Effect Of Alkaline Pretreatment On The Chemical Compositions And Enzymatic Hydrolysis Of Wheat Straw

The production of bioethanol from lignocellulosic biomass offers a great potential for theabundant resources and reducing the raw material cost. An effective pretreatment is necessaryto liberate the cellulose from the lignin seal and its crystalline structure so as to render itaccessible for a subsequent hydrolysis step. Since the characteristics of straw materials, such asloosen structure, and high ash content, a mild alkaline pretreatment can be considered.Alkaline pretreatment is an effective way to improve the enzymatic saccharification of fibermaterials. The effects of weak alkaline pretreatment on the chemical compositions andenzymatic hydrolysis of wheat straw for ethanol production was investigated at differenttemperature, total titrateable alkali (TTA) charges and so on. The green liquor (sodiumcarbonate and sodium sulfide), sodium carbonate, oxygen-sodium carbonate, two-stepoxygen-sodium carbonate and oxygen-sodium hydroxide pretreatment were discussed in thisarticle. The final pH was controlled so as to be lower than9.5in order to prevent the sever lossof carbohydrates and to ensure little silica was dissolved in the black liquor. The pretreatedmaterials were hydrolyzed by using the enzyme cocktail which was mixed by cellulose,xylanase and β-glucosidase at50C to evaluate the improvement of sugar recovery. Comparedwith the cellulase with an enzyme loading of10FPU/g cellulose, sugar yield was significantlyincreased by the cellulase of20FPU/g cellulose. During the green liquor pretreatment, lignincan be effectively removed with the increament of TTA charge, sulfidity and tempraturepretreatment, while the pulp yield drops. After48h enzymatic hydrolysis with an enzymeloading of10FPU/g substrate, the total sugar yield of pretreated green liquor pulp by8%TTAand sulfidity40%at130C is the highest among all pretreated pulps, while the lignin removal is51.1%at this condition. An enzymatic hydrolysis total sugar recovery over73%can beachieved. Compared to the green liquor pretreatment, the total sugar recovery of the wheatstraw pulp treated by Na₂CO₃pretreatment (8%TTA charge,130C) was60%with an enzymeloading of10FPU/g substrate, while the lignin removal is24.2%. The total sugar recovery is63.8%at12%TTA charge with an enzyme loading of20FPU/g cellulose in singleoxygen-sodium carbonate pretreatment. It can be improved to67.0%with an enzyme loading of20FPU/g cellulose if wheat straw is treated by two-stage oxygen-sodium carbonatepretreatment (12%+6%TTA charge) at the same oxygen press and temperature. The enzymatichydrolysis efficiency of the wheat leaf is better than the stem in the mild conditions(sodium-carbonate pretreatment). The highest total sugar recovery of SC pretreated pulps canbe achieved is74.5%(8%Na₂CO₃,120C, leaf), while the highest total sugar recovery of thewheat stem is only58.7%(10%Na₂CO₃,130C, stem). Compared to green liquor and sodiumcarbonate pretreatment, the total sugar recovery of oxygen-sodium hydroxide pretreated pulpscan be achieved is71.9%with an enzyme loading of20FPU/g cellulose.As all said above, the enzymatic hydrolysis efficiency of cellulose and hemicellulose fromlignocelluloses can be significantly improved by weak alkaline pretreatment, and with a certainamount of delignification. Compared to acid pretreatment, there is no formation of inhibitors inalkaline pretreatment, so the increment of bioethnol production is expected. Besides, the blackliquor mixed with the enzymatic residues can be treated by the mature alkali recovery processto recycle the chemicals and heat energy and minimize the pollution caused by the black liquor and enzymatic residues. The weak alkaline pretreatment such as green liquor andsodium-carbonate pretreatment is an efficiency and eco-friendly pretreatment, which has a greatpotential for the production of bioethanol from lignocellulosic biomass.