JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III COAL GASIFICATION, COAL BED METHANE AND SHALE GAS Previous Question Paper (February/March - 2022 - R16)

Underground Coal Gasification (UCG) involves the in-situ conversion of coal into a combustible gas mixture through a series of thermochemical reactions. The primary reactions occurring within the gasification zone are:

1. Combustion (Oxidation): Coal reacts with injected oxygen to provide the necessary heat for the endothermic gasification reactions.
   C + O₂ → CO₂ + 393.5 kJ/mol
2. Boudouard Reaction: Carbon dioxide reacts with hot coal to produce carbon monoxide.
   C + CO₂ → 2CO - 172.6 kJ/mol
3. Water-Gas Reaction: Steam reacts with hot coal to produce hydrogen and carbon monoxide.
   C + H₂O → CO + H₂ - 131.4 kJ/mol
4. Water-Gas Shift Reaction: Carbon monoxide reacts with steam to adjust the H₂/CO ratio.
   CO + H₂O → CO₂ + H₂ + 41.2 kJ/mol
5. Methanation: Hydrogen reacts with carbon to produce methane, which increases the calorific value of the product gas.
   C + 2H₂ → CH₄ + 74.8 kJ/mol

The environmental impacts of UCG are primarily associated with the subsurface nature of the process and the potential for contamination. Key concerns include:

• Groundwater Contamination: The most significant risk is the migration of gasification products, such as phenols, polycyclic aromatic hydrocarbons (PAHs), and heavy metals, into surrounding aquifers. This occurs if the pressure in the gasification cavity is not properly managed or if the geological seal is breached.
• Subsidence: The removal of coal and the thermal degradation of the surrounding rock strata can lead to surface subsidence, potentially damaging surface infrastructure and altering local hydrology.
• Air Emissions: While UCG eliminates traditional coal mining and transport emissions, the surface processing facility must manage gas cleaning and byproduct disposal to prevent the release of hydrogen sulfide, ammonia, and particulate matter.
• Seismic Activity: Although rare, the injection of gases at high pressure and the creation of large underground cavities can induce minor seismic events, requiring careful site selection and pressure monitoring.