JP2011125814A - Exhaust gas treatment method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exhaust gas treatment method for high-S oil burning boiler without using ammonia. <P>SOLUTION: The exhaust gas treatment method includes: denitrating oil combustion exhaust gas discharged from an oil burning boiler 2; introducing the denitrated exhaust gas into an air preheater 4 for heat recovery; introducing the exhaust gas from which the heat is recovered into an electric dust collector 5 and collecting dust contained in the exhaust gas; and desulfurizing the exhaust gas from which the dust is captured. SO<SB>3</SB>removing agent composed of an alkaline material is added to the exhaust gas from which heat is recovered from the upstream side of the electric dust collector 5. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

The present invention relates to an exhaust gas treatment method, and in particular, a high sulfur oil fired boiler (hereinafter referred to as a “high S oil fired boiler”) that uses high sulfur oil fuel (hereinafter referred to as “high S oil fuel”) having a high sulfur content. It relates to the removal of SO 3 contained in the exhaust gas of "hereinafter) or the like _(sulfur trioxide).

Dust contained in the exhaust gas of the oil fired boiler is collected by an electric dust collector. However, since the ratio of moisture and SO ₃ contained in the dust is high, there is a problem that the acid dew point of exhaust gas becomes high, equipment corrosion and ash clogging occur, and a strongly acidic smut is generated. Furthermore, carbon dust, which is abundant in soot dust, is a very fine particle and has a low electrical resistivity. Therefore, even if it is collected on the electrode plate of the electrostatic precipitator, it is likely to be scattered again and the dust collection efficiency is reduced. There was also.

Therefore, as shown in FIG. 9, an ammonia (NH ₃ ) supply device 9 is provided upstream of the electrostatic precipitator, and ammonia gas is injected to neutralize SO ₃ to deal with problems of equipment corrosion and ash clogging. Was. The SO ₃ gas reacts with moisture in the exhaust gas at the outlet of the air preheater 4 to become sulfuric acid (H ₂ SO ₄ ) gas or mist. By injecting ammonia gas here, ammonium sulfate [(NH ₄ ) ₂ SO ₄ ] solid is produced and collected by an electric dust collector.

The ammonia injection amount is desirably an amount corresponding to the amount of SO ₃ produced in the oil-fired boiler, but the SO ₃ concentration in the exhaust gas needs to be periodically measured by manual analysis or the like. Therefore, Patent Document 1 describes that an ammonia amount proportional to the fuel consumption is injected based on the measurement result of the SO ₃ concentration in the exhaust gas at the time of trial operation or the like. Further, it is described that the S content in the fuel is periodically checked and the injection ratio is changed in accordance with the change.

JP 2002-191935 A

  By the way, in oil-fired boilers, low-quality, high-S oil fuel, for example, super heavy oil called oil coke or residual oil (the S content in the fuel is about 3% or more) is used to reduce operating costs. Is increasing.

In the case of a high S oil fired boiler, the change in the amount of SO ₃ produced is unstable and does not necessarily match the fuel consumption. As a result, when ammonia gas is injected into the exhaust gas upstream of the electrostatic precipitator as in the prior art, there is a problem that the injection amount becomes excessive or insufficient. When the ammonia injection amount is insufficient, unreacted SO ₃ tends to cause equipment corrosion and ash clogging as described above. Furthermore, the produced ammonium sulfate is decomposed to produce acidic ammonium sulfate (NH ₄ SO ₄ ). Acidic ammonium sulfate has a low melting point of about 147 ° C. and dissolves in the gas temperature range in the electrostatic precipitator, which causes electrode contamination of the electrostatic precipitator and ash clogging in the hopper. In order to avoid this problem, ammonia may be excessively injected. However, excessive ammonia injection is not economical, and a large amount of unreacted ammonia is discharged from the electrostatic precipitator, causing new pollution.

  The problem to be solved by the present invention is to treat the exhaust gas of a high-S oil-fired boiler without using ammonia.

