JPH0322531B2 - - Google Patents

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention provides a novel combustion method in a fluidized bed combustion boiler in which solid fuel is burned in a fluidized bed to extract steam, and more specifically, In so-called circulating fluidized bed boilers, which collect dust containing unburned coal and return it to the fluidized bed, high combustion efficiency can be achieved even with coal with a high fuel ratio, such as anthracite or oil coke with poor combustibility. This relates to the combustion method used.

[Conventional technology]

Circulating fluidized bed boilers are classified into the following two types. The first method is a type in which there is no dense particle layer at the bottom of the main body of the fluidized bed, and the gas flow rate is 7.
~8 m/s is adopted and is generally called a high-speed circulating fluidized bed method. The second method is a bubbling fluidized bed method in which a so-called bed (fluidized bed) having a distinct layer of concentrated particles exists at the bottom of the fluidized bed main body. There is a clear difference in the particle concentration distribution in the device height direction between the two, and the particle concentration distribution is shown in FIG.

Figure 3 shows the relationship between the height from the air dispersion plate and the particle concentration. The a curve in the figure shows the particle concentration in the high-speed circulating fluidized bed system, and as it goes towards the top of the device, As a result, the particle concentration decreases almost linearly. On the other hand, the b curve in the figure shows the bubbling type fluidized bed system, in which there is an almost uniform particle concentration region of 1 to 2 m height at the bottom of the device where the particle concentration is higher than that of the high-speed circulating fluidized bed system, so-called Betsudo exists. After passing this bed, the particle concentration rapidly decreases.

When solid fuel, such as coal, is supplied to the fluidized bed, it becomes fine during the combustion process, and the ash or unburned coal (char) is blown away from the fluidized bed by the gas flow and enters the rear heat transfer surface where it is cooled. .

In the conventional fluidized bed described above, heat transfer tubes are generally provided within the bed for heat recovery.

[Problem that the invention seeks to solve]

However, a fluidized bed with heat transfer tubes inside the fluidized bed
There are disadvantages such as corrosion of the heat transfer tubes, thinning due to erosion, bed temperature decreases as the load decreases, and especially when performing in-furnace desulfurization, the load range is narrow, about 100 to 70%.

Furthermore, even with a water-cooled wall structure, when steam coal is burned, the temperature of the fluidized bed under normal combustion conditions is approximately
The temperature will exceed 1500℃. The fluidized bed temperature is limited by the melting point of coal ash, and the operating limit temperature is around 1100℃.Furthermore, when performing in-furnace desulfurization with _CaCO3 , there is an optimum temperature for desulfurization, so must be controlled at 800-900℃.

The present invention has been developed in view of the above points, and is made by returning the medium-temperature ash below 500°C after heat absorption (including the scattered desulfurizing agent when performing in-furnace desulfurization) to the fluidized bed. The bed temperature can be kept at a predetermined temperature, and by actively using the freeboard as a reaction area, the freeboard temperature can be kept high and the necessary reaction time can be secured, allowing combustion reactions, desulfurization reactions, and Coal with a high fuel ratio, such as anthracite, promotes the NOx reduction reaction.
The object of the present invention is to provide a method that allows even oil coke, which has poor combustibility, to be combusted with high combustion efficiency.

[Means and actions for solving problems]

The fluidized bed combustion method of the present invention is a method for burning solid fuel in a fluidized bed, in which heat transfer tubes are not provided inside the fluidized bed, but heat transfer tubes are provided above the freeboard, and the heat transfer tubes are placed below the freeboard. Temperature to layer temperature ~
1000℃, the average gas residence time from the fluidized bed to the heat transfer tube of the freeboard is 2 seconds or more, and the heat is collected so that the gas temperature at the freeboard outlet is 500℃ or less. The outlet gas is guided to a dust collector to collect dust, and after the heat has been collected,
Most of the medium-temperature ash below 500℃ is circulated above or above the fluidized bed, and the remainder of the medium-temperature ash is discharged outside the system.
The fluidized bed temperature is controlled by the circulating amount of medium-temperature ash or the discharge amount of medium-temperature ash.

