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 (similar to 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 (R) 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 degrees C and 600 degrees 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 redirecting 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.