In order to solve the above problems, the exhaust gas treatment method of the present invention performs denitration treatment of oil combustion exhaust gas discharged from an oil fired boiler, introduces the denitration exhaust gas into an air preheater, recovers heat, and In the exhaust gas treatment method of introducing the recovered exhaust gas into an electric dust collector, collecting the particulate matter contained in the exhaust gas, and desulfurizing the exhaust gas from which the particulate matter has been collected, heat is recovered from the upstream side of the electrostatic precipitator. An SO ₃ remover made of an alkaline substance is added to the exhaust gas.

According to the present invention, since the SO ₃ remover made of an alkaline substance is used, unreacted ammonia is not generated. As the alkaline substance, alkali metal or alkaline earth metal carbonate, bicarbonate, sulfite, hydroxide can be used, for example, Na ₂ CO ₃ , NaHCO ₃ , NaHSO ₃ , Na ₂ SO ₃ , NaOH, CaCO ₃ , Ca (OH) _{2 and the} like can be used. Moreover, these mixtures and the mineral containing these can also be used. The action of these alkaline substances is to neutralize with SO ₃ gas, sulfuric acid gas, or mist to produce sulfate and fix it as particles. Sulfate is collected by the dust collector together with the granular material. Moreover, since unreacted alkaline substance is also collected with a granular material, it does not affect downstream equipment.

In addition, carbon dust contained in a large amount of exhaust gas particles is very fine particles and has a low electrical resistivity, so that it is likely to be scattered again even if collected on the electrode plate of the electrostatic precipitator. Therefore, by adding a regulator that adjusts the electrical resistivity, the electrical resistivity of the dust can be increased and the dust collection efficiency of the dust collector can be increased. As the regulator, for example, coal ash discharged from a coal combustion boiler can be used. Furthermore, it is possible to use SiO ₂ , Al ₂ O _3, and inorganic minerals based on it. The electrical resistivity of these regulators is considerably higher than that of carbon dust, and the amount added is very small compared to the carbon dust concentration, so there is almost no influence of dust load on the electrostatic precipitator, and the capacity of the electrostatic precipitator and fan No improvement such as up is required. Furthermore, since the modifier has higher fluidity than carbon dust, its addition ensures the fluidity of the powder and prevents deterioration of fluidity due to sulfate, which is a neutralized product of SO ₃ and alkaline substances. it can. Note that the granular material includes soot and dust in the exhaust gas, a SO ₃ remover added to the exhaust gas, a compound of the SO ₃ remover, and a modifier added to the exhaust gas, and hereinafter referred to as dust.

In this case, it is desirable to adjust the amount of the adjusting agent so that the electrical resistivity of the dust is within the set range. Most of the coal ash used as a regulator has a high electric resistance of 5 × 10 ¹⁰ [Ωcm] or more. (When the electrical resistivity is represented by ρ, there is a relationship of ρ = RA / L [Ωm]. R is the electrical resistance [Ω], A is the cross-sectional area of the conductor [m ² ], and L is the length of the conductor [m. The electrical resistivity is a unit indicating an index of electrical resistance that does not depend on the size of the substance.) Therefore, if the amount of the adjusting agent to be added increases and the electrical resistivity of the dust becomes too high, the electrode of the dust collector A large voltage drop occurs due to the current flowing through the coal ash layer on the plate, and dielectric breakdown occurs in the coal ash layer. For this reason, streamer discharge extends from within the coal ash layer, and spark discharge frequently occurs in the dust collector. As a result, the charge of the dust collector becomes unstable, the discharge current is reduced, and the dust collection efficiency is lowered. Further, in a region where the electrical resistivity of dust is 10 ¹² [Ωcm] or more, the dust layer on the surface of the dust collecting electrode is ionized in a wide range and emits light. Since the dust layer is in a discharge state and a large amount of ions are released toward the dust collection space, the current increases rapidly and the voltage drops at the same time. Since the charged dust in the dust collection space is supplied with ions of opposite polarity from the dust collection electrode, it is electrically neutralized and loses its charge, resulting in a decrease in dust collection efficiency. This phenomenon is called reverse corona. Therefore, by adjusting the amount of the adjusting agent so that the electrical resistivity of the dust is within the set range, such a phenomenon can be suppressed and the dust collection efficiency can be increased.

