Catalytic subcritical and supercritical water gasification as a resource recovery approach from waste tires for hydrogen-rich syngas production

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.

Technoeconomic Feasibility of Hydrogen Production from Waste Tires with the Control of CO2 Emissions

The worldwide demand for energy is increasing significantly, and the landfill disposal of waste tires and their stockpiles contributes to huge environmental impacts. Thermochemical recycling of waste tires to produce energy and fuels is an attractive option for reducing waste with the added benefit of meeting energy needs. Hydrogen is a clean fuel that could be produced via the gasification of waste tires followed by syngas processing. In this study, two process models were developed to evaluate the hydrogen production potential from waste tires. Case 1 involves three main processes: the steam gasification of waste tires, water gas shift, and acid gas removal to produce hydrogen. On the other hand, case 2 represents the integration of the waste tire gasification system with the natural gas reforming unit, where the energy from the gasifier-derived syngas can provide sufficient heat to the steam methane reforming (SMR) unit. Both models were also analyzed in terms of syngas compositions, H₂ production rate, H₂ purity, overall process efficiency, CO₂ emissions, and H₂ production cost. The results revealed that case 2 produced syngas with a 55% higher heating value, 28% higher H₂ production, 7% higher H₂ purity, and 26% lower CO₂ emissions as compared to case 1. The results showed that case 2 offers 10.4% higher process efficiency and 28.5% lower H₂ production costs as compared to case 1. Additionally, the second case has 26% lower CO₂-specific emissions than the first, which significantly enhances the process performance in terms of environmental aspects. Overall, the case 2 design has been found to be more efficient and cost-effective compared to the base case design.

Chemical Recycling of High-Molecular-Weight Organosilicon Compounds in Supercritical Fluids

The main known patterns of thermal and/or catalytic destruction of high-molecular-weight organosilicon compounds are considered from the viewpoint of the prospects for processing their wastes. The advantages of using supercritical fluids in plastic recycling are outlined. They are related to a high diffusion rate, efficient extraction of degradation products, the dependence of solvent properties on pressure and temperature, etc. A promising area for further research is described concerning the application of supercritical fluids for processing the wastes of organosilicon macromolecular compounds.

Innovations in applications and prospects of bioplastics and biopolymers: a review

Non-biodegradable plastics are continually amassing landfills and oceans worldwide while creating severe environmental issues and hazards to animal and human health. Plastic pollution has resulted in the death of millions of seabirds and aquatic animals. The worldwide production of plastics in 2020 has increased by 36% since 2010. This has generated significant interest in bioplastics to supplement global plastic demands. Bioplastics have several advantages over conventional plastics in terms of biodegradability, low carbon footprint, energy efficiency, versatility, unique mechanical and thermal characteristics, and societal acceptance. Bioplastics have huge potential to replace petroleum-based plastics in a wide range of industries from automobiles to biomedical applications. Here we review bioplastic polymers such as polyhydroxyalkanoate, polylactic acid, poly-3-hydroxybutyrate, polyamide 11, and polyhydroxyurethanes; and cellulose-based, starch-based, protein-based and lipid-based biopolymers. We discuss economic benefits, market scenarios, chemistry and applications of bioplastic polymers.

Utilization of Plastic Wastes for Sustainable Environmental Management: A Review

The advancement and modernization of industries have provided numerous benefits to human life including diversification of manufacturing a wide range of products made from plastic materials, thereby leading to the generation of huge quantities of plastic waste. Owing to the increasing issues related with plastic waste, recycling methods have attracted much interest. Recycling not only protects the environment and resources for future generations but also reduces energy consumption and greenhouse gas emissions. A wide range of valuable products including char, oil, fuels, sorbent materials, and chemicals can be obtained through different techniques. This Review highlights various sustainable research avenues and potential routes to reduce the environmental impact of plastic waste based on both traditional and potential approaches for its utilization.

Effects of Process Parameters on Sulfur Migration and H2S Generation during Supercritical Water Gasification of Sludge

The generation of sulfur-containing pollution products affects the quality of biofuels obtained from the supercritical water gasification (SCWG) of sludge. This study investigates the effects of the gasification temperature, moisture content, and reaction atmosphere on the evolution of sulfur-containing compounds. The results showed that temperature was the key parameter causing the migration of sulfur from sludge to biogas and liquid products. The sludge decomposition reaction was dominated by ionic reactions at 360 °C, while the decomposition of organic matter was converted to free radical reactions as the temperature increased from 380 °C to 440 °C. The mercaptan and thioether contents of the bio-oil decreased to 0.3% at 440 °C. Correspondingly, the concentration of H₂S increased from 6.7 ppm to 38.0 ppm. The decomposition of organic sulfur with an unstable structure (S-H bond and S-C bond) was the main cause of the increase in the content of H₂S. Additionally, the solubility and oxidation properties of supercritical water were extremely strong. Some sulfur-containing organic compounds were converted into SO₄^2- via hydrolysis and oxidation reactions, forming sulfate crystals with heavy metals in the bio-char, which aided in achieving the synergistic immobilization of sulfur and heavy metals.

Eco-friendly Transformation of Waste Biomass to Biofuels

Background:

The development of viable alternative fuel sources is assuming a new urgency in the face of climate change and environmental degradation linked to the escalating consumption of fossil fuels. Lignocellulosic biomass is composed primarily of high-energy structural components such as cellulose, hemicellulose and lignin. The transformation of lignocellulosic biomass to biofuels requires the application of both pretreatment and conversion technologies.

Methods:

Several pretreatment technologies (e.g. physical, chemical and biological) are used to recover cellulose, hemicellulose and lignin from biomass and begin the transformation into biofuels. This paper reviews the thermochemical (e.g. pyrolysis, gasification and liquefaction), hydrothermal (e.g. subcritical and supercritical water gasification and hydrothermal liquefaction), and biological (e.g. fermentation) conversion pathways that are used to further transform biomass feedstocks into fuel products.

Results:

Through several thermochemical and biological conversion technologies, lignocellulosic biomass and other organic residues can produce biofuels such as bio-oils, biochar, syngas, biohydrogen, bioethanol and biobutanol, all of which have the potential to replace hydrocarbon-based fossil fuels.

Conclusions:

This review paper describes the conversion technologies used in the transformation of biomass into viable biofuels. Biofuels produced from lignocellulosic biomass and organic wastes are a promising potential clean energy source with the potential to be carbon-neutral or even carbonnegative.

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.