Preventing the Self-Heating of Particulate Solids During Drying Operations
Recognizing that a product in powder or granular form can self-heat is the first step in controlling the risks associated with self-heating.
Many particulate solid materials can exhibit self-heating that, if unchecked, can progress to a fire or explosion. Even in less dangerous instances, self-heating is likely to affect the output of the process through product quality or other issues. Recognizing that a product in powder or granular form can self-heat is the first step in controlling the risks associated with self-heating.
Whenever self-heating incidents are investigated, a common root cause is a lack of understanding of the self-heating phenomena. Materials such as silica, limestone, sand, cement, fly-ash, etc., are inert materials in their pure form (i.e., they will neither burn nor support combustion and do not pose a risk of fire or dust explosion). However, during the ceramic manufacturing process, other ingredients such as organic materials and metallic powders are often added to create the final product. The additional ingredients are generally in powder/dust form and may be combustible. Hence, it is important to screen representative samples to determine and document if such a mixture of materials is explosible or not.
Self-heating can arise through one of the following different mechanisms:
Exothermic (Heat-Releasing) Chemical Reaction
The chemical reactions are often oxidation reactions with air, similar to what occurs during a fire or explosion. At the start of the self-heating process, the reaction may be very slow, like steel that oxidizes (corrodes) with atmospheric oxygen to form rust.
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 without detailed experimental studies.
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 reactions may produce flammable gases, which can lead to gas/air explosions or pressure/volume explosions in closed process vessels, and also compromise product quality. The self-heating of solids and powders may also result in smoldering, which can then ignite the material or cause an explosion of dust suspensions, particularly when a “smoldering nest” is disturbed and is 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, “black spots” might be visible 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.
The exothermic onset temperature for self-heating is influenced by the materials’ chemical and physical properties, such as chemical-reaction kinetics, thermal conductivity and specific heat, and heat of reaction or heat of decomposition, as well as other factors, including:
• Dimensions and shape of the material (solid or powder)
• Ambient airflow over the material
• Availability of oxygen within the bulk (porosity)
• Additives or contaminants
• Volume of the container
The material usually has to be exposed to an air or ambient temperature that is near the onset temperature for a characteristic induction time, which is reduced by a higher temperature.
Simulating Self-Heating Behavior
Several laboratory tests have been developed to simulate the conditions where the powder could be heated above the onset temperature. In testing “bulk” conditions, air diffuses into the sample through the open top of a test cell (diameter 50 mm, height 80 mm) and through the bottom of the cell, which is closed with a sintered glass disk. The sample temperature is measured continuously at various locations along the height of the cell.
When testing in the layer form (with air flowing over the powder) and aerated form, air passes through the bulk of the product (see Figure 1). 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 container dimensions on the onset temperature can be assessed (see Figure 2). All tests are carried out in temperature-controlled ovens that allow screening tests with the temperature increasing at a specified rate, or isothermal testing with a constant temperature that is controlled within a narrow range (see Figure 3). Because of the potential for violent reactions during the self-heating process, all equipment is equipped with explosion protection.
Hazards of Refractory Powders
Many ceramics are based on non-combustible aluminum oxide, silicon dioxide or titanium dioxide and thus do not present a fire or explosion hazard. However, several other types of compounds are considered to be ceramics and are combustible; thus, they present possible fire and explosion hazards.
These materials include many metal carbides and nitrides and some metal borides and silicides. A rather large concentration of non-combustible oxides is required to render the mixtures non-hazardous. Thus, testing is advisable to determine the degree of self-heating and oxidation hazards in drying, sintering, and plasma-spraying operations.
Prevention is Key
Many solid materials can exhibit self-heating, which may affect the quality of the product or progress to a fire or even an explosion. The self-heating hazard of solid materials that are subjected to heat should be determined by conducting appropriate laboratory test(s). The test(s) should be selected based on the type of heating/drying process that the solid material undergoes (e.g., tray drying, fluidized-bed drying, spray drying or sintering). The test results can then be used to determine safe heating/drying temperatures and durations using sufficient safety margins.
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For Further Reading
• Prevention of Fire & Explosions in Dryers, ed. J.A. Abbott, 2nd Edition, 1990, The Institution of Chemical Engineers, Rugby, UK.
• Bretherick’s Handbook of Reactive Chemical Hazards, ed. P. G. Urben, 6th Edition, 1999, pages 127, 226, 360, and 382.
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