RUUSKANEN, Marko (Lähdekatu 14, Varkaus, FI-78500, FI)
| Claims 1. Method of recovering heat from bottom ash of a combustion process, the process comprising at least • combusting fuel for generating heat energy to produce steam or hot water in a boiler arrangement so that flue gases and bottom ash are formed, • providing the boiler arrangement with at least a furnace ( 10) having heat transfer surfaces having water tubes for leading the heat energy in the form of high temperature steam or hot water into use, • feeding combustion air into the furnace ( 10), • feeding fuel into the furnace ( 10), • discharging bottom ash from the furnace ( 10), • collecting heat from the flue gases, • utilizing the high temperature steam or hot water, • condensing the steam to a condensate (C), • circulating the condensate (C), the method being characterized in • recovering heat to a bottom ash cooling water circuit (42, 42') from the bottom ash discharged from the furnace ( 10) for the purpose of utilizing the recovered heat elsewhere within the boiler arrangement. 2. The method as recited in claim I , characterized in utilizing the recovered heat in an appropriate position within the boiler arrangement, like for preheating the combustion air, or preheating the condensate (C). 3. The method as recited in claim I or 2, characterized in arranging a combustion air heating means (52') in the bottom ash cooling water circuit (42') for exchanging heat from the cooling water in the combustion air. 4. The method as recited in claim I or 2, characterized in exchanging the heat from the bottom ash cooling circuit (42) to a combustion air heating circuit (48). 5. The method as recited in any one of claims I - 4, characterized in exchanging heat from the bottom ash cooling circuit (42') or the combustion air heating circuit (48) to the combustion air before preheating the combustion air by the heat recovered from flue gases. 6. The method as recited in any one of claims I - 5, characterized in arranging a bypass economizer (62) in parallel with means ( 18) preheating the combustion air for decreasing the amount of flue gases introduced into the preheating means ( 18) and thereby improving the flue gas temperature control. 7. The method as recited in claim 6, characterized in recovering heat from the flue gases in the condensate (C) by means of the bypass economizer (62). 8. The method as recited in any one of claims I - 7, characterized in that said means (20) for separating solids from the flue gases is an electrostatic separator, and in recovering heat from the flue gases after the electrostatic precipitator (20) for heating the combustion air. 9. The method as recited in claim 8, characterized in recovering heat from the flue gases after the electrostatic precipitator (20) into the bottom ash cooling circuit (42') or the combustion air heating circuit (48) for heating the combustion air. 10. The method as recited in any one of claims I - 9, characterized in arranging additional cooling means (54) in the bottom ash cooling circuit (42, 42'). I I . The method as recited in any one of claims I - 10, characterized in arranging additional heating means (60) in the bottom ash cooling circuit (42') or in the combustion air heating circuit (48). 12. Arrangement for recovering heat from bottom ash of a combustion process where fuel is combusted for generating heat to produce steam or hot water, and flue gases and bottom ash are formed in a boiler arrangement, the boiler arrangement at least comprising a furnace ( 10) provided with heat transfer surfaces having water tubes for leading the heat in the form of high temperature steam into use, means (34) for feeding combustion air into the furnace, means for feeding fuel into the furnace, means (32) for discharging bottom ash from the furnace ( 10), means ( 14, 16, 18) for recovering heat from the flue gases, means for utilizing the high temperature steam or hot water, means condensing the steam to a condensate (C), and means (28) for circulating the condensate (C), characterized in that the bottom ash is discharged to means (38) adapted to recover heat from the bottom ash to a bottom ash cooling water circuit (42, 42') for the purpose of utilizing the recovered heat elsewhere within the boiler arrangement. 1 3. The arrangement as recited in claim 12, characterized in that the bottom ash cooling circuit (42') is provided with a heat exchanger (52') exchanging heat from the circuit (42') to the combustion air. 14. The arrangement as recited in claim 12, characterized in that the bottom ash cooling water circuit (42) is provided with a heat exchanger (46) exchanging heat from the cooling water circuit (42) to a combustion air heating circuit (48) for heating the combustion air. 15. The arrangement as recited in claim 14, characterized in that the combustion air heating circuit (48) is provided with a heat exchanger (52) for heating the combustion air. 16. The arrangement as recited in any one of claims 12 - 15, characterized in that a bypass economizer (62) is arranged in parallel with the combustion air pre- heater ( 18) for heating the combustion air for allowing part of the flue gases bypass the combustion air preheater ( 18). 