Toshiaki Yamamoto

Control of volatile organic compounds by an AC energized ferroelectric pellet reactor and a pulsed corona reactor

Two laboratory-scale plasma reactors, an alternating-current-energized ferroelectric (high-dielectric ceramic) packed-bed reactor and a nanosecond pulsed corona reactor, were constructured. The aim was to develop baseline engineering data to demonstrate the feasibility of applying plasma reactors to the destruction of various volatile organic compounds (VOCs). Gas retention time, concentration, and corona power were varied to determine the effect on destruction efficiency, using gas chromatography for each VOC. Complete destruction was obtained for toluene. Conversions of methylene chloride at 95% and trichlorotrifluoroethane (known as CFC-113) at 67% were achieved. The conversion was dependent on the electron energy of the reactor and was also related to how strongly the halogen species were bonded with carbon.<<ETX>>

The dependence of nonthermal plasma behavior of VOCs on their chemical structures

Abstract The dependence of nonthermal plasma behavior of volatile organic compounds (VOCs) on their chemical structures was investigated, using methane, ethane, ethylene, butane, 1,1,2-trichloroethane, trichloroethylene, and tetrachloroethylene as target VOCs. It has been shown that the above VOCs decompose homolytically via their excited states, irrespective of their electron affinities. Efficiencies of energy transfer from hot electrons to the VOCs could be important in the initial steps of their decomposition. Active oxygen species can be involved in the decomposition of nonchlorinated hydrocarbons under humid conditions. Byproduct distributions have been affected by residence time, carrier gas, and humidification, depending on VOC structures.

Catalysis-assisted plasma technology for carbon tetrachloride destruction

Catalysis-assisted plasma technology is a new concept developed to decompose carbon tetrachloride (CCl/sub 4/), one of the greenhouse gases. A laboratory-scale plasma reactor used a packed-bed reactor with 1 to 3 mm spherical BaTiO/sub 3/ pellets. One configuration of the system used a two-stage process in which the packed-bed reactor was followed by a 1% Pt/Al/sub 2/O/sub 3/ catalyst reactor. The other configuration employed a unique one-stage catalysis/plasma process in which the BaTiO/sub 3/ pellets were coated by active catalysts such as Co, Cu, Cr, Ni, and V. The power supplies used for these experiments were either a 50 Hz neon transformer or an 18 kHz invertor neon transformer. Experiments were performed to characterize the catalytic plasma reactor performance on CCl/sub 4/ destruction. Special attention was focused on the enhancement of the CCl/sub 4/ destruction and the reduction of byproduct CO production by catalyst selection in the one-stage reactor. >

Byproduct identification and mechanism determination in plasma chemical decomposition of trichloroethylene

Plasma chemical behavior of trichloroethylene (TCE) was investigated with a packed-bed ferroelectric pellet reactor and a pulsed corona reactor. Volatile byproducts were identified by gas chromatography and mass spectrometry (GC-MS), and it was shown that reactor type, TCE concentration, flow rate, background gas, and moisture affected TCE decomposition efficiency and product distribution. Byproduct distributions in nitrogen and the negative effect of oxygen and moisture on TCE decomposition efficiency show that TCE decomposition proceeds via initial elimination of chlorine and hydrogen atoms, the addition of which to TCE accelerates its decomposition. Active oxygen species like OH radical is less likely involved in the initial step of TCE decomposition in plasma. Triplet oxygen molecules (/sup 3/O/sub 2/) scavenge intermediate carbon radicals derived from TCE decomposition to give much lower yields of organic byproducts.

VOC decomposition by nonthermal plasma processing — a new approach☆

Abstract A new approach to describe VOC decomposition by nonthermal plasma processes is presented. In contrast to the conventional energy balance concept, this approach takes into account the influence of electron impact that is transferred to VOC molecules during decomposition process which time scale is involved. According to this new concept, energy maintains the VOC molecular structure in motion within a potential well. The VOC molecule is decomposed when it accumulates enough energy to overcome the potential well and to form radicals, which leads to the VOC decomposition homolitically via their excited states. A new approach was described using the decomposition rate constant. Based on this new theory, VOC can be decomposed at much lower energy level than the conventional energy balance concept, which was validated with the experimental study.

