Seasonal Effect on Complexation of NOM and Alkaline Earth Metal Ions in Raw and Cooling Water to Study its Impact on Scaling Potentials in Eskom Power Plant

Rising costs of power generation as a result of increased condenser tube scaling and corrosion have become critical areas that need intervention at Eskom's power stations in South Africa. In the past few years, the quality of cooling and raw water has deteriorated with increased concentrations of scale formation metals such as calcium (Ca) and magnesium (Mg) as well as Natural Organic Matter (NOM). The Dissolved Organic Matter (DOC) fraction of NOM in the water forms complexes with metals Ca and Mg under various conditions. In this study, the Cooling Water (CW) and Raw Water (RW) at Lethabo and Duvha power stations were sampled and were analyzed for metals as well as organics and the data obtained was fitted into a Visual MINTEQ chemical model and Langelier Saturation Index (SI) calculations coupled with calcium carbonate precipitation potential models. The data obtained indicated that as the DOC concentration increased, the SI values decreased which may be attributed to the to complexation of Ca and Mg (the scaling metals) bound to DOC, leaving relatively lower concentrations of the free Ca and Mg ions until the reactive sites of DOC were saturated. The seasonal changes affect pH, DOC and concentrations of other metals present in the water that influenced complex formation and scaling.