A Review on Technologies of Removal of Dioxins and Furans from Incinerator Flue Gas

Regulatory compliance of PCDD/F emissions by a municipal solid waste incinerator. A case study in Sant Adrià de Besòs, Catalonia, Spain

Despite incineration is an important emission source of toxic pollutants, such as heavy metals and polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs), it is still one of the most widely used methods for the management of municipal solid waste. The current paper summarizes the results of a 20-year follow-up study of the emissions of PCDD/Fs by a municipal solid waste incinerator (MSWI) in Sant Adrià de Besòs (Catalonia, Spain). Samples of ambient air, soils and herbage were periodically collected near the facility and the content of PCDD/Fs was analyzed. In the last (2017) survey, mean levels in soil were 3.60 ng WHO-TEQ/kg (range: 0.40-10.6), being considerably higher than the mean concentrations of PCDD/Fs in soil samples collected near other MSWIs in Catalonia. Moreover, air PCDD/F concentrations were even higher than those found in a previous (2014) survey, as they increased from 0.026 to 0.044 pg WHO-TEQ/m3. Ultimately, the PCDD/F exposure would be associated to a cancer risk (2.5 × 10-6) for the population living in the surrounding area. Globally, this information indicates that the MSWI of Sant Adrià de Besòs could have had a negative impact on the environment and potentially on public health, being an example of a possible inappropriate management for years. The application of Best Available Techniques to minimize the emission of PCDD/Fs and other chemicals is critical.

Grouping strategies for assessing and managing persistent and mobile substances

Background

Persistent, mobile and toxic (PMT), or very persistent and very mobile (vPvM) substances are a wide class of chemicals that are recalcitrant to degradation, easily transported, and potentially harmful to humans and the environment. Due to their persistence and mobility, these substances are often widespread in the environment once emitted, particularly in water resources, causing increased challenges during water treatment processes. Some PMT/vPvM substances such as GenX and perfluorobutane sulfonic acid have been identified as substances of very high concern (SVHCs) under the European Registration, Evaluation, Authorisation and Restriction of Chemicals (REACH) regulation. With hundreds to thousands of potential PMT/vPvM substances yet to be assessed and managed, effective and efficient approaches that avoid a case-by-case assessment and prevent regrettable substitution are necessary to achieve the European Union's zero-pollution goal for a non-toxic environment by 2050.

Main

Substance grouping has helped global regulation of some highly hazardous chemicals, e.g., through the Montreal Protocol and the Stockholm Convention. This article explores the potential of grouping strategies for identifying, assessing and managing PMT/vPvM substances. The aim is to facilitate early identification of lesser-known or new substances that potentially meet PMT/vPvM criteria, prompt additional testing, avoid regrettable use or substitution, and integrate into existing risk management strategies. Thus, this article provides an overview of PMT/vPvM substances and reviews the definition of PMT/vPvM criteria and various lists of PMT/vPvM substances available. It covers the current definition of groups, compares the use of substance grouping for hazard assessment and regulation, and discusses the advantages and disadvantages of grouping substances for regulation. The article then explores strategies for grouping PMT/vPvM substances, including read-across, structural similarity and commonly retained moieties, as well as the potential application of these strategies using cheminformatics to predict P, M and T properties for selected examples.

Conclusions

Effective substance grouping can accelerate the assessment and management of PMT/vPvM substances, especially for substances that lack information. Advances to read-across methods and cheminformatics tools are needed to support efficient and effective chemical management, preventing broad entry of hazardous chemicals into the global market and favouring safer and more sustainable alternatives.

Enzymatic polyethylene biorecycling: Confronting challenges and shaping the future

Polyethylene (PE) is a widely used plastic known for its resistance to biodegradation, posing a significant environmental challenge. Recent advances have shed light on microorganisms and insects capable of breaking down PE and identified potential PE-degrading enzymes (PEases), hinting at the possibility of PE biorecycling. Research on enzymatic PE degradation is still in its early stages, especially compared to the progress made with polyethylene terephthalate (PET). While PET hydrolases have been extensively studied and engineered for improved performance, even the products of PEases remain mostly undefined. This Perspective analyzes the current state of enzymatic PE degradation research, highlighting obstacles in the search for bona fide PEases and suggesting areas for future exploration. A critical challenge impeding progress in this field stems from the inert nature of the C-C and C-H bonds of PE. Furthermore, breaking down PE into small molecules using only one monofunctional enzyme is theoretically impossible. Overcoming these obstacles requires identifying enzymatic pathways, which can be facilitated using emerging technologies like omics, structure-based design, and computer-assisted engineering of enzymes. Understanding the mechanisms underlying PE enzymatic biodegradation is crucial for research progress and for identifying potential solutions to the global plastic pollution crisis.

