Search

Oat hulls are a promising feedstock annually renewable at an industrial scale for the production of second generation ethanol. Chemical pretreatment of oat hulls with a 4% (w/w) nitric acid solution at atmospheric pressure was performed in pilot production to give a pulp in which hydrolyzables account for 79.9%. Acid-insoluble lignin totaling 12.5% in the pulp does not deteriorate the efficiency of enzymatic hydrolysis using an enzyme cocktail of commercial BrewZyme BGX and CelloLux-A. At an initial solid loading of 33.3 g/L, a hydrolyzate was obtained with a reducing sugar yield of 93.0% of the overall hydrolyzables. The high yield is assured only by removal of residual, water-soluble, nitrated lignin by water washing. Alcoholic fermentation was run using Saccharomyces сerevisiae Y-1693. Simultaneous saccharification and fermentation with delayed inoculation (dSSF) at an initial pulp loading of 90 g/L was found to improve the ethanol yield by 1.2 times compared to separate hydrolysis and fermentation (SHF), i.e. 0.428 versus 0.343 g/g cellulose, respectively. The ethanol samples obtained from oat hulls contain low percentages of esters and methanol.

In this work, a solid simultaneous saccharification and fermentation (SSSF) system with N2 stripping was constructed for ethanol production, and the glucose inhibition to enzymatic saccharification of cellulose and the ethanol inhibition to anaerobic fermentation of yeast cells could be relieved. First, effects of flow rate of carrier gas and initial yeast concentration on mass transfer of ethanol and performance of SSSF were experimentally investigated. It is found that the peaks of total ethanol amount (6.32 g), cellulose consumption amount (13.29 g), ethanol yield (0.48 g/g substrate) and change rate of porosity of the packed bed (57.45 %) were obtained at an initial yeast concentration of 2% and a N2 flow rate of 30 mL/min. The results reveal that mass transfer efficiency during SSSF could be improved at an appropriate N2 flow rate and an optimal initial yeast concentration. Then, a novel kinetic model was established to simulate mass transfer of ethanol during SSSF. The data predicted by the model were in good accordance with the experimental results, which shows that the established model could reasonably reflect the characteristics of ethanol mass transfer in the packed bed. This work demonstrates that the produced ethanol could be effectively stripped out of the packed bed by convection of the sparged N2, leading to improving performances of enzymatic hydrolysis and ethanol fermentation.

A bacterial cellulose–alginate (BCA) sponge, fabricated by a freeze-drying process, was successfully used as a yeast cell carrier for ethanol fermentation. The BCA sponge exhibited several advantageous properties, such as high porosity, appropriate pore size, strong hydrophilicity and high mechanical, chemical and thermal stabilities. BCA has an asymmetric structure, with a thin, dense outer layer covering an interior of interconnected macropores that are distributed throughout the sponge, which is effective for yeast immobilization. At 48 h of the fermentation, the maximum ethanol concentration produced by the immobilized culture (IC) in the BCA carrier was about 100 g/L, which was approximately 13% and 45% higher than that from the suspended culture (SC) and from IC in Ca-alginate matrix, respectively. Repeated-batch ethanol productions using IC in BCA carriers were also more stable than those using SC or IC in Ca-alginate matrix. The results of a 15 cycle repeated batch operation demonstrated that the system with IC in BCA exhibited superior long-term stability for ethanol fermentation with the average ethanol productivity at 1.9 g/L h and the immobilized yield at 86%. The improved ethanol fermentation performance was mainly due to the water uptake ability and properly interconnected pore structure, which help to overcome limiting mass transfer.