Fang Huang and Arthur J. Ragauskas

Abstract Lignocellulosic materials, such as wood, grass, and agricultural and forest residues, are potential resources for the production of bioethanol. The biochem­ical process of converting biomass to bioethanol typically consists of three main steps: pretreatment, enzymatic hydrolysis, and fermentation. During the whole pro­cess, pretreatment is probably the most crucial step since it has a large impact on the efficiency of the overall bioconversion. The aim of pretreatment is to disrupt recalcitrant structures of cellulosic biomass to make cellulose more accessible to the enzymes that convert carbohydrate polymers into fermentable sugars. Physical, physical-chemical, chemical, and biological processes have been used for pretreat­ment of lignocellulosic materials. This chapter summarizes the leading technologies in chemical pretreatment on softwood, particularly pine species, which generally show relatively higher recalcitrance than hardwood, grass, and other lignocellu — losic materials. Different chemical pretreatment techniques, including dilute acid pretreatment, alkaline hydrolysis, wet oxidation, sulfite pretreatment to overcome recalcitrance of lignocellulose (SPORL), organosolv, ionic liquids pretreatment, and ozonolysis process are intensively introduced and discussed. In this chapter, the key points are focused on the structural changes primarily in cellulose, hemicellulose, and lignin during the above leading pretreatment technologies.

Keywords Chemical pretreatment technology ■ Biofuel ■ Biorefinery ■ Softwood

8.1 Introduction

In order to cope with growing demand for energy, the depletion of fossil fuel re­sources, and environmental concerns raised by fossil fuel use, countries wishing to limit their energy dependence on petroleum exporting countries are developing

A. J. Ragauskas (H) ■ F. Huang School of Chemistry and Biochemistry,

Institute of Paper Science and Technology,

Georgia Institute of Technology, Atlanta, Georgia, 30332-0440, USA e-mail: arthur. ragauskas@ipst. gatech. edu

Z. Fang (ed.), Pretreatment Techniques for Biofuels and Biorefineries,

Green Energy and Technology,

DOI 10.1007/978-3-642-32735-3_8, © Springer-Verlag Berlin Heidelberg 2013

alternative energy sources, such as bioethanol produced from renewable biomass [1-4]. Cellulosic bioethanol is regarded as one of the most promising renewable biofuels in the transportation sector for the coming next few decades [5]. Current pro­duction of bioethanol relies on sugars that are obtained from starch-based agricultural crops by using first-generation conversion technologies [6]. Nowadays bioethanol produced from lignocellulosic biomass using second-generation technologies has become an interesting alternative, mainly because lignocellulosic raw materials do not compete with food crops or productive agricultural land, and they are also less expensive than conventional agricultural feedstocks [7, 8].

The biological process of converting biomass to bioethanol typically consists of three main steps: pretreatment, enzymatic hydrolysis, and fermentation. During the whole process, pretreatment is the most crucial step since it has a large impact on the efficiency of the overall bioconversion. In lignocellulosic biomass, cellulose and hemicellulose are densely packed together with lignin, which serves several func­tions including protection against enzymatic hydrolysis [9]. The aim of pretreatment is to disrupt recalcitrant structures of cellulosic biomass to make cellulose more ac­cessible to the enzymes that convert carbohydrate polymers into fermentable sugars (Fig. 8.1). During the pretreatment, the extent of removal of lignin and hemicellulose depends on the pretreatment conditions and severity. For example, acidic chemical pretreatment removes most of hemicellulose. The lignin is condensed when pretreat­ing temperature reaches above 170 °C. On the contrary, the ammonia fiber explosion (AFEX) pretreatment does not significantly remove hemicellulose.

Numerous pretreatment strategies have been developed to enhance the reactiv­ity of cellulose and to increase the yield of fermentable sugars. Typical goals of pretreatment include [11]:

• Production of highly digestible solids that enhances sugar yields during enzyme hydrolysis.

• Avoiding the degradation of sugars (mainly pentoses) including those derived from hemicellulose.

• Minimizing the formation of inhibitors for subsequent fermentation steps.

Among the numerous types of biomass, softwoods (SW) are generally recognized as being much more refractory than hardwoods (HW) or agricultural residues in the pretreatment process. This is, in part, due to the fact that SW have a more rigid structure and contains more lignin [12].

The goal of this paper is to review promising chemical pretreatments technologies on softwood, particularly pine species, and to discuss recent developments which have greatly aided the production of bioethanol. For each technology, a brief process description is first given with recent developments, and then the feedstocks on which these technologies are used are highlighted, followed by discussion of the technol­ogy’s advantages and disadvantages. The key points will be focused on the structural changes primarily in cellulose, hemicellulose, and lignin during the above leading pretreatment technologies.