K. Urashima

Removal of volatile organic compounds from air streams and industrial flue gases by non-thermal plasma technology

Gaseous pollution control technologies for acid gases (NO/sub x/, SO/sub x/, etc.), volatile organic compounds (VOC), greenhouse gases, ozone layer depleting substance (ODS), etc., have been commercialized based on catalysis, incineration and adsorption methods. However, non-thermal plasma techniques based on electron beams and corona discharges become significant due to advantages such as lower cost, higher removal efficiency, smaller space volume, etc. In order to commercialize this new technology, the following needs faster investigation: pollution gas removal rates, energy efficiency of removal, pressure drop of reactors, usable byproduct production rates, identification of major fundamental processes, and optimization of reactor and power supply for an integrated system. In this work, recent development of gaseous pollution control technology based on discharge plasmas is reviewed critically and the principle of processes and reactor technologies are outlined.

Removal of NF/sub 3/ from semiconductor-process flue gases by tandem packed-bed plasma and adsorbent hybrid systems

A tandem hybrid gas cleanup system, consisting of a BaTiO/sub 3/ packed-bed plasma reactor and a CaCO/sub 3/ adsorbent filter, was used to study the removal of NF/sub 3/ from semiconductor-process flue gases. Plasma-chemical kinetics of N/sub 2/-NF/sub 3/-O/sub 2/-H/sub 2/ gas mixtures suggested byproducts observed in the experiments. The laboratory-scale system showed NF/sub 3/ removal at atmospheric pressure. Typically, 100% NF/sub 3/ abatement was achieved with an inlet concentration of 5000 ppm and a gas residence time in the reactor less than 10 s.

Reduction of NO/sub x/ from combustion flue gases by superimposed barrier discharge plasma reactors

NO/sub x/ reduction from combustion flue gases by superimposed barrier discharge plasma reactors is experimentally investigated. The experiments are conducted for applied voltages from 0 to 28 kV, flue gas rates from 0.5 to 2 L/min, ammonia mixture concentrations from 0.7 to 2.65 stoichiometry, and applied voltage phase differences from 0/spl deg/ to 180/spl deg/, where two 60-Hz AC power supplies are used. The results show the following: (1) NO/sub x/ reduction rate decreases with increasing discharge power for surface discharge operations, however, NO/sub x/ reduction rate increases with increasing discharge power for silent and superimposing discharge operating modes; (2) NO/sub x/ reduction rate increases with increasing discharge power, gas flow rate and ammonia stoichiometry under in-phase operations; (3) NO/sub x/ reduction rate for out-of-phase operations is much higher compared with in-phase operations, however, NO/sub x/ reduction rate has an optimum condition on ammonia stoichiometry, discharge power, and gas flow rate; and (4) energy efficiency of NO/sub x/ reduction increases with increasing ammonia mixture and gas flow rate and decreases with increasing discharge power.

Ferro-electric pellet shape effect on C/sub 2/F/sub 6/ removal by a packed-bed-type nonthermal plasma reactor

Ceramic dielectric material pellet shape effects on the performance of perfluoroethane (C/sub 2/F/sub 6/) gas removal from simulated semiconductor process gas using packed-bed reactor are experimentally investigated. The bench-scale cylindrical shaped (plasma part: 30-mm inner diameter and 20-mm length) plasma reactor consists of two metal mesh electrodes packed with spherical, cylindrical, or hollow cylindrical shaped ferro-electric pellets with various dielectric constants. The 60-Hz ac high voltage was applied to the mesh electrode. The 3000 ppm C/sub 2/F/sub 6/ gas diluted with nitrogen was used as simulated gas with flow rate of 30 mL/min. The C/sub 2/F/sub 6/ concentration was monitored using Fourier transform Infrared absorption spectroscopy measurements. The results show that the packed-bed plasma reactor with the hollow cylindrical-shaped pellets removed the C/sub 2/F/sub 6/ gas with energy efficiency of 3.7 g/kWh. This value was almost 1.5 times higher than the efficiency 2.5 g/kWh in case of the spherical pellets. The discharge characteristics in the reactor were also changed with the pellet shape. The discharge onset voltage decreases by changing the pellets shape from sphere to hollow cylinder. The quantity of charges accumulated with the microdischarge currents increases by changing the pellet shape from sphere to hollow cylinder in spite of fact that the energy consumed in the reactor decreases.

Reduction of NOx from combustion flue gases by corona discharge activated methane radical injections

Abstract An experimental investigation has been conducted to reduce NOx from combustion flue gases by means of corona discharge activated methane radical injection methods. Two types of corona radical injectors, the hollow needle type and the multihole type, are used in the present investigations. Experiments are conducted for the simulated flue gas (Nx:Ox:COxNO = 83.996:8:8:0.004) flow rate from I to 20L/min., the activation voltage from 0 to 40 KV and the Ar-methane mixture gas (95:5) flow rate from 0 to 200 mL/min. The results show that the NOx reduction increases with increasing activation voltage and nonmonotonically depends on methane stoichiometry. Trace amounts of N2O and NH3 were observed as a by product

Removal of C/sub 2/F/sub 6/ from a semiconductor process flue gas by a ferroelectric packed-bed barrier discharge reactor with an adsorber

