Catalytic hydrothermal co-gasification of canola meal and low-density polyethylene using mixed metal oxides for hydrogen production

Canola meal is a low-value agricultural residue obtained after oil extraction from canola, the utilization of which requires further attention. On the other hand, plastic waste disposal is also another leading issue that creates severe environmental challenges. Supercritical water gasification is considered an environmentally friendly technology to produce hydrogen from plastic residues and organic wastes. This study deals with hydrothermal co-gasification of canola meal and plastic wastes (i.e., low-density polyethylene) while exploring the influence of temperature (375–525°C), residence time (15–60 min) and plastic-to-biomass ratio (0:100, 20:80, 50:50, 80:20 and 100:0) on hydrogen yield. Maximum hydrogen yield (8.1 mmol/g) and total gas yield (17.9 mmol/g) were obtained at optimal temperature and residence time of 525°C and 60 min, respectively. A change in the gas yield with variable plastic-to-biomass ratio showed synergistic effects between both feedstocks. The trend of catalytic performance towards improving hydrogen yield was in the following order: WO₃–TiO₂ (18.5 mmol/g) > KOH (16.9 mmol/g) > TiO₂ (9.5 mmol/g) > ZrO₂ (7.8 mmol/g) > WO₃–ZrO₂ (7.4 mmol/g).     

Assessment of supercritical water gasification of food waste under the background of waste sorting: Influences of plastic waste contents

Waste sorting is being gradually implemented as a key measure for circular and sustainable development in China, food waste will be separately collected and separated from municipal solid waste (MSW), thus the plastic content in food waste also will be reduced. In this study, supercritical water gasification (SCWG) of food waste with different contents of plastic (0–3.5 wt%) was experimentally investigated to simulate the influence of waste sorting on the food waste treatment. The results showed that lower plastic content in food waste favored higher gas yield and gasification efficiencies. The highest H₂ yield and total gas yield were 3.11 mol/kg and 8.41 mol/kg in the plastic-free case, respectively. When the plastic content decreased from 3.5 wt% to 0 wt%, the cold gas efficiency (CGE), carbon conversion efficiency (CE) and hydrogen gasification efficiency (HE) increased by 125.97%, 173.48% and 94.09%, respectively. However, lower plastic content negatively affected the quality of produced syngas through decreasing H₂ mole fraction and LHV. The solid residues from SCWG of food waste with lower plastic content had higher ratio of fixed carbon to volatile matter (FC/VM). Based on the analysis of pyrolysis properties and combustion behavior, decreasing the plastic content in food waste helped to improve the thermal stability of solid residues. Moreover, lower plastic content resulted in a decrease of total organic carbon (TOC) concentration in liquid effluent, which is favorable for further treatment of liquid effluent.     

H2-enriched gaseous fuel production via co-gasification of an algae-plastic waste mixture using Aspen PLUS

Thermo-chemical conversion of biomass is a promising technological alternative for producing renewable fuel and reducing waste disposal. This simulation study includes the first attempt to perform co-gasification of algae-plastic waste for H₂-enriched gaseous fuel production. An Aspen Plus-based simulation model was developed to evaluate the influence of gasifier temperature and equivalence ratio on the syngas composition, heating value, and carbon conversion efficiency. Simulation results indicated that the rise in gasifier temperature favoured the H₂ and CO formation, and further, plastic loading enhanced H₂ production to a greater extent. It was observed that the product (H₂ and CO) yield decreased significantly with the rise of the equivalence ratio. At the same time, CO₂ formation increased due to more carbon conversion after enhancing O₂ content in the gasifier. It was also noticed that the synergy of biomass and plastic waste significantly enhanced H₂ content and improved heating value, leading to a produced energy-efficient gaseous product. It is inferred that H₂-enriched feedstock acts as an H₂ donor to the H₂ deficient biomass. Based on the findings, consistency in the simulation results was observed compared with the previous literature. Hence, a mixture of biomass and plastic waste favours obtaining an energy-efficient renewable fuel that could be utilized for different applications.     

