Minimum ignition energy (MIE) measures the minimum energy required to ignite a dispersed fuel or combustible compound in air. Chemicals with low MIE values, such as acetylene and hydrogen, must be handled with care. The TWA 800 airline accident was attributed to a possible MIE explosion. Fine particulate matter, including powdered metals and solid fuels, also tend to have low MIE values and can cause dust explosions. Propane-type fuels, iron dust, and carbon disulfide have MIE values well below 1 millijoule.
Minimum ignition energy (MIE) is a measure of the minimum amount of energy transmitted by an electric spark that will ignite a dispersed fuel or otherwise combustible compound in air. Conditions must be ideal for the explosion to take place with MIE values, but otherwise only slightly higher ignition values may be needed to ignite the mixture. The spark that causes an explosion in these cases can be caused by an electrostatic discharge (ESD) where there is a latent difference in electrical potential between two objects in close proximity, or it can be caused by a live electrical circuit undergoing an arc or a short circuit discharge event. Understanding the chemical processes of combustion that lead to minimal energy ignition events is important in the field of firefighting as well as in industries involving the storage and use of volatile compounds, such as in aircraft operation.
Volatile fuels and solvents and other chemicals known to have very low minimum ignition energies include acetylene and hydrogen at 0.017 millijoule, ethylene at 0.08 millijoule, and methane at 0.3 millijoule. These chemicals must be handled with extreme care in confined spaces as all also have low flash points at normal atmospheric pressure and temperature. One of the costliest and deadliest US commercial airline accidents, the flight of TWA 800 12 minutes after takeoff from New York City, New York, in July 1996, was attributed to a possible minimum ignition energy explosion in the fuel tank of the aircraft. central wing. Approximately 50 gallons (189 liters) of jet fuel known as Jet A that was stored in the tank at the time was inadvertently heated by air conditioning units directly under the tank area, which vaporized some of the fuel. As the aircraft climbed, the reduced air pressure increased evaporation to the point that the overpressure and a possible unknown spark condition created an explosion that ruptured the fuel tank itself and destroyed the airplane.
Many other types of compounds that are dispersed into the air as fine particulate matter, from powdered metals to grain flour and solid fuels such as coal and sulfur, also tend to have low minimum ignition energy values. The MIE for aluminum is 15 millijoules and magnesium is 40 millijoules. Charcoal ranges from 30 to 60 millijoules, and grain-based flour will ignite at 240 millijoules. A dust explosion can also occur with a random mixture of airborne floor debris when well dispersed in a heated environment, with the potential to ignite in the range of 0.2 millijoules to 10 joules if the humidity is below 80% and ambient temperature is 59° to 77° Fahrenheit (15° to 25° Celsius).
Other compounds known to have low minimum ignition energy values well below 1 millijoule include propane-type fuels, iron dust, and carbon disulfide. These low values are significant when compared to the energy level generated by common spark sources. A static electric spark typically discharges about 22 millijoules of energy, and a spark plug in an automobile will discharge about 25 millijoules of energy with each burn cycle.
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