Hydrothermal liquefaction of municipal solid wastes for high quality bio-crude production using glycerol as co-solvent

Hydrothermal dechlorination strategy for high-quality oil recovery from polyvinyl chloride

The global production of PVC is around 3.5 million tons each year. Unfortunately, the disposal of PVC waste releases toxic substances such as hydrochloric acid, polychlorinated dioxins, and furans, which can harm the environment. Therefore, there is an urgent need for a safe and environmentally friendly thermochemical treatment method that reduces the damage caused by HCl gas produced during PVC pyrolysis and improves the quality of pyrolysis oil. Hydrothermal treatment technology is one of the potential dechlorination strategies for PVC. However, its efficiency is reduced in the supercritical region, while the additives used result in secondary pollution and increased operating costs. This study is pioneering in its approach, aiming to produce high-quality oil with reduced chlorine through low-temperature hydrothermal treatment of PVC, all without additives. The results are promising, indicating that by administering steam at 250 °C with a 2.0-3.0 g-steam/g-feed ratio, we can significantly reduce chlorine content to 0.13 % while achieving an oil yield of up to 14.9 % from PVC. The hydrothermal process can reduce CO2 emissions by 15-43 % compared to pyrolysis methods, providing a simultaneous opportunity for carbon neutrality and resource recovery.

Hydrothermal liquefaction of composite household waste to biocrude: the effect of liquefaction solvents on product yield and quality

The hydrothermal liquefaction (HTL) of composite household waste (CHW) was investigated at different temperatures in the range of 240-360 °C, residence times in the range of 30-90 min, and co-solvent ratios of 2-8 ml/g, by utilising ethanol, glycerol, and produced aqueous phase as liquefaction solvents. Maximum biocrude yield of 46.19% was obtained at 340 °C and 75 min, with aqueous phase recirculation ratio (RR) of 5 ml/g. The chemical solvents such as glycerol and ethanol yielded a biocrude percentage of 45.18% and 42.16% at a ratio of 6 ml/g and 8 ml/g, respectively, for 340 °C and 75 min. The usage of co-solvents as hydrothermal medium increased the biocrude yield by 35.30% and decreased the formation of solid residue and gaseous products by 19.82% and 18.74% respectively. Also, the solid residue and biocrude obtained from co-solvent HTL possessed higher carbon and hydrogen content, thus having a H/C ratio and HHV that is 1.01 and 1.23 times higher than that of water as hydrothermal medium. Among the co-solvents, HTL with aqueous phase recirculation resulted in higher carbon and energy recovery percentages of 9.36% and 9.78% for solid residue and 52.09% and 56.75% for biocrude respectively. Further qualitatively, co-solvent HTL in the presence of obtained aqueous phase yielded 33.43% higher fraction of hydrocarbons than the pure water HTL and 7.70-17.01% higher hydrocarbons when compared with ethanol and glycerol HTL respectively. Nitrogen containing compounds, such as phenols and furfurals, for biocrudes obtained from all HTL processes, were found to be present in the range of 8.30-14.40%.

Microwave-Assisted Pyrolysis-A New Way for the Sustainable Recycling and Upgrading of Plastic and Biomass: A Review

The efficient utilization of organic solid waste resources can help reducing the consumption of conventional fossil fuels, mitigating environmental pollution, and achieving green sustainable development. Due to its dual nature of being both a resource and a source of pollution, it is crucial to implement suitable recycling technologies throughout the recycling and upgrading processes for plastics and biomass, which are organic solid wastes with complex mixture of components. The conventional pyrolysis and hydropyrolysis were summarized for recycling plastics and biomass into high-value fuels, chemicals, and materials. To enhance reaction efficiency and improve product selectivity, microwave-assisted pyrolysis was introduced to the upgrading of plastics and biomass through efficient energy supply especially with the aid of catalysts and microwave absorbers. This review provides a detail summary of microwave-assisted pyrolysis for plastics and biomass from the technical, applied, and mechanistic perspectives. Based on the recent technological advances, the future directions for the development of microwave-assisted pyrolysis technologies are predicted.

