FI122040B - Process and arrangement for feeding fuel into a boiler with circulating fluidized beds - Google Patents

Description

The present invention relates to a method and an arrangement for feeding fuel to a circulating fluidized bed boiler. A particular object of the invention is the supply of fine, light, easily volatile and / or moist fuel to the boiler.

A circulating fluidized bed boiler (CFB) generally has a furnace having a bottom, si-10 baffle walls and roofs with at least one particle separator which is in fluid communication with the top of the furnace. At least a portion of the walls of the furnace bottom portion may be inclined so that the cross-sectional area of the furnace increases upwardly, i.e., the inclined wall portion of the furnace may be referred to as a shrinkable bottom portion. Virtually all the walls and ceiling of the boiler and the separator comprise water pipes to recover heat from the furnace. There is a grate at the bottom of the furnace for feeding combustion, suspension or fluidizing gas into the furnace and for removing ash and other debris from the furnace. The sidewall of the furnace has means for supplying fuel to the furnace and means for supplying secondary air to the furnace. Arrangements are also provided in the furnace for feeding inert bed material, usually 20 sand.

The particulate separator separates the solid particles into the separator from the flue gas solid particles suspension coming from the top of the furnace. Separated solids are recycled back to the bottom of the furnace via a recirculation port, wherein the recirculation port has a closure device, such as a knee structure, which is intended to prevent gas from flowing into the separator through the recirculation port. Thus, at least one additional opening for the supply of solids to the furnace wall is required. This cycle of solids is called the external cycle. The separator inlet end | In addition to the flow of the vertical flue gas solid particulate suspension g 30 from the incoming furnace, there is a vertical downward flowing particulate stream near the furnace walls. This cycle of solids is called the internal cycle.

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Very often, in the case of internal or external solids circulation, or both, the fluidized bed heat exchange chamber is arranged to transfer heat from the bed of the fluidized solid particles to the heat transfer medium. The heat exchange chamber may be located in the furnace of the circulating fluidized bed boiler near at least one wall of the furnace. The preferred location of the heat transfer chamber (s) is near the bottom of the furnace where the chamber is fixedly connected to the inclined wall (s). The fluidized bed heat exchanger can also be arranged in an external circulation such that the solids leaving the solids separator are removed to the heat exchange chamber as they return to the furnace (see, for example, prior art Fig. 1). Inside the heat exchange chamber, there are heat exchange means for transferring heat from the solid to the heat transfer medium flowing in the heat exchange means.

Light, finely divided and / or moist fuels such as fine carbon powder or peat or sawdust or finely divided lignite are problematic in two different aspects. Light, low density and particle size fuels are easily transported with the fluidizing gas and rise rapidly so that the combustion process begins a few meters above the grate, whereby only a small amount of fuel is not sufficient to maintain the bed material temperature at a sufficient level. . In particular, this can result in lower bed temperatures and higher temperatures in the upper part of the furnace, especially at lower loads, which in turn can lead to problems with boiler emissions and load change rate.

Similarly, the use of moist fuel can lead to similar problems, but for somewhat different reasons. Although the moist fuel may not be too light, it will take some time for the fuel to dry, so that when the fuel is dry and still "incapable" of igniting, the fluid is again lifted with a suction gas into the upper part of the furnace. When the fuel is finally dry enough, ignites and finally burns, there may not be enough in the lower bed area

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30 combustible fuel, which can again cause the bed temperature to be low and lead

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^ the above problems.

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A further problematic fuel is a fuel that contains mostly volatile components and a smaller amount of solid carbon. The volatile components form flammable gases such as CO, CH4, H2, etc. very close to the fuel inlet, after which the flammable gases move upward with the fluidizing gas. This upward flow provides a gas column with which oxygen cannot mix rapidly and efficiently, resulting in the combustion of the gases in the upper parts of the furnace.

