A review on hydrothermal pre-treatment technologies and environmental profiles of algal biomass processing

Enhancing sustainable waste management: Hydrothermal carbonization of polyethylene terephthalate and polystyrene plastics for energy recovery

Hydrothermal carbonization (HTC) of single plastic polymers such as polyethylene terephthalate (PET) and polystyrene (PS) has not yet been explored on a large scale, particularly their thermal behavior, chemical transformations under subcritical conditions, and the energy properties of the resultant hydrochar. This study investigated these aspects by employing techniques, such as thermogravimetric analysis (TGA), Fourier transformed infrared spectroscopy (FTIR), elemental and calorific analysis. The results show that PET hydrochar has a superior energy densification (1.37) and energy yield (89 %) compared to PS hydrochar (1.13, 54 %). Hydrothermal carbonization modifies the chemical structure of the polymers by increasing the number of carbonyl groups (CO) in PET and forming new ones in PS, and by enhancing hydroxyl groups (OH) in PET while retaining them in PS. Both materials preserve their aromatic and aliphatic structures, with the introduction of alkenes groups (CC) in the PET hydrochar. PET hydrochar begins to decompose at lower temperatures (150-270 °C) than PS hydrochar (242-283 °C) but reaches higher peak temperatures (420-585 °C vs. 390-470 °C), with both types achieving similar burnout temperatures (650-800 °C). PET hydrochar recorded a higher activation energy (121-126 kJ/mol) than PS hydrochar (67-74 kJ/mol) with the Mampel first-order reaction model as the best fit.

Thermodynamic Model for Hydrogen Production from Rice Straw Supercritical Water Gasification

Supercritical water gasification (SCWG) technology is highly promising for its ability to cleanly and efficiently convert biomass to hydrogen. This paper developed a model for the gasification of rice straw in supercritical water (SCW) to predict the direction and limit of the reaction based on the Gibbs free energy minimization principle. The equilibrium distribution of rice straw gasification products was analyzed under a wide range of parameters including temperatures of 400-1200 °C, pressures of 20-50 MPa, and rice straw concentrations of 5-40 wt%. Coke may not be produced due to the excellent properties of supercritical water under thermodynamic constraints. Higher temperatures, lower pressures, and biomass concentrations facilitated the movement of the chemical equilibrium towards hydrogen production. The hydrogen yield was 47.17 mol/kg at a temperature of 650 °C, a pressure of 25 MPa, and a rice straw concentration of 5 wt%. Meanwhile, there is an absorptive process in the rice straw SCWG process for high-calorific value hydrogen production. Energy self-sufficiency of the SCWG process can be maintained by adding small amounts of oxygen (ER < 0.2). This work would be of great value in guiding rice straw SCWG experiments.

Hydrochar-nanoparticle integration for arsenic removal from wastewater: Challenges, possible solutions, and future horizon

Arsenic (As) contamination poses a significant threat to human health, ecosystems, and agriculture, with levels ranging from 12 to 75% attributed to mine waste and stream sediments. This naturally element is abundant in Earth's crust and gets released into the environment through mining and rock processing, causing ≈363 million people to depend on As-contaminated groundwater. To combat this issue, introducing a sustainable hydrochar system has achieved a remarkable removal efficiency of over 92% for arsenic through adsorption. This comprehensive review presents an overview of As contamination in the environment, with a specific focus on its impact on drinking water and wastewater. It delves into the far-reaching effects of As on human health, ecosystems, aquatic systems, and agriculture, while also exploring the effectiveness of existing As treatment systems. Additionally, the study examines the potential of hydrochar as an efficient adsorbent for As removal from water/wastewater, along with other relevant adsorbents and biomass-based preparations of hydrochar. Notably, the fusion of hydrochar with nanoparticle-centric approaches presents a highly promising and environmentally friendly solution for achieving the removal of As from wastewater, exceeding >99% efficiency. This innovative approach holds immense potential for advancing the realms of green chemistry and environmental restoration. Various challenges associated with As contamination and treatment are highlighted, and proposed solutions are discussed. The review emphasizes the urgent need to advance treatment technologies, improve monitoring methods, and enhance regulatory frameworks. Looking outlook, the article underscores the importance of fostering research efforts, raising public awareness, and fostering interdisciplinary collaboration to address this critical environmental issue. Such efforts are vital for UN Sustainable Development Goals, especially clean water and sanitation (Goal 6) and climate action (Goal 13), crucial for global sustainability.

