•Microwave-metal discharge process was optimized by using tungsten as electrodes.•The toluene destruction efficiencies were studied at different gas flow rates.•MW-m discharge can effectively destruct toluene with an efficiency of over 90%.•The generated coke can be in-situ eliminated by steam reforming to produce syngas.•This work can offer important reference for tar removal in biomass gasification.Electrical discharges triggered by microwave-metal interactions are important phenomena in microwave heating processes with the generation of plasma. In this study, microwave-metal (MW-m) discharge was developed for tar destruction. Toluene was used as a tar model compound. Uniform tungsten electrodes was adopted and optimized to provide a fast-ignition, relatively-stable and sustainable discharge process. The conversions of toluene were investigated at different gas flow rates and preliminary tests were conducted to in-situ eliminate the generation of solid carbon. Microwave-tungsten discharge can effectively destruct toluene into useful gases (H2, C2H2 and CH4) and solid carbon, with a high conversion efficiency of more than 90%. The generated solid carbon can be effectively eliminated by introducing water steam into the discharge reaction to achieve a comparable toluene conversion efficiency of 92.3% and a production of syngas (H2 to CO ratio is around 1.6). This work can offer important reference for developing a new technology for tar cracking in a biomass gasification process.

•Sulfoaluminate cementitious materials were produced by using 100% industrial solid wastes.•Influence of raw mix composition and calcination temperature on the properties of Sulfoaluminate clinker was discussed.•Sulfoaluminate cementitious materials were produced in a 1 t/d industrial rotary kiln.Sulfoaluminate cement exhibits a good dimensional stability, an early strength, a high strength, a high resistance to chemical attack (from seawater, sulfates and chlorides), a low energy consumption and low carbon-dioxide emissions during production. However, sulfoaluminate cement factories traditionally use limestone, bauxite and gypsum as materials, which places significant pressure on these natural resources. This study aimed to investigate the feasibility of substituting traditional raw materials with industrial solid wastes in sulfoaluminate clinker production. It was achieved by sintering a mixture of industrial solid wastes (coal gangue: 8%–15%, flue gas desulfurization gypsum: 25%–35%, aluminum slag: 30%–35%, carbide slag: 30%–35%) at 1150–1300 °C. The mineralogical composition of the clinker was C4A3S‾, C2S and C4AF. The compressive strength of hydrated specimens reached as high as 75 MPa after a 28 d curing. This research provides a feasible and promising way to produce high-value products using industrial solid wastes and may promote their large-scale utilization.

•Metal discharge presents a novel method to enhance the degradation of biomass tar.•Heating effect of discharge was successfully isolated from the coupling effect.•Heat release from discharge was measured by the temperature rise of paraffin oil.•Direct characterization of the transient phenomena was realized.•The factors influenced the metal discharge density have been studied and discussed.Routine microwave heating and processing are established technologies and have been successfully applied to various fields. However, the discharge phenomenon caused by the interaction between microwave and metals or semiconductors (with sharp edges, tips or submicroscopic irregularities) has not yet been thoroughly studied. The discharge induced by microwaves is a complicated process, which may contain a variety of physical and chemical phenomena. Because of the lack of trapping and characterization tools, it is quite difficult to directly study the instantaneous process of microwave-metal discharge. In this study, the heat generation was isolated by liquid paraffin oil and measured with a direct calorimetric method, and as a result, direct characterization of the transient phenomena was realized. The factors that influence the microwave-metal discharge density, such as metal type, metal mass, metal size and microwave powers, have also been studied and characterized. The results showed that the discharge induced by microwave and metal could cause a significant rise in the temperature of the liquid paraffin, and that the discharge process is accompanied by a large quantity of heat release. In addition to this, the properties of the inserted metal and microwave power all play important roles in the discharge density. In general, this study will help us to develop a better understanding of microwave-metal discharge. It would be beneficial to extend this special phenomenon and their significant effects into applications, such as microwave-assisted pyrolysis, pollutant removal, organic synthesis and many other technological and scientific fields.

