Introduction
Spontaneous combustion of coal is a serious natural disaster in the world’s coal industry. Coal when reacts with atmospheric oxygen, undergoes an oxidation reaction which is highly exothermic in nature. If the heat released during this process is not dissipated, the temperature rises continuously resulting in fire and this phenomenon is known as spontaneous combustion of coal. Mine fire due to spontaneous combustion of coal is a serious hazard that develops during mining and storage of coal. It leads to environment pollution, huge economic loss and personal injury.Spontaneous combustion of coal mainly depends on two factors – intrinsic and extrinsic. Intrinsic parameters are mainly related to the inherent nature of coal, like physico-chemical characteristics, petrographic distribution and mineral make up whereas the extrinsic parameters are associated with geological, atmospheric and mining conditions. Many researchers worked on understanding the mechanism of spontaneous combustion using R70 test, crossing point temperature determination etc. In this study, we tried to determine the coal’s self-heating potential using thermo-gravimetric analysis by taking into consideration two inflection points T1 and T2 which denotes the moisture loss temperature and ignition point temperature respectively, the latter being preceded by a weight gain stage.
A typical thermo-gravimetric experiment curve
Results and discussions
Three bituminous coals samples were taken for TGA analysis and heating rate was varied at 1°C/ min, 5 °C/ min, 10 °C/ min and 15 °C/ min, keeping the gas flow rate of zero air and nitrogen at 20 ml/min and sample weight at 10-20 mg.
Sample 1
Sample 2
Sample 3
Red- 1°C/min, Blue- 5°C/min, Green-10 °C/min, Cyan-15°C/min
Table showing the shift in moisture loss temperature and ignition point temperature with heating rate
| Sample | Heating rate(°C/min) | T1 (°C) | T2 (°C) |
| 1 | 1 | 120.28 | >200 |
| 5 | 140.21 | >200 | |
| 10 | 145.36 | >200 | |
| 15 | 150.45 | >200 | |
| 2 | 1 | 124.61 | >200 |
| 5 | 145.82 | >200 | |
| 10 | 148.21 | >200 | |
| 15 | 144.63 | >200 | |
| 3 | 1 | 110.31 | >200 |
| 5 | 130.35 | >200 | |
| 10 | 136.42 | >200 | |
| 15 | 145.25 | >200 |
All the three bituminous coal samples initially showed a weight loss due to moisture evaporation and gas desorption with increase in temperature denoted by T1. The second stage reflects an increase in weight because of oxygen absorption. Water evaporation and gas desorption caused an increase in the number of pores and free surfaces. As a result the rate of oxygen absorption increases. Thus, the weight of coal gradually increases. When the weight reaches its peakthe coal begins to burn, the corresponding temperature is denoted as T2, ignition point temperature. From the data it can be predicted that the coal samples have very less propensity to undergo spontaneous combustion as the ignition point temperature is well above 200 °C. Moreover, the heating rate has very little influence on the two temperatures.
Conclusion
In this work we have established a method based on thermogravimetric analysis that gives sensitive, reliable and reproducible results related to coal’s propensity to undergo spontaneous combustion. We have identified two temperatures namely moisture loss temperature and ignition point temperature (which is preceded by a weight gain stage) which are good predictors of spontaneous combustion. If a coal has the propensity to undergo self-combustion, the weight gain process would be significant and ignition temperature would be low. Thus TGA provides an easy and convenient method, which can be satisfactorily used in the near future, to determine the self heating potential of coal.
CONTRIBUTED BY DR. SATIRTHA SENGUPTA AND DR. ARIJIT GOSWAMI UNDER THE GUIDANCE OF PROFESSOR BARUN KUMAR GUPTA