This invention provides processes and apparatus to convert biomass, including wood and agricultural residues, into low-ash biomass pellets for combustion, alone or in combination with another solid fuel. Some embodiments provide processes for producing hemicellulosic sugars and low-ash biomass from cellulosic biomass, comprising providing an aqueous extraction solution with acetic acid; extracting the feedstock to produce an extract liquor containing soluble ash, hemicellulosic oligomers, acetic acid, dissolved lignin, and cellulose-rich solids; dewatering and drying the cellulose-rich, lignin-rich solids to produce a low-ash biomass; hydrolyzing the hemicellulosic oligomers to produce fermentable hemicellulosic sugars, wherein additional acetic acid is generated; removing a vapor stream comprising vaporized acetic acid from the extract; recycling the vapor or its condensate to provide some starting acetic acid for the extraction solution; and recovering fermentable hemicellulosic sugars. The disclosed processes can produce clean power from biomass. Co-products include fermentation products such as ethanol, fertilizers, and lignin.