## 5 Hydrothermal Pre-Treatment: Energy and Value-Added Products Recovery
### Bioethanol  
Bioethanol is a non-reactive and environment-friendly reactant to produce biodiesel. Bioethanol production through biomass fermentation makes it a sustainable energy resource, which can be a substitute to fossil fuels [81]. Use of bioethanol as a fuel in the transportation sector contributes to reduce GHG emissions thus helps in climate change mitigation. Being a liquid biofuel produced from the fermentation of corn, wheat, cane, beet and wood, bioethanol is mixed with gasoline with a gasoline: bioethanol ratio ranging from 90:10 to 15:8 [82]. [PERSON] et al. [83] optimised the hydrothermal pre-treatment conditions for bioethanol production at various temperature (130-220 \({}^{\circ}\)C), reaction times (15-180 min) and water-to-solid ratio of 15. Hemicellulose was totally solubilised and the lignin was only partly delignified. The solubilisation of the ligno-cellulosic biomass led to an increase in surface area and pore volume to nearly 2.5 times. [PERSON] et al. [84] studied the hydrothermal pre-treatment, where heating of biomass under pressurised steam of 20-50 bar and temperature of 160-270 \({}^{\circ}\)C for few minutes followed by pressure release to the atmosphere was carried out, which leads to the desegregation of lignocellulosic biomass. The results were: hydrolysis of hemicellulose, transformation of lignin owing to high temperatures, and increase in crystallinity of cellulose. [PERSON] and [PERSON] [85] studied the anaerobic digestion of hydrothermally pre-treated corn stover at 200 \({}^{\circ}\)C for 5 min with 2% sulphuric acid, which led to the four times enhanced enzymatic conversion of cellulose to glucose, and 90% higher yield of ethanol as compared to untreated raw materials. In another study, hydrothermal pre-treatment of wheat straw at 180 \({}^{\circ}\)C for 10 min with 0.9% sulphuric acid, gave a sugar yield of 85% of the total sugar present in the raw material.  
[PERSON] et al. [86] studied the hydrothermal pre-treatment of wheat straw for production of bioethanol and concluded that the optimum temperature and time for pre-treatment was 195 \({}^{\circ}\)C and 6-12 min. Under optimized conditions, 70% of hemicellulose and 93% cellulose were solubilized, where 89% of cellulose was converted to bioethanol. [PERSON] et al. [68] conducted the experiments on hydrothermal pre-treatment of switchgrass and corn-stover for the production of ethanol and carbon microspores. Upon pre-treatment at 190 \({}^{\circ}\)C for 20 min, more than 80% of glucan was digested. Adding 0.4-0.9% of potassium carbonate could allow the digestion to occur at lower temperature whereby pre-treatment efficiency was also enhanced. On increasing the temperature from 150 \({}^{\circ}\)C to 190 \({}^{\circ}\)C, the hydrolysis of hemicellulose to water-soluble products showed an increase from 30% to 77%. 40% lignin was solubilized, however, it remained constant at temperature ranges from 150 to 190 \({}^{\circ}\)C. Hence, bioethanol generated from the digestion of glucan, hydrolysis of hemicellulose and delignification of lignin, fulfils the aim of hydrothermal treatment of the lignocellulosic biomass.