The effect of residence time on the CO/sub 2/ reduction from combustion flue gases by an AC ferroelectric packed bed reactor

An investigation has been conducted to reduce CO/sub 2/ from combustion gases using an AC ferroelectric packed bed reactor. This ferroelectric packed bed reactor consists of two mesh electrodes packed with ferroelectric particles between them. An AC voltage is applied to the reactor to generate partial or spark discharges. The results show that: (1) The CO/sub 2/ gas reduction rate increases with increasing flue gas residential time and primary applied power; (2) The CO/sub 2/ gas reduction rated increases with decreasing gas flow rate and dielectric constant ( xi /sub s/) of packed ferroelectric particles; (3) The CO/sub 2/ concentrations are reduced by up to 18000 p.p.m., and 108 g of CO/sub 2/ are reduced by 1 kWh of primary applied energy used in the packed bed reactor.<<ETX>>

Effects of turbulence and electrohydrodynamics on the performance of electrostatic precipitators

Abstract Numerical simulations of the turbulent diffusion equation coupled with the electrohydrodynamics (EHD) are carried out for the plate-plate and wire-plate ESPs. The local particle concentration profiles and fractional collection efficiencies have been evaluated as a function of three dimensionless parameters: the electric Peclet number (Pe), the EHD number (NEHD), and the reduced particle migration velocity, ϵ ∗ . The collection efficiency for the wire-plate ESP is significantly lower than that for the plate-plate ESP. For the wire-plate ESP, when the turbulent diffusion coefficient is less than 1×10−3 m2/s, the collection efficiency is not affected by any turbulence or EHD level. However, when the turbulent diffusion coefficient exceeds this critical value, the collection efficiency drops sharply, regardless of the values of the EHD number and particle migration velocity. The critical turbulent diffusion coefficient of the plate-plate ESP is higher, 5×10−3m 2/s, indicating that the collection efficiency is less sensitive to turbulence. The results emphasize the importance of controlling the quality of flow characteristics in the ESP.

Numerical Simulation of Three-Dimensional Tuft Corona and Electrohydrodynamics

The importance of high-voltage and low-current operation in the wire-duct precipitator has focused attention on collecting high-resistivity dust. The local current density of individual tufts is considerably higher even at a low average current level and therefore could contribute to both the formation of back corona in the collected-dust layer and the generation of the secondary flow. Numerical simulation for three-dimensional tuft corona is, successfully solved. The electrical characteristics of the tuft corona is investigated and the structure and role of the three-dimensional secondary flow and electrohydrodynamics in relation to transport of the fine particles is described.

Modification of surface energy, dry etching, and organic film removal using atmospheric-pressure pulsed-corona plasma

A laboratory-scale atmospheric-pressure plasma reactor, using a nanosecond pulsed corona, was constructed to demonstrate potential applications ranging from modification of surface energy to removal of surface organic films. For surface modification studies, three different substrates were selected to evaluate the surface energies: bare aluminum, polyurethane, and silicon coated with photoresist. The critical surface energy for all materials studied significantly increased after the plasma treatment. The effects of gas composition and plasma treatment time were also investigated. Photoresist, ethylene glycol, and Micro surfactant were used as test organic films. The etching rate of a photoresist coating on silicon was 9 nm/min. Organic film removal using atmospheric pressure plasma technology was shown to be feasible. >

Nonthermal Plasma Processing for Controlling Volatile Organic Compounds

Power consumption and byproducts analysis are two key issues that users must address in determining which nonthermal plasma technology is the most appropriate for certain applications. We compared the operating characteristics and power consumption for scaled-up nonthermal plasma devices: pulsed-corona, packed-bed, silent corona, and surface discharge plasma technologies. Advantages and disadvantages of each nonthermal plasma technology are discussed. Understanding of plasma chemistry or byproducts is also essential for the development of nonthermal plasma technologies. Plasma chemical reactions of trichloroethylene (TCE) and alkyl acetates were investigated using pulsed-corona and packed-bed reactors. The effects of excited electrons, background gas, moisture, and reactor-dependent phenomena on product distribution and chemical interaction were studied. The initial step of plasma chemical decomposition of TCE can be ascribed to the electron attachment (not to active oxygen species or OH radicals), followed by homolysis and/or heterolysis reactions. Extremely high decomposition of TCE was obtained in nitrogen. Byproduct formation was significantly suppressed under aerated conditions. Alkyl acetate decomposition was affected by reactor type and alkyl chain length.