JPH0735883B2 - Combined circulating fluidized bed boiler - Google Patents

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to an internal circulating fluidized bed boiler that burns combustion materials such as various types of coal, low-grade coal, coal washing sludge, and oil coke in a so-called swirl-type fluidized bed, while recovering heat from a circulating fluidized bed and heat transfer sections provided in the freeboard and downstream of the freeboard section.

[Background technology]

In recent years, coal has been in the spotlight as an alternative energy source to oil. However, in order to expand the use of coal, which has inferior physical and chemical properties as a fuel compared to oil, there has been an urgent need to develop technologies for processing, distributing, and promoting coal use. In terms of combustion technology, research and development has been actively carried out on pulverized coal-fired boilers and fluidized bed boilers, but these combustion technologies are still limited by the type of coal used in terms of combustion efficiency, low NOx, and low SOx. In addition, it has become clear that the coal supply system is complex and that it is difficult to control load fluctuations; this tendency is particularly pronounced when it comes to small and medium-sized boilers.

Regarding fluidized bed boilers, there are two types, which differ in the arrangement of the heat transfer section and the method for taking into account the combustion of unburned particles that fly out of the fluidized bed.

(1) Non-circulating fluidized bed boiler (also called conventional fluidized bed boiler or bubbling fluidized bed boiler).

(2) In a non-circulating type circulating fluidized bed boiler, heat transfer tubes are placed in the fluidized bed, and heat exchange is performed with high heat transfer efficiency through physical contact between the fuel and the bed material during combustion at high temperatures, whereas in a circulating type, fine unburned fuel, ash, or part of the bed material (circulating solid) is carried along in the combustion gas flow and led to a heat exchange section located independently of the combustion chamber, where the combustion of unburned particles continues, and
The circulating solids are returned to the combustion chamber after heat exchange.
The name comes from the fact that the solid is circular.

The non-circulating fluidized bed boiler and the circulating fluidized bed boiler will be explained with reference to FIGS. 3 and 4.

3 shows a non-circulating fluidized bed boiler, in which fluidizing air, compressed by a blower (not shown), is ejected from an air chamber 74 through a distributor plate 72 into a boiler 71, forming a fluidized bed 73. Fuel, such as granular coal, is supplied to the fluidized bed 73 and burned. Heat transfer tubes 76 and 77 are provided in the fluidized bed 73 and at the exhaust gas outlet of the freeboard section to recover heat.

The exhaust gas, which has now become relatively cold, is led from the exhaust gas outlet of the freeboard section to the convection heat transfer section 78, where further heat is recovered, and then the fine particles contained therein are recovered in a cyclone 79 before being discharged outside the system. The ash recovered in the convection heat transfer section is drawn out through pipe 81 and, together with the ash drawn out through pipe 80, is drawn out of the system via pipe 82, and a portion of the gas is returned to the fluidized bed 73 through the air chamber 74 or fuel supply port 75 and re-burned.

FIG. 4 shows a circulating fluidized bed boiler. Fluidizing air is pumped from a blower (not shown) through an air chamber 104 and a dispersion plate 102 before being blown into a furnace 101. The air is used to fluidize and burn fine coal, which may contain lime as a desulfurizing agent if necessary.

Unlike a non-circulating fluidized bed boiler, the velocity of the fluidizing air blown in through the distributor plate 102 is greater than the terminal velocity of the fluidized particles, so the particles and gas mix more vigorously, and the particles are blown up along with the gas, forming a fluidized bed and a spouted bed in sequence from below throughout the entire combustion furnace. The particles and gas undergo some heat exchange in the water-cooled furnace wall 107 along the way before being led to the cyclone 108. After leaving the cyclone 108, the combustion gas undergoes heat exchange in the convection heat transfer section 109 installed in the rear flue.

Meanwhile, the particles collected by the cyclone 108 are returned to the combustion chamber via flow path 113, and some of the particles are led to an external heat exchanger 115 via flow path 114 to control the temperature inside the furnace, where they are cooled and returned to the combustion chamber again, but some of them are discharged outside the system as ash. This circulation of particles to the combustion chamber is a distinctive feature of this system. The circulating particles mainly consist of limestone, which is supplied as a desulfurization agent, and combustion ash and unburned ash from the supplied coal.

Due to the characteristics of the combustion method, these fluidized bed boilers can be used with a wide range of materials, but on the other hand, some drawbacks have begun to be pointed out.