The resource utilization of ABS plastic waste with subcritical and supercritical water treatment

Nowadays, the massive accumulation of plastic wastes has caused serious environmental problems, and supercritical water treatment provides a promising way for the clean and efficient utilization of plastic wastes. In this work, the acrylonitrile-butadiene-styrene (ABS) plastic was selected as the feedstock, and the supercritical water gasification experiments for fuel gas production were firstly conducted from 450 °C to 700 °C, at 23 MPa. The increase of reaction time, temperature and material ratio (water/ABS) can significantly promote the gasification reaction, and the whole reaction process was obviously divided into three stages: the gasification efficiency rapidly increased firstly, maintaining nearly unchanged then, and restarted to grow. The subcritical water hydrolysis for oil products recovery was also investigated from 375 °C to 450 °C at 21 MPa, and results show that most of the monomers were converted into more stable substances at long residence time. The optimal reaction condition for monomer recovery was determined to be 400 °C and 3 min through the experimental results.     

Hydrogen production by supercritical water gasification of lignin over CuO–ZnO catalyst synthesized with different methods

In this study, lignin was gasified in supercritical water with catalysis of CuO–ZnO synthesized by deposition precipitation, co-precipitation and sol-gel methods. Sol-gel synthesized CuO–ZnO showed the highest catalytic performance, and the gasification efficiency was increased by 37.92% with it. The XRD, SEM-EDS and N₂ adsorption/desorption analysis showed that the priority of the sol-gel catalyst was the smallest crystallite size, largest specific surface area and high dispersion. For sol-gel synthesized CuO–ZnO, the increase of CuO/ZnO ratio improved the gasification efficiency but reduced H₂ selectivity. And the catalytic activity was reduced with the calcination temperature above 600 °C due to enlarged crystallites and reduced pores. During sol-gel preparation, both the addition of ethanol and PEG in the solvent reduced the agglomeration and improved the catalytic activity. With CuO–ZnO prepared with 1 g PEG + water as the solvent, the highest H₂ yield of 6.86 mol/kg was obtained, which was over 1.5 times of that without catalyst.     

Hydrogen production from the gasification of lignin with nickel catalysts in supercritical water

Hydrogen production from the gasification of lignin with Ni/MgO catalysts in supercritical water was conducted using stainless steel tube bomb reactor. Ni/MgO catalysts were prepared by impregnation method and were calcined at 773–1173 K in air for 8 h. The results of characterization for reduced Ni/MgO catalysts showed that Ni metal and NiO–MgO phase are formed after the reduction of calcined catalyst by _H2${\mathrm{H}}_2$. Furthermore, Ni metal surface area, which was calculated by CO chemical adsorption technique, decreased with increase in calcination temperatures. It was found that the carbon yield of gas products was increased with increase in Ni metal surface area except 10 wt% Ni/MgO (773 K) catalyst. Thus, it can be supposed that there is an optimal Ni particle size for the gasification of lignin in supercritical water. It should be noted that 10 wt% Ni/MgO (873 K) catalyst showed the best catalytic performance (carbon yield 30%) under reaction condition tested. It was concluded that Ni/MgO catalyst is a promising system for the gasification of lignin in supercritical water.     

H2 and CO production through coking wastewater in supercritical water condition: ReaxFF reactive molecular dynamics simulation

The degradation mechanism of benzo[a]pyrene (BaP), a representative component of coking wastewater, and the pathway for the production of H₂ and CO in supercritical water have been investigated via ReaxFF reactive molecular dynamics simulations. The BaP molecules in the SCWG, SCWPO and SCWO systems show different degradation pathways. The maximum H₂ yield is obtained at the oxygen ratio of 0.2. There are three routes for the generation of H₂ molecules and production from H radical-rich water is the main route. CO molecules are formed by the CC bond breakage and CO bond breakage in the reforming fragments. There is a time delay between the fuel gas generation reaction and the side reactions due to the change of the instantaneous concentrations of H₂ and CO, providing a possible pathway to increase the amount of the produced fuel gases by designing a suitable reactor and recovering the gas fuel in time. Finally, kinetic behaviors of coking wastewater have been analyzed.     

Gasification of guaiacol in supercritical water: Detailed reaction pathway and mechanisms

Supercritical water gasification of guaiacol as a model compound for lignin was conducted in quartz reactors. The formation and degradation pathways of intermediates were discussed. The results show that the gasification efficiency of guaiacol in supercritical water increased as the reaction time increased. The intermediates in the residual liquid consisted mainly of phenols, arenes, cyclopentanones, alcohols and organic acids. Phenols and arenes were difficult to be gasified in supercritical water and easily turned into char and tar while cyclopentanones, alcohols and organic acids could be easily gasified and turn into hydrogen-rich gas. The Ru/Al₂O₃ catalyst promoted the degradation of phenols and arenes and thus inhibited the formation of char and tar.     

