Patent Description:
Boilers are used to produce steam by burning fuel. The steam can be used for heating and/or for producing mechanical energy in a turbine, which may be converted to electricity. In combustion, some compounds of the fuel are released or reacted to produce flue gas. Some compounds of the flue gas corrode the heat transfer surfaces of the boiler, which reduces service life and increases maintenance needs.

To this end, it is known that the corrosive compounds are most corrosive at a certain temperature window in a molten form. Thus, to avoid corrosion, a surface temperature of a heat transfer tube can be, in some cases, arranged to be above a higher critical temperature or below a lower critical temperature. Such critical temperatures may be e.g. about <NUM> and about <NUM>, respectively. However, precise values of these critical temperatures depend on the composition of the flue gas, which depends on the fuel that is used. To further reduce corrosion, it is known to apply different types of coatings onto heat transfer surfaces.

However, nowadays the needed steam temperature has gone up, and the content of the fuel has changed so as to form such compounds, mainly alkali chlorides, that are comprised in the smelt form on heat transfer surfaces in typical use. Particularly some types of fuel comprises a lot of chlorine, which seems to pose corrosion problems. Therefore, other solutions are required in addition or alternatively.

In this field, the document <CIT>, on which the two-part-form of the independent claims is based, discloses burning or gasifying material containing combustible components in a circulating fluidized bed comprising a turbulence chamber, a solids separator, and a cooling device for the indirect cooling of solids arriving from the solids separator. A first fluidized bed of the cooling device, into which the hot solids arriving from the solids separator are introduced first, is situated in a dechlorinating chamber. Fluidizing gas and at least one of the following dechlorinating additives: a) gaseous SO<NUM> or a material which contains sulfur and releases SO<NUM> in an oxidizing atmosphere, b) silicates and aluminium silicates, c) activated silicate, or d) other alkali-binding and HCl-releasing additives, are also introduced into the dechlorinating chamber in at least stoichiometric quantities so as to convert the alkali and metal chlorides contained in the arriving solids.

Moreover, the document <CIT> discloses a method of and a system for maintaining steam temperature and therefore electricity production efficiency with decreased loads of a steam turbine power plant comprising a fluidized bed boiler and a fluidized bed superheater. In that solution, the steam temperature may be maintained by providing, outside a furnace, additional heating to the fluidized bed material in its outer circulation, thereby increasing the amount of thermal energy available in the fluidized bed material.

In addition, the document <CIT> discloses an exhaust gas recirculation system, which comprises a circulating fluidized bed boiler. The recirculation system is connected to a draught fan, the draught fan is connected to a dust remover, the dust remover is connected to a chimney through a desulfurizer, and a cooling circulation system is arranged on the bottom of the circulating fluidized bed boiler.

It has been found that corrosion can be reduced by feeding sulphur-containing reagent to the process. The sulphur-containing reagent may be e.g. elementary sulphur or in a form of a sulphate. However, in line with the background, it has also been found that sulphur-containing reagent needs not be fed to such heat exchange surfaces that a reasonably cool, i.e. so cool that the alkali chlorides are not in a form of a smelt therein. Moreover, it has been found that when high-temperature steam needs to be produced, the high-temperature superheater(s) can be arranged to such a location in which there is only a minor amount of the corrosive components. Thus, a lesser amount of sulphur-containing reagent is needed compared to feeding to some other locations. It has been found that in a circulating fluidized bed boiler, such a location is arranged after a particle separator, which separates a flue gas fraction and a bed material fraction; whereby most of the corrosive components flow with the flue gas fraction. Correspondingly the bed material fraction comprises only a minor amount of the corrosive compounds. Therefore, the high-temperature heat exchanger (hereinafter a first heat exchanger) is configured to recover heat from the bed material fraction, and the sulphur-containing reagent is fed upstream from a heat transfer tube of the first heat exchanger, to the bed material fraction (i.e. downstream from the separation of the bed material fraction). At such a location only a minor amount of corrosive compounds would be present without the sulphur addition, because of the separation of the flue gas fraction, and with the sulphur addition, the amount of corrosive compounds decreases even further.

The invention is disclosed in more specific terms in the independent claims <NUM> and <NUM>. Preferable embodiments are disclosed in dependent claims. These and other embodiments are explained and disclosed in the description and the figures.

<FIG> shows a fluidized bed boiler <NUM>. The fluidized bed boiler is of the circulating type. Even when not in use, such a fluidized bed boiler may be called a circulating fluidized bed boiler. A purpose of a boiler is to recover heat and to produce steam. Therefore, the circulating fluidized bed boiler comprises a first heat exchanger <NUM> comprising a heat exchanger tube <NUM>. In use, heat transfer medium (in an embodiment water/steam) is fed into the heat exchanger tube <NUM> and heat received by an outer surface of the heat exchanger tube <NUM> is transferred to the heat transfer medium. In this way, heat is recovered to the heat transfer medium.

The heat that is recovered is produced by combustion. Therefore, fuel and air are fed to a furnace <NUM> of the circulating fluidized bed boiler <NUM>. In addition, bed material is fed to the furnace <NUM>. Bed material may be supplied from a return channel <NUM>; and in addition additional bed material may be added e.g. with the fuel and/or to the return channel <NUM> and/or through other means (not shown).

Fluidizing air is supplied through nozzles <NUM> to the furnace. In a circulating fluidized bed boiler <NUM> so much air is supplied that the fuel and the bed material fluidizes, and they flow, with the air, as a mixture M comprising the air, fuel, and bed material towards a particle separator <NUM>. Typically, the mixture M flows in the furnace <NUM> substantially upwards. In <FIG>, the direction Sz is upwards, i.e. reverse to gravity. Because the furnace <NUM> is hot, the fuel burns thereby producing heat, ash, and flue gas. Thus, in the furnace <NUM>, the mixture M comprises at least bed material and gas (air and/or flue gas). The directions Sx and Sy are perpendicular to Sz and to each other.

In <FIG>, the air is fed to the nozzles <NUM> through windboxes <NUM>. In <FIG>, in between the windboxes <NUM>, channels <NUM> for removing bottom ash are arranged. Bottom ash may be removed through the channels <NUM> in use, because otherwise the ash resulting from combustion accumulates to the bed material. However, bottom ash may be removed in addition or in the alternative from the heat exchanger <NUM> (such ash removal not shown in <FIG>).