High Performance of Commercial PAC on the Simultaneous Desulfurization and Denitrification of Wastewater From a Coal-Fired Heating Plant

The flue gas desulfurization wastewater is highly saline and has too many refractory pollutants to be recycled during the desulfurization process of the coal-fired heating plant. Given that waste heat is abundant in coal-fired heating plants, a thermal treatment method was developed to simultaneously remove sulfates and nitrates from the wastewater, with the production of chemical-grade natroalunite and recycled water. The results showed that sulfates and nitrates were 50.3 and 10 g/L in the wastewater, respectively, and only 2.8% and 9.1% were removed after direct treatment at 270°C for 7 h; but these rates increased to 99.3% and 99.9%, respectively, with the addition of commercial poly aluminum chloride. Mass balance summarized that the treatment of 1 ton wastewater needed 0.1 ton PAC and produced 0.11 ton natroalunite and 0.92 ton recycle water. The removal of sulfates and nitrates was mainly done by the precipitation reaction of sulfates such as natroalunite and the redox reaction between nitrates and organics, respectively. Thermodynamic analysis demonstrated that the precipitate reaction occurred at 45°C and accelerated in the temperature range of 45–270°C, but became slow with the decrease of sulfate and Al concentrations in wastewater. Four other reagents were also used for wastewater treatment in comparison with PAC and showed the following order of performance: PAC > citrate calcium > limestone > subacetate aluminum > citrate ferric. This method provided a practical route to treat wastewater from flue gas desulfurization without generating secondary waste.

Dioxin Formation in Biomass Gasification: A Review

The amount of PCDD/F emissions produced by gasification operations is often within standard limits set by national and international laws (<0.1 ng TEQ/Nm3). However, a recent assessment of the literature indicates that gasification cannot always reduce PCDD/Fs emissions to acceptable levels, and thus a common belief on the replacement of incineration with gasification in order to reduce PCDD/Fs emissions seems overly simplistic. A review that summarizes the evidence on when gasification would likely result in environmentally benign emissions with PCDD/F below legal limits, and when not, would be of scientific and practical interest. Moreover, there are no reviews on dioxin formation in gasification. This review discusses the available data on the levels of dioxins formed by gasifying different waste streams, such as municipal solid wastes, plastics, wood waste, animal manure, and sewage sludge, from the existing experimental work. The PCDD/Fs formation in gasification and the operational parameters that can be controlled during the process to minimize PCDD/Fs formation are reviewed.

Suppressing the formation of chlorinated aromatics by inhibitor sodium thiocyanate in solid waste incineration process

Suppressing the formation of chlorinated aromatics (Cl-aromatics) by chemical inhibitors is an important measure to reduce dioxin emission from the solid waste incineration plants. In this study, we first investigated the reduction effect of a novel inhibitor sodium thiocyanate (NaSCN) on the emission of dioxins in 2 full-scale solid waste incineration systems. Injection of NaSCN solution into the higher temperature flue gas resulted in about 60% reduction in the concentration of total tetra- to octa-chlorinated dibenzo-p-dioxins and dibenzofurans in stack flue gas. The suppression effect was further verified by a laboratory study on the chlorination of naphthalene over model fly ashes with or without NaSCN addition. By characterizing the reaction products between NaSCN and key catalysts Cu and Fe chlorides, two main suppression mechanisms were proposed: (i) reduction of highly active cupric chloride (CuCl₂) and ferric chloride (FeCl₃) to less active cuprous chloride (CuCl) and ferrous chloride (FeCl₂), (ii) sulfidation of Cu chlorides. The laboratory study indicated that the unreacted NaSCN in the combustion flue gas could be mainly decomposed into Na₂S, C₃N₄, Na₂S₂O₃, NaS₂, Na₂SO₄, CO₂, SO_2, NO₂ and COS. These decomposition products are low toxic or can be effectively removed by the air pollution control devices. CAPSULE: NaSCN suppressed the formation of chlorinated aromatics in combustion flue gas mainly through inducing the reduction of highly active Cu (II) and Fe (III) chlorides.

