Application of stainless steel in desulfurization equipment

When designing flue gas desulfurization equipment, the materials that can be used include: carbon steel with organic anti-corrosion protection, glass fiber reinforced plastic (GI) and stainless alloy. The principles for selecting these materials are technical suitability, such as corrosion, erosion, heat resistance, workability and price. In flue gas desulfurization scrubbers, stainless steel and nickel-based alloys are used as structural materials. Compared with GI or organic anti-corrosion materials, they have many advantages:
1. No life limit;
2. No heat diffusion;
3. Anti-scouring;
4. No temperature limit;

5. No risk of mechanical damage (simple maintenance).

Using stainless steel alloys is economical as there is no service life limit. In other words, the service life is longer than the entire life of the flue gas desulfurization device. On the contrary, the organic anti-corrosion 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 alloys is a characteristic requirement of flue gas desulfurization parameters, and its material and manufacturing costs are also lower. The application of stainless steel alloys in flue gas desulfurization equipment requires the selection of corresponding stainless alloys according to the different corrosive media in each area of the absorber.
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 metastable 18Cr10Ni steel with a small amount of added elements. Because their resistance to pitting corrosion and crevice corrosion is too low, they are usually not used in flue gas desulfurization. The pitting corrosion resistance equivalent PRE of this type of steel is less than 20.
The flue gas enters the scrubber through the flue gas inlet. The corrosive environment inside the inlet is formed by the condensed sulfuric acid and chloride produced by the flue gas. The first wash is performed 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 quenching liquid. In this cycle, saturated flue gas is formed and harmful substances are pre-separated. At this time, hydrochloric acid precipitates and the pH value decreases (pH = absorption tower absorption storage box, high chloride content). After the flue gas passes through the annular channel between the scrubber wall and the collection tray, it enters the second cycle, also called the absorption cycle. Here, most of the SO2 reacts with the limestone. In this area the absorber solution is less corrosive due to the higher pH and low chloride content. After the absorber solution is collected in the pan, it flows into the absorbent 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 sulfuric acid condensed in the flue gas; the other is the corrosion of chloride in the solution. The main idea in the design is resistance to sulfuric acid corrosion at flue gas temperatures. By studying the corrosion contour chart, it is concluded that the only material that can be used is Ni-Cr-Mo alloy. Make sure you use C-276 and 59 alloys. If the flue gas is cooled through a heat exchanger to a temperature between 90°C and the dew point temperature of the acid (the dew point temperature range is 120 to 135°C), even nickel-based alloys may be corroded.
Through research on previous flue gas desulfurization devices, structural changes have reduced the risk of pitting corrosion at the bottom of the pan. The principles of material selection will also change accordingly. Since super austenitic is slightly more expensive than 904L alloy but has high corrosion resistance, 904L alloy is replaced by super austenitic steel. In the quench solution, only alloy 625 has the same corrosion resistance as super austenitic steel, but its price is almost twice that of super austenitic steel. Therefore, since 1996, the two materials 904L and 625 alloy have been canceled from the Noell-KRC material selection criteria.
Summarizing the above applications of stainless steel in flue gas desulfurization, new material selection criteria can be determined. The corrosion conditions of flue gas desulfurization solution are:
1. The temperature is between 50℃ (anthracite power plant) and 70℃ (lignite power plant);
2. The pH value in the quench cooler cycle is between 4 and 5;
3. The pH value in the absorber cycle is 6.
As the investment cost of scrubbers decreases, more and more people are turning to austenitic stainless steel and nickel-based alloys as structural materials for flue gas desulfurization scrubbers. The new material selection criteria are a big step forward in the direction of stainless alloys. In addition, the application of stainless steel or nickel-based alloy coatings and linings can also reduce material costs. Stainless alloys require little maintenance compared to organic materials. In fact, if the process parameters of flue gas desulfurization are fully taken into account 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.