The particle separator <NUM> is configured to separate from the mixture M, which comprises at least bed material and flue gas, both a bed material fraction BM and a flue gas fraction FG. The flue gas fraction FG is conveyed (optionally through cleaning phases) eventually to a chimney. The bed material fraction BM is circulated back to the furnace <NUM>. However, as detailed below, heat is recovered from at least part of the bed material fraction BM using the first heat exchanger <NUM>. The bed material as well as the bed material fraction BM comprises solid particulate material that is heat resistant, such as sand. In use, at least a part of a bed material fraction BM is guided to (i.e. onto) the heat exchanger tube <NUM> so that heat is recovered from the part of the bed material fraction.

As detailed in the background, burning the fuel results in such flue gases that are corrosive at least within a certain temperature window. It has been found that typically alkali chlorides are very corrosive at a temperature window of about <NUM> to about <NUM> or even up to <NUM>. A rate of corrosion depends also on a content of the corrosive compounds in the flue gases. Thus, when a steam temperature inside the heat exchanger tube is more than <NUM> (e.g. around <NUM>) and the environment is hotter, such conditions easily occur on outer surface of the heat exchanger tube <NUM>. Moreover, even if at a higher temperature the corrosiveness goes down, the gases may be somewhat corrosive. Thus, reducing corrosion also by other means is needed at least at such locations.

It has been found that alkali chlorides can be captured by feeding sulphur. More precisely, the alkali chlorides react with sulphur oxide (SOx) to form alkali sulphate, which is less corrosive than alkali chloride. Some of the relevant reactions can be expressed as:.

<NUM>aCl + SO<NUM> + ½O<NUM> + H<NUM>O → Ma<NUM>SO<NUM> + <NUM> HCl     (<NUM>).

<NUM>aCl + SO<NUM> + H<NUM>O → Ma<NUM>SO<NUM> + <NUM> HCl     (<NUM>).

, wherein Ma is an alkali ion (e.g. Na or K).

In the hot environment of the furnace, substantially all sulphur-containing materials react with oxygen to form SO<NUM> (or other SOx), which is needed in the reaction (see the reaction above). Moreover, the alkali sulphate Ma<NUM>SO<NUM> is much less corrosive than the alkali chloride MaCl. Moreover, the hydrogen chloride is in the gaseous form, whereby it also is not corrosive.

Feeding large amounts of sulphur is expensive and results in other problems, mainly the production of excess sulphur oxides. The inventors have found that most of the corrosive alkali chlorides do not enter the bed material fraction BM, because the most of the alkali chlorides flow with the flue gas fraction FG away from the heat exchanger tube <NUM> (see <FIG>). Thus, if the first heat exchanger <NUM> in which the steam temperature is high (see above) is configured to recover heat from the only bed material fraction BM (or a part thereof), the sulphur-containing reagent R can be fed to the bed material fraction BM, from which most of the flue gas have been removed by the particle separator. However, the sulphur-containing reagent R should be fed upstream from the furnace <NUM>, because in the furnace <NUM>, flue gases are generated again. Thus, to convert the chloride to the sulphate before the heat exchanger tube <NUM> of the first heat exchanger, the sulphur-containing reagent R is fed upstream from the heat exchanger tube <NUM>.

It is noted that, not according to the invention, as an alternative to a sulphur-containing reagent R, from the technical point of view aluminium silicates also are capable of reducing corrosion problems. Thus, not according to the invention, as an alternative to sulphur-containing reagent R, a reagent comprising only aluminium silicates can be fed using the same principles as detailed in this description. However, the inventors have noticed that a sulphur-containing reagent R functions much better, and the operational costs remain at a much lower level when a sulphur-containing reagent R is used instead of aluminium silicates. However, the sulphur-containing reagent R of the present invention may comprise, in addition to a sulphur compound, e.g. aluminium silicate.

Hereinbelow, unless otherwise indicated, the terms "upstream" and "downstream" refer to the bed material circulation defined by the following components of the fluidized bed boiler: the particle separator <NUM>, the first heat exchanger <NUM>, and the furnace. The bed material (or a part thereof) flows from the particle separator <NUM> to the first heat exchanger <NUM>; from the first heat exchanger <NUM> to the furnace <NUM>; and from the furnace <NUM> to the particle separator <NUM> (see <FIG>). As will be detailed below, downstream from the particle separator, a splitting chamber may divide the bed material fraction BM to a first part and a second part such that only the first part is conveyed to the heat exchanger tube <NUM>, while the second part bypasses the heat exchanger tube <NUM>.

Moreover, because of the role of alkali chlorides, the method is particularly useful when such a fuel that has a high content of chlorine is burnt. Typically fuels derived from residue comprise a lot of chlorine. Correspondingly, the method and the fluidized bed boiler <NUM> are particularly useful with such a fuel. Thus, in an embodiment of the method, the fuel comprises a component derived from residue. In an embodiment, the fuel comprises biodegradable material and/or plastic material. In an embodiment, the fuel comprises at least one of refuse derived fuel (RDF), recovered fuel (REF), and solid recovered fuel (SRF). Correspondingly, in an embodiment, the fluidized bed boiler <NUM> comprises means for feeding such fuel that has been derived from residue into the furnace <NUM>.

Thus, an embodiment of a method for operating a circulating fluidized bed boiler comprises feeding air ("Air" in <FIG>), fuel ("Fuel" in <FIG>), and bed material ("Bed material" in <FIG>) to a furnace <NUM> of the circulating fluidized bed boiler <NUM> and combusting the fuel in the furnace <NUM>. As a result of the combustion, flue gas is produced. The method comprises conveying a mixture M comprising the flue gas and the bed material from the furnace <NUM> to a particle separator <NUM> and separating from the mixture M both (i) a bed material fraction BM and (ii) a flue gas fraction FG. The bed material fraction BM and the flue gas fraction FG are separated with the particle separator <NUM>. The method comprises conveying at least a part of the bed material fraction BM from the particle separator to a first heat exchanger <NUM> comprising a heat exchanger tube <NUM>, and recovering heat from the at least part of the bed material fraction BM with the heat exchanger tube <NUM>. As indicated above, the heat exchanger tube <NUM> is arranged upstream from the furnace <NUM>. More specifically, the heat exchanger tube <NUM> is arranged upstream from the furnace <NUM> in the direction of bed material flow, but not further upstream from the furnace <NUM> than the particle separator <NUM>. In other words, the heat exchanger tube <NUM> is arranged, in the direction of flow of the bed material fraction in the bed material circulation between the particle separator <NUM> and the furnace <NUM> to such a location that a distance between heat exchanger tube <NUM> and the particle separator <NUM> is less than a distance between the furnace <NUM> and the particle separator <NUM>, wherein the distances are measured along a path that starts from the particle separator <NUM>, runs from the particle separator <NUM> along the path the bed material flows, and ends to the particle separator <NUM> after one full round.

