Did you know that coal can burn on its own without an external heat source? It’s a natural process known as coal self-combustion, and it poses serious risks to both safety and the environment. What you need to know and how you can protect your coal in stockpiles, mines, or even during transportation in a shipment.
🌡️ The science behind self-combustion
Coal contains various organic and mineral components. Despite low ambient temperatures, coal oxidizes over time when exposed to air, generating heat. This self-generated heat can lead to coal ignition. Depending on the conditions, including exposure to air and moisture content, this warming-up process can result in a fire. In addition, the lower-grade coals, the higher susceptibility to spontaneous combustion.
Spontaneous coal combustion is a complex process of various chemical and physical reactions between coal and oxygen. Several factors influence this phenomenon, including coal-related factors (such as coal rank,volatile substances, moisture, surface area, and sulfur content), geological factors (like coal seam slope, thickness, cracks, and fissures), and mining factors (including advancing speed, pillar and roof conditions, mining methods, ventilation, and old roadways), environmental conditions (dry and rainy seasons, wind and ambient temperature), storage conditions (cover and uncover, stockpiles or silos). This process evolves gradually, characterized by exothermic reactions, heat accumulation, and warming, ultimately leading to combustion.
🔥 Stages of Self-Combustion
The entire combustion process can be divided into three phases: Heat Generation rate, Evaporation Moisture and rapid temperaturee rise.
The inital stage or Heat generation consists on the partial temperature incease producing volatiles and releasing some moisture. After that occurs the evaporation of the moisture and production of CO and CO2. Later a rapid temperature raise ocurrs producing some methane and smokes generation.
It may be observed that spontaneous combustion never initiates on the surface of a stockpile; instead, it consistently occurs at depths ranging from 60 cm to 3 meters (depending on the pile’s size). This phenomenon can be explained by two factors. Firstly, at the surface, the rate at which heat is dissipated exceeds the rate at which heat is generated. Secondly, wind that enters the stockpile evaporates water from the lower portions of the pile, creating preferential channels (reminiscent of a chimney effect) through which this water can flow. This results in further condensation of the water somewhere below the surface of the pile, and the heat generated during this condensation is absorbed by the coal, leading to the formation of hot spots.
🌧 Factors Leading to Self-Combustion
High moisture content
High sulfur content (pyrites oxidize leading self-combustion)
Poor ventilation
High exposure of coal stockpiles to intermittent rains can exacerbate
🛡️ Preventive Measures
Frequent temperature monitoring:Temperature readings between 60 cm and 3 meters below the surface are crucial it is highly recommended to use thermo-probes. Having infrared cameras is also useful. A reading of around 50°C indicates imminent self-combustion.
Adequate ventilation: Avoid excess wind by installing wind barriers if needed.
Moisture control
Proper stockpile management: Ensuring stocking method, cover storage, FIFO management and best practices to avoid coal deterioration.
Coal quality control
Isolate coal from heat sources
Train operators to recognize and respond to fires
Using chemical aditives (Encapsulating agents) to control fires in silos, in the case of PRB Coals.
Understand and socialize the Coal MSDS to coal handling in stockpiles, trucks and shipments.
Some references for futher detail:
Diaconu, B.M.; Cruceru, M. and Popescu, L.G. Mathematical modelling of the coal stockpile self-heating process.A case study “Constantin Brâncuşi” University from Targu Jiu.ISBN: 978-960-474-284-4
Hossfeld, R.J. and Hatt, R.”PRB Coal degradation – Causes and cures.
Ozdeniz, H., Sivrikaya, O., Sensogut, C. (2014). Investigation of Spontaneous Combustion of Coal in Underground Coal Mining. In: Drebenstedt, C., Singhal, R. (eds) Mine Planning and Equipment Selection. Springer, Cham. https://doi.org/10.1007/978-3-319-02678-7_61