Open Cut » Environment
The long-term behaviour of spoil piles subject to spontaneous combustion has been evaluated at three hunter valley mines over an 18 month period. Spontaneous combustion has emerged in recent years as a significant issue in the Muswellbrook region of the Hunter Valley. Spontaneous combustion can cause open and smouldering fires, which can emit carbon dioxide, carbon monoxide, nitrogen dioxide, sulphur dioxide and the "tarry" emission products associated with incomplete coal combustion. There is also the danger of fire spreading to surrounding land, destabilisation of the landform and vegetation dying near the hot spoil. Monitoring the long-term behaviour of spoil piles subject to self heating can more accurately define the processes important for heating, predict spoil behaviour and assess the effectiveness of control methods.
The overall objective of the project is to continue to measure the temperature and oxygen profiles from probes already installed in spoil piles at Drayton, Muswellbrook and Ravensworth at three monthly intervals over 18 months. The project will help to maintain a data set on the long-term behaviour of self heating spoil piles.
Researchers also monitored the behaviour of an area at Drayton grouted with flyash.
The data from Ravensworth probes highlights the heterogeneous nature of spoil pile heating with large fluctuations in the temperature profiles measured. The data was derived from probes near the edge of a spoil pile batter undergoing active spontaneous combustion. The data was obtained after an inert cover layer was applied to the surface with the batters left exposed.
The data shows that:
- Covering the surface of a burning spoil pile with a thick layer of inert material without covering the batters will not control fires in spoil.
- Active combustion can occur ~90m in from the batter edge.
The intermittent and heterogeneous nature of the heating at Ravensworth Site 2 shows that individual circumstances (location of reactive materials, presence of cracks, access of oxygen through the batter and transport of water through channels and cracks) are important in determining the nature and extent of spontaneous combustion fires.
The hot Drayton spoil, which has been capped by clay, continues to show a temperature decline at the 19.5m level, consistent with the cessation of combustion and the loss of heat by conduction and water flow through the spoil. The temperature at the hottest location decreased from ~260oC when the probe was installed to ~55oC in June 1999, a loss of more than 200oC over seven years, averaging 32oC per year. While control has been successful to date, careful monitoring is required to ensure it continues.
The Muswellbrook data shows that the small heating induced in the marine conglomerate remains low and does not proceed to combustion in the absence of carbonaceous material.
Data from Drayton shows that the role of grouting is limited to specific applications rather than for use as a general method of controlling spontaneous combustion. Such application might include grouting into highwalls to control in-seam fires or grouting to control fires in well defined volumes of carbonaceous material so that a complete grout curtain can be produced to limit oxygen ingress.
Where To From Here
Further work on spontaneous combustion is being carried out under the ACARP project (C8059 Greenhouse Gas Emissions from Spontaneous Combustion in Coal Mining). This project is due for completion in early 2002.