Use of lignin side-streams from biorefineries as fuel or co-product? Life cycle analysis of bio-ethanol and pulp production processes

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.