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Hydrothermal liquefaction (HTL) is in a closed oxygen-free reactor by pressurizing inert gases (e.g., N2 or He) or reducing gases (e.g., H2 or CO), at a certain temperature (250–380°C) and pressure (5–28 MPa). During HTL, the hot compressed water is used as both solvent and reaction medium. HTL using hot compressed water as the solvent has the adva...
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... The chemical compound, type, and proportion of biocrude derived from digestate at a 1:1 ratio is presented in Table 6. The primary chemical compounds like ester and hydrocarbon were detected in 53.5% and 17.7%, respectively, representing the hydrothermal mechanism of esterification and decarboxylation [50]. Other minor compounds such as phenol (0.5%), aromatics (5.3%), and heterocyclic compounds (7.6%) indicated the hydrothermal mechanism of deamination and Maillard reactions [24]. ...
Higher energy recovery from wet-waste is a challenging task while considering the waste-to-energy approach to tackle the huge wet-waste biomass generated around the globe. This study took the opportunity to explore the combined approach of biohythane (e.g., bioH 2 and bioCH 4) and biocrude generation using two-stage anaerobic digestion coupled with hydrothermal liquefaction (TSAD-HTL). The finding of this study suggests that the highest biohythane productivity of 268.7 mL/g-VS with energy recovery of 9.95 MJ/kg-VS was produced from sludge-microalgae co-digestion, which was 17% and 205% higher than mono-digestion of microalgae and sludge, respectively. In addition, the highest biocrude yield of 45.4% was obtained from sludge-microalgae-derived digestate. The lighter fraction (<C20) of biocrude was observed at 91.8 % (20 % naphtha, 58 % jet fuel, and 13 % diesel) with an HHV of 35.85 MJ/kg. The higher energy recovery of 81 % and lower energy consumption ratio of 0.25 suggests that the integrated TSAD-HTL is an innovative energy-positive approach for producing biohythane-biocrude from wet wastes as a sustainable concept of zero waste.
... Direct use of sludge as a fertilizer might be possible because of the high organic matter content, nitrogen, and phosphorus. However, local legislation has made this use inappropriate due to possible risks caused by pathogenic organisms, heavy metals, and toxic substances derived from industry (Zhang et al., 2017). Including photoautotrophs for WW purification is a promising approach to minimize the problems of traditional WWT plants mentioned above. ...
... • The presence of a catalyst and/or an organic solvent is essential. Tian et al. (2017) gae bio-oil. Karpagam et al. (2021) stated that Zeolite has successfully been applied as an absorbent during the transesterification process; also biodiesel yield was increased. ...
The growing world population, rapid industrialization, and intensive agriculture have increased environmental impacts such as wastewater discharge and global warming. These threats coupled the deficiency of fossil fuel and the rise in crude oil prices globally cause serious social, environmental and economic problems. Microalgae strains can withstand the harsh environments of modern industrial and municipal wastes. The shift toward a circular bio-economy that relies on resource diversification has also prompted the reorganization of traditional wastewater treatment (WWT) processes into a low-carbon, integrated biorefinery model that can accommodate multiple waste streams. Therefore, microalgae-based WWT is now a serious competitor to conventional WWT since the major bottlenecks of nutrient assimilation and high microalgae population have been partially mitigated. This review paper aims to collate advances and new knowledge emerged in recent years for microalgae-based WWT and related biofuel technologies.
... However, is a great advance in the microalgae cosmetic field. In addition, a series of procedures can be applied to the whole microalgae to convert them into biogas or bio-oil, such as hydrothermal treatment, anaerobic digestion, liquefaction, gasification, and pyrolysis [84,[155][156][157]. These approaches will be discussed in depth in the next section. ...
... Hydrothermal liquefaction is a technique that enables the conversion of wet biomass into crude bio-oil, reducing the costs with drying and metabolite extraction [84]. This process combines the extraction and the conversion of the material to oil [156]. Also, it converts not just the lipids but also the carbohydrates and a fraction of the proteins in the biomass [84,158]. ...
