A comprehensive energy and economic assessment of biofuels: When "green" is not enough

Biofuel production systems are sometimes claimed to be able to fill in for future fossil fuel shortages as well as to decrease carbon dioxide emission s and global warming. As such, they are often promoted as a "green" alterna tive to fossil fuels. I present a comprehensive, system-based case study of biofuel production from maize or corn (Zea mays L.) and evaluate it critic ally in this review. The case study is taken as an example of the comprehen sive approach that I suggest for any energy crop. I conclude that the biofu el option on a large scale is not a viable alternative based on economic, e nergy and eMergy (amount of available energy [exergy] of one form [usually solar] that is directly or indirectly required to provide a given flow or s torage of exergy or matter) analyses of the case study data and estimated p ossible improvement of yield and efficiency. This is true for developed cou ntries due to their huge energy demand compared with what biofuel options a re able to supply as well as for developing countries due to the low yield of their agriculture and competition for land and water for food production . However, biofuels may contribute to optimizing the energy and resource ba lance of agricultural, livestock, or industrial production systems at an ap propriate scale. I present a proposal to integrate ethanol production with industrial activities within a "zero emission framework" as a suggestion fo r optimization strategies capable of making the biofuel option more sustain able and profitable in those cases where it is appropriate.