Yanpeng Mao

Numerical simulation of hot-spot effects in microwave heating due to the existence of strong microwave-absorbing media

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.

Pyrolysis of tyre powder using microwave thermogravimetric analysis: Effect of microwave power.

The pyrolytic characteristics of tyre powder treated under different microwave powers (300, 500, and 700 W) were studied via microwave thermogravimetric analysis. The product yields at different power levels were studied, along with comparative analysis of microwave pyrolysis and conventional pyrolysis. The feedstock underwent preheating, intense pyrolysis, and final pyrolysis in sequence. The main and secondary weight loss peaks observed during the intense pyrolysis stage were attributed to the decomposition of natural rubbers and synthetic rubbers, respectively. The total mass loss rates, bulk temperatures, and maximum temperatures were distinctively higher at higher powers. However, the maximum mass loss rate (0.005 s-1), the highest yields of liquid product (53%), and the minimum yields of residual solid samples (43.83%) were obtained at 500 W. Compared with conventional pyrolysis, microwave pyrolysis exhibited significantly different behaviour with faster reaction rates, which can decrease the decomposition temperatures of both natural and synthetic rubber by approximately 110 °C–140 °C.

Experimental Study of Microwave-Induced Discharge and Mechanism Analysis Based on Spectrum Acquisition

When conductor or semiconductor materials are exposed to microwave radiation in different atmospheres (Ar, He, N2, and O2+N2) intense discharge phenomena are observed. The discharge phenomena, generated when strips of the metals Fe, Al, and Zn or particles of the semiconductor SiC are irradiated with microwaves, are characterized experimentally using spectrum acquisition and analysis. Filamentary discharge is observed in an Ar atmosphere while spark discharge is observed in He, N2, and O2+N2 atmospheres. The spectral lines of the discharges are concentrated mainly in the visible region, but there are also peaks in the ultraviolet region. The nature of the discharge and the specific details of the spectra are influenced by: 1) the target metal or semiconductor used; 2) the atmosphere; and 3) the microwave field characteristics. The spectra always consist of two parts: one corresponding to the target metal or semiconductor irradiated by the microwaves and the other due to the formation of the high-energy excitation states of atoms, molecules, and ions induced in the gaseous atmosphere. The microwave-induced discharge and the corresponding luminous and plasma effects have potential uses as energy sources in many applications including chemical or photocatalytic enhancement of reactions and the destruction of volatile organic compounds for which preliminary results are encouraging.

Coal fly ash/CoFe2O4 composites: a magnetic adsorbent for the removal of malachite green from aqueous solution

A coal fly ash/CoFe2O4 (CFA/CFO) magnetic composite was synthesized by a facile hydrothermal synthesis method, and then used as an adsorbent for the removal of malachite green (MG) dye from water. The structure, morphology, and properties of the composite were characterized by XRF, SEM, XRD, FT-IR, BET and VSM techniques. A batch adsorption experiment on the removal of MG was performed with CFA/CFO, and the effect of various experimental parameters, such as the adsorbent dosage and contact time of MG at 25 °C, were investigated. Three isotherm models, namely the Langmuir, Freundlich, and Dubinin–Kaganer–Radushkevich (DKR) models, were applied to describe the adsorption process. The results indicated that the Freundlich and DKR models fit quite well with the experimental data. Thermal and microwave methods were used and compared in the regeneration experiment. All the results indicated that the as-prepared composite should be considered as a potential low-cost adsorbent for easy and efficient removal of MG from water.

Self-Heat Recuperative Microwave and Hot-Air-Combined Drying of Lignite

ABSTRACT In this study, experiments on hot-air drying of lignite were conducted with some important parameters. The moisture content of lignite decreased from 33.40% to 20.84% when dried by hot air (temperature: 80°C, velocity: 4 m/s) alone for 30 min; the thermal efficiency of the hot-air dryer was about 82%. Later on, a self-heat recuperative microwave and hot-air-combined drying system for lignite was designed. The system was divided into three stages: The first stage was the preheat stage, using waste heat of steam evaporated in the microwave dryer to preheat the lignite; the second was the microwave drying stage, using microwaves to dry the preheated lignite; the third was the cooling stage, where dried lignite was cooled by the drying medium of the preheater. Assuming the final moisture content of dried product was 10.00%, the moisture content of lignite would reduce from 33.00% to 26.50% at the preheater and continue to reduce from 26.50% to 10.00% at the second stage. If half of the energy needed by the preheater was provided by the flue gas introduced, the moisture content of lignite would reduce from 33.00% to 21.55% at the preheater and continued to reduce from 21.55% to 10.00% at the microwave dryer.