The disadvantages of the bubbling fluidized bed boiler include problems with the load characteristics, the complexity of the fuel supply system, and the wear of the immersed heat transfer tubes.

The circulation type is attracting attention as a solution to these inherent problems, but there are still elements that need to be developed in order to maintain the temperature of the circulation system, including the combustion furnace cyclone, at an appropriate value, and there are also problems with handling the circulating solids, and it is said that there are difficulties in making it compact for small and medium-sized applications.

[Technical Problem of the Invention]

The present inventors have been conducting various studies to solve the above problems, and have found that in an internal circulating fluidized bed boiler in which a swirling flow is created inside the fluidized bed by using different fluidization velocities and the swirling flow is used to form a circulating flow of bed material between the heat recovery chamber and the fluidized bed, a heat recovery section such as an evaporation tube is provided in the freeboard section above the fluidized bed or downstream of the freeboard section, and the low-temperature exhaust gas after heat recovery is guided to a cyclone, and the particles collected by the cyclone are returned to the downward moving bed of bed material in the heat recovery chamber, thereby making it possible to make the boiler more compact, improve combustion efficiency, and reduce NOx.
Because it can be completely combusted, there are no restrictions on the type of coal used, and silica sand can be used as a bed material. By also using limestone, it is possible to reduce SOx emissions. When these effects are combined, it has been discovered that it is possible to solve all of the problems that have previously been associated with coal boilers.

[Configuration of the invention]

The features of the present invention are as follows: First, the fluidized bed section is largely divided into a main combustion chamber and a heat recovery chamber, and the main combustion chamber is provided with at least two types of air chambers in its lower part, one of which provides a high fluidization velocity and the other of which provides a low fluidization velocity, and a swirling circulating flow is imparted to the fluidized medium in the main combustion chamber by combining these different fluidization velocities, and a heat recovery circulating flow of the fluidized medium is formed between the main combustion chamber and the heat recovery chamber, i.e., an internal circulating fluidized bed boiler provided with air chambers providing a low fluidization velocity in the lower part of the heat recovery chamber and on the opposite side of the main combustion chamber from the heat recovery chamber,
The exhaust gas is directed into a cyclone and the collected particles are returned to either directly above or within the downward moving bed of the heat recovery chamber.

The collected particles do not necessarily have to be returned to the downward moving bed of the heat recovery chamber; particles collected by a bag filter or the like can also be returned to the downward moving bed of the heat recovery chamber. By returning the collected particles to the downward moving bed of the heat recovery chamber, the unburned fuel (char) in the collected particles rides the circulating flow from the heat recovery chamber to the main combustion chamber and is uniformly dispersed inside the fluidized bed of the main combustion chamber, creating a reducing atmosphere throughout the bed, thereby reducing NOx from the main combustion chamber fluidized bed to the freeboard section.

The effect of returning char to the descending moving bed of the heat recovery chamber is that when it is returned to the fluidized bed of the main combustion chamber, the char is fine particles so it immediately scatters into the freeboard section and there is almost no residence time in the bed, and the char itself is burned and low NO
x Although it does not function as a catalyst, when it is returned to the descending moving bed of the heat recovery chamber, even fine particles settle and diffuse in the descending moving bed, and then become a circulation flow from the heat recovery chamber to the main combustion chamber, and are supplied to the bottom of the main combustion chamber fluidized bed, so coal, etc. is burned in the main combustion chamber fluidized bed and generated. The char is sufficiently distributed to the place where NOx is generated, which is extremely effective in reducing NOx, and because the char is sufficiently burned,
Combustion efficiency is improved.

There are two possible reactions involved in reducing NOx: C + 2NO → CO ₂ + N ₂ (oxidation reaction of char) 2CO + 2NO → 2CO ₂ + N ₂ (catalytic reaction of char).

Char is involved in both reactions, and its oxidation reactivity and catalytic effect are thought to govern the low NOx function.

The second feature is that two types of fluidized beds with different particle sizes and specific weights are created in the combustion furnace by returning the collected particles to a heat recovery chamber such as a cyclone.

That is, while the average particle diameter in the fluidized bed of the main combustion chamber is about 0.6 mm, the average particle diameter of the particles collected by a cyclone or the like and returned to the heat recovery chamber is about 0.05 to 0.06 mm. Furthermore, the fluidization mass velocity of the heat recovery chamber is 0 to 2 Gmf, which is less than one-third of the average fluidization mass velocity of the main combustion chamber, so that the amount of particles entrained with the gas and re-scattered in the heat recovery chamber is small, and the average particle diameter in the heat recovery chamber is around 0.4 mm.

