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.
Ammonia/flyash interactions and their impact on flue gas treatment technologies
Abstract Ammonia/flyash interactions were investigated to assess their importance in flue gas treatment processes. Theoretical calculations are presented for the upper bound of ammonia uptake by flyash during selective catalytic reduction. Assuming diffusion-controlled uptake, significant ammonia loss to flyash is predicted; this is not supported by industrial experience. Thus, experiments were conducted to measure ammonia adsorption and reaction on flyash as a function of temperature. Based on the experimental results, ammonia/flyash interactions are not expected to interfere with either current or proposed low-temperature (110–160°C) operation. When adding ammonia as an electrostatic precipitator (ESP) conditioning agent, however, the extent of adsorption onto flyash will be very sensitive to operating conditions. This may explain the erratic performance of ammonia in ESP conditioning reported in the literature. Finally, the saturation capacity for ammonia adsorption onto flyash exceeds the maximum threshold established for flyash reuse as a cement additive and thus could limit this large-volume opportunity for mitigating flyash disposal.
The measurement of the resistivity of power station flue dust
The difficulties in obtaining realistic values of the resistivity of power station flue dust are outlined. A new apparatus, which is thought to overcome most of these problems, is described. The results obtained show that the resistivity of flue dust undergoes a rapid change with time after its removal from the flue gas stream. Many of the anomalous effects obtained in the course of previous measurements of resistivity are probably due to this effect.