10.1021 acs.oprd .0c00459

Self-heating reactions present a significant challenge in process safety, requiring thorough evaluation and analysis to prevent potential hazards. These reactions, which generate heat as they progress, can escalate into uncontrolled fires, explosions, or thermal runaway if not appropriately managed. Incidents of self-heating in processing equipment like dryers, dust collectors, silos, and FIBCs (super sacks) have resulted in numerous fires and explosions in various industries.

To effectively assess potential risks and implement appropriate safety measures, it is crucial to understand the underlying mechanisms and kinetics that govern self-heating reactions. Two widely recognized theories, namely Semenov theory and Frank-Kamenetskii theory, offer different approaches for analyzing and studying self-heating reactions.

Semenov Theory

Semenov theory, developed by Russian scientist Nikolay Semenov, provides a detailed understanding of self-heating in gas and low-viscosity liquid systems. The theory is based on the following three assumptions that restrict its applicability:

  1. Spatially uniform temperature within the reacting body. This assumption implies that either the material of the body is well stirred, such as in a liquid or gas system, or the resistance to heat flow within the body is significantly lower compared to that within the container or boundary. In the latter case, there would be a temperature discontinuity at the boundary of the material. This assumption is a good approximation in deliberately stirred fluids.
  2. The heat generation reaction can be simplified to a single chemical reaction. This approximation is often valid, especially when a “lumped” or empirically determined rate law has been independently measured. This empirical approximation proves to be quite accurate in many cases that do not involve single-step reactions.
  3. Both the heat of reaction and activation energy are sufficiently high to facilitate self-heating and the resulting ignition behavior. These assumptions are based on the intuitive understanding that without any heat of reaction, self-heating would not be possible. Similarly, without any activation energy (the rate at which the reaction accelerates with an increase in temperature), ignition would not occur.

Frank-Kamenetskii Theory

While Semenov theory focuses on the self-heating problems in well-stirred systems, the Frank-Kamenetskii theory, proposed by Soviet scientist Zeldovich Frank-Kamenetskii, highlights the importance of thermal conductivity heat transfer inside the self-heating materials. This theory offers a fundamental understanding of the thermal processes involved in exothermic reactions, especially when heat transfer governs the reaction rate. As a result, it can be practically applied to self-heating systems that are solid or highly viscous, as it takes into account the temperature distribution within the materials. Thus, only are the assumptions 2 and 3 of the Semenov theory applicable to the Frank-Kamenetskii theory.

Both the Semenov theory and the Frank-Kamenetskii theory revolve around key parameters such as heat generation, which is related to reaction kinetics and reaction heat, and heat loss, which is related to factors like heat transfer area, heat transfer coefficient, and surrounding temperature. If the rate of heat generation exceeds the rate of heat removal, a self-heating event has the potential to escalate into a runaway or ignition event. The critical condition, which represents the well-defined boundary between these two behaviors, plays a pivotal role in this domain, both conceptually and pedagogically.

The critical condition is a crucial set of parameters that greatly impact the behavior being studied. These parameters include the surrounding temperature, the size and shape of the material being analyzed, and the temperature of the material itself, as below:

  1. Critical Ambient Temperature (CAT): CAT is conceptually similar to the Self-Accelerating Decomposition Temperature (SADT). It refers to the lowest surrounding temperature at which thermal runaway or ignition could occur for a given size and shape of self-heating material. Alternatively, it can also be defined as the highest surrounding temperature at which thermal runaway or ignition would not occur, or the average value between the two conditions.
  2. Critical Size and/or Shape: In situations where a varying amount of material is stored in a constant temperature environment, the critical size or critical diameter of the body is determined. This refers to the maximum size of the material at which thermal runaway or ignition would not occur, or the minimum size at which thermal runaway or ignition would occur, or the average value between the two conditions.
  3. Critical Stacking Temperature (CST): The critical stacking temperature refers to the maximum temperature at which a material stack can be safely maintained, without the risk of thermal runaway or ignition. This temperature is influenced by various factors including the properties of the material, its thermal characteristics, potential for degradation, and chances of exothermic reactions. It is crucial to take into account both the chemical reactions happening within the material and the heat transfer mechanisms involved when determining the CST.

