Analysis of the chosen thermal and flammability parameters of dried sewage dust

The hazardous sludge disposal process in the form of landfills requires the determination inter alia of the flammable and explosion properties of dried sewage sludge dust, which has the ability to ignite and spontaneously combust when stored in silos. At a constant furnace surface temperature, the minimum ignition temperature of the sludge dust layer with a layer thickness of 5 mm is 270 °C, and for a layer thickness of 12.5 mm it is 250 °C. Two selected fire extinguishing powders for Class A, B, C and D fires were used in the study to determine the possibility of reducing the susceptibility of dried wastewater to ignition from heated surface, self-ignition and explosion parameters. The most effective extinguishing powder was ABC Favorit, which increased the value of the minimum ignition temperature of the layer (5 mm thick) to 360 °C and the spontaneous ignition temperature of the sludge with this powder increased by 22 °C at 169.6 cm³ in comparison to the sludge without extinguishing powder, respectively. The lowest self-ignition temperature of 136 °C was recorded for the largest tested volume (169.6 cm³) for dried sewage dust without any fire extinguishing powders. The biggest values of p_max and (dp/dt)_max dried sewage dust were recorded 4.8 bar and 113 bar/s respectively. By analysing the obtained test results, it can be assumed that dried sewage dust is a combustible material with properties similar to biomass.     

Minimum ignition temperatures and explosion characteristics of micron-sized aluminium powder

To forestall, control, and mitigate the detrimental effects of aluminium dust, a 20-L near-spherical dust explosion experimental system and an HY16429 type dust-cloud ignition temperature test device were employed to explore the explosion characteristics of micron-sized aluminium powder under different ignition energies, dust particle sizes, and dust cloud concentration (C_dust) values; the minimum ignition temperature (MIT) values of aluminium powder under different dust particle sizes and C_dust were also examined. Flame images at different times were photographed by a high-speed camera. Results revealed that under similar dust-cloud concentrations and with dust particle size increasing from 42.89 to 141.70 μm, the MIT of aluminium powder increased. Under various C_dust values, the MIT of aluminium dust clouds attained peak value when concentrations enhanced. Furthermore, the increase of ignition energy contributed to the increase of the explosion pressure (P_ex) and the rate of explosion pressure rise [(dP/dt)_ex]. When dust particle size was augmented gradually, the P_ex and (dP/dt)_ex attenuated. Decreasing particle size lowered both the most violent explosion concentration and explosive limits.     

Effect of particle size and dust layer size on ignition characteristics of PMMA dust layer on hot surface

Deposition of combustible dust on a hot surface is a hidden danger of fire. In this work, polymethylmethacrylate (PMMA) dust was selected to analyse the influence of dust layer diameter, dust particle size and dust layer thickness on the ignition characteristics of PMMA dust layer. Critical heating temperatures and ignition time had been measured. The STA-GC/MS-FTIR analysis was used to determine that the main products of PMMA pyrolysis were MMA, CO, CO₂, and C₂H₄, of which CO and C₂H₄ were transported to the ambient to cause gas phase combustion on the surface of the dust layer. For 10 mm thick dust layer, the critical heating temperatures of 5 μm PMMA, 100 nm PMMA, and 30 μm PMMA were 300 °C, 330 °C, and 320 °C. As the thickness of the dust layer increased, the gas transport path became longer, the critical heating temperature and ignition time increased. The characteristic particle size (D [3,2]) was utilized to represent the true particle size, and the ignition time increased with the increase of the characteristic particle size. The increase in the diameter of the dust layer had a slight effect on the temperature history and ignition time of the dust layer.     

The effects of phosphorus-free inhibitors on the ignition of lycopodium dust

The minimum ignition temperature of dust suspension (MIT) and the hot surface ignition temperature of the dust layer (LIT) are essential safety parameters for the process industry. However, the knowledge of the ignition behavior when solid mixtures of flammable fuels and phosphorous-free inhibitors are considered is still scarce and further experimental and theoretical analyses are requested. In this work, the ignition temperature of phosphorous-free inhibitors (coal fly ash and calcium carbonate) mixed with lycopodium dust have been studied in terms of LIT analysis (hot plate thickness: 5 mm, 12.5 mm and 15 mm), and by the Godbert-Greenwald test for the MIT. Both coal fly ash and calcium carbonate have been tested at different concentrations and particle sizes.Results show that the effects of the inhibitor can be counter-productive when layer ignition temperature is considered even if the minimum ignition temperature of the dust suspension shows a positive effect from the safety point of view. This behavior has been analyzed in the terms of thermal conductivity and diffusivity of the mixture, by using Maxwell's equation for two-phase solid mixtures. Standard empirical correlations for the ignition temperature of solid mixtures have been also tested, showing their weakness in reproducing mixture behavior.     

