Preventing self-heating of particulate solids
14 July 2016
Many particulate solid materials can exhibit self-heating, which - if unchecked - at the very least is likely to affect the quality of the product or even progress to a fire or an explosion. In this article, Dr. Vahid Ebadat of Chilworth Asia Pacific looks into the mechanisms behind this process.
Mechanisms of self-heating
Self-heating can arise by one of the following different mechanisms:
• Exothermic (heat releasing) chemical reaction:
The chemical reactions are often an oxidation reaction with air, similar to what occurs during a fire or explosion. At the start of the self-heating process, the reaction is very slow, like steel that oxidizes (corrodes) with atmospheric oxygen to form rust.
• Exothermic decomposition:
Figure 1 - Test cell for bulk conditions (diameter 50 mm, height 80 mm)
For unstable materials, decomposition results in less complex molecules and sometimes gases, while releasing heat. However, unlike an exothermic reaction, decomposition does not require additional reactants and is therefore largely independent of the environment making it more difficult to predict its occurrence without detailed experimental studies.
Some materials can self-heat at ambient temperatures and spontaneously ignite in large-scale storage, such as sawdust, coal, sewage sludge, and grain.
How does self-heating occur?
When a material undergoes exothermic chemical reaction(s) or decomposes exothermically, the temperature of the material will rise if the rate of heat generation exceeds the rate of heat loss to the environment. Further, the temperature rise of the material due to the exothermic reaction will exponentially increase the chemical reaction rate, resulting in a faster increase in temperature. This unstable process is referred to as self-heating. Self-heating begins at a temperature at which the rate of heat generation is greater than the rate of heat loss, and this temperature is called the exothermic onset temperature.
Self-heating of solids and powders may result in smoldering which can set the material on fire or cause dust explosions, particularly when a “smouldering nest” is disturbed and exposed to air. Many plants that have experienced self-heating incidents have had a history of “near misses” where some self-heating occurs but does not progress to full-blown ignition. In such cases, there may be “black spots” in an otherwise light-colored product, or a lump of charred product is found. It is important to recognize such occurrences as indications of a potentially serious problem, and not just a “near-miss”.
Self-heating reactions may also produce flammable gases, which may lead to gas/air explosions or pressure/volume explosions in closed process vessels, and also compromise product quality.
Figure 2 - Various test cells for thermal stability testing
The exothermic onset temperature for self-heating is influenced by the chemical and physical properties such as chemical-reaction kinetics, thermal conductivity, and heat of reaction, as well as by other factors, including:
• Dimensions and shape of the material - solid or powder;
• Ambient airflow over the material,
• Availability of oxygen within the bulk or porosity,
• And additives or contaminants
Usually, the material has to be exposed to an air or ambient temperature near the onset temperature for an induction time, which is reduced with an increase in temperature.
Laboratory tests to simulate self-heating behaviour
Figure 3 - Basket test sample holders for testing at different scales
Several laboratory tests have been developed to simulate the conditions where the powder could be heated above the onset temperature: bulk form (Photo 1, above), layer form with air flowing over the powder (left of Photo 2, above) and aerated form (bottom of Photo 2, above) where air flows through the sample from top to bottom. This increases the oxygen availability for the reaction, but also removes heat from the reacting material.
For large-scale storage situations, tests are carried out on different scales so that the effect of the size of the bulk material can be assessed (Photo 3, left).
All tests are carried out in temperature-controlled ovens (Photo 4, below) that allow screening tests (with the temperature ramped up at a defined rate) and isothermal testing (with a constant temperature controlled within narrow margins). Because of the potential for violent reactions during the self-heating process, all equipment is equipped with explosion protection.
Closing comments
Many solid materials can exhibit self-heating, which can affect the quality of the product or progress to a fire or even an explosion. Whenever self-heating incidents are investigated, we find that a common root cause is a lack of understanding of the self-heating phenomena. The self-heating hazard of solid materials that are subjected to heat should therefore be determined by conducting appropriate laboratory test(s). The test(s) will be selected based on the type of heating/drying process that the solid material undergoes; for example, tray drying or fluidized-bed drying. The test results can then be used to determine safe heating/drying temperatures and durations, using sufficient safety margins.
Literature
Figure 4 - Basket test sample holder inside a laboratory oven.
Prevention of Fire & Explosions in Dryers, J.A. Abbott (Technical Editor), 2nd Edition, 1990, The Institution of Chemical Engineers, Rugby U.K.
About the author
Dr. Vahid Ebadat Ph.D., M.Inst.P, MIEE, C.Eng., C.Phys. is the Chief Executive Officer, Chilworth Asia Pacific. He has worked extensively as a process and operational hazards consultant for the chemical, pharmaceutical and food industries. Dr. Ebadat is a regular speaker at training courses on gas and vapor flammability, dust explosions, and controlling electrostatic hazards.
He is a member of NFPA 77 Technical Committee on Static Electricity, NFPA 654 Standard for the Prevention of Fire and Dust Explosions from the manufacturing, Processing, and Handling of Combustible Particular Solids and ASTM E27 Committee on Hazard Potential of Chemicals. Dr. Ebadat’s research has culminated in the publication of numerous technical articles and papers.
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