopseal. In the embodiment ofFIG.1a, the second heat exchanger320is arranged in the loopseal140of the of the fluidized bed boiler100. This is also a preferable location for the second heat exchanger320for multiple reasons:less erosion than in the first furnace, because of smaller particle velocity,less corrosion than in the first furnace, because the cyclone132separates the corrosive first flue gas163to a flue gas channel120, andeasier integration with the second furnace172and the third heat exchanger330.

The bed material may be conveyed from a bottom part of the cyclone132to a loopseal heat exchanger400arranged in the loopseal140via a dipleg channel134. The term dipleg refers to a channel, in which the bed material is configured to flow mainly downwards. From the loopseal heat exchanger400, the bed material is configured to return to the first furnace162via the return channel136.

The fluidized bed boiler100(either circulating of bubbling) and/or a loopseal heat exchanger400for a circulating fluidized bed boiler100may comprise a damper, i.e. a third damper475as shown inFIG.2c. If a third damper475is used, in the first mode, i.e. during the first period, the third damper475is in an open position to enable circulation of second flue gas173.

Referring toFIG.2b, in the second mode, i.e. at the second period of time, the method comprises recovering heat from the fluidized bed that is arranged to contact at least a part of the second heat exchanger320, by the second heat exchanger320, to the heat exchange medium.

Preferably, the fluidized bed boiler100is designed in such a way that the circulation of the heat exchange medium needs not be controlled when switching from the first mode to the second mode. Therefore, in an embodiment, also in the second mode, the method comprises conveying the heat exchange medium from the second heat exchanger320to the third heat exchanger330, and conveying the heat exchange medium through the third heat exchanger330. Thus, the heat exchange medium may circulate in a similar manner in the second mode as in the first mode. While this may not be needed to heat the heat transfer medium, this reduces investment costs and improves robustness of the fluidized bed boiler100.

However, in the second mode, the heat exchange medium is conveyed through the third heat exchanger330without burning the second fuel175in the second furnace172during the second period of time. This is may be done to diminish use of the second fuel175. In the alternative, a lesser amount (in terms of mass per time, on the average) of the second fuel175may be burnt during the second period than during the first period. For example a consumption (in terms of mass per time on the average) of the second fuel175during the second period of time may be less than half, less than a quarter, or less than a tenth, of the a consumption (in terms of mass per time) of the second fuel175during the first period of time.

Because the heat exchange medium circulates through the third heat exchanger330also in the second mode, i.e. during the second period, the fluidized bed boiler100and/or a loopseal heat exchanger400for a fluidized bed boiler100, may comprise the third damper475. A purpose of the third damper is to prevent circulation of air through the third heat exchanger330in the second mode. Thus, at the second period, the third damper475may be in a closed position, as indicated inFIG.2c. In this way heat loss to circulating air is minimized at the second period. However, depending on design details, natural convection of air through the third heat exchanger330may be so small that a third damper475is not needed. The third damper475may be e.g. slidable. The third damper475may be e.g. pivotable about an axis476(seeFIG.2c). Further or other dampers may be used for the purpose as detailed below. InFIG.2c, the third damper475is arranged downstream from the third heat exchanger330(downstream in the direction of flow of the second flue gas173during the first period). Even if not shown, the third damper475may be arranged upstream from the burner176. Even if not shown, the third damper475may be arranged downstream from the burner176and upstream from the third heat exchanger330. It is noted that during the second period, the steam may slightly cool down while propagating through the third heat exchanger330even if a damper is used.

The second mode may correspond to an operating condition, wherein a load of the fluidized boiler100is high. Thus, in an embodiment, during the second period of time, a load of the fluidized boiler100more than during the first period of time. For example, during the second period of time, a load of the fluidized boiler100may be at least equal to the threshold. Reference is made to the threshold of load discussed in connection with the first mode. A precise value for the threshold depends on details of the case, but in a typical circulating fluidized bed boiler the threshold may be e.g. from 30% to 70% of the maximal load of the fluidized bed boiler, such as 50% of the maximal load of the fluidized bed boiler.

In a preferable embodiment, the third heat exchanger330is not in contact with the first flue gas163. This has the effect that even if low-quality fuel is used as the first fuel165, which produces the first flue gas163, the corrosive compounds of the first flue gas163do not corrode the third heat exchanger330.

In a preferable embodiment, the third heat exchanger330is not in contact with a fluidized bed of the fluidized bed boiler100. In particular, in an embodiment, the third heat exchanger330is not in contact with the same fluidized bed that is in contact with the second heat exchanger320. Preferably, the third heat exchanger330is not in contact with any fluidized bed that is constituted by fluidizing solid, heat resistant, and particulate material. This has the effect that heat of the second fuel175can be directly utilized at the third heat exchanger330. Thus, heat of the second flue gas173is not consumed to heat bed material of a fluidized bed. Moreover, control of the process becomes more rapid, since bed material needs not be heated when changing from the second mode to the first mode; and bed material needs not be cooled when changing from the first mode to the second mode. Furthermore when the third heat exchanger330is not in contact with a fluidized bed of the fluidized bed boiler100, problems related to agglomeration and/or sintering of bed material are avoided at least near the third heat exchanger330. Moreover, when the third heat exchanger330is not in contact with a fluidized bed of the fluidized bed boiler100, problems related to erosion of the heat exchanger surfaces by the bed material are avoided at the surfaces of the third heat exchanger330.

