Influence of NH4H2PO4 powder on the laminar burning velocity of premixed CH4/Air flames

The comprehensive inhibition mechanism of hydrocarbon/air premixed flames by monoammonium phosphate (NH4H2PO4) powder is studied in this research work using theoretical, experimental and numerical simulation approaches. The critical diameter of the NH4H2PO4 powder particles in the premixed CH4/air flame was calculated to be 30 mm using a predictive mathematical model. Using a Bunsen burner apparatus, the laminar burning velocity of the CH4/air/NH4H2PO4 mixtures, entrained with NH4H2PO4 powder (D90 < 30 mm), was measured over a gas phase equivalence ratio range of 0.9-1.5, powder concentration range of 0-33.8 g/m3, and temperature and pressure conditions of 298 K and 1 bar, respectively. The experimental results show that the laminar burning velocity of the CH4/ air/NH4H2PO4 premixed flames decreased non-linearly with the increase of the NH4H2PO4 powder concentration for a constant equivalence ratio. When the NH4H2PO4 powder concentration exceeded a critical powder concentration under a fixed equivalent ratio, a saturation phenomenon appeared. To study this saturation phenomenon and the inhibition mechanism of NH4H2PO4 powder, a gas-phase kinetic model was developed of the NH4H2PO4 powder on the CH4/air premixed flame. Comparing the simulated laminar burning velocity from the kinetic model with the measurements revealed a reasonably good agreement that proves the dominant chemical inhibition characteristics of NH4H2PO4 powder on the CH4/air premixed flame. The gas-phase chemical kinetic analysis shows that H3PO4 plays a major role in reducing the laminar burning velocity of the CH4/air premixed flames by NH4H2PO4 powder; however, the effect of NH3 was negligible. In addition, the catalytic cycles of small phosphorus species (i.e., HOPO2 < = > PO2 and HOPO < = > PO2) scavenged the H and OH radicals in the flame, resulting in disappearance