An integral study was carried out in Brazil regarding the ecological, economic, and social aspects of fuel ethanol production including the agricultural and indus­trial steps. Three ethanol production plants were analyzed and the results were quantified through the environmental efficiency of each process. According to the analysis performed, none of the three plants achieved the highest level estab­lished for environmental efficiency (Borrero et al., 2003). Prakash et al. (1998), in turn, proposed an indicator called figure of merit, which is expressed as the ratio between the net energy yield of a fuel (the difference between the gross energy produced during its combustion and the energy needed to produce it) and the CO2 emissions produced by that fuel. Using this figure, it was concluded that for anhy­drous ethanol production from cane molasses in India the net energy yield is about 2% the potential of ethanol as a gasoline substitute in road transport has been estimated to be as high as 28%. A similar indicator based on the figure of merit has been proposed by Hu et al. (2004a) as well. Lynd and Wang (2004) developed a methodology for evaluating fossil fuel displacement for biological processing of biomass in the absence of product-specific information other than the product yield and whether fermentation is aerobic or anaerobic. With the help of this meth­odology, the authors answer affirmatively the question: Are there biomass-based processes and products with fossil fuel displacement sufficiently large that they could play a substantial role in a society supported by sustainable resources?

Another methodology that has been used to measure the environmental fea­sibility and sustainability of bioethanol production processes over a long term is the so-called emergy analysis (see Chapter 2, Section 2.2.5). Emergy is the abbreviation of embedded energy. This methodology takes into account the dif­ferent system inputs, such as the renewable and nonrenewable energy sources, goods, labor, and all the materials involved in a process. Each input is assessed under the same physical condition, the equivalent solar energy (emergy) concen­trated to supply the given input. In this regard, the emergy is a measure of the global convergence of energy, time, and space needed to make available a given resource. Bastanioni and Marchettini (1996) analyzed four systems for bioethanol production and concluded that mankind is far from a sustainable production of biofuels because it appears that the biomass energy cannot be the basic energy source in countries having a high level of energy consumption, with arable land being the major constraint. Other methodologies for environmental assessment of fuel ethanol production from lignocellulosic biomass have been reported as the application of the sustainable process index (SPI), a highly aggregated indicator of the environmental pressure of a process, to the bioenergy assessment model (BEAM), which have allowed showing that the bioenergy integrated systems are superior to the fossil fuel systems in terms of their environmental compatibility (Krotscheck et al., 2000).

Berthiaume et al. (2001) propose a method to quantify the renewability of a biofuel taking as an example the ethanol production from corn. This method con­siders the exergy (useful or available energy) as the quantitative measure of the maximum amount of work that can be obtained from an imbalance between the physical system and the environment surrounding it. The exergy was accounted for evaluating the departure from ideal behavior of a system caused by a con­sumption of nonrenewable resources through the so-called restoration work. These authors point out that ethanol production is not renewable, though they emphasize that this evaluation was performed employing many simplifications. Thus, further research is needed to improve the accurateness of the results and the validity of the conclusions.

Undoubtedly, the unification of the environmental assessment criteria is required in such a way that the main aspects of fuel production and utilization, including both fossil fuels and biofuels, are taken into account. The large number of consideration, supposition, assumption, approximation, and methodological approaches limit the use and comparison of the environmental indicators that have been applied to fuel ethanol production. An example is the contradictory results about the environmental and energy performance of corn ethanol that have been published (Patzek et al., 2005; Pimentel, 2003; Shapouri et al., 2003; Wang et al., 1999). This difficulty imposes some constraints when different pro­cess alternatives for bioethanol production are being evaluated, especially if the optimization of an objective function considering not only the technoeconomic indicators but also the environmental performance indexes of the different tech­nological configurations proposed is performed.