Ignition sources for dust fires and explosions can take many forms. When powders are heated, however, new subtle sources of ignition hazards are revealed: Those associated with the inherent thermal instability properties of the powder being heated or dried, and the nature of the drying process.

Self-heating of powders due to exothermic reaction (exothermic decomposition or exothermic oxidation) can result in smoldering, fire and production of hot or incandescent particles that can travel along conveying equipment and act as an ignition source to dust clouds in downstream process equipment. Sometimes, exothermic decomposition causes rapid evolution of large quantities of toxic or flammable gases. Even if there is no smoldering or fire, self-heating can at times compromise product quality (charred particles).

Industrial operations that are prone to fires and possibly explosions due to self-heating include powder drying/heating and their subsequent processing, handling, packaging, storage and transportation.

Dry, bulk powder functions as a thermal insulator, preventing cooling and promoting the generation and buildup of heat energy from its core. At a temperature called the onset temperature, an exothermic reaction process (self-heating) starts somewhere within the powder accumulation. This reaction can continue to grow even if no more heat is applied to the powder from the dryer.

Self-heating is a complicated phenomenon consisting of both an exothermic chemical reaction and a heat-loss process. Simply, when the rate of heat generation exceeds the rate of heat loss, temperatures can rise at an accelerating rate and can result in smoldering, fire and, sometimes, explosion. Many factors can affect the onset temperature of self-heating, including:

• Powder composition.
• Presence of impurities.
• Geometry and size of the accumulated powder.
• Air availability.
• Duration of the powder exposure to a given temperature.

This article discusses some well-established methods for evaluating the thermal instability properties of powders, taking into account the actual powder conditions in dryers and downstream equipment, with the objective of ensuring the safety of operators, community and business.

Evaluation of Self-Heating Hazards of Powders

Ensuring safety from fires and explosions in drying operations first requires the proper understanding of the thermal instability properties of the powder, including its potential for gas generation and dust-cloud explosibility. In this article, I will focus on the powder’s thermal instability hazards and an evaluation of the self-heating hazards of powders[¹]. Any discussion of dust-cloud explosibility and vapor flammability is beyond this article’s scope.

Powder self-heating hazards can occur when the temperature of the powder in bulk or layer is raised to a level at which the heat-generation rate by the exothermic reaction exceeds the rate of heat lost to the surroundings. Temperature increase follows, which frequently results in smoldering and, eventually, fire. One of the factors that can affect the onset temperature for self-heating is the geometry and size of the accumulated powder. The onset temperature for self-heating of a powder in bulk is expected to be lower than the same quantity of powder spread into a layer. This is simply because the surface area per unit mass and, therefore, the rate of heat loss per unit mass, will be lower for bulked powder. Other variables such as airflow through the bulk powder or over the powder surface also have influence on the transition from smoldering to glowing and flaming as well as the onset temperature for self-heating.

For laboratory testing to provide useable indication of the hazards, a representative powder sample must be selected for testing. Also, the tests must reasonably simulate the conditions that the powder experiences during drying and subsequent downstream processes. Often, a number of different tests may need to be considered to obtain all the necessary data for proper evaluation of the self-heating hazards during all process stages. This is particularly important if the initial screening test results suggest an onset temperature for self-heating that is close to the drying temperature.

The results of the tests that I will be discussing in this section provide useful data to assess your powder’s potential for self-heating. Knowing your powder’s potential for self-heating will allow you to specify operating, storage, packaging and transportation conditions that will reduce the risk of self-heating.

Isothermal Basket Test.[^2,3] Measurement of exothermic activity involves heating the sample under controlled conditions to determine the point at which its temperature starts to increase independently of the external heat source.

Isothermal basket testing is performed by heating the powder samples in cubical wire baskets of varying sizes (typically, three sizes) to determine the minimum temperature at which each sample size self-heats. During each trial, a stainless-steel mesh basket filled with the powder sample is placed in an oven. The oven is heated and maintained at a preselected temperature until self-heating is detected, or for a duration of 24 hours (or longer, depending on the actual heating or storage cycle under study), whichever occurs first. The trials are repeated at various temperatures and basket sizes until the sample’s minimum-onset temperature for self-heating is determined for each basket size.

A plot of the onset temperature for self-heating as a function of basket dimensions will allow for the extrapolation to either the maximum safe material temperature for a given-size container, or the maximum safe container size for a given material temperature.

As you can appreciate, the isothermal basket test produces fundamental information with wide application. However, this method is time consuming and, therefore, can be relatively expensive. Although it has some limitations, the bulk powder test may offer a simpler option.

Bulk Powder Test.[⁴] The bulk powder test is used to evaluate self-heating properties of bulk powder in quantities not exceeding one ton. Examples of where it might be used include powder accumulations in bulk in some dryers, hoppers, silos or packaging.

A glass cylinder with a height of ~3.15” (80 mm) and diameter of ~1.97” (50 mm) is used. The cylinder is closed at the base by  sintered glass and filled with the test powder. Then, the cylinder is placed in an oven, and the powder temperature is monitored.