The method and fluidized bed boiler allows for using the fluidized bed boiler100in low loads (by operating in the first mode), and also when low-quality fuel is used as the first fuel165. Herein the term quality of fuel refers at least to a total content of alkalis and halogens of the fuel (applies to first165and second175fuel). The term alkali refers to elements of group 1 of the IUPAC periodic table of elements excluding hydrogen, and the term halogen refers to elements of group 17 of the IUPAC periodic table of elements. The alkalis and halogens are typically comprised by compounds of the fuel. When burnt, at least some of the alkalis and the halogens end ups in the flue gas (applies to first163and second173flue gases; however, the second fuel may be substantially free from alkalis and halogens). In the flue gas, these elements typically form alkali halides, i.e. a compound comprising an alkali element and a halogen element, examples including NaCl, NaF, NaBr, KCl, KF, and KBr. Some of the alkalis and the halogens may remain in the ash. The corrosion problems related to low-quality fuels are avoided by using, as the second flue175, a high quality fuel.

Therefore, in an embodiment, the second flue gas173comprises less alkali and halogen than the first flue gas163. The alkali and/or the halogen may be comprised by alkali halides of the flue gas (163,173). These compounds result from the quality of the fuel. Therefore, in the same or in another embodiment, the second fuel175comprises less alkali and halogen than the first fuel165. More specifically, a content of the compounds comprising at least one of alkali and halogen in the second fuel175is less than a content of the compounds comprising at least one of alkali and halogen in the first fuel173.

In a preferable embodiment, the second fuel175comprises less than 500 ppm or less than 100 ppm on weight basis alkali atoms and halogen atoms. Naturally the atoms are not free, but parts of chemical composition(s) of the second fuel175. Moreover, in an embodiment, the second flue gas173comprises less than 500 ppm, preferably less than 100 ppm, on weight basis alkali halides.

Typically, the high-quality fuels are gases or liquids, such as natural gas or a light oil. Therefore, in an embodiment, the second fuel175is a liquid or gas at a temperature of 20° C. and a pressure of 1 atm, such as a gas comprising natural gas or a liquid comprising oil, e.g. light fuel oil. More preferable, to ensure smooth feed of the second fuel175, the second fuel is free from solid particles at the aforementioned temperature and pressure.

However, as indicated above, the first fuel165needs not be of high quality. Moreover, in order to save operational costs, preferably, the first fuel164is of low quality. Therefore, in an embodiment, the first fuel165comprises solid material, such as biomass and/or residue derived fuel, at a temperature of 20° C. and a pressure of 1 atm. Even more preferably, at the aforementioned temperature and pressure, [A] the second fuel175is gas or liquid and [B] the first fuel165comprises solid material.

As detailed above, preferably, the second furnace172is arranged in a loopseal142of a circulating fluidized bed boiler. More preferably, the second furnace172and the third heat exchanger330are arranged as part of a loopseal heat exchanger400.FIGS.3to7show details of a loopseal heat exchanger400. The loopseal heat exchanger400is, when installed in a loopseal of a circulating fluidized bed boiler100, configured to:burn the second fuel175in the second furnace172of the loopseal heat exchanger400to produce second the flue gas173;convey the heat exchange medium from the second heat exchanger320to the third heat exchanger330of the loopseal heat exchanger400; andrecover heat from the second flue gas173to the heat exchange medium using the third heat exchanger330.

Other components of a fluidized bed boiler100, shown inFIG.1afor the purpose of understanding the context of the embodiments, include a feeder164configured to feed the first fuel165into the first furnace162; and air channels104for feeding combustion air106to the first furnace162. The combustion air106functions as an oxygen source for combustion and as (at least a part of) fluidizing gas. As detailed below, the term “combustion air” may refer to a mixture of air and some other gas, in particular a mixture of air and the second flue gas173. As detailed below, also other gas, in particular second flue gas, may be used to further fluidize the material within the first furnace162.

In between the air channels104, ash channels112are provided for removing bottom ash from the first furnace162. A bottom ash cooler114is configured to receive hot bottom ash and to recover heat therefrom. The first heat exchanger310is arranged in a flue gas duct120. The fluidized bed boiler may comprise further heat exchangers, such as an economizer122configured to recover heat from the first flue gas163. In general, an economizer is a heat exchanger receiving liquid heat exchange medium, in particular water. Typically the water does not boil in an economizer, even if heated. In a direction of flow of the heat exchange medium, the economizer122is arranged upstream from the first heat exchanger310. A drum124may be provided in between the first heat exchanger310and the economizer122to separate a liquid part of the heat exchange medium (e.g. water) from a gaseous part of the heat exchange medium (e.g. steam). Moreover other heat exchangers (not shown) may be connected to the drum124to boil the water and generate saturated steam.