Usually, fuel is supplied to the furnace through one or more openings in the furnace wall. Depending on the type of fuel, the fuel is dispensed into the furnace either as a fuel air suspension, i.e. pneumatically or by means of 10 feed screws or some other mechanical supply means. Usually, the inlet (s) is located in the (shrinking) base of the furnace walls.

Solids from the external circulation of the furnace, i.e. directly from the separator or through the fluidized bed heat exchanger, are also fed through one or more openings in the wall of the furnace.

It is an object of the invention to provide the light, fine, volatile and / or moist fuel with a sufficient residence time for drying and / or burning in the lower bed area of the furnace to optimize the entire combustion process. According to the present invention, the fuel residence time at the lower bed area is increased by slowing the fuel movement to the upper bed area by combining the fuel and solids flowing from the inner and / or outer rotation towards the lower bed area.

Another object of the invention is to prevent the formation of a statue of upwardly burning combustible gases above the fuel inlet by introducing a circulating bed material into the gas column so that the statue is broken and turbulence is increased so that the oxygen reaches the previously burning gases. looser and the combustion takes place lower in the furnace.

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Numerous documents are known in the prior art which speak of the mixing of fuel and recoverable bed material.

It is known, for example, to supply moist fuel and bed material to a separate mixer / dryer prior to feeding the fuel solid mixture to the boiler. U.S. Patent Nos. 4,259,911,4,690,076 and 5,419,267 discuss various alternatives for separating mixers / dryers. However, separate mixer / dryer units increase costs both in the investment / construction phase and in the operating phase, and form another part of the equipment which requires special maintenance.

It is also known to supply fuel to the external circulation between the particulate separator and the furnace, as described, for example, in U.S. Patent 4,442,795.

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However, the above-mentioned patents do not mention the above-mentioned problems or, if they are (wet fuels), lead to very complicated arrangements with a separate particle dryer outside the boiler.

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It is a further object of the present invention to propose some alternative solutions to the above problems related to the supply and combustion of fine, light, volatile and / or moist fuels for circulating fluidized bed boilers.

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All alternative solutions are based on fuel supply to the furnace so that the heat transfer process between the fuel and the recycled solid is improved by improving the mixing between the fuel and the circulating hot bed material.

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A first preferred embodiment of the present invention is based on collecting bed material downwardly filed along the walls of the furnace to be fed in a collected form onto the incoming fuel Z.

Other features of the method and apparatus of the present invention are apparent from the appended claims.

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Section 05 The present invention has solved at least some of the circulating fluidized bed

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35 reactor problems with the supply and combustion of finely divided, light, volatile and / or moist fuel by a simple and efficient device for feeding fuel to the furnace of a circulating fluidized bed boiler. For example, there is no need for the design and construction of exterior drying chambers known in the art for drying wet fuel. Also, there is no longer a need to mix light, powdery fuel with hot bed material in a separate mixing chamber.

The present invention enables the bed temperature to be kept higher, even in the lower parts of the furnace, when burning light, fine, volatile and / or moist fuel. This is especially true with lower boiler loads when the bed temperature tends to decrease for natural reasons. Compared to prior art arrangements, the introduction of the present invention results in: • the wet fuel starting to dry earlier, · the dried fuel igniting and burning earlier, • a greater proportion of light and fine fuel being lowered into the bed, raising or staying acceptable bed temperature even under low load conditions; • mixing combustible fuel with a high density recyclable solid stream raises the temperature of the solid stream, which then transfers heat to the bottom of the furnace, which further raises the bed temperature; further improves combustion in the lower portions of the furnace and ^ leads to smoother combustion and heat flow distribution across the transverse direction, and io · fuel supply can be provided to the higher hearth wall, with lower back pressure.

oy 30 ^ The last two advantages are not necessarily related to light and fine fuels, but to all types of fuel.