In addition, the particles collected by a cyclone or the like and returned to the heat recovery chamber also contain combustion ash, which has a smaller specific gravity than the initial bed material, silica sand, so the apparent specific gravity of the bed material in the heat recovery chamber is smaller than that of the main combustion chamber.

The average particle size of the bed material in the heat recovery chamber is about 0.6 mm to about 0.4 mm.
The minimum fluidization mass velocity is reduced by approximately 2.5 times due to the decrease in apparent specific weight and the resulting
The wear rate of the immersed heat transfer tubes is reduced by approximately 20 times.

Furthermore, heat recovery control, a feature of the internal circulating fluidized bed boiler, is performed by controlling the air volume supplied to the heat recovery chamber. However, if the collected particles are returned to the heat recovery chamber as described above, the air volume required for control is reduced to less than half, which significantly reduces the impact on combustion caused by changes in air volume due to control, contributing to the stabilization of combustion.

In other words, the feature of this application is that by creating two types of fluidized beds with different particle sizes and specific weights in one combustion furnace, the fluidized bed with relatively large particle size and specific weight can support and fluidize large combustion materials while burning them, while the heat recovery chamber fluidized bed with relatively small particle size and specific weight can be controlled with less fluidizing air, thereby minimizing the effect of fluctuations in the amount of air on combustion and making it possible to operate while keeping wear on the in-bed heat transfer tubes to a minimum.

The third feature is that heat transfer tubes are arranged in the freeboard section above the fluidized bed or downstream of the freeboard section, and heat recovery is carried out mainly by convection.

Conventionally, the convection heat transfer section is installed separately from the freeboard section, but in order to make the system more compact, it is installed integrally with the freeboard section either at the top of the freeboard or downstream of the freeboard section, after ensuring the freeboard section volume necessary for secondary combustion. This not only simplifies the dust treatment and char recycling that were previously required around the boiler, but also eliminates the need for a castable lining for the cyclone, as the gas temperature entering the cyclone is 250-400°C, making it possible to make the cyclone lightweight and compact using steel.

The fourth feature is that by installing the convection heat transfer section at the top of the freeboard and by constructing the furnace walls with water-cooled pipes, the combustion gas temperature in the freeboard is prevented from dropping due to the radiation effect by lining the convection heat transfer section and the water-cooled furnace walls with heat insulating material such as fireproofing on the combustion chamber side. This maintains the combustion gas temperature and is effective in reducing CO emissions.

When the convection heat transfer section is installed downstream of the freeboard section, the fireproof heat insulating material only needs to be attached to the water-cooled wall that constitutes the freeboard section.

In other words, the present invention is a combined circulating fluidized bed boiler that combines three circulation systems: a swirling circulation flow in the main combustion chamber, a heat recovery circulation of the bed material between the main combustion chamber and the heat recovery chamber, and an external circulation (char circulation) that returns unburned char to the downward moving bed of the bed material in the heat recovery chamber.

Next, the present invention will be described in detail with reference to the drawings.

In FIG. 1, a distributor plate 2 for fluidizing air introduced from a fluidizing air inlet pipe 15 by a blower 16 is provided at the bottom of the boiler body 1. The side edges of this distributor plate 2 are higher than the center, and the bottom of the boiler body is formed in a concave shape.

The fluidizing air sent by the blower 16 is then
The mass velocity of the fluidizing air ejected from the central air chamber 13 is sufficient to form a fluidized bed of the fluidized medium within the boiler body, i.e., 4 to 20 Gmf.
The mass velocity of the fluidizing air ejected from the air chambers 12, 14 at both sides is preferably in the range of 6 to 12 Gmf, but is generally in the range of 0 to 3 Gmf, and it is preferable that the air be ejected from the air chamber 12 located at the bottom of the heat recovery chamber 4 in which the heat transfer tubes 5 are arranged at a mass velocity of 0 to 2 Gmf, and from the air chamber 14 forming the bottom of the main combustion chamber 3 at a mass velocity of 0.5 to 2 Gmf.

