Explosibility of cork dust in methane/air mixtures

Explosibility studies of hybrid methane/air/cork dust mixtures were carried out in a near-spherical 22.7 L explosibility test chamber, using 2500 J pyrotechnic ignitors. The suspension dust burned as methane/air/dust clouds and the uniformity of the cork dust dispersion inside the chamber was evaluated through optical dust probes and during the explosion the pressure and the temperature evolution inside the reactor were measured. Tested dust particles had mass median diameter of 71.3 μm and the covered dust cloud concentration was up to 550 g/m³. Measured explosions parameters included minimum explosion concentration, maximum explosion pressures and maximum rate of pressure rise. The cork dust explosion behavior in hybrid methane/air mixtures was studied for atmospheres with 1.98 and 3.5% (v/v) of methane. The effect of methane content on the explosions characteristic parameters was evaluated. The conclusion is that the risk and explosion danger rises with the increase of methane concentration characterized by the reduction of the minimum dust explosion concentration, as methane content increases in the atmosphere. The maximum explosion pressure is not very much sensitive to the methane content and only for the system with 3.5% (v/v) of methane it was observed an increase of maximum rate of pressure rise, when compared with the value obtained for the air/dust system.     

Experimental analysis of gas explosions at non-atmospheric initial conditions in cylindrical vessel

Accidental gas explosions in industrial equipment are seldom initiated at atmospheric conditions. Furthermore, fuel–air mixtures are generally turbulent due to rotating parts or flows. Despite these considerations, few studies have been devoted to the analysis of explosion properties at conditions of temperature and pressure different from ambient and in the presence of turbulence; therefore, experiments are still needed, even at lab-scale, e.g. for the design of mitigation system as venting devices.In this work, experimental explosion tests have been performed in 5 l, cylindrical tank reactor with stoichiometric methane–air mixtures at initial pressure and temperature up to 600 kPa and 400 K, centrally ignited or top ignited, and with the effect of initial turbulence level by varying the velocity of the mechanical stirrer.     

Vented confined explosions in Stramberk experimental mine and AutoReaGas simulation

Research Mining Institute, Inc., Ostrava-Radvanice, in cooperation with Dept. of Theory And Technology of Explosives of University of Pardubice and Klokner Institute of CTU in Prague, has performed three series of experiments examining methane–air mixture explosions and their impact on 14 and 29 cm thick wall. The project was named ‘Modeling Pressure Fields Effects on Engineering Structures During Accidental Explosions of Gases in Buildings’ and was sponsored by Grant Agency of Czech Republic (project No. 103/01/0039). The project is aimed at deeper understanding of pressure field effect upon the structures. Methane-air mixture explosion was used to generate the blast wave. The geometrical configuration of the environment resembled a room of an average size, such as larger kitchen. Preliminary simulations were made by AutoReaGas code (Century Dynamics and TNO). The design phase was followed by tests in an experimental mine in Stramberk. Two masonry dams were build in the mine, with cross-section areas of 10.2 m² and longitudinal distance of 5.7 m, creating an explosion chamber with a volume of 58 m³. Two vent openings with an adjustable free cross-section were used to control the maximum overpressure inside the chamber. The concentration of methane-air mixture was approximately 9.5% (vol.) and the volumes of the clouds were 5.25, 10.2 and 15.3 m³ respectively. The generated blast wave overpressures inside the chamber ranged between 1 and 150 kPa. According to experimental results a calibration of the code was performed. After the calibration it is possible to make relatively accurate simulations in similar geometry and to calculate the pressure loading of the structure at any spot in the simulated space. This paper describes the experiments performed and compares experimental and computational results.     

Ignitability assessment of shredder dusts of refrigerator and the prevention of the dust explosion

Explosibility of polyurethane dusts produced in the recycling process of refrigerator and the ways to prevent the dust explosion were studied. In recent years, cyclopentane is often used as the foaming agent and this produces explosive atmosphere in the shredding process. The minimum explosive concentration of polyurethane dust, influence of coexisting cyclopentane gas on the explosibility, effect of relative humidity on the minimum explosive concentration of polyurethane dusts, the minimum ignition energy, influence of cyclopentane mixture on the explosion severity, etc. were investigated.The minimum explosive dust concentration decreased with the increase of cyclopentane concentration and increased with the increase of relative humidity. The minimum ignition energy was about 11 mJ. The ignition energy decreased with the increase of the cyclopentane gas concentration. The cyclopentane gas concentration up to about 5300 ppm did not influence too much on the explosion index (K_st) and maximum explosion pressure. From these, it would be a good way to increase the relative humidity and to regulate the cyclopentane concentration in the shredding process to prevent the dust explosion hazard.     

