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

Abstract

Chemical inhibition of laminar propane flames by organophosphorus compounds has been studied experimentally, using a laboratory Mache Hebra nozzle burner and a flat flame burner with molecular beam mass spectrometry (MBMS), and with a computational flame model 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. The experiments examined a wide variety of organophosphorus compounds. We report on the experimental species flame profiles for tri-methyl phosphate (TMP) and compare them with the species flame profile results from modeling of TMP and di-methyl methyl phosphonate (DMMP). Both the experiments and kinetic modeling support and illustrate previous experimental studies in both premixed and non-premixed flames that inhibition efficiency is effectively the same for all of the organophosphorus compounds examined, independent of the molecular structure of the initial inhibitor molecule. The chemical inhibition is due to reactions involving the small P-bearing species HOPO 2 and HOPO that are produced by the organophosphorus compounds (OPCs). The ratios of the HOPO 2 and HOPO concentrations differ between the lean and rich flames, with HOPO 2 dominant in lean flames while HOPO dominates in rich flames. The resulting HOPO 2 and HOPO species profiles do not depend significantly on the initial source of the HOPO 2 and HOPO and thus are relatively insensitive to the initial OPC inhibitor. A more generalized form of the original Twarowski mechanism for hydrocarbon radical recombination is developed to account for the results observed, and new theoretical values have been determined for heats of formation of the important P-containing species, using the BAC-G2 method.

Chemical Transformations in Inhibited Flames over Range of Stoichiometry

“…Later, the model was refined by altering the rate constants of six key steps and was successfully applied to calculating the structure of flames at atmospheric pressure. The enthalpies of formation of phosphorus oxyacids were calculated by various quantum-chemical methods (Glaude et al, 2000;Mackie et al, 2002;Korobeinichev et al, 2005). At the same time, the enthalpies of formation of some activated complexes were calculated by varying their structures (intermediate states) for different pathways of the key reactions.…”

Promotion and inhibition of a hydrogen—oxygen flame by the addition of trimethyl phosphate

“…The thermochemistry of the reactions involving organophosphorus compounds (OPCs) was studied by Glaude et al [15] using ab initio quantum-chemical calculations for many new substances. The mechanism of the reactions involving OPCs was further developed in [16] and used to explain the inhibition of propane flames.…”

Influence of organophosphorus inhibitors on the structure of atmospheric lean and rich methane-oxygen flames

“…In the last 10-15 years, a number of researchers have shown that the mechanism of flame inhibition by phosphorus-containing compounds involves their influence on the recombination rate of atoms and free radicals in the flame [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15]. The recombination reactions are catalyzed by phosphorus oxides and phosphoric acids.…”

“…The results showed that none of the models describe the concentration profiles of these compounds adequately. The structure and propagation velocity of atmospheric premixed propane-oxygen and propaneair flames doped with TMP was studied in [11,12]. It was established that a change in the stoichiometric composition of the combustible mixture resulted in a change in the composition of phosphorus-containing products of TMP combustion, which, in turn, changed the inhibition efficiency.…”

“…The latter occurred because the catalytic cycle involving HOPO eliminated radicals from the flame at a higher rate compared to the cycle involving HOPO 2 . The flame inhibition mechanism by phosphorus compounds was significantly modified and supplemented in [11,12]. On the one hand, the thermochemical parameters for the main phosphoruscontaining compounds were refined, and some stages were replaced by a set of elementary reactions with a lower activation energy using the Mackie approach [14].…”