SK48798A3 - Method and apparatus for controlling the temperature of the bed of a bubbling bed boiler - Google Patents

Abstract

The present invention relates to a method and assembly for controlling the bed temperature in a fluidized-bed boiler, particularly in boilers fired with coal or other fuels of high heat value but difficult to gasify. Above the fluidized bed (6), but below the freeboard (8), is arranged a cooling zone (19) and the fluidizing gas is injected into a portion of the bed at an excess rate per unit area, whereby over the area of increased fluidizing gas injection, the bed is expanded upward into the cooling zone, wherefrom unburned combustible material and the particulate matter of the bed can fall back in the bed. The gas injection (9) with the higher flow rate is arranged to the center of the fluidized bed, whereby the return circulation can occur past the waterwall portions enclosing the cooling zone, thus facilitating efficient heat transfer from the refluxed bed material in the form of radiation absorbed by the heat transfer surfaces of the cooling zone waterwalls.

Description

Technical field

The present invention relates to a method according to the preamble of claim 1 for controlling the bed temperature in a fluidized bed boiler, in particular in boilers heated with coal or other fuels of high calorific value but difficult gasification. The invention also relates to an assembly suitable for carrying out this method.

BACKGROUND OF THE INVENTION

A fluidized bed boiler is a boiler structure in which a given fuel is combusted and partially gasified substantially above the bottom of the boiler in a fluidized bed layer formed by a mixture of bed material of non-combustible particles with the respective fuel. This bed is maintained fluidized by injection into it at a high rate of fluidizing gas, usually air, from the nozzles located in the bottom of the bed. Fluidized bed boilers are foreseen in the combustion of solid fuels and are particularly effective in the combustion of easily gasified fuels such as wood and peat, whereby the bed temperature can be controlled by adjusting the degree of gasification by means of controlled air distribution.

The present invention is applicable to fluidized bed bubbling boilers, usually operated by means of two zones, a bottom zone formed by an approximately 1 meter high bed in which a certain fuel is combusted and partially gasified. The upper zone is a free space for afterburner. The fuel gasified in the fluidized bed rises to the upper free zone where it is completely combusted, whereby the post-combustion process is controlled by blowing secondary (post-air) into this post-combustion zone to ensure complete combustion of the fuel. The walls of this free zone are traditionally cooled by water and heat is transferred to the water walls, mainly by absorbing the radiation emanating from the gases of the combustion process.

The temperature inside the fluidized bed is approximately 750-900 ° C. On the one hand, in order to avoid loss of combustion efficiency, the bed temperature must not drop too low. On the other hand, if the bed temperature is allowed to rise above a certain upper limit, sintering in the bed begins to occur rapidly, as molten ash connects the bed material into lumps.

When readily gasified fuels are combusted, bed temperature can be controlled by varying the amount of air stream injected into the bed, whereby less air flow favors gasification of the fuel over combustion in the bed, resulting in lower bed temperatures. Most of the fuel is then combusted in said free zone. Since fuels with problematic gasification properties such as coal often cause an uncontrolled rise in bed temperature, these types of fuels cannot be used as the main fuel. Bed temperature problems will also occur when fuel quality and boiler output change.

Therefore, coal is traditionally burned in a circulating fluidized bed boiler where a portion of the bed material and fuel is returned to the bed through the top of the boiler. Because this arrangement requires a cyclone or other efficient particle collector to capture the circulating bed material from the flue gas, its construction becomes more expensive than traditional fluidized bed boilers. Circulating fluidized bed boilers are disclosed, for example, in U.S. Pat. Nos. 5,054,436 and 4,766,851.

