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

In pursuit of more sustainable and competitive biorefineries, the effective valorisation of lignin is key. An alluring opportunity is the exploitation of lignin as a resource for chemicals. Three technological biorefinery aspects will determine the realisation of a successful lignin-to-chemicals valorisation chain, namely (i) lignocellulose fractionation, (ii) lignin depolymerisation, and (iii) upgrading towards targeted chemicals. This review provides a summary and perspective of the extensive research that has been devoted to each of these three interconnected biorefinery aspects, ranging from industrially well-established techniques to the latest cutting edge innovations. To navigate the reader through the overwhelming collection of literature on each topic, distinct strategies/topics were delineated and summarised in comprehensive overview figures. Upon closer inspection, conceptual principles arise that rationalise the success of certain methodologies, and more importantly, can guide future research to further expand the portfolio of promising technologies. When targeting chemicals, a key objective during the fractionation and depolymerisation stage is to minimise lignin condensation (i.e. formation of resistive carbon–carbon linkages). During fractionation, this can be achieved by either (i) preserving the (native) lignin structure or (ii) by tolerating depolymerisation of the lignin polymer but preventing condensation through chemical quenching or physical removal of reactive intermediates. The latter strategy is also commonly applied in the lignin depolymerisation stage, while an alternative approach is to augment the relative rate of depolymerisation vs. condensation by enhancing the reactivity of the lignin structure towards depolymerisation. Finally, because depolymerised lignins often consist of a complex mixture of various compounds, upgrading of the raw product mixture through convergent transformations embodies a promising approach to decrease the complexity. This particular upgrading approach is termed funneling, and includes both chemocatalytic and biological strategies.

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Chemicals from lignin: an interplay of lignocellulose fractionation, depolymerisation, and upgrading

Pyrolysis is one of the thermochemical technologies for converting biomass into energy and chemical products consisting of liquid bio-oil, solid biochar, and pyrolytic gas. Depending on the heating rate and residence time, biomass pyrolysis can be divided into three main categories slow (conventional), fast and flash pyrolysis mainly aiming at maximising either the bio-oil or biochar yields. Synthesis gas or hydrogen-rich gas can also be the target of biomass pyrolysis. Maximised gas rates can be achieved through the catalytic pyrolysis process, which is now increasingly being developed. Biomass pyrolysis generally follows a three-step mechanism comprising of dehydration, primary and secondary reactions. Dehydrogenation, depolymerisation, and fragmentation are the main competitive reactions during the primary decomposition of biomass. A number of parameters affect the biomass pyrolysis process, yields and properties of products. These include the biomass type, biomass pretreatment (physical, chemical, and biological), reaction atmosphere, temperature, heating rate and vapour residence time. This manuscript gives a general summary of the properties of the pyrolytic products and their analysis methods. Also provided are a review of the parameters that affect biomass pyrolysis and a summary of the state of industrial pyrolysis technologies.

Lignocellulosic biomass pyrolysis: A review of product properties and effects of pyrolysis parameters

A review on pyrolysis of plastic wastes

Lignocellulosic biomass is the most abundant and bio-renewable resource with great potential for sustainable production of chemicals and fuels. This critical review provides insights into the state-of the-art accomplishments in the chemocatalytic technologies to generate fuels and value-added chemicals from lignocellulosic biomass, with an emphasis on its major component, cellulose. Catalytic hydrolysis, solvolysis, liquefaction, pyrolysis, gasification, hydrogenolysis and hydrogenation are the major processes presently studied. Regarding catalytic hydrolysis, the acid catalysts cover inorganic or organic acids and various solid acids such as sulfonated carbon, zeolites, heteropolyacids and oxides. Liquefaction and fast pyrolysis of cellulose are primarily conducted over catalysts with proper acidity/basicity. Gasification is typically conducted over supported noble metal catalysts. Reaction conditions, solvents and catalysts are the prime factors that affect the yield and composition of the target products. Most of processes yield a complex mixture, leading to problematic upgrading and separation. An emerging technique is to integrate hydrolysis, liquefaction or pyrolysis with hydrogenation over multifunctional solid catalysts to convert lignocellulosic biomass to value-added fine chemicals and bio-hydrocarbon fuels. And the promising catalysts might be supported transition metal catalysts and zeolite-related materials. There still exist technological barriers that need to be overcome (229 references).

http://www.chtf.stuba.sk/~szolcsanyi/education/files/Chemia%20heterocyklickych%20zlucenin/Prednaska%207/Doplnkove%20studijne%20materialy/Hydroxymethylfurfural/Catalytic%20conversion%20of%20lignocellulosic%20biomass%20to%20fine%20chemicals%20and%20fuels.pdf

Catalytic conversion of lignocellulosic biomass to fine chemicals and fuels

Composition of products from the pyrolysis of polyethylene and polystyrene in a closed batch reactor: Effects of temperature and residence time

Glucose, as a representative biomass model compound, was gasified under supercritical water conditions in a batch reactor. The influences of temperature in the subcritical and supercritical regimes of water, oxidant concentration, glucose concentration, and residence time were examined in relation to the yield and composition of the product gases and oils. The product gases were analyzed by packed-column gas chromatography, and the oils were extracted using solid-phase extraction and analyzed by coupled gas chromatography/mass spectrometry. The main gases produced were carbon dioxide, carbon monoxide, methane, and hydrogen, and there was significant production of oil and char. As the temperature was increased, there was an increase in the yield of gas; similarly, gas yield also increased as the concentration of the oxidant, hydrogen peroxide, was increased. The influence of residence time on the yield of products was small, with a slight decrease in oil yield and an increase in char yield. The oils were c...

Composition of Products from the Supercritical Water Gasification of Glucose: A Model Biomass Compound

Catalytic hydrothermal gasification of algae for hydrogen production: composition of reaction products and potential for nutrient recycling.

Role of sodium hydroxide in the production of hydrogen gas from the hydrothermal gasification of biomass

The subcritical and supercritical water gasification of cellulose, starch, and glucose as representative biomass model compounds and biomass in the form of Cassava waste has been investigated in a heated batch reactor. Cellulose and starch are two polysaccharides which have identical chemical compositions based on the monomer glucose but which have different chemical structures and physical properties. The influence of temperature in the subcritical and supercritical regimes of water were examined in relation to the yield of the product gases, oil, char, and water. For the model compounds and the Cassava waste, the main gases produced were carbon dioxide, carbon monoxide, hydrogen, methane, and other hydrocarbons, and there was significant production of oil and char. There were, however, distinct differences between the yields of the different products and the trends in relation to temperature. Cellulose produced a higher yield of char, carbon monoxide, and C1−C4 hydrocarbons compared to starch and glucos...

Jude A. Onwudili

Papers

Composition of products from the pyrolysis of polyethylene and polystyrene in a closed batch reactor: Effects of temperature and residence time

Composition of Products from the Supercritical Water Gasification of Glucose: A Model Biomass Compound

Catalytic hydrothermal gasification of algae for hydrogen production: composition of reaction products and potential for nutrient recycling.

Role of sodium hydroxide in the production of hydrogen gas from the hydrothermal gasification of biomass

Subcritical and Supercritical Water Gasification of Cellulose, Starch, Glucose, and Biomass Waste