Pretreatment and Fractionation of Wheat Straw for Production of Fuel Ethanol and Value-added Co-products in a Biorefinery

Xiu Zhang,Nhuan P Nghiem

doi:10.3934/bioeng.2014.1.40

Abstract

An integrated process has been developed for a wheat straw biorefinery. In this process, wheat straw was pretreated by soaking in aqueous ammonia (SAA), which extensively removed lignin but preserved high percentages of the carbohydrate fractions for subsequent bioconversion. The pretreatment conditions included 15 wt% NH4OH, 1:10 solid:liquid ratio, 65 oC and 15 hours. Under these conditions, 48% of the original lignin was removed, whereas 98%, 83% and 78% of the original glucan, xylan, and arabinan, respectively, were preserved. The pretreated material was subsequently hydrolyzed with a commercial hemicellulase to produce a solution rich in xylose and low in glucose plus a cellulose-enriched solid residue. The xylose-rich solution then was used for production of value-added products. Xylitol and astaxanthin were selected to demonstrate the fermentability of the xylose-rich hydrolysate. Candida mogii and Phaffia rhodozyma were used for xylitol and astaxanthin fermentation, respectively. The cellulose-enriched residue obtained after the enzymatic hydrolysis of the pretreated straw was used for ethanol production in a fed-batch simultaneous saccharification and fermentation (SSF) process. In this process, a commercial cellulase was used for hydrolysis of the glucan in the residue and Saccharomyces cerevisiae, which is the most efficient commercial ethanol-producing organism, was used for ethanol production. Final ethanol concentration of 57 g/l was obtained at 27 wt% total solid loading.

Highlights

Ethanol has attracted considerable attention worldwide as a clean and renewable liquid fuel
It can be seen that the pretreatment of wheat straw with aqueous ammonia resulted in high degree of delignification
High degree of delignification during soaking in aqueous ammonia (SAA) pretreatment would result in improvement of enzymatic hydrolysis of the pretreated lignocellulosic material as previously observed, for example, for barley straw [25]

Summary

Introduction

Ethanol has attracted considerable attention worldwide as a clean and renewable liquid fuel. Whereas in the U.S corn stover is considered the most important LCF wheat straw is its counterpart in other regions such as Europe and Asia. In 2012 China produced 121 million MT of wheat, which accounted for 18% of the total world production [1]. Aside from the potential economic benefit of generating a suite of products with higher profit margins than ethanol, a major technical advantage is, with glucose as the substrate, the yeast Saccharomyces cerevisiae, which is the most efficient commercial ethanol-producing organism, can be used to carry out the fermentation. The biorefinery process recently developed at the Eastern Regional Research Center (ERRC) using wheat straw as feedstock is described in this report. The proposed conversion process for wheat straw consisted of the following steps: 1. Pretreatment of wheat straw to improve subsequent enzyme hydrolysis

Methods

Results

Discussion

Conclusion