FIG. 2 shows a simplified apparatus for carrying out the method of the invention. In Fig. 2, 1 is the main body of the fluidized bed combustion furnace, 2 is the heat exchanger tube, 3 is the dust collector,
Among the ash and desulfurization agent in the supplied coal, dust collector 3
Particles of the size collected by the dust collector are circulated within the system, and the amount of circulation depends on the performance of the dust collector. If the amount of circulation is insufficient, it is necessary to further circulate the ash collected by the air preheater downstream, and in some cases, the ash collected by the final dust collector. The amount of air preheater repair ash or/and final dust collector collected ash is indicated by W ₃ . In any case, the circulating amount W ₁ is inevitably determined by the amount of retained circulating particles (retained circulating particle amount). The amount of retained circulating particles can be increased or decreased by changing the amount of circulating particles discharged from the circulation system to the outside of the system.
Now, when the temperature of the fluidized bed is low, if the amount withdrawn from the system is increased, the amount of retained circulating particles decreases, the amount of circulating particles decreases, and the fluidizing temperature increases. On the other hand, if the fluidized bed temperature is high, the temperature can be lowered by reducing the amount of waste discharged from the system or by supplying air preheater-collected ash and/or final dust collector-collected ash. . W ₂ indicates the amount of extraction.

In a so-called bubbling-type fluidized bed combustion furnace having a fluidized bed, most of the combustion occurs within the fluidized bed, but some chirping is scattered, so freeboard combustion conditions are important. Figure 4 shows an overview of this, and shows that the higher the freeboard temperature, the lower the amount of char, but the amount of Nox and So ₂ tend to increase when the temperature exceeds 1000℃. Furthermore, if the temperature exceeds 1000°C, the temperature will approach the melting point of ash in the case of coal combustion, and coaching (adhesion) of ash and desulfurization agent will occur on the freeboard, so it is necessary to limit the temperature to below 1000°C.
Therefore, the freeboard temperature is the fluidized bed temperature ~
It can be seen that 1000°C, preferably 850 to 950°C is good.

If the temperature drops in a water-cooled wall structure, cover the water-cooled wall with insulation. In addition, if the temperature exceeds 1000℃ in the case of a fireproof insulation structure, it is necessary to install a conductive surface.

In the method of the present invention, the average residence time of the gas in the freeboard up to the heat transfer tube is preferably 2 seconds or more.
Limited to 2.5-3.5 seconds. If it is less than 2 seconds, the combustion efficiency is poor because unburned matter is not combusted;
If it exceeds 3.5 seconds, the combustion efficiency will improve, but
This is because the equipment becomes expensive and the equipment ratio increases.

The amount of cooled ash returned to the fluidized bed is determined by the fluidized bed setting temperature and the ash temperature, and the circulation ratio R is calculated as follows: Circulation ratio R = ash amount returned to the fluidized bed (Kg/h) / coal supplied to the fluidized bed (Kg/h), the relationship between circulation ratio and bed temperature is shown in Figure 5. Tr indicates the temperature of the ash.

Now, if you want the fluidized bed temperature to be 850℃, if the ash temperature is 100℃, the circulation ratio = 14, and the ash temperature is 400℃.
It is shown that in the case of °C, the circulation ratio = 21.

In this way, by using a fluidized bed temperature control method that uses means to return the scattered ash to the fluidized bed, it is possible to control the temperature even when the boiler load decreases, and it is possible to maintain a stable bed temperature even at low loads. Can be set.

Medium-temperature ash in the method of the present invention refers to temperatures below 500°C;
Preferably refers to ash at a temperature of 400 to 300°C. The above-mentioned high-speed circulation fluidized bed boiler circulates high-temperature ash at around 900°C, and is different in this respect.