As a result, the mass velocity of the fluidizing air ejected from the air chamber 13 inside the main combustion chamber 3 is larger than the mass velocity of the fluidizing air ejected from the air chambers 12 and 14.
At the top of the chambers 12 and 14, the air and fluidized medium form a jet that moves rapidly upward inside the fluidized bed, dispersing into the surrounding area when it leaves the surface of the fluidized bed, and the fluidized medium falls onto the surface of the fluidized bed above the air chambers 12 and 14.

On the other hand, in the upper fluidized layer of the air chamber 13, in order to fill the space left behind by the fluidized medium moving upward, the fluidized medium from the slow fluidized layers on both sides, i.e., the bottom of the upper fluidized layers of the air chambers 12 and 14, moves to the center, i.e., to the top of the air chamber 13.
As a result, a strong upward flow is formed in the center of the fluidized bed, while a gentle downward flow is formed in the peripheral area.

The heat recovery chamber 4 utilizes this downward moving layer.
As shown in Figure 7, which shows the relationship between the overall heat transfer coefficient and fluidization mass velocity for the bubbling system, a large overall heat transfer coefficient can be obtained at a fluidization mass velocity of 1 to 2 Gmf, as shown in Figure 8, without the need for intense fluidization (generally 3 to 5 Gmf) as in the bubbling system, and sufficient heat recovery can be achieved.

A vertical partition wall 18 is provided inside the fluidized bed above the boundary of the air chambers 12 and 13.
The heat recovery chamber is formed by arranging heat transfer tubes 5 inside the fluidized bed above the air chamber 12, i.e., between the back of the partition wall 18 and the water-cooled furnace wall. The partition wall 18 is high enough to allow the bed material to enter the heat recovery chamber 4 from the top of the air chamber 13 during operation, and an opening 19 is provided between the partition wall 18 and the air dispersion plate at the bottom to allow the bed material in the heat recovery chamber 4 to return to the main combustion chamber 3. Therefore, after forming a jet and rising violently in the main combustion chamber, the bed material spreads on the surface of the fluidized bed, crosses the partition wall 18, enters the heat recovery chamber, and gradually descends while being gently fluidized by the air blown in from the air chamber 12, during which time heat exchange takes place via the heat transfer tubes 5.

The amount of settling and circulation of the bed material in the heat recovery chamber is controlled by the amount of air diffused from the air chamber 12 into the heat recovery chamber 4 and the amount of fluidizing air from the air chamber 13 in the main combustion chamber.
As shown in Fig. 5, the amount _G1 of bed material entering the heat recovery chamber 4 increases as the amount of fluidizing air blown out from the air chamber 13 increases. Also, as shown in Fig. 6, when the air flow rate diffused into the heat recovery chamber 4 is changed in the range of 0 to 1 Gmf, the amount of bed material settling within the heat recovery chamber changes almost proportionally, and becomes almost constant when the air flow rate diffused into the heat recovery chamber is 1 Gmf or more.

This constant amount of bed material is approximately equal to the amount _G1 of bed material entering the heat recovery chamber 4, and the amount of bed material settling within the heat recovery chamber is an amount corresponding to _G1 . By adjusting these two airflow rates, the settling amount of bed material settling within the heat recovery chamber 4 can be controlled.

On the other hand, heat is recovered from the settling bed material through the heat transfer tubes 5. As shown in FIG. 8, the heat transfer coefficient changes almost linearly when the volume of air diffused from the air chamber 12 into the heat recovery chamber 4 is changed from 0 to 2 Gmf. Therefore, by changing the volume of air diffused, it is possible to arbitrarily control the amount of heat recovered and the temperature of the fluidized bed in the main combustion chamber 3.

That is, when the amount of fluidizing air from the air chamber 13 in the main combustion chamber 3 is constant, increasing the air diffusion rate in the heat recovery chamber 4 increases the amount of circulating bed material and at the same time increases the heat transfer coefficient, resulting in a synergistic effect that significantly increases the amount of heat recovered. If this increase in heat recovery is balanced with the increase in the amount of heat generated in the main combustion chamber, the fluidized bed temperature will be maintained constant.

The wear rate of the heat transfer tubes in a fluidized bed is said to be proportional to the cube of the fluidization velocity, and the relationship between the fluidization mass velocity and the wear rate is shown in Figure 9. That is, by setting the volume of the diffused air blown into the heat recovery chamber to 0 to 3 Gmf, preferably 0 to 2 Gmf, it is possible to minimize wear on the heat transfer tubes and improve their durability.

