JPH0126447B2 - - Google Patents

Description

[Detailed description of the invention] The present invention relates to a pulverized coal combustion boiler.

Some conventional pulverized coal combustion systems have a pulverized coal machine directly connected to a burner through a pulverized coal pipe.
This is called the single direct method, but in this method the pulverized coal transport medium is the drying air supplied to the pulverizer, and the pulverized coal is fed into the furnace from the coal nozzle together with this transport air. Ru. The operating conditions of this conveying air (primary air) are determined by the pulverizer operating conditions (pulverizer load, coal properties), and are treated independently of low NOx operation, improved combustion stability, and improved boiler efficiency. was.

Another conventional example is a method in which a pulverizer and burner are separated by a pulverized coal storage tank, and a pulverized coal production system and pulverized coal transportation and combustion system are independent, and this is called a storage method. However, in this method, the hot gas used to dry the coal in the pulverizer is recirculated or fed into the furnace. In addition, the primary air for transporting pulverized coal from the pulverized coal storage tank is provided by an independent primary ventilator, and in these systems, oxygen concentration control of the pulverized coal transport medium or circular feeding medium for each burner (or burner group) is required. It is not possible to control operations such as switching (combustion gas, air), etc.

The present invention has been made to solve the conventional problems as described above. However, in the conventional method, these control operations were not possible, so it was not possible to perform optimal operation that matched the coal properties. The purpose of this project is to effectively utilize coal for drying coal to inert the pulverized coal production system and improve boiler efficiency.

According to the present invention, two types of fine powder transport media, combustion exhaust gas (low oxygen concentration) and combustion air (high oxygen concentration), are used singly or mixed to obtain an appropriate oxygen concentration to form a primary air-fuel mixture. The oxygen concentration is controlled to achieve optimal low NOx operation and stable combustion according to the boiler operating conditions (coal properties, boiler load), and the drying heat source for the pulverizer makes effective use of air preheater outlet exhaust gas. This gas is then discharged to the induced draft fan inlet side to improve boiler efficiency.

Embodiments of the present invention will be described in detail below with reference to the drawings.

In FIG. 1, pulverized coal is charged into a furnace 1 of a steam generator (hereinafter referred to as a boiler) from a burner 12, mixes with secondary air introduced from a wind box 2, and burns to become combustion gas. The air is discharged from the air preheater 5, where it exchanges heat with the combustion air, and then passes through the dust collector 7.
The dust is removed from the air, the pressure is increased by an induced draft fan 8, and the air is discharged into the chimney. A part of this combustion exhaust gas is taken from the outlet of the dust collector 7 and is supplied to the upper and lower temperature side combustion gas common duct 20 where the pressure is increased by the booster fan 19 . Further, combustion exhaust gas for temperature adjustment is taken from the upstream side of the air preheater 5, removed by a dust remover 15, boosted in pressure by a booster fan 16, and then supplied to the high temperature side combustion gas common duct 17. From here, the low-temperature and high-temperature combustion gases are automatically controlled by the air volume/temperature control damper 21 in accordance with the operating conditions of the pulverizer, and are supplied to the pulverizer 22 as a drying heat source. In addition, the coal is transported from the coal storage tank 24 to the coal feeding machine 23.
The coal is supplied to the pulverizer 22, and after pulverization, the pulverized coal pipe 2
5, the pulverized coal is transported to a cyclone 26 for separation, and the fine powder is further collected by a back filter or an electric precipitator 27 and stored in a pulverized coal storage tank 28. The separated combustion gas is disposed of through an exhaust flue 29 connected to the inlet side of the induced draft fan 8. This combustion exhaust gas contains a large amount of moisture (including almost all of the surface moisture of the coal and a part of the inherent moisture), and has a temperature of 50°C to 70°C.

Next, a system for transporting and supplying pulverized coal produced and stored in the pulverized coal production system to the burner 12 will be described.

