Efficient and economical technologies are essential to the control of SO2, the emission of which poses serious health concerns and environmental risks. Photocatalysis is an attractive method for reducing SO2 emissions. To reduce energy consumption for excess moisture evaporation, a dry photocatalytic oxidation (DPCO) system was used instead of a traditional gas–liquid process in this study. Considering that TiO2 is a widely applied photocatalyst for the purification of gaseous pollutants, this study investigated the photocatalytic removal of SO2 over different TiO2-based nanofibers. Results show that the reduction of SO2 was mainly due to oxidation. Under ultraviolet irradiation, the removal of SO2 was enhanced by the presence of NO2, which was formed by the oxidation of NO. More interestingly, the SO2 removal efficiency remains 100% over cerium-based titania nanofibers with an increase in gas humidity, indicating that this sample has excellent resistance to H2O. This is very beneficial for application in an actual flue gas atmosphere, where H2O is inevitable. In contrast, H2O played a bifacial effect in the photocatalytic removal of SO2 over copper-based titania nanofibers. Under low levels of H2O (<4%), competitive adsorption for active sites leads to the deactivation of photocatalytic activity, while addition of 8% H2O resulted in more SO2 dissolution. Nevertheless, the promoting effect was limited; competitive adsorption was the major factor. Accordingly, the main reaction products are H2SO4 and H2SO3. These indicate that combining photocatalysis technology with TiO2-based nanofibers is a promising strategy for oxidizing SO2 during a DPCO process.

•Three types of typical MSWI fly ash samples were investigated using carbonation.•The CO2 uptake with oxy-fuel combustion flue gas was analyzed.•The leaching toxicity of heavy metals with oxy-fuel combustion flue gas was reduced.•Effects of temperature on CO2 uptake and leaching of heavy metals were explored.•The relationship between CO2 uptake and leaching of Pb showed a negative correlation.Due to the high cost of pure CO2, carbonation of MSWI fly ash has not been fully developed. It is essential to select a kind of reaction gas with rich CO2 instead of pure CO2. The CO2 uptake and leaching toxicity of heavy metals in three typical types of municipal solid waste incinerator (MSWI) fly ash were investigated with simulated oxy-fuel combustion flue gas under different reaction temperatures, which was compared with both pure CO2 and simulated air combustion flue gas. The CO2 uptake under simulated oxy-fuel combustion flue gas were similar to that of pure CO2. The leaching concentration of heavy metals in all MSWI fly ash samples, especially in ash from Changzhou, China (CZ), decreased after carbonation. Specifically, the leached Pb concentration of the CZ MSWI fly ash decreased 92% under oxy-fuel combustion flue gas, 95% under pure CO2 atmosphere and 84% under the air combustion flue gas. After carbonation, the leaching concentration of Pb was below the Chinese legal limit. The leaching concentration of Zn from CZ sample decreased 69% under oxy-fuel combustion flue gas, which of Cu, As, Cr and Hg decreased 25%, 33%, 11% and 21%, respectively. In the other two samples of Xuzhou, China (XZ) and Wuhan, China (WH), the leaching characteristics of heavy metals were similar to the CZ sample. The speciation of heavy metals was largely changed from the exchangeable to carbonated fraction because of the carbonation reaction under simulated oxy-fuel combustion flue gas. After carbonation reaction, most of heavy metals bound in carbonates became more stable and leached less. Therefore, oxy-fuel combustion flue gas could be a low-cost source for carbonation of MSWI fly ash.

•V and W codoped TiO2 catalysts were easily synthesized and well characterized.•CO2 photoreduction was performed in an internal-illuminated honeycomb photoreactor.•V and W codoped TiO2 exhibited enhanced photocatalytic activity under simulated sunlight.•The roles of V and W dopants during photocatalytic reaction were identified.In this paper, V and W codoped TiO2 catalysts were prepared and coated on the surface of honeycomb support. The V4+ doped in TiO2 lattice enhanced the visible light absorption of TiO2 while the V2O5 on the surface of TiO2 and W6+ in TiO2 lattice could trap photogenerated electrons and transfer them to CO2 and water adsorbed on the surface of catalyst, resulting in effective separation of photogenerated charges. CO2 photocatalytic reduction over V and W codoped TiO2 catalyst was conducted in an internal-illuminated honeycomb photoreactor under simulated sunlight irradiation. Due to the strong visible light absorption and effective separation of photogenerated charges caused by V and W codoping and stable chemical states of V and W during photocatalytic reaction, the V and W codoped TiO2 catalysts exhibited stable and enhanced photocatalytic activity comparing with pristine TiO2 and single metal doped TiO2.Download high-res image (178KB)Download full-size image

•Pt and Cu2O co-deposited TiO2 nanocrystals were synthesized.•Pt and Cu2O co-deposited TiO2 exhibited high selectivity for CH4.•The interaction between Pt and Cu2O cocatalysts was investigated.•A mechanism for the selective reduction of CO2 was proposed.Photocatalytic reduction of CO2 with water is one of the most popular and challenging technologies to produce renewable energy. In this paper, Pt and Cu2O nanoparticles (NPs) were deposited on the surface of anatase TiO2 nanocrystals, and their effects on the photocatalytic performance of TiO2 were thoroughly studied. Based on experimental results, Pt tended to promote the production of CH4 and H2. However, Cu2O suppressed H2 production and exhibited lower CH4 selectivity than that of Pt. Furthermore, when Pt and Cu2O were co-deposited on TiO2 crystals, H2 production was inhibited and CO2 was selectively converted into CH4. From characterization, we found that Pt could not only capture photogenerated electrons and but also increase the electrons density on the catalyst, which were beneficial for selective CH4 formation. In addition, co-deposited Cu2O enhanced the CO2 chemisorption on TiO2 while inhibited that of water, resulting in enhancement of CO2 reduction and lower H2 production. On account of the above-mentioned contribution of Pt and Cu2O NPs, Pt-Cu2O/TiO2 catalyst showed high CH4 selectivity towards all products from reductive reaction.Download high-res image (102KB)Download full-size image

•Palygorskite modified by six kinds of active substances significantly improved the mercury removal efficiency.•CuCl2/CuBr2-impregnated palygorskite (Cu-Pal) performed better than others in mercury removal.•O2 and HCl acted as promoters while SO2 and NO played the role of inhibitors in mercury removal.•In reaction process, Cu2 + was reduced into Cu+ and halogen migrated into new compounds on the surface of Cu-Pal.Several modified adsorbents were developed by impregnating palygorskite (Pal) with the active substances CuCl2, CuBr2, NaBr, sulfur (S), MnO2 and Co3O4, which were used to conduct experiments via a bench-scale fixed-bed reactor system in simulated flue gas for evaluating their elemental mercury removal capacity. In addition, a variety of characterization methods were applied to understand the physicochemical properties of these adsorbents. Furthermore, adsorbents, namely, CuCl2/CuBr2-impregnated Pal (Cu-Pal), were chosen for deep exploration under various gas conditions. The results showed that the mercury removal capability of Pal was greatly improved after impregnation. At 120 °C in pure N2, the mercury removal efficiency of the adsorbents modified by CuCl2 and CuBr2 could reach 90.9% and 95.2%, respectively, while it could be > 80% for the adsorbents modified by the others. The overall trend showed that O2 and HCl were beneficial to increasing the mercury removal efficiency of Cu-Pal. To be specific, for CuCl2-Pal and CuBr2-Pal, when adding in 8% O2, their efficiencies could be increased by 6.6% and 1.9% respectively, while 50 ppm HCl increased their efficiencies by 2.8% and 2.1%, respectively. Different from O2 and HCl, SO2 and NO had negative effects. The removal efficiencies could be reduced by 6.5% for CuCl2-Pal and 4.7% for CuBr2-Pal with 1200 ppm SO2, while they could be reduced by 4.2% and 2.6% with 300 ppm NO. Compared with CuCl2-Pal, CuBr2-Pal performed better. Combined with the characterization results, Cu2 + was reduced to Cu+ and halogen migrated into new compounds on the surface of Cu-Pal after reaction. Eventually, the mercury removal mechanism of Cu-Pal was analysed and proposed.

