JPH0147685B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for preventing material corrosion of a heat exchanger due to sulfur trioxide contained in combustion exhaust gas. High-temperature combustion exhaust gas, such as lime-fired boiler exhaust gas, containing air pollutants such as dust, nitrogen oxides (NO _x ), and sulfur oxides (SO _x ) is generally comprehensively treated as follows. Ammonia (NH ₃ ) is first injected into the exhaust gas discharged from the boiler, and NO _x is reduced to water and nitrogen and rendered harmless by passing through a denitrification reactor filled with a catalyst. Next, an air heater is used to recover the heat from the exhaust gas to a retrievable temperature, generally 130 to 180°C, and then dust is removed using an electrostatic precipitator or cyclone, followed by wet desulfurization treatment. In wet desulfurization methods such as the wet lime and gypsum method, the gas after treatment becomes a moisture-saturated gas at 45 to 70°C, and the low temperature causes problems with diffusion, so a method of reheating the low-temperature exhaust gas has not been put into practical use. ing. Reheating methods include indirect heating using a heat medium such as hot water, oil, or steam, and direct heating using a heat storage body such as a Ljungström heat exchanger, but the latter has recently been put into practical use from the perspective of energy saving. Among them, the method called gas heating method is said to be the most desirable. [Seo et al. “Thermal Nuclear Power Generation”
Vol.30, No.2, P29-35 (1979) and Mitsubishi Heavy Industries Technical Report Vol.17, No.2, P217-222 (1980) The high-temperature gas (130-180℃) and the low-temperature gas (45-70℃) at the outlet are circulated (rotated) through a heat storage element to exchange heat, lowering and increasing the exhaust gas temperature just before wet processing. This is a so-called energy-saving and resource-saving method that reduces the amount of wet cooling water and raises the temperature of the gas exiting the wet desulfurization equipment. However, since this method uses the high-temperature gas at the inlet of the desulfurization equipment as a heat source, the amount of heat exchanged depends on the temperature of the high-temperature gas. That is, if the inlet gas temperature is low, the temperature of the exhaust gas after wet desulfurization treatment will not be raised sufficiently, the exhaust gas discharged from the chimney will not reach a predetermined temperature, and the white smoke will not disappear.
In order to avoid this, a method has been proposed in which a steam heater is installed in addition to the gas-gas heating method to raise the temperature of the exhaust gas after wet treatment to a predetermined temperature, but this is not preferred from the viewpoint of energy saving. Another possibility is to raise the exhaust gas temperature at the inlet of the wet desulfurization apparatus, that is, the temperature at the outlet of the air heater in the previous stage, but this poses the following problems.
That is, gas discharged from a furnace generally contains sulfur trioxide (SO ₃ ), which is produced when part of the sulfur contained in the fuel is oxidized during combustion. Furthermore, when denitrification equipment is installed as is the case recently, 0.5~0.5~ of sulfur dioxide (SO ₂ ) gas is generated accompanying the denitrification reaction.
4% was oxidized to produce SO ₃ in the furnace.
Added to SO ₃ . Therefore, the SO ₃ concentration that reaches the air heater inlet varies depending on the sulfur content in the fuel, but is approximately 5 to 50 ppm. When the SO ₃ that reaches the end passes through the air heater, the exhaust gas temperature drops below the acid dew point, so some of it becomes sulfuric acid mist (H ₂ SO ₄ ) through the condensation reaction described below.
It is converted into a dust, which is then attached to the entrained dust, and is collected together with the dust in a subsequent dust collector. SO ₃ + H ₂ OH ₂ SO _4If the exhaust gas temperature at the air heater outlet is high due to equilibrium conditions, the above condensation reaction will occur.
Since the amount of H ₂ SO ₄ condensed is small, the amount of SO ₃ gas remaining in the air heater outlet gas is large. Air
The heater outlet _SO3 gas passes through the precipitator, and most of it reaches the desulfurization equipment inlet. The SO ₃ gas becomes H ₂ SO ₄ mist due to temperature drop in the gas/gas heater, which causes so-called sulfuric acid corrosion of the constituent materials of the gas/gas heater. Also, generate