Hybrid Selection Method of Feature Variables and Prediction Modeling for Municipal Solid Waste Incinerator Temperature

It is difficult to establish an accurate mechanism model for prediction incinerator temperatures due to the comprehensive complexity of the municipal solid waste (MSW) incineration process. In this paper, feature variables of incineration temperature are selected by combining with mutual information (MI), genetic algorithms (GAs) and stochastic configuration networks (SCNs), and the SCN-based incinerator temperature model is obtained simultaneously. Firstly, filter feature selection is realized by calculating the MI value between each feature variable and the incinerator temperature from historical data. Secondly, the fitness function of GAs is defined by the root mean square error of the incinerator temperature obtained by training SCNs, and features obtained by MI methods are searched iteratively to complete the wrapper feature selection, where the SCN-based incinerator temperature prediction model is obtained. Finally, the proposed model is verified by MSW incinerator temperature historical data. The results show that the SCN-based prediction model using the hybrid selection method can better predict the change trend of incinerator temperature, which proves that the SCNs has great development potential in the field of prediction modeling.

Effect of urea on chlorinated aromatics formation mediated by copper and iron species in combustion flue gas

Urea ((NH₂)₂CO) is widely applied to the reduction of NO_X in modern full-scale solid waste incineration systems, but there is a lack of knowledge about how urea affects the formation and emission of Cl-aromatics. In this study, we investigated the formation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) and polychlorinated polychlorinated naphthalenes (PCNs) via electrophilic chlorination and precursor pathway mediated by model fly ashes containing Cu and Fe species with or without urea addition. The results indicated that the addition of urea promoted the direct chlorination of parent aromatics over Cu (Ⅱ) chlorides and the coupling reaction of chlorophenols over Fe species, while suppressed the catalytic chlorination of parent aromatics over Fe (Ⅲ) chlorides and the coupling reaction of chlorophenols over Cu species. The diverse effects should be mainly attributed to the formation of complex salts containing NH₃ and NH₄⁺. The formation of complex salts of Fe chlorides and NH₄Cl could hinder the oxidization of Fe chlorides, and thus maintain the high activity of Fe species for catalyzing the coupling reaction of chlorophenols. The formation of complex salts of Cu (Ⅱ) chloride and NH₃ could prevent the chemical sorption of phenoxyl groups, and thus suppress the coupling reaction of chlorophenols. NH₃ released from the thermal decomposition of urea could not only react with Cl₂ to suppress the catalytic chlorination of aromatics, but also neutralize HCl to accelerate the direct chlorination of aromatics. In general, urea should act as inhibitor for suppressing the formation of Cl-aromatics in solid waste incineration systems.

Methodological Approaches to Optimising Anaerobic Digestion of Water Hyacinth for Energy Efficiency in South Africa

Anaerobic digestion has been identified as a feasible fragment of a bioeconomy, yet numerous factors hinder the adoption of the technology in South Africa. Apart from its energy recovery, other nonmarket advantages support the technology. Though it may be challenging to have a price tag, they provide clear added worth for such investments. With a growing energy demand and global energy transitions, there is a need to sustainably commercialise the biogas industry in South Africa. Most studies are at laboratory scale and under specific conditions, which invariably create gaps in using their data for commercialising the biogas technology. The key to recognising these gaps depends on knowing the crucial technical phases that have the utmost outcome on the economics of biogas production. This study is a meta-analysis of the optimisation of anaerobic digestion through methodological approaches aimed at enhancing the production of biogas. This review, therefore, argues that regulating the fundamental operational parameters, understanding the microbial community’s interactions, and modelling the anaerobic processes are vital indicators for improving the process stability and methane yield for the commercialisation of the technology. It further argues that South Africa can exploit water hyacinth as a substrate for a self-sufficient biogas production system in a bid to mitigate the invasive alien plants.