Role of acidic hydrochar on dechlorination of waste PVC in high temperature hydrothermal treatment and fuel properties enhancement of solid residues

In this study, the chlorine mitigation from waste polyvinyl chloride (WPVC) during high temperature co-hydrothermal treatment (co-HTT) and the properties of the generated solid products were assessed. WPVC was co-fed with acidic hydrochar (AHC), which was produced via hydrothermal carbonization of pineapple waste in the presence of citric acid water solution. High temperature co-HTT experiments were performed at 300-350 °C, 0.25-4 h of reaction time, and 0-20 wt% AHC loading. Co-HTT solid products (co-HTT_SP) were characterized via proximate analysis, ultimate analyses, combustion analysis, and ash analysis. The results show that the addition of 5% AHC enhances the dechlorination efficiency (DE) of WPVC from 89.35% to 97.66% at 325 °C and 0.5 h. The highest DE, reaching 99.46%, was achieved at 350 °C and 1 h in the presence of 5 wt% AHC. Furthermore, loading 5% AHC improved the higher heat value (HHV) of the solid products from 23.09 to 31.25 MJ/kg at 325 °C and 0.5 h. The maximum HHV (34.77 MJ/kg) of a solid product was achieved at 350 °C, 4 h, in the presence of 5 wt% of AHC. The co-HTT solids shown low slagging indices, fouling indices, alkali indices, and medium chlorine contents. These findings support the viability of WPVC conversion into clean solid fuel via co-HTT.

Recent advances in lignocellulosic and algal biomass pretreatment and its biorefinery approaches for biochemicals and bioenergy conversion

As the global demand for sustainable energy increases, lignocellulosic (such as agricultural residues, forest biomass, municipal waste, and dedicated energy crops) and algal (including macroalgae and microalgae) biomass have attracted considerable attention, because of their high availability of carbohydrates. This is a potential feedstock to produce biochemical and bioenergy. Pretreatment of biomass can disrupt their complex structure, increasing conversion efficiency and product yield. Therefore, this review comprehensively discusses recent advances in different pretreatments (physical, chemical, physicochemical, and biological pretreatments) for lignocellulosic and algal biomass and their biorefining methods. Life cycle assessment (LCA) which enables the quantification of the environmental impact assessment of a biorefinery also be introduced. Biorefinery processes such as raw material acquisition, extraction, production, waste accumulation, and waste conversion are all monitored under this concept. Nevertheless, there still exist some techno-economic barriers during biorefinery and extensive research is still needed to develop cost-effective processes.

Hydrothermal processing of microalgal biomass: Circular bio-economy perspectives for addressing food-water-energy nexus

Hydrothermal processing of microalgae is regarded as a promising technology to generate multitude of energy based and value-added products. The niche of hydrothermal technologies is still under infancy in terms of the technical discrepancies related to research and development. Thus, the present review critically surveyed the recent advancements linked to the influencing factors governing the algal hydrothermal processing in terms of the product yield and quality. The sustainability of hydrothermal technologies as a standalone method and in broader aspects of circular bio-based economy for energy and value-added platform chemicals are comprehensively discussed. Process optimization and strategic integration of technologies has been suggested to improve efficiency, with reduced energy usage and environmental impacts for addressing the energy-food-water supply chains. Within the wider economic transition and sustainability debate, the knowledge gaps identified and the research hotspots fostering future perspective solutions proposed herewith would facilitate its real-time implementation.

Valorization of winery and distillery by-products by hydrothermal carbonization

This work aims at finding an alternative strategy to manage the waste generated by the winemaking industry to obtain a solid biofuel and phenolic compounds. The effect of temperature (180-260 °C), residence time (1-7 h), and biomass-to-liquid ratio (0.05-0.25) on the co-hydrothermal carbonization of vine pruning and exhausted grape pomace, by using vinasse as moisture source, is studied. The effect of the variables is investigated and optimized using the Box-Behnken design of response surface methodology to maximize mass yield, fuel ratio, energy densification yield and phenols extraction yield and to minimize energy consumption. The statistical analysis shows that the carbonization temperature is a crucial parameter of the process, decreasing the product yield on one hand and improving the quality of hydrochar on the other. At the optimal conditions (246.3 °C, 1.6 h, 0.066), an hydrochar yield of 52.64% and a calorific value of 24.1 MJ/kg were obtained. Moreover, the analysis of the H/C and O/C ratios of hydrochars demonstrates that carbonisation significantly improves the fuel properties of solid biofuel. Liquid by-products obtained from the HTC process are found to contain high concentrations of organic matter but the BOD/COD ratios suggest their potential valorization by biological methods.

