Ignition by Electrical Discharges

Abstract

The ignition of combustible/air mixtures by electrical discharges includes several physical and chemical processes. In process safety often the total available electrical energy is taken as a risk measure. However, to get a more detailed insight in the ignition process, also energy losses to the surrounding have to be considered. Additionally, for specific discharges not only the dissipation leading to thermal heating is of importance. Especially in the case of non-equilibrium plasma, a specific fractional amount of the discharge energy is used for electron impact dissociation, excitation, and ionization reactions, producing active radicals and ions during the discharge phase. While the electrical energy can be measured easily, it is difficult to determine energy losses. In this paper three different electrical discharges are examined experimentally and numerically to yield a better understanding of the ignition by electrical discharges.

Ignition by Low-Voltage Electric Discharges of Diluted and Undiluted C3H8–Air Mixtures

“…Several papers have examined the sources of error in measuring the minimum ignition currents. − ,, Among them, one can find the variable geometry of the separation distance between electrodes, which influences the growth of the plasma kernel around the spark along with the degree of ionization of metal vapor. The destruction of the cadmium disk, the blunting and the superficial oxidation of the tungsten electrodes are also disturbing factors, determining the difficulty of obtaining reproducible results with this ignition configuration, even with a careful fitting of contact separation. , …”

“…The minimum ignition currents measured for break sparks and the critical ignition energies for hydrogen–, alkane–, and alkene–air mixtures using the standard test cell and the standard method recommended by IEC were previously reported, − ,,− mostly at ambient initial conditions. Experimental studies on critical conditions of ignition by low-voltage electric sparks, conducted on n -butane–air, , liquefied petroleum gas (LPG)–air, ethylene–air, and propene–air mixtures delivered MICs over an extended range of initial pressures, down to 0.25 bar and outlined the MIC variation with pressure.…”

“…The electric circuit connected to the spark test apparatus contains a stabilized adjustable DC 24 V power supply and a 96 mH calibrated inductance, which affords a current in the range of the sought MIC values. A similar configuration of separating contacts was used by Uber et al and Markus et al for igniting the hydrogen–air mixtures.…”

“…Several types of electrostatic discharges were found to produce the ignition of flammable mixtures such as spark/capacitive, corona, brush, lichtenberg/propagating brush, maurer/cone, and lighting-like discharges. , The most common electrostatic discharge encountered in both domestic and industrial plants is the discharge obtained at every switch of a low-voltage electric circuit. The major incendive arcs in flammable atmospheres are produced when the electrical contacts are interrupted.…”

“…Experimental studies on critical conditions of ignition by low-voltage electric sparks, conducted on n -butane–air, , liquefied petroleum gas (LPG)–air, ethylene–air, and propene–air mixtures delivered MICs over an extended range of initial pressures, down to 0.25 bar and outlined the MIC variation with pressure. The ignition of hydrogen–air and methane/hydrogen–air by break sparks was experimentally studied by monitoring the transient flame kernel using high-resolution interferometry, and numerically, assuming a three-dimensional (3-D) reactive flow along with detailed chemical kinetics. − ,, To validate the computational model, which uses the discharge at the contact arc as a heat source, the ignition delay and critical currents were measured. Then, the temporal and spatial distributions of the energy were correlated by an empirical model using the measured current, the intensity of radiation, and the voltage of the produced arc.…”