Partial and complete methane oxidation in supercritical water

Abstract

The paper represents results on investigation of methane oxidation in supercritical water (SCW) in autoclave and flow conditions. In the autoclave, oxidation is realized under uniform heating of a CH₄/O₂/H₂O and CH₄/O₂/N₂ mixture to 873 K (the water and nitrogen density ≈ 3.2 mmol/cm³, the molar ratio [O₂]₀/[CH₄]₀ ≈ 1 and 2). In the composition of the oxidation products we detected H₂ (only at [O₂]₀/[CH₄]₀ ≈ 1), CO and CO₂. Based on time dependences of the reaction mixture temperature we have found that temperature of the onset of self-heating of the CH₄/O₂/H₂O mixture is lower by 23 K than that of the CH₄/O₂/N₂ mixture and grows as the CH₄ concentration decreases. For comparable values of self-heating the average power in CH₄ combustion in the H₂O medium has appeared to be about two orders lower than in the N₂ medium, which evidences inhibition of SCWmethane oxidation. In the boiler-reactor, oxidation was realized while mixing CH₄ and O₂ in counter-propagating jets in the cocurrent upflow of SCW at 673–874 K, 30 MPa (molar ratio [O₂]/[CH₄] ≈ 2.2). Unsteady combustion was observed only at a reaction mixture temperature of 678 K, which became steady at 700 K after a series of flashes. The carbon-bearing methane oxidation products in the boiler-reactor contain only CO₂ (≥ 97.5%) and CO (≤ 2.5%mole).