Fractal scaling of coated soot aggregates

Recent field observations have shown soot aggregates (SAs) to contain significant amounts of surface coatings of organic compounds which obfuscate their native fractal morphology and make them visually appear as “near-spherical.” Morphologies of these aggregates are currently parameterized using fractal dimension (D_f) values greater than the universal 1.8. This is done to account for the supposedly morphological restructuring of an aggregate to a more compact form upon condensation of organic materials. Using multiple-angle light scattering analysis, it has been experimentally shown that restructuring of SA morphology only takes place during the evaporation process, not condensation. Based on this seminal finding, here we formulate the correct parameterizations to describe the morphology of surface coated aggregates. We perform detailed three-dimensional morphological characterization of computer simulated coated aggregates that mimic atmospheric SAs and show that their D_f remains invariant at 1.8 with increasing coating mass by as much as 18 fold. We find coating to affect only the fractal prefactor k₀, an understudied parameter which controls the aggregate shape anisotropy and local packing fraction of monomers. Specifically, k₀ was observed to scale with the ratio of aggregate's total (coating + bare) mass M_total to bare mass M_bare as k₀ = 1.34*(M_total/M_bare)^0.56.

Copyright © 2017 American Association for Aerosol Research     

High-throughput approach to the catalytic combustion of diesel soot

A methodology for the evaluation of diesel soot oxidation catalysts by high-throughput (HT) screening was developed. The optimal experimental conditions (soot amount, catalyst/soot ratio, type of contact, composition and flow rate of gas reactants) ensuring a reliable and reproducible detection of light-off temperatures in a 16 parallel channels reactor were set up. The temperature profile measured in the catalyst/soot bed under TPO conditions when the exothermic combustion of soot takes place was shown to provide an accurate measurement of the ignition. Its reproducibility and relevance were checked. The results obtained with a reference noble metal free catalyst (La_0.8Cr_0.8Li_0.2O₃ perovskite) agree very well with literature data. Qualitative mechanistic features could be derived from these experiments, stressing the likely limiting step of oxygen transfer from catalyst surface to soot particulates to ignite the soot combustion. Ceria material was shown to be more appropriate than perovskite one. From an HT screening of a large diverse library (over 100 mixed oxides catalysts) under optimized conditions, about 10 new formulations were found to perform better than selected noble metal free reference materials.     

Measurement and simulation of soot agglomerates and aggregates

Soot clusters produced by partial oxidation of acetylene show three distinct phases - primary particles, agglomerates of the primary particles, and aggregates of the agglomerates. The dominant mode of soot formation can be controlled by the concentration of acetylene. Comparison of optical microscopy with inertial measurements indicate that the aggregates are not fractal (f_HB = 3) but porous with a density of 0.094. Those simulations for the smallest aerodynamic diameter suggest a transition from aggregates which are fractal to agglomerates. Algorithms based on generalized Menger sponges are developed to simulate the transition from agglomerates to aggregates.     

Polycyclic aromatic hydrocarbons (PAHs) in soot produced by combustion of polystyrene,polypropylene, and wood

The smoke and soot produced by the combustion of plastics or wood in a domestic stove or fireplace contain many poisonous compounds, including the polycyclic aromatic hydrocarbons (PAHs), many of which are carcinogenic. PAHs were selected as the subject of our study to gain a better sense of the hazards of burning plastics. Small samples of polystyrene, polypropylene, and wood underwent combustion in a tubular oven at 700°C; in addition, polystyrene and wood were combusted at room temperature. After their extraction and purification, the PAHs were analyzed by gas chromatography and mass spectrometry. Conditions in the hot oven promoted soot production, whereas combustion at room temperature led to somewhat more complete combustion. The PAH profiles of the examined materials resembled each other to some extent, though the original chemical structure of the polymeric materials varied a great deal. However, clear differences between the materials could be detected from the soot extracts, the soot of polystyrene being especially rich in compounds containing remnants of the polymer structure. Carcinogenic activity caused by the PAHs can be assumed to be of the same order of magnitude as soot from the combustion of wood.     

Spouted bed combustion of wood charcoal: performance comparison of three different designs

The combustion efficiency of a conventional spouted bed reactor with a central spout inlet is compared with two alternative reactors having different spout inlet arrangements. For one of the designs, air was introduced into a square duct column through a rectangular slit across the whole cross-section of the column, while in the other the air was introduced into a cylindrical column through a circular slit. Column cross-sectional area and air inlet area for all the reactors were approximately the same. The highest gas–solid contacting and lowest carbon monoxide (CO) emission was achieved in the design with circular slit air inlet while the lowest gas–solid contacting and highest CO emission was observed in the design with rectangular slit air inlet.     

Mechanism of wood combustion: 2. Weight loss analysis

The combustion of moisture-free wood cylinders was studied by measuring weight loss at 750 °C. The rate of weight loss increased for an initial period of ≈ 60s; thereafter it decreased asymptotically in an overall first-order reaction. Particles with protected ends showed a characteristic total burning time that was exponentially proportional to the initial diameter of the particle.     