Meanwhile, the coal fuel is supplied to the beginning of the downward moving bed in the main combustion chamber 3. This allows the coal to swirl and circulate inside the high-temperature fluidized bed, making it possible to completely combust even coal with a high fuel ratio, and high-load combustion is possible, which not only makes it possible to downsize the boiler, but also contributes to the widespread use of boilers as there are no restrictions on the type of coal.

The exhaust gas leaves the boiler and is led to the cyclone 7. On the other hand, the particles collected by the cyclone 7 pass through the double damper 8 at the bottom of the boiler shown in FIG. 1 and are discharged into the hopper 10.
The collected particles are introduced into the fuel cell 10 and fed by the screw feeder 11 through the feed port 51 into the downward moving bed of the heat recovery chamber, thereby contributing to the combustion of unburned fuel (char) in the collected particles and the reduction of NOx.

On the other hand, in the upper part of the freeboard, a convection heat transfer surface 6 is provided to recover heat and function as an economizer and evaporation tube, but the combustion temperature in the freeboard is kept constant.
Preferably, to maintain the temperature at 900°C, heat insulating material 17 such as refractory material is attached as necessary to the lower part of the convection heat transfer surface 6 and to the combustion chamber side of the water-cooled furnace wall. In the case of a convection heat transfer surface, each heat transfer tube near the freeboard section is wrapped in heat insulating material, but it goes without saying that the pitch of the heat transfer tubes must be taken into consideration so as not to obstruct the flow path of the exhaust gas.

In this way, by providing the heat insulating material 17, it is possible to maintain a high temperature in the lower part of the freeboard section, and the air blown in from the air blowing inlet 20 for secondary combustion in the freeboard section is effective in reducing CO, etc.

FIG. 2 shows another embodiment of the present invention.

Basically, it has the same structure as the boiler shown in Figure 1.
The main difference is that the lower part of the partition wall 38 separating the main combustion chamber 23 and the heat recovery chamber 24 is inclined to block the upward flow of the air chamber 33, which has a high fluidization velocity, in the main combustion chamber, and to redirect the flow toward the air chamber 34, which has a low fluidization velocity. The angle of inclination is 10 to 60 degrees, preferably 25 to 45 degrees, from the horizontal.
The horizontal projection length l of the inclined portion of the partition wall relative to the hearth is 1/6 to 1/2, preferably 1/4 to 1/2, of the horizontal length L of the hearth.

The bottom fluidized bed of the boiler body 21 is divided into the heat recovery chamber 24 and the main combustion chamber 23 by the partition wall 38, and a fluidizing air distribution plate 22 is provided at the bottom of the main combustion chamber 23.

The fluidization air distribution plate 22 is low in the center and high on the side opposite the heat recovery chamber. There are two types of air chambers 33 and 34 below the distribution plate 22.

The mass velocity of the fluidizing air ejected from the central air chamber 33 is a velocity necessary and sufficient for the fluidized medium in the main combustion chamber to form a fluidized bed, i.e., 4 to 20 Gmf, preferably 6 to 12 Gmf.
However, the mass velocity of the fluidizing air ejected from the air chamber 34 is smaller than the former, within the range of 0 to 3 Gmf, and at this time the fluidized medium above the air chamber 34 does not move violently up and down, forming a descending moving layer in a weak fluidized state. This moving layer spreads downward and reaches above the air chamber 33, so it is blown up by the fluidizing air ejected from the air chamber 33 at a high mass velocity. Then, part of the fluidized medium at the bottom of the moving layer is removed, and the moving layer descends under its own weight. On the other hand,
The fluidized medium blown up by the fluidizing air jet from the air chamber 33 hits the inclined partition wall 38 and is reflected and redirected, and most of it falls to the top of the moving bed, replenishing the fluidized medium in the moving bed that has fallen down.
The upper part of the air chamber 34 becomes a layer of slow downward movement, and as a whole, the bed material in the main combustion chamber 23 forms a swirling flow. On the other hand, part of the bed material blown up by the fluidizing air from the air chamber 33 and reflected and turned by the inclined partition wall 38 goes over the inclined partition wall 38 and enters the heat recovery chamber 24.
The bed material that has entered the heat recovery chamber 24 forms a gentle downward moving layer due to the air blown in from the air diffuser 32.

