High Soot Loadings Research Articles

Effects of gas preheat temperature on soot formation in co-flow methane and ethylene diffusion flames

This study reports the effects of gas preheat temperature on soot formation in a series of methane (CH4), ethylene (C2H4) and nitrogen-diluted C2H4 co-flow diffusion flames with a constant mass flow rate of carbon element. The flames were established in a Santoro-type burner and the temperatures of co-flow air and fuel were varied simultaneously from 293 K to 723 K. Planar distributions of soot volume fraction and soot temperature were measured using laser-induced incandescence and two-color pyrometry based on incandescence, respectively. The results show that the preheat temperature of gases has no significant effect on the shape of the CH4 and nitrogen-diluted C2H4 flames, but significantly influences that of the C2H4 flames with high soot loadings. With an increase in gas preheat temperature, both soot formation and oxidation are enhanced and soot loading increases for all flames. The magnitude of enhancement correlates with the soot loading, while the effect is more significant in the flames with lower soot loading. Soot temperature in the CH4 and nitrogen-diluted C2H4 flames also increases but is not as much as the change in the gas preheat temperature. However, for the C2H4 flames, an inverse tend is found in the central region of the flames, i.e., that lower soot temperatures are found in the flame with the higher gas preheat temperature.

The effect of fuel composition and Reynolds number on soot formation processes in turbulent non-premixed toluene jet flames

The soot formation processes in three different turbulent prevaporized non-premixed toluene jet flames stabilized on a jet-in-hot-coflow (JHC) burner were investigated in this study. The jet Reynolds number and the stoichiometric mixture fraction were varied in order to manipulate the flow time scales and the chemistry, respectively. Time-resolved laser-induced incandescence (TiRe-LII), non-linear two-line atomic fluorescence of indium (nTLAF), and OH planar laser induced fluorescence (PLIF) were simultaneously applied to yield spatially resolved and instantaneous fields of soot volume fraction, primary particle size, temperature, and OH. The mean distributions of the detected quantities are used to identify major differences among the flames. The highest soot loading is observed for the low Reynolds number and low stoichiometric mixture fraction flame. However, this flame features also the lowest temperature and primary particle size. Based on these observations, the simultaneously detected data sets and flamelet computations are employed to elucidate differences in the soot formation pathways in the flames. The analyses reveal that the high soot loading causes greater heat losses in the low Reynolds number and low stoichiometric mixture fraction flame. This has a significant impact on the soot formation pathways and causes a reduction in the particle size.

Planar laser-induced incandescence of turbulent sooting flames: the influence of beam steering and signal trapping

The influence of beam steering and signal trapping on the accuracy of soot volume fractions measured using planar laser-induced incandescence (LII) has been investigated in turbulent non-premixed sooting flames at atmospheric pressure. In turbulent non-premixed C2H4/air flames, the influence of local de-focusing/focusing of the laser sheet from beam steering can result in the underestimate of the averaged LII signal by 30 %, even when operating within the so-called plateau regime of laser fluence. Beam steering was also found to be significant in both the upstream region of C2H4/air flames and non-reacting C2H4 flows, because the fuel has a relatively high refractive index compared with ambient air. The extent of beam steering at different heights of reacting and isothermal flows as well as its dependence on exit Reynolds number (Re) has been measured. The measurements reveal that even at low turbulence levels (2000 < Re < 3000), beam steering effects can be significant. Also found is that the LII signal at a 450 nm wavelength can be attenuated by a few per cent at high soot loading regions in turbulent flames due to signal trapping. Finally, the feasibility of directly evaluating the signal attenuation via planar LII results was assessed by comparing the virtual soot attenuation calculated based on the planar LII result with that measured using light-of-sight extinction.

