Dr. Changqing Cao
Associate Professor, State Key Laboratory of Multiphase Flow in Power Engineering, Xi'an Jiaotong University, China.

Speech Title: Supercritical Water Gasification for Hydrogen Production—An Innovative Treatment Method of Black Liquor

Abstract: Black liquor is a kind of waste water generated from pulp mill, which possess great threat to the environment and human health. Supercritical water gasification (SCWG) is a new treatment of black liquor, which can reduce its pollution and produce hydrogen-rich gases. Compared with conventional treatment method (Tomlinson recovery boiler), SCWG has several advantages, such as no emission of SO₂ NO_x and fine particles, free of energy-intensive evaporation process and no safety and operating problems brought by alkali melting. Previously, we conducted a series of study on SCWG of black liquor and found that relatively high temperature was needed to obtain high gasification efficiency. For example, maximum carbon gasification efficiency of 94.10% for black liquor with a concentration of 9.5 wt% was achieved at 750^°C. To lower the gasification temperature and the construction cost of the reactors, the catalysts were suggested to be used in SCWG. In the present study, we studied the catalytic activity of the metal oxides on SCWG of black liquor because it is a kind of catalyst with high-temperature duration, low price and easy regeneration by oxidation. Firstly, we studied the catalytic effect of 14 kinds of metal oxides (ZnO, TiO₂ SnO₂, V₂O₅, WO₃, MoO₂, Fe₃O₄, Fe₂O₃, MnO₂, Cr₂O₃, CeO₂, CuO, ZrO₂, Co₂O₃) on SCWG of black liquor. Among them, V2O5, WO3 and Co2O3 showed the greatest effect on the gasification efficiency. Most metal oxides favoured H2 production, whose yield reached 21.67 and 21.03mol/kg when Co2O3 and ZnO was used as the catalyst respectively. The influence of residence time, temperature and catalyst dosage on the gasification were also studied with V2O5. The increase of these parameters improved the gasification performance. We also studied the valence state of metal oxides after gasification by X-ray photoelectron spectroscopy (XPS). It was found that the valence of the metal in the oxides was decreased after gasification, revealing that the metal oxides was reduced in gasification. Based on this result, we proposed the catalytic mechanism of metal oxides on SCWG of black liquor. The reduction of the oxides can provide oxidant and promote the decomposition and gasification of black liquor. And the reduced metal can also be served as the catalyst for the reaction involved in SCWG, such as water-gas shift reaction, to increase the H2 production. This study paves the way for the utilization of metal oxides as catalyst on the hydrogen production from SCWG of black liquor.

Acknowledgements: The financial support from the National Natural Science Foundation of China (No. 51606150) and Natural Science Basic Research Plan in Shaanxi Province of China (No. 2018JQ5172) was acknowledged.