... Also, it converts not just the lipids but also the carbohydrates and a fraction of the proteins in the biomass [84,158]. This method has gained prominence on the biofuel pathway, being considered a promising strategy for economic viability [155,156]. Richardson et al. [155] performed financial feasibility analyses of biofuel from microalgae and indicated that hydrothermal liquefaction could provide a 69% higher yield than solvent extraction. The authors also affirm that a combination of hydrothermal liquefaction, catalytic hydrothermal gasification, and electrocoagulation harvesting can reduce the total cost by around 90% when compared with centrifugation and lipid extraction [53,103]. ...
Botryococcus terribilis is a peculiar species that directs a higher portion of the consumed CO2 to metabolite biosynthesis. It results in a low growth rate associated with high oil productivity - compared to other species. In addition to its oils (mainly composed of lipids and hydrocarbons), this microalgae is also a promising source of proteins, carbohydrates, and pigments. The biosynthesis of these products depends on the cultivation conditions. Moreover, the extraction method applied has a direct influence on the quality of the products and their possible applications. Botryococcus terribilis metabolites can be used as fertilizers, biomaterials, pharmaceuticals, cosmetics, food supply, and in a range of biofuels (biodiesel, bioethanol, and biogas). However, biorefinery with this microalgae still needs to overcome economic and technical hurdles to reach a competitive market value. This work analyzed the evolution of the studies with the Botryococcus terribilis, approaching it main metabolites and relating it to the cultivation conditions, extraction methods, and the final products that can be obtained. For the first time, the studies with Botryococcus terribilis were summarized, creating a guide for further research on this microalgae by presenting the recommended conditions for its cultivation and extraction, its possible applications, the information missing in the literature, and the prospects for scale-up.
... Currently, there are three main routes for the production of liquid biofuels from microalgae: (1) biodiesel by extraction or transesterification, (2) bio-oil by pyrolysis, and (3) biocrude oil by hydrothermal liquefaction (HTL) (Tian et al., 2017). In this study, the method used was extraction, followed by transesterification for biodiesel production. ...
Microalgae are one of the most promising feedstocks for biofuel production that can solve the energy crisis, climate change, and the depletion of fossil fuels. Biorefineries have production capacity bottlenecks that prevent them from being economically profitable, without leaving aside the environmental safety of by-products. This research aims to analyze critical stages such as harvesting or lipid extraction from two microalgae species currently unknown, such as Thalassiosira pseudonana and Skeletonema costatum. Inorganic flocculation with a low concentration of iron or aluminum salts (FeCl3 and Al2(SO4)3) was achieved to recover >60% biomass in just 20 min in both cases. Lipids extractions through chloroform: methanol (solvent ratio 2:1) obtained low performance due to the ionic strength medium. The fatty acid composition of the algae extracts showed that stearic acid (C18:0) and palmitoleic acid (C16:1) were predominant in both species. In addition, residues from the lipid extraction process were used for the manufacture of pellets. The data collected showed that these solid biofuels should be combined with other biomass typologies if the end-use are biomass boilers. The development of these studies provides new information on different microalgae species and their potential to use their biomass through an integrated utilization.
... It is clear that without ACA treatment, the algae growth was inhibited since day 2, due to toxic compounds (phenolics, cyclic nitrogenous compounds or heavy metals etc.) that were generated by the HTL process. 47 Nevertheless, the ACA treated AP supported an even better growth than the ones with standard culture, indicating that ACA treatment can remove the undesired compounds effectively and promote algae growth. This is possibly caused by a particularly high content of K and Na after ACA treatment. ...
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 yieldnon-DCM ≈ 0.818 × yieldDCM. 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.
