Environmental Challenges and Opportunities of Lignin
Conference contribution, 2019
Lignin is the most abundant bio-polymer on Earth, gluing the cellulose and hemicellulose fibers together in the tree stem and providing stiffness. Lignin is presently mostly available in side streams of biorefineries and pulp mills (e.g. in the black liquor of the Kraft pulp mill). It is almost always combusted for internal energy use at the plant due to its present low market value. Though lignin can be extracted and valorized, for example via acidification followed by precipitation and membrane filtration or by solvent extraction, this is seldom done today. Nevertheless, lignin is gradually being considered an interesting raw material for various products and applications, ranging from biofuels to carbon fibers. However, in environmental life cycle assessments of lignin-based products, the environmental impacts of lignin production typically have not been accounted for with the rational that lignin is a waste that needs to be taken care off.This argument will no longer be valid when lignin extraction processes are further developed and various markets for lignin open up.
Conducting a life cycle assessment of lignin production is complicated. As lignin is a product of a multi-output process, there will be inherent challenges regarding the choice of system expansion or how to allocate environmental impacts to this product and to other products. Allocation based on economic values is particularly challenging as the market price of lignin and of other products from the process is the basis for allocating impacts, and the future market development for lignin is still rather unexplored and uncertain. The same challenge can be applied to system expansion, as what is being substituted could change in the future.
The aim of the presented study is to provide guidance on how to better assess lignin production in life cycle assessments. The cradle-to-gate environmental impacts of 1 kg of lignin from a Kraft pulp mill is assessed. Different allocation methods as well system expansion by substitution will be applied to the multi-output process to assess and illustrate what influence these will have on the final environmental impacts of the lignin.