Assessment of interferences to nonlinear two-line atomic fluorescence (NTLAF) in sooty flames

Nonlinear excitation regime two-line atomic fluorescence (NTLAF) is a laser-based thermometry technique that has application in turbulent flames with soot. However, no assessment of the various interferences from soot or its precursors in flames with high soot loadings on the technique is available. To examine these issues, both on- and off-wavelength NTLAF measurements are presented and compared for laminar nonpremixed ethylene-air flames. Laser-induced incandescence (LII) measurements were used to determine the corresponding soot concentration and location in the investigated flames. The measurements indicate that interferences, such as spurious scattering and laser-induced incandescence from soot, are not significant for the present set of flame conditions. However, interferences from soot precursors, predominantly condensed species (CS) and perhaps polycyclic aromatic hydrocarbons (PAH), can be significant. Potential detection schemes to correct or circumvent these interference issues are also presented.

Planar measurements of soot volume fraction and OH in a JP-8 pool fire

The simultaneous measurement of soot volume fraction by laser induced incandescence (LII) and qualitative imaging of OH by laser induced fluorescence (LIF) was performed in a JP-8 pool fire contained in a 152 mm diameter pan. Line of sight extinction was used to calibrate the LII system in a laminar flame, and to provide an independent method of measuring average soot volume fraction in the turbulent flame. The presence of soot in the turbulent flame was found to be approximately 50% probable, resulting in high levels of optical extinction, which increased slightly through the flame from approximately 30% near the base, to approximately 50% at the tip. This high soot loading pushes both techniques toward their detection limit. Nevertheless, useful accuracy was obtained, with the LII measurement of apparent extinction in the turbulent flame being approximately 21% lower than a direct measurement, consistent with the influence of signal trapping. The axial and radial distributions of soot volume fraction are presented, along with PDFs of volume fraction, and new insight into the behavior of soot sheets in pool fires are sought from the simultaneous measurements of OH and LII.

Simulation of Laser-Induced Incandescence Measurements in an Anisotropically Scattering Aerosol Through Backward Monte Carlo

Laser-induced incandescence (LII) measurements carried out in aerosols having a large particle volume fraction must be corrected to account for extinction between the energized aerosol particles and the detector, called signal trapping. While standard correction techniques have been developed for signal trapping by absorption, the effect of scattering on LII measurements requires further investigation, particularly the case of highly anisotropic scattering and along a path of relatively large optical thickness. This paper examines this phenomenon in an aerosol containing highly aggregated soot particles by simulating LII signals using a backward Monte Carlo analysis; these signals are then used to recover the soot particle temperature and soot volume fraction. The results show that inscattered radiation is a substantial component of the LII signal under high soot loading conditions, which can strongly influence properties derived from these measurements. Correction techniques based on Bouguer’s law are shown to be effective in mitigating the effect of scatter on the LII signals.

Oxygen diffusion modeling in diesel particulate filter regeneration

AbstractThis study introduces a coupled reaction–diffusion model able to account for O2 diffusion in the wall flow honeycomb particulate filter. The diffusion model is incorporated on a previously published single‐channel model for the calculation of flow distribution, heat transfer, and soot oxidation reaction. A computational study is carried out aiming at evaluating the importance of O2 diffusion effects as the function of operating conditions similar to those encountered in the real world. During controlled regeneration conditions, the effect of oxygen diffusion is minor and observable only at high soot loadings. On the other hand, the role of oxygen diffusion is shown to be critical in uncontrolled regeneration conditions, which are important for filter thermal failure. Traditional models neglecting diffusion effects are shown to strongly underpredict the maximum temperatures during uncontrolled regeneration with moderate to high soot loadings. © 2004 American Institute of Chemical Engineers AIChE J, 50: 2008–2019, 2004

Intra-layer temperature gradients during regeneration of diesel particulate filters

Wall-flow filters are worldwide recognized as the most efficient devices for the abatement of particulate emissions from automotive diesel engines. Mathematical models simulating the particulate thermal oxidation process in the filters are already applied for system optimization. This paper deals with the appropriateness of a specific assumption inherently used in all relevant published models, namely the temperature uniformity in the soot and wall layer. A new mathematical model is developed to predict the temperature gradients under various operating conditions. Based on the model results, it is shown that significant temperature gradients inside the soot layer may exist under some practical operating conditions. These conditions are associated with high flow rates and high soot loadings. In these cases, the uniform temperature assumption may lead to erroneous results for the prediction of the overall regeneration process. The error of this assumption is assessed as function of the soot porosity.

