Jiann-Yang Hwang

Synthesizing mullite from beneficiated fly ash

In the United States, approximately 50 million tonnes of fly ash are generated by electrical utilities annually. The current consumption rate for fly ash materials is less than 20% because most fly ash materials do not meet market specifications and the quality of the ash is inconsistent. A beneficiation process that produces quality-controlled fly ash components has been developed. The synthesis of mullite as a refractory material is one industrial application for the silicate sphere component from the beneficiated fly ash. As-received (untreated) fly ash did not produce a usable mullite refractory. This article discusses the fly ash beneficiation process and mullite synthesis.

Magnetic Loss in Microwave Heating

A simplified equation for quantifying magnetic loss in materials under microwave irradiation was derived to demonstrate the importance of magnetic loss in microwave heating. The magnetic losses for five ferrites, namely, BaFe12O19, SrFe12O19, CuFe2O4, CuZnFe4O4, and NiZnFe4O4 were calculated at 2.45 GHz using the derived equation. It is found that magnetic loss is up to approximately four times greater than dielectric loss in the microwave heating of ferrites. These results, through the calculations, theoretically demonstrate that magnetic dielectric materials are heated much faster in a magnetic field than in an electric field of the microwave applicator.

Microwave Power Absorption Characteristics of Ferrites

The microwave power absorption characteristics of five ferrites, BaFe₁₂O₁₉, SrFe₁₂O₁₉, CuFe₂O ₄, CuZnFe₄O₄ and NiZnFe₄O₄, have been studied by determining the average power absorption patterns of the ferrite slabs using the derived power equations with consideration of both microwave dielectric and magnetic losses. It is shown that microwave power dissipates rapidly in the ferrites with a thickness of microwave penetration depth <i>Dp</i>. The magnetic loss <i>QH</i>, <i>av</i> is much larger than the dielectric loss <i>QE</i>,av for these ferrites in the slab thickness range of 0-3<i>Dp</i>. The ratio of <i>QH</i>,av to <i>QE</i>,av remains above 1.5 as the slab thickness varies from <i>Dp</i>/4 to 3<i>Dp</i> although the difference between losses decreases with increasing thickness. The power absorption patterns of these ferrites also demonstrate that microwave resonance caused by magnetic loss occurs within the materials having a thickness less than a half-wavelength in the medium λ<i>m</i>/2 . It is anticipated that microwave magnetic field heating of ferrites with a thickness smaller than λ<i>m</i>/2 can be accelerated by magnetic loss resonance which leads to a significant increase in the power absorption in these ferrites.

Absorber Impedance Matching in Microwave Heating

A mathematical function for evaluating the impedance matching degree of microwave absorbers was proposed to increase microwave absorption in microwave heating. For nonmagnetic dielectrics, the value of the function depends on the ratio of the sample thickness to the microwave penetration depth and the dielectric loss tangent phase angle. The maximum sample thickness corresponding to perfect impedance matching will decrease with increasing dielectric loss tangent phase angle, and thus, the highest microwave absorption can be achieved. The calculations of impedance matching for hematite indicate that hematite of less than 0.10 m thickness can obtain the maximum absorption throughout the microwave heating process.

Maximum Sample Volume for Permittivity Measurements by Cavity Perturbation Technique

The maximum sample volume for accurate permittivity measurements of dielectric materials having various geometries (rod/bar, strip/disk, and sphere) by cavity perturbation technique has been investigated by determining the maximum volume ratio of sample to cavity (V_s/V_c)max based on analysis of the measurement theory. It is demonstrated that (V_s/V_c)max of a dielectric rod/bar with the height equal to that of resonant cavity relies exclusively on the relative dielectric constant, whereas (V_s/V_c)max of a dielectric strip/disk or sphere depends on both the relative dielectric constant and the dielectric loss factor. There is a relatively weak permittivity dependence of (V_s/V_c)max for dielectric property measurements of dielectric strips/disks compared with rods/bars or spheres. The maximum sample volume used in the measurements for different sample geometries follows the order: . Comparison between (V_s/V_c)max of low-loss Al₂O₃ and high-loss SiC reveals that low-loss materials can have a larger sample volume than high-loss materials for measurement. High-loss materials may require a strip/disk geometry to meet the measurement requirements. The variation in (Vs/Vc)max of Al₂O₃ having different geometries in a broad temperature range up to ~ 1400^°C shows that (V_s/V_c)max of the sample decreases with increasing temperature and the change in (V_s/V_c)max should be considered during the high-temperature permittivity measurements.