In addition, it is desirable to adjust the temperature of the exhaust gas that has been heat-recovered upstream of the electrostatic precipitator so that the electrical resistivity of the dust falls within the set range. The electrical resistivity of dust depends on its temperature. Therefore, the electrical resistivity can be adjusted by adjusting the temperature of the exhaust gas. In particular, it is possible to prevent the electrical resistivity from temporarily exceeding the appropriate range when the adjusting agent is added excessively or the carbon dust concentration changes due to load fluctuation on the boiler side. Moreover, if the exhaust gas temperature is adjusted and the temperature in the electrostatic precipitator is slightly shifted to the low temperature side (90 to 100 ° C.), SO ₃ gas can be almost misted, so neutralization with an alkaline substance in the SO ₃ remover. The effect goes up.

In addition, it is desirable to adjust the ratio of the SO ₃ removing agent and the adjusting agent so that the electrical resistivity of the granular material is within the set range. Thereby, it becomes easy to control the respective addition amounts of the SO ₃ removing agent and the adjusting agent according to the dust collection rate according to the SO ₃ concentration.

Moreover, it is desirable that the setting range of the electrical resistivity of the granular material is 10 ⁴ to 10 ¹⁰ [Ωcm]. As described above, the dust collection efficiency decreases if the electrical resistivity of the granular material is too high or too low. Therefore, dust collection efficiency can be maintained by keeping the electrical resistivity in the range of 10 ⁴ to 10 ¹⁰ [Ωcm].

It is also possible to supply an alkaline substance as an aqueous solution, add a regulator to the evaporated alkaline substance, and add it to the exhaust gas. That is, when the adjusting agent is added to the evaporated alkaline substance, the adjusting agent is less likely to condense, and the particle size of the alkaline substance is refined to increase the reactivity with SO ₃ . The carbon dust contained in the exhaust gas is considered to be condensed by the aqueous solution spray, but the dust collection efficiency is improved.

Further, the amount of the alkaline substance contained in the SO ₃ removal agent is desirably an equivalent ratio of 1 or more with respect to the amount of SO ₃ contained in the exhaust gas. Thereby, SO ₃ can be almost neutralized. For example, in the case of SO ₃ = 10 [ppm], the alkaline substance is about 50 [mg / m 3 N], and even if a regulator added at the same time is added, it is about 20 to 1/100 of the dust load in a general boiler. It can be suppressed. As described above, even if an unreacted alkaline substance remains, it is collected as dust together with the adjusting agent.

Further, in the present invention, the dust concentration of the exhaust gas is detected upstream and downstream of the electric dust collector, the dust collection rate of the electric dust collector is measured, and the addition amount of the SO ₃ removal agent is controlled based on the dust concentration and the dust collection rate. It can also be configured. As a result, as in the prior art, the relationship between the S concentration of fuel, the combustion amount, and the SO ₃ concentration in the exhaust gas is obtained in advance, and the SO ₃ removing agent or the adjusting agent is determined according to the S concentration and the fuel consumption amount. The amount of addition of can be controlled.

Further, sodium sesquicarbonate having both functions of SO ₃ removing agent and adjusting agent can be used. The sesquisodium carbonate can be used after being pulverized as it is.

  According to the present invention, exhaust gas from a high-S oil-fired boiler can be treated without using ammonia.

It is the figure which showed the structure of the waste gas processing system of Example 1 of this invention. SO ₃ is a graph showing the results of testing the amount of Ca _{(OH) 2} for concentration. It is a figure which shows the general relationship of the electrical resistivity of dust, the dust collection rate (eta) in an electrical dust collector, the load voltage V, and the electric current value mA. It is the figure which showed the relationship between the mixing ratio of ash, and an electrical resistivity. It is the figure which showed the relationship between the electrical resistivity of two types of coal ash, and temperature. It is the figure which showed the structure of the waste gas processing system of Example 2 of this invention. It is the figure which showed the structure of the waste gas processing system of Example 3, 4 of this invention. It is the figure which showed the relationship between the electrical resistivity of coal ash from which temperature differs, and temperature. It is the figure which showed the system configuration by the conventional method.

