Air pollution control by electrical discharges

Air pollution caused by gas emission of pollutants produced from a wide ran ge of sources including coal, oil and gas burning power plants, diesel engi nes, paper mills, steel and chemical production plants must be reduced dras tically and urgently as mandated by recent worldwide national legislation w hich recently are being reinforced increasingly by international agreements . Non-thermal plasmas in which the mean energy of the electrons is substant ially higher than that of the gas offer advantages in reducing the energy r equired to remove the pollutants. The electrical energy supplied into the d ischarge is used preferentially to create energetic electrons which are the n used to produce radicals by dissociation and ionization of the carrier ga s in which the pollutants are present. These radicals are used to decompose the pollutants. There are two technologically promising techniques for gen erating non-thermal plasmas in atmospheric gas pressure containing the poll utants, namely electron beam irradiation and electrical discharge technique s. Both techniques are undergoing intensive and continuous development worl dwide. This is done to reduce the energy requirement for pollutant removal, and therefore the associated cost, as well as to obtain a better understan ding of the physical and chemical processes involved in reducing the pollut ants. In the present paper only electrical discharge techniques are reviewe d and emphasis is given to the more recent published work. The paper summar izes the chemical reactions responsible for the removal of the major pollut ing constituents of NO, NO2 and SO2 encountered in hue gases and exhaust em issions. The constructional features of the various types of electrical dis charge reactors commonly employed in the removal of gas pollutants as well as pilot systems used in industrial plants are described briefly The result s on the removal efficiency of the various pollutants including hydrocarbon s and volatile compounds and their dependency on the type of discharge reac tor, the type and the magnitude of the applied voltage (dc, ac and pulsed), the polarity of the voltage (dc and pulsed), the effect of the pulse width , the initial concentration of the pollutants, the addition of ammonia, arg on and other hydrocarbons, the gas now rate, the residence time of the poll utants in the discharge reactor, the gas temperature and on the type of the gas constituents will be reviewed. The removal of pollutants using are pla smas will be discussed. The specific energy density which is supplied into various forms of electrical discharges to reduce the pollutants will be dis cussed. The energy required to remove the pollutants is expected to be one of the main considerations in selecting the technology to be used to remove the pollutants and therefore it is of prime importance.