The importance of coal as the main energy source has posed the challenge to use this fuel in a sustainable way while new clean energy technologies are developed. Carbon capture and storage is one of the main short-term solutions to the problem of CO2 emissions and involves a wide range of technologies. Oxy-fuel combustion is one of them, and even though it has emerged as an attractive alternative, a better understanding of the performance of the process is still required. The current work was aimed at enhancing the understanding of the different thermodynamic equilibrium behaviors of metallic trace elements in coal combustion under air and oxy-fuel environments. The effect of coal composition on the speciation of these elements was analyzed, and a sensitivity analysis of the effect of various concentrations of Cl, S and the mineral content of coal was performed. It was found that the increase in the concentration of CO2 and H2O did not have a significant effect on the thermodynamically stable forms of trace element compounds that were predicted to form. The recycling of a larger amount of trace elements did not affect the speciation either but only increased the concentration of trace elements inside the boiler. The speciation and volatility of the species predicted by thermodynamic modeling in the majority of the trace elements considered in this study was found to be sensitive mainly to changes in the concentration of Cl, Ca, S, and Si. In terms of species that may enhance corrosion at tube wall temperatures, the thermodynamic calculations predicted the condensation of the majority of trace elements as sulfates as well as the formation of V2O5.