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Rehabilitation of Spontaneous Combustion - Prone Spoil Piles

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Published: January 04Project Number: C9031

Get ReportAuthor: A Saghafi, J Cheng, C Perrier, John Carras, Clive Roberts | Clive Roberts Consulting, CSIRO Energy Technology, ACIRL

The control of spontaneous combustion in spoil piles is an area of concern for a number of open-cut coal mines. Spontaneous combustion in spoil occurs when carbonaceous waste materials are exposed to the air. In large piles, the relatively high voidage (20-40%) within the pile may allow sufficient movement of air through the pile to sustain oxidation and heat generation.

Economic spontaneous combustion prevention practices must focus on modification of the near-surface material with respect to reducing oxygen transport into the spoil. This requires reduction in air-filled voidage of the near-surface material and long-term stability of the surfaces. The most practical techniques are:
  • compaction, ie. increasing the bulk density of the near-surface material, and
  • application of a final cover layer with good water-retention properties.
The objectives of this project were:
  • To develop methods to assess the suitability of overburden strata as cover layer materials.
  • To demonstrate the validity of these methods.
  • To develop simple 'cover effectiveness' indices for overburden materials and spoil surface preparation methods.

The objectives were addressed by a work program that involved field and laboratory measurements and numerical simulations.

A method was developed that combined the two existing computer models SPLGOF and SWIM to model the effect of water content from rainfall on the cover layer. SPLGOF solves the equations of heat and mass transfer for a porous body and accounts specifically for oxygen transport, reaction with the carbonaceous material and release and transport of heat. SWIM accounts for the flow of water within the cover layer and the spoil pile, including the effects of evaporation from the surface.

The most significant properties of a cover material were found to be the water retention characteristic and saturated hydraulic conductivity of the unconsolidated ('minesoil') material. These hydraulic properties can be estimated from the size distribution analysis (also known as 'soil texture' analysis), using published correlations which relate the size analysis to the water retention characteristic and hydraulic conductivity. The relevant characteristics of the cover layer were estimated for four overburden materials commonly found at two Hunter Valley mines.

Model results were validated by three main methods. These were:
  • The results of the computer simulations were compared with analytical solutions for well known problems. Good agreement was achieved.
  • The output from the coupled SPLGOF/SWIM model was compared with the output for a similar model 'UNSATH' for the same modelling scenario. Good agreement was achieved.
  • The results from the SWIM simulations were compared with the results from the field studies of spoil rehabilitation in the region, carried out by the NSW Soil Conservation Service. These latter results included details of rainfall and evaporation, but did not include all the data required for input to the computer model SWIM. In a number of cases values had to be assumed for some parameters. However, the agreement between the SWIM results and the field data was considered acceptable.

In order to obtain an empirical ranking of cover materials, and as a further method to provide a test of the computer model predictions, an area of spoil pile at a Hunter Valley mine was used to create 8 cover layer plots from 6 different materials. An oxygen fluxmeter was used to measure the oxygen flux into the spoil both prior to and after a period of 6 months after the covers had been emplaced. The values of oxygen flux into the spoil showed wide variation reflecting the high variability of spontaneous combustion activity and of the properties of the near surface spoil. When an average over all the oxygen flux values for bare spoil was taken and compared with the average values of oxygen flux through the covers, a reduction in the flux of oxygen into 7 of the 8 cover plots was observed. However this result is sensitive to the value of oxygen flux attributed to bare spoil and given the large variability this result cannot be taken as conclusive at this stage. The magnitude of the measured oxygen fluxes through the different covers, and their ranking, were in broad agreement with the computer model predictions.

A cover effectiveness index was developed through the detailed computer simulations of the self heating behaviour of a model spoil pile under the influence of a single bare (nonvegetated) cover layer of different materials and different thicknesses.

Cover Stability Index = Cover Effectiveness x Cover Thickness

The above expression states that the thicker the cover the less the rate of self-heating. Also the more effective the cover, ie. the greater the water retention properties and the greater the thermal diffusivity, the less the rate of self-heating. The simulations suggested that, for the spoil reactivities and weather conditions found in the Hunter Valley, the Cover Stability Index should be greater than a critical value to avoid run-away heating.

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