The invention is directed to a process for preparing a mixture comprising CO and H2 by operating a partial oxidation process of a solid carbonaceous feed, which process comprises the steps of: (a) supplying the solid carbonaceous feed and an oxygen-containing stream to a burner, wherein a CO2 containing transport gas is used to transport the solid carbonaceous feed to the burner; (b) partially oxidizing the carbonaceous feed in the burner wherein a gaseous stream comprising CO and H2 is being discharged from said burner into a reaction zone, wherein the temperature in the reaction zone is from 1200 to 1800° C. and wherein said reaction zone is at least partly bounded by a wall, which wall comprises conduits in which steam is prepared by evaporation of water, resulting in a flow of steam being discharged from said reaction zone; (c) monitoring the conditions in the reaction zone by continually or periodically measuring the rate of the steam flow and using said flow rate as input to adjust the oxygen-to-coal (O/C) ratio in step (a).