The choice of treatment method for different gaseous pollutants depends on multiple factors such as the chemical properties of the pollutants, their concentration, gas volume, temperature and humidity, emission standards, and cost budget.

Main treatment methods are divided into two categories: "recycling" and "conversion".

Technology	Core Principles	Advantage	Shortcoming	Applicable Scenarios
Recycling method	Physical or chemical methods are used to separate, concentrate, and recycle pollutants from waste gas.	Maximize resource utilization and prevent secondary pollution	The process is complex and the cost is high.	The pollutant concentration is high and it has recycling value.
Conversion method	Pollutants are transformed into harmless or less harmful substances through chemical reactions.	The process is relatively simple ; only emission compliance needs to be considered.	Consuming energy or chemicals may generate new pollutants .	The concentration is low and there is no recovery value , so the pollutants need to be thoroughly treated .

 

The following are some common gaseous pollutants and their corresponding main treatment methods. 

1. Particulate Matter

Pollutants, PM10, PM2.5, smoke, dust, etc.

Processing Technology

Mechanical: Gravity settling chamber, cyclone dust collector , used for pre-treatment to remove large particles.

Filtration type: bag filter dust collectors and cartridge dust collectors can achieve a removal efficiency of over 99% for small particulate matter .

Electrostatic precipitator: Suitable for high temperature and large flue gas volume (such as power plants).

Wet type: Venturi scrubber and spray tower can remove dust and desulfurize at the same time.

2. Acidic Gases

Pollutants, sulfur dioxide, hydrogen chloride, hydrogen fluoride, hydrogen sulfide, nitrogen oxides, etc.

Processing Technology

Wet absorption: An alkaline spray tower is used to generate a salt solution by spraying NaOH, Ca(OH)₂, or Na₂CO₃ solutions into the target gas through a chemical reaction . Another method is ammonia-based desulfurization , where the core material is NH₃·H₂O, and the product is (NH₄)₂SO₄ (fertilizer).

Dry /Semi-dry methods: Dry injection primarily uses Ca(OH)₂ powder to treat acidic exhaust gases from waste incineration . Another approach is circulating fluidized bed , which uses CaO/Ca(OH)₂ particles to achieve efficient desulfurization and deacidification.

For NOx reduction: There are two main methods: selective catalytic reduction , using V₂O₅-WO₃/TiO₂ catalysts in medium-to-high temperature environments to reduce and refine nitrogen oxides , and Cu/Fe-molecular sieve catalysts in relatively low temperature environments . Another method is selective non-catalytic reduction , where ammonia or urea is injected into the nitrogen oxides at high temperatures (850-1100°C) to treat them.

3. Volatile Organic Compounds

Pollutants, toluene, xylene, ketones, esters, alkanes, etc.

Processing Technology

Recovery methods include: Adsorption , using activated carbon (granular/fiber) or zeolite molecular sieves. After adsorption saturation , desorption and regeneration (using steam or hot nitrogen) are required. Another method is absorption , using specialized absorbents, suitable for high concentrations of specific VOCs. Finally, condensation involves treating the waste gas in a refrigeration unit to below the VOCs dew point before liquefaction and recovery; this is suitable for gases with high concentrations and recovery value.

Destruction methods: Thermal combustion/catalytic combustion are the mainstream end-of- pipe gas treatment technologies. Thermal combustion involves direct incineration at temperatures above 750°C . Catalytic combustion involves passing high-temperature waste gas through a bed of precious metal catalysts or transition metal oxide catalysts at 250-400°C . Another option is biological methods , which mainly use microorganisms and biological packing materials , suitable for low-concentration, large-volume waste gas (such as wastewater treatment plants and food processing plants). Finally, plasma /photocatalysis is suitable for high-standard treatment of low-concentration, small-volume waste gas in specific industries .

4. Foul-smelling Gases

Pollutants, ammonia, hydrogen sulfide, methanethiol, skatole, etc.

Processing Technology

Chemical washing: acid solutions ( such as H₂SO₄ used against ammonia), alkaline solutions ( such as NaOH used against H₂S), sodium hypochlorite , etc.

Biological deodorization: using microorganisms and organic/inorganic fillers .

Activated carbon adsorption: Impregnated activated carbon is used , which has both strong adsorption and strong catalytic oxidation capabilities.

Advanced oxidation: using ozone, hydrogen peroxide , and ultraviolet light to generate hydroxyl radicals to strongly oxidize and decompose odor molecules.

5. Respiratory Hazardous Gases

Carbon Monoxide

Catalytic oxidation: Hogarth catalyst ( effective at room temperature , used in gas masks), Pt/Pd catalyst ( used in automobile exhaust).

Formaldehyde

Catalytic oxidation: using precious metal catalysts , primarily used in air purifiers at room temperature . Another method is adsorption , which mainly uses activated carbon.

Ozone

Catalytic decomposition method: composite metal catalyst ( effective at room temperature , used for ozone exhaust gas decomposition ).

Mercury Vapor

Adsorption method: Impregnated activated carbon is used. Another method is chemical oxidation , in which halides are injected into the flue gas to convert them into Hg²⁺, which is then absorbed by a wet desulfurization unit .

Suggestions for option selection

The common gaseous pollutant treatment methods mentioned above , the final solution also needs to be considered based on various variable factors in actual working conditions.

1. Concentration and air volume

High concentrations with small air volumes are suitable for recycling or combustion treatment methods, while low concentrations with large air volumes are suitable for adsorption or biological methods.

2. Temperature and humidity

Heat recovery can be considered for high-temperature exhaust gases, high-humidity waste gas may cause adsorbents and catalysts to become ineffective, so pre-treatment is required.

3. Handling object complexity

Single component or complex component? When multiple pollutants coexist, technological compatibility or combined processes ( e.g., alkaline washing + demisting + activated carbon adsorption / catalytic combustion) must be considered.

4. Operating costs

The appropriate solution should be selected by considering the overall effect and cost, including equipment energy consumption, reagent costs, and by-product treatment costs .