17. The arrangement as recited in claim 16, characterized in that the bypass economizer (62) is arranged in flow communication with the condensate (C) flow path for preheating the condensate (C). 18. The arrangement as recited in claim 17, characterized in that the bypass economizer (62) is arranged in the condensate (C) flow path upstream of a feed water tank (26). 19. The arrangement as recited in any one of claims 12 - 18, characterized in that a further heat exchanger (66) is arranged in the bottom ash cooling circuit (42') or the combustion air heating circuit (48) for recovering heat from the flue gases after the last solids separation means, i.e. an electrostatic precipitator (20). 20. The arrangement as recited in any one of claims 12 - 19, characterized in that the bottom ash cooling circuit (42, 42') is provided with means (54) for cooling the water in said circuit (42, 42'). 21. The arrangement as recited in any one of claims 12 - 20, characterized in that the combustion air heating circuit (48) is provided with means (60, 66) for heating the water in said circuit (48). 22. The arrangement as recited in any one of claims 12 - 21 , characterized in that the bottom ash cooling means (38) comprises one of a cooled transport screw, a water-cooled drum cooler, and a water-cooled scraper conveyor. |
[001 ] The present invention relates to a method and an arrangement for recovering heat from bottom ash of a combustion process performed in a combustion device from which bottom ash is removed in high temperature. The present invention is specifically applicable to bubbling bed boilers, fluidized bed boilers and circulating fluidized bed boilers.
[002] An ordinary boiler arrangement of prior art is comprised of a furnace to which the fuel, the bed material and the combustion air are introduced. When combusting the fuel heat is generated and both bottom ash and flue gases are formed. The flue gases are taken to a separator which separates solid particles from the gases, and returns the solid particles back to the furnace. Both the furnace and the separator are provided with heat exchange surfaces comprising water or steam tubes to collect heat from the flue gases and the solids moving in the boiler. The flue gases are taken from the separator to further heat recovery devices, such as superheaters or reheaters, where the heat still available in flue gases is used to further heat the steam. The heat is transferred into use, like for instance for generating electricity by means of steam turbines and generators, in the form of high temperature steam. After the superheaters and reheaters the flue gases flow through an economizer that, again, collects heat from the flue gases to boiler feed water i.e. water or condensate that is returning to the boiler from the use, for instance from the turbines. The most often final step of collecting heat from the flue gases takes place in the combustion air preheater, where the flue gas heat is used to heat the air used as combustion air in the furnace. The preheater is normally a rotary or tubular preheater. The combustion air preheater is followed in the flue gas path by an electrostatic filter/precipitator that separates any solid particles left in the flue gases before the flue gases are vented to the atmosphere by means of a flue gas fan via the chimney.
[003] The boiler feed water entering the economizer originates typically, as was already mentioned above, from the use in steam turbines and a condensor downstream of the steam turbines. The condensate is first heated by steam ex- tracted from the steam turbines by means of one or more low-pressure preheaters until the condensate is introduced into the feed water tank, which is used to deae- rate the water, and sometimes to heat the water further before pumping it towards the economizer. The feed water pumped from the feed water tank by means of a pump may further be heated by means of a high pressure preheater before entering the economizer.
[004] The bottom of the furnace is provided with a grid for introducing combustion or suspending or fluidizing gas, called as primary or combustion air, into the furnace, and for removing ash and other debris from the furnace. The common name for the material discharged from the boiler furnace through the grid is called bottom ash. It contains non-burning material, clinker, unburnt fuel particles etc. Normally the bottom ash is discharged in a water filled trough or to water- or air-cooled conveyors where it gets cooled. The cooled bottom ash is then taken out of the plant to be dumped, or sometimes used as construction material.
