Drexel University is conducting research with the overall objective of developing technologies to reduce freshwater consumption at coal-fired power plants.  The goal of this research is to develop a scale-prevention technology based on a novel filtration method and an integrated system of physical water treatment in an effort to reduce the amount of water needed for cooling tower blowdown.  This objective is being pursued under two coordinated, National Energy Technology Laboratory sponsored research and development projects.  In both projects, pulsed electrical fields are employed to promote the precipitation and removal of mineral deposits from power plant cooling water, thereby allowing the water to be recirculated for longer periods of time before fresh makeup water has to be introduced into the cooling water system.

	Schematic Diagram of Drexel University’s Laboratory Cooling Tower Test Facility

The specific objective of the first project (Application of Pulsed Electrical Fields for Advanced Cooling in Coal-Fired Power Plant - DE-FC26-06NT42724) is to use the spark discharge technology to develop a self-cleaning metal membrane filter, using pulsed electric fields to dislodge particles on the filter.  The filtration method utilizes electrical pulses to rapidly polarize water molecules on the filter membrane such that the water molecules are pulled to the membrane, pushing out the attached particles, which will then be removed by reject flow.  Drexel University utilizes a flow loop consisting of a cooling tower, a rectangular heat transfer test section with a window for visualization of crystal growth, electric heater for hot water, main circulating loop, and side-stream loop.  The development of the system will be followed by validation testing.

In addition to its beneficial effects in filter cleaning, introduction of the spark discharges into the recirculating cooling water has been found to promote the dissociation of the bicarbonate ion (HCO_3-) to the carbonate ion (CO₃ ^2- ), resulting in the spontaneous precipitation of calcium/magnesium carbonate solids.  The second project (Application of Pulse Spark Discharges for Scale Prevention and Continuous Filtration Methods in Coal-Fired Power Plant - DE-NT0005308) seeks to take advantage of this effect by accelerating the precipitation of excess calcium and magnesium ions directly in the recirculating cooling water and subsequently removing the precipitated solids via the self-cleaning filter developed under the initial project.  The combined system will prevent the buildup of mineral scale on condenser tubes, thereby increasing the cycle of concentration in the cooling water system from the present operational value of 3.5 to at least 8.

Laboratory testing will first be conducted to optimize the design of the spark discharge electrode to maximize the energy efficiency of the calcium ion precipitation process.  The laboratory cooling tower and self-cleaning filter system will be operated to determine if the technology can continuously precipitate and remove scale-forming solids.  A simulated condenser tube apparatus will also be employed to verify that mineral scale buildup on condenser tubes can be reduced via the use of the spark discharge technology.

The primary benefit of these coordinated projects is that the higher cycle of concentration will minimize cooling tower blowdown water requirements, which in turn will reduce the amount of freshwater make-up needed for power plant operation.  Additional environmental benefits are expected due to the reduction in the use of chemicals for scaling prevention and the simultaneous prevention of bio-fouling via the spark discharge system.