H. Jander

Laser-induced incandescence for soot diagnostics at high pressures

The influence of pressure on laser-induced incandescence (LII) is investigated systematically in premixed, laminar sooting ethylene/air flames at 1-15 bar with wavelength-, laser fluence-, and time-resolved detection. In the investigated pressure range the LII signal decay rate is proportional to pressure. This observation is consistent with the prediction of heat-transfer models in the free-molecular regime. Pressure does not systematically affect the relationship between LII signal and laser fluence. With appropriate detection timing the pressure influence on LII signals proportionality to soot volume faction obtained by extinction measurements is only minor compared with the variation observed in different flames at fixed pressures. The implications for particle sizing and soot volume fraction measurements using LII techniques at elevated pressures are discussed.

The influence of pressure and temperature on soot formation in premixed flames

The formation of soot has been investigated in flat C2H4-air and C6H6-air flames, with special reference to low flame temperatures. Measurements of the threshold for soot formation show that as well as the “high temperature threshold” (slightly temperature dependent) there exists a low temperature threshold. For a given C/O ratio the mass of soot formed passes through a maximum between low and high temperature threshold at a temperature near 1600 to 1650 K. These soot yield curves are similar to those obtained in shock wave pyrolysis of benzene etc. by Graham, Frenklach et al. With increasing pressure the soot thresholds widen slightly. Their minima shift towards lower C/O ratio and the maxima of the soot yield curves rise. For the pressure dependence of the soot volume fraction finally formed, a P2 dependence has been found at constant flame temperature for flame temperatures above 1650 K and C/O ratios from 0.65 to 0.75. For benzene-air flames the pressure dependence is almost the same as for C2H4-air flames. The dependence of soot yield on the C/O ratio at constant flame temperature is a little weaker than the data obtained for constant flow velocity. Measurements of light absorption and light scattering along the flame axis show that coagulation and mass growth of the soot particles in the low temperature flames proceed in a similar way to that in flames at temperatures above 1600 K.

PAH growth and soot formation in the pyrolysis of acetylene and benzene at high temperatures and pressures: Modeling and experiment

The growth of high molecular polycyclic aromatic hydrocarbons (PAHs) before soot inception is modeled during C 2 H 2 and C 6 H 6 pyrolysis at temperatures between 1600 and 2400 K, p =60 bar, and C-atom concentrations of (3.8–4.0)×10 −6 mol/cm 3 . The formation of high molecular PAH is mainly computed by two different reaction pathways: (1) successive H-abstraction C 2 H 2 additions and (2) a combinative ring-ring condensation of aromatics. The calculations are compared with measurements obtained from C 2 H 2 and C 6 H 6 pyrolysis by the shock wave method. From reaction flux analysis, it is deduced that the combinative route plays a major role in forming PAHs, especially at early reaction times. The calculated induction times of higher PAHs reasonably reflect the experimental trends of soot inception. The slope of the computed and measured induction times in dependence on the temperature resulted in a similar activation energy for high molecular PAH and soot formation. It is concluded that soot mass growth rates during the pyrolysis of C 2 H 2 and C 6 H 6 are strongly related to PAH formation. Thus, the decrease of the soot mass growth rates during C 2 H 2 pyrolysis for T >2000 K seems to be substantially influenced by a change in the nucleation process of soot precursors.

The effect of metal additives on the formation of soot in premixed flames

The influence of low concentrations of alkali and alkaline-earth metals on soot particle size and number density in atmospheric-pressure premixed flat flames is investigated using laser light scattering and extinction measurements. Additions of the alkali metals (Na, K, Cs) only weakly suppress the total amount of soot formed, but they do result in a greater number (up 25 times) of particles than in the unseeded flame. The number of soot particles present correlates with the observed ability of the additive to promote ionization (Cs>K>Na) in the early stages of soot formation. It is proposed that incipient soot particles are charged earlier in the presence of these metals—once charged, these particles resist further collisional growth, so that their number density remains high and their size small. Of the alkaline-earth metlas (Ca, Sr, Ba), only Ba produces enough ions to suppress coagulation in the manner of the alkali metals. However, these metals, Ba included, do reduce the amount of soot formation. These results provide evidence that the influence of metallic additives involves both electrical and chemical factors.

Soot formation in a laminar diffusion flame

Light scattering and extinction measurements have been made to measure profiles of soot volume fraction, particle number density and average size in a laminar ethylene diffusion flame. Velocity profiles by laser Doppler anemometry and temperature profiles were also obtained in order to solve the species conservation equations and deduce the soot formation and particle generation rates. Soot volume fraction and particle diameter profiles peak some 5 mm on the fuel side of the peak temperature and increase with height. Number densities drop steeply from the reaction zone to the sooting zone and are relatively invariant with height. Soot particle trajectories show strong entrainment of particles from the reaction zone into the fuel side of the flame. The soot formation rates and particle generation rates peak in different regions. The picture that emerges is that particles are generated in the reaction zone and move into the fuel side of the flame where the main increase in soot volume takes place.

Soot formation in premixed C2H4 flat flames at elevated pressure

The influence of pressure on flat C2H4-air flames has been investigated for elevated pressures by optical methods and by chemical analysis. It was found that the threshold of soot formation is shifted towards lower C/O ratios with increasing pressure. At 70 bar the minimum C/O ratio drops to 0.4. In the burned gases (P≤10 bar) the final soot volume fraction rises with pressure as fvx∼p2 (T≥1700 K). The fraction of fuel carbon appearing in CO + CO2 changes little with pressure. CH4 becomes the dominant hydrocarbon, the C2H2 concentration is nearly independent of pressure and the PAH rise with P2. The soot carbon content exceeds that of hydrocarbons with increasing pressure. The dependence of fvx on temperature and C/O at elevated pressure is similar to that at 1 bar. The formal rate constants kv for the soot mass growth fit to the data obtained at 1 bar and depend for P≤10 bar only on temperature. Flame concentration profile measurements show that the maximum C2H2 concentration at the main oxidation zone (representative for other reactive hydrocarbons) increases roughly proportional to P and the distribution of the fuel carbon into CO+CO2 and other components is essentially finished behind the main reaction zone of the flame.