Traditionally, the temperature control of the boiler bed has been accomplished, for example, by varying the air coefficient by adjusting the amount thereof

- supplying the fluidized bed or circulating flue gases back to the bottom of the boiler to cool the bed. In some cases, the heat exchangers used have been submerged, causing problems from their rapid erosion. In terms of wear of the components of a particular boiler, the bed is an extremely stressed place where conditions change from reduction to oxidation, causing exceptionally severe erosion and corrosion. In particular, the combination of the glow and several of the above factors with the erosive effect of the fuel and the results of the circulation of the bed material lead to rapid wear of the structures located in the bed. Due to the multifaceted nature of the various bedding mechanisms, it is almost impossible to find any heat exchanger material which will withstand the combined effect of all of the above wear mechanisms and could simultaneously offer a sufficiently high heat transfer efficiency. The use of additional air injection is hampered by the limited bed temperature control range thus applicable. Although a satisfactory result can be achieved by controlling the combustion combustion variable by circulating the flue gas, this arrangement requires the construction of a separate circulation system.

It is an object of the present invention to provide a method capable of controlling the temperature of a fluidized bed of a boiler in a manner that is superior to the traditional method described above.

SUMMARY OF THE INVENTION

The object of the invention is achieved by arranging a cooling zone above the bed but below the free post-combustion zone, and then supplying the fluidizing gas to a portion of the bed with an excess amount per unit area, thereby expanding the bed above the increased fluidized gas injection area. from the bubbling bed to said cooling zone from where unburned combustible material and particulate matter from the bed may fall (fall) back into the bed.

In particular, the injection of gas with a higher flow rate is arranged in the center of the fluidized bed, whereby recirculation can occur. around a portion of the water wall enclosing the cooling zone, thereby facilitating efficient heat transfer from the bed material in the form of radiation absorbed by the heat transfer surfaces of the cooling wall water walls.

The method of the invention is more particularly characterized by what is set forth in the characterizing part of claim 1.

The assembly according to the invention is furthermore characterized by what is stated in the characterizing part of claim 8.

The present invention offers significant advantages. The invention makes it possible to burn high calorific value fuels in a fluidized bed system without the need for expensive particulate matter collectors such as those used in circulating fluidized bed techniques. Temperature in the post-combustion zone, i. in the free zone above the fluidized bed, it can be raised high enough to allow the elimination of noxious nitrogen compounds such as nitrogen dioxide from the flue gas in the normal fluidized bed combustion process. Thus, this method is well suited for, for example, converting pulverized-coal-fired boilers to fluidized-bed boilers. Without abandoning the degree of sufficient cooling, the installation of expensive and rapidly worn heat exchangers into the fluidized bed zone becomes unnecessary.

Due to the variable amount of air injection in the different fluidized bed zones, the combustion process parameters can be controlled in this way easily and in a wider range than was the case with the prior art. Oxidation conditions are created in the high-speed fluidization bed in the fluidized bed, which facilitate desulfurization with limestone in the same manner as in a circulating fluidized bed boiler.

With the help of injection of secondary (auxiliary) air or circulating gas, or both, the cooling wall can be equipped with

-5 gas vortex, which stabilizes the gas flow pattern of the boiler. As a result, nitrogen oxides are reduced inter alia. The size of the fluidized bed gasification region can be reduced and the maximum temperature in the boiler is reduced due to the increased heat transfer rate. This factor also contributes to easier removal of nitrogen and sulfur oxides.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described in more detail with reference to the accompanying drawings, in which:

Fig. 1 shows a schematic block diagram of an arrangement of a fluidized bed boiler according to the invention

Fig. 2 shows a more detailed view of the fluidized bed of the boiler arrangement according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the respective schemes, the boiler arrangement of the present invention is substantially similar to that of conventional fluidized bed boilers. The boiler 1 has a furnace region 11 above which is located a heat exchange region 2 in which heat released by the combustion of certain fuel is transferred by means of heat exchangers 3 to the heat-carrying medium. This heat-carrying medium is traditionally water that is vaporized and superheated in heat exchangers for use in steam turbines and other equipment. From the heat exchange region 2, the cooled flue gas is taken into the chimney either directly or via a scrubber, depending on the need for additional scrubbing. According to the present process, nitrogen and sulfur oxides can be substantially removed already in the boiler 1 during the combustion process.

The boiler furnace region is divided into a number of zones. The lowest in the boiler 1 are the nozzles 4y 5 of the fluidizing gas injection, over which the fluidized bed 6 is maintained.