Since the H ₂ SO ₄ mist is fine particles, some of it passes through the desulfurization equipment and is discharged from the chimney, which may cause acid smud and white smoke. Conventionally, as a method to prevent these problems, NH ₃ is injected into the exhaust gas at the air heater outlet, and a combination of SO ₃ and NH ₃ such as ammonium sulfate (((NH ₄ ) ₂ SO ₄ )) or acidic ammonium sulfate (NH ₄ HSO ₄ ) is used. A method has been put into practical use in which a reactant (hereinafter referred to as NH ₃ -SO ₃ reactant) is collected in a subsequent dust collector.
[“Mitsubishi Heavy Industries Technical Report” Vol.10, No.5, P211-P218
(1973)] However, when this method is applied to remove SO ₃ from exhaust gas containing a large amount of dust, such as from coal-fired boilers, there are the following disadvantages. That is, since both the NH ₃ --SO ₃ reactant and the dust are collected in the dust collector, it becomes a problem to treat a large amount of dust mixed with the NH ₃ --SO ₃ reactant. Conventionally, dust from coal-fired boilers is effectively used for flyash cement or dumped in landfills, but in the former case, NH ₃ odor is generated and the strength is reduced when mixed with water, and in the latter case, it is released into NH ₃ odor and underground water, etc. The leakage of information is considered to be a problem. The amount of dust in conventional heavy oil-fired combustion systems is extremely small compared to coal-fired combustion systems, and the amount of dust mixed with NH3 _{- SO3} reactants is small, making treatment relatively easy. As mentioned above, this becomes a problem when the amount of dust is large. As a result of intensive studies to solve the above-mentioned problems, we have arrived at the present invention.The gist of the invention is to solve the following problems: The dust concentration at the exit of the dust collector is controlled so that the ratio falls within an appropriate range. Specifically, in the exhaust gas treatment method, the fuel exhaust gas is subjected to dust removal treatment and then supplied to the high temperature side of a heat exchanger for cooling, and the exhaust gas after wet desulfurization treatment is further supplied to the low temperature side of the heat exchanger to increase the temperature.
Detecting the concentration of dust contained in the exhaust gas after dust removal treatment and sulfur trioxide gas contained in the exhaust gas, and controlling the dust concentration at the exit of the dust collector so that the concentration ratio is 4 or more (weight ratio) The present invention provides corrosion prevention for an exhaust gas heat exchanger characterized by the following. The present inventors have discovered that even when exhaust gas containing SO ₃ gas is supplied to a heat exchanger and cooled below the thermal dew point, the heat exchanger material is severely corroded by sulfuric acid mist.
We discovered the fact that if more than an appropriate amount of dust coexists in the exhaust gas, the corrosive environment can be significantly suppressed.
This led to the present invention. The present invention can be applied to conditions where the exhaust gas supplied to the heat exchanger contains a large amount of _SO3 , that is, when a denitrification device is installed, as is the case in many future coal-fired power generation plans. In addition, when it is necessary to raise the exhaust gas temperature at the air heater outlet in order to raise the exhaust gas temperature at the smoke inlet to a specified temperature using a heat exchanger, it is necessary to raise the exhaust gas temperature at the _{air heater} outlet. Therefore, it is possible to use a low-grade material such as carbon steel, which is extremely advantageous in terms of equipment cost. Next, embodiments will be described based on the drawings in order to clarify the effects of the present invention. In FIG. 1, the exhaust gas discharged from the coal-fired boiler 1 has _NOx removed by the denitrification device 2, and then
The air is supplied to the air heater 3. In denitrification equipment 2, part of the SO ₂ in the exhaust gas is removed as a result of the denitrification reaction.
At the air heater 3 inlet for conversion to SO ₃ ,
An amount of SO ₃ added to the SO ₃ contained in the exhaust gas from the boiler 1 will arrive. For example, assuming that the SO ₂ concentration is 1000 ppm and the oxidation rates in boiler 1 and denitration equipment 2 are 1% and 2%, respectively,
The SO ₃ concentration at the air heater 3 inlet is 30 ppm. The exhaust gas temperature at the air heater 3 outlet is generally
However, as is clear from Figure 2, which shows the relationship between sulfuric acid dew point and temperature, if the gas temperature, that is, the metal temperature of the air heater element, is high, the dew point temperature will not be reached, and therefore the air heater 3 Most of the SO ₃ gas will remain at the outlet.
