The TwoStage Viking gasifier from the Technical University of Denmark is being further developed for biofuel synthesis applications. In order to optimize the gasification process, it is suggested to apply an O-2-CO2 gas mixture as gasification medium, instead of air, to limit N-2-dilution of the product gas. It is found through a modeling study that the system is expected to achieve operating conditions in the range of air blown operation, when 21v% O-2 in CO2 is applied, and nearly identical parameters as the concentration is increased to 30v%. An experimental campaign with the 80kWth Viking pilot plant using 21v% oxygen confirms this, as operation temperatures are seen to slightly decrease the partial oxidation (PDX) temperature by 52-69 degrees C and grate temperature by 31-36 degrees C. Tests with 25v% oxygen were also carried out with slightly higher temperatures. Detailed gas analysis showed that N-2 had effectively been reduced to a few percent and that tar and sulphur levels were similar to the very high standards of the air-blown operation: only a few mg/Nm(3) of tar and <3 ppm sulphur were detected. The lone gas cleaning, a bag filter, was found to be virtually inactive for capturing these impurities. Hence, the gasifier had been successfully demonstrated with O-2-CO2 mixtures and is expected to be able to maintain its simple design, whilst enabling very high system efficiency. (C) 2018 Elsevier Ltd. All rights reserved.