In the selective catalytic reduction (SCR) denitrification system, the expansion joint acts as a flexible compensation element connecting the flue and the reactor, and is exposed to the flue gas environment with complex composition for a long time. Many engineers ask a core question when selecting a model: Which medium is suitable for SCR expansion joints? The answer is not single, because flue gas temperature, chemical composition, dust content and phase changes directly affect the tolerance limit of expansion joint materials. From media classification, corrosion mechanism to selection comparison table, this paper systematically analyzes the compatibility of SCR expansion joints with various media, helping you to accurately select and avoid early failure.

1. Which medium is suitable for SCR expansion joint? Classification and analysis by working condition

In order to answer which medium is suitable for SCR expansion joint, it is first necessary to clarify the media characteristics of different sections of SCR system. Generally speaking, the SCR expansion joint should cope with the combined erosion of gas phase medium, solid phase medium and possibly liquid phase medium.

1. Gas phase medium: high temperature corrosive flue gas

The core medium processed by the SCR system is the flue gas produced by coal/gas/biomass boilers, and its main gas phase components include:

• Nitrogen oxide (NOx): denitration reaction object, itself is weak to metal oxidation, but when the reducing agent (NH₃) is excessive, it will produce escape ammonia.
• Sulfur dioxide/sulfur trioxide (SO₂/SO₃): The SO₃ concentration is typically 10 – 50 ppm and reacts with escape ammonia to form ammonium bisulfate (NH₄ HSO₄). SO₃ dissolves in condensed water to form sulfuric acid, and the pH can be as low as 1.0.
• Hydrogen chloride/chlorine (HCl/Cl₂): When burning high chlorine coal or refuse-derived fuel, the concentration of Cl₂ can reach over 100ppm, which is the main culprit of inducing stress corrosion cracking of stainless steel.
• Water vapor (H₂O): The moisture content of flue gas is 8%-15%, and it will condense into an acidic liquid film in the low temperature zone (

Media Compatibility Conclusion: Conventional 304 stainless steel is only suitable for clean, dry, chlorine-free flue gas; For media containing SO₃ + water vapor, it must be upgraded to 316L or super austenitic stainless steel containing ≥6% molybdenum (e.g. 254SMO); When the chloride ion concentration is> 50 ppm, Inconel 625 or C-276 Hastelloy is required.

2. Solid phase medium: fly ash and catalyst dust

The solid particulate matter carried by the flue gas has the dual effects of erosion wear and ash accumulation and blockage on the expansion joint:

• Fly ash: The concentration of fly ash of coal-fired units is 15-30g/Nm³, and the particle hardness of Mohs is 6-7 (the main components are SiO₂ and Al₂O₃). Under high-speed impact, the trough thinning rate of metal bellows can reach 0.3mm/year.
• Denitrification catalyst wear dust: honeycomb or plate catalyst will produce trace V₂O₅/TiO₂ powder during operation, although the hardness is slightly lower than fly ash, but it is weakly acidic.

Media Compatibility Conclusion: For high-dust media, the expansion joint must have a built-in wear-resistant guide tube (thickness ≥4mm, material 16Mn or Hardox400). The outer layer of fluororubber of non-metallic expansion joint can withstand particle impact, but the inner layer of glass fiber blanket is easy to be pierced by dust, so stainless steel wire mesh protective layer needs to be added.

3. Liquid medium: condensate and rinse water

During the start-off furnace or low load operation phase, the smoke temperature may drop below the acid dew point (approximately 110-150℃), and the following liquid phase media appear:

• Sulfuric acid/sulfurous acid condensate: pH 1.5-3.5, produces uniform corrosion and pitting on carbon steel and low grade stainless steel.
• Ammonium bisulfate (NH₄ HSO₄) solution: viscous liquid, pH approximately 4.0-5.5, but extremely permeable, can penetrate into bellows trough microcracks and cause stress corrosion cracking.
• Denitrification system flushing water: On-line flushing may contain chloride ions (from process water). If the water quality is not properly controlled, the concentration of Cl⁻¹ can reach more than 200ppm.

Medium compatibility conclusion: For the outlet flue in the low temperature zone where condensate may appear, the expansion joint material should be resistant to acid corrosion. The full PTFE diaphragm expansion joint is almost completely inert to the above liquid media, but has limited temperature resistance (≤230℃). The metal expansion joint needs to be equipped with electric heat tracing to maintain the wall temperature> 160℃ to avoid liquid phase precipitation.

