JNTU B.Tech IV COAL GASIFICATION COAL BED METHANE, AND SHALE GAS Previous Question Paper (July/August - 2022 - R16)

Underground Coal Gasification (UCG) is an in-situ gasification process that converts coal seams into combustible gas without mining. The process involves injecting gasifying agents (air, oxygen, or steam) into a coal seam through an injection well, where it reacts with the coal to produce a synthetic gas (syngas) that is recovered through a production well.

The fundamental principle relies on creating a permeable reaction zone within the coal seam. The process typically follows these stages: 1) Drilling injection and production wells into the coal seam, 2) Establishing a linkage path between wells using hydraulic fracturing or directional drilling, 3) Initiating combustion to heat the coal, and 4) Sustaining the gasification reactions while controlling the cavity growth.

The chemistry of UCG involves a series of simultaneous thermochemical reactions occurring in the coal seam:

1. Combustion (Exothermic): C + O₂ → CO₂ + Heat. This provides the energy required to sustain the gasification process.
2. Boudouard Reaction (Endothermic): C + CO₂ → 2CO.
3. Water-Gas Reaction (Endothermic): C + H₂O → CO + H₂.
4. Water-Gas Shift Reaction: CO + H₂O → CO₂ + H₂.
5. Methanation: C + 2H₂ → CH₄.

Diagrammatically, the process is visualized as a gasification cavity formed within the seam. The injection well delivers the oxidant to the 'active face' of the coal. As the coal is consumed, a cavity expands, and the syngas (comprising CO, H₂, CH₄, and CO₂) flows through the coal seam or a dedicated channel toward the production well. The structural integrity of the surrounding rock (overburden) is critical to prevent gas leakage and ensure the containment of the reaction zone.

Underground Coal Gasification (UCG) is an in-situ gasification process that converts coal into combustible gas while it remains in the seam. The suitability of a site for UCG depends on specific geological and hydrogeological conditions:

• Coal Seam Thickness: Ideally, seams should be between 2 and 20 meters thick to ensure sufficient gas production and structural stability during the burn.
• Depth: The depth typically ranges from 100 to 1000 meters. Shallower seams risk surface subsidence, while deeper seams face challenges with high hydrostatic pressure and drilling costs.
• Coal Quality: While UCG can process lower-rank coals (lignite and sub-bituminous), the coal should have low ash content and high permeability to allow for the injection of gasification agents.
• Hydrogeology: The site must have low groundwater inflow to prevent the quenching of the reaction zone and to minimize the risk of groundwater contamination.
• Structural Integrity: The overburden must be impermeable and stable to prevent gas leakage and surface collapse.

The merits and demerits of UCG are as follows:

Merits:

• Accesses coal deposits that are otherwise uneconomical or unsafe for traditional mining.
• Significantly lower capital expenditure compared to conventional mining and surface gasification plants.
• Reduces surface environmental impact by eliminating the need for large-scale mining, coal handling, and waste disposal.
• Improved safety as it eliminates the need for underground personnel.

Demerits:

• High risk of groundwater contamination due to the migration of pollutants and gasification byproducts into surrounding aquifers.
• Potential for surface subsidence as the coal is consumed, which can damage overlying structures or land.
• Difficulty in controlling the gasification front, leading to unpredictable gas quality and potential loss of the resource.
• Limited site selection due to stringent geological and hydrogeological requirements.