  Hereinafter, an exhaust gas treatment system to which the exhaust gas treatment method of the present invention is applied will be described with reference to the drawings. As shown in FIG. 1, the exhaust gas treatment system of the present embodiment performs denitration by reducing and removing nitrogen oxides from oil combustion exhaust gas discharged from an oil-fired boiler 2 that burns high-S oil fuel 1 with a denitration device 3. Treatment, introducing the denitrated exhaust gas into the air preheater 4 for heat recovery, introducing the heat recovered exhaust gas into the electric dust collector 5 to collect dust contained in the exhaust gas, The exhaust gas in which the gas is collected is introduced into the desulfurization device 7 by the fan 6 and subjected to desulfurization treatment for removing sulfur oxide (SOx) in the exhaust gas, and then discharged from the chimney 8. The exhaust gas temperature at the outlet of the air preheater 4 is generally in the range of 150 to 170 ° C.

Here, the features of the present embodiment will be described. The feature of this embodiment is that an SO ₃ remover made of an alkaline substance and a regulator that adjusts the electrical resistivity of dust are added to the exhaust gas heat-recovered from the upstream side of the electrostatic precipitator 5.

The SO ₃ removing agent and the adjusting agent are filled with powder in the additive supply apparatus 10. The additive supply device 10 is connected to the carrier gas supply device 11 and the carrier gas line 12, and the additive supply device 10 is further connected to the carrier line 13 through the flue between the air preheater 4 and the electrostatic precipitator 5. It is connected to the. The SO ₃ remover is supplied after being mixed with a regulator. In this example, Ca (OH) ₂ is used as the SO ₃ removing agent, and coal ash is used as the adjusting agent.

Here, the results of testing the amount of Ca (OH) ₂ added to the SO ₃ concentration are shown in FIG. As can be seen from FIG. 2, the SO ₃ removal rate tends to increase up to an equivalent ratio of 1 with respect to the amount of Ca (OH) _{2 under} the condition of the inlet temperature of the electrostatic precipitator 5 of 160 [° C.], but the equivalent ratio of 1 or more. In, it becomes a constant value. In this example, Ca (OH) ₂ is added so that the equivalent ratio to the SO ₃ concentration is 2 with a margin.

On the other hand, the adjusting agent is selected in such an amount that the electric resistivity of the dust falls within an appropriate range according to the carbon dust concentration in the exhaust gas. Fig. 3 (Journal of Plasma and Fusion Research, Vol. 74, No. 2, Electric Dust Collection Toshiaki Misaka) shows the electrical resistivity of dust, the dust collection rate in electric dust collector 5 (η in Fig. 3), load voltage V, and current value. The general relationship of mA is shown. In general, carbon dust tends to be too low in electrical resistivity (10 ³ to 10 ⁴ Ωcm) and coal ash tends to be too high (5 × 10 ¹⁰ Ωcm). As can be seen from FIG. 3, the proper electrical resistivity is 10 ⁴ to 10 ¹⁰ [Ωcm].

Here, in a general coal-fired boiler, the following measures are taken as a method for increasing the dust collection efficiency of the electric dust collector.
1) The electric dust collector temperature is set to the high temperature side (300 to 370 ° C.) or the low temperature side (90 ° C.), and the electric resistivity of the dust is reduced to 10 ¹⁰ [Ωcm] or less.
2) Although the electrostatic precipitator is continuously charged, it is changed to intermittent charge, pulse charge, etc., and the reverse corona phenomenon is suppressed to increase the dust collection efficiency.
3) Maintaining the thickness of the dust layer below the limit thickness where reverse corona occurs, the dust collection efficiency is increased while suppressing the reverse corona phenomenon.

