JPH0322529B2 - - 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.

FIG. 7 shows a conventional general bubbling type circulating fluidized bed. An air distribution plate 2 is provided at the bottom of the fluidized bed combustion furnace main body 1. Air is supplied from an air introduction pipe 4 to a wind box 3 at the bottom of the air distribution plate 2. , iron oxide, etc.), and the fuel (for example, coal, oil coke, etc.) supplied from the fuel supply pipe 6 is combusted. The fluidized bed temperature is
A part of the combustion heat is absorbed and controlled by the heat transfer tube 7.

The combustion exhaust gas accompanied by unburned coal generally reaches a temperature of 600 to 700°C at the outlet of the free board 8 at the top of the fluidized bed, passes through the rear heat transfer section 9, and enters the dust collector 10 at 300 to 350°C. The dust is separated. The separated dust containing unburned coal is sent to the circulation line 11
from there to the fluidized bed 5.

[Problem that the invention seeks to solve]

In such conventional fluidized beds, it is difficult to obtain high combustion efficiency, especially when using coal with a high fuel ratio (fixed carbon/volatile matter), for example, when the fuel ratio is ≧2, the fuel efficiency is 95 to 97. % was the limit. For this reason, in order to adopt the conventional fluidized bed, there were restrictions on the coal that could be used.

The present invention was made in view of the above points, and by actively using the freeboard as a reaction area, the freeboard temperature is maintained at a high temperature, the necessary reaction time is secured, and combustion reactions, desulfurization reactions, and
The object of the present invention is to provide a method that can burn coal with a high fuel ratio, such as anthracite coal, and oil coke with poor combustibility, with high combustion efficiency by promoting the NOx reduction reaction.

[Means and actions for solving problems]

The fluidized bed combustion method of the present invention is a method of burning solid fuel in a fluidized bed, in which a heat exchanger tube is provided inside the fluidized bed so that the temperature of the fluidized bed is 800 to 900 ° C.
And the freeboard outlet gas temperature is the fluidized bed temperature ~
1000℃, the average gas residence time in the freeboard is 2 seconds or more, and the freeboard outlet gas is guided to the rear heat transfer section to collect heat so that the rear heat transfer section outlet gas temperature is 500℃ or less, and then collected. Most of the medium-temperature ash below 500℃ after heat collection is circulated above or above the fluidized bed, and the remainder of the medium-temperature ash is discharged outside the system and circulated to the fluidized bed. It is characterized by controlling the amount of ash to be less than 25 times the amount of solid fuel supplied.

FIG. 2 shows an example of an apparatus for carrying out the method of the present invention, and also shows an apparatus used in the tests described below.
2 is provided, and the lower side thereof serves as a wind box 23. A fluidizing medium 24 (consisting of limestone and coal ash) is accommodated above the distribution plate 22. The fuel (anthracite) in the fuel hopper 25 is fed by a fuel supply device (screw feeder) 26.
It is supplied to the upper part of the bed 27. Combustion air is supplied from the combustion air pipe 28 to the bed 27 via the air distribution plate 22 to combust the fuel. A part of the air is supplied to the freeboard 29 as secondary air, if necessary. A portion of the generated combustion heat is recovered by a heat transfer tube 30 provided inside the bed to control the bed temperature. The freeboard temperature, more precisely, the freeboard outlet temperature, can be set to a predetermined temperature by the heat exchanger tube 31 provided in the freeboard. The combustion gas enters the rear heat transfer section 32 from the freeboard outlet, is cooled to below 500° C., and is separated from dust by a dust collector 33. The separated dust containing unburned coal is returned to the fluidized bed combustion furnace main body 21 from the circulation line 34. A part of the dust is discharged from the external discharge line 35 as necessary.

Usually, limestone is used as a desulfurization agent, and a predetermined amount is mixed with coal and supplied. Furthermore, in order to consider the height of the freeboard 29, the secondary air supply position can be changed. That is, a plurality of pipes 36 are connected at positions at different heights.

A test was conducted using a fluidized bed combustion furnace configured as described above. The furnace body used in the test has a cross section of 500 mm.
At the corner, the height from the air distribution plate was approximately 7 m.

The test was conducted by changing the freeboard outlet temperature and changing the secondary air supply position. The conditions were as follows.