As shown in <FIG>, the transfer tube <NUM> is arranged upstream from the furnace <NUM> and downstream from the particle separator <NUM>. Moreover, in the direction of flow of the bed material fraction in the bed material circulation, the the heat exchanger tube <NUM> is arranged between the particle separator <NUM> and the furnace <NUM>. However, the heat exchanger tube <NUM> is not arranged in the primary first channel <NUM>, which is arranged upstream from the particle separator. Therefore, the heat exchanger tube <NUM> is arranged to such a location as discussed above. in <FIG> the distance of the heat exchanger tube <NUM> from the particle separator <NUM> is about <NUM>/<NUM> of a full round. However, a distance of the furnace <NUM> from the particle separator <NUM> (as defined above) is about a third of a full round. Moreover, if the heat exchanger tube was arranged in the primary first channel <NUM> after the furnace <NUM>, the distance would be at least <NUM>/<NUM> of a full round.

In line with these measures, the heat exchanger tube <NUM> is arranged upstream from the furnace <NUM> in the direction of bed material flow, but not further upstream from the furnace <NUM> than the particle separator <NUM>.

When the flue gas fraction FG and the bed material fraction are separated from the mixture M, typically only about <NUM> % to <NUM> % of the flue gases flow with bed material fraction BM. This indicates why it is much more efficient to feed the reagent R to the material fraction BM than e.g. to the furnace <NUM> or to the flue gas fraction FG.

An embodiment of the method comprises conveying only a part of the bed material fraction BM from the particle separator <NUM> to heat exchanger tube <NUM>. In this case, the other part may be conveyed through a bypass <NUM> (see <FIG>). However, in an embodiment and/or at least at certain point of time, an embodiment comprises conveying all the bed material fraction BM from the particle separator <NUM> to a heat exchanger comprising the heat exchanger tube <NUM>.

An embodiment of the method comprises conveying only a part of the bed material fraction BM from the heat exchanger tube <NUM> to the furnace <NUM>. In this embodiment, another part of the bed material fraction BM may be removed from the circulation e.g. from the heat exchanger <NUM> e.g. to a bottom ash cooler for further processing (not shown). An embodiment of the method comprises conveying the whole bed material fraction BM from the heat exchanger tube <NUM> to the furnace <NUM>. In this embodiment, a part of the bed material (i.e. the bottom ash) may be removed from the furnace <NUM> as detailed above.

To convert the alkali chlorides to alkali sulphates upstream from the heat exchanger tube <NUM>, the method comprises feeding the sulphur-containing reagent R to the bed material fraction BM upstream from the heat exchanger tube <NUM>. More specifically, the method comprises feeding the sulphur-containing reagent R to the bed material fraction BM upstream from the heat exchanger tube <NUM>, but not further upstream than the particle separator <NUM>. What has been said about the term "upstream" applies.

Referring to <FIG>, in use, the bed material (or a part thereof) runs from the particle separator <NUM> through a primary second channel <NUM> to the heat exchanger tube <NUM>, and through a primary third channel <NUM> at least a part thereof runs to the furnace <NUM>. In the furnace <NUM> the mixture comprising the bed material forms and flow to and through a primary first channel <NUM> in this way ending up again to the particle separator <NUM>. The primary first channel <NUM> may be an opening in a wall of the furnace <NUM> allowing for the mixture to travel to the particle separator <NUM> (see <FIG>). The primary third channel <NUM> may be an opening in a wall of the furnace <NUM> allowing for the bed material fraction, optionally together with fluidizing gas fed to a heat exchanger <NUM>, to travel from the heat exchanger <NUM> to the furnace <NUM> (see <FIG>).

According to the invention, a fluidized bed boiler <NUM> of the circulating type comprises a furnace <NUM>, in which a fluidized bed comprising a mixture M of solid bed material and gas (air and/or flue gas) is configured to form.

The fluidized bed boiler <NUM> comprises a particle separator <NUM>, which is configured to separate from the mixture M, (i) the solid bed material fraction BM and (ii) the flue gas fraction FG. The fluidized bed boiler <NUM> comprises the primary first channel <NUM> configured to convey the mixture M from the furnace <NUM> to the particle separator <NUM>.

In order to recover heat from the bed material fraction BM or the part thereof, the fluidized bed boiler comprises the first heat exchanger <NUM> comprising the heat exchanger tube <NUM>. What has been said about the location of the heat exchanger tube <NUM> in connection with the method, applies. Moreover, as detailed above, the heat exchanger tube <NUM> is configured to recover heat from the bed material fraction BM or the part thereof. Therefore, the fluidized bed boiler comprises a primary second channel <NUM> configured to convey at least a part of the bed material fraction BM from the particle separator <NUM> to the first heat exchanger <NUM>.

Moreover, to return at least a part of the bed material to the furnace <NUM>, the fluidized bed boiler comprises a primary third channel <NUM>, which is configured to convey at least a part of the bed material fraction BM from the first heat exchanger <NUM> to the furnace <NUM>. What has been said about conveying a part directly to a bottom ash cooler from the heat exchanger <NUM> in connection with the method applies.

In order to feed the sulphur-containing reagent R to such a location detailed above, the fluidized bed boiler <NUM> comprises means <NUM> for feeding a sulphur-containing reagent R to the bed material fraction BM upstream from the heat exchanger tube <NUM>.