  The method of increasing the temperature of the electrostatic precipitator (1) (high temperature dust collection) in 1) has problems such as a decrease in dust collection efficiency over time, an increase in the amount of processing gas, and an increase in heat loss. . Although low-temperature dust collection has a problem of cost, the electrical resistivity of dust decreases to an appropriate value, and dust removal is sufficiently performed.

  The intermittent charge of 2) has a problem that the dust collection efficiency is inferior to that of the pulse charge system. Further, the pulse charging method has a problem of increasing the cost of the power supply device. The dust layer removal method of 3) requires a change in the electric dust collector device itself. Since the electrostatic precipitator has a large device scale, enormous costs are required for existing equipment.

  As described above, problems occur when excessive amounts of high electrical resistivity such as coal ash are added, so when using coal ash as a regulator, it is necessary to select an appropriate addition amount. There is.

  Therefore, in this embodiment, by adjusting the addition amount of the adjusting agent so as to be in an appropriate electrical resistivity range at the normal operating temperature (150 to 170 ° C.) of the electrostatic precipitator 5, The dust collection efficiency of the dust collector is maximized to improve the dust collection efficiency.

  In this embodiment, after obtaining the carbon dust concentration in the exhaust gas at the inlet of the electrostatic precipitator 5 in accordance with JIS method Z8808, the dust adhering to the filter paper is sampled, and the electric dust collector recovered ash (coal ash) of the coal-fired power plant is collected. In addition to quantification, the electrical resistivity was measured. The correlation diagram at that time is shown in FIG. In addition, the appropriate range A in FIG. 4 is a desirable range when using the coal ash A as the adjusting agent, and the appropriate range B is a desirable range when using the coal ash B as the adjusting agent.

In this embodiment, coal ash is added up to a ratio (point 14 in FIG. 4) that increases the electrical resistivity to 10 ⁸ [Ωcm]. In addition, since it will become more than an appropriate range by the change of boiler load if it adds to a regulation value full, it is set as the quantity slightly less than the upper limit of addition amount. This ratio is not limited to this condition because it is affected by carbon dust, characteristics of coal ash (coal ash A, coal ash B), and use temperature. The required amount of coal ash was determined so that the above ratio was obtained, and the mixture of the above-mentioned required amount of SO ₃ removing agent and the required amount of coal ash was set in the additive supply device 10 and added. The SO ₃ removal rate before and after the electrostatic precipitator 5 in this example was 92 [%].

As described above, according to the present embodiment, since the SO ₃ removing agent made of an alkaline material is used, unreacted ammonia is not generated. In addition, as the alkaline substance, Na ₂ CO ₃ , NaHCO ₃ , NaHSO ₃ , Na ₂ SO ₃ , NaOH, CaCO _{3 and the} like can be used. Moreover, these mixtures and the mineral containing these can also be used. Sulfate is collected by the electric dust collector 5 together with dust. Moreover, since unreacted alkaline substance is also collected with a granular material, it does not affect downstream equipment.

Moreover, by adding a regulator that adjusts the electrical resistivity, the electrical resistivity of the dust can be increased and the dust collection efficiency of the electrostatic precipitator 5 can be increased. The electrical resistivity of these regulators is considerably higher than that of carbon dust, and the amount added is very small compared to the concentration of carbon dust, so there is almost no influence of the dust load on the electrostatic dust collector 5, and the electrical dust collector 5 and fan No improvement such as 6 capacity increase is necessary. Furthermore, since the modifier has higher fluidity than carbon dust, its addition ensures the fluidity of the dust, and prevents the fluidity from being lowered by sulfate, which is a neutralized product of SO ₃ and an alkaline substance.

Moreover, since the amount of the adjusting agent is adjusted so that the electrical resistivity of the dust is within the set range (10 ⁴ to 10 ¹⁰ [Ωcm]), the reverse corona phenomenon and the like can be suppressed, and the dust collection efficiency can be increased. The amount of the alkaline substances contained in the SO ₃ removal agent to the amount of SO ₃ contained in the flue gas, SO ₃ because the equivalent ratio 2 can substantially neutralized. In addition, even if an unreacted alkaline substance remains, it is collected as dust together with the adjusting agent.

