WO1998010037A1

WO1998010037A1 - Method and assembly for increasing the capacity of a boiler plant

Info

Publication number: WO1998010037A1
Application number: PCT/FI1997/000518
Authority: WO
Inventors: Jukka Hirvenoja
Original assignee: Imatran Voima Oy
Current assignee: Imatran Voima Oy
Priority date: 1996-09-04
Filing date: 1997-09-04
Publication date: 1998-03-12
Anticipated expiration: 1999-03-04
Also published as: AU4120397A; FI963459A0; SE520040C2; SE9900653L; SE9900653D0

Abstract

A method for increasing the capacity of a boiler plant both in energy production and as regards fuel processing capacity. The invention is, among other things, suited for increasing the share of electric power particularly in plants using fluidized-bed boilers and for increasing the capacity of soda recovery boilers. The invention is based on exhausting gas by suction from an air-deficient zone in phased combustion, e.g. from the fluidized bed of a fluidized-bed boiler, whereafter the gas is separately combusted in a gas turbine or a separate boiler. An amount of additional fuel corresponding approximately to the amount of exhaust gas can then be fed into the boiler or burner.

Description

Method and assembly for increasing the capacity of a boiler plant

The present invention relates to a method according to the preamble of claim 1 for increasing the capacity of a boiler plant in both energy production and as regards the capacity for processing fuel. The invention is suited for, among other things, adding the proportion of electric power particularly in plants using fluidized-bed boilers, and for raising the capacity of soda recovery boilers.

The invention also relates to an assembly for carrying out the method.

In steam boiler plants electric power is produced in a steam turbine by means of steam heated with thermal power. A great part of the thermal energy, however, can not be used for spinning the steam turbine, but instead, the low- temperature steam from the turbine is condensated or the heat contained therein is led to the district heating network. If the need for heat is sufficient the plant attains a good total efficiency because all energy produced can be exploited. In general, however, the need for electricity exceeds the need for thermal energy, and as heat cannot be led very far from the plant, thermal energy is wasted. Thus it would be of advantage to increase the portion of electricity production so as to attain better compatibility of the type of energy produced by the plants with the existing need. Furthermore, it would be beneficial to be able to control the proportion between the electric power and thermal energy produced in accordance with consumer needs.

One arrangement for combustion comprises a fluidized-bed boiler. In a fluidized-bed boiler, typically solid fuel is fired in the furnace of the boiler in stages. The conditions in the fluidized bed in the lower part of the furnace, where the fuel and inert bed material are intermixed and airborne by air jets from the bottom part of the boiler, are air-deficient, and thus, the fuel is not completely burnt in the fluidized bed, and instead, the volatile components are gasified. In the part above the fluidized bed, called the freeboard, the gasified fuel is then completely burnt by means of secondary and tertiary air fed above the fluidized bed. The temperature of the fluidized bed is typically 700 to 900 degrees and the fuel is fed by dropping or by means of a pusher screw. The fluidized bed is kept at fluidized state by means of a primary amount of air introduced from below the bed, this amount typically being about 40 % of the total amount fed into the boiler. The remaining 60 % of air is introduced into the freeboard above the fluidized bed at a height which is typically about 4 to 8 m above the surface of the fluidized bed. In the fluidized bed, the fuel is gasified whereby volatile matter will rise to the freeboard where it is burnt by means of air inlet in steps.

Burning can in fluidized-bed boilers be divided into four zones in the following manner:

- The fluidized bed where the fuel is gasified and the residual coke is burnt at a temperature of about 750 to 900 degrees. The fluidized bed is kept fluidized by blowing a sufficient amount of air through the bed by means of a grate.

- A 3 to 5 meter splash area above the fluidized bed, into which additional air is usually not introduced.

In this area, no significant changes in the gas composition occur, but the remaining free oxygen in the primary air will react with the fuel thus generating heat. - Secondary air is fed into the area above the splash area, whereby gases are burnt and the temperature rises. The temperature in this area is typically 800 to 1400 degrees.