超细三七粉最小引燃温度实验研究

为研究三七粉着火燃烧的参数,用粉尘云引燃温度装置和粉尘层引燃温度装置,对三七粉的最小引燃温度(MIT)进行实验研究。分别研究喷吹压力、质量浓度、粉尘层厚度对MIT的影响。结果表明:三七粉尘云的质量在0.2 g时最小引燃温度随着喷尘压力的增加先减小再增大,在0.3 g到0.6 g时最小引燃温度随着喷尘压力的增加而增大;在压力20 kPa、30 kPa时随着质量浓度的增大,粉尘云引燃温度先减小后增大,在40 kPa到60 kPa时,随着质量浓度的增大,粉尘云引燃温度增大;粉尘云最小引燃温度高于粉尘层最小引燃温度;三七粉尘云的最小引燃温度399℃,粉尘层最小引燃温度240℃。     

不同挥发分煤尘层最低着火温度变化规律研究

针对煤化工等行业的沉积煤尘热自燃问题,运用煤尘层最低着火温度测试系统,研究了不同挥发分煤尘层的着火状态、不同挥发分及不同厚度煤尘层最低着火温度的变化规律。结果表明:煤尘层厚度为5 mm时,挥发分质量分数大于35%的煤尘在较低温度便出现着火现象,肉眼很容易观察到火星的出现,温度曲线波动剧烈,而对于挥发分质量分数小于15%的煤尘,通过煤尘层内部"温度达到450℃"来判断其着火;在灰分质量分数相当的情况下,煤尘层最低着火温度随挥发分增加呈严格递减的趋势变化;得到了煤尘层厚度和最低着火温度的函数关系式,通过试验得到了挥发分质量分数为37.45%煤尘的重要常数M和N。     

Testing of dust clouds for the electrostatic-spark ignition hazard in industry. Need for a modified approach?

Current standard test methods for electric-spark minimum ignition energies (MIEs) of dust clouds in air require that a series inductance of at least 1–2 mH be included in the electric-spark discharge circuit. The reason is to prolong the spark discharge duration and thus minimize the spark energy required for ignition. However, when assessing the minimum electrostatic energy ½CU² for dust cloud ignition by accidental electrostatic-spark discharges, current testing standards require that the series inductance of at least 1–2 mH be removed from the spark discharge circuit. No other changes of apparatus and test procedure are required. The present paper questions whether this simple approach is always adequate. The reason is that in practice in industry accidental electrostatic-spark discharge circuits may contain large ohmic resistances due to corrosion, poor electrical grounding connections, poorly electrically conducting construction materials etc. The result is increased spark discharge durations and reduced mechanical disturbance of the dust cloud by the blast wave emitted by the spark. Therefore, testing for minimum ½CU² for ignition by accidental electrostatic spark discharges may not only require removal of the series inductance of 1–2 mH from the standard MIE spark discharge circuit. Additional tests may be needed with one or more quite large series resistances R_s inserted into the spark discharge circuit. The present paper proposes a modified standard test procedure for measurement of the minimum electrostatic-spark ignition energy of dust clouds that accounts for these effects.     

Effect of pyrolysis and oxidation characteristics on lauric acid and stearic acid dust explosion hazards

The effect of pyrolysis and oxidation characteristics on the explosion sensitivity and severity parameters, including the minimum ignition energy MIE, minimum ignition temperature MIT, minimum explosion concentration MEC, maximum explosion pressure P_max, maximum rate of pressure rise (dP/dt)_max and deflagration index K_st, of lauric acid and stearic acid dust clouds was experimentally investigated. A synchronous thermal analyser was used to test the particle thermal characteristics. The functional test apparatuses including the 1.2 L Hartmann-tube apparatus, modified Godbert-Greenwald furnace, and 20 L explosion apparatus were used to test the explosion parameters. The results indicated that the rapid and slow weight loss processes of lauric acid dust followed a one-dimensional diffusion model (D1 model) and a 1.5 order chemical reaction model (F1.5 model), respectively. In addition, the rapid and slow weight loss processes of stearic acid followed a 1.5 order chemical reaction model (F1.5 model) and a three-dimensional diffusion model (D3 model), respectively, and the corresponding average apparent activation energy E and pre-exponential factor A were larger than those of lauric acid. The stearic acid dust explosion had higher values of MIE and MIT, which were mainly dependent on the higher pyrolysis and oxidation temperatures and the larger apparent activation energy E determining the slower rate of chemical bond breakage during pyrolysis and oxidation. In contrast, the lauric acid dust explosion had a higher MEC related to a smaller pre-exponential factor A with a lower amount of released reaction heat and a lower heat release rate during pyrolysis and oxidation. Additionally, due to the competition regime of the higher oxidation reaction heat release and greater consumption of oxygen during explosion, the explosion pressure P_m of the stearic acid dust was larger in low concentration ranges and decayed to an even smaller pressure than with lauric acid when the concentration exceeded 500 g/m³. The rate of explosion pressure rise (dP/dt)_m of the stearic acid dust was always larger in the experimental concentration range. The stearic acid dust explosion possessed a higher P_max, (dP/dt)_max and K_st mainly because of a larger pre-exponential factor A related to more active sites participating in the pyrolysis and oxidation reaction. Consequently, the active chemical reaction occurred more violently, and the temperature and overpressure rose faster, indicating a higher explosion hazard class for stearic acid dust.     