A cooling zone 7 according to the invention is provided by the bed 15, and above that there is a free post-combustion zone 2. The fluidizing gas, traditionally air or flue gas, gas or any mixture thereof, is blown into the nozzles 5, 5 by means of a fan 10 along a duct 11. This duct 11 is branched on its path into a duct 12 leading to lateral nozzles 5 and duct 13, The fuel is supplied to the bed through a channel 14 using a traditional supply arrangement.

In the region between the cooling zone 2 ^{and the} free afterburning zone 8, secondary (after) air nozzles 15 are provided, the function of which may be supplemented by additional placement, in the cooling zone T, the return nozzle 16 for circulating flue gas or particulate material from ° of the heat transfer region 2.

According to the invention, the fluidizing gas injection nozzles 25 are divided into two regions. In a certain region of the boiler 1, preferably at the center, as shown in this embodiment, the nozzles are tilted more densely than at the periphery, thereby more centrally injected into the bed 6 in proportion to the larger number of nozzles 4, 5 in the center. The amount of air nozzles £ 5 can be regulated through the control dampers 17 are placed in air ducts 12, 22 · K ^{e <J} ° be such damper 17 fully open, the amount of air injected into the bed 6 will set the numerical ratio nozzles central area to the number of nozzles on the perimeter. If the flow rate of air drawn into the nozzles of the central region 6 is regulated to be less than the rate of air drawn into the nozzles 5 of the peripheral region in inverse proportion to the number of nozzles in both regions, the air flow injected into the entire surface of will behave like a traditional fluidized bed boiler. The nozzles may be positioned such that, for example, the number of nozzles is the same in the central and peripheral regions, but the area of the high-speed fluidization region is designed to be approximately 1/9 of the peripheral region. This invention, however, is not to be construed as limited by a certain ratio of nozzles in different areas of the bed.

When air is injected into the fluidized bed 6, the operation of the boiler is as follows. The total amount of air injected into the fluidized bed can be controlled as in traditional fluidized bed combustion. As more air is now injected into the high-speed fluidization area, the bed material will rise therein as a central column or pillar 9 reaching the cooling zone 7. The cooling zone T. is a region formed above the fluidized bed, but below the free post-combustion zone 8y, whereby the height of this zone is determined by the maximum height from the upper surface of the fluidized bed 6 reached by particles ejected upward therefrom. In the boiler 1 this zone extends partially into the upper part of the protective lining 18 of the fluidized bed zone 6, partly above it. Above this protective lining 18, the walls of this boiler are formed into heat transfer walls 19, cooled by water or steam. The secondary (additional) air nozzles 15 are located in the region of the heat transfer walls 19.

In the fluidized bed 6 and the cooling zone there is internal circulation in the form of a central column or pillar extending from the central zone of the high-speed fluidization of the bed 6, by which the column is formed by combustibles and gases entrained with the mass of bed particles. In the area where the mass of the particles rises above the bubbling bottom, the rate of air supply is adjusted so that the particles can at least reach their final velocity. In this context, the term finite velocity refers to the rate at which the particulate matter of the furnace bed begins to rise, entrained in an upwardly moving gas stream. The particle size of the bed material is selected such that the particles can fall back into the bed without becoming fully supported up in the furnace. Thus, the bed material is forced to ascend the center column 11 upwardly and then returned back to the bed as backflow along the inner walls of the boiler 1, thus effectively transferring heat by radiation to the surfaces of the wall 19 of the heat transfer zone. Since the emissivity of the bed material is approximately three times that of the furnace gases, the internal circulation of the bed material provides extremely efficient bed cooling.

The temperature in the cooling zone 7 is approximately 850 ° C, while the temperature in the afterburner 2 free zone is approximately 1100 ° C. These values are given merely as an example to explain the operating conditions used in the present invention. Each boiler will probably have different optimal temperatures for its respective components and operating conditions.

The secondary (additional) air required to ensure complete combustion of the fuel is injected into the boiler 2 preferably tangentially in the region of the cooling zone 2, thereby causing its current to cause a corresponding vortex in the center column or pillar and reflux passing back down the boiler walls. . The vortex flow forces thus induced push the particles of the backflow bed material close to the heat transfer surfaces 19, thereby achieving increased heat transfer. Moreover, the vortex balances the partial gas streams in the different parts of the furnace. The injection of secondary air may be accompanied by the injection of flue gas, which may also be injected into the cooling zone when additional cooling is desired.

