Patent Application
20120237883
The present invention relates to a method and an arrangement for feeding fuel into a circulating fluidized bed boiler (CFB). The invention is specifically concerned with feeding fine, light, volatile and/or moist fuel into the boiler.
A CFB generally includes a furnace having a bottom, side walls, and a roof, and at least one particle separator connected in flow communication with the upper portion of the furnace. At least some of the walls of the bottom portion of the furnace may be inclined such that the cross section of the furnace increases upwardly, i.e., the portion of the furnace having the inclined walls may be called a converging bottom part. In practice, all of the walls and the roof of the boiler and the separator comprise water tubes to collect heat from the furnace. The bottom of the furnace is provided with a grid for introducing combustion or suspending or fluidizing gas into the furnace, and for removing ash and other debris from the furnace. The side walls of the furnace are provided with means for introducing fuel into the furnace, as well as means for introducing secondary air into the furnace. The furnace is also equipped with means for feeding inert bed material that is normally sand.
The particle separator separates solid particles from a flue gas-solid particles suspension entering the separator from the upper portion of the furnace. Separated solids are recycled back to the lower portion of the furnace via a recycling conduit that includes a sealing device, such as a loopseal, the purpose of which is to prevent gas flow from the furnace to the separator via the recycling conduit. Thus, at least a further opening in the furnace wall is needed for the solids introduction. This solids circulation is called an external circulation. In addition to vertical upflow of flue gas-solid particles suspension in the furnace entering finally into the separator inlet, there is a vertical downflow of particles near the furnace walls. This solids circulation is called an internal circulation.
Very often, in connection with the internal or the external circulation of solids material or both, a fluidized bed heat exchange chamber has been arranged to transfer heat from the bed of fluidized particulate solids to a heat transfer medium. The heat exchange chamber may be located inside the furnace of the circulating fluidized bed boiler adjacent to at least one of the furnace walls. A preferred location for the heat exchange chamber (or chambers) is adjacent to the bottom portion of the furnace where the chamber is integrated with the inclined wall (or walls). The fluidized bed heat exchanger may also be arranged in the external circulation, so that the solids leaving the solids separator are discharged into the heat exchange chamber on their way back to the furnace (see, for example, the prior art shown in
Light, fine and/or moist fuels, such as, for example, fine coal powder or peat or sawdust or fine lignite, are problematic in two different aspects. Light, small density and fine particle size fuels are easily entrained with the fluidizing gas, and rise rapidly upwards, so that the combustion process starts a few meters above the grid, whereby only a small amount of fuel, not sufficient to maintain the bed temperature at a sufficient level, is combusted in the lower bed area, with most of the fuel being combusted higher up in the furnace. This may result, especially in low load conditions, in too low a bed temperature, and a higher temperature in the upper portion of the furnace, which, again, may lead to problems in emissions and in the load change rate of the boiler.
In a similar manner, the use of moist fuel may result in similar problems, but for a somewhat different reason. Though the moist fuel may not be too light, the drying thereof requires some time, such that the fuel is again (while it is drying and not yet capable of igniting) lifted by the fluidizing gas in the upper portion of the furnace. When the fuel is finally dry enough, ignites and is finally combusted, there may not be enough combustible fuel in the lower bed area, whereby the bed temperature may, again, be low, and result in the problems already discussed above.
A further problematic fuel type is a fuel that, for the most part, contains volatile components and a smaller amount of solid carbon. The volatile components form combustible gases such as CO, CH4, H2, etc., very close to the fuel feed opening, whereafter, the combustible gases move upwards with the fluidizing gas. This upward flow creates a gas column with which the oxygen is not able to mix quickly and efficiently, resulting in combustion of the gases in the upper portions of the furnace.
Normally, the fuel is introduced into the furnace via one or more openings in the wall of the furnace. The fuel is, depending on the type of fuel, proportioned in the furnace either as a fuel-air suspension, i.e., pneumatically, or by means of a screw feeder or some other mechanical feed means. Normally, the fuel opening is or the fuel openings are located in the (converging) bottom portion of the furnace walls.
