Pulsed electrical discharges in saline solutions: Possibilty of supercritical water production for water treatment

Abstract

The field of aqueous-based plasma water treatment has a history of about two and one-half decades, starting with so-called electrohydraulic discharges (EHDs) and corona-streamer discharges. Aqueous-phase electrical discharges have been powered by DC, pulsed DC, AC and RF sources. The figure below shows a pulsed-power circuit which we have used to generate aqueous-phase plasmas in saline solutions. A capacitor was charged to a high DC voltage (typically 5-25 kV, depending on electrode geometry) and switched into electrodes placed in water by a spark gap. The resulting electrical discharge is a pulse, whose properties depend on the water conductivity and the circuit parameters. Initially, there is a surface-discharge current that is much smaller than the eventual peak current. The bulk-plasma discharge current starts after a delay time characteristic of the voltage rise time, solution conductivity, and electric field across the electrodes. In our experiments, we deposited 15-20 J of energy (-10 kV, -500 A) into a thin layer of water in several microseconds and observed Mach 4-8 shock waves and transonic bulk fluid flow in our closed test vessel. Supercritical water can be produced by such shock waves. From our saline-solution pulsed discharge experiments, we propose the possibility of producing supercritical water at high temperatures and densities, as well as the potential for using such discharges for water treatment and co-power generation. We will discuss our results, including the scenario that exothermic ion chemistry in the ablated supercritical water releases enough energy on a short enough time-scale to produce a detonation front and propagating shock wave, thus producing supercritical water.