In order to be able to clarify the problem of condensation of smoke, let us first understand the fuel combustion process, and the conditions of condensation of smoke.
Combustion refers to a severe oxidation reaction that occurs when the fuel temperature rises. The fuel here mainly refers to various hydrocarbons (CXHY). When the fuel temperature rises during the combustion process, the carbon-carbon or carbon-hydrogen bonds in the fuel break and release heat energy. The following is the chemical equation for the combustion of hydrocarbon fuels in the air:
CXHY+bO2+(79/21)bN2=xCO2+y/2H2O+(79/21)bN2
In the equation, b=x+(y/4) is the amount of oxygen required for the combustion of hydrocarbon CXHY.
It should be noted that in the combustion process, the fuel is usually not purified, and the impure fuel will form SO2 or SO3 during the combustion process.
During the combustion process, hydrogen in the fuel reacts with oxygen in the air to form H2O, and exists as water vapor at high temperatures. The fuel contains hydrogen, and the ratio of hydrogen to carbon in different fuels (hydrocarbons) is different, so the content of moisture in the flue gas is different after combustion. If natural gas is used as fuel, the moisture content in the flue gas can be as high as 25%; brown coal is higher than the content of anthracite after burning.
The temperature at which water vapor starts to condense in the flue gas is called the dew point. The dew point temperature of the flue gas generated during the actual combustion process is related to many factors. If the content of water vapor in the flue gas is high, that is, the partial pressure of water vapor is high, the dew point temperature is high. With high sulfur content in the fuel, the dew point temperature is also high. Sulfur dioxide is generated in the combustion of sulfur in the fuel, and sulfur dioxide is further oxidized into sulfur trioxide. Sulfur trioxide and the steam in the flue gas generate sulfuric acid vapor. The presence of sulfuric acid vapor can greatly increase the dew point. If the sulphur content in the fuel is high, more SO2 will be generated after combustion. If the excess air factor increases at this time, the amount of SO2 converted to SO3 will increase. If the coal contains 1% sulfur, 600 ppm (about 1700 mg/m3) of SO2 may be generated in the combustion flue gas. If the moisture content in the flue gas is 10% and the SO3 content reaches 10 ppm, the dew point temperature of the flue gas will reach about 135 degrees.
Also, different combustion methods, different fuels, and different heavy metal content in the fuel (even if the fuel has the same sulfur content) will change the dew point temperature of the flue gas.
For normal operation of the bag filter, the operating temperature of the filter must be higher than the dew point of the flue gas by more than 20°C! Once the flue gas forms condensation on the filter bag, it will cause the dust bag of the filter bag to be stuck in the bag and knot, which will cause the resistance of the dust bag to rise rapidly and affect the normal operation of the system. What’s more, SO3 in the flue gas reacts irreversibly with water to produce H2SO4 sulfate. Sulfuric acid will continue to play the role of desiccant in the bag or dust on the fog, then oxidized corrosion of the filter bag and equipment metal parts. If H2SO4 is to be converted back into gaseous state, the temperature must rise above 315 degrees before it is possible.
Therefore, when purifying the flue gas from the tail end of the combustion process, the temperature of the flue gas must be controlled to prevent condensation of the flue gas.
Precipitator prevents condensation
In use, it is necessary to prevent the gas from cooling in the bag room to below the dew point, especially when bag filters are used under negative pressure. Because the shell often leaks air, so that the bag room gas temperature is lower than the dew point, the filter bag will be damp, resulting in dust is not loose, but sticky to the filter bag, the fabric hole blocked, resulting in clear Ash failure, so that the filter pressure drop is too large, can not continue to run, and some produce a paste bag can not dust.
To prevent condensation, the temperature of the gas must be kept above the dew point of 25 to 35°C above the deduster and its system (such as the kiln mill’s dew point temperature of 58°C and the operating temperature should be above 90°C). Guarantee the good effect of dust filter bag. The measures are as follows:
(1) Add raw material sheds. In the production of cement, the raw materials, fuels, and admixtures have different water contents. If they are placed in a fixed pile, preventing rain can greatly reduce the water content of the materials. This is an effective measure to reduce the moisture content of the materials. Cement plants in the south of China are relatively common. However, some of the material piles are too small and others are not. Therefore, the use of bag filter has caused certain difficulties.
(2) Reduce air leakage. The air leakage of the gap in the dust collector body should be controlled below 3.5%. In the precipitator system, the leakage of process equipment such as the closed unloader of the discharge port of the ball mill, the air leakage of the closed ash valve under the precipitator, and the flange joint of the pipeline are all ignored by the maintenance managers. , Increased unnecessary air leakage, deteriorated the bag filter operating conditions.
(3) Dust-laden gas should be evenly distributed in the dust collector to prevent eddy currents from occurring at corners to reduce the amount of gas passing through here and cause local low temperatures to cause condensation problems.
(4) Do a good job of insulation and rain protection in various places such as dust collectors and pipes. It has been proved that good insulation measures can make the difference between the inlet and outlet temperatures of the bag filter very small. This is an effective measure to prevent condensation.
(5) Take appropriate warming measures. If far-infrared electric heaters and electric heaters are installed in the dust collector, or if additional radiators are installed in the bag room, the temperature of the host gas may be appropriately increased.
(6) Strengthen the temperature monitoring of the dust removal system of the precipitator so as to grasp the conditions of use of the bag filter and prevent condensation.