The solids entering the furnace from the external circulation, i.e., directly from the separator or via a fluidized bed heat exchanger, are introduced into the furnace via one or more openings in the furnace walls, too.
An object of the invention is to ensure, for the light, fine volatile and/or moist fuel, sufficient residence time for drying and/or combustion in the lower bed area of the furnace for optimizing the entire combustion process. According to the present invention, the residence time of the fuel in the lower bed area is increased by retarding the movement of the fuel to the upper bed area by means of combining the fuel and the solids flowing towards the lower bed area from the internal and/or external circulation.
Another object of the invention is to prevent the formation of the upward flowing column of combustible gases above the fuel feed opening by introducing circulating bed material to the gas column, so that the column breaks, and turbulence increases, whereby oxygen reaches the combustible gases more efficiently than before, and the combustion takes place lower in the furnace.
Several prior art documents discuss the mixing of the fuel and the returning bed material.
It is, for instance, known to introduce moist fuel and bed material into a separate mixer/drier device before feeding the fuel-solids mixture into the boiler. U.S. Pat. No. 4,529,911, No. 4,690,076, and No. 5,419,267 discuss various alternatives for separate mixers/driers. The separate mixer/drier units, however, increase costs in both the investment/building phase and running phase, and form yet one more piece of equipment that requires special maintenance.
It is also known to feed fuel into the external circulation between the particle separator and the furnace, as discussed, for instance, in U.S. Pat. No. 4,442,795.
However, the above-mentioned patent documents either do not address the problems discussed above, or if they do (moist fuel), they result in a very complicated arrangement with a separate particle dryer outside the boiler.
A further object of the present invention is to suggest a few alternative solutions to the problems discussed above relating to the feed and combustion of fine, light, volatile and/or moist fuels in circulating fluidized bed boilers.
All the alternative solutions are based on feeding the fuel in the furnace such that the heat transfer process between the fuel and the recirculating solids is improved by improving the mixing between the fuel and the circulating hot bed material.
A first preferred embodiment of the present invention is based on collecting bed material flowing downwardly along the furnace wall to be fed in a consolidated form on top of the incoming fuel.
A second preferred embodiment of the present invention is based on arranging the feed of the recirculated material from the external circulation to take place on top of the fuel feed.
A third preferred embodiment of the present invention is based on arranging a fluidized bed heat exchange chamber to discharge bed material flow on top of the incoming fuel.
A fourth preferred embodiment of the present invention is based on arranging the exit of a bed material flow from the solids separator or from an external heat exchange chamber to introduce the bed material flow on top of the incoming fuel.
A fifth preferred embodiment of the present invention is based on taking a side flow of bed material from the internal circulation temporarily outside of the furnace, and to arrange such in communication with the fuel just prior to the fuel entering the furnace.
Other features of the method and the apparatus of the present invention can be seen in the appended claims.
By means of the present invention, at least some problems relating to the feeding and combustion of fine, light, volatile and/or moist fuel in a circulating fluidized bed boiler have been solved by means of a simple and an effective means of feeding fuel into a furnace of a circulating fluidized bed boiler. For instance, there is no need to design and to build external drying chambers, known from the prior art, to dry moist fuel. Also, the need of mixing light, powdery fuel with hot bed material in a separate mixing chamber has been obviated.
The present invention makes it possible to keep the bed temperature higher even in the lower portions of the furnace when light, fine, volatile and/or moist fuel is combusted. This is particularly true with lower boiling loads when the bed temperature tends to decrease for natural reasons, too. Compared to prior art arrangements, the present invention, when taken into use, accomplishes that:
The last two advantages are not necessarily related to light and fine fuels, but are related to all types of fuels.
