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Quasi-static dispersion of dusts for the determination of lower explosion limits of hybrid mixtures
(2022)
Knowledge of explosion limiting concentration of explosible materials is necessary forthe design of explosion protection measures. Currently employed methods of testing MEC of a dust cloud or LEL of a hybrid mixture are based on arbitrary assumptionsand possess technical limitations that often lead to values of MEC/LEL, which are unrealistically low or poorly reproducible. This contribution presents an improved method for experimental determination of MEC of a combustible dust cloud or LEL of a flammable gas or hybrid mixture. The new setup operates under laminar conditions and allows a uniform suspension of dust particles in an open top acrylic glass tube. Dust concentration is measured with the help of infrared sensors installed a few centimeters above and below the ignition source. In order to evaluate the dependence of MEC on flow front velocity, MEC of lycopodium was determined at four flow velocities. The results show that the flow field intensity does not significantly influence the MEC of lycopodium for the flow ranges tested in this work. Moreover, LEL of hybrid mixtures of lycopodium and methane was also tested at flow velocities of 4.7 cm/s, 5.8 cm/s, 7 cm/s and 11 cm/s and compared with the values obtained from other sources. The results suggest that the requirement of high energy pyrotechnical igniter may be relinquished, provided that a truly homogeneous suspension of dust particles could be achieved. Moreover, the effect of relative amount of dust and gas, on the course of ignition and flame propagation in hybrid mixtures at their LEL, was studied by the help of high speed videos. For hybrid mixtures of carbonaceous dusts (like lycopodium) at their LEL, ignition occurs in the gas phase, however, flame propagation is only possible through a two-way interaction of dust and gas during the course of combustion.
The European Committee for Standardization (CEN) set up the Technical Committee 352 (CEN/TC 352) in 2006 to develop and maintain up to date standards in the field of nanotechnologies. Part of the work group (WG 3) dedicated to Health, Safety and Environment, the CEN/TC 352/WG 3/PG 3 led by INERIS was constituted to develop a Technical Specification (TS) for the determination of explosivity and flammability properties of manufactured nano-objects in powder form (2013-2018).
In practice, occurring explosive dust/air mixtures are usually not homogeneous and in many cases do not spread over the whole enclosure. For the safety measure explosion venting, a smaller venting area might be sufficient as calculated according to venting standards (EN 14491 'Dust Explosion Venting Protective Systems'). The tendency of bulk materials to form dust clouds is not taken into account so far. To optimize the design of explosion protection measures, a new safety characteristic the so-called 'dustiness' (see VDI 2263 part 9 'Determination of Dustiness of Bulk Materials'.) could be useful. Dustiness means the tendency of dust to form dust clouds. There are six dustiness groups (DG) from one to six (one means little tendency to stay airborne, six the opposite). The paper describes first results on the reasons for different dustiness and on dust explosion venting for several dusts of different dustiness. A 75 L vertical tube apparatus for vented dust explosion experiments was created. Pressure, pressure rises and flame speeds were measured to evaluate the course of the dust explosions. In addition the paper includes CFD simulations (ANSYS CFX) about the possibilities to model the dust/air mixtures with the Euler/Lagrange approach. In order to simulate the settling of dust clouds more realistic the particle surface area was adjusted and the particle size distribution was taken into account. Results were compared to experimentally determined terminal velocities.
Quasi-static dispersion of dusts for the determination of lower explosion limits of hybrid mixtures
(2020)
Knowledge of explosion limiting concentration of explosible materials is necessary for the design of explosion protection measures. Currently employed methods of testing MEC of a dust cloud or LEL of a hybrid mixture are based on arbitrary assumptions and possess technical limitations that often lead to values of MEC/LEL, which are unrealistically low or poorly reproducible.
This contribution presents an improved method for experimental determination of MEC of a combustible dust cloud or LEL of a flammable gas or hybrid mixture. The new set-up operates under laminar conditions and allows a uniform suspension of dust particles in an open top acrylic glass tube. Dust concentration is measured with the help of infrared sensors installed a few centimeters above and below the ignition source.
In order to evaluate the dependence of MEC on flow front velocity, MEC of lycopodium was determined at four flow velocities. The results show that the flow field intensity does not significantly influence the MEC of lycopodium for the flow ranges tested in this work. Moreover, LEL of hybrid mixtures of lycopodium and methane was also tested at flow velocities of 4.7 cm/s, 5.8 cm/s, 7 cm/s and 11 cm/s and compared with the values obtained from other sources. The results suggest that the requirement of high energy pyrotechnical igniter may be relinquished, provided that a truly homogeneous suspension of dust particles could be achieved.
Moreover, the effect of relative amount of dust and gas, on the course of ignition and flame propagation in hybrid mixtures at their LEL, was studied by the help of high speed videos. For hybrid mixtures of carbonaceous dusts (like lycopodium) at their LEL, ignition occurs in the gas phase, however, flame propagation is only possible through a two-way interaction of dust and gas during the course of combustion.