[005] Thus, in conventional prior art boilers the loss of heat energy in the discharge of the bottom ash forms a significant portion of boiler losses. This is even more so with certain high ash content fuels, i.e. where the estimated bottom ash content of the fuel is high or when there is a need to remove or to circulate coarse or otherwise inappropriate bed material from the furnace. The reason for the high loss of energy is that the bottom ash to be removed from the furnace is in high temperature, usually in about 700 - 800 °C. For example, if the bottom ash flow from the boiler is 10 kg/s in a temperature of 725 °C, using the reference temperature of 25 °C and the heat capacity for ash I kj/kg, an energy loss of 7 MW while discharging bottom ash can be expected.
[006] Prior art knows, however, a couple systems where some heat of the bottom ash is recovered. EP-B I -0471055 discusses a boiler arrangement where the bottom ash is discharged from the grid area onto a moving specifically designed steel belt. The heat recovery is arranged such that cooling air is made to flow countercur- rent to the bottom ash flow and to the steel belt movement so that the heated air finally enters the furnace.
[007] WO-A I -2007/ 1 34874 discusses a boiler arrangement where the steel belt bottom ash discharge of the above referenced patent is still used, as well as the countercurrent air flow. However, this document teaches further that, for cooling the bottom ash, water is sprayed on the bottom ash so that the generated steam finally enters the furnace together with the heated air flow. [008] The heat recovery technology discussed in the above mentioned patent documents, though teaching the possibility of recovering heat from the bottom ash discharge, is not that efficient. Especially when a larger amount of bottom ash is discharged, it is clear that the countercurrent air flow is not able to cool the ash sufficiently. And even if the additional water spraying is used there are two risks. The first being that the ash will not be cooled sufficiently, and secondly, if the ash is cooled sufficiently, there may be too much steam to be introduced into the furnace. Additionally, mostly due to the inefficiency of air acting as the heat transfer medium the size of the devices functioning even in a satisfactory manner are very large, and expensive. And further, to be able to cool the bottom ash sufficiently, huge amounts of air is needed the use of which as combustion air may interfere negatively the combustion air system of the boiler.
[009] Thus, an object of the present invention is to suggest a novel and efficient method of cooling down the bottom ash discharged from the furnace.
[0010] Another object of the present invention is to take the heat recovered from the bottom ash into use in one or more appropriate position in the power plant. [001 1 ] The above and other objects of the present invention are met with the method of recovering heat from bottom ash of a combustion process, the process comprising at least combusting fuel for generating heat energy to produce steam or hot water in a boiler arrangement so that flue gases and bottom ash are formed, providing the boiler arrangement with at least a furnace having heat transfer surfaces having water tubes for leading the heat energy in the form of high temperature steam or water into use, feeding combustion air into the furnace, feeding fuel into the fur- nace, discharging bottom ash from the furnace, collecting heat from the flue gases, utilizing the high temperature steam, condensing the steam to a condensate, circulating the condensate, where heat is recovered to a bottom ash cooling water circuit from the bottom ash discharged from the furnace for the purpose of utilizing the recovered heat elsewhere within the boiler arrangement.
[0012] Similarly, the above and other objects of the present invention are met with the arrangement for recovering heat from bottom ash of a combustion process where fuel is combusted for generating heat to produce steam or hot water, and flue gases and bottom ash are formed in a boiler arrangement, the boiler arrangement at least comprising a furnace provided with heat transfer surfaces having water tubes for leading the heat in the form of high temperature steam into use, means for feeding combustion air into the furnace, means for feeding fuel into the furnace, means for discharging bottom ash from the furnace, means for recovering heat from the flue gases, means for utilizing the high temperature steam or hot water, means for con- densing the steam to a condensate, and means for circulating the condensate, where the bottom ash is discharged to means adapted to recover heat from the bottom ash to a bottom ash cooling water circuit for the purpose of utilizing the recovered heat elsewhere within the boiler arrangement. [0013] Other features of the method and the apparatus of the present invention can be seen in the appended claims.
