Study on the detailed reaction pathway and catalytic mechanism of a Ni/ZrO₂ catalyst for supercritical water gasification of diesel oil

Diesel oil, as a petroleum product, can be used as a model of oilfield wastewater to be gasified in supercritical water (SCW). This paper is aimed to obtain the gasification characteristics, detailed reaction pathway of diesel oil and the catalytic mechanism of a Ni/ZrO₂ catalyst in SCW, providing some guidance for supercritical water gasification (SCWG) of oilfield wastewater. The SCWG of 6.8 wt% diesel experiments were carried out in a quartz reactor system with different temperatures (460, 500, 540 °C), reaction time range of 0–90 min. The carbon gasification efficiency achieved 97.5% at 500 °C with the Ni/ZrO₂ catalyst, which was over 3 times higher than 27.6% under non-catalytic condition. The liquid intermediate was analyzed by gas chromatography-mass spectrometry, and naphthalene was the most difficulty intermediate to be gasified in diesel SCWG. A kinetic model was established to describe quantitatively the variation of gas yields with experimental conditions. The production and consumption of gases by SCWG of diesel oil under non-catalytic and catalytic conditions were analyzed in detail, and the mechanism of the Ni/ZrO₂ catalyst was proposed. The Ni/ZrO₂ catalyst effectively enhanced the steam reforming and pyrolysis reactions as well as inhibited the polymerization and aromatization reaction.