Food waste hydrothermal carbonization: Study on the effects of reaction severities, pelletization and framework development using approaches of the circular economy

Food waste (FW) is difficult to manage during thermal treatment. In this study hydrothermal carbonization (HTC) of FW was carried out at increasing temperatures and retention times using the approach of reaction severities (logR0 = 5.31-7.09). The hydrochar sample with the best-obtained energy yield was further pelletized using molasses as a binder at different ratios (5%, 10%, 20% and 30%). A conceptual framework was proposed using the circular economy concept. As severity increases, hydrochar yield declines while its fuel properties improve. Decarboxylation and dehydration allow functional groups to become impaired, including C-O and -OH. Carbon microspheres were observed on the hydrochar surface due to extensive FW carbonization. The pellets with 30% molasses as binder showed the highest mass density (1683.24 kg/m3), while the energy density for it was 37.54 GJ/m³. Food waste management will generate local employment and new business prospects by integrating HTC and pelletization.

Hydrothermal Carbonization (HTC) of Seaweed (Macroalgae) for Producing Hydrochar

Waste seaweed that is collected at coastal regions of maritime provinces in Canada is creating ecological problems as it promotes an anoxic event, which produces nearly zero dissolved oxygen in water along with hydrogen sulfide emission. The work done in this study attempts to address this issue by producing a coal-like solid hydrochar and nutritious liquid slurry (processed water) by employing a rather recent thermo-chemical process called hydrothermal carbonization (HTC) on the seaweed. The HTC was carried out in a batch reactor system for three different reaction temperatures, 180 °C, 200 °C, 220 °C, and three different reaction times, 30, 60, and 120 min. Each of the produced hydrochars was characterized by different analytical methods. The effects of the process conditions on the yield and the properties of the hydrochar and process water were examined. The hydrochar produced at 220 °C and 120 min showed the highest carbon content (48.5%) and heating value (18.93 MJ/kg). The energy density and carbon to nitrogen (C/N) ratio in the hydrochar increased significantly as compared to raw seaweed. Moreover, HTC reduced the ash yield and volatile compounds of the seaweed. Thus, hydrochar can be used as a fuel for direct combustion, in soil remediation, or in carbon sequestration applications.

Reduction of particulate matter and volatile organic compounds in biorefineries: A state-of-the-art review

A biorefinery is an efficient approach to generate multiple bio-products from biomass. With the increasing demand for bioenergy and bio-products, biorefineries are essential industrial platforms that provide needed demand while significantly reducing greenhouse gas emissions. A biorefinery consists of various conversion technologies where particulate matter (PM) and volatile organic compounds (VOCs) are emitted. The released PM and VOCs pose detrimental health and environmental risks for society. Moreover, the projected rise of global bioenergy demand may lead to an increase in PM and VOCs from biorefineries. With the use of cleaner technologies and approaches, PM and VOCs can be avoided in biorefineries. The study presents the landscape of the research field through a bibliometric review of emissions from a biorefinery. A comprehensive review of works on the reduction of PM and VOCs in a biorefinery is outlined. The study includes a perspective of cleaner technologies and approaches utilized in biorefineries to mitigate these hazardous materials. The results reveal that the employment of life cycle assessment, safety assessment, and green chemistry processes can significantly reduce PM and VOC emissions as well as the consumption of hazardous substances in the biorefinery.

Production, properties, and catalytic applications of sludge derived biochar for environmental remediation

Environment-friendly and cost-effective disposal and reutilization of sludge wastes are essential in wastewater treatment processes (WWTPs). Converting activated sludge into biochar via thermochemical treatment is a promising technology for waste management in WWTPs. This review summarizes the compositions of sludge, the dewatering methods, and the thermochemical methods whichinfluence the structures, chemistry, and catalytic performances of the derived biochar. Moreover, the physiochemical characteristics and chemical stability of sludge biochar are discussed. Catalytic applications of biochar are highlighted, including the reaction mechanisms and feasibility for catalytic removal of organic contaminants. High-temperature carbonized sludge biochar exhibits excellent performance for persulfate activation in advanced oxidation processes due to the graphitic carbon structure, newly-created active sites, and fine-tuned metal species. Therefore, the sludge biochar can be produced via cost-effective and eco-friendly approaches to immobilize harmful components from sludge and remediate organic pollution in wastewater, offering a sustainable route toward sludge reutilization into value-added products for water purification.

