(115d) Kinetic and Environmental Impact Modeling of Reaction Products From Supercritical Water Gasification of Phenol

Biomass gasification is a potential source of sustainable fuel gas that can help mitigate growing global energy needs and concerns over fossil fuel depletion and greenhouse gas emissions. Hydrothermal gasification in supercritical water offers improved thermal efficiencies for wet biomass, which otherwise requires energy intensive dewatering. This technology has been demonstrated on many starting feedstocks, including seaweed, microalgae, wood, straw, switchgrass, corn stover, cellulose, and lignin, but, in each case, phenol and/or other aromatic compounds persisted in the liquid phase as undesired byproducts. Phenolic compounds are notoriously difficult to gasify and represent one of the main obstacles to complete gasification of biomass, but little is known about the chemistry that leads to such liquid phase byproducts. In order to better understand this underlying chemistry, kinetic and temperature studies were conducted using phenol as a model compound. Reactions were carried out in quartz batch reactors to eliminate catalytic effects from metal walls. These experiments led to the production of little gas, but instead a swath of heavier molecular weight compounds formed. A reaction network was constructed based on additional experiments and careful analysis of chemical pathways. Many of the non-gaseous byproducts identified in these reactions are polycyclic aromatic hydrocarbons (PAHs), EPA priority pollutants, and/or have known human health and environmental effects. The UNEP/SETAC environmental toxicity model “USEtox” was employed to characterize the human and ecotoxic impacts due to a hypothetical emission of this byproduct stream into freshwater. Coupling environmental impact modeling with kinetic modeling of the formation chemistry of these byproduct molecules can provide the insight needed to minimize their production, decrease the risk associated with an unintended environmental release, and simultaneously increase gas yields.