The chemical fiber industry is one of the important basic material industries. However, during solvent spinning, melt spinning, and finishing processes, large amounts of organic solvents and auxiliaries volatilize to form volatile organic compound (VOC) emissions. These exhaust gases often contain sulfur-containing organic compounds such as mercaptans, sulfides, and sulfoxides. If discharged directly, they cause serious atmospheric pollution. Regenerative Thermal Oxidizers (RTOs) are widely used as end-of-pipe VOC treatment equipment due to their high thermal efficiency and purification rate. However, after high-temperature oxidation in RTOs, sulfur-containing organics are almost entirely converted to sulfur dioxide (SO₂). If discharged without post-treatment, this not only leads to equipment corrosion but also becomes a precursor to acid rain, causing secondary damage to the ecological environment. Therefore, constructing a complete treatment chain of "RTO high-temperature oxidation + quench cooling + alkaline scrubbing absorption" has become the key technical pathway for meeting emission standards for sulfur-containing VOCs in the chemical fiber industry.

1. VOC Emission Characteristics and Sources of Sulfur-Containing Organics in the Chemical Fiber Industry

1.1 VOC Emission Sources in Chemical Fiber Production

Chemical fiber (synthetic fiber) production mainly includes three processes: polymerization, spinning, and post-treatment. During solvent spinning, large amounts of sulfur-containing or nitrogen-containing organic solvents such as dimethylformamide (DMF), dimethylacetamide (DMAC), and dimethyl sulfoxide (DMSO) are used; during melt spinning and finishing stages, the use of spin finishes, antistatic agents, and dyeing auxiliaries also releases VOCs such as formaldehyde and acrylates from spin finishes and finishing agents; in acrylic fiber and PAN-based carbon fiber production, residual acrylonitrile monomer is also emitted during polymerization and solution spinning. In addition, sulfur-containing solvents such as DMSO can thermally degrade during solvent recovery, releasing low-molecular-weight sulfur compounds such as methanethiol, dimethyl sulfide, and dimethyl disulfide.

1.2 Types and Characteristics of Typical Sulfur-Containing Organics

Sulfur-containing organics in chemical fiber industry exhaust gases can be classified into two categories by structure: first, low-valence sulfur-containing organics (sulfur at -2 valence), such as mercaptans (R-SH), thiophenols (Ar-SH), and sulfides (R-S-R); second, high-valence sulfur-containing organics (sulfur at +2 to +6), such as sulfoxides, sulfones, sulfinic acids, and sulfonic acid derivatives. Among them, dimethyl sulfoxide (DMSO) is a typical solvent in polyacrylonitrile carbon fiber production, while methyl sulfide, methyl mercaptan, and dimethyl disulfide are representative components of sulfide compounds. These organics generally have foul odors, strong volatility, and high toxicity, and the C-S bonds in their molecules have low bond energy, making them prone to cleavage under high-temperature conditions.

2. RTO Oxidation Process and Mechanism of Sulfur Oxide Formation

Fig. 2-1 Process Diagram for RTO Treatment of Sulfur-Containing VOCs

2.1 RTO Working Principle

The core mechanism of the regenerative thermal oxidizer (RTO) lies in using high-temperature oxidation to decompose organic exhaust gases and achieving efficient heat recovery through ceramic regenerative media. As shown in Fig. 2-1, organic exhaust gases collected from the front end are sent to the RTO by the main fan, first flowing through the ceramic regenerative bed where they are preheated to above 760°C before entering the top combustion chamber. In the combustion chamber, VOCs are mixed with combustion air and completely oxidized under the ignition of auxiliary fuels such as natural gas, decomposing into CO₂ and H₂O. The high-temperature flue gas produced by oxidation then flows reversely through another set of ceramic regenerative beds, transferring heat to the regenerative media and significantly reducing its own temperature before discharge. In the next cycle, the airflow direction switches, and new low-temperature exhaust gases flow reversely through the heated ceramic bed, absorbing heat before entering the combustion chamber. Through this periodic airflow switching and heat exchange, the heat recovery efficiency of RTOs can typically reach over 95%, significantly reducing auxiliary fuel consumption.

2.2 Oxidation Pathways of Sulfur-Containing Organics in RTO

In the oxygen-rich environment of the RTO combustion chamber (temperature typically maintained at 800–900°C), sulfur-containing organics undergo complete oxidation reactions. Taking common components as examples:

Mercaptans/Sulfides: The C-S and S-H bonds in methyl mercaptan (CH₃SH) and methyl sulfide (CH₃SCH₃) cleave at high temperatures, and sulfur atoms are oxidized to SO₂.

