Patent Description:
The present invention relates to a circulating fluidized bed reactor having a vertical combustion chamber and a convection part which at least partly are formed by tube walls, and a vertical cyclone separator, the gas inlet channel of which is connected to the upper part of the combustion chamber and a return path for separated solids is connected to the lower part of the combustion chamber. Fluidized bed boilers, and particularly circulating fluidized bed boilers are known to be advantageous for combustion of great variety of solid fuels, such as fuels derived from various waste material while being configured to produce steam.

Document <CIT> discloses a furnace for combusting solid refuse in a fluidised bed. The flue gases pass from the furnace into a withdrawal chute, which constitutes the input to a return flue, and where from the gases pass onwards through a rear chute out of the boiler. The flue gases subsequently pass through gas cleaning equipment, which comprises a cyclone and a filter, before they are allowed to escape through a chimney to the air.

Document <CIT> discloses a fluidized bed boiler comprising a primary particle trap and a pass containing convective heat exchangers. The particle trap is located at the transition between the top of the reactor and a first vertically extending pass. The first vertically extending pass is an empty pass which does not include any inserted heat exchangers. Instead the boiler comprises an upward pass after the first vertically extending pass where a convective heat exchanger is located. There is a cyclone arranged to follow the upward pass.

<CIT> discloses a circulating fluidized bed boiler system using municipal solid waste as a single fuel. The boiler system a combustion chamber and a cyclone separator connected to the combustion chamber. There is disclosed a vertical heat exchange flue for the exhaust gas connected to the cyclone separator and a horizontal heat exchange flue provided with superheaters and economizers.

<CIT> discloses a fluidized bed combustion boiler for RDF fuel. The boiler has cyclone separator connected through a flue gas path to an empty pass and a convective heat transfer pass. The heat transfer pass comprises a superheater, an economizer and an air preheater.

All of the above mentioned publications disclose an in-line layout of the plant. This results in a considerably long, and cumbersome arrangement since the different parts of the boiler are located laterally one after the other in the course of exhaust gas flow.

An object of the invention is to provide a circulating fluidized bed boiler which enhances the compactness of the boiler considerably compared to the prior art solutions.

Objects of the invention can be met substantially as is disclosed in the independent claim and in the other claims describing more details of different embodiments of the invention.

According to the invention a circulating fluidized bed boiler comprises a vertically extending furnace, a separator unit and a cross over duct and an exhaust gas channel connected to the separator unit via the cross over duct. The exhaust gas channel comprises a first vertically extending pass and a horizontally extending pass and a second vertically extending pass, wherein the first vertically extending pass and the horizontally extending pass and the second vertically extending pass are arranged successively in the gas flow direction, when in use, such that the horizontally extending pass is configured to connect the first vertically extending pass and the second vertically extending pass with each other, and the horizontally extending pass is arranged below the separator unit which is arranged between the first vertically extending pass and the second vertically extending pass.

This way, the need of particularly floor space of the circulating fluidized bed boiler is minimized and/or utilized very efficiently.

According to an embodiment of the invention the furnace has a rectangular cross section and the horizontally extending pass extends parallel with a rear wall of the furnace.

According to an embodiment of the invention the furnace has a rectangular cross section and the horizontally extending pass extends parallel with a rear wall of the furnace and the cross over duct extends parallel with the rear wall of the furnace.

Rectangular shape of the cross section of the furnace together with the horizontally extending pass and the cross over duct directed parallel with the rear wall of the furnace decreases the required footprint of the boiler.

According to an embodiment of the invention a solids return system is arranged at least partly between the first vertically extending pass and the second vertically extending pass.

When the solids return system is fitted between the vertically extending exhaust gas passes the occupied volume of the circulating fluidized bed boiler is not increased by the solids return system but is mainly defined by the furnace, cross over duct and the vertical gas passes.

According to an embodiment of the invention the circulating fluidized bed boiler is provided with a fluidized bed solid material cooler in the solids return system providing a path of a return channel of separated solids, which fluidized bed solid material cooler is at least partially arranged between the first vertically extending pass and the second vertically extending pass.

