Prevention and Control Module for Spontaneous Combustion of Coal at Coal Yards

Posted on November 08, 2011
Posted By: S. Deepak Kumar
Spontaneous combustion of coal is a common concern within the coal stockyard of thermal power plants due to the direct effect that energy losses have on financial performance. As coal is the primary fuel for a thermal power plant, adequate emphasis needs to be given for its proper handling and storage. It's also essential because of related safety and environmental implications of spontaneous combustion of coal. The problem is a natural phenomenon that is aggravated by improper management of coal. A lot of research has been done on spontaneous combustion of coal but still there is a lack of a comprehensive approach which can be applied to deal with the problem.

The costs involved due to the energy and quantity loss of coal as an outcome of self combustion have been well established in the past and shown to be considerable. Also accelerated weathering due to prolonged self heating leads to loss of the gross calorific value of coal, and is considerable.

The objective of the article is to compile prevention strategies and provide a holistic ready reference methodology to reduce the loss of reserves from coal stockyards and significantly decrease environmental pollution occurring because of the phenomenon.

For better understanding of the process, vital factors contributing to the phenomenon of spontaneous combustion of coal has been discussed.

Spontaneous combustion of coal is the process of self heating resulting eventually in its ignition without the application of external heat. Coal when exposed to air absorbs oxygen at the uncovered surface. Some fraction of the exposed coal substance absorbs oxygen at a faster rate than others and the oxidation results in the formation of gases. Mainly CO, CO2, water vapor along with the evolution of heat during the chemical reaction. If the rate of dissipation of heat is slow with respect to the evolution of heat by oxidation there is a gradual build up of heat and temperature reaches the ignition point of coal thereby causing fire.

Favorable conditions for spontaneous heating are accumulation of heat caused by a rise in temperature and hence an increase in the reaction rate. Although, at ambient temperature, the reaction can be so slow that it is unnoticed, when heat accumulates the temperature is raised and, the reaction rate increases exponential. The increased rate of reaction can be described by Arrhenius law,

There are number factor which contribute to the process of spontaneous combustion of coal. The most important parameters involved in the process of spontaneous combustion of coal are:

Factors inherent to coal:

  • Size of the coal particles and surface area

  • Moisture content

  • Coal composition, quality and rank of coal

  • Heat conductivity of the particles

Extrinsic conditions:

  • Degree of compaction

  • Temperature

  • Barometric pressure

  • Oxygen concentration

  • Dimensions and Shape of stockpile

A three-stage approach is proposed and is centered on the use of concepts like risk identification, sensing, spotting, monitoring, control and stockpile management. In summary, this methodology will ensure that coal in the stockyard is used in the most efficient and effective manner, concentrating effort on areas where large scope of improvement exist.

Process Overview

Stage 1: Prevention -- Mitigating the effects of factors responsible for the augmentation of spontaneous combustion of coal:

  • Proper selection of particle size distribution. If the stockpile is of mainly medium to fine particle then making it finer or adding fines will help. If the pile consists of coarse particles of relatively low reactivity then removing the finer material will help.

  • Avoiding segregation. Improper stacking might result in segregation of stockpiles and hence increased propensity to oxidation.

  • Proper attention must be given to the preparation and maintenance of the stockpile. Pile maintenance might be crucial in avoiding channels where air can easily creep into the dump.

  • Proper compaction. Air circulating within the stockpile should be restricted by proper compacting and dozing off.

  • Moisture contributes the spontaneous combustion as it aids in the oxidation process. Moisture content should be limited to about 3 percent to avoid enhanced oxidation. Measures must be taken to keep stored coal from being exposed to moisture.

  • Dimensions of stockpile. Size and area of stockpile should be based not only on estimated tonnage but also on design principles of stockpile management. Proper dimensioning of stockpiles helps negate weathering of coal. Just dumping the coal in a big pile might lead to problems. Rather coal should be packed in horizontal layers of about 1.3 to 3 feet high followed by leveling and compaction by dozers. It helps in evenly distribution of coal thus avoiding segregation of fine coal. Pile unlayered, uncompacted high grade coal should be limited to about 15 feet and while layered and packed coal pile height should be limited to about 26 feet.

