Abstract
PurposeLignin extraction in pulp mills and biorefineries are emerging technologies. Lignin is always the product of a multi-output process. Assessing such processes using life cycle assessment (LCA) requires the environmental impacts to be divided between the co-products of the system, referred to as allocation. This article explores different allocation approaches for lignin and illustrates the influence of the choice of allocation approach on the climate impact in a case study.MethodTen different applicable allocation methods were found in literature and two more were developed. Lignin production in a Kraft pulp mill using the LignoBoost process for lignin extraction was selected as a study object for the case study, and due to limited data availability only climate impact was considered. A cradle-to-gate LCA was done for the study object, and all of the twelve allocation approaches were applied; for eight of the methods, factors that strongly influence the results were identified and varied. Finally, the results were put in the context of cradle-to-grave LCAs from literature for different possible uses of lignin to give an indication of how important the choice of allocation approach can be when assessing lignin as a substitute for other raw materials.Results and discussionResults show that all allocation approaches tested were applicable to the special case of lignin, but each one of them comes with inherent challenges. Factors that often have a large impact on the results are (1) market and price of different outputs; (2) what is seen as the main product or the driver of the system or system changes; (3) what the surrounding system looks like and hence what other products will be displaced by outputs. These factors can be particularly challenging in prospective studies as such studies are future-oriented and consider systems that do not yet exist. Finally, the results show that the choice of allocation could have a significant influence on the climate impact on the cradle-to-grave climate impact of the final product.ConclusionsWe recommend for LCAs of lignin-based technologies that allocation methods are very carefully selected based on the goal and scope of the study and that when relevant, several methods are applied and factors are varied within them in a sensitivity analysis. In particular, the driver(s) of the system’s existence or of changes to it, sometimes reflected in market prices of outputs, should be carefully considered.
Highlights
Introduction and backgroundLignin is the world’s most common natural aromatic polymer and is found in plants where it provides stiffness to the stems (Ragauskas et al 2014)
It has previously been recognized that the choice of allocation approach in Life cycle assessment (LCA) of lignin is crucial (Secchi et al 2019) because it can strongly affect the environmental performance of the end product, as was shown for lignin-based carbon fibers by Hermansson et al (2019)
The results show that the climate impact of lignin varies significantly depending on the choice of allocation method: System expansion by substitution exhibits a climate impact close to zero when replacing reading a magazine on a tablet and a negative climate impact, the lowest of all applied methods, when pulp is replacing cotton
Summary
Introduction and backgroundLignin is the world’s most common natural aromatic polymer and is found in plants where it provides stiffness to the stems (Ragauskas et al 2014). Lignin has received more and more attention as a possible future renewable raw material for products such as adhesives (e.g. replacing urea-formaldehyde, see Yuan and Guo (2017)) and carbon fibers (e.g. replacing polyacrylonitrile (PAN), see the LIBRE (2016) and GreenLight (2016) projects) Assessing emerging technologies using LCA (sometimes referred to as prospective LCA (Arvidsson et al 2018)) is challenging as data availability for the technology is low and it is difficult to predict both how the technology and its surrounding systems will develop, especially if many years can be expected to be needed before its large-scale introduction on the market This makes the choice of allocation method in LCAs of lignin even more difficult as some of these uncertain factors affect how allocation should be done
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