[0014] The present invention has solved at least some problems relating to the prior art bottom ash cooling systems. For instance, the present invention makes it possible to recover heat efficiently from the bottom ash, and to utilize the recovered heat wherever needed in the power plant. [0015] The present invention thus makes it possible to improve the efficiency of the boiler significantly. Also, the present invention takes the, in any case required, bottom ash cooling devices or arrangements into more efficient use whereby the required investment is very low.
[0016] In the following the method and the arrangement of the present invention will be explained in more detail with reference to the following drawings, of which
Fig. I is a schematic representation of a circulating fluidized bed boiler arrangement of prior art,
Fig. 2 is a schematic representation of a first preferred embodiment of the present invention arranged in connection with a circulating fluidized bed boiler,
Fig. 3 is a schematic representation of a second preferred embodiment of the present invention arranged in connection with a circulating fluidized bed boiler,
Fig. 4 is a schematic representation of a third preferred embodiment of the present invention arranged in connection with a circulating fluidized bed boiler, and
Fig. 5 is a schematic representation of a fourth preferred embodiment of the present invention arranged in connection with a circulating fluidized bed boiler. [0017] Fig. I illustrates schematically, and as an example only, an ordinary boiler arrangement of prior art. A furnace to which the fuel, the bed material and the combustion air are introduced is referenced with numeral I 0. When combusting the fuel in the furnace I 0 heat is generated and both bottom ash and flue gases are formed. The gases are taken to a separator 12, which separates solid particles from the gases, and recirculates the solid particles back to the furnace I 0. Both the furnace I 0 and the separator 12 are provided with heat exchange surfaces comprising water tubes to collect heat from the flue gases and the solids moving in the furnace and the separator. The flue gases are taken from the separator 12 to further heat recovery devices, superheaters and reheaters 14, where the heat still available in flue gases is used to further heat steam. The heat is transferred into use, like for instance for generating electricity by means of steam turbines and generators, in the form of high temperature steam. After the superheaters and reheaters 14 the flue gases flow through an economizer 16 that, again, collects heat from the flue gases to boiler feed water. The often final step of collecting heat from the flue gases takes place in the combustion air preheater 18, where the flue gas heat is used to heat the air used as combustion air in the furnace 10. The air preheater 18 is followed in the flue gas path by an electrostatic filter/precipitator or a bag filter 20 that separates any solid particles left in the flue gases before the flue gases are vented to the atmosphere by means of a flue gas fan 22 via the chimney.
[0018] The boiler feed water entering the economizer originates from the use in steam turbines and a condensor downstream of the steam turbines. The condensate C is first heated by steam by means of one or more low-pressure preheaters 24 until the condensate is introduced into the feed water tank 26, which is used to deaerate the water, and sometimes to heat the water before pumping it to the economizer. The feed water pumped from the feed water tank 26 by means of a pump 28 may further be heated by means of a high pressure preheater 30 before entering the economizer 16.
[0019] The bottom of the furnace I 0 is provided with a grid 32, on the one hand, for introducing combustion or suspending or fluidizing gas, called as primary air or combustion air, into the furnace I 0 pumped by the fan 34 via the air preheater 18, and, on the other hand, for removing ash and other debris from the furnace I 0. The material discharged from the boiler furnace I 0 through the grid 32 is commonly called bottom ash. It contains non-burning material, clinker, unburnt fuel ash particles etc. Normally the bottom ash is discharged in a water filled trough 36 where it gets cooled. The cooled bottom ash is then taken out of the plant to be dumped, or sometimes used as construction material.
[0020] Figures 2 - 5 illustrate schematically several variations of the present invention arranged, in an exemplary manner, in connection with a circulating fluidized bed boiler, though the bottom ash cooling arrangement of the invention may as well be used in other boiler types where bottom ash is discharged. A common feature to all embodiments of the present invention is that the bottom ash is discharged through the grid 32 to bottom ash cooling means 38, preferably a cooled transport screw arrangement driven by means of a motor 40. The cooled transport screw is normally provided with a water-cooled casing and a hollow shaft allowing cooling water flow therethrough. Other applicable bottom ash cooling means are water-cooled drum coolers, and water-cooled scraper conveyors, just to name a couple alternatives. Another common feature to all illustrated embodiments of the present invention is that the water used for cooling the bottom ash in the cooling means 38 is used for preheating the combustion air.
