High gas emissions from the longwall goaf are a significant problem at several highly gassy Australian coal mines. Even with a large concentration of closely spaced goaf holes, many of these mines find methane concentration at the return end of the face exceeds operational gas levels, resulting in a significant number of face stoppages. Conventional solutions such as bleeder ventilation systems have also failed to control the problem.
To address this issue an ACARP-funded research project aimed at developing optimum goaf gas drainage and control strategies for longwall panels was successfully carried out by the CSIRO, and Central and Appin Collieries.
The main objective of the project was to develop and demonstrate optimum goaf gas drainage and control strategies to improve the overall efficiency of gas drainage systems and to reduce gas delays on the longwall face.
The project combined extensive field studies with computational fluid dynamic (CFD) models of gas flow in order to characterise goaf gas flow patterns and develop effective control strategies.
The project specifically involved the review of gas control issues at field sites, comprehensive goaf gas monitoring, tracer gas investigations, CFD simulations and extensive field trials and demonstration studies at Central and Appin Collieries, both highly gassy underground coal mines.
Initially, a detailed review of the goaf gas issues and the control methods in place at the field sites was carried out to understand the effect of various mining and operational parameters on goaf gas behaviour. The two collieries presented wide variations in geological conditions, ventilation systems and gas emission zones.
Central Colliery is located in the Bowen Basin of Queensland. The mine extracts coal from the two to three metre thick German Creek seam at a depth of about 400m to produce about 2.5 million tonnes annually. German Creek seam gas is predominantly methane, although nitrogen and carbon dioxide are also present. The total gas content over the mine lease ranges from 0-17 cubic metres per tonne, with about 12-14m3/t in the current workings. The longwall panels are 230m wide and approximately 2000m long with a cutting height of about 2.8m.
Over the years goaf gas emissions at Central Colliery have increased substantially, with almost a two-fold increase in two to three years, and reached over 1600 litres per second in LW309. The total number of goaf holes in the longwall panels was about 20-25, with goaf holes drilled at a close spacing of 100m to control the high goaf gas emissions. However, even after this extensive gas drainage, LW308 and LW309 panels suffered delays due to gas outages with return gas concentration levels rising above operational gas limits.
Appin Colliery is located in the Wollongong region of New South Wales. All current mining operations are in the 2.6-3.6m thick Bulli seam and the mine produces about 3- 3.5Mtpa. The Bulli seam is the upper coal seam of the Illawarra Coal Measures, which consists of a number of coal seams. The depth of the current workings is about 450m. The seam gas at Appin is predominantly methane, with total gas content in the range of 10-14m3/t. The longwall panel's width is 255m and the length is about 2800m.
Longwall goaf gas emissions at Appin Colliery have also increased substantially over the years, with gas emissions in LW403 and 404 panels currently in the range of 3-4000l/s. Appin Colliery employed underground cross measure holes (no surface goaf holes), which drained up to only 50% of the gas. Bleeder ventilation was one of the main control measures employed to manage the remaining goaf gas emissions and drained up to 700-1200l/s. However, even with these control measures, gas remained a major issue in the longwall panels.
Analysis of the gas data from the two collieries showed longwall gas emissions were rising steadily over the years and increasing the number of goaf holes in the panel or decreasing hole spacing was not an effective control measure. The standard practice of draining gas from just one to two goaf holes near the face was similarly ineffective and bleeder ventilation seemed to work only up to 600-800m of face retreat from the panel start-up area, due to compaction of the goaf.
A series of detailed goaf gas flow characterisation studies were conducted at both collieries to obtain a better understanding of the goaf gas distribution and flow patterns.
Gas monitoring results at Central showed oxygen concentration was above 15% even 300m behind the face on the intake side of the goaf. At Appin, it was observed there was about 8-10% difference in methane gas concentration between two sides of the goaf, which clearly indicated the effect of buoyancy on goaf gas distribution.
Tracer gas investigations were then carried out at both mines to provide more insight into the gas flow dynamics in longwall goafs. The tracer gas studies involved the release of tracer gas into the goaf at a designated point and collection of gas samples at various locations in and around the goaf.
Tracer gas was detected at sampling points located 150m behind the face within two minutes and at 1000m behind the face within one hour. All three goaf holes behind the face recorded very high levels of tracer gas concentration, even though it took four hours for the tracer gas to reach the third hole. These results indicate that even the goaf holes located far behind the face help to keep gas away from the face.
Test results at Appin detected tracer gas at all goaf seals on both the maingate and tailgate sides of the goaf, as well as in the bleeder roadway in less than one hour. The sampling tubes located at 1800-2000m behind the face also recorded tracer gas concentrations.
When the airflow bleeder was closed off during the second test the arrival time of tracer gas to various sampling points had increased substantially. Arrival times to...click here to read on.