In-Depth Study on the Effects of Impurity Ions in Saline Wastewater Electrolysis.

Concentration followed by electrolysis is one of the most promising ways for saline wastewater treatment, since it could produce H2, Cl2, and an alkaline solution with deacidification potential. However, due to the diversity and difference of wastewater, knowledge on the suitable salt concentration for wastewater electrolysis and the effects of mixed ions are still lacking. In this work, electrolysis experiments of mixed saline water were conducted. The salt concentration for stable dechlorination was explored, with in-depth discussions on the effects of typical ions such as K+, Ca2+, Mg2+, and SO42-. Results showed that K+ had a positive effect on the H2/Cl2 production of saline wastewater through accelerating the mass transfer efficiency in the electrolyte. However, the existence of Ca2+ and Mg2+ had negative effects on the electrolysis performance by forming precipitates, which would adhere to the membrane, reduce the membrane permeability, occupy the active sites on the cathode surface, and also increase the transport resistance of the electrons in the electrolyte. Compared to Mg2+, the damaging effect of Ca2+ on the membrane was even worse. Additionally, the existence of SO42- reduced the current density of the salt solution by affecting the anodic reaction while having less of an effect on the membrane. Overall, Ca2+ ≤ 0.01 mol/L, Mg2+ ≤ 0.1 mol/L and SO42- ≤ 0.01 mol/L were allowable to ensure the continuous and stable dechlorination electrolysis of saline wastewater.

Density Functional Study on Adsorption of NH3 and NOx on the γ-Fe2O3 (111) Surface.

γ-Fe2O3 is considered to be a promising catalyst for the selective catalytic reduction (SCR) of nitrogen oxide (NOx). In this study, first-principle calculations based on the density function theory (DFT) were utilized to explore the adsorption mechanism of NH3, NO, and other molecules on γ-Fe2O3, which is identified as a crucial step in the SCR process to eliminate NOx from coal-fired flue gas. The adsorption characteristics of reactants (NH3 and NOx) and products (N2 and H2O) at different active sites of the γ-Fe2O3 (111) surface were investigated. The results show that the NH3 was preferably adsorbed on the octahedral Fe site, with the N atom bonding to the octahedral Fe site. Both octahedral and tetrahedral Fe atoms were likely involved in bonding with the N and O atoms during the NO adsorption. The NO tended to be adsorbed on the tetrahedral Fe site though the combination of the N atom and the Fe site. Meanwhile, the simultaneous bonding of N and O atoms with surface sites made the adsorption more stable than that of single atom bonding. The γ-Fe2O3 (111) surface exhibited a low adsorption energy for N2 and H2O, suggesting that they could be adsorbed onto the surface but were readily desorbed, thus facilitating the SCR reaction. This work is conducive to reveal the reaction mechanism of SCR on γ-Fe2O3 and contributes to the development of low-temperature iron-based SCR catalysts.

Gas-Pressurized Torrefaction of Lignocellulosic Solid Wastes: Deoxygenation and Aromatization Mechanisms of Cellulose.

A novel gas-pressurized (GP) torrefaction method at 250 °C has recently been developed that realizes the deep decomposition of cellulose in lignocellulosic solid wastes (LSW) to as high as 90% through deoxygenation and aromatization reactions. However, the deoxygenation and aromatization mechanisms are currently unclear. In this work, these mechanisms were studied through a developed molecular structure calculation method and the GP torrefaction of pure cellulose. The results demonstrate that GP torrefaction at 250 °C causes 47 wt.% of mass loss and 72 wt.% of O removal for cellulose, while traditional torrefaction at atmospheric pressure has almost no impact on cellulose decomposition. The GP-torrefied cellulose is determined to be composed of an aromatic furans nucleus with branch aliphatic C through conventional characterization. A molecular structure calculation method and its principles were developed for further investigation of molecular-level mechanisms. It was found 2-ring furans aromatic compound intermediate is formed by intra- and inter-molecular dehydroxylation reactions of amorphous cellulose, and the removal of O-containing function groups is mainly through the production of H2O. The three-ring furans aromatic compound intermediate and GP-torrefied cellulose are further formed through the polymerization reaction, which enhances the removal of ketones and aldehydes function groups in intermediate torrefied cellulose and form gaseous CO and O-containing organic molecules. A deoxygenation and aromatization mechanism model was developed based on the above investigation. This work provides theoretical guidance for the optimization of the gas-pressurized torrefaction method and a study method for the determination of molecular-level structure and the mechanism investigation of the thermal conversion processes of LSW.