- The uppermost zone before the first heat exchangers penetrating the flue gas flow comprises a zone for burning up wherein the air ratio of the combustion is raised to a value above 1 and by means of the excess air, the gas and particles are burnt up.

The amount of fluidizing air in the furnaces of the boilers to a large extent corresponds to the amounts of air used in typical fluidized-bed carburettors, and thus, the composition of the gas above the surface of the fluidized bed and below the secondary air corresponds to gas produced in special carburettors, the caloric value of which is still relatively good when combusted. This is evident from a steep temperature rise after secondary air has been introduced.

In circulating fluidized-bed boilers the fluidizing medium circulates via the upper part of the boiler to a cyclone at the side of the furnace, in which the combustion gases and fluidizing medium are separated from each other. The fluidizing medium is returned to the lower part of the boiler and the fuel used is fed into the returning inlet. The gases are led into the convection part after separation of solid particles. Only the amount of gas required by the gas seal is admixed into the stream of particles returning to the furnace. Thus, the mixture of fuel, fluidizing medium and circulating gas is very air-deficient, and accordingly, the fuel is strongly gasified before it ends up in the furnace. In circulating fluidized-bed boilers the combustion air is introduced into the furnace in a similar manner as in the case of fluidized-bed boilers, that is, by blowing from the lower part of the furnace upwards and through inlets in the lower part of the furnace. The phasing of the combustion air is almost exclusively the result of poor horizontal mixing in the furnace. In circulating fluidized-bed boilers the fuel is admixed with air mainly in the vertical direction, and all mixing in the horizontal direction is slow wherefore there are strongly air-deficient areas in the vicinity of the fuel inlet points where the fuel is gasified and the burning up of the fuel is carried out while it moves upward in the furnace.

The phasing makes it possible to exhaust gas from the gasification zone of the furnace and use it for increasing the inlet temperature of a gas turbine by burning the exhausted gas in the combustion chamber of the gas turbine. Since the maximum temperature of the fluidized bed furnace is 900°C, the efficiency of the gas turbine would otherwise be poor because of the low temperature of the flue gases and the burning of the exhaus gases increases the efficiency notably by rising the inlet temperature. This method is called "topping" and it is used chiefly in composite power stations. In this method the furnace has to be pressurized so that the flue gases and exhausted gas can be used in the gas turbine. Topping does not increase the maximum efficiency of the furnace and no extra fuel corresponding the combustion value of the exhausted gas can be fed into the furnace. Topping processes are described for example in EP 340 351 and WO 94/02711.

In oil burners and pulverized fuel burners the phasing takes place in different steps in the flame of the burner by means of air currents led to the flame. In wall combustion burners, these being mainly used today, the fuel is usually fed from the center of a rotation symmetrical burner, and two air inlet channels are usually arranged around the fuel channel or nozzle. Close to the fuel inlet point, the flame is very air-deficient and reducing, and strong gasification occurs in the area. The fuel is completely combusted by secondary and tertiary air led into the flame in stages. In addition, additional air can be introduced into the upper part of the furnace, if need be. In a soda recovery boiler black liquor is combusted by injecting it into the boiler at a suitable height. The dro of black liquor is dried and partly combusted while falling, and a glowing hill is formed on the bottom of the boiler. Even in a soda recovery vessel, air is introduced in phases at different heights, and combustion is only complete after all air registers. Thus the lower part of a soda recovery boiler functions according to the same phase principle as the fluidized-bed boiler.

The aim of the present invention is to achieve a method which can be used to raise the efficiency of a boiler with phased function, and in particular, the proportion of the electric power it produces can be increased.

The invention is based on the concept of exhausting gas from an air-deficient zone of the furnace by suction in phased burning, the gas being then separately combusted in a separate gas turbine or a separate boiler. An amount of additional fuel approximately corresponding to the amount of heat of gas exhausted by suction is then fed into the furnace or burner in order to compensate the heat volume removed from the furnace.

More specifically, the method according to the invention i characterized by what is stated in the characterizing part of claim 1.