... The chemical compound, type, and proportion of biocrude derived from digestate at a 1:1 ratio is presented in Table 6. The primary chemical compounds like ester and hydrocarbon were detected in 53.5% and 17.7%, respectively, representing the hydrothermal mechanism of esterification and decarboxylation [50]. Other minor compounds such as phenol (0.5%), aromatics (5.3%), and heterocyclic compounds (7.6%) indicated the hydrothermal mechanism of deamination and Maillard reactions [24]. ...
... The majority of studies suggest that the range of time for HTL is between 30 and 60 min. At residence time of 60 min yielded highest oil content [122]. At high temperatures, if the residence time is short, the yield of oil is high; however, at high temperatures and with an extended residence time, the yield of solid residue is high [123]. ...
Algae, a potential biomass feedstock with a faster growth rate and capability of greenhouse gas absorption, mitigates the limitations of the first- and second-generation feedstock in bio-oil production. hydrothermal liquefaction (HTL) is known to be an active method capable of producing substantial energy resources. In HTL, biomass undergoes thermal depolymerization in the presence of water, at around 280 °C–350 °C following subcritical and near supercritical conditions to produce chemical compounds such as alkanes, nitrogenates, esters, phenolics, etc. The primary product, “Biocrude/Bio-oil” obtained from the reaction, is identified as the essential fuel source after processing and also as a distinct value-added chemical source, along with biochar and biogas as co-products. This review outlines a range of routes available for thermochemical conversion of the algal biomass. It also provides a better understanding of the reaction mechanism like depolymerization, decomposition, and re-polymerization, operating conditions like temperature, pressure, the quantity of catalyst required, and the solvent used in the process. The review also highlights the yield achieved by altering the aforementioned parameters, comparing and presenting them as a collective result.
... In HTL, char formation stems from incomplete conversion of biomass at low temperature, and repolymerization of free radicals at high temperature [41]. A large portion of the ash in the feedstock usually partitions into the char phase during HTL [24,31]. ...
Hydrothermal liquefaction (HTL) of wastewater microalgae is constrained by the high ash content in the biomass. High ash content causes slagging, fouling, and catalyst deactivation. In this study, batch experiments for hydrothermal liquefaction (HTL) of filamentous algae were carried out in a 1.8-L autoclave reactor to measure the effects of reaction conditions and demineralization on the yields and quality of the products. Nitric acid was the most efficient demineralization acid, halving ash content (27.1 to 13.5 wt%). HTL of untreated algae showed that, as temperature increased from 310 to 350 • C, total bio-crude oil (light + heavy) yield slightly increased (25.4 to 28.0 wt%, dry ash-free). HTL of demineralized algae gave higher total bio-crude oil yields (27.2 to 43.4 wt%, dry ash-free), greater energy recovery (29.5 to 50.9%), higher char yields (22.1 to 32.5 wt%, dry ash-free), and decreased energy consumption ratios (from 1.0 to 0.6) compared to HTL of untreated algae. Fourier transform infrared spectroscopy (FT-IR) of the feedstocks revealed slightly higher carbohydrate content in the acid-treated biomass compared to the untreated biomass. Both FT-IR and FT ion cyclotron resonance mass spectroscopy of the bio-crude oils showed no significant difference in the functional groups and heteroatoms in the hexane-soluble bio-crude oil derived from untreated and acid-treated biomass. These results demonstrate that demineralization of wastewater algae is relevant for improving the feasibility of algal wastewater-treatment-to-biofuel systems.
... HTL main product is an organic liquid referred to as biocrude having an energy content in the range of 30 to 40 MJ/kg. Biocrude is produced through different reaction pathways such as depolymerisation, decomposition and reformation [6,7]. The resultant biocrude is characterised with undesired high heteroatoms such as oxygen and nitrogen; hence, upgrading is necessary in order to meet standard fuel specifications [8]. ...