A critical review on lead migration, transformation and emission control in Chinese coal-fired power plants.

Coal is widely utilized as an important energy source, but coal-fired power plant was considered to be an important anthropogenic lead emission source. In the present study, the distribution characteristics of lead in coal and combustion by-products are reviewed. Specifically, lead is mainly transferred to ash particles and the formation and migration mechanisms of particulate lead are summarized. Also, targeted measures are proposed to control the formation of fine particulate lead as well as to increase the removal efficiency during the low-temperature flue gas clean process. In detail, interactions between gaseous lead and some coal-bearing minerals or added adsorbents could obviously suppress the formation of fine particulate lead. On the other hand, some efforts (including promoting capture of fine particles, reducing resistivity of particles and strengthening the gas-liquid contact) could be made to improve the fine particulate lead removal capacity. Notably, the formation mechanism of fine particulate lead is still unclear due to the limitations of research methods. Some differences in the removal principles of fine particles and particulate lead make the lead emission precisely control a great challenge. Finally, the environmental potential risk of lead emission from flue gas and ash residues is addressed and further discussed.

Modeling Study of Selenium Migration Behavior in Wet Flue Gas Desulfurization Spray Towers.

Wet flue gas desulfurization (WFGD) system is the core equipment for removing SO2 from coal-fired power plants, and it also has an important synergistic effect on the removal of selenium. However, the removal efficiency of Se across WFGD systems is not as expected, and it varies greatly in different coal-fired units (12.5-96%). In this study, a mathematical model was established to quantitatively describe the selenium migration behavior in WFGD spray towers, including the conversion of gaseous selenium to particulate selenium and the capture of gaseous SeO2 and particles by droplets. The calculation results show that the behavior of selenium in the spray tower can be divided into three stages: preparation, condensation, and removal. The condensation stage significantly affected the selenium distribution and its total removal efficiency. Furthermore, five factors which may affect the selenium behavior were investigated. Among them, the inlet particle size distribution and the droplet temperature had great impacts on the outlet selenium concentration, which may be the reason for the unstable selenium removal efficiencies. This study can help in understanding the migration process of selenium in WFGD spray towers and provide some guidance for the development of specific selenium control technologies.

In-Furnace Control of Arsenic Vapor Emissions Using Kaolinite during Low-Rank Coal Combustion: Influence of Gaseous Sodium Compounds.

Using additives in the in-furnace control of arsenic emissions is promising for reducing the impact on the downstream selective catalytic reduction system and blocking the spread of arsenic pollutants into the environment. The study quantifies the arsenic adsorption capacity of kaolinite at high temperature and clarifies its fixation pathway with and without the existence of sodium vapor, which is easily adsorbed by kaolinite. Experiments about Al-coordination and acid sites of products, as well as calculations of thermodynamic equilibrium and the adsorption energy based on density functional theory were performed. During separated arsenic adsorption, nearly 40% of trivalent arsenic [As(III)] is oxidized to pentavalent arsenic [As(V)] and bonded to kaolinite, forming an As-O-Al structure. In this respect, the arsenic adsorption capacity of kaolinite is 200 µg g-1, with 24% of arsenic shown to be well-crystallized Al-bound. During the co-adsorption process, 82% of As(III) is oxidized to As(V) and connected to the Al surface of kaolinite, and the O-Na groups bond to As around the As-O-Al structure, thereby forming Na-O-As-O-Al. The arsenic adsorption capacity increased to 878 µg g-1 with well-crystallized Al-bound arsenic accounting for 56%. This study demonstrates the potential for the application of kaolinite as an arsenic adsorbent in the actual furnace.

