Jawaharlal Nehru Technological University Hyderabad B.Tech IV Year GEOLOGICAL AND TECHNOLOGICAL FACTORS OF COAL GASIFICATION CBM, SHALE GAB Previous Question Paper (December - 2020 - R15)

Underground Coal Gasification (UCG) is a complex thermo-chemical process that converts in-situ coal into combustible syngas through the injection of gasifying agents (air, oxygen, or steam) into a coal seam. The process occurs within the reaction zone, which is divided into distinct thermal and chemical regions.

The primary thermo-chemical processes are:

1. Drying and Devolatilization: As the heat from the reaction zone propagates, moisture is first evaporated from the coal. Subsequently, at temperatures between 300°C and 700°C, the coal undergoes pyrolysis, releasing volatile matter including methane, hydrogen, carbon monoxide, and tars.
2. Combustion Zone: Near the injection point, oxygen reacts with coal (carbon) in an exothermic reaction to provide the heat necessary for the endothermic gasification reactions. The primary reactions are:
   C + O₂ → CO₂ + 394 kJ/mol
   C + 0.5O₂ → CO + 111 kJ/mol
3. Gasification Zone: In the oxygen-depleted region, the carbon reacts with steam and carbon dioxide produced in the combustion zone. These are highly endothermic reactions:
   C + H₂O → CO + H₂ - 131 kJ/mol
   C + CO₂ → 2CO - 172 kJ/mol
4. Water-Gas Shift Reaction: This secondary reaction occurs in the gas phase, adjusting the ratio of hydrogen to carbon monoxide:
   CO + H₂O ↔ CO₂ + H₂ + 41 kJ/mol
5. Methanation: At higher pressures and lower temperatures, hydrogen reacts with carbon to form methane:
   C + 2H₂ ↔ CH₄ + 75 kJ/mol

The efficiency of the syngas generation depends on the residence time of the gases, the temperature profile within the cavity, and the contact surface area of the coal seam. The resulting syngas composition is a mixture of H₂, CO, CO₂, CH₄, and nitrogen (if air is used), which is then extracted through production wells for industrial use.

Underground Coal Gasification (UCG) is an in-situ gasification process that converts coal into combustible gas while it remains in its geological seam. The fundamental principle involves injecting gasifying agents—typically air, oxygen, or steam—into an underground coal seam through an injection well, initiating a controlled combustion process that produces synthesis gas (syngas), which is then recovered through a production well.

The core principles governing the UCG process are as follows:

• Access and Linkage: Two or more wells are drilled into the coal seam. A hydraulic or thermal connection (linkage) is established between the injection and production wells to allow the flow of gases through the coal bed.
• Gasification Reactions: Once ignited, the coal undergoes a series of thermochemical reactions. The primary reactions include:
  • Combustion: C + O₂ → CO₂ + Heat (provides energy for the process).
  • Boudouard Reaction: C + CO₂ → 2CO (endothermic).
  • Water-Gas Reaction: C + H₂O → CO + H₂ (endothermic).
  • Methanation: C + 2H₂ → CH₄ (exothermic).
• Controlled Cavity Growth: The process relies on the development of a reaction zone or cavity. As coal is consumed, the cavity expands. Managing the pressure and flow rates is critical to ensure the flame front moves predictably through the seam without causing roof collapse or groundwater contamination.
• Product Recovery: The resulting syngas, composed primarily of hydrogen, carbon monoxide, methane, and carbon dioxide, is brought to the surface through the production well, where it is cleaned and processed for use in power generation, chemical synthesis, or as a fuel source.

The efficiency of UCG depends on geological factors such as seam thickness, depth, dip, permeability, and the integrity of the surrounding strata, which act as a natural pressure vessel for the gasification process.