The invented process separates main components in lignocellulosic biomass, specifically hardwoods, softwoods into lignin and fractions of high purity sugars which are used for ethanol production. The invented process comprises of treatment stages at high temperature and high pressure with hydrochloric acid or sulfuric acid. Residual lignin and extractives in the cellulosic solid fraction are selectively removed by chemical treatments of sodium chlorite, anhydrous acetic acid, chlorine and chlorine dioxide to enhance the purity and biological conversion of cellulose to ethanol. The pre-hydrolysate generated from the acid treatment stage, containing xylose, arabinose, galactose, glucose and the purified cellulosic fraction are enzymatically hydrolyzed and fermented to produce ethanol. Significant amount of lignin from the process is recovered as a by-product.