Written by: Wienke Reynolds – CTO at Lignopure
Lignin sources
Wood, grasses, cereals and many more – lignocellulosic biomass is all around us, and so is lignin! Even if most have never heard of it, lignin is a big part of the bio-based polymers that surround us and that we unknowingly use or “produce” in our daily lives. So it is very likely that you have encountered lignin many times already without even knowing it!
In the complex matrix with cellulose and other sugar polymers of wood or herbaceous plants, lignin provides plants with their strength and resilience. Take, for example, wooden houses and furniture: Their longevity speaks for themselves. And for every sheet of paper or other cellulose pulp product we use, there is an almost equal amount of lignin to cellulose that has been produced as a side stream in the industry.
A byproduct of biorefineries and the pulp and paper industry
Additionally, impressive biorefinery projects are being implemented all around the globe as part of bioeconomy programs that will soon be capable of providing large amounts of 2G (second generation, non-food) bioethanol. In this case, lignin is also a massive side stream which should be valorised for the amazing resource it is. In the early stages of 2G biorefining, lignin is mainly burned to provide energy for the biorefining and pulping processes and to recover valuable chemicals used in the process. Lignin has a good heating value and energetic use definitely has its legitimate place in lignocellulose circularity.
However, we believe that lignin can do much more than that, and thus also prefer not to call lignin a waste product, but rather a by-product of biorefining and pulping. In R&D, the possible lignin applications seem almost endless, ranging from concrete, asphalt and bitumen to resins, bioplastics, food, pharmaceuticals, coatings and even cosmetics, just to name a few.
Lignin categorization
The beauty of lignin is that its versatility is almost as great as its possible applications. Depending on the regional ecosystem and available biomass, the isolation technology as well as downstream processing, lignin can show completely different properties – not only in chemical composition and structure, but also in very trivial properties like color or odor. This is often overlooked when people start working with lignin, but we at Lignopure have committed ourselves to promoting the use of lignin by have explicitly committed processing the lignins for applications that match their properties.
Basically, lignin can be categorized based on both biomass source and its extraction process: Biomass sources can be hardwood (e.g. beech, birch, oak, ash, etc.), softwood (pine, spruce, fir, larch, cedar, etc.) and/or herbaceous plants (mainly poaceae such as cereals, bamboo, rice, reeds, maize or sugarcane). Many of these biomasses are already being utilized by the bioeconomy on a (pre-) industrial scale.
Different extraction methods lead to different lignin biomasses
From these biomass types, different processing methods can be used. These are commonly grouped into pulping and biorefining processes, although there are also some hybrid processes of both groups. Furthermore, lignin can even be found in more exotic side streams such as biomass pyrolysis oil or biogas digestate.
In pulping, cellulose pulp is the main product (polymeric cellulose), requiring the lignin to be dissolved from the lignocellulosic matrix. The most prevalent process in this group by far is the Kraft process, which is complemented by sulphite and soda pulping, for example. In all cases, if the lignin is to be used as a product, it is precipitated from the pulping liquor and further purified and processed. Due to the strong smell, sulphur-containing lignins such as Kraft lignin or lignosulfonate have not yet been successfully used in consumer products such as cosmetics. However, advanced deodorization technologies may be able to change this.
In contrast, a major goal of biorefining is not to produce pulp, but to obtain chemicals and products from glucose. Consequently, the cellulose is depolymerized, yielding hemicellulose and macromolecular lignin as side products. Typically, 2G biorefineries operate by using a three-step process, comprised of a mechanical disintegration/pre-treatment of the biomass, a thermal treatment (e.g., steam extraction, steam explosion, hot water extraction, etc., with or without additional catalysts such as acids) and a subsequent enzymatic depolymerization of the cellulose from the pre-treated biomass. The glucose is further processed into bioethanol or other chemicals such as glycols, whereas the lignin remains as a solid residue which is then further processed. Biorefinery lignins are the closest to the native structure of lignin in plants and have great/exciting potential for applications in the life sciences.
Besides pulping and biorefining, there are technological processes that combine the working principles of both process groups, making them able to yield either pulp, sugars, or in some cases, even both. In the context of these hybrid processing groups, organosolv and acetosolv processing come to mind. Some of these processing groups even combine biorefinery pre-treatments with a subsequent pulping step, not to forget about novel technologies such as ionic liquid, deep eutectic solvent or even supercritical fluid processes.
Beyond the technological differences of the isolation or extraction processes and lignin qualities, criteria like pricing, availability and political or social circumstances are also essential to making lignin-based innovation possible. We at Lignopure believe that we can only be successful in building novel lignin-based business models if we have a comprehensive understanding of both sides: the various lignin sources and qualities as well as the requirements for its high-value applications.
References
https://doi.org/10.1016/j.rser.2020.109768
https://doi.org/10.1515/9783110658842
https://link.springer.com/book/10.1007/978-3-030-10961-5
https://www.sciencedirect.com/book/9780323852692/biofuels-and-bioenergy
https://doi.org/10.1002/cite.202200079