In an embodiment, the particle separator <NUM> comprises a cyclone. In an embodiment, the particle separator <NUM> is a cyclone. This has the benefit that the particle separator <NUM> can be continuously operated at a high temperature. Filters, such as perforated plates, e.g. perforated metal plates, could be used for the purpose of separating bed material and flue gas from their mixture in addition or as an alternative to the cyclone.

Even if the method is defined as being a method for operating a circulating fluidized bed boiler, the method is also a method for reducing corrosion of a fluidized bed heat exchanger of a circulating fluidized bed boiler, because the first heat exchanger <NUM> is a fluidized bed heat exchanger. As detailed below, a (bubbling) fluidized bed comprising the bed material fraction BM (or the part of the bed material fraction) may be arranged at a vicinity of the heat exchanger tube <NUM>.

Concerning the sulphur-containing reagent R and, correspondingly, the means <NUM> for feeding the reagent, in an embodiment, the sulphur-containing reagent R comprises at least one of elementary sulphur (S) and a sulphate (i.e. a compound comprising the ion SO<NUM><NUM>-). Elementary sulphur may be used in solid form, e.g. in the form of granules or powder. As for the sulphates, they may be supplied in liquid form. However, it has been found that ferric sulphate (Fe<NUM>(SO<NUM>)<NUM>) increases agglomeration of the particles of the bed material fraction BM. Therefore, if a sulphate is used as or in the reagent R, preferably, the sulphate is not ferric sulphate. As for the sulphates, diethyl sulphate ((C<NUM>H<NUM>)<NUM>SO<NUM>) and/or dimethyl sulphate ((CH<NUM>)<NUM>SO<NUM> are most preferable. However, in general, elementary sulphur is even more preferable, as it is cheaper to use. Therefore, in an embodiment, the sulphur-containing reagent R comprises at least one of elementary sulphur S and a sulphate that is not ferric sulphate, e.g. diethyl sulphate ((C<NUM>H<NUM>)<NUM>SO<NUM>) or dimethyl sulphate ((CH<NUM>)<NUM>SO<NUM>). More preferably, the sulphur-containing reagent R comprises solid elementary sulphur (S). All these compounds produce sulphur oxide SOx (e.g. SO<NUM> and/or SO<NUM>, see Eqns. <NUM> and <NUM>) in the hot environment.

Solid elementary sulphur, e.g. in the form of granules and/or powder, may be fed to the fluidized bed boiler <NUM> through a pipeline. A carrier gas, such as air, may be used to facilitate feeding. For these reasons, in an embodiment, the means <NUM> for feeding the sulphur-containing reagent R is configured to feed the sulphur-containing reagent R in a form of solid granules and/or powder. In a preferable embodiment, the means <NUM> for feeding the sulphur-containing reagent R is configured to feed the sulphur-containing reagent R in a form of solid granules and/or powder by using a carrier gas, e.g. air, with which the solid granules and/or powder flow to a feeding location FL. In this embodiment, the means <NUM> comprises a pipeline configured to convey the granules and/or the powder. In addition or as an alternative to the carrier gas, transfer of the granules and/or powder within the means <NUM> may be facilitated by a conveyor, e.g. a conveyor screw. Thus, in an embodiment, the means <NUM> for feeding the sulphur-containing reagent R comprises a pipeline. In an embodiment, the means <NUM> for feeding the sulphur-containing reagent R comprises a conveyor, such as a screw conveyor.

Referring to <FIG> and <FIG>, the sulphur-containing reagent R is fed to a feeding location FL. Correspondingly, the means <NUM> for feeding the sulphur-containing reagent R is configured to feed the sulphur-containing reagent R to the feeding location FL.

Typically the particle separator <NUM>, e.g. a cyclone, functions in such a way that the separated bed material fraction BM accumulates to and/or escapes from the particle separator <NUM> to/from a lower part of the particle separator <NUM>. Correspondingly, the reagent R can be fed [i] downstream from the particle separator <NUM>, but not further downstream than the heat exchanger tube <NUM> and/or [ii] to a lower part of the particle separator <NUM>. Therefore, in an embodiment of the fluidized bed boiler <NUM>, the feeding location FL is arranged in a lower part of the particle separator <NUM> or in a direction of a flow of the bed material fraction BM, downstream from the particle separator <NUM> and upstream from the heat exchanger tube <NUM>. As detailed above, the heat exchanger tube <NUM> is arranged upstream from the furnace <NUM>. More specifically, the feeding location FL is arranged as follows:.

Naturally, a part of the reagent R may be supplied to the lower part of the particle separator <NUM> (see option [A] above) and another part or other parts may be supplied between the particle separator <NUM> and the heat exchanger tube <NUM> (see option [B] above).

In terms of distances along the flow path of the bed material, the feeding location FL is arranged [A] in a lower part of the particle separator <NUM> or [B] such that in the direction of flow of the bed material fraction in the bed material circulation between the particle separator <NUM> and the furnace <NUM> a distance between feeding location FL and the particle separator <NUM> is less than a distance between the heat exchanger tube <NUM> and the particle separator <NUM>, wherein the distances are measured along a path that starts from the particle separator <NUM>, runs from the particle separator <NUM> along the path the bed material flows, and ends to the particle separator <NUM> after one full round.

As detailed above, the consumption of the sulphur-containing reagent R is diminished because the reagent R is fed only to the separated bed material fraction BM, which comprises only a little flue gas, and thereby only a little alkali chlorides. Referring specifically to <FIG> and <FIG>, the separation of bed material fraction BM and flue gas fraction FG can be enhanced by providing the circulating fluidized bed boiler <NUM> with a loopseal. In general, a loopseal is used in circulating fluidized bed systems to convey particles from a low pressure region to a high pressure region. One of the principal functions of a loopseal is to avoid the undesirable inverse gas/particle flow and to provide gas tightness. Typically, a loopseal comprises at least two chambers, i.e. a first chamber <NUM> and a second chamber <NUM> (see <FIG>, <FIG>, <FIG>, and <FIG>). To ensure proper circulation of bed material fraction, the loopseal, if arranged, is arranged downstream from the particle separator <NUM>, but not further downstream than the furnace <NUM>.