  Moreover, the coal ash collect | recovered from a coal burning boiler can be used as a regulator, and this can be obtained cheaply and does not become a cost increase factor.

  Although the present embodiment has been described above, the present invention is not limited to these, and can be applied by appropriately changing the configuration. For example, whether or not the electrical resistivity is appropriate is determined based on whether or not the charge (load voltage V and current mA in FIG. 3) of the electrostatic precipitator 5 is stable or whether the dust collection efficiency η is stable. Therefore, if a means for measuring the dust concentration upstream and downstream of the electrostatic precipitator 5 is provided, it becomes possible to monitor online.

Moreover, as shown in FIG. 5, the electrical resistivity of coal ash changes with temperature. Although it affects the type of coal, in the vicinity of the normal operating temperature (150 ° C.) of the electrostatic precipitator 5 shown in II, even if the electrical resistivity is as high as 10 ¹² [Ωcm] or higher, the treatment temperature is shown on the low temperature side I (90 to 100 ° C.) or the high temperature side (370 ° C.) shown in III decreases the electrical resistivity. In particular, if the temperature of the electrostatic precipitator is slightly shifted to the low temperature side (90 to 100 ° C.), the SO ₃ gas can be almost misted, so that the neutralizing effect by the alkaline substance in the SO ₃ remover increases. Further, when the adjusting agent is excessively added or the carbon dust concentration changes due to load fluctuation on the furnace side, it is possible to prevent the electrical resistivity from temporarily exceeding the appropriate range. Of course, as in the prior art, the relationship between the S concentration in the fuel, the combustion amount, and the SO ₃ concentration in the exhaust gas is obtained in advance, and depending on the S concentration and the fuel consumption, the SO ₃ remover or regulator It is also possible to control the amount of addition.

Next, Embodiment 2 will be described with reference to FIG. In this embodiment, the carrier gas line 12 is connected to the EP ash carrier line 15 from the carrier gas supply device 11, and a part of the EP ash collected by the electric dust collector 5 is put into the additive supplier 10. After being transported and mixed in the apparatus, it is supplied into the flue upstream of the electrostatic precipitator 5 via the transport line 13. In this example, sesquisodium carbonate (trade name Trona) having both properties of SO ₃ removing agent and adjusting agent is used in powder form. The SO ₃ removal rate in this example was 92%.

In the present embodiment, the high temperature ash in the electric dust collector 5 is circulated and used, so that the unreacted SO ₃ removal agent can be reused, and the amount of use can be saved. However, when the moisture concentration in the exhaust gas is high, condensation may occur in the additive supply device 10 and the alkali components and the like may absorb moisture. Therefore, the EP ash transport line 15 is not directly connected to the additive supply device 10. Further, it may be connected in the middle of the transport line 13, or it can be handled by keeping the transport line 13 and the EP ash transport line 15 warm so as not to absorb moisture.

Further, dust concentration measuring devices 16 and 17 are provided before and after the electrostatic precipitator 5, the dust collection rate is calculated by the controller 18, and the additive supply device 10 is controlled based on the calculated value to control the additive amount of the SO ₃ removal agent. Increase or decrease. By doing so, the electrical resistivity can be controlled within an appropriate range. The dust concentration measuring device 16 (on the inlet side of the electrostatic precipitator 5) is preferably downstream of the supply position of the SO ₃ removing agent and the adjusting agent.

Next, Embodiment 3 will be described with reference to FIG. In the present embodiment, the additive supply device 10 supplies only the adjusting agent, and the upstream side thereof supplies the alkaline solution 20 of the SO ₃ removing agent by the sprayer 19. As a configuration of the sprayer 19, the alkali solution 20 and the carrier gas 21 may be premixed in the sprayer 19 or may be postmixed at the nozzle outlet of the sprayer 19. The alkaline solution 20 is preferably water-soluble, but in the case of an insoluble material, it can be sprayed by selecting an appropriate nozzle. Although it is possible to use a one-fluid nozzle, there is a possibility that the nozzle may be clogged. Therefore, it is more preferable to provide another line that can be washed with water or to use a two-fluid nozzle.

