Hydrothermal upgrading of algae paste: Inorganics and recycling potential in the aqueous phase

Influence of alkali catalysts on product yield and Si-containing products from hydrothermal liquefaction of corn stover

This study investigated the effects of different alkali catalysts (K2CO3, KOH, NaOH, and Na2CO3) on the yield and composition of biocrude oil and aqueous products obtained from hydrothermal liquefaction (HTL) of corn stover. HTL was performed in a laboratory-scale tubular reactor at 320 °C for 90 min and catalyst loading of 5.0 and 7.5 % (by weight of biomass). The composition of the biocrude oil and aqueous products was determined using GC-MS. Results revealed that hydroxide catalysts are more effective than carbonate catalysts in increasing biocrude oil yield. Notably, NaOH achieved a high conversion rate of 92-94 % daf (dry and ash-free basis), significantly surpassing the uncatalyzed HTL (69.4 % daf). The highest biocrude oil yield of 22.12-22.57 % daf was obtained using KOH. Si-containing compounds (e.g., silanes and siloxanes) were identified as the most abundant components in the biocrude oil, suggesting potential for further exploration in producing platform chemicals from these compounds.

Current challenges of hydrothermal treated wastewater (HTWW) for environmental applications and their perspectives: A review

Hydrothermal treatment (HT) is an emerged thermochemical approach for the utilization of biomass. In the last decade, intense research has been conducted on bio-oil and hydrochar, during which extensive amount of hydrothermal treated wastewater (HTWW) is produced, containing large amount of organic compounds along with several toxic chemicals. The composition of HTWW is highly dependent on the process conditions and organic composition of biomass, which determines its further utilization. The current study provides a comprehensive overview of recent advancements in HTWW utilization and its properties which can be changed by varying different parameters like temperature, residence time, solid concentration, mass ratio and catalyst including types of biomasses. HTWW characterization, parameters, reaction mechanism and its application were also summarized. By considering the challenges of HTWW, some suggestions and proposed methodology to overcome the bottleneck are provided.

Biomass hydrothermal conversion under CO2 atmosphere: A way to improve the regulation of hydrothermal products

In this study, batched hydrothermal experiments on corn stalk were conducted at 240-330 °C under CO₂ or inert (N₂) atmosphere. The distribution and characteristics of gaseous, solid, and liquid products were analyzed in detail to comprehensively investigate the effects of CO₂ on the hydrothermal conversion of biomass, especially on the cellulose and lignin in biomass. The results demonstrate that compared with N₂, CO₂ slightly increased the liquid and gas yields and significantly improved the control effect of temperature on bio-oil components. Under CO₂ atmosphere, bio-oil achieved effective enrichment of ketones and phenols at 240 °C and 300 °C, respectively, and their highest relative contents reached 44.8% and 62.0%, respectively. In addition, the hydrochar obtained under CO₂ atmosphere showed higher crystallinity, which is conducive to its subsequent utilization. This study explored the feasibility of introducing CO₂ into the biomass hydrothermal process to realize the high-value utilization of biomass waste and the reuse of CO₂.

The Role of Catalysts in Biomass Hydrothermal Liquefaction and Biocrude Upgrading

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.

Biomass pre-treatments of the N2-fixing cyanobacterium Tolypothrix for co-production of methane

Tolypothrix, a self-flocculating, fast growing, CO₂ and nitrogen-fixing cyanobacterium, can be cultivated in nutrient-poor ash dam waters of coal-fired power stations, converting CO₂ emissions into organic biomass. Therefore, the biomass of Tolypothrix sp. is a promising source for bio-fertiliser production, providing micro- and macronutrients. Energy requirements for production could potentially be offset via anaerobic digestion (AD) of the produced biomass, which may further improve the efficiency of the resulting biofertilizer. The aim of this study was to evaluate the effectiveness of pre-treatment conditions and subsequent methane (CH₄) production of Tolypothrix under out-door cultivation conditions. Pre-treatments on biogas and methane production for Tolypothrix sp. biomass investigated were: (1) thermal at 95 °C for 10 h, (2) hydrothermal by autoclave at 121 °C at 1013.25 hPa for 20 min, using a standard moisture-heat procedure, (3) microwave at an output power of 900 W and an exposure time of 3 min, (4) sonication at an output power of 10 W for 3.5 h at 10 min intervals with 20 s breaks and (5) freeze-thaw cycles at -80 °C for 24 h followed by thawing at room temperature. Thermal, hydrothermal and sonication pre-treatments supported high solubilization of organic compounds up to 24.40 g L^-1. However, higher specific CH₄ production of 0.012 and 0.01 L CH₄ g^-1 volatile solids_added. was achieved for thermal and sonic pre-treatments, respectively. High N- and low C-content of the Tolypothrix biomass affected CH₄ recovery, while pre-treatment accelerated production of volatile acids (15.90 g L^-1) and ammonia-N-accumulation (1.41 g L^-1), leading to poor CH₄ yields. Calculated theoretical CH₄ yields based on the elemental composition of the biomass were ~55% higher than actual yields. This highlights the complexity of interactions during AD which are not adequately represented by elemental composition.