The loopseal forms a part of the circulating fluidized bed boiler <NUM>. Thus in an embodiment, the circulating fluidized bed boiler <NUM> comprises the first chamber <NUM> and the second chamber <NUM>. Moreover, the loopseal is used to prevent the inverse circulation of the bed material fraction, whereby the loopseal is provided in between the particle separator <NUM> and the furnace <NUM> such that in use, the bed material fraction flows from the particle separator <NUM> to the loopseal and from the loopseal to the furnace (not vice versa). Therefore, the first chamber <NUM> is arranged downstream from the particle separator <NUM> (but not further downstream than the second chamber <NUM>) in a direction of a flow of the bed material fraction BM, and the second chamber <NUM> is arranged downstream from the first chamber <NUM> but not further downstream than the furnace <NUM> in a direction of a flow of the bed material fraction BM.

Concerning an embodiment of the method, i.e. in a use of the circulating fluidized bed boiler <NUM>, the bed material fraction BM enters the first chamber <NUM> of the circulating fluidized bed boiler <NUM> at a first level I1 and exits the first chamber <NUM> at a second level I2. The term "level" refers to distance from ground level, thereby determining a height at which the bed material enters/exits the chamber <NUM>, <NUM>. The second level I2 is arranged lower than the first level I1. The levels I1 and I2 are shown in <FIG>. It is noted that the direction arrow Sz in these figures points upwards, i.e. reverse to gravity. Furthermore, in use, the bed material fraction BM enters a second chamber <NUM> of the circulating fluidized bed boiler <NUM> at a third level I3 and exits the second chamber <NUM> at a fourth level I4. The fourth level I4 is arranged higher than the third level I3. The levels I3 and I4 are shown in <FIG>.

Concerning an embodiment of the circulating fluidized bed boiler, the bed material fraction is configured to enter and exit the chamber <NUM>, <NUM> as discussed above.

In a preferable embodiment, as shown in <FIG>, <FIG>, and <FIG>, the second chamber <NUM> is arranged upstream from the heat exchanger tube <NUM> in the direction of a flow of the bed material fraction BM. More specifically, the second chamber <NUM> is arranged upstream from the heat exchanger tube <NUM> in the direction of a flow of the bed material fraction BM, but not further upstream than the first chamber <NUM>. To some extent this improves the separation of the bed material fraction already upstream from the heat exchanger tube <NUM>.

Preferably, in such an embodiment, the feeding location FL is arranged in the second chamber <NUM> (as shown in <FIG>) or in a direction of a flow of the bed material fraction BM, downstream from the second chamber <NUM> (as shown in <FIG>), but not further downstream than the heat exchanger tube <NUM>. The direction "downstream" refers to the direction of flow of the bed material fraction BM.

Concerning the position of the chambers <NUM>, <NUM>, in an embodiment, the bed material fraction BM is configured to flow substantially downwards in the first chamber <NUM> and substantially upwards in the second chamber <NUM>. Reference is made to <FIG>.

As detailed above, the corrosive effect of the alkali chlorides is most prominent at a certain temperature window, which most often would occurs only in a last heat exchanger configured to heat the heat transfer medium. Herein the term "last" refers to the direction of flow of heat transfer medium within a circulation of the heat transfer medium. Such circulation is shown e.g. in <FIG>, the heat transfer medium therein flowing from the heat exchanger <NUM> to the heat exchanger <NUM>, therefrom to the heat exchanger <NUM>, and therefrom to the first heat exchanger <NUM>, which is also the last heat exchangers in this circulation. After the last heat exchanger, the heat transfer medium is conveyed for use; most typically at this stage, the heat transfer medium is superheated steam, which is conveyed to a steam turbine combined with a generator for the production of electricity.

Thus, in an embodiment, the first heat exchanger <NUM> comprising the heat exchanger tube <NUM> is such a last heat exchanger that no heat exchanger that is arranged to be in contact with flue gas is arranged to a location that is downstream from the first heat exchanger <NUM> (but not further downstream than a steam turbine, if a steam turbine is used), the term "downstream" here referring to the direction of flow of the heat transfer medium flowing within the circulation of the heat transfer medium. Thus, in such a case, the first heat exchanger <NUM> is 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 exchanger <NUM> is arranged to be in contact with flue gas. Naturally, when the system comprises a steam turbine, downstream from the steam turbine the steam may condense and be recirculated back to an economizer, e.g. the heat exchanger <NUM>.

In other words, in such heat exchangers that are arranged upstream from the last heat exchanger (in the direction of flow of the heat transfer medium), the heat transfer medium may be cooler, whereby also a temperature of an outer surface of a tube of that heat exchanger may be, in use, cooler than the lower critical temperature, above which the corrosion is most prominent. Reference is made to the section "Background". Notable, reducing corrosion e.g. by feeding sulphur onto heat exchanger tubes of such other heat exchangers is not needed. Correspondingly feeding the sulphur-containing reagent R only to one or more locations as discussed above reduces the consumption of the reagent R. Thus, an embodiment of the method comprises feeding the sulphur-containing reagent R only to the bed material fraction BM (or only to a part of the bed material fraction BM) and only to one or more location(s) that is/are upstream from the heat exchanger tube <NUM>. The proper location has been discussed in more specific terms above. In case the reagent R is fed to multiple locations, what has been said above for the feeding location FL applies to each of the multiple locations.

Naturally, a minor amount of the reagent R (or other sulphur-containing reagent) can be fed also elsewhere, but that seems not to have a technical effect on corrosion, at least provided that the temperature window of the other heat transfer surfaces remain outside the temperature window, in particular lower than the lower critical temperature. Furthermore, typically a fluidized bed boiler is equipped with multiple heat exchanger to effectively recover heat from the flue gases and/or from the bed material and/or received by radiation at the other heat exchanger.

Thus, an embodiment of the method comprises feeding a first amount M1 of the sulphur-containing reagent R to the bed material fraction BM (i.e. the separated bed material), to one or more location(s) that is/are upstream from the heat exchanger tube <NUM>. In case the reagent R is fed to multiple locations, what has been said above for the feeding location FL applies to each of the multiple locations. Moreover, the amount M1 in such a case refers to the total amount of the sulphur-containing reagent R fed to the multiple locations. The first amount M1 refers to a rate (e.g. kg/h). The first amount M1 may be an average value taken e.g. over an hour of operation.

