Supercritical Water Gasification of Model Alcohols

Thesis (Ph.D.)--University of Washington, 2020Supercritical water is a unique reaction medium with emerging applications in waste destruction, fuel generation, and material synthesis. The high temperatures and pressures necessary to bring water to the supercritical state create a kinetically and thermodynamically advantageous environment for redox reactions to occur, with minimized mass transfer limitations. Supercritical water acts both as reactant and reaction medium, facilitating the rapid decomposition of many organic molecules into hydrogen-rich gas. As a waste-to-energy technology, supercritical water gasification shows promise for converting organic waste, especially wet waste, into green hydrogen. Despite decades of interest in supercritical water gasification and oxidation, fundamental knowledge of reaction chemistry in supercritical water remains limited. It is relevant to study the chemistry of simple model compounds in supercritical water as a means of elucidating reaction phenomena occurring with common compound classes or functional groups. A continuous, tubular supercritical water reactor facilitates novel studies of reaction kinetics in supercritical water. In situ Raman spectroscopy allows for real-time monitoring of reaction products, short experimentation times, and rapid identification and quantification of reaction products. Through multiple studies investigating reaction kinetics of model compounds in the gasification environment, chemical reaction routes and mechanisms of several model compounds are determined under varied conditions. As a proof-of-concept study to demonstrate the validity of using Raman spectroscopy for determining product yields, the reaction kinetics of formic acid (HCOOH) in 25 MPa water are quantified for subcritical and supercritical temperatures between 300 and 430 °C. In situ Raman spectroscopy facilitates precise, time-resolved data collection. First-order reaction behavior is not observed during gasification of methanol, ethanol, or isopropyl alcohol in supercritical water between 500 and 560 °C. Chain-branching free radical reaction mechanisms are inferred to drive the decomposition of alcohols in supercritical water, and the heterogeneous catalyst of the reactor wall surface is significant for initiating radical reactions. Global reaction pathways are proposed, and mechanisms for free radical initiation, propagation, and termination are discussed. Ethanol is also studied under partially oxidative conditions at 500 to 530 °C, where intermediate reaction products counterintuitively polymerize to form char. A deeper understanding of reaction mechanisms in supercritical water is gained, lending insight towards the conditions necessary to maximize gaseous yields and minimize char buildup