JPS58120020A - Disposal of exhaust smoke - Google Patents

Abstract

PURPOSE:To prevent the corrosion of a heat exchanger due to sulfuric acid by a method wherein a low pressure supply water heater is provided between an air preheater and an electrical dust collector so that the temperature of the gas at the outlet of the air preheater is lowered and SO3 contained in the gas is removed by making it condensed and adsorbed by fly ash. CONSTITUTION:The combustion gas thermally collected by combustion air at an air preheater is thermally collected additionally by the gas heating low pressure supply water heater 101 and reaches the electrical dust collector 8 after the temperature thereof is lowered. The coal combustion gas contains fly ash and unburned carbon and more than about 99% of the contents is deprived of dust by the dust collector 8. After that, the gas is desulfurized, flows from the gas- gas heat exchanger 14 to a line 20 where it is prevented from generating white smoke and heated to a temperature sufficient for it to be diffused into the atmosphere, and finally reaches a chimney 12. As a consequence, it is possible to prevent the heat exchanger 14 from corrosion due to sulfuric acid.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for treating coal-fired boiler flue gas, and particularly to a method for preventing low-temperature corrosion due to 803 in the flue gas.

It was held in

In FIG. 1, combustion gas from a boiler, etc. passes through a carbon saver and a denitrification device (not shown), enters the combustion gas flow path B of the air preheater from line 575, is pressurized by forced draft fan 1, and is pressurized by line 2. After exchanging heat with the combustion air sent from the air to the air passage 3 of the air preheater, it enters the electrostatic precipitator 8 from the line 7, where it is dedusted, and then passes through the line 9, the induced draft fan 10°, and the line 11. It enters the booster fan 12, is pressurized, and enters the gas-gas heat exchanger 14 through the line 13. After heat exchange with the processing gas, which will be described later, the line 15 leads to the humidifying cooling tower 16, and after humidifying and cooling, the line 17 leads to the absorption tower 18.
, sulfur dioxide gas is removed, and then a dehumidifier 19 removes water droplets. The treated gas from which the water droplets have been removed enters the gas-to-gas heat exchanger 14, is pressurized by the booster fan 12, and is heat exchanged with the gas. The processing gas heated to high temperature by this heat exchange is discharged into the atmosphere through a line 20 and a chimney 21. Note that 22 is a switching damper for bypassing the processing system from line 12 to line 20 described above.

By the way, the desulfurization treatment gas (that is, the gas after sulfur dioxide gas is removed in the absorption tower 18 and dehumidified in the dehumidifier 19) contains saturated moisture at approximately 50 to 60°C, so the gas-gas heat It must be heated in the exchanger 14 to a temperature (usually +00-130°C) necessary for preventing white smoke and dispersing it into the atmosphere. The gas entering the gas-gas heat exchanger 14 from line 15 is therefore required to maintain a temperature of 130-140°C. However, in systems that must maintain such a temperature, there are the following inconsistencies.

(1) Since the temperature of the gas output from the air preheater 6, that is, the temperature of the gas from the gas-gas heat exchanger 14 (gas from the line 13) must be increased, heat loss is large. For example, if the air preheater 6 output log gas temperature can be lowered by 10 degrees Celsius in total, the air preheater 3 output log gas temperature will increase by 10 degrees Celsius, and the boiler efficiency will improve by about 1%.

(11) If the temperature of the gas output from the air preheater 6 is high, S03 in the combustion generated gas will not condense and will not adhere to the fly ash. Therefore, S03 is not removed by the electrostatic precipitator 8 and is directly carried into the gas-gas heat exchanger 14.
The low temperature end metal temperature of the gas-gas heat exchanger 14 is 70
Due to the low temperature of 0.degree. C. and the high humidity, the above-mentioned SO3 condenses and adheres to heat transfer surfaces, causing severe sulfuric acid corrosion not only on the heat transfer surfaces but also on structural members.

The present invention was made to solve these problems. (1) A low-pressure feed water heater (gas-heated low-pressure water heater) is installed between the air preheater and the electrostatic precipitator, and An exhaust system characterized by lowering the temperature, condensing 803 in the gas, adsorbing it to fly ash, and removing it together with the fly ash in an electrostatic precipitator to prevent sulfuric acid corrosion of gas-gas heat exchangers. smoke treatment method, and (2) installing all the low-pressure feed water heaters in (1) above to prevent sulfuric acid corrosion of gas-to-gas heat exchangers;
A heater (steam type heater) using steam obtained from the above-mentioned low-pressure feed water heater is installed between the treated gas outlet of the gas heat exchanger and the chimney to reduce the amount of heat exchanged in the gas-gas heat exchanger. This relates to a flue gas treatment method, which is characterized in that the temperature of the gas is maintained at the required gas temperature at the smoke inlet, without supplementing the temperature with this steam heater. It is a diagram. In FIG. 2, the same reference numerals as in FIG. 1 indicate the same functional parts as in FIG.

In FIG. 2, the combustion generated gas whose heat has been recovered by the combustion air in the air preheater 6 is transferred to the gas-heated low-pressure water heater 101.
The heat is further recovered and the temperature is reduced before reaching the electrostatic precipitator 8. Coal combustion gas usually contains 5 to 50 S'/Nm3 of fly ash and unburned carbon, of which 99% or more is removed by the dust collector 8. Thereafter, it is desulfurized through the same thread path as shown in Figure 1, flows out from the gas-gas heat exchanger 14 to the line 20, and is heated in the steam heater +02 to a temperature necessary for preventing white smoke and dispersing it into the atmosphere. , leading to the chimney 21.

