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

Study on kinetics and thermsodynamics of municipal solid waste incineration fly ash in air and N2 atmospheres

This study attempted to investigate the thermal behavior and reaction mechanisms of municipal solid waste incineration fly ash under air and N2. Mass loss patterns at temperatures from 30ºC to 1100ºC were obtained through thermogravimetric analysis. Based on mass loss patterns, the behavior of fly ash under high temperature was divided into three stages. Mass loss in Stage I (30ºC-500ºC) amounted to 3.0%-6.2%. The majority of mass loss concentrated in Stage II (500ºC-800ºC) and Stage III (800ºC-1100ºC). Kinetic parameters of fly ash in Stage II and Stage III were evaluated using Flynn-Wall-Ozawa (FWO), Kissinger-Akahira-Sunose (KAS), and Friedman methods. By comparison, the iso-conversional FWO method exhibited the highest correlation coefficient with R2 > 0.99. Activation energy (E) values in Stage II calculated via the FWO method indicate that reaction in air showed considerably higher hurdle (E = 171.11 kJ/mol) than reaction in N2 (E = 124.52 kJ/mol). This difference was partly attributed to the presence of carbonation process in air. In contrast, E values in Stage III were similar with E of 373.38 kJ/mol in air and 382.25 kJ/mol in N2. Mechanistic analysis via the Coats-Redfern (CR) model, employing 15 kinetic functions, identified dominant mechanisms of one-dimensional diffusion and contracting sphere for Stage II in air and N2 respectively. At the same time, three-dimensional diffusion could best explain the reaction mechanism in Stage III in both air and N2. Moreover, calculations of thermodynamic parameters (ΔH, ΔG, and ΔS) revealed that major reactions of fly ash during thermal treatment were endothermic and non-spontaneous, with Stage III exhibiting heightened complexity. This multi-stage characterization elucidates the degradation mechanisms of fly ash under varying thermal conditions and provides useful insight into the fly ash thermal treatment processes.

Steam washing for MSWI-FA treatment

This study investigates steam washing (SW) as an innovative pretreatment for municipal solid waste incineration fly ash (MSWI-FA) dechlorination, useful for a more effective stabilization in cementitious matrix. By using a detailed analytical approach (XRPD, XRF, ICP-MS, IRMS, SEM) and geochemical modeling, great focus is dedicated on pollutant leaching reduction and changes in ash physicochemical characteristics as a function of exposure time. The research demonstrates that SW removes up to 70 % cadmium, 17 % zinc, and 10 % lead, primarily by dissolving the soluble and carbonate/hydroxide fractions and promoting the reprecipitation and adsorption of heavy metals into more stable compounds. Chloride, sulfate, and heavy metal leaching are reduced by 85 %, 50 %, and 90 %, respectively, with even short treatment time (8 min) performing better than conventional water washing at a liquid-to-solid ratio of 2. However, antimony leaching remains above regulatory thresholds, controlled by soluble Ca-antimonate phases, thus requiring supplementary tailored treatments. Further optimization of the energy recovery during the exposure and a comprehensive life-cycle assessments to evaluate its long-term environmental and economic impact, may contribute significantly to propose SW as a sustainable strategy for MSWI-FA treatment and valorization.

Degradation of PCDD/Fs from incineration of waste at low temperatures for resource utilization of fly ash

Municipal solid waste incineration fly ash (MSWIFA) is of great value in resource utilization. Harmless pretreatment is a crucial prerequisite for the resource utilization of MSWIFA. The detoxification process is a crucial step in the harmless pretreatment of MSWIFA. This includes polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs), which are among the most toxic substances to humans and other living organisms. Low-temperature degradation technology has broad prospects in engineering applications due to the advantages of low technical difficulty and operating costs. This work conducts a pilot test on the degradation of 17 toxic PCDD/Fs in MSWIFA at low temperatures. The pilot test investigates the effects of reaction temperature and oxygen content on the degradation of PCDD/Fs in MSWIFA. Furthermore, based on the perspective of MSWIFA resource utilization, an analysis and a proposal are made to judge the degradation effect of low-temperature thermal treatment technology on PCDD/Fs in MSWIFA. Further, taking the soil sludge field as the application scenario, the application feasibility of MSWIFA after detoxification is analyzed. The flotation process markedly reduces both the carbon content and the levels of PCDD/Fs in MSWIFA. The hydrothermal method facilitates the degradation of dioxins in fly ash while introducing oxygen significantly lowers the reaction temperature required for fly ash treatment. This can enhance the degradation rate and reduce the demands on reaction equipment. The results indicate that the low-temperature thermal treatment technology can effectively degrade PCDD/Fs in MSWIFA, satisfying the requirements of some application scenarios. Notably, evaluating the effect of low-temperature thermal treatment technology on the degradation and detoxification of PCDD/Fs in MSWIFA should satisfy the residue requirements of different industries and achieve a certain detoxication efficiency.

Degradation and regeneration inhibition of PCDD/Fs in incineration fly ash by low-temperature thermal technology

Low-temperature thermal degradation of PCDD/Fs for incineration fly ash (IFA), as a novel and emerging technology approach, offers promising features of high degradation efficiency and low energy consumption, presenting enormous potential for application in IFA resource utilization processes. This review summarizes the concentrations, congener distributions, and heterogeneity characteristics of PCDD/Fs in IFA from municipal, medical, and hazardous waste incineration. A comparative analysis of five PCDD/Fs degradation technologies is conducted regarding their characteristics, industrial potential, and applicability. From the perspective of low-temperature degradation mechanisms, pathways to enhance PCDD/Fs degradation efficiency and inhibit their regeneration reactions are discussed in detail. Finally, the challenges to achieve low-temperature degradation of PCDD/Fs for IFA with high-efficiency are prospected. This review seeks to explore new opportunities for the detoxification and resource utilization of IFA by implementing more efficient and viable low-temperature degradation technologies.