•Comprehensive experimental parameters in chemical agglomeration were studied.•High removal efficiency was obtained in iso-electric environment.•Chemical solution kappa-carrageenan achieved a removal efficiency of 47.1%.•Synergism showed great advantage: the mixture had a removal efficiency of 59.3%.Fly ash fine particles emitted from coal-fired power plants are the primary atmospheric pollutant in China. The pressure, temperature, and velocity distribution of the flow field in a chamber was simulated to evaluate chemical agglomeration. The effect on the removal efficiency of composition, concentration, pH, K+ content, and zeta potential of the chemical agglomeration solutions as well as surfactant and flue gas temperature were evaluated. The results suggested that suitable solutions could significantly improve the removal efficiency of fine particles. Among these, kappa-carrageenan performed best and achieved a removal efficiency of 47.1%. Synergism was detected as two different chemical agglomeration solutions were mixed together: a mixture of kappa-carrageenan and konjac glucomannan attained a removal efficiency of 59.3%, higher than their individual values. The efficiency improved initially and then decreased as the solution K+ concentration increased. Also surfactants affected agglomeration: cationic and non-ionic surfactants enlarged the removal efficiency by 9.0% and 3.7%, respectively, whereas anionic surfactants lessened average removal efficiency by 5.6%. Zeta potential influenced the removal efficiency as well: it peaked as the zeta potential was around 0 mV.

•MnOx/TiO2 exhibits high activity for NO and Hg0 removal in SCR atmosphere.•NO conversion in air-SFG is higher than that in oxy-SFG.•Hg0 removal efficiency in oxy-SFG is higher than that in air-SFG.•SO2 and H2O in coal combustion flue gas have inhibition on the catalytic activity.•SCR atmosphere has negligible effects on Hg0 removal under low space velocity.In this study, manganese oxides supported on titania was prepared by sol-gel method and employed as SCR catalyst for simultaneous NO and mercury removal from coal combustion flue gas. The simultaneous NO and Hg0 conversion rate on MnOx/TiO2 was evaluated in different atmospheres, and the interaction between denitration and demercuration was investigated. The results indicated that MnOx/TiO2 had good activity in simultaneously removing NO and Hg0 in SCR atmosphere (NH3 + NO + O2 + N2). However, both NO conversion and Hg0 removal efficiency were inhibited in simulated coal combustion flue gas compared to that in SCR atmosphere because of the presence of SO2 leading to the generation of sulfite and sulfate on the catalyst. H2O in simulated coal combustion flue gas had negative effect on the catalytic performance as well. NO conversion in oxy-fuel combustion flue gas was lower than that in air combustion flue gas, while Hg0 removal efficiency in oxy-fuel combustion flue gas was higher than that in air combustion flue gas. Hg0 displayed no obvious influence on NO conversion and N2 selectivity over SCR process. And SCR atmosphere showed almost no inhibition to Hg0 removal when the temperature was below 300 °C. At the optimal temperature for NO conversion (250 °C), the inhibition of SCR atmosphere on Hg0 removal became stronger as the space velocity (GHSV) increased, but the inhibitive effect could be ignored when GHSV slowed down to 50,000 h− 1 or less.Download high-res image (192KB)Download full-size image

•The effect of sulfite on the Hg2 + reaction with typical desulfurization parameters was studied.•A buffer solution was used to investigate Hg2 + reduction and re-emission without Cl −.•A reaction pathway of Hg2 + and SO32 − with SO42 −, O2 and different pH was proposed.Wet flue gas desulfurization (WFGD) systems have several benefits, including SO2 removal and Hg pollution control. However, the absorbed ionic mercury may be transformed into insoluble elemental mercury because of the chemical interaction with the aqueous scrubbing solution, which causes mercury re-emission and reduces the mercury capture efficiency of the scrubber. This study investigates the effects of operating temperatures, pH, and O2 and SO42 − concentrations on the reduction of Hg2 + in the presence of SO32 − in a simulated desulfurization aqueous solution. The results indicate that excess SO32 − inhibits Hg2 + reduction due to the formation of the stable Hg(SO3)22 − complex, whereas a low SO32 − concentration (< 2 mM) leads to enhanced mercury re-emission due to the formation of more redox-unstable HgSO3. The Hg0 release increases from 18.6% to 59.6% when the operating temperature increases from 45 °C to 55 °C. A pH decrease in the slurry from 5 to 3 causes a significant increase in the Hg0 emission from 6.9 to 127.9 μg/m3 in a simulated desulfurization slurry. In addition, O2 caused a secondary emission of Hg0 by damaging the stable redox complexes of Hg2 + and SO32 −. In the presence of oxygen, the Hg0 emission decreases; subsequently, a HgSO3SO42 − complex is formed. Sulfate (HgSO4 and HgSO3SO42 −) inhibit the emission of Hg0.

Novel magnetic biochars (MBC) were prepared by one-step pyrolysis of FeCl3-laden biomass and employed for Hg0 removal in simulated combustion flue gas. The sample characterization indicated that highly dispersed Fe3O4 particles could be deposited on the MBC surface. Both enhanced surface area and excellent magnetization property were obtained. With the activation of FeCl3, more oxygen-rich functional groups were formed on the MBC, especially the C═O group. The MBC exhibited far greater Hg0 removal performance compared to the nonmagnetic biochar (NMBC) under N2 + 4% O2 atmosphere in a wide reaction temperature window (120–250 °C). The optimal pyrolysis temperature for the preparation of MBC is 600 °C, and the best FeCl3/biomass impregnation mass ratio is 1.5 g/g. At the optimal temperature (120 °C), the Fe1.5MBC600 was superior in both Hg0 adsorption capacity and adsorption rate to a commercial brominated activated carbon (Br-AC) used for mercury removal in power plants. The mechanism of Hg0 removal was proposed, and there are two types of active adsorption/oxidation sites for Hg0: Fe3O4 and oxygen-rich functional groups. The role of Fe3O4 in Hg0 removal was attributed to the Fe3+(t) coordination and lattice oxygen. The C═O group could act as act as electron acceptors, facilitating the electron transfer for Hg0 oxidation.

It is known that the combination of TiO2 and graphene and the control of TiO2 crystal facets are both effective routes to improve the photocatalytic performance of TiO2. Here, we report the synthesis and the photocatalytic CO2 reduction performance of graphene supported TiO2 nanocrystals with coexposed {001} and {101} facets (G/TiO2-001/101). The combination of TiO2 and graphene enhanced the crystallinity of TiO2 single nanocrystals and obviously improved their dispersion on graphene. The “surface heterojunction” formed by the coexposed {001} and {101} facets can promote the spatial separation of photogenerated electrons and holes toward different facets and the supports of graphene can further enhance the separation through accelerated electron migration from TiO2 to graphene. The G/TiO2-001/101 exhibited high photocatalytic CO2-reduction activity with a maximum CO yield reaching 70.8 μmol g−1 h−1. The enhanced photocatalytic activity of the composites can be attributed to their high surface area, good dispersion of TiO2 nanoparticles, and effective separation of excited charges due to the synergy of graphene supports and the co-exposure of {001} and {101} facets.

