Experimental study of the supercritical water oxidation of recalcitrant compounds under hydrothermal flames using tubular reactors

The hydrothermal flame is a new method of combustion that takes place in supercritical water oxidation reactions when the temperature is higher than the autoignition temperature. In these conditions, waste can be completely mineralized in residence times of milliseconds without the formation of by-products typical of conventional combustion. The object of this work is to study the hydrothermal flame formation in aqueous streams with high concentrations of recalcitrant compounds: an industrial waste with a high concentration of acetic acid and various concentrated solutions of ammonia. A tubular reactor with a residence time of 0.7 s was used. Oxygen was used as the oxidant and isopropyl alcohol (IPA) as co-fuel to reach the operation temperature required. The increase of IPA concentrations in the feeds resulted in a better TOC removal. For mixtures containing acetic acid, 99% elimination of TOC was achieved at temperatures higher than 750 °C. In the case of mixtures containing ammonia, TOC removals reached 99% while maximum total nitrogen removals were never higher than 94%, even for reaction temperatures higher than 710 °C. Ignition was observed at concentrations as high as 6% wt NH3 with 2% wt IPA while at IPA concentrations below 2% wt IPA, the ammonia did not ignite.

► Novel method for wastewater treatment in less than 1 s using hydrothermal flames.
► Recalcitrant compounds: ammonia and acetic acid: 99% mineralization at 750 °C in concentrations up to 4% wt of acetic acid using IPA as a co-fuel.
► Lower temperatures required for lower concentration of recalcitrant compounds. Maximum 99% removals of TOC and 94% of N in feeds NH3 + Isopropanol.
► No NOx detected in the gas effluent.
► Necessity of a co-fuel for ignition.