Flameless oxy-combustion is a very efficient technology for treating low-ranking fuels with energy recovery, when operating in the high temperature range of 1200-1600 degrees C. However, the thermal valorization of these low-ranking fuels, containing a significant amount of vanadium, causes some problems due to the volatility and corrosiveness of V2O5 with ceramic and metal alloys at high temperatures. The formation of mixed V/Mg or V/Ni compounds, which are stable at high temperatures (1300 degrees C), was investigated in a tubular lab-scale furnace with a residence time of 2 s, starting from sprayed aqueous VOSO4, MgSO4, and NiSO4 solutions. It was seen that MgSO4 and/or NiSO4 led first to the formation of MgO and NiO, which then reacted with V2O5 to produce fully inert Mg2V2O7, Mg3V2O8, and Ni3V2O8. The experimental data were combined with thermodynamic equilibrium calculations, to predict the stability of the vanadates obtained. (C) 2015 Elsevier B.V. All rights reserved.