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

Abstract

Computational studies have been performed for methane-air cup-burner (i.e., co-flow diffusion) flames with addition of phosphorous containing compounds (PCCs). The PCCs are: phosphoric acid, trimethyl phosphate (TMP), and dimethyl methylphosphonate (DMMP), and these are added to the oxidizer or fuel flow. Calculations (time-dependent) employing a 2D axisymmetric domain, detailed gas-phase chemistry and mixture-averaged diffusion, are employed to unravel the chemical inhibition and flame structure. A CH4O2 chemical mechanism is combined with one for phosphorous in flames, and has 77 species and 886 reactions. Radiation heat losses are modeled in the energy equation via an assumption of optically-thin behavior, with plank-mean averaged emission from CO2, H2O, CO, CH4, and soot. The effect of the PCC on the CO2 volume fraction required for extinction (the minimum extinguishment concentration [MEC]) is used as the metric for inhibitor effectiveness, when agent is added to either the air or fuel stream. For flames inhibited by only CO2, the MEC from calculations and experiment are in good agreement; however, with DMMP added to the air (< 1%), the calculated MEC is significantly greater. Moreover, addition of the PCC to the air stream as opposed to the fuel stream is about ten times more effective. As flame inhibitors, DMMP, TMP, and phosphoric acid behave similarly, lowering the volume fraction of radicals and the rate of heat release at the reaction kernel (i.e., the flame-stabilizing peak reactivity spot) in the flame base, promoting flame blow-off. The three compounds behave differently, however, with regard to the trailing flame. While all three increase the peak 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 two-zone flame structure, whereas phosphoric acid does not.