Determination of explosion parameters of methane-air mixtures in the chamber of 40 dm3 at normal and elevated temperature

The experimental results of the measurements of the explosion pressure and rate of explosion pressure rise as a function of molar methane concentration in the mixture with air in the 40 dm³ explosion chamber are presented. The research was aimed at determination of the explosion limits, according to the EU Standard. The influence of initial temperature of the mixture (changing in the range of 293–473 K) on the fundamental explosion parameters was also investigated. The ignition source was an induction electrical spark of the power equal to approximately 10 W. It was stated, that the increase of initial temperature of the methane-air mixture causes a significant increase of the explosion range.     

Investigations of flame front propagation between interconnected process vessels. Development of a new flame front propagation time prediction model

For the case where a dust or gas explosion can occur in a connected process vessel, it would be useful, for the purpose of designing protection measures and also for assessing the existing protection measures such as the correct placement, to have a tool to estimate the time for flame front propagation along the connecting pipe. Measurements of data from large-scale explosion tests in industrially relevant process vessels are reported. To determine the flame front propagation time, either a 1 m³ or a 4.25 m³ primary process vessel was connected via a pipe to a mechanically or pneumatically fed 9.4 m³ secondary silo. The explosion propagation started after ignition of a maize starch/air mixture in the primary vessel. No additional dust was present along the connecting pipe. Systematic investigations of the explosion data have shown a relationship between the flame front propagating time and the reduced explosion over-pressure of the primary explosion vessel for both vessel volumes. Furthermore, it was possible to validate this theory by using explosion data from previous investigations. Using the data, a flame front propagation time prediction model was developed which is applicable for:

• •gas and dust explosions up to a K value of 100 and 200 bar m s⁻¹, respectively, and a maximum reduced explosion over-pressure of up to 7 bar;
• •explosion vessel volumes of 0.5, 1, 4.25 and 9.4 m³, independent of whether they are closed or vented;
• •connecting pipes of pneumatic systems with diameters of 100–200 mm and an air velocity up to 30 m s⁻¹;
• •open ended pipes and pipes of interconnected vessels with a diameter equal to or greater than 100 mm;
• •lengths of connecting pipe of at least 2.5–7 m.

     

Propagation of ethylene–air flames in closed cylindrical vessels with asymmetrical ignition

A study of explosions in several elongated cylindrical vessels with length to diameter L/D = 2.4–20.7 and ignition at vessel's bottom is reported. Ethylene–air mixtures with variable concentration between 3.0 and 10.0 vol% and pressures between 0.30 and 1.80 bara were experimentally investigated at ambient initial temperature. For the whole range of ethylene concentration, several characteristic stages of flame propagation were observed. The height and rate of pressure rise in these stages were found to depend on ethylene concentration, on volume and asymmetry ratio L/D of each vessel. High rates of pressure rise were found in the early stage; in later stages lower rates of pressure rise were observed due to the increase of heat losses. The peak explosion pressures and the maximum rates of pressure rise differ strongly from those measured in centrally ignited explosions, in all examined vessels. In elongated vessels, smooth p(t) records have been obtained for the explosions of lean C₂H₄–air mixtures. In stoichiometric and rich mixtures, pressure oscillations appear even at initial pressures below ambient, resulting in significant overpressures as compared to compact vessels. In the stoichiometric mixture, the frequency of the oscillations was close to the fundamental characteristic frequency of the tube.     

Influence of the ignition source location on the determination of the explosion pressure at elevated initial pressures

Explosion pressures are determined for rich methane–air mixtures at initial pressures up to 30 bar and at ambient temperature. The experiments are performed in a closed spherical vessel with an internal diameter of 20 cm. Four different igniter positions were used along the vertical axis of the spherical vessel, namely at 1, 6, 11 and 18 cm from the bottom of the vessel. At high initial pressures and central ignition a sharp decrease in explosion pressures is found upon enriching the mixture, leading to a concentration range with seemingly low explosion pressures. It is found that lowering the ignition source substantially increases the explosion pressure for mixtures inside this concentration range, thereby implying that central ignition is unsuitable to determine the explosion pressure for mixtures approaching the flammability limits.     

