Integrated thermal behavior and compounds transition mechanism of municipal solid waste incineration fly ash during thermal treatment process

Mitigating heavy metal volatilization during thermal treatment of MSWI fly ash by using iron(III) sulfate as a chlorine depleting agent

In the thermal treatment of municipal solid waste incineration fly ash (FA), the presence of chlorides leads to the pronounced volatilization of heavy metals at high temperature, making heavy metals stabilization challenging. Conventional washing processes struggle to remove chlorides completely, and even minor residual chlorides can lead to significant heavy metal volatilization. This study innovatively applied iron(III) sulfate as a chlorine depleting agent, which can form FeCl3 (boiling point 316 °C) and volatilize to remove the residual chlorides at below 500 °C, thus preventing the chlorination and volatilization of heavy metals at 600-1000 °C. Using water-washed FA to produce lightweight aggregate (LWA) preparation, after adding iron(III) sulfate, the volatilization rates of Pb and Cd at 1140 °C decreased to 5.4% and 9.3%, respectively, a reduction of 82.8% and 84.1% compared to before its addition. The LWA met standard requirements in both performance and heavy metal leaching toxicity. The mechanism was further studied through thermodynamic equilibrium calculations and heating experiments of pure chemicals. This study presents novel approaches and insights for suppressing the volatilization of heavy metals in FA at high temperature, thereby promoting the advancement of thermal treatment techniques and the safe, resourceful disposal of FA.

Removal of dioxins from municipal solid waste incineration fly ash by low-temperature thermal treatment: Laboratory simulation of degradation and ash discharge stages

Dioxins in municipal solid waste incineration fly ash (MSWIFA) can cause significant risks to the environment and human health. In this study, the low-temperature thermal treatment of MSWIFA under industrial conditions was simulated in the laboratory to investigate the process parameters for dioxin degradation and ash discharge stages. Correlation analysis and dioxin fingerprint characterization were used to analyze the degradation and ash discharge processes. The degradation efficiency of low-temperature thermal treatment was influenced by multiple factors. At 400℃ for 90 min and 1% O2, the dioxin removal rate was 95.80%, the detoxification rate was 91.73%, and the residual dioxin toxicity in MSWIFA was 22.7 ± 17.8 ng I-TEQ/kg, which was in line with the limit value of 50 ng I-TEQ/kg in the "Technical specification for pollution control of fly-ash from municipal solid waste incineration" (HJ1134-2020). The increase in dioxins during ash discharge did not follow a linear relationship with the process parameters. This was assumed to be related to the MSWIFA composition, as some components containing P, Si, and Al at 150 °C may inhibit dioxin formation. The dioxin increased only by 0.79 ± 2.65 ng/kg, an increase in toxicity of 0.42 ± 0.10 ng I-TEQ/kg, when treated at 150 °C for 30 min and 10% O2.

Review of thermal treatments for the degradation of dioxins in municipal solid waste incineration fly ash: Proposing a suitable method for large-scale processing

Dioxin degradation is considered essential for the environmentally sound management of municipal solid waste incineration fly ash (MSWIFA). Among the many degradation techniques, thermal treatment has shown good prospects owing to its high efficiency and wide range of applications. Thermal treatment is divided into high-temperature thermal, microwave thermal, hydrothermal, and low-temperature thermal treatments. High-temperature sintering and melting not only have dioxin degradation rates higher than 95 % but also remove volatile heavy metals, although energy consumption is high. High-temperature industrial co-processing effectively solves the problem of energy consumption, but with a low fly ash (FA) mixture, and the process is limited by location. Microwave thermal treatment and hydrothermal treatment are still in the experimental stage and cannot be used for large-scale processing. The dioxin degradation rate of low-temperature thermal treatment can also be stabilized at higher than 95 %. Compared to other methods, low-temperature thermal treatment is less costly and energy consumption with no restriction on location. This review comprehensively compares the current status of the above-mentioned thermal treatment methods and their ability to dispose of MSWIFA, especially the potential for large-scale processing. Then, the respective characteristics, challenges, and application prospects of different thermal treatment methods were discussed. Finally, based on the goal of low carbon and emission reduction, three possible approaches for improvement were proposed to address the challenges of large-scale processing of low-temperature thermal treatment, namely, adding a catalyst, changing the FA fraction, or supplementing with blockers, providing a reasonable development direction for the degradation of dioxins in MSWIFA.