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The method and arrangement of the present invention will now be described in more detail with reference to the following drawings, in which:

Fig. 1 schematically illustrates a prior art circulating fluidized bed boiler, Fig. 2a schematically illustrates a front view of a first preferred embodiment of the present invention,

Fig. 2b schematically illustrates a side view of a first preferred embodiment of the present invention,

Fig. 3 schematically illustrates another preferred embodiment of the present invention,

Fig. 4 schematically illustrates a third preferred embodiment of the present invention,

Fig. 5 schematically illustrates a fourth preferred embodiment of the present invention, and Figs. 6a-6c schematically illustrate numerous alternatives of a fifth preferred embodiment of the present invention.

Figure 1 schematically illustrates a prior art circulating fluidized bed boiler. The boiler 10 comprises a furnace 12 having substantially vertical walls 32 for discharging flue gas and solid particles therein at the upper end of the furnace 12 into the solids separator 16, to extract the purified exhaust gas 16 from the duct 18 arranged at the upper end of the solid separator 16, a recirculation unit 20 provided at the lower part of the separator 16 to at least partially return the separated solids back to the lower part of the furnace 12, the fuel feeds arranged on the side wall 32 of the furnace;

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working means 22 and means 24 and 26 arranged on the lower part of the furnace 12 for supplying primary and secondary air respectively. The fuel supply means may comprise: a screw feeder, a drop pipe, a pneumatic feeder

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to mention alternatives. Primary air 24 is a primary oxygenated S 30 gas, which is also used to fluidize and feed bed material

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g through the grate provided on the bottom of the furnace 12. Secondary w gas 26 is introduced into furnace 12 through its side wall 32 just above the grate. A gas lock 28 is provided at the recirculation line to prevent the flow of gas 7 from the furnace 12 through the recirculation line 20 to the solid separator 16. This recycle connection 20 further includes a fluidized bed heat exchange chamber 30 for recovering heat from the recycled solids to the heat transfer medium. The sidewalls 32 of the boiler 10, as well as the sidewalls of the solids separator, usually comprise water pipes so that the water acts as a heat transfer medium.

Figures 2a and 2b illustrate a first preferred embodiment of the present invention showing the bottom end of the furnace such that the inclined side wall 32 of the furnace is assumed to have two fuel supply means. 10 The inside surface of the sidewall 32 of the boiler 10 has two inclined baffles 34 above each fuel supply member 22. The baffles 34 are preferably, but not necessarily, made of refractory material and secured to wall 32 such that they are substantially perpendicular to wall 32 and inclined toward the fuel supply means, which may in fact be shown as aperture 36 in step 32. preferably corresponding to the diameter of the fuel inlet 36. The purpose of the baffles 34 is to collect solids, i.e. petal material flowing down the walls 32 of boiler 10 and to assemble a bed of petite material above the fuel inlet 36. As the bed material now flows in assembled form onto the fuel feed 20, it will carry at least a portion of the fuel down and mix the fuel with the actual bed material at the bottom of the furnace 12. Figures 2a and 2b show, in addition to the baffles 34 attached to the furnace wall 32, the cover 38 of the fuel inlet 36 is intended to guide the fuel supply down the wall 32 of the furnace 12, thereby promoting the flow of fuel 25 towards the bottom of the furnace 12. Another purpose of the cover is to form a kind of curtain between the fuel and the turbulent bed material.

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^ at the bottom of the furnace so that as much fuel as possible could flow

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track down the grate. The baffles 34 preferably have substantially vertical extensions 34 'extending along the sides of the lid 38 so as to direct the petal stream B to the fuel F entering the furnace below the lid 38, such as the arrows F and B are Fig. These extensions further promote the formation of a solid tax on the side of the fuel stream facing the interior of the furnace. Thus, now that the residence time of the fuel in the lower part of the furnace has increased 8 on the one hand, in the case of a humid fuel, the fuel has more time to dry so that combustion can occur in the bed area, or, on the other hand, the fuel is light, volatile and / , it has more time to burn back at the bottom of the furnace compared to the prior art fuel-fuel feed arrangement where fuel is fed into the furnace at a distance above the grate so that the fuel enters an upwardly moving fluidized bed and is quickly transported to the top of the furnace. For both alternatives, the end result is that the fuel burns lower in the furnace, whereby the temperature of the bed material in the lower furnace rises.

