## 2 Mineral Acid Fractionation
### Industrial Applications  
Dilute sulfuric acid fractionation under fairly mild conditions received the main focus from manufacturers and research institutes. For instance, the National Renewable Energy Laboratory (NREL), from the U.S. department of Energy, established an exhaustive public report with the technical and economic feasibility of such fractionation [19]. The process described uses co-current dilute acid fractionation of lignocellulosic biomass (corn stover) to liberate the hemicelluloses, followed by enzymatic hydrolysis (saccharification) of the remaining cellulose, followed by fermentation of the resulting glucose and xylose to ethanol. Overall, the total acid loading was 22.1 mg/g dry biomass, and the effective sulfuric acid concentration in the fractionation reactor was estimated at 0.3-0.4% _w/v_, which may allow for the use of lower cost metallurgies in the reaction zone. All acetate groups bound to the hemicelluloses were released under the form of acetic acid; 5% of the xylose was converted to furfural; and 5% of the lignin was solubilized (Table 2). Industrial development of the concentrated acid fractionation is at its beginnings. Some companies such as Biosulfurol Energy attempted to commercialize the concentrated acid process for the fractionation of lignocellulosic biomass in order to produce ethanol [63], but eventually failed (the company was dissolved in 2016). The market appears to be led by three companies: BlueFire Renewables, Virdia, and Renmatix [64]. The companies claim to be economically competitive with the production of sugars from traditional agricultural sources such as sugarcane. For instance, the process from Virdia claims to produce monomeric sugars at 0.25 $/kg from lignocellulosic wastes compared to 0.45 $/kg from sugarcane [64]. Arkenol gave detailed conditions in two patents regarding the fractionation with concentrated acid followed by dilute acid hydrolysis. Sulfuric acid 70-77% (_w/w_) should be added to the biomass in order to achieve a ratio of acid to cellulose and hemicelluloses of at least 1.25:1 (_w/w_) [65; 66]. After the decrystallization or solubilization stage, if the concentrated acid fractionation did not lead to a 100% yields of monomeric sugars, the concentrated acid can be diluted, to a concentration of about 20-30% for a second hydrolysis to completely convert the solubilized cellulose and hemicelluloses into monomeric sugars [66]. BlueFire Renewable's process is based on the Arkenol patent [66]; their plant, in operation since 2002 in Izumi, Japan, was already producing over 80,000 L of ethanol 95% in 2004 ([[https://bfreinc.com/production-plant/](https://bfreinc.com/production-plant/)]([https://bfreinc.com/production-plant/](https://bfreinc.com/production-plant/)), accessed on 25 November 2017). Another patent gave similar hydrolysis conditions and provided detailed solubilization yields of polymeric sugars and conversion yields into their monomeric forms depending on the conditions (acid concentration, S:L ratio, temperature, and experiment duration) used during the concentrated acid hydrolysis, then the dilute acid hydrolysis [20]. Similar processes have been used in other studies; for instance, [PERSON] et al. (2011) produced an acid hydrolysate from bamboo with successive concentrated acid, then dilute acid hydrolysis and obtained a monomeric sugar yield of 81.6% [67]. The combination of concentrated acid hydrolysis and diluted acid hydrolysis for the saccharification of cellulose and hemicelluloses has been optimized and proven to be so efficient that it became a standard method for the characterization of lignocellulosic biomass carbohydrates and lignin, developed by the National Renewable Energy Laboratory, Colorado, USA [23]. A first hydrolysis with concentrated sulfuric acid (72% _w/w_) and an S:L ratio of 1:10 (_w/v_) at 30 \({}^{\circ}\)C for 1 h, followed by a dilution to reach 4% (_w/w_) sulfuric acid to run a second hydrolysis at 121 \({}^{\circ}\)C for 1 h, led to a complete saccharification of the cellulose and hemicelluloses.