Modelling of a Brazilian ethanol plant: impact of the bagasse use and the ethanol dehydration on energy efficiency and sustainability

Abstract

Ethanol has a large share in Brazilian primary energy consumption and plays a significant role in the mitigation of the emissions from combustion engines, as national regulations advance towards increasing anhydrous ethanol content in gasoline or by the direct use of hydrated ethanol as fuel. A simulation of a sugarcane ethanol plant with a regular yield of 85.3 L of ethanol per ton of cane was carried out. The investigated process starts from the cultivation of sugarcane to the production of hydrated and/or anhydrous ethanol. The study considered that bagasse can be either burnt for cogeneration or used as feedstock for the enzymatic hydrolysis system. Additionally, alternative energy sources (such as natural gas (NG) burnt in a boiler or electricity from the Brazilian National Interconnected System (NIS)) are considered in order to cover the domestic demand in cases where biomass lacked for energy purposes. Net energy ratio (NER), fossil energy ratio (FER) and total CO2 emission were modelled through central composite designs to assess scenarios in which the fraction of bagasse sent to hydrolysis (x1) and hydrated ethanol sent to dehydration (x2) both varied between 0 and 100%. The application of statistical tools (ANOVA, Student’s t test and R2) showed that x1 caused significant variations on the investigated parameters, while x2 proportioned mild effects. NER varied in the range 0.795–0.984, with an optimal value being found at (x1, x2) equal to (81.82, 0) and (36.67, 0) when the system is coupled to NIS and a NG boiler, respectively. FER was verified as high as 11.910 with energy from NIS to produce hydrated ethanol with maximal cogeneration from the bagasse. It diminished to 3, when around 40% of bagasse is sent to enzymatic hydrolysis instead for the cogeneration system and NG complements the domestic demand.