Lignin – the second most abundant natural polymer on earth – was designed by nature to keep plant tissue together and protect it from damage and decay. Lignin presence is particularly significant in woody (lignocellulosic) biomass used as raw material by biobased industries to manufacture goods and commodities. Importantly, such goods and commodities do not compete with food applications.
From a chemical perspective, lignin is an irregular and complex amorphous polyphenol with high carbon content. The nature of lignin is directly related to its role in the plant tissue: it is hydrophobic, flame-retardant, possesses antibacterial and antioxidant properties and is capable of absorbing UV-radiation.
Much more than waste
In industries involving chemical processing of biomass, cellulose and sometimes hemicelluloses are purposefully isolated and converted into products while lignin is typically found in a side stream dissolved in pulping liquor or as solid residue after sacharification.
Throughout the history, lignin has been notorious for its complexity, resistance to treatments, peculiar brown colour and, in case of kraft lignin, strong odour originating from sulphur containing functionalities.
Fortunately, the future looks brighter.
The perception of lignin is about to change
Climate action, the scarcity of fossil energy and material sources, and the world political situation require rapid change and the development of new solutions to reduce the use of non-renewable natural resources.
Biomass is a traditional but at the same time novel renewable source of materials and energy which surprises us with its endless potential. However, to be able, in the long run, to preserve biodiversity and make the most of biomass, we need to be smart, inventive and broad-minded. This is where lignin steps into the picture.
The abundancy of lignin as a biomass component multiplied by the amount of biomass processed chemically to isolate cellulose and lignin into separate fractions yield an impressive quantity of lignin potentially available for conversion into products. More importantly, actual lignin availability as raw material is gradually increasing as its economy and isolation technologies improve.
It is especially gratifying that, in addition to academic researchers, small start-up companies and large industrial players have taken lignin into account in their production, and the future looks very promising.
From macromolecule to products
There are several general approaches to lignin utilization based on how much the molecule is processed, namely, lignin can be used as macromolecule, or, alternatively, it can be depolymerized or degraded.
Lignin’s intrinsic natural properties can be best employed when using it as a macromolecule. The macromolecule can also be carbonized to obtain a variety of technical and specialty carbons.
Chemical and enzymatic depolymerization can reduce the size of the macromolecule or yield monomeric units that can be used as such OR converted into basic chemicals. Depolymerization of lignin is a non-trivial task due to its high chemical resistance.
Finally, lignin can be degraded using thermochemical treatments like combustion, gasification and pyrolysis to produce energy, biofuels and basic chemicals.
Lignin brings new business – from food to batteries
Potential products made from lignin range from energy and bulk products to specialty products with precise properties and performance.
Though still relevant today, energy applications bring little added value to the unique and versatile biopolymer.
On the other hand, lignin can be converted into products with very high revenue. The most fascinating potential high-tech applications, for example, structural and non-structural carbon fibres, sensors, and energy storage electrodes all of which are possible to obtain by carbonization and upgrading of a lignin precursor. The viability of such applications is rapidly increasing.
Other exciting segments are cosmetics, pharma and food. Would you fancy lignin-based sunscreen or vanillin? In fact, you might have had biobased vanillin in your pot of yoghurt for quite some time.
Using lignin as an additive is also very interesting. In such applications, lignin partially or fully replaces fossil-based components, for example, in phenolic resins, polyurethane foams, paints, thermoplastics and bitumen. In many applications, however, full replacement without compromising quality and performance is still a significant challenge.
Whatever the application, the main prerequisites for success in terms of bioeconomy are sustainability of the source and the processing steps and commercial feasibility. Is completely possible that lignin can become a full member of the bioeconomy society.
We at Sweco help small and large businesses implement bioprocesses from a concept to start-up and operation, from pilot to commercial scale. We are most willing to take a leap into sustainable future together with our collaborators and partners.
Lidia Testova-Färlin, prosessiasiantuntija, Sweco, lidia.testova@sweco.fi