Hydrothermal Treatment of Vegetable Oils and Fats Aiming at Yielding Hydrocarbons: A Review

According to the International Air Transport Agency (IATA), the aviation industry causes 2% of GHG emissions. As a result, goals such as improving aircraft efficiency by 1.5% per year and achieving carbon-neutral growth by 2020 were established. In this circumstance, fuels produced from biomass seem to be a promising route. There are many routes available to convert biomass into renewable fuels such as pyrolysis, hydroprocessing, transesterification, hydrothermal processes, and steam reforming. In this study, one reports a review of hydrothermal technologies. This review reports recent information about hydrothermal processes using water in sub- and supercritical states. This article introduces some concepts of the hydrothermal processes, advantages, and different types of feedstock adopted. The parameters which have an influence on hydrothermal processes such as temperature, pressure, particle size, catalyst, biomass/water ratio, and reaction time are illuminated. Water characteristics in sub- and supercritical conditions are discussed as a highly reactive medium to increase the affinity for the extraction of value-added compounds. Additionally, this review splits and details the reaction schemes that take place under hydrothermal conditions. Finally, it introduces recent research and development (R&D) trends in the hydrothermal process of fatty acids and triglycerides.

Synergy of Hydrothermal and Organic Acid Washing Treatments in Chinese Fir Wood Vinegar Preparation

Pretreatment is an effective method to change the pyrolysis behavior and improve the product properties of biomass. In this study, the effects of hydrothermal treatment (HTT) and hydrothermal treatment combined with organic acid washing (HTT-A) on Chinese fir waste (CF) pyrolysis and preparation of wood vinegar (WV) were investigated. The results indicated that HTT promoted the decomposition of hemicellulose and disrupted the chemical structure, while HTT-A partly removed the lignin as well as hemicellulose. HTT-A showed a more effective removal efficiency of alkali/alkaline earth metals (AAEMs) than HTT. Both HTT and HTT-A delayed the initial decomposition temperature but promoted the pyrolysis process. The yields of WVs increased after HTT and HTT-A, while the moisture contents reduced, obviously. HTT increased the relative contents of phenols from 47.04 to 59.85% but reduced the relative contents of acids from 24.31 to 18.38%, whereas HHT-A reduced the relative contents of phenols but increased those of aldehydes. In addition, HTT and HTT-A showed the different effects on chemical compositions of WVs, especially for phenolic and acid compounds. This study indicated that HTT and HTT-A were the efficient methods to produce WVs with target chemical components, which would be conducive to the efficient application of WVs.

Microbial aging of hydrochar as a way to increase cadmium ion adsorption capacity: Process and mechanism

Microbially-aged hydrochar were prepared to investigate how aging affected their ability to remove Cd²⁺ from aqueous solutions. Based on aging time in an anaerobic fermenter, four samples were produced: HC, M20-HC, M40-HC, and M60-HC. Results indicated increases in specific surface area, pH, and negative charge on hydrochar surface with aging process. Also, there were a decrease in O/C and an increase in surface functional groups, such as -COOH. The adsorption experiments confirmed the positive correlation between aging time and adsorption performance. The 60-day-aged M60-HC treatment displayed the maximum adsorption capacity, which was 3.8 times higher than that of HC. The Langmuir and pseudo-second-order kinetic equations fitted well with isothermal and kinetic data, respectively. Thermodynamic study indicated that Cd²⁺ adsorption is dominated by chemisorption. This study showed that microbial aging process is an effective and promising measure to improve hydrochar adsorption capacity for Cd²⁺.

A conceptual scenario for the use of microalgae biomass for microgeneration in wastewater treatment plants

Microalgae are a potential source of biomass for the production of energy, which is why the amount of research on this topic has increased in recent years. This work describes the state of the art of microalgae production from wastewater treatment plants (WWTP), its potential to generate electricity and the scale in which it is possible. The methodology used was a systematic review of the gasification of microalgae from 49 articles selected. Based on the review, a conceptual scenario for microgeneration in WWTP using as feedstock microalgae for thermal gasification was developed. The most consistent assumptions for a real scale microgeneration are microalgae production in open ponds using domestic sewage as a nutritional medium; the use of the flocculation process in process of harvesting; microalgae to energy through thermal gasification process using a downdraft gasifier. Considering a WWTP with a 3000 m³/d flux capacity, 860 kg/d of dry microalgae biomass might be produced. For which, gasification has a production potential of 0.167 kWh/m³ of treated sewage, but the energy balance is compromised by the drying process. However, when the biogas produced in anaerobic treatment enter in the model, it is possible to add a surplus of electricity of 0.14 kWh/m³ of treated sewage. Finally, a cost estimate is made for the acquisition of drying and gasification-electricity generation systems. For this scenario, the results suggest that the investments may be financially returned after five years, with additional potential for further optimization.