The assembly according to the invention, then, is characterized by what is stated in the characterizing part of claim 13.

The invention offers considerable benefits.

By means of the invention, the production capacity of an existing power plant can easily be increased, as well as its electricity production efficiency, as combustable gas formed during the initial burning stage of the fuel is removed from the furnace by suction such that it can be utilized for power production. Combustible gas is always first formed during combustion, said gas being completely combusted given that a sufficient amount of oxygen is provided and the temperature is sufficiently high. The maximum amount of gas exhausted depends on the fuel but is in an order corresponding to the amount of gas formed by the volatile components in the fuel. The changes made in the previous process are small and mainly related to automation. More benefits can also be gained in the form of cheaper district heat power where the plant has a gas or an oil vessel for the combustion of the product gas. The method is well suited for evening out the consumption peaks in small power plants because it will provide cheap additonal energy from the same vessel if need be, and there is no need to use expensive fuel in gaseous or liquid form. The invention can be applied to many kinds of plants as long as the combustion takes place either in a boiler or a burner in sufficiently many phases.

The function of the system and, in particular, the gasification part of the system is balanced by the fact that the gasification process runs as a secondary line in a big and stable process and is therefore not as sensitive to failure as are small carburettors where the gasification process also needs to produce the heat required for gasification, whereby the quality of the gas produced is easily subject to compromising. In the process of the invention, the heat required by gasification is obtained by means of the combustion process because heat is emitted from the zone for complete burning downstream into the fluidized bed, keeping up the gasification of the fuel.

Gas suction from the vessel does not reduce the efficiency of the vessel; instead, more fuel can be fed into the inlet area for primary air in the same vessel, i.e., a greater amount of fuel can be gasified, but only an amount of gas corresponding to that previously combusted is combusted in the upper part of the furnace. Additional efficiency obtained by gasification is limited by the temperature decrease in the fluidized bed and the fact that the suspension velocity or, in a soda recovery boiler, the amount of the lowermost air (primary air) can only be increased within certain limits so as not to disturb the function of the boiler. However, from a big boiler, significant additional fuel efficiency can be obtained which can be used to replace expensive gas turbine or boiler fuel. The increase in fuel efficiency can be as high as 20 to 30 %.

The purification and cooling of gas under atmospheric pressure emerging from the unpressurized boiler for compression is cheap, and thus, the cost of the additional equipment needed is reasonable. In cellulose mills the gas produced by a soda recovery unit can be used as fuel for the lime sludge reburning kiln, the superheating furnace gas turbine or an auxiliary boiler.

In the following, the invention is depicted in more detail with reference to the accompanying drawings.

Fig. 1 illustrates one embodiment of the invention.

Fig. 2 illustrates a second embodiment of the invention.

Fig. 3 illustrates a third embodiment of the invention.

Fig. 4 illustrates a fourth embodiment of the invention.

Fig. 5 illustrates a fifth embodiment of the invention.

Fig. 6 illustrates a sixth embodiment of the invention. The invention is based on exhausting gas by suction during combustion in phases from an area where the flame is strongly air-deficient and where gas is formed, the gas being further burnt by feeding air. In a fluidized-bed boiler the gas is suctioned from an area in the furnace 24 of the boiler 1 where the quality of the gas is at its best. The suctioned gas is purified and compressed to be under high pressure, such that it can be used as gas turbine fuel, for example. Thus, an atmospheric-pressure carburettor is formed by very simple measures, and the efficiency of electricity production and fuel efficiency can be raised effectively also in old plants.

The invention can be implemented, e.g., in connection with fluidized-bed boilers. Air nozzles 2 are provided in the lower part of the furnace 24 in a fluidized-bed boiler 1, air being blown upward from the nozzles into the fluidized bed 1 above the nozzles. The fluidizing medium is usually sand. The fuel is fed through inlets 4 and forms an airborne bed together with the fluidizing medium, the fuel and air being efficiently intermixed in the bed. The velocity of the fluidizing air should be sufficiently low sow as to allow the fluidized bed to remain sufficiently shallow. The fluidizing air is also primary air of the boiler 1, the proportion of primary air being about 40 % of the total amount of air in the boiler, and thus, the amount of air is insufficient for completely combusting the fuel. Due to efficient mixing, however, the fuel reacts well with the amount of air available and is effectively gasified. Gasification efficiency is increased by heat emitted from the top part of the boiler.

