Abstract:
A boiler ( 1 ) comprising at least: a furnace ( 2 ), in whose lower part the combustion used as the primary source of thermal energy of the boiler is configured to take place; devices ( 6 ) for supplying fuel into the furnace; devices ( 3, 7, 8, 26 ) for supplying combustion air into the furnace; one or more flue gas ducts ( 11 ); at least one chamber ( 17 ) accommodating at least one steam superheater ( 15 ) for recovering thermal energy. Said chamber is configured to allow a direct line of sight ( 16 ) between said superheater and said primary source, to enable the reception of thermal energy by means of thermal radiation. Said chamber is further configured to prevent the entry of said flue gases ( 19 ) to said superheater either totally or almost totally, to avoid the reception of thermal energy by convection.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to a boiler for producing and recovering thermal energy. 
       BACKGROUND OF THE INVENTION 
       [0002]    In boilers, a lot of compounds are formed which are detrimental to the materials of the heat exchange surfaces of the boiler. Especially when burning biofuel and refuse fuel, corrosion of the heat exchange surfaces of the boiler has been detected, especially corrosion of superheaters and their heat exchange surfaces. In addition, it has been detected that ash produced during combustion deposits on the heat exchange surfaces, which reduces the heat transfer and thereby the recovery of thermal energy. 
         [0003]    The above-mentioned biofuels include botanical materials from nature, such as wood chips, bark, agro-biomass, sawdust, black liquor, and the like. Refuse fuels include, for example, sorted household refuse, industrial waste and waste from businesses, as well as demolition wood. These fuels include significant amounts of chlorine. Together with sodium and potassium released from fuel they form gaseous alkaline chlorides in flue gases, which are condensed and deposited on heat exchange surfaces, especially on superheater surfaces. Deposition and condensation takes places especially in places where the surface temperature of the heat exchange surfaces is below 650° C. When the surface temperature of a heat exchange surface is above 450° C., the alkaline chlorides cause chlorine corrosion. 
         [0004]    Supplying various additional materials to the furnace has been suggested in order to eliminate corrosion problems caused by chlorides. Publication WO 2006/134227 A1 discloses the spraying of a liquid sulphate-containing to the superheater area of a steam boiler, to bind the alkaline chlorides formed in the furnace. According to publication WO 02/059526 A1, a liquid sulphate compound or sulphuric acid is added to flue gases before the superheaters. Publication EP 2071239 A2, in turn, discloses that additional material needed for preventing corrosion is fed to the flue gases of a boiler by means of at least one cooled pipe. 
         [0005]    It is also known to decrease the nitrogen oxide emissions of different types of boilers by supplying their furnace with various additional materials which decrease the amount of nitrogen oxides in the flue gases formed during combustion. This kind of a solution is presented, for example, in publication WO 9813649 A1, in which cooled pipe panel surfaces are installed in the furnace of a fluidized bed boiler, which include separate additional material channels for the additional material. 
         [0006]    Publication WO 96/02792, in turn, discloses a heat exchanger placed in a pocket-like compartment for collecting particles in the central part of the boiler, separated from the fluidized bed in the lower part of the boiler. Material in the pocket is fluidized with a gas which is non-aggressive and substantially oxygen-free, to avoid corrosion problems in the heat exchanger. Publication WO 03/104547 A1, in turn, discloses a boiler with a separate compartment which accommodates a superheater and where combustion also takes place. The aim is the combustion of such fuels which do not cause problems of corrosion in the superheater placed in the chamber. 
         [0007]    According to the prior art, the superheaters of the boiler are placed either in the furnace of the boiler, typically at the top of the furnace, or in the flue gas duct downstream of the furnace, where the flue gases from the furnace are led. The superheaters may be placed either in the same flue gas duct or in parallel flue gas ducts. The superheaters are placed in the flue gas flow, and the thermal energy of the flue gas is transferred to the superheater by means of both thermal radiation and convection of heat, in which case one can refer to combination superheaters. It is also possible to use special radiant superheaters, whose utilization is primarily based on recovering thermal radiation from the flame, and special convection superheaters, whose utilization is primarily based on the convection of thermal energy by means of contact between the superheater and flue gases. The radiant superheater is normally placed in the upper part of the furnace, for example on the wall of the furnace, and it is in direct contact with the thermal radiation from the flame. Thus, there is a direct line of sight between the flame and the superheater. A convection superheater is normally protected from the thermal radiation of the flame and is placed outside the furnace, for example in the flue gas duct. The type of the superheater also affects the construction of the superheater, wherein for example in a radiant superheater the pipes are fitted very close to each other, forming a plate-like surface or plane. 