The heated heat exchange medium is preferably used to generate mechanical energy in a steam turbine152. Thus, in an embodiment of the method, the heat exchange medium comprises steam, and the method comprises conveying steam from the third heat exchanger330to a steam turbine152. Preferably, the steam is conveyed from the third heat exchanger330to the steam turbine152such that no heat exchanger is provided in between the third heat exchanger330and the steam turbine152. Thus, preferably, the third heat exchanger330is the last heat exchanger before the steam turbine152. In a corresponding manner, a power plant comprises the fluidized bed boiler100and the steam turbine152, as indicated inFIGS.1aand1b. The power plant further comprises a third pipeline332configured to convey heat exchange medium from the third heat exchanger330to the steam turbine152. Preferably, the third pipeline332does not comprise a heat exchanger configured to heat or cool the heat exchange medium in between the third heat exchanger330and the steam turbine152.

Preferably, the method further comprises operating an electricity generator155using the steam turbine152and in this way producing electricity. A corresponding power plant comprises an electricity generator155arranged in a mechanical connection with the steam turbine152. E.g. a shaft may be configured to be rotated by the steam turbine152and to operate the electricity generator, i.e. to rotate parts of the electricity generator155.

FIG.3shows a cross section of an embodiment of a loopseal heat exchanger,400as seen from top (seeFIG.1afor the cut line III-III). In use, the loopseal heat exchanger400is arranged in the loopseal140of a circulating fluidized bed boiler100. Thus, the loopseal heat exchanger400is suitable for such a purpose, even if not part of a boiler. The first chamber412of the fluidized bed boiler100is, in this embodiment, a first chamber412of the loopseal heat exchanger400. Moreover, in this embodiment, the third heat exchanger330is arranged in a second chamber422of the loopseal heat exchanger400.

Walls of the loopseal heat exchanger400limit (i.e. the loopseal heat exchanger400comprises) the first chamber412, the second chamber422, and the second furnace172, of which functions have been detailed above. The second heat exchanger320is arranged in the first chamber412, the third heat exchanger330is arranged in the second chamber422, and the second pipeline322connects the second320and third330heat exchangers. The second pipeline322may run only within the chambers412,422as inFIG.3, or the a part of the second pipeline may run outside the chambers412,422, as inFIG.9b.

The loopseal heat exchanger comprises an entrance chamber431, to which the circulating bed material enters from the dipleg channel134(seeFIG.1a). From the entrance chamber431the bed material flows to at least one of an inlet chamber433and a bypass chamber432(seeFIG.3).

The bypass chamber432has two functions. First, the heat recovery by the second heat exchanger320can be controlled by controlling the amount of bed material that flows through the bypass chamber432. The bypass chamber432does not have heat exchanger surfaces for heating the heat exchange medium. Thus, by guiding the flow of bed material only or mainly through the bypass chamber instead of the first chamber412, the heat exchange medium becomes heated to a lesser extent in the second heat exchanger320. Second, the bypass chamber432serves as a gas lock. The bypass chamber432is an upleg, i.e. the bed material flows mainly upwards in the bypass chamber432. This, in connection with the dipleg channel134provides for a gas lock that prevents the bed material from flowing in a wrong direction, i.e. from the first furnace162to the loopseal heat exchanger400.

The inlet chamber433is also designed as an upleg. Thus, the inlet chamber433serves mainly as a gas lock in combination with the dipleg channel134. Moreover, another function of the inlet chamber433is to feed the bed material to the first chamber412. From the first chamber412, which is formed as a dipleg, the bed material flows to an outlet chamber435, which is designed as an upleg.

In the embodiment ofFIG.3, the second heat exchanger320is arranged inside the first chamber412of a loopseal heat exchanger400and the third heat exchanger330is arranged inside a second chamber422of the loopseal heat exchanger400. Having the heat exchanges arranged within the chambers is preferable, since this increases an area of the heat transfer surfaces and helps maintenance. However, the second heat exchanger320can be arranged within the walls of the first chamber412. In a similar manner, the third heat exchanger330can be arranged within the walls of the second chamber422.

From the outlet chamber435and/or from the bypass chamber432, the bed material flows to the return channel136. The direction of flow of bed material is shown by the arrows451,452,453,454,455, and457inFIG.3. Geometric details of the loopseal heat exchanger400may vary. For example, as indicated inFIG.14, a part of the return channel136may be arranged between parallel walls of the bypass chamber432and the outlet chamber435, and the bed material is configured to flow through apertures of these parallel walls to the return channel136. Other types of loopseal heat exchangers will be detailed below.

In this way, the invention also relates to a new type of a fluidized bed heat exchanger400. As detailed above, and shown inFIGS.3to10band14, an embodiment of a fluidized bed heat exchanger400comprises the first chamber412and the second chamber422. The second heat exchanger320is arranged inside (i.e. in) the first chamber412or in (i.e. as a part of) a wall that limits the first chamber412. The third heat exchanger330arranged inside (i.e. in) the second chamber422or in (i.e. as part of) a wall that limits the second chamber422. The loopseal heat exchanger400may comprise the second pipeline322for conveying the heat exchange medium from the second heat exchanger320to the third heat exchanger330; and comprises first nozzles462(seeFIG.7) configured to fluidize the bed material in the first chamber412. In order to further heat the heat exchange medium already within the loopseal heat exchanger400, the loopseal heat exchanger400comprises the second furnace172for burning the second fuel175to produce the second flue gas173; and a passage178for conveying the second flue gas173to the second chamber422.

