Flame behaviors and pressure characteristics of vented dust explosions at elevated static activation overpressures

Dust explosion venting experiments were performed using a 20-L spherical chamber at elevated static activation overpressures larger than 1 bar. Lycopodium dust samples with mean diameter of 70 μm and electric igniters with 0.5 KJ ignition energy were used in the experiments. Explosion overpressures in the chamber and flame appearances near the vent were recorded simultaneously. The results indicated that the flame appeared as the under-expanded free jet with shock diamonds, when the overpressure in the chamber was larger than the critical pressure during the venting process. The flame appeared as the normal constant-pressure combustion when the pressure venting process finished. Three types of venting processes were concluded in the experiments: no secondary flame and no secondary explosion, secondary flame, secondary explosion. The occurrence of the secondary explosions near the vent was related to the vent diameter and the static activation overpressure. Larger diameters and lower static activation overpressures were beneficial to the occurrence of the secondary explosions. In current experiments, the secondary explosions only occurred at the following combinations of the vent diameter and the static activation overpressure: 40 mm and 1.2 bar, 60 mm and 1.2 bar, 60 mm and 1.8 bar.     

Application of CFD on the sensitivity analyses of some parameters of the modified Hartmann tube

The aim of this work is to determine the influence of operating parameters such as the dispersion pressure, the ignition delay and height on the dust flammability. A Computational Fluid Dynamics (CFD) simulation, based on an Euler–Lagrange approach, was developed with Ansys Fluent™ and validated experimentally. Such analysis will facilitate the choice of the most conservative conditions for a flammability test. This paper is focused on a case study performed on wheat starch with the modified Hartmann tube. The dispersion process of the powder was studied with granulometric analyses performed in situ and high speed videos. Tests were performed with injections at gas pressure ranging from 3 to 6 bars and the evolution of the particle size distribution (PSD) was recorded at different ignition heights (5, 10 and 15 cm over the dispersion nozzle). The observations highlighted the presence of agglomeration/deagglomeration processes and dust segregation. Besides, a CFD simulation analysis was aimed at evaluating the impact of a set of parameters on the PSD and the local turbulence, which are closely linked to some flammability parameters. For this computational analysis, the CFD simulation was coupled with a collision treatment based on a Discrete Element Method (DEM) in order to consider the cohesive behavior of the combustible dust. Thus the results suggest performing the injection of the gases at approximately 5 bars for the flammability tests of wheat starch in order to obtain the finest PSD at a given ignition height. It is also shown that the finest PSD are obtained at 5 cm over the dispersion nozzle. However, the local instabilities and turbulence levels are so high during the first stages of the dispersion that the flame growth can be disturbed for short ignition delays. Moreover, the stabilization of the bulk of the dust cloud requires longer periods of time when the ignition sources are located at 15 cm. As a result, the recommended height to perform a flammability test is 10 cm in this case. Finally, this study proposes some tools that might improve the procedure of dust flammability testing.     

Classification of dispersibility for combustible dust based on Hausner ratio

Mixing of combustible dust and oxidant is one of five essential prerequisites in the dust explosion pentagon, requiring that particles originally in mutual contact within the deposits be separated and suspended in the air. However, dust dispersion never proceeds with 100% efficiency, with inevitable particle agglomeration, and an inherent trend toward settling out of suspension. Dispersibility is defined to describe the ease of dispersion of a dust and the tendency of the particulate matter to remain airborne once a dust cloud has been formed. Pioneers made contributions to classify dust dispersibility by introducing dustiness group (DG), dustability index (DI), NIOSH dispersion chamber and in-situ particle size analysis. Issues remained including the difficulty in comparing results from different methods, as well as the availability of some high-tech testing apparatus.This study aims to provide a quick and universal testing method to estimate the dispersion property of combustible dust. A new dispersibility classification was developed based on dimensionless numbers Hausner ratio and Archimedes number. Four dispersibility classes (DCs) were proposed from one to four, with a larger number meaning better dispersibility. Results for more than a dozen dust samples and mixtures thereof showed the new method is useful in dust explosion research. The consistency in classifying dust dispersion properties between the DC method and previous methods was good. Changes in DC well explained our earlier findings on suppressant enhanced explosion parameter (SEEP) phenomenon attributed to the improvement in dust dispersibility. Hausner ratio and Archimedes number, as easily measured parameters, can be quite advantageous to assess dust dispersibility, permitting a proper risk assessment for the formation of explosible dust clouds.     

Numerical investigation of powder aerosolization in a mining rock dust dispersion chamber

We have conducted numerical simulations of dust dispersion within the NIOSH Rock Dust Dispersion Chamber. The apparatus consists of a low-speed background ventilation flow down a long box in which is placed a tray containing a rock dust powder. A nozzle upstream of the tray introduces a short pulse of a turbulent horizontal jet flow just above the powder surface. We have utilized an incompressible Reynolds-Averaged Navier-Stokes k-ω model for the turbulent flow; particles are incorporated within a one-way Euler-Lagrangian formalism. The Rock Dust Dispersion Chamber ventilation flow exhibits a recirculation zone just above the powder-containing tray. Aerosolization proceeds via the interplay of the jet pulse flow with the background recirculation flow. The air flow is not well-mixed. The aerosolized dust is convected as a concentration cloud downstream towards the detection zone. For larger particles, gravitational settling depletes the convected cloud, so the instrument behaves as a horizontal elutriator. The instrument is robust with respect to misalignment of the jet nozzle. However, reduced streamwise drift velocity allows mixing to disperse the optically detected dust cloud concentration pulse. Our large particle simulation results compare favorably with published experimental results for large, polydisperse calcium carbonate rock dust.     

