Many solid materials can exhibit self-heating, which — if unchecked — can progress to a fire or even explosion. And even if the situation does not get that far, it is likely to affect the output of the process, in terms of product quality degradation, for example. Recognizing that your 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, we find that a common root cause is a lack of understanding of self-heating phenomena. This article provides an introduction to self-heating phenomena and suggests measures to control this type of ignition source.
What is self-heating?
Not all particulate solids that are classified as combustible dust (in other words, pose a dust explosion hazard) will self-heat at normal processing temperatures, and conversely, some of the materials that do self-heat react too slowly to pose a dust explosion hazard. Some materials can self-heat at ambient temperatures, especially in large-scale storage, but for most materials the hazards arise when they are heated.
Self-heating can arise by one of two different mechanisms: by exothermic (heat releasing) chemical reactions and by exothermic decomposition. The chemical reactions are often the same as what occurs during a fire or explosion: an oxidation reaction with the oxygen in the air. At the start of the self-heating process, the reaction is very slow, like steel that oxidizes with atmospheric oxygen to form rust. Decomposition happens in a material that is unstable, and the material will fall apart while releasing heat. A significant difference between the two mechanisms is that decomposition does not require additional reactants and is therefore largely independent of the environment, while an oxidation reaction only happens if certain conditions are present, making it more difficult to predict its occurrence without detailed experimental studies.
What happens in self-heating?
Step 1. Rate of heat generation exceeds rate of heat loss.If a material undergoes an exothermic chemical reaction (or multiple reactions) or decomposes exothermically, the temperature of the material will rise due to the heat released from the exothermic reaction or decomposition. In the meantime, some of the heat is lost to the environment. If the rate of heat loss exceeds the rate of heat generation, the temperature of the material will be the same as the ambient temperature, otherwise, it will increase. Due to the poor thermal conductivities of many solids, a large portion of the re- action heat is retained in the powder.
Step 2. Resulting temperature rise further increases chemical reaction rate exponentially. The temperature rise of the material due to the exothermic reaction will further increase the chemical reaction rate, which in turn will cause the temperature to increase further. The increase of material temperature also results in an increase in the rate of heat loss. However, the rate of heat loss increases linearly with temperature, while the chemical reaction rate, and thus the heat generation rate, increases exponentially with temperature. Consequently, the heat generation rate will exceed the rate of heat loss and the temperature of the material will rise higher. This 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.
Potential smoldering. Self-heating of solid materials usually results in smoldering, which can set the material on fire or cause dust explosions, particularly when the smoldering material is disturbed and exposed to air (Figure 1). Many plants that experience self-heating incidents have 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 may be found, a so-called “smoldering nest”. It is important to recognize such occurrences as indications of a potentially serious problem, rather than to learn to live with it.
Potential release of flammable gases. Self-heating reactions may also produce flammable gases, which may lead to gas explosions in process vessels or compromise product quality (Figure 2).
Testing self-heating behavior
The exothermic onset temperature is influenced not only by the chemical and physical properties, such as chemical reaction kinetics and heat of reaction, but also by other factors, including the following:
• Dimension and geometry of the solid bulk
• Ambient airflow
• Availability of oxygen in the bulk
• Additives and contaminants
Usually, the material has to be exposed to an elevated temperature for a period of time before it self-heats. This time is referred to as the induction time, which is dependent on temperature; and usually the higher the temperature, the shorter the induction time will be.
Because of the influencing factors mentioned, a single test is usually unable to predict self-heating behavior for all different drying and storage conditions. Instead, separate tests have been developed to simulate the conditions where the powder is in bulk form (Figure 3), layer form (with air flowing over the powder; Figure 4) and aerated form (Figure 4), where air is passing through the bulk of the product, increasing the oxygen availability for the reaction and also helping to remove heat from the reacting material. For large-scale storage situations tests are carried out at different scales so that the effect of the size of the bulk material can be assessed (Figure 5). All tests are carried out in temperature-controlled ovens (Figure 6) 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 testing equipment needs to be fitted with explosion protection.
| Figure 6. This “basket test” sample holder is prepared for testing inside
a laboratory oven
Learning from a real incident
In one incident, the powder in a fluidized bed dryer caught fire when the powder conveyer in the dryer was turned off in order to fix clogging in an upstream wet-product conveyer. During this period, the hot air supply was continued.
