Explosive properties of dust

How does a dust explosion occur?

Combustible gases and fluids have known explosion properties which are stated in their material safety data sheets. For dust and powder, it is not as simple and many are unaware that most organic materials such as wood, sugar, paint pigment and plastics but also some inorganic materials like various metal dusts, are able to create an explosion.

Compared to the traditional fire triangle two additional dimensions are added to demonstrate the requirements for a dust explosion. To form an explosive atmosphere the dust needs to consist of a large enough fraction of fine particles, which are mixed in the air, and for a pressure rise to occur the explosion needs to be in a confined space. The dust needs energy to ignite. The energy can origin from a small source of static electricity or larger source, such as an open flame, electrical malfunction, or by self-heating if the dust is resting on a hot surface. The particle size distribution is important, smaller particles are more easily ignited and will be better dispersed in air. The concentration is also important and needs to be within a given range in order to create an explosive dispersion. To support and maintain the combustion an oxidant is required. Usually, the oxygen content in ambient air is enough to create an explosive atmosphere. The dust needs to be airborne. Inside various process equipment the particles are frequently dispersed into the air. Furthermore, also dust which is not usually airborne can be lifted due to a primary explosion or other circumstances. If the explosion occurs in a confined area the pressure rise may develop in a very fast manner, which can cause severe damage to personnel and property.

Two factors that have a major impact on dust explosions

The particle distribution and the moisture content are of eminent importance for the dusts’ explosive characteristics. Generally smaller particles of dust generate a stronger reaction, since finer particles have a larger specific area (i.e. area per mass) and combustion more easily spread between smaller particles.

Specific surface area is easily visualized by dividing a cube with the sides 1 x 1 x 1 cm

In the first case, the oxygen reaches 6 sides which corresponds a surface area of 6 cm². In the second case the same figure has 48 faces and a surface area of 12 cm² while the third figure consists of 384 faces and a surface area of 24 cm².

Similarly, the shape also affects the rate of combustion. An irregular or flakey particle has a larger surface area than a spherical, for example. The two sides that first meet during combustion, determine the thinnest part of a particle. The combustion rate is thus proportional to the smallest dimension of a particle divided by two.

The moisture content of the dust is also an important parameter, and a dry dust is more reactive than a moist one. Multiple studies confirm a clear relationship between the dusts’ reactivity and its particle size and moisture content. Currently there is much available data on different materials explosive characteristics, but finding a comparable match could be difficult since the results are so widely dependent on moisture content and particle size - and this data is often left out.

According to the EU regulations testing is performed on particles with diameter <500 μm. Moisture content shall be declared but there is no recommended range specified. The American standards are more conservative and recommends a maximum moisture content of 5 % and a maximum particle size of <75 μm. Testing may be performed on as received samples to match the dust which is normally found in the process. However, there is a risk of accumulated fine dust even at industries generally handling coarser materials, as the handling itself can give rise to a fine fraction.

How can we prevent dust explosions?

RISE has six pieces of equipment which can be used to determine nine different explosive parameters. This includes two Hartmann tubes, one to perform a screening test, a yes/no test to see if the dust is combustible or not and one for defining the minimum ignition energy required to ignite a dust cloud. A 20 L sphere where the most common test indicates the maximum pressure achieved from an explosion in a contained space and how fast it develops (p_max, dp/dt_max and K_st). Two apparatuses define at which temperature a dust layer and a dust cloud will ignite (minimum ignition temperature). 

The parameters achieved through testing can i.a be used to dimension vent sizing and to specify maximum surface temperature that may be achieved on equipment to be installed. Information which is vital for many industries to comply with the user directive 1999/92/EC, linked to ATEX. 

Like flammable gases, dust has an explosive range with an upper and lower limit. The crucial difference is that these limits are difficult to use in practice due to the mobility of the dust. Dust concentrations can change rapidly due to deposition and entrainment caused by swirling. As a rule of thumb, when a 25 W light bulb at two meters distance is obscured by a dust cloud, the lower explosive limit has been surpassed. Another general rule is that it is time to clean when footprints are visible on the floor. 1 mm dust layer is sufficient to create a hazard if the dust swirls up and forms an explosive mixture. 

RISE is focusing on improving process safety by performing research projects using advanced numerical tools. One example is a collaboration between RISE and Chalmers University of Technology where an open source numerical tool, based on OpenFOAM toolbox, is developed which can be used to simulate dust explosions. Data from one of the test equipments, the 20 L sphere, can be used for validating the numerical model. This project will give an open source numerical tool with detailed documentation for predicting dust explosion hazards and giving recommendations for plant designs regarding dust explosion risks.