Energy recovery from effluents of supercritical water oxidation reactors

Abstract

Supercritical water oxidation (SCWO) reactors can process waste effluents achieving high conversions, but the required extreme pressure and temperature operational conditions entail high-energy operational expenditure. SCWO has the potential to be considered a clean energy generation process, as the process effluent is a high temperature, high pressure stream with a high enthalpy content that can be converted to heat and shaft work. This ensures the self-sustained reaction and can generate excess shaft power to drive both the high-pressure pump and the air compressor. On the contrary, an efficient heat and power recovery from SCWO reactors outlet streams using conventional procedures presents several problems. First, Rankine cycles impose indirect heat transfer to the working fluid and are unable to recover the pressure energy and second, direct expansion of the effluents entails costly development of specific, efficient expansion equipment.

In this work, we investigate the options for energy recovery of SCWO reactors coupled with commercial gas turbines (GT). SCWO outlet streams are mainly composed of water, nitrogen and carbon dioxide. These operating values nearly resemble the well-known and already-implemented GT steam injection procedures. The temperature of the flue gases (approx. 500 °C) and the direct shaft work usage offers adequate energy integration possibilities for both feed preheating and compression. The wide range of commercially available GT sizes enables process scaling.