Abstract
Lignin is an abundant, renewable, and relatively cheap biobased feedstock that has potential in energy, chemicals, and materials. Kraft lignin, more specifically, has been used for more than 100 years as a self-sustaining energy feedstock for industry after which it has finally reached more widespread commercial appeal. Unfortunately, hardwood kraft lignin (HWKL) has been neglected over these years when compared to softwood kraft lignin (SWKL). Therefore, the present work summarizes and critically reviews the research and development (R&D) dealing specifically with HWKL. It will also cover methods for HWKL extraction from black liquor, as well as its structure, properties, fractionation, and modification. Finally, it will reveal several interesting opportunities for HWKL that include dispersants, adsorbents, antioxidants, aromatic compounds (chemicals), and additives in briquettes, pellets, hydrogels, carbon fibers and polymer blends and composites. HWKL shows great potential for all these applications, however more R&D is needed to make its utilization economically feasible and reach the levels in the commercial lignin market commensurate with SWKL. The motivation for this critical review is to galvanize further studies, especially increased understandings in the field of HWKL, and hence amplify much greater utilization.
Highlights
Lignin is the main source of renewable aromatic structures on Earth [1,2]
There are several modification techniques reported in the literature, the present review only addresses the most common and relevant methods to hardwood kraft lignin
It is unfortunate that most lignin work deals with softwood kraft lignin, while hardwoods have been neglected
Summary
Lignin is the main source of renewable aromatic structures on Earth [1,2]. The lignocellulose biorefinery is an interesting approach to maximize the value of lignin and its products. Rather than just producing steam and energy, a portion of the black liquor would be utilized as a feedstock for production of value-added products. Removal of such a feedstock could be used for increasing pulp production since many recovery boilers are limited by total solids loading or steam generation (circulation limit), and they represent the capacity-limiting step in several pulp mills. A gap exists between the kraft lignin producers and market off-takers, due to the current lignin processes having difficulties meeting stringent quality control constraints and market specifications which end-uses require.
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