Longwall methane limit before a production increase
A longwall coal face 220 m wide and 2.8 m high produces 5,000 t/d. Methane emission from the seam and goaf is 4.5 m3 per tonne of coal mined. Intake air contains 0.05% CH4. Current airflow is 30 m3/s and the return measures 0.92% CH4. The main fan can deliver at most 36 m3/s at the existing mine resistance; a second fan would take six months to install. The statutory limit is 1.0% CH4 in the return airway. Production is planned to rise to 7,000 t/d next quarter. The ventilation engineer proposes raising airflow to the fan limit, while the operations manager wants to keep the production increase and rely on the existing margin. The mine also has a goaf drainage borehole that is currently unused.
1. Calculate the current methane concentration and the airflow required for 7,000 t/d. Can the existing fan support the increase?
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Current methane emission is 5,000 x 4.5 / 86,400 = 0.260 m3/s. The return concentration is 0.260 / 30 + 0.0005 = 0.00918, i.e. 0.92%, matching the measurement.
At 7,000 t/d, emission becomes 7,000 x 4.5 / 86,400 = 0.365 m3/s. To keep the return at 1.0% with 0.05% intake, required airflow is 0.365 / (0.010 - 0.0005) = 38.4 m3/s. The fan maximum is 36 m3/s, so it cannot support 7,000 t/d. At 36 m3/s the concentration would be 0.365 / 36 + 0.0005 = 0.0106, i.e. 1.06%, above the statutory limit. The maximum production at 36 m3/s is about 36 x 0.0095 x 86,400 / 4.5 = 6,570 t/d.
2. Evaluate the options: raise airflow to 36 m3/s, use goaf drainage, or cap production. Recommend a sequence.
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Raising airflow to 36 m3/s alone supports only about 6,570 t/d, so it is not sufficient for 7,000 t/d. Goaf drainage must reduce emission to at most 36 x 0.0095 = 0.342 m3/s, a reduction of 0.023 m3/s or about 6.3% of the 7,000 t/d emission. A working borehole should achieve more than 10% capture, but its reliability and gas quality must be proven.
Recommended sequence: cap production at about 6,500 t/d immediately; commission and test the goaf drainage borehole, targeting at least 15% capture; raise airflow to 36 m3/s; then increase production in steps while monitoring return CH4. If drainage capture is below 10%, keep the production cap or install the second fan. Do not rely on the existing 0.92% reading as margin, because goaf gas can spike suddenly.
3. What controls and monitoring are needed for goaf gas and methane spikes?
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Install continuous CH4 sensors in the return, at the goaf edge and in bleeder airways, with automatic power trip at the statutory limit. Monitor airflow, fan pressure and drainage flow and quality. Maintain seals and ventilation doors, and control the bleeder system so goaf gas is directed away from the face. Record drainage capture rate and CH4 concentration; if capture falls, reduce production before the return concentration reaches 1.0%. Include a formal trigger-action-response plan for gas spikes, and test it with the workforce.