Supercritical water gasification of Kraft black liquor: Process design, analysis, pulp mill integration and economic evaluation

Supercritical water is a practical processing medium for the treatment of weak black liquor (WBL) produced from pulp digesters in Kraft mill operations. WBL is characterized as a bio-feed with considerable thermal potential, but also a challenging high water content (~82 wt.%) and high inorganic to organic ratio for thermochemical conversion. The advantageous thermo-physical properties of water near to and beyond the critical point allow for the valorization of the organic content into a product gas, while enabling the efficient recycle of the inorganic pulping chemicals. Detailed process models were developed on Aspen Plus® and commercial spreadsheet software to examine the impact of an integrated Sub/supercritical water (SCW) reactor system on the mill material and energy flows. When considering the three energy co-products: gas, solids and hot water: the stand-alone SCW reactor system had a system efficiency of 83% and 80% for a 450 °C and 600 °C operating reactor temperature, respectively. The inorganic fraction of the solid SCW co-product and the aqueous by-product provide a synergetic effect as drop-in material streams within the chemicals recovery cycle. By re-directing the WBL to the SCW reactor system, pulp production capacity could be increased by 75%, while matching mill energy requirements and, with minimum disruptions to the mill chemistry. Under the economic assumptions of this study, a 30–50% WBL split fraction to the SCW reactor system improves the minimum selling price of the pulp product compared to a reference Nordic softwood mill with 800 k air-dried ton pulp capacity per year.