To make an accurate assessment on transformation behaviors of arsenic in coal at a high temperature, a typical high-arsenic coal collected from southwest China has been chosen in the study and a series of high-temperature experiments in different atmospheres have been conducted with the help of a lab-scale drop-tube furnace. Fine particulate matter were collected by a low-pressure impactor, for obtaining their mass size distributions and further quantifying for arsenic distributions and speciation in fine particles of different sizes. The results indicated that the bleeding ratios of arsenic in air combustion, CO2 gasification, and N2 pyrolysis were 85, 65, and 45%, respectively, at 1300 °C. The ratio was found to remain relatively constant when the temperature increased from 1200 to 1400 °C. Organically associated arsenic would be more inclined to vaporize in N2 pyrolysis, while both organic and inorganic associated arsenic would vaporize in CO2 gasification and air combustion. Pyrite-associated arsenic would vaporize together with sulfur in pyrite inclusions. The decomposition of pyrite followed the principle of an unreacted core model and was mostly controlled by the surface sulfur vapor pressure. Mass size distributions of fine particulate matter generated from coal gasification presented a bimodal distribution, and two major peaks appeared at 0.4 and 5 μm. Particles in the size range of 5 μm were presented as a round shape with pores and cracks on the surface, while particles in the size range of 0.4 μm were confirmed to be soot. Arsenic was obviously enriched in fine particles with a size of around 0.1–0.2 μm in both combustion and gasification. The major speciation of arsenic identified in fine particles generated from coal combustion was As2O5 and Ca3(AsO4)2, while that in fine particles generated from coal gasification was As2O5, As, AsO, and Ca3(AsO4)2.

•Ca-doped Li4SiO4 exhibited enhanced CO2 absorption.•The transformation of Ca species during reactions was identified.•The influence of the transformation of Ca species was investigated.•A possible CO2 absorption mechanism was proposed.Ca-doped Li4SiO4 (LiCa) sorbent was synthesized using a solid-state reaction method. The CO2 sorption capacity of the sorbents was thermogravimetrically analyzed in the presence of a pure CO2 flux. The cyclic performances of the sorbents were investigated in a twin fixed-bed reactor with absorption and desorption temperatures of 700 °C and 800 °C, respectively. The results indicate that the surface area of Li4SiO4 increased from 0.064 m2 g−1 to 0.314 m2 g−1 as the Ca doping content increased to 32 mol%, which enabled the sufficient contact between CO2 and Li4SiO4, and enhanced its CO2 sorption. The transformation of the Ca species from Ca2SiO4 to Li2CaSiO4 during the absorption process was helpful for the transfer of CO2 to Li4SiO4, and the transformation of Li2CaSiO4 to Ca2SiO4 in the desorption process favors CO2 desorption. Due to the special composition and structure, Li4SiO4 doped with 6 mol% of Ca had a maximum CO2 sorption of 35.1 wt% at 700 °C for 1 h and exhibited an excellent cyclic performance for CO2 sorption/desorption.

The Hg0 adsorption and oxidation sites on the CuCl2-MF catalyst as well as the role of atomic Cu and Cl in Hg0 removal were identified by a temperature programmed desorption (TPD) experiment. The reaction mechanism with the participation of O2 and HCl was investigated. The changes in surface chemistry of fresh, spent and in situ pretreated catalyst with O2 and/or HCl were investigated by EPR and XPS to better understand the intermediate reaction products and steps. The results suggested that different mercury adsorption sites are existed on the catalyst: Cl adsorption sites and Cu adsorption sites. The binding energy of mercury on the Cu adsorption sites is higher than that on the Cl adsorption sites. O2 and HCl significantly affected the state of Cu and Cl on the spent catalyst. The interaction between Hg0 and CuCl2 with the participation of O2 and/or HCl follows three steps mechanism: (1) the reduction of CuCl2 to CuCl for the interaction with Hg0, (2) the reoxidation of CuCl for the interaction with O2 forming an intermediate copper oxygen chloride species, (3) the rechlorination of oxychloride species resulting in the restoration of CuCl2. This demonstrated that the interaction between Hg0 and CuCl2 is a cycle when O2 and HCl are contained in the reaction system.

•The release and speciation transformation of uranium during coal combustion were studied.•Alkaline and alkaline-earth metal play an important role in the release of uranium.•The interaction mechanism of uranium and alkaline/alkaline-earth metals was proposed.Uranium is one of the typical naturally occurring radioactive materials in coal. The release and speciation transformation of uranium was investigated at various combustion temperatures, and the thermodynamic modelling was performed to complement the experimental work. The results showed that the uranium release ratio did not increase consistently with the combustion temperature increasing, where the highest release ratio occurred at 500 °C. At the temperature range of 500–900 °C, the uranium release ratio obviously decreased, which could be attributed to the formation of uranate with the interaction of alkaline/alkaline-earth metal compounds in coal. However, some of the thermal unstable uranate was decomposed and released at the temperature above 1000 °C, while part of them remains stable in the combustion product even when the sample was heated at 1200 °C. Further, the interaction mechanism of uranium and alkaline/alkaline-earth metals during coal combustion was proposed based on the experimental and modelling results. This study will provide valuable information for understanding the primary factors and processes that affect the release of uranium during coal combustion.

•Different copper coordinations are existed in the catalyst with different Cu loading.•At low Cu loading, isolated Cu2+ ions existed in a chlorine-free coordination.•At high Cu loading, associated Cu2+ ions existed in a chlorine-enriched coordination.•The chlorine-enriched coordination acted as the active adsorption sites for Hg0.•The chlorine-free coordination is inactive for Hg0 removal.To remove Hg0 from coal combustion flue gas and eliminate secondary pollution of spent sorbent, a novel magnetic catalyst based on CuCl2 modified magnetospheres from fly ash (CuCl2-MF) was developed. The solubility tests, electron paramagnetic resonance (EPR), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and physical property measurement system with vibrating sample magnetometer (PPMS-VSM) were employed to characterize the catalyst. The effects of CuCl2 loading value and reaction temperature on Hg0 removal performance were studied using a laboratory-scale fixed-bed reactor. The results suggested that the CuCl2 loaded on the support is present mostly in an amorphous phase. Different copper coordinations are existed in the catalyst with different Cu loading: at low Cu loading, isolated Cu2+ ions existed in a chlorine-free coordination; at high Cu loading, associated Cu2+ ions existed in a chlorine-enriched coordination. The catalyst with optimal loading 6% CuCl2 attained the highest Hg0 removal efficiency (90.6%) at 150 °C. The chlorine-enriched coordination acted as the active adsorption sites for Hg0, while the chlorine-free coordination is inactive for Hg0 removal. The CuCl2-MF catalyst showed good resistance to SO2 for Hg0 removal in the presence of HCl. Further, the management method for spent catalyst containing mercury was proposed, and the economic analysis of the technology was also conducted.

