Conversion of a resistant pollutant, phenol, into green fuels by gasification using supercritical water compressed up to 1000 bar

Abstract

For green sustainable chemistry, it is crucial to investigate the destruction of such a common pollutant as phenol. This study reports the gasification of phenol with steam and supercritical water (SCW) and shows that gasification under high-pressure SCW is a method that destroys and efficiently converts phenol into valuable products. To the best of our knowledge, the widest pressure range ever investigated in this field is utilized, i.e., from atmospheric pressure steam to SCW at 1000 bar. The high temperature used (700 °C) leads to fast degradation of phenol under all the conditions studied but the amount of phenol gasified does strongly depend on pressure. During gasification, polymeric compounds such as naphthalene and phenanthrene are generated. They play a key role in the proposed degradation–gasification mechanism since they are difficult to degrade and can lead to the formation of char. High-pressure SCW can more efficiently degrade and gasify such polymeric compounds compared to steam. Then, the supercritical fluid leads to the conversion of a greater amount of phenol into gas than steam: 68% of the pollutant is gasified at 750 bar and 700 °C after 16.5 min with no generation of pollutant by-products, except CO₂. Furthermore, the gaseous stream contains the valuable green gases H₂ and CH₄. The use of highly compressed SCW implies not only the production of more gases but also the enrichment of the gas mixture in H₂ and CH₄. H₂ and CH₄ concentrations up to 35 and 30%, respectively, are obtained at 1000 bar.