Study into influence of different types of igniters on the explosion parameters of dispersed nitrocellulose powder

Nitrocellulose is a flammable compound produced by cellulose nitration. The nitrocellulose production and handling are associated with a risk of fire and explosion. Nitrocellulose is used as either collodion cotton (<12.5% N) or as an explosive (>12.5% N). Nitrocellulose is a fibrous or powdered substance and may detonate or burn upon certain conditions. The article compares the combustion parameters of dry nitrocellulose in the KV-150M2-UIBE explosion chamber at the concentrations of 250, 500 and 750 g m⁻³. To ignite a nitrocellulose sample, six different types of igniters were used. A commercially available 5 kJ pyrotechnic igniter was used as the standard. Also used were a nitrocellulose igniter, a pyrotechnic igniter with magnesium powder and KNO₃/KClO₃, and an exploding wire (Kanthal and tungsten wire). The examined igniters were found to affect the explosion parameters of dispersed nitrocellulose. The deviation of the explosion constant K_st reached 50% of the standard value. The highest pressure of 12.73 bar g was reached at a concentration of 750 g m⁻³ and an igniter exploding wire with Kanthal wire. The highest K_st value of 287.9 bar.m.s⁻¹ was achieved at a concentration of 750 g m⁻³, when using the pyrotechnic igniter with KClO₃ and magnesium powder.     

Brominated flame retardants in dust from UK cars--within-vehicle spatial variability, evidence for degradation and exposure implications

Polybrominated diphenyl ethers (PBDEs), hexabromocyclododecanes (HBCDs), and tetrabromobiphenol-A (TBBP-A) were measured in a preliminary study of dust from passenger cabins and trunks of 14 UK cars. Concentrations in cabin dust of HBCDs, TBBP-A, and BDEs 47, 85, 99, 100, 153, 154, 183, 196, 197, 202, 203, 206, 207, 208, and 209 exceeded significantly (p < 0.05) those in trunk dust. Sampling cabin dust thus appears to provide a more accurate indicator of human exposure via car dust ingestion than trunk dust. Elevated cabin concentrations are consistent with greater in-cabin use of BFRs. In five cars, while no significant differences (p > 0.05) in concentrations of HBCDs and most PBDEs were detected in dust sampled from four different seating areas; concentrations of TBBP-A and of PBDEs 154, 206, 207, 208, and 209 were significantly higher (p < 0.05) in dust sampled in the front seats. Possible photodebromination of BDE-209 was indicated by significantly higher (p < 0.05) concentrations of BDE-202 in cabin dust. In-vehicle exposure via dust ingestion to PBDEs, HBCDs and TBBP-A exceeded that via inhalation. Comparison with overall exposure via diet, dust ingestion, and inhalation shows while in-vehicle exposure is a minor contributor to overall exposure to BDE-99, ΣHBCDs, and TBBP-A, it is a significant pathway for BDE-209.     

Implications of high altitude desert dust transport from Western Sahara to Nile Delta during biomass burning season

The air over major cities and rural regions of the Nile Delta is highly polluted during autumn which is the biomass burning season, locally known as black cloud. Previous studies have attributed the increased pollution levels during the black cloud season to the biomass or open burning of agricultural waste, vehicular, industrial emissions, and secondary aerosols. However, new multi-sensor observations (column and vertical profiles) from satellites, dust transport models and associated meteorology present a different picture of the autumn pollution. Here we show, for the first time, the evidence of long range transport of dust at high altitude (2.5-6 km) from Western Sahara and its deposition over the Nile Delta region unlike current Models. The desert dust is found to be a major contributor to the local air quality which was previously considered to be due to pollution from biomass burning enhanced by the dominant northerly winds coming from Europe.     

制定粉尘卫生标准的新趋势——介绍欧洲的经验并提出几点建议

介绍了近年来欧洲国家新的粉尘卫生标准及其监测、管理方法,分析其制定粉尘卫生标准的经验,并与我国的粉尘卫生标准情况做了初步的比较,提出了建立我国新分级分类粉尘卫生标准体系的建议。     

青岛港前湾一期煤系统环保设备运行效果监测与评价

通过对青岛港前湾一期煤系统环保设备除尘效果实地监测，分析论证了煤系统环保防污措施的实效性和可行性，为环保设备管理提供科学依据。     

闽江口秋茄红树林凋落物产量及分解动态

于2017年以闽江口粗芦岛秋茄(Kandelia obovata)红树林为研究对象,分别采用凋落物收集框和分解袋法,研究秋茄凋落物产量及其碳(C)、氮(N)、磷(P)含量月动态及凋落叶分解过程中C、N、P含量与水解酶活性.结果表明:①秋茄凋落物年产量为618.79 g·m~(-2)·a~(-1),其中,叶占61.2%,果、枝和花分别占23.7%、10.5%和4.6%.②凋落叶总氮(TN)含量8月显著高于其他月份(p0.05),而TP含量在1—3月显著高于其他月份;C/N在8月显著低于其他月份(p0.05),而C/P及N/P在9月显著升高(p0.05).③在凋落叶分解过程中,C、N、P含量及其化学计量比随时间存在明显差异(p0.01),并且地上组TC、TN、C/N和C/P明显不同于地下组(p0.05).④在分解过程中,4种水解酶随时间存在明显差异(p0.01).⑤凋落叶酸性磷酸酶活性与土壤温度、电导率和凋落叶TP含量存在显著相关关系(p0.01).这些结果说明,秋茄凋落物产量及元素含量随季节变化存在明显差异;沉积作用对凋落叶分解过程中元素含量有显著影响,水解酶活性主要受凋落叶元素含量和土壤环境因子的控制.