The gas-heated low-pressure feedwater heater 101 is a turbine cycle low-pressure feedwater heater that uses the air preheater 6 output gas as a heating source instead of turbine bleed air, and has a temperature lower than the gas temperature (usually 100 This is a low-pressure water supply for use in a temperature range (below ℃).

Moreover, as the heating steam of the above steam heater 102,
The steam obtained by the gas-heated low-pressure water supply heater 101 described above is extracted and used.

By the way, the humidifying cooler 16 adjusts the combustion generated gas to the humidity and temperature (usually 50°C) necessary for desulfurization in the absorption tower 18, but if the humidity and temperature of the humidifying cooler 16 is lowered, As the heat loss in the humidifying cooler 16 decreases, the amount of heat recovered in the gas-heated low-pressure feed water heater 101 increases, so the amount of steam generated increases accordingly, and the amount of heating steam for the steam heater 102 increases. This improves plant efficiency.

As described above, the method of the present invention applies to the gas-gas heat exchanger 1
This method can be said to be extremely effective in this type of technology because it prevents the sulfuric acid corrosion described in No. 4 and also makes full use of the heat reheated within the system for reheating the processing gas.

Below, the effects of the method of the present invention will be summarized, including specific data.

(1) Figure 5 shows the gas temperature and SO3 on the low-temperature heat transfer surface.
It is a chart showing the relationship with the passage rate.

According to the figure, the lower the gas temperature, the more 803 in the gas condenses and adheres to the low-temperature heat transfer surface, resulting in SO3 on the low-temperature heat transfer surface.
Although the passage rate will be low, in the method of the present invention, the gas is cooled in advance, 5o31 condensed and adsorbed on fly ash, and the fly ash is removed using an electrostatic precipitator, so that it is sent to the low-temperature heat transfer surface. The SC2 concentration in the gas is significantly reduced, and there is no problem even if the SO3 passage rate on the low-temperature heat transfer surface is low.

Moreover, FIG. 4 is a diagram showing the relationship between the So3 concentration in the gas and the acid dew point temperature.

According to the figure, the lower the SC2 concentration, the lower the acid dew point temperature. However, in the method of the present invention, as described above, the temperature is reduced and dust is removed in a high dust area, so the condensation adsorption rate of SO3 to fly ash and the SO3 High removal rate of adhered fly ash, SO
Since the gas from which most of 3 has been removed is sent to the low-temperature heat transfer surface, there is very little SO2 to condense even if the acid dew point temperature is low, and there is no risk of low-temperature corrosion.In addition, it contains a large amount of gas phase 803. In the conventional method of sending the gas to the low-temperature heat transfer surface after dust removal, it is not possible to remove gaseous SO3 during dust removal, so a large amount of SO2 condenses on the low-temperature heat transfer surface.
Severe cold corrosion occurs.

(11) FIG. 5 is a chart showing an example of the relationship between the temperature of the exhaust gas to be treated and the plant efficiency.

As is clear from the figure, the lower the temperature of the exhaust gas to be treated, the higher the plant efficiency. Therefore, the method of the present invention, which uses a gas-heated low-pressure water heater to recover heat and treat the gas whose temperature has been reduced, significantly improves the plant efficiency. It can be seen that it will improve.

011) Since there is no risk of low-temperature corrosion in the gas-gas heat exchanger, the temperature of the untreated gas introduced into the heat exchanger (i.e., the gas introduced from line 13) can be lowered, resulting in humidification and cooling. The amount of temperature loss in the humidifier can be reduced (i.e., the heat loss in the humidifying cooler can be reduced)
0 (IV) If reheating using only a gas-gas heat exchanger does not raise the temperature to the temperature required for preventing white smoke or dispersing it into the atmosphere, further heating can be performed using a steam heater, and this steam If the steam obtained by the gas-heated low-pressure feedwater heater of the turbine cycle is extracted and used as heated steam for use in the type heater, the efficiency of the turbine will also be improved.

(V) Since the temperature of the gas to be treated is low, the volumetric flow rate of the combustion gas is reduced, the induced draft fan and the boost draft fan can be downsized, and their power consumption can also be reduced.

[Brief explanation of the drawing]

Fig. 1 is a system diagram showing a conventional flue gas treatment method, Fig. 2 is a system diagram showing an embodiment of the method of the present invention, and Fig. 6 is the relationship between gas temperature and SO3 passage rate on a low-temperature heat transfer surface. FIG. 4 is a chart showing the relationship between SC2 concentration and acid dew point temperature, and FIG. 5 is a chart showing an example of the relationship between the exhaust gas temperature to be treated and plant efficiency. Sub-agents 1) Meifuku agent Ryo Hagiwara - Horse 3 illustrations 5 illustrations 4 illustrations

Claims (2)

[Claims] (1) In the coal-fired boiler flue gas treatment system, a low-pressure feed water heater is installed between the air preheater and the electrostatic precipitator to lower the air preheater outlet gas temperature and adsorb it to the 5o3i fly ash in the combustion gas. A flue gas treatment method comprising: removing the fly ash with an electrostatic precipitator to reduce the concentration of SO in the combustion gas, and then sending it to a gas-gas heat exchanger of a desulfurization facility. (2) In the coal-fired boiler flue gas treatment system, a low-pressure feed water heater is installed between the air preheater and the electrostatic precipitator to lower the air preheater outlet gas temperature and adsorb it to the 5o3f fly ash in the combustion gas. After removing the fly ash with an electrostatic precipitator to reduce the concentration of SO in the combustion gas, it is sent to the gas-to-gas heat exchanger of the desulfurization equipment, and the treated gas after desulfurization is passed through the chimney. A post-desulfurization treatment device that installs a steam heater before releasing into the atmosphere.