Xiaorong Wu, James McLaren, Ron Madl, and Donghai Wang

Abstract Biomass feedstock, which is mainly lignocellulose, has considerable potential to contribute to the future production of biofuels and to the mitigation of carbon dioxide emissions. Several challenges exist in the production, harvest­ing, and conversion aspects of lignocellulose, and these must be resolved in order to reach economic viability. A broad array of research projects are underway to address the technical hurdles, however, additional research may be required to reach commercial sustainability. Gasification and enzymatic hydrolysis are the main technologies being investigated for the conversion of lignocellulosic biomass into material for the production of biofuels. While each approach has pros and cons, both are being explored to determine the optimum potential commercial method for particular feedstock situations, and to better understand the requirements for the massive scale required to contribute to biofuel volume.

Keywords Lignocellulosic biomass ■ Biofuels ■ Syngas ■ Enzymatic hydrolysis ■ Pretreatment ■ Fermentation ■ Gasification

1 Introduction

As the world population increases from the current 6.7 billion to over 8 billion by 2030 [1], and supporting economic growth expands, energy consumption is pro­jected to increase by 42% to 695 quadrillion (1015) British thermal units (Btu, 1 Btu = 1055 joule) in 2030 [2]. Most of the required energy will still be acquired from fossil fuels, with around 6% being from nuclear sources and about 8% from other renewable energy sources. Carbon dioxide (CO2) emission from such widespread industrial consumption of fossil fuels (coal, oil, and natural gas) is

D. Wang (B)

Department of Biological and Agricultural Engineering, Kansas State University, Manhattan, KS 66506, USA e-mail: dwang@ksu. edu

O. V. Singh, S. P. Harvey (eds.), Sustainable Biotechnology,

DOI 10.1007/978-90-481-3295-9_2, © Springer Science+Business Media B. V. 2010 likely to continue to be a major contributor to anthropogenic greenhouse gases [3, 4]. Mitigation of CO2-based contributions to the global warming process requires specific actions, including capture and sequestration of CO2 during the consump­tion of fossil fuels and expanded utilization of carbon-neutral and carbon negative renewable energy sources (wind, solar, nuclear, geothermal, and various biomass sources) [3-6]. Most of the types of renewable energy (wind, solar, etc.) can be uti­lized to generate electricity, but not liquid transport fuels. Consequently, biomass has received much attention as a feedstock for biofuels, both in the existing com­mercial industry (e. g. ethanol from grains or sugar) and in the research realm where lignocellulose is the current focal feedstock material [7-11]. To avoid confusion, we adapt the common definition for biomass and biofuels as follows:

• Biomass: Organic, non-fossil material of biological origin (plant parts including grains, tubers, stems/leaves, roots/tubers, agricultural residues, forest residues, animal residues, and municipal wastes arising from biological sources) poten­tially constituting a renewable energy source (basically originating from primary capture of solar energy).

• Lignocellulosic biomass: Organic material derived from biological origin which has a relatively high content of lignin, hemicellulose, cellulose, and pectin com­bined into a molecular matrix with a relatively low content of monosaccharides, starch, protein, or oils. Typically refers to plant structural material with high cell wall content. Sometimes referred to as “cellulosic” biomass, which is techni­cally inaccurate, but is (mis)used due to the typical 40%+ cellulose content in lignocellulose.

• Biofuels: Liquid fuels and blending components produced from biomass (plant) feedstocks, used primarily for transportation. Technically, biogas (e. g. methane from anaerobic digestion of biological residues) is a “biofuel” but tends to be utilized in stationary combustion units and is typically referred to separately as biogas.

Survey reports suggest that the annual world biomass yield contains sufficient inherent energy to contribute 20-100% of the world’s total annual energy con­sumption of 500 EJ (1 EJ = 1 x 1018 Joule), with annual and regional variations [4, 10, 12]. Currently, commercial biofuels are generated from harvestable compo­nents of known crops (starch, sucrose, and oils), while a relatively small amount of the lignocellulosic biomass is used for combustion (cooking/heating fires or co­firing to create steam for electricity generation). The large potential of lignocellulose as an energy feedstock remains to be utilized, and is dependent on the development of economic, sustainable production, and processing systems [11].

Two platforms have been set up to transform the energy in lignocellulosic biomass into liquid fuels or chemicals: the sugar platform and thermochemical platform. In the sugar platform, the lignocellulosic material is first pre-treated to facilitate separation into the major components, then the polymeric celluloses and hemicelluloses are enzymatically hydrolyzed into sugars (hexoses and pentoses), after which these sugars can be fermented into biofuels or converted into other valuable intermediate chemicals. The residual lignin may be utilized as a specialty intermediate or, more commonly, is combusted for heat or power. In the thermo­chemical platform, biomass is degraded into small gas molecules (hydrogen, carbon monoxide, carbon dioxide, methane, etc.) under high temperature and certain pres­sure conditions, then these gas molecules are converted chemically or biologically into Fischer-Tropsch (FT) liquid fuel, alcohols, or other intermediate chemicals. This chapter focuses on the processes, potential, and challenges associated with each of these platforms.