Test fuel: Anthracite fuel supply rate: 50 kg/h Bed temperature: 850°C Oxygen in exhaust gas: 3 to 4% Ca/S molar ratio: 2 Figure 4 shows the test results with different freeboard outlet temperatures. Unburned loss decreases as the freeboard temperature increases, but the NOx concentration reaches 900℃
It is lowest in the vicinity, and the SO ₂ concentration is 850-950℃
It was the lowest at temperatures above 1000°C, but increased rapidly above 1000°C.

Furthermore, when the test was continued under the condition that the freeboard outlet temperature exceeded 1000℃, a deposit (coating) consisting mainly of coal ash was observed to grow on the furnace wall.

From the above combustion efficiency, SO ₂ concentration, NOx concentration and coating, the freeboard outlet temperature is 1000℃
Hereinafter, it can be seen that 850 to 950°C is preferable.

The bed temperature when performing in-furnace desulfurization is controlled at 800 to 850°C by heat transfer tubes, so the freeboard outlet temperature is set to 800 to 1000°C, preferably 850 to 850°C.
By controlling the temperature to 950℃, high combustion efficiency is achieved.
Low NOx and low _SO2 can be achieved.

Other tests include changing the secondary air supply position.
NOx concentration, combustion efficiency and CO concentration were investigated. The longer the average gas residence time to the secondary air supply position, the lower the NOx concentration, but if it is made longer by 1.5 seconds or more, there is little effect. The average gas residence time up to the secondary air supply position was required to be at least 1 second. Furthermore, judging from the change in CO concentration as after-combustion after secondary air supply, an average gas residence time of 1.5 seconds or more was required.

Based on the above results, the freeboard conditions for the circulating fluidized bed are set at 800 to 1000°C, preferably 850 to 950°C,
By ensuring the above temperature with an average gas residence time of 1 second or more until secondary air is supplied and an average gas residence time of 1.5 seconds or more after secondary air is supplied, a significant performance improvement, i.e., high combustion can be achieved. efficiency,
Low NOx and low SO ₂ were achieved. Furthermore, when two-stage combustion is not performed, an average gas residence time of 2 seconds or more is required.

Therefore, in the method of the present invention, the average gas residence time on the freeboard is 2 seconds or more, preferably
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 equipment costs increase.

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 determined by the circulation ratio R=
When defined as the amount of ash returned to the fluidized bed (Kg/h)/the amount of 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.

Figure 6 shows that the circulation ratio is defined as the ratio of the amount of ash collected by the cyclone returned to the bed and the amount of coal supplied.
That is, the relationship between bed temperature and circulation ratio (R) is determined by defining the above equation. Curve a in Figure 6 shows the case where ash at 500°C is circulated in the bed when a transmission surface is provided in the bed so that the bed temperature is 850°C when ash is not circulated. This is the calculated bed temperature. Curve b shows the relationship between the circulation ratio of ash at 500°C and the bed temperature when no transmission surface is provided in the bed.

The maximum circulation ratio when circulating ash at 500℃ is 25% from curve b in Figure 6, assuming the bed temperature is 850℃.
is calculated, and the circulation ratio in the method of the present invention is determined when the bed temperature is 850
In the case of °C, 25 or less will be adopted.

On the other hand, by controlling the combustion gas leaving the freeboard at a temperature of 500° C. or less, preferably 400 to 300° C., in the rear heat transfer section, the following advantages can be obtained.

(1) The volume of gas is smaller, so the dust collector can be smaller.

(2) There is no clinker trouble because there is no after combustion.

(3) Those made of steel plates can be used instead of fireproof or heat-insulating structures.

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 in order to bring the bed temperature to a predetermined temperature, which improves performance, but causes disadvantages such as increased circulation power cost and severe damage to the transmission surface.

The main body of a fluidized combustion furnace is usually constructed with a water-cooled wall, so in small and medium-sized boilers, the freeboard outlet temperature will drop, so it is lined with insulating material to limit the amount of heat absorption to maintain a predetermined temperature. I need to. Furthermore, in the case of a large boiler or a fireproof or heat-insulating structure, it is necessary to provide a conductive surface on the freeboard to maintain a predetermined temperature.

