MINERAL PRECIPITATION AND DEPOSITION IN COOLING SYSTEMS USING IMPAIRED WATERS: MECHANISMS, KINETICS, AND INHIBITION

摘要

Given the increasing water shortage and growing energy demand, novel approaches to water reuse is critical to ensuring sufficient water supply for cooling in thermoelectric power plants, the single largest use of water. Major challenges arise from complicated chemistries of the waters under consideration and changing operating conditions in open recirculating cooling systems, both of which lead to accelerated and complex mineral precipitation/deposition (mineral scaling). This study evaluated three impaired waters for their use in cooling water systems, namely, municipal wastewater, abandoned mine drainage, and coal-ash pond effluent. The focus was on understanding the mechanisms, kinetics, and inhibition of scaling from both fundamental and applied perspective.Scaling inhibition with a variety of antiscalants was investigated and polymaleic acid (PMA) was the most effective in all three waters. Antiscaling with PMA was achieved through two mechanisms: retardation of mineral precipitation via PMA's competitive interactions with crystallizing minerals, and stabilization of suspended mineral particles via surface adsorption of negatively-charged polymer molecules. Nevertheless, biofouling and corrosion, two other main technical challenges in water reuse for cooling, compromised the effectiveness of scaling control by PMA.Equilibrium-based chemical modeling for scaling prediction exhibited limited success in cooling systems using impaired waters. The modeling was useful for describing the pH behavior of the cooling waters. MINEQL+ modeling that considered CO2 degassing, NH3 stripping, and kinetically-limited solids formation captured the underlying mechanisms dictating the pH changes observed in pilot-scale cooling towers.An in situ, sensitive approach for monitoring scaling rates was developed by measuring the electrical impedance of the metal-mineral-water interface using Electrochemical Impedance Spectroscopy (EIS). The EIS capacitance correlated very well with the mass of mineral deposits. The sensitivity provides means for early scaling detection. The applicability of the method was successfully expanded to broader water chemistries and multiple minerals—conditions more relevant to cooling water systems.The key findings of this study indicate that it is possible to control scaling of several impaired waters used as cooling makeup water. The chemical treatment approach demonstrated in this study can very likely replace costly pre-treatment that is often suggested when impaired waters are used in cooling systems.