Air pollution control by electrical discharges

Air pollution caused by gas emission of pollutants produced from a wide range of sources including coal, oil and gas burning power plants, diesel engines, paper mills, steel and chemical production plants must be reduced drastically and urgently, as mandated by recent worldwide national legislation which recently are being reinforced increasingly by international agreements. Non-thermal plasmas in which the mean energy of the electrons is substantially higher than that of the gas offer advantages in reducing the energy required to remove the pollutants. The electrical energy supplied into the discharge is used preferentially to create energetic electrons which are then used to produce radicals by dissociation and ionization of the carrier gas in which the pollutants are present. These radicals are used to decompose the pollutants. There are two technologically promising techniques for generating non-thermal plasmas in atmospheric gas pressure containing the pollutants, namely electron beam irradiation and electrical discharge techniques. Both techniques are undergoing intensive and continuous development worldwide. This is done to reduce the energy requirement for pollutant removal, and therefore the associated cost, as well as to obtain a better understanding of the physical and chemical processes involved in reducing the pollutants. In the present paper only electrical discharge techniques are reviewed and emphasis is given to the more recent published work. The paper summarizes the chemical reactions responsible for the removal of the major polluting constituents of NO, NO/sub 2/ and SO/sub 2/ encountered in flue gases and exhaust emissions. The constructional features of the various types of electrical discharge reactors commonly employed in the removal of gas pollutants as well as pilot systems used in industrial plants are described briefly The results on the removal efficiency of the various pollutants including hydrocarbons and volatile compounds and their dependency on the type of discharge reactor, the type and the magnitude of the applied voltage (dc, ac and pulsed), the polarity of the voltage (de and pulsed), the effect of the pulse width, the initial concentration of the pollutants, the addition of ammonia, argon and other hydrocarbons, the gas flow rate, the residence time of the pollutants in the discharge reactor, the gas temperature and on the type of the gas constituents will be reviewed. The removal of pollutants using arc plasmas will be discussed. The specific energy density which is supplied in various forms of electrical discharges to reduce the pollutants is discussed. 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.