Life-cycle assessment of bioethanol production from sweet sorghum stalks cultivated in the state of Yucatan, Mexico

Bioethanol is being promoted in Mexico to be used in a blend with gasoline. On the north of the Yucatan peninsula, bioethanol could be produced from sweet sorghum, as it can grow efficiently on this land; it can be harvested 2-3 times in a year and possesses a better agronomical stability than sugarcane with low nitrogen requirements and high productivity. In this work , the potential environmental impacts and energy efficiency of bioethanol production from sweet sorghum were evaluated using life-cycle assessment. Four scenarios were evaluated: scenario PI considered only bioethanol production from the stalk juice; scenarios PII and PIII added cogeneration from the dry-stalk biomass in single and combined cycle, respectively. Scenario PIV considered bioethanol production from both stalk juice and dry-stalk biomass. Scenario PI demanded more fossil energy than what was generated as bioethanol, while scenarios PII and PIII were fossil energy independent. Scenario PIII showed the higher net energy ratio (1.89) and a better environmental performance in all CML-IA baseline impact categories. In terms of global warming potential, the scenario PIII showed a mitigation potential of 16% with respect to the fossil reference. In the categories where the sweet sorghum scenarios represented larger emissions than the fossil reference, it was due mainly to the use of fertilizers and the conventional energy consumption in the various processing steps of the biomass. Scenario PIV showed the highest energy demand and worst environmental performance due to large demands of energy and chemicals in the bagasse pretreatment step.