The explosion overpressure field and flame propagation of methane/air and methane/coal dust/air mixtures

The explosion of the methane/air mixture and the methane/coal dust/air mixture under 40 J center spark ignition condition was experimentally studied in a large-scale system of 10 m³ vessel. Five pressure sensors were arranged in space with different distances from the ignition point. A high-speed camera system was used to record the growth of the flame. The maximum overpressure of the methane/air mixture appeared at 0.75 m away from the ignition point; the thickness of the flame was about 10 mm and the propagation speed of the flame fluctuated around 2.5 m/s with the methane concentration of 9.5%. The maximum overpressure of the methane/coal dust/air mixture appeared at 0.5 m. The flame had a structure of three concentric zones from outside were the red zone, the yellow illuminating zone and the bright white illuminating zone respectively; the thickness and the propagation speed of the flame increased gradually, the thickness of red zone and yellow illuminating zone reached 3.5 cm and 1 cm, the speed reached 9.2 m/s at 28 ms.     

Large-scale dust explosions in vessel-pipe systems

This study investigates dust explosions in vessel-pipe systems to develop a better understanding of dust flame propagation between interconnected vessels and implications for the proper application of explosion isolation systems. Cornstarch dust explosions were conducted in a large-scale setup consisting of a vented 8-m³ vessel and an attached pipe with a diameter of 0.4 m and a length of 9.8 m. The ignition location and effective dust reactivity were varied between experiments. The experimental results are compared against previous experiments with initially quiescent propane-air mixtures, demonstrating a significantly higher reactivity of the dust explosions due to elevated initial turbulence, leading to higher peak pressures and faster flame propagation. In addition, a physics-based model developed previously to predict gas explosion dynamics in vessel-pipe systems was extended for dust combustion. The model successfully predicts the pressure transients and flame progress recorded in the experiments and captures the effects of ignition location and effective dust reactivity.     

Explosibility of fine graphite and tungsten dusts and their mixtures

To evaluate the explosion hazard of ITER-relevant dusts, a standard method of 20-l-sphere was used to measure the explosion indices of fine graphite and tungsten dusts and their mixtures. The effect of dust particle size was studied on the maximum overpressures, maximum rates of pressure rise, and lower explosive concentrations of graphite dusts in the range 4 μm to 45 μm. The explosion indices of 1 μm tungsten dust and its mixtures with 4 μm graphite dust were measured. The explosibility of these dusts and mixtures were evaluated. The dusts tested were ranked as St1 class. Dust particle size was shown to be very important for explosion properties. The finest graphite dust appeared to have the lowest minimum explosion concentration and be able to explode with 2 kJ ignition energy.     

Experimental study on vented gas explosion in a cylindrical vessel with a vent duct

A study of vented explosions in a length over diameter (L/D) of 2 in cylindrical vessel connecting with a vent duct (L/D = 7) is reported. The influence of vent burst pressure and ignition locations on the maximum overpressure and flame speeds at constant vent coefficient, K of 16.4 were investigated to elucidate how these parameters affect the severity of a vented explosion. Propane and methane/air mixtures were studied with equivalence ratio, Φ ranges from 0.8 to 1.6. It is demonstrated that end ignition exhibited higher maximum overpressures and flame speeds in comparison to central ignition, contrary to what is reported in literature. There was a large acceleration of the flame toward the duct due to the development of cellular flames and end ignition demonstrated to have higher flame speeds prior to entry into the vent due to the larger flame distance. The higher vent flow velocities and subsequent flame speeds were responsible for the higher overpressures obtained. Rich mixtures for propane/air mixtures at Φ = 1.35 had the greatest flame acceleration and the highest overpressures. In addition, the results showed that Bartknecht's gas explosion venting correlation is grossly overestimated the overpressure for K = 16.4 and thus, misleading the impact of the vent burst pressure.     

