Memory effect driven emissions of persistent organic pollutants from industrial thermal processes, their implications and management: a review

Unveiling the formation and influence mechanisms of PCDD/F memory effect in wet scrubbers: Fundamental research and field verification

The "memory effect" of polychlorinated dibenzo-p-dioxin and dibenzofurans (PCDD/Fs) in wet scrubber (WS) has become a frequent negative phenomenon in waste incineration field. This work focuses on studying the major influence factors and pathways of memory effect of PCDD/Fs in WS from the aspects of PCDD/F carriers and operating conditions. The PCDD/F contents of fillings used for over three years is 0.098 ng I-TEQ/g, which performs as a stable source of PCDD/Fs for thousands of hours with PCDD/F desorption rates ranged in 0.023-0.116 pg I-TEQ/g·h at 65 °C-93 °C. On the one hand, the filling layer has been the biggest PCDD/F storage part in WS (6845.1 μg). On the other hand, the generated yellow wrapping layer in long-term operation can limit the desorption of inner PCDD/Fs. The solubility of PCDD/Fs in scrubbing water (SW) performs a positive correlation with the content of suspended substances, and the increased temperature and pH value of SW both lead to a higher toxic concentration of PCDD/Fs dissolved from the fly ash to solutions. In addition, the built mass balance of PCDD/Fs around WS suggests the incomplete SW refreshing and sludge cleaning also contribute to the memory effect of PCDD/Fs through enhancing the liquid-phase PCDD/Fs in flue gas from SW. Based on this study, three suggestions are propounded on the operation of WS. The results of this study will provide essential evidence and guidelines for optimizing operation and inhibiting the PCDD/F memory effect in WS.

Mitigation of PBDE net discharge in hazardous waste thermal treatment system through reintroducion of sludge and fly ash into GASMILD operations

The presence of polybrominated diphenyl ethers (PBDEs) in consumer products, waste treatment processes, and treated ashes poses a significant environmental threat. Due to the lack of research on the removal of PBDEs during waste incineration, this study investigated the effectiveness of a Hazardous Waste Thermal Treatment System (HAWTTS) utilizing reburning of sludge and fly ash (SFA) with gasification-moderate or intense low-oxygen dilution (GASMILD) combustion for PBDE removal. The closed-loop treatment of sludge and ash within the HAWTTS provides a potential pathway for near-zero PBDE emissions. The GASMILD combustion addresses potential combustion issues associated with fly ash recirculation. The system achieved an impressive overall removal efficiency of 98.4% for PBDEs, with minimal stack emissions (2.45 ng/Nm³) and a negative net discharge rate (-1.02 μg/h). GASMILD combustion played a crucial role (92.7%-97.6% destruction) in addressing challenges associated with high-moisture feedstocks and SFA residues. Debromination of highly brominated PBDEs occurred within the incinerator, resulting in an increased proportion of lower brominated PBDEs in the bottom slag compared to the feedstock. Air Pollution Control Devices (APCDs) achieved a total PBDE removal efficiency of 74.4%. However, the hydrophobic nature of PBDEs limited removal efficiency in scrubbers (36.0%) and cyclonic demisters (37.86%). This study demonstrates that reintroducing SFA into the GASMILD combustion process offers an effective and environmentally sustainable strategy for reducing net PBDE levels in hazardous waste. This approach also provides additional benefits such as energy conservation, reduced carbon emissions, and lower operating costs associated with secondary treatment of thermally treated byproducts.

Eco-Friendly Inorganic Binders: A Key Alternative for Reducing Harmful Emissions in Molding and Core-Making Technologies

Many years of foundry practice and much more accurate analytical methods have shown that sands with organic binders, in addition to their many technological advantages, pose risks associated with the emission of many compounds, including harmful ones (e.g., formaldehyde, phenol, benzene, polycyclic aromatic hydrocarbons, and sulfur), arising during the pouring of liquid casting alloys into molds, their cooling, and knock-out. The aim of this research is to demonstrate the potential benefits of adopting inorganic binders in European iron foundries. This will improve the environmental and working conditions by introducing cleaner and more ecological production methods, while also ranking the tested binders studied in terms of their harmful content. The article pays special attention to the analysis of seven innovative inorganic binders and one organic binder, acting as a reference for emissions of gases from the BTEX (benzene, toluene, ethylbenzene, and xylenes) and PAHs (polycyclic aromatic hydrocarbons) groups and other compounds such as phenol, formaldehyde, and isocyanates (MDI and TDI) generated during the mold pouring process with liquid metals. The knowledge gained will, for the first time, enrich the database needed to update the Reference Document on The Best Available Techniques for the Smitheries and Foundries Industry (SF BREF).