SYNTHESIS OF MONODISPERSED IRON OXIDE PARTICLES BY A LARGE-SCALE MICROWAVE REACTOR

Well-defined mondispersed iron oxide particles can be synthesized in conventional hydrolysis approaches, but the synthesis generally takes two to seven days. Microwave hydrothermal approaches offer the possibility of significantly reducing synthesis time. This work demonstrates the feasibility of microwave synthesis in large scale. The batch size was 5 L, and the holding time was two hours. The crystallinity of the powders and the morphology and the monodispersity of the synthesized particles were comparable to those produced from conventional approaches. The particle size was in the nanosize range. The results indicated that for synthesis of monodispersed Fe2O3 nanopowders, it is possible to achieve the production rate of a 2.1 m3 conventional reactor with a 5 L microwave accelerated reactor.

Microwave Absorption Characteristics of Tire

The recycling of waste tires has been a big environmental problem. About 280 million waste tires are produced annually in the United States and more than 2 billion tires are stockpiled, which cause fire hazards and health issues. Tire rubbers are insoluble elastic high polymer materials. They are not biodegradable and may take hundreds of years to decompose in the natural environment. Microwave irradiation can be a thermal processing method for the decomposition of tire rubbers. In this study, the microwave absorption properties of waste tire at various temperatures are characterized to determine the conditions favorable for the microwave heating of waste tires.

UTILIZATION OF LOW NOx COAL COMBUSTION BY-PRODUCTS

Low NO{sub x} combustion practices are critical for reducing NO{sub x} emissions from power plants. These low NO{sub x} combustion practices, however, generate high residual carbon contents in the fly ash produced. These high carbon contents threaten utilization of this combustion by-product. This research has successfully developed a separation technology to render fly ash into useful, quality-controlled materials. This technology offers great flexibility and has been shown to be applicable to all of the fly ashes tested (more than 10). The separated materials can be utilized in traditional fly ash applications, such as cement and concrete, as well as in nontraditional applications such as plastic fillers, metal matrix composites, refractories, and carbon adsorbents. Technologies to use beneficiated fly ash in these applications are being successfully developed. In the future, we will continue to refine the separation and utilization technologies to expand the utilization of fly ash. The disposal of more than 31 million tons of fly ash per year is an important environmental issue. With continued development, it will be possible to increase economic, energy and environmental benefits by re-directing more of this fly ash into useful materials.

Characterization of Low-Zinc Electric Arc Furnace Dust

Electric arc furnace (EAF) dust is an important secondary resource that should be recycled to enhance its considerable economic value and potential environmental benefit. In this study, a low-zinc EAF dust was characterized by various techniques, including chemical titration, X-ray diffraction, granulometric analysis, scanning electron microscopy and thermogravimetry. It is shown that the dust contains 2.08 wt% Zn, 23.16 wt% Fe and 19.84 wt% Ca, accompanying small amounts of Cr, Pb, etc. Magnetite, calcium ferrite and zinc ferrite are the main phase constituents. The majority (90%) of particles have size less than 137.862 μm. According to these characteristics, it is expected that the use of microwave energy for intensification of the reduction of EAF dust in the presence of biochar will succeed in the dust recycling by promoting the processing efficiency with elimination of secondary hazardous pollutants.

Waveguides for long-distance energy transport in microwave heating

ABSTRACT Performance of waveguides for long-distance energy transport in microwave heating was evaluated by comparing the microwave propagation characteristics, including maximum transmitted power Ptrans, attenuation constant α and percentage of power loss ploss proposed in this study, of commercially available rectangular waveguides (WRs) constructed of various metals (Ag, Cu, Ag, Al and Fe) for microwave heating at 0.915 and 2.45 GHz with those of different types of circular waveguides, namely circular waveguides with the same cut-off frequency as rectangular waveguides (denoted by WCFs), circular waveguides with the size equal to rectangular waveguide (denoted by WCDs), and circular waveguides with the same circumference as rectangular waveguides (denoted by WCCs). It is demonstrated that WCFs exhibit the highest values of maximum Ptrans together with lowest α and ploss due to their characteristics of microwave field configuration. Circular waveguides (e.g. WCF1150) made of Cu or Al with large dimensions operating at 0.915 GHz are suitable for microwave heating through long-distance microwave energy transport over the distance compared to traditional rectangular waveguides, with up to ∼50% reduction of power loss. Optimal waveguides for long-distance uses can be obtained by assessing microwave propagation characteristics together with manufacturing cost, depending on specific applications.