PHASE BEHAVIOR AND COMBUSTION OF HYDROCARBON-CONTAMINATED SLUDGE IN SUPERCRITICAL WATER AT PRESSURES UP TO 822 MPa AND TEMPERATURES UP TO 535°C

摘要

Phase behaviors of cellulose, naphthalene (NT), and benzo(a)pyrene (BaP) in subcritical and supercritical water were studied with a diamond anvil cell technique and optical microscopy at heating rates of 8-10°C/s. The homogeneous phases were obtained for cellulose at 329.5°Cand 345.1 MPa, for NT at 383.2°C and 419.7 MPa, and for BaP at 508.1°C and 770.2 MPa. Establishing the homogeneous conditions was important for the combustion study of NT- and BaP-contaminated cellulose-based sludge in supercritical water (SCW). A batch reactor (6 rnL volume) was used in the SCW combustion experiments. It was found that 99.2% carbon, 99.86% BaP, and 100% NT were converted within the SCW during 300 s reaction time under 450°C, 30.6 MPa, and 17.1% excess oxygen. Thus, even residues with high ash content (～20%) and stable polycyclic aromatic hydrocarbons (PAHs) could become almost completely oxidized to CO_2 and H_2O in this novel type of "incineration." The conversion rates increased at longer reaction times (up to 30 min.) and higher oxygen concentration (65.7%). During the SCW combustion, water, oxygen, sludge (mainly cellulose), and PAHs may become a single phase before their decomposition via pyrolysis or oxidation. A transformation pathway of the major components of the sludge during SCW combustion is proposed. The major trends in the suggested mechanism are (1) the rapid hydrolysis of cellulose to oligo-mers and glucose, (2) the dissolution of naphthalene and its oxidation to quinones, (3) the cleaving of benzo(a)pyrene and formation of acetylene and cycloalkenes with benzylic rings, and (4) the homogeneous oxidation of dissolved organic species to light hydrocarbons → acids → acetate, which is transformed to carbon dioxide and water.