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Show Burning low sulphur coal is one of the options being considered to reduce Ontario Hydro's acid gas emissions. Ontario Hydro's coal fired stations are equipped with electrostatic precipitators (ESP) for collecting ashes from flue gases. These precipitators were designed for collecting ashes from bituminous coals wi th a sulphur content ranging between 1.5 2.2%. Collection efficiencies drop significantly when these ESP's are used for 8011ecting ashes from low sulphur coals with resistivities exceeding 5 x 101 ohm.cm. High resistivity ashes are being produced at Nanticoke TGS, where low sulphur coal blends are being burned. High resistivity wastes (a mixture of coal ash and partially sulphated calcine) are also produced from sorbent injection and spray drier technologies for controlling S02 emissions, currently under development at Ontario Hydro. Fly ash collection in the ESP involves ash deposition on the collector plates followed by its removal and collection in the hopper. The various factors affecting ESP efficiency given below are related to the electrical environment in the precipitator and ash properties: - electric field strength and ion density in the ESP; - electrical resistivity of the ash; - adhesion and cohesion characteristics of the ash; - particle size distribution of the ash; The conduction of electric current through an ash layer is essential for good precipitator performance, and is related to ash resistivity. The two most important conduction mechanisms are termed volume conduction and surface conduction. Volume conduction is determined by the bulk composition of the ash, and surface conduction depends on an absorbed film on the dust surface, which is related to the composition of the gas and the dust particle surface/2/. High resistivity ash collection by existing precipitators can be improved by changing various parameters in the precipitator design, such as: collection area, residence time, power density, and the use of hot side ESP's. With old ESP's, however, it is either impossible or too expensive to change most of these parameters and ash conditioning may be the method of choice. A conditioning agent added to the flue gas usually in trace amounts, may affect some or all of the factors influencing ESP performance. Many conditioning chemicals such as S03' sulphuric acid, sodium sulphate, sodium carbonate and water provide conductive surface films and enhance surface conductivity (thus reduce ash resistivity). Some chemical modify the crystal lattice of ash particles and improve volume conductivity/3,4/. Resistivity modifications can also be accomplished by the use of an agent that increases the cohesiveness of the deposited fly ash through inter-particle binding forces. Agglomeration of small fly ash particles in the flue gas stream is a potential mechanism for enhancing ESP performance: small particles wi th |