Mechanism of wood combustion: 1. Flue gas analysis

The mechanism of open system combustion of dry wood, a solid carbonaceous fuel of a high volatile matter content, was studied through a quantitative monitoring of the stack flue gas. Combustion is in three stages. The first stage is an accelerated burning during which the chemical composition of the burned fractions is virtually constant, approximates that of the initial dry wood and is independent of air flow rate. The second stage involves a rapid change in chemical composition of the burned fractions. In the third stage the chemical composition of the burned fractions is again virtually constant and independent of air flow.     

柴油车排气碳微粒催化燃烧研究新进展

主要介绍了国内外柴油车排气碳微粒催化燃烧的研究现状和最新进展,重点分析了贵金属、金属氧化物、金属氧化物碱金属复合型以及过渡金属复合型催化剂在去除柴油车排气中碳微粒中的特点、存在问题以及在实际中的应用,指出未来柴油车排气后处理的发展趋势是同时去除CO、HC、NOx和微粒的四元催化剂。     

Deposition of fractal-like soot aggregates in the human respiratory tract

Deposition of diesel exhaust particles in the human respiratory tract is calculated in terms of the equivalent mobility diameter while accounting for the aggregate's number of primary spherules, N_p, and its mass mobility fractal dimension. The size and shape of the soot particles studied correspond to emissions from diesel engines under different loading conditions. The aggregate's morphology, characterized by the aggregate mass mobility fractal dimension D_fm, is shown to significantly affect its age- and ventilation-specific deposition patterns in the human respiratory tract and hence, the exposure experienced by the receptor. Reporting respiratory tract deposition of diesel soot solely in terms of the aggregate mobility diameter, which lumps together size and shape, precludes a close look at deposition patterns of real particles and does not provide a complete picture for exposure inference.For sedentary adults, soot aggregates tend to deposit in the pulmonary region, with large open aggregates depositing to a greater extent than large compact aggregates. In most cases, and for open structured aggregates in particular, the aggregate deposition exceeds the deposition of their equivalent volume spheres. Whereas adults experience higher total respiratory tract soot deposition, infants are more susceptible to proximal deposition of open aggregates (D_fm<2.2). For an adult performing intense activity both the tracheobronchial and the alveolar deposition fractions are smaller than at rest, whereas extrathoracic deposition increases during intense activity. Alveolar deposition of soot aggregates is evident mainly during rest and is more pronounced for small aggregates (N_p<50).     

Thermal stability of Cu-K-V catalyst for diesel soot combustion

This paper reports a study on the thermal stability of a Cu-K-V catalyst, which showed particular promise for low temperature combustion of diesel particulate. Prolonged treatments were performed at high temperatures (400–1000°C) for periods up to 15 days under different gaseous atmospheres. The effect of such treatments on the catalyst composition was investigated by means of weight-decrease measurements and composition analysis (atomic absorption, X-ray diffraction, etc.), whereas the catalyst activity towards soot combustion was determined via thermogravimetric analysis (TGA) and differential thermal analysis (DTA). The apparent activation energy of the soot combustion process was calculated for a collection of catalyst samples, thermally treated according to several different representative conditions, by the Ozawa method on the basis of the DTA results. Some of the thermal treatments (especially those performed at high temperatures: 900–1000°C) resulted in a reduction of the catalyst activity as shown by the increase of both the activation energy and the soot ignition temperature, as a consequence of the volatilisation of at least some of the active compounds of the catalyst itself (KCl, CuCl₂, etc.). Any periodic thermal regeneration of a catalytically-activated trap for diesel emissions (leading to such high temperatures) performed to eliminate any accumulated soot, has thus to be avoided by designing a trap capable of burning out all the soot produced at the diesel exhaust temperatures (< 400°C).     

Behavior features of soot combustion in diesel particulate filter

Infra-red measuremnts of the combustion of particular matter (PM) deposited on the surface of a single layer diesel particulate filter (DPF) showed that it may proceed in three different modes: either by a moving hot zone emanating from a single ignition point, or hot zones generated at several different ignition points or uniform combustion all over the surface. The velocity of the downwards moving temperature front exceeds that of the upstream front bounding the hot zone. The number of ignition points increases as the PM loading is decreased. The highest temperature rise is obtained by a downward moving hot zone. Avoiding this mode of combustion decreases the probability of excessive hot zone formation during the PM regeneration.     