If the descending speed is slow, the bed material that has entered the heat recovery chamber forms an angle of repose at the top of the heat recovery chamber, and the excess falls from the top of the inclined partition wall 38 back into the main combustion chamber.

In the heat recovery chamber, the fluidized medium flows slowly downward and exchanges heat through the heat transfer tubes 25, and then flows back through the opening 39 into the main combustion chamber.

The amount of circulating fluidized bed material in the heat recovery chamber and the amount of recovered heat are controlled by the amount of diffused air blown into the heat recovery chamber, as in the embodiment shown in Figure 1. In the case of the boiler shown in Figure 2, they are controlled by the amount of air blown in from the air diffuser 32, and the mass velocity is 0 to 3 Gmf, preferably 0 to 2 Gmf.
The range is.

Although not shown, coal, which is the fuel, is supplied to the upper part of the air chamber 34, which is a downward moving bed in the main combustion chamber 23, and swirls and circulates within the fluidized bed of the main combustion chamber, resulting in very good combustibility.

On the other hand, after leaving the boiler, the exhaust gas is led to the cyclone 27. The particles collected by the cyclone 27 pass through a double damper 28 and are introduced into a hopper 30, and are mixed into the downward moving layer of the heat recovery chamber by a screw feeder 31 through a supply port 51, and after passing through an opening 39, are introduced into the main combustion chamber to contribute to the combustion of unburned char in the collected particles and the reduction of NOx.

Although not specifically shown, the particles collected by the cyclone 27 can be conveyed by air instead of a screw feeder, unlike the feeding device shown in FIG.

On the other hand, a convection heat transfer surface 26 is provided above the freeboard for heat recovery. In order to maintain the combustion temperature in the freeboard at a constant temperature, preferably 900°C, heat insulating material 37 such as refractory material is attached to the bottom of the convection heat transfer surface 26 and to the combustion chamber side of the water-cooled furnace wall, as necessary. In addition, providing an air inlet 40 for secondary combustion is effective in reducing CO, etc.

Next, with reference to Figs. 10 and 11, another embodiment of the present invention will be described in which heat recovery from exhaust gas is performed by a group of heat transfer tubes provided integrally with the freeboard section on the downstream side of the freeboard section.

Fig. 10 shows a longitudinal cross section of a combined circulating fluidized bed boiler showing one embodiment of the present invention in which heat is recovered from exhaust gas by a group of heat transfer tubes provided integrally with the freeboard section on the downstream side of the freeboard section, and Fig. 11 shows a cross section taken along line A-A in Fig. 10. In Figs. 10 and 11, reference numeral 201 denotes the boiler body, 202 denotes the fluidizing air distribution nozzle, 203 denotes the main combustion chamber,
204, 204' are heat recovery chambers, 205, 205' are heat transfer tubes, 207 is a cyclone, 208 is a rotary valve, 209 is a fuel supply pipe, 21
0 is a hopper, 211 is a fuel supply screw feeder,
212, 213 and 214 are air chambers, 218, 218' are partition walls, 219, 21
9' is the opening at the bottom of the heat recovery chamber, 228 is the secondary air inlet pipe, 22
9 is the exhaust gas outlet, 230 is the steam drum, 231 is the water drum, 2
32 denotes a convection heat transfer chamber, 233, 234, and 235 denote partition walls within the convection heat transfer chamber, 236 denotes an evaporation tube, 237 denotes a water tube wall, 238 and 238' denote the bottom of the convection heat transfer chamber, 239 and 239' denote screw conveyors, 240 denotes an exhaust pipe for the convection heat transfer chamber, and 242, 242', 243, and 243' denote air diffusers of a different type from the air diffuser shown in FIG. 2.

10 and 11 are basically formed by integrating the heat recovery chambers of the embodiment shown in Fig. 2 in a symmetrical position. As a result, the air chamber 213, where the mass velocity of the blown air is small, is located in the center, and the air chambers 212 and 214, where the mass velocity is large, are located. Therefore, the flow of the bed material caused by the air blown from the air chambers 212 and 214 is reversed by the inclined partition walls 218 and 218' and flows to the center (air chamber
213) and becomes a downward moving layer.
The flow reaches the top of the main combustion chamber, where it splits into two and is blown up again. Therefore, there are two symmetrical swirling flows in the main combustion chamber fluidized bed.