[0021 ] Figure 2 illustrates a bottom ash heat recovery arrangement in accordance with a first preferred embodiment of the present invention. The bottom ash cooling means 38 is a part of a bottom ash cooling circuit 42, which further comprises at least a circulation pump 44, and a heat exchanger 46. Thus, the heat from the bottom ash cooling water circulating in the cooling circuit 42 is transferred in a heat exchanger 46 to a closed water circuit i.e. a heating circuit 48 including, in addition to the heat exchanger 46, at least a circulation pump 50, and another heat exchanger 52. The heat collected in the heating circuit 48 is used for heating the combustion air by means of the heat exchanger 52. The heat exchanger 52 is positioned between the combustion air fan 34 and the air preheater 18 such that the preheater 18, preferably a rotary air preheater, receives air that has already been heated in the heat exchanger 52.
[0022] Since the combustion air preheater 18 receives now air in a higher temperature than before the bottom ash heat recovery, the combustion air preheater 18 may need some kind of an adjustment. If the combustion air preheater 1 8 were kept the same it would not be able to cool down the flue gas temperature as efficiently as before the bottom ash heat recovery, and some heat contained in the flue gases might be lost. Thus, in practice, to be able to recover heat from the flue gases as efficiently as before (equals to maintaining the flue gas temperature after the combustion air preheater same as before the bottom ash heat recovery) a more efficient combustion air preheater is needed. [0023] Figure 2 shows also additional equipment arranged in both the cooling circuit 42, and the heating circuit 48. Optionally, the bottom ash cooling circuit 42 is provided with a second heat exchanger 54 connected to the side of (or in parallel with) the first heat exchanger 46. In practice, the cooling water coming from the bottom ash cooling means 38 is divided, in this additional embodiment, by means of a valve (not shown) between the two heat exchangers 46 and 54 depending on the amount of heat needed for heating the combustion air. Whenever more heat from bottom ash is recovered, or should be taken to ensure low enough temperature of the bottom ash in its discharge, than needed in the combustion air preheating, a part of the cooling water flow is guided to the second heat exchanger 54 so that the excess heat is transferred by means of the second heat exchanger 54 from the cooling water circuit 42 to another cooling water circuit 56. In other words, the operation of the valve (not shown) opening the cooling water flow path to the second heat exchanger 54 is preferably controlled by at least one of the temperature of the bottom ash in its discharge to the hopper 58, the temperature of the combustion air exiting the heat exchanger 52, the temperature of the cooling water after the heat exchanger 46, and the temperature of the combustion air exiting the combustion air preheater 1 8. This connection ensures low enough temperature for the bottom ash discharged in a hopper 58. The heating circuit 48 is also provided with a second heat exchanger 60 in addition to the first one 52 heating the combustion air. The second heat exchanger 60 is arranged in series with the first heat exchanger 52 for providing the heating circuit 48 with additional heat for heating the combustion air. The heat exchanger 60 may, for instance, be used in start-up or partial load situations when neither the flue gas heat recovery nor the bottom ash heat recovery nor such together are able to provide sufficient amount of heat for the heating of the combustion air. The use of the second heat exchanger 60, i.e. the steam flow therein, may preferably, but not necessarily, be controlled by at least one of the temperature of the flue gases upstream of the combustion air preheater 1 8, the temperature of the bottom ash in its discharge to the hopper 58, the temperature of the combustion air exiting the heat exchanger 52 and the temperature of the combustion air exiting the combustion air preheater 1 8. [0024] Another option for arranging the heat exchanger in the cooling circuit is to position the heat exchanger 54 of the second cooling circuit 56 in series (and not in parallel to) with the heat exchanger 46 of the first cooling circuit. This connection functions in practice so that the hot water from the bottom ash heat recovery arrangement 38 first heats the water in the combustion air heating circuit 48 by means of the heat exchanger 46, and then flows further to the heat exchanger 54 to be cooled further, whereafter the water is returned to the bottom ash heat recovery arrangement 38 by means of the pump 44.
