W.C. Gardiner

COMBUSTION CHEMISTRY OF PROPANE: A CASE STUDY OF DETAILED REACTION MECHANISM OPTIMIZATION

Detailed chemical reaction mechanisms describing hydrocarbon combustion chemistry are conceptually structured in a hierarchical manner with H2 and CO chemistry at the base, supplemented as needed by elementary reactions of larger chemical species. While this structure gives a logical organization to combustion chemistry, the degree to which this organization reflects actual reactive fluxes in flames is not known. Moreover, it has not been tested whether sets of rate parameters derived by optimizing fits to small-hydrocarbon combustion data are secure foundations upon which to optimize the rate parameters needed for modeling the combustion of larger hydrocarbons. In this work, a computer modeling study was undertaken to discover whether optimizing the rate parameters of a 258-reaction C3 combustion chemistry mechanism that was added to a previously optimized 205-reaction C3 mechanism would provide satisfactory accounting for C3 flame speed and ignition data. The optimization was done with 21 optimization targets, 9 of which were ignition delays and 12 of which were atmospheric pressure laminar flame speeds; 2 of the ignition delays and 2 of the flame speeds, all for methane fuel, had served as optimization targets for the C3 rate parameters. It was found in sensitivity studies that the coupling between the C3 and the C3 chemistry was much stronger than anticipated. No set of C3 rate parameters could account for the C3 combustion data as long as the previously optimized (against C3 optimization targets only) C3 rate parameters remained fixed. A reasonable match to the C3 targets could be obtained, without degrading the match between experiment and calculation for the C3 optimization targets, by reoptimizing six of the previously optimized and three additional C3 rate parameters.

Refractivity of combustion gases

Abstract A comprehensive survey of the refractive indices and dispersions of gases that are found in practical or laboratory combustion experiments is reported. A critical evaluation was used to obtain recommended values where experimental data are available; where they are not, sums of atomic and bond refractivities were used. The results are tabulated as molar refractivities at common laser wavelengths and as the constants of Cauchy dispersion formulas.

Mechanism of soot formation in acetylene-oxygen mixtures

Abstract A computational study of the chemical kinetic effects of oxygen addition on the process of soot formation from hot acetylene is reported. The results, which are supported by trends observed in shock-tube experiments, reveal that the reaction pathway to soot identified previously for acetylene pyrolysis remains essentially unchanged in an oxidative environment. The main effects of oxygen are: (1) promotion of fuel decomposition, which alters the initiation route to soot; (2) supplementary rapid production of hydrogen atoms in the initial, small-molecule reactions, which drives the concentration of hydrogen above the equilibrium value with respect to H2 and thus enhances polymeric growth of polycyclic aromatics; (3)oxidation of aromatic radicals by molecular oxygen, which removes them from the polymeric growth. The computational results indicate a crucial need for kinetic studies of high temperature reactions between molecular oxygen and hydrocarbon radicals.

Oxidation of hydrogen sulfide

An experimental and modeling study of the oxidation of H2S is reported. The experiments entailed induction time measurements in reflected shock waves, in 4 to 22% H2S in air, some of which contained from 1.6 to 13% H2O, with P1 = 1 atm and 950 < T5 < 1200°K. Regression analyses gave the induction time formulas τ = 10−7.4 exp (13000T)[H2S]0.31 (with τ in seconds, T in degrees Kelvin, and [H2S] in mol cm−3) for dry test gas and τ = 10−8.2 exp (13400T)[H2S]0.18 for moist test gas. Computer modeling using a 17-species, 57-reaction mechanism led to reasonably satisfactory agreement between model calculations and experiments, including earlier studies on H2S combustion.

Combustion of methane in fuel-rich mixtures☆

Abstract The combustion of CH 4 in fuel-rich, CH 4 /O 2 /Ar = 9/1/90, mixtures was studied by infrared (IR) laser kinetic absorption spectroscopy behind incident shock waves with 1800 T K at total densities of ∼1.2 × 10 −6 mol cm −3 . Computer simulations using a 63-reaction mechanism were used to identify the elementary reactions that determined the data parameters and to investigate the consequences of various rate-constant assumptions. An experimental data base from the literature was also used to test the mechanism and rate constants over a wide variety of conditions of temperature, pressure, and equivalence ratio. Rate-constant expressions are suggested for CH 2 + CH 3 = C 2 H 4 + H and CH 3 + O 2 = CH 2 O + OH.

