Flue gas heat exchanger tube bundle corrosion
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The picture below is of a fin-type heat exchange tube bundle in a GGH of a thermal power plant. The material is ND steel. It was scrapped after more than one year of use. Corrosion and scaling are severe, which is a bottleneck problem in the development of GGH. This problem has not been solved well, and many flue gas systems do not use such a heat exchange structure. What kind of anti-corrosion measures can best solve this problem so that this energy-saving product can contribute to the industry. This system is mainly caused by flue gas dew point corrosion. Since the sulfur element in the fuel generates SO2 and SO3 during combustion, when the outer surface temperature of the heat exchange surface is 50 to 120 degrees lower than the flue gas dew point temperature, sulfuric acid mist and dewdrops will form on the heat exchange surface, causing corrosion of the heat exchange surface.
Fluorine plastic can better solve this kind of problem, and there are many users.
The anti-corrosion coating of heat exchange tubes is required to be thin, generally less than 250 microns.； The thermal conductivity of the coating should be close to that of carbon steel metal ； The coating has good penetration ability and corrosion resistance. In addition, the coating is required to have good paintability. Although fluorine paint has good anti-corrosion properties, the disadvantage of fluorine paint is that it has no recoatability. In other words, after one layer of coating is applied and the second layer is applied, the adhesion between them is very poor and cannot meet the needs. In addition, fluorine paint has poor thermal conductivity. If it is necessary to paint on heat pipes, PTFE powder can only be used and then plasticized at high temperature. This coating is complicated and costly, and is not suitable for heat exchange pipes. In addition, there is a kind of fluorine resin that can be made into fluorine paint after etherification, which can be painted multiple times. However, such coatings are used outdoors for anti-corrosion and decoration, and are not good at high temperature resistance.
Did the fins fall off a lot? It’s not clear from the picture. Salt formation is quite severe. We also encounter this situation here. We use high-pressure water to clean off the salt and the fins are intact. In fact, this is a temporary solution rather than a permanent solution. The fundamental thing is to control the sulfur content of the fuel gas, the low-nitrogen combustion of the furnace, and the dew point of the flue gas.
Such finned heat pipes have no use value.
Petrochemical air preheater fins can be treated in this way. However, the GGH heat pipes in the thermal power plant were severely corroded, and the ND steel fins were basically scrapped in less than a year.