Application of Subcritical Water to Dechlorinate Polyvinyl Chloride Electric Wires

Hydrothermal carbonisation of polyvinyl chloride in ethanol-water/water system for solid fuels: Dechlorination, characteristics analysis of hydrochar, and reaction path

Polyvinyl chloride (PVC) waste plastic is a typical solid waste. In this paper, the dechlorination and carbonization behavior of PVC in ethanol-water/water system under different process parameters (temperature, residence time, solid-liquid ratio) was studied, and hydrothermal carbon was characterized by SEM, elemental analysis, TG-DTG, XPS, Py-GC/MS. The results show that temperature is the key to the hydrothermal dechlorination of PVC, and the dechlorination efficiency of PVC is the highest by parameter optimization (220°C-90 min-10% S/D-80% E/D), which can reach 96.33 %. With the removal of Cl, the surface of the PVC matrix changed from full and smooth flocculent to honeycomb with uniform pore size distribution. Thermogravimetric analysis shows that the combustion of hydrochar can be divided into three stages: HCl precipitation and volatile combustion, semi-coke and coke combustion, and fixed carbon combustion. The combustion parameters and kinetic parameters of hydrochar were measured, and it was found that the hydrothermal carbonization of PVC at higher temperatures and ethanol-water ratio could improve the combustion performance of hydrochar. The highest calorific value can reach 36.68 MJ/mol. Py-GC/MS analyzed the distribution of the pyrolysis products, and alkylbenzene and aliphatic were the main products of pyrolysis. The structural analysis of hydrochar showed that C-C and CC accounted for the largest proportion, accompanied by a small amount of C-O and CO and trace C-Cl. The possible reaction mechanism of the hydrothermal carbonization of PVC was analyzed based on the distribution of functional groups and compound composition. This work provides an effective and sustainable method for the recycling of refractory chlorinated plastics.

Insights into the evolution of chemical structure and pyrolysis reactivity of PVC-derived hydrochar during hydrothermal carbonization

Hydrothermal carbonization (HTC) is emerging as an effective technology to convert PVC into highly valuable materials via the removal of chlorine. This means that an in-depth understanding of HTC requires the hydrochar structure, thermal degradation behavior, and relationship between structure and thermal reactivity to be understood. In this work, two typical PVC waste materials were selected for HTC experiments at different temperatures. The structure of the hydrochar was characterized in detail by compositional analysis, FTIR spectroscopy, and 13C NMR analysis. Furthermore, the thermal degradation behavior of the hydrochar was analyzed. The changes after thermal degradation were used to establish a correlation with pyrolysis reactivity. The results showed that the C content and chemical structure of the hydrochar approached that of bituminous coal with increasing HTC temperature. Compared with the untreated PVC feedstock, the hydrochar exhibited higher levels of oxygen-containing functional groups on its surface, and its carbon skeleton structure changed from polymeric straight chains to short-chain paraffins, cycloalkanes, and aromatics. A negative correlation was observed between the CPI value of the hydrochar derived from SPVC and the HTC temperature. The structural evolution path of the hydrochar was altered by additives, which improved its thermal reactivity. These findings are expected to play a significant role in bridging the gap from the creation of a theoretical potential energy source to the development of a sustainable alternative renewable fuel.

Hydrothermal treatment of polyvinyl chloride: Reactors, dechlorination chemistry, application, and challenges

Polyvinyl chloride (PVC) plastic wastes can bring a series of problems during pyrolysis or incineration such as the emission of dioxins, corrosion, slagging in the reactors, etc. Hydrothermal treatment of PVC plastics has been intensively studied as it can efficiently remove chlorine from PVC plastics under relatively mild reaction conditions (220-300 °C) to provide value-added products. Meanwhile, the research progress, knowledge gaps, and challenges in this field have not been well addressed yet. This paper gives a comprehensive review of hydrothermal dechlorination of PVC plastics regarding reactors, process variables and fundamentals, possible applications, and challenges. The main pathways of hydrothermal dechlorination of PVC plastics are elimination and -OH nucleophilic substitution. Catalytic hydrothermal and co-hydrothermal optimize the chemical reactions and transportation, boosting the dechlorination of PVC plastics. Hydrochar derived from PVC plastics, on the one hand, is coalified close to sub-bituminous and bituminous coal and can be used as low-chlorine solid fuel. On the other hand, it is also a porous material with aromatic structure and oxygen-containing functional groups, with good potential as adsorbent or energy storage materials. Further studies are expected to focus on waste liquid treatment, revealing the energy and economic balance, reducing the dechlorination temperature and pressure, expanding the application of products, etc. for promoting the implementation of the hydrothermal treatment of PVC plastic wastes.