A splash area 9 is provided upstream of the fluidized bed 3, injections of fluidizing medium and ungasified fuel particles rising into the splash area. No additional area is introduced into this area, and therefore, only the fuel will react with the rest of the free oxygen. The actual combustion of gasified fuel takes place further up in the boiler. Upstream from the splash area 9, additional air is fed into the boiler through secondary air inlets 5, whereb the gasified fuel is ignited and the temperature in this bed reaches the range from 800 to 1400 °C. Secondary air i fed in an amount causing the total air ratio to rise to a value close to 1, and accordingly, air is almost completel combusted in this bed. The rest of the air is fed into a zone 11 for complete combustion through tertiary air inlet 6, whereby combustion of all of the remaining gases and particles is aimed at in said zone.

From the zone 24 for complete combustion, combustion gases rise into the top part of boiler 1 provided with the first heat exchangers usually comprising steam circuit superheaters 7. From the superheaters 7 the combustion gases are brought to heat exchangers 8 in the convection channel and from there, further to purification and a chimney.

Gas used as fuel in a gas turbine or a separate boiler is exhausted from the boiler 1 via outlets 12 arranged above the fluidized bed. The outlets can be positioned in various ways but the gas must be exhausted evenly from the boiler so as not to disturb the streams in the furnace 24 too much. The suction nozzles are preferably positioned evenly around the walls of the boiler similarly to the positioning of the air infeed nozzles. In order to obtain the desired gas from the furnace 24 and in order not to suction the completely combusted gas from further up in the furnace, the combustion of which is no longer possible, the suction nozzles 12 for gas must be positioned below the secondary air infeed nozzles 5. The suctioned gas contains impurities and fluidizing medium which cannot be allowed to enter, e.g., a gas turbine. The gas is purified in a manner known per se whereby the first purification step in at least a fluidized-bed boiler preferably comprises a cyclone 14 or other apparatus capable of separating rough particles. The impurities in particle form separated from the cyclone 13 as well as the fluidizing medium are led back to the fluidized bed 3 through an inlet 14.

For turbine use the purification result is finished off by means of a washer 16 to which the gas is led along the line 15 or else, it is led to heat-proof filters. The advantage with a washer 16 lies therein that in it gas can be cooled at the same time. After the washer 16, the gas is led into a compressor along line 17, wherein the gas is compressed to service pressure, and the product gas is led to combustion along line 20 or is utilized is another manner. If the gas is combusted in the auxiliary combustion boiler, cyclone separation may alone suffice as purification and compression is not necessarily needed. The compressor 18 is rotated by means of a separate motor 19 or the compressor may be the compressor of the gas turbine, if the product gas is led directly into the turbine suction air.

In the solution of Fig. 2, the cyclone 21 is located in the fluidized bed 3 inside the furnace 24. Gas is suctioned into the cyclone 21 through duct 22 and the airborne material and solid particles are removed and conducted back to the bed via duct 23. In other respects the equipment corresponds to the above description.

Fig. 3 illustrates a circulating fluidized-bed boiler 25. In a circulating bed boiler combustion air is blown from the bottom of the boiler 25 upwards by means of inlets 27 at the bottom and by secondary air inlets 26 in the lower part of the furnace 24. The air flow rate in the furnace is kept so high that the fluidizing medium will rise to the upper part of the furnace where it is transferred into the cyclone 32 together with the combustion gases. In the cyclone 32 the fluidizing medium is separated from the combustion gases which are taken to the convection part 35 which is equipped with heat exchangers 8 in the conventional manner. The fluidizing medium falls into the lower part of the cyclone 32 from where it is fed back to the lower part of the furnace 24 and into the zone of the air inlets 26. Fuel is fed into the boiler via a separate fuel feeder 29 directly into the zone of the air inlets 26 or into the reflux channel 34 for fluidizing medium.