•A novel IL-based technology is developed for removing VOCs from the atmosphere.•[Bmim][NTf2] is shown to be a promising absorbent for VOC pollutants.•Absorptivity of [Bmim][NTf2] for toluene is >94% over a wide range of conditions.•Toluene is easily removed from [Bmim][NTf2] permitting IL reuse in multiple cycles.A novel method of removal of volatile organic compounds (VOCs) using the ionic liquid [Bmim][NTf2] as an absorbent is developed as a contribution to dealing with recent severe smog incidents in China. The effects of concentration, temperature and flow rates on the ability of [Bmim][NTf2] to absorb VOCs were studied using toluene as a model volatile organic pollutant. The potential of the use of [Bmim][NTf2] as an absorbent for VOCs is shown by the solubility of toluene in the ionic liquid; the absorptivity of the ionic liquid for toluene; and the fact that absorbed toluene can be removed easily from [Bmim][NTf2], permitting recycle of the ionic liquid in multiple reuse phases. The solubility of toluene in [Bmim][NTf2] is 61.5% at 20 °C and atmospheric pressure; the highest absorptivity of [Bmim][NTf2] for toluene is 98.3%, achieved at a toluene concentration of 300 ppm and a flow rate of 50 mL min−1 at 20 °C; and the absorptivity of the ionic liquid is >94% over a wide range of conditions. The ionic liquid can be recovered and recycled in the absorption process at least five times, reducing the reagent cost in the VOC removal process.

Hot spots can occur in microwave heating when the heated materials have different microwave absorbing properties, resulting in non-uniform temperature distribution. Understanding the features and extent of hot-spot effects can be essential in microwave-assisted processes, but little has been reported quantitatively due to the difficulty in direct determination. The issues are measured experimentally and numerically simulated using silicon carbide (SiC) particles dispersed in paraffin oil as a representative case here. Hot spots are definitively shown to exist and may trigger temperature gaps between surrounding substances at the magnitude of several hundred degrees Celsius or even higher in certain cases. The temperature gaps are enhanced for larger sized SiC particles, with a higher heat generation rate and increasing heating time. The extent of hot-spot effects substantially depends on how much and how quickly heat generated by the strong microwave absorbing media can be transferred to the weak ones. The findings have great practical value. By choosing materials with strong microwave absorption, or where discharges occur due to microwave–metal interactions, prominent hot spots can be intentionally forged and the temperature gradient may be tailored to enhance chemical reactions and catalytic processes for specific scientific and engineering applications.

The coupling mechanism between wave absorption and microwave metal discharges is very important in the application of microwave heating for the treatment of waste electronic scraps, tires, and other refuse of a similar nature. In this work, microwave absorbing material, metal strips, and their mixture embedded in quartz sands were irradiated by microwaves and an indirect calorimetric method was used to measure their heating effect to explore their coupling mechanism. Numerical study was also carried out to validate the experimental results and explain the mechanism in theory. The results indicate that the amount of microwave absorber directly influences the stimulation and the intensity of microwave metal discharges. An appropriate amount of microwave absorber is advisable for an efficient heating process by microwaves. The coupling mechanism of wave absorption and microwave metal discharges varies from collaborative to competitive with the increased amount of microwave absorber. Overuse of microwave absorber can counteract the improvement of microwave energy efficiency.

