Control of mineral scaling in power plant recirculating cooling systems using treated municipal wastewater

摘要

The global energy demand is projected to increase by 77% from 2006 to 2030 along with a projected 38% increase in freshwater withdrawal for cooling in power industry. Finding alternative sources of water for cooling has become essential for future energy generation in thermoelectric power plants because of water scarcity in many parts of the US. Treated municipal wastewater is considered as one of the most promising alternative water sources because of its geographic distribution and abundant quantity. However, its impaired water quality makes the cooling tower management more challenging. Therefore, effective approaches are required to prevent scaling, corrosion, and biological growth to promote the reuse of treated municipal wastewater as cooling water in power plants. udThis study focuses on understanding mineral scale formation and developing effective mitigation methods when using tertiary treated municipal wastewater as power plant cooling makeup. Two types of tertiary-treated municipal wastewater that were evaluated included secondary-treated water with pH adjustment (MWW_pH) and water from secondary-treatment followed by nitrification and sand filtration (MWW_NF). Laboratory-scale studies and pilot-scale cooling systems were used to evaluate mineral scaling formation and inhibition on non-heated surfaces (e.g., pipelines, tower packing, etc.) under conditions relevant to full-scale cooling systems. Results showed that pH adjustment to 7.8 plus the addition of 5 ppm polymaleic acid (PMA) could reduce the scaling significantly with MWW_pH. MWW_NF exhibited little scaling potential, which is related in part to the lower pH and alkalinity in this water. Amorphous calcium phosphate (ACP) was the primary form of mineral scale on non-heated surface with the above two-types of tertiary-treated municipal wastewater. udA bench-scale experimental system was designed to simulate the condenser surface to study the impacts of mineral scaling on the heated surfaces and the effectiveness of proposed scaling control strategies under these conditions. Heated surface favored the formation of hydroxyapatite (HAP), the most thermodynamically stable calcium phosphate, was the main reason for the crystalline fouling with MWW_pH at pH 7.8. 10 ppm PMA addition could suppress the crystalline fouling of MWW_pH at 7.8 to a low level by inhibiting the transformation of amorphous calcium phosphate to hydroxyapatite during the test period. Significant crystalline fouling was identified with MWW_NF at pH 7.2 while pH adjustment to 7.8 resulted in negligible fouling. udThe impact of flow velocity on particle deposition was analyzed in a quantitative model, showing positive deposition potential for bulk precipitates at flow velocity of 0.5 and 0.4 m/s while little particulate fouling was theoretically predicted at 0.6 m/s in the test situation. Bench-scale studies were consistent the model prediction, confirming that the model could be used to identify optimal hydrodynamic conditions to control depositions of bulk precipitates. udThe mechanism of calcium phosphate scale control by common antiscalants included PMA and 1-hydroxyethane 1,1-diphosphonic acid (HEDP) was elucidated to provide scientific background for the effective scaling mitigation when treated municipal wastewater is used as make-up in thermoelectric power plant cooling systems. Both PMA and HEDP inhibited the transformation of ACP to HAP by preventing the aggregation of ACP particles. However, PMA dispersed the ACP particles mainly through electrostatic repulsive force while hydration force was hypothesized to be the reason for the function of HEDP in dispersion.udThe key findings of this study indicate that it is possible to control mineral scaling through direct chemical addition at proper operating conditions when treated municipal wastewater is used as makeup water in the recirculating cooling system. This study not only evaluated scaling control methods in cooling systems, but also revealed the fundamentals of scaling formation and inhibition.ud