Biomethane production through anaerobic co-digestion with Maize Cob Waste based on a biorefinery concept: A review

Maize Cob Waste (MCW) is available worldwide in high amounts, as maize is the most produced cereal in the world. MCW is generally left in the crop fields, but due to its low biodegradability it has a negligible impact in soil fertility. Moreover, MCW can be used as substrate to balance the C/N ratio during the Anaerobic co-Digestion (AcoD) with other biodegradable substrates, and is an excellent precursor for the production of Activated Carbons (ACs). In this context, a biorefinery is theoretically discussed in the present review, based on the idea that MCW, after proper pre-treatment is valorised as precursor of ACs and as co-substrate in AcoD for biomethane generation. This paper provides an overview on different scientific and technological aspects that can be involved in the development of the proposed biorefinery; the major topics considered in this work are the following ones: (i) the most suitable pre-treatments of MCW prior to AcoD; (ii) AcoD process with regard to the critical parameters resulting from MCW pre-treatments; (iii) production of ACs using MCW as precursor, with the aim to use these ACs in biogas conditioning (H₂S removal) and upgrading (biomethane production), and (iv) an overview on biogas upgrading technologies.

Environmentally friendly synthesis of photoluminescent biochar dots from waste soy residues for rapid monitoring of potentially toxic elements

Single-step environmentally friendly synthesis of biochar dots (BCDs) was developed using hydrothermal treatment of waste biomass. Using soy residue as the carbon precursor, the resultant BCDs had strong and stable photoluminescence. Characterization by atomic force microscopy (AFM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and infrared spectroscopy indicates that the BCDs prepared were water soluble, spherical, oxygenous and nitrogen-doped carbon nanoparticles with 10-20 nm in diameter. The fluorescence quantum yield of the BCDs was 3.7%. The use of the BCDs as a very effective fluorescent probe for label-free, rapid, and selective detection of Hg2+ and Fe3+ ions was further demonstrated with good linear relationships at 0-50 μM and 10-50 μM, respectively. The minimum detection limits of Hg2+ and Fe3+ were 100 nM and 30 nM. Furthermore, the feasibility of using the BCDs for monitoring of Hg2+ and Fe3+ in open waters was also established.

Comparative Life Cycle Assessment of HTC Concepts Valorizing Sewage Sludge for Energetic and Agricultural Use

In many countries, sewage sludge is directly used for energy and agricultural purposes after dewatering or digestion and dewatering. In recent years, there has been a growing interest in additional upstream hydrothermal carbonization (HTC), which could lead to higher yields in the energetic and agricultural use. Twelve energetic and agricultural valorization concepts of sewage sludge are defined and assessed for Germany to investigate whether the integration of HTC will have a positive effect on the greenhouse gas (GHG) emissions. The study shows that the higher expenses within the HTC process cannot be compensated by additional energy production and agricultural yields. However, the optimization of the HTC process chain through integrated sewage sludge digestion and process water recirculation leads to significant reductions in GHG emissions of the HTC concepts. Subsequently, nearly the same results can be achieved when compared to the direct energetic use of sewage sludge; in the agricultural valorization, the optimized HTC concept would be even the best concept if the direct use of sewage sludge will no longer be permitted in Germany from 2029/2032. Nevertheless, the agricultural valorization concepts are not generally advantageous when compared to the energetic valorization concepts, as it is shown for two concepts.

Optimization of a pretreatment and hydrolysis process for the efficient recovery of recycled sugars and unknown compounds from agricultural sweet sorghum bagasse stem pith solid waste

Background

Sweet sorghum bagasse (SSB), comprising both a dermal layer and pith, is a solid waste generated by agricultural activities. Open burning was previously used to treat agricultural solid waste but is harmful to the environment and human health. Recent reports showed that certain techniques can convert this agricultural waste into valuable products. While SSB has been considered an attractive raw material for sugar extraction and the production of value-added products, the pith root in the SSB can be difficult to process. Therefore, it is necessary to pretreat bagasse before conventional hydrolysis.

Methods

A thorough analysis and comparison of various pretreatment methods were conducted based on physicochemical and microscopic approaches. The responses of agricultural SSB stem pith with different particle sizes to pretreatment temperature, acid and alkali concentration and enzyme dosage were investigated to determine the optimal pretreatment. The integrated methods are beneficial to the utilization of carbohydrate-based and unknown compounds in agricultural solid waste.

