Structure of atmospheric pressure premixed hydrogen-oxygen flame doped with iron pentacarbonyl

Transcription

1 Structure of atmospheric pressure premixed hydrogen-oxygen flame doped with iron pentacarbonyl I.E. Gerasimov, D.A. Knyazkov, A.G. Shmakov, A.A. Paletsky, V.M. Shvartsberg, T.A. Bolshova, O.P. Korobeinichev Institute of Chemical Kinetics & Combustion, Novosibirsk, Russia 7 th International Seminar on Flame Structure, Novosibirsk, Russia, July 11-15, 2011

2 Background Combustion chemistry of iron-containing compounds, including iron pentacarbonyl Fe(CO) 5, is of considerable interest. Fe(CO) 5 is known to be an effective flame inhibitor (due to formation from Fe(CO) 5 in flames FeO and FeOH species, which catalyze the recombination of H and OH radicals). Additive of Fe(CO) 5 (or other ICS) reduces soot concentration in the post-flame zone under certain conditions. Synthesis of iron-containing nanoparticles in flames is a promising line of research. These issues are interrelated because they deal with chemical transformations of ICS in flames. Dr. Linteris, et al proposed a detailed chemical kinetic mechanism for flame inhibition by Fe(CO) 5. Atakan et al made a major experimental contribution to study of ICS transformation in flames.

3 Motivation Earlier developed mechanism of Fe(CO) 5 transformation in flames (Babushok, Linteris et al.) was validated by comparing measured and simulated burning velocities of premixed flames and extinction strain rates of opposed-jet diffusion flames. Atakan and coworkers validated it against measured concentration profiles of atomic iron. Therefore, an urgent need emerged to validate the mechanism by comparing measured and predicted flame structure, especially concentration profiles of iron-containing products of Fe(CO) 5 combustion.

4 Objective to validate the mechanism for iron pentacarbonyl transformation in flames by comparison of chemical kinetic model predictions with new experimental data on spatial variations of mole fractions of Fe-containing products of Fe(CO) 5 oxidation in atmospheric-pressure H 2 /O 2 /N 2 flame

5 Premixed, near-stoichiometric (f=1.1) H 2 /O 2 /N 2 flame stabilized on a flat burner at P=1 bar and T 0 =60 0 C was studied. The gas velocity near the burner was 107 cm/s (75% of the flame speed). Perforated plate Experimental approach Fresh mixture Steel beads Bubbler with Fe(CO) 5 Flame temperature ~ 1600 K Thermostat 60 0 С Flame reactants Component H 2 O 2 N 2 Fe(CO) 5 Content, %

6 Ion pump, 400 L/s Turbomolecular pump, 500 L/s Turbomolecular pump, 500 L/s Diffusion pump, 1100 L/s Experimental approach Molecular beam mass spectrometric setup for measuring ICS mole fraction profiles Flame 5x10 torr -8 3x10 torr torr -5 6,7 мм MS-7302 Liquid-nitrogen trap Sonic probe Disk chopper Skimmer Burner with burner positioning mechanism The quartz probe was clogged by Fecontaining particles for 5 minutes of experiment and underwent irreversible changes. Mean-square error ±50% as a result of low concentration of Fe species

7 Modeling Flame structure was simulated using PREMIX code. Chemical kinetic mechanism for Fe(CO) 5 transformation in flames proposed by Babushok, Linteris et al. (59 reactions involving 12 ICS) and mechanism GRI 3.0 were used for the simulation. Calculation and visualization of Fe-element fluxes from species to species was performed using the KINALC and the Flux Viewer codes

8 Results and Discussion Simulated spatial variations of major ICS concentration in H 2 /O 2 /N 2 flame doped with 100 ppm Fe(CO) 5 stabilized on a flat burner at 1 bar.

9 Results and Discussion Spatial variations of Fe mole fraction in H 2 /O 2 /N 2 flame doped with 100 ppm Fe(CO) 5 stabilized on a flat burner at 1 bar; symbols experiment, curves modeling.

10 Results and Discussion Spatial variations of FeOH mole fraction in H 2 /O 2 /N 2 flame doped with 100 ppm Fe(CO) 5 stabilized on a flat burner at 1 bar; symbols experiment, curves modeling.

11 Results and Discussion Spatial variations of FeO 2 mole fraction in H 2 /O 2 /N 2 flame doped with 100 ppm Fe(CO) 5 stabilized on a flat burner at 1 bar; symbols experiment, curves modeling.

12 Results and Discussion Spatial variations of Fe(OH) 2 mole fraction in H 2 /O 2 /N 2 flame doped with 100 ppm Fe(CO) 5 stabilized on a flat burner at 1 bar; symbols experiment, curves modeling.

13 Results and Discussion Where is FeO? The intensity of peak at 72 AMU (FeO) was very low and we failed to measure it along the flame zone. It may be explained by 2 reasons: 1. a low FeO concentration in the flame (contrary to the model prediction). 2. a low sensitivity of mass spectrometer to FeO. In our practice we met examples when two parent compounds have essentially different calibration coefficients, e.g. k(po2)/k(po)=0.77/0.02=38.5. By the same argument we failed to measure FeH and FeOOH.

14 Results and Discussion Spatial variations of total ICS concentration in H 2 /O 2 /N 2 flame doped with 100 ppm Fe(CO) 5 stabilized on a flat burner at 1 bar; symbols experiment, curves modeling. Fe + FeO 2 + FeOH + Fe(OH) 2 50%-deficit of iron is observed at mm above burner. Possible explanation: Fe 2 O 3, Fe 3 O 4 and other species, which were not measured, are present in the flame.

15 Results and Discussion Schematic diagram of ICS reaction pathways in H 2 /O 2 /N 2 flame doped with 100 ppm Fe(CO) 5 at 0.3 mm from the burner. Fe(CO) 5 -CO Fe(CO) 4 +H/ -H 2 O FeOH Fe(OH) 2 -CO Fe(CO) 3 +H/ -H 2 +H 2 O -CO Fe(CO) 2 -CO +O/ -O 2 +H/ -OH +H 2 / -H 2 O FeO +O/ -O 2 Fe(CO) Fe FeO 2 +O/ -CO 2 +O 2

16 Conclusions 1. Concentrations of iron-containing products of Fe(CO) 5 combustion (Fe, FeO 2, FeOH, and Fe(OH) 2 ) in a H 2 /O 2 /N 2 flame at atmospheric pressure using the MBMS method were measured. 2. A kinetic model for flame inhibition by Fe(CO) 5 proposed earlier by Rumminger, Reinelt, Babushok, Linteris was validated basing on the experimental data. 3. The kinetic model satisfactorily predicts FeO 2 and Fe(OH) 2 in the flame. FeOH and Fe(OH) 2 were shown to be the main iron-containing products of Fe(CO) 5 oxidation in the flame studied. In the flame reaction zone, atomic iron was shown to be the main iron-containing compound produced by the reduction of iron oxides via interaction with H, O, and H 2.

17 Thank you!

18 H 2 O a) Major stable species Neat flame and flame doped with 230 ppm of Fe(CO) H Neat H2 flame мольная доля O 2 H H 2 O б) Doped flame O расстояние от горелки, мм

19 Reactions responsible for flame inhibition by Fe(CO) 5 FeOH + H FeO + H 2 FeO + H 2 O Fe(OH) 2 Fe(OH) 2 + H FeOH + H 2 O Total: H + H H 2 Fe + O 2 + M FeO 2 + M FeO 2 + O FeO + O 2 FeO + O Fe + O 2 Total: O + O O 2