The Effect of Dichloromethane on Product Separation during Continuous Hydrothermal Liquefaction of Chlorella vulgaris and Aqueous Product Recycling for Algae Cultivation

Abstract

Dichloromethane (DCM) is a solvent commonly used in laboratories for microalgae hydrothermal liquefaction (HTL) product separation. The addition of DCM would lead to an "overestimation effect" of biocrude yield and diminish biocrude quality. However, it is currently not clear to what extent this overestimation effect will impact a continuous HTL process. In this study, Chlorella vulgaris microalgae was processed in a continuous stirred tank reactor at different temperatures (300, 325, 350, 375, and 400 °C) at 24 MPa for 15 min holding time. Two separation methods were applied to investigate the effect of using DCM in a cHTL product separation procedure in terms of product yield, biocrude elemental content, and aqueous product (AP) composition. Subsequently, the feasibility of reusing AP for algae cultivation has been evaluated. Results suggest that 350 °C is the optimal temperature for cHTL operation, leading to the highest biocrude yield, and an average increase in biocrude yield of 9 wt % was achieved when using DCM in cHTL product separation. Within the temperature range investigated, an average biocrude yield estimation can be proposed by yield non-DCM ≈ 0.818 × yield DCM . The AP has been characterized by total organic carbon and total nitrogen, high-performance liquid chromatography, and inductively coupled plasma optical emission spectroscopy. Results show that at 350−375 °C more nitrogen and other ions were directed into the AP, which could be advantageous in nutrient recovery. With the help of optical density testing, algae was shown to exhibit a better growth using AP with activated carbon absorption purification treatment as compared to the standard medium. The recovery of water and nutrients from the HTL-AP could improve the economics of a microalgae biorefinery process.

Hydrothermal Treatment of Biomass Feedstocks for Sustainable Production of Chemicals, Fuels, and Materials: Progress and Perspectives

“…HTL is the most widely used hydrothermal technique to convert algae to biofuels. ,, The main processes and transformation pathways involved in the HTL of algae are shown in Figure . The algae slurry obtained from algae cultivation was added in HTL batch or continuous reactors for hydrothermal transformation (Figure a) . Then, gaseous products were collected and analyzed for potential usage (Figure a) .…”

“…The algae slurry obtained from algae cultivation was added in HTL batch or continuous reactors for hydrothermal transformation (Figure a) . Then, gaseous products were collected and analyzed for potential usage (Figure a) . Subsequently, solid and liquid products were separated as precursors of solid fuels and bio-oil (Figure a) for upgrading, respectively .…”

“…309 Then, gaseous products were collected and analyzed for potential usage (Figure 31a). 309 Subsequently, solid and liquid products were separated as precursors of solid fuels and bio-oil (Figure 31a) for upgrading, respectively. 309 The processing water (aqueous solution) was purified or reformed in algae cultivation or in HTL processes as a circular process (Figure 31a).…”

“…309 Subsequently, solid and liquid products were separated as precursors of solid fuels and bio-oil (Figure 31a) for upgrading, respectively. 309 The processing water (aqueous solution) was purified or reformed in algae cultivation or in HTL processes as a circular process (Figure 31a). 309 The transformation pathway of algae covers the changes of lipids, proteins, and carbohydrates as shown in Figure 31b.…”

Review on Nitrogen Transformation during Microalgae Thermochemical Liquefaction: Recent Advances and Future Perspectives

“…For example, ammonia and amine are derived from the deamination 122 and decarboxylation 123 of amino acids, which eventually enter the biocrude and aqueous phase as amides through acylation or ammonolysis reactions. 124 Meanwhile, reducing sugars produced by the hydrolysis of carbohydrates undergo Maillard reactions with amino acids/NH 3 to produce an intermediate with a pungent odor but still structurally unclear�melanoidins, 6,72,125 which can be described as repeating units of furans and pyrroles or polymers constituted by sugar degradation products and branched amino compounds. 126 One of the common N-containing heterocyclic compounds (DKP) 37 was observed in biocrude when the temperature was around 200 °C.…”