JPS5864144A - Method for tempering fly ash contained in combustion gas of boiler - Google Patents

Abstract

PURPOSE:To lower the resistivity of fly ash to resistivity facilitating electric dust collection, by directly spraying a tempering agent into the combustion chamber of a boiler. CONSTITUTION:When an electric dust collecting apparatus generates inverse ionization caused by high resistivity of fly ash, a charging state becomes unstable and the current and the voltage of a charging installation show abnormal values. A control apparatus 17 receives this abnormal output to increase the opening degree of a flow amount control valve 15. By this mechanism, the spray amount of a tempering agent such as sodium carbonate or sodium hydroxide is increased and the resistivity of fly ash is lowered. Because of the lowering of resistivity, inverse ionization in the electric dust collecting apparatus 12 is dissolved. As the result, the fly ash in the combustion gas of a boiler can be effectively tempered and the capacity of the electric dust collecting apparatus can be maintained stably.

Description

Detailed Description of the Invention The present invention relates to a method for refining fly ash contained in boiler combustion gas, particularly when the fly ash has a high electrical resistivity. It relates to a thermal refining method suitable for reducing the temperature to .

Boiler combustion gas from thermal power plants, etc. is normally discharged at a temperature range of 150 to 250°C after undergoing a prescribed heat exchange, and in order to remove fly ash contained in the gas,
Introduced into electrical industrial dust equipment. However, in the above temperature range, the apparent electrical resistivity (hereinafter referred to as electrical resistivity) of fly ash may exhibit a high value that is unsuitable for electrical industry dust.

For example, the electrical resistivity of fly ash contained in the combustion exhaust gas of a coal-fired boiler is usually as high as 1012 to 1013 Ω, which causes abnormal charging conditions such as reverse ionization in electrical industrial dust equipment, making dust collection difficult. As one method for avoiding this abnormal phenomenon, attempts have been made to refine the combustion gas. Tempering here refers to directly or indirectly adding water, steam, or a conditioning agent (referring to a specific agent other than water or steam) to the combustion gas to improve the quality of the fly ash contained in the combustion gas. This refers to reducing the electrical resistivity to a value suitable for electrostatic precipitate. By the way, the above method of adding water and steam has a small refining effect, so
It is necessary to add a large amount of water or steam, which results in large energy consumption and has the disadvantage of lowering the dew point temperature of the exhaust gas. Therefore, this method has a narrow scope of application and has limitations as a refining method.

Further, as methods for adding the refining agent, there are a method of adding the refining agent to the boiler fuel in advance, and a method of adding the refining agent to the flue connecting the boiler and the electrostatic precipitator. These methods do not have the drawbacks of the above-mentioned refining method that adds water or steam, but in the former method, the refining method is difficult and it is not possible to make fine adjustments depending on the operating status of the electrostatic precipitator. It has its drawbacks. In the latter case, the refining agent is not sufficiently dispersed into the combustion gas, and the refining effect is low.

An object of the present invention is to provide a method for refining fly ash contained in boiler combustion gas, which eliminates the drawbacks of the conventional methods, is easy to perform, and can achieve effective refining.

As a means for achieving the above object, the present invention is characterized in that a refiner is directly sprayed into the combustion chamber of a boiler.

As the conditioning agent, for example, sodium carbonate, sodium hydroxide, etc. are preferably used. The refiner is usually sprayed in the form of a solution or slurry, but it may also be sprayed directly into the combustion chamber of the boiler in the form of a powder. The spraying position is preferably near the burner in order to increase the residence time of the refiner in the combustion chamber, and it is most preferable that the refiner is sprayed into the flame of the burner if possible. The amount of spraying agent is
Normally, control is performed according to the operating status of an electrostatic precipitator installed downstream of the boiler. That is, the spark frequency or charging state such as current or voltage appearing in the charging equipment of the electrostatic precipitator is outputted, and this output is used as a signal to control the amount of spraying agent.

Alternatively, the soot and dust concentration at the outlet of the electrostatic precipitator is automatically measured, and the amount of image quality agent sprayed is controlled based on this signal.

If the properties of the fuel for the boiler are stable and there are few variables in the operating conditions of the boiler, the refiner may be sprayed in a fixed amount and adjusted as necessary.

The reason why the method of the present invention is suitable as a method for reducing the electrical resistivity of fly ash has not been sufficiently elucidated. The following points can be mentioned.

