AbstractsEngineering

The overall objective of this study was to develop a comprehensive methodology to identify viable treatment strategies for utilization of degraded waters for cooling in thermoelectric power systems. To achieve this objective a process simulation model was developed using Aspen Plus® with the OLI (OLI System, Inc.) water chemistry model to predict water quality and the rate of fouling in the recirculating cooling loop utilizing secondary-treated municipal wastewater (MWW) and tertiary-treated municipal wastewater as the sources of makeup water. This process simulation model includes sub- models for pre-treatment units; the cooling tower with water, CO2, and NH3 evaporation; as well as the recirculating cooling system and condenser with salt precipitation and fouling. The input parameters of the model, including CO2 mass transfer coefficients in the cooling tower and kinetics of salts precipitation reactions, were determined by developing mathematical models and calibrating the models with the experimental data obtained from literature. The process simulation module was used to predict the water quality in the recirculating cooling loop and the results were compared with pilot-scale experimental data from literature on makeup water alkalinity, loop pH and ammonia evaporation. The effects of various parameters including makeup water quality, salt formation, NH3 and CO2 evaporation mass transfer coefficients, heat load and operating temperatures were investigated. The results indicate that stripping of CO2 and NH3 in the cooling tower can significantly affect the cooling loop pH. x viii The model was also used to determine the rate of fouling in the condenser. The results indicate that the fouling rate of MWW as makeup water is significantly higher than that expected with fresh water, and tertiary treatment of MWW such as nitrification and/or softening can significantly reduce the fouling potential. Finally, the rate of fouling obtained from this study was integrated into the existing cost model developed earlier (at Illinois Institute of Technology) to perform the overall economic analysis. The results show that the use of municipal wastewater (MWW) to replace freshwater as makeup for the recirculating cooling loops of thermoelectric power plants is economically viable when tertiary treatments such as nitrification or softening are applied. Among various treatment strategies studied, nitrification of MWW has the lowest cost of 0.29 $/m3 for utilization in a 550 MW power plant. Furthermore, it was concluded that utilization of secondary treated municipal wastewater (MWW) without tertiary treatments such as nitrification or softening is not economically viable due to its significant fouling costs.