Towards accurate methane emission reporting from coal mine ventilation: a direct measurement approach at the shaft

Emissions from coal production represent one of the major anthropogenic sources of atmospheric methane, particularly CH₄ released through underground mine degassing and discharged via the ventilation system. Ventilation air methane (VAM) is of great interest in terms of safe mining exploitation and GHG emission reduction targets. The direct measurements and quantification of CH₄ emission from mining activities and its controlling factors are often subject to considerable uncertainty. Therefore, it is necessary to perform continuous monitoring of CH₄ content and estimate methane emission rates across mine ventilation shafts into the atmosphere.

This study investigates methane concentration variability in ventilation air, evaluates the performance of a low-cost TDLAS analyzer (Axetris LGD), and estimates a site-specific methane emission factor for an underground coal mine ventilation system. The analysis is based on continuous one-year VAM measurements conducted at the exhaust ventilation shaft of a hard coal mine located in the western part of Upper Silesian Coal Basin (USCB), Poland. During the study period, the mine operated three active longwall panels at a depth of ⁓700 m b.g.l.

Methane monitoring conducted between July 2024 and August 2025 revealed pronounced temporal variability in concentration and volume of exhausted VAM in the ventilation shaft on daily and weekly timescales. These variations reflect episodic gas release events, changes in airflow rates, and operational dynamics associated with mining activities. Seasonal fluctuations in shaft methane concentrations, ranging from 0.15 to 0.45 %, were generally associated with intensified mining activity, particularly during the pre-winter period, whereas downward trends corresponded to a reduced number of active longwalls.

A comparison of the Axetris LGD analyzer with in-mine thermocatalytic Pellistor gas detector at 1-minute resolution revealed systematic offsets in the Pellistor measurements, which consistently underestimated CH₄ content under low-concentration conditions. Following recalibration of the Pellistor sensor using the higher-resolution Axetris measurements as a reference, a strong agreement between the two instruments was achieved, characterized by convergent concentration trends and a substantially reduced measurement bias (relative RMSE of ⁓7 %). These results demonstrate the necessity of regular low-range calibration to ensure the reliability of long-term Pellistor-based CH₄ monitoring in mine ventilation air.

Analysis of methane concentrations from coal mine ventilation shaft identified three distinct emission trends. Period 1 (August–December 2024) exhibited the highest CH₄ emission rates, averaging 1060 ± 140 tons ∙ month^–1, Period 2 (January–March 2025) showed slightly lower emissions, with an average of 1034 ± 80 tons ∙ month^–1, while Period 3 (April–June 2025) was characterized by the lowest methane release, averaging 720 ± 40 tons ∙ month^–1. Hourly emission rates ranged from 1.0 to 2.5 tons CH₄ ∙ h^–1. Methane emission rates correlated with mining activity indicators, including longwall advance (R² = 0.55) and coal production (R² = 0.38). A site-specific methane emission factor of 5.1 ± 1.0 m³ ∙ ton^–1 coal was determined for the studied mine.

Acknowledgment:

This work was funded by the Polish Ministry of Science and Higher Education under Grant No. 2022/44/C/ST10/00112. The authors want to thank the coal mine company for permission to access the Pellistor sensor and airflow records.