Further Insight into the Formation and Oxidation of CaCr2O4 during Solid Fuel Combustion.

The control of toxic chromate (Cr6+) formation is still a significant challenge in solid fuel combustion. In particular, the mechanism of chromium transformation from Cr3+ to chromate or other unoxidized forms remains unclear. The present study confirms the formation of a significant unoxidized Cr-containing compound CaCr2O4(Cr3+) during solid fuel combustion. Experiments were conducted, for the first time, to clarify the mechanism of CaCr2O4 oxidation, which is quite different from Cr2O3 oxidation. The findings demonstrate that CaCr2O4 was formed at temperatures above 1200 K, through rapid decomposition of CaCrO4 or slow and direct interaction between CaO and Cr2O3. Compared to Cr2O3, CaCr2O4 could be oxidized at lower temperatures under the influence of free CaO. In the absence of free CaO, the oxidation of CaCr2O4 was minimal; however, in the presence of CaSO4, calcium in the form of CaCr2O4 participated in the oxidation of CaCr2O4. Thus, chromium in the form of CaCr2O4 was more likely to be oxidized when CaCr2O4-containing fly ash was reheated. Fortunately, CaCr2O4 showed slight basicity on the surface, allowing it to react with acidic gases. Accordingly, measures were proposed to suppress the oxidation of CaCr2O4 by stimulating the reactions between CaCr2O4 and acidic substances, like SO2 and Si/Al-compounds. These compounds competed with chromium at high temperatures to react with calcium in the fly ash and in CaCr2O4. As a result, the unoxidized chromium was transformed into highly stable Cr2O3 or Ca3Cr2 (SiO4)3.

Evolution and speciation transformation of chlorine during automobile shredder residue pyrolysis.

Disposal of automobile shredder residue (ASR) via pyrolysis enables the recovery of valuable products; however, the production of hazardous pollutants and low-value products is inevitable due to its high chlorine content. In this work, chlorine evolution behavior and the conversion mechanism during ASR pyrolysis between 480 and 600 °C were systematically studied. The experimental results for organic chlorine (Org-Cl) showed that released chlorinated gases were complex, and HCl only accounted for 35% of the gas phase products, while short-chain hydrocarbons with carbon atoms between two and four accounted for 52%. Chlorine was predominantly retained in the char, and Org-Cl was the primary contributor to the residual chlorine, accounting for over 50% of the char. The content of inorganic chlorine (InO-Cl) was low in the raw sample but significantly increased in the char. Through the distinction between organic and inorganic chlorine content in char, it was confirmed that Org-Cl could be converted to InO-Cl due to complex secondary reactions with metallic compounds. The conversion was favored by increasing the Org-Cl content and the temperature. Our findings clarified the evolution mechanism of chlorine and the transformation from Org-Cl to InO-Cl, thus providing guidance for chlorine regulation and the efficient recycling of metal resources.

Fate of sulfur and chlorine during co-incineration of municipal solid waste and industrial organic solid waste.

In China, the co-incineration of municipal solid waste (MSW) with industrial organic solid waste (IOSW) is increasingly adopted. Compared with MSW, IOSW contains higher levels of sulfur (S) and chlorine (Cl), presenting significant challenges for controlling S/Cl emissions in MSW incineration plants. In this study, the impact of co-incinerating IOSW was investigated in a 500 t/d incinerator grate, focusing on the emissions and transformation behaviors of S/Cl. IOSW, with a consistent sulfur content of about 0.22 wt% and a more variable chlorine content averaging 0.53 wt%, contains over 40 % organic sulfur and >90 % organic chlorine, higher than in MSW. The results of co-incineration experiments showed that the median SO2 concentration in the flue gas was stable at 50 mg/m3, while HCl concentration decreased initially and then increased as the co-incineration ratio of IOSW rose from 20 % to 40 %. Furthermore, the concentrations of SO2 and HCl were not significantly influenced by wind flow but were positively affected by the rising furnace temperatures. Besides, the co-incineration ratio had minimal impact on sulfur in fly ash before deacidification, primarily derived from the gas stream. However, the (Na + K)/Cl ratio in fly ash progressively increased from 1.5 to 1.9, and the Ca content decreased from 0.35 % to 0.15 % as the co-incineration ratio rose to 40 %, indicating more chlorine migration into the fly ash at higher co-incineration rates. This research offers essential guidance for effectively controlling pollutant emissions during the co-incineration of IOSW, specifically the S/Cl pollutants.