Even if, in addition to a solids return channel, a fluidized bed solid material cooler or the solids return system is fitted between the vertically extending exhaust gas passes the occupied volume of the circulating fluidized bed boiler is still mainly defined by the furnace, cross over duct and the vertical gas passes.

According to an embodiment of the invention the furnace has a rectangular cross section and the horizontally extending pass extends parallel with a rear wall of the furnace, and the cross over duct extends parallel with the rear wall of the furnace, and a solids return system is arranged between the first vertically extending pass and the second vertically extending pass.

According to an embodiment of the invention the furnace has a rectangular cross section and the horizontally extending pass extends parallel with a rear wall of the furnace, and the cross over duct extends parallel with the rear wall of the furnace, and a solids return system is arranged between the first vertically extending pass and the second vertically extending pass, and the circulating fluidized bed boiler is provided with a fluidized bed solid material cooler in the path of a return channel of separated solids, which fluidized bed solid material cooler is at least partially arranged between the first vertically extending pass and the second vertically extending pass.

According to an embodiment of the invention the first vertically extending pass has a first end connected to an outlet of cross over duct, and a second end, and the second vertically extending pass has a first end and a second end, and the horizontally extending pass between the first and the second vertically extending pass is connected to the second end of the first vertically extending pass and the first end of the second vertically extending pass.

According to an embodiment of the invention the first vertically extending pass has a first end connected to an outlet of cross over duct, and a second end, and the second vertically extending pass has a first end and a second end, and the horizontally extending pass between the first and the second vertically extending pass is connected to the second end of the first vertically extending pass and the first end of the second vertically extending pass, and the first vertically extending pass and the second vertically extending pass are at right angle the horizontally extending pass.

According to an embodiment of the invention the first vertically extending pass is free from heat exchangers in its internal space. This provides an effect that the exhaust gases are cooled only by heat transfer to the walls of the first vertically extending pass from which any deposit may be easily removed by suitable rapping systems, compared to cleaning of internal heat exchanger bundles. This embodiment is feasible in connection with any other embodiment of the invention since the first vertically extending pass is an integral part of the invention.

According to an embodiment of the invention there are heat exchangers arranged to transfer heat from the exhaust gas in the exhaust gas channel as follows: in the first vertically extending pass is an empty pass free from heat exchangers in its internal space, the horizontally extending pass comprises at least one heat exchanger in its internal space and the second vertically extending pass comprises at least one heat exchanger in its internal space.

According to an embodiment of the invention a steam generation system is arranged in connection with the circulating fluidized bed boiler comprising economizer heat exchangers, evaporating heat exchangers and superheater heat exchangers, wherein the superheater heat exchangers are arranged in connection with the fluidized bed solid material cooler and the cross over duct, the evaporating heat exchangers are arranged in connection with the furnace, the separator unit and the horizontally extending pass and the economizer heat exchangers are arranged in connection with the second vertically extending pass, and the first vertically extending pass is free from heat exchangers in its internal space.

According to another embodiment of the invention a steam generation system is arranged in connection with the circulating fluidized bed boiler comprising economizer heat exchangers, evaporating heat exchangers and superheater heat exchangers, wherein the superheater heat exchangers are arranged in connection with the fluidized bed solid material cooler and the cross over duct, the evaporating heat exchangers are arranged in connection with the furnace, the separator unit, the horizontally extending pass and the second vertically extending pass, and the economizer heat exchangers are arranged in connection with the second vertically extending pass, and the first vertically extending pass is free from heat exchangers in its internal space.

Generally, an advantage of the design is that the gas passes are close to the furnace and the solid material fluidized bed cooler. This way, since there is a functionally empty gas pass involved, the overall design reduces the required room of the boiler and the length of steam piping. The layout provides also advantages in terms on connecting different steam generating stages in the steam system.

According to an embodiment of the invention the horizontally extending pass comprises independently supported modules each comprising an evaporating heat exchanger, which facilitates the service of the heat exchangers in the horizontally extending gas pass.

