Co-firing of biomass and coal may increase short-term renewable fuel usage. However, the lower heating value and higher reactivity (at lower temperatures) of raw biomass can result in fuel segregation in boilers, causing burnout at lower temperatures, lower steam generation efficiency and fouling. In addition, the relatively high water content of some biomasses requires extensive drying prior to combustion. These issues may be addressed by hydrothermally carbonizing moist biomasses to produce hydrochars that more closely resemble coal's properties prior to co-firing. In the present work, we probe the co-oxidation behavior of a series of hydrothermally carbonized biomass samples over a range of hydrochar-coal blend ratios to determine the degree of carbonization necessary to reduce fuel segregation. However, due to the presence of an extractable amorphous secondary char, even highly carbonized hydrochars have considerably higher oxidative reactivity than a representative bituminous coal sample. When blended, the hydrochars and coal display distinct derivative thermogravimetric oxidation ranges, in which a low-temperature peak is dominated by the secondary char oxidation, followed by a high-temperature char oxidation peak of the primary solid hydrochar. After extraction of the secondary char, the primary char displays a lower reactivity than the coal. As a co-fired fuel, it appears that blending hydrochars up to 10 wt% with a bituminous coal is possible as a partial fuel substitution. To increase the percentage of hydrochars blended with coal, it may be necessary to extract this secondary char (which contains valuable biofuels and platform chemicals) before blending.