Supercritical water gasification (SCWG) is a new treatment of black liquor (BL) for both energy recovery and pollution management. To provide more energy for the pulp mill, it is proposed to use the pulping raw material as supplementary energy source because it is readily available, inexpensive and renewable. In this study, co-gasification of BL and wheat straw (WS) in supercritical water was investigated. The synergistic effect was observed in the co-gasification because the addition of wheat straw can make better use of the alkali in BL. The maximum improvement of the gasification by the synergistic effect was obtained with the mixing ratio of 1:1. The influences of the temperature (500-750 degrees C), reaction time (5-40 min), mixture concentration (5.0-19.1 wt%), mixing ratio (0-100%) and the wheat straw particle diameter (74-150 mu m) were studied. It was found that the increase of temperature and reaction time, and the decrease of concentration and wheat straw particle size favored the gasification by improving the hydrogen production and gasification efficiency. The highest carbon gasification efficiency of 97.87% was obtained at 750 degrees C. Meanwhile, the H-2 yield increased from 12.29 mol/kg at 500 degrees C to 46.02 mol/kg. This study can help to develop a distributed energy system based on SCWG of BL and raw biomass to supply energy for the pulp mill and surrounding communities. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.