Plastic waste management through liquefaction in hydrogen donating solvents: A review

Plastic pollution poses a significant environmental threat, particularly to marine ecosystems, as conventional plastics persist without degradation, accumulating plastic waste in landfills and natural environments. A promising alternative to address this issue involves the use of hydrogen donor solvents in plastic liquefaction, offering a dual benefit of waste reduction and the generation of valuable liquid products with diverse industrial applications. This review delves into plastic recycling methods with a specific focus on liquefaction using hydrogen donating solvents as an innovative approach to waste management. Liquefaction, conducted at moderate to high temperatures (280-450 °C) and pressures (7-30 MPa), yields high oil conversion using various solvents. This study examined the performance of hydrogen-donating solvents, including water, alcohols, decalin, and cyclohexane, in enhancing the oil yield while minimising the oxygen content. Supercritical water, recognised for its effective plastic degradation and chemical production capabilities, and alcohols, with their alkylating and hydrogen-donating properties, have emerged as key solvents in plastic liquefaction. The use of hydrogen donor solvents stabilizes the free radicals, enhancing the conversion of plastic waste into valuable products. In addition, this review addresses the economic efficiency of the liquefaction process.

Production of biodiesel from waste fish fat through ultrasound-assisted transesterification using petro-diesel as cosolvent and optimization of process parameters using response surface methodology

Biodiesel is a highly promising and viable alternative to fossil-based diesel that also addresses the urgent need for effective waste management. It can be synthesized by the chemical modification of triglycerides sourced from vegetable origin, animal fat, or algal oil. The transesterification reaction is the preferred method of producing biodiesel. However, the non-miscibility of alcohol and oil layer causes excessive utilization of alcohol, catalyst, and a substantial reacting time and temperature. In the current investigation, transesterification of waste fish oil was performed with petro-diesel as cosolvent, under the influence of ultrasound energy. The combination of both techniques is a unique and efficient way to minimize the mass transfer limitations considerably and hence reduces the parameters of the reaction. It is also a sincere effort to comply with the principles of green chemistry. The optimum reaction conditions were obtained using response surface methodology (RSM) that were as follows: molar ratio of methanol to oil 9.09:1, catalyst concentration of 0.97 wt%, cosolvent concentration of 29.1 wt%, temperature 60.1℃, and a reacting time 30 min. Under these listed conditions, 98.1% biodiesel was achievable, which was in close agreement with the expected result. In addition, the cosolvent removal step from the crude biodiesel was also eliminated as it could be employed as a blended fuel in CI engines.

Hydrothermal Liquefaction: How the Holistic Approach by Nature Will Help Solve the Environmental Conundrum

Hydrothermal liquefaction (HTL) represents a beacon of scientific innovation, which unlocks nature's alchemical wonders while reshaping the waste-to-energy platform. This transformative technology offers sustainable solutions for converting a variety of waste materials to valuable energy products and chemicals-thus addressing environmental concerns, inefficiencies, and high costs associated with conventional waste-management practices. By operating under high temperature and pressure conditions, HTL efficiently reduces waste volume, mitigates harmful pollutant release, and extracts valuable energy from organic waste materials. This comprehensive review delves into the intricacies of the HTL process and explores its applications. Key process parameters, diverse feedstocks, various reactor designs, and recent advancements in HTL technology are thoroughly discussed. Diverse applications of HTL products are examined, and their economic viability toward integration in the market is assessed. Knowledge gaps and opportunities for further exploration are accordingly identified, with a focus on optimizing and scaling up the HTL process for commercial applications. In conclusion, HTL holds great promise as a sustainable technology for waste management, chemical synthesis, and energy production, thus making a significant contribution to a more sustainable future. Its potential to foster a circular economy and its versatility in producing valuable products underscore its transformative role in shaping a more sustainable world.

Biofuel production by hydro-thermal liquefaction of municipal solid waste: Process characterization and optimization

The significant growth of the global population, as well as the increase in energy demand and the limitations of energy generation from fossil fuels, have become a serious challenge over the world. To address these challenges, renewable energies like biofuels are recently found as a proper alternative to conventional fuels. Although biofuel production by using various techniques such as hydrothermal liquefaction (HTL) is considered one of the most promising methods to provide energy, the challenges correlated to its progression and development are still striking. In this investigation, the HTL method was employed to produce biofuel from municipal solid waste (MSW). In this regard, the effect of various parameters such as temperature, reaction time and waste-to-water ratio on mass and energy yield were assessed. It should be stressed that the optimization of biofuel production has been accomplished by the Box-Behnken method using Design Expert 8 software. Based on the results, biofuel production has an upward trend by increasing temperature to 364.57 °C and reaction time to 88.23 min Whereas, there is an inverse relationship between the biofuel waste-to-waterater ratio, in both the context of mass and energy yield.

