Patent Application
20090293818
The present invention relates to a fluidized bed heat exchanger for a circulating fluidized bed boiler (CFB boiler), and to a circulating fluidized bed boiler with such a heat exchanger. More specifically, the invention relates to the external hot circulation of the CFB boiler, in other words, to an efficient heat exchanger arranged in the return channel for solids, which have been separated from the exhaust gas of the CFB boiler by a particle separator, and which are to be returned to the furnace. The invention especially relates to arranging an efficient heat exchanger in a supercritical once-through utility boiler (OTU boiler), which is provided with reheating.
In CFB boilers, generation of hot steam from feed water takes place in several stages, for example, by means of heat exchangers arranged in the backpass of the boiler, by means of water tube panels of the furnace and backpass walls, and in heat exchange chambers arranged in the external hot circulation. As larger and more efficient CFB boilers are developed, heat exchange chambers in the external hot circulation become increasingly important. Therefore, ways must be found to advantageously provide the boilers with heat exchange chambers that are capable of producing a sufficiently high heat transfer power, while still operating flexibly in various operating conditions.
OTU boilers have the advantage of not needing a density different between water and steam, to provide the driving force for water circulation which cools the evaporator tubes of the furnace walls of the boiler. Instead of the density difference, the feed water pump of the boiler acts as the driving force for the water circulation. Therefore, in OTU boilers, it is possible to heat the steam to high temperatures at pressures above the critical point of water (220 bar), which improves the efficiency of the water vapor generation process of the boiler. In suspension-fired boilers, in operation and having capacities of about 1000 MWe, in which the temperature of the flue gas exiting the furnace may be about 1300° C., the achieved end temperature of steam at about 300 bar pressure has been 610° C. In CFB boilers, in which the furnace temperature is typically 850° to 900° C., achieving corresponding steam values and, especially, a high reheat temperature, e.g., 620° C., calls for new solutions in the designing of boiler heat exchangers.
A heat exchanger has a high efficiency when a large amount of solids, having a high inlet temperature and low outlet temperature, flows through it. In general, it is possible to raise the efficiency of the heat exchanger by increasing its heat exchange surface, which requires that the volume of the fluidized bed in the heat exchange chamber is large enough. Increasing the height of the fluidized bed increases the pressure loss of the fluidizing gas, and increasing its width and depth may lead to disadvantageous solutions in view of the structure or space consumption. To avoid these problems, it is advantageous to use at least two separate heat exchange chambers, instead of one large heat exchange chamber.
U.S. Pat. No. 5,275,788 discloses a heat exchanger of a CFB boiler comprising two heat exchange chambers arranged in association with the furnace wall, one on top of the other, but in parallel, in view of the particles flow. Desirable portions of the solids separated from the boiler exhaust gas by means of a particle separator may be introduced into these heat exchange chambers. With this kind of a heat exchanger, the solids to be introduced into both heat exchange chambers have the same temperature, and the end temperature of the solids remains very high. Thus, the heat exchange efficiency of the heat exchanger, and the adjustability of the heat exchange efficiency, may be inadequate, especially at low loads.
U.S. Pat. No. 5,537,941 discloses a heat exchanger with two stacked sections, an upper and a lower section, connected with each other in series, both sections having two heat exchange chambers connected in parallel. Both the upper section and the lower section also comprise a bypass channel through which a portion of the solids entering each section may be passed in a non-cooled condition, past the heat exchange chambers into the solids exiting the section. The adjustability of this kind of a heat exchanger is quite good, but even here, the efficiency and flexibility of the heat exchanger are not necessarily sufficient in all operational conditions of the boiler.
An object of the present invention is to provide a heat exchanger, to be arranged in the external hot circulation path of a circulating fluidized bed boiler, for reducing the above-mentioned drawbacks of prior art heat exchangers of the circulating fluidized bed boiler.
Another object of the invention is to provide a heat exchanger to be arranged in the external hot circulation of the circulating fluidized bed boiler, which heat exchanger is applicable to high efficiency once-through utility circulating fluidized bed boilers provided with reheating.
A further object of the invention is to provide a circulating fluidized bed boiler with a heat exchanger as described above.
To solve the problems involved in the prior art, the present invention provides particular aspects of a heat exchanger and a circulating fluidized bed boiler as set forth in the claims.