As the gases emerging from the cyclone together with the fluidizing medium comprise completely burned combustion gases and air needed for the aeration of the gasification, the amount of which is so small that the small fuel particles having entered with the fluidizing medium consum a large part of the oxygen derived therefrom, a very small amount of oxygen is admixed with the fuel fed into the reflux channel 34 before it is led into the furnace 24. Thus, the mixture of fuel, fluidizing medium and circulating gas is highly air-deficient, and accordingly, before ending up in the furnace 24, the fuel is strongly gasified due to the heat of the fluidizing medium and the hot combustion gases. The phasing of fuel fed directly into the furnace 24 takes place close to the feed point for fuel. In a circulating fluidized-bed boiler the fuel is mainly mixed with air in the vertical direction, and poor horizontal mixing takes place, wherefore there are highly air-deficient areas in the vicinity of the fuel feeding points, gasification occurring in these areas, and phasing and complete combustion of the fuel takes place while the fuel is transferred upward in the furnace. Thus, gasified fuel can be removed by suction from the immediate vicinity of the feed nozzles 29 in a similar manner as from the fluidizing medium reflux channel 34.

In a boiler using coal dust or oil as fuel phasing takes place in the burner and not in the boiler area. In present- day boilers, the so called wall combustion method is applied whereby the burners 36 are placed in the boiler walls and their flames are directed toward the boiler center. The combustion air is fed mainly through the burners in phases. In the typical coal burner shown in Fig. 4, the fuel is fed from the center of the burner and a two- part air channel 38, 39 is arranged around it. The structure of oil burners is similar to that of the burner in Fig. 4. Air fed in with the coal dust is effectively intermixed with the fuel and fuel is gasified under reducing conditions in the gasification area 41 near the root of the flame. Additional air is brought into this gasified fuel by means of an inlet, whereby the gasified fuel is ignited and partly burnt in the area 42 surrounding the gasification area 41. The rest of the air is brought to the area surrounding the flame by means of the inlet 38 and the fuel is completely combusted in the area 43.

Gas formed by the fuel can be removed from the reducing area 41 of the flame by suction in a similar manner as from fluidized-bed boilers. A suction pipe 40 is placed near the burner tip such that it reaches the gasification area 41. As the flow of air and fuel in modern phase burners is carefully planned, the position and size of suction pipe 40 must be carefully determined for each burner type so as not to disturb the function of the burner. Suction pipes can be placed in all burners in a boiler, or in just some burners.

In Fig. 5, a soda recovery boiler is shown. As for its structure, the upper part of the boiler with heat exchangers corresponds approximately to the upper part of a fluidized-bed boiler or a pulverized fuel boiler with their superheaters 7 and convection parts 35. The black liquor used as fuel in the soda recovery boiler is injected into the furnace 24 via inlet nozzles 45 positioned around the furnace. The fuel drops are dried and burnt while falling and all unburnt matter is gathered on the boiler bottom and forms a hill 48. Green liquor is removed from the edges of the hill 48 via outlets 49 and is the returned to the pulping process. Air is fed into the furnace in three phases through inlets 44, 46 and 47. At the first air inlets 47 the mixture of black liquor and air is extremely air-deficient, wherefore black liquor is merely gasified in this area. Thus, combustible gas can be suctioned from the furnace below the lowermost air inlets 47 or approximately at said inlets. Gas is suctioned through duct 50 which is cleaned for use by means of a cleaner 51. By means of the invention, the liquor processing capacity of an expensive boiler investment can be increased in a most economical manner. Soda recovery boilers being extremely expensive, they are dimensioned as small as possible, and there is clear need for additional capacity.