According to an embodiment of the invention the horizontally extending pass comprises independently supported modules each comprising a heat exchanger, which facilitates the service of the heat exchangers in the horizontally extending gas pass.

The exemplary embodiments of the invention presented in this patent application are not to be interpreted to pose limitations to the applicability of the appended claims. The verb "to comprise" is used in this patent application as an open limitation that does not exclude the existence of also unrecited features. The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims.

<FIG> depicts schematically a circulating fluidized bed (CFB) boiler <NUM> according to an embodiment of the invention. The circulating fluidized bed boiler <NUM> comprises a furnace <NUM>, a solids separator <NUM>, which may generally be referred to as a separator unit, and solids return system <NUM>, as well as a cross over duct <NUM> which connects the separator unit <NUM> with an exhaust gas channel <NUM>. The cross over duct <NUM> and the exhaust channel <NUM> are configured to lead the exhaust gases generated by combustion of fuel in the CFB boiler to further processing and eventually to the atmosphere, in most usual case. The circulating fluidized bed (CFB) boiler <NUM> is supported by a separate support structure which is not shown here for clarity reasons. In practical cases, relatively large boiler is arranged top-supported, i.e. it is supported so that the boiler is arranged to hang from a conventional rigid support steel structure extending around and above the boiler pressure body. Relatively small boilers may be arranged bottom-supported, wherein vertical load of the boiler is supported solely by a rigid support steel structure arranged below the boiler. The main difference between top-supported and bottom-supported constructions is that when the temperature of the boiler increases, thermal expansion of a top-supported boiler takes place mainly downwards whereas in a bottom-supported boiler thermal expansion takes place mainly upwards. A third alternative of supporting the boiler <NUM> in practise is to support it to a rigid support steel structure at its middle-section. Thereby, the lower portion of the boiler, below the middle section, is top-supported, and the upper portion of the boiler pressure body, above the middle section, is bottom supported. Middle-supported construction is advantageous while it reduces the size of the support steel structure from that needed for the top-supported boiler. Simultaneously such a middle-supported construction decreases the need for very strong walls of the boiler as is the case in bottom-supported boilers.

In the following the circulating fluidized bed boiler <NUM> is explained with a reference to the <FIG>. <FIG> shows the circulating fluidized bed boiler <NUM> from a side where the solids separator <NUM> is assembled, which is called here a back side of the circulating fluidized bed boiler <NUM>, the direction of which is indicated in the <FIG> by the arrow I. <FIG> shows a sectional view II-II of the <FIG> and the <FIG> show a sectional view III-III of the <FIG>. The furnace <NUM> of the CFB <NUM> is extending vertically and it has advantageously a rectangular cross section with tapering lower section thereof. There is a windbox <NUM> at the lower end of the furnace <NUM> for introducing fluidization gas into the furnace <NUM> through a grid. The cross section of the furnace <NUM> shown in the figure is of rectangular shape and it has a front wall <NUM>, a rear wall <NUM>, left wall <NUM> and a right wall <NUM>. The furnace <NUM> is connected to the solids separator <NUM> at its upper region by a flow channel <NUM> coupled to the rear wall <NUM> of the furnace <NUM>. The solids separator <NUM> advantageously consists of one cyclone separator having its central pipe as a gas outlet. In some practical applications the separator unit may be provided with more than one parallel cyclone separators instead of one or with other type of separator, such as an impingement separator. The separator is cooled comprising evaporating heat exchangers or surfaces <NUM> integrated to its walls, as is customary in the art. Advantageously the evaporating heat exchangers are coupled such that natural circulation of evaporating water based solution is obtained. In operation of the CFB <NUM> major part of solid materials entrained by the gases flowing from the furnace to the separator <NUM> is separated from the gas flow. At least part - usually a major portion - of separated solid material, herein referred to as solids, is returned back to furnace <NUM> via the solids return system <NUM>.