InFIGS.3,11, and14, the second chamber422is arranged next to the first chamber412. Moreover, inFIGS.3,11, and14, the second furnace172is arranged either next to the inlet chamber433(FIGS.3and14) or next to the entrance chamber431(FIG.11). Both these features, in isolation and in combination, are beneficial from the point of view of recovering energy. Because of this arrangement, the second furnace172is heated through a first wall441. Thus, the second furnace172is not only heated by burning the second fuel, but also with the hot bed material arranged in the neighbouring chamber. In a similar manner, the second chamber422is heated through the first wall441. Thus, the second chamber is not only heated by burning the second fuel, but also with the hot bed material. In this way, in an embodiment, a first side of a wall of the loopseal heat exchanger400(i.e. a first side of the first wall441) limits the first chamber412, and an opposite second side of the wall (i.e. the first wall441) of the loopseal heat exchanger limits the second chamber422. In addition, preferably, the first wall441comprises heat transfer tubes for recovering heat to the heat exchange medium. This applies both to the circulating fluidized bed boiler100and the loopseal heat exchanger400. Moreover, when the second320and third330heat exchangers are arranged on opposite sides of the first wall441and in such a way that a normal N (seeFIG.3) of the first wall441runs through both the second heat exchanger320and the third heat exchanger330, the second320and third330heat exchanger can be easily integrated. In other words, the connecting second pipeline322can be made short. This simplifies the structure of the loopseal heat exchanger and reduces manufacturing costs.

Moreover, the second chamber422and the first chamber412share a common wall. Referring toFIG.3, the second chamber422and the first chamber412commonly share a second wall443of the loopseal heat exchanger400. Referring toFIG.3, the second chamber422and the first chamber412commonly share a third wall445of the loopseal heat exchanger400. In addition, preferably, the second wall443comprises heat transfer tubes for recovering heat to the heat exchange medium. In addition, preferably, the third wall445comprises heat transfer tubes for recovering heat to the heat exchange medium.

As for the term “wall” herein the term refers to a planar object delimiting the chambers. Thus, the first wall441delimits the chambers433,172,422, and412(seeFIG.3). As indicated inFIG.3, the second wall443delimits the chambers422,412, and435; and the third wall445delimits all the chambers of the loopseal heat exchanger ofFIG.3.

Therefore, in an embodiment a first side of a wall (i.e. the second wall443) of the loopseal heat exchanger limits the first chamber412, and the first side of the wall (i.e. the second wall443) of the loopseal heat exchanger limits the second chamber422. This is also beneficial from the point of view of keeping the outer shape of the loopseal heat exchanger400simple for installation.

While the position of third wall445is not critical, for manufacturing reasons it also preferably limits both the first and second chambers412,422. Thus, inFIG.3, a first side of a wall (i.e. the third wall445) of the loopseal heat exchanger limits the first chamber412, and the first side of the wall (i.e. the third wall445) of the loopseal heat exchanger limits the second chamber422.

As shown in the figures, the second and third walls (443,445) are arranged a distance apart from each other; i.e. they are not different parts of the same wall.

In the structure as discussed above, the second pipeline322can be made short or dispensed with. For example, referring toFIG.15, the second heat exchanger320may comprise an inlet header320aand an outlet header320b, whereby the steam coming from the first superheater310may be distributed to the tubes of the second superheater320through the inlet header320a, and the steam that has run through the tubes of the second superheater320may be collected in the outlet header320bof the second heat exchanger320. In a similar manner, the third heat exchanger330may comprise an inlet header330afor distributing the steam to the tubes of the third heat exchanger330; and an outlet header330bfor collecting the steam that has run through the tubes of the third heat exchanger330.

Referring toFIG.15, the outlet header320bof the second heat exchanger320and the inlet header330aof the third heat exchanger may be arranged to form parts of a steam chamber335. A part of the steam chamber335may be seen as forming the second pipeline322. Having a steam chamber335that serves both as the outlet header320bof the second heat exchanger320as well as the inlet header330aof the third heat exchanger330further simplifies the structure of the loopseal heat exchanger400thereby reducing manufacturing costs. In addition, such a structure is mechanically sturdy. Such structure is easily achievable, when the second and third heat exchangers320,330are arranged in neighbouring chambers412,422, as shown in the Figures. This relates also to the issue that a normal N of a first wall441of a loopseal superheater runs through both the second320and third330heat exchangers; an issue further detailed below.

