As one of the most abundant natural polymers on Earth, Lignin is found in all cell walls of vascular plants. Its name is derived from Latin Lignum (meaning of wood). There is increasing concern about the quality of Lignin from researchers and industrial buyers, because it is perceived as the core raw materials for next-generation sustainable products. In the following, knowledge about Lignin, ranging from basic biology to industrial applications, will be explained.
As a natural substitute of petrochemicals, Lignin attracts increasing investment and research with the global concerns of plastic pollution and carbon emission.
1. What is Lignin?
Lignin is a type of complex natural phenolic biopolymers, and it accounts for 15-30% of the dry weight of the wood. It functions as the skeleton frame of a plant to support the weight. It also enables the efficient transport of water from roots to treetops. Besides, it can strengthen the cell wall, prevent water loss, and protect the plant from bacterial or fungal infection.
Lignin is the essential component of the global carbon cycle as it accounts for around 30% of all organic carbon on Earth. There are more than 300 billion tons of Lignin stored in global living plants, which makes it one of the richest organic materials on Earth.
The plant cell wall is composed of Lignin and cellulose, in which cellulose provides structural support and Lignin provides rigidity and water resistance. The natural composite structure is more effective than most artificial composite materials.
2. Where Does Lignin Come From?
Lignin exists in the cell walls of vascular plants, including trees, bamboo, and agricultural residue such as straw and corn straw. The content and structure of Lignin in hardwood and softwood are completely different. There is 25-35% of Lignin in softwood, such as pines, and 18-25% Lignin in hardwood, such as oaks. The content of Lignin in non-woody plant such as herbaceous plant is the lowest, which accounts for 15-20% only.
The difference in content and structure affects the application in industrial use directly. Other than woods, there are billions of tons of Lignin-containing wastes produced by agriculture annually. The potential sources of lignin, including rice husk, sugarcane bagasse, and wheat straw, are not fully utilized nowadays.
The cost of the extraction of Lignin is low because it is originally an agricultural waste. There are 700 million tons of straw produced annually, and they provide raw materials almost free of charge if the Lignin can be extracted efficiently.
3. Who Discovered Lignin?
A chemical manufacturer in France named Anselme Payen accidentally discovered two different substances after the treatment of wood using nitric acid and alkaline solutions in 1838. One of them was called cellulose, and the other one was described as a coating material. The secret behind this discovery took a century to unravel.
In 1865, a German botanist, Frank Schulze, named the coating material Lignin, which is derived from Latin Lignum. Some scientists, such as Karl Freudenberg, unraveled the complex chemical structure of Lignin in the 20th century. The complete deconstruction from initial discovery took more than 100 years.
The discovery of Payen is a significant chemical breakthrough in the 19th century. It laid the foundation for the papermaking industry, and also initiated the systematic research of plant cell wall, which propelled the development of biofuels and biomaterials subsequently.
4. Types of Lignin
Lignin can be categorized by plant origin or extraction technique. It is more practical for buyers and engineers to categorize Lignin by extraction technique as it decides the purity, solubility, and applicable range.
Categorized by plant origin |
● Softwood Lignin is mainly composed of Guaiacyl (G) units, while hardwood Lignin is composed of Guaiacyl (G) and Syringyl (S) units. ● Herbaceous plants contain p-hydroxyphenyl (H) units additionally. |
Categorized by extraction technique |
● Kraft Lignin is the most common and accounts for 85% global industrial Lignin. It is the byproduct of the papermaking industry, and is mainly combusted for electricity generation. ● Lignosulfonate is produced by the sulfite process, which is highly soluble in water. It is highly commercialized with the lowest price of approximately $50 a ton. ● Organosolv Lignin is the type of Lignin with the highest purity. It is suitable for high-end applications such as carbon fiber and pharmaceuticals, but at a high price of $750 a ton. ● Soda Lignin is made from non-wood plants such as straw and sugarcane bagasse. It does not contain sulphur and is suitable for special use. |
The choice of Lignin depends on the budget and the use. Lignosulfonate can be the economical choice if it is used for concrete additive or animal feed. For the use of nanoparticle drug delivery systems or high performance composite materials, Organosolv Lignin is recommended because of its high purity. However, the cost can be 10-fold.
5. How Lignin is Produced
Industrial Lignin is a byproduct of the papermaking industry. The most popular method is the Kraft process. After decomposing wood at high temperatures using sodium hydroxide and sodium sulfide, Lignin is extracted, and cellulose is left for papermaking.
Approximately 50 million tons of Lignin are produced by the Kraft process annually. However, 98-99% of Lignin was combusted for electricity generation, and it caused waste. This situation is changing in the industry by Deep Eutectic Solvent and Ionic Liquid Extraction, which can produce Lignin with higher purity. Although the cost is high, more Lignin is moving to the laboratory and market with the maturation of technology.
The reason for combusting Lignin in a papermaking factory is reasonable. It is because Lignin is a high-energy fuel that can provide enough steam and electricity for the factory during combustion. The factory needs to invest extra extraction and purification units to change the current system, which do not provide sufficient financial incentives in the short term.
6. Applications of Lignin
Even though most of the Lignin was combusted, the commercialized part is already widely used. One of the most successful example is Vanillin, in which 15% of Vanillin is extracted from Lignin. The estimated market value of Vanillin had reached US$150 million, and it is one of the most mature commercial applications of Lignin.
In the aspect of construction industry, Lignosulfonate can be used as a water reducer for concrete, which can improve the construction performance. In the aspect of agriculture, Lignin can be used as a soil conditioner and a slow-release fertilizer binder. In the field of carbon fiber, Lignin precursor may reduce the cost of carbon fiber from $20–30 per kg to $5–10 per kg. It is beneficial to the automotive and aerospace Industries after commercialization.
Besides, there are potential applications in the textile industry. Lignin can be modified to natural dyes and textile modifiers. In the electronics industry, Lignin-derived carbon materials are being investigated as electrode materials for supercapacitors and batteries, which may become a sustainable substitute for lithium batteries in the future.
7. Why Lignin Matters for Sustainability
Approximately 50 million tons of Lignin are burned in paper factories annually, which is equivalent to a waste of $25 billion in potential chemical raw materials. Carbon emissions can be greatly reduced with only a small portion of petroleum-based plastics and chemicals substituted by Lignin-derived alternatives.
The issue is getting more attention from governments. The EU Bioeconomy Strategy and the Bioenergy Technologies Office put Lignin as a material for priority development. The importance of Lignin as a renewable, biodegradable raw material will continue to grow because of the global trend of phasing out fossil fuels.
For the general consumers, the development of Lignin means that the package, automotive parts, and medication capsules may be derived from wood or agricultural waste, but not petroleum, in the future. Some products derived from Lignin are already on the market.
8. Future of Lignin-Based Materials
The estimated market value of the global Lignin market was approximately US$ 800 million in 2023, and it is estimated to grow to US$ $1.6 billion in 2030. The compound annual growth rate is approximately 10%. The fields of highest growth are carbon fiber, bioplastics, and nanoparticle drug delivery systems.
The biggest challenge of Lignin development is the instability in structure. The content of Lignin from different plant origin or extraction technique varies significantly, which makes it difficult to standardize for high-end use. It will be one of the core materials in the bioeconomy if the problem of quality control can be addressed.
The closest to large-scale commercialization currently is the Lignin-based carbon fiber. The Oak Ridge National Laboratory had successfully produced automotive-grade carbon fiber from Lignin. Ford Motor Company and General Motors Company are testing the parts. The weight of the vehicle can be reduced by up to 10% if it can achieve mass production, and the fuel efficiency can be improved accordingly.
