• Energy Research
• 6. Clean water close

Abstract The removal of nitrogen (N) and sulphur (S) from biocrude oil produced using hydrothermal liquefaction (HTL), is important for the production of high quality renewable fuels. Here the effect of co-liquefaction of bagasse and algae was analysed. Algae (Chlorella vulgaris and Cyanobacteria) were mixed with bagasse (1:1) subjected to HTL at 250–350 °C for 10–60 min. Higher HTL temperatures had a positive effect in increasing the biocrude yield and slightly reduced N content; S did not show a consistent trend. Most of the nitrogen (~66%) and sulphur (~80%) were recovered in the aqueous phase rather than in the biocrude phase, opening the opportunity to recycle these nutrients for algae cultivation. Co-liquefying bagasse with algae improved the biocrude yield (54 wt%) compared to pure Cyanobacteria (47.5 wt%). It also reduced N content from 7 wt% (Cyanobacteria biocrude) to 4.2 wt% (Cyanobacteria: Bagasse) and S from 0.7 wt% to 0.4 wt%. Principal Component Analysis (PCA) analysis identified that biocrude yield is positively correlated with the initial lipid content and anti-correlated with the carbohydrates fraction. Biocrude N content is closely related to the initial amount of proteins in the algae. The Preference Ranking Organization METHod for Enrichment of Evaluations and its descriptive complement Geometrical Analysis for Interactive Aid (PROMETHEE and GAIA) analysis ranked the co-liquefaction of Chlorella vulgaris and bagasse (1:1) at 350 °C and 60 min as one of the best overall combination in terms of biocrude yield, N and S content.

To mitigate global warming, humankind has been forced to develop new efficient energy solutions based on renewable energy sources. Hydrothermal liquefaction (HTL) is a promising technology that can efficiently produce bio-oil from several biomass sources. The HTL process uses sub- or supercritical water for producing bio-oil, water-soluble organics, gaseous products and char. Black liquor mainly contains cooking chemicals (mainly alkali salts) lignin and the hemicellulose parts of the wood chips used for cellulose digestion. This review explores the effects of different process parameters, solvents and catalysts for the HTL of black liquor or black liquor-derived lignin. Using short residence times under near- or supercritical water conditions may improve both the quality and the quantity of the bio-oil yield. The quality and yield of bio-oil can be further improved by using solvents (e.g., phenol) and catalysts (e.g., alkali salts, zirconia). However, the solubility of alkali salts present in black liquor can lead to clogging problem in the HTL reactor and process tubes when approaching supercritical water conditions.

Abstract The brown algae Fucus vesiculosus, Laminaria saccharina and Alaria esculenta were subjected to hydrothermal liquefaction (HTL) for 15 min at 350 °C in batch microautoclaves. Further optimization was carried out in view of optimizing the biocrude oil yield, varying the temperature from 330 to 370 °C. The maximum conversion to biocrude was 29.4 ± 1.1 wt.% at 360 °C for A. esculenta. The reaction pathways for macroalgae HTL and its capability for recycling nutrients were also investigated. The aqueous phase showed potential for a partial recovery of the nitrogen (21.2–28.6 wt.%) and sulfur (25.8–34.6 wt.%) from the initial biomass, and an almost total recovery of potassium and sodium. Results indicate that HTL as a sole conversion method to produce biofuel as single product is not recommended for macroalgae due to the low conversion to biocrude oil. At such conditions, its use as post-treatment for the remaining biomass after extracting valuable compounds (especially from the carbohydrate fraction) might be more interesting, and is suggested as the future direction for research.

Abstract The microalgae species Nannochloropsis gaditana (marine) and Scenedesmus almeriensis (freshwater) were subjected to hydrothermal liquefaction (HTL) at 350 °C in small microautoclaves for 15 min to study the separation of the aqueous and biocrude oil products, either by gravity or assisted by an organic solvent (dichloromethane). The vast majority of the research available for microalgae HTL determines the product yields by separating the HTL phases with an organic solvent. This study shows that its utilization affects the product distribution, increasing the amount of biocrude oil produced and reducing the concentration of organic molecules in the aqueous phase. The increase in the biocrude oil yield comes at the expense of a higher nitrogen and oxygen content. This harms the quality of the biocrude oil in view of its application as biofuel, due to undesired emissions upon combustion. The results herewith presented indicate that the yields of the HTL products strongly depend on the separation method applied. As the use of large amounts of organic solvents for separating the products at industrial scales is unlikely, their use is also discouraged in laboratory experimentation in order to forestall creating false expectations about the biocrude oil yields obtained by means of microalgae HTL.

Abstract With the rapid growth in population and urbanization, a development in sustainable treatment of sewage sludge has become an urgent environmental concern globally. Lipid extraction has been investigated in order to valorize waste sewage sludge treatment through a pathway that leads to biodiesel. In this work, an integrated approach that combines lipid extraction of sewage sludge with hydrothermal liquefaction of the lipid-extracted sludge was studied in order to maximize valorization. The hydrothermal process was performed at temperatures ranging from 250 to 350 °C with 20 min. Regarding the bio-crude: below 300 °C, similar values are found with and without lipid-extraction, with the former variant containing more nitrogenated compounds stemming from Maillard reactions, while the latter more hydrocarbons; at 350 °C, higher bio-crude is obtained from raw sewage sludge owning to the conversion of lipids. Palmitic acid was selected as a model lipid to elucidate the role of lipids during the process, as well as to provide an improved understanding of the reaction network. Energy recovery reached values of 85.4% for hydrothermal liquefaction of sewage sludge and 98.3% for integrated approach considering the whole range of biofuel products. The energy consumption ratio was applied to estimate energetic efficiency for the combined process, making it possible to estimate the breakeven point of the process, plus the efficiency of both the hydrothermal process on its own in comparison with the combined option.