WO1998025075A1

WO1998025075A1 - Method and arrangement for separating bed material in a circulating fluidized bed boiler

Info

Publication number: WO1998025075A1
Application number: PCT/FI1997/000712
Authority: WO
Inventors: Tommy Jacobson; Jukka Hirvenoja
Original assignee: Imatran Voima Oy
Current assignee: Imatran Voima Oy
Priority date: 1996-12-04
Filing date: 1997-11-20
Publication date: 1998-06-11
Anticipated expiration: 1999-06-04
Also published as: AU5054698A; FI964839A0; FI101102B

Abstract

A method and an arrangement for separating fluidized bed medium from flue gas flow in a circulating fluidized bed boiler. The invention is based on forming at least one vigorous vortex (19) in the top part (2) of the furnace (1) by means of at least one gas jet blown toward the centre of the furnace (1), the vortex bringing the particles into furnace (1) corners or to separate separator devices in the vicinity of the walls by means of centrifugal force. The vortex or vortices (19) formed must be shifted sideways from the centre axis of the furnace (1) such that no calm area with no particle separation is formed in the middle of it.

Description

Method and arrangement for separating bed material in a circulating fluidized bed boiler

The present invention relates to a method according to the preamble of claim 1 for separating fluidized bed medium from flue gas flow in a circulating fluidized bed boiler .

The invention also relates to an arrangement for implementing the method.

In circulating fluidized bed boilers, the fluidized bed medium usually circulates via the top part of the boiler into a cyclone adjacent to the furnace, the flue gases and the fluidized bed medium being separated from each other in the cyclone. The fluidized bed medium is returned to the bottom part of the boiler and the fuel used is fed into a reflux inlet or the bottom part of the furnace. In circulating fluidized bed boilers, combustion air is led into the furnace upwards from its bottom part by blowing and through inlets arranged in the bottom part of the furnace blowing towards the centre of the furnace. The fuel is fed into the bottom part of the furnace or admixed with the flow of fluidized bed material returning from the furnace, said flow being fed into the bottom part of the furnace.

As stated above, in a circulating fluidized bed boiler combustion air is blown upwards from the boiler bottom by means of inlets in the bottom and inlets for combustion air arranged in the bottom part of the furnace. The air flow rate in the furnace is kept at such a high level that a part of the fluidized bed medium rises into the top part of the furnace from where it is conveyed to a cyclone adjacent to the furnace together with the flue gases. In the cyclone, the fluidized bed medium is separated from the flue gases which are led into a convection part which in a conventional manner is equipped with heat exchangers. The fluidized bed medium is separated into the lower part of the cyclone from where it is fed back to the bottom part of the furnace into the air inlet area. Fuel is fed into the boiler through a separate fuel inlet straight into the air inlet area or into a reflux channel for fluidized bed medium.

In circulating fluidized bed combustion a separate particle separator is usually connected to the furnace. Previously known and widely used methods comprise various kinds of cyclone separators. These are described, for instance, in FI Patent Applications No. 905070, A. Ahlstrδm; 910809, A. Ahlstrδm; and 850973, A. Ahlstrδm. However, it is characteristic of the known methods that the mixture of fluidized bed material and flue gases is led into a separator outside the furnace, from which separator the flue gases are then forwarded to the following heat delivery surfaces and the particles are returned to the furnace, usually into its bottom part. The cyclone can also be constructed inside the furnace. However, in these solutions there are always massive structures inside the furnace, and even a cyclone constructed inside the furnace is a traditional one as regards its operating principle even though it may comprise several inlets.

Particle separation can also be carried out be means of flows inside the furnace. Separation in the furnace by means of gas jets has been described in a number of references. In these solutions, the particles are separated onto the walls of the furnace by means of a vortex formed in the furnace, usually in a vertical furnace. The separating vortex has been formed in the furnace by means of combustion air jets from several walls. One developer of such a unit is Delft University of Technology. In this unit the combustion air jets are tangential and the vortex is formed in the middle of the furnace wherefore the particles cannot freely pass the air jets and reach the furnace walls, which explains the poor separation capacity which is insufficient for commercial energy production. This being the case, the unit is intended as a test unit for monitoring the combustion process.