Numerical investigation of coal gasification in supercritical water with the ReaxFF molecular dynamics method

Studies on the coal gasification process in supercritical water (SCW) were carried out with the ReaxFF molecular dynamics (MD) method, in which the Wiser model of the coal molecule was adopted. The results show that hydrogen production increases with increase of temperature and water–coal mass ratio. It is also found that the coal molecule breaks into small fragments before it reacts with water molecules. The detailed chemical reactions and pathways of hydrogen generation during the gasification process are disclosed. H ions are found to be the main source of hydrogen generation, and C–H–O compounds or radicals are the most essential reactants throughout the reactions producing H₂ and H ions. OH ions can significantly accelerate the oxidization of organic fragments to produce C–H–O compounds and radicals, which explains how catalysts of alkali salts such as NaOH and KOH improve hydrogen production.     

Experimental study on the energy conversion of food waste via supercritical water gasification: Improvement of hydrogen production

In this study, the model food waste was gasified to hydrogen-rich syngas in a batch reactor under supercritical water condition. The model food consisted of rice, chicken, cabbage, and cooking oil. The effects of the main operating parameters including temperature (420–500 °C), residence time (20–60 min) and feedstock concentration (2–10 wt%) were investigated. Under the optimal condition at 500 °C, 2 wt% feedstock and 60 min residence time, the highest H₂ yield of 13.34 mol/kg and total gas yield of 28.27 mol/kg were obtained from non-catalytic experiments. In addition, four commercial catalysts namely FeCl₃, K₂CO₃, activated carbon, and KOH were employed to investigate the catalytic effect of additives at the optimal condition. The results showed that the highest hydrogen yield of 20.37 mol/kg with H₂ selectivity of 113.19%, and the total gas yield of 38.36 mol/kg were achieved with 5 wt% KOH addition Moreover, the low heating value of gas products from catalytic experiments with KOH increased by 32.21% compared to the non-catalytic experiment. The catalytic performance of the catalysts can be ranked in descending order as KOH > activated carbon > FeCl₃ > K₂CO₃. The supercritical water gasification (SCWG) with KOH addition can be a potential applied technology for food waste treatment with production of hydrogen-rich gases.     

Synergistic effect of supercritical water and nano-catalyst on lignin gasification

Understand the microscopic mechanism of supercritical water catalytic gasification is of great significance for more efficient and convenient utilization of biomass energy. In this work, Pt and Ni nano-particles (NPs) were used as catalysts to accelerate the SCWG of guaiac-based lignin dimer with γ-O-4 linkages for the first time, and the SCWG processes at different conditions were simulated by reaction molecular dynamics simulation to understand the degradation mechanism and the path of gas generation. The simulation results indicate that PtNPs and NiNPs apparently reduce the temperature at which the gasification reaction can take place and the by-products of γ-O-4 lignin by increasing the degradation rate into monomer, accelerating the aromatic ring opening, and adsorbing CO, CO₂ and CH₄. Compared with NiNPs, the synergy of PtNPs and SCWG shows more excellent properties.     

Catalytic gasification of food waste in supercritical water over La promoted Ni/Al2O3 catalysts for enhancing H2 production

Ni/Al₂O₃ catalyst is the one of promising catalysts for enhancing H₂ production from supercritical water gasification (SCWG) of biomass. However, due to carbon deposition, the deactivation of Ni/Al₂O₃ catalyst is still a serious issue. In this work, the effects of lanthanum (La) as promoter on the properties and catalytic performance of Ni/Al₂O₃ in SCWG of food waste were investigated. La promoted Ni/Al₂O₃ catalysts with different La loading content (3–15 wt%) were prepared via impregnation method. The catalysts were characterized using XRD, SEM, BET techniques. The SCWG experiments were carried out in a Hastelloy batch reactor in the operating temperature range of 420–480 °C, and evaluated based on H₂ production. The stability of the catalysts was assessed by the amount of carbon deposition on catalyst surface and their catalytic activity after reuse cycles. The results showed that 9 wt% La promoter is the optimal loading as Ni/9La–Al₂O₃ catalyst performed best performance with the highest H₂ yield of 8.03 mol/kg, and H₂ mole fraction of 42.46% at 480 °C. La promoted Ni/Al₂O₃ catalysts have better anti-carbon deposition properties than bare Ni/Al₂O₃ catalyst, resulting in better gasification efficiency after reuse cycles. Ni/9La–Al₂O₃ catalyst showed high catalytic activity in SCWG of food waste and had good stability as it was still active for enhancing H₂ production when used in SCWG for the third time, which indicated that La promoted Ni/Al₂O₃ catalysts are potential additive to improve the SCWG of food waste.     

