Decontamination and gasification of aqueous organic waste by submerged thermal plasma

摘要

No suitable disposition routes are currently available for some wasteforms with high moisture content, such as sewage sludge, pharmaceutical effluents, or agroprocess waste arising from wine or olive oil production. The waste is spread or dumped on fields with or without precautions, although its carbon content can be reused for animal feeding, composting, burning after drying, etc. Other methods such as gasification producing syngas (H_2 + CO) are of considerable interest in the chemical and energy industries because syngas does not generate any more greenhouse gases than natural decomposition. However, the waste cannot be gasified without drying.This paper describes a novel process using a nontransferred arc plasma torch operating directly inside the aqueous organic waste solution. Immersion allows a high quench rate (about 10~7 K·s~(-1)), preventing the formation of dioxins and Polycyclic Aromatic Hydrocarbons (PAH). The first step was to treat aqueous solutions contaminated by dye molecules (methylene blue) and chlorinated molecules (chlorophenol). If these molecules in aqueous solution can be decomposed by UV and oxidizing species (OH°, O°, etc.) produced by submerged plasma, the process could also be used to clean solutions of living species (bacteria, viruses). Treatment with argon and oxygen plasma resulted in the complete mineralization of pollutant species. Nevertheless, although the results show that the process is relatively efficient, no energy can be produced with complete mineralization. The process described here therefore partially oxidizes most concentrated aqueous organic solutions such as sucrose to produce syngas (a mixture of H_2 and CO). Preliminary results identified the main parameters and areas for optimization. The underwater plasma jet is already capable of gasifying several kilograms of sucrose in aqueous solution per hour. Analysis of the degradation products revealed no production of PAH.