In the calculations inherent in the data for figure 4.2, some interesting conclusions are reached. Although wheat straw has undoubted advantages, other feedstocks out­perform wheat straw for some key parameters:

• Wheat straw has a lower gravimetric ratio of total carbohydrate (cellulose, starch, xylan, arabinan) to lignin than barley straw, corn stover, switch — grass, or even wheat chaff.

• Wheat straw has a lower cellulose:lignin ratio than all of these nonsoftwood sources (with the exception of switchgrass).

• Of the seven quoted examples of lignocellulosic feedstocks, wheat chaff and switchgrass have the highest total pentose (xylan and arabinan) con­tents — quoted as an important quantitative predictor for ethanol yield from cellulose because less cellulase is required (or, conversely, more of the cellulose glucose is made available for fermentation).1,2

An important consideration included in Iogen’s deliberations on feedstock suit­ability was the reproducible and predictable supply of wheat straw. The USDA Agricultural Service also itemized sustainable supply as one of their two key fac­tors for biomass feedstocks, the other being cost-effectiveness.6 Financial models indicate that feedstocks costs are crucial, and any managed reduction of the costs of biomass crop production, harvesting, and the sequential logistics of collec­tion, transportation, and storage before substrate pretreatment will all impact the viability of biofuel facilities — economic aspects of feedstock supply chains are discussed in the next chapter. The Energy Information Administration has con­structed a National Energy Modeling System to forecast U. S. energy production, use, and price trends in 25-year predictive segments; the biomass supply schedule includes agricultural residues, dedicated energy crops, forestry sources, and wood waste and mill residues, and wheat straw (together with corn stover, barley straw, rice straw, and sugarcane bagasse) is the specified component of the agricultural residue supply.7

A brief survey will, therefore, be made of wheat straw and other leading candi­date lignocellulosics, with special emphasis on how different national priorities place emphasis on different biomass sources and on what evolving agricultural practices and processing technologies may diversify bioethanol facilities on scales equal to and larger than the Iogen demonstration facility.