Moreover, as detailed above, an embodiment comprises not feeding the sulphur-containing reagent R to any one of: the flue gas fraction FG, the primary first channel <NUM>, or the furnace <NUM>.

However, if, for some reason, some sulphur-containing reagent (same reagent R or other reagent) was fed elsewhere, preferably that amount would not be significant compared to the first amount M1. More specifically, such an embodiment comprises feeding a second amount M2 of the sulphur-containing reagent R and/or other sulphur-containing reagent to the flue gas fraction FG, the primary first channel <NUM> and/or the furnace <NUM>, wherein a ratio (M1/M2) of the first amount M1 to the second amount M2 is at least one third <NUM>/<NUM>. Preferably the ratio M1/M2 is at least one, more preferably at least <NUM>. As detailed above, a non-zero second amount M2 seems not to have any beneficial technical effect when the operating temperatures are proper. If the sulphur-containing reagent R and/or other sulphur-containing reagent is fed to more than one of the flue gas fraction FG, the primary first channel <NUM> and/or the furnace <NUM>, the second amount refers to the total amount of the reagent fed to these places. The second amount M2 refers to a rate (e.g. kg/h). The second amount M2 may be an average value taken e.g. over an hour of operation.

However, feeding a lot of sulphur to the fluidized bed boiler has the disadvantage of increasing the need for removing sulphur from the flue gas fraction FG. Therefore, preferable the second amount M2 is zero (i.e. the sulphur-containing reagent R is not fed to any one of: the flue gas fraction FG, the primary first channel <NUM>, or the furnace <NUM>).

To have a suitably low temperature at a surface of the second heat exchanger (<NUM>, <NUM>, <NUM>) the circulating fluidized bed boiler <NUM> according to the invention comprises a pipeline <NUM> for conveying heat transfer medium from the second heat exchanger (<NUM>, <NUM>, <NUM>) to the first heat exchanger <NUM>, optionally via another heat exchanger. For example, in <FIG>, the pipeline <NUM> conveys water/steam also from the heat exchanger <NUM> to the first heat exchanger <NUM>, via the heat exchanger <NUM>. In contrast, in in <FIG>, the pipeline <NUM> conveys water/steam from the heat exchanger <NUM> to the first heat exchanger <NUM> directly. In use, i.e. in an embodiment according to the invention of the method, the pipeline <NUM> conveys heat transfer medium from the second heat exchanger (<NUM>, <NUM>, <NUM>) to the first heat exchanger <NUM>. Because of the pipeline, the second heat exchanger (<NUM>, <NUM>, <NUM>) is arranged upstream (in the direction of heat transfer medium circulation) from the first heat exchanger, whereby a temperature of an outer surface of a tube of the second heat exchanger (<NUM>, <NUM>, <NUM>) remains sufficiently low.

As for the proper temperature, in an embodiment, a temperature of an outer surface of a second heat exchanger tube of the second heat exchanger (<NUM>, <NUM>, <NUM>) of is less than <NUM>. In such a case, the temperature is sufficiently low for not having significant corrosion problems on the an outer surface of a second heat exchanger tube of the second heat exchanger (<NUM>, <NUM>, <NUM>).

However, the feeding of the sulphur-containing reagent R is particularly beneficial, when a temperature of an outer surface of the heat exchanger tube <NUM> is within a critical temperature window, as detailed above. Therefore, in an embodiment, a temperature of an outer surface of the heat exchanger tube <NUM> is more than <NUM>, such as from <NUM> to <NUM>. It is possible to also increase a surface temperature of the heat exchanger tube <NUM> by applying an insulator 201c, as detailed below.

To efficiently recover heat, according to the invention, the circulating fluidized bed boiler <NUM> comprises the second heat exchanger (<NUM>, <NUM>, <NUM>) to recover heat from the flue gas fraction FG, as shown in <FIG>. In an embodiment, the second heat exchanger <NUM> is configured to recover heat from the mixture M as shown in <FIG>. The fluidized bed boiler <NUM> may comprise multiple second heat exchangers (<NUM>, <NUM>, <NUM>) configured to recover heat from the flue gas fraction FG or from the mixture M, as shown in <FIG>.

Using the fluidized bed boiler <NUM> should only have as minor environmental effects as possible. This applies at least to emissions of sulphur oxides SOx. In order to reduce sulphur oxide emissions, according to the invention, the circulating fluidized bed boiler <NUM> comprises a sulphur removal unit <NUM> and a channel <NUM> configured to convey the flue gas fraction FG to the sulphur removal unit <NUM> (see <FIG>) for removing sulphur from the flue gas fraction FG. Sulphur can be removed by from the flue gas fraction means known as such. For example, an embodiment comprises removing sulphur oxide (SOx) from the flue gas fraction FG in a wet scrubber. The wet scrubber may be comprised by the sulphur removal unit <NUM>. The sulphur removal unit <NUM> may be a wet scrubber. An aqueous and alkaline scrubbing solution may be used in the wet scrubber for removing the sulphur oxides.

The effect of not feeding sulphur e.g. to flue gas (i.e. the second amount M2 being zero), or feeding only a minor amount of sulphur e.g. to flue gas (i.e. the ratio M1/M2 being as discussed above) has the further effect that a smaller sulphur removal unit <NUM> suffices, compared to the case where relatively large amounts of sulphur was fed also to other locations that those discussed above. Thus, feeding the reagent R as discussed above diminishes the investment and operating costs related to the sulphur removal unit <NUM>.

According to the invention, the circulating fluidized bed boiler <NUM> comprises a second heat exchanger (<NUM>, <NUM>) configured to recover heat from the flue gas fraction FG to the heat transfer medium; and the second heat exchanger (<NUM>, <NUM>) is arranged downstream from the particle separator <NUM> in the direction of flow of the flue gas fraction FG, but not further downstream than the sulphur removal unit <NUM>.