This paper is a study on effects of separation procedures on yield and characteristics of biocrude derived from hydrothermal liquefaction (HTL) of Tetraselmis sp. microalgae. The algae was grown and cultivated in outdoor open raceway ponds. The HTL experiments were performed using 1 l custom built high pressure–temperature reactor with inbuilt magnetic stirrer. HTL experimental studies were conducted at reaction temperature of 350 °C and 15 min holding time using alga solids loading of 16 w/v%. HTL product mixture diluted with dichloromethane (ratio 1:1) was allowed to stand for 1 h, 3 h, 6 h, 7 h, 8 h, 9 h, 10 h, 11 h, 12 h and 15 h at room temperature. The result showed that varying stand times for product mixture separation influenced yields in biocrude, solid residue and dissolved aqueous solids. Biocrude yields were in the range of 30 wt% to 56 wt% characterised with higher heating value of ~ 35 to 37 MJ/kg and hydrogen to carbon atomic ratios of 1.56 to 1.95. Maximum yield of biocrude was obtained after 9 h stand time for product mixture and dichloromethane (PM–DCM) mixture. Although, varying PM–DCM mixture stand times showed variation in product yields, there was no clear trend in distribution of elemental contents. Majority of alkali metals distributed in aqueous phase and solid residue, which could be used as nutrients, an alternative to conventional fertiliser.
... Applying Equations (12) and (13), the AOSc was determined to be −0.3255 and the HHV of the energy dense biocrude product was subsequently estimated to be 34 MJ/kg. The calculated HHV of the biocrude product is within the range expected for the typical biocrude product from the HTL processing of biomass, which was reported to be in the range from 30 MJ/kg to 38 MJ/kg [80]. Applying Equations (14)- (20) for different digestate moisture contents (fractional) ranging from 0.999 wt. to 0.5 wt., the NER was determined and is presented in Figure 10. ...
... Applying Equations (12) and (13), the AOS c was determined to be −0.3255 and the HHV of the energy dense biocrude product was subsequently estimated to be 34 MJ/kg. The calculated HHV of the biocrude product is within the range expected for the typical biocrude product from the HTL processing of biomass, which was reported to be in the range from 30 MJ/kg to 38 MJ/kg [80]. Applying Equations (14)- (20) for different digestate moisture contents (fractional) ranging from 0.999 wt. to 0.5 wt., the NER was determined and is presented in Figure 10. Figure 10 shows that the NER negatively correlates with increments in the moisture content of the digestate, with the fractional moisture content of the digestate necessary for environmental sustainability being less than 0.733 wt. ...
In line with global efforts at encouraging paradigm transitions from waste disposal to resource recovery, the anaerobic co-digestion of substrates of wet hydrolyzed meat processing dissolved air flotation sludge and meat processing stock yard waste was investigated in the present study. It was demonstrated that the co-digestion of these substrates leads to the introduction of co-digestion synergizing effects. This study assessed biomethane potentials of the co-digestion of different substrate mixtures, with the preferred substrate mixture composed of stockyard waste and wet hydrolyzed meat processing dissolved air flotation sludge, present in a 4:1 ratio on a volatile solid mass basis. This co-digestion substrate mix ratio presented an experimentally determined cumulative biomethane potential of 264.13 mL/gVSadded (volatile solid). The experimentally determined cumulative biomethane potential was greater than the predicted maximum cumulative biomethane potential of 148.4 mL/gVSadded, anticipated from a similar substrate mixture if synergizing effects were non-existent. The viability of integrating a downstream hydrothermal liquefaction processing of the digestate residue from the co-digestion process, for enhanced resource recovery, was also initially assessed. Assessments were undertaken via the theoretical based estimation of the yields of useful products of biocrude and biochar obtainable from the hydrothermal liquefaction processing of the digestate residue. The environmental sustainability of the proposed integrated system of anaerobic digestion and hydrothermal liquefaction technologies was also initially assessed. The opportunity for secondary resource recovery from the digestate, via the employment of the hydrothermal liquefaction process and the dependence of the environmental sustainability of the integrated system on the moisture content of the digestate, were established. It is anticipated that the results of this study will constitute an invaluable basis for the future large-scale implementation of the proposed integrated system for enhanced value extraction from organic waste streams.
