Environment, Ventilation & Hazards

Methane Drainage

Pre- and post-drainage of seam gas, capture efficiency and the dilution airflow needed to keep methane below statutory limits.

PART 1

Topic Breakdown & Traps

The Engineering Principle

Methane (firedamp) is released from coal as it is mined and is explosive between roughly 5% and 15% in air (most violent near 9.5%). Two controls work together: ventilation dilution carries gas away in the airstream, and methane drainage captures gas at source through boreholes (pre-drainage ahead of mining, post-drainage from the relaxed/goaf zone) and pipes it away. Drainage efficiency (or capture ratio) is the fraction of total emission removed by the drainage system; the remainder must be diluted by ventilation. The dilution airflow follows directly from a mass balance: enough fresh air to hold the general-body concentration below the statutory limit.

The Core Formula Matrix

Drainage (capture) efficiency:

η = \frac{Q _{d r ain e d}}{Q _{t o t a l e mi ss i o n}} \times 100%

Gas to dilute by ventilation:

Q_{v e n t g a s} = Q_{t o t a l} (1 - η)

Dilution airflow:

Q_{ai r} = \frac{Q _{g a s}}{C _{l imi t}}

(

C_{l imi t}

as a fraction, e.g.

1.25% = 0.0125

)

Explosive range of methane in air:

\approx 5%

(LEL) to

15%

(UEL).

The ‘IIT Traps’

⚠
Use the concentration *limit* as a fraction in the dilution formula, not a percentage — $0.0125$ , not $1.25$ .
⚠
Drainage reduces the gas the air must carry, so the ventilation duty is on $(1 - η)$ of the emission.
⚠
The explosive range matters even below the LEL — layering at the roof can be locally rich; measure near the roof.
⚠
Capture efficiency is of total emission, not just the goaf release; define the denominator clearly.

PART 2

Progressive 3-Tier Question Suite

Q1BASIC1 Mark · MCQ

The approximate lower explosive limit (LEL) of methane in air is:

Q2MEDIUM2 Marks · NAT

A district emits

5.0 m^{3} / min

of methane, of which the drainage system captures

3.5 m^{3} / min

. The drainage (capture) efficiency is ______ %. (Round to the nearest whole number.)

Q3HARD2 Marks · NAT

After drainage,

1.5 m^{3} / min

of methane enters the general body of air. To hold the concentration at the statutory limit of

1.25%

, the dilution airflow required is ______

m^{3} / min

. (Round to the nearest whole number.)

m³/min