Goaf gas flow behaviours uncovered

Until recently, knowledge of goaf gas flow behaviour has been scanty. A research project due to be wrapped up in March next year will deliver some important solutions for future gas management of longwall goafs.

Staff Reporter

Until recently, knowledge of goaf gas flow behaviour has been scanty. A research project due to be wrapped up in March next year will deliver some important solutions for future gas management of longwall goafs. Already, the ideas that have developed out of this research have improved drainage rates at the Dartbrook mine in New South Wales by 200%, according to researchers.

Current gas drainage rates are 4000-4500 litres per second. Being able to achieve this kind of high capacity goaf drainage in a spontaneous combustion prone mine has been a major achievement and has important implications for the planning of gas prone mines in the future.

Incorporating data from five longwall panels, and taking place over three years, the research project was conducted by the Brisbane-located CSIRO exploration and mining division with the support of JCOAL of Japan, Dartbrook Colliery and the Australian Coal Association Research Program (ACARP) for technology transfers. Primary researcher Dr Rao Balusu, senior mining engineer at CSIRO exploration and mining, spoke to ILN about some of the findings of the project.

What is unique about this study is that it incorporated a range of techniques and modelling tools in an attempt to take an integrated approach to the problem. These included, among others, goaf gas distribution monitoring with face retreat, ventilation studies, tracer gas studies, adopting various drainage strategies and caving characterisation. The project also involved a number of modelling studies using computational fluid dynamcis (CFD) codes.

Balusu said the aim of the project was to understand what is happening inside the goaf with gas flow behaviour where access is not possible. It is not surprising that this integrated approach has revealed that many traditional methods of ventilation and gas drainage may in fact be partly responsible for generating problems within a goaf.

In order to characterise goaf gas flow dynamics a range of techniques was used, such as extensive gas monitoring with up to 12 tube bundle points for monitoring gas on both sides of the goaf. Samples were gathered from all seals to check gas build-up in the goaf with face retreat.

Tracer gas studies were also carried out with SF6 gas being introduced into the longwall goaf. The travel time, concentration, distance travelled, and behaviour of the SF6 gas were then successfully monitored to gain a better understanding of gas flow dynamics. This data was used to generate a gas model. Changes in various parameters were then introduced to investigate their impact on goaf gas distribution.

The most important findings were that goaf gas is affected by a number of parameters, previously thought to have little or not affect on gas behaviour. Changing ventilation practices, for instance, impacts on goaf gas behaviour, as does the speed of retreat, the number of cut-throughs left open behind the face, and the presence of geological disturbances, according to Balusu.

The usual gas drainage practice is to drain from surface goaf holes located immediately behind the retreating longwall face, with no draining continuing in deep goaf and adjacent goafs. "The normal perception is that gas drainage at more than 1000m behind the face has no effect on face gas levels, but deep goaf holes affect goaf gas dynamics and come onto the face, perhaps not immediately, but eventually they do," Balusu said. In addition, Balusu pointed out that not draining gas from deep goaf holes increases the gas emissions substantially in the next longwall panels.

In addition, the studies have shown that gas migration from one goaf to another can occur, through seals and fractured zones. Balusu said the research pointed to two immediate actions which could be implemented in a gas drainage program.

The first is to keep goaf holes running as long as possible, provided oxygen content is under 3%, and secondly, cut-throughs behind the face should be closed as early as possible to reduce oxygen ingress into the goaf. Reducing the number of cut-throughs open behind the face was shown to reduce the oxygen penetration distance into the goaf, which helps with spontaneous combustion control.

Other changes implemented at Dartbrook have been changing the start-up area hole location from 50m to 150m for drainage of CO2 gas; increasing goaf hole spacing from 120m to 300m in the second half of the panel, increasing the goaf drainage suction and flow capacity and draining from adjacent goafs. Currently, eight to 10 goaf drainage holes are being used for longwall panels with 2km length.

The researchers faced some unusual problems at Dartbrook, which has a gas content ranging from 6cu.m/t to 11cu.m/t with CO2/CH4 ratios ranging from 90:10 to 60:40. With longwall goaf gas emissions averaging 7000-9000l/s, the mine is the gassiest mine in Australia and has a medium to high propensity for spontaneous combustion. This also means bleeder return ventilation systems cannot be used for goaf gas management.

The requirement for high goaf gas drainage is at odds with the need to prevent oxygen ingress into the goaf, Balusu said. Draining the higher percentage (80%) of CO2 gas proved difficult using traditional surface goaf hole methods, with around 50% air dilution because of negative buoyancy of the gas. In the past, surface goaf holes had to be decommissioned within four to five weeks. To allow adequate drainage of CO2, several design changes were incorporated into the methodologies for gas capture. This included reducing goaf holes slotted casing section from 48m to 12m and finally to 6m.

These changes reduced oxygen concentration in goaf holes from over 10% to below 5% and thus allowed gas flow from each hole to be increased from 1000 to 2500 litres per second and also allowed the operation of goaf holes for longer periods. This has had a major effect in reducing longwall return gas levels, by up to 50% even when goaf holes were over 1000m deep in the goaf.

To date, surface goaf holes have not however proved successful for draining CO2 in the start-up area of longwall panels. Further research needs to be carried out to address this problem and to investigate the effect of changes in caving characteristics on goaf holes and casing in deeper longwall panels to further improve the goaf gas drainage system, according to Balusu.

At Dartbrook, the major positive outcomes of the project are a major reduction in mine gas drainage cost, and a shift in focus from pre-drainage to post-drainage.

By March 2001, final data compilation will be complete and Balusu will present the findings of the project to the coal mining industry.

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