Enhanced mitigation of inhalable particles and fine particle-bound PAHs from a novel hazardous waste-power plant candidate

Emissions of the inhalable particle (dp < 10 μm, PM10) and their harmful compositions from combustion sources have high potential on health risk with nearly no regulation. This study investigates the particle size distribution (PSD), as well as the removal mechanism of PM10 and fine particle (FP)-bound polycyclic aromatic hydrocarbons (PAHs) from the flue gas of a hazardous waste thermal treatment system. It has ultralow regulated emission and becomes a candidate of power generation module. A series of the advanced scrubbers, cyclonic demister, and baghouse was equipped for multi-pollutant control. The moderate or intense low oxygen dilution (MILD) combustion effectively inhibited the PM2.5 generation by volumetric oxidation. Advanced scrubbers removed PM1, PM2.5, and PM10 by 85.24, 68.68, and 97.60%, respectively, which achieved by local supersaturation, heterogeneous condensation of water vapor, and the growth of fine PM. Moreover, the scrubbers effectively scavenged the course PM10 containing the high-molecular-weight PAH homologs onto the water phase but promoted the condensation and absorption of the lighter homologs onto the fine particle surface (dp ∼5.3 μm). The size window (dp = 0.3-1.0 μm) of the minimum efficiency reporting value of a BH filtration led to the peak of FP-PAH mass and BaP equivalent (BaPeq) toxicity at dp = 0.1-0.4 and 0.1-0.8 μm, respectively. Consequently, the synergy of MILD combustion and the SCB-CYC-BH system effectively inhibited the PM2.5, PM10, PM2.5-PAHs, and FP-PAH levels from a waste thermal treatment process and further mitigated the potential health risk.

Assessment of PCDD/Fs formation and emission characteristics at a municipal solid waste incinerator for one year

Municipal solid waste incineration (MSWI) has become a predominant emission source of polychlorinated dibenzo-p-dioxins and furans (PCDD/Fs). Research focusing on the impact of operating conditions, environmental changes, and operating time on the generation and emissions of PCDD/Fs has not been resolved. To this end, this study tracked and investigated the PCDD/Fs and 17 congener emissions of a typical grate incinerator (800 t/d) continuously for one year. Results showed that the PCDD/Fs concentration at the boiler outlet, stack inlet, and bag filter, including normal and abnormal operation conditions, ranges from 2.11E-02-41.86 ng I-TEQ/Nm³, 7.00E-04-6.76 ng I-TEQ/Nm³, and 1.12-2.90E+03 ng I-TEQ/Nm³, respectively. The 2,3,4,7,8-P₅CDF has the highest contribution in all samples, in which a proportion of TEQ ranged from 30 % to 77.73 %. Moreover, by applying the correlation analysis between PCDD/Fs and operating parameters, the emission characteristic is mainly affected by incinerators and boilers during the normal period, and it is affected by the whole MSWI process under abnormal conditions. In addition, the PCDD/Fs emission from the MSWI plant gradually increases from spring to winter. This study is beneficial for supporting the control of PCDD/Fs emission reduction and assisting the operators to optimize the relevant operating parameters of the MSWI plant to achieve a stable and up-to-substandard emissions during the operation period.

Investigation on dioxins emission characteristic during complete maintenance operating period of municipal solid waste incineration

The dioxins (DXN) are a set of pollutants encompass polychlorinated dibenzo-p-dioxin/dibenzofuran (PCDD/F), their emissions from municipal waste incineration processes (MSWI) are normally detected under steady operating conditions. However, limited studies have focused on the PCDD/F emission characteristics under a complete maintenance operating period (CMOP), which includes shut-down, cooling, maintenance, heating, startup, and normal operations. In this article, the shutdown process (SDP) starts from the normal operation, followed by shutdown, and then cooling; while the startup process (SUP) commences from heating, followed by startup, and then normal operation. The detection and analysis were conducted at the SDP and SUP stages. The PCDD/F mass and total toxic equivalent quantity (TEQ) concentrations were measured in the flue gas and bag filter fly ash (BF-FA) during a CMOP of Beijing MSWI plant. The highest PCDD/F concentrations in the flue gas were found in the "cooling" and "startup" phases; in the FA, this condition occurred in the "startup" phase. Further, the results show that the most heightened concentrations were observed for 5-6 chlorinated PCDF and 4-5 chlorinated PCDD among the 17 PCDD/F congeners in most cases. More importantly, the air pollution control devices (APCDs) which include activated carbon, lime, and BF, have high removal efficiency for PCDD/F (especially PCDD) during the "startup" phase. APCDs also easily release a considerable amount of PCDD/F because of the memory effect, which emits more PCDD/F at the "shutdown" phase than at the "startup" one. Besides, the annual PCDD/F emission in the flue gas of the MSWI plant was estimated to be 67.72 mg I-TEQ, of which the emission accounts for approx. 20% during the CMOP. Moreover, the experiment shows that the PCDD/F emissions of the MSWI plant in Beijing under unsteady conditions are more miniature than those reported earlier in other areas.

