SPONTANEOUS COMBUSTION OF COAL – A SHORT REVIEW

Coal mine fire, due to spontaneous combustion of coal, is a major health and safety hazard affecting the environment by releasing toxic fumes such as carbon monoxide and sulphur dioxide and cause subsidence of surface infrastructure such as roads, pipelines, electric lines, bridge supports, buildings and homes. The spontaneous heating of coal is defined as the process by which coal starts burning by itself just by absorbing oxygen from atmosphere and not by any external fire. Some fraction of the exposed coal absorbs free oxygen at a faster rate than others which leads to oxidation with the formation of gases (CO, SO2 and CO2), water vapour and some heat during the reaction. Combustion occurs when the rate of heat generation exceeds the rate of heat loss. Two basic factors that leads to self combustion of coal are:

Intrinsic factors:

  • Coal composition, rank and petrographic constituents
  • Coal friability, particle size and surface area
  • Moisture content
  • The presence of iron pyrites
  • Volatile matter content

Extrinsic factors:

  • Climatic conditions (temperature, relative humidity, barometric pressure and oxygen concentration)
  • Stockpile compaction, as related to height and method of stockpiling
  • Dump consolidation, influenced by height, method of formation and equipment used
  • Presence of timber or other organic waste material in abandoned areas or dumps
  • Excavation stability and maintenance (for open-cut high wall faces)
  • Strata conditions, method of working and ventilation (for shallow underground workings).

The result of interactions between these factors in a given situation can lead to open fires.

Mechanism of self-oxidation

At atmospheric temperature, freshly exposed coal has affinity for oxygen of the air in contact with it. The oxygen is absorbed by coal on its surface by a purely physical process which, however, rapidly gives place to a chemical chain reaction resulting in oxidation of certain constituents of coal. Like all other oxidation reactions, the interaction of oxygen with coal is an exothermic with production of a small quantity of heat.

The oxygen absorption reaction is considered to take place as follows:

Coal + O2 → Coal-O2 complex → Oxidized coal + CO, CO2, H2O + Heat

The mechanisms involved in the oxidation of coal leading to spontaneous combustion form a complicated process, consisting of the following four overlapping stages.

First Stage: Adsorption and chemisorption of oxygen: During this stage, the coal increases in weight as the oxygen is adsorbed and heat is generated. This process rises temperature up to 70°C and may continue up to 350°C. However, the oxygen-enriched adsorption complex formed is unstable which decomposes and react further.

Second Stage: At the second stage, i.e. above 70°C, the adsorption complex decomposes, and the weight of the coal decreases. This process is common in temperature range of 80 and 150°C. The decomposition of this complex yields mainly carbon monoxide. Inherent moisture is driven off between 100 and 150°C.

Third Stage: During the third stage between 150 and 230°C, further chemical reaction leads to the formation of the stable oxygen-carbon compound, oxy-coal from unstable adsorption-chemisorption complex evolving excessive heat. This process also leads to the evolution of carbon monoxide.

Final Stage: At even higher temperatures, the formation of oxy-coal stops and combustion begins. With the sharp rise in temperature, there is a rapid decrease in weight, with the excessive formation of soot, and the coal substance begins to incinerate.

The stages can be summarised in a table as follows:

Stage Reaction Weight Temperature   (° C) Heat Generated (cal/g) Remarks
Adsorption Water adsorption Gain Any temperature 2-25 Physical process and large amounts of heat produced
Chemisorption Oxygen adsorbed forming peroxides Gain 70 2-16 Traces of water required and CO is typical product
Peroxide decomposition Decomposition of peroxides and release of water Loss 70-150 4-18 Wet spots in the stockpile are visible and steam evolves.
Oxy-coal formation Formation of stable oxygen complexes Gain 150-230 6-27 Much heat
Onset of Combustion Devolatilization and combustion Loss 230 10-58 Much Heat
Active burning Combustion Loss >230 3500-7800 Much Heat

Factors Affecting Spontaneous Combustion of coal:

  1. Intrinsic Factors:
  • Coal composition, rank and petrographic constituents: Coal properties mainly depend on their petro graphic composition and rank of coal. The petro graphic composition of coals mainly depends on facial conditions and composition of organic matter (mainly vitrinite) in a pale swamp. Lower rank coals containing higher moisture, oxygen and volatile content are more prone to self-oxidation.
  • Coal friability, particle size and surface area: The smaller the coal particle, the greater the exposed surface area in contact with the air and greater will be the tendency towards spontaneous combustion. Friable coals tend to produce coal fines which due to larger surface area have a greater tendency for spontaneous combustion.
  • Moisture content: Changes in moisture content; i.e., the drying or wetting of coal, have obvious effects on spontaneous heating of coal. A small quantity assists oxidation slightly whilst large quantities of moisture prevent the heating, although, as in a surface stockpile reserve, drying and wetting of the coal speeds up the heating process.

