Y.-H. Percival Zhang and Lee R. Lynd

16.1 Introduction

Accumulation of the greenhouse gas, CO2, mainly from burning of fossil fuels, and the depletion of finite fossil fuels are vital threats to the sustainable development of humans (1-3). Lignocellulose is the most abundant renewable biological resource today (ca. 2 x 1011 tons/year) and is produced by photosynthesis (i. e., plants fix atmospheric CO2) (4-8). Development of technologies for effectively converting less-costly agricultural and forestry residues for use in bio-based chemical and fuels production offers potential benefits to the national interest by improving strategic security, decreasing trade deficits, encouraging healthier rural economies, and improving environmental quality by moving closer to zero net greenhouse gas emissions and sustainable resource supplies (1-3, 9-19).

Lignocellulosic feedstock is far less costly than other energy feedstocks (i. e., crude oil, natural gas, corn kernels, and soy oils) based on energy content ($/GJ) in Figure 16.1. For example, when crude oil prices vary from $40 to $70 per barrel, equaling $7.1 to $12.1/GJ, they are much higher than those of lignocellulose ($0 to $3/GJ). Similarly, corn kernels costing from $2.25 to $4.0 per bushel equal $6.3 to $11.5/GJ. During the past 2 years (2004-2006), corn kernel prices have risen by >70% from the historically low prices (~$2.25 per bushel) to ~$4 per bushel. With high demands of corn kernels for ethanol production, higher prices of corn kernels have resulted in rising prices of animal feed and human food. For example, the Chinese government banned the establishment of new ethanol production facilities based on grains in 2006. As expected, less costly feedstock and the most abundant supplies make production of fuels and chemicals from lignocellulose appealing.

Production of commodity products (i. e., fuels, chemicals, and materials) from renewable biomass is distinct from biotechnology motivated by health care at many levels, including economic driving forces, importance of feedstock prices, processing costs and capital invest­ment, the scale of applications, and feedstock availability (10). Biocommodities have low selling prices so that raw material costs are often dominant factors in determining prices of commodity products (~30-70%), whereas raw materials usually account for a very small fraction of high selling prices of pharmaceuticals (10, 13). The production costs, including capital recovery and processing costs, are usually another dominant factor determining the

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

Energy price ($/GJ)

price of commodity products, whereas they are not nearly so important for pharmaceuticals. Market sizes for individual biocommodity products and biopharmaceuticals are of relatively similar magnitude. However, tremendous differences exist with respect to a product mass basis, e. g., the largest commodity markets exceed pharmaceutical markets by approximately 11 orders of magnitude (10). The production of high-volume/low-value biocommodity products has an absolute requirement for high-volume/low-cost feedstock and must be re­sponsive to the availability and characteristics of feedstock, whereas no such requirement exists for the production of pharmaceuticals.

Overcoming the recalcitrant structure of lignocellulose is still among the greatest chal­lenges for the emerging biofuel and bio-based chemical industries (20, 21). Currently, high conversion costs, large investment risks, and a narrow economic margin between feed­stock costs and product prices slow the realization of cellulosic ethanol production on a large scale (22-24). Effective biological conversion of recalcitrant lignocellulose to bio­commodity products involves four main sequential steps: 1) biomass size reduction, 2) pretreatment/fractionation, 3) enzymatic cellulose and hemicellulose hydrolysis, and 4) fer­mentation (11,24,25). For most types of lignocellulose biomass, the enzymatic digestibility of cellulose is very low (<20%) without some type of pretreatment that opens up the struc­ture and makes it accessible to attack by enzymes (11, 24, 25). A number of biological, chemical, and physical pretreatment techniques have been investigated (22, 23, 26, 27).