Application of machine learning in prediction of Pb2+ adsorption of biochar prepared by tube furnace and fluidized bed.

Data mining by machine learning (ML) has recently come into application in heavy metals purification from wastewater, especially in exploring lead removal by biochar that prepared using tube furnace (TF-C) and fluidized bed (FB-C) pyrolysis methods. In this study, six ML models including Random Forest Regression (RFR), Gradient Boosting Regression (GBR), Support Vector Regression (SVR), Kernel Ridge Regression (KRR), Extreme Gradient Boosting (XGB), and Light Gradient Boosting Machine (LGBM) were employed to predict lead adsorption based on a dataset of 1012 adsorption experiments, comprising 422 TF-C groups from our experiments and 590 FB-C groups from literatures. The XGB model showed superior accuracy and predictive performance for adsorption, achieving R2 values for TF-C (0.992) and FB-C (0.981), respectively. Contrasting inferior results were observed in other models, including RF (0.962 and 0.961), GBR (0.987 and 0.975), SVR (0.839 and 0.763), KRR (0.817 and 0.881), and LGBM (0.975 and 0.868). Additionally, a hybrid dataset combining both biochars in Pb adsorption also indicated high accuracy (0.972) as obtained from XGB model. The investigation revealed that the influence of char characteristics and adsorption conditions on Pb adsorption differs between the two biochar. Specific char characteristics, particularly nitrogen content, significantly influence lead adsorption in both biochar. Interestingly, the influence of pyrolysis temperature (PT) on lead adsorption is found to be greater for TF-C than for FB-C. Consequently, careful consideration of PT is crucial when preparing TF-C biochar. These findings offer practical guidance for optimizing biochar preparation conditions during heavy metal removal from wastewater.

The adsorption mechanism of arsenic in flue gas over the P-doped carbonaceous adsorbent: Experimental and theoretical study.

The utilization of carbon-based sorbent has gained extensive attention for arsenic removal from flue gas due to their high specific surface area, sufficient active sites and abundant sources. This study proposes that the addition of phosphorous could be used as an effective promoter for the activation and modification of carbonaceous sorbent to enhance their arsenic fixation capacity. Both experimental and density functional theory (DFT) methods were employed to systematically investigate the adsorption characteristics of arsenic over different carbon based sorbents. The results reveal that the modification of H3PO4 generated C-O-P, C-P-O, and C3-P-O functional groups on the surface of activated carbon, and the adsorption ability of H3PO4-modified activated carbon for gaseous arsenic was significantly improved compared with the untreated activated carbon. DFT calculations indicate that unsaturated C atoms on carbonaceous surface served as active sites during arsenic adsorption, the electronegativity of which could be enhanced by phosphorous functional group, thereby facilitating the adsorption of gaseous arsenic species. Additionally, the positive effect of the phosphorous functional group on arsenic adsorption is more pronounced on zigzag carbonaceous surface than on armchair carbonaceous surface. This work provides a theoretical basis of the development of high-performance biochar preparation for arsenic adsorption by explaining the promoting effect of phosphorous functional group on gaseous arsenic adsorption on carbonaceous surface.

Migration and emission characteristics of trace elements in coal-fired power plant under deep peak load regulation.