An all-in-one strategy for municipal solid waste incineration fly ash full resource utilization by heat treatment with added kaolin

Resourcization has become a popular research topic for the final disposal of municipal solid waste incineration fly ash (MSWI FA). However, the current research is limited to building material preparation or valuable chloride recovery, which may cause resource waste and secondary pollution. A unique process, heat treatment with the addition of kaolin (KL), was presented to achieve complete resource utilization of MSWI FA. The physical properties of ceramsite could be improved by adding KL, and dioxin removal, heavy metals, and valuable chloride separation could be achieved via sintering at 1150 °C. The separation and purification of dust carried by the flue gas during thermal treatment (secondary fly ash) was achieved via wet separation. A building ceramsite with a compressive strength of 24.8 MPa was obtained, whereas dioxin and heavy metal toxicity were far below the standard limits. Heavy metal content was enriched by 12 times, approximately 59.6%, achieved after secondary fly ash separation and purification. A heavy metal product containing 39.5% Zn, 19.1% Pb, and chloride salt containing 41.8% KCl were obtained. This showed a high potential for the developed process to separate multiple valuable elements from ashes. This novel process will further promote the development and application of harmless and resourceful technologies for MSWI FA.

Recycling municipal solid waste incineration fly ash in super-lightweight aggregates by sintering with clay and using SiC as bloating agent

Municipal solid waste incineration (MSWI) fly ash is classified as hazardous waste and requires proper treatments. Sintering of MSWI fly ash for the production of lightweight aggregate (LWA) is a promising treatment technology, while the dependence on natural bloating clay to produce high quality LWA has limited its wide application. In this study, by using SiC as a bloating agent, normal clay could be used to produce super-lightweight aggregate (bulk density <500 kg/m³) with MSWI fly ash. Effects of SiC addition amount, sintering temperature and duration on LWA performance were studied. The results showed that LWA with SiC addition of 0.1-0.5 wt% had significant expansion at sintering temperature of 1120 °C-1160 °C. The optimal conditions were 0.3 wt% SiC addition and sintering at 1120 °C for 30 min, and the bulk density could reach 212 kg/m³ with other properties meeting the LWA standard (GB/T 17431.1-2010). Further, the heavy metal leaching toxicity was significantly decreased after sintering and met the MSWI fly ash utilization standard (HJ 1134-2020). The X-ray diffraction results revealed the formation of a complex diopside-based phase after sintering. This study provides a new approach for recycling MSWI fly ash in LWA without dependence on specific clay resources, and makes this technology wider applicability.

Silica-assisted pyro-hydrolysis of CaCl2 waste for the recovery of hydrochloric acid (HCl): Reaction pathways with the evolution of Ca(OH)Cl intermediate by experimental investigation and DFT modelling

The chlorine evolution mechanism remains unclear during the thermal treatment of CaCl₂/Ca(OH)Cl-containing solid waste. In this paper, we have conducted both experimental investigation and density functional theory (DFT) calculation to elucidate the mechanism of pyro-hydrolysis of CaCl₂ with and without SiO₂ in the temperature ranges of 400-900 °C. It was determined that pyro-hydrolysis of CaCl₂ alone generated a maximum of 12% HCl by decomposition into Ca(OH)Cl, which is a stable intermediate that can be reverted to CaCl₂ at 800 °C. Upon the addition of SiO₂ at an equimolar ratio to CaCl₂, the HCl release extent was accelerated to 50% at 900 °C. Both experiments and DFT calculations prove that the added SiO₂ can promote the dissociation of water molecules which provides hydroxyl ions that enable the conversion of CaCl₂ into Ca(OH)Cl at low temperatures. The resulting Ca(OH)Cl can also quickly react with SiO₂ to form Cl-bearing silicates such as Ca₂SiO₃Cl₂ and Ca₃SiO₄Cl₂ with weakened CaCl bond that are relatively easy to cleave into Cl-free CaSiO₃ and HCl_(g) from 800 °C.

