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

We converted the marine microalga Nannochloropsis sp. into a crude bio-oil product and a gaseous product via hydrothermal processing from 200 to 500 °C and a batch holding time of 60 min. A moderate temperature of 350 °C led to the highest bio-oil yield of 43 wt %. We estimate the heating value of the bio-oil to be about 39 MJ kg−1, which is comparable to that of a petroleum crude oil. The H/C and O/C ratios for the bio-oil decreased from 1.73 and 0.12, respectively, for the 200 °C product to 1.04 and 0.05, respectively, for the 500 °C product. Major bio-oil constituents include phenol and its alkylated derivatives, heterocyclic N-containing compounds, long-chain fatty acids, alkanes and alkenes, and derivatives of phytol and cholesterol. CO2 was always the most abundant gas product. H2 was the second most abundant gas at all temperatures other than 500 °C, where its yield was surpassed by that of CH4. The activation energies for gas formation suggest the presence of gas-forming reactions other than steam...

Hydrothermal liquefaction and gasification of Nannochloropsis sp.

The current status of biomass gasification in near- and supercritical water (SCWG) is reviewed. There are two approaches to biomass gasification in supercritical water. The first: low-temperature catalytic gasification, employs reaction temperature ranging from 350 to 600 °C, and gasifies the feedstock with the aid of metal catalysts. The second: high-temperature supercritical water gasification, employs reaction temperatures ranging from 500 to 750 °C, without catalyst or with non-metallic catalysts. Reviews are made on reaction mechanism, catalyst, and experimental results for these two approaches. Engineering technologies for SCWG gasification, and an example of process analysis are also introduced. Finally, the authors’ prognostications on the future prospects of this technology are offered.

Biomass gasification in near- and super-critical water: Status and prospects

Hydrogen is defined as an attractive energy carrier due to its potentially higher energy efficiency and low generation of pollutants, which can replace conventional fossil fuels in the future. The governments have invested huge funds and made great efforts on the research of hydrogen production. Among the various options, supercritical water gasification (SCWG) is a most promising method of hydrogen production from biomass. Supercritical water (SCW) has received a great deal of attention as a most suitable reaction medium for biomass gasification because it is safe, non-toxic, readily available, inexpensive and environmentally benign. However, high temperature and pressure are required to meet the minimum reaction condition. Therefore, the high operating cost has become the biggest obstacle to the development of this technology. To overcome this bottleneck, many researchers have carried out intensive research work on the catalytic supercritical water gasification (CSCWG). Based on the previous studies stated in the literature, the authors try to give an overview (but not an exhaustive review) on the recent investigations of CSCWG. Besides, the physicochemical properties of SCW and its contributions in subcritical and supercritical water reaction are also summarized.

Review of catalytic supercritical water gasification for hydrogen production from biomass

Hydropyrolysis of glucose as a model compound for cellulose in hot compressed water in the presence of K2CO3 at temperatures of 400 and 500 °C and pressures of 30 and 50 MPa in the range of 1.8−16.3 min reaction time is conducted to characterize key compounds of the thermochemical conversion {phenols (phenol and cresols), furfurals [methylfurfural and (hydroxymethyl)furfural], organic acids, aldehydes}. To compare the effect of alkali salts on the pyrolysis of glucose and biomass, the experiments are conducted in the presence of K2CO3, while biomass includes usually alkali salts. Hydrothermolysis of glucose in the presence of K2CO3 increases the hydrogen yield and the amount of phenols under these conditions. The formation and the degradation pathways of phenols and the other compounds that are identified are discussed. A simplified reaction scheme for glucose decomposition is given and used to discuss the role of these compounds. The effect of varying properties of supercritical water such as density, di...

Ali Sınaǧ

Papers

Key Compounds of the Hydropyrolysis of Glucose in Supercritical Water in the Presence of K2CO3