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Black liquor, an industrial waste, also generally called spent liquor is a promising feedstock for production of bio-carbon due to its high lignin and energy contents. Mild processing conditions and sustainable resource utilization can drive commercial utilization of this industrial waste. A novel oxidative procedure using hydrogen peroxide as a green liquid oxidant at room temperature for processing of paper recycling neutral sulphite semi-chemical spent liquor was established and evaluations were carried out for its efficiency. Standard hydrothermal carbonization was carried out for comparing bio-carbon production qualitatively and quantitatively using the same spent liquor. The oxidation procedure was performed at room temperature using 5% of H2O2 followed by washing in diluted sulfuric acid (0.15 N) while hydrothermal carbonization was performed at 250 °C. The bio-carbon produced were similar in terms of ash percentage and ranged from 1.25 ± 0.05 to 1.48 ± 0.05% for oxidation method and from 1.11 ± 0.03 to 1.45 ± 0.04% for hydrothermal carbonization. The carbon contents for the oxidation method ranged from 60.54 to 61.50% and 66.81–67.31% for hydrothermal carbonization while higher heating values ranged from 25.32 to 26.11 MJ/kg and from 28.68 to 29.34 MJ/kg respectively. The mass yield ranged from 29.98 to 31.88% by the oxidation, while it was 15.02–16.08% by hydrothermal carbonization.

Black liquor (spent cooking liquor) is one of the major byproducts of pulp and paper manufacturing. Black liquor contains 10–50% lignin, which is the main organic matter found within that liquor. Different types of black liquors are obtained as per the type of feedstock, pulping process and cooking method adopted by industries. In recent years, industries have been required to accommodate newer varieties of feedstock such as non-wood and recycled fibers during the delignification process, which can save plenty of trees and hence reduce their carbon footprint. Therefore, the newer black liquors being generated differ in their physical characteristics, chemical composition, and energy content from that of traditional processes.Currently, black liquor is seen as a platform for the production of many renewable materials for industrial applications that can be environmentally friendly with the potential to be used substitute for fuel and commercial materials. However, most of the published review articles focus on the kraft spent liquor and its derived kraft lignin that is obtained from kraft pulping process at the pulp and paper as a source of bio-fuel and biomaterials. Meanwhile, several other black liquors such as soda, and neutral sulfite spent (NSSC) liquor and their derived lignin are not highlighted as sources of biofuel and biomaterials. Therefore, this review highlights all the types of black liquors including soda, and neutral sulfite spent (NSSC) liquor in terms of their sources, physical and chemical characterization, purification processes, and the potential applications of black liquor and its derived lignin.

The application of black (spent) liquors for bio-carbon production is a promising method to add value to this industrial renewable waste stream. Further valorization by conversion to higher grade bio-carbon products may significantly enhance their adoption to various industrial uses. To increase the purity of the bio-carbon by selective removal of minerals and ash, a novel oxidative procedure performed at room temperature was developed and its potential was evaluated. The procedure was carried out with hydrogen peroxide (H2O2) as oxidant on three different types of spent liquor, namely neutral sulphite semi-chemical spent liquor (NSSCSL) sourced from recycled paper, softwood (pine and spruce) kraft spent liquor (PSKSL), and hardwood (aspen and balsam poplar) kraft spent liquor (APKSL). The efficiency of the oxidation procedure on each raw material was studied and reported. The maximum mass yield of bio-carbon was 31.88%, 46.27% and 43.93% after the ash removal processing of NSSCSL, PSKSL, and APKSL spent liquors respectively. The maximum efficiency for mineral and ash removal from spent liquors achieved were 94.63%, 98.08 %, and 94.93% respectively for the three types of spent liquors. The highest heating values (HHV) demonstrated in the bio-carbon produced were 26.11 MJ/kg, 24.74 MJ/kg, and 26.89 MJ/kg respectively. Empirical indices were also calculated to establish and characterize bio-carbons potential for its combustion quality as a function of this oxidation process. The upgraded bio-carbon appears to enhance their potential as environment friendly, green substitutes for solid fuel and bio-material applications.