The regeneration performance of spent catalyst after mercury removal as well as the involved reaction mechanism was investigated. The effects of restoration with or without O2 and/or HCl, O2 and HCl concentration, regeneration temperature, and heating rate on regeneration performance were studied, respectively. The results suggested that two steps were essential in the regeneration process: (1) releasing the sulfur and mercury species adsorbed on the catalyst; (2) restoring the surface chemistry of catalyst by O2 and HCl. Optimal regeneration performance of spent catalyst was attained after thermal desorption at 400 °C and following by the restoration of O2 and HCl. The changes in surface chemistry of catalyst revealed that the sulfur and mercury species could be fully released by heating at 400 °C, and the deactivated adsorption sites was exposed. With the assistance of O2 and HCl, the surface chemistry of Cu and Cl was restored close to original CuCl2. Four cycles of mercury adsorption−catalyst regeneration showed that the mercury removal capacity did not vary appreciably with respect to fresh catalyst after multiple regeneration.

Cobalt oxide loaded magnetospheres catalyst from fly ash (Co–MF catalyst) showed good mercury removal capacity and recyclability under air combustion flue gas in our previous study. In this work, the Hg0 removal behaviors as well as the involved reactions mechanism were investigated in oxyfuel combustion conditions. Further, the recyclability of Co–MF catalyst in oxyfuel combustion atmosphere was also evaluated. The results showed that the Hg0 removal efficiency in oxyfuel combustion conditions was relative high compared to that in air combustion conditions. The presence of enriched CO2 (70%) in oxyfuel combustion atmosphere assisted the mercury oxidation due to the oxidation of function group of C–O formed from CO2. Under both atmospheres, the mercury removal efficiency decreased with the addition of SO2, NO, and H2O. However, the enriched CO2 in oxyfuel combustion atmosphere could somewhat weaken the inhibition of SO2, NO, and H2O. The multiple capture–regeneration cycles demonstrated that the Co–MF catalyst also present good regeneration performance in oxyfuel combustion atmosphere.

•The CH3OH yield of CT reached 188 μmol g− 1 h− 1 under UV light irradiation.•The impact of Ce doping on the structural properties of CT was discussed.•The role of Ce species in the CO2 photocatalytic reduction was investigated.•The influence of product desorption on the deactivation of CT catalyst was discussed.Cerium doped TiO2 nanoparticles were synthesized through a simple sol–gel auto-ignited method. Ce doping inhibited the growth of TiO2 particles and increased the surface area of the catalysts. The catalyst was well characterized and used as photocatalysts to convert CO2 and H2O into hydrocarbons, including CH3OH, HCHO, and CH4, in liquid phase. CH3OH was found to be the primary product with a highest yield of 188 μmol/g obtained by 1% Ce doped TiO2 catalyst after UV irradiation for 8 h, which was much higher than that of pure TiO2 and P25. The selective formation of CH3OH can be ascribed to the compromise between charge transfer and thermodynamics. The introduction of CeO2 can enhance the chemisorbed oxygen on the surface of the catalyst and the existence of Ce3 +/Ce4 + mixture can effectively inhibit the recombination of photogenerated electron–hole pairs, resulting in remarkable activity of cerium doped TiO2 catalyst. Desorption of the reaction products plays an important role on the deactivation of the catalyst.

•TiO2 modified by Pt2+ and Pt0 simultaneously was prepared by a sol–gel method.•The CH4 yield reached 264.5  μmol g−1 after UV light irradiation for 7h.•Doping of Pt2+ led to new energy level within the band gap of TiO2.•Deactivation of the catalyst during the photocatalytic process was discussed.TiO2 nanoparticles modified by Pt2+ ions and Pt nanoparticles (Pt2+–Pt0/TiO2) were synthesized through a simple sol–gel method. The physical-chemical characteristics of the catalysts were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscope, UV–Vis absorbance spectroscopy, and photoluminescence spectroscopy. Pt2+ ions were doped into the lattice of TiO2 and Pt nanoparticles were deposited on the surface of TiO2. Photocatalytic reduction of CO2 with water vapor under ultraviolet (UV) and visible light irradiation was conducted in a continuous-flow reactor. Loading of Pt2+ and Pt0 with appropriate ratio effectively enhanced the yields of H2 and CH4 but showed no significant effect on the production of CO. The highest yield of CH4 reached 264.5 and 138.6 μmol g-cat−1 after UV and visible light irradiation for 7 h, respectively. The enhanced activity of Pt2+–Pt0/TiO2 catalyst may be due to the low recombination rate of photogenerated electron–hole pairs caused by Pt nanoparticles and the strong visible light absorption, as well as the high surface area induced by Pt2+ ions.

To remove Hg0 in coal combustion flue gas and eliminate secondary mercury pollution of the spent catalyst, a new regenerable magnetic catalyst based on cobalt oxide loaded magnetospheres from fly ash (Co–MF) was developed. The catalyst, with an optimal loading of 5.8% cobalt species, attained approximately 95% Hg0 removal efficiency at 150 °C under simulated flue gas atmosphere. O2 could enhance the Hg0 removal activity of magnetospheres catalyst via the Mars-Maessen mechanism. SO2 displayed an inhibitive effect on Hg0 removal capacity. NO with lower concentration could promote the Hg0 removal efficiency. However, when increasing the NO concentration to 300 ppm, a slightly inhibitive effect of NO was observed. In the presence of 10 ppm of HCl, greater than 95.5% Hg0 removal efficiency was attained, which was attributed to the formation of active chlorine species on the surface. H2O presented a seriously inhibitive effect on Hg0 removal efficiency. Repeated oxidation–regeneration cycles demonstrated that the spent Co–MF catalyst could be regenerated effectively via thermally treated at 400 °C for 2 h.

Sorption-enhanced steam reforming, assisted by membrane separation of H2 in a fluidized bed reactor, is simulated numerically based on a kinetic two-phase model. A residence time distribution function method is implemented to account for CO2 capture in continuous operation. The effects of operating pressure, total gas feed rate, solid recycle rate, fresh sorbent feed rate, effective membrane area, and permeate pressure on the performance of a continuous fluidized bed reactor are investigated. A CH4 conversion of >91% for operation at 0.6 MPa and 550 °C is predicted to be possible with the assistance of the sorbent and membranes. The reforming performance is very sensitive to the effective surface area of membranes. A sorbent fraction of >0.7 (by mass) is necessary to achieve a product with H2 selectivity of >98%, free of CO and CO2, for realistic membrane effectiveness factors. Adding fresh sorbent or increasing the sorbent mass fraction improves the H2 productivity for a moderate solids recycling rate. Effective CO2 capture rate depends greatly on the sorbent feed rate.

•The detailed mineralogy components of magnetospheres were investigated using Mössbauer spectrometer.•The saturation magnetization of magnetospheres is proportion to their Fe3O4 content.•The possible existence forms of trace elements enriched in magnetospheres were proposed.Nine magnetospheres samples were recovered by magnetic separation from fly ashes of typical coal-fired power plants in China and Russia, respectively. The physical–chemical characteristics of magnetospheres were investigated using particle size distribution instrument, BET surface area analyzer, physical property measurement system with vibrating sample magnetometer (PPMS-VSM), field emission scanning electron microscopy with energy-dispersive X-ray spectroscopy (FE-SEM-EDX), X-ray fluorescence (XRF), inductively coupled plasma mass spectrometry (ICP-MS), X-ray diffraction (XRD), and Mössbauer spectroscopy. The potential application and possible environmental concern of magnetospheres was also discussed. The results suggest that magnetospheres are superparamagnetism with a minimized coercivity and a negligible magnetization hysteresis. The iron species in magnetospheres mainly include Fe3O4, α-Fe2O3, γ-Fe2O3, Fe2+-silicate, Fe3+-silicate, and FeSi, while the content of each iron species are varied from different power plants. Magnetite (Fe3O4) is the dominant iron-bearing mineral, which governs the magnetic property of magnetospheres. The saturation magnetization of magnetospheres are proportion to their Fe3O4 content, and it can be described by the linear regression equations of [Ms] = −21.4 + 0.71 [Fe3O4] (R = 0.89). The magnetism of magnetospheres are also determined by the quantity of elements substituted for Fe and their respective magnetic moments in the spinel structure. The siderophile elements (Cr, V, Co, Ni), chalcophile elements (Cu, Zn) and lithophile elements (In, U) are obviously enriched in magnetospheres.

