In a previous work (Sanchez et al., 2007), the environmental impacts of fuel ethanol production processes from sugarcane or lignocellulosic biomass were estimated. The evaluation of the environmental performance was performed for three techno­logical configurations employing sugarcane juice as feedstock (stand-alone facili­ties). The first one comprised the utilization of sugarcane bagasse for co-generation of process steam and power and included the production of concentrated stillage for fertilization purposes. The second option did not consider the co-generation though it did consider the production of the concentrated stillage as a fertilizer, thus the bagasse was accumulated as a solid residue. The third variant involved neither cogeneration nor production of concentrated stillage. The simulation data for these alternative flowsheets were taken from a preceding work (Cardona et al., 2005b). In addition, the production of fuel ethanol from lignocellulosic biomass through a process including the dilute-acid pretreatment of biomass, simultaneous saccharifi­cation and co-fermentation (SSCF) and ethanol dehydration using molecular sieves was also analyzed. The simulation data for production of biomass ethanol were taken, in turn, from a previously published work (Cardona and Sanchez, 2006). For the biomass process, two variants were considered: with and without cogenera­tion using the lignin recovered from the whole stillage. For all five technological configurations, the normalized values of the total impact expressed as PEI units per kilogram of ethanol produced were obtained (Figure 10.6).

The values of the total PEI were calculated from the weighted sum of the eight impact categories evaluated by the software WARGUI. The PEI for each category is presented in Figure 10.7. The weighted factors employed for six of the eight impact categories were taken from the work of Chen et al. (2002) as follows:

• Global warming 2.5 (equivalent to GWP in the WAR algorithm)

• Smog formation 2.5 (equivalent to PCOP)

• Acid rain 10.0 (equivalent to AP)

• Human noncarcinogenic inhalation toxicity 5.0 (equivalent to HTPE)

• Human noncarcinogenic ingestion toxicity 5.0 (HTPI)

• Fish toxicity 10.0 (equivalent to ATP)

The two remaining impact categories in the WAR algorithm methodology were assigned the following weights: TTP 2.5 and ODP 2.5.

The results obtained indicate that the use of sugarcane presents a higher envi­ronmental friendliness than the use of lignocellulosic biomass. This is explained by the complexity of the conversion process from biomass to ethanol. For such conversion, a pretreatment step involving the use of inorganic acids and high pres­sure is required. In addition, the hydrolysis of cellulose and fermentation of formed

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FIGURE 10.6 Total output rate of potential environmental impact (PEI) per mass of products for five process configurations for fuel ethanol production: Cane A = production of ethanol and fertilizer (concentrated stillage) from sugarcane employing bagasse for co-generation, Cane B = production of ethanol and fertilizer (concentrated stillage) from sugarcane without co-generation, Cane C = production of ethanol from sugarcane without co-generation, Biomass A = production of ethanol from lignocellulosic biomass without co-generation, Biomass B = production of ethanol from lignocellulosic biomass employ­ing the recovered lignin for co-generation.

FIGURE 10.7 Potential environmental impact (PEI) per mass of product streams for different ethanol production configurations according to the eight impact categories considered by the WAR algorithm (the denominations of the categories are presented in Figure 10.5): Cane A = production of ethanol and fertilizer (concentrated stillage) from sugarcane employing bagasse for co-generation, Cane B = production of ethanol and fer­tilizer (concentrated stillage) from sugarcane without co-generation, Cane C = production of ethanol from sugarcane without co-generation, Biomass A = production of ethanol from lignocellulosic biomass without co-generation, Biomass B = production of ethanol from lignocellulosic biomass employing the recovered lignin for cogeneration.

glucose should be accomplished. These processes imply higher energy expendi­tures. To supply this amount of energy, the combustion of lignin is considered lead­ing to the release of atmospheric emissions containing CO2, CO, particulate matter, and polycyclic aromatic hydrocarbons, which generate important environmental impacts. In fact, if lignin is not burned, the environmental impacts are appreciably reduced (see Figure 10.6).

In the case of the process using sugarcane, the co-generation and the production of concentrated stillage as a fertilizer (configuration Cane A in Figures 10.6 and 10.7) is a good option to diminish the burdens to the environment. However, the configuration considering the cane bagasse as a solid residue and the concentrated stillage as co-product (configuration Cane B in Figures 10.6 and 10.7) shows indi­cators slightly more favorable. The gases released during the bagasse combustion have a higher contribution to the aquatic toxicity calculated by the WARGUI soft­ware than the components of the bagasse itself (see Figure 10.7). This difference is mostly responsible for the better environmental performance of this configuration compared to the scheme involving the co-generation using the bagasse as a solid fuel. Nevertheless, the economic considerations do indicate the evident benefit of burning the bagasse because no money is spent for acquiring the fossil fuels needed to supply the thermal energy for the overall process. If the stillage is not treated and considered as a liquid effluent, the environmental impact potential remarkably increases, as shown in Figures 10.6 and 10.7. The total PEI per kilogram of products is increased by 430% related to the Cane A case. This fact is explained by the very high organic load of the stillage that raises the potential impacts corresponding to the four toxicological impact categories.

It should be noted that the apparent better environmental performance of the configuration Cane B is due in a higher degree to the weighting factors chosen. If equal weighting factors are selected for the four local toxicological impact catego­ries (using a value of 10) related to the global atmospheric impact categories (using a value of 2.5), a lower PEI/kg for the Cane A case is obtained: 0.43 for configura­tion Cane A and 0.74 for configuration Cane B. This selection of weighting factors favors the local effects on human health, flora, and fauna during the environmental analysis more than the effects on the biosphere, which is logical considering that the former effects are more significant in the short and middle terms.