Fuels from bio-based resources hold the potential to supply fossil fuel alternatives at reasonable economic and environmental expenses that can assist emission reduction in the transportation sector. However, apart from a general need to reduce fossil-based emissions, the shift from fossil energy carriers to renewables is likely to cause mismatches of supply and demand of electricity. This paper investigates the potential of integrated biorefineries to assist in balancing electricity demand and supply while co-producing fuels from biomass. At the example of a pulp mill and a nearby residential district, synergies between biomass valorization at an industrial site and residential energy demands are explored regarding defossilization potential and economic impacts. A Kraft pulp mill is integrated with thermochemical conversion processes, converting residuals of the pulping process to fuel. Power-to-fuel and fuel-to-power processes are enabled for increasing flexibility and enhancing additional fuel production in times of electricity oversupply. Multi-objective optimization is coupled with systematic solution generation and exploration approaches for deriving viable system configurations. Results indicate that direct emissions of the mill and the residential district can be reduced by up to 90% compared to the non-integrated system. Optimization of the prices of internal exchanges between mill and district reveals that this reduction comes at added expenses for all stakeholders, but that a reduction of emissions by approximately 50% provides economic benefits. Extrapolating the analysis to the European level reveals that the benefits of integration depend largely on the energy portfolio present in the considered region.