Mechanisms Involved in Fly Ash Precipitation in the Presence of Conditioning Agents - A Review
Conditioning agents are usually used to improve the collection efficiency of electrostatic precipitators by causing a reduction in the resistivity of the fly ash. However, certain conditioning agents may have other effects including changing the electrical breakdown strength of the flue gas, space charge enhancement of the collecting field, increasing the mean particle size, changing the adhesion and cohesion properties of the ash, and changing the acid dew point of the flue gas. These mechanisms are reviewed and related to results reported in the literature for ammonia and triethylamine, two conditioning agents which appear able to affect one or more of the mechanisms mentioned above, in addition to the resistivity.
A comparison of chemical additives as aids to the electrostatic precipitation of fly-ash
Abstract Pilot plant electrostatic precipitator experiments have shown that the collection of pulverized fuel ash was aided by a number of additives, including sulphur trioxide, sulphuric acid, ammonium sulphate, ammonium bisulphate, sulphamic acid and, in certain circumstances, ammonia. The first three are economically the most attractive for use in power station precipitators. Sulphuric acid and ammonium sulphate are easily injected in the form of aqueous sprays and the effectiveness of the latter has been confirmed in full-scale tests. Other additives studied were sodium chloride, calcium chloride, sodium sulphate, lithium iodide and hydrogen chloride. Additives improved precipitator performance by increasing the flashover voltage. It is suggested that changes in resistivity due to an additive affected the electron charge on the ash layer, which in turn influenced the surface work function and raised the flashover voltage. Nevertheless, some additives increased the flashover voltage and efficiency without altering resistivity. Where performance correlated with resistivity a better correlation was obtained at an arbitrary bulk density rather than at that of the precipitated layer. The cohesive properties of ash were related to efficiency only when the resistivity was low.
The effective collection of fly ash at pulverized coal-fired plants.
The relationship between sulfur in coal, boiler exit gas temperature, and the carbon portion of fly ash have a major effect on the electrical properties of fly ash. Whether effective collection of fly ash is obtained by the electrostatic precipitator installation alone or the precipitator—mechanical combination depends primarily on a knowledge of this relationship. Fly ash electrical properties can range from a highly "resistive" to a highly "conductive" state which can appreciably alter the precipitator collection performance. A correlation of coal sulfur and boiler exit flue gas temperature is given to indicate the probability of expecting an optimum voltage—current relationship with different combinations of these factors. Carbon affects the electrical conditioning of fly ash by providing parallel paths of current leakage through the deposited dust layer. Therefore, removal of the carbon particles in a mechanical collector placed before the precipitator can alter the precipitator electrical characteris...