The heat value of the gas obtained by above described methods is relatively low since it contains plenty of inert components such as nitrogen and water. The heat value of the gas can be increased by decreasing the amount of nitrogen and/or water in the gas. In the method shown in fig. 6 the gas is enriched by decreasing its nitrogen content. This is accomplished by dividing the furnace partially by a wall 52 in two spaces and feeding extra fuel into one of the spaces. The space wherein the extra fuel is fed is smaller than the rest of the furnace space, whereby less air comes from the bed into this space. Since the amount of air in relation to the fuel is smaller in the space wherin the extra fuel is fed, the heat value of the gas in this space is greater than that of the gas in the neighbouring space. The amount of air that comes from the bed in the separated spaces of the furnace can be determined by the areas (x, y) occupied by the spaces over the bed 3.

The gasified excess fuel is removed from the furnace as high as possible because the flames in the main combustion zone above radiates energy downwards and also some clusters of bed material travel high in the furnace. The energy from the main combustion zone increases the energy of the top part of the smaller gasification space and facilitates the decomposition of tars of the fuel.

In addition to the above, the present invention has other embodiments, too.

In addition to a gas turbine the suctioned product gas can be burnt in an auxiliary boiler, a lime sludge reburning kiln or other boiler where the additional energy obtained by burning gas is needed. One way of utilizing the gas could be cementation performed within the mechanical engineering industry and the steel industry. In the method, steel is subjected, at an elevated temperature, to a reducing gas containing a lot of carbon monoxide, whereby coal is diffused on the surface of the steel, the coal considerably increasing the hardness of the steel during quick cooling of the steel by a martensite reaction. In this case the cementation furnace would be placed right next to the cyclone in the line, whereby the cyclone may first be followed by normal combustion or purification and compression into a high pressure. The method could be implemented to reduce fuel costs within the mechanical engineering industry.

The method according to the invention is particularly well suited for old boilers but can also be advantageously exploited in new boilers whereby greater than usual efficiency is achieved with a boiler of a certain size. The parts relating to the use of gas and to the combustion of solid material can naturally be selected more freely in new boilers than in already existing boilers because the assembly can be designed to function in a certain manner from the outset. The method can be implemented in connection with an additional or an initiation burner and for damp fuel, if the fuel is dried before combustion so as to improve gas quality.

Claims

Claims :

1. A method for increasing the capacity of a boiler plant, comprising steps of

- feeding air and fuel are into a furnace (24) ,

- feeding the air into the fuel during a first phase in a lesser amount than is required for complete combustion of the amount of fuel fed, whereby the fuel is gasified,

- feeding air into the fuel during at least one later phase for firing and combusting the gasified fuel,

- keeping the pressure of the furnace essentially at ambient pressure,

characterized by

- removing gas formed by the fuel from a mixture of fuel and air by suction from a reducing zone deficient in air before mixing additional air into the mixture, and,

- using gas removed from the furnace in a separate apparatus .

2. A method as defined in claim 1, characterized by feeding an amount of additional fuel approximately corresponding to the amount of heat of gas exhausted by suction into the furnace or burner in order to compensate the heat volume removed from the furnace.

3. A method as defined in claim 1 or 2, characterized in that the gas removed by suction is purified and used as gas turbine fuel.

4. A method as defined in claim 1 or 2 , characterized in that the gas removed by suction is combusted in a separate boiler.

5. A method as defined in claim 1 or 2, characterized in that the gas removed by suction is fired in a lime sludge reburning kiln.

6. A method as defined in any foregoing claim wherein the boiler is a fluidized-bed boiler and coprising steps of

- feeding air and fuel into the lower part of the combustion chamber,

- feeding less air into the furnace during a first phase at a first air inlet level than is required for complete combustion of the amount of fuel fed, whereby the fuel is gasified, and

- feeding air into the furnace during at least one later phase on a second air inlet level above the fuel inlet level and the first air inlet level,

characterized in that

- exhausting the gas formed by the fuel from the combustion chamber by suction below the second air inlet level and in the area of the fluidized bed.

7. A method as defined in claim 1 whereby the boiler is a soda recovery boiler, characterized in that gas is exhausted by suction from an area between a second air inlet level (46) and a green liquor removal point (49) .