A Win-Win Combination to Inhibit Persistent Organic Pollutant Formation via the Co-Incineration of Polyvinyl Chloride E-Waste and Sewage Sludge

Persistent organic pollutant inhibition in the combustion process of polyvinyl chloride (PVC) by prior addition of an inhibitor is currently being studied, reducing the emission of pollutants, and thus reducing the large amount of waste PVC destined for landfill. In this work, the use of sewage sludge (SS) as an alternative to chemical inhibitors to improve the quality emissions of the incineration of polyvinyl chloride waste (PVC e-waste) was studied and optimized. Different combustion runs were carried out at 850 °C in a laboratory tubular reactor, varying both the molar ratio R_i (0.25, 0.50, 0.75) between inhibitors (N + S) and chlorine (Cl) and the oxygen ratio λ (0.15, 0.50) between actual oxygen and stoichiometric oxygen. The emissions of several semivolatile compounds families such as polycyclic aromatic hydrocarbons (PAHs), polychlorobenzenes (ClBzs), and polychlorophenols (ClPhs), with special interest in the emissions of the most toxic compounds, i.e., polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/Fs) and dioxin-like polychlorinated biphenyls (dl-PCBs), were analyzed. A notable decrease in PCDD/F and dl-PCB formation was achieved in most of the experiments, especially for those runs performed under an oxygen-rich atmosphere (λ = 0.50), where the addition of sludge was beneficial with inhibition ratios Ri ≥ 0.25. An inhibition ratio of 0.75 showed the best results with almost a 100% reduction in PCDD/F formation and a 95% reduction in dl-PCB formation.

Chemical reactions of organic compounds in supercritical water gasification and oxidation

Supercritical water is a benign reaction medium to convert organic matters through supercritical water gasification and supercritical water oxidation into flammable gaseous and harmless substances, respectively. This work systematically summarizes main chemical reactions of some typical organic compounds in supercritical water with or without oxidant for the first time. These compounds include hydrocarbons, proteins, cellulose, lignins, phenols, alcohols, aldehydes, ketones, organic acids, and some N-, Cl-, Br-, F-, S- and P-containing organic matters. Their main conversion pathways, reaction processes, intermediate products, final products and influence factors are analyzed deeply. This information helps to understand and predict corresponding reaction mechanisms and to better achieve objective products in supercritical water gasification and supercritical water oxidation.

Co-hydrothermal carbonization of food waste with yard waste for solid biofuel production: Hydrochar characterization and its pelletization

In this study, Co-HTC of food waste with yard waste was conducted for biofuel pellets production, and also to understand any possible synergy between two feedstock types. The calorific value of blended raw feedstock was 13.5 MJ/kg which increased to 27.6 MJ/kg after Co-HTC at 220 °C for 1 h. Energy yield and fuel ratio calculated was 45% and 0.65 respectively. Hydrochar produced demonstrated a stable combustion profile as compared to reactive combustion profile for raw samples. The blend of food and yard waste hydrochar was easily pelletized, and its pellets showed improvement in mechanical properties as compared to pellets made from mono-substrate((food waste) hydrochar. Pellets produced from the blend of food and yard waste hydrochar showed higher energy (46.4 MJ/m³) and mass density (1679 kg/m³) as compare to the pellet produced from food waste hydrochar alone. Tensile strength obtained for the blended hydrochar pellet was 2.64 MPa while same for the pellets produced from food waste hydrochar alone was 1.30 MPa. In addition to improving hydrophobicity, soften lignin from yard waste also helped in binding the food waste hydrochar particles together within the pellets matrix during heated pelletization. The results presented in the study indicated that in the presence of all favorable conditions, there is a potential that approximately 11% of the global coal consumption could be replaced by the combustion of hydrochar produced from food and yard waste globally.

Dechlorination of polyvinyl chloride electric wires by hydrothermal treatment using K2CO3 in subcritical water

Polyvinyl chloride (PVC) waste generation has significantly increased in recent years and their disposal is considered a major environmental concern. Removal techniques of chlorine from PVC waste are being studied to minimize a negative environmental impact. In this work, the use of K₂CO₃ as an alkaline additive to improve the dechlorination efficiency (DE) in the hydrothermal degradation of PVC wires was studied. Different experiments were carried out varying both temperature (175, 200, 225, 235 and 250 °C) and K₂CO₃ concentration (0.025, 0.050 and 0.125 M), using a solid/liquid ratio of 1:5 in order to determine the evolution of the dechlorination efficiency with time. About 4.66, 21.1, 24.4, 45.7 and 92.6 wt% of chlorine in PVC wire was removed during hydrothermal dechlorination (HTD) with an additive/chlorine ratio of 1:25 (K₂CO₃ solution of 0.050 M) at 175, 200, 225, 235 and 250 °C, respectively. Optimal additive/chlorine ratio decreased to 1:50 (K₂CO₃ solution of 0.025 M) at 250 °C, obtaining a dechlorination degree of 99.1% after 4 h without the need of metallic catalysts. Concerning the solid phase behavior during dechlorination, a linear correlation between the DE reached and the weight loss of PVC was found.

Co-hydrothermal carbonization of lignocellulosic biomass and waste polyvinyl chloride for high-quality solid fuel production: Hydrochar properties and its combustion and pyrolysis behaviors

The rigid polyvinyl chloride (PVC) and pinewood sawdust (PS) were selected for co-hydrothermal carbonization (Co-HTC) process. The effects of hydrothermal reaction temperatures and the mixing ratios of raw materials were fully investigated. The results showed that hydrothermal reaction temperature increased could significantly promote the dechlorination efficiency at the mixing ratio of 1:1, which was 92.98% at 280 °C. The experimental HHV were higher than theoretical value and increased by 4.04%, 8.21% and 2.81% at the mixing ratios of 3:1, 1:1 and 1:3. The combustion behavior and the thermodynamic parameters of hydrochar were determined, and the activation energy tended to decrease. The Py-GC/MS analysis showed the changes of the distribution for the pyrolysis product. Aliphatic and aliphatic cyclic hydrocarbons were the main products of hydrochar pyrolysis, and the yield could be promoted by Co-HTC process. According to the FTIR spectrum, elimination and substitution were the primary mechanisms of dechlorination.