Kraft delignification of energy crops in view of pulp production and lignin valorization

Abstract

The hardwoods Acacia dealbata (mimosa), Salix spp. (willow), and the perennial plants Arundo donax (giant reed) and Miscanthus giganteus (giant Chinese silver grass) are important energy crops, with low requirements and high productivity. The polysaccharide content of these species has drawn attention as source of sugars for biobased products. In this work, biomasses were deconstructed by kraft pulping, evaluating final pulp yields as well as residual lignin (kappa number, KN) as indicators of the pulp quality for conversion to sugars. The black liquors and lignin in the isolated materials were evaluated concerning composition and structural characterization parameters with relevance to further valorizations routes, such as production of vanillin and syringaldehyde. The best trade-off between pulp yield and KN were achieved for 160 degrees C and pulping time 210 min, with active alkali (AA) 22% for mimosa and willow and AA 18% for giant reed; these three biomasses presented similar pulp yields (45-47%) and KN (13-15). For silver grass a lower AA (16%) and the combination of higher temperature/lower pulping time (170 degrees C/180 min) were selected. Among the four, silver grass presented the highest pulp yield (51.5%) and the lowest KN (10), which is the first advantage identified for this species. Another advantage is the higher production of lignin by isolation from-black liquor (128 g/kg of biomass), compared to the other materials (88-119 g/kg), with the additional benefit of low contamination with inorganic compounds. The isolation of lignins is favorable for the valorization route involving oxidative depolymerization. The structural analysis of lignins and the comparison between the frequency of the main interlinkages and functional groups allowed drawing remarks about their suitability for some applications. The data presented are a tool for decision about the best exploitation route for lignin, contributing for the valorization of the streams generated in biorefining processes of energy crops.