Two types of pulverized coal transportation media are prepared: combustion exhaust gas and combustion air. First, the combustion exhaust gas system will be explained. As for the combustion exhaust gas, low-temperature combustion exhaust gas and high-temperature combustion exhaust gas are extracted from the combustion exhaust gas system for the pulverizer through flues 36 and 37, respectively, and mixed to a predetermined temperature using a temperature control damper 38, and then transferred to a combustion exhaust gas common duct 39. supply to.

Next, the air system will be explained. High-temperature air is taken from the air preheater 5 and outlet secondary air duct 11, or low-temperature air is taken from the forced draft fan 9 outlet air, adjusted with a temperature control damper 31, and then boosted in pressure by the primary air fan 32, and the primary air It is supplied to the common duct 33.
A pulverized coal pipe (main pipe) 41 is installed for each burner or burner group, and as shown in FIG.
5 and 42 are connected to a combustion exhaust gas common duct 39 and a primary air common duct 33.

The inlet of the pulverized coal pipe 41 is equipped with a flow meter and an oxygen concentration meter, and a control mechanism (FC, AC, S, CD)
The conveying medium is controlled to have a predetermined oxygen concentration and flow rate.

The pulverized coal is delivered to the pulverized coal feeder 3 from the pulverized coal storage tank 28.
0, the pulverized coal pipe 4 is controlled according to the boiler output.
1, is mixed with a conveying medium and transported by an ejector 43, etc., and reaches a burner.

In addition, when the pulverized coal pipes 41 are installed for each burner group, the air-fuel mixture is distributed to the pulverized coal pipes for each burner by a distributor 44 (see FIG. 3) and supplied to the burners. The example in FIG. 3 shows an example in which the burners are divided into five groups and installed in five stages from top to bottom in the furnace, but in this case, the conveyance medium can be arbitrarily changed for each burner stage.

Next, the effect will be explained.

Most of the NOx generated during coal combustion is NOx (fail NOx) generated by the oxidation of nitrogen in the coal, and since most of this is due to volatile nitrogen, it is NOx reduction can be effectively achieved by controlling the atmosphere, and this can be done efficiently without adversely affecting combustion by controlling the oxygen concentration in the pulverized coal carrier medium. For example, as shown in FIG. 4, the amount of nitrogen oxides (NOx) generated changes depending on the oxygen concentration of the transport medium (injected into the furnace together with pulverized coal). in this case,
When operated in region A or B, NOx can be reduced.

Further, FIG. 5 shows the relationship between the flame propagation speed of a coal combustion flame and the oxygen concentration in the pulverized coal transport medium, which changes depending on the coal properties (particularly the volatile content). If the flame propagation speed is too fast, there is a risk of flashback and explosion within the pulverized coal pipe. Conversely, if the flame propagation speed is too slow, there is a risk that flame stability will be lost and combustion instability will occur. In such cases, by controlling the oxygen concentration in the transport medium,
Stable and good combustion is possible for a wide range of coal properties.

Furthermore, in the system according to the present invention, the oxygen concentration can be controlled for each burner stage, so by varying the oxygen concentration for each burner stage, the burner stages can be divided and combined into combustion stabilization and low NOx operation, and low NOx operation can be performed.
It is also possible to perform stable NOx combustion.

The effects of the present invention described above and other effects are listed below.

(1) NOx reduction operation can be performed by controlling the oxygen concentration of the pulverized coal conveying medium to the region A or B in Fig. 4.

(2) By adjusting the oxygen concentration of the pulverized coal transport medium according to the coal properties shown in Figure 5, stable and good combustion can be achieved under a wide range of operating conditions.

(3) When aiming to reduce NOx using coal with poor combustibility, for example, by varying the concentration of oxygen in the pulverized coal carrier medium for each burner stage, it is possible to achieve low NOx operation while achieving stable combustion.