Molecular dynamics investigation on the gasification of a coal particle in supercritical water

Coal gasification technology in supercritical water provides a clean and efficient way to convert coal to H₂. In the present paper, the whole supercritical water（SWC）gasification process of a coal particle is studied with the reactive force field (ReaxFF) molecular dynamics (MD) method for the first time. First, the detailed reaction mechanism which can't be clearly illustrated in experiments, such as the evolution of the carbon structure during the gasification process and the detailed reaction mechanism of the main products, is obtained. According to the generation mechanism of H₂, it is found that the supercritical water gasification process of a coal particle can be divided into two stages with different reaction mechanisms, namely the rapid reaction stage and the stable reaction stage. Then, the effects of temperature and coal concentration in the reaction system on the yield of H₂ are studied. Finally, the transition of N in the coal particle is revealed, in which the precursors of NH₃ such as CN, CHN, and CHON are the basic molecular structures for nitrogen atoms during the gasification process at high temperature.     

A molecular dynamics simulation study on solubility behaviors of polycyclic aromatic hydrocarbons in supercritical water/hydrogen environment

Violates containing polycyclic aromatic hydrocarbons (PAHs) were precipitated in the process of fast pyrolysis and gasification of coal and organic substances. PAHs are one of bottlenecks of entire coal gasification for hydrogen production. In current work, the solubility of PAH oil droplets in supercritical water/hydrogen circumstances were investigated based on molecular dynamics simulation, which was beneficial for understanding the solubility behaviors of PAHs in supercritical water/hydrogen environment. The results showed that heavy PAHs were rather stable in the water phase. Supercritical water along with hydrogen promoted the miscibility of PAHs compared with that of pure supercritical water. Furthermore, high density and high temperature facilitated the rapid solvation of PAHs in supercritical water/hydrogen environment. This paper is expected to provide a theoretical support for the development of complete coal gasification technology for hydrogen production.     

Supercritical water gasification of food waste: Effect of parameters on hydrogen production

Food waste is a type of municipal solid waste with abundant organic matter. Hydrogen contains high energy and can be produced by supercritical water gasification (SCWG) of organic waste. In this study, food waste was gasified at various reaction times (20–60 min) and temperatures (400 °C-450 °C) and with different food additives (NaOH, NaHCO₃, and NaCl) to investigate the effects of these factors on syngas yield and composition. The results showed that the increase in gasification temperature and time improved gasification efficiency. Also, the addition of food additives with Na⁺ promoted the SCWG of food waste. The highest H₂ yield obtained through non-catalytic experiments was 2.0 mol/kg, and the total gas yield was 7.89 mol/kg. NaOH demonstrated the best catalytic performance in SCWG of food waste, and the highest hydrogen production was 12.73 mol/kg. The results propose that supercritical water gasification could be a proficient technology for food waste to generate hydrogen-rich gas products.     

Study on gasification mechanism of biomass waste in supercritical water based on product distribution

Supercritical water gasification technology is widely applied to convert organic waste into valuable substances as a clean and efficient method. Biomass gasification in SCW is a complex process and complicated chemical reactions like decomposition and poly-condensation take place, thus, reaction mechanism of real biomass needs to be further investigated. In this paper, experimental study on cornstalk gasification in SCW was conducted at the temperature of 500–800 °C, reaction time of 1–15min and feedstock concentration of 1–9%. The effects of various operating parameters on evolution of gas, liquid and solid products were conducted. It was discovered that pore structure and carbon microspheres appeared successively on the surface of solid residue. Mechanism study showed that the biomass was first depolymerized into monomer and its derivatives, then cracked and poly-condensed into a nuclear to generate carbon microspheres as its concentration reached the critical concentration. As the reaction proceeds, reduction reaction, coke combustion and secondary reaction occurred, thus carbon microspheres decreased. The results indicated that higher reaction temperature, longer reaction time and lower feed concentration were conducive to improving reaction performance of biomass. Finally, it was discovered that carbon gasification efficiency reached 99% at the temperature of 700 °C, reaction time of 15 min and biomass concentration of 3%.     