Remediation and recovery of Kariba weed as emerging contaminant in freshwater and shellfish aquaculture system via solvothermal liquefaction

Fast growing Kariba weed causes major problems and pollution on freshwater and shellfish aquaculture systems by interfering with nutrient uptake of crops, restricting sunlight penetration, and decreasing water quality due to massive biomass of Kariba weed remnants. Solvothermal liquefaction is considered an emerging thermochemical technique to convert waste into high yield of value-added products. Solvothermal liquefaction (STL) of Kariba weed as an emerging contaminant was performed to investigate the effects of different types of solvents (ethanol and methanol) and Kariba weed mass loadings (2.5-10 % w/v) on treating and reducing the weed via conversion into potentially useful crude oil product and char. Up to 92.53 % of Kariba weed has been reduced via this technique. The optimal conditions for crude oil production were found to be at 5 % w/v of mass loading in methanol medium, resulting in a high heating value (HHV) of 34.66 MJ/kg and yield of 20.86 wt%, whereas the biochar production was found to be optimum at 7.5 % w/v of mass loading in methanol medium, resulting in 29.92 MJ/kg of HHV and 25.38 wt% of yield. The crude oil consisted of beneficial chemical compounds for biofuel production such as hexadecanoic acid, methyl ester (65.02 peak area %) and the biochar showed high carbon content (72.83 %). In conclusion, STL as a remediation for emerging Kariba weed is a feasible process for shellfish aquaculture waste treatment and biofuels production.

Effect of Acidic Hydrochar on Plastic Crude Oil Produced from Hydrothermal Liquefaction of Waste PVC

In this study, the effect of hydrothermal liquefaction (HTL) of waste PVC was investigated in the presence of acidic hydrochar. The hydrochar was prepared by hydrothermal carbonization of pineapple waste at 250 °C and at 1 h in the presence of citric acid. Hydrochar was acidic, stable, and porous and contained acidic functional groups. Hydrochar was co-fed with PVC during HTL to enhance HTL conversion and quality of the plastic crude oil. HTL experiments were performed at 300–350 °C, 0.25–4 h of reaction times, and 0–20 wt% hydrochar-to-PVC ratio. The plastic crude oil was separated from the solid residue to evaluate HTL conversion and to analyze elemental compositions, boiling point distribution, alteration of chemical bonds, and chemical compositions. The results showed that acidic hydrochar enhances HTL conversion with a maximum value of 28.75 at 5 wt% hydrochar content at 350 °C and 0.5 h. Furthermore, plastic crude oils contained no chloride but contained significantly high carbon and hydrogen, resulting in a higher heating value of up to 36.43 MJ/kg. The major component of the plastic crude oil was 3, 5 dimethylphenol produced ranging from 61.4 to 86.4% (percentage of total identified area) according to gas chromatography mass spectroscopy (GCMS) data.

Citrus limetta fruit waste management by liquefaction using hydrogen-donor solvent

The liquefaction of Citrus limetta fruit waste (both pulp and peel) in a hydrogen-donor solvent has not been reported in the literature and authors considered the same as the objective of this work. Thus, results on waste management of this potential fruit wastes by liquefaction in hydrogen-donor methanol solvent at 260 °C temperature, residence time of 30 min, and 1 : 2, 1 : 3 and 1 : 4 biomass-to-solvent ratios were reported in this work. The aim was to achieve biofuels of high quantity and quality from this waste, which would otherwise be disposed of without any value addition. A maximum of 12.5 wt% of biocrude yield was found from Citrus limetta peel biomass, which was higher than that from the thermochemical conversion of other citrus fruit waste biomasses. Biocrude having higher heating value (HHV) of 26.76 MJ kg-1 from Citrus limetta pulp, when the feed biomass pulp-to-solvent ratio was 1 : 4, found to be the best outcome of this study. Biocrude and biochar have also been extremely characterized using several advanced techniques such as Gas Chromatography Mass Spectrometry (GC-MS), Fourier Transform Infrared Spectroscopy (FTIR), and Field Emission Scanning Electron Microscopy (FESEM), etc. Finally, the novelty of this work is not only obtaining higher yield of biocrude but also better HHV compared to other similar studies in the literature.