The trace elements (TEs) have caused great harm to the environment due to the large consumption of coal, and their emission from the coal-fired power plant (CFPP) has become a hot issue. The deep peak load regulation (DPLR) become a trend in the CFPP, which will affect the migration and emission of TEs. To explore the effect of the DLPR on the migration and emission characteristics of typical TEs in a 330 MW CFPP, the TEs field tests were carried out during the regulation period. Results showed that a higher load enhanced the migration of Pb, Mn, and Cr from bottom ash to fly ash, while it had little effect on the other TEs. More importantly, >99 % of TEs (93 % of Se) could be captured by air pollution control devices (APCDs), and the emission risk of Se and Mn increased with the load. Compared with the other TEs, it is particularly noteworthy that Se has a higher gaseous proportion in the flue gas, and the emission factor sharply increased from 165 MW to 297 MW. In addition, part of the particulate selenium transformed into a gaseous state across the ESP. This work contributes to understanding the migration characteristic of TEs during the DPLR process of CFPP and provides guidance for TEs control in the CFPP.

Gas-pressurized torrefaction of lignocellulosic solid wastes: Low-temperature deoxygenation and chemical structure evolution mechanisms.

A novel gas-pressurized (GP) torrefaction realizes deeper deoxygenation of lignocellulosic solid wastes (LSW) to as high as 79% compared to traditional torrefaction (AP) with the oxygen removal of 40% at the same temperature. However, the deoxygenation and chemical structure evolution mechanisms of LSW during GP torrefaction are currently unclear. In this work, the reaction process and mechanism of GP torrefaction were studied through follow-up analysis of the three-phase products. Results demonstrate gas pressure causes over 90.4% of cellulose decomposition and the conversion of volatile matter to fixed carbon through secondary polymerization reactions. Above phenomena are completely absent during AP torrefaction. A deoxygenation and structure evolution mechanism model is developed through analysis of fingerprint molecule and C structure. This model not only provides theoretical guidance for optimization of the GP torrefaction, but also contributes to the mechanism understanding of pressurized thermal conversion processes of solid fuel, such as coal and biomass.

Effects of molten salt thermal treatment on the properties improvement of waste tire pyrolytic char.

Pyrolysis is a technical means for waste tires recycling, which can promote the enrichment of carbon black and facilitate the subsequent recovery. However, carbon black particles aggregated and the inorganic impurities tended to be enriched in pyrolytic char during the waste tire pyrolysis process, which is not conducive to the substitution of commercial carbon black by pyrolytic char. In the present study, a novel method using molten salts thermal treatment was proposed for the impurities removal from pyrolytic chars with different characteristics. In addition, the proper thermal treatment conditions were further estimated to obtain better performance for the physical-chemical properties improvement of pyrolytic char. Six kinds of char samples were chosen to conduct molten salts thermal treatment (MSTT) experiments at 350, 400, and 450 °C. The experimental results show that MSTT can effectively remove the impurities of different pyrolytic chars, and the most optimum reaction conditions are at 400 °C, 2 h of reaction time, and molten salt/char ratio of 10:1. In addition, after MSTT, the pyrolytic char was depolymerized, and the average particle size reduced from 36.63 µm to 19.08 µm, the specific surface area increased from 49 m2/g to 73 m2/g. At the same time, the graphite carbon content of the pyrolytic char increased from 24.41% to 70.90%, and the hydroxyl content on the pyrolytic char surface increased significantly. In summary, the physical-chemical properties of waste tire pyrolytic char were improved by MSTT, which is close to the carbon black N550 level.

Feasibility study on co-processing of automobile shredder residue in coal-fired power plants via pyrolysis.

