(557b) Supercritical Water Gasification of Phenol

Biomass gasification is a clean and renewable energy alternative that can help mitigate growing global energy needs and concerns over fossil fuel depletion and greenhouse gas emissions. Hydrothermal gasification in supercritical water has recently received attention as a promising technology, but little is known about the chemistry that leads to the formation of gas and undesirable byproducts such as char under these harsh conditions. In order to better understand biomass gasification chemistry in supercritical water, a systematic study of the effects of process variables on product composition and yield was conducted. The process variables tested were temperature, water density, reactant loading, and reaction time. Phenol was studied as a model compound because phenolic compounds are difficult to gasify and represent one of the main obstacles to complete gasification of biomass. Phenolic lignin is a major constituent of lignocellulosic biomass, and phenols are known to form from both lignin and cellulose under high temperature and pressure conditions. Reactions were carried out at 500?700°C and 22?67 MPa in quartz batch reactors to eliminate catalytic effects from metal walls. Gas- and liquid-phase reaction products were identified and quantified using gas chromatography with mass spectrometry, flame ionization, and thermal conductivity detection. Reaction of phenol in supercritical water led to the production of little gas, but instead a swath of heavier molecular weight compounds, primarily polycyclic aromatic hydrocarbons (PAHs), formed, and more than 20 of these were identified for the first time in this study. These compounds represent char precursor molecules, and it is expected that even larger molecular weight compounds are formed via ring-fusion, polymerization, and cross-linking reactions, which ultimately leads to the char that was observed on reactor walls. Studying the formation of these molecules provides insight into how to suppress char formation and improve gas yield.