Safety Issues and Preventive Measures of Catalytic Combustion Method in the Treatment of VOCs

VOCs (volatile organic compounds) refer to organic compounds with a boiling point between 50 and 260 ℃ at normal pressure and a saturated vapor pressure greater than 133.32 Pa at room temperature, or any volatile organic liquid and solid at normal temperature and pressure [1]. The category of VOCs is complex, mainly including alkenes, alkanes, alcohols, aromatic hydrocarbons, aromatic hydrocarbon derivatives, aldehydes, halogenated hydrocarbons, etc., and they are mainly produced in industries such as pharmaceuticals, footwear manufacturing, petroleum refining, and spray painting [2]. Some VOCs have a destructive effect on the ozone layer, such as hydrogen chloride fluorocarbons (HCFCs) and chlorofluorocarbons (CFCs); VOCs are also important precursors to haze formation and are an important source of toxic and harmful gases. The Ministry of Environmental Protection has listed them as the biggest air pollution culprit besides fine particulate matter [3]. With the acceleration of industrialization, the health and environmental problems caused by VOCs have attracted more and more attention, and the treatment of VOCs is also urgent.

I. The Important Position of Catalytic Combustion Method in VOCs Treatment

There are many measures for VOCs treatment, and the treatment methods include source reduction, intermediate control, and end-of-pipe treatment [4]. Currently, China mainly adopts end-of-pipe treatment. End-of-pipe treatment technologies are generally divided into destructive treatment and recovery treatment. Destructive treatment mainly includes catalytic combustion and incineration treatment methods. Recovery treatment includes absorption, condensation, adsorption, and membrane separation methods [5]. Due to the immature technology and high cost of recovery treatment, it has not been widely applied. Incineration is to directly introduce VOCs into the incinerator for complete combustion, generating carbon dioxide and water [6]. This method has a low cost, wide application range, and relatively mature technical route. Catalytic combustion is a chemical reaction involving gas-solid two-phase, and its essential mechanism is a deep oxidation reaction under the participation of active oxygen [7]. During the reaction, adding a catalyst can significantly reduce the activation energy required for the reaction, and VOCs accumulate on the catalyst surface, burning without flame at a lower combustion temperature, ultimately decomposing into carbon dioxide and water, while releasing a large amount of heat. The chemical reaction equation is as follows:

CnHm + (n + m/4) O2 → n CO2 + m/2 H2O + energy

Under the action of the catalyst, VOCs can be fully combusted at a lower temperature, generating carbon dioxide and water, with an efficiency of up to 90%. Catalytic combustion method has the advantages of low energy consumption, reliable and stable operation, and no secondary pollution. It has been widely applied in European and American countries. In China, catalytic combustion method is also the main means of treating VOCs, and its application proportion is approximately 55%. Currently, catalytic combustion method has become the mainstream for treating VOCs. 

II. Common Safety Issues and Prevention Measures of Catalytic Combustion Method

During the treatment of VOCs by catalytic combustion method, the following several safety hazards exist: the phenomenon of active carbon catching fire in the adsorption-decomposition-catalytic combustion method, the explosion problem of catalytic oxidation furnace, and the fire and explosion problem of the entire catalytic combustion treatment device. 

1. Active Carbon Catching Fire Phenomenon and Prevention Measures

During the early stage of VOCs enrichment process, due to the low ignition point of activated carbon and the excessively high desorption temperature, when desorbing saturated activated carbon, the desorption box would catch fire due to the excessively high temperature inside. 

To solve this problem, two approaches can be taken: one is to use activated carbon with a higher ignition point; the other is to strictly control the desorption temperature, making it much lower than the ignition point of activated carbon. Therefore, the following measures can be adopted: strictly control the desorption temperature, select high-quality desorption temperature sensors, and install two temperature sensors at appropriate positions in the activated carbon adsorption box; add a fire protection water pipe above the activated carbon adsorption box and connect smoke alarm and automatic sprinkler devices to prevent accidental fires. 

2. Explosion problem of catalytic oxidation furnace and preventive measures

When dealing with high-concentration VOCs, due to the presence of a large amount of oxygen in the furnace, when the concentration of the waste gas reaches the explosion limit of the components of the waste gas, there will be an explosion risk. Therefore, it is necessary to constantly monitor the VOCs concentration in the furnace. Install concentration dilution devices on the waste gas pipeline entering the catalytic oxidation furnace to dilute the high-concentration waste gas to below the explosion limit. At the same time, add pressure exhaust valves to the catalytic oxidation furnace to automatically open the valves to reduce pressure and exhaust gas when the pressure is too high, to prevent explosion caused by excessively high temperature and pressure in the furnace during the desorption catalytic combustion process (as the waste gas rapidly releases a large amount of heat energy during the thermal oxidation process, causing excessive pressure of hot air in the furnace). 

3. Fire and explosion of the entire catalytic combustion treatment device and preventive measures

Fire and explosion of the entire catalytic combustion treatment device mostly occur in systems with only one adsorption device. Due to the accumulation of a large amount of high-concentration organic waste gas particles on the pipe walls and inside the equipment, the gap between the pipeline air valve is too large, and the desorption catalytic combustion process is not stopped during production, and the workshop intake valve cannot be closed, and the entire pipeline is fully open. At this time, the desorption catalytic combustion is very likely to cause fire and explosion of the entire system under the high-temperature effect. For the above situation, the following measures should be taken: install high-quality air valves, frequently check for leaks, and in the system with only one adsorption device, during the desorption catalytic combustion process, the treatment should be stopped; set up safety distances and safety barriers and related colors, air flow direction, valve opening and closing directions, power identification, operation specifications, etc. as safety warning measures according to the requirements of safety production norms. 

3. Conclusion

With the continuous development and improvement of industrialization and modernization, the health and environmental problems caused by VOCs will attract more and more attention, and the treatment of VOCs is urgent. Catalytic combustion, due to its high catalytic efficiency and low energy consumption, has a broader application prospect and will gradually become one of the main methods for treating VOCs. However, when using catalytic combustion to treat VOCs, we still need to pay attention to possible safety issues at all times, prevent problems from escalating, and solve the danger at the source. 

The article is excerpted from the website of "Arctic Star VOCs Online". 

The pictures are from the Internet. 

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