In many industries, dust fires and explosions pose serious risks. Therefore, it is necessary to assess dust explosibility and ignitability in industrial facilities to recognize hazards and implement the relevant safety measures.
What is Minimum Ignition Energy MIE?
The minimum energy needed to ignite an explosive dust-air combination with a high-voltage spark discharge is known as the minimum ignition energy MIE of a dust cloud.
Dust cloud MIE measurements are crucial for determining plant equipment’s grounding and bonding specifications and predicting the possibility of dust cloud igniting during solids handling operations.
MIKE3 is a dust cloud MIE measurement device similar to other vertical explosion tube devices.
Factors that Effect MIE Measurements
MIE measurements are affected by several particle characteristics such as morphology, polydispersity, and size distribution.
MIE results are also influenced by the testing environment conditions, including oxidizer composition and the spark discharge method.
A concept similar to the fire triangle is a dust explosion pentagon that includes five essential causation factors: ignition, confinement, mixing, oxidizer, and fuel.
Advanced Measurement Techniques and MIE
Advanced measurement techniques complement MIE data and help explore the underlining chemistry and physics of dust cloud combustion.
Advanced experiments in vertical explosion tube setups use lasers, cameras and several other sensors to extract high-resolution data from the dust cloud. For instance, researchers have used digital in-line holography (DIH) in several experiments to help collect quantitative particle data inside the MIKE3 vertical tube setup.
Another study combined digital in-line holography with particle image velocimetry (PIV) which enabled macro-scale flow and micro-scale particle measurement in a MIKE3 glass tube.
What is Chemiluminescence?
Chemiluminescence is described as light emission from chemically stimulated species returning to their electronic ground state. Species-specific data is obtained from the dust cloud combustion reaction zone using chemiluminescence imaging.
Excited-state radicals such as methylidyne (CH*) and hydroxyl (OH*) in hydrocarbon flames aid in identifying the flame front and the reaction zone, respectively. As a result, flame chemiluminescence may be used to more precisely evaluate the flame structure in burning dust clouds.
Comparison with Previous Studies
The experiment described in this study combines high-speed broadband, OH*, and CH* imaging for in-situ and non-intrusive measurements of the flame kernel within the MIKE3 device. It exhibits advanced measurements of dust cloud ignition and combustion that are entirely compatible with the functioning of the MIKE3 device and the generation of typical MIE data.
Most earlier investigations on flame propagation in comparable configurations and spatial domains lacked sufficient temporal and spatial resolution and were less sensitive. Past studies have concentrated on measuring the leading flame edge's speed rather than the flame kernel's development and motion. Therefore, within the framework of an industry-standard testing environment, this study constitutes a thorough assessment of dust cloud ignitability.
How the Experiment was Conducted
The experimental setup included a UV lens, high-speed camera, nozzle cup, glass tube, ignition electrode, and MARK3 device. A dust cloud MIE testing device was used to create and ignite the dust clouds.
MIKE3 device's components included a couple of tungsten electrodes for spark discharge generation, a vertical glass tube constraining the flow, a mushroom-shaped nozzle for dust dispersion with compressed air burst, and a cup to hold dust material. The spark ignition energies needed for aluminum and niacin were 300 mJ and 20 mJ, respectively. A moving-electrode discharge circuit was used to deliver these spark energies.
An ultraviolent-sensitive lens fitted on a high-speed camera was used to image the resulting dust flames. Using a 5-volt trigger signal from the solenoid valve that actuates the burst of air within the MIKE3 device, it was possible to synchronize the high-speed camera and the MIKE3 device using a digital delay generator.
Significant Findings of the Study
This study implements high-speed and species-specific (OH* and CH*) imaging for spark-ignited dust clouds measurement inside the MIKE3 device.
The researchers focused on flame kernel development from aluminum and niacin dust clouds after spar ignition for 10 ms in the central ignition region.
An intensity thresholding algorithm extracts velocity, position and kernel size measurements from high-speed image patterns. In addition, aluminum and niacin dust samples investigated metal and organic fuel combustion properties.
A non-uniform continuous reaction zone composed of excited-state species and particle clusters was shown by niacin flame kernels which increased from 5 to 17 mm and traveled with 5 m/s velocities from the central electrode region.
An unresolved internal structure with a bright reaction zone was shown by aluminum flame kernels comprising distinct burning particles near its boundary. The aluminum flame kernels increased from 7 to 10 mm and traveled with a 3 m/s velocity from the central electrode region.
In future studies, chemiluminescence from other species such as oxides of aluminum (AlO) can be incorporated to better understand substance-specific spark-ignited dust clouds.
The current study can be extended to flame propagation in the overall combustion process instead of focusing only on the early growth and motion of the flame kernel.
Christian Schweizer, Chad V. Mashuga, Waruna D.Kulatilaka (2022) Investigation of niacin and aluminum dust cloud ignition characteristics in an explosion hazard testing device using high-speed imaging. Process Safety and Environmental Protection. https://www.sciencedirect.com/science/article/pii/S0957582022006966