This paper investigates the changes in reactivity and physicochemical characteristics of char and tar produced from severe heat treatment in either inert or CO2-rich atmospheres of a synthetic fuel doped with Fe&Mg and K&Mg sulfates. The mineral-free model fuel was obtained by hydrothermal carbonization (HTC) of cellulose. The comparison of Py-GC/MS-GC/TCD, heated strip reactor (HSR), and drop tube reactor (DTR) results highlighted that mineral doping, heating rate, temperature, residence time and atmosphere all interacted and influenced the pyrolysis products. Fe&Mg increased the light permanent gases during flash pyrolysis. The minerals catalytically affected the decomposition of the levoglucosan fraction in the tars. In N2, the addition of K&Mg influenced the oxy-aliphatic tars, while Fe&Mg had a stronger impact on oxy-aromatic compounds. Depending on the pyrolysis temperature and residence time (and reactor type), CO2 inhibited (700 °C in the HSR) or enhanced (1027 °C in the DTR) the formation of polycyclic aromatic hydrocarbon (PAHs) in the tars. Chars doped with Fe&Mg always exhibited higher reactivity than the chars doped with K&Mg. While the presence of CO2 during pyrolysis favored the aromatization of the chars especially in combination with doped minerals, alkali mineral interaction with CO2 decisively altered char reactivity.