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With regard to the dimensioning of the baffles 34 and 34 ', the basic dimensions depend on the dimensions of the furnace on the one hand and on the properties of the bed material on the other. Normally, the height of the baffles (measured outwards from the surface of the furnace wall) is of the order of 200 to 500 mm and the heeling angle (measured from the horizontal 15 directions) is approximately 30 to 70 degrees. However, if the baffles form a trough that leads the recycled bed material from, for example, a fluidized bed heat exchanger to the bottom of the furnace, the baffles may also be vertical so that their inclination can actually vary between 30 and 90 degrees. In other words, it is possible that the trough collects circulating solids not only from the fluidized bed heat exchanger but also from the furnace wall, whereby it will be appreciated that the trough sidewalls function in the same manner as the aforementioned baffles 34. and two planar side plates as shown in Figures 2a and 2b, but the lid may also conform to the shape of the circumference of the opening 36, wherein the lid is a r-curved plate. Alternatively, baffles 34 and 34 '(if used) can either feed the solid substantially to the width of the fuel inlet 36, or they may also feed the solid over a substantially larger area o, that is, extending to either side of the orifice | 36, thereby ensuring that the fuel entering the furnace is covered on all sides (of 30 pitches) by the fuel, that is to say, forming a curtain covering the entire S. However, if, for some reason, it is desired that the fuel be allowed to enter the furnace more freely, the bed material may be fed onto the fuel in a narrower region, whereby the fuel may escape from the sides of the solid stream. And finally, the amount of bed material collected by the baffles and fed onto the fuel feed 9 can be designed on a case-by-case basis.

Figure 3 schematically illustrates another preferred embodiment 5 of the present invention. Here, the assembled solid stream B from the solids separator 16 (see Figure 1) or heat exchange chamber 30 (see Figure 1) is circulated through the fuel outlet 20 to the fuel F through the inlet port 32 in the furnace wall 32. Feeding the external recycle solid and fuel into the furnace as described above ensures that the fuel is first effectively blended with the recyclable solids, unless they form a curtain overlay between the fuel and the turbulent bed, allowing the fuel furnace to descend deeper into the furnace. Second, the mixture of fuel and solids, or fuel and recovered solids, is effectively mixed with bed material at the bottom of the furnace. The solids supply channel 20 is designed to be located as close as possible above the fuel supply means 22, taking into account structural requirements and limitations.

Naturally, arrangements similar to those illustrated in the embodiments of Figures 2a and 2b, such as, for example, the fuel inlet cover, may also be used here. In this embodiment, it may be advantageous to arrange such a cover at each of the inlet openings so that the flow direction of both the solid inlet and the fuel inlet is aligned with the wall. In this way, the curtain effect of the solid flow is assured.

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Figure 4 schematically illustrates a third preferred embodiment x of the present invention. Here, recycled solids are collected internally

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30 turns in a trough or hopper 21 arranged in the lower chamber of the furnace

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£ man to the inclined wall 32 '. The gutter 21 can collect solids not only from the gasket wall 32 'but also, for example, from a fluidized bed heat exchanger.

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mesta, which is arranged in the upper furnace. Preferably, the trough 21 is arranged to terminate near the bottom of the furnace such that the bottom surface of the trough is inclined so that it is parallel to the fuel supply means 22 entering the furnace. A further feature of the trough is a baffle plate 23 which is parallel to the fuel supply means 22 and disposed above the fuel supply means 22 so as to direct the solid stream over the fuel stream entering the furnace 5. Alternatively, the trough may terminate in the plate 23 and the fuel inlet 36 or more generally, the fuel supply means may be located in a wall 32 'below the trough bottom plate 23. In either case, the plate 23 secures a proper solid curtain over the fuel. Thus, the operation of the trough 21 and the cover plate 23 is in principle similar to that described above with respect to the embodiments of Figures 2a and 2b. And, the aforementioned fuel supply member 22 may be only an opening in the inclined wall of the trough 21, or it may be a tubular duct extending a distance to the trough 21 as shown in Figure 4, or an opening in the furnace wall 32 'immediately below the trough 21.

