Combustion method
Direct combustion method: The direct combustion of VOCs as fuel is called direct combustion method. Direct combustion method requires a relatively high temperature, generally to reach more than 1100 ℃. And there are certain restrictions on the oxygen concentration, low oxygen concentration will lead to incomplete combustion of VOCs, easy to cause secondary pollution; oxygen concentration is too high indirectly lead to a reduction in the concentration of combustibles can not reach the threshold of ignition concentration.
Thermal combustion method is generally used when the concentration of VOCs is low. The difference with the direct combustion method is that the need for organic waste gas preheating treatment, combustion temperature is greatly reduced, generally in 350 ~ 600 ℃, is a flameless combustion, reducing energy consumption, increasing safety.
The equipment commonly used in industry can be divided into thermal oxidizer without heat recovery, thermal oxidizer with inter-wall heat exchanger, and regenerative thermal oxidizer (RTO).
Catalytic combustion method: Catalytic combustion method refers to the use of catalysts to reduce the activation energy required for the oxidation of volatile organic compounds, improve the reaction rate, so that the oxidation reaction at a lower temperature (200 ~ 400 ℃).
Photocatalytic degradation method
Photocatalytic degradation refers to the oxidation of target pollutants adsorbed on the surface of photocatalysts under light conditions, which will be oxidized and decomposed into CO2 and H2O to achieve the degradation of VOCs.
Photocatalysts undergo electron (e-) leap when exposed to light. The electron (e-) leaps from the low-energy valence band (VB) to the high-energy conduction band (CB), while the low-energy valence band (VB) forms electron holes (h+) due to the lack of electrons.
When O2 and H2O are adsorbed on the photocatalyst surface, electron holes will react with H2O on the photocatalyst surface to form hydroxyl radicals (-OH), and also combine with hydroxide ions (OH-) to form hydroxyl radicals (-OH).
Photogenerated electrons react with O2 to form superoxide anion radicals (-O-2), which combine with hydrogen ions (H+) to form superoxide radicals (HO2-), which then undergo a series of reactions to form O2, hydroxide ions (OH-) and hydroxyl radicals (-OH). VOCs will react with the radicals generated in the above reactions.
TiO2, Fe2O3, ZnO, CdS, WO3, SnO2 and ZrO2 are several common photocatalysts in the industrial field, among which TiO2 has the advantages of high activity, low price, stable reaction conditions and non-toxic and harmless, which makes it widely used, but it also has the disadvantages of low utilization of visible light.
Therefore, researchers often modify them, and the common modification methods include metal doping, non-metal doping, noble metal deposition, compound semiconductor, surface photosensitization, and TiO2 immobilization. Table 2 lists the effects of several modified TiO2-based catalysts on the treatment of VOCs.
Low-temperature plasma method
The low-temperature plasma method is a process that uses high-energy electrons or free radicals to react with organic waste gas to generate CO2 and H2O. The high-energy electrons collide inelastically with VOCs to break the molecules and decompose them; meanwhile, the high-energy electrons are excited to generate free radicals such as -OH and -O, and react with VOCs molecules, thus degrading and removing VOCs. This method has good treatment effect and is suitable for the treatment of low and medium concentration of exhaust gas, but the energy consumption is high and the degradation process is easy to cause secondary pollution.
Composite purification treatment technology
In recent years, a single organic waste gas treatment technology has failed to meet the requirements of the market, so a variety of treatment technologies combined to become a research hotspot.
At present, there are more composite processes such as condensation and adsorption integrated technology, photocatalytic-absorption technology, water spraying combined with activated carbon adsorption method, low-temperature plasma synergistic catalytic technology.