Phenol Gasification in Supercritical Water: Chemistry, Byproducts, and Toxic Impacts.

Abstract

In order to better understand the chemistry underlying supercritical water gasification (SCWG) of biomass, phenol was processed with supercritical water in quartz reactors while systematically varying the temperature, water density, reactant concentration, and reaction time. Both the gas and liquid phases were analyzed post-reaction to identify and quantify the reaction intermediates and products, including H2, CO, CH4, and CO2 in the gas phase and many different compounds—mainly polycyclic aromatic hydrocarbons (PAHs)—in the liquid phase. Higher temperatures promoted gasification and resulted in a product gas rich in H2 and CH4 (33% and 29%, respectively, at 700 °C), but char yields increased as well. Dibenzofuran and other identified phenolic dimers were implicated as precursor molecules for char formation pathways, which can be driven by free radical polymerization at high temperatures. Two different reaction pathways emerged from the kinetic modeling of phenol conversion: a water-inhibited thermal pathway in which rate ~ [phenol]^1.73 [water]^-16.60 and a water-accelerated hydrothermal pathway in which rate ~ [phenol]^0.92 [water]^1.39. Benzene and dibenzofuran form directly from phenol and account for nearly all phenol consumption during SCWG at 500–700 °C. Experiments with dibenzofuran as the starting reactant generated the same array of products—typically in comparable quantities—as that observed with phenol as the reactant. When benzene was the reactant, biphenyl was the main product and some H2 formed. Information about the reaction pathways obtained from these experiments served as the basis for constructing and optimizing a kinetic model that describes the reaction rates of phenol and its primary and gaseous products in supercritical water. Arrhenius parameters are reported, and the formation and consumption rates for each species as calculated by the model are analyzed. Since many of the identified PAHs are EPA priority pollutants and have known human health and environmental effects, the UNEP/SETAC toxicity model, “USEtox,” was employed to characterize the human toxic and ecotoxic impacts due to a hypothetical emission of this byproduct stream into freshwater. Total toxic impact increased with gasification temperature up to a maximum at 650 °C but then decreased at 700 °C.