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Implications of removing straw from soil for bioenergy: An LCA of ethanol production using total sugarcane biomass

Caldeira-Pires A., Benoist A., Da Luz S.M., Chaves Silverio V., Silveira C.M., Machado F.. 2018. Journal of Cleaner Production, 181 : p. 249-259.

DOI: 10.1016/j.jclepro.2018.01.119

Changes in biotic parameters, primarily in those associated with land use, are a fundamental issue for assessing the greenhouse gas (GHG) balance of bioenergy production. From 2002 to 2017, the transition from manual to mechanized sugarcane harvesting in Brazil resulted in 10¿20¿Mg¿ha-1 y-1 of biomass, increasing incomes by 150% via bioelectricity. This study is a life cycle assessment (LCA) of using straw along with bagasse for bioethanol production. The assessment focused on the global warming potential (GWP), characterizing the biotic and fossil carbon flows for a functional unit of 1¿MJ of 96% hydrated bioethanol production as modeled in GaBi. The model is based on the primary data for a specific cradle-to-gate Petrobras agro-industrial unit. The biotic flow includes the CO2 uptake and the agricultural and industrial emissions. The parametric equations depict the influence of leaving straw in the field on the sugarcane yield and the soil carbon emissions. The remaining CML impact categories are then assessed. Two straw utilization scenarios were studied as follows: the reference system (RS), in which straw is left in the field, and the straw-to-boiler system (SS), in which part or all of the straw is exported for energy conversion. Two sensitivity analyses were performed by considering the effect of the straw on the amount of sugar in the sugarcane juice (SA) and considering the use of a second-generation ethanol unit (2G) on the basis of the hydrolysis of both bagasse and straw. In the RS, 55% of the carbon is captured in the sugarcane (bagasse + juice), and 45% is captured in the straw. The agricultural GHG output is distributed between vinasse fertigation (9.2%) and the straw left on the soil (7.6%). The industrial GHG emissions include the burning of bagasse (46%), fermentation emissions (11%) and the pre-emissions embedded in EtOH 96% (26%). In the SS 100% scenario, the GWP increases by 161%, with the biotic GWP shifting from soil to boiler. In addition, the fossil fuel GWP decreases by 39% due to the use of only 86% of the initial planting area. These results highlight the fact that removing straw to use as extra biomass for electricity is associated with a consequent higher sugarcane yield with an overall higher environmental performance and efficiency when using the captured carbon to produce the same functional unit or the combined ethanol and electricity technological products. In the SA scenario, the GWP is directly correlated with the imposed ±10% variation in the sugar that is transformed into ethanol. In the 2G scenario, the GWP results show that second-generation bioenergy must optimize the amount of straw. The other categories show similar qualitative variations among the scenarios, which are related to fossil and mineral reserve processing or to additional boiler emissions.

Mots-clés : saccharum officinarum; paille; canne à sucre; bioénergie; production énergétique; analyse du cycle de vie; Éthanol; biomasse; bagasse; matière organique du sol; émission atmosphérique

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