Tunable diode laser absorption measurements of carbon monoxide and temperature in a time-varying, methane/air, non-premixed flame

A time-varying, axi-symmetric, methane/air, nonpremixed flame is created by imposing a sinusoidal modulation to the fuel flow rate, which creates flames that “pinch off” and then detach from the burner. Carbon monoxide concentrations and temperatures are measured using tunable diode laser absorption spectroscopy (TDLAS) and are compared with those observed in a nonperturbed flame. The measurements indicate that at most phases of the oscillation, the time-varying flame is both broader and taller than the steady flame. Just before the flame detaches, high levels of CO are observed at the flame tip in regions of the flame that have high soot loading. In the detached flames, relatively high levels of CO remain and the time-averaged emission index for CO is larger in the oscillating flame.

A study on the combustion characteristics of PVC, poly(styrene), poly(ethylene), and poly(propylene) particles under high heating rates

The combustion characteristics of four commonly encountered plastics: poly(styrene), PVC, poly(ethylene) and poly(propylene) were studied under conditions pertinent to incinerators, that is, high heating rates (in the order of 10,000 K/s) and elevated gas temperatures (1200–1500 K). Batches of spherical and monodisperse particles of these plastics were generated in the size range of 53–300 μm. Combustion of single particles, of known size and mass, was conducted in a laminar-flow drop-tube furnace, at controlled atmospheres. The radiation emitted from burning particles was monitored, along their flight path, by simultaneous three-color optical pyrometry and high-speed cinematography. With these techniques the total particle/flame combustion duration, as well as the flame temperature and diameter were measured. Results indicate that polymer particles (plastics) burned expediently with burntimes similar to those of light oil drops such as kerosene, hexadecane, etc. Both PVC and poly(styrene) burned with very luminous yellow flames, which were attributed to high soot loadings. The flame combustion of PVC was the brightest and fastest with steadily decreasing temperature and flame diameter, while that of poly(styrene) occurred mostly at constant flame diameter and mildly decreasing temperature. Combustion of both poly(ethylene) and poly(propylene) was dimmer and somewhat lengthier. Furthermore, in this temperature region, it is argued herein that the combustion of poly(styrene), poly(ethylene), and poly(propylene) occurred concurrently with, and was partially controlled by, pyrolysis reactions. PVC seemed to undergo significant pyrolysis prior to ignition; thereafter, combustion occurred in a premixed-like flame mode and, finally, dimly glowing combustion of the remaining char was observed. An energy balance during the flame combustion period enabled the calculation of the instantaneous burning rate and the average soot loading of the flame. PVC exhibited the highest soot volume fraction in its flame (3 × 10 −5), followed by poly(styrene) (3 × 10 −6), poly(propylene) (2.5 × 10 −6), and poly(ethylene) (1 × 10 −6). Moreover, it was observed that the rate of burning was the highest for PVC particles and the lowest for poly(ethylene).

Effect of Soot Build-Up While Sampling with the Volatile Organic Sampling Train (VOST)

The U.S. Environmental Protection Agency's Method 00301, the Volatile Organic Sampling Train (VOST), is used to determine the destruction and removal efficiencies of volatile organic emissions from industrial boilers co-firing hazardous waste. Previous reports detailing a hysteresis effect2,3,4 for volatile organic compounds (VOCs), resulting from soot build-up on the interior surfaces of boilers and industrial furnaces, raised concerns of possible VOST measurement biases due to soot deposits within the VOST. This possibility required laboratory investigation of the method under sooty conditions. Method collection efficiency was evaluated by comparing volatile organic compound levels collected in a control VOST (using a soot-free particulate filter in the sampling probe), to VOC recoveries while using a soot-laden particulate filter in the probe. Emphasis was directed to substances in the upper range of VOC boiling points (120 to 130° C). Statistical evaluation of the data collected indicated that recoveries for two of the higher boiling VOCs (chlorobenzene and octane) appeared to be negatively influenced by the presence of soot on the VOST filter. It is not likely, however, that these findings will have a major impact on previously collected hazardous waste incinerator data, since the VOST-Soot effect was determined at moderately high soot loadings, which are atypical of properly operating hazardous waste incinerators.