Referring toFIG.15, the inlet header320aof the second heat exchanger320may be mechanically connected to the outlet header330bof third heat exchanger330. However, in such a case, a plug325may be used for preventing the steam from flowing directly from the inlet header320aof the second heat exchanger320to the outlet header330bof the third heat exchanger330, e.g. if the connection between the headers320aand330bis a pipeline. The inlet header320aof the second heat exchanger320and the outlet header330bof the third heat exchanger330may be parts of the same tubular structure provided with the plug325.

As for the circulation of bed material within the loopseal heat exchanger400and through the first chamber412, a first opening451′ is provided in a lower part of the entrance chamber431for allowing bed material to flow to the inlet chamber433, as shown by the arrow451(seeFIGS.3and4). A second opening453′ is provided in an upper part of the inlet chamber433for allowing bed material to flow to the first chamber412, as indicated by the arrow453(seeFIGS.4and7). A third opening455′ is provided in a lower part of the first chamber412for allowing bed material to flow to the outlet chamber435as indicated by the arrow455(seeFIG.7). Finally, a fourth opening457′ is provided in an upper part of the outlet chamber435for allowing the bed material to exit the loopseal heat exchanger400, as depicted by the arrow457.

As for the circulation of bed material within the loopseal heat exchanger400and through the bypass chamber432, a fifth opening452′ is provided in a lower part of the entrance chamber431for allowing flow of bed material to the bypass chamber432, as indicated by the arrow452(seeFIGS.3and4). A sixth opening454′ is provided in an upper part of the bypass chamber432for allowing the bed material to exit the loopseal heat exchanger400, as depicted by the arrow454.

The bed material is fluidized within the chambers of the loopseal heat exchanger400, excluding the chambers without bed material, i.e. at least the first furnace172and the second chamber422. However, the bed material need not be fluidized in all the chambers simultaneously. E.g. fluidizing of the material in the bypass chamber432may be stopped for controlling the flow through other chambers. For the purpose of fluidizing, the loopseal heat exchanger400comprises nozzles460(seeFIGS.4and5) arranged at a bottom of the chambers wherein the material is fluidized400. As for more detailed function of the nozzles460, the loopseal heat exchanger400comprises first nozzles462(seeFIGS.5and7) configured to fluidize the bed material in the first chamber412. In this way, a fluidized bed is formed in the first chamber412so that the second heat exchanger320makes a contact with a fluidized bed of the fluidized bed boiler.

The loopseal heat exchanger400comprises second nozzles464(seeFIG.4) configured to fluidize the bed material in the bypass chamber432. The loopseal heat exchanger400comprises third nozzles466(seeFIG.4) configured to fluidize the bed material in the inlet chamber433. By controlling the amount of fluidizing air in the bypass chamber432and in the inlet chamber433one can control how much bed material is directed to bypass chamber432and how much to the inlet chamber433. Typically, the more fluidizing gas is fed, the easier the bed material flows, and in this way the bed material flow increases; and vice versa.

Since the purpose of the third heat exchanger330is to superheat the steam to the final temperature during the first period (seeFIG.2a), the temperature of the heat exchange medium within the third heat exchanger330is at its highest during the first period. Thus, the second flue gas173has also downstream from the third heat exchanger330such a high temperature, that it can be used in at least the following waysto heat the heat exchange medium upstream from the third heat exchanger330; e.g. by using the second heat exchanger320and/or the first heat exchanger310and/oras fluidizing gas or as a part of a fluidizing gas of the fluidized bed boiler100.

Concerning the former, the second flue gas173may be mixed with the first flue gas163e.g. upstream from the first heat exchanger310. However, in terms of heat recovery, it may be beneficial to feed the second flue gas173much upstream from the first heat exchanger, whereby it may e.g. become used as fluidizing gas.

Concerning the latter, since the temperature of the second flue gas173is very high, using it as a fluidizing gas or as a part of a fluidizing gas implies that the remaining thermal energy of the second flue gas173is transferred to the bed material of the fluidized bed, and can thus be utilized in the process and recovered by conventional means. Examples of a fluidized bed, wherein the second flue gas173can be utilized as fluidizing gas, include the first furnace162(seeFIGS.1aand1b) and the first chamber412(seeFIG.1a).

The fluidizing gas can comprise also other gas than the second flue gas173. Not all the second flue gas needs to be used as at least a part of a fluidizing gas. Thus, a preferable embodiment comprises fluidizing a fluidized bed within the fluidized bed boiler100using fluidizing gas that comprises at least some of the second flue gas173. For reasons of process simplicity, preferably all the second flue gas173is used in a same way. Thus, a more preferable embodiment comprises using at least 75% (by volume) of the second flue gas173to fluidize one or more fluidized beds of the fluidized bed boiler100. Herein the fluidized bed may be the fluidized bed of the first furnace162or of the first chamber412; and the second flue gas173may be used as at least a part of the fluidizing gases in both the fluidized beds.

Correspondingly, the fluidized bed boiler100preferably comprises a channel179for conveying at least some of the second flue gas173to such a chamber of the fluidized bed boiler, wherein a fluidized bed is configured to form in use. As an example, the fluidized bed boiler100may comprise a second flue gas channel179for conveying at least some of the second flue gas173to such a part of the first furnace162, wherein a fluidized bed is configured to form. Referring toFIG.1a, in use, in a circulating fluidized bed boiler, the material within substantially the whole first furnace162is fluidized. Referring toFIG.1b, in use, in a bubbling fluidized bed boiler, the material within a lower part of the first furnace162is fluidized.