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Figure 5 schematically illustrates a fourth preferred embodiment of the present invention. Here, as in previous embodiments, the internal circulation solids are effectively mixed with fine, easily volatile and / or light and / or moist fuel, bring the fuel down into the furnace and mix the fuel with the actual bed material. Here, the invention is again located in the lower part of the furnace in connection with the inclined side wall 32 'of the furnace.

In this embodiment of the present invention, the fuel supply means is a fuel supply opening 40, which is arranged in a special manner

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wall or furnace wall 32 '. The fuel inlet port 40 is provided

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a vertically oriented trough 42 on a substantially vertical bottom which projects

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ru is an integral part of the inclined side wall 32 'of the furnace. Gutter 42 is a special

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In the 30th embodiment, formed somewhat sloping (almost

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<£ vertical) from the bottom wall 43 and substantially vertical side walls

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In Fig. 5, the bottom wall 43 of the trough 42 is shown slightly inclined, but the wall 43 may equally well be completely vertical or even more inclined.

The substantially vertical trough 42 brings the solids back, for example, from the inner thread 11 towards the bottom of the furnace. This structure operates independently in the same manner as previous embodiments, i.e., by feeding a solid stream of aggregate form into the furnace over the fuel from the orifice 40. In other words, a vertical, substantially U-shaped trough can receive a downward flow of solids from a fluidized bed heat exchanger arranged at an upper level in the furnace, or an internal rotation along the furnace wall. Trough 42 is capable of efficiently collecting solids from turbulent bed material in the furnace because turbulence levels in the trough are significantly lower. The substantially deep cross-section of the trough forms a resting space in which the solid may collect and flow downward in an assembled form, i.e., a flow which is denser than the internal circulation on the wall 32 '.

However, in this preferred embodiment, the internal fluidized bed heat exchange chamber 44 is provided in fluid communication with the aforementioned substantially sidewall 46 of the vertical chute 42. The fluidized bed heat exchange chamber 44 receives solids from the internal circulation above it through an opening 48 arranged in the furnace wall 32 '. The solids entering the fluidized bed heat exchange chamber are fluidized by the flow of air through the bottom 50 of the chamber. On the side of the chamber, in fact, between the fluidized bed heat exchange chamber 44 and the substantially vertical trough 42, there is a so-called lifting tube 52 which connects the heat exchange chamber 44 to the vertical trough 42. The lifting tube 52 is in the chamber and at the top of the opposite side wall, an opening for solids flow out of the riser tube 52 to 25 in a substantially vertical trough 42. Thus, both the internal rotation downstream of the substantially vertical trough 42 and the downstream flow vertical gutter 42 to the bottom 54.

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It should be understood at this point that the fluidized bed heat exchange chambers 05 § 44 may be provided on both sides of a substantially vertical trough 42. The fluidized bed

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the position of the heat exchange chambers may also be higher or lower with respect to the fuel inlet 40 than shown in Figure 12 5. Similarly, the substantially vertical trough side wall (s) 46 may be inclined such that the side walls 46 collect internal rotation over a wider area than 5 is shown. It is also possible to provide sloping baffles substantially above the vertical trough, such as that shown in FIG. 2 5, to collect internal rotation in the trough 42. Likewise, it is possible to provide an additional opening above the fuel inlet 40 to feed solids from the external

Thus, it is clear that the present embodiment is primarily intended to use the solid stream from the fluidized bed heat exchanger to force the fuel flow down into the bed area, regardless of the location of the fluidized bed heat exchanger. That is, recycled bed material may be fed from one or more fluidized bed heat exchange chambers along a trough to be fed onto the fuel feed 15. In its simplest form, the fuel inlet is arranged below the solid outlet of the fluidized bed heat exchange chamber, whereby a separate trough may not be required.

