Cheng-Hsien Tsai

Formation of fluorine for abating sulfur hexafluoride in an atmospheric-pressure plasma environment

In this study, a large amount of toxic and reactive fluorine (F(2)) was produced in the atmospheric-pressure microwave discharge environment by adding additives to abate sulfur hexafluoride (SF(6)). When H(2) was added, the selectivity of F(2) was as high as 89.7% at inlet H(2)/SF(6) molar ratio (R(H2)) = 1. Moreover, the conversion of SF(6) significantly increased from 33.7% (without additive) to 97.7% (R(H2) = 5) at [SF(6)]=1%, and 0.8 kW because the addition of H(2) inhibited the recombination of SF(6). With the addition of O(2), H(2)+O(2) or H(2)O, the selectivity of F(2) was still greater than 81.2%, though toxic byproducts, including SO(2)F(2), SOF(2), SOF(4), SO(2), NO, and HF, were detected. From optical emission spectra, SF(2) was identified, revealing the SF(6) dissociation process might be carried out rapidly through an electron impaction reaction: SF(6)-->SF(2)+4F. Subsequently, F(2) was formed via the recombination of F atoms.

Decomposition of SF6 in an RF Plasma Environment

Abstract Sulfur hexafluoride (SF6)-contained gas is a common pollutant emitted during the etching process used in the semiconductor industry. This study demonstrated the application of radio-frequency (RF) plasma in the decomposition of SF6. The decomposition fraction of SF6 [ηSF6 (Cin–Cout)/Cin x 100%] and the mole fraction profile of the products were investigated as functions of input power and feed O2/SF6 ratio in an SiO2 reactor. The species detected in both SF6/Ar and SF6/O2/Ar RF plasmas were SiF4, SO2, F2, SO2F2, SOF2, SOF4, S2F10, S2OF10, S2O2F10, and SF4. The results revealed that at 40 W, ηSF6 exceeded 99%, and the reaction products were almost all converted into stable compounds such as SiF4, SO2, and F2 with or without the addition of oxygen. Sulfur oxyfluorides such as SO2F2, SOF2, SOF4, S2OF10, and S2O2F10 were produced only below 40 W. The results of this work can be used to design a plasma/chemical system for online use in a series of a manufacturing process to treat SF6-containing exhaust gases.

Difference in Conversions Between Dimethyl Sulfide and Methanethiol in a Cold Plasma Environment

This study compared the conversion of two malodorous substances, dimethyl sulfide (CH3SCH3, DMS) and methanethiol (CH3SH) in a cold plasma reactor. The DMS and CH3SH were successfully destroyed at room temperature. DMS decomposed less than CH3SH at the same conditions. In oxygen-free condition, CS2 and hydrocarbons were the major products, while SO2 and COx were main compounds in oxygen-rich environments. The DMS/Ar plasma yielded more hydrocarbons and less CS2 than that of CH3SH/Ar plasma. In the CH3SH/O2/Ar plasma, rapid formation of SO and CO resulted in the yields much more amounts of SO2 and CO2 than those in the DMS/O2/Ar plasma; and remained only a trace of total hydrocarbons, CH2O, CH3OH, CS2, and OCS. The major differences between the reaction mechanisms of DMS and CH3SH were also proposed and discussed.

Chemical and physical properties of plasma slags containing various amorphous volume fractions

In this study, municipal solid waste incinerator fly ash was vitrified using a plasma torch. The fly ash contained rich Ca, causing a high basicity of 2.43. Pure quartz was used as an additive to adjust the basicity. BET surface area analysis, X-ray diffraction analysis, and a scanning electron microscope were used to examine the physical properties of slags. The chemical stability and the acid resistance of slags were evaluated using the toxicity characteristics leaching procedure and tests of acid bathing. The results indicate that the plasma torch effectively vitrified the fly ash. Anthropogenic metals with low boiling points, such as Cd, Pb, and Zn, were predominately vaporized into flue gas. Most of the metals with high boiling points, such as Cr, Cu, and Mn, remained in the slag. After the vitrification, hazardous metals were noticeably immobilized in all slags. However, the slags with higher amorphous volume fractions were more effective in metal immobilization and in resisting acid corrosion. This indicates that SiO(2) enhanced the formation of the glassy amorphous phase and improved the resistance of acid corrosion and the immobilization of hazardous metals.

Effects of Burnings of Wax Apple Stubble and Rice Straw on Polychlorinated Dibenzo-p-Dioxin and Dibenzofuran Concentrations in Air and Soil

Abstract Measurements of the concentrations of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) were made in ambient air, ash, and soil impacted by the open burning of wax apple and rice straw residues. Measurements showed that the mean PCDD/F concentration (0.458 pg I-TEQ/Nm3; international toxicity equivalence) in air at two wax apple orchards during open burning increased markedly, ~8.1 times higher than that (0.057 pg I-TEQ/Nm3); before open burning. In addition, the mean PCDD/F concentration (0.409 pg I-TEQ/Nm3) in ambient air at a rice straw field was 4.6 times higher than that (0.089 pg I-TEQ/Nm3) before open burning. After burning the residues of wax apple stubble and rice straw, the contents of PCDD/F in ashes were 1.393 and 1.568 ng I-TEQ/kg-ash, respectively, and the contents of PCDD/F in soil were 2.258 and 2.890 ng I-TEQ/kg-soil, respectively. Therefore, the turnover of soil with the ash after open burning over years will result in the accumulation of PCDD/Fs in farm soils.

