Application of a Two Stage Heat Integrated Cooling Tower to Reduce Energy Costs in CO2 Capture Systems

Application of a Two Stage Heat Integrated Cooling Tower to Reduce Energy Costs in CO₂ Capture Systems

Bethany Clontz^a, Kunlei Liu^a,b, Michael Manahan^c, David Link^c

^a Center for Applied Energy Research, University of Kentucky, Lexington, KY, USA

^b Department of Mechanical Engineering, University of Kentucky, Lexington, KY USA

^c LG&E and KU Energy LLC, Louisville, KY USA

The United States Department of Energy (DOE) has set a cost goal of less than $40/tonne of CO₂ captured to be reached by 2020, with industrial integration by 2025 for second-generation capture technologies.¹ A partnership between the DOE, other industrial partners, and the University of Kentucky Center for Applied Energy Research (UKy-CAER) has resulted in a 0.7 MWe CO₂ capture system (CCS) constructed at the E.W. Brown Generating Station, a Kentucky Utilities Company commercial coal-fired plant. This CCS is currently operational and has a parametric monoethanolamine (MEA) campaign underway. The UKy-CAER process is set apart from other CCS projects because of three main design differences: the use of a two-stage stripping process, the use of a two-stage heat integrated cooling tower, and the use of an advanced Hitachi solvent. This presentation focuses on the two-stage integrated cooling tower and how it contributes to the goal of lowering the energy costs by recovering waste heat, as well as how is contributes to the cooling of the process.

This integrated cooling tower has two stages. The top stage functions as a typical cooling tower and the bottom stage utilizes a liquid desiccant to pre-dry the ambient air before is passes through to the top stage. The pre-dried ambient air provides additional cooling of the water in the top stage. The water-saturated desiccant is warmed by a series of heat exchangers, using heat normally lost in the process. The waste heat is recovered from the lean MEA prior to entering the absorber and from the CO₂ exiting the primary stripper. Additional heat (provided by process steam) may be needed depending on the desired operating conditions of the system. The water-saturated desiccant stream is then used to condition a new supply of ambient air through a water evaporator column, which is used in the secondary air stripper to increase solvent working capacity.  The desiccant stream is then cooled via a series of heat exchangers including an ethylene glycol chiller and returned to the sump of the cooling tower.

Technical and economic analysis (TEA) studies show that in the UKy-CAER process, cooling water is projected to cool two to five degrees Celsius cooler than a conventional cooling tower design for ambient conditions common to the Midwest and other regions.² The TEA data also shows the UKy-CAER process has 38.1% less heat rejection to the atmosphere (2104 MBtu/hr) associated with the CCS compared to the DOE Case 10 (3398 MBtu/hr).²

This presentation will further discuss these topics and elaborate on them by comparing to the projected degree of cooling. It will also include data referring to the amount of waste heat recovered by the desiccant loop and its effect on the energy efficiency of the project.

References

1. DOE/NETL, “DOE/NETL CO₂ Capture R&D Program,” presented at 2014 NETL CO₂ Capture Technology Meeting, Pittsburg, PA, July 2014.
2. A. Brown, et al, “Preliminary Technical and Economic Feasibility study on the Application of a Heat Integrated Post-Combustion CO₂ Capture System With Hitachi Advanced Solvent into Existing Coal-Fired Power Plant,” Topical Report, December 2012