One way of enhancing particle separation in circulating fluidized bed combustion is to feed even as much as 75 % of the combustion air into the cyclone whereby the flow of combustion air intensifies the vortex. Even then, the actual circulating fluidized bed furnace and the cyclone are separate devices. Research work on this has been carried out by Donlee Technologies Inc. The unit is described in the ASME Fluidized Bed Combustion conference publication of 1991, p. 203 to 209. The disadvantage involved in a separate cyclone separator and furnace lies in the complicated and large construction of the unit. Due to heat expansion, the arrangement must be made to contain several expansion joints, and in particular the implementation of the passage between the cyclone and the furnace provides problems.

The gas flows treated in separators are of relatively great volume and must withstand very abrasive and hot conditions. Accordingly, they must be protected by abrasion and heat resistant materials and they must also be thermally insulated. For these reasons, a separate separator entails a significant increase in the cost of a boiler plant.

The aim of the present invention s to achieve a method and an arrangement where particle separation is carried out m the actual furnace m a circulating fluidized bed reactor by means of a gas et and appropriate design of the furnace walls.

The invention is based on the notion of forming an intense vortex m the top part of the furnace by means of at least one gas net whose blow is directed toward the centre part of the furnace, the vortex transferring the particles to the vicinity of the walls m the furnace corners or to separate separator devices by centrifugal force. The vortex or vortices formed must be arranged shifted sideways from the centre axis of the furnace such that no calm area is formed m the middle of it leaving the particles unseparated.

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

The arrangement according to the invention, then, is characterized by what is stated m the characterizing part of claim 11.

The invention offers considerable benefits.

The entire boiler plant construction can be much simpler because no external structures and no passage between the furnace and the separator are needed. This results in reduced boiler size and, hence, extensive savings in the entire plant. The heat transfer surfaces of the furnace walls can be increased in number compared to a boiler equipped with a separate particle separator. Fewer motion joints are required and the boiler weight is reduced, and accordingly, the set-up investments required are reduced. In the arrangement according to the invention, the particle velocity is moderate wherefore erosion problems are smaller than in cyclones where the velocity of the particle/flue gas mixture is considerable.

As the particles are separated in the furnace, combustion continues in the top part of the furnace even after separation, whereby small particles which are not necessarily separated by the separator due to the possibly insufficient efficiency thereof are further reduced in size before they end up in means for flue gas deposition and as flue dust. Thus, a small concentration of incombustibles is achieved, and flue gas deposition is facilitated. It is possible to affect the conditions in the furnace and thus to optimize the combustion by means of the choice of jet gas and by the choice of its temperature and the amount thereof. Air or circulation gas is advantageously used as the jet gas.

In the following, the invention is described in closer detail with reference to the annexed drawings.

Fig. 1 is a side sectional view of the boiler according to the invention.

Fig. 2 is a sectional view of the furnace of the boiler of Fig. 1. Fig. 3 is a sectional view of Fig. 2.

Fig. 4 is a sectional view of Fig. 2.

Fig. 5 is a sectional view of Fig. 2.

Fig. 6 is a sectional view of a second embodiment of the invention .

Fig. 1 depicts the boiler according to the invention including a convection part and a heat exchanger. The furnace 1 consists of a fluidized bed part 2 and a separation part 3. In this context, the term 'fluidized bed part' is used to refer to the part of the furnace where the fuel, combustion air and fluidized bed medium form a mixed suspension and which contains a considerable amount of fluidized bed medium. The term 'top part of the furnace' or 'separation part' is used to refer to that part of the furnace where the mixture of fuel and air contains essentially less fluidized bed medium and where exhaustive combustion of the fuel takes place essentially without the presence of fluidized bed medium. The top part of the furnace is also termed the freeboard. In the embodiments according to the figures, the bottom part and the top part of the furnace are clearly divided into separate parts and their centre axes have been shifted in relation to one another. The lower section of the fluidized bed part 2 comprises air inlets 4 and a fire grate 5. The upper section of the fluidized bed part 2 turns slightly away from its centre axis and ends in the particle separation part which forms the top part of the furnace 1. The separation part 3 of the furnace 1 comprises discharge means 6 for flue gas, leading to the convection part 7 of the boiler, the convection part being furnished with heat exchangers 8 for recovering the thermal energy of the flue gases. Furthermore, the furnace may in the conventional manner have water-cooled walls (not shown) through which most of the heat transfer from the fuel occurs, or walls furnished with fire- resistant brickwork. In the example depicted in the Figures, the convection part 8 ends in a cyclone 9 from which the flue gases are taken to purification through a pipe 10 and then to a chimney. The solid particles contained in the flue gases are separated in the cyclone 9 and returned to the furnace 1 through a return passage 11. At this stage the solid particles mainly comprise light non-combusted fuel particles and incombustible solid material. The cyclone as such is not always necessary because of the good separation in the furnace; instead, the flue gases can be purified using a conventional electric filter.