For example, in the context of the embodiments ofFIGS.1aand3to7, as depicted inFIG.8a, the second flue gas173can be conveyed to the return channel136through the second flue gas channel179. At some point within the second flue gas channel179, the second flue gas173is mixed with the bed material of the return channel136. Thereafter, through a combined second flue gas channel179and a return channel136both the bed material and the second flue gas173are conveyed into the first furnace162. Therein the second flue gas173becomes mixed with the other fluidizing gas (i.e. combustion air), and is thus used as part of the fluidizing gas of the first furnace. Thus, the second flue gas173needs not be fed through the combustion air channel104of the fluidized bed boiler. However, as indicated inFIG.8b, the second flue gas173can be mixed with the combustion air to be fed to the first furnace through the combustion air channel104. In case the combustion air is preheated, as it typically is, the second flue gas is preferably mixed with the preheated combustion air (i.e. downstream from the combustion air preheaters) to improve efficiency of the preheating. In this way, a part of the combustion air piping forms a part of the second flue gas channel179.

As discussed above, the third heat exchanger330can be arranged side-by-side (in a horizontal direction Sx) with respect to second heat exchanger320. In other words, the first wall441may be a vertical wall.

Referring toFIGS.9aand9b, the third heat exchanger330may be arranged on top of the second heat exchanger320. In such a case, the first wall441is substantially horizontal, as detailed inFIG.9b. As discussed above, the second heat exchanger320is arranged on a first side of the first wall441and the third heat exchanger is arranged on a second, opposite, side of the first wall441. Moreover, the first wall441limits the first chamber412, in which the second heat exchanger320is arranged, and the first wall441limits the second chamber422, in which the third heat exchanger330is arranged. Finally, the second320and third heat exchangers330are arranged such that a normal N of the first wall441runs through both the second320and third330heat exchangers. For technical effects, see above. The outlet header320bof the second heat exchanger320may be connected to the inlet header of the third heat exchanger330. Reference is made toFIG.15. However, the steam chamber335may be vertical in such a case.

The second and third walls443and445are shown inFIG.9a. As discussed above, the second heat exchanger320is arranged on a first side of the second wall443and the third heat exchanger330is arranged on the first side of the second wall443. Moreover, the second wall443limits the first chamber412, in which the second heat exchanger320is arranged, and the second wall443limits the second chamber422, in which the third heat exchanger330is arranged. As discussed above, the second heat exchanger320is arranged on a first side of the third wall445and the third heat exchanger330is arranged on the first side of the third wall445. Moreover, the third wall445limits the first chamber412, in which the second heat exchanger320is arranged, and the third wall445limits the second chamber422, in which the third heat exchanger330is arranged.

In the embodiment ofFIGS.9aand9b, the second flue gas173can be used as fluidizing gas in the first furnace162e.g. by providing a flue gas passage on top of an outlet chamber435. As readable fromFIGS.3to7,9a, and9b, the loopseal heat exchangers400ofFIGS.3to7andFIGS.9aand9bmay be similar with respect to the chambers through which the bed material is configured to flow, i.e. the chambers431,433,412,435, and432. Thus, in the embodiment ofFIGS.9aand9b, the second flue gas may flow through a flue gas passage provided in a roof of the outlet chamber435, through the outlet chamber435to the return channel136, and finally through the return channel136to the first furnace162. Thus, the outlet chamber435forms a part of the channel179for conveying at least some of the second flue gas173to such a chamber of the fluidized bed boiler, wherein a fluidized bed is configured to form in use. For other parts of the channel179, reference is made toFIG.9b.

What has been said in connection withFIGS.9aand9bapplies both to a loopseal heat exchanger400and a fluidized bed boiler100.

Referring toFIGS.10ato10f, the third heat exchanger330may be arranged below the second heat exchanger320. In such a case, the first wall441is substantially horizontal, as detailed inFIG.10b. As discussed above, the second heat exchanger320is arranged on a first side of the first wall441and the third heat exchanger330is arranged on a second, opposite, side of the first wall441. Moreover, the first wall441limits second chamber422, in which the third heat exchanger330is arranged. Finally, the second320and third heat exchangers330are arranged such that a normal N of the first wall runs through both the second and third330heat exchangers. For technical effects, see above. The outlet header320bof the second heat exchanger320may be connected to the inlet header of the third heat exchanger330. Reference is made toFIG.15. However, the steam chamber335may be vertical in such a case.

The second and third walls443and445are shown inFIG.10a. As discussed above, the second heat exchanger320is arranged on a first side of the second wall443and the third heat exchanger330is arranged on the first side of the second wall443. Moreover, the second wall443limits the first chamber412, in which the second heat exchanger320is arranged, and the second wall443limits the second chamber422, in which the third heat exchanger330is arranged. As discussed above, the second heat exchanger320is arranged on a first side of the third wall445and the third heat exchanger330is arranged on the first side of the third wall445. Moreover, the third wall445limits the first chamber412, in which the second heat exchanger320is arranged, and the third wall445limits the second chamber422, in which the third heat exchanger330is arranged.

