The Influence of SO₃ conditioning on the resistivity of selected South African fly-ashes

Abstract

Particulate emission legislation, as instructed by the Department of Environment, Forestry and Fisheries, implies a particulate emission limit of 50 mg/Nmᶟ (10% O2, dry basis) for all new and existing power stations by 2020. Generally, Eskom’s power stations, that uses Electrostatic Precipitators (ESP’s) for particulate emissions control, were not designed to comply with this stringent legislation. Fly ash resistivity is key factor in ESP design and performance with South African ashes generally having highly resistive ash compared to e.g. American ashes. Ash resistivity can be divided into two regions namely volume and surface resistivity where atmospheric conditions such as moisture and SO₃ concentration influence electron transfer across the surface of the particle. The aim of this study was to experimentally quantify the relation between SO₃ ambient concentration and fly ash resistivity for industrially generated fly ashes, and to relate the resistivity to the composition of the fly ash. Industrially sampled South African fly ashes were tested in a laboratory test rig consisting of a temperature controlled oven, humidifiers and a range of flow controllers. The test rig was adjusted to allow for SO₃ conditioning to evaluate the resistivity at various SO₃ concentrations. It was found that the injection of SO₃ at ambient concentrations of between 10 and 20 ppm and constant moisture concentrations yielded a reduction of the resistivity by a ratio of between 2.7·10ᶟ to 1.5·10⁶ at 150 ºC. These results indicated that it would be possible to reduce the resistivity of these highly resistive ashes to well within the desired ESP operating range of between 10⁹ to 10¹¹ Ω∙cm. Evaluation against the standard Bickelhaupt model for resistivity prediction showed a significant deviation from the experimental results, primarily over-predicting the resistivities. The differences were attributed to the elemental composition, where South African ashes show considerably lower levels of alkali metals and more specifically sodium. Modifications to the model were proposed to describe the South African ashes more accurately with differences between predicted and measured values within a ratio 2. These findings can aid in the understanding, design of new and retrofitting of existing ESPs.