A variety of natural mineral sorbents were synthesized and tested on a lab-scale fixed-bed system to evaluate mercury removal efficiencies under a simulated flue gas condition that contains 84% N2, 4% O2, and 12% CO2 in volume fraction. Three types of natural minerals, bentonite (Ben), mordenite (Mor), and attapulgite (Atp), were selected as raw sorbents, and several chemical promoters, such as CuCl2, NaClO3, KBr, and KI, were employed to enhance mercury removal abilities of the raw sorbents. The physical-chemical characteristics of these minerals were analyzed by an X-ray diffractometer (XRD), an accelerated surface area and porosimeter (ASAP) using the N2 isotherm adsorption/desorption method, and X-ray fluorescence (XRF) spectrometry. The mercury concentration was detected continuously using a VM3000 online mercury analyzer. The results showed that CuCl2-impregnated Atp (Cu-Atp) and CuCl2-impregnated Ben (Cu-Ben) presented about 90% average Hg0 removal efficiencies at 120 °C, respectively. In addition, as the temperature increased, the removal efficiencies decreased. Although NaClO3-impregnated Atp showed an average Hg0 removal efficiency more than 90% at 120 °C, its performance was limited by the testing temperature, and that was probably due to the high iron oxide content in Atp. For the KI-impregnated sorbents, high mercury removal efficiencies could be observed, and the efficiencies increased steadily with the temperature increased from 70 to 150 °C. The three natural minerals presented poor adsorption abilities for bromine, which resulted in the disappointing mercury removal efficiencies. Generally, Cu-Atp, Cu-Ben, and KI-impregnated sorbents were promising cost-effective mercury sorbents.

Wet flue gas desulfurization, heterogeneous condensation, and chemical agglomeration have been reported to be promising methods for controlling fine particulate matter (PM) from coal combustion flue gas. The effectiveness of these processes is affected by the wettability of the fly ash particles. A novel method based on the flotation mechanism using a laser particle size analyzer was used to measure the wettability of fly ash particles in water, sodium dodecylbenzene sulfonate (SDBS), and Triton X-100 (TX100) solutions. Fly ash samples were collected from four coal-fired power plants in China [Xuanwei (XW), Jungar (JG), Xiaolongtan (XLT), and Yangzonghai (YZH)], representing high-silicon, high-aluminum, high-calcium, and high-iron fly ashes, respectively. The particle size distributions, surface areas, zeta (ζ) potentials, densities, chemical compositions, mineral compositions, and morphologies of the fly ash samples were investigated as well. PM0.1 and PM10+ were readily wetted in water, but most particles within the 1–10-μm range exhibited lower wettabilities. The wettability of fly ashes in water is related to other physical or chemical characteristics and follows the trend YZH > XLT > XW > JG. For YZH fly ash, PM0.1 and PM1+ can be completely wetted in water. For XW, JG, and XLT fly ashes, only PM0.1 and PM10+ can be wetted completely in water. Addition of SDBS or TX100 promoted most wetting processes, and the promotional effect is also related to other characteristics of fly ash particles such as surface roughness, surface area, and ζ potential.

The fate of mercury in coal gasification systems is closely relevant to its transformation in fuel gases and subsequent release into the environment. This paper presents a study of the volatility and speciation of mercury during pyrolysis and gasification of five typical Chinese coals commonly used in gasification stations. Experiments were conducted on a bench-scale fixed bed under atmospheric pressure at final temperatures from 400 to 1200 °C, in 200 °C increments. The mercury concentration in coals ranged from 0.259 to 8.339 μg/g. The gas-phase mercury was analyzed by an atomic fluorescence mercury analyzer according to the Ontario Hydro Method. It was observed that mercury volatility increased monotonically with temperature over 30 min. Hg0(g) was the dominant species at most of the temperatures and holding times examined. It was also observed that high temperatures and long times enhanced mercury oxidation. With the temperature increasing from 800 to 1200 °C, Hg2+(g) increased from 5% to 35% during pyrolysis, and from 20% to 60% during gasification. The ratio of Hg2+/HgT reached its minimum at 800 °C for one coal (GZA) during pyrolysis, whereas for two other coals (BS and SF) the maximum value of that ratio was higher than the other coals and occurred during high temperature steam gasification at 1200 °C, respectively. The maximum value of the ratio of Hg2+/HgT was achieved during the high temperature steam gasification. Comparative studies on mercury emission under different conditions indicated that the volatility and speciation of mercury may correlate with the halogen concentration in the coals. However, no obvious correlation between Hg2+/HgT and basic oxides or acidic oxides in the ash was observed.

An exergy life cycle assessment (ELCA) model based on life cycle assessment (LCA) and exergy methodology was developed to assess a 2×300 MW coal-fired power plant, and the results indicated that the exergy input in operation phase of power plant accounts for 99.89% of the total input and only 0.11% in construction and decommission phases. Direct and indirect exergy inputs account for 93.03% and 6.97%, respectively. Compared with coal-fired power generation system before carbon emission reduction, exergy input-output ratio of life cycle “CO2 zero-emission” energy system and exergy efficiency are about 5.563 and 17.97%, respectively, which increases by 62.47% and declines by 11.21% approximately. The model quantifies the energy, resource consumption and pollutant emissions of system life cycle using exergy as the basic physical parameter, which will make the assessment more objective and reasonable.

Ash deposition has a major impact on safe and economic operation of coal-fired boiler. A new method for ash melting thermo-analysis based on X-ray diffraction mineral quantitative analysis was developed; the classical thermal analysis theory was used to describe the dynamic behavior of ash melting. The low-temperature ash melting process curve was acquired. Compared with the conventional method of ash fusibility, the new method of ash melting characteristic curve reflects the ash melting dynamic better. The ash melting characteristic curve reveals the multi-stage reaction process of minerals melting, explains the gradual increase of mineral melting process in theory.

To understand the formation mechanism of high-calcium fly ashes, the mineralogical, physical, and chemical properties of several high calcium fly ashes and their different density fractions (<1.0, 1.0−2.5, 2.5−2.89, and >2.89 g/cm3) from a coal-fired power plant were characterized by X-ray diffractometry (XRD), field scanning electron microscopy equipped with energy dispersive X-ray analysis (FSEM-EDX), and X-ray fluorescence spectroscopy (XRF). The occurrence of calcium in coal was determined using sequential extraction tests. The results show that the carbonate-bonded calcium is the dominant species in Xiaolongtan coal, and the ion-exchangeable calcium only occupies 19.2% of total calcium. The major calcium-bearing minerals in low temperature ash (LTA) of the feed coal, lignite from the Yunnan province, include calcite, bassanite, and dolomite. The fly ashes examined contained aluminosilicates with a high concentration of calcium oxide. The major minerals include mullite, quartz, lime, anhydrite, and gehlenite, and the minor minerals are comprised of hematite, magnetite, akermanite, portlandite, and larnite. Minerals in the density faction less than 1.0 g/cm3 consist of lime, calcite, anhydrite, and clay; between 1.0−2.5 g/cm3, quartz, mullite, anhydrite, and gehlenite; between 2.5−2.89 g/cm3, anhydrite, lime, gehlenite, hematite, and quartz; and greater than 2.89 g/cm3, larnite, gehlenite, anhydrite, brownmillerite, and some heavy minerals. In accordance with the microstructural characteristics of the fly ash particles, high-calcium fly ash can be classified into several groups, namely hollowed smooth particles, dense particles, agglomerate particles, porous particles, plerosphere, and other particles with complex surface characteristics. On the basis of chemical composition, high-calcium fly ashes can be classified into four groups namely: calcium oxide, calcium sulfates, Ca−Al−Si compounds, and Ca−S−X (X: Fe, Al, Si, Mg, etc.) compounds. Calcium oxide and calcium sulfates are mainly derived from the original calcium-bearing minerals in coal, whereas Ca−Al−Si and Ca−S−X compounds are formed by the secondary reaction of CaO and CaSO4.