In the embodiment ofFIGS.10ato10d, the second flue gas173can be used as fluidizing gas in the first chamber412. The principle is shown inFIG.10b, wherein the second flue gas173flows through the heat exchanger pipes of the third heat exchanger330and therefrom, through the first nozzles462, to the first chamber412. In this embodiment, air (“Air” inFIG.10b) is used both as oxidizing medium for the second fuel175(i.e. combustion air in the second furnace) and a fluidizing gas for the first chamber412. Thus, in the second operating mode, i.e. during the second period (seeFIG.2b), the air could cool the heat exchange medium flowing through the third heat exchanger330.

A solution to this problem has been shown inFIGS.10cand10d. The loopseal heat exchanger ofFIGS.10cand10dcomprises a first damper471and, optionally, a first pivot472. The first damper471is pivotable about the first pivot472. However, the damper471could be construed using two slidable dampers (one horizontal and one vertical corresponding to the positions of the first damper471) without the need for a pivot. Referring toFIG.10c, during the first period, the first damper471is arranged to such a position that it is configured to guide the second flue gas173to the third heat exchanger330. Referring toFIG.10d, during the second period, the first damper471is arranged to such a position that it is configured to guide fluidizing air or the second flue gas173as the case may be to bypass the third heat exchanger330. In this embodiment, there is no need to burn even a small amount of the second fuel175during the second period. Thus, typically, in this embodiment, no second flue gas would be produced, and only the air would be guided by the first damper [A] to bypass the third heat exchanger330and [B] to the first nozzles462.

In order to further secure that the air does not cool down the third heat exchanger330during the second period, the loopseal heat exchanger400may comprise a second damper473and, optionally, a second pivot474, as shown inFIGS.10cand10d.FIGS.10cand10dshow two second dampers473and a second pivot474for each second damper473.

The second damper473is pivotable about the second pivot474. As detailed above, Slidable dampers could be used instead or in addition. Referring toFIG.10c, during the first period, the second damper473is arranged to such a position that the second damper473is configured to guide second flue gas173from the third heat exchanger330to the to the first nozzles462. Referring toFIG.10d, during the second period, the second damper473is arranged to such a position that it is configured to prevent the fluidizing air or the second flue gas173as the case may be from flowing to the third heat exchanger330. In combination, the first and second dampers471,473serve for the same purpose as the third damper475discussed in connection withFIG.2c.

Therefore, an embodiment of a fluidized bed boiler100or a loopseal heat exchanger for a circulating fluidized bed boiler comprises a damper arrangement comprising at least one damper (471,473,475), wherein the damper arrangement is configured toenable circulation of second flue173gas from the second furnace172to the third heat exchanger330andprevent air circulation through the third heat exchanger330.

The former applies at the first period (seeFIGS.2a,2c, and10c); and the latter applies at the second period (seeFIGS.2b,2c, and10d). Concerning the latter, the air therein may comprise second flue gas, provided that a minor amount of second flue is burned also during the second period.

It has further been found that in the first mode, i.e. at the first period, typically more combustion air is needed to burn the second fuel175than what is needed for fluidizing the bed material in the first chamber412. It has been found that the excess heat can be recovered to the combustion air that is used for combustion in the second furnace172.

Referring toFIG.10e, in embodiment, the loopseal heat exchanger400comprises a fourth heat exchanger340. The fourth heat exchanger is configured to recover heat from the second flue gas173to air, in particular the combustion air use in the second furnace172. The embodiment also comprises a fourth pipeline342configured to convey the heated air from the fourth heat exchanger340to the second furnace172, to be used as an oxidizing medium therein. The embodiment also comprises a valve344configured to limit a flow of the second flue gas through the fourth heat exchanger340. Thus, the valve344can be used to control how much second flue gas is used as the fluidizing gas in the loopseal heat exchanger400. The second flue gas may be used to fluidize bed material also in another chamber (or other chamber) of the loopseal heat exchanger than the first chamber412.

To recover heat from the excess second flue gas173, it is also possible to convey a part of the second flue gas173to the first furnace162and to use only a part of the second flue gas173as fluidizing medium in the loopseal heat exchanger400. An example of such a solution is shown inFIG.10f, wherein a portion of the second flue gas173is taken from a location upstream from the fluidizing nozzles460and conveyed through a part of the channel179to the return channel136. Through the return channel136the second flue gas173flows to the first furnace162as detailed above in connection with e.g.FIG.8a. A valve (not shown) may be applied to control the division of the second flue gas173to the part conveyed to the nozzles460and to the part conveyed to the first furnace162. Even if not shown, the excess second flue gas173could be mixed with the combustion air of the first furnace162as detailed inFIG.8b.

