The term dust explosion defines the rapid combustion occurs when dust dispersed in air, within the closed medium or confined space. Its depending on the following conditions, the dust must be combustible, it should be dispersed in air, oxygen availability, ignition source and confined space. For example, wooden or solid material can ignite in air and speed that increase with increasing degree of sub-division of the material.

The particles size less than 0.1mm are suspended in a sufficiently large volume of air to give each particle enough space for its unrestricted burning, the combustion rate will be very fast, and the energy required for ignition very small. The rapid burning dust can give the pressure rise in the confined space, for the boundaries of the confinement to burst or collapse, and a “bang” being emitted into the surroundings. 

Generally, when compared to bigger particle size the smaller particles are easily get ignite and it may burn more violently. If such an explosive combustion of a dust cloud takes place inside process equipment or work rooms, the pressure in the fully or partly enclosed explosion space may rise rapidly and the process equipment or building may burst, and life, limb and property can be lost. In both hazard evaluation and in accident investigation, it is helpful to conceptualize the necessary conditions for a deflagration. With flammable gases and vapours, these conditions are symbolized as the fire triangle: fuel, oxidizer, and an ignition source make up the three sides of the triangle. For a combustible dust flash fire, a fourth condition is necessary: the dust (fuel) must be suspended creating a dust concentration within the ignitable range. This is often symbolized as the combustible dust flash fire square. A dust explosion requires one more condition beyond the flash fire: containment of the dust cloud such that the combustion process causes an increase in the pressure of the enclosure. Since a dust explosion requires five necessary conditions, it is symbolized as a pentagon as

The heat of combustion of a material is an important parameter because it determines the amount of heat that can be liberated in an explosion. When comparing various materials in terms of their potential hazard, it is useful to relate the heat of combustion to the same amount of oxygen consumed. This is because the air in each volume of dust cloud contains a given, limited amount of oxygen irrespective of the kind of dust. This limited amount of oxygen also limits how much heat can be released in an explosion per unit volume of dust cloud.

Factors influencing ignitability and explosibility of dust clouds,

1. Chemical composition of the dust, including its moisture content
2. Chemical composition and initial pressure and temperature of the gas phase
3. Distributions of particle sizes and shapes in the dust, determining the specific surface area of the dust in the fully dispersed state
4. Degree of dispersion, or agglomeration, of dust particles, determining the effective specific surface area available to the combustion process in the dust cloud in the actual industrial situation
5. Distribution of dust concentration in the actual cloud
6. Distribution of initial turbulence in the actual cloud
7. Possibility of generation of explosion-induced turbulence in the still unburnt part of the cloud (location of ignition source an important parameter)
8. Possibility of flame front distortion by other mechanisms than turbulence
9. Possibility of significant radiative heat transfer (highly dependent on flame temperature, which in turn depends on particle chemistry) Factors a, b, c and perhaps also i can be regarded as basic parameters of the explosive dust cloud.

However, factors 4 to 8 are influenced by the actual industrial dust cloud generation process and explosion development. These in turn depend on the nature of the industrial process (flow rates, etc.) and the geometry of the system in which the dust cloud is burning.

Also, the location of the ignition point can play an important role in deciding the course of the explosion. In view of the wide spectrum of dust cloud concentrations, degrees of dust dispersion and turbulence, and of locations of potential ignition sources in industry, a correspondingly wide spectrum of possible dust cloud ignition sensitivities and combustion rates must be expected.

However dust explosion plays the vital role in every industry, which is handling and processing the dust particles. Also, it causes more damage to human lives, industry and environment. It will also lead the financial downfall in industry, so before identifying and assess the source of hazards helps the preventing all riddle.

Dust combustibility group (A/B)

Dust combustibility group (A/B) is introductory test to determine the dust cloud or powder cloud (ie., below 63 microns) whether combustible or not. This is a screening test for dust explosibility studies.

The dust concentration range between 500-1000g/m3 was usually taken for the conducting the test and that dust concentration range can create the perfect cloud for accessing the dust combustibility, when it was contact with the ignition source.