Formation behavior of PCDD/Fs during waste pyrolysis and incineration: Effect of temperature, calcium oxide addition, and redox atmosphere

The clean and efficient utilization of municipal solid waste (MSW) has attracted increasing concerns in recent years. Pyrolysis of MSW is one of the promising options due to the production of high-value intermediates and the inhibition of pollutants at reducing atmosphere. Herein, the formation behavior of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) during MSW pyrolysis and incineration was experimentally investigated and compared. The influence of reaction temperature, CaO addition, and redox atmosphere on PCDD/Fs formation were compared and discussed. The results showed as the pyrolysis temperature increased, the mass concentration and international toxicity equivalence quantity of PCDD/Fs initially peaked at ∼750 °C before declining. Most of the generated PCDD/Fs were concentrated in the liquid and gaseous products, accounting for ∼90% of the total. Among liquid products, octachlorodibenzo-p-dioxin (O8CDD), 2,3,4,7,8-pentachlorodibenzofuran and 1,2,3,4,6,7,8-heptachlorodibenzofuran (H7CDF) were the most crucial mass concentration contributors, while in gas products, high-chlorinated PCDD/Fs, such as O8CDD, octachlorodibenzofuran (O8CDF) and 1,2,3,4,6,7,8-H7CDF were predominant. Compared to incineration, the formation of PCDD/Fs was 7-20 times greater than that from pyrolysis. This discrepancy can be attributed to the hydrogen-rich and oxygen-deficient atmosphere during pyrolysis, which effectively inhibited the Deacon reaction and the formation of C-Cl bonds, thereby reducing the active chlorine in the system. The addition of in-situ CaO additives also decreased the active chlorine content in the system, bolstering the inhibiting of PCDD/Fs formation during MSW pyrolysis.

Incineration-source fingerprints and emission spectrums of dioxins with diagnostic application

Despite increasing waste-to-energy (WtE) capacities, there remain deficiencies in comprehension of 136 kinds of tetra- through octa-chlorinated dibenzo-p-dioxin and dibenzofurans (136 PCDD/Fs) originating from incineration sources. Samples from twenty typical WtE plants, encompassing coal-fired power plants (CPP), grate incinerators (GI), fluidized bed incinerators (FBI), and rotary kilns (RK), yielded extensive PCDD/F datasets. Research was conducted on fingerprint mapping, formation pathways, emission profiles, and diagnostic analysis of PCDD/Fs in WtE plants. Fingerprints revealed a prevalence of TCDF, followed by PeCDF, while CPP and RK respectively generated more PCDD and HxCDD. De novo synthesis was the predominant formation pathway except one plant, where CP-route dominated. DD/DF chlorination also facilitated PCDD/F formation, showing general trends of FBI > GI > CPP > RK. The PCDD/F emission intensities emitted in air pollution control system inlet (APCSI) and outlet (APCSO) followed the statistical sequence of RK > FBI > GI > CPP, with the average I-TEQ concentrations in APCSO reaching 0.18, 0.08, 0.11, and 0.04 ng I-TEQ·Nm-3. Emission spectrum were accordingly formed. Four clusters were segmented for diagnosis analysis, where PCDD/Fs in GI and FBI were similar, grouped as a single cluster. PCDD/Fs in CPP and RK demonstrated distinctive features in TCDD, HxCDD, and HxCDF. The WtE plants exceeding the limit value tended to generate and retain fewer TCDD and TCDF yet had higher fractions of HxCDD and HxCDF. The failure of APCS coupled with the intrinsic source strength of PCDD/Fs directly led to exceedance, highlighting safe operational practices. This study motivated source tracing and precise evaluation of 136 PCDD/Fs based on the revealed fingerprint profiles for WtE processes.

Preparation of municipal solid waste incineration fly ash/ granite sawing mud ceramsite and the morphological transformation and migration properties of chlorine

Municipal solid waste incineration (MSWI) fly ash is a hazardous waste containing high chlorine and harmful substances generated during the waste incineration disposal, and its resource utilization has a positive effect on reducing environmental pollution. In this study, the feasibility of preparing lightweight MSWI fly ash/granite sawing mud ceramsite (MG ceramsite) was investigated by evaluating the influence of Al2O3 addition, MSWI fly ash content and sintering temperature on the ceramsite properties. The microstructure of MG ceramsite was investigated by using SEM, the chlorine morphological transformation and migration behaviors were simultaneously explored by using the tube furnace experiment, XRD and XRF analyses. The experimental results show that the maximum MSWI fly ash content is about 30 wt%∼35 wt%, with the Al2O3 addition of at least 10 %. By controlling the MSWI fly ash content of 30 wt%, MG ceramsite can be obtained with bulk density of 986 kg/m3, cylindrical compressive strength of 19.67 MPa, 1 h water absorption of 0.31 %, and chlorine content of 0 after sintering at 1150 °C for 20 min. Chlorine in MG ceramsite enters into the tail gas or secondary fly ash in the form of chlorine salts and chlorine-containing gas when the sintering temperature is above 800 °C. The MG ceramsite prepared from MSWI fly ash meets the lightweight aggregate standard and are environmentally friendly. However, the disposal of tail gas and secondary fly ash needs attention when the MSWI fly ash is used as one of the main raw materials to prepare ceramsite.