Characterizing the Opportunity Space for Sustainable Hydrothermal Valorization of Wet Organic Wastes.

Resource recovery from wet organic wastes can support circular economies by creating financial incentives to produce renewable energy and return nutrients to agriculture. In this study, we characterize the potential for hydrothermal liquefaction (HTL)-based resource recovery systems to advance the economic and environmental sustainability of wastewater sludge, FOG (fats, oils, and grease), food waste, green waste, and animal manure management through the production of liquid biofuels (naphtha, diesel), fertilizers (struvite, ammonium sulfate), and power (heat, electricity). From the waste management perspective, median costs range from -193 $·tonne-1 (FOG) to 251 $·tonne-1 (green waste), and median carbon intensities range from 367 kg CO2 eq·tonne-1 (wastewater sludge) to 769 kg CO2 eq·tonne-1 (green waste). From the fuel production perspective, the minimum selling price of renewable diesel blendstocks are within the commercial diesel price range (2.37 to 5.81 $·gal-1) and have a lower carbon intensity than petroleum diesel (101 kg CO2 eq·MMBTU-1). Finally, through uncertainty analysis and Monte Carlo filtering, we set specific targets (i.e., achieve wastewater sludge-to-biocrude yield >0.440) for the future development of hydrothermal waste management system components. Overall, our work demonstrates the potential of HTL-based resource recovery systems to reduce the costs and carbon intensity of resource-rich organic wastes.

Evaluation on hydrothermal gasification of styrene-butadiene rubber with oxidants via ReaxFF-MD simulation.

Styrene-butadiene rubber (SBR) is widely used in tires, which brings great challenge to the disposal and reclaiming of the used tires. The ring-opening reaction pathways of benzene rings in hydrothermal gasification of styrene-butadiene rubber were revealed based on reactive force field molecular dynamics (ReaxFF-MD) simulation. H-abstraction reaction that OH radicals capture H atom from the vinyl group of styrene was critical to the degrading of the styrene monomers. The energy barrier of H2O2 converted to OH radicals was lower than that of O2 and pure water converted to OH radicals. The oxidants that can urge OH radical formed in reaction were beneficial to SBR degradation, which could be assigned to confirm that SBR degradation with H2O2 was better than that with oxygen at the same concentration. The addition of oxidant could be helpful for decreasing the degradation temperature of styrene monomers. At oxidant equivalent ratio (ER) of 0.1, H2 yield at 2500 K lifted after 135 ps and increased by 75% at 500 ps compared with that without oxidants. According to the chemical equilibrium analysis, the optimal ER for H2 was 0.4 between 350 and 600 °C (real temperatures). The results could provide theoretic support and experiment guidance for adding oxidants in reclaiming waste rubber products.

Hybrid Prediction-Driven High-Throughput Sustainability Screening for Advancing Waste-to-Dimethyl Ether Valorization.

Assessing the prospective climate preservation potential of novel, innovative, but immature chemical production techniques is limited by the high number of process synthesis options and the lack of reliable, high-throughput quantitative sustainability pre-screening methods. This study presents the sequential use of data-driven hybrid prediction (ANN-RSM-DOM) to streamline waste-to-dimethyl ether (DME) upcycling using a set of sustainability criteria. Artificial neural networks (ANNs) are developed to generate in silico waste valorization experimental results and ex-ante model the operating space of biorefineries applying the organic fraction of municipal solid waste (OFMSW) and sewage sludge (SS). Aspen Plus process flowsheeting and ANN simulations are postprocessed using the response surface methodology (RSM) and desirability optimization method (DOM) to improve the in-depth mechanistic understanding of environmental systems and identify the most benign configurations. The hybrid prediction highlights the importance of targeted waste selection based on elemental composition and the need to design waste-specific DME synthesis to improve techno-economic and environmental performances. The developed framework reveals plant configurations with concurrent climate benefits (-1.241 and -2.128 kg CO2-eq (kg DME)-1) and low DME production costs (0.382 and 0.492 € (kg DME)-1) using OFMSW and SS feedstocks. Overall, the multi-scale explorative hybrid prediction facilitates early stage process synthesis, assists in the design of block units with nonlinear characteristics, resolves the simultaneous analysis of qualitative and quantitative variables, and enables the high-throughput sustainability screening of low technological readiness level processes.

Machine learning for hydrothermal treatment of biomass: A review.