The feasibility of using tributylamine as a pH regulator to enhance CO2 mineralization in Ca2+-rich aqueous solutions was demonstrated. The influencing factors, such as the makeup of the extraction system and the operating parameters, were investigated experimentally. With N-butyl alcohol as the diluent of tributylamine, the mineralization ratio could reach approximately 93% for an initial concentration of 3.4 g/L when the dilution ratio was 2:1 and the ratio of organic to water was 4:1. A stirring speed of 500 rpm was observed to be sufficient to ensure good mixing of organic and water phases. A rise in the reaction temperature results in a lowering of the mineralization ratio. Characterization using X-ray diffraction (XRD) and scanning electron microscopy (SEM) indicated that the precipitation product is pure CaCO3 that is sufficiently good for use in commercial filling agents. Moreover, it was determined that Ca(OH)2 powder acted as an excellent regenerant to turn N(C4H9)3·H+ in the organic phase back into tributylamine. The extraction system exhibited stable performance for acting as the enhancing medium for CO2 mineralization reactions in Ca2+-rich aqueous solutions, after 11 rounds of regeneration cycles. The results of this study provide information that is relevant for practical application of this CO2 fixation concept.

Desulfurization gypsum, the byproduct from wet flue gas desulfurization, and red mud, from the production of aluminum oxide, are two bulk industrial solid wastes that trigger many local environmental problems in China. This study aims to jointly utilize them. Through experimentation and modeling using FactSage, it has been found to be feasible to prepare sulfoaluminate cement using these wastes. The calcination temperature in the preparation was as low as 1250–1300 °C, and the main mineral phases of the cement clinker were 3CaO·3Al2O3, CaSO4, β-2CaO·SiO2, and 2CaO·Fe2O3. The cement clinkers tested showed excellent mechanical strength performances. This process was found to be an efficient way to consume industrial solid wastes, with the total proportion of desulfurization gypsum and red mud over 70–90% by mass in the raw materials. The sulfoaluminate cement products have outstanding cost superiority over Portland cement because of their low material costs, low material pretreatment costs, and low calcination temperature. Moreover, this technology could bring about immense environmental and social benefits in terms of waste consumption, energy conservation, and CO2 reductions. This technology has considerable prospects, and it is worth undertaking further research into its potential applications.

With the increasing use of biomass resources, the recycling or disposal of biomass ash should be given more attention. An idea is put forward in this paper to study the possibility of CO2 fixation with biomass ash through the bicarbonation reactions of carbonate minerals. Four typical biomass resources, corn stalk, wheat stalk, cotton branch, and poplar, in a rural area of northern China, were sampled. Their ashes were thoroughly characterized for their chemical and mineral compositions through the joint adoption of three analysis tools, X-ray diffraction, X-ray fluorescence, and thermogravimetric analysis. It was found that the percentages of carbonates, such as CaCO3, K2CO3, MgCO3, etc., could amount to 25–40% by weight in the ashes of wheat stalk and corn stalk and to about 70% in the cotton branch and poplar. The high carbonate contents give them a theoretically good potential to absorb CO2 through bicarbonation reactions. Experiments of bubbling CO2 into solutions made from the ashes showed that 65–97% by weight of the carbonates could, in practice, be transformed into bicarbonates. This enhanced method can make the biomass ashes fix much more CO2 than they would under natural absorption conditions. Therefore, proper use of biomass ash may help to mitigate climate change in a distributed way if the fertilization process is improved.

When high-sulfur coal is burnt with lime, the mineral phases of fly ash may be discussed within a SiO2−Al2O3−CaO−CaSO4 system. By obtaining the absent thermochemical property data of the key mineral 3CaO·3Al2O3·CaSO4, the quaternary system was studied thermodynamically. It has been concluded that 2CaO·SiO2 and 3CaO·3Al2O3·CaSO4 may become the dominant minerals of the fly ash if sufficient solid phase reactions can be ensured. Because 3CaO·3Al2O3·CaSO4 and 2CaO·SiO2 are both important cement minerals, there will be great potential to improve the ash characteristics in combustion of high-sulfur coal. This finding gives substantial support to our previous studies of cement clinker cogeneration in pulverized coal combustion boilers. The results will also be very helpful to study the sulfur fixation in high-temperature combustion for which the formation of 3CaO·3Al2O3·CaSO4 is critical. In addition, the knowledge of the mechanism for involved chemical reactions may benefit the production of sulfoaluminate cement as well as the modification of portland cement.

