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

Conversion of sewage sludge to clean solid fuel using hydrothermal carbonization: Hydrochar fuel characteristics and combustion behavior

Marine macroalgae Enteromorpha prolifera, one of the main algae genera for green tide, was converted to bio-oil by hydrothermal liquefaction in a batch reactor at temperatures of 220−320 °C. The liquefaction products were separated into a dichloromethane-soluble fraction (bio-oil), water-soluble fraction, solid residue, and gaseous fraction. Effects of the temperature, reaction time, and Na2CO3 catalyst on the yields of liquefaction products were investigated. A moderate temperature of 300 °C with 5 wt % Na2CO3 and reaction time of 30 min led to the highest bio-oil yield of 23.0 wt %. The raw algae and liquefaction products were analyzed using elemental analysis, Fourier transform infrared (FTIR) spectroscopy, gas chromatography−mass spectrometry (GC−MS), and 1H nuclear magnetic resonance (NMR). The higher heating values (HHVs) of bio-oils obtained at 300 °C were around 28−30 MJ/kg. The bio-oil was a complex mixture of ketones, aldehydes, phenols, alkenes, fatty acids, esters, aromatics, and nitrogen-cont...

Hydrothermal Liquefaction of Macroalgae Enteromorpha prolifera to Bio-oil

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.

The hydrothermal gasification is a promising process to produce hydrogen from biomass with high water content. In a lot of cases, this biomass may contain proteins, for example, in residues from the food industry or sewage sludge. In Part I of this serial, experiments on hydrothermal gasification of protein containing biomass (zoo mass) have been reported. This biomass produces lower gas yields than biomass originating exclusively from plants (phyto mass). To understand these findings, experiments with model compounds are necessary. Here, such experiments with model compounds in a tubular and a batch reactor are described. The model system for the phyto mass is glucose with a potassium salt, and the model system for the zoo mass is glucose, potassium salt, and the amino acid alanine. The model systems show a lower gas yield in the presence of alanine. So the presence of alanine in the model system has a similar effect to the presence of proteins in biomass. Additionally, the gas composition and the concentration of selected key compounds are slightly changed by alanine addition. Likely, consecutive products of carbohydrate and protein degradation react with each other. In such Maillard reactions, free radical scavengers might be formed, reducing the reaction rate of free radical chain reactions that are highly relevant for gas formation. Therefore, the gas yield is lower in the presence of proteins or amino acids compared with systems without these compounds. In addition, experiments with real biomass in a batch reactor were reported to verify the assumption of Maillard products reducing free radical reactions. As an example, the addition of urea to phyto mass leads to a decrease of the gas yield to a value similar to that found for zoo mass.

Influence of Proteins on the Hydrothermal Gasification and Liquefaction of Biomass. 2. Model Compounds

The reaction of intermediates formed during hydrothermal biomass gasification (HBG) with each other or with hydrogen produced by the water-gas shift reaction has a significant influence on the process. To understand these reactions, the conversion of different C4 compounds (1-butanol, 1-butanal, cis-butendiol) was investigated in a batch reactor. These compounds carry different functional groups also found in intermediates of HBG. All compounds react to make products with aromatic ring systems, which shows that the intermediates can react with each other and aromatic rings are formed independently of the functional groups. The HBG intermediates can also react with hydrogen formed via the water-gas shift reaction (CO + H2O ⇔ CO2 + H2). This is shown by the reaction of deuterated glucose in H2O. The reaction of hydrogen originating from water leads to the formation of C–H bonds not present in the feedstock.

Hydrothermal gasification of biomass: consecutive reactions to long-living intermediates

In this study, a comparison of the hydrothermal behavior of various biomasses such as phyto mass, zoo mass, phyto mass/K2CO3 and glucose K2CO3 (model system for the phyto mass) is given. The experiments have been carried out in a batch reactor at 500 °C, 30 MPa with a reaction time of 1 h. The phenol content of the aqueous phase obtained by biomass conversion was remarkably high. Actually, phenols and furfurals are the most important intermediates formed during the hydrothermal conversion of biomass. No furfural and its derivatives were observed in this study. In order to investigate the hydrothermal biomass conversion in detail, the behavior of the phenols and furfurals, under the same experimental conditions was also studied. The differences between the feedstocks have been discussed. Likely variations of the products and intermediates formed are a consequence of differences in the composition of the feedstock, namely, salts and proteins which seem to influence the chemistry of hydrothermal conversions.

Palanikumar Maniam

Papers

Influence of Proteins on the Hydrothermal Gasification and Liquefaction of Biomass. 2. Model Compounds

Hydrothermal gasification of biomass: consecutive reactions to long-living intermediates

Hydrothermal conversion of biomass and different model compounds

Hot compressed water treatment of biomass