Exergy and CO2 Analyses as Key Tools for the Evaluation of Bio-Ethanol Production
The background of bioethanol as an alternative to conventional fuels is analyzed with the aim of examining the efficiency of bioethanol production by first (sugar-based) and second (cellulose-based) generation processes. Energy integration is of paramount importance for a complete recovery of the processes' exergy potential. Based upon literature data and our own findings, exergy analysis is shown to be an important tool in analyzing integrated ethanol production from an efficiency and cost
... ciency and cost perspective. The paper will finally conclude by highlighting the most important identified observations, together with an identification of the current shortcomings and the specific topics where further research is recommended. Bioethanol: A Promising Biofuel Within first (sugar-based), second (lignocellulosic-based) and third (algae-based) generation processes of bio-ethanol [19, 20] , only the first and second generations are today considered at the industrial and pilot scales, respectively. The traditional feedstocks used for first generation ethanol production are corn, wheat, cassava, sweet sorghum, molasses and sugarcane  . The feedstocks for the second generation are based on non-food, cheap and abundant plant waste biomass (agricultural and forest residue, grass, etc.)  . In this case, all parts of the plant such as leaves, bark, fruits, and seeds can be transformed into useful products. Lignocellulosic biomass is the most abundant on earth [21, 23] . Consequently, lignocellulosic biomass offers significant advantages over the first generation feedstock for ethanol production (e.g., do not compete with agricultural land needed for food and feed production, lower costs are involved, better engine performance is achieved, and new markers for the agricultural sector are stimulated)  . As can be seen in Table 1 , the major fraction of such biomass materials is represented by cellulose, followed by hemicellulose and lignin [24, 25] .