Methane exceedance on a longwall return with limited fan capacity
A 1.6 Mt/y longwall coal mine in a gassy seam is producing 4,800 t/day from one face. Intake airflow is 24 m³/s; the return airway measures 1.35% methane on a calibrated fixed monitor, and the statutory trip is set at 1.0%. The main fan is rated at 30 m³/s at 1.8 kPa and is already running at 92% of its permitted motor current. Methane drainage boreholes have been drilled from the surface but are not yet connected to the vacuum pump. The forecast is for a 4 kPa barometric pressure fall over the next 12 hours, and two development headings are ventilated by the same split. The mine has no spare air capacity without changes to the fan or the ventilation network. The ventilation officer needs a defensible plan for the next shift, not a general recommendation.
1. Calculate the current methane release rate and the airflow required to hold the return at 1.0%. Compare this with the fan capability and state whether increasing airflow alone can solve the problem.
Read the worked answer
Use a steady-state methane balance. Methane release q = Q × C = 24 × 0.0135 = 0.324 m³/s. To hold 1.0%, Q = q / 0.010 = 0.324 / 0.010 = 32.4 m³/s. The fan can deliver only 30 m³/s, so at full output the concentration would be 0.324 / 30 = 1.08%, still above the trip. Airflow alone therefore cannot restore compliance. Even if the fan reached 32.4 m³/s, that would give exactly 1.0% with no operating margin; a defensible target is 0.8% or lower, which would require 40.5 m³/s. At 30 m³/s the methane release must fall to 0.30 m³/s just to reach 1.0%, and lower still to leave margin. The practical conclusion is that the mine needs both a reduction in methane release and a network or fan change, not simply a higher fan speed.
2. Recommend an ordered action plan for the next 24 hours. Justify the sequence and identify what must not be done.
Read the worked answer
First verify the reading with a handheld instrument and check the monitor calibration and sample line; a false high reading must be excluded before production changes. If the reading is confirmed above 1.0%, stop the longwall and withdraw persons from the return, because the trip level is a limit, not a target. Next reduce methane release: cut or stop production on the face and minimise disturbance of the gob. Connect the surface drainage boreholes to the vacuum pump and bring them on suction, since captured gas is more effective than diluted gas. Then raise fan output only if motor current, fan vibration and the ventilation network can accept it; at 30 m³/s the return is still about 1.08%, so fan action alone is insufficient. Check regulators and seals to ensure the split is not short-circuiting, and confirm that the two development headings still receive their required air. Do not increase airflow by opening a regulator that robs the headings, and do not rely on a higher trip setting or on continuing production while methane trends upward. Re-enter only after methane is stable below the limit with a margin.
3. Explain how the forecast barometric pressure fall changes the methane hazard and what monitoring and controls should be in place.
Read the worked answer
A falling barometric pressure lowers the absolute pressure in the gob and workings. Gas stored in the gob expands and migrates into the return, so methane concentration can rise even when production and airflow are unchanged. The effect is greatest where gob seals are weak, where the gob is large, or where drainage boreholes are not on suction. A 4 kPa fall is about 4% of atmospheric pressure and should not be treated as negligible. Monitor barometric pressure, gob gas pressure at the seals, methane at the return and at each development heading, and airflow. Maintain seals and stoppings, keep the gob drainage on suction, and avoid changing regulators in a way that unbalances the gob. If methane approaches the trip, reduce production or stop the face. The plan should specify who reads the trend, at what interval, and what action follows a defined rise in methane or a defined fall in barometric pressure.