In the present study, wood biomass from Acacia mangium and its main components (hemicellulose (xylan), cellulose, and lignin) were blended with a sub-bituminous coal in 20:80 wt % ratio and subsequently were heat-treated at 900 °C using CO2 or N2 atmospheres. The reactivity (ignition temperature and activation energy) under oxy-fuel conditions (21% O2–79% CO2) was studied by thermogravimetry (TGA). It has been observed that adding biomass or its main components to coal improved the combustion efficiency. Coal/biomass and coal/xylan/cellulose/lignin char blends showed lower ignition temperatures when they were devolatilized using CO2 instead of N2. In addition, the activation energies were lower for blends thermally treated with CO2. Differences in reactivity are discussed considering changes in physical-chemical properties characterized by SEM, N2 adsorption at −196 °C, Raman spectroscopy, and XPS.