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Minimum explosion concentration (MEC) of three agriculture dusts and two coal dusts was studied via a 20-L explosion chamber to reveal the role of gaseous inhibitors. Both active method (CO2 diluting air) and passive method (CO2 replacing O2) were used. The TG and DTG thermal analysis tests were conducted to study the pyrolysis and combustion characteristics of dust samples. An alternative explosion criterion based on combustion duration time was used to determine MEC, and compared with the standardized overpressure method. Under 10-kJ ignition condition, as oxygen mole fraction (XO2) decreased from 21% to 10%, MEC of agriculture dusts and coal dusts respectively increased by around 5 times and 2 times. The active inerting method with a lower N2/CO2 ratio was found to have a better suppression effect on the explosion of the five carbonaceous dusts because the blend has a higher specific heat and a lower oxygen diffusion rate.
According to international standards several safety characteristics of dusts are determined in the 20-L sphere (also known as SIWEK-Chamber). Dust cloud ignition is carried out using pyrotechnical igniters. Due to various disadvantages of pyrotechnical igniters the need for alternative ignition sources arises again and again. An alternative ignition source could be the so-called 'exploding wire' or 'fuse wire'. The paper presents test results of a comparative study between both ignition sources for the determination of the safety characteristics Maximum Explosion Pressure and Maximum Rate of Explosion Pressure Rise of selected dusts in the 20-L sphere. In addition the propagation of flame and electrical arc was analysed by high speed camera recordings. Tests in a windowed autoclave were performed to get information on the influence of turbulence during ignition on the shape of the generated flame (electrical arc) and its propagation. Turbulence measurements with a LDA system in the windowed autoclave as well as in the 20-L sphere allowed investigation if the ignition sources led to an additional turbulence increase at the moment of ignition in comparison to tests without ignition.
Validation of the new ignition source 'exploding wire' for dust explosion testing in the 20-L-sphere
(2014)
The safety characteristics Maximum Explosion Pressure pniB, Maximum Explosion Pressure Rise (dp/dt)™* and Lower Explosion Limit LEL are determined in closed vessels such as the 20-L-sphere according to international Standards. Dust ignition is carried out using pyrotechnical igniters which are defined in the Standards. Due to various disadvantages of pyrotechnical igniters the need for alternative ignition sources arises again and again. Studies at the Federal Institute for Material Research and Testing (BAM) with ignition units which were able to generate ignition energies up to 2000 J showed that the so-called “exploding wire” or “fuse wire“ is suitable as an alternative ignition source. The paper presents further test results for the Validation of the exploding wire for the determination of Pm» and (dp/dt)mx in the 20-L-sphere. The tests were performed with a new ignition unit and improved electrodes which allowed ignition energies up to 10000 J. The paper also analyses propagation of flame and electrical arc on basis of high speed camera recordings. Turbulence measurements with a LDA System in the 20-L-sphere allowed investigation whether the activation of the ignition sources has an influence on the turbulence field generated during dust dispersion and whether the influence differs depending on the ignition source.