Solid fuel production from co-hydrothermal carbonization of polyvinyl chloride and corncob: Higher dechlorination efficiency and process water recycling

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.

Catalytic hydrothermal carbonization of wet organic solid waste: A review

Hydrothermal carbonization has gained attention in converting wet organic solid waste into hydrochar with many applications such as solid fuel, energy storage material precursor, fertilizer or soil conditioner. Recently, various catalysts such as organic and inorganic catalysts are employed to guide the properties of the hydrochar. This review presents a summarize and a critical discussion on types of catalysts, process parameters and catalytic mechanisms. The catalytic impact of carboxylic acids is related to their acidity level and the number of carboxylic groups. The catalysis level with strong mineral acids is likely related to the number of hydronium ions liberated from their hydrolysis. The impact of inorganic salts is determined by the Lewis acidity of the cation. The metallic ions in metallic salts may incorporate into the hydrochar and increase the ash of the hydrochar. The selection of catalysts for various applications of hydrochars and the environmental and the techno-economic aspects of the process are also presented. Although some catalysts might enhance the characteristics of hydrochar for various applications, these catalysts may also result in considerable carbon loss, particularly in the case of organic acid catalysts, which may potentially ruin the overall advantage of the process. Overall, depending on the expected application of the hydrochar, the type of catalyst and the amount of catalyst loading requires careful consideration. Some recommendations are made for future investigations to improve laboratory-scale process comprehension and understanding of pathways as well as to encourage widespread industrial adoption.

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.

Coupled microwave hydrothermal dechlorination and geopolymer preparation for the solidification/stabilization of heavy metals and chlorine in municipal solid waste incineration fly ash

To improve the degradation efficiency of persistent organic pollutants (POPs) in municipal solid waste incineration fly ash (MSWIFA), as well as to overcome the difficulties of subsequent hydrothermal liquid and hydrothermal slag treatment, a two-step treatment strategy of microwave hydrothermal degradation coupled with geopolymer immobilization was proposed. Results showed that the optimal process parameters for microwave hydrothermal dechlorination were a temperature of 220 °C, a time of 1 h, and NaOH addition of 10 wt%. Microwaves accelerated the OH^- mediated hydrolysis reactions and promoted the breaking of CCl bonds, leading to dechlorination. The compressive strength of the 20 % MSWIFA-based geopolymers reached 75.79 MPa, and the immobilization rate of the heavy metals (HMs) and Cl^- surpassed 90 %. Alkaline environment provided by microwave hydrothermal promoted the formation of Ca(OH)₂, which subsequently formed Friedel's salt (3CaO•Al₂O₃•CaCl₂•10H₂O) with Cl^- in the geopolymer. The charge density difference and density of states (DOS) of Friedel's salt were analyzed by first-principles calculations, confirming that the existence of strong interactions between Ca-s, Al-p, O-p, and Cl-p states was the chemical mechanism of Cl^- immobilization. The Friedel's salt and HMs were encapsulated by geopolymers with dense silica-alumina tetrahedral frameworks, achieving the solidification/stabilization (S/S) of HMs and Cl^-. This work provided a new approach for the environmentally sound and resourceful treatment of MSWIFA.