Next, the exhaust gas at the outlet of the air heater 3 is supplied to an electrostatic precipitator (EP) 4. In EP4, although some of the H ₂ SO ₄ mist generated by the air heater 3 in the previous stage is collected and removed while adhering to dust, SO ₃
Almost no gas is collected and reaches the gas/gas heater 5 (untreated side) in the latter stage. On the other hand, the dust concentration at the EP4 outlet is an appropriate amount to prevent corrosion of the heat exchanger material in the latter stage, that is, the dust/ _SO3 weight ratio is 4.
The above is preferably adjusted to 10 or more by the following method. The dust concentration is detected by the dust concentration meter A installed at the EP4 outlet flue, and the concentration signal is input to the controller B through line a. Controller B sends EP from line B to power pack C according to the deviation between the input concentration signal and the preset dust concentration value.
Outputs the charging condition (eg voltage) setting value. Electric power is supplied to the EP 4 from the power pack C through the line c under the set charging conditions, and the dust concentration in the exhaust gas at the outlet of the EP 4 is adjusted. In the above control example, it is necessary to set the concentration setting value of controller B in advance according to the SO ₃ concentration, but in general, if the type of coal, that is, the S content in the fuel is constant, the SO ₃ concentration will change depending on the boiler load conditions. It is determined. Therefore, there is a strong correlation between the SO ₃ concentration and the boiler load signal such as the amount of power generation, so this relationship is determined in advance, and the load signal from the boiler 1 is input to the calculator D via line d, and the dust/SO ₃
A dust concentration setting value is output so that the weight ratio becomes a predetermined value, and this can be input as a setting value to the controller B through line e. By the above method, dust/
After the SO ₃ weight ratio is adjusted to an appropriate value, the exhaust gas is supplied to the subsequent gas/gas heater 5 (untreated side). Gas/gas heater 5 (untreated side) removes exhaust gas.
As it cools down to 70-90℃, _SO3 gas is
The reaction of SO ₃ + H ₂ O → H ₂ SO ₄ converts it into H ₂ SO ₄ mist, but at this time, due to the effect of the dust adjusted and entrained in the previous stage, the condensed H ₂ SO ₄ causes the gas/gas heater 5 to This has the effect of significantly suppressing sulfuric acid corrosion of the constituent materials. Possible mechanisms of suppression include adsorption of sulfuric acid onto the dust surface, coating of the material surface with dust to reduce contact between H ₂ SO ₄ and the material, and sulfuric acid neutralization effect due to the alkaline content of the dust. Judging from the mechanism, it is thought that the higher the dust concentration, that is, the higher the dust/SO ₃ weight ratio, the higher the corrosion prevention effect. In view of the dust removal performance of the desulfurization equipment, etc., the dust concentration is preferably 300 mg/m ³ N or less. Now, the outlet gas of the gas/gas heater 5 (untreated side) is supplied to the desulfurization device 6 to remove SO ₂ and dust, and then heated again by the gas/gas heater 5 (treated side) and discharged from the chimney 7.

【table】

[Table] This example confirmed that the amount of corrosion on the test piece was significantly reduced when the dust/SO ₃ weight ratio was approximately 10 or more, and therefore the dust/SO ₃ weight ratio at the dust collector outlet was adjusted and controlled. The effectiveness of this approach has become clear.

[Brief explanation of drawings]

FIG. 1 is a diagram showing an embodiment of the present invention, and FIG. 2 is an equilibrium diagram showing the relationship between the dew point of H ₂ SO ₄ and temperature.

Claims (1)

[Claims] 1. In an exhaust gas processing method in which after dust removal treatment, the flue gas is supplied to the high temperature side of a heat exchanger for cooling, and after further wet desulfurization treatment, the exhaust gas is supplied to the low temperature side of the heat exchanger to raise the temperature. Detecting the concentration of dust contained in the exhaust gas after treatment and sulfur trioxide gas contained in the exhaust gas, and controlling the dust concentration at the exit of the dust collector so that the concentration ratio is 4 or more (weight ratio). Features: Corrosion prevention method for exhaust gas heat exchangers.