• 2 CH₃SH + 6O₂ → 2 CO₂ + 4 H₂O + 2 SO₂
• CH₃SCH₃ + 9/2 O₂ → 2 CO₂ + 3 H₂O + SO₂

Sulfoxides/Sulfones: the molecular skeleton fragments at high temperature, and the sulfur atom — already partially oxidized (0 or +2) — is likewise converted to SO₂ (+4).

• (CH₃)₂SO + 4O₂ → 2 CO₂ + 3 H₂O + SO₂

Hydrogen Sulfide (H₂S): If H₂S is present in the exhaust gas, it is directly oxidized to SO₂ under oxygen-rich conditions.

• 2 H₂S + 3 O₂ → 2 H₂O + 2 SO₂

In summary, regardless of the valence or structure of sulfur-containing organics, in the high-temperature, oxygen-rich environment of the RTO, their sulfur elements are almost entirely converted to SO₂. If the sulfur content in the exhaust gas is high, the SO₂ concentration at the RTO outlet may significantly exceed standards, and direct discharge will exacerbate the risk of regional acid rain formation and cause severe corrosion to downstream equipment.

3. Sulfur Oxide Treatment Solution: Quench + Alkaline Scrubbing Combined Process

3.1 Quench Tower (Rapid Cooling)

The temperature of RTO outlet flue gas is typically high. If it enters the alkaline scrubber directly, the high temperature will accelerate alkali volatilization, increase water consumption, and potentially damage the packing inside the tower. The quench tower adopts an empty-tower spray structure, where circulating cooling water directly contacts the high-temperature flue gas, rapidly reducing the flue gas temperature to the range suitable for alkaline scrubbing processes. The quenching process can also preliminarily wash out some soluble acidic gases, reducing the load on the subsequent alkaline scrubber.

3.2 Post-Alkaline Scrubber (SO₂ Neutralization and Absorption)

After quench cooling, the flue gas enters the post-alkaline scrubber (Alkaline Scrubber), where it fully contacts with sodium hydroxide (NaOH) solution. SO₂ is neutralized and absorbed to form sodium sulfite or sodium sulfate. The reaction equations are as follows:

• SO₂ + 2 NaOH → Na₂SO₃ + H₂O
• 2 SO₂ + 4 NaOH + O₂ → 2 Na₂SO₄ + 2 H₂O

The alkaline scrubber typically adopts a packed-tower structure with spray layers and demisting layers installed inside to ensure sufficient gas-liquid contact and prevent droplet entrainment. Through online monitoring of the circulating liquid pH, alkali is automatically added to ensure stable scrubbing efficiency. After alkaline scrubbing treatment, the flue gas is drawn by the induced draft fan to the exhaust stack for high-altitude discharge, where the SO₂ concentration can meet the limit requirement of SO₂ ≤ 50 mg/m³.

3.3 Supporting Circulation and Dosing System

The alkaline scrubbing system is equipped with a circulating liquid tank and an automatic dosing device. The circulating liquid is pumped back to the top of the tower for spraying, achieving closed-loop circulation. The system automatically adds alkali or discharges part of the waste liquid through online monitoring of the circulating liquid pH and salt concentration to maintain stable absorption efficiency. Waste alkaline liquid is discharged to the plant wastewater treatment station for unified treatment. In addition, the low-temperature piping between the RTO outlet and the quench tower requires anti-corrosion measures to resist corrosion from acidic condensate.

3.4 Safety and Emergency Measures

The RTO system is equipped with multiple safety interlocks. When the exhaust gas concentration exceeds 25% of the lower explosive limit (LEL), the system automatically opens the fresh air damper for dilution or opens the emergency vent valve; when the furnace temperature exceeds the limit, the high-temperature bypass valve automatically opens to release excess heat. The activated carbon adsorption unit serves as an emergency backup measure to ensure that exhaust gases are not discharged directly during RTO malfunction or maintenance.

4. Conclusion

The treatment of sulfur-containing VOCs in the chemical fiber industry is a systematic engineering task. As an efficient oxidation device, the RTO can thoroughly decompose sulfur-containing organics such as mercaptans, sulfides, and sulfoxides, but simultaneously converts the sulfur element into SO₂. Through the combined process of "quench tower rapid cooling + post-alkaline scrubber alkali absorption," SO₂ can be effectively intercepted, preventing acid rain precursors from being discharged into the atmosphere, while protecting downstream equipment and chimneys from corrosion.