Safety issues of activated carbon in VOCs waste gas treatment

Activated carbon adsorption is the most common and universal technology in the VOCs treatment industry. Most combined waste gas treatment processes are equipped with activated carbon adsorption systems. In addition, carbon boxes are widely applied as terminal emergency treatment equipment for exhaust systems. Although activated carbon adsorption technology is extensively used in the VOCs industry, most users cannot fully master its potential safety risks. This chapter systematically sorts out the safety problems of activated carbon waste gas treatment.

Activated carbon adsorption devices are widely used in the pharmaceutical and chemical industry. Due to the high investment cost of RTO and other thermal oxidation equipment, many enterprises with limited funds choose activated carbon adsorption for cost savings. However, numerous users ignore potential safety hazards while pursuing economic benefits.

Activated carbon is specially processed carbon material. Organic raw materials such as fruit shells, coal and wood are carbonized under an oxygen-free environment to remove non-carbon components. Then the materials react with specific gas for surface etching to form abundant microporous structures. The microscopic etching forms countless tiny pores on the carbon surface.

The micropore diameter of activated carbon is generally between 2 nm and 50 nm. Even a small amount of activated carbon possesses an extremely large specific surface area of 500~1500 m² per gram. Particle activated carbon is commonly used for organic waste gas purification. Waste gas flows through the carbon bed to realize physical adsorption and concentration reduction. Adsorption is an exothermic process. Pollutant molecules adhere to the solid surface with reduced free energy, releasing adsorption heat.

Apart from physical adsorption, slow oxidation occurs between oxygen, activated carbon and adsorbed organic substances. The large specific surface area further accelerates oxidation. Moreover, incompatible chemical components in complex waste gas will trigger exothermic chemical reactions under the catalytic effect of activated carbon. All exothermic reactions lead to heat accumulation and potential safety hazards.

According to previous fire and explosion cases of activated carbon tanks, most accidents have the following characteristics:

1. Most accidents occur in high-temperature summer seasons with direct sunlight and an average daily temperature above 32℃;

2. The activated carbon has not been replaced for a long time with excessive ash and impurities;

3. The waste gas composition is complex, containing acetone, ethyl acetate, alcohols, methylene chloride, organic amines, acidic gas and alkaline gas;

4. The VOCs concentration fluctuates sharply. Accidents usually occur under low-load working conditions with excessive air mixing into the waste gas pipeline.

Under high ambient temperature, complex waste gas releases physical and chemical reaction heat during adsorption. Long-term service leads to excessive ash accumulation and poor heat dissipation inside the carbon bed, resulting in local hot spots. The continuous temperature rise reaches the autoignition point of activated carbon or the flash point of mixed organic gas. Excessive air enters the pipeline and forms explosive mixed gas with combustible VOCs. Static electricity is also regarded as a potential ignition source.

Adsorption and organic oxidation continuously release heat. Complex chemical components may cause incompatible exothermic reactions on the carbon surface, especially under high concentration working conditions. The temperature variation depends on heat balance. Convection heat dissipation is the main cooling method for carbon beds. Insufficient air volume leads to heat accumulation, temperature surge, organic gas ignition and carbon autoignition.

1. For reactors, storage tanks and filters that generate high-concentration organic waste gas, nitrogen sealing and positive-pressure conveying systems are recommended to prevent explosive gas mixture formation.

2. The condensation device shall meet production load requirements to reduce organic concentration. Incompatible waste gas must be pretreated separately before entering carbon adsorption tanks.

3. Select low-ash and high-ignition-temperature activated carbon, and implement regular carbon replacement schedules.

4. Install auxiliary cooling devices for adsorption equipment if conditions permit.

5. Regularly inspect pipelines and equipment for air leakage to ensure airtightness.

6. Install detonation-resistant flame arresters on waste gas pipelines. Bursting discs shall be installed on segmented pipelines and buffer tanks with fixed protective structures.

7. Equip the adsorption device with manual or automatic water spray or steam fire-extinguishing systems to suppress initial fire hazards.

8. Arrange temperature sensors inside the carbon bed for real-time temperature monitoring and early risk warning.

9. Strengthen staff safety training to improve emergency disposal capability and prevent fire spread.