Analysis and discussion on formation and control of dioxins generated from municipal solid waste incineration process

Figures & data

Figure 1. Harmless disposal of MSW in China from 2014 to 2019.

Figure 2. Molecular structure of dioxins.

Figure 3. Formation pathway of high-temperature gas-phase reaction for PCDFs.

Figure 4. Schematic diagram of “de novo” synthesis reaction.

Table 1. Representative models for homogeneous and heterogeneous catalytic reactions. (Ficarella and Laforgia 2000)

Reaction	Reaction formula	Reaction constants (homogeneous)	Reaction constants (heterogeneous)
Generation	Ps+Pg→Ds	K = 2.8*10^14 exp (−42,000/RT)	K = 5*10^9 exp (−20,800/RT)
Desorption	Ds→Dg	K = 10^13 exp (−53,400/RT)	K = 10^13 exp (−53,400/RT)
Dechlorination	Ds→Pr	K = 5*10^5 exp (−15,000/RT)	K = 5*10^5 exp (−15,000/RT)
Decomposition	Ds→DPr	K = 10^13 exp (−37,500/RT)	K = 10^13 exp (−37,500/RT)

Table 2. Effects of various influencing factors on the formation of PCDD/Fs.

Influencing factors	Effect on the formation of PCDD/Fs	References
Temperature	Temperature range of formation: 200–500°C; Optimal formation temperature: 300–320°C.	(Stieglitz and Vogg 1987, Vogg, Metzger, and Stieglitz 1987)
Catalyst	Cu-Fe compounds: The strongest catalytic effect of CuCl2; Activated Carbon: Catalytic dechlorination and condensation reactions of PCDD; Fly ash: Chlorophenol reaction is significantly promoted by CuCl2 and FeCl3 in the fly ash.	(Vogg, Metzger, and Stieglitz 1987, Kawamoto 2009)
Chlorine source	Necessary conditions for the formation of PCDD/Fs: The formation quantity of PCDD/Fs increases significantly with increasing Cl2 concentration; The effect is more pronounced when the chlorine concentration in the fuel is above 1.1%.	(Zheng and Qi 2008, Chen et al. 2007)
Carbon source	Prerequisites for the formation of PCDD/Fs: Provide macromolecular carbon structure and small molecular substances for de novo reaction and precursor synthesis reaction; The formation quantity of PCDD/Fs increases with the increase of carbon content in the fly ash.	(Wikström et al. 2003, Zeng et al. 2011)
Oxygen concentration	The effect on the formation of PCDD/Fs is more complex: No PCDD/Fs are formed without oxygen, and the formation quantity of PCDD/Fs in anoxic conditions is small; As the oxygen concentration increases, the combustion efficiency increases and the concentration of PICs decreases, which in turn leads to a decrease in the formation quantity of PCDD/Fs; The formation quantity of PCDD/Fs increases considerably when the oxygen concentration is higher than 10%.	(Yan et al. 2010)
Moisture	The effect on the formation of PCDD/Fs is more complex: Provide hydrogen source, oxygen source and hydroxyl radical for the formation of PCDD/Fs; The formation of PCDD/FS is inhibited by adsorption of precursors on the surface of fly ash; The increase of water content will change the balance of de novo reaction and increase the Cl2 content, which will inhibit the formation of PCDD/Fs.	(Jin, Tian, and Nie 2003)

Figure 5. Technical routes of waste classification and pretreatment.

Figure 6. Technical routes for controlling dioxin emissions in MSWI process.

Figure 7. Schematic diagram of semiconductor photocatalytic degradation of PCDD/Fs.

Figure 8. Schematic diagram of PCDD/Fs degradation by plasma technology.

Figure 9. Schematic diagram of microbial mineralized PCDD/Fs process.

Table 3. Summary for biodegradable POPs.

Microorganism	Contaminants	Degradation efficiency	Reference
Dehalococcoides sp.strain CBDB1	1,2,3-TrCDD	60%	(Lechner 2001)
	1,2,4-TrCDD	37%
Oyster mushroom	Benz(a)anthracene	77.4%	(Song 1999)
Cladosporium and Fusarium	PAHs	26.5–27.5%	(Olivier et al. 2004)
Sphingomonas wittichii RW1	1,2,3,4,7,8-HCDD	10.2%	(Nam et al. 2006)
Pseudomonas veronii PH-03	1,2,3,4-TeCDD	18%	(Hong et al. 2004)
Proteobacteria,Actinobacteria,Bac teroidetes,Firmicutes	PCDD/Fs	75%	(Huang et al. 2019)
Stropharia rugosoannulata	1,2,3,4,6,7,8-HxCDF	65%	(Anasonye et al. 2014)
Pseudomonas aeruginosa	Dibenzofuran,DF	98.4%	(Jiao et al. 2017)
Rhodococcus sp.strain P52	Dibenzo-p-dioxin,DD	48.86%	(Zhang 2020)
	Dibenzofuran,DF	61.5%

Figure 10. Schematic diagram of water-washing coupled cement kiln co-processing process.

Table 4. Advantages, disadvantages and technical maturity of PCDD/Fs degradation technologies in MSWI fly ash.

Degradation technology	Advantages	Disadvantages	Technology maturity
High-temperature melting technology	Fast efficiency, complete degradation, good effect of capacity and volume reduction, and can be utilized as a resource.	High energy consumption and cost, secondary fly ash will be resynthesized into PCDD/Fs.	Industrial applications
Low- temperature heat treatment technology	Fast efficiency, complete degradation and no problem of PCDD/Fs resynthesis.	Higher energy consumption, must be carried out under nitrogen atmosphere, high practical difficulty.	Industrial applications
Hydrothermal treatment technology	Lower energy consumption, simple operation, small footprint, can stabilize heavy metals in fly ash at the same time, and can be utilized as a resource.	High equipment cost and waste liquid needs to be reprocessed.	Experimental and theoretical research phase
Photodegradation technology	Faster efficiency, low energy consumption and cost.	Incomplete degradation, secondary contamination of degradation products.	Experimental and theoretical research phase
Plasma technology	Fast efficiency, complete degradation, good effect of capacity and volume reduction, and can be utilized as a resource, no heavy metal leaching.	Extremely high energy consumption and cost, unfavorable by-products are generated.	Abroad: mature Domestic: development stage
Biodegradation technology	Extremely low energy consumption and cost.	Long degradation cycle, lower efficiency, toxic intermediate products.	Experimental and theoretical research phase
Water-washing coupled cement kiln co-processing technology	large treatment capacity, saving cement raw materials, effective degradation of dioxins and solidification of heavy metals	high energy consumption, high cost of water-washing pretreatment	Industrial applications
Iron ore sintering process co-processing technology	Simple operation conditions, low cost	heavy metal-containing flue gas and secondary fly ash are generated.	Experimental and pilot phase

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Data availability statement

The data that support the findings of this study are openly available in [China Statistical Yearbook] at [http://www.stats.gov.cn/].