A study on synergies of combined pulp and fuel production

To limit climate change by reducing global greenhouse gas emissions, a transition away from fossil fuels is required. Biofuels represent a promising low carbon alternative for sectors that are hard to electrify, such as aviation. The integration of bio refineries in pulp mill can result in a better resource use and higher energy efficiency, while diversifying the product portfolio and providing bio-based fuel products to the market. This work investigates pathways for increasing the value of biomass at a kraft pulp mill, focusing on the black liquor and bark streams. Mathematical programming coupled with process simulation and optimization tools are used to describe the process superstructure and generate solutions. For making use of synergies, dry gasification of woody biomass for Fischer-Tropsch (FT) fuel synthesis combined with high temperature supercritical water gasification (SCWG) of black liquor to produce hydrogen are integrated in the pulping process. Dry gasification of bark for FT fuel production is compared to the conventional case of bark combustion in a boiler. Furthermore, addition of hydrogen from SCWG of black liquor is made available. It was found that incorporating the dry gasification unit increased the overall carbon conversion efficiency of the mill, and decreased overall GHG emissions with a minimal change to total costs. By incorporating hydrogen from gasifying a portion of the black liquor stream, the yield of FT syncrude could be increased. Higher SCWG gasification temperatures promote hydrogen formation improving the performance of the SCWG unit, while the inclusion of a Selexol unit improves CO2 recovery at the expense of higher electricity requirements which decrease the overall mill exports and therefore revenue. Overall, it was found that biofuel production at integrated product biorefineries can play an important role in the energy transition by offering alternatives to conventionally produced fuels, while improving resource efficiency and providing plant owners with a more diverse product range and minimal disturbance to profitability