In this example, by supplying only the alkaline component that reacts with SO ₃ by spraying with an aqueous solution, the alkali can be atomized to increase the reactivity, and when a highly hygroscopic alkaline compound is used. , The SO ₃ remover can be prevented from solidifying in the additive supply device 10 due to moisture absorption. In this example, a 5 wt% Na ₂ CO ₃ solution was added so that the equivalent ratio was 2 with respect to the SO ₃ concentration. FIG. 4 shows the result of obtaining the relationship between the addition amount and the SO ₃ removal rate. In particular, although removal performance of 95 [%] or more was obtained even with an equivalent ratio of 1, this was also set to an equivalent ratio of 2 with a margin. Since the time required for the evaporation was 0.1 second, the Na ₂ CO ₃ solution was introduced from the upstream flue corresponding to a residence time of 2 seconds from the inlet of the electrostatic precipitator 5 with a margin, and the residence time was reached from the solution introduction position. A regulator (coal ash) was introduced into the downstream portion corresponding to 1 second. The residence time required for evaporation varies depending on conditions such as gas temperature, and is not limited to this value. By doing so, it is possible to prevent the adjusting agent from getting wet and clogged by the alkaline solution 20.

Similarly to the second embodiment, dust concentration measuring devices 16 and 17 are installed at the entrance and exit of the electric dust collector 5, and the additive supply device 10 is controlled by the controller 18 according to the dust collection rate. Since the adjusting agent mainly controls the electrical resistivity, it is possible to control only the amount of the adjusting agent added to an appropriate value, and the dust collection efficiency can be increased more reliably. Furthermore, as shown in FIG. 7, a controller 18 for detecting the amount of fuel combustion can be provided on the furnace load side, and the supply amount of the alkaline solution 20 can be controlled by calculating the SO ₃ concentration by proportional calculation. In this embodiment, control is always performed to add an amount that gives an equivalent ratio of 2. The SO ₃ removal rate in this example was 95%.

Next, Embodiment 4 will be described with reference to FIGS. This example has the same configuration as that of Example 3 in FIG. 7, but instead of coal ash, NaOH is used as the SO ₃ removing agent and SiO ₂ is used as the adjusting agent. FIG. 8 is a diagram showing the relationship between the mixing ratio of coal ash having different temperatures and the electrical resistivity. In addition, the appropriate range (150 degreeC) in FIG. 8 is a desirable range at the time of using 150 degreeC coal ash as a regulator, and the appropriate range (100 degreeC) uses 100 degreeC coal ash as a regulator. This is a desirable range.

In this example, NaOH is added as an aqueous solution in an equivalent ratio of 2, and SiO ₂ is added to carbon dust so that the electrical resistivity of the mixture becomes 10 ¹⁰ [Ωcm] at a temperature of 150 ° C. (point of FIG. 8). 23). However, under this condition, the electrical resistivity may exceed 10 ¹⁰ [Ωcm] due to the change in the amount of carbon dust due to the furnace load change, so that the exhaust gas temperature at the inlet of the electric dust collector 5 is further increased by the air preheater 4. The temperature is decreased to 100 ° C., and the electrical resistivity is decreased to 10 ⁸ [Ωcm] (corresponding to the point 24 in FIG. 8). The SO ₃ removal rate in this example was 96%.

Table 1 shows the SO ₃ removal rates of Examples 1 to 4 described above. In Comparative Example 1 shown in Table 1, in Example 4, the SiO ₂ with respect to the carbon dust, in which the electrical resistivity of the mixture was added to a 10 11 ^[Ωcm] at a temperature 0.99 ° C. . As the SO ₃ remover, the same amount of NaOH was added. Moreover, the comparative example 2 adds ammonia from the electrostatic precipitator 5 (temperature 150 degreeC) upstream as a conventional method.