8. A method as defined in claim 1, wherein the boiler is a circulating fluidized-bed boiler, characterized in that gas is exhausted by suction from the combustion chamber (24) from a reducing area in the vicinity of the fuel inle point (29) .

9. A method as defined in claim 1 wherein the boiler is a circulating fluidized-bed boiler where the circulating bed material is returned to the combustion chamber (24) along return line (34) , characterized in that fuel is fed into the return line (34) and gas is exhausted by suction from the area between a fuel inlet point (30) and the furnace (24).

10. A method as defined in claim 1, wherein the boiler is pulverized fuel boiler or an oil vessel with a phased burner, characterized in that gas is exhausted from the reducing area of the burner flame.

11. A method as defined in claim 1, wherein at least one initiation burner or additional burner is used in the boiler, characterized in that gas is exhausted by suctio from the reducing area of the flame of the initiation or additional burner.

12. A method as defined in any foregoing claim, characterized in that damp fuel is used in the boiler which is dried prior to burning so as to improve the quality of the gas exhausted by suction.

13. An assembly for increasing the capacity of a boiler plant, the assembly comprising:

- a boiler (1) with a furnace (24) ,

- means (4) for feeding fuel into the combustion chamber and for burning the fuel, - means (2) for mixing air into the fuel being fed into the furnace (24) during a first phase in order to gasify the fuel, and

- at least one second means (5, 6) for feeding air into the mixture of air and fuel during at least one later phase for lighting and burning the fuel,

characterized by

- at least one means for exhausting gas from the reducing area beyond the fuel inlet point prior to mixing air into the fuel during the second phase, and

- at least one apparatus capable of using the exhausted fuel.

14. An assembly as defined in claim 13, comprising

- a fluidized-bed boiler (1) with a furnace (24) ,

- means (4) for feeding fuel into the furnace (24) ,

- means (2) for feeding air into the furnace (24) during the first phase, and

- at least one means (5, 6) above the first air feeding means (2) for feeding air into the furnace (24) during at least one succeeding phase,

characterized by

- at least one outlet located below the air feeding means (5) for exhausting gas from the furnace (24) by suction.

15. An assembly as defined in claim 14, characterized in that the suction outlets (12) for gas are located in the fluidized bed (3).

16. An assembly as defined in claim 15, characterized by a cyclone (21) arranged inside the furnace (24) , the cyclone being connected to the suction outlets (22) for gas.

17. An assembly as defined in claim 13, characterized by a cyclone (13) arranged outside the combustion chamber (24) as well as a washer (16) for purifying the gas exhausted from the combustion chamber (24) by suction.

18. An assembly as defined in claim 13, wherein the boiler is a soda recovery boiler, characterized by at least one outlet (50) for removing gas by suction from the area between the second air feeding point (46) and the green liquor discharge point (49) .

19. An assembly as defined in claim 13, wherein the boiler is a circulating fluidized-bed boiler, characterized by at least one outlet for exhausting gas from the combustion chamber (24) by suction from the reducing area in the vicinity of the fuel feeding point (29) .

20. An assembly as defined in claim 13, wherein the boiler is a circulating fluidized-bed boiler comprising a return line (34) for returning circulating bed material into the combustion chamber (24) , characterized by means (30) for feeding fuel into the return line (34) and for exhausting gas by suction from the area between the fuel feeding point (30) and the combustion chamber (24) .

21. An assembly as defined in claim 13, wherein the boiler is a powdered-coal boiler or an oil vessel using a phased burner (36) , characterized by means (40) for exhausting gas by suction from the reducing area of the burner (36) flame.

22. An assembly as defined in claim 13, wherein the boiler comprises at least one ignition or additional boiler, characterized by means for exhausting gas by suction from the reducing area of the additional or ignition burner.

23. An assembly as defined in any of the claims 13 - 22, characterized by a wall (52) which divides the furnace partially in two spaces having different volume and means for feeding extra fuel into one of the spaces.