(1) The conditioning agent and fly ash must be mixed sufficiently. That is, the combustion chamber of the boiler has a volume and structure that is extremely effective in dispersing the refiner into the combustion gas. For this reason, the refiner sprayed into the combustion chamber is given sufficient residence time to be dispersed in the combustion gas and sufficiently mixed and contacted with the fly ash.

(2) A phenomenon in which the conditioning agent or its decomposed product envelops the fly ash can be expected. That is, the refiner sprayed into the combustion chamber is exposed to the high temperature atmosphere of the combustion chamber, and often melts or decomposes.

This molten tempering agent and decomposed products adhere to the fly ash with the fly ash as a core, and eventually envelop the fly ash. Since the tempering agent and its decomposed products are originally selected to have low electrical resistivity, in an electrostatic precipitator, the discharge current depends on the electrical resistance of the tempering agent and its decomposed products that surround the surface of the fly ash. It flows through a layer with a low rate of conduction, and a normal charge can be obtained without causing problems such as reverse ionization. In this sense, as mentioned above, the tempering agent is preferably sprayed into or near the flame of the burner in order to sufficiently progress its melting and decomposition.

FIGS. 1 and 2 show the results of comparative experiments conducted by the present inventors. Figure 1 shows the electrical resistivity of fly ash in coal-fired boiler combustion gas under various conditions as a function of temperature.
It is shown as a ξ parameter. The term "fly ash" in FIG. 1 includes the state in which a conditioning agent or its decomposition products are mixed into or attached to the fly ash. In the figure, curve A is the case where no tempering agent is added.
Fly ash exhibits the highest electrical resistivity. Curve B is a conditioner applied to the flue on the inlet side of the electrostatic precipitator.

Curve C is the case when the same amount of sodium carbonate solution as in curve B is sprayed into the combustion chamber of the boiler. As is clear from FIG. 1, in the method according to the present invention (curve C), the electrical resistivity of fly ash is significantly reduced. On the other hand, in the method according to the conventional technology (curve B), there is no significant difference in the electrical resistivity of fly ash compared to the case without adding a tempering agent (curve A), and the effect of adding a tempering agent is small. .

FIG. 2 shows a model of the distribution state of the tempering agent in the fly ash obtained in the above experimental example.

Figure 2 (A) corresponds to curve B in Figure 1 above, and when the conditioning agent is sprayed into the flue, Figure 2 (B) corresponds to curve C in Figure 1 above. This is the case when the refining agent is sprayed directly into the combustion chamber of the boiler. In the fly ash according to the conventional method shown in Figure 2 (a),
Ash particles 1 originating from the boiler fuel and IJum particles 2, which are carbon dioxide as a refining agent, are independently mixed. On the other hand, in the fly ash according to the method of the present invention shown in FIG. 2 (b), ash particles l
A layer 3 of sodium carbonate or a reactant thereof is formed in a state surrounding the carbonate. The composition of layer 3 is mainly composed of sodium oxide, which is a decomposition product of sodium carbonate, and various sodium salts are also mixed therein. Figure 2 (b)
Although the production process of fly ash shown in Figure 1 is not fully understood, it is estimated to be roughly as follows. That is, in the combustion chamber of a boiler, most of the carbonic acid, which is a refining agent, is decomposed into IJum and carbon dioxide gas.

On the other hand, the ash component in the fuel is composed of silicon oxide, alumina, or the like having a high melting point, and the ash component having a relatively low melting point adheres to the core, and ash particles 1 are generated. During the cooling process of the combustion gas, the decomposed sodium oxide and sodium salt having a low melting point solidify on the surface of the ash particles, forming a layer 3. The electrical resistivity of sodium oxide is at a temperature of 150℃
, in an atmosphere containing 8% moisture by volume, is approximately 100-α, and the electrical resistivity of ash particles in the same atmosphere is 10 to 10.
Ω - significantly lower than rust. Therefore, when collecting the fly ash shown in Figure 2 (b) with an electric collector and dust device, the discharge current is oxidized, which has a low electrical resistivity.
It flows through the layer 3 whose main component is IJium, and a relatively normal charge state is obtained. On the other hand, in the fly ash shown in FIG. 2(a), the refining agent does not work effectively, and it is necessary to add a large amount of refining agent to prevent reverse ionization.

The experimental results were described above when sodium carbonate was selected as the tempering agent, but similar results can be obtained when an alkali metal compound or a substance with a low melting point is used as the tempering agent.

Note that even when a tempering agent is added to the boiler fuel in advance, the electrical resistivity of fly ash exhibits the same tendency as the method according to the present invention shown by curve C in FIG.

