JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III MINE ENVIRONMENTAL ENGINEERING – II Previous Question Paper (September - 2021 - R16)

The process of spontaneous combustion of coal occurs in three distinct stages: the incubation stage, the self-heating stage, and the ignition stage.

1. Incubation stage: This is the initial period where coal absorbs oxygen at ambient temperatures, leading to the formation of unstable oxygen-coal complexes.
2. Self-heating stage: As the temperature rises, the rate of oxidation increases, generating more heat than is dissipated, which further accelerates the reaction.
3. Ignition stage: The temperature reaches the critical ignition point of the coal, resulting in active combustion and the production of flames.

The incubation stage is characterized by the physical and chemical adsorption of oxygen onto the coal surface. During this phase, oxygen molecules react with the active sites of the coal matrix, specifically the aliphatic side chains and functional groups. This process is exothermic, but at low temperatures, the heat generated is typically dissipated into the surrounding environment. If the ventilation is restricted or the coal is finely divided, the heat accumulates, marking the transition from simple adsorption to the self-heating phase.

Extrinsic factors are environmental and operational conditions external to the coal's inherent properties that influence its susceptibility to spontaneous combustion. The primary extrinsic factors include:

• Ventilation conditions: The rate of air leakage into goaves or abandoned areas is critical. Insufficient airflow provides enough oxygen to sustain oxidation but not enough to carry away the heat, creating ideal conditions for self-heating.
• Mining method: Methods that result in high coal losses or leave behind pillars (like bord and pillar mining) increase the risk of spontaneous combustion in the goaf.
• Temperature of the environment: Higher ambient temperatures in deep mines accelerate the initial chemical reaction rates between coal and oxygen.
• Pressure differences: Variations in pressure across coal barriers or seals, caused by main fan operations or barometric changes, drive oxygen-rich air into fractured coal zones.
• Moisture content: While coal moisture is often considered intrinsic, the humidity of the ventilating air affects the rate of evaporation and the adsorption of oxygen, influencing the overall thermal balance.
• Degree of fragmentation: The extent to which coal is crushed during extraction increases the surface area available for oxidation, significantly enhancing the reaction rate.