Mismanagement of Plastic Waste through Open Burning with Emphasis on the Global South: A Systematic Review of Risks to Occupational and Public Health

Large quantities of mismanaged plastic waste threaten the health and wellbeing of billions worldwide, particularly in low- and middle-income countries where waste management capacity is being outstripped by increasing levels of consumption and plastic waste generation. One of the main self-management strategies adopted by 2 billion people who have no waste collection service, is to burn their discarded plastic in open, uncontrolled fires. While this strategy provides many benefits, including mass and volume reduction, it is a form of plastic pollution that results in the release of chemical substances and particles that may pose serious risks to public health and the environment. We followed adapted PRISMA guidelines to select and review 20 publications that provide evidence on potential harm to human health from open burning plastic waste, arranging evidence into eight groups of substance emissions: brominated flame retardants; phthalates; potentially toxic elements; dioxins and related compounds; bisphenol A; particulate matter; and polycyclic aromatic hydrocarbons. We semiquantitatively assessed 18 hazard-pathway-receptor combination scenarios to provide an indication of the relative harm of these emissions so that they could be ranked, compared and considered in future research agenda. This assessment overwhelmingly indicated a high risk of harm to waste pickers, a large group of 11 million informal entrepreneurs who work closely with waste, delivering a circular economy but often without protective equipment or a structured, safe system of work. Though the risk to human health from open burning emissions is high, this remains a substantially under-researched topic.

Wooden Activated Carbon Production for Dioxin Removal via a Two-Step Process of Carbonization Coupled with Steam Activation from Biomass Wastes

A two-step process of carbonization coupled with steam activation was proposed for wooden activated carbon production from four kinds of biomass waste materials. The TG-FTIR results show that the carbonization process started at around 250 °C and finished at 500 °C for the coconut shell, pinewood, and plywood. The carbonization temperature of corn straw was lower than those of the other three samples, which was attributed to the higher concentration of ash content. FTIR results for the volatile compounds during carbonization show that CH₄, CO, CO₂, and hydrocarbons are the main detected gaseous species. The CH₄ and C _m H _n yields of pinewood and plywood are higher than those of the coconut shell and corn straw. The carbonization results on the tubular furnace reactor show that furfural and phenol and its derivatives are the main tar compounds in waste carbonization. Carbonization experiments show that a temperature of 500 °C and residence time of 30 min are the optimized parameters for the three biomass wastes. The char yields are 26.4, 25.73, and 30.38% for pinewood, plywood, and coconut shell, respectively. CFD modeling has proven that using 20% of the volatiles could achieve lowest pollution and provide heat for carbonization of biomass waste. The steam activation results show that an activation temperature of 800 °C and activation time of 30 min are suitable for all three biomass samples, which could obtain optimized AC yields and adsorption quality for dioxin.

Cocaine degradation using a rotating biological disc reactor: Techno-economic and environmental analysis using experimental data

A bacterial mixed culture that utilizes cocaine as the sole carbon and energy sources was isolated and used in a Rotary Disc Reactor as an alternative method for the final disposal of seized cocaine. This study aimed to compare the performances of cocaine incineration (oven) and biodegradation (Rotary Disc Reactor), considering economic and environmental aspects. There was a 99.4% cocaine removal efficiency when bacterial C1T consortium was grown in a Rotary Disc Reactor for 42 h. The economic analysis allowed determining the high potential of the biotechnological cocaine degradation to be evaluated at higher scales. Indeed, the unit disposition price of the biotechnological degradation pathway was 58% higher than the calculated value for the incineration process considering an initial cocaine concentration of 30 g/L. Moreover, the economic sensitivity analysis demonstrated a price reduction of 20% in the unit disposition price of the biotechnological degradation using a rotary disc reactor. Further, cocaine degradation using a rotary disc reactor system presented a better environmental performance than the incineration process considering atmospheric and toxicological impact categories because of the low release of hazardous materials to the atmosphere.

Online measurement of 1,2,4-trichlorobenzene as dioxin indicator on multi-walled carbon nanotubes

Polychlorinated dibenzo-p-dioxin and dibenzofuran (PCDD/F) emission is one of main concerns for the secondary pollution of municipal solid waste incinerators (MSWI). For timely response to emission, 1,2,4-trichlorobenzene (1,2,4-TrClBz) as dioxin indicator can be monitored via online measurement techniques. In this study, multi-walled carbon nanotubes (MWCNTs) were investigated for their suitability as a 1,2,4-TrClBz sorbent for MSWI stack gas analysis. The tests include, batch adsorption, continuous adsorption-desorption of 1,2,4-TrClBz via thermal desorption coupled with gas chromatography (TD-GC-ECD), temperature and concentration stability of MWCNTs, and adsorption performance of the system. Thermogravimetric/derivative thermogravimetric (TGA/DTG) analysis reveals that MWCNTs has higher capacity in terms of weight loss (14.34%) to adsorb 1,2,4-TrClBz compared to Tenax TA (9.46%) and also shows fast desorption of adsorbate at temperature of 87 °C compared to Tenax TA (130 °C). Interestingly, carbon nanotubes and Tenax TA gave almost similar adsorption-desorption response, and from TD-GC-ECD analysis it was found that with increasing mass flow of 1,2,4-TrClBz (7.42 × 10^-6 - 44.52 × 10^-6 mg ml^-1) through sorbent traps, average peak areas increased from 2.86 ± 0.02 to 13.54 ± 0.26 for MWCNTs and 2.89 ± 0.02 to 13.38 ± 0.12 for Tenax TA, respectively. The stability of MWCNTs for temperature was 400 °C and for concentration of 1,2,4-TrClBz was 50 ppbv. However, regeneration of sorbent at 100 ppbv (1,2,4-TrClBz) was not possible. TD-GC-ECD system showed high adsorption performance with 3.86% and 3.59% relative standard deviation at 250 °C and 300 °C, respectively. Further Fourier Transform Infrared Spectroscopy (FTIR) analysis confirmed that adsorbate can be fully desorbed at 300 °C.