The exhaust gas channel <NUM> of the circulating fluidized bed boiler <NUM> is connected to the separator unit <NUM> through a substantially horizontally extending cross over duct <NUM>. The cross over duct <NUM> is arranged to connect the solids separator <NUM>, at its gas outlet, and the first vertically extending pass <NUM> at above the aforementioned parts. The walls of the cross over duct <NUM> are cooled, preferably steam cooled. So, the cross over duct <NUM> comprises cooled walls which are arranged as heat exchangers surfaces <NUM> and the internal gas space of the cross over duct <NUM> is empty i.e. free from internal heat exchangers.

The exhaust gas channel <NUM> comprises a first vertically extending pass <NUM> to which the cross over duct is connected. The exhaust gas channel <NUM> comprises further a horizontally extending pass <NUM> and a second vertically extending pass <NUM>. One end of the horizontally extending pass <NUM> is arranged in connection with a lower portion of the first vertically extending pass <NUM>. Another end of the horizontally extending pass <NUM> is arranged in connection with a lower portion of the second vertically extending pass <NUM>. The general flow direction of the gas in the circulating fluidized bed boiler is depicted by the arrows A in the figures, and the general flow direction of the separated solid is shown by the arrows B. The horizontally extending pass <NUM> has a length which is at least equal to the width of the rear wall <NUM> of the furnace <NUM> such that the first and the second vertically extending passes leave a space for the fluidized bed heat exchanger <NUM>.

The first vertically extending pass <NUM> has a first end <NUM>' connected to an outlet of the cross over duct <NUM> and a second end <NUM>" which is at opposite end part to the first end <NUM>'. The gas flow opening in the first end <NUM>' of the first vertically extending pass <NUM> is at its top, the circumference of the opening formed by the ends of the side walls of the first vertically extending pass <NUM>. The gas flow opening in the second end <NUM>" of the first vertically extending pass <NUM> is arranged to one of its side walls, such that the very end of the pass is closed by a wall. The second vertically extending pass <NUM> has a first end <NUM>' and a second end <NUM>". The horizontally extending pass <NUM> between the first and the second vertically extending passes is connected to the second end <NUM>" of the first vertically extending pass <NUM> and the first end <NUM>' of the second vertically extending pass <NUM>. There is a gas outlet <NUM> at the second end of the of the second vertically extending pass <NUM>. Advantageously there is a rigid connection between the furnace <NUM>, the separator unit <NUM> and the solids return system <NUM> and cross over duct, and expansion joint between the cross over duct <NUM> and first vertically extending pass <NUM> and the first vertically extending pass <NUM> and the horizontally extending pass <NUM>. The first vertically extending pass <NUM> and the second vertically extending pass <NUM> are parallel with each other and at right angle with the horizontally extending pass <NUM>. Each one of the passes form a straight conduit for the exhaust gas. The cross over duct is supported on the separator unit roof, which means that it does not need any additional support from steel structure.

As it becomes clear from the <FIG> the exhaust gas channel <NUM> is configured such that the cross over duct <NUM>, the first vertically extending pass <NUM> and the horizontally extending pass <NUM> as well as the second vertically extending pass <NUM> are arranged successively in the gas flow direction A such that the cross over duct <NUM> connects the separator unit <NUM> to the first vertically extending pass <NUM>, and the horizontally extending pass <NUM> is configured to connect the first vertically extending pass <NUM> and the second vertically extending pass <NUM> with each other. In other words, the gas is arranged to flow from the cross over duct <NUM> to the first vertically extending pass <NUM>, where, when the fluidized bed boiler is in use, the flue gas flows substantially downwards. The flue gas flows further from the first vertically extending pass <NUM> to the horizontally extending pass <NUM>, where the flue gas flows substantially horizontally. The flue gas flows still further from the horizontally extending pass <NUM> to the second vertically extending pass <NUM> where the flue gas flows substantially upwards. The cross over duct <NUM> is also extending horizontally and there the flue gas flows substantially horizontally, but substantially opposite direction to the gas flow direction in the horizontally extending pass <NUM>.

