Ethanol from wood. An experimental study of steam pretreatment, enzymatic hydrolysis and enzyme recovery

摘要

This thesis deals with pretreatment of the raw material, enzymatic hydrolysis of the cellulose fraction and fermentation of the sugars obtained in the bioconversion of lignocellulosics to fuel ethanol. The success of such a process is dependent on high conversion yields in all process steps, together with efficient use of the enzymes. The steam-pretreatment methods was found to be highly efficient in making lignocellulosic materials accessible to cellulolytic enzymes. Impregnation with sulphur dioxide prior to steam pretreatment at 200 degrees C was found to be the most efficient pretreatment method for willow, with which it was possible to reach 95% of the theoretical glucose yield, after subsequent hydrolysis, and a xylose yield of 62%. A temperature of 40-45 degrees C and an enzyme concentration of about 20 FPU/g cellulose were found to be optimal for the hydrolysis of steam-pretreated willow. Delignification with alkali and peroxide increased the accessibility of the material and increased the maximum glucose concentration obtained in the enzymatic hydrolysis from 62 g/l to 83 g/l. However, the treatment reduced the overall yield of glucose, partly due to end-product inhibition in the hydrolysis. A continuous desorption/re-adsorption procedure was developed to recover spent cellulases after hydrolysis, with which it was possible to recover 18% of the original cellulases from the hydrolysate and 40% from the solid hydrolysis residue. The conditions for simultaneous saccharification and fermentation using either S. cerevisiae or Z. mobilis were optimised and the results show that 88% of the theoretical ethanol yield could be reached in 72 hours, compared with 120 hours when the hydrolysis and fermentation were performed separately. The nutrient addition could be substantially reduced compared with literature data. 85 refs, 12 figs, 6 tabs