Investigating graphite oxidation via real-time offgas analysis

Quantitative gas analysis has been used to study the oxidation behaviour of a nuclear graphite block using real-time monitoring of the CO₂ and CO evolution. The investigation was carried out across the temperature range: 800 – 1473 K, using partial pressures of oxygen between 20 – 100 %. A rapid increase in oxidation rate is seen at T ~ 850 K with CO₂ as the only product. The reaction kinetics in this range conform to Arrhenius behaviour with reaction kinetics limited by a combination of surface reaction rate and the diffusion of oxygen into the graphite pores. In this regime we observe a measured activation energy, E_a = 236 ± 30 kJ mol^-1and anassociated reaction order of ~ 1. At T ≥ 1050 K, the oxygen supply becomes limiting, and the system reaches steady state. At T ≥ ~ 1250 K, the onset of CO evolution is observed, and CO₂ production reduces. Kinetic analysis suggests that in this temperature range, evolved CO₂ reacts further with the bulk carbon in the thermodynamically favourable Boudouard reaction with a measured E_a~182 ± 26 kJ mol^-1.