The Effects Of Sulfite Pretreatment On The Saccharification Of Wheat Straw

Fossil energy, as a form of unrenewable source of energy, has a massive influence on the progress of the global economy and human society. The leading position of the economy based on the fossil energy is being under great pressure for the depletion of the fossil energy and environmental crisis due to the increasingly expanded consumption of fossil energy. Therefore the development of renewable energy is urgent, especially depriving energy from lignocellulosic materials. Bioethanol from lignocellulosic material was touted as a key solution to both the global mushrooming energy demands and the environmental burden. Four processes are employed in the conversion of lignocellulosic material to bioethanol:pretreatment, hydrolysis, fermentation and separation, the pretreatment is the most important and the most difficult point. The orthogonal design was adopted to investigate the effects of dilute acid pretreatment and sulfite pretreatment on the saccharification of wheat straw.The four parameters, temperature, dosage of sulphuric acid, retention time and solid to liquor ratio were selected in the L16(44) designed dilute acid pretreatment. The performance of the pretreatment showed that dosage of sulphuric acid and temperature occupied the first place, while the retention time and solid to liquor ratio came second. The optimal condition of the dilute acid was determined:temperature180℃, sulphuric acid dosage1.5%(w/w), solid to liquor ratio1:6and retention time10min. The results of the optimal condition were:enzymatic hydrolysis efficiency72.76%, glucose yield61.33%, dissolution ratio of lignin14.18%, dissolution ratio of pentosan41.22%and retention ratio of pentosan21.20%.The four factors, temperature, dosage of sodium bisulfite and sulphuric acid, retention time were chose in the L16(44) designed sulfite pretreatment. The analysis signified that temperature and dosage of sodium bisulfite were major factors to the performance, while the dosage of sulphuric acid and retention time were minor factors. The optimal condition was selected:temperature180℃, sodium bisulfite dosage3%(w/w), sulphuric acid dosage1.5%(w/w) and retention time20min. The final results of the optimal condition were:enzymatic hydrolysis efficiency91.00%, glucose yield72.45%, dissolution ratio of lignin24.52%, dissolution ratio of pentosan45.53%and retention ratio of pentosan25.02%. All the parameters indicated that the sulfite pretreatment was a much more effective method than the dilute acid pretreatment. On the basis of the optimal condition, the effects of solid to liquor ratio on the saccharification of wheat straw were investigated. The data showed the dissolution ratio of pentosan and lignin, as well as the enzymatic hydrolysis efficiency and glucose yield, can be prompted by increasing the solid to liquor ratio. The ratio had limited effect on the glucose concentration during the hydrolysis of the substrate.The effects of pH value on the saccharification of wheat straw were investigated in the pretreatments with or without sodium bisulfite. The results indicated that the addition of sodium bisulfite favored the pretreatment and the performance of the sulfite process under acidic condition was much better than the alkalic ones, while the neutral condition can not get a satisfactory level of pretreatment. The reaction course of the sulfite pretreatment from100℃to180℃following retention time40min was studied on the condition of sodium dosage4%(w/w), sulphuric dosage1.86%(w/w) and solid to liquor ratio1:4. The results showed that just limited amount of the pentosan and lignin degraded into the liquor, and the enzymatic hydrolysis efficiency climbed up with rising of the dissolution ratio of lignin and pentosan. The consequences of the research proved the saccharification efficiency can be significantly improved with the extension of the retention time, so were the dissolution of pentosan and lignin sulfonation. The negative effect of the time extension was the lignin condensation, which hinder the dissolution of lignin.