Overview of dust explosibility characteristics

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Abstract

This paper is an overview of and introduction to the subject of dust explosions. The purpose is to provide information on the explosibility and ignitability properties of dust clouds that can be used to improve safety in industries that generate, process, use, or transport combustible dusts. The requirements for a dust explosion are: a combustible dust, dispersed in air, a concentration above the flammable limit, the presence of a sufficiently energetic ignition source, and some confinement. An explosion of a fuel in air involves the rapid oxidation of combustible material, leading to a rapid increase in temperature and pressure. The violence of an explosion is related to the rate of energy release due to chemical reactions relative to the degree of confinement and heat losses. The combustion properties of a dust depend on its chemical and physical characteristics, especially its particle size distribution. In this paper, the explosion characteristics of combustible dusts will be compared and contrasted with those of flammable gases, using methane as an example. These characteristics include minimum explosible concentration, maximum explosion pressure, maximum rate of pressure rise, limiting oxygen concentration, ignition temperature, and amount of inert dust necessary to prevent flame propagation. The parameters considered include the effects of dust volatility, dust particle size, turbulence, initial pressure, initial temperature, and oxygen concentration. Both carbonaceous and metal dusts will be used as examples. The goal of this research is to better understand the fundamental aspects of dust explosions.

Introduction

In industries that manufacture, process, generate, or use combustible dusts, an accurate knowledge of their explosion hazards is essential. Various books have been published since 1980 on the general subject of the explosion hazards of dusts and powders (Bartknecht, 1981, Bartknecht, 1989, Bartknecht, 1993, Field, 1982, Nagy & Verakis, 1983, Cashdollar & Hertzberg, 1987, Eckhoff, 1991). The present paper is an update of a previous Pittsburgh Research Laboratory1 (PRL) paper (Hertzberg & Cashdollar, 1987) on the general topic of the explosion hazards of dusts. The basic variables that influence the characteristics of a dust explosion will be discussed in general terms without specific reference to particular practical systems. One purpose of this paper is to provide assistance and guidance to the practising safety engineer at a plant regarding the important variables in dust explosibility. Both carbonaceous and metal dusts are used as examples of combustible dusts. Although many of the examples in this paper use coal dust, the concepts are applicable to other dusts as well.

This paper is not meant to be an overview of the many areas of dust explosion research throughout the world. Instead, it is only meant to be an overview of some of the dust explosibility characteristics that are important for safety engineers to consider at industrial plants.

Section snippets

Dust explosion requirements

The three requirements for combustion are a fuel, an oxidizer (usually air), and an adequate heat or ignition source. This is often called the “fire triangle”. The fuel can be any material capable of reacting rapidly and exothermically with an oxidizing medium. In this case, the fuel is a combustible dust. For a dust explosion, the dust must be dispersed in the air at the same time that the ignition source is present. The resulting rapid oxidation of the fuel dust leads to a rapid increase in

Laboratory equipment for dust explosibility evaluation

The explosibility characteristics of dust clouds are often measured in closed volume chambers. The 1.2-L Hartmann tube (Nagy & Verakis, 1983; Dorsett et al., 1960) is often used for preliminary screening tests and for minimum ignition energy (MIE) measurements. However, it may yield false negatives for dusts that are difficult to ignite with a spark but that are ignitable by stronger ignition sources. It is also not recommended (ASTM, 1999a) for measuring rates of pressure rise. The 20-L

Pressures and rates of pressure rise

Examples of the pressure data for a weak and a moderate coal dust explosion are shown in Fig. 3, Fig. 4. The absolute pressure (Fig. 3, Fig. 4) and rate of pressure rise (Fig. 3, Fig. 4) are plotted versus time. Fig. 3 shows the data for a 20-L chamber explosion test of a low volatile bituminous coal at a dust concentration of 125 g/m3, which is just above the minimum required for an explosion. The pressure trace in Fig. 3A starts at the partially evacuated value of 0.14 bar,a. The blast of air

Conclusions

The data examples reported in this paper show that laboratory test chambers are useful in studying a wide range of explosion characteristics of dusts. For both carbonaceous and metal dusts, the finer sized dusts are the more hazardous. Because of the importance of particle size, it is critical that representative samples of dusts be collected for explosibility evaluation. Because of the possible accumulation of fines at some location in a processing system, ASTM E1226 (ASTM, 1999a) and E1515 (

Acknowledgments

The author acknowledges the assistance of G.M. Green in the conduction of the size analyses and the 20-L chamber explosion tests and C.E. Lucci for the computer program for data acquisition and analysis. Both are from the Pittsburgh Research Laboratory of NIOSH. The author also thanks M. Hertzberg (retired from the Bureau of Mines) for introducing him to the subject of dust explosions and helping him to understand combustion theory.

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