Thus, the method of the present invention for cooling a fluidized bed is based on concentrated injection of a stream of air drawn into the bed such that a region of the bed is formed into a high-speed fluidized area in which the particulate matter is ejected upwards substantially higher than the particles surrounding the surface. bed. Through the arrangement above the tangential flow bed with the aid of gas streams that can be supplied by the secondary or circulating gas, the total gas flow in the furnace is forced into a vortex movement that transports entrained particles in the gas flow to the inner walls of the boiler. Further, the lower part of the furnace is provided, in the vicinity of its water-cooled walls, with a dense slurry area in which the fluidized-bed material is effectively cooled during its flow

-9 direction down. In this way, a turbulent internal circulation is formed into the fluidized bed in which the particles located in the central surface of the bed are forced to ascend higher and return back in the bed after being cooled in the reflux flow near the furnace walls, thereby lowering the bed temperature. , especially when high calorific value fuels are burned. As a result, the bed temperature can be controlled by varying the internal reflux circulation, thereby increasing the cooling effect through increased gas injection in the center of the bed. In the above-described embodiment for controlling the temperature of the bed, it occurs by adjusting the position of the gate 17 in the air ducts 12 and 13.

In this method, the primary air flow through the fluidized bed is maintained substantially the same as in a typical fluidized bed bubbler. Under favorable conditions, the system can be deactivated, whereby fuels with a high moisture content can be combusted, as in a traditional fluidized bed process. Thus, the present invention makes it possible to burn a variety of different fuels in a single boiler. As the need for bed temperature control increases, a major portion of the injection gas is directed to the central region of the fluidized bed, simultaneously reducing flow through the peripheral surfaces of the bed. At all times, however, a minimum amount of fluidizing gas flow must be maintained in all parts of the bed to be sufficient to keep the bed fluidized.

In addition to what has been described above, the present invention may have an alternative embodiment. In principle, fluidizing gas injection nozzles could be divided into a plurality of control areas, but such an arrangement is hardly practical due to the complicated design and the minimum particular benefit associated therewith. The gas that is injected into separate regions of the nozzles can be taken using separate ducts and fans, thereby mixing the air and the circulating flue gas.

The principle of the present invention, based on the controlled injection of different amounts of air into different regions of the bed, can be accomplished in different ways. When using nozzles of the same size, evenly distributed, the injection pressure in some nozzles may be supplied with higher pressure, thereby increasing the flow rate of the air flow. Alternatively, larger diameter nozzles may be used in certain areas of the nozzle field or a higher nozzle density may be provided by supplementing the traditional nozzle system in a particular area with circulating gas nozzles, thereby operating the fluidized bed in a traditional manner while the particulate material backflow is driven by circulating. gas. The cooling zone may be formed in existing boilers by adjusting the height of the center column of the fluidized bed at the bottom of the free post-combustion zone, whereby the respective cooling effect is achieved through the cooled walls of the free zone.

Claims (13)