Combustibility of dust cloud or powder cloud was grouping by,

1. Combustibility (Group A) – Flammable or Explosible as a dust cloud
2. Non-Combustible (Group B) – Non-Flammable or Explosible as a dust cloud.

Test conditions:

The standard atmospheric conditions for determination of characteristics of combustible dust
are:

1. Temperature –20 °C to +60 °C,
2. Pressure 1-4 bar and
3. Zero Air

The test equipment consists of a vertical tube closed at the bottom with a dispersion cup (volume approximately 1.2 L, internal diameter (70 ± 5) mm). As ignition source a continuous induction spark (electrode gap of approximately 4 mm, with a transformer rated approximately 15 kV, and approximately 0,2 kVA), the vertical separation between the bottom of the dispersion cup and the ignition source is approximately 100 mm.

Test standards:

Both British standards and European standards are employed for determining the dust combustibility.

British Standards BS EN ISO IEC 80079-20-2

ISO/IEC 80079 describes the test methods for the identification of combustible dust and combustible dust layers to permit classification of areas where such materials exist for the purpose of the proper selection and installation of electrical and mechanical equipment for use in the presence of combustible dust.

American Society for Testing and Materials (ASTM) E1226

ASTM Standard E1226 – 19 describes the Particulate solids of combustible materials present a significant risk of dust explosion if suspended in air and subjected to an ignition source. The methods of this standard can be used to determine if a dispersed dust cloud is “explosible” and, if so, to what degree it is explosible, that is, its “explosibility.” Knowledge that a dust may be explosible if dispersed as a dust cloud is important in the conduct of a process hazard safety review. Contained herein is an explosibility or go/no-go screening test procedure for the purpose of determining whether a dust sample is explosible.

If a dust is explosible, the explosibility parameters such as maximum explosion pressure, Pmax; maximum rate of pressure rise, (dP/dt) max; and explosibility index, KSt, are measured using 20L Sphere or 1m3 Equipment,

Test Procedure

A Hartmann tube made of glass with a volume of 1 litres is used as the explosion vessel. The dust dispersion system at the base of the tube is of the "mushroom-shaped" type around which the sample is loosely scattered. A blast of compressed air at 2.5-3.5 bar is used to disperse the dust in the glass cylinder where ignition trials are conducted by passing a spark of known energy, nominally 1000 mJ, between two electrodes and 10J using Coil.

The test sample is deposited in the dispersion cup and dispersed with a blast of air (50 cm3, 700 kPa to 800 kPa gauge). The dust concentration is varied over a wide range from 250 g/m3 to 1500 g/m3 (typically 250 g/m3, 500 g/m3, 750 g/m3, 1 000 g/m3 and 1 500 g/m3) and the behavior is visually observed. The different quantities are each tested once, but repeated dispersions are made for at least 3 attempts.

If a flame propagates from the ignition source the test material is combustible dust or combustible flying. If no ignitions are observed with the spark ignition source, then the coil ignition source is used. Testing may be stopped immediately after an ignition is observed. If it is unclear whether ignition has been observed, then the 20-litre sphere test shall be used.

Importance of dust combustibility Group(A/B)

1. Hazard identification and assessment: Dust combustibility group A/B testing is helps to identify the source of hazards and prioritize the risk level and those data also helps to prevent the dust explosion.

2. Safety aspects: Test results provide the engineering controls like preventive measures to be taken for eliminate the ignition possibilities and helps to prevent the dust explosion.

3. Material Characteristics: Determining the properties of dust particles like, particle size analysis, and moisture content, those are important factors for determining the dust combustibility.

4. Regulatory Compliance: Many industries are required to comply with safety standards such as NFPA 652, which mandates a Dust Hazard Analysis (DHA) for operations involving combustible dust. Proper classification aids in meeting these requirements.

5. Risk Management: Understanding the combustibility of dust allows for better risk assessment and management. It helps in implementing appropriate safety measures and emergency response plans.

6. Process Optimization: Knowing the properties of dust can lead to improved process design and operation, reducing the likelihood of hazardous incidents.