Recently, A group led by Prof. CHEN Jiping and Prof. ZHANG Haijun in the Dalian Institute of Chemical Physics (DICP) of the Chinese Academy of Sciences revealed the formation mechanism of highly toxic chlorinated aromatic compounds during solid waste incineration. Their findings were published in Environmental Science and Technology, and it was selected as the current Supplementary Cover.
A large number of aromatic compounds, such as benzene, phenol, biphenyl, polycyclic aromatic hydrocarbons and analogues, can be formed and emitted during solid waste incineration, accompanied by the formation and volatilization of chloride, chlorine and chlorine radicals.
In the incineration flue gas, these aromatic compounds can react with chlorine-containing species in a complex way to form chlorinated aromatic compounds, such as dioxins, polychlorinated biphenyls, polychlorinated naphthalene and chlorinated polycyclic aromatic hydrocarbons. These chlorinated compounds, which is known as their carcinogenic, teratogenic and mutagenic effects, have been partly listed as persistent organic pollutants by United Nations Environment Program. Therefore, their emission is specially monitored by environmental protection authorities.
Chlorination of aromatic hydrocarbons is a key step to control the formation of chlorinated aromatic compounds in incineration flue gas. Since too many substances and elementary reactions are involved, the formation mechanism of highly toxic chlorinated aromatic compounds during solid waste incineration has not been comprehensively and intensively understood, which limits the development of emission reduction technology of chlorinated aromatic compounds.
Dechlorination-oxychlorination cycles of Cu and Fe species drive naphthalene chlorination. (Image by WANG Dan and ZHANG Haijun)
The researchers investigated the effects of a series of metal oxides and chlorides, such as copper, iron, manganese, aluminum, chromium, nickel, etc., on the electrophilic chlorination of aromatics by simulating the reaction atmosphere of incineration flue gas and using naphthalene as a model compound.
“We found that copper and iron chlorides were the key active substances to promote the chlorination of naphthalene, and we revealed the mechanism of electrophilic chlorination of aromatic compounds driven by dechlorination–oxychlorination cycles of copper and iron species” said Prof. CHEN.
Copper chloride and ferric chloride could act as chlorinating agents to directly induce chlorination of aromatic compounds. In the dechlorination-oxychlorination cycle of copper species, the direct chlorination induced by copper chloride was the main mechanism of chlorination of aromatic compounds, while the direct chlorination and catalytic chlorination played the same role on the chlorination of aromatic compounds driven by dechlorination-oxychlorination cycle of iron species.
Based on the in-depth understanding of the chlorination mechanism of chlorinated aromatic compounds during solid waste incineration, the researchers developed a technology to inhibit the formation of chlorinated aromatic compounds such as dioxins from incineration sources, which has been applied in three large-scale municipal solid waste incineration power plants.
The above work was supported by DICP Innovation Fund the National Natural Science Foundation of China and the National Key Basic Research Development Plan (973). (Text by WANG Dan and ZHANG Haijun)