The problem of dioxin emissions from combustion and thermal process

Dioxin is the most poisonous substance discovered and produced by humans. Dioxin and its harmful effects have been the subject of interest and research by many scientists for decades. These are also serious problems in Vietnam because the consequences of the herbicide in the Vietnam war left by the American colonialists still have effects on present and future generations. In the following article, we will discuss a different viewpoint on possible sources of dioxin emissions. It is a matter of dioxin emissions from combustion and thermal processes.

1. Mechanism of dioxin formation from combustion and thermal processes

Thermal processes, especially incineration processes, are considered the main sources of dioxin that emits into the environment. Dioxins are compounds accidentally formed as a by-product in several chemical processes, especially those involving the combustion of elements such as carbon, oxygen, hydrogen, and chlorine. The parameters affecting the burning process are the type of fuel, the kind of trash, the technology and efficiency, the incinerator’s running pollution control system, and its waste treatment technology. The ability to handle wastes after combustion is crucial in determining the level of dioxin released. The following are three mechanisms of dioxin formation during combustion:

(1) Incomplete destruction of dioxin compounds already present in the composition of combustion materials such as fuel, waste, etc.
The dioxin will release into the environment according to the waste sources of the incinerator in the following situations:

• The combustion process is not efficient;
• The combustion technology and pollution control systems during the operation of the incinerator are poor,
• Fail to ensure necessary conditions for complete combustion, such as combustion temperature, time, and the miscibility with oxygen (Temperature, Time, and Turbulence – 3T).

(2) Formation of dioxins in an incinerator through a chemical reaction between pro-dioxin compounds.
The pro-dioxin compounds usually contain aromatic and chlorine heteroatom compounds (chlorobenzenes, chlorophenols, and clobiphenyls). If combustion occurs incompletely due to insufficient 3T conditions, the mentioned precursors are formable as intermediate products. In these circumstances, the chlorine will cause the precursors to react with the chlorine to produce dioxins.

(3) Formation of dioxins by initial fusion.
Dioxin is formed by the oxidation and conversion of macromolecular carbon (coal, charcoal, soot) into cyclic compounds. Then it is combined with chlorine and hydrogen. Factors affecting the synthesis of dioxins include:
(1) Temperature: 250 – 400°C, at 1000°C the reactions are still possible to happen;
(2) Carbon source from fly ash of flue gas;
(3) Oxygen in the exhaust gas is a necessary component. The higher the oxygen content, the easier it is to form dioxin/furan;
(4) The source of hydrogen and chlorine is mainly from inorganic compounds bound to solid carbon particles;
(5) Cu2+ strongly catalyze this process;
(6) The quick cooling of the exhaust gas stream and the presence of certain additives will delay or prohibit this process.
For example:
Incineration is believed to be the main source of dioxin in the environment. The more backward incinerator technology leads to poorer treatment technology and a higher level of dioxin emissions (especially the problem of ensuring the temperature of the combustion chamber). Many studies often classify the level of dioxin emissions in incineration activities by burning materials such as urban waste, industrial waste, medical waste, and biomass. The level of dioxin emissions is also classified by incineration-controlled burning (using incinerators) and uncontrolled burning (waste burning, open-air biomass).

2. Industrial activities that use high temperatures and incineration to release dioxins

Thermal processes, especially incineration processes, are considered the main sources of dioxin that emits into the environment. Dioxin is unintentionally formed by incomplete combustion of various materials, including fuel, municipal solid waste, medical waste, hazardous waste, sewage sludge, and biomass burning; activities using high-temperature cement calcination, metallurgy, and metal recycling. In addition, dioxin is also formed from uncontrolled combustion, especially uncontrolled processes such as forest fires, house fires in residential areas, and spontaneous and smoldering fires in landfills. The following table lists the thermal and incinerator processes most likely to release dioxins.

No.	Production process	Characteristics
1	Urban waste incineration	Outdated technology, not equipped with exhaust pollution control
2	Industrial waste incineration	Outdated technology, not equipped with exhaust pollution control
3	Hazardous waste incineration	Outdated incinerator, not equipped with exhaust pollution control
4	Mud incineration	Outdated incinerator, not equipped with exhaust pollution control, manual incinerator
5	Hospital waste incineration	Outdated incinerator, not equipped with exhaust pollution control, manual incinerator
6	Wood waste incineration	Wood treated with chlorinated organic compounds
7	Crematoriums	Outdated incinerator, not equipped with exhaust pollution control
8	Landfill emissions, biogas burning	Lack of exhaust pollution control
9	Burning coal	Burning brown coal, lignite, mud, etc.
10	Coke production	Using lignite, brown coal
11	Burning biomass	Uncontrollable: burn the rest of the forest, bushes, straw, etc,..
12	Fire from accidents	Uncontrollable: fire accidents in industries, warehouses, residential houses, etc
13	Landfill smoldering fires	Uncontrollable process
14	Burning PVC	Plastics contain halogen
15	Using a blast furnace to preheat iron ore	Rotary scattering of ash
16	Primary smelting of copper metal	Cu2+ ions play a catalytic role in the formation of dioxin
17	Scrap metal recycling	Burning wire, recovering metal from dust, ash
18	Cement kilns	Using hazardous waste containing halogen as fuel
19	Mineral production (lime, ceramics, glass, bricks, etc.)	Small scale, no dust and emission control system
20	Cooking asphalt	The processes of smelting and laying asphalt are carried out at high temperature and without emission control
21	Electrical industry	Power plants run on coal, oil, gas, biomass, etc.
22	Transportation	Lead is present in the fuel for internal combustion engines.

Reference: Report on the status of dioxin contamination in the environment in Vietnam – Steering Committee 33/Project Dioxin treatment in heavily polluted areas in Vietnam
Author: Do Thi Hue