In Examples 1 and 2, since the SO ₃ removal agent was supplied in powder form, the removal rate remained at about 90% even if the amount of the adjusting agent was controlled by the dust collection rate, but an alkaline substance was added as an aqueous solution. In Examples 3 and 4, a removal rate of 95% or more was obtained. In addition to the effect of reducing the size of the alkaline substance particles, the removal rate was improved due to the effect of controlling the addition amount of the alkaline substance by the furnace load.

2 Oil fired boiler 3 Denitration device 4 Air preheater 5 Electric dust collector 6 Fan 7 Desulfurization device 9 Ammonia supply device 10 Additive supply device 12 Carrier gas line 13 Carrier line 15 EP ash carrier line 16 Dust concentration measuring device 17 Dust concentration measuring device 18 Controller 19 Nebulizer 20 Alkaline solution 21 Carrier gas

Claims (13)

Oil-fired exhaust gas discharged from an oil fired boiler is denitrated, the denitrated exhaust gas is introduced into an air preheater for heat recovery, and the heat-recovered exhaust gas is introduced into an electric dust collector and included in the exhaust gas. In the exhaust gas treatment method of collecting the granular material to be collected and desulfurizing the exhaust gas from which the granular material has been collected,
An exhaust gas treatment method, comprising adding an SO ₃ remover made of an alkaline substance to the exhaust gas recovered from the upstream side of the electric dust collector. The exhaust gas treatment method according to claim 1,
An exhaust gas treatment method comprising adding, to the exhaust gas recovered from the upstream side of the electrostatic precipitator, a regulator that adjusts the electrical resistivity of the granular material in addition to the SO ₃ removing agent. The exhaust gas treatment method according to claim 2,
An exhaust gas treatment method, wherein the amount of the adjusting agent is adjusted so that the electrical resistivity of the granular material is within a set range. In the exhaust gas treatment method according to claim 2 or 3,
An exhaust gas treatment method characterized by adjusting the temperature of the heat-recovered exhaust gas upstream of the electric dust collector so that the electrical resistivity of the granular material is within a set range. In the exhaust gas treatment method according to any one of claims 2 to 4,
An exhaust gas treatment method, wherein a ratio of the SO ₃ removing agent and the adjusting agent is adjusted so that an electric resistivity of the granular material is within a set range. The exhaust gas treatment method according to any one of claims 2 to 5,
The exhaust gas treatment method according to claim 1, wherein a setting range of the electrical resistivity of the granular material is 10 ⁴ to 10 ¹⁰ Ωcm. In the exhaust gas treatment method according to any one of claims 2 to 6,
An exhaust gas treatment method, wherein an alkaline substance of the SO ₃ removing agent is supplied as an aqueous solution, and the regulator is added to the evaporated alkaline substance and added to the exhaust gas. In the exhaust gas treatment method according to any one of claims 2 to 7,
The exhaust gas treating method, wherein the adjusting agent is coal ash. In the exhaust gas treatment method according to any one of claims 2 to 8,
The exhaust gas treating method, wherein the adjusting agent contains SiO ₂ or Al ₂ O ₃ . The exhaust gas treatment method according to any one of claims 2 to 9,
An exhaust gas treatment method using sodium sesquicarbonate having the functions of the SO ₃ removing agent and the adjusting agent. The exhaust gas treatment method according to any one of claims 1 to 10,
The exhaust gas treatment method according to claim 1, wherein an amount of the alkaline substance of the SO ₃ removing agent is an equivalent ratio of 1 or more with respect to an amount of SO ₃ contained in the exhaust gas. The exhaust gas treatment method according to any one of claims 1 to 11,
The particulate matter concentration of the exhaust gas is detected upstream and downstream of the electric dust collector, the dust collection rate of the electric dust collector is measured, and the SO ₃ removing agent or the above based on the particulate matter concentration and the dust collection rate An exhaust gas treatment method characterized by controlling an addition amount of a regulator. The exhaust gas treatment method according to any one of claims 1 to 12,
The exhaust gas treatment method according to claim 1, wherein the alkaline substance of the SO ₃ removing agent includes at least one of alkali metal or alkaline earth metal carbonate, bicarbonate, sulfite, and hydroxide.

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