The mineral phase evolution of ash was studied when high-sulfur coal was burnt with CaO at different proportions. Experiments were carried out in a drop-tube furnace. Modeling via FACTSAGE was performed by obtaining the absent thermochemical property data of the key mineral 3CaO·3Al2O3·CaSO4 and expanding the database. It was found that the ash mineral phases follow a certain rule to evolve and have an immense potential for modification. At low CaO proportion, 2CaO·Al2O3·SiO2 might be the dominant mineral phase in ash. However, with the increase of CaO addition, the amount of 2CaO·Al2O3·SiO2 may decrease and the amount of 2CaO·SiO2 and 3CaO·3Al2O3·CaSO4 may increase noticeably. Owing to the excellent hydraulic characteristics of the latter two minerals, coal ash is very possible to become cement-like. The findings may open up an efficient way to burn the high-sulfur coal. The burning of high-sulfur coal with lime may result in an attractive sulfur fixation as well as a high-quality byproduct.

In this work, the possibility of achieving fixation of CO2 using Ca and Mg ions was tested and verified. Concentrated seawater from desalination plants, subsurface brines, industrial effluents with high hardness, and/or natural seawaters that are rich in Ca2+ and Mg2+ could all be potential aqueous sources. Theoretical analyses indicated that the carbonation reaction could be enhanced by raising the pH or the CO2 partial pressure. Experiments using synthesized seawater confirmed this possibility. Over 90% of the Ca2+ and Mg2+ ions in the seawater could be converted by precipitation in the forms of MgCO3 and dolomite [MgCa(CO3)2], and the kinetics of the process was found to be quite acceptable. It was found that 1 m3 of natural seawater could fix about 1.34 m3 or 2.65 kg of CO2 (gas volume, standard conditions), and the potential of concentrated seawater is 2–3 times this value. Even if the annual CO2 emissions of the entire world were captured in this way, the concentration of Ca2+/Mg2+ in natural seawater would change at only the part-per-million scale, such that the ecological effects could be negligible. This idea has great potential for application. It might be able to realize not only the permanent fixation of CO2 but also the production of large amounts of carbonate byproducts.

The combination of microwave-induced pyrolysis and mechanical processing is a promising way to recycle the waste printed circuit boards (WPCBs). In pyrolysis, WPCBs yield an average of 78.6 wt.% solid residues, 15.7 wt.% oil, and 5.7 wt.% gas. The solid residues are rich in metals; the oil is abundant with phenol and substituted phenols which can be reclaimed as chemicals or fuels; and the gas is combustible with a caloric value of 4504 kcal/m3. Our featured mechanical processing, including crushing and specially designed sink-float separation, is very suitable for metal reclamation from the pyrolysis residues. Over 99 wt.% of metals can be dissociated by the crushing step; the final recycling rate and grade of metals in the separation step can amount to 95 wt.% and 96.5%, respectively. The economic assessment reveals that the combined treatment is amazingly profitable and very promising to tackle the challenges posed by the electronic scraps.

In view of the ambiguity concerning the effect of CO2 on the consumption of desulfurizing agent and based on our previous findings when studying flue gas desulfurization (FGD) byproduct, a specific study on the influence of CO2 has been carried out here with Ca(OH)2 being employed as a desulfurizing agent. This study is based on compositional analyses of the reaction products, and combined measurements have provided relatively precise compositions of the reaction products derived from Ca(OH)2 and flue gases. It has been found that the presence of CO2 does have an effect on the desulfurization reaction and on the consumption of desulfurizing agent when Ca(OH)2 is employed as the sorbent for SO2. Also, the formation of CaCO3 is inevitable at about 70 °C and is enhanced under conditions of high humidity. Although over 90% SO2 removal efficiency could be achieved with a relatively high Ca/S mole ratio, the effective utilization of desulfurizing agent would be low because CaCO3 is not the targeted product. Therefore, in order to improve semidry FGD technology, measures must be taken to prevent the reaction between CO2 and Ca(OH)2 so as to increase the utilization ratio of the desulfurizing agent.

