Application of Stainless Steel in Flue Gas Desulfurization Equipment
When designing flue gas desulfurization equipment, the available materials are carbon steel with machine anti-corrosion protection, glass fiber reinforced plastic (GI), and stainless alloy. The principle of choosing these materials is technical adaptability, such as corrosion, erosion, heat resistance, workability, and price. In the flue gas desulfurization scrubber, stainless steel and nickel based alloys are used as structural materials.
Compared with GI or organic anticorrosive materials, they have much advantages:-no life limit;-no thermal diffusion;-erosion resistance;-no temperature limitation;-No danger of mechanical damage (simple maintenance).
Because there is no life limit, it is economical to use stainless steel alloy. That is, their service life is longer than the entire life of the flue gas desulfurization device. On the contrary, the organic anticorrosive materials used in flue gas desulfurization equipment must be updated within a certain period as the characteristics of the flue gas desulfurization process change. On the other hand, the selection of stainless steel alloy is a characteristic requirement of flue gas desulfurization parameters, and it’s material and manufacturing costs are also relatively low. The application of stainless steel alloy in flue gas desulfurization equipment should select the corresponding stainless alloy according to the different corrosive media in each area of
The stainless steel alloys used in flue gas desulfurization can be divided into four groups, standard austenitic stainless steel, fully austenitic stainless steel, super austenitic stainless steel, and nickel-based alloys containing chromium and molybdenum. Standard austenitic stainless steel is a metastable 18Cr10Ni steel with a small amount of added elements. Because their pitting resistance and crevice corrosion resistance are too low, they are usually not used in flue gas desulfurization. The pitting corrosion equivalent PRE of this type of steel is less than 20.
The flue gas enters the scrubber through the flue gas inlet, and the corrosive environment inside the inlet is formed by the condensation of sulfuric acid and chloride from the flue gas. The first washing is carried out in the first cycle, which is also called the quench cycle. Here, the flue gas is quenched to the process temperature of 45-70°C by the sprayed quench liquid. In this cycle, saturated flue gas is formed and the harmful substances are pre-separated.
At this time, hydrochloric acid is precipitated and the pH value decreases (pH=absorption tower suction collection The box and the content of chloride are high. After the flue gas passes through the annular channel between the washing wall and the collecting pan, it enters the second cycle, also called the absorption cycle. Here, most of the SO2 reacts with limestone.
Due to the high pH value and low chloride content in this area, the absorber solution is less corrosive. After the absorber solution is collected in the pan, it flows into the absorber supply tank. The flue gas is purified after passing through the saturated aqueous solution of the mist eliminator.
The flue gas inlet is the transition zone between the flue and the scrubber. It is subject to two kinds of corrosion: one is the corrosion of the sulfuric acid condensed in the flue gas; the other is the corrosion of the chloride in the solution.
The main idea in the design is to resist sulfuric acid corrosion at flue gas temperature. By studying the corrosion contour chart, it is concluded that the only material that can be used is Ni-Cr-Mo alloy. Make sure to use C-276 and 59 alloys. If the flue gas is cooled to between 90°C and the dew point temperature of the acid through a heat exchanger (the dew point temperature range is 120 to 135°C), even the nickel-based alloy may be corroded.
Through the research of the previous flue gas desulfurization device, the structural change reduces the risk of pitting at the bottom of the plate. The selection principle will also change accordingly. Since the super austenitic alloy is slightly more expensive than the 904L alloy but has high corrosion resistance, 904L alloy is replaced by super austenitic steel. In the quencher solution, only 625 alloy has the same corrosion resistance as super austenitic steel, but its price is almost twice that of super austenitic steel. Therefore, since 1996, 904L and 625 alloys have been eliminated from Noell-KRC’s selection criteria.
Summarizing the application of the above stainless steel in flue gas desulfurization, new material selection criteria can be determined. The corrosion conditions of the flue gas desulfurization solution are: the temperature is between 50°C (anthracite power plant) and 70°C (lignite power plant); the pH value in the quencher cycle is between 4 and 5; the pH value in the absorber cycle is 6.
As the investment cost of scrubbers decreases, more and more people use austenitic stainless steel and nickel based alloys as structural materials for flue gas desulfurization scrubbers. The new selection criteria is a big step forward in the direction of stainless alloys. In addition, the application of stainless steel or nickel-based alloy coatings and liners can also reduce the cost of materials.
Compared with organic materials, stainless alloys are rarely maintained. In fact, if the process parameters of flue gas desulfurization are fully considered when selecting materials, coupled with long-term operating experience on various types of equipment, the service life of these materials can be extended indefinitely. In other words, in power plant technology, industrial The life cycle of flue gas desulfurization equipment can be greatly improved.