         [0008]    The superheaters are used as heat exchangers which typically comprise an assembly constructed of pipes connected to each other, by means of which thermal energy is transferred to a medium, such as gas, liquid, or a mixture of these, flowing inside the pipe. 
         [0009]    The above-presented methods to eliminate corrosion problems of the superheaters are often not practical, particularly if a separate supply has to be provided for the additional material or a separate supply of gas to protect the superheaters, in order to avoid the detrimental effects of flue gases. 
       BRIEF SUMMARY OF THE INVENTION 
       [0010]    The aim of the present invention is thus to provide a system to avoid the above mentioned problems which relate particularly to the corrosion and chemical attacks caused expressly by flue gases. 
         [0011]    The boiler according to the invention is presented in claim  1 . 
         [0012]    The principle of the invention is to utilize thermal radiation in the superheater and simultaneously to prevent the detrimental effects of flue gases for the superheater, wherein the aim is not to utilize heat transfer by means of convection. 
         [0013]    In one embodiment, the principle is to place the superheater in a chamber, in which the entry of flue gas and thereby also the detrimental compounds, corrosive or aggressive substances contained in the flue gas is substantially limited, so that the entry is totally or almost totally prevented. The aim is to prevent or strongly limit the entry of these compounds and substances to the heat exchange surfaces of the superheater. The aim is to avoid the circulation of flue gases at the superheater in various ways. 
         [0014]    According to one example, the chamber is constructed in such a way that no continuous flow of flue gases through the chamber takes place past the superheater. The transfer of thermal energy into the superheater is primarily based on thermal radiation, and the possibility of transfer of thermal energy by convection to the superheater is totally or almost totally prevented. 
         [0015]    In an example, the chamber is made to be open from below only or on only one side in such a way that the flow of flue gases into the chamber and particularly through the chamber, past the superheater, is prevented as well as possible. 
         [0016]    In one example, the chamber is constructed in such a way that the gas in the chamber remains in the chamber by the effect of dynamic pressure of the ascending flue gas flow in the furnace. The gas stagnates in the chamber and remains in the chamber. In this way, no flue gases flow continuously past the superheater, nor is the gas in the chamber replaced. Preferably, the chamber is placed in the upper part of the furnace and is open from below. Preferably, a radiation superheater is placed in the chamber. Also preferably, the superheater and its heat exchange surfaces have a direct line of sight to the flame, to recover thermal energy on the basis of thermal radiation, because the aim is to avoid the transfer of heat by convection. Thus, the aim is not to enable convection by the flue gas flow and thus not by means of, for example, particles or bed material in the furnace either. 
         [0017]    In this description, the direct line of sight does not refer merely to the visible flame of combustion in the furnace but also to the source of thermal radiation, of which the visible flame constitutes only a part, wherein the thermal radiation is invisible and primarily in the infrared range. 
         [0018]    Also, the chamber is not one in which the combustion of fuel would take place separately and whose flue gases would be combined with the flue gases from the combustion in the furnace of the boiler in such a way that the flue gases from the separate combustion would be led through the superheater or the chamber to the furnace. It is primarily the thermal radiation and the flue gases developed in connection with the combustion in the furnace that tend to find their way into the chamber. Said combustion is the primary source of thermal energy. 
         [0019]    In an example, the flow of flue gases and simultaneously also the flow of various particles is guided by a jet of gas, powder or liquid from one or more nozzles at the open bottom or side of the chamber in such a way that a curtain of gas, power or liquid is formed in front of the open entrance of the chamber, to guide the flow of flue gases away from the open entrance of the chamber and to prevent the flue gases from entering into the chamber. The gas, powder or liquid to be sprayed may also be an additional material of prior art, known as such, to prevent corrosion problems caused by flue gases. The gas to be sprayed may be a gas of prior art, known as such, which is used to reduce corrosion problems, or, for example, a gas that is free from corrosive substances, for example an inert gas. It may also be air or gas from the boiler. 