In the method of the present invention, the reason for the above limitations is that when the ash temperature is less than 300°C, the amount of circulation is small to bring the bed temperature to a predetermined temperature, resulting in a decrease in performance; If the temperature exceeds 500℃,
This is because the amount of circulation is too large to bring the bed temperature to a predetermined temperature, and although performance is improved, there are disadvantages such as an increase in circulation power cost and severe damage to the transmission surface.

〔Example〕

Hereinafter, a preferred embodiment of the present invention will be exemplarily described with reference to FIG. However, unless there is a specific description, this example does not limit the scope of the present invention, and is merely an illustrative example. Without providing a heat transfer tube inside the fluidized bed 4,
The heat exchanger tubes 2 are provided on the upper part of the freeboard 5, and the average gas residence time up to the heat exchanger tubes 2 on the upper part of the freeboard 5 is set to 2 seconds or more, preferably 2.5 to 3.5 seconds. The temperature of the lower side is between fluidized bed temperature and 1000°C, preferably between 850 and 950°C. The outlet gas temperature of the heat transfer tube 2 of the freeboard 5 is 500℃ or less,
After generating steam, preferably at a temperature of 400 to 300°C, the gas is led to a dust collector 3 to collect dust, and the temperature collected in the dust collector 3 is preferably 500°C or lower, preferably Most of the medium-temperature ash at 400 to 300°C is circulated to the upper side or upper part of the fluidized bed 4 through the circulation line 6, and the remaining medium-temperature ash is discharged to the outside of the system through the discharge line 7.
The fluidized bed temperature is controlled by the circulating amount of medium-temperature ash or the discharge amount of medium-temperature ash. 8 is an air distribution plate, 9 is a wind box, 10
is the primary air supply pipe, 11 is the secondary air supply pipe, 12
is a damper, and 13 is a discharger.

When the fluidized bed combustion furnace main body 1 has a refractory heat insulating material structure, the fluidized bed temperature can be maintained at a predetermined temperature at a circulation ratio that is approximately the same as the circulation ratio at 100% load. In the case of a water-cooled wall structure, since the heat absorption rate by the water-cooled wall increases slightly, a predetermined fluidized bed temperature can be set by slightly decreasing the circulation ratio.

Further, it is preferable that coal is fed from a position higher than at least the height of the stationary bed, and that secondary air is supplied from a position higher than the coal feeding position.

The overall temperature can be kept at a predetermined temperature by returning the circulating ash to a plurality of positions, such as a lower part of the freeboard 5 that is higher than the static layer height and a central part of the freeboard 5.

It goes without saying that the present invention can also be applied to two-stage combustion and in-furnace desulfurization.

A rear heat transfer section 14 is provided downstream of the dust collector 3, in which steam is further generated, the dust is collected in a second dust collector 15, and then combustion air is preheated in an air preheater 16. Then, the final dust collector 17
In some cases, it is configured to collect dust. In this case, most of the ash collected in the rear heat transfer section 14, second dust collector 15, air preheater 16, and final dust collector 17 is circulated within the fluidized bed 4, and the rest is discharged outside the system. Do it like this. 18 is an air fan.

In addition, if the freeboard 5 has a water-cooled wall structure and the temperature at the bottom of the freeboard's heat transfer tubes drops below the layer temperature, the freeboard 5 should be lined with a heat insulating material and the bottom side of the freeboard's heat transfer tubes should be lined with a heat insulating material. The temperature should be between the fluidized bed temperature and 1000℃.On the other hand, if the freeboard 5 has a fireproof insulation structure and the temperature below the heat transfer tube of the freeboard exceeds 1000℃, the heat transfer tube should be placed on the freeboard. so that the temperature below the freeboard heat transfer tube is between the fluidized bed temperature and 1000°C.

〔Effect of the invention〕

Since the present invention is configured as described above, it has the following effects.

(1) Since the freeboard outlet gas is cooled to below 500°C, the volume of the gas is reduced, making the downstream dust collector smaller and further increasing the dust collection efficiency. In addition, since there is no after-combustion, clean car troubles and coaching troubles do not occur, and continuous operation can be continued for a long period of time. Furthermore, there is no need for the dust collector to have a fireproof or insulating structure, and one made of steel plate can be used.
Cost reduction can be achieved.

