Study into parameters of the dust explosion ignited by an improvised explosion device filled with organic peroxide

Abstract

Dust explosion poses a significant hazard in industry. An explosion of dispersed dust can be ignited by an improvised explosive device. The article deals with the study of explosion parameters of representative samples of dispersed dust ignited by an improvised explosive device. Hexamethylene triperoxide diamine was used as an igniter. Lycopodium clavatum spores were used as a standard sample. Furthermore, dust samples were selected from those types of operations that may be endangered if an improvised explosive device is used as an igniter. Wheat flour and beech wood dust were selected as representative samples. The achieved parameters of explosion pressure and explosion constant K_st were on average by 5–15% lower than the parameters achieved when using a commercial igniter. The P_max value and the inflection point of the explosion record were reached 7–12 ms earlier than those achieved with a commercially available igniter. The findings may be relevant in the design of explosion prevention devices. New types of explosion prevention devices can be designed to reduce the risk of explosion of dispersed combustible dust caused by improvised explosive devices, e.g. in a case of terrorist attack on the objects with the occurrence of dust clouds.

Introduction

The article is based on the idea that the industrial plants generating flammable dispersed dust may become a target of terrorist attacks. The dispersed combustible dust generated during industrial operations can cause serious consequences, typically an explosion, even if the minimum conditions are met (Fumagalli et al., 2016, Eckhoff, 2002). Combustible dust thus poses a serious threat in industry, and therefore the issue needs to be addressed.

Terrorist attacks have become a global phenomenon nowadays (Schmid, 2020). The attacks on the World Trade Center in 2001 can be considered a modern beginning (Bullock et al., 2021). Terrorists use the means of transport (Milazzo et al., 2009, Patrick and Radi, 2019); the attacks are often carried out using either industrially produced explosives (the well-known Lockerbie attack (Smart, 1997)) or improvised explosive devices (Manelici, 2017, Draca et al., 2011).

In addition to these well-known ways of conducting a terrorist attack, explosives can also be employed in other ways; e.g. small quantities (grams) of explosive can initiate explosion of another substance, such as flammable dust. That way of a terrorist attack is quite inexpensive and difficult to detect, and may significantly affect the process of safety, causing human injury and property damage. Despite the high risk of handling, an improvised explosive device can be simply used to cause an explosion of a dispersed dust cloud (Morley and Leslie, 2007, Zoli et al., 2018).

Explosion of dispersed dust can occur under five minimum conditions: flammable dust, oxidizer, concentration of dispersed dust above the lower explosive limit, confined space and an ignition source (Ogle, 2017a, Eckhoff, 2019, Abbasi and Abbasi, 2007).

Ignition source can significantly affect the dynamics and course of the dispersed dust explosion (Bartknecht, 1989, Yuan et al., 2015). In the industry, there may be several types of significant ignition sources that can make dust explode. In practice, it is usually a spark (static, capacitive), hot surface, overheating, direct fire, etc. Those ignition sources are characterized by the basic parameter of "the ignition energy" (the energy of the ignition source that ignites a cloud of dust) and the dynamics of the ignition process (spark is an extremely local source, fire/burning is a "slow" source, regarding the burning rate of dust) (Amyotte, 2014, Eckhoff, 2002).

The EN 14034 Standard specifies the energy of an ignition source for the study of explosion parameters of dispersed dusts as 2 × 5 kJ (EN 14034:2006 + A1, 2011). The ASTM E1226-12a Standard specifies the exact composition and amount of the mixture in a chemical igniter (ignitor weight 1.2 g, 40 wt% zirconium powder, 30 wt% barium nitrate, 30 wt% barium peroxide (Bartknecht, 1989; ASTM E1226-12a, 2012)). In practice, a chemical igniter (e.g. of the Soebbe manufacturer) with an ignition energy value of 10 kJ (2 × 5 kJ) is the most commonly used to measure explosion parameters of dispersed dusts (Eckhoff, 2002, Eckhoff, 2019, Bartknecht, 1989). Use of an igniter with a lower energy value results in a lower Kst value (Field, 1983). Other scientific types of igniters also encountered in scientific publications are: chemical igniter, induction spark, surface-gap spark, exploding wire (Spitzer et al., 2021). The authors compare those types of igniters with the commercially available ones that meet the above-mentioned standards (e.g. Soebbe). Spitzer et al. compare selected parameters of igniters (calorimetric energy of the igniter, visual course of activation of the igniter, etc.) Spitzer et al. (2021) Several authors assess the impact of different types of igniters on the explosion parameters of dispersed dust (Zhen and Leuckel, 1997, Zhu et al., 1988, Marc et al., 2013). Theoretical influence of the shock wave on the explosion of dispersed dust is dealt with in (Ogle, 2017b). In scientific publications, we have not encountered a study of the effect of an explosive igniter on dispersed dust and its explosion parameters.

In the case of using various ignition sources, the initiated explosion is characterized by typical dynamics and course (Eckhoff, 2019). It is then possible to determine the explosion parameters by using the measurements according to the EN 14034 and ASTM 1226 Standards. The characteristic features of the dynamics and the course of the dispersed dust explosion can also be the values of the time of reaching the inflex point and the maximum value of the pressure.

In this article, we deal with the use of an improvised explosive device filled with explosive. The course and dynamics of the ignition process using organic peroxide can be significantly different from the ignition caused by a conventional ignition source (Ogle, 2017b). For the purposes of detonation, HMTD combines the properties of the ignition sources listed above. Owing to the high detonation velocity (Matyáš and Pachmáň, 2013), it is a local source that produces rapid combustion (the parameters and behaviour of HMTD are given in the Materials and Methods section). The detonation of HMTD can be an igniting source the properties of which can have a significant effect on the dynamics and course of the explosion of dispersed dust. Explosion of the dust cloud activated by HMTD can significantly affect both, the activation of the explosion prevention devices as well as the achieved values of the explosion parameters.

Probably the largest number of industrial plants with the potential of generating explosive dust clouds operate in the processing of food products and wood. For this reason, samples of beech wood and flour were selected to study the explosion parameters and the dynamics of explosion. Exhibiting a low value of the minimum initiation energy (igniter does not produce hot particles), those materials are commonly available. The experiments described below were conducted to investigate the recorded explosion parameters and the time parameters determining the course and dynamics of the explosion after the use of an improvised explosive device.