Facing the challenges of organic industrial solid waste (OISW) disposal, co-processing of OISW by power plants has become a developing trend. In order to avoid feeding problems of OISW and enhance the combustion adaptability of the furnace, pyrolysis coupled with incineration technology is proposed as a potential method. Among various OISW, automobile shredder residue (ASR) is regarded as a promising fuel due to its high heating value. In view of engineering application, the researches focused on the products' properties and economic evaluation under a wide range of heating rates which are insufficient. In this study, regarding the rapid pyrolysis by conducting the high-temperature flue gas as heating source in power plants, the pyrolysis behavior of ASR was correspondingly studied under a wide range of heating rates. The formation of volatiles and property's improvement were further investigated for generating high-valued oil. Results showed that the high heating rate is not only beneficial to the homogenization of pyrolytic products but also the aromatization in oil and radical generation in gases. Importantly, it also contributed to the cleavage of the single bond connected to the benzene ring and carbon-oxygen single bond for esters. By conducting the enhanced cracking of volatiles, the wax-like fraction was significantly reduced. In addition, the deoxygenation in oil (oxygen content decreased by 20 wt%) and high heating value of gases (increased by 73%) were improved. Our findings demonstrated the feasibility and economic efficiency for the co-processing of ASR in coal-fired power plants via pyrolysis and thus provide guidance for future commercial application.

Identifying functional brain abnormalities in migraine and depression comorbidity.

Background: Migraine and major depressive disorder (MDD) are both highly prevalent brain disorders and are often comorbid. However, the common and distinctive neural mechanisms underlying these disorders and the brain function alterations associated with their comorbidity are largely unknown. We aimed to explore the functional abnormalities of the brain associated with the co-occurrence of migraine and depression. Methods: High-resolution T1-weighted and resting-state functional magnetic resonance images (MRI) were acquired from 93 well-matched patients with comorbid migraine and depression, patients with migraine, patients with MDD, and healthy controls. Voxel-wise analysis of variance (ANOVA) and a two-sample t-test of multiple functional variables were performed among the groups. Furthermore, correlation analysis was conducted to detect the clinical significance of the altered functional regions in the brain. Results: Migraine patients with and without depression revealed widely shared regional networks of functional changes. Brain function changes in the right paracentral lobule and fusiform were specific to patients with comorbid migraine and depression [P<0.05, cluster-level familywise error (FWE)-corrected], while changes in the left thalamus, medial orbital of superior frontal gyrus and triangular part of the inferior frontal gyrus were specific to patients with migraine (P<0.05, cluster-level FWE-corrected). Importantly, the brain activity of the right paracentral lobule, left calcarine, and left dorsolateral superior frontal gyrus was associated with emotional symptoms in the pooled migraine data (P<0.05). Conclusions: These findings help to identify the neural correlates underlying patients with migraine and those with comorbid migraine and depression. These shared and distinct brain changes could be used as potential image markers to decipher the comorbidity of the 2 disorders.

Products distribution and sulfur fixation during the pyrolysis of CaO conditioned textile dyeing sludge: Effects of pyrolysis temperature and heating rate.

Textile dyeing sludge (TDS) is a typical industrial solid waste whose amount surged with the textile industry's development. Pyrolysis treatment is a promising technique for TDS to realize harmless disposal and resource reuse. However, the high content of organic compounds would cause sulfurous pollutants emission, reducing the economic feasibility during pyrolysis. This study aimed to fill the knowledge gaps about the thermal behavior, products distribution, kinetics, and sulfur transformation during TDS pyrolysis in 350-575 â„ƒ with the heating rate of 60, 600, and 6000 â„ƒ/min, then investigate the sulfur fixation effect of CaO under representative conditions (350 â„ƒ, 650 â„ƒ with 60 â„ƒ/min, 6000 â„ƒ/min). The primary decomposition stage of TDS is observed in 127-557 â„ƒ, following the Avrami-Erofeev (n = 3) model, while the activation energy presents a convergent tendency with the increased heating rate. The pyrolysis temperature and heating rates impact the cracking of organic compounds, while a weakening effect is found for the sulfur distribution. CaO addition could efficiently realize sulfur fixation in char by absorbing sulfurous gas products, but SO2 escape appeared with the increased CaO fraction. Pyrolysis condition at 650 â„ƒ-60 â„ƒ/min with 10 wt% CaO addition is recommended to achieve high sulfur retention, and the sulfur transformation mechanism in char during the TDS pyrolysis with and without CaO is proposed. Our findings provide novel and fundamental insights into the efficient disposal and pollution control during TDS pyrolysis.