A preferred additional way of using a fluidized bed heat exchanger is associated with low load conditions where there is practically no external circulation and internal circulation is marginal. By now feeding the bed material to the heat exchanger through an overflow or auxiliary channel (described in more detail in US-B2-7,240,639), it is possible to remove particulate material over the incoming fuel provided the heat exchanger outlet is above the fuel inlet, 25

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As a further improvement over the prior art, the fuel solid mixture is brought down to the grate for fluidization. According to a further preferred embodiment of the present invention, a vertical trough 42 is provided

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30 at the bottom 54 is a grate forming a so-called fluidization zone in which the S fuel-solid mixture is fluidized at a velocity of 10 to

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g 20 Umf for the particle (Umf corresponds to the minimum fluidization rate) to effect effective mixing between fuel and solids. There is another grate area 56 near the bottom grate of the trough, which forms the so-called grate. a low-13-zone zone for a portion of the bottom of the furnace at a speed of about 40-50 Umf for a D50 particle. In this zone, material is transported and sprayed to other areas of the furnace, but at a lower speed than the terminal speed, so that the fuel particles have time to dry (wet fuel mode) and warm enough to start the ignition and combustion process at the bottom. Both the first and second grate regions have either directional nozzles or a so-called stair grating for moving the solid horizontally along the grate. Outside of the second grate area 56 is the normal grate velocity range, where the gas flow below the grate is sufficient to initiate the recycling of fuel and bed material in the furnace.

Such a stepped grate velocity arrangement also provides internal circulation of the solid material from the wall (where the fuel feeds are mounted) towards the lower part of the furnace along the grate and up the wall and somewhat higher in the fluidized bed heat exchanger. This increases the amount of solid material going into the fluidization zone and also recycles some of the fuel particles.

Figures 6a-6c illustrate three different variants of a fifth preferred embodiment of the present invention. In the arrangement of Figures 6a to 6c, the basic principle is to take a portion of the downstream internal rotation along the furnace walls 32 temporarily out of the furnace 12 as a side stream and to supply a side stream to the fuel stream prior to feeding fuel to the furnace 12.

Fig. 6a shows a first alternative for taking some o bed material from the internal rotation IC from the furnace 12 through at least one opening 58 in the inclined wall 32 'of the furnace o 12 and feeding the channel 60 | along the fuel supply channel 22, where the bed material and the fuel material 30 are effectively mixed by the operation of the fuel supply means, for example S screw feeder, pneumatic feeder, or the like.

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Figure 6b shows another alternative whereby some of the bed material is again taken from the internal circulation IC from the furnace 12 through an opening 58 in at least one inclined wall 32 'of the furnace 14 and fed through a duct 60 in connection with fuel flowing towards the furnace. However, in this embodiment, the flow of recycled bed material is controlled by a knee-type adjustment 62, whereby the amount of bed material supplied to the fuel supply can be controlled.

Figure 6c shows a third alternative which is very close to the second alternative. In fact, the only visible difference is in the way the fuel and recyclable material are mixed. In the embodiment of Figure 6b, the bed material is supplied to the fuel flow passage 10, while in the alternative of Figure 6c, the fuel flow channel 22 feeds fuel to the bed material channel 60 'after a knee-type adjustment 62.

The bed material-fuel mixture formed as shown in Figs. 15a-6c may be fed as such into the furnace, but may equally well be fed to the furnace such that the mixture supply port is provided in a trough of the type shown in Figures 4 and 5. The mixture may also be treated in the same manner as in the fuel feed in the previous embodiments described with reference to Figures 2a to 5. In other words, a lid may be provided in its inlet 20 for directing the mixture stream towards the furnace grate and also any other solid circulating in the furnace (either through its walls or trough) or returning to the furnace may be fed over the mixture stream to force the mixture downward.

