JPS62255712A - Combustion of fluidized bed - Google Patents

Abstract

PURPOSE:To obtain a high combustion efficiency even for a fuel poor in burning property by returning scattered unburnt substances into a fluidized bed. CONSTITUTION:Heat transfer tubes 30 are provided in a fluidized bed 27 to obtain the fluidized bed temperature of 800-900 deg.C and the stagnating time of average gas in a free board 29 is designed so as to be 2.5-3.5sec while the average gas is introduced into the rear gas heat transfer section 32 of the free board 29 whose outlet temperature is 850-950 deg.C. Steam is generated and the outlet gas with the temperature of 400-300 deg.C is sent into a dust collector 33. Most of ashes in the rear heat transfer section 32 and the dust collector 33, which are provided with the temperature of 400-300 deg.C, are circulated to the upper part of the fluidized bed 27 through a circulating line 37 while remaining ashes are discharged to the outside of a system through a discharging line 38 by a discharger 40. The amount of intermediate temperature ash, circulating through the fluidized bed 27, is controlled by a damper 39 so as to become 25 times or less as compared with supplying solid fuel.

Description

[Detailed Description of the Invention] [Industrial Application Field] 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;
Specifically, in a so-called circulating fluidized bed boiler, which collects dust containing unburned coal scattered from the fluidized bed and returns it to the fluidized bed, coal with a high fuel ratio, such as anthracite or oil coke with poor combustibility, is used. However, it is related to a combustion method that provides high combustion efficiency.

[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 a gas flow rate of 7 to h/s is used.
It is generally called the high speed Wi ring flow force method. The second method is a bubbling fluidized bed method in which a so-called head (fluidized bed) having a distinct dense particle layer is present 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 8 curves in the figure show the concentration of the high-speed vF1 reflux reflux system, and as you move toward the top of the device, , the particle concentration decreases almost linearly. On the other hand, curve 5 in the figure shows the bubbling fluidized bed system.
In the lower part of the apparatus, there is a generally uniform particle concentration region with a height of 1 to 2111, where the particle concentration is higher than in the high-speed circulating fluidized bed system, and a so-called head exists. After passing this bed, the particle concentration rapidly decreases.

FIG. 7 shows a conventional general bubbling type WJ circulating fluidized bed. An air distribution plate 2 is provided at the bottom of the fluidized bed combustion furnace main body 1, and a wind box 3 at the bottom of the air distribution plate 2 is provided.
Air is supplied from the air introduction pipe 4 to fluidize the fluidized medium (e.g. limestone, iron oxide, etc.) through the air distribution plate 2, and the fuel (e.g. coal, oil coke, etc.) supplied from the fuel supply pipe 6. is burned. 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 has a temperature of 600 to 700°C at the outlet of the freeboard 8 in the upper part of the fluidized bed, passes through the rear heat transfer section 9, and enters the collector 10 at 300 to 350°C. The dust is separated. The separated dust containing unburned coal is returned to the fluidized bed 5 from the circulation line 11.

[Problem that the invention seeks to solve]

In such a conventional fluidized bed, it is difficult to obtain high combustion efficiency, and especially in the case of coal with a high fuel ratio (fixed carbon/volatile matter), for example, when the fuel ratio is ≧2, the fuel efficiency is 9.
The limit was 5-97%. For this reason, in order to employ 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 kept at a high temperature and the necessary reaction time is secured.
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 combustion reactions, desulfurization reactions, and NOx reduction reactions. It is.

[Means and effects for solving the 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, and the temperature of the fluidized bed is 800°C.
~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 Logus temperature for hot part is 500
After collecting the heat to a temperature below ℃, the dust is collected by guiding it to a dust collector, and most of the medium-temperature ash after heat collection, which is below 500℃, is circulated above or above the fluidized bed, and the remaining medium-temperature ash is removed. The amount of medium-temperature ash discharged outside the system and circulating in the fluidized bed is 25% of the solid fuel supplied.
The feature is that it is controlled so that it is less than double.

FIG. 2 shows an example of an apparatus for carrying out the method of the present invention.
It also shows the apparatus used in the test described below, in which an air distribution plate 22 is provided at the bottom of the fluidized bed combustion furnace main body 21,
The lower side is a wind box 23, and the upper side of the dispersion plate 22 is a fluidizing medium 24 (made of limestone and coal ash).
is stored.

Fuel (anthracite) in the fuel hopper 25 is supplied to the upper part of the bed 27 by a fuel supply device (screw feeder) 26 . 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 heat transfer tubes 30 provided in 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 the dust is separated by the dust collector 33. The separated dust containing unburned coal is sent from the circulation line 34. It is returned to the fluidized bed combustion furnace main body 21. 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. In other words, multiple tubes 3 are placed at different heights.
6 is connected.