(2) Top-loading coal feeding method reduces combustion efficiency,
The adoption of the conventional method was limited due to the increase in NOx and SO ₂ , but in the method of the present invention, even if the top-filling method is adopted, the ash containing the char and desulfurization agent can be circulated, and the free By keeping the board temperature high, NOx and SO ₂ can be reduced and sufficient performance can be ensured. In addition, since the circulating ash is returned to the upper side of the fluidized bed where there is less back pressure, sufficient performance can be ensured and energy loss is reduced.

[Brief explanation of drawings]

Fig. 1 is a flow sheet showing an example of an apparatus for carrying out the fluidized bed combustion method of the present invention, Fig. 2 is a flow sheet showing a simplified apparatus for carrying out the method of the present invention, and Fig. 3 is a flow sheet showing an example of an apparatus for carrying out the method of the present invention. Figure 4 is a graph showing the relationship between particle concentration and height from the air dispersion plate for the layer method and bubbling _fluidized bed method. The figure is a graph showing the relationship between circulation ratio and layer temperature when the ash temperature is changed. 1...Fluidized bed combustion furnace main body, 2...Heat transfer tube, 3...
...Dust collector, 4...Fluidized bed, 5...Freeboard, 6...Circulation line, 7...Discharge line, 8...
... Air distribution plate, 9 ... Wind box, 10 ... Primary air supply pipe, 11 ... Secondary air supply pipe, 12 ... Damper, 13 ... Discharge machine, 14 ... Rear heat transfer section, 15
...Second dust collector, 16...Air preheater, 17
...Final dust collector, 18...Air fan.

Claims (1)

[Claims] 1. A method for burning solid fuel in a fluidized bed,
Instead of installing a heat transfer tube inside the fluidized bed, a heat transfer tube is provided above the freeboard, and the temperature below the freeboard heat transfer tube is set to the bed temperature ~1000℃, and the average temperature from the fluidized bed to the freeboard heat transfer tube is After making the gas residence time 2 seconds or more and collecting heat so that the gas temperature at the freeboard outlet is 500℃ or less, the collected freeboard outlet gas is guided to a dust collector to collect dust and collect heat. Most of the remaining medium-temperature ash below 500℃ is circulated above or above the fluidized bed, and the remainder of the medium-temperature ash is discharged outside the system, and the fluidized bed temperature is controlled by the circulation amount of medium-temperature ash or the amount of medium-temperature ash discharged. A fluidized bed combustion method characterized by: 2. If the freeboard has a water-cooled wall structure and the temperature at the bottom of the freeboard's heat transfer tubes drops below the layer temperature, line the freeboard with a heat insulating material to reduce the temperature at the bottom of the freeboard's heat transfer tubes. Fluidized bed temperature ~
The fluidized bed combustion method according to claim 1, wherein the temperature is 1000°C. 3. If the freeboard has a fireproof insulation material structure and the temperature below the freeboard's heat transfer tube exceeds 1000°C, place another heat transfer tube below the freeboard's heat transfer tube. The fluidized bed combustion method according to claim 1, wherein the temperature below the heat exchanger tube is between the fluidized bed temperature and 1000°C. 4. The fluidized bed combustion method according to claim 1, wherein the coal is fed from a position higher than the stationary bed height, and the secondary air is fed from a position higher than the coal feeding position. 5. The fluidized bed combustion method according to claim 1, wherein the ash is returned to a position higher than the static bed height. 6. The fluidized bed combustion method according to claim 1, wherein ash from the air preheater and final dust collector is also returned to the fluidized bed. 7. The fluidized bed combustion method according to claim 1, which performs two-stage combustion. 8. The fluidized bed combustion method according to claim 1, which performs in-furnace desulfurization. 9. The fluidized bed combustion method according to claim 1, wherein a rear heat transfer section is provided next to the dust collector to recover heat.