Biocarbons are promising alternatives to fossil reductants and have, for decades, been considered for metallurgy to allow for the reduction of CO2 emissions without changing today’s industrial processes and furnaces. The chemical, structural, and mechanical properties of biocarbon are different from those of fossil carbon. The approach to overcome this challenge is often to modify biocarbon or produce designed biocarbon with properties similar to those of coke or other fossil carbon sources. In this work, the approach has been to investigate how different properties of the carbon source such as reactivity, density, and onset temperature of the Boudouard reaction will affect furnace operation. A simulation tool for FeMn process, based on HSC Sim flowsheet which incorporated knowledge of mass and energy balance and kinetic data for Mn-ores, has been developed in earlier work. This HSC Sim simulation was further developed to incorporate knowledge about kinetic data for reactions involving carbon and to allow for distinct behaviour of the carbon from different sources. This gives the possibility to evaluate the effect of carbon properties on the Boudouard reaction, the energy consumption, and the CO2 emissions. Overall, the gas composition and the temperature distribution in different zones in the furnace will be altered. Mixtures of carbon sources are also considered, as well as the effect of potassium circulation in the furnace, known to accelerate the Boudouard reaction. The simulation results are discussed against current knowledge.