Effects of obstacles and deposited coal dust on characteristics of premixed methane–air explosions in a long closed pipe

An experimental system including pressure transducer, electric spark ignition device, data acquisition and control unit was set up to investigate methane–air explosions in a horizontal pipe closed at both ends with or without the presence of obstacles and deposited coal dust. The experimental results show that explosion characteristics depended on the methane content, on the layout of obstacles, and on the deposited coal dust. Pressure fluctuation with a frequency of 150 Hz appeared in its crest when the methane content was close to the stoichiometric ratio (9.5% methane percentage by volume). The pressure rise rate increased locally when a single obstacle was mounted in the pipe, but it had little effect on the pressure peak. Repeated obstacles mounted in the pipe caused the pressure to rise sharply, and the mean maximum explosion overpressure increased with the increase of the obstacle’s number. The amplitude of pressure fluctuation was reduced when deposited coal dust was paved in the bottom of the pipe. However, when repeated obstacles were arranged inside, the maximum overpressures were higher with coal dust deposited than pure gas explosions.     

Flammability of hydrocarbon and carbon dioxide mixtures

The effect of carbon dioxide (CO₂) concentration on the ignition behaviour of hydrocarbon and CO₂ gas mixtures is examined in both jets and confined explosions. Results from explosion tests are presented using a 20 l explosion sphere and an 8 m long section of 1.04 m diameter pipeline. Experiments to assess the flame stability and ignition probability in free-jets are reported for a range of different release velocities. An empirically-based flammability factor model for free-jets is also presented and results are compared to ignition probability measurements previously reported in the literature and those resulting from the present tests.The results help to understand how CO₂ changes the severity of fires and explosions resulting from hydrocarbon releases. They also demonstrate that it is possible to ignite gas mixtures when the mean concentration is outside the flammable range. This information may be useful for risk assessments of offshore platforms involved in carbon sequestration or enhanced oil recovery, or in assessing the hazards posed by poorly-inerted hydrocarbon processing plant.     

Experimental simulation of airbag deployment for pipeline closing

Small scale tests were carried out at ISL's shock tube facility STA (100 mm inner diameter) to study the problem of closing a pipeline by means of an airbag in case of explosions or gas leakages. Experiments were carried out to simulate the flow in a pipeline at velocities and gas pressures as present in pipeline flows. In this study the gas used was nitrogen at static pressures of 0.2 up to 5 MPa and at flow velocities of 25 m/s up to 170 m/s. A special Nylon airbag, deployed from the tube wall into the pipe, was used to simulate the airbag inflation in a real pipeline. For this purpose a special gas filling system consisting of a gas generator with a reservoir volume of up to 500 cm³ which permits air pressures up to 17 MPa to be generated inside the airbag was developed at ISL. With a fast pyrotechnically opened valve the reservoir gas was released for airbag filling. The airbag inflation was triggered in such a way that it opened in nearly 3 ms into the pipe flow generated by the shock tube and continued for about 10 ms. For this application a special measuring chamber was designed and constructed with 20 measuring ports. Through two window ports, located one in front of the other, the airbag inflation could be visualized with up to 50 successive flash sparks illuminating a fast rotating film inside a drum camera. Pressure measurements using commercially available PCB pressure gauges at 9 measuring ports placed along the inner tube surface gave some hints on the behaviour of the wall pressure during airbag deployment. As a result from the experiments performed it is to conclude, that, with the Nylon airbag samples available, the pipe flow cannot be blocked by the inflating airbag. The flow forces acting on the airbag during deployment are in the shock tube experiments of the order of about 1000 N, which are not balanced by the airbags' neck, fixing it to the shock tube wall. This outcome suggests that a mechanical support is required to fix the airbag in its place during inflation.     

The investigation of correlated factors of external explosion during the venting process

Experimental investigations were done in the paper for the process of venting explosion in a ϕ200 mm×400 mm cylindrical vessel. Compared with the normal venting process, the phenomenon of external explosion was observed and discussed first. Moreover, when CH₄–air mixture gases were used and the vent diameter was 55 mm, three kinds of condition were selected: ϕ=0.8, ϕ=1.0 and ϕ=1.3. And two ignition positions were selected: at the vessel center and at the bottom. Then the venting processes influenced by these factors were experimented and discussed, too.     

