Experiment-based investigations of magnesium dust explosion characteristics

Abstract

An experimental investigation was carried out on magnesium dust explosions. Tests of explosion severity, flammability limit and solid inerting were conducted thanks to the Siwek 20 L vessel and influences of dust concentration, particle size, ignition energy, initial pressure and added inertant were taken into account. That magnesium dust is more of an explosion hazard than coal dust is confirmed and quantified by contrastive investigation. The Chinese procedure GB/T 16425 is overly conservative for LEL determination while EN 14034-3 yields realistic LEL data. It is also suggested that 2000–5000 J is the most appropriate ignition energy to use in the LEL determination of magnesium dusts, using the 20 L vessel. It is essential to point out that the overdriving phenomenon usually occurs for carbonaceous and less volatile metal materials is not notable for magnesium dusts. Trends of faster burning velocity and more efficient and adiabatic flame propagation are associated with fuel-rich dust clouds, smaller particles and hyperbaric conditions. Moreover, Inerting effectiveness of CaCO₃ appears to be higher than KCl values on thermodynamics, whereas KCl represents higher effectiveness upon kinetics. Finer inertant shows better inerting effectiveness.

Introduction

Magnesium is an excellent engineering material due to its light weight and high mechanical strength. As one of the most important magnesium products, magnesium powders are widely used in metallurgy, aeronautics, fireworks, painting and chemical industries. Nevertheless, magnesium is combustible and explosive. There could be severe hazards in production, storage, transportation and handling. While the concentration of suspended magnesium dust in a confined space exceeds the lower explosion limit (LEL), explosions will occur with the effect of ignition sources. In fact, dust explosions represent significant damages in industries. As far as China is concerned, accidents happened with losses of human lives and destruction of industrial facilities. For example, a severe magnesium dust explosion described by Zhang, Jiang, and Zheng (2005) resulted in the death of five people in 1978. Another case described by Kuai, Li, and Chen (2010) caused one death in 2008.

Safety handling of magnesium powders first requires data on explosion hazards. Thus it is urgent to increase the understanding of magnesium dust explosion characteristics and explosion mitigation. Several studies were carried out on this field and data in previous articles mostly focused on the determination of explosion sensitivity parameters and explosion severity characteristics of magnesium dust (Li et al., 2009a, Li et al., 2008, Nifuku et al., 2007). Unfortunately, (i) few studies included a systematic study showing influences of dust concentration and particle size, above all, influences of ignition energy or initial absolute pressure are hardly mentioned. (ii) The comparison between Chinese and European LEL determination procedures has not been practiced and the most appropriate ignition energy for LEL determination of magnesium dust has not been discussed yet. (iii) Inerting of magnesium dust with chloride (widely used as the powder extinguishing agent for light metal fire) has not been practiced yet.

This work aims to present data about the overall characteristics of magnesium dust ignitability and explosibility. (i) Information about the flammability limit (i.e. LEL) and severity characteristics (maximum explosion pressure p_max, maximum rate of pressure rise (dp/dt)_max and combustion time t_c) was given. (ii) Influences of some factors, like dust concentration, particle size, ignition energy, initial absolute pressure, and added inertant, were analyzed. (iii) The most appropriate ignition energy for LEL determination of magnesium dust was presented. (iv) The inerting effectiveness of different inertants was compared.