VOC flue gas treatment

Volatile organic compounds (VOCs) are defined as organic substances with a saturated vapor pressure greater than 133.32 Pa at room temperature and a boiling point below 50–260°C at normal atmospheric pressure, as well as any volatile organic solids or liquids under standard temperature and pressure conditions.

1. Classification of Volatile Organic Compounds (VOC) Exhaust Gases
Volatile organic compounds (VOCs) are defined as organic substances with a saturated vapor pressure greater than 133.32 Pa at room temperature and a boiling point between 50–260°C under standard atmospheric pressure, as well as any volatile organic solids or liquids at normal temperature and pressure.
According to their chemical structure, VOCs can be divided into eight categories: alkanes, aromatic hydrocarbons, alkenes, halogenated hydrocarbons, esters, aldehydes, ketones, and others. Their main components include hydrocarbons, halogenated hydrocarbons, oxygenated hydrocarbons, and nitrogen-containing hydrocarbons, such as aromatic series compounds, organic chlorides, freons, organic ketones, amines, alcohols, simple and complex esters, acids, and petroleum hydrocarbon compounds.

Specifically, this includes:
• Aromatic hydrocarbons (benzene, toluene, xylene, styrene, etc.)
• Aliphatic hydrocarbons (butane, n-butane, gasoline, etc.)
• Halogenated hydrocarbons (carbon tetrachloride, chloroform, vinyl chloride, etc.)
• Freons
• Alcohols, aldehydes, ketones, polyols (methanol, propanol, isobutanol, formaldehyde, acetaldehyde, acetone, cyclohexanone, etc.)
• Simple ethers, phenols, epoxides (ether, methylphenol, phenol, ethylene oxide, propylene oxide, etc.)
• Esters and organic acids (ethyl acetate, butyl acetate, etc.)
• Amines, nitrile compounds (dimethylformamide, acrylonitrile, etc.)
• Others (methyl bromide, chlorofluorocarbons, etc.)
These eight categories encompass a broad range of VOCs commonly found in industrial emissions.

2.Sources of Volatile Organic Compound (VOC) Exhaust Gases

1. Sources of VOC Gases at Coal Chemical Enterprises
Exhaust gases generated in the coal chemical industry mainly result from two processes: coal-to-coke conversion and coal-to-gas conversion.

1.1 Coal and Coke Exhaust Gases

These are primarily generated during coal charging, coking, and downstream chemical processing. During coal loading, the raw coal comes into direct contact with high-temperature air, producing large volumes of smoke, dust, and various harmful polycyclic aromatic hydrocarbons. VOCs during the coking stage mainly originate from chemical production zones and wastewater treatment areas. The chemical production zone is subdivided into the cold drainage section, desulfurization section, ammonium sulfate section, and crude benzene section, each with different pollutants and emission characteristics.
1. Cold Drainage Section:
Volatile gaseous compounds mainly include ammonia, hydrogen sulfide, naphthalene, and small amounts of VOCs. Emissions originate from tar tanks, ammonia water tanks, intermediate tar tanks, tar pipelines, and underground seal tanks, characterized by continuous emission, high concentration, and both low and high temperature conditions.
2. Desulfurization / Ammonium Sulfate Section:
Raw gas emissions mainly contain ammonia and hydrogen sulfide with fewer VOCs. Sources include mother liquor tanks, regeneration tanks, and foam quenching tanks. Emissions are continuous and ammonia-rich.
3. Crude Benzene Section:
Unorganized emissions are rich in benzene series compounds and come from crude benzene storage tanks, de-benzolized oil tanks, washing oil tanks, underground tanks, and dosing tanks. These emissions are low in volume but highly concentrated.
4. Wastewater Treatment Area:
Emissions include volatile gases and water vapor. Major pollutants are benzene series compounds, hydrogen sulfide, and nitrogen. Sources include emergency combustion tanks, regulating tanks, anaerobic tanks, filter press areas, etc., characterized by large volume, low concentration, and high moisture content.

3.Sources of VOC Emissions in Packaging and Printing Enterprises
Packaging and printing can be classified in two ways:
1. By material type: printing on paper-based packaging, plastic packaging, metal packaging, and packaging made from other materials.
2. By printing technique: lithography (offset printing), gravure printing, flexographic printing, embossing, screen printing, and others.

According to statistical data, the total VOC emissions from the packaging and printing industry exceed 2 million tons. These emissions are mainly concentrated in the printing, drying, laminating (compounding), and cleaning processes. The primary sources include the use of inks, adhesives, coating solutions, dampening fluids, cleaning agents, and various organic solvents, which release VOCs through both natural evaporation and drying-related volatilization.

The color printing segment on plastic and flexible packaging is the largest contributor to VOC emissions in this sector. To achieve desired marketing and visual effects, the manufacturing process requires the use of organic solvents such as ethyl acetate, toluene, butanone (MEK), and isopropanol.
As a result, industrial exhaust gas treatment systems are most widely needed and applied in the flexible packaging industry involving color printing on plastic materials.

4. Sources of VOC Emissions in Coating Production and Application Enterprises

Coating production and application typically involves industrial processes such as paint mixing, coating application (including primer, intermediate, and top layers, as well as varnish), leveling layers, drying, and other related steps.

According to the National Economic Industry Classification Standard (GB/T 4754-2017), industries involving coatings and associated VOC emissions typically include:
• Furniture manufacturing (C21)
• Metallurgical industry (C33)
• General-purpose equipment manufacturing (C34)
• Special-purpose industrial equipment (C35)
• Automotive industry (C36)
• Railway, shipbuilding, aerospace, and other transportation equipment (C37)
• Metal products, repair and maintenance of machinery and equipment (C43)

In these industries, VOC emissions are mainly generated during the paint formulation, spraying, curing, and drying processes. These emissions come from solvents, thinners, and additives contained in paints and coatings. Therefore, environmental protection policies, pollution control strategies, and VOC management practices are crucial for coating production enterprises.