Process and plant for producing synthesis gas with variable composition

A process for the continuous conversion of a hydrocarbonaceous feed gas into a synthesis gas comprising carbon monoxide and hydrogen, comprising the following process steps carried out one after the other:

• a) providing a feed gas mixture which comprises hydrocarbonaceous gas and steam, as well as a heating gas mixture which comprises a fuel gas and an oxygen-rich gas;
• b) splitting the feed gas mixture under reforming conditions by catalytic steam reformation in a tubular reformer fired by burners into a raw synthesis gas containing hydrogen, carbon monoxide, carbon dioxide and methane, wherein heat necessary for splitting, which is indirectly transmitted to the feed gas mixture, is produced by combustion of the heating gas mixture, wherein in normal operation of the process a base H2/CO ratio (VB) is obtained,
• c) separating the carbon dioxide from the raw synthesis gas;
• d) fractionating the raw synthesis gas into a hydrogen-rich gas, a carbon monoxide-rich gas and a methane-rich gas, wherein the hydrogen-rich gas optionally is supplied to a further cleaning step and otherwise is discharged from the process as H2 product gas, and the carbon monoxide-rich gas is supplied to the further use outside the process as CO product gas;
• e) optionally cleaning of the hydrogen-rich gas by separating gaseous impurities and admixing the same to the heating gas mixture and discharging the cleaned hydrogen-rich gas as H2 product gas for the further use outside the process;
• f) admixing at least a part of the methane-rich gas obtained according to step d) to the feed gas mixture;
• and
• g) controlling the mass flows of first gases to be admixed with the feed gas mixture and second gases to be admixed with the heating gas mixture,
• wherein the process is carried out for the purpose of setting a target value V1 of the H2/CO ratio according to a first mode of operation or a second mode of operation,
• wherein the first mode of operation is selected when the target value V1 is less than the base ratio VB,
• wherein the second mode of operation is selected when the target value V1 is greater than the base ratio VB, wherein the step of controlling the mass flows comprises increasing the flow rate of the first gases admixed with the feed gas mixture during the first mode of operation, and wherein the step of controlling the mass flows comprises increasing the flow rate of the second gases admixed with the heating gas mixture during the second mode of operation,
• wherein the first gases to be admixed with the feed gas mixture comprise at least a part of the H2 product gas and/or a fraction of the hydrogen-rich gas,
• wherein the second gases to be admixed with the heating gas mixture comprise at least a part of the CO product gas and/or a fraction of the carbon monoxide-rich gas.