Coupling Exergy with the Emission of Greenhouse Gases in Bioenergy: A Case Study Using Biochar

Abstract

The loss in the quality of energy throughout any process can be assessed by the thermodynamics magnitude related to its entropic performance—the exergy. This indicator has been suggested as an environmental index, as an alternative to life cycle assessment (LCA), which is a classic tool for this purpose. This study assesses the potential of coupling the life cycle approach and exergy in a bioenergy supply chain environmental performance characterization, examining two scenarios in the sugarcane agroindustry. The first one, the reference scenario, is a classical production, and the second includes the returning of a portion of residual biomass from the plant, in the form of biochar, to agricultural soil. The use of biochar engendered an increase in sugarcane productivities and a reduction of nitrous oxide emissions. These changes resulted in scenarios 1 and 2, reducing the exergy destroyed from 390 to 355 MJ/MJ ethanol (9.0%) and decreasing the greenhouse gases emissions (GHG) from 11.8 to 11.0 g CO2-equivalent/MJ ethanol (6.8%). The latter represents an improvement in the use of carbon. A sensitivity analysis showed that the effect of changing productivity was quite significant: The exergy showed a sensitivity of −0.49, and in total emissions, this figure was slightly lower, at −0.41. By changing the emissions of N2O in the soil, the sensitivity of exergy was almost null, and the total emissions were 0.077.