As detailed above, the corrosion problem of the heat exchanger tube <NUM> can be further reduced by increasing a temperature of an outer surface of the heat exchanger tube <NUM>. It has been found that the temperature of an outer surface of the heat exchanger tube <NUM> can be increased by reducing thermal conductivity of the heat exchanger tube <NUM>. This can be done by providing thermal insulation in between the inner and outer surfaces of the heat exchanger tube <NUM>. Therefore, and with reference to <FIG>, in an embodiment, the heat exchanger tube <NUM> comprises an inner tube 201a, an outer tube 201b radially surrounding the inner tube 201a, and thermally insulating material 201c arranged in between the inner tube 201a and the outer tube 201b. Preferably, the thermal conductivity of the thermally insulating material 201c is from <NUM> W/mK (Watt per metre Kelvin) to <NUM> W/mK at <NUM>.

To promote the bed material circulation particularly near the heat exchanger tube <NUM>, the bed material fraction BM in a vicinity of the heat exchanger tube <NUM> is fluidized. Referring to <FIG> and <FIG>, an embodiment of the circulating fluidized bed boiler <NUM> comprises nozzles <NUM> configured to fluidize the bed material fraction BM in the vicinity of the heat exchanger tube <NUM>. On one hand the fluidizing of the bed material fraction BM facilitates the flow of the bed material fraction BM. On the other hand, when the fluidizing gas comprises oxygen O<NUM>, the fluidizing gas enhances the reactions of the Equations (<NUM>) and (<NUM>) and in this way helps to reduce the corrosion problems. Thus, an embodiment comprises fluidizing at least a part of the bed material fraction BM in a vicinity of the heat exchanger tube <NUM> by feeding fluidizing gas, wherein the fluidizing gas comprises oxygen O<NUM>. Preferably the fluidizing gas further comprises humidity H<NUM>O. Air may be used as the fluidizing gas. When the system comprises a bypass <NUM>, not all the bed material needs to be fluidized in the vicinity of the heat exchanger tube <NUM>. As an alternative to fluidizing gas, oxygen-containing gas may be fed to a location that is arranged downstream from the particle separator <NUM> but not further downstream than the heat exchanger tube <NUM>. As detailed above, the oxygen-containing gas may further be used for fluidizing the bed material fraction BM. The oxygen-containing gas may comprise humidity (H<NUM>O). Preferably, the oxygen-containing gas does not comprise a lot of corrosive compounds of the flue gas, because this would increase the consumption of the sulphur-containing reagent R.

More specifically, an embodiment of the circulating fluidized bed boiler <NUM> comprises a first compartment <NUM>, in which the heat exchanger tube <NUM> is arranged. The term "compartment" refers to a space surrounded by walls. Moreover, the nozzles <NUM> are configured to fluidize the bed material fraction BM in the first compartment <NUM>. Preferably, the nozzles <NUM> are arranged at a bottom of the first compartment <NUM>. Thus, by feeding fluidizing gas, e.g. air, through the nozzles <NUM> the circulation of bed material fraction BM can be enhanced. Naturally, corrosive compounds, e.g. alkali chlorides, should not be fed with the fluidizing gas to the first compartment <NUM>. In such an arrangement, the means <NUM> for feeding the sulphur-containing reagent R to the bed material fraction BM may be configured to.

What has been said above about the content of fluidizing gas, applies.

Referring to <FIG>, a preferable embodiment comprises:.

Even if not shown in the figures, nozzles for feeding fluidizing air for improving the bed material circulation may be provided also to other compartments or chambers <NUM>, <NUM>, if considered feasible. In particular, in the embodiments of <FIG> and <FIG>, both the chambers <NUM>, <NUM> may be provided with such nozzles. In the embodiment of <FIG>, a bottom of the channel <NUM> may be provided with such nozzles.

In the embodiments of <FIG>, <FIG>, and <FIG> the fluidizing gas fed through the nozzles <NUM> may be conveyed with the bed material fraction BM to the furnace through the primary third channel <NUM> (which may be called a return channel).

In the embodiment of <FIG>, the circulating fluidized bed boiler <NUM> is provided with an auxiliary channel <NUM> and an opening <NUM>. Nozzles for feeding fluidizing air may be provided at a bottom of the auxiliary channel <NUM>. The auxiliary channel <NUM> may be used to guide some of the bed material to the first heat exchanger <NUM> from the furnace <NUM> directly. It may be possible that a content of corrosive compounds at a bottom of the furnace <NUM> is so low that only a little alkali chlorides of the flue gases pass from the furnace <NUM> to the heat exchanger <NUM> through the auxiliary channel <NUM>. The opening <NUM> can be used is some operating conditions for guiding bed material directly from the furnace <NUM> to the first heat exchanger. Thus, a part of the bed material in the furnace will not propagate to the particle separator <NUM>, but can be circulated to the first heat exchanger <NUM> via the opening <NUM>, which may serve the purpose of a particle separator. Typically also the opening <NUM> decreases an amount of flue gases near the heat exchanger tube <NUM> compared to the furnace. Moreover, in regular use of the circulating fluidized bed boiler, the bed material circulates at least from the furnace <NUM> to the particle separator <NUM>, therefrom to the first heat exchanger <NUM>, and therefrom back to the furnace <NUM>. Because the sole purpose of the first heat exchanger <NUM> is to recover heat, the bed material is not conveyed to the furnace only through the bypass opening <NUM>.

In the embodiments of <FIG>, <FIG>, and <FIG>, the first heat exchanger <NUM> only receives bed material through the particle separator <NUM>. Thus, in an embodiment, no other bed material than the bed material fraction BM that has been separated in the particle separator <NUM> is conveyed to the first heat exchanger <NUM>. As detailed below, not all the separated bed material needs to be conveyed to the heat exchanger tube <NUM>. Fluidizing gas fed through the nozzles <NUM> may be conveyed with the bed material fraction BM to the furnace <NUM> through the primary third channel <NUM> (which may be called a return channel).

It is noted that in the embodiment of <FIG>, the auxiliary channel <NUM> could be used, in the alternative, to expel bed material fraction BM from the heat exchanger <NUM> so that the bed material fraction BM or a part of the bed material fraction BM bypasses the heat exchanger tube <NUM>. It may be beneficial to be able to let a part of the bed material fraction BM bypass the heat exchanger tube <NUM>, because the temperature of the heat transfer medium downstream from the first heat exchanger <NUM> (in the direction of the flow of the heat transfer medium) needs often be carefully controlled.

