Ageing effect on the self-heating incubation behaviour of lignite

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.