Flame inhibition by phosphorus-containing compounds in lean and rich propane flames

Chemical inhibition of laminar propane flames by organophosphorus compounds was studied experimentally and computationally using a detailed chemical kinetic reaction mechanism. Both fuel-lean and fuel-rich propane flames were studied to examine the role of equivalence ratio in flame inhibition. Both experiments and kinetic modeling indicated that inhibition efficiency is effectively the same for all of the organophosphorus compounds examined, independent of the molecular structure of the initial inhibitor molecule. Chemical inhibition was due to reactions involving small P-bearing species HOPO₂ and HOPO produced by the organophosphorus compounds (OPC). Ratios of HOPO₂ and HOPO concentrations differed between lean and rich flames, with HOPO₂ dominant in lean flames while HOPO dominated in rich flames. Resulting HOPO₂ and HOPO species profiles did not depend significantly on the initial source of the HOPO₂ and HOPO and were relatively insensitive to the initial OPC inhibitor. A more generalized form of the Twarowski mechanism was developed to account for the results observed, and new theoretical values were determined for heats of formation of the important P-containing species, using the BAC-G2 method.