Inhibiting effects by Fe2O3 on combustion and explosion characteristics of ABS resin

The global increase in the use of, and reliance on, plastics has prompted the demand for acrylonitrile-butadiene-styrene (ABS) resin in various fields. With this increased requirement, numerous failures have occurred in the ABS process. Those incidents, resulting from electrostatic discharge, powder accumulation, heat accumulation, construction sparks, and plant fires, have caused dust fire and explosions.In this study, the ABS resin was gleaned from the site and tested for its explosion parameters, including minimum ignition temperature of dust cloud (MIT_C), minimum ignition energy (MIE), and minimum explosion concentration (MEC). To improve loss prevention in the manufacturing process, ferric oxide (Fe₂O₃) as an inert additive was added in the ABS powder. According to the MIE test, Fe₂O₃ has an apparent inhibiting effect on dust explosion for the ABS dust. With the proportion of Fe₂O₃ increased from 25 to 50 mass% in ABS, the MIE increased from 67 to 540 mJ. The explosion tests via 20-L apparatus indicated that Fe₂O₃ mixed with ABS could not increase the MEC significantly. However, the explosion pressure dropped by increasing in the ratio of Fe₂O₃ in ABS. This inerting strategy of ABS was deemed to substantially lessen the probability and severity of fire and explosion.     

Applications of dust explosion hazard and disaster prevention technology

Powdered materials are widely used in industrial processes, chemical processing, and nanoscience. Because most flammable powders and chemicals are not pure substances, their flammability and self-heating characteristics cannot be accurately identified using safety data sheets. Therefore, site staff can easily underestimate the risks they pose. Flammable dust accidents are frequent and force industrial process managers to pay attention to the characteristics of flammable powders and create inherently safer designs.This study verified that although the flammable powders used by petrochemical plants have been tested, some powders have different minimum ignition energies (MIEs) before and after drying, whereas some of the powders are released of flammable gases. These hazard characteristics are usually neglected, leading to the neglect of preventive parameters for fires and explosions, such as dust particle size specified by NFPA-654, MIE, the minimum ignition temperature of the dust cloud, the minimum ignition temperature of the dust layer, and limiting oxygen concentration. Unless these parameters are fully integrated into process hazard analysis and process safety management, the risks cannot be fully identified, and the reliability of process hazard analysis cannot be improved to facilitate the development of appropriate countermeasures. Preventing the underestimation of process risk severity due to the fire and explosion parameters of unknown flammable dusts and overestimation of existing safety measures is crucial for effective accident prevention.     

Prediction of minimum ignition energy of aerosols using flame kernel modeling combined with flame front propagation theory

Flammable aerosols have created many fire and explosion hazards in the process industry, but the flammability of aerosols has not been fully understood. The minimum ignition energy has been widely used as an indicator for flammability of combustible mixtures, but the amount of experimental data on the minimum ignition energy of aerosols is very limited. In this work, the minimum ignition energy of tetralin aerosols is predicted using an integrated model. The model applies the flame front propagation theory in aerosol systems to the growth of the flame kernel, which was created during the spark discharge in the ignition process. The aerosol minimum ignition energy was defined as the minimum level of energy in the initial flame kernel to maintain the kernel temperature above the minimum ignition temperature of 1073 K specific for tetralin aerosols during the kernel growth. The minimum ignition energy obtained in the model is influenced by the fuel-air equivalence ratio and the size of the aerosol droplets. For tetralin aerosols of 40 μm diameter, E_min decreases significantly from 0.32 mJ to 4.3 × 10 e⁻³ mJ when the equivalence ratio rises from 0.57 to 1.0. For tetralin aerosols of 0.57 equivalence ratio, E_min increases from as 0.09 mJ to 0.32 mJ when the droplet diameter rises from 10 μm to 60 μm. The trends are in agreement with previous experimental observations. The method used in current work has the potential to prediction of the minimum ignition energy of aerosol.     

