A process for the pretreatment of
lignocellulosic biomass is disclosed. The process is intended for use in connection with
biomass to
ethanol processes and is directed in particular to an economical removal of inhibitory compounds generated in
biomass pretreatment, which are inhibitory to downstream
hydrolysis and
fermentation steps. The process includes the steps of heating the
lignocellulosic biomass with steam to a preselected temperature, at a preselected pressure and for a preselected time to hydrolyze and solubilize hemicelluloses in the
biomass; explosively decomposing the biomass into fibers; and extracting from the resulting reaction mixture a liquefied portion of the
lignocellulosic biomass before or after explosive
decomposition. The liquefied portion is extracted to remove compounds from the lignocellulosic biomass which are inhibitory to enzymatic
cellulose hydrolysis and
sugar fermentation to
ethanol. For improved efficiency and economy, the inhibitory compounds are not completely removed. Furthermore,
xylose has been found to be a good indicator compound for the
general level of inhibitory compounds in the reaction mixture and the extraction step is therefore controlled on the basis of the
xylose content in the reaction mixture. In particular, the extracting step is discontinued once a
dry matter (dm) content of
xylose, as
monomer or
oligomer, in the reaction mixture of 4% to 8% (w / w dm) is achieved. This most economically balances the practical need for inhibitory compound removal with the economical need to control and preferably minimize the costs of the overall
ethanol production process.