SESSION 4

KINETICS


INITIATION OF PREMIXED FLAMES IN H2-AIR-H2O MIXTURES

A. Yu. Kusharin, O. E. Popov, G. L. Agafonov and B. E. Gelfand

Semenov Institute of Chemical Physics, Russian Academy of Sciences, Moscow, Russia

ABSTRACT

The dynamics of laminar spherical flame kernels in hydrogen/air mixtures diluted with steam are studied numerically for hydrogen percentage ranging from lean to rich flammability limits at atmospheric pressure and temperature of 373 K. Ignition is simulated by means of a one-dimensional time dependent computational model (conservation of mass, energy, momentum) taking into account detailed chemistry, multicomponent transport properties, and thermal radiation of water molecules. The analysis is focused on the transient processes leading to the development or extinction of lean, stoichiometric, and rich flame kernels initiated by an ignition source with variable energy. The formation of a propagating flame kernel is used as a criterion for mixture flammability. The extinction turning-point behavior is demonstrated for lean hydrogen-air mixtures. The turning point corresponds to about 3.5% hydrogen at 373 K. Both dynamics and critical size of a flame kernel are strongly affected by hydrogen diffusion and thermal radiation. The specific effect of hydrogen diffusion is important for a range of steam diluted lean mixtures extending to the inerting point. The amount of steam required for quenching is 56.5% by volume. The characteristic modes of behavior of developing and extinguishing flame kernels are demonstrated and analyzed for near-limit lean hydrogen/air/steam mixtures confined in a spherical laboratory-scale explosion vessel. The flammability limits obtained by numerical simulations are in reasonable agreement with available experimental data.


FLAME STRUCTURE STUDIES OF SEVERAL PREMIXED ETHYLENE - OXYGEN - ARGON FLAMES AT EQUIVALENCE RATIOS FROM 1.00 TO 2.00

M. Musick, P. J. Van Tiggelen and J. Vandooren

Laboratoire de Physico-Chimie de la Combustion, Universite Catholique de Louvain, Place Louis Pasteur 1, 1348 Louvain la Neuve Belgium

ABSTRACT

An exhaustive experimental investigation has been performed on the structures of five low pressure premixed C2H4/O2/Ar flames at equivalence ratios from 1.00 to 2.00. Stable species, atoms and radicals have been monitored by using molecular beam mass spectrometry (MBMS). This study has shown that some maximum concentrations of C2-, C3-, C4-, C5- and C6- species increase drastically with equivalence ratio. Some of these species like C2H2, C3H4 and C4H2 remain in non negligible amounts in burnt gases of the richest flames .(f = 1.75 and 2.00). We can consider they are soot precursors. It has been noticed for some C3- (C3H3 and C3H4) and C4- (C4H2, C4H4, C4H6, and C4H8) species a dependence in the form (Xi)max = ai fni in the equivalence ratio (f) range investigated. The exponent ni of f depends on the species considered and decreases along the following sequence:

C4H2 > C4H4 ~ C4H6 > C4H8 > C3H3 > C3H4

By using the net reaction rate of C2H4 , the rate coefficient of the reaction " C2H4 + O ->Products " has been deduced (k = 1.93 1013 cm3mol-1s-1 at 1400K). It is the main process leading to the formation of the first C1- compound in these flames, CH3 or less likely CH2. Moreover, it has been established that the building-up of C3H4 must proceed along the formation of the propenyl radical (C3H5) by the reaction between C2H2 and CH3 . The deduced experimental rate coefficient of this reaction is 3.15*1011exp(-1422/T) cm3mol-ls-1 from 1000K to 1670K.

In addition, the rate coefficient of reaction:

C3H3 + C2H4 -> C5H6 + H

has been deduced and amounts to 1.4*1012 cm3 mol-1 s-1 at 1530K.

