Are eucalyptus harvest residues a truly burden-free biomass source for bioenergy? A deeper look into biorefinery process design and Life Cycle Assessment

Abstract

With the advent of RenovaBio, Brazil has cast a new light towards the life cycle of cellulosic ethanol. Once considered a resource intensive alternative pathway to achieve the same biofuel, second-generation approaches can now provide an economic advantage due to their potentially lower carbon footprint. The exploration of lignocellulosic harvest residues to this end can be beneficial, since productivity can be increased while not expanding cultivated areas. Eucalyptus forest residues are an example, result of logging and harvest procedures, being a low-cost and readily available biomass. Through an integrated biorefinery process simulation and a Life Cycle Assessment of the coproduction of ethanol and electricity, it was analyzed whether forestry burden is truly relevant when exploring this material, identifying technical and environmental bottlenecks. The biorefinery design implementation of anaerobic digestion and energy integration allowed a productivity boost of 20% for ethanol and 115% for electricity. With a 80 km collection radius, an annual production capacity of 30.3 ML could be achieved in the Campinas region. Enzymes were identified as the main environmental hotspot, but inconsistent published datasets and lack of transparency lead to inconclusive results regarding this input. While the burden associated with the lignocellulosic feedstock is relevant in most impact categories, the main bottleneck resides within the biorefinery itself, with inputs related to pretreatment and hydrolysis, boiler emissions and water consumption. Nevertheless, eucalyptus harvest residues cannot be considered a burden-free resource, since additional operations such as retrieval and transportation cannot be dismissed and often surpasses the impact potential of the aforementioned forestry activities.