PATENT CLAIMS A method for controlling the bed temperature in a fluidized bed bubbling boiler, wherein: - a fluidizing gas is injected from the bottom of the boiler (1) containing the bed material by means of a fluidising gas nozzle (4, 5) to suspend the bed material as a fluidized bed (6) above the bottom of the boiler, - the fluidized bed (6) is supplied with fuel which is allowed to be partially gasified and combusted in the bed, - the gasified fuel and the rising combustion gas are allowed in the boiler (1) to rise upwards to the free zone (8) for afterburning, and - air is injected into the boiler (1) above the fluidized bed to effect complete combustion of the gasified fuel in the free afterburning zone (8), characterized in that: - the fluidizing gas is injected into the boiler (1) so that the amount of injected gas per unit area is greater in at least one area (4) of the nozzle area (4, 5) than in other areas equipped with nozzles (5). (i) injecting the fluidizing gas and bed particles located above this region (4) with a higher amount of injected gas is supplied at least at a final velocity which forces these particles to rise up (9) from the actual fluidized bed (6), and - the amount and injection rate of the fluidizing gas supplied are maintained such that higher ascending bed material particles can fall back in the fluidized bed (6) after being directed to a surface (5) supplied with a smaller amount; a fluidizing gas, and - at least a portion of the inner wall of the boiler (1) is used as a heat exchanger through the region of the furnace extending from the upper surface of the fluidized bed (6) to the maximum ascension level of the bed material bed. Method according to claim 1, characterized in that a relatively larger amount of fluidizing gas is injected through a region located in the center of the boiler (1). Method according to claim 1 or 2, characterized in that air is injected into the furnace of the boiler (1) tangentially from the inner walls of the boiler. Method according to claim 1 or 2, characterized in that circulating gas is supplied to the region of the furnace remaining between the upper surface of the fluidized bed (6) and the level of maximum pitch of the bed material particles. The method of claim 2, wherein the fluidizing gas is air. The method of claim 2, wherein the fluidizing gas is air and a circulating gas. the 'rhyme of' Method according to any one of claims 1, 2, 5 or 6, characterized in that limestone is removed to the injection area of the relatively larger amount of fluidizing gas to remove sulfur. - 138. Apparatus for mounting the temperature of the bed in a fluidized bed boiler, comprising: - boiler (1) containing bed material, - fluidizing gas nozzles (4,5) adapted to the bottom of the boiler (1) for injecting fluidizing gas into the bed material to suspend the bed material as a fluidized bed (6) above the bottom of the boiler, - means (14) for supplying fuel to the fluidized bed (6), - free zone / 8) for afterburning, placed on the fluidized bed (6) of the boiler (1), and - at least one nozzle (15) for supplying secondary (additional) air to the free zone (8), characterized in that the fluidizing gas means control the amount injected by area so that the amount of injected gas per unit area through at least one area (4) the area equipped with the nozzles (4, 5) of the fluidizing gas injection is larger than in other areas equipped with the nozzles (5) of the fluidising gas, thereby providing at least such a final velocity to the bed particles located above this area (4). which forces these particles to rise upward (9) from their own fluidized bed (6), and the particle size of the bed material is selected such that higher ascending bed material particles can be in the fluidized bed - 14/6 / descend backwards as they are directed to an area (5) supplied with a smaller amount of fluidizing gas. - at least one heat transfer surface (19) formed into that wall of the furnace of the boiler (1) that extends from the top to the top of the fluidized bed (6) up to the maximum pitch height of the bed material particles; bed temperature control particles. Method according to claim 8, characterized in that said means for controlling the amount of fluidizing gas injected comprise a first nozzle region (4) in the middle of the boiler bottom and a second nozzle region (5) in the peripheral regions of the boiler bottom, and that nozzle density per unit area in the central region of the high speed fluidization is relatively higher than in the peripheral region of the low speed fluidization. Method according to claim 8, characterized in that said means for controlling the amount of fluidizing gas injected comprise a first nozzle region (4) in the middle of the boiler bottom and a second nozzle region (5) in the peripheral regions of the boiler bottom and that the flow rate The nozzle injection rate per unit area in the central region of the high-speed fluidization is relatively higher than in the peripheral region of the low-speed fluidization. Method according to claim 8, characterized in that said means for controlling the amount of fluidizing gas injected comprise a first nozzle region (4) in the central surface of the high-speed fluidization of the boiler bottom and a second nozzle region (5) in the peripheral regions of the boiler bottom, supplemented by means for controlling the pressure of the gas supplied to the nozzles. - 15 Method according to any one of the preceding claims 8 - 12, characterized in that the nozzles for supplying the secondary (auxiliary) air are adapted to the boiler (1) tangentially, at approximately the level of the maximum pitch of the bed material particles. Method according to any one of the preceding claims 8 - 12, characterized in that at least one nozzle (16) is adapted for supplying the circulating gas to the region between the upper surface of the fluidized bed (6) and the maximum pitch level of the bed material particles.