Encapsulation Behaviors of Metals in Slags Containing Various Amorphous Volume Fractions

Abstract In this study, a melting process with addition of SiO2was applied to treat incinerator fly ash. To describe the encapsulation behaviors of metals quantitatively, the amorphous volume fraction (AVF) of slags was initially determined. Vitrification appeared to reduce the mobility of Cr, Cu, Mn, and Ni instead of significantly immobilizing Cd, Pb, and Zn. It was verified that SiO2enhanced the formation of an amorphous glassy structure. With the increase of SiO2, the crystalline phases would gradually diminish and transform into a higher silica-connected species. During the formation of slag matrix, Al, Ca, and Mg could modify the glass network, and consequently the encapsulation behaviors of these species would noticeably affect the chemical stability of slags. Significant immobilization of crust metals could be achieved only when a more compact and interconnected amorphous glass network was formed. Hence, it indicated that a higher AVF silica-based slag had a better potential to resist acid attack. In conclusion, for environmental protection, it is important to investigate the correlation between the encapsulation behaviors of metals and the crystalline characteristics of slag structure.

Size distribution of airborne fungi in vehicles under various driving conditions.

ABSTRACT Circulation or air conditioning (AC) system was proven to improve the air quality inside the vehicles; however, the quantified study was limited. In this study, fungal concentration under various driving mode inside the vehicle was proposed. The driving conditions were classified into 4 states: (1) window closed without AC and circulation, (2) window open without AC and circulation, (3) window closed with only circulation on, and (4) window closed with only AC on. Results show that at state 4, the mean respirable fraction was 83.3%, with a number median diameter of the fungi being 1.73 μm. More attention should be paid for these smaller fungi easily penetrating into the alveoli and probably lead to allergic alveolitis. Turning on AC for reducing the normalized concentration for each size range of fungi was suggested; however, the respirable fraction increased. Those who are prone to allergies or asthma are suggested to switch between AC and the circulation mode while driving a long time.

Interactive analysis of waste recycling and energy recovery program in a small-scale incinerator.

Abstract Conflicting goals affecting solid waste management are explored in this paper to find the best implementation of resource recovery with a small-scale waste-to-energy process. Recycling paper and plastic material often leaves a shortage of thermal energy to support incineration that forces operators to supplement the process with auxiliary fuels. Although there are considerable profits to be made from material recovery, the increase of fuel usage causes conflict given that it is cost prohibitive. A series of trials performed on a small-scale 1.5-t/day incineration plant with a cyclone heat recovery system found that material recycling can impede performance. Experimental results are expressed as empirical regression formulas with regard to combustion temperature, energy transfer, and heat recovery. Process optimization is possible if the waste moisture content remains <30%. To test the robustness of the optimization analysis, a series of sensitivity analyses clarify the extent of material recycling needed with regard to plastic, paper, and metal. The experiments also test whether the moisture in the waste would decrease when recycling paper because of its exceptional capacity to absorb moisture. Results show that recycling paper is strongly recommended when the moisture content is >20%, whereas plastic recycling is not necessary at that moisture condition. Notably, plastic recovery reduces the heat needed to vaporize the water content of the solid waste, thus it is recommended only when the moisture content is <10%. For above-normal incineration temperatures, plastic recycling is encouraged, because it removes excess energy. Metal is confirmed as an overall priority in material recycling regardless of the moisture content of the incoming waste.

Effects of additives on the selectivity of byproducts and dry removal of fluorine for abating tetrafluoromethane in a discharge reactor

The removal efficiency of tetrafluoromethane (CF(4)) was significantly enhanced by adding additives (H(2), O(2), H(2)+O(2), H(2)O) in an atmospheric-pressure microwave plasma reactor. However, large amounts of fluorine (F(2)) were produced in this study. Moreover, the selectivity of F(2) was apparently greater than that of HF (in H(2)-based condition) or COF(2) (in O(2)-based abatement). Notably, in an O(2)-rich environment, more F(2) and a larger amount of CO(2) were produced. Subsequently, F(2) can be effectively removed by reacting with CaO to form CaF(2) at 200 degrees C via an in situ dry, chemical absorption process in the low-temperature afterglow discharge zone within the same plasma reactor.

Deodorization of Dimethyl Sulfide Using a Discharge Approach at Room Temperature

Abstract The traditional technologies for odor removal of thiol usually create either secondary pollution for scrubbing, adsorption, and absorption processes, or sulfur (S) poisoning for catalytic incineration. This study applied a laboratory-scale radio-frequency plasma reactor to destructive percentage-grade concentrations of odorous dimethyl sulfide (CH3SCH3, or DMS). Odor was diminished effectively via reforming DMS into mainly carbon disulfide (CS2) or sulfur dioxide (SO2). The removal efficiencies of DMS elevated significantly with a lower feeding concentration of DMS or a higher applied rf power. A greater inlet oxygen (O2)/DMS molar ratio slightly improved the removal efficiency. In an O2-free environment, DMS was converted primarily to CS2, methane (CH4), acetylene (C2H2), ethylene (C2H4), and hydrogen (H2), with traces of hydrogen sulfide (H2S), methyl mercaptan (CH3SH), and dimethyl disulfide. In an O2-containing environment, the species detected were SO2, CS2, carbonyl sulfide, carbon dioxide (CO2), CH4, C2H4, C2H2, H2, formal-dehyde, and methanol. Differences in yield of products were functions of the amounts of added O2 and the applied power. This study provided useful information for gaining insight into the reaction pathways for the DMS dissociation and the formation of products in the plasmolysis and conversion processes.