Influence of process parameters on the hydrothermal decomposition and oxidation of glucose in sub- and supercritical water

How to identify suitable ways for the hydrothermal treatment of wet bio-waste? A critical review and methods proposal

A considerable amount of wet biogenic residues and waste has no resource-efficient use in several European countries yet. Hydrothermal processes (HTP) seem to be promising for treating such biomass as they best work with substrates with 70% to 90% water content. However, thus far the suitability of HTP for this purpose has not been sufficiently evaluated, for which this work aims to identify suitable multi-criteria analysis (MCA) methods that can be used to identify promising ways for the hydrothermal treatment of wet bio-waste. A review on 31 recent MCA studies in bio-waste management was conducted with the aim of comparing them to methodological requirements for evaluating HTP. Furthermore, an MCA approach for HTP based on the review findings is proposed. Results show that no observed MCA method is directly transferable for assessing HTP, for which a customized approach combining the analytical hierarchy process and the technique for order preference by similarity to ideal solutions is proposed and preliminarily validated with literature data. These preliminary calculations indicate that hydrothermal gasification seems most promising under consideration of multiple criteria using the available average and exemplary data. However, needless to say there is still a long way to go to obtain the sufficient adequate data to validate and use the model appropriately, for which further studies are necessary to acquire more reliable data and to assess also future technology developments of HTP.

Governing chemistry of cellulose hydrolysis in supercritical water

At extremely low reaction times (0.02 s), cellulose was hydrolyzed in supercritical water (T=400 °C and P=25 MPa) to obtain a sugar yield higher than 95 wt%, whereas the 5-hydroxymethylfurfural (5-HMF) yield was lower than 0.01 wt %. If the reaction time was increased to 1 s, the main product was glycolaldehyde (60 wt%). Independently of the reaction time, the yield of 5-HMF was always lower than 0.01 wt%. To evaluate the reaction mechanism of biomass hydrolysis in pressurized water, several parameters (temperature, pressure, reaction time, and reaction medium) were studied for different biomasses (cellulose, glucose, fructose, and wheat bran). It was found that the H(+) and OH(-) ion concentration in the reaction medium as a result of water dissociation is the determining factor in the selectivity. The reaction of glucose isomerization to fructose and the further dehydration to 5-HMF are highly dependent on the ion concentration. By an increase in the pOH/pH value, these reactions were minimized to allow control of 5-HMF production. Under these conditions, the retroaldol condensation pathway was enhanced, instead of the isomerization/dehydration pathway.