Abstract
This paper reports on a study of calcium carbonate (CaCO3) precipitation potentials of cooling water (CW) at Eskom power stations. It is important to understand the complexation reaction between calcium and dissolved organic carbon (Ca-DOC) because the natural organic matter (NOM) affects the precipitation potentials of CaCO3. Secondly, it is necessary to understand the nature of the organic matter in the cooling and raw water. This study was carried out in raw and cooling water samples collected from Lethabo and Kriel power stations. These power stations obtain their cooling water from the Vaal River and Usuthu schemes, respectively. The DOC concentration in both water systems was higher in winter (little/no rainfall) as compared to spring (more rainfall). Specific ultraviolet absorption (SUVA) was calculated for the raw and cooling water for both stations. The SUVA results obtained for the raw water (RW) indicated that the NOM had a high degree of aromaticity (humic substances), whereas for the recycled CW the NOM present was found to be less aromatic. The Visual MINTEQ data for the winter sampling indicated that both Lethabo and Kriel were supplied by raw water that was under-saturated with respect to CaCO3, with saturation indices (SI) of -0.29 and −0.43, respectively. In addition, the % Ca-DOC in the raw water at Lethabo was 2.01 compared to 3.08 in the raw water at the Kriel plant. Therefore this study gives an indication that % Ca-DOC is one of the factors that affect the CaCO3 precipitation potentials. The difference in SUVA-254 values for both stations has been identified and requires further investigation. The SUVA values help in identifying the type of fractions that make up the NOM. The interaction between calcium/magnesium and NOM determines the potential for CaCO3 to form scale in condenser tubes carrying cooling water in the power generation plants at Eskom.
Highlights
The presence of natural organic matter (NOM) in raw water (RW) is mainly due to the degradation of organic substances arising from agricultural as well as human activity (Zazouli et al, 2007)
The pH and temperature are known to affect precipitation and dissolution of a number of compounds. These processes are the main causes of the scale that forms on condenser walls and will depend on the concentration of cations, anions and the dissolved organic carbon (DOC)/TOC levels
These power stations obtain their cooling water from the Vaal River and Usuthu schemes, respectively. These main physico-chemical parameters govern the interaction between scale-forming metals (i.e. Ca and Mg) and the anions in water to form various mineral assemblages
Summary
The presence of natural organic matter (NOM) in raw water (RW) is mainly due to the degradation of organic substances arising from agricultural as well as human activity (Zazouli et al, 2007). Over the past 5 to 10 years, the concentration of NOM in natural water sources has greatly increased, due to an increase in domestic, industrial and agricultural activities in most catchments; raw water quality has deteriorated (Nkambule et al, 2012). The effects of this phenomenon have been experienced by Eskom, South Africa’s national power utility. Make-up water (raw water) is added to the cooling water system to control the chemistry of the cooling system This concentration mechanism is referred to as cycles of concentrations (COC) and is governed by the system design and the supplied raw water quality
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