Pyrolytic characteristics of fine materials from municipal solid waste using TG-FTIR, Py-GC/MS, and deep learning approach: Kinetics, thermodynamics, and gaseous products distribution

Fine materials (FM) from municipal solid waste (MSW) classification require disposal, and pyrolysis is a feasible method for the treatments. Hence, the behavior, kinetics, and products of FM pyrolysis were investigated in this study. A deep learning algorithm was firstly employed to predict and verify the TG data during the process of FM pyrolysis. The results showed that FM pyrolysis could be divided into drying (<138 °C), de-volatilization (138-570 °C), and decomposition stage (≥570 °C above). The de-volatilization can further be divided into stage 2 and stage 3, with values of activation energy estimated by Flynn-Wall-Ozawa and Kissinger-Akahira-Sunose methods as 123.35 and 172.95 kJ/mol, respectively. The gas products like H₂O, CO₂, CH₄, and CO, as well as functional groups like phenols and carbonyl (CO), were all detected during the process of FM pyrolysis by thermogravimetric-fourier transform infrared spectrometry at a heating rate of 10 ^°C/min. The main species detected by pyrolysis-gas chromatography-mass spectrometry analyzer included acid (41.98%) and aliphatic hydrocarbon (22.44%). Finally, the 1D-CNN-LSTM algorithm demonstrated an outstanding generalization capability to predict the relationship between FM composition and temperature, with R² reaching 93.91%. In sum, this study provided a reference for the treatment of FM from MSW classification as well as the feasibility and practicability of deep learning applied in pyrolysis.

Comprehensive understanding the transition behaviors and mechanisms of chlorine and metal ions in municipal solid waste incineration fly ash during thermal treatment

Municipal solid waste incineration fly ash is classified as the hazardous waste because of its high levels of heavy metals alkali chlorides, and polychlorinated dibenzo-p-dioxins. Thermal treatment is widely used for fly ash treatment because of its advantages of reduction and harmless. The transformation behaviors of chlorine and metal ions during the thermal treatment of fly ash has a significant impact on the harmless and resource of fly ash. At present, the migration behaviors of chlorine and metal ions during thermal treatment of fly ash is not clearly demonstrated. In this manuscript, the phase compositions, transformation behaviors, the variation of mass and content of chlorine and various metal ions were analyzed through diverse characterization methods under different sintering temperatures to understand the migration behaviors of chlorine and metal ions during thermal treatment. Roasting experiments showed that the migration behaviors of heavy metals and chlorides were consistent. The chlorine, sodium, potassium and heavy metal ions can be removed sharply while the calcium, aluminum, magnesium and iron were decreased slightly when the roasting temperature was above 750 °C. The findings also suggested that removed chlorides were soluble chlorides and unstable crystals in municipal solid waste incineration fly ash were inclined to formed steady structure under high temperature. The structure of roasted fly ash became denser and generated ceramic-like particle due to thermal agglomeration and chemical reactions.

Mechanical property and heavy metal leaching behavior enhancement of municipal solid waste incineration fly ash during the pressure-assisted sintering treatment

The conventional sintering process of municipal solid waste incineration (MSWI) fly ash is always energy intensive. The process forms a cracked structure because of the difficulty in forming the liquid phase to enhance the mass transfer process. Therefore, exploring a new disposal method to simultaneously decrease the sintering temperature and improve the mechanical and heavy metal leaching properties of sintered samples is necessary. In this study, a pressure-assisted sintering treatment was introduced to dispose fly ash by varying the chemical composition and mechanical pressure at relatively low temperatures (300-500 °C). The results revealed that the compressive strength of treated samples increased with the CaO/SiO₂ molar ratio increasing from 0.5 to 1.0, and a maximum value of 238.28 ± 8.50 MPa was obtained. The heavy metal leaching concentration results demonstrated a low risk of contamination in the treated samples. Microstructure analyses suggested that the densification process was enhanced with increased mechanical pressure, and the formed calcium silicates and aluminosilicates positively affected the compressive strength. Moreover, smaller crystal lattices were observed during aggregation formation, suggesting the restraint of anomalous crystal growth, which accelerated the densification process and increased the compressive strength. Moreover, the mass transfer process during the pressure-assisted sintering process was enhanced compared with the conventional thermal process, which was reflected by the transformation of elements from homogeneous to heterogeneous distribution. Therefore, the improved mechanical properties and leaching behavior of heavy metals were attributed to the densified microstructure, formation of new minerals, and enhanced driving force during the pressure-assisted sintering process. These findings suggest that pressure-assisted sintering is a promising method for maximizing the reutilization and minimizing the energy consumption simultaneously to dispose fly ash.