Bin Yang, Yanpin Lu, and Charles E. Wyman

Abstract

Cellulosic biomass is inexpensive and abundant and provides a unique resource for large-scale and low-cost solar energy collection and storage. Agricultural residues are particularly promising for initial commercial applications because of their potential low cost and near-term availability. Because the rapidly evolving tools of biotechnology can radically lower conversion costs and enhance yields, biological processing presents a particularly promising approach to converting these solids into liquid fuels that better fit our transportation infrastructure while providing unparalleled environmental, economic, and strategic benefits. Yet breakdown of the cellulose and hemicellulose in these naturally resistant cellulosic materials to release fermentable sugars is projected to be the most expensive processing step. In addition, the pretreatment step needed to realize high yields has pervasive impacts on all other major operations from choice of feedstock through product recovery and residue processing. Thus, knowledge of how agricultural residues respond to pretreatment and integrate with other operations is vital to successful applica­tions. This chapter begins with an overview of biological processing of agricultural resi­dues to ethanol followed by a summary of environmental considerations in their use and some estimates of availability based on these factors. Information is also given on the composition of major agricultural residues and reported yields of sugars from many such materials to provide a perspective on their suitability for ethanol production. Then, approaches and needs for harvesting, transporting, and storing agricultural residues are discussed. The chapter closes with a simplified analysis of the cost of processing cel- lulosic biomass to ethanol to point out key cost factors and the importance of employing low-cost feedstocks and realizing high yields. In addition, opportunities for advanced technologies to lower the cost of biological processing to ethanol and other products are outlined.

Biofuels from Agricultural Wastes and Byproducts Edited by Hans P. Blaschek, Thaddeus C. Ezeji and Ju rgen Scheffran 175 © 2010 Blackwell Publishing. ISBN: 978-0-813-80252-7

Introduction

Cellulosic biomass provides a truly unique resource for large-scale sustainable production of liquid fuels that integrate into our existing transportation infrastructure, and no other raw material can match its potential impact and cost (Lynd et al. 2008). For example, cellulosic biomass costing about $63/dry ton is as inexpensive as petroleum at $20/barrel on an equiva­lent energy content basis (Lynd et al. 1999). Furthermore, the U. S. Departments of Agriculture and Energy estimate that over 1.3 billion dry tons of biomass could be available annually, of which about 1 billion dry tons per year are agricultural resources available on the sustainable basis (Perlack et al. 2005; Long 2008). This quantity is enough to make a major impact on energy supplies (Lynd et al. 2008), with the result that conversion of agricultural biomass to organic liquid fuels (e. g., ethanol) can enhance energy security, reduce trade deficits, enhance global competitiveness, and create rural employment. In addition, processing cellulosic biomass to ethanol can continue to employ the power of biotechnology to simplify technology and realize high yields vital to low costs that address concerns about mounting petroleum prices (Wyman 1993, 1994b, 1999a). Because transportation is the single largest contributor to carbon dioxide (CO2) emissions in the United States (Tyson 1993; Wyman 1994a; Farrell et al. 2006; U. S. DOE 2006), the promise for cellulosic fuels to reduce greenhouse gas (GHG) emissions by about 90% and more compared to gasoline coupled with the low-cost potential and large resource base are vital as we seek avenues to abate increasing temperatures and deterioration in the climate. In fact, the only other potentially cost-effective energy options to power mobility with a low carbon footprint are through energy storage in batteries, hydro­gen, or compressed air, provided the electricity required to power each is derived from sus­tainable technologies at low cost. Even then, liquid fuels will be essential for long distance transport and aircraft. Combining cellulosic fuels with plug-in hybrids, more public transpor­tation, and better fuel efficiency will likely prove the most cost-effective avenue to affordable local and long distance mobility with low carbon emissions.

Despite its great promise and tremendous progress in improving cellulosic conversion technology, no commercial facilities are yet in place, with a vital challenge being to overcome the perceived risk of implementing the technology for the first time (Wyman 1999a). Once commercialized, costs are expected to drop dramatically through the learning curve effect, as clearly demonstrated for cane sugar ethanol in Brazil and corn ethanol in the United States, and projects will become both more profitable and less risky as more capacity comes on line (Wyman 2007). The current situation presents a classical chicken-and-egg challenge of how to overcome the greater risk and lower returns associated with first commercial plants to realize lower costs and higher returns of mature projects.

Because feedstock costs are dominant in processing economics, it is critical to seek those that are low in cost for first applications while being sufficiently abundant. However, high product yields and ease of processing are also vital to minimizing costs, while sufficient amounts must be available to support a large enough facility to achieve reasonable econo­mies of scale. Agricultural residues are expected to serve as a major biofuels feedstock, and their potential low cost and current availability can be particularly important in the near term (Perlack et al. 2005) . Thus, this chapter will summarize estimated amounts of leading agricultural residues and their potential for making ethanol. However, first an over­view will be presented of the biological conversion of these materials to ethanol to provide a context on key feedstock and processing considerations. The economics of converting residues to ethanol will then be outlined to demonstrate the importance of feedstock com­position, availability, and cost to good returns on capital. In addition, some other important considerations in process economics and financing will be summarized. Finally, strategies will be discussed to introduce technologies for biological conversion of agricultural residues to ethanol.