There has been an increase in the identification of cases of coal workers' pneumoconiosis (CWP) in recent years around the world. While there are a range of possible explanations for this, studies have implicated the pyrite content of coal as a key determinant of CWP risk. However, experimental studies to support this link are limited. The aim of this study was to assess the association between the pyrite content, and subsequent release of bioavailable iron, in coal particles and the response of lung cells involved in the pathogenesis of CWP (epithelial cells, macrophages and fibroblasts). Using real-world Australian coal samples, we found no evidence of an association between the pyrite content of the coal and the magnitude of the detrimental cell response. We did find evidence of an increase in IL-8 production by epithelial cells with increasing bioavailable iron (p=0.01), however, this was not linked to the pyrite content of the coal (p=0.75) and we did not see any evidence of a positive association in the other cell types. Given the lack of association between the pyrite content of real-world coal particles and lung cell cytotoxicity (epithelial cells and macrophages), inflammatory cytokine production (epithelial cells, macrophages and fibroblasts), and cell proliferation (fibroblasts) our data do not support the use of coal pyrite content as a predictor of CWP risk.

The Leigh Creek Coal Mine, located approximately

Laboratory testing was undertaken to examine the spontaneous combustion propensity of some of the coal seams being mined at Leigh Creek. Tests indicated that while carbonaceous rocks retained a fuel load, in isolation, these did not have the capacity to reach thermal runaway. However, the potential existed for heat from another source, such as coal, to raise the temperature of these rocks to above the threshold for thermal runaway (>100 °C). The mine closure plan submitted to the regulator (Department of Premier and Cabinet, South Australia) incorporated a monitoring trial of the selected spontaneous combustion management rehabilitation strategy to demonstrate its effectiveness. The strategy included reducing batter slopes of waste spoil piles and the application of an inert cover. The trial was established in June 2017 in a location with active combustion immediately prior to rehabilitation treatment. Measurements of temperature and oxygen concentrations within the spoil pile over twelve months show that oxygen is consumed within 1 m of the outer surface of the waste, while maximum spoil pile temperatures have been decreasing, indicating a net heat loss from the trial spoil pile area. No spontaneous combustion outbreaks have occurred in the trial area since the trial commenced. Characteristics of the trial area materials, the management strategy, and outcomes from the field trial measurements are presented.

Coal spontaneous combustion continues to pose a significant hazard to mining operations. It is a complex process that ranges from low-temperature oxidation in the normal mine environment to thermal runaway once temperatures exceed 120 degrees C. At the thermal runaway stage, the coal becomes dry locally after moisture liberation and evaporation and a well-defined hot spot forms. This process takes place over a period of time, which can be referred to as the incubation period. Assessing the spontaneous combustion hazard likelihood has normally relied on the use of laboratory testing to produce index parameters that give a propensity rating. These are often a single value on a relative rating scale, which gives no indication of the nature of the coal self-heating with respect to time as it would occur under various mine site conditions. To overcome this deficiency it is necessary to consider the incubation behaviour of the spontaneous combustion process, particularly as it relates to coal self-heating at initial mine ambient temperatures. This has recently been achieved using adiabatic oven testing, which shows the modifying influences of moisture content, initial start temperature, seam gas content and reactive pyrite content on the low temperature coal self-heating rate. The incubation testing procedure is able to determine whether self-heating can reach thermal runaway and if so in what timeframe this can take place for the environmental conditions present at the mine site.

The same coal at different moisture contents will reach thermal runaway at different times. There is also the possibility that if the coal has a sufficiently high moisture content it cannot reach thermal runaway due to the moisture moderating effects combined with the non-Arrhenius kinetics behaviour at low ambient temperatures. The moisture removal and evaporation processes that occur during coal self-heating are remodeled in this paper based on the definitions of moisture in the coal. These model projections of self-heating behavior of a specific coal in an adiabatic environment have been compared with the experimentally measured time-temperature curves of the coal. The results obtained emphasise the importance of the shape of the self-heating curve in terms of understanding the incubation behaviour of the coal that either leads to thermal runaway or continues to remain in a safe state for practical purposes.

The accelerating effect of reactive pyrite on coal self-heating was measured experimentally using an adiabatic oven. Reaction rate data obtained from the experimental results were applied to a numerical model of coal self-heating. The model results showed reasonable agreement with the measured time taken to reach thermal runaway. However, the shape of the self-heating curves showed subtle variations. Since the pyrite oxidation reaction consumes moisture, there was a mutual effect of accelerated heating as less heat was used up in moisture evaporation. (C) 2014 Elsevier Ltd. All rights reserved.

Adiabatic oven testing of coal to assess the propensity for self-heating produces data on the kinetics of the coal oxidation reaction. Repeat tests on the same coal at different starting temperatures ranging from 20 to 57 degrees C shows that there is a strong time-temperature effect on the reaction kinetics. At these low temperatures the initial reaction rate is predominantly non-Arrhenius, but converges to Arrhenius behaviour at temperatures above 70 degrees C. A new rate equation model has been derived that accounts for the change in kinetic behaviour in the low temperature region. This model is based on the concept of reactive site availability. (C) 2015 Elsevier Ltd. All rights reserved.