Hence, in accordance with one aspect of the present invention, a heat exchanger comprises a first heat exchange chamber and a second heat exchange chamber arranged in association with a furnace of a circulating fluidized bed boiler, a first inlet channel for introducing hot solids from the particle separator of the external circulation of the circulating fluidized bed boiler into the first heat exchange chamber, which is provided with a first means for fluidizing solids, a second inlet channel for introducing solids into the second heat exchange chamber, which is provided with a second means for fluidizing solids, a first discharge for removing a first portion of the cooled solids from the first heat exchange chamber into the second inlet channel, a second discharge for removing the cooled solids from the second heat exchange chamber into the furnace, and an inlet for introducing hot solids from the internal circulation of the furnace directly into the second heat exchange chamber.
Thus, the present invention offers a new solution for providing an efficient heat exchanger, wherein the heat exchanger comprises two heat exchange chambers, connected in series in the external hot circulation of a CFB boiler, and a means for introducing hot solids from the internal circulation of the furnace directly into the latter heat exchange chamber. In this kind of a heat exchanger, it is possible to obtain a sufficiently high flow of solids, a sufficiently high inlet temperature of the solids and, at the same time, a relatively low outlet temperature of the solids in both heat exchange chambers.
In accordance with a preferred embodiment of the present invention, the heat exchanger also comprises a third discharge for removing a second portion of the cooled solids from the first heat exchange chamber directly into the furnace. Preferably, the third discharge and the first discharge, referred to above, comprise a controller for controlling the amounts of the first and second portions of the cooled solids. Thus, it is possible, for example, to prevent the solids cooled in the first exchange chamber from flowing into the second heat exchange chamber, if necessary, in which case, solids flow to the second heat exchange chamber directly from the furnace only. Thus, the inlet temperature of the solids in the second heat exchange chamber is the highest possible, and in the second heat exchange chamber, for example, reheating of the steam returning from the high-pressure turbine to a sufficiently high temperature can be accomplished.
A heat exchanger arrangement similar to the one described in the above embodiment is also applicable to a circulating fluidized bed boiler driven by different combustion modes, in which one combustion mode requires efficient cooling of solids in two heat exchanger chambers connected in series, and another combustion mode requires cooling of the solids in one heat exchange chamber only. In the latter case, it is possible to return the solids to the furnace directly from the first heat exchange chamber, the second heat exchange chamber being not used at all. Of these two combustion modes, the first may correspond, for example, to combustion in which the oxidizing gas is air-enriched with oxygen, or even pure oxygen, whereas, the latter corresponds to combustion with ordinary air.
The two heat exchange chambers of the exchanger, in accordance with the present invention, which are connected in series, may be arranged adjacently in connection with the furnace wall of the CFB boiler, but, in accordance with an especially advantageous embodiment of the invention, the first heat exchange chamber of the heat exchanger is arranged above the second heat exchange chamber. This embodiment is especially advantageous in large CFB boilers, which comprise several efficient and relatively small particle separators, in which case, two superposed and separate heat exchange chambers may be arranged in the space remaining below them. In two superposed heat exchange chambers, the pressure loss of the fluidizing gas in the fluidized bed of the solids remains lower than that in a corresponding undivided high chamber.
When the heat exchanger in accordance with the invention is connected to a supercritical OTU boiler, the end temperature required for superheating may be considerably high, for example, 610° C., and the end temperature required for reheating may be even higher, for example, 620° C. In this case, when the temperature in the CFB boiler furnace is, for example, 850° to 900° C., the heat exchanger containing the last heat exchange surface of the boiler steam cycle must be arranged in a very effective manner, in order to gain the desired superheating temperatures. It is possible to advantageously implement this kind of a heat exchanger, in accordance with the present invention, so that the last superheater of the steam cycle is arranged in the first heat exchange chamber of the heat exchanger, and the last reheater of the boiler steam cycle is arranged in the second heat exchange chamber, which is most preferably arranged below the first heat exchange chamber. When, according to the invention, hot solids are fed from the furnace directly to one of the heat exchange chambers connected in series, it is possible to achieve a sufficient reheating temperature in this heat exchange chamber, at all loads of the boiler. This results, above all, from the inventor's surprising discovery that, in large once-through utility circulating fluidized bed boilers, the temperature in the lower section of the boiler is usually higher than the temperature in the upper section of the boiler, because of the considerable height of the boiler, and the heat exchange surfaces being arranged in the furnace. Therefore, solids fed directly from the internal circulation of the furnace, the temperature of which is close to the temperature of the lower section of the boiler, are hotter than the solids separated from the exhaust gases of the furnace, the temperature of which corresponds to the temperature in the upper section of the boiler. It has especially been found that the temperature difference between the upper and lower sections of the CFB boiler furnace becomes more prominent at low loads, when reaching a sufficient reheat temperature with normal modes is especially difficult. Achieving a sufficient superheating temperature, on the other hand, is no problem, because, at all loads, the efficiency of the boiler is raised so high that the desired superheating temperature will be reached.