粉尘云浓度对HDPE粉尘云最低着火温度的影响

为准确评价高密度聚乙烯（HDPE）粉尘爆炸敏感性和开展有效的粉尘防爆工作,采用Godbert-Greenwald恒温炉标准实验装置研究了典型HDPE粉尘云最低着火温度的分布特性,着重探讨了粉尘云浓度对不同喷尘压力条件下HDPE粉尘云最低着火温度的影响规律。研究表明:测试条件下HDPE粉尘云最低着火温度的变化处于360~445 ℃范围,随粉尘云浓度的增加呈现先降低后升高的总体趋势,粉尘云浓度为1.111 kg/m3时出现拐点,且粉尘云最低着火温度随喷尘压力的增加而降低。     

Experimental determination of dust cloud deflagration parameters of selected hydrogen storage materials: Complex metal hydrides,chemical hydrides,and adsorbents

This paper discusses the results of an experimental program carried out to determine dust cloud deflagration parameters of selected solid-state hydrogen storage materials, including complex metal hydrides (sodium alanate and lithium borohydride/magnesium hydride mixture), chemical hydrides (alane and ammonia borane) and activated carbon (Maxsorb, AX-21). The measured parameters include maximum deflagration pressure rise, maximum rate of pressure rise, minimum ignition temperature, minimum ignition energy and minimum explosible concentration. The calculated explosion indexes include volume-normalized maximum rate of pressure rise (K_St), explosion severity (ES) and ignition sensitivity (IS). The deflagration parameters of Pittsburgh seam coal dust and Lycopodium spores (reference materials) are also measured. The results show that activated carbon is the safest hydrogen storage media among the examined materials. Ammonia borane is unsafe to use because of the high explosibility of its dust. The core insights of this contribution are useful for quantifying the risks associated with use of these materials for on-board systems in light-duty fuel cell-powered vehicles and for supporting the development of hydrogen safety codes and standards. These insights are also critical for designing adequate safety features such as explosion relief venting and isolation devices and for supplementing missing data in materials safety data sheets.     

Influence of different ignition delay times on the pressure rise rate in hybrid mixture explosions in the 20-L sphere

For the development of a standardized method for measuring the explosion safety characteristics of combustible hybrid dust/vapor mixtures, the influence of the ignition delay time needs to be investigated. The ignition delay time, defined as the time between the injection of dust and the activation of the ignition source, is related to the turbulence of the mixture and thus to the pressure rise rate. The ignition source for pure vapors, however, has to be activated in a quiescent atmosphere according to the standards. Nevertheless, when measuring the explosion safety characteristics of hybrid mixtures, it is important that the dust be in suspension around the igniter. Like pure dust/air mixtures, hybrid dust/vapor/air mixtures need to be ignited in a turbulent atmosphere to keep the dust in suspension.This work will therefore investigate the influence of ignition delay times on the severity of hybrid explosions. It was generally found that at shorter ignition delay times, (dp/dt)_ex increased due to higher turbulence and decreases as the dust sinks to the bottom of the 20 L-sphere. This effect is more pronounced for hybrid mixtures with higher vapor content compared to dust content.     

Effect of superfine KHCO3 and ABC powder on ignition sensitivity of PMMA dust layer

In order to evaluate the flame-retardant capacity of KHCO₃ and ABC on the ignition of PMMA dust layer accumulation on hot surfaces, the ignition time and critical heating temperature of PMMA/KHCO₃ and PMMA/ABC dust layer were experimentally investigated. The thermal stability of the mixed dust, the condensed phase products and gas phase products of the mixed dust combustion were analyzed to reveal the flame-retardant mechanism. The ignition time of 30 μm PMMA was obviously longer than that of 5 μm PMMA, and the critical heating temperature was close to that of 5 μm PMMA. KHCO₃ and ABC could greatly extend the ignition time of the PMMA dust layer and increase the critical heating temperature of the dust layer. ABC was more effective than KHCO₃. The decomposition of KHCO₃ and ABC absorbed the heat and inhibits the pyrolysis of PMMA. The HPO₃ and P₂O₅ generated by the decomposition of ABC would cover the surface of PMMA aggregates or particles and act as a physical barrier. The main light combustible gas produced by PMMA pyrolysis were CO and C₂H₄. The CO₂ generated during the decomposition of KHCO₃ could dilute the combustible gas in the ambient to inhibit the combustion of PMMA.     

