Application of Super Austenitic Stainless Steel in Wet Flue Gas Desulfurization Process System

Super Austenitic Stainless Steel is currently the most suitable material for wet flue gas desulfurization process

This process is also the most commonly used with super austenitic stainless steel in the world today. Of the many desulfurization technologies, it accounts for 80% of the total installed capacity of desulfurization. It is always above a constant range.

Limestone-gypsum wet flue gas desulfurization is the most commonly used desulfurization process in wet flue gas desulfurization.

The corrosive environment inside the wet flue gas desulfurization system is harsh, and the internal equipment of the system is easily corroded. Doing good anticorrosion work is an important measure to ensure the safe and stable operation of the factory and control the power plant to prevent emissions.

Super austenitic stainless steel is rich in a large amount of Cr, Ni, Mo and appropriate amounts of alloying elements such as N and Cu, resistant to dilute sulfuric acid and pitting corrosion, crevice corrosion resistance is equivalent to nickel-based alloys, has excellent mechanical properties and corrosion resistance performance. Among them, Mo can improve the overall corrosion resistance of super austenitic stainless steel, especially the resistance to chloride ion pitting corrosion. 6% Mo super austenitic stainless steel has better corrosion resistance in acid solutions containing chloride ions and is more suitable for application In the flue gas desulfurization system.


Process flow of wet flue gas desulfurization process system

The main basic technological process of limestone-gypsum wet flue gas desulfurization is: after the boiler flue gas is dedusted by an electric precipitator and boosted by a booster fan, and then cooled by a flue gas heat exchanger, it enters the absorption tower. The limestone slurry is sprayed in the absorption tower to react with the flue gas to remove SO2, SO3, HCl and HF in the flue gas. The limestone slurry is transferred to the spray layer through the slurry circulation pump, and then atomized through the nozzle, so that the slurry and the flue gas can be completely contacted and fully reacted. The desulfurized flue gas heats up through the flue gas heat exchanger to increase its diffusivity, and finally is discharged into the chimney, which is discharged into the atmosphere.

In the absorption tower, the complex chemical reaction between the limestone slurry and the sulfur dioxide in the flue gas produces gypsum. This part of the gypsum slurry passes through the absorption tower discharge pump and enters the gypsum dehydration system. The dehydration system mainly includes gypsum cyclone, vacuum belt dehydrator and slurry distributor. The flue gas stream after the reaction is defogged by a demister to remove slurry mist droplets in the flue gas. At the same time, the process water of the power plant should be used to flush the demister from time to time. There are two main purposes for flushing the demister, one is to prevent the demister from clogging, and the other is to use the flushing water as supplementary water to stabilize the liquid level of the absorption tower.

At the outlet of the absorption tower, the flue gas is generally cooled to about 50°C and contains a large amount of saturated water vapor. After passing through the flue gas heat exchanger, the flue gas is heated to above 80 ℃, mainly to improve the lifting height and diffusion capacity of the flue gas, thereby eliminating the phenomenon of gypsum rain. Finally, clean flue gas that meets the flue gas emission standards of the power plant is discharged to the atmosphere through the chimney.


Application of Super Austenitic Stainless Steel in wet flue gas desulfurization process system

In some power plants, the coal-fired flue gas passes through the absorption tower and is discharged directly into the chimney without a flue gas heater (WGGH). This type of chimney becomes a wet chimney. Some power plants do not install a flue gas heater after the absorption tower but a wet electric dust collector (WESP). The flue gas passes through the wet electric dust collector and is discharged through the chimney. It can be seen that the corrosive environment of the wet chimney, wet electrostatic precipitator and flue gas heater is equivalent.

Wet chimneys are lined with anti-corrosion metal materials including super austenitic stainless steel, nickel-based alloys, titanium steel composite plates, etc.:

  • Nickel-based alloys have good corrosion resistance. Nicrofer 5923 h Mo-59 alloy, Nicrofer 3127h Mo-31 alloy, Cronifer 1925 h Mo-926 alloy, etc. are typical.
  • Titanium has good resistance, its welding performance is poor and its strength is low.
  • The price of super austenitic stainless steel is much lower than that of nickel-based alloys. It has excellent welding performance and can be directly welded with carbon steel. It has excellent mechanical properties and processing performance, and is a more suitable material

254SMo is a kind of commonly used in 6Mo steel. It has excellent pitting corrosion resistance. It is widely used in industrial processing equipment rich in halogen ions such as seawater. It can be used to replace expensive materials such as nickel-based alloys or titanium. There are also a few applications in flue gas desulfurization systems.