Results

Acid (1.5−4.5%, v/v) and alkali (5−8%, w/v) reagents were used to collect cellulose from different meshes of pith at 25–100 °C. The results showed that the use of 100 mesh pith soaked in 8% (w/v) NaOH solution at 100 °C resulted in 32.47% ± 0.01% solid recovery. Follow-up fermentation with 3% (v/v) acid and 6.5% (w/v) alkali at 50 °C for enzymolysis was performed with the optimal enzyme ratio. An analysis of the surface topography and porosity before and after pretreatment showed that both the pore size of the pith and the amount of exposed cellulose increased as the mesh size increased. Interestingly, various compounds, including 42 compounds previously known to be present and 13 compounds not previously known to be present, were detected in the pretreatment liquid, while 10 types of monosaccharides, including D-glucose, D-xylose and D-arabinose, were found in the enzymatic solution. The total monosaccharide content of the pith was 149.48 ± 0.3 mg/g dry matter.

Discussion

An integrated technique for obtaining value-added products from sweet sorghum pith is presented in this work. Based on this technique, lignin and hemicellulose were effectively broken down, amorphous cellulose was obtained and all sugars in the sweet sorghum pith were hydrolysed into monosaccharides. A total of 42 compounds previously found in these materials, including alcohol, ester, acid, alkene, aldehyde ketone, alkene, phenolic and benzene ring compounds, were detected in the pretreatment pith. In addition, several compounds that had not been previously observed in these materials were found in the pretreatment solution. These findings will improve the transformation of lignocellulosic biomass into sugar to create a high-value-added coproduct during the integrated process and to maximize the potential utilization of agricultural waste in current biorefinery processing.

Effect of acidic, neutral and alkaline conditions on product distribution and biocrude oil chemistry from hydrothermal liquefaction of microalgae

Hydrothermal liquefaction (HTL) of microalgae produces high amount of water-insoluble organic compounds, the biocrude oil. Using high-growth-rate Spirulina platensis as feedstock, product fraction distribution and biocrude oil chemistry from HTL at a temperature of 240-300 °C under acidic, neutral and alkaline condition were studied. Positive effects on biocrude oil yield were only found with KOH and acetic acid, and these effects were stronger under milder HTL conditions. FT-ICR MS showed that O₂ class in the biocrude was high due to higher carbohydrate in the biomass, numbers of N₃O_5-6 species present in the sample from acetic acid run, indicating its less decarboxylation ability. GC-MS showed more ketones and amides were formed from fatty acids in catalytic HTL, and this effect was sensitive toward reaction temperature. GPC suggested more light volatiles were in biocrude from KOH run, while analysis from NMR, FT-IR and elemental confirmed its high oil quality.

Emerging techniques for cell disruption and extraction of valuable bio-molecules of microalgae Nannochloropsis sp

Microalgae of Nannochloropsis sp. present valuable source of bio-molecules (pigments, lipids, proteins) that have nutritional potential for the prevention and treatment of human diseases. Moreover, some species of Nannochloropsis are the promising sources of biofuels and excellent candidates for the replacement of classical biofuel crops. This review describes and compares the efficiency of different conventional and novel techniques that can be used for cell disruption and recovery of bio-molecules from Nannochloropsis sp. Classification of different extraction techniques includes chemical, enzymatic, mechanical and other physical methods. The detailed analysis of extraction efficiency assisted by pressure and temperature (subcritical and supercritical fluids, hydrothermal liquefaction), ultrasound, microwaves, and pulsed electric energy (pulsed electric fields and high voltage electrical discharges) is presented. The general discussion includes comparison between techniques, their effectiveness for cell disruption and selectivity of bio-molecules extraction from Nannochloropsis sp. The cost-effectiveness, benefits and limitations of different techniques are also analyzed.

Highly abrasion resistant thermally fused olivine as in-situ catalysts for tar reduction in a circulating fluidized bed biomass gasifier

Olivine synthesized by wetness impregnation (WI) and thermal fusion (TF) methods were investigated as in-situ catalysts to reduce the tar content during air-blown biomass gasification in a circulating fluidized bed (CFB). The results showed that the tar content decreases with the increase of reaction temperature. Raw-olivine reduced tar content by 40.6% compared to non-active bed material (silica sand) experiments; after calcination, the catalytic activities of olivine catalysts were further improved due to the migration of Fe from olivine grain to the surface. 1100-WI-olivine could reduce the tar content by up to 81.5% compared with that of raw-olivine because of the existence of Fe₂O₃, NiO and NiO-MgO. For 1400-TF-olivine, due to the formation of NiFe₂O₄, the tar content decreased to 0.77 g/Nm³, an 82.9% reduction compared to raw-olivine. Moreover, the TF-olivine had a stronger anti-attrition performance and was more suitable for using in a circulating fluidized bed.

