The project, focused on developing post-mining goaf gas drainage technology, began four years ago as a collaborative project between the Japan Coal Energy Center (JCOAL) and CSIRO. Much of the work was undertaken at the Anglo Coal Dartbrook mine in the NSW Hunter Valley as a model mine.
Dartbrook is a very gassy mine, with a high risk of spontaneous combustion in the goaf area. Goaf gas emissions from the longwall face measure 7000l/s to 9000l/s and to control goaf gas emissions in the panel, about 100cu.m/s of airflow was supplied to the face and another 100cu.m/s of airflow was added to the longwall return to dilute return gas concentration. Even intensive pre-drainage programmes were not totally successful because of the low permeability of the coal.
The project has combined extensive field studies with computational fluid dynamic (CFD) models of gas flow in order to characterise goaf gas flow mechanics and develop effective gas and spontaneous combustion (sponcom) control strategies.
Speaking at the Australia-Japan Technology Exchange workshop, held in December in NSW, Gota Deguchi of JCOAL reported that the project attained its objective in March.
The overall scope of the project was as follows:
1. Field experiments with gas drainage, underground gas drainage including long horizontal holes and cross measure holes, and surface goaf drainage to design variations and new gas control techniques,
2. Other field experiments including ventilation and goaf pressure monitoring,
3. A tracer gas study to identify the goaf gas flow,
4. A geomechanical study, and
5. Computational Flow Dynamics (CFD) modeling.
The first part of the project carried out field investigations to improve the performance of surface goaf drainage methods in longwall panels No.1 to No.5 during the project. The number of holes, position, slotted casing length and capacity of drainage station on the surface were changed. As a consequence, though total gas flow from the goaf holes in longwall No 1 reached 90-100cu.m/min, which represents about 20-25% of total goaf gas emissions, a total gas flow of 240-300cu.m/min, which represents a drainage ratio of about 50-60%, was achieved in longwalls No 4 and No 5. As air leakage into drainage holes, which increases the risk of spontaneous combustion, could be reduced, the operating time of each drainage hole could be extended substantially.
The development and use of the CFD model has made it possible to analyse the actual distribution of oxygen and CO2 concentration in a goaf. The model could also estimate the effect of ventilation methods, position of surface gas drainage holes, seam gradients and existence of dykes/faults on air leakage into a goaf.
“The performance of goaf gas drainage was improved considerably at the model mine,” Deguchi said. “Much information on gas flow in a goaf area was collected in the field investigation. The fracture mechanism around a longwall face and gas flow dynamics in a goaf area were also understood through geomechanical modeling and CFD modeling, respectively. Examination of ventilation measures and the location of goaf gas drainage holes could be estimated by the CFD modelling.”
The results of the joint project will be directly connected with mine gas control technology in the other Australian coal mines. Deguchi said a comprehensive review and analysis of the 4-year research project would be carried out during 2001-02 and the results of the entire project will be summarized in a final report. Project results will be presented at industry workshops and conferences to achieve technology transfer to the other mines in Australia and other coal producing countries.
Part of the project used tracer gas tests to help gain an understanding of gas flow in an active longwall goaf.
Some findings were:
Gas flow velocity varies considerably inside a longwall goaf, in this trial from 0.15-0.02m/s in the different parts of the longwall goaf.
Gas was shown to migrate from one goaf to another goaf all along the panel length. Analysis suggests that gas moves from an active longwall to the adjacent sealed longwall goaf through the interconnected bed separation zones in the roof, rather than through the seals in the working section.
Even with a 50m thick barrier pillar and strong seals, gas leaks from one panel to the other, but at a much reduced intensity. Tracer gas results showed that 0.04% to 1% of the tracer gas released in one panel goaf leaked to the other panel goaf, depending on the ventilation pressure differentials.
CSIRO researchers said new control strategies at Dartbrook have tripled the performance of the goaf gas drainage system, with current drainage rates in the order of 4000-5000l/s. Researchers said this had considerably reduced gas related delays on the longwall face and greatly contributed to production increases at the mine.