[0025] Figure 3 illustrates a bottom ash heat recovery arrangement in accordance with a second preferred embodiment of the present invention. In fact, the embodiment of Fig. 3 is a simplification of the embodiment of Fig. 2. Here the bottom ash cooling circuit 42 and the combustion air heating circuit have been combined to a single circuit 42' such that the bottom ash cooling water is taken to heat the combustion air directly in the heat exchanger 52'. The circuit 42' includes the additional cooling water circuit 56 with its heat exchanger 54 in the manner of the embodiment of Fig. 2. Naturally, as already discussed above, an additional heat exchanger (for instance heat exchanger 54) arranged in a certain circuit (42, 42') for cooling the heat exchange medium in said specific circuit (42, 42') may be arranged either in series or in parallel with the heat exchanger (46, 52') used for heating a heat exchange medium in another circuit or the combustion air directly. The cooling/heating circuit 42' need not be provided with the heat exchanger 60 of Fig. 2, but a corresponding heat exchanger may be arranged in the combustion air line either before or after heat exchanger 52' for the start-up and partial load situations when this is the only way of heating the combustion air.
[0026] Figure 4 illustrates a bottom ash heat recovery arrangement in accordance with a third preferred embodiment of the present invention. As can be seen from Fig. 4 the main arrangement is the same as in Fig. 2. The only exception is that a bypass economizer 62 has been added in the flue gas path between the economizer 16 and the electrostatic precipitator/bag filter 20 in parallel with the combustion air preheater 18. The bypass economizer 62 is used for preheating the condensate by means of connecting it to the condensate C flow path between the low-pressure preheaters 24' such that a part of the condensate is made to flow by means of pump 64 via the bypass economizer 62 to the feed water tank 26 whereby the economizer 62 may as well be called condensate preheater. Here it has to be understood that there are several options for the condensate flow in relation to the bypass economizer. As has already been discussed earlier there may be only one condensate preheater or more than two preheaters before the feed water tank. Thereby it is obvious that the condensate flow to the bypass economizer may be taken already before the first preheater, between the preheaters (as shown in Fig, 4), or after the preheaters. Another option for the use of the bypass economizer is to connect it in parallel with the high pressure preheater 30. And naturally, a third option is the combination of the above two options, i.e. to circulate both the low-pressure and the high-pressure condensate in the bypass economizer. It is even possible that the entire condensate flow is heated in the bypass economizer, with or without one or more preheaters arranged in series with the economizer, before the feed water tank.
[0027] The reason why the condensate preheater/bypass economizer 62 is used is the fact that sometimes while using the bottom ash heat recovery arrangement of Fig. 2 the combustion air preheater may not be able to reduce the flue gas temperature sufficiently, but the temperature remains excessively high (wasting heat energy). The ultimate cause is that due to the heat exchanger 52, the difference between the temperature of the flue gas entering the combustion air preheater 18 and that of the combustion air exiting it is decreased. To maintain the flue gas temperature at an acceptably low level after the combustion air preheater 18 a part of the flue gases is taken to the condensate preheater 62 instead of the combustion air preheater 18, whereby the heat duty of the combustion air preheater 18 is reduced and the flue gas temperature after the air preheater 18 is maintained the same or lower as before the new bottom ash heat recovery arrangement. Alternatively, in some applications it may be preferable to arrange an additional economizer in series with the air preheater 18, either upstream or downstream of the air preheater. I I
[0028] The above discussed embodiment of the present invention offers a number of different ways to control both the temperature of the bottom ash exiting the cooling arrangement and the heating of the combustion air in the preheater 18. Naturally, the controls discussed in connection with the embodiment of Fig. 2 are applicable here, too, but since the bypass economizer 62 is taken into use, its controls have to be made compatible with the ones discussed in Fig. 2. In other words, the valve (not shown) governing the flue gas introduction to the bypass economizer 62 may be controlled by the temperature of the flue gases exiting the combustion gas preheater 1 8. In other words, the higher the temperature of the flue gases exiting the combustion air preheater 18 is, the larger part of the flue gases is allowed to enter the bypass economizer 62. The condensate C flow to the bypass economizer 62 has to be controlled, too. One feasible way of controlling is to make the condensate flow to the bypass economizer dependent on the flue gas flow into the economizer 62. And finally both the bottom ash temperature after cooling thereof, the combustion air preheating, and the temperature of the flue gas entering the chimney should be controlled such that the energy losses are kept minimal.