Effect of different air pollution control devices on the gas/solid-phase distribution of PCDD/F in a full-scale municipal solid waste incinerator

The emission of polychlorinated dibenzo-p-dioxins and -furans (PCDD/F) from full-scale municipal solid waste incinerators (MSWI) is harmful to human and environmental health. This study analyzes the effect of different units of an air pollution control devices (APCDs), i.e. the semi-dry scrubber, fabric filter (FF), selective catalytic reduction (SCR), and wet scrubber (WS), on the removal characteristics and gas- and solid-phase distributions of PCDD/F in MSWI flue gas. APCDs reduce PCDD/F concentrations from 24.9 ng Nm^-3 to 0.979 ng Nm^-3 (2.16 ng I-TEQ Nm^-3 to 0.0607 ng I-TEQ Nm^-3), with a total removal efficiency (RE) of 96.1% (97.2% I-TEQ). Specifically, APCDs remove more than 95% of both gas- and solid-phase PCDD/F. The FF coupled with active carbon injection (FF + ACI) substantially reduces both gas- and solid-phase PCDD/F concentrations with an RE of 97.2% (98.7% I-TEQ). Additionally, FF + ACI exhibits a better RE of PCDF (98.9%) than PCDD (94.6%) and leads to PCDD congeners dominating the gas-phase. Both desorption and destruction of PCDD/F occur in the SCR, which favors removal of gas-phase PCDD/F but increases solid-phase PCDD/F. Therefore, SCR only decreases PCDD/F with a low RE of 27.6% (16.9% I-TEQ). However, SCR reduces NO_x with a high RE of 82.3%, which could inhibit the RE of PCDD/F because of their different reaction mechanisms. WS increases PCDD/F in both the gas and solid-phase by 1.95 times (2.57 times for I-TEQ) due to the memory effect, which typically increases the total mass concentration of PCDD/F and the proportions of lower-chlorinated gas-phase PCDD/F. Migration of gas- and solid-phase PCDD/F are also analyzed according to temperature. The results of this study can contribute to the optimized design of industrial APCDs for controlling PCDD/F emissions from MSWI.

Relative Environmental, Economic, and Energy Performance Indicators of Fuel Compositions with Biomass

The present study deals with the experimental research findings for the characteristics of ignition (ignition delay times, minimum ignition temperature) and combustion (maximum combustion temperature, concentration of anthropogenic emission), as well as theoretical calculations of integral environmental, economic, and energy performance indicators of fuel compositions based on coal processing waste with the most typical types of biomass (sawdust, leaves, straw, oil-containing waste, and rapeseed oil). Based on the results of the experiments, involving the co-combustion of biomass (10% mass) with coal processing waste (90% mass) as part of slurry fuels, we establish differences in the concentrations of NOx and SOx in the gaseous combustion products. They make up from 36 to 218 ppm when analyzing the flue gases of coal and fuel slurries. Additionally, the values of relative environmental, economic, and energy performance indicators were calculated for a group of biomass-containing fuel compositions. The calculation results for equal weight coefficients are presented. It was shown that the efficiency of slurry fuels with biomass is 10%–24% better than that of coal and 2%–8% better than that of filter-cake without additives. Much lower anthropogenic emissions (NOx by 25%–62% and SOx by 61%–88%) are confirmed when solid fossil fuels are partly or completely replaced with slurry fuels.