Comparison of combustion properties of native wood species used for fire pots in Korea

In Korean rural homes, wood is used as a fuel in fire pots for cooking and heating. The process of combustion generates products which may be harmful to occupants. This paper investigates the combustion characteristics of four species of Korean native wood using the cone calorimeter (ISO 5660-1). The peak HRR of the Oak at 50 kW/m² was 375.7 kW/m² in comparison with 287.7, 370.5 and 193.7 kW/m² for the Pitch Pine, Chestnut Tree and Zelkova, respectively. In addition, Oak has low CO_peak yield (0.0581 kg/kg) compared to that of Pitch Pine and Chestnut Tree except for Zelkova and lower CO/CO₂ yield (0.013144) than Chestnut Tree and Zelkova except for Pitch Pine which had for 0.000012. Also, the Oak relatively has the low specific extinction area (SEA_peak, 79.80 m²/kg) except for Chestnut Tree. With respect to the reduction of incidents of gaseous poisoning cause headache and sickness attributed to CO production, Oak showed excellent properties compared with that of Pitch Pine, Chestnut Tree and Zelkova.     

3种支原体检测方法的比较

目的比较PCR法、酶法和DNA荧光染色法3种支原体检测方法的灵敏度。方法支原体阳性的BSR细胞培养上清10倍系列稀释后(10-1~10-8),接种至支原体阴性的Vero细胞上,盲传培养5代,每代每个稀释度的样品分别采用PCR法、酶法和DNA荧光染色法检测支原体,并以支原体阴性的Vero细胞作为阴性对照。结果 10倍系列稀释的阳性样品盲传1代,PCR法能检测到10-4,酶法和DNA荧光染色法能检测到10-3;盲传2代,3种方法均能检测到10-4;盲传3代后,3种方法均能检测到10-5,且检测的支原体滴度不随盲传代次的增加而增加。结论待检样本盲传3代后的支原体用3种方法均可检出,检测灵敏度一致。PCR法与酶法检测支原体准确、快速、简便易行,可作为DNA荧光染色法(支原体检测的金标准)的补充手段。     

三种灰分测定方法比较研究

用单一因素三水平方差分析方法,对三种不同测定方法测定的煤灰分实验数据进行统计分析,比较了三种灰分测定方法存在的差异;研究表明快速灰分测定法A更接近于仲裁方法,而快速灰分测定法B有一定的局限性。     

Comparison of methods of predicting the higher thermal combustion of coal

Formulas for predicting the higher heat of combustion of coal on the basis of technical analysis (the yield of volatiles), elementary analysis, and petrographic analysis are verified. It is found that the formulas are only valid for the samples used in their derivation. Statistical analysis of data for coal from the Donetsk and Lvov-Volynsk basins shows that the higher heat of combustion of such coal is most accurately described by a mathematical formula in which the predictors are petrographic characteristics of the initial coal components, including the reflection coefficient of vitrinite. The mean higher heat of combustion may be determined for vitrinite at different metamorphic stages (for ranks from D to T) and also for liptinite and the sum of fusinized (lean) components in Ukrainian Donetsk and Lvov-Volynsk coal.     

Comparison of methods of flight simulation using a combustion heated facility

This paper gives the results of a comparative analysis of methods of simulating airflow for specified flight conditions at the inlet of a high-speed scramjet engine using test facilities with combustion heating of the working fluid. Conditions of the adequacy of the compared heating methods were determined from the obtained values of the engine thrust.     

The source identification and classification study of soot after combustion

Solid phase microextraction‐gas chromatography‐mass spectroscopy technology was used to extract and analyze three kinds of accelerant soot including diesel soot, gasoline soot, and diesel–gasoline mixture soot. A total of 60 spectrograms have been obtained, analyzed, and compared. It was found that these kinds of soot were quite different with each other in the difference of major target compounds and retention time span, and could be visually identified by the profile of the corresponding spectrogram. A data matrix of 60 * 41 was reached by the characteristic substances corresponding with the retention time in all these 60 spectrograms. With principal component analysis method, two major component variables were obtained to classify the attribution of soot, with perfect classification efficiency of 100%. Hierarchical cluster analysis was further applied for the dendrogram analysis. In spite of the absence of the training set, the classification of 100% accuracy of these kinds of soot could be achieved. Copyright © 2012 John Wiley & Sons, Ltd.     

Impact of necking and overlapping on radiative properties of coated soot aggregates

The impact of necking and overlapping on the radiative properties of coated soot aggregates was investigated numerically by using the Discrete Dipole Approximation (DDA). The present study concerns the situations of slight overlapping between primary particles and small to moderate necking. The effects of overlapping, necking, and coating on the aggregate volume equivalent radius were presented. To show the overlapping effect, the radiative properties of aggregates consisting of N = 200 particles were evaluated with and without coating at refractive indices of m = 1.60 + 0.60i for the soot core and m = 1.46 for the coating material at four different wavelengths in the visible and near-infrared. The radiative properties of coated soot aggregates with three overlapping values of 0.05, 0.1, and 0.2 were calculated. In addition, the relationship between absorption cross-section and wavelength was illustrated at overlapping values of 0.05, 0.1, and 0.2 for uncoated and 25%, 50%, 75%, and 100% coated aggregates. As overlapping and necking increased, the calculated extinction, absorption, and scattering properties also increased in the visible and near-infrared regions. It was found that the volume equivalent radii of coated aggregates increased linearly with coating thickness when the necking values were 0.40–0.50.

Copyright © 2017 American Association for Aerosol Research