Modeling of microalgal biodiesel production integrated to a sugarcane ethanol plant: Energy and exergy efficiencies and environmental impacts due to trade-offs in the usage of bagasse in the Brazilian context

Abstract

The Brazilian sugarcane mills already provide significant amounts of renewable products (ethanol and bioelectricity) through self-sufficient bagasse cogeneration. Gaseous outputs of the sector can provide streams rich in biogenic carbon dioxide (CO2), suitable for the autotrophic cultivation of microalgae and aim, hereinafter, their conversion into biodiesel. This work modeled the integration of an ethanol plant with a cogeneration system, covering the enzymatic hydrolysis of bagasse and the production of microalgae biodiesel. A standard ethanol plant yielding 96 L ethanol/t cane may produce an additional 7.5–30 L microalgae biodiesel/t cane but at the expense of its energy self-sufficiency; the biorefinery must recur to fossil resources unless other biomass (besides bagasse) and coproducts by the plant's neighborhood can be exploited. The process simulation is conducted in nine cases derived from a central composite design with two factors – the fraction of bagasse destined for the hydrolysis system (x1) and the fraction of ethanol sent to produce anhydrous ethanol (x2) – and three levels for each factor (0, 50, 100%). Energy ratios between representative outputs and inputs – Net energy ratio (NER) and Fossil energy ratio (FER) –, direct CO2 emission, and exergy efficiencies were calculated as representative responses to evaluate the plant's technical and environmental efficiencies. Statistical tools (t-Student test, ANOVA, and R2) showed that the constructed models were robust and that the responses varied significantly with x1, while x2 produced mild or negligible effects. The highest achievable NER and FER were 0.83 and 2.09, respectively. The whole system resulted in a FER equal to 1 when bagasse usage is split in half between the hydrolysis and cogeneration systems. This result guarantees that the energy related to the useful products is higher than the requirements of fossil energy inputs, within an acceptable range. The optimum CO2 emission and exergy efficiency were found when 6–8% of the bagasse is hydrolyzed, which is also a range covering attractive energy ratios. The proposed integration benefits the sector, but resorting to renewable sources to supply energy deficits, developing the technological exploitation over wasted coproducts, and studying heat and mass integration of processes are highly recommended to enhance the global-system performance.