Co-combustion of coal processing waste, oil refining waste and municipal solid waste: Mechanism, characteristics, emissions

This experimental research studies co-combustion of wet coal processing waste (filter cakes) with typical municipal solid waste (wood, rubber, plastic, cardboard) and used turbine oil, as combustible components of composite liquid fuel. Ignition mechanisms and characteristics of single droplets of three fuel composition groups have been investigated in a motionless heated air with using a high-speed video recording system. Analyzing video frames, a physical model of the process under study was developed. The values of the guaranteed ignition delay times have been determined for three fuel groups with different compositions at the ambient temperature 600-1,000 °C. The minimum values of ignition delay times are about 3 s, the maximum ones are about 25 s. In addition to the established difference in the ignition delay times, the various fuel compositions also differ in combustion temperatures. Maximum values reaching 1,300 °C for compositions with 10% of used oil. It has also been determined that fuels with municipal solid waste are notable for lower nitrogen and sulfur oxide concentrations in flue gases as compared to filter cakes in initial state. Adding used oil to such fuel compositions increases the anthropogenic emissions but these worsening environmental characteristics do not exceed the regulated allowable limits of pollutants for solid fossil fuel combustion by thermal power plants. The obtained results are the backbone for the development of an environmentally friendly, cost- and energy-efficient co-combustion technology for municipal solid waste recovery by burning it as part of composite fuels, e.g., in boiler furnaces instead of coal.

Bio-fixation of flue gas from thermal power plants with algal biomass: Overview and research perspectives

Rate of energy production is reflecting growth of nations and most of energy produced from the coal and natural gas-based thermal power plants (TPPs). Flue gas (point sources of emission) are main exhaustible form of gases that come from thermal power plants and are continuously promoting climate change and various environmental problems in global scenario. The present available technologies of flue gas treatment are energy and cost-intensive process. Among the available techniques for fixation of flue-gases at sustainable part, microalgal bio-fixation of flue gas is an alternative promising and competent technology with assurance of eco-friendly path of low energy and low-cost solution for pollution abetment with production of value added products. According to mechanism involves during photosynthetic process of microalgae, it utilizes atmospheric CO₂ and CO₂ from flue gases for their growth. Past, present and future treatment technologies for flue gas with their challenges are discussed. Recent experimental studies and commercially available bioreactors are very particular for bio-fixation of flue gas from thermal power plants are also reviewed with their future perspectives. The commercial viability of process with specific microalgal strains and utilized biomass for further value-added products are suggested with future limitations.

Atline measurement of 1,2,4-trichlorobenzene for polychlorinated dibenzo-p-dioxin and dibenzofuran International Toxic Equivalent Quantity prediction in the stack gas

A home-made analytical instrument based on thermal desorption gas chromatography coupled to resonance enhanced multiphoton ionization time-of-flight mass spectrometry (TD-GC-REMPI-TOFMS) was applied for atline measurement of 1,2,4-trichlorobenzene for the prediction of polychlorinated dibenzodioxin and dibenzofuran (PCDD/F) concentrations in the stack gas of a municipal solid waste incinerator (400 ton/day). Conventional high resolution gas chromatography/high resolution mass spectroscopy (HRGC/HRMS) measurements for the determination of PCDD/F concentrations were performed to compare with TD-GC-REMPI-TOFMS measurements. 1,2,4-Trichlorobenzene correlated with I-TEQ at r = 0.867, 0.953 and 0.944 in unstable, stable and integrated conditions. The correlation was independent of the facility operating conditions observed in this study. Using a linear model to predict I-TEQ by 1,2,4-trichlorobenzene over the test, the average of the relative difference between predicted and measured I-TEQ was 18.9%. 1,2,4-Trichlorobenzene measured by TD-GC-REMPI-TOFMS can be used as a robust indicator of I-TEQ in stack gas.

Experimental study on dioxin formation in an MSW gasification-combustion process: An attempt for the simultaneous control of dioxins and nitrogen oxides