  • Use of protective covering. Inert covering material such as tarpaulin sheets with sufficient heat resistivity can be used to cover the openly kept stockpile to reduce the loss of calorific value and further oxidation of coal. It helps in cutting off oxygen to come in contact with coal.

Stage 2: Monitoring -- An early warning system to prevent the onset of fire remains the best and most reliable solution. Stockpiles are monitored initially to check temperature, presence of gases and sufficient compaction. Following methods are proposed for the monitoring process:

  • Temperature recordings. Calibrated thermocouples can preferred over fixed probe thermometers as these tend to disrupt because of routine stockpiling activities. Temperature below 50°C can be assumed considerably safe. Temperature level at the range of 70°C and more should seek for regular monitoring and deployment of control measures.

  • Continuous thermographic monitoring. Where a storage facility is large and difficult to access, a continuous and fully automatic monitoring device can be employed. A fixed thermal imaging system located at a suitable viewing point can successfully monitor over a large area, and importantly detect hot spots. A scanning pan-and-tilt unit adds even greater flexibility to cover a larger area.

  • Compaction testing. Dynamic cone penetrometer can be checked for consistent compaction. Also, one of the better ways to measure the compaction of piles is to employ densimetric tests on to find below surface voids on air-dry basis. Voids below 15% are considered ideal and voids from 15 to 20% are generally considered as acceptable.

  • Gas tests. To find the inertness of the surroundings and extent of depletion of oxygen subsurface gas tests are useful. About 50 ppm of Carbon monoxide above the residual carbon monoxide level indicates that internal temperature is in excess of 70°C.

Stage 3: Controlling -- Monitoring is essential as a first measure for controlling spontaneous combustion of coal. Control measures need to be implemented when the temperature of the stockpile reaches to the level of 60°C -- 70°C:

  • After the first indication of presence of hotspot, earth moving equipment can be used to excavate the hot material. The excavated material should then allowed to cool and re-compacted accordingly.

  • Use of sealant. The sealant combines an inhibitor of coal oxidation, (CaCl), with a binding agent and filler, (bentonite). Most of the known oxidation inhibitors have poor stability when applied as either a coating on coal surfaces or filler in cleavages and fissures in pillars. Clays such as bentonite flow readily into cracks, and swell and fill the cracks. Therefore bentonite and CaCl are homogenized to form a mixture having long-term stability.

  • Inert gas flow. Inert gases can be used to cool off the temperature of the surface. The favored gas is nitrogen as it is cheaper and more easily expelled from the mine after the heating has been controlled than is carbon dioxide. Out of all the methods it least effective and most expensive.

  • Water sprinklers. Though use of water should be carried out sensibly as there is always risk of production of water gas by the mixture of carbon monoxide and hydrogen. Water should not used in thick jet. Sprinklers are suggested instead of water jets.

Taken together, these actions can and will help in saving coal as a commodity. Implementation of these kinds of systematic proactive programs should be the first step in changing the way we use our increasingly scarce sources of energy.

This methodology provides a comprehensive approach to control spontaneous combustion of coal and would make good targets for further investigation. Coal costs represent a large percentage of operating budgets of power plants. Therefore, Coal savings in power plants provide a real opportunity to improve the financial performance of the overall organization.


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Authored By:
S. Deepak Kumar is doing his Master's from Indian Institute of Forest Management in Environmental and Development Management. He also holds a Bachelor's degree in Mechanical Engineering from National Institute of Technology, Raipur, India. He has been involved in projects related to energy efficiency and environmental compliances in Reliance Infrastructures Ltd and NTPC Ltd.

Other Posts by: S. Deepak Kumar

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