Numerical study of gas-phase interactions of phosphorus compounds with co-flow diffusion flames

The effects of phosphorus-containing compounds (PCCs) on the extinguishment and structure of methane-air coflow diffusion flames, in the cup-burner configuration, is studied computationally. Dimethyl methylphosphonate (DMMP), trimethyl phosphate (TMP), or phosphoric acid is added to either the air or fuel flow. Time-dependent axisymmetric computation is performed with full gas-phase chemistry and transport to reveal the flame structure and inhibition process. A detailed chemical-kinetics model (77 species and 886 reactions) is constructed by combining the methane-oxygen combustion and phosphorus inhibition chemistry. A simple model for radiation from CH₄, CO, CO₂, H₂O, and soot based on the optically thin-media assumption is incorporated into the energy equation. The inhibitor effectiveness is calculated as the minimum extinguishing concentrations (MECs) of CO₂ (added to the oxidizer) as a function of the PCC loading (added to the oxidizer or fuel stream). The calculated MEC of CO₂ without an inhibitor is in good agreement with the measured value. For moderate DMMP loading to the air (<1 %), the measured value becomes significantly smaller, presumably due to particle formation in the experiment. An inhibitor in the oxidizer flow is an order of magnitude more effective compared to that in the fuel flow in gas-phase inhibition of co-flow diffusion flames. The three PCCs studied behave similarly with regard to flame inhibition, lowering radical concentrations and the heat-release rate at the flame-stabilizing peak reactivity spot (i.e., reaction kernel) in the base, promoting flame blow-off. The three compounds behave differently, however, with regard to the trailing flame. While all three raise the maximum temperature in the trailing flame, DMMP and TMP, which contain three methyl groups, result in higher maximum flame temperature and combustion enhancement there, with a unique two-zone flame structure, whereas phosphoric acid does not.