A theoretical model for the prediction of the minimum ignition energy of dust clouds

In this study, a physical model of the dust cloud ignition process is developed for both cylindrical coordinates with a straight-line shaped ignition source and spherical coordinates with a point shaped ignition source. Using this model, a numerical algorithm for the calculation of the minimum ignition energy (MIE) is established and validated. This algorithm can evaluate MIEs of dusts and their mixtures with different dust concentrations and particle sizes. Although the average calculated cylindrical MIE (MIE_cylindrical) of the studied dusts only amounts to 63.9% of the average experimental MIE value due to reasons including high idealization of the numerical model and possible energy losses in the experimental tests, the algorithm with cylindrical coordinates correctly predicts the experimental MIE variation trends against particle diameter and dust concentration. There is a power function relationship between the MIE and particle diameter of the type MIE ∝ d_p^k with k being approximately 2 for cylindrical coordinates and 3 for spherical coordinates. Moreover, as dust concentration increases MIE(conc) first drops because of the decreasing average distance between particles and, at fuel-lean concentrations the increasing dust cloud combustion heat; however, after the dust concentration rises beyond a certain value, MIE(conc) starts to increase as a result of the increasingly significant heat sink effect from the particles and, at fuel-rich concentrations the no longer increasing dust cloud combustion heat.     

Inerting effect of CaCO3 powder on flame spread of wood dust layer

The effect of CaCO₃ powder, a typical inert dust, on the flame spread characteristics of wood dust layers was studied using an experimental device to understand the ignition characteristics of and develop inert explosion-proof technology for deposited wood dust. The results showed that the flame spread velocity (FSV) of the mixed dust layer was affected by the dispersion effect of CaCO₃ powder and physical heat absorption. As the CaCO₃ powder mass fraction increased, the FSV of the dust layer first increased and then decreased, reaching a peak at a 50% mass fraction. Moreover, the front-end temperature of the flame gradually decreased, and the red spark faded. The combustion reaction of the mixed dust layer could be more completed, and the colour of the combustion residue changed from charcoal black to charcoal grey. The coupling effect of the initial temperature and wind speed can promote an increase in the FSV in the mixed dust layer. The Gauss–Amp model of the FSV of the wood dust layer and mass fraction of CaCO₃ powder showed that the peak of the FSV occurred when the mass fraction of CaCO₃ powder was between 40 and 50%. Thus, a good inerting and explosion-proof effect can be achieved by using CaCO₃ powder with a mass fraction of more than 50%; it can improve the whole inerting process. Inert explosion-proof technology should be considered when assessing fire and explosion risk of dust in real process industry situations.     

Ignition tests for electrical and mechanical equipment subjected to hot surfaces

This paper reports some experimental work on hot surface ignition temperatures of dust deposits. Dust layers up to 75 mm in depth were ignited using a modified version of the standard 5 mm layer apparatus. The measured ignition temperatures show good agreement with predictions using the method given in EN 50281-2-1. Ignition temperatures of conical dust deposits over an electrically heated box were not predictable, but were not too dissimilar from the ignition temperatures of the thick layers. Both tests gave adequate reproducibility in round robin tests. Rotating steel wheels in contact, immersed in a dust deposit produce the frictional hot surfaces. A relation between the power lost by friction and the surface temperature developed has been derived. The surface temperatures leading to ignition were close to the ignition temperatures for the conical deposits on the heated box. The similarities between the ignition temperatures of dust deposits in several configurations indicate that a simple test for ignition temperature measurement could have wide application in dusty environments.     

A numerical model for the prediction of the minimum ignition temperature of dust clouds

This paper presents a numerical model for the prediction of the minimum ignition temperature (MIT) of dust clouds. First, a physical model is developed for the dust cloud ignition in the Godbert-Greenwald furnace. A numerical approach is then applied for the MIT prediction based on the physical model. The model considers heat transfer between the air and dust particles, the dust particle reaction kinetics, and the residence times of dust clouds in the furnace. In general, for the 13 dusts studied, the calculated MIT data are in agreement with the experimental values. There is also great accordance between the experimental and numerical MIT variation trends against particle size. Two different ignition modes are discovered. The first one consists in ignition near the furnace wall for bigger particles characterized by rather short residence times. In the second mode, the ignition starts from the center of the furnace by self-heating of the dust cloud for smaller particles with longer residence times. For magnesium, as dust concentration increases, the lowest ignition temperature of the dust cloud IT(conc) decreases first, then transits to increase at a certain point. The transition happens at different dust concentrations for different particle sizes. Moreover, the MIT of the magnesium dust cloud generally increases as particle size increases, but the increasing trend stagnates within a certain medium particle size range.