Hypercycles and compartments: Compartments assists—but do not replace—Hypercyclic organization of early genetic information

Abstract The development of the first protocells proceeded by mechanisms which were governed by three principles: 1. (1) Self-reproduction of molecules identical or similar to modern RNAs was required to transmit information from mother to daughter molecules. 2. (2) Evolution of these molecules to improve their phenotypic qualities—stability, error-proneness, and reproduction rate—proceeded in hypercyclically organized systems in which the total information content was distributed over a number of information carriers. 3. (3) Compartmentation, insofar as it was not already necessary for protection against parasitic infection, became necessary for utilization of genotypic qualities, i.e. the nature of the macromolecules formed as translation products of the information carriers. With reśpect to their different goals, principles (2) and (3) are not interchangeable. A contrary claim advanced by Bresch, Niesert & Harnasch is based on a particular model for evolution that ignores the appearance of mutants with differing selective values. In such a non-realistic model, error threshold is of no importance for the stability of the wild-type.

Reactions of sodium species in the promoted SNCR process

Abstract Selective Non-Catalytic Reduction (SNCR) is a well-known commercial NOx control process based on injecting a nitrogen agent into combustion products containing NO at temperatures near 1250 K. A serious limitation of the SNCR processes is that the temperature range over which nitrogen agents are effective is relatively narrow. In this work, we show that adding small amounts of sodium salts significantly improves the performance of the SNCR process. Parts per million levels of sodium compounds enhance NO removal and extend the effective SNCR temperature range in comparison with use of a nitrogen agent alone. When added in the same sodium atom amounts, the efficiencies of different sodium compounds are similar. Kinetic modeling suggests that the performance improvement can be explained as a homogeneous chain reaction ensuing after the sodium compounds are converted into NaOH. The overall result of introducing sodium compounds is conversion of H2O and inactive HO2 radicals into reactive OH radicals, with the effective stoichiometry H2O + HO2 → 3 OH, which enhances the SNCR performance of nitrogen agents by increasing the production rate of NH2 radicals.

Thermal decomposition of ethylene

Abstract Laser-schlieren profiles of incident shock waves in 2.5, 5, and 10% C 2 H 4 in Ar were recorded for the ranges of shock front conditions 2000 T 2 −6 ϱ −6 mol/cm 3 . Data analysis was accomplished by computer modeling using a 14-reaction mechanism. Most, but not all, previous observations could be accounted for with the final rate constant set. For C 2 H 4 + M → C 2 H 2 + H 2 + M the expression log( k 1 /cm 3 mol −1 s −1 ) = 17.47 – 340 kJ/RT, for C 2 H 4 + M → C 2 H 3 + H + M the expression log ( k 2 /cm 3 mol −1 s −1 ) = 17.49 – 400 kJ/RT, and for C 2 H 3 + H → C 2 H 2 + H 2 the rate constant k 6 = 10 13 cm 3 mol −1 s −1 were obtained.

Measurement and modeling of shock-tube ignition delay for propene

Abstract Propene ignition was studied behind reflected shock waves at postshock temperatures ranging from 1270 to 1820 K and postshock pressures from 0.95 to 4.7 atm. Reactant concentrations were varied from 0.8 to 3.2% propene and from 3.6 to 15.1% oxygen diluted in argon, giving equivalence ratios ranging from 0.5 to 2.0. The pressure-based ignition delay correlation equation τ(s) = 4.2 × 10−15 [C3H6]0.378[O2]−1.043 exp(48800/RT5) for mol/cm3 and cal units was derived. The data could be accounted for using a reaction mechanism with 463 elementary reactions.

Molecular evolution of RNA in vitro

Experimental studies of RNA evolution in vitro are reviewed in the context of Eigens 1971 theory and its subsequent extensions. Current research activity and future prospects for using automated molecular biology techniques for in vitro evolution experiments are surveyed.