Behaviors of rice straw two-step liquefaction with sub/supercritical ethanol in carbon dioxide atmosphere

This study extended previous work investigating two-step liquefaction by supercritical ethanol of rice straw under CO₂ atmosphere at temperatures of 270-345 °C. Subcritical CO₂-subcritical ethanol (SubCO₂-SubEtOH) pretreatment decreased the content of lignin in the rice stalk from 22.94 to 21.43 wt%. The results showed that although oxygen-transfer reaction, transesterification, carbonylation, and other reactions may occur with the supercritical CO₂-supercritical ethanol (ScCO₂-ScEtOH) liquefaction reactions, transesterification was the main reaction. The "de-oxygen-transfer" reaction mainly comprised de-oxygenation and decarboxylation. For temperatures exceeding 320 °C, the bio-oil yield decreased because the effects of esters decreased. The residence time affected the H/C and O/C ratios to a minor extent. It was shown that the nucleophilic and hydrolytic functions of ethanol might be strengthened, generating higher amounts of ester, phenolic, acidic, and hydrocarbon derivatives in the bio-oil fraction.

The effects of temperature and color value on hydrochars' properties in hydrothermal carbonization

In order to investigate the influence of hydrothermal carbonization (HTC) on the properties of the hydrochars, sawdust with a particle size below 0.45mm was treated in an autoclave at 200-260°C. The physical and chemical characteristics of products were studied, including proximate analysis, elemental composition, fiber content, surface area, bulk density, energy yield, color value, combustion activities and pyrolysis kinetics, etc. It showed that the color of hydrochars turned blacker, greener, and bluer after HTC. The ash, carbon, hydrogen and lignin contents showed a good correlation (R²>0.96) with color coordinates. The decrement in stage 1 and increment in stage 2 of temperature intervals were attributed to the volatile matters removal and fixed carbon accumulation, improving the stability and safety of hydrochars combustion. As shown by the Kissenger-Akahira-Sunose (KAS) and Coats-Redfern calculations, the HTC process can also make the pyrolysis more stable.

Characteristics of co-hydrothermal carbonization on polyvinyl chloride wastes with bamboo

The PVC waste and bamboo were treated by co-hydrothermal carbonization (co-HTC) at three different temperatures. The inorganic-Cl could be removed from the carbon rich solid products (hydrochar) in the form of HCl via hydrolysis, elimination, substitution and aromatization. Due to the high carbon content, the hydrochar could be applied as premium fuel. Bamboo had a synergistic effect on dechlorination with PVC in the HTC process. The bamboo could accelerate the HTC dechlorination of PVC at 200°C because it strengthened the substitution of Cl with OH. While at 230 and 260°C, the existence of bamboo hindered the dechlorination of PVC in HTC. Thermogravimetric analysis showed the combustion performance of hydrochar was better than the raw samples at 200°C. Owing to the low chlorine content, low ignition temperature and the superior combustion performance, the M-260 can be adopted as alternative fuels for coal.

Effects of hydrothermal treatment on the pyrolysis behavior of Chinese fan palm

The effect of hydrothermal treatment (HTT) on Chinese fan palm pyrolysis was investigated. It indicated that HTT could effectively remove a large portion of alkali/alkaline earth metals and disrupt the chemical structure to a certain extent. HTT delayed the initial decomposition temperature but accelerated the pyrolysis process completely. HTT also increased the relative contents of both sugars and hydrocarbons in pyrolysis. At 210°C, HTT had the most significant promotion effect on the sugars formation with the relative content of 30.58%. While, The relative content of phenols, acids, furans, aldehydes, esters and CO₂ decreased more or less after HTT. With increasing pyrolysis temperature, the relative content of most groups of chemicals except hydrocarbons decreased. Response contours were analyzed to find the optimal reaction conditions for generating acids, phenols, sugars and hydrocarbons, respectively. The results indicated both pyrolysis temperature and HTT temperature had distinct influence on relative contents of products.

Hydrothermal carbonization of typical components of municipal solid waste for deriving hydrochars and their combustion behavior

In this work, five typical components were employed as representative pseudo-components to indirectly complete previous established simulation system during hydrothermal carbonization (HTC) of municipal solid waste. The fuel characteristics and combustion behavior of HTC-derived hydrochars were evaluated. Results clearly illustrated that the energy ranks of hydrochars were upgraded after HTC. For paper and wood, superior combustion performances of their hydrochars could achieve under suitable conditions. While for food, none positive enrichments on combustion loss rate were observed for hydrochars due to its high solubilization and decomposition under hot compressed water. It was noteworthy that a new weight loss peak was detected for paper and food, suggesting that new compounds were formed. For rubber, the HTC process made the properties of styrene butadiene rubber more close to natural rubber. Therefore, the first peak of hydrochars became significantly intense. While for plastic, only physical changes of polypropylene and polyethylene were observed.

