Life Cycle Assessment of Hemicellulose Separation and Utilization from Black Liquor of a Hardwood Kraft Pulp Mill

Separating hemicelluloses from black liquor and producing high-value products have been proposed as a means to utilize residues, reduce carbon emissions, and generate additional revenues for pulp mills. However, the combustion of black liquor is essential to provide the mills with low-cost energies without high input of fossil fuels, so whether the utilization of residual hemicelluloses would actually reduce the overall carbon emissions remains doubtful. In this study, comprehensive energy and environmental impact analyses of the hardwood kraft pulping process incorporating membrane extraction of hemicelluloses and their high-value utilizations were conducted by using life cycle assessment (LCA). A process simulation method was used to study the black liquor treatment and membrane separation process. The LCA indicates that incorporating hemicellulose extraction and utilization leads to a 9.97% reduction in overall greenhouse gas emissions without alternation of pulp production, considering the increase of fossil fuel and electricity usage. Furthermore, hemicellulose extraction and utilization can also enhance the pulp production by approximately 15%, because the recovery boiler can support more production load due to the hemicellulose separation from black liquor. Sensitivity analyses indicate that coal carbon factor, the use of calcium oxide, and high-value utilization of hemicellulose have a significant impact on the Global Warming Potential associated with kraft pulp production activities. These findings indicate the potential of commercial hemicellulose extraction to mitigate the environmental impact of kraft pulp mills.

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