The separator unit <NUM> is situated at least partly between the first vertically extending pass <NUM> and the second vertically extending pass <NUM> and above the horizontally extending pass <NUM>. This becomes clear particularly from the <FIG>. The inner space of the furnace <NUM> and the separator unit <NUM> are arranged adjacently to each other. There is a line L1 running through the cross sections of the furnace <NUM> and the separator unit <NUM>, and a line L2 running through the cross sections of the first vertical pass <NUM> and the second vertical pass <NUM>. More particularly, in the <FIG> the line L1 is running through the centres of the cross sections of the furnace <NUM> and the separator unit <NUM>, and the line L2 running through the centres of the cross sections of the first vertical pass <NUM> and the second vertical pass <NUM>. And, as can be seen, the lines L1 and L2 are at an angle to each other. In the embodiments of figures the position of the separator unit <NUM> in respect to the furnace, and the vertical passes is such that there is a right angle between the lines L1 and L2, but if, for example, the separator unit <NUM> would be, for some practical reason, moved towards either of the vertical passes from its current position, the angle would still be substantially right angle. This means also that the horizontally extending pass <NUM> is extending parallel with both the rear wall <NUM> and the front wall <NUM> of the furnace <NUM> since the furnace <NUM> has rectangular cross section. In practise, it is worth noting that the direction of the cross over duct <NUM> is parallel with the direction of the horizontally extending pass <NUM>. And, as can be seen particularly in the <FIG> and <FIG> the separator unit <NUM> is not only at vertically higher level to the horizontally extending pass <NUM> but also directly above the horizontally extending pass18. <NUM> i.e. it is laterally at same position.

The solids return system <NUM> comprises a return channel <NUM> of separated solids, and it is arranged between the first vertically extending pass <NUM> and the second vertically extending pass <NUM>. There is a fluidized bed solid material cooler <NUM> in the path of a return channel of separated solids. The fluidized bed solid material cooler <NUM> is provided with one or more heat exchangers, according to the invention, for superheating steam, and therefore it can be referred to as a fluidized bed heat exchanger, as well, Also the fluidized bed solid material cooler <NUM> is at least mainly arranged between the first vertically extending pass <NUM> and the second vertically extending pass <NUM>, below the separator unit <NUM>. Even if not shown in the figures the solids return system <NUM> is provided with a loop seal arrangement for preventing back flow of gas from the furnace <NUM> to the separator unit <NUM>.

The circulating fluidized bed boiler according to the invention is particularly advantageous for combustion of solid waste derived fuel, such as biomass, sludges and refuse derived fuels having various compositions. The heat produced by combustion is utilized in steam generation. The compact design provided by the invention is particularly advantageous for CFB boiler for waste derived fuel. The circulating fluidized bed boiler may be a part of a power plant producing electric power. Typically the working medium is a water based solution. The CFB boiler <NUM> comprises a steam generation system, connected to a steam cycle applying for example approximately the rankine cycle. The steam generation system is arranged in connection with the circulating fluidized bed boiler such that it comprises economizer heat exchangers <NUM>, evaporating heat exchangers <NUM> and superheater heat exchangers <NUM>. The superheater heat exchangers <NUM> of the steam generation system are located in the fluidized bed solid material cooler <NUM> and in the cross over duct <NUM>, the evaporating heat exchangers <NUM> are located in the furnace <NUM> (cooled furnace wall), the separator unit <NUM>, in the horizontally extending pass <NUM>, and partly also in the second vertically extending pass18. The economizer heat exchangers <NUM> are located in the second vertically extending pass <NUM>. The first vertically extending pass <NUM> is free from heat exchanger bundles in its internal space <NUM>, and therefore, the first vertically extending pass <NUM> can be called in practice as an empty pass. The flue gas is cooled by the cooled wall of the first vertically extending pass <NUM> so as to avoid or at least mitigate chlorine and heavy metal corrosion.