Understanding self-heating and these theories behind enables industries to assess the potential risks associated with self-heating reactions, implement appropriate safety measures, and design effective control systems. By considering factors such as temperature control, reactant composition, ventilation systems, and heat transfer mechanisms, the likelihood of self-heating incidents can be significantly reduced, ensuring a safer working environment.

Why perform self-heating evaluation and analysis:

By incorporating self-heating evaluation and analysis into process safety practices, industries can significantly reduce the risk of fires, explosions, and accidents. Proactive identification and mitigation of self-heating hazards not only protect personnel and assets but also bolster compliance with safety regulations. Ultimately, such efforts contribute to a safer working environment, improved operational efficiency, and the preservation of a company’s reputation. Embrace self-heating evaluation as a vital component of your process safety strategy and unlock a more secure and productive future.

Why work with Prime Process Safety Center?

Prime Process Safety Center, a leading provider of process safety consulting and testing services, extends its expertise to self-heating evaluation and analysis, empowering industries to navigate the intricacies of exothermic reactions and safeguard their operations.

By prioritizing self-heating evaluation and analysis, Prime Process Safety Center helps industries achieve operational excellence, safeguard their reputation, and contribute to a safer and more sustainable world. Through its expertise in self-heating phenomena, Prime Process Safety Center empowers industries to navigate the intricacies of exothermic reactions, ensuring safety, efficiency, and environmental responsibility, paving the way for a future where exothermic reactions are harnessed for innovation, not destruction.


1. What is self-heating?

Self-heating refers to the process by which a material, typically organic or inorganic solids, undergoes an exothermic reaction and generates heat internally. This heat generation can lead to an increase in temperature, potentially causing a fire or explosion.

2. Why is self-heating evaluation important?

Self-heating evaluation is crucial in process safety because it helps identify materials that may have the potential for self-heating and assess their potential ignition risks. It allows industries to develop appropriate control measures to prevent incidents related to self-heating.

3. How is self-heating evaluation conducted?

Self-heating evaluation involves analyzing the properties and characteristics of materials such as their chemical composition, environmental conditions, heat capacity, and reaction kinetics. Usually, oven-basket test with varying basket sizes is used to deduce the lump-sum kinetic parameters and physical properties. This information is used to evaluate the likelihood and severity of self-heating.

4. Who should perform self-heating evaluations in industries?

Self-heating evaluations are typically conducted by process safety experts, such as chemical engineers, industrial hygienists, or safety consultants who are knowledgeable in the properties and behaviors of hazardous materials.

5. How frequently should self-heating evaluations be carried out?

The frequency of self-heating evaluations depends on various factors, including the type of materials being handled, their storage and handling conditions, as well as any changes in the process or material that may affect self-heating characteristics. It is generally recommended to conduct evaluations regularly or when changes occur.

6. What control measures can be implemented to mitigate self-heating risks?

Control measures for self-heating risks may include maintaining adequate ventilation, controlling temperature and humidity, implementing appropriate storage and handling practices, conducting regular inspections and preventive maintenance, and implementing a robust monitoring system such as  carbon monoxide gas monitoring.

7. What are some common materials prone to self-heating?

Common materials prone to self-heating include oily rags, certain chemical compounds containing unsaturated bond or peroxides, coal, biomass, vegetable oils, and greases. These materials can undergo exothermic reactions and generate heat if not managed properly.

8. How can self-heating evaluations help in preventing incidents?

By identifying materials prone to self-heating and assessing their risks, industries can implement preventive measures such as proper storage, ventilation, and monitoring, reducing the likelihood of incidents caused by self-heating.

9. Can self-heating be completely eliminated?

Complete elimination of self-heating may not always be possible due to inherent properties of certain materials or operational requirements. However, through diligent evaluation, control measures, and monitoring, the likelihood of self-heating can be significantly reduced.

10. How can self-heating evaluation contribute to regulatory compliance?

Self-heating evaluations ensure compliance with safety regulations by identifying and assessing self-heating risks. It helps industries adhere to standards and guidelines set by regulatory bodies that mandate the safe handling and storage of materials prone to self-heating.