Experimental study of destruction of acetone in exhaust gas using microwave-induced metal discharge

Volatile organic compounds (VOCs) are air pollutants that pose a major concern, and novel treatment technologies must be continuously explored and developed. In this study, microwave-induced metal discharge was applied to investigate the destruction of acetone as a representative model VOC compound. Results revealed that metal discharge intensity largely depended on microwave output power and the number of metal strips. Microwave metal discharge exerted the distinct combined effects of intense heat, strong light, and plasma. In the case of MW without metal discharge, the decrease in acetone at 200 ppm was remarkably limited (approximately 5.5% (mol/mol)). By contrast, in the case of microwave-induced metal discharge, a considerably high destruction efficiency of up to 65% (mol/mol) was obtained at low concentrations. This finding highlights the potential of microwave-induced discharge for VOC removal. Initial assessment indicated that energy consumption can be acceptable.

Process of CH 4 -CO 2 reforming over Fe/SiC catalyst under microwave irradiation

In this work, the properties of the CH4-CO2 reforming reaction over the Fe/SiC catalyst during the whole process were studied under microwave irradiation and the reaction process was analyzed by mass spectrometry and Fourier transfer infrared spectrometry in real time. The effects of microwave power on the gas composition, conversion of reactants, and selectivity of products in the reaction were investigated. It was found that the microwave dry reforming reaction can be divided into a rapid reaction stage, slow reaction stage, and reaction equilibrium stage. The conversion of reactants and selectivity of products in the slow reaction stage were both higher than 95% under 90 W/g. In the long-term (~50 h) stability test, a combination of SEM, XRD, BET, and TG analyses found that the catalyst activity did not reduce significantly and the amount of carbon deposits (which was mainly Cγ) was negligible (~0.78 wt%). The results indicate that the cheap Fe-based catalyst has good catalytic activity and stability under microwave irradiation and hence has a promising application.

Rapid degradation of malachite green by CoFe 2 O 4 –SiC foam under microwave radiation

This study demonstrated rapid degradation of malachite green (MG) by a microwave (MW)-induced enhanced catalytic process with CoFe2O4–SiC foam. The catalyst was synthesized from CoFe2O4 particles and SiC foam by the hydrothermal method. X-ray diffraction and scanning electron microscopy techniques were used to confirm that CoFe2O4 particles were settled on the surface of SiC foam. In this experiment, a novel fixed-bed reactor was set up with this catalyst for a continuous flow process in a MW oven. The different parameters that affect the MW-induced degradation rate of MG were explored. The MW irradiation leads to the effective catalytic degradation of MG, achieving 95.01% degradation within 5 min at pH 8.5. At the same time, the good stability and applicability of CoFe2O4–SiC foam for the degradation process were also discussed, as well as the underlying mechanism. In brief, these findings make the CoFe2O4–SiC foam an excellent catalyst that could be used in practical rapid degradation of MG.

Study of microwave-induced metal discharge under high pressure based on spectrum analysis

ABSTRACT Discharge phenomena can occur when metals are exposed to microwave radiation. The mechanism of the discharge can be effectively studied by using spectral analysis. In this study, a reactor that can withstand long-term high pressure was used in combination with a spectrometer to investigate the effects of pressure and other parameters on the microwave metal discharge plasma. It was observed that the increase of radiation power, pressure, and metal wire diameter can have positive effects on the intensity of spectral lines. If other experimental conditions remain unchanged, the spectral lines and discharge intensity exhibit dynamic changes because the wire melts and changes its shape under the high local temperature associated with discharge. Because factors like radiation power and pressure can enhance the intensity of the plasma significantly, they can be reasonably utilized to accelerate the chemical reactions or improve the degradation rate of pollutants, e.g. in microwave wet oxidation applications.

Microwave steam gasification of semi-coke derived from co-pyrolysis of fungus chaff and lignite

Steam gasification of different semi-coke of biomass (fungus chaff) and coal (lignite) has been carried out in a fixed-bed reactor in a microwave. All the semi-coke derived from different pyrolytic...