Gasification of food waste in supercritical water: An innovative synthesis gas composition prediction model based on Artificial Neural Networks

The present study intends to develop multi-layered feed-forward back-propagation algorithm based artificial neural network (FFBPNN) models to predict the synthesis gas (SG) compositions (H₂, CH₄, CO & CO₂) and yields (mol/kg) for supercritical water gasification (SCWG) of food wastes. Such models are trained with Levenberg-Marquardt (L-M) algorithm, minimized using gradient descent approach and tested with real-time experimental datasets obtained from literature. Moreover, to determine an optimal form of the neural network for a typical non-catalytic SCWG process, a trial and error approach involving multiple combinations of transfer functions and neurons in the network layers is performed. The predicted values of SG compositions yield delivered by the FFBPNN models are in line with the experimental datasets converging to a mean squared error (MSE) value below 0.300 range and coefficient of determination (R²) above 98%. Best prediction accuracy is achieved for CO yield prediction characterized by a least MSE of 0.022 and highest train-test R² of 0.9942–0.9939. The performance of the developed FFBPNN models can be arranged on the basis of MSE as (ann7)_CO < (ann6)_CH? < (ann5)_H? < (ann8)_CO? and on the basis of testing R² as (ann7)_CO > (ann6)_CH? > (ann5)_H? > (ann8)_CO?.     

Supercritical water gasification of real biomass feedstocks in continuous flow system

In this study, supercritical water gasification of the selected five biomass samples (cauliflower residue, acorn, tomatoes residue, extracted acorn and hazelnut shell) was investigated. Lignocellulosic feedstocks were gasified in a continuous flow reactor at 600 °C and 35 MPa. The product gas is composed of hydrogen, carbon dioxide, methane, carbon monoxide and a small amount of C₂ compounds. Quantitative analysis of product gas was performed by Gas chromatography device. Potassium carbonate (K₂CO₃) and Trona (Na₂CO₃·NaHCO₃·2H₂O) were used as catalysts. Carbon gasification efficiencies were improved by addition of these catalysts into the reacting system. Moreover, carbon gasification efficiency changes with type of biomass that includes different ratio of cellulose, hemicellulose and lignin. The H₂ yield (mol gas/kg C in feed) of acorn in the presence of Trona was found to be 7 times higher than that of without catalyst.     

Supercritical water synthesized Ni/ZrO2 catalyst for hydrogen production from supercritical water gasification of glycerol

Catalysts are crucial to promote the technical feasibility of supercritical water gasification (SCWG) for H₂ production from wet biomass, yet catalysts prepared by conventional methods normally encounter sintering problems in supercritical water. Herein, a series of ZrO₂-supported Ni catalysts were tried to be prepared by supercritical water synthesis (SCWS) and evaluated for SCWG in terms of activity and property stability. The SCWS was conducted at 500 °C and 23 MPa using metal nitrates as starting materials. Effect of precursor concentration on property and catalytic performance of the SCWS-prepared catalysts for SCWG of 20 wt% glycerol were systematically studied. XRD, SEM-EDS, TEM and TGA were applied for catalyst characterization. Results verified the successful obtaining of Ni/ZrO₂ nanocatalysts with Ni crystals of 30–70 nm and ZrO₂ crystals of ~11 nm by the SCWS process, which were found to be active on the WGSR for SCWG to increase the H₂ yield as high as 155%. Importantly, the SCWS-prepared Ni/ZrO₂ catalysts exhibited excellent property stability and anti-coking ability for SCWG of glycerol.     

Hydrogen generation by gasification of phenol and alcohols in supercritical water

The conversion of phenol, cyclohexanol (a hydrogenated analog of phenol for comparison with phenol), and ethanol into gas products in supercritical water (SCW) was studied with the goal to compare the reactivity of their aqueous solutions with the structural features obtained by the method of classical molecular dynamics. Transformation of phenol and alcohols occurs in different ways. In the case of alcohols, the conversion of 75–100% is achieved at 600 °C with noticeable gasification. At the same time, the conversion of phenol is only 47% and no gas products are formed at all. The complete conversion of phenol is achieved at a temperature of 750 °C, while the degree of gasification does not exceed 30%. It is shown that an increase in the phenol gasification degree is possible by pre-catalytic hydrogenation of phenol into cyclohexanol.