UPPER LIMIT RATE CONSTANTS FOR THE REACTIONS OF NH AND NH2 RADICALS WITH N20 DERIVED FROM THE KINETIC MODELING OF HYDROGEN OXIDATION BY NITROUS OXIDE

A. A. Konnov, J. De Ruyck

Department of Mechanical Engineering, Vrije Universiteit Brussel 1050 Brussels, Belgium

ABSTRACT

A detailed H/N/O reaction mechanism has been validated with respect to the experimental species profiles in the hydrogen oxidation by nitrous oxide (Allen, M.T. et al., Combust. Flame 112: 302311 (1998)). It is shown that incorporation of the reactions between NH and NH2 radicals and N2O with currently adopted rate constants prohibits correct modeling of the minor species, NH3 and NO. A sensitivity analysis is used to reveal which reactions are critical for the quality of the modeling. The choice of the rate constants for these reactions is discussed. It has been concluded that only significant decrease in the rate constant of one or both reactions mentioned can improve the agreement between the modeling and experiment. The upper limit rate constant of the reaction

N2H2 + NO = N2O + NH2

is found to be about 1.0E+11 cm3/mole s. The upper limit rate constant of the reverse reaction
N2O + NH2 = N2H2 + NO

is calculated from the equilibrium data as 6.0E+09 at temperatures close to 1000K. Finally, the upper limit rate constant of the reaction
NH + N2O = N2 + HNO

is found to be about 2.0E+1l.

Ignition delays in the H2 - N2O - Ar mixtures at temperatures below 1400 K are calculated. The modeling is in agreement with the high temperature data, however at temperatures lower than 1200 K the difference is almost 10-fold. It is concluded that the ignition measurements in a static by-pass apparatus are incompatible with the flow reactor data.


PYROLYSIS AND OXIDATION OF UNSATURATED C2 AND C3 SPECIES

E. Ranzi, T. Faravelli, A. Goldaniga, F. Ferrari, M. Lattuada

CIIC Dipartimento di Chimica Industriale e Ingegneria Chimica, Politecnico di Milano, P.za L. da Vinci 32 20133 Milano (Italy)

ABSTRACT

In this work we analyse and study several experimental data on pyrolysis, partial oxidation and combustion of C2 and C3 acetylenics as well as allene and ketene. These data, already presented in the literature, refer to turbulent flow reactors, jet stirred reactors and shock tube experiments and span wide ranges of concentration, pressure and temperature conditions. In order to stress the importance of condensation reactions of unsaturated species some data of ethylene and propylene pyrolysis are also discussed.

C2 and C3 unsaturated species are always present as important intermediates in combustion processes and their reactions play a crucial role in explaining the formation of aromatic and polyaromatic species in sooting flames.

A detailed kinetic scheme able to describe the partial oxidation and the combustion of hydrocarbon mixtures up to commercial gasolines and liquid fuels is extended and more carefully validated on these sets of experimental data. The key reactions are discussed and compared with literature kinetic data. Moreover, this kinetic analysis is very useful for the study of 'mild combustion' in the intermediate temperature region where the 'flameless combustion' seems to offer the great advantage of a reduced formation of toxic components such as nitric oxides, polyaromatics and soot.


THE PREDICTION OF CHAR COMBUSTION KINETICS AT HIGH TEMPERATURE

B.Coda, L. Tognotti

Universit‚ degli Studi di Pisa - Dip. di Ingegneria Chimica, Chimica Industriale e Scienza dei Materiali - Pisa, Italia

ABSTRACT

The optimisation of numerical methods applied to the description of char combustion behaviour is required in order to achieve a good prediction of the amount of unburned carbon in fly ash from pulverised coal fired boilers. A question arising is whether it is necessary to take into account the distribution of the coal properties and reactivity and how to describe them in. such a simplified way in order to be handled in a simulation code.

The reliability of a simple model for char heterogeneous oxidation describing the burning of single char particles has been assessed through the use of available experimental data for a high volatile bituminous coal in a pilot plant, under single stage and staged combustion conditions. Experiments in an electrodynamic chamber effected on the char particles collected in the pilot plant, showed the heterogeneity of particles properties, evaluated by fitting the temperature-time histories of single burning particles.

The model was later used to predict kinetics data at high heating rate to be used in comprehensive code: the results of the prediction provide a reactivity distribution expressed in the form of a pre-exponential factor as a function of particle size.

Future Meetings | Past Meetings | Proceedings on Sale | Related Links

ICHMT World Wide Web Administrator