NOVELTY - System (1) for producing synthesis gas (20, 40) comprises: a first gasification reactor (2), a second gasification reactor (3), a source (4) of an oxygen containing stream (50), and a distributor (25) that is arranged to selectively connect the source to the first or second gasification reactor. Each reactor contains inlet (7, 11) for oxygen containing stream, inlet (6, 9) for carbonaceous stream, and outlet (8, 12).    USE - For producing synthesis gas containing carbon monoxide and hydrogen; for preparing hydrogen from a source of petroleum coke; and for generating power from source of petroleum coke (claimed).    ADVANTAGE - The system produces synthesis gas while ensuring a very high availability of the synthesis gas, even if the gasification reactor intended for the particulate carbonaceous stream is out of order. Combination of the gasification reactor intended for particulate carbonaceous stream and different type gasification reactor is more economic than if two gasification reactors intended for particulate carbonaceous stream would be used. The maximum capacity of air separation unit is less than 80 (preferably lower than 65%). In case the first gasification fails, the second gasification can take over the preparation of synthesis gas, thereby making use of the oxygen manufacturing capacity, which is at that time not used by first gasification reactor. As a result, the availability of synthesis gas is ensured, even if the first gasification reactor or one of the first gasification reactors is out of order or on hot standby, while minimizing the required capacity of the oxygen manufacturing unit, suitably the air separation unit. By switching between the two reactors, if the reactor is to be shut down, the distributor gradually decreases the oxygen stream to reactor, at the same time increasing the oxygen stream to another reactor. As a result, the distributor feeds oxygen to both reactors at the same time for a certain period.    DETAILED DESCRIPTION - System (1) for producing synthesis gas (20, 40) comprises: a first gasification reactor (2), a second gasification reactor (3), a source (4) of an oxygen containing stream (50), and a distributor (25) that is arranged to selectively connect the source to the first or second gasification reactor. The first reactor has an inlet (7) for a first oxygen containing stream (60), an inlet (6) for a first particulate or liquid carbonaceous stream (10), an outlet (8) for raw synthesis gas (20) produced in the first gasification reactor. The second reactor has an inlet (11) for a second oxygen containing stream (70), an inlet (9) for a second carbonaceous stream (e.g. a gaseous and liquid stream (30)), and an outlet (12) for raw synthesis gas (40) produced in the second reactor. The distributor fluidly connects the source to the inlet for the first oxygen-containing stream of the first gasification reactor, and to the inlet for the second oxygen-containing stream of the second gasification reactor.    INDEPENDENT CLAIMS are included for the following:    (1) producing synthesis gas using oxygen containing stream involving injecting a first and second particulate or liquid carbonaceous stream; at least partially oxidizing first and second carbonaceous stream; removing the first and second raw synthesis gas from the first and second reactors, where the first and second gasification reactors are used alternately;    (2) use of a spare gasification reactor in a process to generate power from a source of petroleum coke in parallel operated gasification reactors, where spare reactor is capable of preparing a spare synthesis gas mixture involving partial oxidation of the petroleum coke and a certain second volume of oxygen per hour as obtained from air separation unit to yield synthesis gas mixture, and using the synthesis gas and/or the spare synthesis mixture to generate power, where the maximum capacity of the air separation unit is less than the sum of the first and second oxygen volumes per hour; and    (3) use of spare gasification reactor in a process to prepare hydrogen from a source of petroleum coke involving partial oxidation of the petroleum coke and a certain second volume of oxygen per hour, subjecting the synthesis and/or the spare synthesis gas mixture to a water gas shift step, subjecting the shifted gas to gas separation step (swing absorbing process) to obtain a hydrogen rich mixture.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of the system for producing synthesis gas.    system (1)    gasification reactors (2, 3)    source of oxygen containing stream (4)    inlet for first particulate or liquid carbonaceous stream (6)    inlet for first oxygen containing stream (7)    outlet for raw synthesis gas (8, 12)    inlet for second carbonaceous stream (9)    particulate or liquid carbonaceous stream (10)    synthesis gas (20, 40)    distributor (25)    oxygen containing stream (50)    oxygen containing stream. (60, 70)