2. Comparison table of material selection of SCR expansion joint under different media conditions

To more intuitively answer which media is suitable for SCR expansion joints, the following table summarizes common media combinations and recommended material schemes:

3. Corresponding relationship between medium corrosivity and expansion joint failure

To understand which media is suitable for SCR expansion joint, it is necessary to deduce the media hazard from the failure. According to the field failure analysis, the typical failure modes induced by different media are as follows:

1. Chloride medium → stress corrosion cracking (SCC)

When the concentration of HCl in flue gas is> 20ppm and there is tensile stress (residual stress of bellows forming), SCC is most likely to occur in 304/316L stainless steel in the range of 80-200℃. The cracks are dendritic and propagate intergranular, which often leads to sudden fracture of the whole expansion joint without warning.

Solution: Select austenitic stainless steel (e.g. 254SMO) or nickel-based alloy (Inconel 625) with molybdenum content> 6%, and perform solution treatment after manufacture to relieve residual stress.

2. SO₃ + water vapor → dew point corrosion

When the metal wall temperature is lower than the acid dew point, SO₃ forms sulfuric acid with water, resulting in the uniform thinning of the bellows trough and the appearance of puncture-like corrosion pits on the surface. The wall thickness of 316L bellows was reduced from 1.5mm to 0.7mm after the operation of the flue expansion joint at the outlet of a power plant for one year.

Solution path: maintain wall temperature> acid dew point (by heat preservation + heat tracing); Or switch to non-metallic expansion joints, whose PTFE/fluororubber layer is completely resistant to sulfuric acid corrosion.

3. Fly ash media → erosion wear

The high dust medium impacts the outside of the bellows valley at a certain angle, and the wear morphology is fish-scale. When the deflector is improperly designed or falls off, the wear rate can be increased by 5-10 times.

Solution: A guide tube must be set, and the gap between the guide tube and the bellows should be ≤1/3 of the wave height of the bellows. For extremely high dust conditions (> 50g/Nm³), a ceramic-lined guide can be selected.

4. NH₄ HSO₄ Medium → Adhesion and Clogging

After the liquid or semi-solid ammonium bisulfate adheres to the trough, it continuously traps fly ash to form hard lumps, resulting in the expansion joint losing its compensation function. This failure is directly related to the escape ammonia concentration in the medium (typically starting to be significant at> 3 ppm).

Solution: strictly control the uniformity of ammonia injection, so that the NOx distribution deviation at the outlet is

IV. Selection decision-making process: How to determine which medium is suitable for SCR expansion joint?

In the actual project, the following four steps can be used to determine which medium and the corresponding selection of SCR expansion joint are suitable:

Step 1: Get the full media analysis report
Obtained from boiler design parameters or field tests: flue gas temperature range, SO₂/SO₃ concentration, Cl⁻¹ content, dust content, moisture, escape ammonia concentration. Focus on the minimum operating temperature (to determine whether condensation occurs) and the maximum instantaneous temperature (to determine the temperature resistance limit of the material).

Step 2: Judge phase state and corrosion risk

• If the wall temperature of all working conditions is> acid dew point +20℃: metal expansion joint is preferred, which has high cost performance.
• If there is a risk of condensation or Cl⁻¹> 30ppm: choose a non-metallic or composite expansion joint.
• If high dust (> 20g/Nm³) and acid gas are present at the same time: metal expansion joints with wear-resistant guide tubes must be used, and the material must be at least 316L.

Step 3: Check the displacement requirements
On the premise of confirming that the medium is compatible, calculate the thermal expansion of the flue (axial, transverse and angular) to ensure that the allowable displacement of the selected expansion joint is greater than 1.5 times the actual displacement.

V. CONCLUSIONS AND CALL TO ACTION

Which medium is suitable for SCR expansion joint is not a fixed answer, but needs to be selected according to flue gas temperature, chemical composition (especially Cl⁻¹ and SO₃ concentration), dust content and phase change. High clean gas media can choose economical 316L or non-metallic expansion joints; Highly corrosive media containing sulfur and chlorine must be upgraded to a nickel-based alloy or PTFE fully clad structure; While Gao Chen The medium relies on mechanical protection measures such as flow guide tubes.

Act Now: If you are selecting or replacing the expansion joint for the SCR denitrification system, first complete the full composition test of the flue gas medium. Send your flue gas analysis report (temperature, SO₃, Cl⁻¹, dust content, escaped ammonia) to our technical email or submit it online through official website. The engineer will issue the SCR Expansion Joint Media Compatibility Evaluation and Selection Proposal for you within 24 hours, clearly recommending the material, structure and expected service life. Don't cause unplanned shutdowns because of misjudged media-the first step to proper selection starts with recognizing your media. Contact us today for professional model selection support.