NOVELTY - Preparing mixture of hydrogen and carbon monoxide (i.e. synthesis gas) involves partial oxidation of a hydrocarbon feed using a multi-orifice burner provided with an arrangement of separate passages, where the hydrocarbon flows through one passage of the burner, an oxidizer gas flows through a separate passage of the burner and the passage for hydrocarbon feed and oxidizer gas are separated by a passage through which a moderator gas flows, such that its exit velocity is greater than the exit velocity of the oxidizer gas.    USE - For preparing mixture of hydrogen and carbon monoxide (i.e. synthesis gas) from a hydrocarbon feed stock (claimed) e.g. a liquid, an emulsion or a pumpable slurry comprising a carbonaceous component e.g. butanes, pentanes, hexanes, natural gasolines, kerosenes, gas oils, naphthas, diesel fuels, crude oils, residual. The synthesis gas is used as a feedstock for the synthesis of methanol, ammonia or hydrocarbons via the Fischer-Tropsch process, yielding gaseous hydrocarbons and liquid hydrocarbons such as gasoline, middle distillates, lube oils and waxes.    ADVANTAGE - The method avoids burner damage. The process results in substantial improved lifetime, when compared to burners operating under the conditions of the state of the art process. Furthermore less temperature fluctuations are observed in the reactor when performing the process. The use of a moderator gas having a high velocity as a separate medium between oxidizer gas and hydrocarbon feed results in more stable and less damaging operation of the burner. The moderator gas also breaks up the hydrocarbon feed and acts as a moderator such that reactions in the recirculation zone at the burner tips are avoided. The result is that the hydrocarbon droplets only come in contact with the oxidizer gas at some distance from the burner surface. This results in less burner damage, e.g. burner tip retraction. The process is more flexible in case the properties of the hydrocarbon feed change during operation or when the operator of the process intends to influence the combustion velocity for e.g. if one desires a better break-up of the hydrocarbon feeding a simple increase in moderator gas velocity would suffice. The presence of some moderator gas in the moderator-pure oxygen mixture is advantageous because it allows the operator of the process to increase the velocity of the moderator in an existing burner by reducing the content of moderator gas in the moderator-pure oxygen mixture and increasing the amount of moderator gas as provided to the passages. Increasing the flow of moderator gas is also advantageous when the viscosity of the hydrocarbon feed increases and a higher velocity is required to achieve break-up of the viscous feed. The presence of some moderator gas in the moderator-pure-oxygen mixture is also advantageous because it allows the operator of the process to advantageously decrease the velocity of the moderator in an existing burner by redirecting part of the moderator gas. Such a decrease in velocity of the moderator gas results for the same feed in larger hydrocarbon droplets, which in turn results in larger soot particles or more soot particles in the resultant mixture of carbon monoxide and hydrogen. Larger soot particles can be more easily be separated from this gas.