Reaction pathways of monomers and oligomers during hydrothermal liquefaction of lignin

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Lignin Hydrothermal Liquefaction into Bifunctional Chemicals: A Concise Review

Lignin, the second largest biomass after cellulose is underutilized. Yet, it remains the only natural source of aromatic, and phenolic compounds. It is imperative to, amidst the expanding interest on biomass conversion, to accord the necessary attention towards lignin degradation into value added chemicals. Specifically, its phenyl, guaiacyl, and syringyl derivatives. Understanding lignin degradation chemistry, goes a long way in its selective valorization into fuels and chemicals via thermochemical routes such as hydrothermal liquefaction (HTL). Therefore, development of technologies targeting value addition of products and by-products from lignin, would undoubtedly give way to emerging markets in the industry. Previous review papers focused on the general HTL of biomass, food waste, algae, and their model compounds. However, review on HTL of lignin is scarcely available. This paper presents the detailed literature analyses of the current trend in lignin degradation via HTL. Effect of HTL conditions including temperature, heating rate and catalyst has been reviewed. In-depth discussion on use of ionic liquids as catalyst for HTL of lignin has also been compiled. Other lignin degradation techniques such as pyrolysis and hydrolysis were also discussed. This is aimed at bringing together an up-to-date information on lignin degradation into selected chemical intermediates

Techno‐Economic Assessment of a Microalgae Biorefinery

A preliminary techno-economic assessment of a microalgae biorefinery plant is reported, with pulsed electric field treatment (PEF) hydrothermal liquefaction as core technology. The results indicate that standalone production of microalgae biofuel would lead to an annual loss of 2.615 M€. PEF treatment could improve this scenario by bringing the microalgae biofuel to a competitive level (0.78 € kg$^{-1}$). Assuming that microalgae biofuel would be sold at the price of crude oil (0.44 € kg$^{-1}$), the minimum price of the amino-acid based product should be 7.56 € L$^{-1}$ for positive capital returns

Valorisation of the residual aqueous phase from hydrothermally liquefied black liquor by persulphate-based advanced oxidation

Hydrothermal liquefaction of Kraft black liquor is a promising method for the production of valuable organic chemicals. However, the separation of the biochar and biocrude leaves a residual aqueous phase in large volumes, which needs to be properly managed to make the process profitable. In this work, the persulphate-based advanced oxidation was assessed, for the first time ever, as a pretreatment of this aqueous phase to reduce its content of phenolic compounds and alcohols, which hinder further valorisation strategies. Results revealed that the phenolic compounds and the alcohols were oxidised in presence of low persulphate anion concentrations (<50 mM), mainly to quinone-like compounds and organic acids. At higher oxidant concentrations, these intermediates were subsequently oxidised to valuable acetic acid. When Fe (II) was added as the catalyst, low concentrations (<9 mM) enhanced the degradation of both phenolic compounds and alcohols due to the increase of the sulphate radicals, consequently reducing persulphate requirements for their removal. Nevertheless, higher Fe (II) doses produced the sequestration of sulphate radicals, thus decreasing the oxidation performance and generating undesired parallel reactions

Eutectic solvents for the valorisation of the aqueous phase from hydrothermally liquefied black liquor

The potential valorisation of the aqueous phase obtained after the hydrothermal liquefaction of Kraft black liquor by means of liquid-liquid extraction with new generation solvents was analysed for the first time ever. For this purpose, hydrophobic eutectic solvents (ES), based on combinations of menthol or thymol with octanoic, decanoic or dodecanoic acid, were tested to recover phenolic compounds from this wastewater. All of them showed high affinity for phenolic compounds and ethanol, but low affinity for the rest of the compounds, leaving a more biodegradable raffinate. Regarding phenolic compounds, the average extraction yields ranged from 66% to 91% with menthol-based ES and from 34% to 98% with thymol-based ES. The best solvent in terms of recovery and selectivity for phenolic compounds was 1:1 Menthol:Octanoic acid, with separation factors of 104.2 and 29.2 for phenolic compounds to volatile fatty acids and alcohols, respectively. In this regard, the results obtained open the simultaneous valorisation of the extract as a source of phenolic compounds, regenerating the ES, and the raffinate as a sustainable feedstock for further fermentation or catalytic processes

Understanding the fate of nitrogen during catalytic hydrothermal liquefaction of sewage sludge

In this study, the fate of nitrogen during catalytic hydrothermal liquefaction (HTL) of sewage sludge (SS) is investigated using three different catalysts (CuNi/SiO$_2$, HCOOH, CuSO$_4$) with 5 wt% loading. The bio-crude yields obtained from HTL experiments catalyzed with CuNi/SiO$_2$ are similar to those obtained through non-catalyzed experiments. HCOOH slightly increases the bio-crude yields, while maximum yields of 24.5 wt% is obtained in the presence of CuSO$_4$, which also reduces the nitrogen content by 15 % and enhances the hydrocarbons compared to the non-catalyzed HTL. Mechanistic investigations regarding the interaction of amino acids and carbohydrates by Maillard reactions are carried out using model compounds, namely lysine and lactose. CuSO$_4$ effectively increases the yield with 50 % and reduces the nitrogen content by 24 % in the bio-crude during HTL of lysine alone. In the case of the model mixtures, bio-crude yields, nitrogen content, and Maillard reactions products behaved similarly both for catalytic and non-catalytic HTL, a slight reduction of amines was found in the presence of CuSO$_4$. Hydro-char and some organic compounds are assumed to act highly reductant during catalytic HTL. Coke deposition and adsorbed poisoning by Maillard reaction products are proposed as the main reasons for the deactivation of catalysts