Underground » Ventilation, Gas Drainage and Monitoring
Gas content measurement in a coal seam is critical to design gas drainage systems and assess mine ventilation requirements, directly affecting coal production rates and thus being required to be measured for outburst management compliance. Gas content is a basis for outburst threshold parameters such as desorption rate index (DRI) and threshold limit value (TLV), primarily influencing the probability and size of a gas outburst. The Australian Standard (AS) 3980-2016 covers two direct methods (slow and fast desorption), being involved with crushing subsamples to quickly release the adsorbed gas into a gas phase to determine the gas content (Q3) of coal.
This project has tackled two issues in Q3 measurements by revisiting the fast desorption method. First, along with potential errors associated with gas content measurements, the crushing efficiency and gas release rate are largely affected by crushers and crushing conditions used in each gas testing laboratory. This can lead to a great discrepancy in measuring DRI and Q3. Second, at gas outburst sites, pulverised coal particles (< 100 µm in size) can often be found, instantly releasing a large amount of gas at the very beginning of an outburst. This suggests that the initial gas release from coal particles is of great significance to understand the occurrence of an outburst. As DRI values are based on the first 30 seconds of gas released at Q3 testing and related to the total gas content, they are not directly reflecting the very initial gas release for < 10 seconds.
To tackle the first issue, pulverised coal samples after Q3 testing were obtained from two gas testing laboratories and analysed in terms of particle size distributions (PSDs) and gas adsorption isotherms. The gas content data of those samples from four underground coal mines were also collected. The two laboratories use different types of crushers and crushing conditions. The material portion passing through the 212 µm mesh was found to be 82.8, 50.3, 55.1 and 49.4 % for pulverised coal samples from mines, all of which did not meet the recommended particle sizes (95 % materials passing through a 212 µm) stated in AS 3980-2016. Also, pulverised samples from Lab 2 had some large particles (> 1 cm), generally containing bigger particles than the ones from Lab 1. As gas diffusion is a function of particle size, the crushing conditions can significantly influence the initial rate of gas released at Q3 testing, consequently affecting the DRI values. From adsorption isotherm measurements of CO2 and CH4, samples from mine A were found to have more diffusional restrictions than the others due to the presence of distinctive peaks at micropore sizes between 4 and 5 Å.
A separate set of gas content data obtained from mine A over 6 years indicated that for given total gas contents (Qm), lost gas (Q1) values were found to be very different between CO2 and CH4 rich samples whereas Q2 values were similar. Especially for over 8 m3/t of Qm, the Q1 values of CO2 rich samples were much greater than those of CH4 rich samples and potentially related to Q3 values. The Q3 values of 95 samples in this project were generally a linear function of Qm, regardless of gas compositions.
For the second issue, a Q3 measurement system was established at a CSIRO laboratory, using a conventional water displacement method with graduated cylinders and separately allowing direct measurements of pressure, temperature and the volume of gas released. Four types of crushers were tested while monitoring changes in pressure and temperature. Double pucks were found to be more effective crushing than other types in terms of PSDs with a minimum temperature increase (by 0.95 K for 30 seconds of crushing). This insignificant temperature rise means that the gas pressure rise while crushing is mainly due to the increased volume of gas released. Due to unavoidable gas loss during the sample delivery from mine sites to the CSIRO laboratory, the Q3 values reported by mines were not used. Instead, Q3 testing for as-received 15 sub-core samples from mine A was separately carried out with both graduated cylinders and volume meters at the CSIRO laboratory. The rates of initial gas released over the first 5-10 (R5-10), 10 (R10) and 30 (R30) seconds were overall found to be linearly proportional to the measured Q3 values. Also, for samples with similar Q3 values, R5-10 values were greater than R30 values and the difference was increased with Q3 values. This indicates that the relationship between DRI values and outburst TLVs can be weakened with an increase in Q3 (or Qm) values as R5-10 can be more relevant to outburst occurrence. If the rates of gas released were measured onsite, it is expected that the difference between R5-10 and R30 would be greater, stressing the importance of initial gas release at Q3 testing. Onsite measurements are recommended to understand this observation, relating to the total gas content.
Along with the conventional water displacement method which requires slight vacuum at the start, this project used a volume meter to directly measure the volume of gas released at ambient pressure. The Q3 values were well correlated with but slightly lower than the ones with graduated cylinders due to the initial pressure difference. Unlike the water displacement method, a second rise in the volume of gas released while crushing was found for some samples whereas other samples with similar Q3 values exhibited a gradual increase. This indicates that other factors such as coal characteristics are involved with the second rise. The early indication was that the micropore size distributions (MPSDs) seem to make a difference in the gas desorption behaviour. The reason(s) behind the second rise needs to be further investigated.