         [0020]    By means of one or more jets of gas, powder of liquid, it is also possible to form a curtain or a barrier in front of the open entrance of the chamber, to prevent the flow of flue gases through the entrance into the chamber. 
         [0021]    In another example, one or more nozzles are provided inside the chamber or directly at the entrance of the chamber, for the purpose of filling the chamber with a gas to reduce problems of corrosion and chemical attack, to dilute flue gases entered in the chamber, or to replace all the gas in the chamber with another gas, such as an inert gas. The filling is performed, for example, once when the operation of the boiler is started, or at certain intervals if needed. In this way, no flue gases can stagnate in the chamber at any stage. The gas supplied into the chamber by means of nozzles remains in the chamber preferably by the effect of dynamic pressure of the ascending flue gas flow. It is also possible to spray said additional materials into the chamber by means of a jet of liquid or powder. 
         [0022]    The above-presented nozzles may be placed inside the chamber or in the vicinity of the chamber, as needed. 
         [0023]    In one embodiment, the chamber is placed inside a nose in the upper part of the furnace, wherein there is no need to modify the walls of the boiler. The nose is a structure in the upper part of the furnace, tapering the upper part of the furnace smaller than the lower part of the furnace. Simultaneously, the hose guides the flue gases to the superheaters and into the flue gas duct above the nose. The chamber is typically cubical, comprising side walls consisting of pipes conveying a medium, one or more of the pipes forming simultaneously the side wall of the furnace, or one or more of the pipes forming simultaneously the side wall of the nose. The chamber does not comprise a bottom, or it is open at the bottom. The chamber has a top that can be part of the nose. 
         [0024]    In one example, the chamber is placed outside the furnace in such a way that the entrance of the chamber is in a horizontal or vertical wall of the furnace and extends into the chamber either via the bottom or a side of the chamber. Also in this case, the chamber is provided with a radiation superheater with a direct line of sight to the flame. In one example, the chamber is placed inside the furnace in such a way that one or more of the side walls of the chamber is simultaneously the side wall of the furnace. The top of the chamber may also be the top of the furnace. 
         [0025]    The aim is to avoid the flow of gases in the chamber. With respect to the structure of the chamber, it is possible that the chamber is not fully leak-proof but it allows gases and flue gases to leak through various gaps and holes. However, the aim is to limit such leaks substantially and to keep them insignificant. However, it may be necessary to provide the chamber with continuous ventilation by means of leaks. The chamber can also be provided with a separate ventilation channel which can be opened and dosed as needed, for example by means of a controlled valve, and through which the gases or flue gases in the chamber can be removed from the chamber. 
         [0026]    One or more of the walls of the chamber are made by using pipes conveying a medium, to recover the thermal energy of the furnace by means of radiation and/or convection. The inside of the walls of the chamber is preferably coated with an insulating gunning. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]    In the following, the invention will be described in more detail with reference to the appended drawings, in which: 
           [0028]      FIG. 1  shows a schematic view of a fluidized bed boiler seen from the side, provided with a chamber and a superheater, 
           [0029]      FIG. 2  shows the chamber and the superheater of  FIG. 1  in more detail, 
           [0030]      FIG. 3  shows a schematic view of an example of the layout of the chamber inside the furnace, seen from the side, 
           [0031]      FIG. 4  shows a schematic view of an example of the layout of the chamber outside the furnace, seen from the side, 
           [0032]      FIG. 5  shows a schematic view of a circulating fluidized bed boiler, seen from the side, in which circulating fluidized bed boiler it is possible to apply a chamber and a superheater, 
           [0033]      FIG. 6  shows a schematic view of the lower part of a soda recovery boiler, seen from the side, in which soda recovery boiler it is possible to apply a chamber and a superheater. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0034]    In the drawings, elements with corresponding functions are indicated with the same reference numerals. 
