Properties and composition of LcB

Wooden biomass, also known as LcB, is easily acquired from forests and green waste. In the modern world, 820 million tonnes of dry biomass are generated. As previously stated, LcB bulk is composed of cellulose, hemicellulose and lignin, with tiny quantities of phenolic and acetyl groups, as well as minerals depending on where it originates ¹⁴. Figs. 1 and 2 show the steps of LcB conversion to biomass, and lignin and biofuels composition. Around 50% LcB is made up of cellulose. Asignificant crystalline-shaped molecule disaccharide A repeating unit of the cellulose chain, called cellobiose, is tightly joined to glucose by powerful hydrogen bonds. Along with these three main components, there are traces of proteins, pectin, amino acids, metals and ashes ¹⁵.

Figure 1: Biomass to biofuel conversion 16. 

Wide range of physicochemical approaches available for delignifying LcB

LcB biochemical conversion is significantly hampered by the refractory nature of lignin present in this substance. Since lignin prevents enzymes from accessing hemicellulose and cellulose during saccharification, purifying LcB by pretreatment is an important step. Lignin can be recovered from biopolymers using a variety of chemical solvents, including alkaline reagents, ammonia, organosolv, DES and IL. To achieve a cost-effective bioenergy process, an efficient pretreatment procedure with less carbohydrates loss and maximum delignification is preferred ¹⁴. However, since each component needs a particular set of reaction conditions to be depolymerized, it is difficult to me, et all the aforementioned parameters in a single pretreatment step. Thus, for effective solubilization, multi-stage pretreatment techniques are frequently used. This section gives an overview of a number of delignification treatment techniques, covering both standalone and combined approaches.

Pretreatment methods for LcB

To separate lignin and hemicellulose parts of LcB, there are several pretreatments classified as advanced, biological, microbiological, chemical, physical-chemical or chemical-physical. Fig. 2 shows various LcB pretreatment methods.

Figure 2: LcB valorization for multipurpose fractionation 17. 

This process is a very important preliminary step to hydrolysis. The following pretreatments considerations are important: low cost and energy consumption with high feedstock yield; no harm to sugars; cellulosic compounds preference for non-hazardous chemicals and low pollution emitting treatments; absence of chemicals or agents that prevent fermentation and hydrolysis; and minimal equipment and labour requirements. The types of pretreatment, environmental factors and bioconversion methods for generating hydrogen from LcB are shown in Fig. 3 and Table 1 ¹⁵.

Figure 3: Various LcB pretreatment methods¹⁶. 

Chemical pretreatment method

The pretreatment method using chemicals such as alkalis, acids and various mixtures of chemicals is more efficient.

Alkaline treatment method

Alkaline treatment process successfully uses the base solution to treat and break down the lignin component. LcB swells as a result of this approach, while crystallinity and polymerization rate are reduced. When CaOH, KOH and NaOH are utilized as chemicals to treat the amorphous surface area, the method to reduce lignin amount is more effective. Even when bases prevent it, hydrolysis is used to treat agricultural, corn and sugarcane waste ¹⁷. The alkaline approach basic operating premise is cellulose propensity to swell when immersed in an alkaline solution. The technique produces the highest biomass when lignin is absent. When breaking C atoms bonds in green biomass, they are highly selective. At 30 ºC, where 73% of the product is made, even gaseous ammonia performs better throughout delignification process ¹⁸.

Acid treatment procedure

Acid pretreatment dissolves hydrogen bonds and van der Waals forces in hemicellulose and xylan content. HCl and H₂SO₄ are frequently employed because they accelerate the hydrolysis of cellulose and other organic materials, and reaction speed. Rarely dissolved, lignin is severely harmed by high pH and T conditions. After H₂SO₄ and HNO₃ treatment, glucose output increases. However, these chemicals also release sulfate or nitrate hazardous compounds. Acid therapies are employed to stop recalcitrance reaction, which cleaves Vander wall attraction by deteriorating C bonds and breaking carbohydrates. The initial step in biohydrogen synthesis is pretreatment, by which 77.5% of it is produced ¹⁹.

Polymeric substances based on lignin

Lignin, which takes up between 15 to 40% of the dry biomass weight in plant cell walls, is the second most common biopolymer ²⁰. Given that it is a complex, highly branching polymer with several aromatic moieties and functional groups, including hydroxyl, methoxyl, carbonyl and carboxyl groups, it has the potential to be used in a variety of polymeric materials. For lignin to be reactive, aliphatic and phenolic hydroxyl groups in particular are required ²¹. Still, they are thought of as reactive zones, including open ortho locations, C5 positions on guaiacyl units and C3 positions on p-coumaryl type units on phenolic rings of phenylpropane units. Due to the numerous stiff aromatic groups found in lignin, it has a reinforcing effect, and can provide stiffness and strength ²². Lignin has frequently been used in the past as a filler or physical blending component in plastic composites. We will focus on complex chemical processes in polymers made from lignin ²³. Table 2 summarises lignin materials.