The invention is described in more detail below, with reference to the attached drawings, in which:
In operation of the boiler 10, oxide-containing fluidizing gas, for example, air, fed through nozzles 28 at an adequate velocity, makes the fuel burn in a fluidized bed, typically, at a temperature of about 850° to 900° C., in which case, flue gas and entrained solids, primarily ashes, inert bed material and unburned fuel, exit the upper section of the boiler 10 through the outlet channel 14 and enter the particle separator 16. The particle separator 16 separates hot solids from the flue gas, which hot solids are passed through the return channel 18 to the heat exchanger 30, where heat exchange surfaces 32, 34 arranged in the heat exchanger 30 cool the solids before they are returned to the lower section of the furnace 12. A large CFB boiler 10 is usually provided with several parallel particle separators 16 and heat exchangers 30 connected to their return channel 18, but, for clarity reasons,
Normally, the walls of the furnace 12 are made of water tube panels serving as so-called evaporating surfaces, in which the high-pressure feed water of the boiler steam cycle, heated in an economizer (not shown in
In a preferred embodiment of the present invention, illustrated in
In accordance with a preferred embodiment shown in
In the arrangement of
In accordance with a preferred embodiment of the present invention, shown in
In accordance with a preferred embodiment of the invention, the heat exchange surface 32 of the upper heat exchange chamber 36 is the last superheater of the steam cycle boiler 10 and the heat exchange surface 34 of the lower heat exchange chamber 38 is the last reheater of the steam cycle. As the temperature of the furnace 12 in a large once-through utility circulating fluidized bed boiler 10 is at its highest in the lower section of the boiler 10, especially, at low loads, this arrangement is capable of providing a sufficiently high reheat temperature. The heat exchange surfaces 32, 34 of the heat exchange chambers 36, 38 also may be other heat exchange surfaces, for example, both of them can be either superheaters or reheaters.
The material cooled in the upper heat exchange chamber 70 is preferably passed to the lower heat exchange chamber 78, when it is desirable to recover as much energy as possible from the solids separated, by a particle separator. The material cooled in the upper heat exchange chamber 70 is preferably passed directly to the furnace 12 when it is desirable that the temperature of the solids entering the lower heat exchange chamber 78 be as high as possible. Therefore, only non-cooled solids enter the lower heat exchange chamber 78, either directly from the internal circulation of the furnace 12, exclusively, through inlet openings 80, or possibly, also from the external hot circulation through the overflow channel 82.
This embodiment deviates from the other described embodiments specifically, in that it comprises a first heat exchange chamber 86 and a second heat exchange chamber 88, which are connected in series in view of the solids flow and arranged in parallel in association with the wall of the furnace 12. The solids discharged from the particle separator of the external hot circulation of the circulation fluidized bed boiler 10 are passed along the return channel to the first heat exchange chamber 86, from the lower section, whereby it is possible to return the solids to the furnace 12 through a lifting channel 90. If there is not enough fluidizing gas introduced into the lifting channel 90 through fluidizing gas nozzles 92 arranged in the lower part of the channel, the solids entering the first heat exchange chamber 86, or a portion of it, may end up in the furnace 12 through an overflow channel 94 attached to the upper section of the chamber 86.
A specific feature of the first heat exchange chamber 86, illustrated in
The invention has been described above with reference to a few exemplary arrangements. These arrangements are not intended to limit the scope of the invention, but the invention is limited only by the accompanying claims and the definitions therein.
| Number | Date | Country | Kind |
|---|---|---|---|
| FI20065308 | May 2006 | FI | national |
This application is a U.S. national stage application of PCT International Application No. PCT/FI2007/050264, filed May 9, 2007, published as PCT Publication No. WO 2007/128883 A2, and which claims priority from Finnish patent application number 20065308, filed May 10, 2006.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/FI07/50264 | 5/9/2007 | WO | 00 | 4/16/2009 |
