Comparative analysis of multifunctionality approaches in LCA: a case study on lignin recovery from kraft pulp mills and thermoplastic production.

Lignin is a natural polymer that binds cellulose and hemicellulose. It is one of the most abundant biopolymers in nature, second only to cellulose. Lignin is mainly produced as a by-product of the pulp and paper industry and biorefineries, but despite its abundance, its use has not been implemented on a large scale. In kraft pulp production, lignin is typically burnt in recovery boilers to generate energy. However, since the recovery boiler is the bottleneck of the production process, the possibility of extracting lignin from the process has been becoming more and more attractive in the last ten years, and different methods have been implemented to remove the lignin by precipitation. This not only increases pulp production (marginal tonnage) but also opens up the possibility of using lignin in other applications, such as thermoplastic polymers, which will be discussed in this project. Through the instrument of life cycle assessment (LCA), this study aims to evaluate the potential environmental benefits of lignin recovery and its transformation into thermoplastic polymers, compared to the regular production of kraft pulp and fossil-based plastic production. In an attempt to reduce the environmental burdens, will be discussed also some methods to optimize not only the pulping process but also the production of the thermoplastic from precipitated lignin. LCA is a robust yet flexible framework where methodological decisions, such as how to address multifunctionality, can lead to varying outcomes. Multifunctionality occurs in processes or systems that generate multiple outputs, known as co-products, each serving different functions. While the primary aim of this thesis is the one stated above, an additional objective is examining the consistency of these potential environmental benefits across different methodological approaches used to address multifunctionality. In this project, LCAs of lignin-based thermoplastic material and high-density polyethylene (HDPE) production were compared, and three other cases were analyzed: the regular operation of a kraft pulp mill, a modified kraft mill with lignin recovery, and a case considering marginal lignin production due to lignin recovery. Each case is evaluated using three approaches: Attributional LCA with economic allocation, Attributional LCA with system expansion, and Consequential LCA with system expansion. The study, therefore, wants to assess whether or not it is environmentally beneficial to recover lignin to produce thermoplastic material and to evaluate whether or not it is beneficial to produce the marginal tonnage by freeing up space in the recovery boiler. By examining the impact of different approaches to multifunctionality on LCA results, this project seeks to understand how methodological choices influence conclusions about the environmental performance of lignin recovery.