A Comprehensive Hydrothermal Co-Liquefaction of Diverse Biowastes for Energy-Dense Biocrude Production: Synergistic and Antagonistic Effects

Hydrothermal co-liquefaction (co-HTL) is a promising technology to valorize binary or even ternary biowastes into bioenergy. However, the complex biochemical compositions and unclear synergistic effect prevent the development of this technology. Thus, this study explored a comprehensive co-HTL of representative biowastes to investigate the synergistic and antagonistic effects. An apparent synergistic effect on biocrude yield was observed when sewage sludge was co-liquefied with cow manure or wheat straw. Further, the co-HTL of sewage sludge-cow manure was investigated in a detailed manner. The highest yield (21.84 wt%) of biocrude, with a positive synergistic effect (11.37%), the highest energy recovery (47.48%), and a moderate biocrude HHV (34.31 MJ/kg) were achieved from co-HTL at 350 °C for 30 min. Hydrochar and gas products were also characterized to unravel the reaction pathways. Accordingly, this work indicates that sewage sludge co-liquefied with other biowastes can serve as a multi-purpose solution for biowaste treatment and bioenergy production.

Sustainable management of municipal solid waste through waste-to-energy technologies

Increasing municipal solid waste (MSW) generation and environmental concerns have sparked global interest in waste valorization through various waste-to-energy (WtE) to generate renewable energy sources and reduce dependency on fossil-derived fuels and chemicals. These technologies are vital for implementing the envisioned global "bioeconomy" through biorefineries. In light of that, a detailed overview of WtE technologies with their benefits and drawbacks is provided in this paper. Additionally, the biorefinery concept for waste management and sustainable energy generation is discussed. The identification of appropriate WtE technology for energy recovery continues to be a significant challenge. So, in order to effectively apply WtE technologies in the burgeoning bioeconomy, this review provides a comprehensive overview of the existing scenario for sustainable MSW management along with the bottlenecks and perspectives.

Optimal use of glycerol co-solvent to enhance product yield and its quality from hydrothermal liquefaction of refuse-derived fuel

Refuse-derived fuels (RDF) are rich in resources that make them an attractive feedstock for the production of energy and biofuels. Hydrothermal liquefaction (HTL) is a promising thermochemical conversion technology to handle wet feedstocks and convert them to valuable bio-crude, bio-char and aqueous products. This study highlights the advantages of using glycerol as the co-solvent along with water in different proportions to produce bio-crude from RDF via HTL. The ratio of water:glycerol (vol.%:vol.%) was varied for each experiment (100:0, 90:10, 80:20, 70:30, 60:40, 50:50), and the product yields and their quality were studied. The results demonstrate that increasing the proportion of glycerol until 50 vol.% in the solvent enhances the bio-crude yield (36.2 wt.%) and its higher heating value (HHV) (30.9 MJ kg-1). Deoxygenation achieved in the bio-crude was 42%. The production of bio-char was minimum (9.5 wt.%) at 50 vol.% glycerol with HHV of 31.9 MJ kg-1. The selectivity to phenolic compounds in the bio-crude increased, while that of cyclic oxygenates decreased when the glycerol content was more than 20 vol.%. The gas-phase analysis revealed that the major deoxygenation pathway was decarboxylation. The yield of aqueous products drastically increased with the addition of glycerol. The minimum amount of glycerol in the co-solvent that favours an energetically feasible process with low carbon footprint is 30 vol.%. Using 50 vol.% glycerol resulted in the highest energy recovery in the bio-crude and bio-char (80%), the lowest energy consumption ratio (0.43) and lowest environmental factor (0.1). The mass-based process mass intensity factor, calculated based on only bio-crude and bio-char as the valuable products, decreased with an increase in addition of glycerol, while it was close to unity when the aqueous phase is also considered as a valuable product.