The abatement of greenhouse gases from semiconductor processes is becoming important. Methane and/or nitrous oxide are continuously exhausted from the processes, and high concentrations of per-fluorocarbons (PFCs), such as NF/sub 3/, C/sub 2/F/sub 6/, SF/sub 6/, and CF/sub 4/, are exhausted during wafer etching and clean up of PECVD (plasma enhanced chemical vapor deposition) chambers. The removal of C/sub 2/F/sub 6/ from a simulated semiconductor-process flue gas was studied using a hybrid control system, consisting of a ferroelectric packed-bed barrier discharge reactor and an adsorber. The barrier discharge reactor was composed of BaTiO/sub 3/ ferroelectric pellets and was operated with AC voltages at 60 Hz. The adsorber was either artificial zeolite or activated carbon. Simulated flue gases consisted of N/sub 2/ or N/sub 2/-H/sub 2/O mixtures with 1000 to 3000 ppm C/sub 2/F/sub 6/. The experiments showed: (1) the removal efficiency for C/sub 2/F/sub 6/ increases with increasing applied voltage until the threshold for spark formation is reached; (2) the removal efficiency increases at lower temperatures and by use of the hybrid system, and decreases with increasing gas flow rate; (3) humidity significantly reduces the reactors efficiency as a result of the energy drawn from the discharge for H/sub 2/O molecule dissociation; (4) trace CF/sub 4/, CO, NO/sub 2/, N/sub 2/O, and SiF/sub 4/ are by-products of the control system; and (5) about 13.5 g of C/sub 2/F/sub 6/ is decomposed by 1 kWh of input electrical power to the hybrid system. CF/sub 4/ is a by-product from C/sub 2/F/sub 6/ removal, while the other by-products come from etching the reactors glass wall (SiO/sub 2/) by fluorine released from C/sub 2/F/sub 6/ removal.

Destruction of volatile organic compounds in air by a superimposed barrier discharge plasma reactor and activated carbon filter hybrid system

The superimposed barrier discharge and activated carbon filter hybrid systems are used to remove toluene and trichloro-ethylene (TCE) from air streams. The superimposed barrier discharge consists of silent and surface discharges. Experiments are conducted for the gas flow rate from 1 to 10 L/min., applied power from 0 to 7 W and toluene and TCE initial concentration from 0 to 2,000 ppm for 60 Hz AC applied voltage conditions. Discharge byproducts are measured by FTIR, GC and TLV VOC detector. The results shows that: (1) toluene decomposition rate monotonically increases with increasing applied power; (2) approximately 90% of toluene is removed by plasma reactors alone and up to 98% is removed by hybrid systems; (3) TCE removal rate by hybrid system is 90% and up to 50% is removed by a discharge reactor alone; (4) the pressure drop of the reactor and carbon filter increase with increasing gas flow rate; (5) TCE decomposition to form CO/sub 2/, H/sub 2/O and Cl/sub 2/ and except CO/sub 2/ and H/sub 2/O these discharge byproducts are absorbed in activated carbon filters; (6) no COCl/sub 2/, HCl, CO, NO/sub x/ and O/sub 3/ are observed in a discharge byproducts for the present range of experiments; and (7) the energy yield for toluene decompositions is up to 30 g/kWh, and up to 15 g/kWh for TCE decompositions.

Electrohydrodynamically induced flow direction in a wire-non-parallel plate electrode corona discharge

An experimental investigation has been conducted to study the net flow direction induced by electrohydrodynamic forces in a wire-non-parallel plate electrode corona discharge. The experiments were conducted at two different locations of corona wire electrode for negative and positive applied voltage from 0 to 14 kV at atmospheric pressure and room temperature, where air was used as the working fluid. It was experimentally revealed that the net flow direction of electrohydrodynamically induced gas flow in a wire-non-parallel plate electrode system was changed depending on the location of the corona wire electrode relative to the grounded electrode position.

Reduction of NO/sub x/ from natural gas combustion flue gases by corona discharge radical injection techniques

An experimental investigation has been conducted to reduce NO/sub x/ from natural gas combustion flue gases by means of corona discharge activated ammonia/methane radical injection methods. The multi-hole type corona radical injectors are used in the present investigation. Experiments are conducted for the simulated natural gas combustion flue gas (N/sub 2/:O/sub 2/:CO/sub 2/:NO=83.96:8:8:0.04) flow rate from 1 to 200 l/min., activation voltage (DC or pulse) from 0 to 40 kV and Ar-ammoniathe or Ar-methane mixture gases flow rate from 0 to 200 ml/min. The results show that the NO/sub x/ reduction increases with increasing activation voltage and nonmonotonically depends on ammonia/methane stoichiometry.

Electrohydrodynamic gas flow regime map in a wire-plate electrostatic precipitator

In this paper, the particle image velocimetry measurements of the flow velocity fields in a wire-plate type electrostatic precipitator (ESP) with a single negatively polarized wire were analyzed for wide range of Reynolds and electrohydrodynamic numbers, and their influence on the dust particle collection efficiency was investigated. The results show that the dust particle collection was influenced by electrohydrodynamic (EHD) secondary flow when Ehd/Re/sup 2//spl ges/1 and ReSc/spl ges/F/sub E/, where Sc is the Schmidt number and F/sub E/ is the electric field number.