Formation pathways, gas-solid partitioning, and reaction kinetics of PCDD/Fs associated with baghouse filters operated at high temperatures: A case study

The application of the 3T method during combustion (i.e., a Temperature > 850 °C, a residence Time > 2 s, and sufficient Turbulence) can lead to elevated operating temperature in the baghouse filter for the municipal solid waste incineration (MSWI) systems without sufficient heat exchange capacity, which is potentially detrimental to the emission control of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs). Herein, a field study focusing on the distribution and variation of PCDD/Fs in gaseous and solid phases in a baghouse filter with high operating temperature (225-230 °C) was carried out. The concentration of PCDD/Fs in gases at the outlet of the baghouse filter was around 1 order of magnitude higher than that in inlet gases (i.e., noticeable memory effect of PCDD/Fs), because of the significant PCDD/Fs formation in filter fly ash (primarily contributed by the precursor pathway) followed by PCDD/Fs desorption. In addition, the mechanisms and factors resulting in the memory effect of PCDD/Fs were identified based on a laboratory study that carefully investigated the formation and desorption of PCDD/Fs at potential operating temperatures of baghouse filters (i.e., 180, 200, and 225 °C). The temperature was identified as the key factor inducing the memory effect of PCDD/Fs, because: i) PCDD/Fs memory effect was not observed for baghouse filters with low operating temperatures of ~150 °C in previous studies; ii) both the formation and desorption of PCDD/Fs were noticeably favored by rising temperature from 180 to 225 °C; iii) increasing temperature appeared to facilitate the transformation from inorganic Cl to organic Cl and the conversion from aliphatic carbon to aromatic carbon or unsaturated hydrocarbons, both of which were favorable to PCDD/Fs formation; and iv) the release rate of PCDD/Fs from fly ash was exponentially dependent on temperature based on the modeling results of reaction kinetics.

PCDD/Fs from a large-scale municipal solid waste incinerator under transient operations: Insight formation pathways and optimal reduction strategies

Polychlorinated dibenzo-p-dioxin and dibenzofurans (PCDD/Fs) emissions from the transient operation of municipal solid waste incinerators can reach up to 690 ng/Nm³, as measured in this study. To control the extreme emissions to meet the national standard, the formation pathways of PCDD/F were investigated under transient operations (cold start-up, hot start-up, and after start-up) and normal operations. Compared with normal operations, transient operations facilitate the formation of low-chlorinated congeners rather than highly chlorinated congeners. Statistically, for transient operations, strong correlations were found among tetrachlorodibenzo-p-dioxin or tetrachlorodibenzofuran isomers. An abundant carbon matrix is an important carbon source for PCDD formation. Moreover, the comprehensive study revealed that the oxidation of deposited soot is the main source of PCDD/F emissions, relative to de novo synthesis, chlorobenzene-route synthesis, chlorophenol-route synthesis, and chlorination of dibenzo-p-dioxin/dibenzofuran. In addition, the optimal start-up procedure was constructed by analyzing main formation pathways and operating conditions. The relationship between the international toxic quantity (I-TEQ) values (C_I-TEQ) and the reaction time can be assigned as C_I-TEQ = 11.72t^-0.65 (R² = 0.97) for the circulating fluidized bed. The relationship of C_I-TEQ = 4.61t^-0.59 (R² = 0.85) was also proven on the dataset with a grate furnace. Then, the optimal feeding rate of activated carbon was further proposed by the relationship between the reaction time and I-TEQ, and the semi-empirical equation for PCDD/Fs adsorption. Finally, the PCDD/Fs emissions can be reduced to 0.1 ng I-TEQ/Nm³ under transient operations according to the time since start-up.