Current Status and Future Prospects of Biolubricants: Properties and Applications

Biolubricants generated from biomass and other wastes can reduce the carbon footprint of manufacturing processes and power generation. In this paper, the properties and uses of biolubricants have been compared thoroughly with conventional mineral-based lubricants. The biolubricants, which are currently based on vegetable oils, are discussed in terms of their physicochemical and thermophysical properties, stability, and biodegradability. This mini-review points out the main features of the existing biolubricants, and puts forward the case of using sustainable biolubricants, which can be generated from agro-residues via thermochemical processes. The properties, applications, and limitations of non-edible oils and waste-derived oils, such as bio-oil from pyrolysis and bio-crude from hydrothermal liquefaction, are discussed in the context of biolubricants. While the existing studies on biolubricants have mostly focused on the use of vegetable oils and some non-edible oils, there is a need to shift to waste-derived oils, which is highlighted in this paper. This perspective compares the key properties of conventional oils with different oils derived from renewable resources and wastes. In the authors’ opinion, the use of waste-derived oils is a potential future option to address the problem of the waste management and supply of biolubricant for various applications including machining, milling applications, biological applications, engine oils, and compressor oils. In order to achieve this, significant research needs to be conducted to evaluate salient properties such as viscosity, flash point, biodegradability, thermo-oxidative and storage stability of the oils, technoeconomics, and sustainability, which are highlighted in this review.

Valorization of red macroalgae biomass via hydrothermal liquefaction using homogeneous catalysts

Hydrothermal liquefaction of red macroalgae species, Kappaphycus alvarezii (KA) and Eucheuma denticulatum (ED), was performed at 350 °C in the presence of 5 wt% neutral and alkali catalysts like Na₂CO₃, K₂CO₃, CaCO₃, Na₂SO₄, NaOH, and KOH. The maximum bio-crude yield of 26.7 wt% and 18.5 wt%, on a dry ash-free basis, was obtained from Na₂CO₃ treatment of KA and KOH treatment of ED, respectively. The bio-crude from both feedstocks mainly consisted of cyclic oxygenates, whose selectivities were maximum in K₂CO₃ and CaCO₃ treatments. The calorific value of the bio-crude was 38.5 MJ/kg from KA and 30.8 MJ/kg from ED, while that of biochars was 20-24 MJ/kg. A high degree of deoxygenation (64.2%) was observed in bio-crude produced from Na₂SO₄ treatment of KA biomass. Salts of Cl^-, SO₄^2- and K⁺ constituted the major inorganic portion of the aqueous phase. Maximum energy recovery (99%) was observed from the Na₂CO₃ treatment of ED.

Fast hydropyrolysis of biomass Conversion: A comparative review

Recent studies show that fast hydropyrolysis (i.e., pyrolysis under hydrogen atmosphere operating at a rapid heating rate) is a promising technology for the conversion of biomass into liquid fuels (e.g., bio-oil and C₄₊ hydrocarbons). This pyrolysis approach is reported to be more effective than conventional fast pyrolysis in producing aromatic hydrocarbons and also lowering the oxygen content of the bio-oil obtained compared to hydrodeoxygenation (a common bio-oil upgrading method). Based on current literature, various non-catalytic and catalytic fast hydropyrolysis processes are reviewed and discussed. Efforts to combine fast hydropyrolysis and hydrotreatment process are also highlighted. Points to be considered for future research into fast hydropyrolysis and pending challenges are also discussed.

Biocrude oil production via hydrothermal liquefaction of food waste in a simplified high-throughput reactor

In this study, the hydrothermal liquefaction of food waste was investigated for generating bio-oils or biocrudes using a simplified high-throughput reactor. Different operating conditions were parametrically tested for obtaining maximum yield of biocrudes at temperatures from 280 to 340 °C and sample-to-water mixture ratios from 1:3 to 1:7, with 30 min residence time. The biocrude yields were found to increase significantly with increasing temperature and mixture ratio. A maximum biocrude yield of approximately 40% w/w dry basis (from a total liquid product yield of over 56% w/w) and energy recovery of over 70% were obtained at 340 °C and a mixture ratio of 1:7. High heating values of the resulting biocrude and solid residue were remarkably better than those of the raw material. Fatty acids, amides, N-containing compounds, and cyclic ketones were identified to be the main components of the biocrudes using advanced GCxGC/TOF-MS.