JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III MINE ENVIRONMENTAL ENGINEERING - II Previous Question Paper (March - 2021 - R18)

Spontaneous heating is the process of self-heating of coal due to the oxidation of coal at ambient temperatures, which, if left unchecked, leads to a rise in temperature, thermal runaway, and eventually spontaneous combustion. It occurs when the rate of heat generated by the oxidation of coal exceeds the rate of heat dissipated into the surrounding environment.

The coal-oxygen complex theory is the most widely accepted explanation for the initiation of this process:

• Adsorption: When coal is exposed to air, oxygen molecules are physically adsorbed onto the surface of the coal particles.
• Formation of Complex: The adsorbed oxygen reacts chemically with the active sites of the coal to form an unstable coal-oxygen complex (often described as a peroxide-like surface compound). This reaction is exothermic.
• Decomposition: As the temperature increases, this unstable complex decomposes, releasing heat and forming more stable oxidation products like carbon dioxide (CO₂), carbon monoxide (CO), and water.
• Autocatalysis: The heat released during the decomposition of the complex further accelerates the oxidation rate, creating a self-sustaining cycle of increasing temperature. If the ventilation is insufficient to carry away this heat, the temperature rises until the ignition temperature of coal is reached.

Graham's index, also known as the CO/O₂ deficiency ratio, is a critical diagnostic tool used in underground coal mines to detect the early stages of spontaneous heating. Unlike absolute gas concentrations, which can be influenced by ventilation fluctuations, this ratio provides a more reliable indicator of the oxidation state of coal.

Usage:

• It serves as an early warning system for detecting heating in goaf areas or sealed-off regions.
• An increasing trend in the index value over time indicates active oxidation and potential spontaneous heating.
• It helps mine management differentiate between normal strata gas emission and active combustion.
• Generally, a value below 0.5% suggests normal conditions, while values exceeding 1.0% to 2.0% are often treated as indicators of incipient heating requiring immediate investigation.

Formula:

The index is calculated as the ratio of the volume of carbon monoxide produced to the volume of oxygen consumed, expressed as a percentage:

Graham's Index = (CO produced / O2 absorbed) * 100

Where:

• CO produced = (CO in sample) - (CO in intake air)
• O₂ absorbed = (O₂ in intake air * (N₂ in sample / N₂ in intake air)) - (O₂ in sample)

Note: The nitrogen (N₂) is used as an inert tracer to calculate the oxygen absorbed because it does not participate in the oxidation reaction.