Effect of algae (Scenedesmus obliquus) biomass pre-treatment on bio-oil production in hydrothermal liquefaction (HTL): Biochar and aqueous phase utilization studies

Environmental concerns due to fossil fuel usage has turned the research interest towards biomass and bioenergy field. Renewable biomass such as microalgae provides numerous advantages as they can grow in wastewater; sequester carbon dioxide, economical and eco-friendly. In this study, effect of pretreatment of microalgae (Scenedesmus obliquus) biomass using post-hydrothermal liquefaction wastewater (PHWW) for bio-oil production through hydrothermal liquefaction at a temperature of 300 °C was studied. Results showed liquefaction of pre-treated biomass yielded 48.53% bio-oil whereas 28.35% was resulted from biomass without pretreatment. The analysis of higher heating value of bio-oil showed that pretreated biomass oil has 36.19 MJ.Kg^-1 against non-pretreated biomass oil, which has 28.88 MJ.Kg^-1. Bio-oil (pretreated biomass) analysis revealed that 60% of compounds are in diesel and gasoline range with 58.09% of energy recovery. Bio-oil was rich in hydrocarbons of C₇-C₂₁ range with less oxygenated compounds. Carbon balance showed that an increase of 13% of carbon was sequestered in solid residue obtained from pretreated biomass and about 146% of increase also obtained in bio-oil.

Beneficial migration of sulfur element during scrap tire depolymerization with supercritical water: A molecular dynamics and DFT study

Complete depolymerization of scrap tires (ST) to valuable oil products and fuel gas could be achieved by supercritical water (SCW) technology. For implementing this promising technology, migration mechanism of sulfur element during the entire ST-SCW depolymerization process was identified to reduce the sulfur pollutions. In the depolymerization process of ST, OH radicals released from SCW molecules could enhance cleavage of CS bonds, resulting sulfur-containing intermediates. The intermediates could be further oxidized by free OH radicals and transformed into inorganic sulfur molecules mainly consisting of SO₄^2-, S₂O₃^2-, SO₃^2- and S^2-. In this study, a combined ReaxFF-MD and DFT method was performed to study the detailed sulfur migration mechanism during ST depolymerization in the presence of SCW and provided a strategy to fix low-valent sulfur in aqueous solution for separation of sulfur from the oil & gas products. This work provides a guidance to make ST-SCW technology cleaner and cheaper.

A review of biochemical and thermochemical energy conversion routes of wastewater grown algal biomass

Microalgae are recognized as a potential source of biomass for obtaining bioenergy. However, the lack of studies towards economic viability and environmental sustainability of the entire production chain limits its large-scale application. The use of wastewaters economizes natural resources used for algal biomass cultivation. However, desirable biomass characteristics for a good fuel may be impaired when wastewaters are used, namely low lipid content and high ash and protein contents. Thus, the choice of wastewaters with more favorable characteristics may be one way of obtaining a more balanced macromolecular composition of the algal biomass and therefore, a more suitable feedstock for the desired energetic route. The exploration of biorefinery concept and the use of wastewaters as culture medium are considered as the main strategic tools in the search of this viability. Considering the economics of overall process, direct utilization of wet biomass using hydrothermal liquefaction or hydrothermal carbonization and anaerobic digestion is recommended. Among the explored routes, anaerobic digestion is the most studied process. However, some main challenges remain as little explored, such as a low energy pretreatment and suitable and large-scale reactors for algal biomass digestion. On the other hand, thermochemical conversion routes offer better valorization of the algal biomass but have higher costs. A biorefinery combining anaerobic digestion, hydrothermal carbonization and hydrothermal liquefaction processes would provide the maximum possible output from the biomass depending on its characteristics. Therefore, the choice must be made in an integrated way, aiming at optimizing the quality of the final product to be obtained. Life cycle assessment studies are critical for scaling up of any algal biomass valorization technique for sustainability. Although there are limitations, suitable integrations of these processes would enable to make an economically feasible process which require further study.