Normally, a screening test is first performed during which the oven temperature is raised at a rate of 0.9°F/min (0.5°C/min) for a 14-hour period. If the results of the screening tests indicate a temperature difference higher than 90°F (50°C) between the onset temperature for self-heating and the intended drying temperature, hazardous exothermic reaction is unlikely to occur. However, additional testing will be required if any of the following conditions exist:

1. The temperature difference between the onset temperature for self-heating and the dryer operating temperature is less than 90°F.
2. The operating cycle is longer than the test period above the process temperature.
3. The onset temperature for self-heating is less than 392°F (200°C).

If any one of the above conditions exists, then an isothermal bulk powder test is conducted at 90°F above the intended drying temperature for a duration longer than the maximum drying or heated storage time. Alternatively, an isothermal basket test may be conducted.

Aerated Powder Test.[⁴] An aerated powder test simulates conditions during heating operations of powder in quantities not exceeding one ton. During the test, a hot airstream diffuses through the powder — for example at the end of a fluid-bed drying cycle, when the dryer fan is off but induced airflow is still present.

This test is identical to the bulk powder test except that in the aerated powder test, an airstream at the same temperature as the oven temperature flows at a rate of 0.1585 gal/min (0.61 liter/min) through the sample during the entire test cycle. The sample temperature is measured at several locations in the cell to detect the onset temperature for any exothermic activity.

Normally, testing starts with a screening test. Additional isothermal testing at 90°F above the intended drying temperature — for a duration longer than the maximum drying or heated storage time — will be required if any of the following conditions exist:

1. The temperature difference between the onset temperature for self-heating and the dryer operating temperature is less than 90°F.
2. The operating cycle is longer than the test period above the process temperature.
3. The onset temperature for self-heating is less than 392°F.

Powder Layer Test.[⁴] Also called the air-over-layer test, the powder layer test simulates the conditions in which hot air passes above a layer or deposit of powder in a dryer. Examples include tray dryers and powder deposits on the internal surfaces of all dryer types.

A metal tray with dimensions 3 by 1.6 by 0.6” (75 by 40 by 15 mm) is filled with the test powder. The tray and powder are heated by air passing at a velocity of 14 ft/min (4.3 m/min). The temperature in the powder bed and around the outside of the tray is monitored.

Normally, a screening test is first performed. Additional isothermal testing at 90°F above the intended drying temperature — for a duration longer than the maximum drying or heated storage time — will be required if any of the following conditions exist:

1. The temperature difference between the onset temperature for self-heating and the dryer operating temperature is less than 90°F.
2. The operating cycle is longer than the test period above the process temperature.
3. The onset temperature for self-heating is less than 392°F.

Such tests can effectively determine the powder’s thermal instability hazards and allow evaluation of the self-heating hazards.

Avoiding Self-Heating Hazards

Preventing self-heating hazards in drying operations requires proper understanding of the thermal instability properties of the powder through laboratory tests. Such tests reasonably simulate the conditions experienced by the powder during the drying and subsequent downstream storage, packaging and transportation stages. The tests discussed in this article form an important route to achieving safe powder drying.

Precautions for avoiding smoldering, fires and explosions caused by powder self-heating include:

• Keeping the powder temperature at a safe margin below the temperature for the onset for self-heating, obtained by appropriate test methods.
• Employing facility and equipment designs to avoid ledges, corners, dead zones, etc., where powder could inadvertently build up inside process equipment.
• Avoiding the accumulation of hazardous levels of powder deposits on the inside surfaces of process equipment.

Additionally, each plant working with bulk powders should:

• Identify and control other ignition sources that might be present during normal and foreseeable abnormal conditions.
• Consider fire- and explosion-protection measures when preventive measures alone may not ensure acceptable level of safety.

In conclusion, self-heating occurs when the powder temperature is raised to a level at which the heat generated by an exothermic reaction exceeds the heat lost to surroundings, resulting in a thermal runaway.

Preventing thermal runaway and subsequent fires and explosions in drying operations requires proper understanding of the self-heating properties of the power through specific and tailored laboratory tests that reasonably simulate the conditions experienced by the powder during the drying and subsequent downstream storage, packaging and transportation stages. 

References

1. John Abbott, Prevention of Fires and Explosions in Dryers, 2^nd Edition, The Institution of Chemical Engineers, 1990.
2. P. C. Bowes, “Self-Heating: Evaluating and Controlling the Hazards,” Building Research Establishment, HMSO.
3. P. F. Beever and P. F. Thorne, “Isothermal Methods for Assessing Combustible Powders – Theoretical and Experimental Approach,” Symposium Series No. 68, Institution of Chemical Engineers, 1981.
4. N. Gibson, D. J. Harper and R. L. Rogers, “Evaluation of the Fire and Explosion Risk in Drying Powders,” Plant/Operations Progress, Vol. 4, No. 3, July 1985, pp. 181-189.