Methane/coal dust/air explosions and their suppression by solid particle suppressing agents in a large-scale experimental tube

Methane/coal dust/air explosions under strong ignition conditions have been studied in a 199 mm inner diameter and 30.8 m long horizontal tube. A fuel gas/air manifold assembly was used to introduce methane and air into the experimental tube, and an array of 44 equally spaced dust dispersion units was used to disperse coal dust particles into the tube. The methane/coal dust/air mixture was ignited by a 7 m long epoxypropane mist cloud explosion. A deflagration-to-detonation transition (DDT) was observed, and a self-sustained detonation wave characterized by the existence of a transverse wave was propagated in the methane/coal dust/air mixtures.The suppressing effects on methane/coal dust/air mixture explosions of three solid particle suppressing agents have been studied. Coal dust and the suppressing agent were injected into the experimental tube by the dust dispersion units. The length of the suppression was 14 m. The suppression agents examined in this study comprised ABC powder, SiO₂ powder, and rock dust powder (CaCO₃). Methane/coal dust/air explosions can be efficiently suppressed by the suppression agents characterized by the rapid decrease in overpressure and propagating velocity of the explosion waves.     

Investigation on removal of p-nitrophenol using a hybridized photo-thermal activated persulfate process: Central composite design modeling

The aim of this work is the study of p-nitrophenol (PNP) removal, as a nitroaromatic compound, using a hybridized photo-thermally activated potassium persulfate (KPS) in a fully recycled batch reactor. Response surface method was used for modeling the process. Reaction temperature, KPS initial dosage and initial pH of the solution were selected as variables, besides PNP degradation efficiency was selected as the response. ANOVA analysis reveals that a second order polynomial model with F-value of 41.7, p-value of 0.0001 and regression coefficient of 0.95 is able to predict the response. Based on the model, the process optimum conditions were introduced as initial pH of 4.5, [KPS]₀ = 1452 mg/L and T = 66 °C. Also experiments showed that using thermolysis and photolysis of the persulfate simultaneously, the role of thermolysis is not considerable. A pseudo first order kinetic model was established to describe the degradation reaction. Operational cost, as a vital industrial criterion, was estimated so that the condition of initial pH of 4.5, [KPS]₀ = 1452 mg/L and T = 25 °C showed the highest cost effective case. Under the preferred mild condition, the process will reach to 84% and 89% of degradation and mineralization efficiencies, after 60 and 120 min, respectively.     

Fire and Explosion Characteristics of 3-Methyl Pyridine at 270°C with High Oxygen Concentrations

The mixture of 3-methyl pyridine (3-picoline) and steam is used in the production of vitamin B3 in the gas phase. The aim of this study was to investigate the influence of inert steam (H₂O) on the flammability characteristics of 3-picoline in the manufacturing process. Four practical vapour mixing ratios of 3-picoline/steam mixtures, 5, 10, 30 and 100 vol% 3-picoline, were selected in this study. A series of flammability tests were employed for determining their fire and explosion characteristics. Fire tests H₂O: 3-picoline 5,10, 30 and 100 vol% were carried out in a 20-L-Apparatus under simulated conditions of 760 mmHg, 270°C, together with high oxygen concentrations (42 and 21 vol%) used in the real process.The experimental results showed that the safety-related parameters and flammability hazard degrees were all able to be significantly reduced while substantial amount of steam was infused into the 3-picoline/steam system. While the steam proportion was up to 97 vol%, 3-picoline/steam would be non-flammable. As a result, dosing steam to the process is one of the effective methods to prevent the relevant processes from incurring fire and explosion hazards, not to mention its economical benefit.     

Effect of spark duration on explosion parameters of methane/air mixtures in closed vessels

The paper outlines an experimental study on influence of the spark duration and the vessel volume on explosion parameters of premixed methane–air mixtures in the closed explosion vessels. The main findings from these experiments are: For the weaker ignition the spark durations in the range from 6.5 μs to 40.6 μs had little impact on explosion parameters for premixed methane–air mixtures in the 5 L vessel or 20 L vessel; For the same ignitions and volume fractions of methane in air the explosion pressures and the flame temperatures in both vessels of 5 L and 20 L were approximately the same, but the rates of pressure rises in both vessels of 5 L and 20 L were different; The explosion indexes obtained from the measured pressure time histories for both vessels of 5 L and 20 L were approximately equal; For the weaker ignition with the fixed spark duration 45 μs the ignition energies in the range from 54 mJ to 430 mJ had little impact on the explosion parameters; For the same ignition and the volume fractions of methane in air, the vessel volumes had a significant impact on the flame temperatures near the vessel wall; The flame temperatures near the vessel wall decreased as the vessel volumes increased.