In wet flue gas desulfurization system, the selection of material of flue expansion joint directly determines the service life of equipment and the safe operation of the system. After desulfurization, the flue gas temperature drops to 45-55℃, which is completely saturated. The pH value of the condensate is as low as 2-3, and the concentration of Cl⁻¹ is as high as 20,000-60000ppm, which causes extremely serious corrosion to the expansion joint. Traditional materials often perforate and leak in one to two years in this environment, which becomes the hardest hit area of desulfurization system. In this paper, the performance characteristics and applicable scenarios of three kinds of mainstream desulfurization flue expansion joint materials will be systematically compared from the corrosion mechanism.

1. Corrosion mechanism and material challenges of desulfurization flue

The corrosive environment in the desulfurization flue is extremely harsh. After electrostatic dust removal, the high temperature flue gas (100~150℃) mainly contains SO₂, SO₃, HCl, HF, NOx, smoke and water vapor, etc. After spraying and washing by the desulfurization absorption tower system, the main components of the flue gas are saturated water vapor, residual SO₂, SO₃, HCl, HF, SO₃²⁻¹ carried by NOx, etc. It is generally believed that there are three main causes of corrosion in desulfurization flue: chemical corrosion caused by SO₂ and SO₃; Electrochemical corrosion caused by Cl⁻¹ and F⁻¹; Mechanical corrosion caused by fluid flushing。

It is particularly noteworthy that the exhaust temperature of some ultra-low emission units after desulfurization has been as low as 45℃. At this temperature, the corrosiveness of desulfurization wet flue gas to flue system is further strengthened, which puts forward higher requirements for the anti-corrosion performance of desulfurization flue expansion joint materials。

2. Materials 2: Non-metallic composite materials-an economical and flexible choice

1. Structural composition

The non-metallic expansion joint is composed of multi-layer composite materials, which is the most flexible choice of desulfurization flue expansion joint materials. Typical structures include, from the outside to the inside:

2. Principles of material selection

The selection of non-metallic materials in the expansion joint of desulfurization flue shall be determined according to the flue gas temperature and corrosivity：

3. Special requirements

According to the technical specification, the non-metallic expansion joint skin needs to be equipped with multi-layer structure from outside to inside: fluororubber composite material, PTFE membrane, PTFE cloth, alkali-free glass fiber cloth and other multi-layer composite, and built-in aluminum silicate insulation cotton for heat insulation。 Fluoropolymer fiber felt has extremely high resistance to high concentration acidic condensate in flue of desulfurization process, and its service life can be as long as 6 years under normal use conditions。

III. Material III: Upgrading selection of nickel-based alloy steel-metal expansion joint

1. Performance comparison of each material

The nickel-based metal expansion joints are mainly 316L, 2205 and 2507. The performance of different materials in desulfurization wet flue is significantly different:

Enlightenment: If metal expansion joint is selected for desulfurization wet flue, 316L is difficult to meet the long-term operation requirements, so it should be upgraded to 2205 duplex stainless steel or higher grade material.

2. Wear-resistant and corrosion-resistant patented technology

A patent technology for wear-resistant and corrosion-resistant metal expansion joint for desulfurization adopts a metal bellows and a flow guide pipe, fills an anti-corrosion sealing material between the flow guide pipe, improves the anti-corrosion ability and is beneficial to eliminate vibration generated during work, and improves the life of the rubber expansion joint by 5 times compared with that of the rubber expansion joint。

4. Comparison of selection of non-metal and metal expansion joints

Compared with metal expansion joints, the advantages of non-metal expansion joints are: under the same length condition, the compensation amount of products exceeds that of metal expansion joints; It is convenient to replace and overhaul. If the metal structure is not damaged, only the non-metal skin part can be replaced; Under normal load conditions, the product cost is less than that of metal expansion joints。

V. Material Selection Decision Guide

The material selection of desulfurization flue expansion joint should be determined according to the specific working conditions:

Decision-making process

1. Define the working condition parameters: flue gas temperature, Cl⁻¹ concentration, pH value, dust content, pressure range
2. Primary by Temperature Type:
   • t ≤250℃, highly corrosive → fluororubber non-metallic expansion joint
   • t ≤400℃, low corrosiveness → silicone rubber non-metallic expansion joint
   • t> 400℃ or high pressure → metal expansion joint (stainless steel/2205)
3. Determine the material by corrosivity:
   • High Cl⁻wet flue (after desulfurization) → pure titanium TA2 or fluororubber non-metal
   • Sulfur-containing raw flue → 2205 duplex steel or fluororubber non-metal
   • General corrosive environment → 316L metal or silicone rubber non-metal
4. Comprehensive economic assessment:
   • Non-metallic expansion joints have low initial cost, but short replacement cycle (3-5 years)
   • The initial cost of titanium expansion joint is the same as that of non-metal, the service life can reach more than 10 years, and the cost advantage of "zero maintenance" is obvious

Summary of Selection Recommendations

sum up

The selection of the material of the expansion joint of the desulfurization flue is the key to ensure the long-term operation of the desulfurization system. Each of the three types of mainstream materials has its own applicable scenarios:

• Non-metallic composite material: It is characterized by multi-level composite of fluororubber/silicone rubber, which can absorb multi-directional displacement at the same time and has no reverse thrust. It is suitable for economic demand and has an estimated service life of 3-6 years
• Nickel-based alloy: 2205 duplex stainless steel has better pitting corrosion resistance than 316L, and has good performance; The average life of 316L in wet flue is not more than two years, so choose carefully

Core selection principle: the material grade is determined according to the flue gas temperature and corrosivity, and pure titanium or fluororubber is preferred for high Cl⁻¹ wet flue; Horizontally mounted expansion joints must be provided with drainage holes; The design and manufacturing standards shall follow JB/T 12235-2015. A desulfurization flue expansion joint with correct material selection and reasonable design can solve the vicious circle of "leakage after just changing" from the root cause.