Hydrothermal technologies are broadly defined as chemical and physical transformations in high-temperature (200–600 °C), high-pressure (5–40 MPa) liquid or supercritical water. This thermochemical means of reforming biomass may have energetic advantages, since, when water is heated at high pressures a phase change to steam is avoided which avoids large enthalpic energy penalties. Biological chemicals undergo a range of reactions, including dehydration and decarboxylation reactions, which are influenced by the temperature, pressure, concentration, and presence of homogeneous or heterogeneous catalysts. Several biomass hydrothermal conversion processes are in development or demonstration. Liquefaction processes are generally lower temperature (200–400 °C) reactions which produce liquid products, often called “bio-oil” or “bio-crude”. Gasification processes generally take place at higher temperatures (400–700 °C) and can produce methane or hydrogen gases in high yields.

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Thermochemical biofuel production in hydrothermal media: A review of sub- and supercritical water technologies

Hydrothermal liquefaction of biomass: A review of subcritical water technologies

Experiments were performed on the products of glucose decomposition at short residence times to elucidate the reaction pathways and evaluate kinetics of glucose and fructose decomposition in sub- and supercritical water. The conditions were a temperature of 300−400 °C and pressure of 25−40 MPa for extremely short residence times between 0.02 and 2 s. The products of glucose decomposition were fructose, a product of isomerization, 1,6-anhydroglucose, a product of dehydration, and erythrose and glyceraldehyde, products of C−C bond cleavage. Fructose underwent reactions similar to glucose except that it did not form 1,6-anhydroglucose and isomerization to glucose is negligible. The mechanism for the products formed from C−C bond cleavage could be explained by reverse aldol condensation and the double-bond rule of the respective enediols formed during the Lobry de Bruyn Alberda van Ekenstein transformation. The differential equations resulting from the proposed pathways were fit to experimental results to obt...

Glucose and fructose decomposition in subcritical and supercritical water: Detailed reaction pathway, mechanisms, and kinetics

This article reviews the work relating to the supercritical water gasifi cation of biomass with a focus on hydrogen production. The high hydrogen yield predicted by thermodynamic calculations and the special properties of near- and supercritical water support the biomass degradation; these were the main reasons why the process of = supercritical water gasifi cation was investigated. The main advantage is that biomass, with a natural water content of 80 wt.% or more, can be converted without drying before. The energy required for heating up the relatively high water amount can be recovered by a compact heat exchanger, which is very important for the overall energy balance. The chemistry of biomass degradation is rather complex: from experiments with model compounds, the main reaction pathways and their dependencies on reaction conditions are identifi ed. This knowledge was applied in studies of biomass conversion. Biomass may include proteins and salts, which have a signifi cant infl uence on the gasifi cation: salts increase and proteins decrease the gas yield at comparable reactions conditions. In addition, the heating-up rate and the reactor type used infl uence the results. For the scale-up in view of a technical application, a bench-scale plant is necessary. This plant exists for some years and demonstrates the process feasibility also in the scale of 100 kg/h. Still challenges for a technical application, like corrosion and solid handling, exist. © 2008 Society of Chemical Industry and John Wiley & Sons, Ltd

Supercritical water gasification.

Glucose decomposition kinetics in subcritical and supercritical water were studied for the temperatures 573, 623, and 673 K, pressures between 25 and 40 MPa, and residence times between 0.02 and 2 ...

Kinetics of glucose epimerization and decomposition in subcritical and supercritical water

The hydrothermal degradation was examined using D-(U-14C) glucose and D-(U-14C) fructose. By thin layer chromatography with methylene chloride, tetrahydrofuran (THF), acetic acid −60∶20∶20 as a mobile phase it was, possible to separate and identify the carbohydrates and their reaction products, glyceraldehyde, dihydroxyacetone, methylglyoxal, glycolaldehyde, 5-hydroxymethylfurfural and furfural. Up to 99% of the initial activity was determined by scintillation counting of the TL-chromatograms. A reaction scheme for the hydrothermal degradation of glucose and fructose was obtained from these results.

O. Bobleter

Papers

Determination of the hydrothermal degradation products of D-(U-14C) glucose and D-(U-14C) fructose by TLC