Comparative cradle-to-grave life cycle assessment of biogas production from marine algae and cattle manure biorefineries

The environmental impacts resulting from the cradle-to-grave life cycles of Enteromorpha prolifera macroalgae and cattle manure biorefineries are assessed and compared. Sensitivity analysis is carried out to evaluate the response of the impacts to changes in biogas application by using Simapro 7.3.3. Three scenarios are considered in the biorefineries. In the first and second scenarios, the biogas produced is considered to be used for electricity production and transportation, respectively. In the third scenario, the biogas is considered to be recycled back to the systems. Process energy requirements and transportation of inputs contribute the largest share of the overall impacts. The cattle manure biorefinery is slightly more eco-friendly than the macroalgae biorefinery in Scenarios 1 and 2 because it requires more eco-friendly inputs. However, the macroalgae biorefinery becomes more eco-friendly than the cattle manure biorefinery in Scenario 3 because macroalgae require less energy and water for biogas production.

Rapid biodiesel synthesis from waste pepper seeds without lipid isolation step

In situ transformation of lipid in waste pepper seeds into biodiesel (i.e., fatty acid methyl esters: FAMEs) via thermally-induced transmethylation on silica was mainly investigated in this study. This study reported that waste pepper seeds contained 26.9wt% of lipid and that 94.1% of the total lipid in waste pepper seeds could be converted into biodiesel without lipid extraction step for only ∼1min reaction time. This study also suggested that the optimal temperature for in situ transmethylation was identified as 390°C. Moreover, comparison of in situ process via the conventional transmethylation catalyzed by H₂SO₄ showed that the introduced biodiesel conversion in this study had a higher tolerance against impurities, thereby being technically feasible. The in situ biodiesel production from other oil-bearing food wastes can be studied.

Effect of residence time on two-step liquefaction of rice straw in a CO2 atmosphere: Differences between subcritical water and supercritical ethanol

This study investigated the influence of temperature and residence time on liquefaction of rice straw in subcritical CO₂-subcritical water (subCO₂-subH₂O) and in subcritical CO₂-supercritical ethanol (subCO₂-scEtOH), considering the final reaction temperatures (270-345°C) and residence times (15 and 30min). Residence time was identified as a crucial parameter in the subCO₂-subH₂O liquefaction, whereas residence time had a marginal influence on subCO₂-scEtOH liquefaction. When reaction conditions were 320°C and 15min, solvents have weak impact on the quality of bio-oil, HHV 28.57MJ/kg and 28.62MJ/kg, respectively. There was an obvious difference between the subCO₂-subH₂O and subCO₂-scEtOH liquefaction mechanisms. In subCO₂-subH₂O, CO₂ promoted the carbonyl reaction. In subCO₂-scEtOH, supercritical ethanol have the function of donating hydrogen and promoting the reaction of hydroxyl-alkylation.

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

Hydrothermal Liquefaction (HTL) for algal biomass conversion is a promising technology capable of producing high yields of biocrude as well as partitioning even higher quantity of nutrients in the aqueous phase. To assess the feasibility of utilizing the aqueous phase, HTL of Nannochloropsis sp. was carried out in the temperature range of 275 to 350°C and Residence Times (RT) ranging between 5 and 60min The effect of reaction conditions on the NO3(-),PO4(3-),SO4(2-),Cl(-),Na(+),andK(+) ions as well as Chemical Oxygen Demand (COD) and pH was investigated with view of recycling the aqueous phase for either cultivation or energy generation via Anaerobic Digestion (AD), quantified via Lifecycle Assessment (LCA). It addition to substantial nutrient partitioning at short RT, an increase in alkalinity to almost pH10 and decrease in COD at longer RT was observed. The LCA investigation found reaction conditions of 275°C/30min and 350°C/10min to be most suitable for nutrient and energy recovery but both processing routes offer environmental benefit at all reaction conditions, however recycling for cultivation has marginally better environmental credentials compared to AD.

Scalable preparation of monodisperse micron-sized carbon microspheres and their application in anion-exchange chromatography

Polyacrylic acid sodium (PAAS), which is an anionic water-soluble polymer, is widely utilized as a thickener in the food industry and flocculant in water treatment owing to its high anion density, low cost and nontoxicity.

Hydrothermal liquefaction and in situ supercritical transesterification of algae paste

This manuscript presents the outcome of simultaneous hydrothermal liquefaction (HTL) and in situ supercritical transesterification (SCT) of algal biomass at reaction temperature of 300–380 °C and reaction times (RT) of 5–30 min.