NOVELTY - Producing fermentable hemicellulosic sugars and low-ash biomass from cellulosic biomass involves providing an extraction solution comprising steam and/or hot water, and acetic acid; treating feedstock comprising cellulosic biomass with extraction solution under effective extraction conditions to produce an extract liquor containing soluble ash, hemicellulosic oligomers, acetic acid, dissolved lignin and cellulose-rich solids; d) separating at least a portion of cellulose-rich solids from extract liquor; and e) hydrolyzing hemicellulosic oligomers contained in extract liquor.    USE - For producing fermentable hemicellulosic sugars and low-ash biomass from cellulosic biomass (claimed).    ADVANTAGE - The process allows preparing low-ash biomass for combustion, while also recovering fermentable sugars from the biomass. The process can prepare biomass, including wood and agricultural residues, into clean, low-ash biomass for improved combustion, with or without first pelletizing the biomass. The low-ash biomass is capable of being fired alone or in combination with another solid fuel. The process also has good potential to recover various co-products, such as fermentable sugars, fertilizers, and lignin. The pretreatment of biomass can be optimized to remove water-soluble ash from the biomass, and therefore reduce volatilized mineral ash in the flue gas when the biomass is combusted. The acids added before or during biomass pretreatment can increase the extent of mineral release and/or increase the mineral solubility in solution, so that high amounts of ash are removed. The low-ash biomass has lower inorganic emissions potential compared to the original cellulosic biomass. Also, the low-ash biomass generally has higher energy density compared to a process that does not extract hemicellulosic sugars from the feedstock prior to combustion.    DETAILED DESCRIPTION - Producing fermentable hemicellulosic sugars and low-ash biomass from cellulosic biomass involves a) providing a feedstock comprising cellulosic biomass; b) providing an extraction solution comprising steam and/or hot water, and starting acetic acid; c) treating feedstock with extraction solution under effective extraction conditions to produce an extract liquor containing soluble ash, hemicellulosic oligomers, acetic acid, dissolved lignin, and cellulose-rich solids; d) separating at least a portion of cellulose-rich solids from extract liquor, to produce dewatered solids containing cellulose and lignin; e) hydrolyzing hemicellulosic oligomers contained in extract liquor, under effective hydrolysis conditions, to produce fermentable hemicellulosic sugars, where hydrolysis conditions release acetyl groups to generate additional acetic acid; f) removing a vapor stream comprising water and vaporized acetic acid from extract liquor in at least one evaporation stage at a pH of less than or equal to 4.8, to produce a concentrated extract liquor comprising fermentable hemicellulosic sugars; g) recycling at least a portion of vapor stream from step (f), or its condensed form, back to step (b) to provide at least some of starting acetic acid for extraction solution and to provide heat for step (c); (h) recovering fermentable hemicellulosic sugars; and (i) drying dewatered solids to produce low-ash biomass.    DESCRIPTION OF DRAWING(S) - The figure shows a simplified block-flow diagram of the process of producing fermentable hemicellulosic sugars and low-ash biomass from cellulosic biomass.