Soot formation in premixed C2H4-air flames for pressures up to 100 bar

The influence of pressure on flat C2H4-air flames has been further investigated for pressures up to 70, resp. 100 bar by optical methods and by chemical analysis. In addition concentration profile computations have been performed for pressures up to 100 bar. For pressures above 10 bar the final soot volume fraction rises proportional with pressure fv∞ ∼ P (T>1650 K). The same holds for the concentration of CH4 and the much smaller concentration of C2H4. The final carbon density present as C2H2 decreases above 10 bar with increasing pressure. There is a change in mechanism of soot formation towards high pressures: The mass growth process ends because of a lack of substance which can be added readily. The final particle number density increases above the value obtained at normal pressures and becomes proportional to fv∞ towards high pressure. Profile calculations show that the maximum C2H2 concentration in the reaction zone grows as p1.3 and the higher PAH grow even more strongly with pressure. This is an indication that the formation of soot precursors can lead to high particle number densities and correspondingly a very high reactivity of the young soot which then causes this change in mechanism.

Spectral and structural properties of carbon nanoparticle forming in C3O2 and C2H2 pyrolysis behind shock waves

The diversity of carbon particles, forming durin pyrolysis of C 3 O 2 and C 2 H 2 behind shock waves in thewide temperature range 1200–3800 K, was investigated. The process of condensed carbon particle formation was observed in situ by the multichannel registration of the time profiles of optical properties of media in the UV, visible, and near-IR ranges. Besides that, the probes of postshock materials, deposited on the walls of the shock tube, were analyzed bylow- and high-resolution transition electron microscopy (TEM) and by electron microdiffraction (MDF) measurements. The comparison of extinction properties of young growting particles with the electron microscopic analysis of solidified substance gave a notion about the peculiarities of carbon particle formation process from the diffeerent carbon-bearing gases at various temperatures. particles, forming from both substances at 1500–200 K, look similar to usual soot, and the absence of hydrogen in C 3 O 2 leads to faster formation and graphitization of particles. At the tempratures 2100–2600 K, the decrease of the paticle formation rate and the fall of final particle yield in all mixtures is observed. After C 3 O 2 pyrolysis experiments, gigantic film-like spheres with the size up to 700 nm were observed on the walls. The peculiarity of the high-temperature (2700–3200 K) process of carbon particle formation in C 3 O 2 pyrolysis is the high degree of crystallization of the final particles.

The influence of gaseous additives on the formation of soot in premixed flames

Laser-light scattering and absorption measurements of soot volume fractions and particle number densities in premixed ethylene-air flat flames at atmospheric pressure show that additions of up to 3%, by volume of the unburned gas, of N 2 , H 2 O, H 2 , NH 3 , CH 3 NH 2 , NO, H 2 S, SO 2 , SO 3 , HCl, CH 4 , C 2 H 4 or O 2 do not influence the coagulation of soot particles although the soot volume fraction may be significantly affected. Pro-soot additives are CH 4 , C 2 H 4 and CH 3 NH 2 while additives which reduce the soot volume fraction are O 2 , NO, NH 3 , H 2 S, SO 2 and SO 3 . Whatever their effects, these additives do not alter the specific surface growth rate for soot particle mass addition. The various additives retain their pro- or anti-soot tendencies when considered in terms of their effects on the critical C/O-ratio for the appearance of soot. Addition of H 2 promote soot under these conditions. Special attention is paid to the effect of SO 3 —no evidence of any pre-soot effects at or beyond the critical C/O-ratio is found in aliphatic or aromatic flames. The mode of action of the additives is not generally understood, although the formal equivalence of the addition of one molecule of NH 3 with removal of one fuel-C atom is consistent with the expected rapid formation of inert HCN in the flame and with the relatively weak effects of CH 3 NH 2 addition.

Rate of soot growth in atmospheric premixed laminar flames

The process of soot formation, the growth of the soot mass, of the soot particles and the change in their number density has been measured in laminar premixed fuel-air flames. The following fuels have been used: acetylene, benzene, n -hexane, n -heptane, cyclohexane, cyclohexene, naphthalene, pyridine, thiophene, and furane. In addition, mixtures of ethylene with hydrogen have been investigated for C/H ratios from 0.48 to 0.30. The various mixtures have been tested for different C/O ratios and flow velocities on a capillary burner with shielded flame and on a porous plate burner. In some cases the initial temperature was varied. Two lasers were used to measure the soot aerosol absorption and scattering; temperatures were measured by the Na-line reversal and the Kurlbaum methods. The experiments show that the particle coagulation process is practically the same for all flames (flame temperatures from 1500 K to 200 K). The growth of the soot volume fraction f c in the later phase (after about 10% of the final volume fraction f M has been reached) can be approximated by df v /dt=k(f M -f v ). The “rate constant ( k )“ for the growth of f v has been found to depend on temperature, but not on the type of fuel, the C/O ratio or the C/H ratio. The amount of soot finally present ( f M ) varies greatly with C/O ratio, approximately with 3rd to 4th power of the carbon surplus (C/O-(C/O) crit ). Moreover, it depends considerably on temperature above 1500 K. Its total value for a given C/O is typical of the type of fuel.