Heat of that part of the second flue gas173that is used for fluidizing the bed material in the second chamber412, and therefore conveyed through the nozzles460, can be recovered by using the gas as a part of the fluidizing gas in the first furnace162or in other ways detailed above. Reference is made toFIGS.8a,8b,9a, and9b. For example, an opening may be provided in an upper part of the outlet chamber435to allow for the used fluidizing gas to flow to the return channel136, even another part of the second flue gas is used as depicted inFIG.10eor10f.

Even if not shown, the excess second flue gas173, i.e. the part not conveyed through the nozzles460, could be mixed with the first flue gas163downstream from a fluidized bed arranged in the first furnace162, e.g. downstream from the cyclone132of a circulating fluidized bed boiler, as the case may be. In this case it would be beneficial to mix the flue gases173,163upstream from the first heat exchanger310. However, from the point of view of recovering energy, it is more beneficial to use the excess second flue gas or all the second flue gas as part of the fluidizing gas of the first furnace162.

What has been said in connection withFIGS.10ato10fapplies both to a loopseal heat exchanger400and a fluidized bed boiler100.

While such chambers (431,432,433,412,435), through which bed material is configured to run, of the loopseal heat exchangers ofFIGS.3to10fhave been arranged in the same manner relative to each other, the second furnace172and the third heat exchanger330can be provided also in simpler loopseal heat exchangers400, as depicted inFIGS.11to13.

In the loopseal heat exchangers ofFIGS.11to13, the bed material enters through the dipleg channel134to the entrance chamber431, as inFIG.3. However, from the entrance chamber431, at least a part of the bed material enters directly the first chamber412as shown by the arrow451. An aperture may be provided in a lower part of the entrance chamber431for this purpose. In the first chamber412, the second heat exchanger320is provided. In the first chamber412, the bed material flows upwards, and also in the negative Sx direction inFIG.11. From the first chamber412, the bed material exits to the return channel136, as indicated by the arrow457. An aperture may be provided in an upper part of the first chamber412for the purpose. The loopseal heat exchanger400ofFIG.11comprises also a bypass chamber432. At least some bed material may bypass the second heat exchanger320by flowing through the bypass chamber432as indicated by the arrows452and454. What has been said about bed material circulation through the bypass chamber432in the embodiment ofFIG.3, applies also here.

The loopseal heat exchanger400ofFIG.11comprises also the second chamber422, in which the third heat exchanger330has been arranged, and the second furnace172provided with the burner176. Even if not shown inFIG.11, the second flue gas is configured to flow from the second furnace172to the second chamber422, in which the third heat exchanger330has been provided. For reference, seeFIG.6, even if inFIG.11, the second chamber422is arranged in the positive Sy direction from the second furnace172.

The loopseal heat exchanger400ofFIG.11comprises the a first wall441. What has been said about the first wall441and the normal N thereof in connection withFIGS.3to7applies. The loopseal heat exchanger400ofFIG.11comprises a second wall443. What has been said about the second wall443in connection withFIGS.3to7applies. The loopseal heat exchanger400ofFIG.11comprises a third wall445. What has been said about the third wall445in connection withFIGS.3to7applies. An outlet header320bof the second heat exchanger320may be connected to an inlet header330aof the third heat exchanger330as depicted inFIG.15.

Even if not shown, the loopseal heat exchanger400is provided with nozzles460for blowing fluidizing gas to the first chamber412(see nozzles462above) and with nozzles460for blowing fluidizing gas to the bypass chamber432(see nozzles464above) in order to fluidize the bed material therein.

InFIG.11, the first chamber412and the second chamber422have been arranged side by side in the horizontal direction (Sx). However, they may be arranged such that the third heat exchanger330is arranged above the second heat exchanger, as depicted inFIG.12. In the embodiment ofFIG.12, the second flue gas173, downstream from the third heat exchanger330, may be conveyed through the first chamber412to the return channel136, to be used as fluidizing medium both in the first chamber412and in the first furnace162. The loopseal heat exchanger400ofFIG.12comprises a first wall441. What has been said about the first wall441and the normal N thereof in connection withFIGS.9aand9bapplies. Even if now shown, inFIG.12, the loopseal heat exchanger400ofFIG.12comprises the second and third walls443,445for the same purpose as the walls443and445disclosed in connection withFIGS.9aand9b.

Moreover, the first chamber412and the second chamber422may be arranged such that the third heat exchanger330is arranged below the second heat exchanger320, as depicted inFIG.13. Therein, the second flue gas173cab be used for fluidizing the bed material in the first chamber412. In case there is excess second flue gas available, a part of the flue gas need not be conveyed through the nozzles460(in particular462). The excess second flue gas may be used in one of the ways discussed above in connection withFIGS.10eand10f. Cooling of the third heat exchanger330by the combustion air for the second fuel175can be prevented by the solution discussed above in connection withFIGS.10cand10d.

The loopseal heat exchanger400ofFIG.13comprises a first wall441. What has been said about the first wall441and the normal N thereof in connection withFIGS.10ato10fapplies. Even if not shown inFIG.13, the loopseal heat exchanger400ofFIG.13comprises the second and third walls443,445for the same purpose as the walls443and445disclosed in connection withFIGS.10ato10f.