Influence of moisture on the resistivity of selected South African fly ashes
Ribberink, Johannes Adrianus
MetadataShow full item record
The coals fired in the pulverised fuel boilers of South African power stations are generally of poor quality, producing numerous emissions, of which particulate matter has the largest impact in South Africa. Recently, the government of South Africa introduced a new Air Quality Management Act, in order to regulate the amount of emissions from pulverised fuel operations. Amongst other, the emission level of particulate matter is addressed. The majority of coal power station in South Africa utilise electrostatic precipitators for the control of fly ash, with fly ash resistivity being an important factor strongly influencing ESP performance. The resistivity of fly ash is influenced by a number of factors; including the chemical composition of the ash and adsorption of physical or chemical species on the surface of the ash particle. The resistivity can be subdivided into two distinct areas, which are the surface resistivity and volume resistivity region. Generally, where ambient moisture is present, electron transfer occurs via the surface of the ash particle at lower temperatures, while at higher temperatures the electron transfer occurs more predominantly via the bulk of the ash particle. In this study, the resistivity of three typical South African coal ashes, collected from operational power stations, is determined at various ambient moisture levels. For this purpose, a resistivity oven, equipped with local climate control, was used. The observed results were compared to a general predictive model that was developed by Roy Bickelhaupt in the 1970s specifically for Northern American coal ashes. The three South African fly ash samples were sourced and the resistivity determined at 0, 4.6, 6.0 and 9.0 vol% moisture, to assess the influence of ambient moisture on the resistivity. Dry ash resistivity showed a linear relation between the log of resistivity and temperature in the volume resistivity region which agreed with literature. When moisture was introduced, the samples showed a reduction in resistivity below temperatures of 200°C. The reduction in resistivity, in the surface resistivity region, is due to the moisture forming a thin conducting layer along the surface of the particle, accelerating electron transfer. An increase in the moisture content resulted in a further reduction in resistivity in the surface resistivity region. At 150°C and 9.0 vol% ambient moisture the resistivity of the ash samples showed a reduction of 89 to 95% when compared to dry resistivities. The elemental composition of the sampled ashes was also used, along with the standard Bickelhaupt predictive model to determine the theoretical resistivity values. The standard Bickelhaupt model showed deviations of several orders between the experimental values and theoretical values, with the model overestimating the resistivity. The overestimation was attributed to the different nature of the American coal ashes when compared to South African ashes. The deviation is most significantly related to the sodium content, which is substantially higher in American coal ashes. For this reason, modifications were proposed to the volume and surface resistivity term of the Bickelhaupt model to better account for the unique chemistry of South African ashes. These modifications improved the predictive accuracy of the model significantly and deviations smaller than a factor of 2.5, a ratio commonly considered acceptable for between-laboratory repeatability by the Institute of Electrical and Electronics Engineering, were obtained. These modified relations can in future be used for more effective ESP design and operation, in the case where South African coals are used for power generation.
- Engineering