The top part or the separation part 3 of the furnace 1 is slightly shifted sideways in relation to the bottom part 2 of the furnace such that a slanting opening 12 is formed between the fluidized bed part 2 and the separation part 3. A wedge-shaped partition wall 13 is arranged below the lower edge of this opening 12, the partition wall forming two receiving hoppers 14 together with the walls 16 of the separation part 3, the hoppers leading to two return passages 15 which in turn lead to the fluidized bed part 2 of the furnace 1. A gas nozzle 18 is arranged in the middle of the furnace 1 wall 17 on the side of the convection part 7 and on the side of the upper edge of the slanting furnace opening 12, the gas nozzle being pointed at the receiving hoppers 14 and return passages 15. In the boiler, fuel is fed into the bottom part 2 of the furnace at a point close to the air inlets 4. Air introduced into the bottom part of the furnace 1 makes the mixture of fuel and fluidized bed medium rise upwards. At this stage, the aim is to keep the velocity of the fluidized bed medium and the fuel low, said velocity preferably being about 5 m/s. When the fluidized bed medium rises to the slanting opening 12 between the bottom part 2 and top part 3 of the furnace, its flow is turned sideways toward the hoppers 14 and return passages 15 because the top part 3 of the furnace is shifted sideways from the centre line of the bottom part 2. At the same time, the lift of the gas flow from the fluidized bed part 2 driving up the fluidized bed medium is weakened and most of the fluidized bed medium falls straight into the hoppers 14 and returns to the bottom part 2 of the furnace via the return passages 15. Only part of the fluidized bed medium and the non-combusted fuel particles as well as the combustible gas formed from the fuel by gasification rise into the top part or separation part 3 of the furnace 1. The fuel is burnt out in the separation part 3.

A gas nozzle 18 is arranged in the middle of the wall facing the hoppers 14 in the separation part 3. A high- velocity gas jet is blown towards the centre of the separation part by means of this nozzle 18, the jet dividing the furnace into two and forming, together with the flue gases, the combustible fuel and the fluidized bed medium, two vortices 19 in the separation part 3. In these vortices heavier particles travel towards the furnace 1 walls, being driven out of the vortex at the corners of the separation part 3, whereby the lift of the gas flow ends and the particles fall into the hoppers 14. The gas flow in the nozzle may be of extremely high velocity but even so, the velocity of the fluidized bed medium remains relatively low and, in particular, the velocity of the particles in the vicinity of the walls is low, the particles thus causing only little erosion. If two parallel vortices are formed in the separation part in accordance with this example, better separation capacity is achieved than when one bigger vortex is used.

The cas jet can be formed using air or circulation gas depending on the desired air distribution arrangement in the furnace. As this embodiment comprises the formation of two separating vortices in the separation part, two flue gas outlets 6 and two return passages 15 and hoppers 14 are provided in the upper section thereof for collecting the circulating fluidized bed medium.

Intense vortices are formed in the top part of the furnace essentially by means of one gas jet, whereby the vortices move the particles close to the walls and into the corners of the furnace by means of centrifugal force, the particles then falling down due to the action of gravity. Separation can be enhanced by means of separator devices on the walls, said devices removing the particles from the gas flow.

Depending on the furnace construction, the gas jet may comprise circulation gas or secondary air depending on the need of regulating the furnace temperature and that of the fluidized bed material.