In the following, the method and the arrangement of the present invention will be explained in more detail with reference to the following drawings.
a is a schematic frontal representation of a first preferred embodiment of the present invention.
b is a schematic side representation of the first preferred embodiment of the present invention.
a through 6c are schematic representations of several alternatives of a fifth preferred embodiment of the present invention.
a and 2b illustrate a first preferred embodiment of the present invention showing the lower end of the furnace 12 such that an inclined side wall 32 of the furnace 12 is supposed to have two fuel feed means 22. The inner surface of the side wall 32 of the boiler 10 has been provided with two inclined guide plates 34 above each fuel feed means 22. The guide plates 34 are preferably, but not necessarily, made of refractory material, and fastened on the side wall 32 such that they are at substantially right angles to the side wall 32, and inclined towards the fuel feed means 22, which, in fact, may appear as an opening 36 in the side wall 32. The lower ends of the guide plates 34 leave a gap therebetween, the gap preferably corresponding to the diameter of the fuel feed opening 36. The purpose of the guide plates 34 is to collect the solids, i.e., the bed material flowing down the side wall 32 of the boiler 10 and to consolidate the bed material flow above the fuel feed opening 36. Now that the consolidated bed material flows on top of the fuel feed, it takes at least a portion of the fuel downwards and mixes the fuel with the actual bed material at the bottom of the furnace 12.
As to the dimensioning of the guide plates 34 and 34′, the basic dimensions are dependent, on the one hand, on the dimensions of the furnace 12, and, on the other hand, on the properties of the bed material. Normally, the height (measured from the surface of the furnace wall outwards) of the guide plates is on the order of about 200 to about 500 mm, and the inclination angle (measured from the horizontal direction) from about thirty to about seventy degrees. However, if the guide plates form a chute leading the recycled bed material from, for instance, a fluidized bed heat exchanger to the bottom portion of the furnace 12, the guide plates may be vertical, too, so that their inclination may, in fact, range from thirty to ninety degrees. In other words, it is possible that the chute collects circulating solids not only from the fluidized bed heat exchanger, but also from the furnace wall, whereby it is understandable that the side walls of the chute act in a similar manner as the guide plates 34 discussed above. As to the cover 38, it may be formed of a planar top plate, and two planar side plates (as shown in
Naturally, similar arrangements as shown in connection with the embodiment of
In this embodiment of the present invention, the fuel feed means 22 is a fuel feed opening 40 arranged in a specific manner in the wall 32′ of the furnace 12 of the boiler 10. The fuel feed opening 40 is arranged in the substantially vertical bottom of a vertically oriented channel 42 integrated in the inclined side wall 32′ of the furnace 12. The channel 42 is, in this specific embodiment, formed of a somewhat inclined (almost vertical) bottom wall 43, and substantially vertical side walls 46. In
In this preferred embodiment, however, an internal fluidized bed heat exchange chamber 44 has been arranged in flow communication with a side wall 46 of the substantially vertical channel 42 discussed above. The fluidized bed heat exchange chamber 44 receives solids from the internal circulation via an opening 48 arranged thereabove in the wall 32′ of the furnace 12. The solids entering the fluidized bed heat exchange chamber 44 are fluidized by means of air current through the bottom 50 of the chamber 44. At the side of the chamber 44, in fact, between the fluidized bed heat exchange chamber 44 and the substantially vertical channel 42, there is a so-called lift-leg 52 connecting the heat exchanger chamber 44 to the vertical channel 42. The lift-leg 52 is a small chamber having, at a lower end of its side wall facing the heat exchange chamber 44, an opening for allowing the solids flow in the chamber 44, and at an upper end of the opposite side wall, an opening for allowing the solids flow out of the lift-leg chamber 52 to the substantially vertical channel 42. Thus, both the internal circulation flowing down the substantially vertical channel 42 and the solids flow from the heat exchange chamber 44 via the lift-leg 52 mix with the fuel and force the fuel downwards to the bottom 54 of the substantially vertical channel 42.
Here, it has to be understood that there may be fluidized bed heat exchange chambers 44 on both sides of the substantially vertical channel 42. Also, the position of the fluidized bed heat exchanger chambers 44 may be, in relation to the fuel feed opening 40, either higher or lower than that shown in
Thus, it is clear that the present embodiment is mainly concerned with using the solids flow from the fluidized bed heat exchanger 44 to force the fuel flow down to the bed area irrespective of the positioning of the fluidized bed heat exchanger 44. In other words, recycled bed material may be introduced from one or several fluidized bed heat exchange chamber (or chambers) 44 along a channel 42 to be introduced on top of the fuel feed. In its simplest form, the fuel feed opening 40 is arranged below the solids exit opening of a fluidized bed heat exchange chamber 44, whereby a separate channel is not necessarily needed.
