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.

To obtain drop-in fuel properties from 3rd generation biomass, we herein report the catalytic hydrotreatment of microalgae biocrude, produced from hydrothermal liquefaction (HTL) of Spirulina. Our contribution focuses on the effect of temperature, initial H₂ pressure, and residence time on the removal of heteroatoms (O and N) in a batch hydrotreating setup. In contrast to common experimental protocols for hydrotreating at batch scale, we devised a set of two-level factorial experiments and studied the most influential parameters affecting the removal of heteroatoms. It was found that up to 350 °C, the degree of deoxygenation (de-O) is mainly driven by temperature, whereas the degree of denitrogenation (de-N) also relies on initial H₂ pressure and temperature-pressure interaction. Based on this, complete deoxygenation was obtained at mild operating conditions (350 °C), reaching a concurrent 47% denitrogenation. Moreover, three optimized experiments are reported with 100% removal of oxygen. In addition, the analysis by GC-MS and Sim-Dis gives insight to the fuel quality. The distribution of heteroatom N in lower (340 °C) fractional cuts is studied by a fractional distillation unit following ASTM D-1160. Final results show that 63–68% of nitrogen is concentrated in higher fractional cuts.

In this contribution, we successfully applied demineralization (i.e., solvent-assisted separation and acid washing) for the removal of carbonaceous solids and inorganics from a biocrude obtained from the catalytic hydrothermal liquefaction (HTL) of Miscanthus. The experimental results of all six employed acids showed that 0.1 M H₂SO₄ was the most effective and significantly reduced metallic (Fe by 93.9%, <15 µg/g and Mg by 95.6%, 2.1 µg/g) and ash content (by 92.7% to 337 µg/g) from the already filtered biocrude. The utilized demineralizing agents caused a loss of nitrogen and of organic carbon (1% total organic carbon (TOC) and 0.058% total nitrogen (TN) in 0.1 M H₂SO₄). Gas chromatography-mass spectrometry (GC–MS) results clarified the nature of this loss, showing that 54% of ketones and 39% of alcohols were removed when 0.1 M H₂SO₄ was employed. Furthermore, FT-IR spectra remained the same before and after acid washing without affecting any functional groups. This work therefore proposes demineralization as a viable route for the removal of high inorganic content from lignocellulosic HTL biocrudes.

This study involves the investigation of municipal solid waste (MSW) based biofuel in order to demonstrate its utilization as a diesel blendstock in a compression ignition (CI) engine. The biofuel was produced from the Hydrothermal Liquefaction (HTL) process. The tested biofuels represented both distilled (known as nonupgraded HTL biofuel) and hydrotreated (known as upgraded HTL biofuel) fuels, obtained from raw bio-crude. The effects of the HTL biofuel and diesel blending on the combustion and emission characteristics were investigated. A comparative study of nonupgraded and upgraded HTL biofuel in terms of combustion and emissions was conducted. The upgraded HTL biofuel was blended with reference diesel (RD) by 5%, 10%, and 40% by weight, respectively, and the nonupgraded HTL biofuel was blended with RD by 10% by weight. The experiments were conducted in an optically accessible compression ignition chamber (OACIC) with engine-like thermodynamic conditions. The parameters were recorded at a constant speed and at fixed thermodynamic conditions. The heat release rate (HRR), in-cylinder pressure, ignition delay (ID), flame lift-off length (FLOL), and in-flame soot were measured. The PM, CO, NOx, and CO₂ were also recorded. In summary, the HTL blends exhibited a close resemblance to the reference diesel across a range of combustion parameters and regulated emissions. Furthermore, the upgraded HTL blends outperformed the nonupgraded blend in terms of both combustion characteristics and emissions.