Investigation of properties change in the reacted molten salts after molten chlorides cyclic thermal treatment of toxic MSWI fly ash.

To realize the thermal detoxification of municipal solid waste incineration (MSWI) fly ash in a relatively mild environment, molten salts thermal treatment technology was proposed in our previous research, which showed good effects. To investigate the properties of molten salts (NaCl-CaCl2) during cycling reusing, the change of the main components and the physical properties of the used molten salts were estimated. Results showed that the salts in fly ash would dissolve into molten salts. During this process, the concentration of K+, SO42- kept increasing while Cl- was decreased. The changing trend of Na+ and Ca2+ was dependent on the ratio of Ca/Na in raw fly ash. Ca(OH)2 in fly ash would react with CaCl2 to form CaClOH. Moreover, the introduction of the salt components on the thermal properties of molten salts were also studied. The melting point hardly changed by NaCl, CaSO4, and SiO2. Nevertheless, it was lowered to 431 °C with 15% CaCO3 addition, while increased to 523 °C with 20% KCl. Besides, there were no significant influences on the viscosity, stability, and thermal diffusivity of molten salts. KCl had the greatest influence on the specific heat capacity of molten salt, with an increase of about 20%.

Condensation and adsorption characteristics of gaseous selenium on coal-fired fly ash at low temperatures.

Gaseous selenium is of high saturated vapor pressure, making its retention in solid phases quite difficult during coal combustion. The selenium transformation from gaseous form into solid phases at low temperatures can be essential to control selenium emission. To understand the migration of SeO2 (g) on ash particles in the low-temperature zone, this study investigated the speciation of selenium in fly ash and simulated the physical retention of SeO2 (g) on fly ash. The results demonstrated that there was a large proportion of physically-bound Se in the fly ash of pulverized-coal-fired boiler (22.62 %-58.03%), while the content of physically-bound Se in fly ash of circulated fluidized-bed boiler was lower (∼6%). The physically-bound Se was formed through selenium condensation and physical adsorption. The decrease of temperature or the increase of cooling rate could promote the transformation of gaseous selenium to solid phase and the presence of HCl might suppress SeO2 transformation into Se in the condensation process. Meanwhile the compositions of fly ash had a great influence on the selenium adsorption process. Among typical coal-fired ash components, mullite showed the best performance in the selenium capture in the temperature range of 90-200 °C, contributing to the high content of physically-adsorbed selenium in PC fly ash. These findings provided new ideas for improving the removal rate of volatile selenium.

Ash formation and the inherent heavy metal partitioning behavior in a 100 t/d hazardous waste incineration plant.

Hazardous waste incineration (HWI) ash is also defined as hazardous waste and its disposal performance depends largely on the ash compositions as well as the potential environmental risk of heavy metals. In this work, HWI ashes of four sampling sites were collected in a 100 t/d hazardous waste incineration plant with rotary kiln over three consecutive days. The formation characteristics of ash samples including heavy metal partitioning were given, with further discussions on the melting disposal of HWI ash mixtures. Results showed significant differences in the ash compositions among the sampling sites. Caused by NaHCO3 injection as de-acidizing adsorbent, the sum of Na, S and Cl content in bag filter ash even exceeded 70%. Cu/Mn/Cr tended to transfer into the bottom ash due to low volatilities, while Zn/Pb/Cd/Se/As were more likely to be enriched in the ash particles. In particular, chemical adsorption at medium- to high- temperature range was dominant for As enrichment in the waste heat boiler ash. Despite the complexity and diversity of raw hazardous wastes, little difference was found in the melting temperature of bottom ash during the sampling period. However, it could vary by more than 200 °C for fly ash due to the fluctuation of alkali components in raw wastes. Moreover, slagging medium was encouraged in order to achieve rapid and complete melting of ash mixtures. The objective of this study is to gain knowledge on the HWI ash formation and inherent heavy metal partitioning behavior, expecting to provide guidelines on the deep harmless disposal of HWI ash in future.