Mild Hydrothermal Pretreatment of Microalgae for the Production of Biocrude with a Low N and O Content

A hydrothermal pretreatment of the microalga Nannochloropsis gaditana at mild temperatures was studied in order to reduce the N and O content in the biocrude obtained by hydrothermal liquefaction (HTL). The work focused on the evaluation of temperature, reactor loading, and time (factors) to maximize the yield of the pretreated biomass and the heteroatom contents transferred from the microalga biomass to the aqueous phase (responses). The study followed the factorial design and response surface methodology. An equation for every response was obtained, which led to the accurate calculation of the operating conditions required to obtain a given value of these responses. Temperature and time are critical factors with a negative effect on the pretreated biomass yield but a positive one on the N and O recovery in the aqueous phase. The slurry concentration has to be low to increase heteroatom recovery and has to be high to maximize the pretreated microalga yields. Response equations were obtained for the analyzed responses, which facilitated the accurate prediction of the operating conditions required to obtain a given value of these responses.

Wastewater To Resource: Design of a Sustainable Phosphorus Recovery System

To enable a more sustainable wastewater treatment processes, a transition towards resource recovery methods that have minimal environmental impact while being financially viable is imperative. Phosphorus (P) is a finite resource that is being discharged into the aqueous environment in excessive quantities. As such, understanding the financial and environmental effectiveness of different approaches for removing and recovering P from wastewater streams is important to reduce the overall impact of wastewater treatment. In this study, a process‐systems modelling framework for comprehensively evaluating these approaches in terms of both economic and environmental impacts is developed. Applying this framework, treatment pathways are designed, simulated and analysed to determine the most suitable approaches for P removal and recovery. The purpose of this methodology is not only to assist with plant design, but also to identify the principal economic and environmental factors acting as barriers to implementing a given technology, incorporating the impact of waste recovery. The results suggest that the chemical and ion‐exchange approaches studied deliver sustainable advantages over biological pathways, both economically and environmentally, with each possessing different strengths. The assessment methodology developed enables a more rational and environmentally sound wastewater plant design approach to be taken.

Improved methane production and energy recovery of post-hydrothermal liquefaction waste water via integration of zeolite adsorption and anaerobic digestion

Hydrothermal liquefaction (HTL) is a promising technology for converting organic wastes into bio-crude oil, with organic-rich post-hydrothermal liquefaction wastewater (PHWW) as by-product. In this study, zeolite adsorption and anaerobic digestion (AD) were integrated to improve the methane production and energy recovery of PHWW from Chlorella 1067. A statistical design for maximum toxicants removal by zeolite was applied before AD process. Zeolite could mitigate the inhibition associated to compounds such as ammonia, N-heterocyclic compounds, etc. in PHWW and thereby shortening the lag phase and increasing methane production by 32-117% compared with that without zeolite adsorption. Zeolite adsorption also increased energy recovery efficiency (up to 70.5%) for this integrated system. Integration of HTL and AD brought higher energetic return from feedstock via oil and biomethane production, which may offer insight into industrial application of microalgae biomass in the circular economy. In addition, carbon and nitrogen flow for the integrated process was determined.

