Evaluating the Effect of Aqueous Calcium Chloride Desiccant on Cooling Tower Performance in a Heat-Integrated Post-Combustion CO2 Capture System

Performance of steam turbines in the power plant is heavily dependent on the achievable recirculating cooling water temperature, a result from the ambient wet bulb temperature and cooling tower approach temperature. As such, the University of Kentucky Center for Applied Energy Research (UKy-CAER) has developed a technology to enhance the performance of cooling towers through an addition of a liquid desiccant dehumidification step in front of the conventional cooling section. In the bottom segment of a two-stage cooling tower, liquid desiccant flows counter-current to air, absorbing moisture from the vapor phase. The endothermic process reduces the air relative humidity and wet bulb temperature (T_wb), resulting in a lower cooling water temperature than otherwise plausible assuming the same approach temperature is maintained in the cooling tower. If scaled-up and integrated with conventional power plant steam condensers, the cooler recirculation water would reduce back pressure of the turbine, and ultimately, improve turbine efficiency.

UKy-CAER has completed a sensitivity study to estimate a T_wb depression of 3-7 degrees due to the dehumidification stage and determined ambient conditions that realize the greatest benefit. After a relative humidity and T_db measurement apparatus was installed and tested, the integrated cooling tower system was evaluated during a carbon capture project campaign to validate the modeling and analyses initially completed. Engineered T_wb improvement compared to ambient T_wb was established during the month of September, confirming the effectiveness of the UKy-CAER cooling tower design on hot, humid days. A standard deviation of only 0.3°F was seen in the cooling water exit temperature during long-term operation, suggesting accurate estimation of turbine and condenser design specifications, resulting in a reliable and economic process. As long as the waste heat needed to drive desiccant regeneration is within the inventory of available waste heat, as is the case for the UKy-CAER demonstration CO₂ capture unit, little additional energy will be required to operate said system.