All of the main parts of the CFB boiler <NUM> are substantially provided with heat insulation cover. Thanks to the layout of the CFB boiler <NUM> according to the invention the economizer heat exchangers <NUM>, evaporating heat exchangers <NUM> and superheater heat exchangers <NUM> are adjacently located with each other and the length of the necessary piping is therefore minimized, which also minimizes the need of insulation of the piping. In order to further ease the maintenance of the CFB boiler <NUM> the horizontally extending pass <NUM> comprises independently supported, assemblable and disassemblable heat exchanger modules <NUM>, each one of which is provided with an evaporating heat exchanger <NUM>. In the embodiment of the figures the horizontally extending pass <NUM> consists of four heat exchanger modules <NUM>, in which the number of the modules is selected as required by the practical application. The modules are configured to be removable in horizontal direction from the gas pass. The second vertically extending pass <NUM> is supported independently from bottom. Both of the vertically extending passes and the horizontally extending pass are equipped with proper amount of ash hoppers that can be designed as cooled or uncooled parts.

<FIG> shows a sectional view II-II according to another embodiment of the invention, which illustrates some possibilities to make adaptations to the boiler system <NUM> according to an embodiment of the invention. There is a line L1 running through the cross sections of the furnace <NUM> and the separator unit <NUM>, and a line L2 running through the cross sections of the first vertically extending pass <NUM> and the second vertically extending pass <NUM>. In this embodiment the second vertically extending pass <NUM> has greater cross sectional area compared to that shown in the <FIG>, having the channel cross section extended towards the furnace <NUM> from a general wall line of the horizontally extending pass <NUM>. Also, the empty pass <NUM> has smaller cross sectional area than that in the <FIG>. And, as can be seen, the lines L1 and L2 are also now at an angle to each other. In the boiler <NUM> according to the invention the furnace and the separator unit <NUM> are adjacent to each other in a first direction, which is generally in the direction of the line L1. The first and the second vertically extending pass and the horizontally extending pass <NUM> are arranged substantially in a vertical plane adjacently to each other in a second direction which is generally the direction of the rear wall <NUM> of the furnace <NUM>. The cross sectional areas of the first and the second vertically extending passes <NUM>, <NUM> are designed suitably according to the need of a specific practical application. It is conceivable that in some cases the cross sectional are of the empty pass <NUM> is greater than that of the second vertically extending pass <NUM>, but is the circumstances so requires the case may be vice-versa. The criteria may be desired gas velocity or pressure drop.

<FIG> depicts an embodiment of the invention by means of which benefits of the invention can be obtained in respect to at least the space saving of the exhaust gas channel <NUM>, while some more room is reserved between the exhaust gas channel <NUM> and the furnace <NUM>. The embodiment of the <FIG> differs from the one in the <FIG> such that the cross over duct <NUM> is at an angle to i.e. non-parallel with the longitudinal direction of the horizontally extending pass <NUM>. This way the horizontal distance between the exhaust gas channel <NUM> and the furnace <NUM> is increased compared to that shown in the <FIG>. It should be noted that the features shown in the <FIG>, <FIG> are technically compatible with each other to freely combine them with each other. Particularly, the feature shown in the <FIG>, providing more horizontal distance between the exhaust gas channel <NUM> and the furnace <NUM>, makes it possible to apply the increased cross section of the exhaust gas channel shown in the <FIG>. This is not shown in the <FIG> because this feature also allows using the space for other purposes, as well.

Claim 1:
A circulating fluidized bed boiler (<NUM>) comprising a vertically extending furnace (<NUM>), a separator unit (<NUM>), a cross over duct (<NUM>) and an exhaust gas channel (<NUM>) connected to the separator unit (<NUM>) via the cross over duct (<NUM>), wherein the exhaust gas channel comprises a first vertically extending pass (<NUM>) and a horizontally extending pass (<NUM>) and a second vertically extending pass (<NUM>), wherein the first vertically extending pass (<NUM>) and the horizontally extending pass (<NUM>) and the second vertically extending pass (<NUM>) are arranged successively in the gas flow direction such that the horizontally extending pass (<NUM>) is configured to connect the first vertically extending pass (<NUM>) and the second vertically extending pass (<NUM>) with each other, characterized in that the horizontally extending pass (<NUM>) is arranged below the separator unit (<NUM>) which is arranged between the first vertically extending pass (<NUM>) and the second vertically extending pass (<NUM>).