A gasification-combustion conversion process has been proposed for MSW disposal, and the positive effect of "homogeneous conversion" and "staged combustion" on the stabilization of Cl atoms and the rupture of CCl has been proved previously in fundamental experiments. To verify and evaluate their inhibition effect on dioxin generation, a bench-scale experimental system of the proposed process was established in the present work and an experimental study on the formation of dioxins in the process was carried out. Since both dioxins and nitrogen oxides should be strictly controlled while applied in industry, the simultaneous control of nitrogen oxides was also considered. Results indicated that "homogeneous conversion" has a clear inhibition effect on dioxins in both the syngas from gasification and the flue gas from combustion, which is a very important process in the stabilization of Cl atoms and the control of dioxins. During the "staged combustion" of the syngas, the increase of SR₁, the rise of temperature and the extension of residence time all have a clear inhibition effect on dioxins in the flue gas. The extension of residence time seems to be more efficient, and while increasing SR₁ and temperature, the regeneration of active chlorine species and the increase of NO need to be concerned. By the combination of "homogeneous conversion" with a temperature of 700 °C and "staged combustion" with a SR₁/SR₂ ratio of 0.7/0.4, a temperature of 900 °C, a residence time of 241 ms, a satisfactory simultaneous control of both dioxins and nitrogen oxides was obtained in the experiments of the present work.

Inhibition of CCl formation during the combustion of MSW gasification syngas: An experimental study on the synergism and competition between oxidation and chlorination

For the safe disposal of MSW, a four-step gasification-combustion conversion process is proposed in this work, consisting of material gasification, ash melting, syngas conversion and combustion. Based on the control method of dioxin in gasification process which has been studied previously, experiments of tar chlorination process under oxidative atmospheres were carried out in a homogeneous flow reaction system, using benzene as the tar model compound, to find a way for the inhibition of CCl formation during the syngas combustion process. Results indicated that Cl₂ reacts with benzene more easily than O₂ under low temperatures, and has a positive effect on both oxidative cracking and polymerization. For chlorination reactions, high temperature enhances the chlorination degree and leads to the formation of perchlorinated hydrocarbons, but also promotes the rupture of the weak CCl bonds. With the rise of temperature, hexachlorobenzene became the major product, the amounts of all chlorinated hydrocarbons decreased rapidly, and the conversion direction depended on the amount of O₂. O₂ generally promotes the formation of hydrogen chloride, and inhibits the chlorination of hydrocarbons. At a temperature above 900 °C, the total amount of chlorinated hydrocarbons was very low under oxidative atmospheres, even only with a equivalence ratio of 0.2. However, during the oxidation process under low temperatures, CCl can also be formed on cyclopentadienyl and 1,3-butadienyl radicals, whose chlorination products were observed. The synergistic and competitive effects between oxidation and chlorination are concluded and the major benzene conversion pathways are summarized according to the products detected.

Economic evaluation of an electrochemical process for the recovery of metals from electronic waste

As the market of electronic devices continues to evolve, the waste stream generated from antiquated technology is increasingly view as an alternative to substitute primary sources of critical a value metals. Nevertheless, the sustainable recovery of materials can only be achieved by environmentally friendly processes that are economically competitive with the extraction from mineral ores. Hence, This paper presents the techno-economic assessment for a comprehensive process for the recovery of metals and critical materials from e-waste, which is based in an electrochemical recovery (ER) technology. Economic comparison is performed with the treatment of e-waste via smelting, which is currently the primary route for recycling metals from electronics. Results indicate that the electrochemical recovery process is a competitive alternative for the recovery of value from electronic waste when compared with the traditional black Cu smelting process. A significantly lower capital investment, 2.9 kg e-waste per dollar of capital investment, can be achieved with the ER process vs. 1.3 kg per dollar in the black Cu smelting process.

Adsorption behavior of 2, 3, 7, 8-tetrachlorodibenzo-p-dioxin on pristine and doped black phosphorene: A DFT study

Polychlorinated dibenzo-p-dioxins (PCDDs) are highly toxic to humans. The search for novel and effective methods and materials for detecting or removing these gas pollutants is becoming more important and urgent. With its high specific surface area, abundance, and variety of potential applications, phosphorene has attracted much research interest. In this study, density functional theory was used to study the interactions between a doped phosphorene sheet and a tetrachlorodibenzo-p-dioxin (TCDD) molecule. The initial configurations of the TCDD and metallic (Ca or Ti) or nonmetallic (S and Se) dopants were investigated during the TCDD-phosphorene interaction study. Adsorption energy, isosurface of electron density difference, and density of states analysis were utilized to explore the interactions between TCDD and phosphorene. The results indicated that Ca dopant effectively improved the interaction between TCDD and phosphorene. Se dopant reduced the interaction between TCDD and phosphorene. Combining interactions between TCDD and the pristine, Ca-doped, and Se-doped phosphorenes, phosphorene could be a promising candidate for TCDD sensing and removal.