Hydrothermal liquefaction is a technique for obtaining clean biofuel from biomass in the presence of a solvent at moderate to high temperature (250–550 °C) and pressure (5–25 MPa). Hydrothermal decomposition of biomass leads to the formation of various compounds depending upon operating parameters. The role of processing conditions including final liquefaction temperature, residence times, rate of biomass heating, size of biomass particles, type of solvent media and hydrogen donor solvents is important for the bio-oil yield and quality of the product. The effect of these parameters on the yield and composition of the liquid products is reviewed in the paper. A brief description about the decomposition mechanism is also included to highlight the product types during hydrothermal liquefaction.

A review on process conditions for optimum bio-oil yield in hydrothermal liquefaction of biomass

Biomass gasification is a widely used thermochemical process for obtaining products with more value and potential applications than the raw material itself. Cutting-edge, innovative and economical gasification techniques with high efficiencies are a prerequisite for the development of this technology. This paper delivers an assessment on the fundamentals such as feedstock types, the impact of different operating parameters, tar formation and cracking, and modelling approaches for biomass gasification. Furthermore, the authors comparatively discuss various conventional mechanisms for gasification as well as recent advances in biomass gasification. Unique gasifiers along with multi-generation strategies are discussed as a means to promote this technology into alternative applications, which require higher flexibility and greater efficiency. A strategy to improve the feasibility and sustainability of biomass gasification is via technological advancement and the minimization of socio-environmental effects. This paper sheds light on diverse areas of biomass gasification as a potentially sustainable and environmentally friendly technology.

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An overview of advances in biomass gasification

The rapid depletion of conventional fossil fuels and day-by-day growth of environmental pollution due to use of extensive use of fossil fuels have raised concerns over the use of the fossil fuels; and thus search for alternate renewable and sustainable sources for fuels has started in the last few decades. In this context biomass derived fuels seems to be the promising path; and various routes are available for the biomass processing such as pyrolysis, transesterification, hydrothermal liquefaction, steam reforming, etc.; and the hydrothermal liquefaction (HTL) of wet biomass seems to be the promising route. Therefore, this article briefly enlightened a few concepts of HTL such as the elemental composition of bio-crude obtained by HTL, different types of feedstock adopted for HTL, mechanism of HTL processes, possible process flow diagrams for HTL of both wet and dry biomass and energy efficiency of the process. In addition, this article also enlisted possible future research scope for concerned researchers and a few of them are setting up HTL plant suitable for both wet and dry biomass feedstock; analysing influence of parameters such as temperature, pressure, residence time, catalytic effects, etc.; deriving optimized pathways for better conversion; and development of theoretical models representing the process to the best possible accuracy depending on nature of feedstock.

A review on hydrothermal liquefaction of biomass

Ethylene glycol (EG) is an important organic compound and chemical intermediate used in a large number of industrial processes (e.g. energy, plastics, automobiles, and chemicals). Indeed, owing to its unique properties and versatile commercial applications, a variety of chemical systems (e.g., catalytic and non-catalytic) have been explored for the synthesis of EG, particularly via reaction processes derived from fossil fuels (e.g., petroleum, natural gas, and coal) and biomass-based resources. This critical review describes a broad spectrum of properties of EG and significant advances in the prevalent synthesis and applications of EG, with emphases on the catalytic reactivity and reaction mechanisms of the main synthetic methodologies and applied strategies. We also provide an overview regarding the challenges and opportunities for future research associated with EG.

Ethylene glycol: properties, synthesis, and applications

In view of the diminishing oil resources and the ongoing climate change, the use of efficient and environmentally benign technologies for the utilization of renewable resources has become indispensible. Therein, hydrogenolysis reactions offer a promising possibility for future biorefinery concepts. These reactions result in the cleavage of C-C and C-O bonds by hydrogen and allow direct access to valuable platform chemicals already integrated in today's value chains. Thus, hydrogenolysis bears the potential to bridge currently available technologies and future biomass-based refinery concepts. This Review highlights past and present developments in this field, with special emphasis on the direct utilization of cellulosic feedstocks.

Hydrogenolysis Goes Bio: From Carbohydrates and Sugar Alcohols to Platform Chemicals

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

Algae is generally regarded as the only source of renewable biodiesel that is capable of meeting the global demand for transport fuels. Hydrothermal liquefaction is the method of dissolving organic compounds in subcritical water at high ion product, which can accelerate the acid-catalyzed and hydrolytic decomposition of algal macromolecules. This promising process converts different algal strains with high moisture content to high bio-oil yields with lower coke and lower energy consumption in comparison to other methods. The properties of the resulting bio-oil are clearly affected by parameters such as temperature, reaction time, algae species, algae concentration, reaction atmosphere, and catalysts, in subcritical water reaction conditions. This paper will provide an extensive review of the updated results on the hydrothermal liquefaction of microalgae. Including summary on the conversion of major compounds in microalgae and products related to the process and the influences of reaction conditions, catalysts and hydrotreating with comparing the variation tendencies and main results reported.

A review of bio-oil production from hydrothermal liquefaction of algae

Treatment of municipal sewage sludge in supercritical water: A review

Over the last two decades, the hydrothermal liquefaction (HTL) of algae has emerged as a promising technology for producing liquid bio-oil to meet increasing energy demands and reduce environmental pollution. In this article, the present research status of the catalytic HTL of algae and the catalytic hydrothermal upgrading of biocrude (crude bio-oil) is systematically reviewed and analyzed. The corresponding catalytic characteristics (such as the catalytic effect on the biocrude yield and quality and the related influencing factors) and catalytic mechanisms (e.g., hydrogenation, deoxygenation, decarboxylation, denitrogenation and desulfurization) during algae HTL as well as the approaches for upgrading of biocrude are summarized and analyzed comprehensively. Another potential technological flow for bio-oil production from algae HTL is proposed, and a comparison between direct catalytic HTL and the two-step production method is presented for the first time. Moreover, contemporary problems and subsequent research directions are presented.

Yang Guo

Papers

Review of catalytic supercritical water gasification for hydrogen production from biomass

A review of bio-oil production from hydrothermal liquefaction of algae

Treatment of municipal sewage sludge in supercritical water: A review

Catalytic hydrothermal liquefaction of algae and upgrading of biocrude: A critical review

Salt deposition problems in supercritical water oxidation