**1. Introduction**

Lignin is a loosely defined term that is used to describe different types of plantbased materials depending on the context. Lignin in different plants can be different in monolignol composition and molecular structure. On top of this variability is the susceptibility of lignins to various lignin extraction processes, as well as the various chemical and structural changes of extracted lignins during processing. In this chapter, the stream of lignins derived from a biorefining process (e.g., kraft pulping, cellulosic ethanol production) are referred to as "technical lignins", which are compositionally and structurally different from the native lignins present in plant cell walls.

Depending on the isolation process, technical lignins contain components other than phenolic polymers. Some of these components (e.g., sodium salt, solvents, carbohydrates, extractives) may complicate the downstream processing of value-added products. The most common types of technical lignin are kraft lignin and sulfite lignin, of which 5 × 107 tons are produced annually by the pulp and paper industry [1]. These lignins contain partially depolymerized plant lignins of various molecular weight, along with sodium salts as well as sulfur-containing organic and inorganic components. Lignins derived from cellulosic biorefining (e.g., biochemical conversion of lignocellulose feedstocks via saccharification and fermentation) contain ash components, undigested plant polysaccharides as well as oligomeric and monomeric sugars [2, 3]. These non-lignin components have various influences on lignin valorization. For instance, ash components introduce defects and negatively affects

the crystallinity of carbon fiber produced from lignins. Sugars and carbohydrates participate in condensation reactions with lignins under thermal treatment, which complicate the processing of lignins into thermoplastic materials.

Fractionation of technical lignins is a necessary step in the upgrading of lignins to high-value products, which has higher requirements in product purity and/or performance compared to other lower-value uses of technical lignins (e.g., boiler fuel, dust control agent). In the conversion process of technical lignins to lignin fiber and carbon fiber, soften spinning lignin is complicated by the high polydispersity of and branching of technical lignins [4]. As for catalytic hydrogenolysis of technical lignins, the noble metal catalysts lose activity in presence of the sulfur impurities in technical lignins [5]. With fractionation processes, these adverse effects of structural heterogeneity and chemical impurity can be addressed as suitable precursors for valorization is derived from technical lignins.
