Figure 1. A test cell for bulk conditions measures 1.96" (50 mm) dia. and 3.15" (80 mm) high. Air diffuses into the sample through the open top of the cell and through the bottom of the cell, which is closed with a sintered glass disc. The sample temperature is measured continuously at various locations along the height of the cell.
Many particulate solid materials can exhibit self-heating that, if unchecked, is likely to affect the quality of the product (at the least) or, more dangerously, progress to the point of fire or explosion. Self-heating can arise by one of the following different mechanisms:
Exothermic (Heat-Releasing) Chemical Reaction. These chemical reactions often are 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 slow, like steel that oxidizes (corrodes) with atmospheric oxygen to form rust.
- Exothermic Decomposition. 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 such as sawdust, coal, sewage sludge and grain can self-heat at ambient temperatures and spontaneously ignite in large-scale storage.
When a material undergoes exothermic chemical reactions 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.
Figure 2. Various test cells for thermal stability testing are shown. The top left example is for aerated conditions, where air flows through the sample from top to bottom, which are both closed by sintered glass discs. The cylindrical section has the same dimensions as the bulk test cell. The top right example is a wire basket for basket testing. The example on the bottom is a sample tray for “air over layer” testing, where warm air flows over the powder layer.
Self-heating of solids and powders may result in smoldering, which can set the material on fire or cause dust explosions, particularly when a “smoldering 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 also may produce flammable gases that can lead to gas/air explosions or pressure/volume explosions in closed process vessels. These gases also compromise product quality.
Laboratory Tests to Simulate Self-Heating Behavior
Several laboratory tests have been developed to simulate the conditions where the powder could be heated above the exothermic onset temperature. Examples include:
Bulk form (figure 1).
Layer form, with air flowing over the powder (figure 2).
Aerated form (figure 2), where air is passed through the bulk of the product. This increases the oxygen availability for the reaction, but it also removes heat from the reacting material.
Figure 3. The basket test sample holders are used for testing at different scales, allowing extrapolation to large-scale storage conditions. The baskets typically have sides of 0.98, 1.96 and 3.94" (25, 50 and 100 mm).
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 (figure 3). All tests are carried out in temperature-controlled ovens (figure 4). They allow screening tests with the temperature ramped up at a defined rate as well as isothermal testing with a constant temperature controlled within narrow margins. Because of the potential for violent reactions during the self-heating process, all equipment has explosion protection.
Figure 4. The basket test sample holder is prepared for testing inside a laboratory oven.
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 tests. The tests 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.