The bottom part 2 of the circulating fluidized bed furnace is dimensioned in compliance with principles known as such and operates in a conventional manner. A vortex space is formed in the top part 3 of the furnace, wherein the separation of solid particles is performed by forming an intense vortex by means of a high-velocity gas jet. The cross section of the furnace may in the conventional manner be shaped as a square, a rectangle or a circle, or its shape may be a mixture of these, and the operation of the arrangement is not dependent on the shape of the furnace. However, an at least partially angular construction or separator devices fitted on the boiler walls and slowing down the flow will enhance separation capacity.

In a preferred embodiment, two parallel vortices are formed in the rectangular top part 3, the vortices whirling into opposite directions . The mixture of gas and particles is brought from the fluidized bed part 2 of the furnace essentially without choking the cross section into the possibly slightly expanded top part where the vortex is formed mainly by means of an external gas jet. The flow of fluidized bed medium from the bottom part 2 of the furnace is preferably turned at least slightly sideways whereby it will flow mainly of itself into the return hoppers 14 and passages 15. This will essentially facilitate separation in the top part 3 of the furnace.

The separation apparatus comprises a nozzle or nozzles, or one bigger nozzle on one wall, arranged in a relatively narrow and tall vertical line. The gas nozzle or the divided gas nozzles are preferably positioned in the furnace such that the gas jet is in the lower section of the separation part, preferably such that the lower edge of the jet is level with the upper edge of the opening from the bottom part of the furnace. In addition, extra nozzles may be arranged in the top part of the furnace. The nozzles preferably point at the centre of the furnace. In some cases it may be of advantage to deviate the j et slightly from the centre axis of the furnace but the et must always be directed from the wall towards the centre of the furnace in order to avoid the formation of a wide vortex formed when a tangential net is used, such a vortex having an essentially non-whirlmg centre. The vortex formed by means of the gas net must be at least slightly eccentric m relation to the bottom part of the furnace, or, advantageously, two vortices are formed m the separation part 3 m accordance with the above example, whereby no calm area with no particle separation is formed m the furnace.

In addition to the above, the invention also has other embodiments .

The flue gas exhaust pipes may enter the top part advantageously either through its top surface or from below such that the gas flow has to be pronouncedly redirected as is the case in the cyclone separator. Fig. 6 depicts a construction where the flue gas exhaust 20 is taken to the furnace 1 at a point in its side at the opening between the bottom part 2 and the top part 3 of the furnace 1. The exhaust 20 points upwards and is taken slightly above the upper edge of the gas nozzle 18 in the vertical direction. In this solution the flue gases flow upwards m vortices such as those described above and at the same time solid particles move to the sides of the separation part 3. A downward flowing air flow is formed inside the separation part 3, flue gases separated from the solid particles flowing down with it. As the flue gas flow has to turn back and flow downwards in the top part of the furnace 1, separation capacity is enhanced because the turning flow pushes the particles towards the boiler walls and their velocity is simultaneously reduced due to the turning, and thus, when ending up on the furnace walls they rapidly move forward to the separation vortices 19. A further benefit of this construction is that it can be given a lower height than a conventional circulating fluidized bed boiler where flue gases are removed from the top part of the furnace .

Claims

Claims :

1. A method for separating fluidized bed material from gases in a circulating fluidized bed boiler, comprising steps of

- feeding fuel into a furnace (1) containing fluidized bed medium,

- feeding air into the bottom part (2) of the furnace (1) to serve as combustion air and to blow the fuel/fluidized bed medium suspension upwards in the furnace, and

- separating the fluidized bed medium from gases in the top part (3) of the furnace (1) by means of at least one gas jet blown into the furnace (1), and it is then taken back to the bottom part (2) of the furnace,

characterized by

- blowing the gas jet into the furnace by directing the jet from a furnace wall (17) toward its inner part such that at least one vortex is formed inside the furnace, so that the centre axis of the vortex is shifted sideways in relation to the centre axis of the top part of the furnace.

2. A method according to Claim 1, characterized in that at least one gas jet is blown into the furnace (1) toward the centre axis of the furnace (1) such that the jet divides the furnace (1) in two parts forming a vortex (19) on its both sides.

