The effect of impurities and pressure on oxidation in CO₂ at 700°-800°C

Direct-fired or open supercritical CO₂ (sCO₂) cycles are expected to have impurities that may greatly alter the compatibility of structural alloys in these environments. However, there are no available laboratory facilities to simulate these environments at operating conditions of 750°C and >100 bar. As an initial assessment of the effects of H₂O and SO₂ on oxidation in a CO₂ environment, 500 h experiments were conducted at 1 and 25 bar at 700° and 800°C in CO₂, CO₂+10%H₂O and CO₂+10%H₂O+0.1%SO₂. Representative structural alloys S30409, N07230, N07208 and superalloy N07247 were exposed and the reaction products characterized after each exposure. The Ni-based alloys generally formed thin protective oxides at both pressures and only minor changes were observed with the addition of SO₂ and/or H₂O. As expected, the Fe-based S30409 showed more attack, especially at 800°C. However, at 25 bar the attack was reduced at 800°C compared to the breakaway oxidation at 1 bar. For the addition of SO₂ at 800°C, a more protective oxide formed at 1 bar but a thick, duplex Fe-rich oxide formed at 25 bar.