Thomas D. Foust, Kelly N. Ibsen, David C. Dayton, J. Richard Hess, and Kevin E. Kenney

2.1 Introduction

A significant challenge for the future will be meeting the world’s mobility and chemical needs as populations and mobility needs grow. Currently, crude oil supplies almost all of the world’s transportations’ fuel need and a significant portion of the material and chemical needs (1). Overdependence on crude oil is leading to concerns about national energy security and short — and long-term price stability for both transportation fuels and commodity chemicals made from crude oil. Additionally, the everincreasing worldwide emissions of CO2 from the transportation sector and the effect this is having on global climate change further increase concerns of over-reliance on crude oil.

Biomass, as the only source of renewable carbon, shows great promise for the large-scale economical production of renewable transportation fuels and chemicals. Biomass is an extremely abundant resource that can be produced in agriculture, forestry, and microbial systems. Biomass can also be captured from waste sources such as urban wood residues. Worldwide production of terrestrial biomass has been estimated to be on the order of 200 x 1012 kg (220 billion tonnes) annually. To put it in perspective, the total energy content from this amount of biomass (via heat of combustion analysis) is approximately five times the energy content of the total worldwide crude oil consumption (2).

A recent study (3) that looked specifically at US production capability showed the po­tential for the sustainable production of 1.3 billion dry tons per year of biomass from forest and agricultural lands without negatively impacting food, feed, and fiber production while still meeting export demands. Figure 2.1 shows the portion of this potential that could be produced from forest and agricultural lands by the middle of the next century with ag­gressive policies and economic incentives to maximize biomass production. The underlying assumptions in Figure 2.1 are that both agricultural resources and perennial energy crops are produced from agricultural land including some currently protected, or reserved, land with the forest resources being produced on forest land. Additionally, the agricultural resource potential includes grains that would be available for biofuels production.

Table 2.1 shows the transportation fuel production potential from the amount depicted in Figure 2.1. Ethanol was chosen as the representative biofuel because very accurate yield data exists for the various categories of feedstocks. To develop the yield numbers listed in Table 2.1, traditional corn dry-mill ethanol technology yields were used for the grain

Biomass Recalcitrance: Deconstructing the Plant Cell Wall for Bioenergy. Edited by Michael. E. Himmel © 2008 Blackwell Publishing Ltd. ISBN: 978-1-405-16360-6

resource potential, biochemical conversion process yields were used for the agronomic resource potential, and thermochemical process yields were used for the forestry resources. Biochemical — and thermochemical-to-ethanol yields are from Foust and coworkers (4).

In 2006, motor gasoline demand in the United States was approximately 143 billion gallons per year (5), so the ultimate potential for biofuels would be to supply approximately 60% of current gasoline demand on an energy basis. Although detailed data on worldwide biofuels potential does not exist, it is reasonable to assume the potential on a worldwide basis would be similar to the United States. However, only looking at biofuels as a replacement for existing transportation fuel usage is somewhat shortsighted and misleading. If the goal is to reduce dependence on crude oil, it is important to look at biofuels as a part of the overall solution to reducing worldwide dependence on imported oil. Scenarios have been developed in which sustainable development of biofuels, in conjunction with vehicle efficiency gains and smart growth, have been shown to be capable of virtually eliminating gasoline demand by the year 2050 (6). Given this significant potential, the challenge is to quickly deploy biofuels technology in an economically viable manner at a large scale.

Table 2.1 Total ethanol production potential

The biorefinery concept is commonly presented as the method for large-scale deployment of biofuels. The term biorefinery was established in the 1990s (7) and has been refined many times over the years. The National Renewable Energy Laboratory’s (NREL) web site defines biorefineries (8) as follows:

A biorefinery is a facility that integrates biomass conversion processes and equipment to pro­duce fuels, power, and chemicals from biomass. The biorefinery concept is analogous to today’s petroleum refineries, which produce multiple fuels and products from petroleum. Industrial biorefineries have been identified as the most promising route to the creation of a new domestic bio-based industry. By producing multiple products, a biorefinery can take advantage of the differences in biomass components and intermediates and maximize the value derived from the biomass feedstock while also being able to adapt to changing market conditions. The high-value products enhance profitability while the high-volume fuel helps meet national energy needs.

Sometimes in the literature the term “integrated biorefinery” is used (9) and is generally used in the context that a number of unit operations or technologies are used in an integrated manner to convert biomass to fuels and chemicals, so essentially the terms “biorefinery” and “integrated biorefinery” are interchangeable in the literature.

A good explanation of the stages or phases of biorefineries is provided by Kamm and Kamm (10) and Van Dyne and coworkers (11). They explain the progression of biorefiner­ies in three phases as the technology develops to move from the simple, easily processed feedstocks at lower volumes to the more difficult to process lignocellulosic biomass feed­stocks at higher volumes. An example of a Phase I biorefinery would be an existing corn dry-mill ethanol plant. It uses corn grain as the feedstock and a fairly straightforward set of conversion technologies that limit capital costs but put fairly confined constraints on production flexibilities and co-product production capabilities. In fact, most corn dry-mill ethanol plants are limited to an animal feed co-product, distiller’s dry grain (DDG), as their only co-product in addition to ethanol. A dry mill has very limited capabilities to change its product mix to adapt to changing market conditions. However, a major advantage of corn dry-mill technology is its low capital cost. A cane sugar-based ethanol plant produc­ing ethanol and food sugar, as currently exists in Brazil, is another example of a Phase I biorefinery.

An example of a Phase II biorefinery is an existing corn wet-mill plant. Although a wet — mill plant processes corn grain, it has more operational flexibility compared to a dry mill to produce a multitude of products such as ethanol, starch, high fructose corn syrup, corn oil, and corn gluten meal. The product mix in a corn wet mill can be varied to provide the highest economic return based on current market conditions. However, this co-product and product flexibility commands higher capital costs so wet mills tend to be twice as large, on average, compared to current dry mills to take advantage of economies of scale. Yet, even with the larger sizes, wet mills tend to have about a 10% higher capital cost per bushel of corn processed than dry mills (12). The Phase I and II biorefinery discussion does not need to be limited to corn wet and dry mills, an existing pulp and paper mill could be considered another example on a Phase I biorefinery that produces primarily a single project. A Phase III biorefinery would be essentially an integrated biorefinery capable of producing fuel(s) and other products from various feedstocks which would include lignocellulosic feedstocks.

Ethanol production has seen tremendous growth since 2000 primarily from new biore­finery construction and, to a lesser extent, from expanding the capacity of the existing
biorefinery industry (13). Essentially, all of the new ethanol production capacity being added is Phase I biorefineries, corn dry mills, suggesting investors favor lower capital costs over product flexibility. As an important historical reference, this indicates that lower capital costs should be favored over product flexibility for developing Phase III biorefineries.

Although corn grain and other starch — or sugar-based feedstocks processed in Phase I or II biorefineries can start the transition from sole dependence on petroleum for transportation fuels, as shown in Table 2.1, grain- and sugar-based biofuels have limited potential. Some studies (14) have listed the ultimate potential of biofuels considerably above the 9.2 billion gallons shown in Table 2.1, but the general consensus is that for biofuels to have significant production potential at the current scales of petroleum-based transportation fuels, the technology must be advanced to Phase III biorefineries that economically convert abundant lignocellulosic feedstocks into fuels and chemicals. The next sections describe the challenges and technology needed to achieve economic viability of Phase III biorefineries.