3. A method according to claim 2 for boilers furnished with an at least partly rectangular furnace, characterized in that the gas jet s blown into the furnace from a point in the middle of the wall constituting the long side (17) of the rectangle.

4. A method according to claim 2, characterized in that the flue gases are exhausted from the top part of the furnace (1) through two exhausts (6) arranged at the vortices (19) in the top part of the furnace and having openings pointed downwards, and the fluidized bed medium is led back to the bottom part (2) of the furnace through two reflux passages (15) .

5. A method according to claim 2, characterized in that the flue gases are removed from the furnace (1) through at least one exhaust (20) taken in from belov; and having an upward directed opening.

6. A method according to any one of the above claims, characterized in that the direction of the flow of fluidized bed medium rising from the bottom part (2) of the furnace into its top part is rerouted sideways in relation to the centre axis of the bottom part (2) whereby the fluidized bed medium falls downwards due to the influence of its own weight after it has risen into the top part (3) .

7. A method according to any one of the previous claims, characterized in that air is used as the jet gas.

8. A method according to any one of the previous claims, characterized m that circulation gas is used as the net gas .

9. A method according to any one of the previous claims, characterized m that the jet is divided in the vertical direction nto sublets from several nozzles.

10. A method according to any one of the previous claims, characterxzed in that one separating net is blown into the furnace (1) in the lower section of its top part and one auxiliary jet above it.

11. An arrangement for separating fluidized bed material from gases m a circulating fluidized bed boiler, comprising

- a furnace (1) containing fluidized bed medium, whereby fuel can be fed into the furnace,

- means (4) for feeding air into the bottom part

(2) of the furnace (1) to serve as combustion air and to blow the fuel/fluidized bed medium suspension upwards m the furnace,

- means (18) for forming at least one gas jet to be blown into the furnace (1) for separating the fluidized bed medium from the gases m the top part (3) of the furnace (1), and

- means (14, 15) for leading the fluidized bed medium back into the bottom part (2) of the furnace,

characterized by - at least one nozzle (18) for blowing the gas jet into the furnace so that the jet is directed from the furnace wall (17) towards its inner part whereby at least one vortex is formed in the furnace (1), the centre axis of the vortex being shifted sideways in relation to the centre axis of the top part (3) of the furnace.

12. An arrangement according to claim 11, characterized in that the gas nozzle (18) is directed towards the centre axis of the furnace (1) such that the jet divides the furnace (1) into two forming a vortex (19) on its both sides.

13. An arrangement according to claim 12, wherein the boiler has an at least partly rectangular furnace, characterized in that the gas nozzle is fitted in the furnace wall in the middle of the wall forming the long side (17) of the rectangle.

14. An arrangement according to claim 12, characterized by two exhausts (6) for removing flue gases, arranged at the vortices (19) in the top part (3) of the furnace, the exhausts having openings pointing downwards, and two reflux passages for leading the fluidized bed medium back into the bottom part (2) of the furnace.

15. An arrangement according to claim 11, characterized by at least one exhaust (20) for flue gases taken into the furnace (1) from below and having an upward directed opening.

16. An arrangement according to any one of the previous claims 11 to 15, characterized in that the furnace (1) is divided into a separate bottom part (2) and a top part (3) and the top part (3) is shifted in relation to the centre axis of the bottom part (2) in order to diverge the flow of fluidized bed medium entering the top part from the bottom part (2) in relation to the centre axis of the bottom part (2), whereby the fluidized bed medium falls downwards due to the influence of its own weight after rising into the top part (3) .

17. An arrangement according to any one of the previous claims 15 to 16, characterized by a number of nozzles arranged on top of one another for forming a narrow vertical gas jet.

18. An arrangement according to any one of the previous claims 11 to 17, characterized by means (18) in the lower section of the top part (3) of the furnace for blowing one separating jet into the furnace (1) and by second means above it for blowing an auxiliary jet above the separating jet.

19. An arrangement according to any one of the claims 11 to 18, characterized in that the means (18) for blowing a gas jet into the furnace (1) are positioned such that the lower edge of the jet is arranged essentially at the upper edge of the opening (12) leading from the bottom part (2) to the top part (3) .