         [0035]      FIG. 1  shows an example of a steam boiler applying the above-presented chamber and superheater configuration. As the steam boiler and the location for the chamber and superheater configuration, it is possible to apply a boiler based on fluidized bed combustion, particularly a bubbling fluidized bed boiler (BFB), as shown in  FIG. 1 , or a circulating fluidized bed boiler (CFB), as shown in  FIG. 5 . In bubbling fluidized bed boilers, a fluidized bed is produced by means of a gas flow. The place of application may also be a soda recovery boiler which is shown in  FIG. 6  and which is based on the combustion of black liquor, or a boiler in which the fuel is burnt on a grate, or a steam boiler of another kind. 
         [0036]      FIG. 1  shows a boiler  1 , which in this example is a bubbling fluidized bed boiler, which comprises a furnace  2 . The walls of the furnace are formed of water-cooled pipes, which are attached to each other by fins. The lower part of the furnace comprises nozzles  3  for feeding fluidizing air, i.e. primary air from an air box  4  to the furnace  2 . By the effect of the fluidizing air, the fluidized bed  5  in the lower part of the furnace is fluidized, i.e. brought into continuous movement in the furnace  2 . Fuel is supplied into the furnace from fuel supply devices  6 , and secondary air is supplied from secondary air nozzles  7 . In this boiler, tertiary air is also supplied into the furnace from tertiary air nozzles  8 . The fuel used is, for example, biofuel and/or refuse fuel. The flame  12  produced in connection with the combustion of the fuel rises above the fluidized bed and extends, for example, above the secondary air nozzles  7  and often also up to the tertiary air nozzles. The combustion of fuel by means of oxygen-containing gas in the lower part of the furnace  2  is the primary source of thermal energy. 
         [0037]    The upper part of the furnace comprises superheaters  9  and  13 , whose function is to provide superheated steam that is typically used in a turbine (not shown in the figure). As seen in the figure, the pipes forming the wall of the furnace are bent inwards from the rear wall  2   b  in such a way that a nose  10  is formed extending towards the front wall  2   a  of the furnace. The purpose of the nose  10  is to direct the flue gases in a desired way to the superheaters  9  and  13 . The superheater  9  is, for example, a radiation superheater, or a combination superheater, whose function is based on thermal radiation and the convection of heat, and the superheater  13  is, for example, a convection superheater. In the figure, the superheaters and the nose are drawn in a reduced manner to illustrate the circulation of the medium. 
         [0038]    The flue gases  19  formed in the furnace are conveyed further via a flue gas duct  11  in connection with the furnace. The flue gas duct may be provided with heat exchange surfaces or heat exchangers  14 . 
         [0039]    In the example of  FIG. 1 , the chamber  17  for the superheater  15  is placed inside the nose  10  placed in the upper part of the furnace. The bottom of the chamber  17  is open, and it simultaneously forms the entrance  18 . The superheater placed in the chamber has a direct line of sight  16  to the primary source of heat, which is represented by the flame  12  in  FIG. 1 . The line of sight is made possible by the entrance  18  or a corresponding opening. 
         [0040]    The entrance  18  may consist of one or more separate openings. If the entrance consists of several openings, it is possible, for example by selecting the size of the openings in a suitable way, to prevent the flow of flue gases into the chamber and simultaneously to allow the entry of thermal radiation into the chamber and onto the heat exchange surfaces of the superheater. 
         [0041]    In the example shown in  FIG. 1 , the entry of flue gases into the chamber is prevented in such a way that the gas in the chamber remains in the chamber, thanks to the ascending flow of flue gases, so that the gas stagnates in the chamber. In this way, no replacement gas, particularly flue gases, can enter the chamber. 
         [0042]    In the example shown in  FIG. 2 , a nozzle  20  is also utilized, which is in this example placed on the wall  2   b  of the furnace, in the vicinity of the chamber. The liquid, powder or gas blown out of the nozzle and directed in a suitable way guides the flue gases  19  away from the chamber and its entrance. Furthermore, it is difficult for the flue gases to penetrate the jet to enter the chamber. 
