Production Of Fuel Ethanol From Corncobs

Corncobs, as a low-cost abundant agricultural residue, has been regarded as a promising lignocellulosic feed stock for bioethanol production. Currently, technologies of lignocellulosic ethanol are still economically unfeasible, but could be improved through proper process optimization.In this study, response surface methodology was used to optimize the dilute acid pretreatment of corncobs. The optimizing experiments were established on the basis of Box-Behnken Design, while the response variable was final ethanol concentration, and factor variables are temperature, time, liquid-to-solid ratio and H2SO4concentration. The optimum conditions were determined to be88.6℃,19.8h, liquid-to-solid ratio of6.5:1and H2SO4concentration of1.86%(v/v). After the pretreatment under the optimized conditions, cellulose and hemicellulose contents of the corncobs were53.36%and7.29%, which were66.49%higher and77.58%lower than those of the untreated corncobs, respectively. The concentrations of xylose, glucose and total sugar were36.88g/L,4.50g/L and59.84g/L, respectively. By-products contained4.88g/L of acetic acid,0.0028g/L of furfural and0.28g/L of5-HMF, respectively. The final ethanol concentration of12.4g/L were obtained after simultaneous saccharification and fermentation (SSF) with SPSCOl, which was very close to the predicted value,12.64g/L.Single factor experiment was used to optimize the SSF process. The optimal strategy was determined to be:using S.cerevisiae6525-flol, substrate concentration of20%(w/v), cellulase dosage of35FPU/g, inoculation quantity of5%(v/v) and temperature of30℃. The final ethanol concentration was increased to be21.75g/L with ethanol yield from substrate of40.23%. A pre-saccharify process was developed which decreased the ferementation time from72h to36h, with compatible ethanol concentration and yield. When the substrate concentration was increased to30%, the final ethanol concentration was increased to30.44g/L. but the ethanol yield from substrate was decreased to37.5%.A secondary pretreatment to remove lignin was further investigated using NaOH, Ca(OH)2, ethanol and peroxyacetic acid, respectively. Experimental results indicated that NaOH was better than other reagents in lignin revomal. After the secondary pretreatment with 2%(v/v) sodium hydroxide and solid/liquid ratio of1:10(w/v) at80℃for6h, the cellulose content of the corncobs was increased to64.77%, which was102%higher than that of the untreated corncobs. Compared to the untreated corncobs, more porous structure and more irregular and corrugated surface were observed in SEM images for H2SO4/NaOH pretreated corncobs. After SSF, ethanol concentration of36.91g/L and ethanol yield of0.2364g/g corncobs were obtained. When the initial substrate concentration was15%, the cellulose conversion ratio reached a hight level of76.34%. Ethanol concentration increased with the increase of cellulase dosage. An overall ethanol yield of0.2431g/g corncobs was achieved with the cellulase dosage of35FPU/g. Pre-saccharification for12h can reduce the time for SSF by twenty-four hours without impacting ethanol production. An SSF fed-batch procedure was further used to increase the substrate concentration up to30%, resulting in final ethanol concentrations about65g/L and an overall ethanol yield of0.2185g/g corncobs.