This article takes 254SMo as an example to study the corrosion resistance of 6Mo super austenitic stainless steel in wet chimney and flue gas heater.

Polarization curve test

Sample preparation

The test materials are 316L, 317L and 254SMo, and their main components are shown in Table 1.

stainless steel 316L, 317L and 254SMo components
Table 1

The stainless steel tubes of the three materials were mechanically cut out to make samples of 20mm×5mm×0.7mm size. The samples were polished to 01# sandpaper around the samples.

After ultrasonic cleaning for 15min and concentrated nitric acid passivation for 30min, the backside was welded with copper wires and then acetone cleans the grease attached to the electrode surface, and finally the non-working surface is encapsulated with epoxy resin, the working surface is 20mm×5mm.

The packaged electrodes are sanded with different roughnesses from coarse to fine, and then polished to 01# sandpaper for use.

Test method

The test uses a three-electrode system. The working electrode is a self-made planar electrode of the measured material, the reference electrode is a saturated calomel electrode (SCE), and the auxiliary electrode is a platinum electrode. The potential scanning speed is 1mV/s and the scanning frequency is 2Hz. The test medium is flue gas condensate (pH=1.84), and then NaCl is added to the flue gas condensate to prepare 1000 mg/L and 40000 mg/L two different Cl-concentration test medium solutions, and the other ion concentrations remain unchanged.

The test temperature is selected to be 50℃, and the temperature control accuracy is 0.1℃. The pitting potential of the material was tested by the dynamic potential scanning method, and the test was started from -400mV relative to the reference electrode until the anode current reached 0.2~1.0mA/cm2.

Each material is tested three times under the same working conditions to ensure that the test results are true and reliable.

Test results

Figure 1 shows the polarization curves of 316L stainless steel and 317L stainless steel in 1000mg/LCl-flue gas condensate and 254SMo super austenitic stainless steel in 40000mg/LCl-flue gas condensate.

Polarization curves of 316L 317L 254SMO in flue gas condensate with different Cl‑ concentrations
Table 1

It can be seen that the pitting potential of 316L is lower than that of 317L when the Cl-concentration is 1000mg/L in the flue gas condensate.

The pitting potential of stainless steel generally decreases with the increase of Cl-concentration in the solution medium. However, when the Cl-concentration in the flue gas condensate reaches 40000, the pitting potential of 254SMo is still very high and is in an over-passivated state. It can be seen that 254SMo has better pitting corrosion resistance in flue gas condensate with higher Cl-concentration.

The content of alloy elements such as Cr and Mo in 254SMo is relatively high. Among them, Cr is the main element to form the passivation film. Increasing the Cr content can make the passivation film more stable; and the higher Mo content can make the steel surface have A MoOCl2 protective film is formed in the medium where Cl- is present, thereby effectively preventing Cl- from penetrating the passivation film.

The magnitude of the dimensional blunt current in the polarization curve can characterize the uniform corrosion rate of stainless steel. According to Faraday's law, there is a strict quantitative relationship between the corrosion current index and the weight index. The greater the corrosion current, the faster the metal corrosion rate, which represents the resistance of the material. The worse the uniform corrosion performance. Explain the performance of uniform corrosion resistance 254SMo>316L>317L.

On-site testing

The corrosion rates of the three stainless steel materials are shown in Figure 2. As can be seen from the figure, the corrosion rate of 317L is slightly higher than that of 316L, respectively 0.081μm/a and 0.069μm/a, which is the same as the polarization curve test results Is consistent.

Corrosion rate of 316L, 317L, 254SMO in actual flue
Table 2

The corrosion rate of 254SMo is much smaller than the previous two, which is 0.013μm per year. Due to the existence of weighing errors, the corrosion of 254SMo is almost negligible. It can be seen that 254SMo has better resistance to uniform corrosion under actual operating conditions.


Summary

  • 1) Super austenitic stainless steel has good resistance to Cl-pitting corrosion in acidic solution, and its price is lower than that of nickel-based alloy, which is more suitable for application in wet desulfurization flue gas system;
  • 2) Gradient curve test and on-site test show that 254SMo has excellent corrosion resistance in flue gas condensate and can meet the working conditions of equipment such as wet chimney and flue gas heater;
  • 3) Super austenitic stainless steel 904L and AL-6XN can be well used in desulfurization absorption towers, and it is a good material selection direction for absorption towers to replace non-metallic materials with alloy materials.

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