With conservation initiatives like the European Union’s new forest strategy for 2030, sustainability is becoming an increasingly important theme in the biorefinery industry. One way to minimize the environmental impact of biorefineries is more efficient utilization of side-streams. Making up around 15 to 30% of the mass of wood, lignin is one of the most prominent biorefinery byproducts whose full potential has yet to be realized.
Approximately 50 million tons of side-stream lignin are produced each year1. Most of this is burned, as lignin’s chemical composition poses considerable challenges to finding practical industrial applications. Nevertheless, interest in lignin tends to resurface every 30 years or so, with one of these high-interest seasons now in full swing. A better understanding of lignin’s structure through comprehensive laboratory testing is key to overcoming the challenges in its processing and bringing commercially successful innovations to the market.
What is lignin, and how is it extracted?
Lignin is nature's second most abundant polymer and one of the major components in the wood lignocellulosic structure. It helps give plants structure as a constituent of cell walls and acts as a natural barrier against microbial attacks.
From a structural perspective, lignin consists of three main components: p-hydroxyphenol (H), guaiacyl (G), and syringyl (S). The exact chemical structure is complicated and varies based on the extraction method and the type of biomass the lignin is separated from2.
Lignin is usually extracted from biomass during the pulping process. The most common separation method is the Kraft pulping method, which uses sodium hydroxide and sodium sulfide as cooking chemicals. The sulfur component results in an unpleasant odor, which complicates further processing. It is possible, however, to remove the odor through oxidation.
Potential commercial uses
Several leading biorefinery companies have invested a considerable amount of resources into lignin processing in recent years. These include Stora Enso3 and UPM4, both of which produce and sell kraft lignin powder to other manufacturers while also developing lignin-based innovations themselves.
Lignin has excellent binding properties, which makes its use as an adhesive in wood construction one of the most potential commercial applications. Significant progress towards producing commercially viable adhesives is currently being made. As an example, a group of researchers at Aalto University recently developed a strong, fire-resistant adhesive with over 90 percent lignin content6.
Lignin may also be used to produce biobased alternatives to plastic, be converted into transportation fuels, or be used as a binder in asphalt or composites5. Being fully organic and currently under-utilized, its successful utilization in such applications would present a considerable sustainability advantage.
Challenges in commercialization
While the potential high-value uses of lignin are various, only a few have been successfully commercialized. This is mainly due to the heterogeneous structure of lignin: as its precise composition varies from one batch to the next, it is challenging to establish effective processing procedures.
Many potential applications also require significant pretreatment to be performed, raising the costs and diminishing the profit potential. Pretreatment is necessary to remove the odor of sulfur and possible impurities, in addition to which the brown color may need to be altered for some applications.
Overcoming the challenges in lignin utilization
Understanding the composition and properties of each batch of lignin is key to its successful utilization. A range of different laboratory analyses is required to determine a lignin sample's precise content and properties.
The first step is often to determine the lignin content of biomass. This is usually achieved through acid hydrolysis, which makes it possible to determine the amounts of both acid-insoluble and acid-soluble lignin. Once lignin has been separated from the biomass, its ash content, carbohydrate composition, and elemental composition can be determined through additional tests.
In-depth analysis of separated lignin is made possible by modern spectroscopy techniques, such as FTIR, Raman, and NIR. They are not as straightforward to use as more traditional analyses, however, as obtaining meaningful results requires fairly complicated statistical analysis and large sample amounts.
If you are interested in laboratory testing of lignin or high lignin content biomass, you can find many testing options in the biorefinery section of our webshop. In the case of novel spectroscopy-based methods, please contact our testing experts at firstname.lastname@example.org to discuss your testing needs.
2Rui Katahira, Thomas J. Elder and Gregg T. Beckham, Chapter 1: A Brief Introduction to Lignin Structure, in Lignin Valorization: Emerging Approaches, 2018, pp. 1-20
5Virpi Raski: Eco-glue can replace harmful adhesives in wood construction
6Uses of lignin on Valmet’s website