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, Ea = 236 ± 30 kJ mol-1 and an associated 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 Ea ~182 ± 26 kJ mol-1.