Biogas liquid digestate grown Chlorella sp. for biocrude oil production via hydrothermal liquefaction

Microalgae can not only purify and recover the nutrients from wastewater, but also be harvested as wet biomass for the production of biocrude oil via hydrothermal liquefaction (HTL). Chlorella sp. cultivated in the ultrafiltration (UF) membrane treated anaerobic digestion (AD) liquid digestate of chicken manure was used as the feedstock in this study. The present study characterized the products and investigated the elemental migration during HTL of Chlorella sp. fed with AD effluent wastewater (WW) and BG11 standard medium (ST) in 100mL and 500mL reactors under different operational conditions. Results showed that the highest oil yield of WW (38.1%, daf) was achieved at 320°C, 60min and 15% TS in 500mL reactor, which was 14.1% higher than that of ST (33.4%, daf) at 320°C, 30min and 20% TS in the same reactor. WW had a similar carbon and hydrogen distribution in the four product fractions under HTL conditions compared with ST. 43.4% and 32.4% of carbon in WW11 and ST11 were released into the biocrude and aqueous phase in 500mL reactor, respectively. As much as 64.5% of the hydrogen was transferred to the aqueous phase. GC-MS results showed that the chemical compounds in the biocrude oil from WW consist of a variety of chemical constituents, such as hydrocarbons, acids, alcohols, ketones, phenols and aldehydes. These two biocrude oils contained 17.5% wt. and 8.64% wt. hydrocarbons, and 63.7% wt. and 79.8% wt. oxygen-containing compounds, respectively. TGA results showed that 69.3%-66.7% of the biocrude oil was gasified in 30°C-400°C. This study demonstrates the great potential for biocrude oil production from microalgae grown in biogas effluent via HTL.

Optimizing the conditions for hydrothermal liquefaction of barley straw for bio-crude oil production using response surface methodology

The present paper examines the conversion of barley straw to bio-crude oil (BO) via hydrothermal liquefaction. Response surface methodology based on central composite design was utilized to optimize the conditions of four independent variables including reaction temperature (factor X₁, 260-340°C), reaction time (factor X₂, 5-25min), catalyst dosage (factor X₃, 2-18%) and biomass/water ratio (factor X₄, 9-21%) for BO yield. It was found that reaction temperature, catalyst dosage and biomass/water ratio had more remarkable influence than reaction time on BO yield by analysis of variance. The predicted BO yield by the second order polynomial model was in good agreement with experimental results. A maximum BO yield of 38.72wt% was obtained at 304.8°C, 15.5min, 11.7% potassium carbonate as catalyst and 18% biomass (based on water). GC/MS analysis revealed that the major BO components were phenols and their derivatives, acids, aromatic hydrocarbon, ketones, N-contained compounds and alcohols, which makes it a promising material in the applications of either bio-fuel or as a phenol substitute in bio-phenolic resins.

Supercritical water gasification of microalgae over a two-component catalyst mixture

Supercritical water gasification (SCWG) of the microalga Chlorella pyrenoidosa was examined with a catalyst mixture of Ru/C and Rh/C in a mass ratio of 1:1. The influences of temperature (380-600°C), water density (0-0.197g/cm³), and catalyst loading (0-20wt%) on the yields and composition of the gaseous products and the gasification efficiency were examined. The temperature and water density significantly affected the SCWG of the microalgae. The hydrogen gasification efficiency was more dependent on the temperature, while the carbon gasification efficiency was more dependent on the water density. The gaseous products mainly consisted of CH₄, H₂, CO, and CO₂, with smaller amounts of C₂-C₃ hydrocarbons. CH₄ made up half of the mole fraction of the gaseous products under most reaction conditions. A synergistic effect between Ru/C and Rh/C existed during the SCWG of the microalgae, and this effect favored the production of CH₄. The role of the catalyst mixture became indistinct at higher temperatures. Hydrogen atoms from the water were transferred to the gaseous products during the SCWG, leading to hydrogen gasification efficiencies that exceeded 100%. The main components of the bio-oil were aromatics and nitrogen-containing compounds, and the main aromatics consisted of azulene and anthracene. The nitrogen-containing compounds are potential poisons to the catalyst mixture.

Use of microalgae to recycle nutrients in aqueous phase derived from hydrothermal liquefaction process

Hydrothermal liquefaction (HTL) of microalgae biomass generates an aqueous phase (AP) byproduct with limited energy value. Recycling the AP solution as a source of nutrients for microalgae cultivation provides an opportunity for a cost-effective production of HTL based biofuel and algal biomass feedstock for HTL, allowing a closed-loop biofuel production in microalgae HTL biofuel system. This paper aims to provide a comprehensive overview of characteristics of AP and its nutrients recycling for algae production. Inhibitory effects resulted from the toxic compounds in AP and alleviation strategies are discussed.