Determination of ignition energies and studies of flame propagation in ammonia-air mixtures

Today, natural refrigerants are replacing CFCs at an increasing rate. New technology should always be accompanied by a risk assessment. However, a correct risk assessment cannot be carried out without proper understanding of the basic input data used in the model. This study investigates input data for the identification of possible explosion hazards of ammonia-air mixtures. An ammonia-air mixture in a test tube was exposed to,electrical sparks from capacitor discharges as well as sparks generated from an induction coil. The flame propagation rate was measured by photodetectors along the inside wall of the tube, and the quenching distance was determined by varying the spark electrode distance. The flame propagation rate was found to be 0.5 m/s and the quenching distance was found to be not less than 3.3 mm. Capacitor spark discharge, characterised by high power of short duration, required an energy of 300 mJ, whereas only 30 mJ was required from an induction coil spark, characterised by low power of long duration. The result presented show that ammonia-air mixtures should be regarded as an explosion hazard when exposed to certain ignition sources. Furthermore, the study also highlights the general difficulty of determining minimum ignition energies and the way that they are used in risk assessments.