In this study, a detailed technical-economic analysis on a O2/CO2 recycle combustion power plant (Oxy-combustion plant) retrofitted from the existing coal-fired plant (with a capacity of 2×300 MW) in China was carried out by using life cycle assessment (LCA) and life cycle cost (LCC) method. The CO2 emissions, investment cost, cost of electricity and CO2 avoidance cost within the life cycle were calculated respectively. The results showed that the CO2 emission avoidance rate of retrofitted Oxy-combustion plant in the life cycle was about 77.09% without taking account of the CO2 compression; the annual cost increased by 5.9% approximately, the net power decreased by 21.33%, the cost of electricity increased by 34.77%, and the CO2 avoidance cost was about 28.93 USD/t. Considering the compression process of CO2, the avoidance rate of CO2 emission was about 73.35% or so; the annual cost increased by 9.35% approximately, the net power decreased by about 26.70%, the cost of electricity increased by 49.13%, and the CO2 avoidance cost was about 45.46 USD/t. The carbon tax (the CO2 tax) should be more than about 24 USD/t and 34 USD/t under the condition of considering CO2 compression or not, respectively, which is beneficial to promote transformation of existing coal-fired plant for reducing the CO2 emissions.

Systematic experiments were conducted on a fixed-bed reactor to investigate the interaction between fly ash and mercury, the results implied that fly ash can capture mercury effectively. Among different fly ashes, the unburned carbon in the FA2 and FA3 fly ashes has the highest mercury capture capacity, up to 10.3 and 9.36 μg/g, respectively, which is close to that of commercial activated carbon. There is no obvious relationship between mercury content and carbon content or BET surface area of fly ash. Petrography classification standard was applied to distinguish fly ash carbon particles. Carbon content is not the only variable that controls mercury capture on fly ash, there are likely significant differences in the mercury capture capacities of the various carbon forms. Mercury capture capacity mainly depends on the content of anisotropy carbon particles with porous network structure.

Hazardous trace element emissions have caused serious harm to human health in China. Several typical high-toxic trace element coals were collected from different districts and were used to investigate the emission characteristics of toxic trace elements (As, Se, Cr, Hg) and to explore preliminary control methods. Coal combustion tests were conducted in several bench-scale furnaces including drop tube furnace (DTF), circulating fluidized bed (CFB) combustion furnace, and fixed-bed combustion furnace. Calcium oxide was used to control the emission of arsenic and selenium. The granular activated carbons (AC) and activated-carbon fibers (ACF) were used to remove mercury in the flue gas from coal combustion. The chemical composition and trace element contents of ash and particulate matter (PM) were determined by X-ray fluorescence (XRF) spectrometry and inductively coupled plasma-atomic emission spectrometry (ICP-AES), respectively. The speciation and concentration of mercury were investigated using the Ontario-Hydro method. X-ray diffraction spectrometry (XRD) was used to determine the mineral composition of production during combustion experiments. With the addition of a calcium-based sorbent, arsenic concentration in PM1 sharply decreased from 0.25–0.11 mg/m3. In fixed-bed combustion of coal, the retention rates of selenium volatiles were between 11.6% and 50.7% using lime. In the circulating fluidized-bed combustion of coal, the content of selenium in ash from the chimney was reduced to one-fourth of its original value and that in leaching water from the chimney decreased by two orders of magnitude using lime. Calcium-based sorbent is an effective additive to control the emission of As and Se during coal combustion. The emission of chromium is influenced by the occurrence mode of Cr in coal. Chromium emission in PM2.5 during coal combustion is 55.5 and 34.7 μg/m3 for Shenbei coal and mixed Pingdingshan coal, respectively. The adsorptive capacity of granular activated carbon for Hg0 is significantly enhanced through ZnCl2-impregnation. The activated carbon fibers showed decent efficiency in mercury adsorption, on which surface oxygen complex showed positive effects on mercury adsorption.

Shanxi province, located in the center of China, is the biggest coal base of China. There are five coal-forming periods in Shanxi province: Late Carboniferous (Taiyuan Formation), Early Permian (Shanxi Formation), Middle Jurassic (Datong Formation), Tertiary (Taxigou Formation), and Quaternary. Hundred and ten coal samples and a peat sample from Shanxi province were collected and the contents of 20 potentially hazardous trace elements (PHTEs) (As, B, Ba, Cd, Cl, Co, Cr, Cu, F, Hg, Mn, Mo, Ni, Pb, Sb, Se, Th, U, V and Zn) in these samples were determined by instrumental neutron activation analysis, atomic absorption spectrometry, cold-vapor atomic absorption spectrometry, ion chromatography spectrometry, and wet chemical analysis. The result shows that the brown coals are enriched in As, Ba, Cd, Cr, Cu, F and Zn compared with the bituminous coals and anthracite, whereas the bituminous coals are enriched in B, Cl, Hg, and the anthracite is enriched in Cl, Hg, U and V. A comparison with world averages and crustal abundances (Clarke values) shows that the Quaternary peat is highly enriched in As and Mo, Tertiary brown coals are highly enriched in Cd, Middle Jurassic coals, Early Permian coals and Late Carboniferous coals are enriched in Hg. According to the coal ranks, the bituminous coals are highly enriched in Hg, whereas Cd, F and Th show low enrichments, and the anthracite is also highly enriched in Hg and low enrichment in Th. The concentrations of Cd, F, Hg and Th in Shanxi coals are more than world arithmetic means of concentrations for the corresponding elements. Comparing with the United States coals, Shanxi coals show higher concentrations of Cd, Hg, Pb, Se and Th. Most of Shanxi coals contain lower concentrations of PHTEs.