Hydrothermal treatment (HTT) (i.e., hydrothermal carbonization, liquefaction, and gasification) is a promising technology for biomass valorization. However, diverse variables, including biomass compositions and hydrothermal processes parameters, have impeded in-depth mechanistic understanding on the reaction and engineering in HTT. Recently, machine learning (ML) has been widely employed to predict and optimize the production of biofuels, chemicals, and materials from HTT by feeding experimental data. This review comprehensively analyzed the application of ML for HTT of biomass and systematically illustrated basic ML procedure and descriptors for inputs and outputs of ML models (e.g., biomass compositions, operation conditions, yield and physicochemical properties of derived products) that could be applied in HTT. Moreover, this review summarized ML-aided HTT prediction of yield, compositions, and physicochemical properties of HTT hydrochar or biochar, bio-oil, syngas, and aqueous phase. Ultimately, future prospects were proposed to enhance predictive performance, mechanistic interpretation, process optimization, data sharing, and model application during ML-aided HTT.

Agricultural waste-derived biochars from co-hydrothermal gasification of rice husk and chicken manure and their adsorption performance for dimethoate.

This study aimed to combine energy utilisation of agricultural wastes with the dimethoate (DT) adsorption from agricultural wastewater via hydrogen and biochar production using co-hydrothermal gasification (CHTG). The gasification behaviour after CHTG of five ratios of rice husk (RH) and chicken manure (CM) and the corresponding adsorption performance of biochars on DT were evaluated. The results demonstrated that the feedstock of 3RH+ 1CM achieved the maximum gas yield and hydrogen gasification efficiency (HGE), and the highest adsorption capacity of the derived biochars was 3.57 mg g-1. Surface characterisation and elemental analysis showed that the biochar derived under different C/N ratios varied considerably. The results of the isotherm and kinetic simulation showed that the Langmuir model and pseudo-first-order model best fitted the experimental data. The superior performance of agricultural waste-derived biochars (AWB) over five cycles of regeneration and adsorption indicated that AWB is a green and stable adsorption material for farmland tailwater. In addition, the degradation pathway of DT during hydrothermal gasification (HTG) regeneration of the spent adsorbent was comprehensively discussed. The CHTG treatment enhanced the yield of gaseous products from RH and CM and produced AWBs with high adsorption capacities for DT. This provides a green and efficient technology for resource utilisation of agricultural waste and treatment of agricultural wastewater using pesticide residues.

Hydrothermal liquefaction of Scenedesmus obliquus using a novel catalyst derived from clam shells: Solid residue as catalyst for hydrogen production.

This study explores the catalytic application of waste clam shell in hydrothermal liquefaction (HTL) of microalgae (Scenedesmus obliquus) for liquid hydrocarbons production. Novel catalyst (calcium hydroxide) was derived from clam shells. Catalytic HTL was performed at varying temperature of 240-320 °C for catalyst load (0.2-1 wt%) at a reaction time of 60 min. Bio-oil yield was maximum (39.6 wt%) at a temperature of 300 °C for catalyst load of 0.6 wt% at a reaction time of 60 min with calorific value of 35.01 MJ/kg. Compounds like phenols, aromatic hydrocarbons, acids and aldehydes were detected in bio-oil through Gas Chromatography Mass Spectrophotometry (GC-MS). Gasification of microalgae with waste solid residue obtained from HTL was carried out for hydrogen production. Valuable hydrogen gas production was maximum (37 wt%) at a temperature of 400 °C for 3 wt% of solid residue. Water-gas shift, methanation and steam reforming reactions favoured the hydrogen gas production.

Co-Hydrothermal gasification of Chlorella vulgaris and hydrochar: The effects of waste-to-solid biofuel production and blending concentration on biogas generation.

This study investigates enhanced biogas production via co-Hydrothermal gasification (co-HTG) of wet Chlorella vulgaris biomass and hydrochar (HC). Hydrothermal carbonization was applied to valorize struvite containing waste microalgae stream into solid bio-fuel with improved combustion properties. The effects of HC quality and mixing ratio are investigated on biogas yield, composition and carbon conversion ratio. The results show that the application of blending components promotes H2, CH4 formation and selectivity in hydrothermal gasification. The total co-HTG gas yield is increased from 19.13 to 46.95 mol kg-1 at 650 °C and 300 bar by applying 5 wt% HC blending concentration and reduced level of volatile matter content (24.61 wt%). The obtained high hydrogen, methane yields and carbon conversion ratio (19.49, 2.98 mol kg-1, 82.31%, respectively) indicate effective hydrothermal upgrading potentials in case of wet and waste biomass feedstocks.

Char and tar formation during hydrothermal gasification of dewatered sewage sludge in subcritical and supercritical water: Influence of reaction parameters and lumped reaction kinetics.