A test was conducted using a fluidized bed combustion furnace configured as described above. The furnace body used in the test had a cross section of 500+n square, and the height from the air distribution plate was about 7-.

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

Test fuel V1'4 Anthracite fuel supply jil 50kg/h Bed temperature
850°C Oxygen in exhaust gas 3-4% Ca/S molar ratio 2 Figure 4 shows the test results when the freeboard outlet temperature was changed. , NoxtQ degree was lowest near 900°C, and SO2 concentration was lowest between 850 and 950°C, but rapidly increased above 1000°C.

Furthermore, if the test is continued under conditions where the freeboard outlet temperature exceeds 1000℃, deposits mainly composed of coal ash (
Coaching) growth was recognized.

From the above combustion efficiency, SO□ concentration, NOx 7m degrees, and coating, it can be seen that the freeboard outlet temperature is preferably 1000°C or less, preferably 850 to 950°C.

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 950°C.
By controlling the temperature to .degree. C., high combustion efficiency, low NOx, and low Sot can be achieved.

Another test is to change the position of the secondary air supply to determine whether NOx?
The temperature, combustion efficiency, and CO concentration were investigated.

The longer the average gas residence time to the secondary air supply position, the higher the N
Although the Ox concentration is lowered, there is almost no effect even if the time is increased by 1.5 seconds or more.At least 1 second or more is required as the average gas residence time to the secondary air supply position. Further, 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.

From the above results, the Wi reflux fluidized bed freeboard condition is
The temperature is 800 to 1000°C, preferably 850 to 950°C, and the average gas residence time until secondary air is supplied is 1 second or more, and the average gas residence time after secondary air is supplied is 1.
By maintaining the above temperature for 5 seconds or more, we were able to achieve significant performance improvements, ie, high combustion efficiency, low Now, and low SOt. 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 in the freeboard is 2 seconds or more, preferably 2.5 to 3.5 seconds.
limited to seconds. If the time is less than 2 seconds, the unburned matter will not be combusted, resulting in poor combustion efficiency, while if it is more than 3.5 seconds, the combustion efficiency will be better, but the equipment will be expensive and the equipment cost will increase. It is.

The amount of cooled ash returned to the fluidized bed is determined by the fluidized bed set temperature and the ash temperature.If the circulation ratio R is defined as coal 11 (kg/h) supplied to the fluidized bed, then the va ring ratio and the bed temperature The relationship with is shown in FIG. Tr indicates the temperature of the ash.

Figure 6 shows the relationship between bed temperature and circulation ratio (R), where the ratio of the ash collected by the cyclone to the bed and the amount of coal supplied is defined as the circulation ratio, that is, defined by the above equation. It was a search for a relationship. Curve 8 in Figure 6 shows the bed temperature when ash at 500°C is circulated in the bed when a Buddhist painting is installed in the bed so that the bed temperature is 850°C when ash is not circulated. This is the calculated temperature. Also, the 5th curve is the It! of 500℃ ash when there are no Buddhist paintings on the bed! This figure shows the relationship between the 2 ratio and the bed temperature.

The maximum circulation ratio when circulating ash at 500℃ is 25, which can be found from curve 5 in Figure 6, assuming the bed temperature is 850℃.
The circulation ratio in the method of the present invention is as follows when the N temperature is 850°C.
Those under 25 will be accepted.

On the other hand, the combustion gas leaving the freeboard is transferred to the rear heat transfer section.
By keeping the temperature below 00°C, preferably from 400 to 300°C, there are the following advantages.

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

(2) Since there is no after-combustion, there is no problem with the combustion engine.

(3) Instead of a fireproof or heat-insulating structure, one made of steel plate can be used.

The medium-temperature ash used in the method of the present invention refers to ash at a temperature of 500°C or lower, preferably 400 to 300°C. The above-mentioned high-speed circulating fluidized bed boiler uses high-temperature ash at around 900° C., and is different in this point.

, in the method of the present invention, the above limitations are:
If the bed temperature is less than 300℃, change the bed temperature.

If the bed temperature exceeds 500°C, the circulation rate is too large to bring the bed temperature to the prescribed temperature, resulting in poor performance. However, there are disadvantages such as an increase in circulating power costs and severe damage caused by displacement.