Polyesters derived from lignin

Common methods for producing polyesters include polyesterifying hydroxycarboxylic acids on their own or dicarboxylic acids with diols or dihalides, and ring-opening polymerization of lactones and cyclic esters. The polymer backbone of polyesters has repetitive ester bonds ²⁴. Grafting polymer chains to lignin hydroxyl group is a standard technique for creating lignin-based copolymers. This method includes grafting-from and grafting-to ²⁵.

Lignin-based epoxy resins

Epoxy resins are thermosetting polymers containing an oxirane ring. They may need extra curing agents, most often amines, or they may self-crosslink on their own. They have a wide range of applications such as in adhesives, high-performance composites, coatings for electronics, due to their highly tuneable characteristics and generally great heat resistance ²⁶. Due to the presence of aromatic rings in its molecular structure, lignin can serve as an appealing bio-based substitute for the vast majority of commercial epoxies made from BPA, which is harmful to human health and environment ²⁷. There are several methods for adding lignin to epoxy resins. The two most frequently used methods are; producing epoxy prepolymers directly from lignin; or making curing agents from lignin or modified lignin ²⁸.

Phenolic-formaldehyde resins based on lignin

When phenol and formaldehyde step-growth polymerize in the presence of an acid or base catalyst, PF resins are created. Due to their many advantageous qualities, such as strong adhesion, thermal stability and resistance to water and chemicals, PF resins are widely utilised in wood industry as a bonding medium for plywood, oriented strand boards and other engineered wood products. Some research look for bio-based substitutes for crosslinkers in lignin-based PF resins, since formaldehyde is a recognised carcinogen. In a recent work, ²⁹ created PF resins through glyoxal, a dialdehyde derived from a range of natural resources, as a replacement for formaldehyde in a ratio of 50%. In OLPG resins that were employed as wood adhesives in plywood samples, up to 50 wt% phenol were substituted with organosolv lignin. In comparison to market PF wood adhesives, plywood made with 50% OLPG showed noticeably greater tensile strength and modulus of elasticity in both dry and wet circumstances. Another biobased substance that is frequently utilised in PF resins as a formaldehyde substitute is an organosolv HMF. It is an aromatic aldehyde generated from lignocellulose and cellulose that is employed to make several compounds ³⁰. Fig. 4 shows a brief overview of lignin applications, according to ³¹.

Figure 4: Brief overview of lignin applications 31. 

Importance of agricultural waste in a circular economy

With the expansion in global population and demand for agricultural products, there is also an increase in agricultural waste. This agricultural waste generation has become a concern that must be addressed. In China, annual agro-waste amounts to roughly 0.9 billion ton. The simple disposal of this agrowaste has the potential to harm the ecosystem by polluting soil and water sources. Various options for using agricultural wastes for energy generation are being studied. Non-renewable fossil fuels have met the world enormous energy need in contemporary era. As an alternative, emerging renewable fuel sources are among the most pressing research priorities today ³².

Among the renewable energy sources, biomass has been proposed as a viable replacement for crude oil-based refineries. C biomass, as is well known, is a C-neutral, readily available and most crucially renewable feedstock for the production of various chemicals and fuels. Circular economy is a new industry paradigm that aims for long-term growth. This new business model is centred on the economy and the environment, as it promotes development, profit and environmental conservation. Industrial residues, e.g., are not wasted; rather, they can be turned into raw materials for new products. Circular economy is defined by two guiding principles: it raises the value of raw materials by improving their conversion to products, and it reduces service time loss via responsible product design. The closed loop concept (circular) attempts to improve the continuous flow of technical and biological resources, such as agricultural waste, by maintaining products, components and resources at their highest value, at all times, and reducing waste to the bare minimum. As well known, circular economy is a developing idea with no consensus on its theoretical formulation in academia or literature ³³, because it is the product of changes in governmental frameworks and laws, rather than academic study. Recycling and reprocessing a product ingredients allows the material utility to be extended even further. In other words, rather of squandering a material potential, it is fully utilised. As a result, companies seek for materials with endless features such as recyclability and reusability, which may be achieved by using circular economy concepts.

According to a recent IEA 2017 estimate, biomass accounts for 9% of overall energy supply ³⁴.

Agricultural activities have produced a variety of biomasses, including animal manure and slurries, postharvest plant residues, non-marketable products, products with no market value, waste from low or unprocessed vegetables, waste from olive and grape processing lines, and milk-based waste, among others ³⁵. The absence of an appropriate reuse strategy leads to unrestricted exploitation of these bioresources for manufacturing and consumption, halting economic restoration. However, the world leading economies are presently shifting their production policy frameworks from unsustainable to more sustainable, eco-friendly and resource-conserving ³⁶.

Fig. 5 shows how agricultural waste streams may be used as a resource in circular economy.

Figure 5: Agricultural waste streams used as a resource in circular economy ³⁷.