The hydrothermal gasification of dewatered sewage sludge converts waste into syngas fuel (gas containing H2, CH4, CO, and CO2), but the formation of char and tar by a side reaction restricts practical applications. In this study, the formation of char/tar by the hydrothermal gasification of dewatered sewage sludge at 300-400 °C for 5-90 min with a heating rate of 8-50 °C/min is investigated. The results showed that the formation of tar/char reached equilibrium after 30 min, and an increase in the temperature and heating rate reduced the char yield. However, a higher tar yield was achieved at the highest temperature. Experiments on sludge with different moisture contents were also conducted, and the formation of char/tar was inhibited at a higher moisture content. The addition of NaOH and H2O2 can effectively reduce the char and tar yields, respectively. A lumped kinetic model for describing carbon conversion during the hydrothermal gasification of sewage sludge was developed to elucidate the char/tar formation mechanism. The experimental data were used to determine the model parameters, and the fitting results showed that solid-solid conversion from organic matter in sludge was the predominant char/tar formation pathway, with an activation energy of 7.1 kJ/mol. The experiments using the model compounds indicated that humus and protein are the main precursors of char and tar, respectively. This study provides insights for understanding the formation of char/tar from sewage sludge following hydrothermal gasification, which is vital for controlling the formation of char and tar to improve the gasification efficiency.

Fate and transport of estrogenic compounds in an integrated swine manure treatment systems combining algal-bacterial bioreactor and hydrothermal processes for improved water quality.

An integrated manure treatment system, including a mixed algal-bacterial bioreactor (MABB) and hydrothermal processing of biomass solids, was found to remove 76.4-97.0% of the total estrogenic hormones (estrone (E1), 17ß-estradiol (E2), and estriol (E3)) from the liquid portion of animal manure (LPAM). The mixed biomass was converted into either biocrude oil with a yield up to 40% via hydrothermal liquefaction (HTL) or syngas with a yield up to 54% yield via catalytic hydrothermal gasification (CHG). Adding granular activated carbon (GAC) in the MABB enhanced the removal of estrogenic hormones (+ 7.2%), cytotoxicity (+ 58%), and heavy metals (+ 10.5%). After the integrated system with the MABB, HTL, and CHG processes, the overall percent removal of heavy metals from the LPAM ranged from 27.1 to 40.3%. The concentrations of potentially toxic heavy metals (lead (Pb), copper (Cu), zinc (Zn), cadmium (Cd), nickel (Ni), chromium (Cr)) in the aqueous phase after HTL and CHG tests ranged from 0.01 to 25.3 mg/L.

Biohythane production of post-hydrothermal liquefaction wastewater: A comparison of two-stage fermentation and catalytic hydrothermal gasification.

Developing efficient methods to recover energy from post-hydrothermal liquefaction wastewater (PHW) is critical for scaling up hydrothermal liquefaction (HTL) technology. Here we evaluated two-stage fermentation (TF) and catalytic hydrothermal gasification (CHG) for biohythane production using PHW. A hydrogen yield of 29 mL·g-1 COD and methane yield of 254 mL·g-1 COD were achieved via TF. In comparison, a higher hydrogen yield (116 mL·g-1 COD) and lower methane yield (65 mL·g-1 COD) were achieved during CHG. Further, a techno-economic and sensitivity analysis was conducted. The capital cost and operating cost for TF varied with the different reactor systems. TF with high-rate reactors suggested its promising commercialized application as it had a lower minimum selling price (-0.71 to 2.59 USD per gallon of gasoline equivalent) compared with conventional fossil fuels under both the best and reference market conditions. Compared with TF, CHG was only likely to be profitable under the best case conditions.

A parametric study on supercritical water gasification of Laminaria hyperborea: a carbohydrate-rich macroalga.

The potential of supercritical water gasification (SCWG) of macroalgae for hydrogen and methane production has been investigated in view of the growing interest in a future macroalgae biorefinery concept. The compositions of syngas from the catalytic SCWG of Laminaria hyperborea under varying parameters including catalyst loading, feed concentration, hold time and temperature have been investigated. Their effects on gas yields, gasification efficiency and energy recovery are presented. Results show that the carbon gasification efficiencies increased with reaction temperature, reaction hold time and catalyst loading but decreased with increasing feed concentrations. In addition, the selectivity towards hydrogen and/or methane production from the SCWG tests could be controlled by the combination of catalysts and varying reaction conditions. For instance, Ru/Al2O3 gave highest carbon conversion and highest methane yield of up to 11 mol/kg, whilst NaOH produced highest hydrogen yield of nearly 30 mol/kg under certain gasification conditions.