Adiabatic oven testing of seven coal samples with a similar rank has been conducted, which demonstrates differences in their self-heating rate behaviours under the mine settings that they are found in. This has been achieved using a new benchmarking adiabatic test that provides an accurate means of establishing if a coal can reach thermal runaway and in what minimum timeframe. Four of the samples reached thermal runaway, but there was a considerable range in the time taken. The shape of the self-heating rate curves also showed a degree of variation. One of the samples displayed gradual self-heating over the duration of the test and would have reached thermal runaway eventually. The other two samples initially self-heated and reached a maximum temperature before the heat loss mechanism from moisture evaporation dominated and the coal temperature steadily decreased. One of these samples was retested at a lower moisture state and was able to reach thermal runaway. These results confirm the importance of testing samples to assess the risk of developing a spontaneous combustion event.

A moist coal adiabatic oven test has been used to quantify the effect of applying an anti-oxidant agent to reactive coals from Australia and the US. For the dosage rate applied, the anti-oxidant significantly reduces the coal self-heating rate and extends the time taken to reach thermal runaway by a factor of three for sub-bituminous coal and by a factor of two for the same application to high volatile C bituminous coal. The laboratory result obtained for sub-bituminous coal from Powder River Basin is in direct agreement with the practical site experience of applying the anti-oxidant product as a spontaneous combustion management control. Consequently, it is now possible to benchmark the application of the anti-oxidant to any reactive coal prior to mining as part of developing a leading practice spontaneous combustion management plan.

The acceleration of coal self-heating has long been attributed to the presence of reactive pyrite. However, a definitive means of quantifying this effect has been lacking, particularly from the low ambient temperatures experienced at mine sites. A recently developed moist coal adiabatic oven test has been used to investigate the influence of reactive pyrite on self-heating of a high volatile bituminous coal containing sulphur concentrations from 0.62% to 17.95%. A relationship exists between the amount of pyritic sulphur in the coal and the time taken to reach thermal runaway. However, simply measuring the pyritic sulphur concentration of a coal is not sufficient to quantify the accelerated self-heating effect, as it is the form of the pyrite that determines the pyrite reactivity. These findings will be expanded on in the paper as they have a major significance for the risk assessment of coal self-heating.

The oxidation process occurring in a coal stockpile is a serious economic and safety problem. In this research, heat and fluid flow within and around a heat generating porous material (coal stockpile) are numerically investigated by both a FORTRAN code and the commercially available software CFD-ACE for a self-heating medium. Transient variation of the maximum temperature inside the coal stockpile, as the main parameter to study self-heating and spontaneous self-ignition is monitored and a threshold is presented. It is shown that the maximum temperature inside the pile may reduce/increase depending on the stockpile average porosity and permeability.

A moist coal adiabatic oven test has been used to quantify the effect of applying a known inhibiting agent to alter the self-heating rate of highly reactive coals. The inhibitor significantly reduces the coal self-heating rate and extends the time taken to reach thermal runaway by a factor of approximately 3, for the dosage rate applied to the coals. A distilled water placebo test on the most reactive coal using the same application amount increased the initial self-heating rate, but the time taken to reach thermal runaway remained unchanged. These results are in agreement with practical experience and have major implications for the mining, handling and transportation of reactive coals.

Many small-scale coal spontaneous combustion tests exist, but none of these appear to be comprehensive and definitive in terms of assessing the self-heating behaviour of coal at low temperatures from ambient to thermal runaway. Within this low temperature region there are a number of competing influences on the tendency of the coal to continue to gain heat. One of the key influences is the presence of moisture in the coal as this contributes to heat loss from evaporation. The presence of disseminated pyrite can contribute to heat gain from the additional exothermic reaction with oxygen in the presence of moisture. A new moist coal adiabatic oven test has been developed that can measure the influence of these parameters on the low temperature self-heating of coal. Examples of the experience gained from using this test clearly show the effects of moisture and pyrite on coal self-heating leading to thermal runaway. The results obtained have major consequences for the operational planning and management of the spontaneous combustion hazard.

Adiabatic self-heating tests have been conducted on subbituminous coal cores from the same seam profile, which cover a mineral matter content range of 11.2–71.1%. In all cases the heat release rate does not conform to an Arrhenius kinetic model, but can best be described by a third order polynomial. Assessment of the theoretical heat sink effect of the mineral matter in each of the tests reveals that the coal is less reactive than predicted using a simple energy conservation equation. There is an additional effect of the mineral matter in these cases that cannot be explained by heat sink alone. The disseminated mineral matter in the coal is therefore inhibiting the oxidation reaction due to physicochemical effects.

Spontaneous combustion of coal is an important problem in mining and storage, in terms of both safety and economics. This is because coal reacts with oxygen in the air and an exothermic reaction occurs, even in ambient conditions. The heat of the reaction accumulates and the reaction becomes progressively faster and thermal runaway may take place to the point of ignition. A detailed computer model has been developed to simulate a bulk-scale, one-dimensional test column. Predictions from this model can then be used to simulate full-scale storage conditions. Model predictions are verified by using the experimental results from the test column at the University of Queensland. A 2-m column is being used in this laboratory to conduct a practical test capable of providing reliable data on coal self-heating. Coal self-heating results produced with the 2-m column are consistent with theory. In particular, the hot spot development in test runs closely matches model predictions. Features of moisture transfer and hot spot migration are clearly visible, both in the model and in tests in the column. Under the specific conditions considered in this study, it is shown that a subbituminous coal can reach thermal runaway in 4.5 days. This result is confirmed by observations made at the mine site, where hot spots have been found to occur within this timeframe. The results obtained in this study indicate that there is a definite need to consider the influence of coal moisture on spontaneous combustion.