The world’s most abundant renewable resource is biomass (wood and nonwood), of which the three main components are cellulose, hemicellulose, and lignin.
In woody biomass, cellulose is the most abundant biopolymer, followed by lignin, which accounts for 15–40 wt% of wood.[1,2] Lignin is also the largest natural resource of aromatic compounds. [3,4] However, only 1–2% of the 50–70 million tons of lignin produced annually is used for the production of value-added products; this implies that it is an underutilized material.[1, 2,5,6] Today, lignin is primarily used for energy generation through combustion in pulping processes.
As such, it is highly advantageous to identify lignin-based value-added products and to develop processes for their production. Integrated forest biorefineries (IFBRs) have been introduced as methods to increase the economic viability of the pulping industry.
This economic advantage can be achieved by utilizing the waste products of the pulping processes, such as lignin, in producing value-added products. Carbon fiber, polymer alloys, fillers, or dispersants could be regarded as lignin based value-added products.
To produce lignin-based value-added products, lignin should be first separated from biomass, for which several methods can be employed. Kraft and sulfite pulping processes are known for their effective lignin separation from wood, and hence, are used worldwide.
Kraft lignin is separated from wood with the help of NaOH and Na2S.Lignins from sulfite pulping processes are denoted as lignosulfonates, and are produced by using sulfurous acid and/ or a sulfite salt containing magnesium, calcium, sodium, or ammonium at varying pH levels.[2,14] Currently, lignosulfonates ac
count for 90% of the total market of commercial lignin,[14–16] and the total annual worldwide production of lignosulfonates is approximately 1.8 million tons.However, lignosulfonates and kraft lignin have different properties.
Lignosulfonates have generally more sulfur groups, and thus, a higher degree of sulfonation than that of kraft lignin. Due to the presence of the sulfonated group, lignosulfonates are anionically charged and water soluble. The molecular weights (Mw) of lignosulfonates can be similar to or larger than that of kraft lignin (Table 1).
Due to their unique properties, lignosulfonates have a wide range of uses, such as animal feed, pesticides, surfactants, additives in oil drilling, stabilizers in colloidal suspensions, and as plasticizers in concrete admixtures.[13, 14,31–34] However, the majority of pulp mills employ kraft technology for pulp production.