Wilson Merchan-Merchan

Soot and NO formation in methane–oxygen enriched diffusion flames

NO and soot formation were investigated both numerically and experimentally in oxygen-enriched counterflow diffusion flames. Two sets of experiments were conducted. In the first set, the soot volume fraction was measured as a function of oxygen content in the oxidizer jet at constant strain rate (20 s−1). In the second set of experiments, the soot volume fraction was measured as a function of strain rate variation from 10 to 60 s−1 and at constant oxygen content on the oxidizer side. A soot model was developed based on a detailed C6 gas phase chemistry. The soot and molecular radiation were taken into account. Numerical results were verified against experimental data. The soot volume fraction was predicted with the maximum discrepancy less than 30% for all cases considered. It was found that oxygen variation significantly modified the diffusion flame structure and the flame temperature, resulting in a substantial increase of soot. The temperature increase promotes aromatics production in the fuel pyrolysis zone and changes the relative contributions of the thermal and Fenimore mechanisms into NO formation. As the strain rate increases, the residence time of incipient soot particles in the high temperature zone is reduced and the total amount of soot decreases. High concentration of soot in the flame leads to enhancement of radiant heat exchange: the reduction of temperature due to radiation was found to be between 10 and 50 K. This caused a reduction of peak NO concentrations by 20%–25%. The increase of oxygen content in the oxidizer stream resulted in a reduction of the distance between the plane of the maximum temperature and the stagnation plane.

Formation of carbon nanotubes in counter-flow, oxy-methane diffusion flames without catalysts

In oxygen enriched methane diffusion flames, carbon nanotubes were discovered to be formed in the region on the fuel-rich side of the flame front at an oxygen enrichment of 50%. No catalyst was employed. An opposed flow diffusion flames with varying strain rate and oxygen content in the oxidizer stream was used. Substantial quantities of nanotube material are produced at atmospheric pressure in this continuous (non-batch) process. Thermophoretic sampling of the flame and collecting the carbon material deposited near the exhaust was done. Both confirm the growth of carbon nanotubes and other carbon clusters.

Metal catalyzed synthesis of carbon nanostructures in an opposed flow methane oxygen flame

Abstract Results of an experimental study on metal-catalyzed synthesis of carbon tubular nanostructures in opposed flow flame are reported. The catalytic support made of Ni-alloy was positioned at the fuel side of the opposed flow flame formed by fuel (96%CH4+4%C2H2) and oxidizer (50%O2+50%N2) streams. The electron microscopy studies reveal the presence of highly organized carbonaceous structures with the configurations showing strong dependence on the flame location. Several typical structures were detected. These include: multiwalled nanotubes (MWNT), MWNT bundles, irregular high-density carbon nanofibers, long (up to 0.2 mm) uniform-diameter (∼100 nm) nanofibers, helical regularly coiled tubular nanofibers, and ribbon-like coiled nanofibers with rectangular cross section. Transmission electron microscopy (TEM) studies performed on long nanofibers revealed the presence of highly organized multiple (∼100) graphene layers. These layers are parallel to the nanofiber axis resembling the structure of MWNT. The TEM studies of coiled nanofibers show internal tubular structure and the presence of regular carbon lattice. The well-aligned bundles of nanotubes were examined by TEM showing tight packing of MWNTs with varying inner and outer diameters. The diversity of formed nanomaterials is attributed to the strong variation of flame properties along the flame axis including temperature, hydrocarbon and radical pool. This provides strong selectivity for formation of different nanoforms even without adjustment of catalyst properties.

Carbon nanostructures in opposed-flow methane oxy-flames

Carbon nanostructures formed in the opposed-flow flames of methane- and oxygen-enriched air are studied experimentally using thermophoretic sampling technique and high-resolution transmission electron microscopy (TEM). Reconstructed evolution of soot particles along the burner centerline shows the existence of two characteristic layers. The narrow layer of polydisperse precursor particles is located on the oxidizer side of the stagnation plane. The precursor particles undergo carbonization and agglomeration as they are driven to the stagnation plane forming the layer of mature soot aggregates on the fuel side of the precursor layer. High-resolution TEM imaging performed on precursors and mature soot particles reveals the presence of highly organized carbon nanostructures formed inside the tarlike amorphous condensate by carbonization process. Two characteristic structures are observed. They resemble shapes of carbon onions and carbon nanopolyhedral particles. Multiwalled carbon nanotubes (MWNTs) appeared to be present in the collected samples along with other carbon particulates. TEM imaging reveals incidence of isolated MWNTs, MWNT clusters, and clusters of MWNT with soot and nanopolyhedral particles. The nanotube growth by elongation of partially carbonized nanopolyhedral particles is considered as a possible mechanism of noncatalytic MWNT formation.

Growth of Carbon Nanotubes and Carbon Nanofibers in Opposed Flow Oxy-Flame of Methane

Carbon nanotubes and carbon nanofibers formed on a Ni-based catalytic support positioned at the fuel side of opposed flow oxy-flame are characterized by electron microscopy. Observed nanoforms include multiwalled carbon nanotubes (MWNTs), MWNT bundles, helically coiled tubular nanofibers, and ribbon-like coiled nanofibers with rectangular cross-section. The electric field method is applied to control structure and purity of formed carbon nanomaterial. A coating layer of nanotubes possessing a thickness of 35 to 40 microns and a high degree of alignment was formed along the surface of the catalytic probe with variation of probe potential. The method shows a great promise in controlling the structure and formation rate of flame generated carbon nanomaterials.Copyright