The production of materials with dimensions in the nanometre range has continued to increase in recent years. In order to ensure safety when handling these products, the hazard potential of such innovative materials must be known. While several studies have already investigated the effects of explosions (such as maximum explosion pressure and maximum pressure rise) of powders with primary particles in the nanometre range, little is known about the ignition temperatures and flame velocities. Therefore, the minimum ignition temperature (MIT) of metallic nano powders (aluminium, iron, copper and zinc) was determined experimentally in a so called Godbert-Greenwald (GG) oven. Furthermore, the flame velocities were determined in a vertical tube. In order to better classify the test results, the tested samples were characterised in detail and the lower explosion limits of the tested dust samples were determined. Values for the burning velocity of aluminium nano powders are higher compared to values of micrometre powd
ers (from literature). While MIT of nanometre aluminium powders is within the range of micrometre samples, MIT of zinc and copper nano powders is lower than values reported in literature for respective micrometre samples.
The production of materials with dimensions in the nanometre range has continued to increase in recent years. In order to ensure safety when handling these products, the hazard potential of such innovative materials must be known. While several studies have already investigated the effects of explosions (such as maximum explosion pressure and maximum pressure rise) of powders with primary particles in the nanometre range, little is known about the ignition temperatures and flame velocities. Therefore, the minimum ignition temperature (MIT) of metallic nano powders (aluminium, iron, copper and zinc) was determined experimentally in a so called Godbert-Greenwald (GG) oven. Furthermore, the flame velocities were determined in a vertical tube. In order to better classify the test results, the tested samples were characterised in detail and the lower explosion limits of the tested dust samples were determined. Values for the burning velocity of aluminium nano powders are higher compared to values of micrometre powders (from literature). While MIT of nanometre aluminium powders is within the range of micrometre samples, MIT of zinc and copper nano powders is lower than values reported in literature for respective micrometre samples.
The maximum explosion overpressure and the maximum rate of pressure rise, which characterize thedust explosion severity, are commonly measured in apparatuses and under specific conditions defined byinternational standards. However, those standards conditions, designed for micropowders, may not befully adapted to nanoparticles. Investigations were conducted on different nanopowders (nanocellulose,carbon black, aluminum) to illustrate their specific behaviors and highlight the potential inadequacyof the standards. The influence of the sample preparation was explored. Various testing procedureswere compared, focusing on the dust cloud turbulence and homogeneity. Dust dispersion experimentsevidenced the importance of the characterization of the dust cloud after dispersion, due to the frag-mentation of agglomerates, using metrics relevant with nanoparticles reactivity (e.g. surface diameterinstead of volume diameter). Moreover, the overdriving phenomenon (when the experimental resultsbecome dependent of the ignition energy), already identified for micropowders, can be exacerbated fornanoparticles due to their low minimum ignition energy and to the high energy used under standardconditions. It was evidenced that for highly sensitive nanopowders, pre-ignition phenomenon can occur.Finally, during severe explosions and due to a too long opening delay of the ‘fast acting valve’, the flamecan go back to the dust container.
Experiments were performed on the influence of pre-ignition turbulence on the course of vented gas and dust explosions. A vertical cylindrical explosion chamber of approximately 100 l volume and a length-to-diameter ratio (l/d) of 4.7 consisting of a steel bottom segment and three glass sections connected by steel flanges was used to perform the experiments. Sixteen small fans evenly distributed within the chamber produced turbulent fluctuations from 0 to 0.45 m/s. A Laser-Doppler-anemometer (LDA) was used to measure the flow and turbulence fields. During the experiments the pressure and in the case of dust explosions the dust concentration were measured. In addition, the flame propagation was observed by a high-speed video camera. A propane/nitrogen/oxygen mixture was used for the gas explosion experiments, while the dust explosions were produced by a cornstarch/air mixture.
It turned out that the reduced explosion pressure increased with increasing turbulence intensity. This effect was most pronounced for small vents with low activation pressures, e.g. for bursting disks made from polyethylene foil. In this case, the overpressure at an initial turbulence of 0.45 m/s was twice that for zero initial turbulence.
Minimum explosion concentrations of coal dusts with CH4/H2/CO below the gas lower explosion limit
(2020)
A 20-L spherical explosion chamber was used to investigate the explosion characteristics of dust clouds in air with the presence of flammable gas which is lower than its lower explosion limit (LEL).
including minimum explosion concentration (MEC) of dust. Explosion pressure (Pex) and explosion pressure rise (dp/dt)ex). Two dust samples (anthracite coal, bituminous coal) and were three flammable gases (CH4, H2 and CO) were tested. Experimental results showed that the explosion of hybrid mixtures occurs when both dust and gas concentrations are lower than the LEL/MEC of the single substances.
Meanwhile. all flammable gases with different volume fractions have distinct effects on the MEC. Pex and (dp/dt)ex of the dust samples.
