There is no air in the containment for combustion, the material is disintegrating slowly.
In this case, the material is coal or carbon. The contraption is heated to above \(T_{\small{1298}}=538.27^oC\), around \(550^oC\) and then allowed to cool to \(T_{\small{1298}}\). The sealed reactor should fluctuate at a temperature above \(T_{\small{1298}}\), as any temperature dip to \(T_{\small{1298}}\) triggers the carbon to disintegrate and the heat generated drives the temperature up again. At above \(T_{\small{1298}}\), the material is not at resonance, disintegration reduces and its temperature drops.
It is like a rubber ball bouncing off a concrete floor. Theoretically, with \(E=mc^2\), the reactor should persist at above \(T_{\small{1298}}=538.27^oC\) for a long time, or until an over-drain causes temperature to drop below \(T_{\small{1298}}\). Below \(T_{\small{1298}}\), the reactor will simply cool off.
Have a nice day.
