Pretreatment of lignocellulosic biomass through removal of inhibitory compounds

Abstract

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.

1 . A process for pretreatment of lignocellulosic biomass, comprising 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 lignocellulosic biomass, explosively decomposing the lignocellulosic biomass by rapidly releasing the pressure to break down the lignocellulosic biomass into fibers and extracting from the resulting reaction mixture a liquefied portion of the lignocellulosic biomass before or after explosive decomposition, for removing compounds from the lignocellulosic biomass which are inhibitory to enzymatic cellulose hydrolysis and sugar fermentation to ethanol, wherein 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. 
     
     
         2 . The process of  claim 1 , wherein the xylose dm content is about 6%. 
     
     
         3 . The process of  claim 1 , wherein the liquid extraction step is carried out by separating fibrous solids from the liquid of the pretreated lignocellulosic biomass. 
     
     
         4 . The process of  claim 3 , wherein an eluent is used to increase the level of inhibitory extraction. 
     
     
         5 . The process of  claim 3 , wherein the eluent extraction step is carried out under a pressure of up to 350 psi. 
     
     
         6 . The process of  claim 5 , wherein the liquid extraction step is carried out with a sealed mechanical compression device. 
     
     
         7 . The process of  claim 3 , wherein the liquid extraction step is carried out under a pressure of up to 350 psi with a sealed mechanical compression device using an eluent to improve inhibitor removal. 
     
     
         8 . The process of  claim 4 , wherein the eluent extraction step is carried out after pretreatment with a screw press, a filter, a filter press, a belt press, a centrifuge or a drainer. 
     
     
         9 . The process of  claim 3 , wherein the liquid extraction step is carried out after pretreatment with a screw press, a filter, a filter press, a belt press, a centrifuge or a drainer with the addition of an eluent. 
     
     
         10 . The process of  claim 3 , wherein the liquid extraction step is carried during the pretreatment under pressure and after pretreatment after pressure reduction. 
     
     
         11 . The process of  claim 3 , wherein the liquid extraction step is carried out during the pretreatment under pressure and after pretreatment with an eluent. 
     
     
         12 . The process of  claim 1 , wherein the removal step is carried out for the removal of water soluble hydrolyzed hemicellulose and hemicellulose hydrolysis and degradation components and water soluble or suspended degradation products thereof. 
     
     
         13 . The process of  claim 12 , wherein the water soluble hydrolyzed hemicellulose and hemicellulose hydrolysis and degradation products include xylo-oligosaccharides, xylose, mannose-, galactose-, rhamnose- and arabinose-based oligomer and monomer sugars, acetic acid and formic acid. 
     
     
         14 . The process of  claim 13 , wherein other compounds inhibitory to downstream cellulose hydrolysis and fermentation processes are removed in the removing steps. 
     
     
         15 . The process of  claim 14  where the other compounds are fatty acids, sterols, esters, or ethers. 
     
     
         16 . The process of  claim 14 , wherein soluble xylose oligomers created in the hemicellulose hydrolysis during pretreatment are 30% to 90% of the hydrolyzed xylan in the pretreated biomass. 
     
     
         17 . The process of  claim 1 , wherein the lignocellulosic biomass is pretreated by auto-hydrolysis or dilute acid catalysis. 
     
     
         18 . The process of  claim 1 , wherein the removing is carried out with counter current washing using water as an eluent. 
     
     
         19 . The process of  claim 5 , wherein eluent origins are from recycle streams or recycled eluent water. 
     
     
         20 . The process of  claim 1 , wherein a combined xylose and xylo-oligosaccharide concentration in the pretreated biomass after the extracting/removing step is between 4% and 6% for minimizing an amount of cellulolytic enzymes required to hydrolyze the reactive cellulose obtained.