With the increase of the flammable gas concentration. either the Pex and (dp/dt)ex increase or the MEC decreases for all the hybrid mixtures of both two dust samples. At the same concentration of coal dusts. the addition of CH4 promotes higher explosion risks than the other two flammable gases. The distribution of (dp/dt)ex is quite different with the restricted area defined by empirical formulas.
These results improve our understanding of the explosion behaviour and the explosion risk of hybrid dust-gas mixtures in air.
Nanotechnology has been identified as a Key Enabling Technology for the European Union (Horizon 2020) and already has a wide field of applications. However, hazardous properties of nanomaterials such as their flammability and explosivity, have to be determined to ensure their sustainable development. This poster gives an overview of the current status in nanodust explosion research, standardization and european collaboration as well as highlighting current and future perspectives.
To reveal and improve our understanding of the ignition behavior and mechanism, G-G furnace experiments of three typical organic dusts were performed to investigate the minimum ignition temperature (MIT) in O2/N2, O2/ Ar and O2/CO2 atmospheres with oxygen mole fraction from 8.4% to 50%. The experimental results were presented in oxygen-lean and oxy-fuel atmospheres to evaluate the ignition sensitivity of dusts in different atmospheres. It was found that CO2 is the strongest in terms of lowing the ignition sensitivity of the three dusts, and the dust explosion risk increases significantly with increasing O2 mole fraction for the three dusts through a logarithmically and significantly reducing MIT. However, for different dusts, inert gases show different suppression effects. In addition, a modified steady-state homogeneous ignition model was proposed and successfully applied to oxygen-lean atmospheres, and in oxy-fuel atmospheres, this model has also been improved to estimate the ignition mechanism. This ignition mechanism model could be used to successfully predict the minimum ignition temperature of high volatile dust under different inert atmospheres controlled by homogeneous ignition, which will provide a reference for the ignition hazard assessment of dust on hot surfaces.
"Ullmann's Encyclopedia of Industrial Chemistry" is the benchmark reference in chemistry and chemical and life science engineering, covering inorganic and organic chemicals, advanced materials, pharmaceuticals, polymers and plastics, metals and alloys, biotechnology and biotechnological products, food chemistry, process engineering and unit operations, analytical methods, environmental protection, and much more.
The present article is part of the topic collection "Environmental Protection and Industrial Safety". It covers plant and process safety, focusing on hazardous materials and process conditions.
Godbert-Greenwald furnace was used to investigate the minimum ignition temperature of dust clouds (MITC) in air with the presence of flammable gas which is lower than its lower explosion limit (LEL). Three flammable gases (CH4, H2 and CO) and three carbonaceous dusts (anthracite coal, bituminous coal and sweet potato starch) were tested. Results showed that all flammable gases have distinct effects on the MITC of the dust samples and volatile matter content of dust plays an important role during the ignition process. Specifically, the MITC of anthracite coal dust decreased from 610 °C to 560 °C, 580 °C and 570 °C with 3% CH4, 3% CO and 2.5% H2, respectively. Moreover, a heterogeneous ignition mechanism model was proposed to verify the equally global ignition characteristic between hybrid anthracite coal-CxHy mixture and bituminous coal. All three gases had an ignorable effect on the MITC of starch dust considering the experimental error. The presence of CO and H2 slightly promoted the ignition of bituminous coal dust, but the addition of CH4 showed a distinct concentration effect on the MITC of bituminous coal: the MITC decreased with 1% CH4 while increased with 2% and 3% CH4.
This negative-effect of flammable gases at such low concentrations on ignition temperature of bituminous coal dusts was found for the first time. Furthermore, the presence of the 2nd flammable gas had a smaller effect on the MITC of dust samples with a higher volatile content, resulted from the competition of heterogeneous and homogeneous ignition mechanisms.
Minimum ignition temperature of dust clouds (MITC) was studied experimentally and theoretically in different atmospheres. Three carbonaceous dusts were tested in both air and O2/CO2 atmospheres with CH4 mole fraction from 0 to 2%. Results showed that the ignition risk of the three dusts significantly increases (decrease of MITC by ~100 ℃) with increasing XO2 from 21% to 50%, but significantly decreases replacing N2 in air with CO2. The inhibition effect of CO2 on MITCs could be diminished by increasing XO2 or adding CH4. The addition of small amount of CH4 has different effects on the MITCs of different dust samples, following the opposite order of volatile matter content: anthracite>bituminous coal>starch. Two modified steady-state ignition models, considering the density of mixture gas and dust cloud, XO2 and its diffusivity, were developed to interpret the experimental observations. The analysis revealed that the global heterogeneous ignition model suits well for the hybrid mixtures of anthracite or bituminous coal dusts. In contrast, the proposed global homogeneous ignition model was found to be only valid for the pure starch dust, and the extra CH4 addition could strongly affect the ignition process of starch, particularly in O2/CO2 atmospheres with higher XO2.