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Ageing effect on the self-heating incubation behaviour of lignite

Basil Beamish, Jan Theiler, Andrew Garvie
Monday, February 18, 2019
First presented: 
Coal Operators' Conference 2019
Published paper
Rock Mechanics

A lignite seam present in overburden strata of an opencut mine poses an interesting question as to its spontaneous combustion hazard likelihood when placed into an overburden spoil pile. Lignite is often assumed to rapidly spontaneously combust when exposed to air due to its low rank. However, lignite also has a high moisture content as-mined, generally in the order of 40% or greater. Self-heating is a balancing act between the intrinsic reactivity of the coal and the moisture present, which can act as a moderating influence to the rate of self-heating. In addition, this balance can be altered by ageing since both the intrinsic reactivity of the lignite and the moisture content decrease over time. This feature of coal selfheating is frequently overlooked in almost all spontaneous combustion test methods.

A new adiabatic oven incubation test method is now routinely used by the Australian coal industry to overcome this deficiency. It assesses the spontaneous combustion hazard for the environmental conditions that exist for each mining situation. Incubation testing of the fresh lignite demonstrates that in an as-mined moisture state of 45.2% the incubation period of the lignite is considerably extended by heat loss from moisture liberation and evaporation to the point of no thermal runaway being achieved in a practical timeframe. However, rehandling or exposure to the atmosphere of aged overburden spoil containing the lignite within a certain timeframe can alter the heat balance in favour of thermal runaway. This paper presents laboratory examples of how this mechanism can occur.

Feature Author

Andrew Garvie

Andrew Garvie has more than 24 years’ experience providing scientific and technical assessments in acid and metalliferous drainage (AMD) and heap leach oxidation.  More recently he has undertaken assessments of self-heating and the potential for spontaneous combustion of coal wastes and carbonaceous black shales associated with sulfide minerals.  Studies have included the quantification of physical processes that support the oxidation of mine wastes and heap leach piles by measurement and predictive modelling.  He has assessed strategies used at mines to control oxygen supply and water flux into dumps and heap leach piles using the same methods.  Andrew’s experience includes use of geostatistics to assess the adequacy of sampling, geochemical characterisation to examine the potential of mine wastes to produce AMD, and assessment of contributors to pit lake water quality, including wall rock oxidation and in-pit waste rock disposal.  He has applied his understanding of the above processes to the development of conceptual waste landform closure strategies for the control of AMD production and spontaneous combustion.

Principal Consultant (Geochemistry)
PhD (Physics), MAusIMM
SRK Sydney
SRK Africa