Experimental study on supercritical water gasification of oily sludge using a continuous two-step method

Supercritical water gasification (SCWG) technology can convert oily sludge into hydrogen-rich gas. To achieve high gasification efficiency of oily sludge with a high oil concentration under mild conditions, a two-step method involving a desorption process and a catalytic gasification process using Raney-Ni catalyst was investigated. High oil removal efficiency (99.57%) and carbon gasification efficiency (93.87%) were achieved. The lowest waste-water total organic carbon, oil content, and carbon content in the solid residues were 4.88 ppm, 0.08% and 0.88%, respectively, using a gasification temperature of 600 degrees C, treatment concentration of 1.11 wt%, gasifi-cation time of 70.7 s, and the optimal desorption temperature of 390 degrees C. The main organic carbon component in the solid residues was cellulose, which is environmentally safe. As the treatment concentration increased, the two-step method outperformed the single-step method. The mechanism for the two-step SCWG of oily sludge was revealed. In the first step, supercritical water is used in the desorption unit to achieve a high oil removal effi-ciency with few liquid products generated. In the second step, the Raney-Ni catalyst promotes efficient gasifi-cation of high-concentration oil at a low temperature. This research provides valuable insights into the effective SCWG of oily sludge at a low temperature.