A screening test was conducted to determine whether this powder could self-heat under the conditions that existed in the dryer before the incident. An typical powder sample was placed in a temperature-programmed oven and the temperature of the oven was increased from 20 to 400°C at a rate of 0.5°C/min (Figure 7). The exothermic onset temperature was identified to be 166°C, which was lower than the hot air temperature of the dryer. The result indicated that self-heating was the most probable ignition source for this incident.
This test provides a useful tool for a quick identification of the self-ignition hazard of materials and should be conducted for materials whose thermal stability characteristics are not known. However, to be of any practical value, the test results obtained in a laboratory-scale apparatus have to be scaled up to plant-size process. In order to establish the relationship between the exothermic onset temperature and powder layer thickness with the aid of thermal explosion theory, the sample of the powder that was being dried in the same incident batch was tested at three different layer thicknesses. In each trial, the powder sample was exposed to a constant temperature to test if self-heating would actually take place. The highest temperature at which self-heating did not occur and the lowest temperature at which self-heating did occur were determined. The average value of these two temperatures was taken as the exothermic onset temperature for each powder layer.
The exothermic onset temperatures were used to determine the unknown constants of the following equation, which expresses the relationship between the exothermic onset temperature and the thickness of the powder layer:
Ta = Exothermic onset temperature for a powder layer, K
r = One half of the powder layer thickness, m
δc = Frank-Kamenetskii-parameter, dimensionless
M, N = Constants determined by properties of the powder material
Using Equation (1), the exothermic onset temperatures of layers of the powder at different thicknesses were calculated and the results are plotted (Figure 8). As the thickness of the powder layer was increased from 1 to 12 in., the exothermic onset temperature decreased by 48°C.
The powder layer in the dryer before the incident was on the order of 4–8 in. This suggests that the exothermic onset temperature for the powder layer was well below the hot air temperature. Ignition would occur if the heating time exceeded the induction time.
The self-heating hazard of solid materials to be dried should be determined. Depending on the drying process, the solid materials can be tested in different shape, heating environment, with or without an airflow through the material and an airflow at the surface of the material. The exothermic onset temperatures can be used to determine safe drying temperatures using sufficient safety margins. However, the relationship between the exothermic onset temperature and the dimension of solid bulk is often needed in order to design a safe drying process. This relationship can be established from self-heating experiments based on thermal explosion theories, as demonstrated by the example introduced above.
J.A. Abbott (technical ed.), “Prevention of Fire & Explosions in Dryers”, 2nd ed., The Institution of Chemical Engineers, Rugby U.K. 1990.
Vahid Ebadat is the CEO of Chilworth North America (Chilworth Global, Princeton, NJ; Phone: 609-799-4449; Email: safety-usa@ chilworthglobal.com; Website: www.chilworth.com). He holds a B.S. in electrical engineering and a Ph.D. from Southampton University. He has worked extensively as a process and operational hazards consultant for the chemical, pharmaceutical and food industries. 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. Ebadat’s research has culminated in the publication of numerous technical articles and papers.
Pieter Zeeuwen, is a senior process safety specialist at Chilworth Global. He holds a M.Sc. in applied physics from Eindhoven University of Technology and has more than 30 years experience in the gas and dust explosion fields, including materials testing, small and large scale explosion research, and consultancy for industry and government agencies in a number of countries. His areas of expertise include gas and dust explosion hazard assessment, gas and dust explosion prevention and protection, electrostatic hazard assessment, hazardous area classification, and gas cloud explosions as well as incident investigations. Over the years, Zeeuwen has served on many working groups including various standards committees, both nationally and internationally, for instance, most recently CEN (European Standards Committee) working groups on explosion protection methods and on test methods. He regularly lectures on various aspects of explosion safety and acts as seminar chairman and course director. Zeeuwen has published numerous articles in scientific journals and presented many papers at international conferences.
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