**4.5 Chemical method**

Chemical method with thermal heating is commonly used for fractionation of biomass into lignin cellulose, and hemicellulose. The function of acids in low concentration break the outer surface and release bulk of extratives and hemicellulose of the biomass. Wheras, the concentrated acids at high thermal treatment disassociate the firm structure of the biomass. This treatment release both lignin and hemicellulose sugars of the biomass simultaniously. Sulfuric acid, hydrocholric acid, and nitric acid with (1–10% w/v) are mostly reported in the acid chemcial treatment method [34]. The alkalin solutions particularly of sodium hydroxide, potassium hydroxide, and calcium hydroxide specifical target linkages bonds in lignin and has very less penetration power for the cellulose of biomass. The alkaline solution with thermal heating pretreatment makes the biomass digestible for enzymes saccharification into sugars [35] allowing maximum entrance to hemicellulose and cellulose molecules [36]. It is hard to classify the most appropriate pre-treatment process for different kinds of biomasses [37]. But, the right choice pre-treatment process needs to enhance porosity of the biomass and reduce the inhibitors generation [37]. Ideally, low acids and alkal concentration has revealed optimmum separation of lignin, hemicellulose and cellulose of the biomass without addition of inhibitors [38]. Further, dilute acids and alkali pre-treatment can are economical for bioenergy and biochemicals production from biomass polysaccharides at large scale. Potassium hydroxide, calcium hydroxide and sodium hydroxide are the highly ested alkalies for lignocellulose degradation, however, sodium hydroxide has a greater potential reaction rate compare to other alaklies [39]. Wherase, in cost perspective, calcium hydroxide has less cost per kg than the other and could prove to the inexpensive pretreatment of biomass [40]. Therefore, for industrial scale process, it is better to select the inexpensive acids and alkali with optimum thermal process for yield of polysaccharides and biofuels production from pretreated biomass through anaerobic fermentation.