〔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. A heat transfer tube 30 is provided inside the fluidized bed 27 to control the fluidized bed temperature to 800 to 900°C, and the average gas residence time in the freeboard 29 is set to 2 seconds or more, preferably 2.5 to 3.5 seconds. , the outlet gas temperature of the freeboard 29 is set to a layer temperature to 1000°C, preferably 850 to 950°C. The outlet gas of the freeboard 29 is guided to the rear heat transfer section 32 to generate steam such that the temperature of the outlet gas at the rear heat transfer section is 500°C or less, preferably 400 to 300°C, and then this gas is Most of the medium-temperature ash collected in the rear heat transfer section 32 and the dust collector 33 at temperatures below 500°C, preferably from 400 to 300°C, is passed through the circulation line 37.
The remainder of the medium-temperature ash is discharged from the system through the discharge line 38, and the amount of medium-temperature ash circulating in the fluidized bed 27 is controlled to be 25 times or less of the supplied solid fuel. . 39 is a damper, and 40 is a discharger.

Note that the amount of medium-temperature ash that is circulated to the fluidized bed 27 may be controlled by changing the amount of medium-temperature ash discharged outside the system.

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 29 that is higher than the static layer height and a central part of the freeboard 29.

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

A second rear heat transfer section 41 is provided downstream of the dust collector 33, where steam is further generated.
The air preheater 42 preheats the combustion air, and then
The final dust collector 43 may be configured to collect dust. In this case, most of the ash collected in the second rear heat transfer section 41, air preheater 42, and final dust collector 43 is circulated within the fluidized bed 27, and the remainder is discharged outside the system. 44 is an air fan.

〔Effect of the invention〕

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

(1) Since the outlet gas of the rear heat transfer section is cooled to below 500℃, the volume of the gas is reduced, the dust collector in the wake can be made smaller, and the dust collection efficiency can be further increased. Furthermore, since there is no after-combustion, clinker 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 heat-insulating structure, and one made of steel plate can be used, so that costs can be reduced.

(2) High combustion efficiency, low NOx, and low SO ₂ by limiting freeboard conditions (temperature, time)
can be achieved.

(3) 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 implementing the fluidized bed combustion method of the present invention, and FIG. 2 is a flow sheet for the apparatus used in the test, Figure 3 is a graph showing the relationship between particle concentration and height from the air dispersion plate for the high-speed circulating fluidized bed method and the bubbling fluidized bed method, and Figure 4 is a graph showing the relationship between the particle concentration and the height from the air distribution plate in the high-speed circulating fluidized bed method and the bubbling fluidized bed method.
Graph showing the relationship with SO ₂ concentration. Figure 5 is a graph showing the relationship between circulation ratio and layer temperature when the ash temperature is changed. Figure 6 is the graph showing the relationship between circulation ratio and layer temperature when the ash temperature is 500℃. FIG. 7 is a flow sheet of a conventional device. 1...Fluidized bed combustion furnace main body, 2...Air distribution plate,
3...Wind box, 4...Air introduction pipe, 5...Fluidized bed,
6... Fuel supply pipe, 7... Heat transfer tube, 8... Free board, 9... Rear heat transfer section, 10... Dust collector,
11...Circulation line, 21...Fluidized bed combustion furnace main body, 22...Air distribution plate, 23...Wind box, 24...
... Fluidized medium, 25 ... Fuel hopper, 26 ... Fuel supply device, 27 ... Fluidized bed (bed), 28 ...
Combustion air pipe, 29...Free board, 30...
Heat exchanger tube, 31... Heat exchanger tube, 32... Rear heat transfer section,
33...dust collector, 34...circulation line, 35...
...Discharge line, 36...Pipe, 37...Circulation line, 38...Discharge line, 39...Damper, 4
0... Ejector, 41... Second rear heat transfer section, 42
...Air preheater, 43...Final dust collector, 44...
...Air fan.

Claims (1)

[Claims] 1. A method for burning solid fuel in a fluidized bed,
A heat exchanger tube is installed inside the fluidized bed, and the temperature of the fluidized bed is 800℃.
~900℃, the freeboard outlet gas temperature is ~1000℃ the fluidized bed temperature, the average gas residence time in the freeboard is 2 seconds or more, and the freeboard outlet gas is guided to the rear heat transfer section to After the heat has been absorbed so that the hot section outlet gas temperature is below 500℃,
The dust is collected by guiding it to a dust collector, and most of the medium-temperature ash below 500℃ after heat collection is circulated above or above the fluidized bed, and the remainder of the medium-temperature ash is discharged outside the system and circulated to the fluidized bed. A fluidized bed combustion method characterized by controlling the amount of medium-temperature ash to be 25 times or less the amount of solid fuel supplied. 2. Claim 1, which controls the amount of medium-temperature ash circulating in the fluidized bed by changing the amount of medium-temperature ash discharged outside the system.
Fluidized bed combustion method described in section.