Role of an additive in retarding coal oxidation at moderate temperatures

The impact of an additive on coal oxidation process is studied from the mechanistic perspective, aiming at development of a guideline in search of effective inhibitors for controlling the coal self-heating phenomena. The salt Na₃PO₄ was chosen as an additive. Behaviors of samples with/without the additive were examined at temperatures up to 400 °C both in oxidative and inert atmosphere using a TGA instrument, and the compounds on coal surface during oxidation and pyrolysis were monitored by FT-IR technique. The TGA data show that the impact of the additive on coal oxidation process can be directly evaluated using a parameter defined as the percentage of mass increase at ∼265 °C, and the addition of Na₃PO₄ slows down the rates of oxygen uptake and decomposition reactions. FT-IR results also indicate that the additive suppresses both the coal oxidation and pyrolysis processes essentially by accelerating the formation of saturated ether linkages. Further analysis suggests that Na₃PO₄ plays a role in modifying the routes for decomposition of hydroxyl, and subsequently improving the coal thermal stability.