In light of the above description, it should be understood that only some of the most preferred embodiments of the present invention are disclosed. Thus, it is to be understood that the invention is not limited to the embodiments shown, but may be modified in many ways within the scope of the appended claims g. It is also to be understood that individual embodiments of the invention

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The features of the 30 embodiments may be applied in conjunction with the features of the other embodiments.

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Within the basic idea of the present invention, or combining features of the various embodiments of Section 05 to provide a workable and technically feasible structure,

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to obtain.

Claims (12)

A method for feeding light, finely divided, readily volatile and / or moist fuel to a circulating fluidized bed boiler, the method comprising feeding a fuel stream into a furnace, combusting the fluidized bed material in a furnace (12), 12) within the internal circulation where the bed material is returned along the furnace walls down to the bottom of the furnace and outside the furnace (12) in an external circulation, i.e. at least 10 through the solid separator (16) provided with the furnace (12); , the flue gases are removed from the separator (16) for further processing and the separated bed material is returned to the furnace (12) of the boiler (10), characterized in that the circulating bed material is collected from the internal flow downwardly flowing down the furnace walls (32). and the assembled return stream is fed to the fuel to be fed to the furnace (12) such that the bed material return stream and the fuel stream are mixed together and filed down the furnace (12) to extend the fuel residence time in the furnace (12). A method according to claim 1, characterized in that the return stream is supplied over the fuel stream when the fuel is supplied to the furnace (12). Method according to claim 1 or 2, characterized in that the return stream is directed to cover the fuel stream entering the furnace (12). δ Method according to one of the preceding claims, characterized in that the flow of current entering the furnace (12) is directed downwards towards the bottom of the furnace. tr 30 σ> CD δ Method according to one of Claims 1 to 4, characterized in that the means (21,34) are arranged on the wall or wall (32) of the furnace (12) to collect the return stream and supply it to the fuel stream. 35 Arrangement for supplying light, finely divided, easily volatile and / or moist fuel to a circulating fluidized bed boiler, the boiler (10) comprising a furnace (12) bounded at least by a grate at the bottom of the boiler, side walls (32) and a roof, solid separator (16) , arranged in flow communication with the upper part of the furnace; means (24, 26) for introducing primary and secondary air into the fire housing (12); means (20) for separating the solids separated from the flue gases in the separator (16) into the furnace (12); and means (22) for supplying a fuel stream to the furnace (12), characterized by means for supplying a solid circulating in the furnace (12) and flowing downwardly along the furnace walls (32) in assembled form 10 to force the fuel stream downwardly towards the grate for longer fuel residence time. An arrangement according to claim 6, characterized in that said means for supplying fuel to the furnace (12) comprise a fuel supply opening (36, 40) in the wall (32) of the furnace 15 (12) and that said means (34, 34 ', 21,42). ) for supplying a solid with the fuel is arranged substantially above said inlet (36, 40). Arrangement according to Claim 6 or 7, characterized in that said means for supplying solids to the fuel comprises a trough (21, 42) arranged on the wall of the furnace (12) or on the wall (32) above said fuel inlet (36, 40). . Arrangement according to Claim 8, characterized in that the roller (21,42) is a sidewall, and that the sidewalls are inclined to collect a solid flowing downwards along the furnace walls (32, 32 '). CM Arrangement according to Claim 6 or 7, characterized in that said means for supplying solids to the fuel comprises 3. inclined baffles (34) arranged on the wall (32) of the furnace (12) above the fuel inlet (36) to collect the fuel. the solid (5) flowing along the wall (32) and feeding it above the fuel feed. o o C \ J Arrangement according to one of Claims 6 to 10, characterized in that said means for supplying fuel to the furnace (12) comprise, in addition to the fuel inlet (36), said opening (38) in the furnace (12) above the fuel supply port (36). 36) for guiding the fuel passing down towards the grate of the furnace (12). An arrangement according to claim 11, characterized in that said means for supplying a solid with fuel comprises at least one baffle plate (34 ') arranged in connection with a cover (38). δ (M i CD O oo o X en CL O) CD δ σ> o o (M