A further advantageous way of using a fluidized bed heat exchanger 44 relates to low load conditions when there is, in practice, no external circulation, and the internal circulation is marginal. Now, by introducing bed material into the heat exchanger 44 via an overflow or auxiliary channel (discussed in more detail in U.S. Pat. No. 7,240,639 B2), it is possible to discharge particulate material on the incoming fuel, provided that the heat exchanger discharge opening is above the fuel feed opening 40.
As a further improvement, compared to the prior art, the fuel-solids mixture is taken down on the grid to be fluidized. In accordance with a further preferred embodiment of the present invention, the bottom 54 of the vertically oriented channel 42 is provided with a grid forming a so-called fluidization zone where the mixture of fuel and solids is fluidized, such that the velocity is in the range of about 10 to about 20 Umf for D50 sized particles (Umf equals the minimum fluidization velocity), to obtain high mixing between the fuel and the solid material. In the neighborhood of the channel bottom grid, there is a second grid area 56 that forms a so-called low velocity zone in a portion of the bottom of the furnace 12, where the velocity is in the range of about 40 to about 50 Umf for D50 sized particles. In this zone, material is transported and sprayed to other parts of the furnace 12, but with the velocity below the terminal velocity, so fuel particles have time to dry (in the case of moist fuel) and to heat enough, such that the ignition and combustion process will start in the lower portion of the combustion chamber. Both the first and second grid areas may be provided with either directional nozzles, or a so-called step grid, for moving the solids in the horizontal direction along the grid. Outside this second grid area 56 is the area of normal grid velocity where the gas flow from below the grid is sufficient for initiating the circulation of the fuel and bed material in the furnace 12.
This kind of stepwise grid velocity arrangement also gives internal circulation of solid material from the wall (where fuel feeds are installed) towards the lower portion of the furnace 12 along the grid and up along the wall, and to the fluidized bed heat exchanger 44 somewhat in the upper portion of the furnace 12. This increases the solid material amount, which is going to the fluidization zone and also recirculates some fuel particles.
a through 6c show a fifth preferred embodiment of the present invention, with three different alternatives. In the arrangement of
a shows a first alternative where some bed material from the internal circulation (IC) is taken out of the furnace 12 through at least one opening 58 in the inclined wall 32′ of the furnace 12, and introduced along conduit 60 directly into the fuel feed conduit 22, where the bed material and fuel are efficiently mixed together due to the operation of the fuel feed means, such as, for instance, a screw feeder, a pneumatic feeder, etc.
b shows a second alternative where some bed material is again taken from the internal circulation IC out of the furnace 12 through at least one opening 58 in the inclined walls 32′ of the furnace 12, and introduced along conduit 60 into communication with the fuel flowing towards the furnace 12 along feed conduit 22. In this embodiment, however, the flow of the circulated bed material is controlled by means of a loop-seal type control 62, whereby the amount of bed material introduced to the fuel feed may be adjusted.
c shows a third alternative, which is very close to the second one. In fact, the only difference that can be seen is in the way the fuel and the recirculating bed material are mixed. In the alternative of
The bed material-fuel mixture formed as shown in
In view of the above description, it has to be understood that only a few most preferred embodiments of the present invention have been discussed. Thus, it is obvious that the invention is not limited only to the embodiments discussed above, but that it can be modified in many ways within the scope of the appended claims. It has to be understood, too, that features of a specific embodiment of the invention may be applied in connection with features of other embodiments within the basic idea of the present invention, or to combine features from different embodiments to result in a working and technically feasible construction.
| Number | Date | Country | Kind |
|---|---|---|---|
| 20096169 | Nov 2009 | FI | national |
This application is a U.S. national stage application of PCT International Application No. PCT/FI2010/050862, filed Oct. 29, 2010, published as International Publication No. WO 2011/058217, and which claims priority from Finnish patent application number 20096169, filed Nov. 10, 2009.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/FI10/50862 | 10/29/2010 | WO | 00 | 5/31/2012 |