         [0043]    In the example shown in  FIG. 2 , a nozzle  21  is also utilized, which is in this example placed inside the chamber  17 . The liquid, powder or gas blown out of the nozzle and directed in a suitable way, if necessary, neutralizes or dilutes the gas in the chamber, for example flue gas entered in the chamber, or fills the chamber with a desired gas in such a way that the gas present in the chamber is replaced with said desired gas. The gas is, for example, a non-corrosive or non-aggressive gas, for example an inert gas. The gas remains in the chamber  17 , thanks to stagnation. By means of the gas, it is also possible to maintain a pressure in the chamber that is higher than outside the chamber in the furnace, wherein the gases only flow out of the chamber, for example via the entrance. 
         [0044]    Of the nozzles  20  and  21 , only one or both are used in different examples. 
         [0045]      FIG. 3  shows an example in which the chamber  17  is placed inside the furnace  2  in such a way that the rear part of the chamber is limited to the wall of the furnace, for example its front wall  2   a,  rear wall  2   b  or a side wall. The chamber is open on at least one side, which simultaneously forms the entrance  18 . The superheater  15  placed in the chamber has a direct line of sight  16  to the primary source of heat. In this example, the nozzle  21  is placed on the wall of the chamber. The operation of the nozzles  20  and  21  corresponds to what has been discussed in connection with FIG..  2 . 
         [0046]      FIG. 4  shows an example in which the chamber  17  is placed outside the furnace  2  in such a way that the front part of the chamber is limited to the wall of the furnace, for example its front wall  2   a,  rear wall  2   b  or a side wall. The chamber is open on at least one side, which simultaneously forms the entrance  18 . The chamber has a connection from the furnace through the wall of the furnace, and the entrance  18  is formed in said wall. The superheater  15  placed in the chamber has a direct line of sight  16  to the primary source of heat. In this example, the nozzle  21  is placed on the wall of the furnace, in the vicinity of the chamber. The operation of the nozzles  20  and  21  corresponds to what has been discussed in connection with  FIG. 2 . 
         [0047]    The presented chamber and superheater configuration can also be applied in a circulating fluidized bed boiler as well as in a soda recovery boiler or in a boiler applying combustion on a grate.  FIG. 5  shows a boiler  1 , which is a circulating fluidized bed boiler. The boiler comprises a furnace  2 , a flue gas duct  11  and a cyclone  25 . The separation of fluidized bed particles entrained in the flue gases takes place in the cyclone. The fluidized bed particles separated from the flue gases are returned back to the furnace  2 . From the lower part of the furnace, fluidizing air is supplied to the furnace. The rate and amount of fluidizing air are adjusted to be such that by its effect, the fluidized bed particles fill substantially the entire furnace. The furnace is supplied with fuel, which may be biofuel, refuse fuel or coal, from fuel supply devices  6 , and with combustion air from air nozzles  7 . Combustion air can be supplied from several levels. The boiler further comprises several superheaters  9 ,  22 ,  23 , and  24 . 
         [0048]    The boiler is provided with the chamber  17  and the superheater  15  shown in  FIG. 3 . Alternatively, chambers according to the examples of  FIGS. 2  or  4  can be applied in said boiler. 
         [0049]      FIG. 6  shows a boiler, which is a soda recovery boiler. In the recovery boiler, the fuel used consists of cooking chemicals produced in pulp manufacture, as well as liquid that contains elements dissolved from wood, i.e. black liquor. The boiler comprises a furnace  2  which is supplied with black liquor from the fuel supply devices  6  and with combustion air from air nozzles  26 ,  7  and  8  placed at different heights of the boiler. Smelt  27  is created at the bottom of the furnace from combusting liquor, which smelt is discharged from the furnace to be processed further. The upper part of the furnace comprises superheaters, and flue gases are discharged from the furnace via flue gas ducts. 
         [0050]    The boiler can be provided with the chamber  17  and the superheater  15  shown in  FIG. 3 . Alternatively, chambers according to the examples of  FIGS. 2  or  4  can be applied in said boiler. 
         [0051]    The invention is not intended to be limited to the embodiments presented by way of examples above, but the invention is intended to be applied widely within the scope of the features defined in the appended claims.