Dry coal + moisture → wet coal + heat

  • The presence of iron pyrites: The presence of pyrite increases the potential of coal for spontaneous combustion. Iron pyrite (FeS2) easily oxidizes by its own (especially when pyrite is more than 2%) in presence of oxygen of the air and moisture at ordinary atmospheric temperature releasing heat according to the following equation.

2FeS2 + 7O2 + 16H2O = 2H2SO4 + 2FeSO4. 7H2O + Heat

  • Volatile matter content: Volatile matter in the coal is generally easier to oxidize than non-volatile content. Hence, increase in volatile content of coal, especially combustible gases and hydrocarbons, increases the rate of oxidation.
  1. Extrinsic factors
  • Climatic conditions: Higher the oxygen concentration in the atmosphere more rapid is the oxidation procedure as oxygen is readily available. Limiting the supply of oxygen to the active surfaces of the solid matter abates the reaction well. Moisture plays an important role on behaviour of coals in stockpiles. The amount of water contained by coal for a given value of relative humidity is described by its adsorption and desorption isotherm. The interaction between water and coal can be exothermic or endothermic depend on whether the water evaporates. Higher neighbouring temperature leads to increase in oxidation process and ultimately in spontaneous heating of coal. There is a pronounced temperature coefficient of oxidation and the average rate of oxidation approximately doubles for every rise of 18 °F.
  • Stockpile compaction and other factors: Spontaneous combustion in a stockpile can be prevented by stacking the coal layer by layer with compaction of each layer because coal stacked in this manner will prevent air movement through the stockpile and also limit moisture percolation. Small particles having higher surface area oxidizes faster but flow of air is restricted in them. Large particles allow free circulation of air and oxidize slowly as heat can get dissipated. However, in case of mixture of sizes, it gets enough oxygen but the heat generated cannot be dissipated fast enough and results in heating of stockpile.

Measurement of Spontaneous Combustion

The common approaches used to predict the spontaneous combustion phenomena are based on thermal analysis. In this case, the main laboratory techniques based on the direct temperature measurement to estimate reactivity include Thermogravimetric Analysis (TGA) Differential Thermal Analysis (DTA), Adiabatic Calorimetry, Differential Scanner Calorimeter (DSC) and isothermal methods.

  1. Thermo-gravimetric Analysis (TGA): TGA experiments have shown a link between weight loss and the self heating behaviour of coal. Researchers reported the increase of weight of the sample between 20 to 300 °C when it is exposed to a heating ramp which is due to increased oxygen adsorption on coal surface. Typically, a complete oxidation process can be divided into three stages according to the changing pattern of the TG curve. In the initial stage, the specimen loses its weight due to the evaporation of the contained moisture and this stage ends at the inflection point T1. Then the specimen could experience a weight gain stage as the low-temperature oxidation occurs before ignition point T2. Point T1 and T2 are two inflection points to define the weight loss and weight gain stages. The third stage is the combustion process of the specimen during which the weight of the specimen decreases rapidly and this stage ends at Tend when all the combustibles in the coal specimen are fully consumed. The most useful part of the resulting T-G curve to assess the propensity of a coal’s spontaneous combustion is that between the inflection points T1 and T2. If a coal has the propensity, the weight gain process would be significant. On the other hand, the coal without potential for spontaneous combustion will experience no or insignificant weight gain during the temperature between T1 and T2.auto-oxidation-coal-1
  2. Differential Thermal Analysis (DTA): This technique has been used extensively to characterize thermal changes in coal samples. The usual experimental procedure of DTA exposes coals to a steady heating rate, measuring the temperature difference between the sample and a reference material heated under same conditions. Then, the difference is used to observe the heat evolution of the sample, which relate to the potential chemical reactions that are occurring. The steps are similar to that mentioned above in TGA.
  3. Adiabatic Calorimetry (R70 test): The R70 self-heating rate index, proposed by Humphreys (1981), is a measure of spontaneous combustion propensity of coal and is defined as the average temperature increase per hour in the duration for the sample’s temperature to increase from 40°C to 70°C. A reaction container holding a certain amount of coal sample is placed in an adiabatic oven. After the sample is preheat and dried in the nitrogen atmosphere, pure oxygen as the major reaction gas is passed through the sample. A thermometer is buried inside the coal sample while another is used to measure the oven temperature. Ideally, the oven temperature should be controlled to be equal to that of the coal sample to avoid heat exchange between the coal sample and its testing surrounding. Through such control, it is ensured that only and all the heat generated by the oxidation of the coal sample is used to sustain the reaction and to raise the temperature of the coal.
  4. Differential Scanner Calorimetry (DSC): The coal sample and a reference material are heated under the same thermal conditions for the same period of time, allowing the difference in energy emitted from each crucible. In this case, the difference between the amount of heat needed to increase the temperature of the sample, and the amount of heat necessary to raise the temperature of a reference material is measured as a function of temperature. Consequently, any difference is therefore directly as a result of physical or chemical transformations taking place.
  5. Isothermal Method: It is also referred to as the oxygen adsorption method, where the potential of self-ignition was designated by the amount of oxygen consumption at a constant starting temperature. At constant temperature (30 °C) and atmosphere pressure (101 kPa), 1 gram of coal sample was dried under nitrogen for 1.5 hour and then placed in oxygen flow condition to absorb the oxygen for 20 min. The more oxygen the coal samples absorbed, the higher potential of spontaneous combustion.
  6. Crossing Point Temperature (CPT) determination: Crossing point temperature (CPT) is a parameter for determination of coal’s susceptibility to self-oxidation and highlights the oxidation behaviour of coal and simulates the situation that prevails in a coal stack. A coal bed is heated in a bath at a particular rate and stream of oxygen is passed through it. The temperature gradient is maintained at 1°C/min and the oxygen flow at 60-80 ml/min. The temperature difference of bath and coal bed narrow down gradually until a stage is reached when the coal bed temperature crosses the bath temperature. This is the crossing point and beyond this stage the slow oxidation rates gain momentum. The lower is the CPT, higher is the tendency of spontaneous fire hazards.

