The work of waste gas treatment is tedious and the process is variable, but in terms of process principles, it can be summarized as activated carbon adsorption method, low-temperature plasma method, photocatalytic oxidation method, biological treatment method, and combustion method, which are the five methods of gas removal!

1、 Activated carbon adsorption method

1. Principle of pollution treatment

The activated carbon adsorption method is currently the most widely used VOCs treatment method due to its low initial investment. It adsorbs VOCs through the natural adsorption capacity of activated carbon. When the adsorption is saturated, the activated carbon is desorbed and regenerated or handed over to a professional hazardous waste company for treatment.

2. Actual application situation

Environmental protection companies that use activated carbon adsorption method for VOCs treatment generally mention that the pollution removal efficiency of such equipment reaches over 90% when it comes to the pollution removal parameters of their equipment. However, in actual pollution removal applications, achieving a pollution removal efficiency of over 90% is only a theoretical value. Moreover, in different working environments, its pollution removal efficiency is much lower than this theoretical value. The main reasons include temperature, working environment humidity, water mist, acidity, dust, and the interaction between adsorbed gases. For example, the humidity and temperature in southern China are relatively high throughout the year, and the actual adsorption capacity of activated carbon is less than 50% of that in laboratories.

3. Main issues

The actual operation and maintenance costs of using activated carbon adsorption method to treat VOCs that meet emission standards are very high. At the same time, natural adsorption and desorption management are difficult, and the applicability is affected by various factors. It is not suitable for the treatment of waste gases containing dust, water vapor, emulsions, etc., and it is difficult to stabilize environmental protection standards. Moreover, the treatment of a large amount of saturated activated carbon is even more costly. This method only involves the adsorption and transfer of pollutants. Without strict monitoring and tracking of the transfer process of saturated activated carbon, it is highly likely to cause secondary pollution. However, due to the low initial investment, enterprises naturally choose more. Although regulation is difficult now (there is no activated carbon in the carbon box, the activated carbon facilities are too rudimentary and almost do not replace the carbon, the selection of activated carbon does not match the actual design, and the usage is too low), the environmental protection department will eventually take action, and there is a huge environmental risk. And it is easy to fabricate to cope with environmental management. For example, there is no activated carbon inside the charcoal box, the activated carbon facilities are too rudimentary and almost do not replace the carbon, the actual engineering of activated carbon does not match the design, and the usage is too low.

2、 Low temperature plasma technology

1. Principle of pollution treatment

During the dielectric barrier discharge process in low-temperature plasma exhaust gas treatment equipment, highly chemically active particles such as electrons, ions, ozone, and excited molecules are generated inside the plasma. In theory, organic waste gas will react with these high-energy active groups, some of which will be cracked and ultimately converted into substances such as carbon dioxide and water, thereby achieving the goal of purifying waste gas.

2. Actual application situation

Domestic pollution control equipment using low-temperature plasma technology, as well as environmental protection companies manufacturing such equipment, generally have a pollution removal efficiency of over 80% according to their pollution removal parameters. A large number of low-energy plasma equipment that can be used for VOCs treatment can only be used to treat oil fume pollution. In the actual process of treating industrial VOCs, this low-temperature plasma technology equipment is basically ineffective in degrading organic waste gas and will generate pollution by-products. Its degradation efficiency is low, and the flammability of VOCs makes its safety a concern.

3. Main issues

The currently widely used low-power low-temperature plasma is used in the kitchen industry for oil fume treatment in the past. It is not suitable for VOCs treatment and generates by-products and a large amount of ozone, which can cause arc ignition of VOCs and other problems.

Because plasma technology has a low efficiency in treating organic waste gases, including aromatic compounds, in a short period of time, mainly generating intermediate products. If high-power plasma is used in stable organic waste gas, the treatment effect also needs to be achieved within a certain period of time. For the VOCs waste gas continuously and rapidly discharged from industrial sources, its treatment efficiency is very low and it will produce many intermediate by-products, resulting in more complex VOCs composition (these by-products may be more harmful), and at the same time, a large amount of useless ozone will be generated during equipment operation. And the vast majority of organic waste gases are flammable and explosive compounds. The arc pulling during plasma operation is highly prone to detonating VOCs, and the Tianjin explosion has raised concerns about its safety in society. Therefore, this technology is increasingly being banned in various regions.

3、 Photocatalytic oxidation

1. Principle of pollution treatment

The technology of photocatalytic waste gas treatment equipment is a treatment method that uses a special ultraviolet band to catalyze the generation of ozone, hydroxyl radicals, and negative oxygen ions from oxygen under the action of a catalyst, and then oxidizes and reduces VOCs molecules.

