Application of TDLAS analysers, challenges and implications for future monitoring of methane emission from the coal mine ventilation shafts.

Tunable diode laser absorption spectrometers (TDLAS) are versatile devices with a wide range of applications. In particular, they can be used to detect methane using an analyzer that operates with an infrared band (1.6 – 1.7 μm) laser beam configured in an open path architecture. This setup is suitable for nonhomogeneous airflow coming from coal mine ventilation shafts. TDLAS devices are compact, robust, easy to install, and require minimal maintenance.

Methane emissions from the coal mining sector in Poland account for approximately 0.5 million tons of methane per year and are significant contributors to the continental budget of this gas. Most of the emissions occur through the ventilation shafts, where the methane content can vary from 0.05% to 0.7%.

During June 2023, a TDLAS analyzer (Unisearch, LasIR) was installed at a selected ventilation shaft air diffuser. The device operated for one month, recording the methane concentration in the ventilated air with a temporal resolution of 1 second. In addition to TDLAS, two other instruments were used to determine methane content: an ICOS analyzer (LGR/ABB, mGGA-918) and a pellistor sensor (EMAG, DCH). The ICOS analyzer was used to cross calibrate the TDLAS instrument across a wide range of methane concentrations. The pellistor sensor is a popular type of sensor used in coal mines for safety reasons. Typically, methane emissions are determined through gas chromatographic analyses conducted using periodically collected samples (e.g., once a month). However, methane content in ventilated air can vary on shorter timescales of hours, days, and weeks. Additionally, pellistor sensors are less precise, and the uncertainty of a single measurement cannot be better than 0.1%. In contrast, TDLAS analyzers can be commonly used by coal mine operators for methane reporting, as their precision is usually better than 0.01%.

The presentation will address the challenges associated with using TDLAS for methane emission calculations and highlight its advantages over other commonly used techniques. It will also provide insights into interpreting pellistor sensor readings for quantifying methane emissions and assessing associated uncertainties. Finally, the presentation will discuss the benefits of deploying TDLAS techniques in the coal mining industry, both in the short term and as a potential long-term solution of the reporting of CH4 release.

 This research was funded by and performed in collaboration with UNEP's International Methane Emissions Observatory. The results presented here are part of the findings from a series of three measurement campaigns performed in Poland’s Upper Silesia coal basin.