For a detailed structure of a suitable heat exchanger having a bypass reference is made to the publications <CIT>, <CIT>, and <CIT>.

The loopseal heat exchangers presented therein can be used as the first heat exchanger <NUM> of the present invention. If a heat exchanger of the publication <CIT> is used, preferably the sulphur-containing reagent R is fed to the feeding chamber <NUM> of the heat exchanger of <FIG> of the publication <CIT>. However, the sulphur-containing reagent R may be fed e.g. to the inlet chamber <NUM> of the heat exchanger of any one of <FIG>, <FIG> of the publication <CIT> and/or to the channel <NUM> of <FIG> of the publication <CIT>. If a heat exchanger of the publication <CIT> is used, preferably the sulphur-containing reagent R is fed to the feeding chamber <NUM> of <FIG> of the publication <CIT> or to the channel <NUM> of <FIG> of the publication <CIT>. If a heat exchanger of the publication <CIT> is used, preferably the reagent R is fed to the feeding chamber A of the heat exchanger of the <FIG> of the publication <CIT>. In addition or alternatively, the reagent R may be fed e.g. to the inlet pipe <NUM> of the <FIG> of the publication <CIT>. For another detailed structure of a suitable heat exchanger reference is made to the publication <CIT>.

If a heat exchanger of the publication <CIT> is used, preferably the reagent R is fed to the feeding upleg <NUM> of the heat exchanger of the Figs. 4a to 5b the publication <CIT>. In addition or alternatively, the reagent R may be fed e.g. to the dip leg <NUM> of the publication <CIT>.

In an embodiment, the circulating fluidized bed boiler <NUM> comprises a bypass <NUM> (see <FIG>) that is configured to guide a part of the bed material fraction BM from the particle separator <NUM> to the furnace <NUM> (e.g. via the primary third channel <NUM> of <FIG>) such that the part of the bed material fraction BM that travels through the bypass <NUM> does not come into contact with the heat exchanger tube <NUM>.

When the circulating fluidized bed boiler <NUM> comprises the bypass <NUM>, the circulating fluidized bed boiler <NUM> comprises a splitting chamber <NUM> configured to guide a first part of the bed material fraction BM to the heat exchanger tube <NUM> and a second part to the bypass <NUM>. Structures of bypasses are disclosed in the publications cited above, and the operating principle is also illustrated in <FIG>. However, for clarity, a splitting chamber <NUM> is shown also in <FIG>. In <FIG>, the primary second channel <NUM> extends downwards (in the negative Sz direction) to the splitting chamber <NUM>, which splits the bed material fraction to two parts. The splitting chamber <NUM> divides the bed material fraction to the first part, which is conveyed to the heat exchanger tube <NUM> (i.e. onto the heat exchanger tube <NUM>, as the water/steam propagates in the tube). The second part flows through the bypass <NUM>. The division of the bed material to the first and second parts can be controlled by fluidizing gas feed as detailed e.g. in the cited publications. Preferably, in such an embodiment, the sulphur-containing reagent R is fed to the first part of the bed material fraction BM and downstream from the splitting chamber <NUM> (or into the splitting chamber <NUM>), but not further downstream than the heat exchanger tube <NUM>. This further reduces the consumption of the sulphur-containing reagent R, because the bypass <NUM> is (or at least may be) free from heat transfer surfaces, whereby the reagent R is not needed in the bypass <NUM>. Thus, the feeding location FL is, in an embodiment, arranged in the splitting chamber <NUM> or downstream from the splitting chamber <NUM>, but not further downstream than the heat exchanger tube <NUM>. Thus, the feeding location FL is, in an embodiment, arranged such that the sulphur-containing reagent R is fed to the first part of the bed material fraction (i.e. not conveyed to the bypass <NUM>).

Therefore, an embodiment comprises dividing the bed material fraction BM to a first part and a second part using a splitting chamber, guiding the first part of the bed material fraction to the to the heat exchanger tube <NUM>, guiding the second part of the bed material fraction through a bypass <NUM> so that the second part of bed material fraction BM does not come into contact with the heat exchanger tube <NUM>, and feeding the sulphur-containing reagent R to the splitting chamber <NUM> or downstream from the splitting chamber <NUM>, but not further downstream than the heat exchanger tube <NUM>. The bypass <NUM> may be arranged as a part of the heat exchanger <NUM> is detailed in the publications listed above and in <FIG>. In the alternative, a bypass may be arranged in parallel with the heat exchanger <NUM>.

Claim 1:
A fluidized bed boiler (<NUM>) of the circulating type, comprising
- a furnace (<NUM>), in which a fluidized bed comprising a mixture (M) of solid bed material and gas is configured to form,
- a particle separator (<NUM>) configured to separate from the mixture (M) a solid bed material fraction (BM) and a flue gas fraction (FG),
- a primary first channel (<NUM>) configured to convey the mixture (M) from the furnace (<NUM>) to the particle separator (<NUM>),
- a first heat exchanger (<NUM>) comprising a heat exchanger tube (<NUM>) configured to recover heat from at least a part of the bed material fraction (BM) and arranged upstream from the furnace (<NUM>),
- a primary second channel (<NUM>) configured to convey at least the part of the bed material fraction (BM) from the particle separator (<NUM>) to the first heat exchanger (<NUM>), and
- a primary third channel (<NUM>) configured to convey at least a part of the bed material fraction (BM) from the first heat exchanger (<NUM>) to the furnace (<NUM>),
- means (<NUM>) for feeding a sulphur-containing reagent (R) to the bed material fraction (BM) upstream from the heat exchanger tube (<NUM>),
characterized by
- a sulphur removal unit (<NUM>),
- a channel (<NUM>) configured to convey the flue gas fraction (FG) to the sulphur removal unit (<NUM>),
- a second heat exchanger (<NUM>, <NUM>) configured to recover heat from the flue gas fraction (FG), the second heat exchanger (<NUM>, <NUM>) being arranged downstream from the particle separator (<NUM>) and upstream from the sulphur removal unit (<NUM>) in the direction of flow of the flue gas fraction (FG), and
- a pipeline (<NUM>) configured to convey heat exchange medium from the second heat exchanger (<NUM>, <NUM>) to the first heat exchanger (<NUM>).