2. Actual application situation

Most of the UV photocatalytic treatment equipment used for VOCs treatment refers to the principle of deodorization and sterilization technology in the past. Usually, dual wavelength ultraviolet tubes are used to convert energy mainly into ozone, and ordinary titanium dioxide materials are used as catalysts. Although the pollution removal efficiency is claimed to be over 80%. The efficiency of the UV photocatalytic treatment equipment currently used for VOCs is relatively low. Without the control of computational technology, a large amount of ozone and intermediate by-products are generated.

3. Main issues

In the application of UV photocatalytic oxidation technology, factors such as the wavelength of UV tubes, photocatalytic materials, reaction time, relative humidity, and dust particles are all bottleneck factors for the success or failure of VOCs treatment. At present, it is widely believed that photocatalytic oxidation can completely degrade VOCs to produce non-toxic and harmless CO2 and H2O. However, due to the short reaction time in use, volatile organic compounds in photocatalytic oxidation reactions will generate more toxic intermediate products such as ketones and aldehydes, as well as a large amount of ozone.

In recent years, one of the factors causing excessive ozone in industrial towns is the misuse of plasma and UV photocatalytic oxidation equipment that produces ozone. Due to the fact that both types of equipment attempt to decompose industrial waste gas through chemical reactions after converting oxygen in the air into ozone, the efficiency of converting the generated ozone into free radicals and negative oxygen ions is extremely low due to reaction conditions. At the same time, due to the short reaction time, most of the ozone generated by the equipment cannot be directly discharged for VOCs treatment.

4、 Biological treatment method

1. Principle of pollution treatment

The use of microorganisms for digestion and metabolism of pollutants in exhaust gas is essentially a biochemical decomposition process. It absorbs organic waste gas through active microorganisms attached to the medium, converting pollutants into harmless water, carbon dioxide, and other inorganic salts.

2. Actual application situation

Using pollutants as a food source for microorganisms, biological treatment methods include various organic compounds composed of carbon, hydrogen, and oxygen, simple organic sulfides, organic nitrides, hydrogen sulfide, ammonia, and other inorganic compounds. The requirements for small gas volume, low concentration, continuous exhaust, and large waste gas treatment containers, although the treatment process is relatively environmentally friendly, the complex operation and maintenance, and the cumbersome biological supplementation make the biological treatment method meaningless. Due to its difficult supervision, it is still ubiquitous.

3. Main issues

Poor applicability: only applicable to specific pollutants, and biological bacteria are prone to death. When dealing with soluble and degradable pollutants, there will be certain limitations; Biological metabolism is prone to blockage; The specific surface area and porosity of the fillers used in biological methods directly affect the biomass of the reactor, as well as the pressure drop of the entire packed bed and whether the packed bed is prone to blockage; Difficulty in achieving automatic control; Difficulty in improving the control ability of various operating parameters, high maintenance costs, and difficulty in managing faults; Difficulty in cultivating strains: it is difficult to screen and select advantageous strains that can efficiently degrade various VOCs gases; Reaction site constraints: The reaction device occupies a large area and takes a long reaction time. Therefore, biological methods are not lacking in their applications.

Ventilation and smoke exhaust installation in Shanghai workshop

5、 Combustion method

1. Principle of pollution treatment

The combustion method is divided into regenerative combustion technology (RTO) and catalytic combustion technology (RCO). The principle is to directly burn or add catalysts for low-temperature combustion, using 'burning' to completely degrade organic waste gas into water and carbon dioxide.

2. Practical application

The combustion method, as a relatively ideal process for current treatment efficiency and effectiveness, although its price is relatively expensive and the operating cost is not low, it has been recognized by most experts and some municipal environmental authorities, and even formulated as the main treatment process.

3. The main problems that exist

Due to the fact that the temperature inside the combustion chamber of the thermal storage combustion (RTO) method is generally not less than 750 degrees, and even up to 1000 degrees, fuel type nitrogen oxides are produced. Nitrogen oxides can be classified into three types based on their generation mechanisms: thermal, rapid, and fuel type, with fuel type accounting for 60% to 95%. In the process of generating fuel type NOx, first, organic compounds containing nitrogen or nitrogen in the air are thermally cracked to produce intermediate product groups such as N, CN, HCN, and then oxidized to NOx. After rough calculation, a set of 200000 m3/h heat storage combustion equipment has a nitrogen oxide emission equivalent to a 35t/h coal-fired fluidized bed boiler.

In the catalytic combustion equipment (RCO) process of organic waste gas, due to the use of tap water as a water spray for pre-treatment, chloride ions in the water and the inherent chloride ions of organic substances are prone to generate dioxins in the catalytic combustion chamber (200-500 degrees). However, there are no high-temperature devices on the VOCs treatment equipment to promote the decomposition of dioxins. Therefore, the dioxins generated during the combustion process will be directly discharged into the atmosphere.