In the gasification of an aquatic biomass with He/CO2/O-2, the effects of the concentration of CO2 and O-2 in the gasifying agent and the feeding rate on the gasification behavior were investigated using a downdraft fixed-bed gasifier at 900 degrees C. Using CO2/O-2 as the gasifying agent led to an increase in the conversion to gas and the syngas (CO + H-2) content because the gasification of char with CO2 (C + CO2 -> 2CO) and the decomposition of tarry compounds were promoted. Increasing CO2 content led to the increase in the conversion to gas and CO content and the decrease in the H-2 content. With increasing O2 content, contents of CO and H-2 increased while the conversions to gas remained almost constant. Especially with CO2/O-2 = 45/55 vol.%, the conversion to gas was 94.0 C-mol% and the syngas content exhibited a maximum value of 69.7 vol.%. As the feeding rate was decreased, contents of CO and H-2 decreased while the conversion to gas remained almost constant. The nitrogen atoms in the feedstock were mainly converted to form N-2. H2S and COS were the main sulfurous gases. The sulfur content in the char was much higher than that in the feedstock. (C) 2013 Elsevier B.V. All rights reserved.