Supercritical water gasification of organic acids and alcohols: the effect of chain length

We report the influence of the molecular structure on the gasification behaviour for a homologous series of linear chain (C1–C8) carboxylic acids and alcohols in supercritical water (600 °C and 250 bar) at two different concentrations (10 and 20 wt%). The initial concentration of carboxylic acids had a significant influence on the gasification efficiency (GE), whereas no such effect was found for the alcohol series studied. Alcohols were found to be much easier to gasify than the organic acids and the carbon GE trend of alcohols was opposite to that of acids, especially for the short chain compounds. With increasing chain length the GE stabilizes to around 50% at the conditions chosen. From gas and liquid phase product analysis decomposition schemes were proposed for the short chain compounds. A remarkable oscillatory behaviour of the methane and CO2 product yield with increasing chain length of the acids and alcohols was found experimentally. A simple model was derived, based on essentially a β-scission cracking mechanism, which was able to explain the observed product spectrum and the oscillatory behaviour. Additionally, the influence of the number and positioning of OH groups on the gasification behaviour was studied