However, in this case, it is difficult to add the refining agent uniformly to the fuel (especially in the case of solid fuel such as coal), and it is difficult to add the refining agent uniformly to the fuel (especially in the case of solid fuel such as coal). Because of the large time difference, it is practically impossible to control the amount of tempering agent added depending on the operating state of the electrostatic precipitator.

In the method of the present invention, the electrical resistivity of fly ash in the combustion gas can be adjusted in response to the operating conditions of the electrostatic precipitator, and as a result, the dust concentration at the outlet of the electrostatic precipitator can be kept almost constant. This allows you to use the conditioning agent effectively without wasting it.

In the present invention, spraying the refining agent directly into the combustion chamber may mean spraying the refining agent directly into the combustion chamber, for example, when adding the refining agent to a fuel supply pipe directly connected to a combustion burner, or when spraying the refining agent directly into the combustion chamber. This also covers the case where the refining agent is sprayed into the duct leading into the combustion chamber.

Hereinafter, an embodiment of the present invention will be described based on FIG. In FIG. 3, combustion gas from a coal-fired boiler IO is introduced into an electrostatic precipitator 12 via a flue 11, where fly ash in the combustion gas is removed. A refining side introduction pipe 14 is connected to the combustion chamber 13 of the coal-fired boiler 1O near the mounting position of a pulverized coal burner (not shown), and a refining side introduction pipe 14 has a refining agent flow rate regulating valve 15. It is provided. The opening degree of the flow rate control valve 150 is determined by the opening degree of the flow control valve 150.
Based on the output from the charging equipment 16 of No. 2, the control device 17
Adjusted by.

In the above configuration, when reverse ionization occurs in the electrostatic precipitator 12 due to the high electrical resistivity of fly ash, the charging state becomes unstable and the current and voltage of the charging equipment 16 become abnormal values. shows.

This output is received by the control device 17, and the opening degree of the flow control valve 15 is increased. This increases the amount of tempering agent sprayed per spray and reduces the electrical resistivity of fly ash. Therefore, reverse ionization in the electrostatic precipitator 12 is eliminated. The electrostatic precipitator 12 is usually a dry type, and the fly ash collected on the dust collecting electrode plate is peeled off from the dust collecting electrode plate by intermittent hammering. Before and after, the current and voltage of the charging equipment 16 change regardless of the electrical resistivity of the fly ash in the introduced gas. Therefore, it is preferable that the control device 17 has a control mechanism for correcting the influence of hammering. Microcomputer control is suitable for this purpose.

FIG. 4 shows the state of oral changes in the soot and dust concentration at the outlet of the electrostatic precipitator and the consumption amount of conditioning agent in the operation results of the above example. In the figure, the solid line indicates the operation results of the above example, and the broken line indicates a comparative example in which a certain amount of refining agent was added to raw coal.

From FIG. 4, it can be seen that in the method according to the present invention, although the consumption amount of the conditioning agent is extremely large during oral fluctuations, the outlet soot and dust concentration is stable. Furthermore, the method according to the present invention requires less total consumption of conditioning agent than the comparative example.

As described above, according to the present invention, fly ash in boiler combustion gas can be effectively refined, and the performance of an electrostatic precipitator can be stably maintained.

[Brief explanation of the drawing]

Figure 1 is a graph showing the correlation between the electrical resistivity of fly ash and temperature based on comparative experiments, Figure 2 is an explanatory diagram modeling the distribution state of tempering agent in fly ash, and Figure 3
The figure is a schematic system diagram showing an embodiment of the present invention, and FIG. 4 is a graph showing how the outlet soot and dust concentration and the consumption amount of conditioning agent change over time. lO...Coal-fired boiler, 12...Electrostatic precipitator,
13... Combustion chamber, 14... Refining side introduction pipe,
15...Flow control valve, 16...Charging equipment, 17
···Control device. Figure 1 Ken1'i ("C) Figure 2

Claims (1)

[Claims] 1. The present invention is characterized in that fly ash contained in boiler combustion gas is reduced to an electric resistivity suitable for electrical industry dust by directly spraying a refining agent into the combustion chamber of the boiler. A method for refining fly ash contained in boiler combustion gas. 2. In the boiler combustion gas according to claim 1, wherein the spray amount of the refiner is controlled according to the charging state of an electric industrial dust device provided at a downstream stage of the boiler. The method of tempering the included fly ash. 3. The boiler according to claim 1, wherein the amount of spraying of the conditioning agent is controlled according to the concentration of soot and dust at the outlet of an electrical industrial dust device provided downstream of the boiler. A method for refining fly ash that is included in combustion gas.