A catalyst composed of manganese oxides supported on titania (MnOx/TiO2) synthesized by a sol–gel method was selected to remove nitric oxide and mercury jointly at a relatively low temperature in simulated flue gas from coal-fired power plants. The physico-chemical characteristics of catalysts were investigated by X-ray fluorescence (XRF), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) analyses, etc. The effects of Mn loading, reaction temperature and individual flue gas components on denitration and Hg0 removal were examined. The results indicated that the optimal Mn/Ti molar ratio was 0.8 and the best working temperature was 240°C for NO conversion. O2 and a proper ratio of [NH3]/[NO] are essential for the denitration reaction. Both NO conversion and Hg0 removal efficiency could reach more than 80% when NO and Hg0 were removed simultaneously using Mn0.8Ti at 240°C. Hg0 removal efficiency slightly declined as the Mn content increased in the catalysts. The reaction temperature had no significant effect on Hg0 removal efficiency. O2 and HCl had a promotional effect on Hg0 removal. SO2 and NH3 were observed to weaken Hg0 removal because of competitive adsorption. NO first facilitated Hg0 removal and then had an inhibiting effect as NO concentration increased without O2, and it exhibited weak inhibition of Hg0 removal efficiency in the presence of O2. The oxidation of Hg0 on MnOx/TiO2 follows the Mars–Maessen and Langmuir–Hinshelwood mechanisms.Download high-res image (76KB)Download full-size image

The emissions of potentially hazardous trace elements from spontaneous combustion of gob piles from coal mining in Shanxi Province, China, have been studied. More than ninety samples of solid waste from gob piles in Shanxi were collected and the contents of twenty potentially hazardous trace elements (Be, F, V, Cr, Mn, Co, Ni, Cu, Zn, As, Se, Mo, Cd, Sn, Sb, Hg, Tl, Pb, Th, and U) in these samples were determined.Trace element contents in solid waste samples showed wide ranges. As compared with the upper continental crust, the solid waste samples are significantly enriched in Se (20x) and Tl (12x) and are moderately enriched in F, As, Mo, Sn, Sb, Hg, Th, and U (2–5x). The solid waste samples are depleted in V, Cr, Mn, Co, Ni, Cu, and Zn. The solid waste samples are enriched in F, V, Mn, Cr, Co, Ni, Cu, Zn, Sb, Th, and U as compared with the Shanxi coals. Most trace elements are higher in the clinker than in the unburnt solid waste except F, Sn, and Hg. Trace element abundances are related to the ash content and composition of the samples. The content of F is negatively correlated with the ash content, while Pb is positively correlated with the ash. The concentrations of As, Mn, Zn, and Cd are highly positively correlated with Fe2O3 in the solid waste. The As content increases with increasing sulfur content in the solid waste. The trace element emissions are calculated for mass balance. The emission factors of trace elements during the spontaneous combustion of the gobs are determined and the trace element concentrations in the flue gas from the spontaneous combustion of solid waste are calculated. More than a half of F, Se, Hg and Pb are released to the atmosphere during spontaneous combustion. Some trace element concentrations in flue gas are higher than the national emission standards. Thus, gob piles from coal mining pose a serious environmental problem.

•Low temperature ashing plus density fraction was applicable for mineral separation in coal.•Pyrite dominates in heavy mineral fractions from the Guizhou coal.•Organic associated and pyrite associated arsenic are the two major forms in the Guizhou coal.•Arsenic occurrence in pyrite was directly identified by the FE/SEM–EDX measurement.The arsenic chemistry, mineral association and distributions in coal were evaluated using density fraction and a low temperature ashing process. A high arsenic coal from Guizhou province in southwestern China was chosen in the study. The results show that light mineral (< 2.89 g/cm3) fraction and heavy mineral (> 2.89 g/cm3) fraction were successfully separated from Guizhou coal by means of the process. Phase-mineral composition in low temperature ash and light mineral fraction are almost the same, mostly quartz and kaolinite. In contrast, pyrite dominates in the heavy mineral fraction. Arsenic concentration in Guizhou coal is as high as 265 μg/g. An obvious enrichment of arsenic in heavy mineral fraction is observed, and the arsenic concentration is approaching to 650 μg/g. Modes of occurrence of arsenic exhibit as multiple forms. Both organic and inorganic associated arsenic in Guizhou coal have been identified. The organic associated arsenic accounts for 28.40% (wt.%) of the total arsenic. The inorganic associated arsenic mostly occurs in pyrite. Direct evidence by using the EDX analysis on 27 pyrite particles confirms that arsenic is occurring in pyrite. Each distinct pyrite particle contains a certain amount of arsenic with an average of 3.5% (wt.%), while the maximum is reaching to 6.7% (wt.%). Endemic arseniasis is highly suspected to be link to the frequent use of coal-burning stove for heating and/or food drying, inhalation of indoor air polluted by arsenic derived from coal combustion in houses, and the contaminated water in Guizhou province, southwestern China.

•The mineralogy during extraction alumina from fly ash was investigated.•Three groups of trace elements association in the plant were identified.•Most elements were volatilized or redistributed in the silicate-calcite residue.•Gallium exhibits a strong correlation with Al2O3 during the extraction process.The mineralogy and trace element redistribution in the processes of extracting alumina from high aluminum fly ash in an industrial-scale production line (ISPL) in Inner Mongolia, China, are systematically investigated. Three samples were collected from the Togtoh power plant and ten samples from different processing sections in the ISPL. The mineralogy, chemical composition and morphology of the samples were characterized respectively by X-ray diffraction (XRD), X-ray fluorescence (XRF) spectrometry, and field scanning electron microscopy combined with energy dispersive X-ray spectrometry (FSEM-EDX). The trace element contents were determined by atomic fluorescence spectroscopy (AFS) and inductively coupled plasma-mass spectrometry (ICP-MS).The results show that the minerals in the pre-desilication fly ash (PDFA) include mullite, corundum, quartz, and nosean, and that amorphous SiO2 substantially disappears after the high aluminum fly ash (HAFA) is desiliconized. The minerals in the sintered fly ash (SFA) are sodium aluminate and larnite; most of the mullite reacts with Na2CO3 in the sintering process. Compared with trace element data for global hard coal ashes, elements that are slightly enriched elements in the HAFA are Li, Ga, Zr, Nb, Hf, Pb, and Th, and elements that are depleted include Ni, Ge, Rb, Sb, Cs, and Bi. The by-product silicon-calcium residue (SCR) is slightly enriched in Li, Zr, Nb, Hf, Ta, and Th, and depleted in Cr, Co, Ni, Cu, Ge, As, Rb, Mo, Cd, Sb, Cs, Ba, W, Hg, Tl, and Bi. Gallium shows a close affinity with Al2O3, and is strongly enriched (97 ppm) in the aluminum hydroxide (AH) product. For the whole ISPL, most of the trace elements are redistributed into the main SCR by-product, including Li, Be, Sc, V, Cr, Co, Ni, Cu, Sr, Zr, Nb, Cd, In, Hf, Ta, Bi, Th, U, and REY (rare earth elements and Y) (relative enrichment factor > 0.6; relative enrichment factor is used to describe the distribution of trace elements that end up in the extraction process). Some of the trace elements are released to atmosphere at high temperature, such as Mo, Hg and Tl, and only small amounts of trace elements become redistributed into the product or other by-products.