The coal is then fed into the downward moving bed in the center of the main combustion chamber, and the particles collected in the cyclone are fed into the upper part of the downward moving bed in the heat recovery chambers 204, 204'.

When the flow of the fluidized medium caused by the air blown out from the air chambers 212, 214 reverses at the inclined partition walls 218, 218', a part of the fluidized medium goes over the partition walls and enters the heat recovery chambers 204, 204'.

The amount of settling and circulation of the bed material in the heat recovery chamber is controlled by the amount of air diffused from the air diffusers 242, 242', 243, 243', as in the device shown in FIG.

After heat exchange with the heat transfer tubes 205 and 205′, the fluid medium flows through the openings 219 and 21
9' and moves to the main combustion chamber 203.

The exhaust gas discharged from the exhaust gas outlet 229 in the freeboard section is introduced into a convection heat transfer chamber 232 having an evaporator tube group disposed between a steam drum 230 and a water drum 231. Due to the action of partition walls 233, 234, and 235 disposed within the convection heat transfer chamber, the exhaust gas exchanges heat with the water in the evaporator tube group while flowing downstream in the direction of the arrow. The exhaust gas is cooled to 250-400°C and then guided through an exhaust pipe 240 to a cyclone 207, where fine particles including char are collected and then discharged into the atmosphere. The fine particles including char collected in the cyclone pass through a rotary valve 208 and are guided to a pipe 241, and then returned via screw conveyors 239 and 239' to just above the downward moving beds in the heat exchange chambers 204 and 204'.

Furthermore, fuel such as coal is returned to just above the descending moving layer of the main combustion chamber 203 via an inlet 209, a hopper 210 and a screw feeder 211.

Furthermore, particles containing the fluidized bed material, desulfurization agent, and char, which are relatively large in particle size and separated in the convection heat transfer chamber 232, are collected in the W-shaped bottom of the lower part of the convection heat transfer chamber and are returned by screw conveyors 239 and 239' to just above the downward moving layer in the heat recovery chambers 204 and 204'.

In the examples shown in Figs. 10 and 11, the bottom of the convection heat transfer section is W-shaped, but the bottom may also be V-shaped.

When the convection heat transfer section is installed downstream of the freeboard section as shown in Figs. 10 and 11, secondary air is blown in the opposite direction to the flow of exhaust gas from the freeboard section to the convection heat transfer section, which causes a swirling flow in the freeboard section as shown in Fig. 10, and oxygen and exhaust gas are efficiently mixed and stirred, resulting in a significant CO reduction effect.

BRIEF DESCRIPTION OF THE DRAWINGS Figures 1 and 2 are schematic diagrams of different types of combined circulating fluidized bed boilers of the present invention, each having heat transfer tubes such as evaporation tubes arranged above the freeboard section; Figure 3 is a schematic diagram of a conventional fluidized bed boiler; Figure 4 is a schematic diagram of the same circulating fluidized bed boiler; Figure 5 is a diagram showing the relationship between the flow rate of fluidizing air below the inclined partition wall and the flow rate of circulating bed material in the heat recovery chamber; Figure 6 is a diagram showing the relationship between the air diffused in the heat recovery chamber and the settling velocity of the descending moving bed in the heat recovery chamber; Figure 7 is a diagram showing the relationship between a general fluidization mass velocity and the overall heat transfer coefficient;
The figure shows the relationship between the volume of diffused air in an internal circulation type heat recovery chamber and the overall heat transfer coefficient, Fig. 9 shows the relationship between the fluidization mass velocity and the wear rate of the heat transfer tubes, and Fig. 10 is a schematic diagram of a combined circulating fluidized bed boiler in which a group of heat transfer tubes such as evaporators integrated with the freeboard section is arranged downstream of the freeboard section, and relatively large particles collected in the group of heat transfer tubes are returned to the left and right heat recovery chambers located at both ends of the main combustion chamber together with fine particles containing char collected by a cyclone.
FIG. 11 is a cross-sectional view taken along the line A-A in FIG.

