Abstract
Hydrothermal liquefaction (HTL) of biomass is establishing itself as one of the leading technologies for the conversion of virtually any type of biomass feedstock into drop-in biofuels and renewable materials. Several catalysis strategies have been proposed for this process to increase the yields of the product (biocrude) and/or to obtain a product with better properties in light of the final use. A number of different studies are available in the literature nowadays, where different catalysts are utilized within HTL including both homogeneous and heterogeneous approaches. Additionally, catalysis plays a major role in the upgrading of HTL biocrude into final products, in which field significant developments have been observed in recent times. This review has the ambition to summarize the different available information to draw an updated overall picture of catalysis applied to HTL. The different catalysis strategies are reviewed, highlighting the specific effect of each kind of catalyst on the yields and properties of the HTL products, by comparing them with the non-catalyzed case. This allows for drawing quantitative conclusions on the actual effectiveness of each catalyst, in relation to the different biomass processed. Additionally, the pros and cons of each different catalysis approach are discussed critically, identifying new challenges and future directions of research.
Highlights
Fossil fuels are declining rapidly due to their excessive use and increase in global industrialization
The results indicated a higher thermal stability of CeO2 catalysts deposited with Ni nanoparticles than the pure CeO2 catalyst, as the Ni/CeO2 catalyst possessed excellent cycling stability after the recovery
As was observed in the previous treatise, the presence of a catalyst turns out to be decisive in order to increase both the yield and the quality of the produced biocrude
Summary
Fossil fuels are declining rapidly due to their excessive use and increase in global industrialization. Biofuels derived from organic matters or plant biomass are the only sustainable carbon source for the production of liquid fuels and well-suited to the existing transport infrastructure [3,4]. Gasification, pyrolysis, and hydrothermal liquefaction (HTL) are the most developed thermochemical technologies, which can produce heat, syngas (H2 and CO), and biocrude oil, respectively, through the conversion of various biomass. Combustion, gasification, and pyrolysis are energy intensive processes due to pre-drying of feedstocks with higher water content [2,8]. Hydrothermal liquefaction (HTL) is a promising technology, which is potentially able to convert higher water content feedstocks into considerably lower oxygen content and higher calorific value biocrude oil without demanding a preliminary drying step of the biomass. Many informative reviews are accessible in the literature in which many research efforts have been dedicated to the production of biocrude through the HTL process and investigated the influence of various process variables such as temperature, pressure, catalyst, retention time, and biomass:water ratio on HTL products [10,11,12,13,14,15,16,17,18]
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