Fate of dioxins in a municipal solid waste incinerator with state-of-the-art air pollution control devices in China

The variation of municipal solid waste (MSW) components and the improvement of incinerators have an obvious effect on dioxin emissions. However, there is a knowledge gap on dioxin distribution characteristics following China's implementation of MSW classification. To reveal the fate of dioxins under ultra-low emission standards in leading cities in China, a systematic investigation was carried out in a typical modern MSW incinerator in Shenzhen, China. The dioxin mass balance was built using improved models, which included expanded samples, e.g., the leachate, the raw gas and raw ash from boiler, and the chemicals and residuals from air pollution control devices (APCDs). The results indicated a positive dioxin balance of 0.88 μg I-TEQ/t MSW according to the conventional method containing fly ash, bottom ash, and stack gas. In the new model revealing dioxin characteristics after APCDs, a higher value of 0.89 μg I-TEQ/t MSW was found due to the leachate, slaked lime, and activated carbon-containing dioxins. The distribution of dioxins in output samples of fly ash, bottom ash, stack gas, and leachate were 149.0 %, 41.8 %, 1.6 %, and 0.6 % of MSW, respectively. For incineration itself, the balance was 0.85 μg I-TEQ/t MSW, which indicated the possible release owing to the "memory effect" for the other two methods. This study provided new insight for the accurate estimation of dioxin emissions and a typical case report of MSW incineration with ultra-low dioxin emissions.

Process tracing of PCDD/Fs from economizer to APCDs during solid waste incineration: Re-formation and transformation mechanisms

The emission of PCDD/Fs is a crucial factor for the aggravation of the Not-In-My-Back-Yard (NIMBY) syndrome, especially for the incineration plants that fail to meet the emission standard. It is well known that physicochemical processes in the boiler can notably affect the discharge of dioxins, especially under transient, non-steady conditions. However, few studies paid attention to the important operational parameters that influence PCDD/Fs formation and transformation in the boiler when an incinerator is in its daily steady operation. In this study, 36 samples were analyzed to achieve process tracing of PCDD/Fs. The concentration, congener profile and vapor/solid partitions of PCDD/Fs from the economizer to air pollution control devices (APCDs) under two typical steady conditions were investigated. Results indicated that increasing air supply aggravated the formation of PCDD/Fs, disturbed the vapor/solid partitions, and triggered a substandard emission. Quantitative structure-activity relationship (QSAR) modeling was firstly performed for the formation mechanism and orbital energy factors were identified as dominating factors. Besides, the removal rates of PCDD/Fs significantly correlated with the saturated vapor pressure and proportions of different isomers. This study is beneficial for operators to optimize relevant operational parameters of the incineration plants so as to get rid of substandard problems.

Partitioning and removal behaviors of PCDD/Fs, PCBs and PCNs in a modern municipal solid waste incineration system

An extensive evaluation on a modern full-scale municipal solid waste incineration system was conducted for characterizing the distribution of highly toxic chlorinated aromatics, polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs), polychlorinated biphenyls (PCBs) and polychlorinated naphthalenes (PCNs), and their corresponding mass fluxes in post combustion zone. It was found that the flue gas/fly ash partitioning behaviors of chlorinated aromatics could be essentially described by their octanol-air partition coefficients (K_OA) and strongly affected by the flue gas temperature. Above 93% of chlorinated aromatics formed in boiler section was partitioned into the flue gas and transported into the subsequent flue gas cleaning system, in which above 92% of Cl_3-8DDs, Cl_3-7DFs, Cl_5-10Bs and Cl_4-8Ns in flue gas was removed by the discharge of fly ash. The results of mass flux calculation indicated that the memory effect in flue gas cleaning system remarkably elevated the emission levels of chlorinated aromatics, especially the less chlorinated ones. The memory effect should mainly result from the direct chlorination mechanism mediated by the deposited particles. In addition, activated carbon injection could cause an obvious increase in PCBs emission. The obtained results provided some important implications for further reducing the emission of highly toxic chlorinated aromatics.

Review of the current state and main sources of dioxins around the world

Polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) are a group of dangerous compounds, emitted mostly from anthropogenic sources, that have negative effects on human health. Therefore, it is interesting to analyze the emission patterns of dioxins proceeding from different sources around the world, to observe the actual trend of the transmission of dioxins and furans into the atmosphere.For that reason, the main objective of the present document is to provide a general assessment about the dioxin problematic, analyzing the main parameters that influence the ambient concentration of dioxins worldwide, and describing the most characteristic features of the fingerprint from different sources, while making emphasis in the importance that non-industrial sources are gaining over the last years in front of the decreasing tendency of industrial sources. The description of the most important abatement technologies for dioxins is also included in this review.

IMPLICATIONS

Given the negative effects of dioxins in human health, it is important to depict and locate the main sources of these dangerous compounds. Emissions proceeding from industrial facilities have decreased over the last years; however, other zones where nonindustrial sources used to be relevant contributors do not show the same decreasing tendency because it is more difficult to control this type of emissions. For that reason, future studies should focus on measuring and regulating this highly uncontrolled source of dioxins.