Explanation of symbols: 1, 21, 201... boiler, 2, 22, 202... air distribution plate, 3, 23, 2
03...Main combustion chamber, 4, 24, 204, 204'...Heat recovery chamber, 5, 25, 2
05,205'...heat transfer tube, 7,27,207...cyclone, 12,13,
14, 33, 34, 212, 213, 214...Air chambers, 18, 38, 218, 218'...
...Partition wall

Claims (12)

[Claims] [Claim 1] The fluidized bed section of a fluidized bed boiler is divided into a main combustion chamber and a heat recovery chamber by a partition, and the lower part of the main combustion chamber is provided with at least two types of air chambers: an air chamber that imparts a high fluidization velocity to the fluidizing medium and an air chamber that imparts a low fluidization velocity to the fluidizing medium;
In an internal circulating fluidized bed boiler, a swirling circulating flow is given to the bed material in the main combustion chamber by a combination of airs having different fluidization velocities ejected from these air chambers, and a circulating flow of bed material is formed between the main combustion chamber and the heat recovery chamber. In this boiler, heat is recovered from the exhaust gas, the exhaust gas is cooled at the boiler outlet, and then introduced into a cyclone, and the particles collected by the cyclone are returned to the heat recovery chamber, with the return port being located immediately above or within the downward moving bed of the bed material in the heat recovery chamber. [Claim 2] A combined circulating fluidized bed boiler as described in claim 1, characterized in that the partition wall separating the main combustion chamber and the heat recovery chamber is inclined so as to block the upward flow of fluidizing air ejected from the air ejection section above the main combustion chamber where the mass velocity is high, and to reflect and redirect the fluidizing air toward the air ejection section above the air ejection section where the mass velocity is low. 3. A combined circulating fluidized bed boiler according to claim 1 or 2, wherein a desulfurization agent is supplied to the descending moving bed of the main combustion chamber. 4. A combined circulating fluidized bed boiler according to claim 1, 2 or 3, characterized in that the exhaust gas is introduced into the cyclone cooled to 250 to 400°C. 5. A combined circulating fluidized bed boiler according to claim 1, 2, 3 or 4, wherein heat recovery from the exhaust gas is carried out by a group of heat transfer tubes provided in the freeboard section above the fluidized bed. 6. A combined circulating fluidized bed boiler according to claim 1, 2, 3 or 4, wherein heat recovery from the exhaust gas is carried out by a group of heat transfer tubes provided integrally with the freeboard section on the downstream side of the freeboard section. [Claim 7] A combined circulating fluidized bed boiler as described in claim 6, characterized in that the bed material, desulfurization agent and char particles having relatively large particle sizes that are collected in the heat transfer tube group that is integrally formed with the freeboard section downstream of the freeboard section are returned to the heat recovery chamber just above or into the downward moving bed of the bed material by a conveying device such as a screw conveyor. [Claim 8] A combined circulating fluidized bed boiler as described in Claim 7, characterized in that particles including fine char collected by the cyclone are returned to a transport device such as a conveyor that returns particles collected in a heat transfer tube group integral with the freeboard section to the heat recovery chamber. 9. The method according to claim 1, wherein secondary air is blown in a direction opposite to the flow direction of exhaust gas flowing from the freeboard section to the convection heat transfer section, thereby generating a swirling flow of exhaust gas in the freeboard section.
The combined circulating fluidized bed boiler according to any one of the preceding claims. [Claim 10] An internal circulating fluidized bed boiler in which the fluidized bed section of the fluidized bed boiler is divided into a main combustion chamber and a heat recovery chamber by a partition, and at least two types of air chambers are provided below the main combustion chamber, one of which gives a high fluidization velocity to the fluidized material and the other of which gives a low fluidization velocity, and a swirling circulating flow is given to the fluidized material in the main combustion chamber by a combination of air with different fluidization velocities ejected from these air chambers, and a circulating flow of the fluidized material is formed between the main combustion chamber and the heat recovery chamber, and a freeboard section is provided downstream of the freeboard section of the main combustion chamber. The boiler is constructed so that particles collected in the convection heat transfer section are returned to the downward moving bed of the fluidized bed material in the heat recovery chamber, either directly above the steam drum or into the downward moving bed. Claim 11: The particles collected in the V-shaped bottom provided at the bottom of the water drum are collected, and the particles are returned to the heat recovery chamber directly above or into the downward moving bed of the fluidized medium by a screw conveyor provided at the V-shaped bottom.
The combined circulating fluidized bed boiler described. [Claim 12] A combined circulating fluidized bed boiler as described in Claim 10, in which the particles collected at the W-shaped bottom provided at the bottom of the water drum are collected and the particles are returned to the downward moving bed of the fluidized medium in the heat recovery chamber provided on both sides of the combustion chamber by two screw conveyors provided at the W-shaped bottom.