Biomass process development depends upon the economics of the conver­sion process, be it chemical, enzymatic, or a combination of both. A number of estimates have been computed based upon existing or potential technolo­gies. One obvious factor is that, regardless of the process, transportation of the biomass material from its source to the site of conversion must be kept to an absolute minimum. Approximately 35% of the expected energy is con­sumed by transporting the substrate a distance of 15 miles [68]. This con­siderable expenditure of energy simply to transport the starting material dictates that any conversion plant be of moderate size and in close proximity to the production source of the starting material.

There are some objections to the production and use of ethanol as a fuel. Most important is the criticism that producing ethanol can consume more energy than the finished ethanol contains. The European analysis takes wheat as the feedstock and includes estimates of the energy involved to grow the wheat, transport it to the distillery, make the alcohol, and transport it to a refinery for blending with gasoline. It allows credit for by-products, such as animal feed from wheat, for savings on gasoline that come from replacing 5% with alcohol, and from the energy gained from the increase of 1.25 octane points.

As fully explained in Chapter 3, a more recent and very extensive assess­ment on the net energy value (NEV) of corn ethanol technology [88] using advanced process technology as well as more realistic industrial data [89, 90] decisively showed that the prevailing corn ethanol process in the United States generates a significantly positive net energy value. As for the cellulosic ethanol, a number of factors and issues including the feedstock diversity and availability, the use of nonfood crops, minimal or no use of fertilizers, non­use of arable land, and more complex but still evolving conversion process technology make such an assessment far more difficult and less meaningful. Yet, to confine debates on biomass fuels solely to the process energy balance would be misleading. Based on the merits of cellulosic biofuels as well as regional strengths, a number of cellulosic biofuel plants based on diverse process technologies are in operation or are under construction throughout the world [91].

Energy requirements to produce ethanol from different crops were evalu­ated by Da Silva et al. [92]. The industrial phase is always more energy inten­sive, consuming from 60 to 75% of the total energy. The energy expended in crop production includes all the forms of energy used in agricultural and industrial processing, except the solar energy that plants use for growth. The industrial stage, including extraction and hydrolysis, alcohol fermen­tation, and distillation, requires about 6.5 kg of steam per liter of alcohol. It is possible to furnish the total industrial energy requirements from the by-products of some of the crops. Thus, it is also informative to consider a simplified energy balance in which only agricultural energy is taken as input and only ethanol is taken as the output, the bagasse supplying energy for the industrial stage, for example.

Furthermore, technological data are often very difficult or nearly impos­sible to compare between different options, due to the wide variety of feed­stock crops as starting lignocellulose. Therefore, the U. S. Department of Energy-sponsored projects chose corn stover as the model feedstock [93]. This selection is based on the fact that corn stover is the most abundant and concentrated biomass resource in the United States and its collection can leverage the existing corn ethanol infrastructure, including corn harvesting and ethanol production [93].

Unlike the cellulosic ethanol technology, sugarcane ethanol technology is far more straightforward and as such the energy balance evaluation is relatively straightforward. The energy balance results for ethanol produc­tion from sugarcane in Zimbabwe have shown that the energy ratio is 1.52 if all the major output is considered and 1.15 if ethanol is considered as the only output. The reported value of the net energy ratio for ethanol produc­tion from sugarcane in Brazil [92] is 2.41 and in Louisiana, United States [94], it is 1.85. The low ratio in Zimbabwe is due to (a) the large energy input in the agricultural phase, arising from a large fertilizer need, and (b) the large fossil-based fuel consumption in sugarcane processing. As shown in this comparative example, the energy balance results are dependent upon a large number of factors including process conversion technology, agricul­tural technology, climate and soil quality, logistical issues, and much more.

The NEV of cellulose ethanol from switchgrass was analyzed by Schmer et al. [95]. In this study, perennial herbaceous plants such as switchgrass were evaluated as cellulosic bioenergy crops. Two major concerns of their investigation were the net energy efficiency and economic feasibility of switchgrass and similar crops. This was a baseline study that represented the genetic material and agronomic technology available for switchgrass production in 2000 and 2001. Their study reported the following.

(a) The annual biomass yields of established fields averaged 5.2-11.1 Mg ■ ha-1 ■ y-1 with a resulting average estimated net energy yield (NEY) of 60 GJ ■ ha-1 ■ y-1.

(b) Switchgrass produced 540% more renewable than nonrenewable energy consumed.

(c) Switchgrass monocultures managed for high yield produced 93% more biomass yield and an equivalent estimated NEY than previous estimates from human-made prairies that received low agricultural input.

(d) Estimated average greenhouse gas emissions from cellulosic etha­nol derived from switchgrass were 94% lower than estimated GHG emissions from gasoline.

Generally speaking, the cost of production of ethanol decreases with an increase in capacity of the production facility, as is the case with most pet­rochemical industries. However, the minimum total cost corresponds to a point of inflection, at which point an increase in the production cost for every increase in the plant capacity is seen [42]. The possibility of the existence of an empirical relationship between plant size or output and the production cost has also been examined using various production functions and the computed F values at a 5% level of significance [96]. It is also imaginable that if the average distance of raw material transportation and acquisition becomes excessively long due to the increased plant capacity, then the pro­duction cost can be adversely affected by the plant size.

Xylose fermentation is being carried out by bacteria, fungi, yeast, enzyme — yeast systems, or genetically engineered micro-organisms. Advanced fer­mentation technology would reduce the cost by 25% or more in the case of herbaceous-type materials, as shown in the study by Schmer et al. [95]. Efforts are being made to achieve the yield of 100% and an increased etha­nol concentration.

Lignin is another major component of biomass, and accounts for its large energy content because it has a much higher energy per pound than carbohy­drates. Because it is a phenolic polymer it cannot be fermented to sugar, and is instead converted to materials such as methyl aryl ethers, which are compat­ible with gasoline as a high-octane enhancer. The combination of the above processes has the potential to produce transportation fuels at a competitive price.