The 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 and maintain a predetermined temperature. I need to do it. Furthermore, in the case of a large boiler or a fireproof or heat-insulating structure, it is necessary to provide a transposition 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, in this example, unless there is a specific description, the scope of the present invention is not limited and is merely an illustrative example. is 800~900℃
and the average gas residence time in the freeboard 29 is set to 2 seconds or more, preferably 2.5 to 3.5 seconds, so that the outlet gas temperature of the freeboard 29 is controlled to be between the layer temperature and 1000.
℃, preferably 850 to 950℃. free board 2
9 is guided to the rear heat transfer section 32 so that the temperature of the outlet gas at the rear heat transfer section is 500°C or less, preferably 400 to 300°C.
After generating steam, the gas is guided to the dust collector 33 to collect dust, and the temperature of the gas collected by the rear heat transfer section 32 and the dust collector 33 is 500°C or less, preferably 400°C or less. 3
Most of the medium-temperature ash at 00° C. is circulated to the upper side or upper part of the fluidized bed 27 through the circulation line 37, and the remainder of the medium-temperature ash is discharged to the outside of the thread through the discharge line 38, and the amount of medium-temperature ash that circulates into the fluidized bed 27 is supplied. Controlled to be 25 times or less than solid fuel.
do. 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 Wi-ring 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 above-mentioned concentrator 33, and after further steam is generated there, the air preheater 42
In some cases, the combustion air is preheated by a final dust collector 43, and then dust is collected by a final dust collector 43. In this case, most of the ash collected in the second rear heat transfer section 41, air preheater 42, and final precipitator 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 500° C. or lower, the volume of the gas is reduced, the dust collector of the 2nd flow becomes smaller, and the dust collection efficiency can be further increased. Furthermore, since there is no after-combustion, there are no problems with the engine or coaching, 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) By limiting the freeboard conditions (1 degree, time), high combustion efficiency, low NOx, and low SO□ can be achieved.

(3) Top-loading coal feeding method reduces combustion efficiency and NO
However, in the method of the present invention, even if the top-filling method is adopted, the ash containing char and desulfurization agent can be circulated, By keeping the freeboard temperature high, NOxSSo
□ can be lowered 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 N combustion method of the present invention, and FIG. 2 is a flow sheet for the apparatus used in the test, showing another example of an apparatus for implementing the method of the present invention. Figure 3 is a graph showing the relationship between temperature and height from the air dispersion plate; Figure 4 is a graph showing the relationship between freeboard temperature and char, N
A graph showing the relationship between Ox and So□ concentration, Figure 5 is a graph showing the relationship between the circulation ratio and the layer temperature when the layer temperature is changed, and Figure 6 is a graph showing the relationship between the circulation ratio and the layer temperature when the layer temperature is 500℃. A graph showing the relationship with layer temperature, FIG. 7, is a flow sheet of a conventional device. DESCRIPTION OF SYMBOLS 1...Fluidized bed combustion furnace main body, 2...Air bone +tJl plate, 3...Wind box, 4...Air introduction pipe, 5...Fluidized bed, 6...Fuel supply pipe, 7 ...heat exchanger tube, 8...free board,? ... 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 equipment surface, 27... Fluidized bed (bed), 28... ... Combustion air pipe, 29 ... Free board, 30 ... Heat exchanger tube, 3
DESCRIPTION OF SYMBOLS 1... Heat exchanger tube, 32... Rear heat transfer part, 33... Dust collector, 34... Circulation line, 35... Discharge line, 36... Pipe, 37... Circulation line , 38... Discharge line, 39... Damper, 40... Discharge machine, 4
1... Second rear heat transfer section, 42... Air preheater, 4
3... Final dust collector, 44... Air fan Applicant Kawasaki Heavy Industries, Ltd. Figure 2 Grain size 3ffLt (season/-t3) Figure 4

Claims (1)

[Claims] 1. In a method of burning solid fuel in a fluidized bed, heat transfer tubes are provided inside the fluidized bed so that the temperature of the fluidized bed is 800 to 900°C.
In addition, the freeboard outlet gas temperature is set to between the fluidized bed temperature and 1000°C, the average gas residence time in the freeboard is set to 2 seconds or more, and the freeboard outlet gas is guided to the rear heat transfer section to be connected to the rear heat transfer section outlet. After collecting heat so that the gas temperature becomes 500°C or less, it is led to a dust collector to collect dust, and most of the medium-temperature ash below 500°C after heat collection is circulated above or above the fluidized bed. A fluidized bed combustion method characterized by discharging the remainder of the ash outside the system and controlling the amount of medium-temperature ash circulating in the fluidized bed to be 25 times or less the amount of solid fuel supplied. 2. The fluidized bed combustion method according to claim 1, wherein the amount of medium temperature ash that is circulated in the fluidized bed is controlled by changing the amount of medium temperature ash discharged outside the system.