PEER-REVIEWED ARTICLE bioresources.com Secchi et al. (2019). “LCA of lignin as fuel or co-product,” BioResources 14(2), 4832-4865. 4832 Use of Lignin Side-Streams from Biorefineries as Fuel or Co-product? Life Cycle Analysis of Bio-ethanol and Pulp Production Processes Michela Secchi, Valentina Castellani, Marco Orlandi,* and Elena Collina Increasing the profitability of lignin side-streams is a challenge in the scientific community. Lignin residue originates from black liquor and lignin cake, which are residues from pulp and bio-ethanol production. This paper presents a life cycle assessment study to investigate how pulp and bio- ethanol processes vary in their environmental performance when a fraction of lignin is removed and to identify the best alternative energy source. Fossil energy, natural gas, and cogeneration were evaluated as heat and power alternative sources. The results showed that lignin removal does not considerably affect the environmental performance of the baseline systems and does not generate a relevant risk of “burdens shifting.” Natural gas was the best alternative of power source in a bio- ethanol system, whereas cogeneration showed better compatibility with pulp mills. For the analyzed systems, the necessary allocation distributed the impact contributions between the main products (bio-ethanol/pulp) and the co-products (lignin-cake/black liquor), counterbalancing the impact increase due to the introduction of the new heat, electricity supply, and additional treatment aimed at lignin extraction. Finally, sensitivity analyses confirmed the low influence on the results of the substitution ratio. Keywords: Lignin; Side-streams valorization; Biorefinery; Life cycle assessment; Allocation criteria; Green chemistry; Industrial symbiosis Contact information: Department of Earth and Environmental Sciences, University of Milano-Bicocca, Piazza della Scienza 1, 20126 Milano, Italy; *Corresponding author: marco.orlandi@unimib.it INTRODUCTION Second generation bio-ethanol conversion and pulp production are both well- known technologies. There is an abundance of literature reviews dealing with these industry processes and their sustainability (Aresta et al. 2012; Aditiya et al. 2016). Many aspects, such as the choice of energy sources for pulp mills, the problem of competition in land use for food crops, or the energy profitability of bio-ethanol compared to traditional fuel sources, have been investigated from a life cycle perspective (Bai et al. 2010; Cherubini and Ulgiati 2010; Gaudreault et al. 2010; Naik et al. 2010; Cherubini and Strømman 2011; Fazio and Monti 2011; Aresta et al. 2012; Tonini et al. 2012). One of the current challenges in the scientific community is the use of lignin residue coming from these activities (Sannigrahi et al. 2010; González-García et al. 2016). Lignin, the second most abundant polymer in nature, is the generic term for a large group of aromatic polymers resulting from the oxidative combinatorial coupling of 4- hydroxyphenylpropanoids (Boerjan 2005). Lignin is one of the main compounds present in black liquor and lignin cake, which are residues from pulp and bio-ethanol production processes. As aromatic structures are still present, they have sustainable and economic