[0029] Figure 5 illustrates a bottom ash heat recovery arrangement in accordance with a fourth preferred embodiment of the present invention. In accordance with Fig. 5 the arrangement of Fig. 5 is a further development of the arrangement discussed in Fig. 4. The starting point in this embodiment of the present invention is that some boiler plants are designed to recover heat from flue gases after the electrostatic precipitator 20 for heating the combustion air. Thus such a known arrangement consists of a water circuit comprising a flue gas cooler 66 located after flue gas fan 22 and a combustion air heater 52 located after combustion air fans 34 in the combustion air flow path. Now the above-mentioned known arrangement has been connected to the combustion air heating circuit 48 that utilizes the heat recovered from the bottom ash by means of the cooling circuit 42 as discussed already above. Heat from both the bottom ash cooling means 38 (with heat exchanger 46) and the flue gas heat recovery means 66 i.e. the flue gas cooler is exchanged to combustion air heating circuit 48. If it is desired that the heat recovery by means of the flue gas cooler 66 is maintained at the same level as before the new bottom ash heat recovery connection, it means that combustion air temperature before the combustion air preheater 18 rises. However, the combustion air temperature after the combustion air preheater 18 is not able to rise equally, since only a certain difference in temperatures of flue gas entering the combustion air preheater 18 and the combustion air exiting it may exist. This is compensated by decreasing the flue gas flow through the combustion air preheater 18 and increasing the flue gas flow through the bypass economizer 62, whereby the heat duty of the air preheater 18 is decreased and the heat duty of the bypass economizer 62 is increased, respectively.
In view of the above description it has to be understood that only a few most preferred embodiments of the present invention has been discussed. Thus it is obvious that the invention is not limited to the above disclosed embodiments only, but that it can be modified in many ways within the scope of the appended claims. It has to be understood, too, that features of a specific embodiment of the invention may be applied in connection with features of other embodiments within the basic idea of the present invention or that features from different embodiments may be combined to result in a working and technically feasible construction. In other words, it is obvious that the additional heating means 60 arranged in the combustion air heating circuit in the embodiments of Figs. 2, 3, and 4 may also be utilized in the embodiment of Fig. 5. Likewise, the utilization of the heat recovered from the bottom ash may not only be used in heating the combustion air, but also other applications within the boiler arrangement or the power plant are possible. It is for instance possible to use the heat for heating the condensate. Further applications relate to using the recovered heat for heating the power plant building, or delivering the heat for district heating purposes. Also, it is possible to connect the heat exchanger 66 recovering heat from the flue gases after the electrostatic precipitator from the embodiment of Fig. 5 to the embodiments of Figs. 2, 3, and 4 to bring additional energy to the combustion air heating circuit 48 or the bottom ash cooling circuit (42') provided that it has been ensured that the connection has been done so that it does not complicate the bottom ash cooling i.e. by means of connecting the flue gas heat recovery in parallel with the bottom ash cooling means. Further, it has to be understood that the heat exchanger 52 used for heating the combustion air may be positioned upstream of the fans 34, and not necessarily thereafter. And finally, it should be understood that the word 'condensate' should be understood broadly to cover also such water returning to the boiler arrangement that has not been in gaseous state (i.e. steam) but has been hot water that has been cooled down in use.