Classification can reduce dioxin emissions from municipal solid waste incineration
The research shows that cities that do a good job in garbage sorting have a lower dioxin concentration per ton of garbage on average. Then, why can waste sorting reduce dioxin emissions from incineration? Li Xiaodong of Zhejiang University and his research team published an article in the journal Waste Disposal & Sustainable Energy, which systematically analyzed the relationship between waste classification and dioxin emission from combustion. It was considered that unclassified municipal solid waste would cause three problems, such as incomplete combustion, high chlorine content and high heavy metal content, which would promote the generation and emission of dioxin from waste incineration[1]. The following is a concrete analysis of the correlation between these three problems and dioxin in garbage incineration.
1. The thermal properties of waste components with great differences in materials and structures are significantly different, and the incomplete combustion products formed by incineration are easy to polymerize into dioxins.
According to the thermal characteristics of different components, they can actually be divided into three categories, namely kitchen waste, paper products and textiles, and plastics. There are obvious differences among these three categories. When the components with great differences in combustion characteristics are mixed, the calorific value will change sharply in the process of garbage incineration, resulting in unstable combustion and reduced combustion efficiency. Incomplete combustion caused by the reduction of combustion efficiency will inevitably form incomplete combustion products such as polycyclic aromatic hydrocarbons, chlorophenol and chlorobenzene.
At present, domestic waste incineration enterprises propose to use "3T+E" (time, turbulence, temperature and excessive oxygen) to control and monitor the combustion conditions more closely, which can reduce the amount of dioxins generated in the process of waste feeding, but the incineration of mixed waste will still produce incomplete combustion products and dioxins in the burned area of the incinerator. This is because at the low temperature of 200-400℃, chlorine-containing aromatic rings such as chlorophenol and chlorobenzene in incomplete combustion products can be used as precursor compounds or polymerized by carbon-carbon matrix existing on the surface of fly ash to form dioxins. Therefore, if the domestic waste incinerator is not classified or not completely classified, it will bring low combustion efficiency, and in the absence of terminal air pollution control devices, the probability of dioxin emission exceeding the standard is still high.
2. Chlorine-containing substances in municipal solid waste promote the formation of dioxins.
This paper cites the research of many research institutions from many countries, and proves from different angles that chlorine-containing substances in municipal solid waste promote the formation of dioxins. Among them, researchers from Sweden found that the threshold for chlorine content to promote dioxin emission from municipal solid waste incinerators is 1% by mass (the ratio of chlorine mass in garbage to total garbage mass is equal to 1%). Below this threshold, the chlorine content has no obvious effect on the formation of dioxins; But above the threshold, with the increase of chlorine content, the formation of dioxins increased [2].
Japanese researchers simulated adding different amounts of PVC and sodium chloride to fly ash produced by incineration to determine whether they promoted the formation of dioxins. The results show that the concentration of dioxins increases with the increase of chlorine content during flue gas cooling, regardless of whether the chlorine source is organic or inorganic [3].
3. Mixed heavy metals in municipal solid waste catalyze the formation of dioxins
The catalytic effect of heavy metals in the fly ash of municipal solid waste incinerator is very important for the formation of dioxins. Many studies show that common metals such as copper, iron, chromium, nickel and zinc in fly ash may play an important role in the formation of dioxins, among which copper has the strongest promotion effect. Dioxin production of fly ash with copper, iron, chromium, nickel or zinc oxides is 2-4 orders of magnitude higher than that of fly ash without these catalytic metals.
Japanese researchers found that the dioxin production rate of incineration of garbage with copper content of 0.007% was five times higher than that of incineration of chlorine-free garbage. Therefore, in order to significantly reduce the formation of dioxins, it is necessary to reduce the content of metal catalysts in imported wastes [4].
When kitchen waste and plastics are mixed with municipal solid waste, they will cause incomplete combustion, increase chlorine content and heavy metal content at the same time, which will promote the production of dioxins. Since the promulgation and implementation of the classification management regulations in various places, the separation of kitchen waste in various places has effectively reduced the amount of dioxins produced. It can be seen from the table that the separation of plastic and electronic waste will be the key step to further promote the waste sorting work in order to further reduce the amount of dioxin generated during waste incineration.
Reference
[1]Li, X. Study on the relationship between waste classification, combustion condition and dioxin emission from waste incineration[J]. Waste Disposal & Sustainable Energy,2019(01): 91–98.
[2]Wikström E. Influence of level and form of chlorine on the formation of chlorinated dioxins, diben- zofurans, and benzenes during combustion of an artificial fuel in a laboratory reactor[J]. Environ Sci Technol, 1996(30):1637–1644.
[3] Hatanaka T. Formation of PCDD/Fs in artificial solid waste incineration in a laboratory-scale fluid- ized-bed reactor: influence of contents and forms of chlorine sources in high-temperature combustion[J]. Environ Sci Technol,2000(34):3920–3924.
[4]Takeshi H. Role of copper chloride in the for- mation of polychlorinated dibenzo-p-dioxins and dibenzofurans during incineration[J]. Chemosphere,2004(57):73–79.
Source：https://mp.weixin.qq.com/s/2Xx81zpW_fHeYFGPP5W6mA