•The mineralogy during extraction alumina from fly ash was investigated.•Three groups of trace elements association in the plant were identified.•Most elements were volatilized or redistributed in the silicate-calcite residue.•Gallium exhibits a strong correlation with Al2O3 during the extraction process.The mineralogy and trace element redistribution in the processes of extracting alumina from high aluminum fly ash in an industrial-scale production line (ISPL) in Inner Mongolia, China, are systematically investigated. Three samples were collected from the Togtoh power plant and ten samples from different processing sections in the ISPL. The mineralogy, chemical composition and morphology of the samples were characterized respectively by X-ray diffraction (XRD), X-ray fluorescence (XRF) spectrometry, and field scanning electron microscopy combined with energy dispersive X-ray spectrometry (FSEM-EDX). The trace element contents were determined by atomic fluorescence spectroscopy (AFS) and inductively coupled plasma-mass spectrometry (ICP-MS).The results show that the minerals in the pre-desilication fly ash (PDFA) include mullite, corundum, quartz, and nosean, and that amorphous SiO2 substantially disappears after the high aluminum fly ash (HAFA) is desiliconized. The minerals in the sintered fly ash (SFA) are sodium aluminate and larnite; most of the mullite reacts with Na2CO3 in the sintering process. Compared with trace element data for global hard coal ashes, elements that are slightly enriched elements in the HAFA are Li, Ga, Zr, Nb, Hf, Pb, and Th, and elements that are depleted include Ni, Ge, Rb, Sb, Cs, and Bi. The by-product silicon-calcium residue (SCR) is slightly enriched in Li, Zr, Nb, Hf, Ta, and Th, and depleted in Cr, Co, Ni, Cu, Ge, As, Rb, Mo, Cd, Sb, Cs, Ba, W, Hg, Tl, and Bi. Gallium shows a close affinity with Al2O3, and is strongly enriched (97 ppm) in the aluminum hydroxide (AH) product. For the whole ISPL, most of the trace elements are redistributed into the main SCR by-product, including Li, Be, Sc, V, Cr, Co, Ni, Cu, Sr, Zr, Nb, Cd, In, Hf, Ta, Bi, Th, U, and REY (rare earth elements and Y) (relative enrichment factor > 0.6; relative enrichment factor is used to describe the distribution of trace elements that end up in the extraction process). Some of the trace elements are released to atmosphere at high temperature, such as Mo, Hg and Tl, and only small amounts of trace elements become redistributed into the product or other by-products.

•A novel method combining low temperature oxygen-plasma ashing and float-sink density separation for minerals separation in coal has been proposed and evaluated.•Mineralogical characteristic of three coals from China have been systematically reported.•Pyrite-rich heavy mineral fractions from coals were successfully separated. Euhedral, framboid and massive pyrite particles presenting as various shapes were observed by FE/SEM-EDX.•Quantitative data on trace elements affinities to specific minerals, including pyrite and clay minerals were obtained.For understanding the mineralogical characteristic and trace element affinities to specific minerals in coals, a combined method has been adopted and evaluated, which is low temperature oxygen-plasma ashing followed by float-sink density separation in bromoform. Mineral matter would be isolated from coals, and light (< 2.89 g/cm3) and heavy (> 2.89 g/cm3) mineral fractions would be separated from coal low temperature ash residues. Coals collected from three coalfields in China, namely Xiaolongtan (XLT), Huolinhe (HLH) and Yangquan (YQ), have been studied. Mineralogical characteristic of coals was characterized by X-ray diffraction and a field emission scanning electron microscope equipped with energy-dispersive X-ray spectroscopy. The distributions of trace elements in coals, low temperature ashes, and light and heavy mineral fractions were determined by inductively coupled-plasma mass spectrometry.The results show that minerals of different densities in coal can be successfully isolated and separated into two mineral fractions with the help of the combined method. Phase-mineral compositions in light mineral fraction of each coal are very similar to those in the respective low temperature ashes as a whole. Phases identified in the heavy mineral fraction are completely different from those in the low temperature ash and the light mineral fraction of each coal. Pyrite dominates in the heavy mineral fractions from the three coals, which constitutes up 48.7 wt.%, 82.9 wt.%, and 100 wt.% in the heavy mineral fraction of YQ, XLT, and HLH coals, respectively. Pyrite particles mostly occur as euhedral crystals, which display well-defined shapes, such as pentagonal dodecahedra, tetrakaidecahedra, cubes, decahedra, and regular octahedra. Framboid and massive pyrite are also clearly observed. Trace elements Pb, Cd, Mo, Cu, Tl, Sn, Sb, and Zn have a strong affinity to pyrite; elements Ni and Co are likely to occur in pyrite or ankerite; elements Sr, Hf, Li, V, Ga, Rb, Ba, Nb, Cs, U, Th, and Zr are associated with clay minerals and quartz; rare earth elements are clearly associated with clay minerals.

Samples of two high-aluminum coals and an associated fly ash were collected from a coal-fired power plant and a coalfield in Inner Mongolia, China. The mineralogy and physicochemical characteristics of low-temperature ash (LTA), high-temperature ash (HTA), and fly ash from those coals were studied by X-ray diffraction (XRD), X-ray fluorescence (XRF), and field scanning electron microscopy with energy dispersive X-ray spectroscopy (FSEM-EDX). The transformation of typical aluminum-bearing minerals at high temperature was investigated by systematic drop tube furnace (DTF) experiments and thermogravimetric analysis. The results show that the aluminum-bearing minerals in the high-Al coal are mainly boehmite and kaolinite. High temperature treatment transforms the aluminum-rich minerals to gamma alumina (γ-Al2O3), corundum (α-Al2O3), and an amorphous phase. γ-Al2O3 is the main mineral in the HTA (17.4 wt.%), while α-Al2O3 and mullite are the main minerals in the fly ash. The high-aluminum fly ash particles are irregular and their shapes are related to their compositions. The degree of irregularity of the high-aluminum fly ash particles is proportional to their aluminum content. The phase transformation of boehmite in the coal during high temperature treatment appears to have involved four stages including: boehmite dehydroxylation, transitional θ-Al2O3 formation, crystal nucleation and α-Al2O3 formation, and growth of α-Al2O3 crystals. The DTF experimental results indicated that the growth of α-Al2O3 crystals has a significant impact on PM emissions. Understanding the mineral transformation mechanism is therefore helpful in reducing PM emissions.Research highlights► Alumina with different crystal structures were found in high temperature ash. ► The occurrences of minerals have significant influence on mineral transformation. ► The growth of α-Al2O3 crystal has a significant impact on PM emission.

Ash deposition is one of the major problems encountered in coal-fired power plants. To understand its formation mechanism, six samples of the deposit were collected from different positions of boiler in the Zhuzhou coal-fired power plant in Hunan of China. The mineralogical, chemical compositions, and microstructure of ash samples were analyzed by X-ray diffraction spectrometry (XRD), X-ray fluorescence spectrometry (XRF), optical microscopy, and field scanning electron microscopy equipped with energy dispersive X-ray spectrometry (FSEM-EDX). The results show that the deposits were mainly composed of silica amorphous phases and their chemical compositions of different layers significantly varied. The identified minerals include mullite, cristobalite, hematite, quartz, hercynite, and anorthite. The silica-rich glass phases are derived from volatilization–recondensation of SiO and the interaction between aluminosilicates and other minerals. Several types of crystals were identified in the deposits, including iron-oxide crystals, Fe–Ca-bearing phase, Si-rich phase, and aluminosilicate phase. The deposition of crystals as well as the following melting is the main reason for the porosity structure of deposit. The interaction and eutectic of minerals in coal led to the serious deposition in the coal-fired power plant.

It is known that the combination of TiO2 and graphene and the control of TiO2 crystal facets are both effective routes to improve the photocatalytic performance of TiO2. Here, we report the synthesis and the photocatalytic CO2 reduction performance of graphene supported TiO2 nanocrystals with coexposed {001} and {101} facets (G/TiO2-001/101). The combination of TiO2 and graphene enhanced the crystallinity of TiO2 single nanocrystals and obviously improved their dispersion on graphene. The “surface heterojunction” formed by the coexposed {001} and {101} facets can promote the spatial separation of photogenerated electrons and holes toward different facets and the supports of graphene can further enhance the separation through accelerated electron migration from TiO2 to graphene. The G/TiO2-001/101 exhibited high photocatalytic CO2-reduction activity with a maximum CO yield reaching 70.8 μmol g−1 h−1. The enhanced photocatalytic activity of the composites can be attributed to their high surface area, good dispersion of TiO2 nanoparticles, and effective separation of excited charges due to the synergy of graphene supports and the co-exposure of {001} and {101} facets.