Supercritical water gasification

Abstract

Supercritical water gasification is a process in which wet biomass is converted to bio-syngas. In this process the temperature and pressure are raised above the critical point of water (374 C, 221 bar), creating a supercritical medium in which a high conversion and energetic efficiency of biomass to bio-syngas is realized. Due to these high efficiencies supercritical water gasification has received much attention as a potential treatment technique for sewage sludge from wastewater treatment plants.
To design a supercritical water gasification process kinetic models are used. They provide predictions on the decomposition products of the organic components of the biomass during treatment. However, kinetic data on lipids, which can make up to 25% of the organic matter in sewage sludge, are not available yet. This study aims to identify main reaction pathways and corresponding kinetic parameters that describe the decomposition of lipids in supercritical water.

Experiments were performed to provide data of decomposition products yields and find the dominant reaction pathways. Oleic acid was used as a model compound for lipids from sewage sludge. Experiments were conducted in a stainless steel batch reactor which was heated by immersion in a fluidized hot sand bath. Investigated temperatures and residence times were 400, 420, 460 and 520 C and 15, 35 and 65 min, respectively. Oleic acid feed concentration was 10 wt% and a pressure of 25 MPa was applied.

From experimental results the decomposition of oleic acid into aliphatic hydrocarbons and shorter chain fatty acids was identified. With increasing time and temperature these products would either gasify or the aliphatic hydrocarbons would dehydrogenate to cyclic and (poly)-aromatic compounds. A remarkably high selectivity towards the light hydrocarbon gases (C2H6, C2H4, C3H8, C3H6) compared to an earlier study into the decomposition of oleic acid in supercritical water was observed for all temperatures and residence times.

Parameters for a kinetic model, build up from the identified reaction paths, were fitted to the experimental data using Matlab. The Arrhenius equation was used to describe the reaction constants as function of temperature. For the oleic acid decomposition an activation energy of 151 kJ/mol was fitted first with a percentage output variation of 82% between 420 C and 520 C. Parameters for the other reactions were fitted using this activation energy as constraint.
Qualitative trends on the gas and liquid decomposition products distribution over time and temperature were predicted well by the model, but predictions on the quantitative yield of them were concluded to be inaccurate. Largest differences between experimental and model yields were observed for CH4 and the light hydrocarbon gases.

One reason for these model errors is the scarcity of data points in the 0-15 min time-scale, where the process was highest in reactivity. Also some of the reaction pathways in the model might have been oversimplified, neglecting certain dominant decomposition reactions.