(4) Boiler combustion exhaust gas is taken from the air preheater entrance and exit as a heat source for coal drying, and the exhaust heat is effectively used.
At this time, the temperature of the pulverized coal-combustion exhaust gas mixture at the outlet of the pulverized coal machine is lowered as much as possible, and the pulverized coal is completely collected and then discarded. Normal air preheater outlet exhaust gas temperature is 150
~130℃, whereas the combustion gas used for coal drying is disposed of outside the system at a temperature of 50℃ to 70℃.
%), the boiler efficiency can be improved by approximately 4.5% x 0.25 x 100 to 60/100 = 1.1% to 0.7%.

(5) Substantially all of the combustion air, including the air for transporting pulverized coal, passes through the air preheater (this is possible by increasing the temperature of the air for transporting pulverized coal), while a portion of the combustion gas is used for coal drying. Since the heat source is supplied to the pulverizer from the inlet of the air preheater, the air preheater is bypassed by that amount, resulting in a large temperature difference between the gas side and the air side of the air preheater, and the heat exchange in the air preheater. The air preheater can be downsized to reduce the amount (assuming a constant air preheater inlet and outlet gas temperature). For example, if the amount of gas passing through the air preheater is reduced by 20% under the conditions of inlet gas temperature 380°C, outlet gas temperature 140°C, and inlet air temperature 40°C, the required heat transfer area of the air preheater will be approximately halved as shown in Figure 6. can do. That is, in FIG. 6, a is the conventional example (convective average temperature difference 66°C), b is the present invention (convective average temperature difference
100℃), the required heat transfer area HS of the air preheater is
HS∝K×Δtm/Q (K is the thermal current rate, Δt
is the logarithmic average temperature difference, and Q is the amount of heat exchange), so HS = 66/100 x 0.8≒53%.

[Brief explanation of drawings]

Figure 1 is a system diagram showing one embodiment of the present invention, Figure 2 is a system diagram showing an embodiment of the present invention.
The figure is a partially detailed view, Figure 3 is a diagram showing an example of grouping of burners, Figure 4 is a diagram showing the relationship between the oxygen concentration of the transport medium and the amount of nitrogen oxides (NOx) generated, and Figure 5 6 is a diagram showing the relationship between oxygen concentration and flame propagation speed, and FIG. 6 is a diagram for explaining one of the effects of the present invention in comparison with a conventional example. 1... Furnace, 2... Wind box, 3... Economizer, 5...
... Air preheater, 7 ... Dust collector, 8 ... Induced draft fan, 9 ... Forced draft fan, 11 ... Secondary air duct,
12... Burner, 15... Dust remover, 16... Boosting ventilator, 17... High temperature side combustion gas common duct,
19...Boost ventilator, 20...Low temperature side combustion gas common duct, 21...Damper, 22...Pulverized coal machine, 23...Coal feeder, 24...Coal storage tank, 25...
...pulverized coal pipe, 26...cyclone, 27...dust collector, 28...pulverized coal storage tank, 29...flue, 30...
...Pulverized coal feeder, 31...Damper, 32...Primary air ventilator, 33...Primary air common duct, 3
4... Damper, 35... Extracting pipe, 36, 37... Flue duct, 38... Damper, 39... Combustion exhaust gas common duct, 40... Damper, 41... Pulverized coal pipe, 4
2... Extubation, 43... Ejector, 44... Distributor.

Claims (1)

[Claims] 1 Use the air preheater outlet exhaust gas and inlet combustion exhaust gas as a heat source for coal drying, lower the pulverizer outlet temperature as much as possible, separate and collect the pulverized coal after producing pulverized coal, and then dispose of the drying combustion gas outside the system. In a pulverized coal combustion boiler that has pulverized coal combustion equipment, the pulverized coal combustion equipment transports pulverized coal supplied from a pulverized coal storage tank to the burners using boiler combustion exhaust gas, combustion air, or a mixture of both. A pulverized coal combustion boiler characterized by controlling the oxygen concentration and amount of the carrier medium for each burner group to reduce NOx and achieve stable combustion, thereby controlling combustion in the primary combustion area of pulverized coal.