Prevention of Spontaneous Combustion

The following steps may be followed in order to prevent spontaneous combustion of coal:

  1. Segregation of coal is to be avoided. Improper stacking might result in segregation of coal particles and hence increased propensity to oxidation.
  2. Differences in air temperature between the pile and the outside air can encourage air movement through the pile, thereby increasing oxidation. Hence, just dumping the coal in a big pile might lead to problems. Hence the stockpile needs to be compacted.
  3. It is also recommended to lightly compact the “toe” of the pile (that is, the outer bottom edge of the coal pile), which is susceptible to air flow. The compacting of this portion of the pile prevents air from flowing into or out of the bottom of the pile when the coal pile is either warmer than or cooler than ambient conditions as shown in the following figure.auto-oxidation-coal-2
  4. Any stockpile should, if practical, be rectangular in shape presenting a minimum face (width) to prevailing wind direction and the ingress (entrance) or egress (exit) ramp constructed on the windward face. It is recommended that height of a high grade uncompacted coal pile should be limited to about 15 feet while height of layered and compacted coal pile should be limited to about 26 feet.
  5. Moisture contributes to the spontaneous combustion as it aids in the oxidation process. As rewetting of coal is exothermic process, it liberates heat. Hence, measures must be taken to keep stored coal from being exposed to moisture. Moreover, dry and wet coal should be stored separately; wet coal should not be piled on dry coal and dry coal should not be rewetted.
  6. Blending of coals to achieve the correct level of sulfur content is being practiced by using high sulfur coal (high in pyrites) and blending it with low sulfur coal (high in moisture). However, blended coal can greatly increase the rate of spontaneous heating. Hence, it is recommended to store them in separate piles and blend them just before feeding them to coal bunkers.

In addition to the above factors the coal must not be stored in stockpiles for long periods. It is also necessary to prevent generation of coal dust as they may lead to explosions. To minimize dust generation due to loss of moisture from large coal particles, their surface moisture level should be maintained. For this, water should be sprayed to keep the piles damp.

Conclusion

Spontaneous combustion of coal depends on several factors. High volatile coals are susceptible to self-oxidation. Initially oxygen attacks the methylene group constituting the hydro-aromatic structure of coal which are further activated by the presence of phenolic (-OH) groups in low rank coals. The presence of deactivating groups would, on the other hand, prevent self-oxidation by offsetting the reactivity towards oxygen. The physicochemical characteristics of coal, its petrographic distribution and its mineral makeup are the endogenous factors for self-oxidation. The environmental conditions i.e. humidity of the surrounding air, ambient temperature, stockpile compaction etc are some of the exogenous factors. The interactions involving both the exogenous and endogenous factors ultimately determine the sequence of reactions leading to spontaneous fire.

Selected References

  1. Wang and Luo, International Pittsburgh Coal Conference, 2011
  2. Naktiyok , Energy Fuels, December 2017
  3. Avila, T. Wu and E. Lester, Energy Fuels 2014, 28, 1765−1773
  4. Khan, A.K. Adak, R. Sen and A. Sarkar, Indian Journal of Chemical Technology, 2013, 20, 52-56.
  5. Spontaneous Combustion of Coal by K.P. Shah

 

CONTRIBUTED BY SATIRTHA SENGUPTA