Method for producing a purified synthesis gas from a biomass including a purification step upstream from the partial oxidation

A process for the production of a synthesis gas that is purified with a controlled H2/CO molar ratio from lignocellulosic biomass, comprising:

• A) severe pyrolysis at a temperature of between 500° C. and 900° C., producing a gas effluent that is sent to purification, and a solid residue that concentrates the majority of impurities, including alkaline compounds, and a portion of sulfur compounds,
• B) purification of the pyrolysis gas that is obtained from A that consists essentially of a removal of dust from the gas and a capture of sulfur compounds on adsorbent compounds based on ZnO or Co—Mo,
• C) partial oxidation carried out by contact of the feedstock that consists of purified gas obtained from B with oxygen at a purity level of more than 90%, at a reaction temperature of between 1000° C. and 1600° C., and under a pressure of between 2.5 MPa and 4 MPa, whereby said oxidation produces a synthesis gas that comprises carbon monoxide and hydrogen in an H2/CO molar ratio of between 0.5 and 1.5 based on the type of feedstock that is used,
• D) rapid cooling of the synthesis gas that is obtained from C at a temperature of between 300° C. and 500° C.,
• E) carbon-monoxide conversion on a portion of the flow of synthesis gas produced in C of between 30% and 70%, making it possible to attain —after mixing with a portion of the flow that has not undergone the carbon-monoxide conversion —an H2/CO molar ratio that corresponds to a value desired for subsequent use of the synthesis gas, further comprising subjecting the synthesis gas to FT synthesis on a catalyst comprising iron or cobalt, said synthesis gas having a CO/H2 ratio of 1.1-1.3 where iron is the catalyst and 2.0-2.3 where cobalt is the catalyst.