The utilization of dry flue gas desulfurization (FGD) residue has become an environmental issue in China. A new method is introduced here to produce sulfoaluminate cement using the dry FGD residue and fly ash without modification of the cement plant equipments. The bench- and pilot-scale experimental studies indicated that the obtained sulfoaluminate cement product had excellent performance. FACTSAGE was improved by a new database and then used for raw material proportion optimization. The mineral phases of a CaO−SiO2−Al2O3−CaSO4 system after calcination were calculated. The theoretical and experimental results are accordant. This technology has good potential to decrease the raw material costs and energy consumption at cement plants. It is also beneficial in CO2 emission reduction and circular economy development. The calculation results may be generally referred in the studies on the chemical processes such as sulfur fixation at high temperature, mineral optimization, and cement production, etc.

•Toluene decomposition was investigated over Fe0/ZSM-5 under microwave irradiation.•Fe0 was doped into a ZSM-5 catalyst by wet impregnation.•Microwave irradiation primarily contributed to the catalytic efficiency.•Discharge occurred at Fe0 micro-tips in the catalyst.A new Fe0/ZSM-5 catalyst was investigated for tar decomposition under microwave irradiation using a model compound (methylbenzene). Systematic thermal imaging, temperature-programmed oxidation, and X-ray photoelectron spectroscopy were used to investigate the decomposition mechanism and the product composition. The microwave cracking of the tar model was efficiently catalyzed by Fe0/ZSM-5 with micro-tip discharging. While the methylbenzene concentration decreased slightly when microwaved over H-ZSM-5, it was significantly reduced, by 50%, when irradiated over Fe0/ZSM-5. These results suggest that micro-tip discharging, which was possibly induced by microwave radiation, is primarily responsible for the catalytic cracking of methylbenzene in this system. These results will benefit future investigations related to thermal cracking.Download high-res image (128KB)Download full-size image

Calcium sulfoaluminate, 3CaO·3Al2O3·CaSO4, has been widely recognized in the realms of high-temperature combustion and cement chemistry. However, the lack of relevant thermodynamic data limits the progress of its research and development. Through comparative calculations using several different approaches, we obtain the thermochemical properties of 3CaO·3Al2O3·CaSO4 in this work, such as its standard formation enthalpy, Gibbs free energy of formation, entropy, and molar heat capacity. With these fundamental data, thermodynamic calculations become possible for reactions involving this mineral. It is found that some reactions proposed in literature to generate this mineral may not proceed thermodynamically.

•Microwave metal discharge was investigated for degradation of biomass tar.•Thermal and luminous effects associated with discharge were studied.•Microwave metal discharge successfully decreased toluene levels by 50%.•A method for studying the mechanism of discharge process was developed.•Discharge process could be used broadly in biomass energy conversion.Considering the global energy and environment crisis, biomass energy is a major research focus. Gasification is a commonly used biomass energy conversion technology, but it inevitably yields tar as a by-product, accompanied by many hazards. This study investigates the use of microwave metal discharge as an energy-efficient alternative to current technologies for processing and treating tar from biomass gasification. The related special effects and factors affecting metal discharge were also investigated. The experimental and analytical results confirmed that microwave metal discharge could easily degrade toluene, reaching more than 50% degradation in the presence of very few (n = 5) discharge points. Compared with the traditionally employed tar-cracking process, the proposed process has distinct advantages and characteristics, particularly regarding speed and efficiency. In addition, microwave metal discharge achieves an excellent combination of multi-physical effects of light, heat, and plasma. In this study, the thermal and luminous effects associated with the discharge process were successfully studied in isolation, and a feasible mechanism research method was obtained by the appropriate test instruments and characterization parameters. This facilitates study on the mechanism of toluene degradation by microwave metal discharge. In-depth studies of the mechanism are necessary to enable the potential applications of microwave-assisted pyrolysis, pollutant removal, organic synthesis, and material preparation and regeneration.