Real Time Return Gas Monitoring for Outburst and Gas Drainage Assessment

The aim of this project was to create a turn key, prototype, stand alone, real time, gas monitoring system capable of providing quantitative assessments of gassiness levels on at least a shift by shift basis and enabling unusual gas emission patterns to be readily flagged. The project involved the design and construction of a return gas monitoring system and its trial at Tahmoor Colliery and Dartbrook Mine.

The System Comprises:

hardware components - high accuracy CH4 and CO2 gas analysers, air velocity meter, belt weigher, PLC, MODEMS and computer; and software for data logging (CITECT) and data processing.

The gas monitoring system (hardware and CITECT software), is a fully functional monitoring system in its own right. It has a number of features not found in other systems currently operating in Australian underground coal mines.

The GeoGAS Return Gas Monitoring System (RGMS) data processing software was written from the ground up to undertake the necessary calculations and establish a process for operators to follow, to arrive at meaningful results. The RGMS software is a fully functional PASCAL language (Borland Delphi 2.0), 32 bit WINDOWS program.

Quantification of the face area gas emission on a shift by shift basis has been achieved with low maintenance and high system up time. Reports are generated quantifying the gas make of CO2 and CH4 (in m3/t), and the gas quantity in m3. A significant part of the process is validation of the data. That process is well defined in the RGMS data processing software. A hard copy report is generated that includes the computer's characterisation of the gas emission in terms of abnormally high, normal above average, normal below average, and abnormally low emission.

Via the documented process, the user is provided with a wide range of data and data checks, so that his final hard copy report contains his verdict on the gas emission in a form that should be readily useable in mine gas management plans.

While there is ample scope for further refinement, and research into devising an intra shift gas emission assessment, the system as it stands has considerable application within the coal industry. Particular areas of application are:

• Characterisation of gassiness in areas of difficult drilling. The minor outburst at South Bulli Colliery provided a good example of the application of this system. Return gas monitoring was the only real option in assessing gassiness during mining. 

• Return gas monitoring is an additional barrier that could make the Inspectorate feel agreeable to raising the gas content threshold for coals with inherently low gas desorption rates. 
• New mines, especially those in Queensland where depth and gas content are progressively increasing with depth, need to know that mining is being carried out in an environment of zero "gas dynamic incidents". Conditions will change with depth. In addition to measures (such as the GeoGAS Desorption Rate Index) aimed at defining when to take action to alleviate outburst potential, return gas monitoring should prove to be an important additional barrier. 
• Existing mines with well developed outburst management strategies can still benefit from this application. Apart from introducing an additional safety barrier, there is the potential to rationalise drilling operations through fine tuning of gas drainage design. In time, there may be scope to increase gas content threshold levels without compromising safety, by placing greater reliance on the return gas monitoring system. 
• In the event of an incident occurring, the return gas monitoring system enables the best possible back analysis, quantification and diagnosis of the event. The "borehole blowout event" at Tahmoor Colliery is one such example.

Miner drivers from time to time report increased (or decreased) gassiness. Ready access to return gas monitoring data can help to understand the environment, alleviate people's concerns and take action as appropriate.

Findings and Conclusions

The aim has been essentially achieved.  The "turn key" aspect of the project has been an overriding consideration. By "turn key", we mean a system that has been thought through, is well documented in its application and is robust.

The gas monitoring system (hardware and CITECT software), is a fully functional monitoring system in its own right. It has a number of features not found in other systems currently operating in Australian underground coal mines:

• High gas analyser accuracy coupled with short response time. 

• High sampling frequency (down to 2 seconds). 
• High level of flexibility in setting sample rates and moving averages, with the user able to change settings in the PLC via the CITECT software interface. 
• Complete automatic calibration of the gas analysers. 
• A high level of PLC controlled condition monitoring to ensure that the system is working.

The decision to use CITECT software was made to create a fully functional, WINDOWS based, operator interface, complete with trending, alarms and user access rights. CITECT is an Australian made and supported product, that is becoming widespread throughout the industry.

CITECT is good for data logging and trending (real time functions), but less amenable to intensive data processing. The GeoGAS Return Gas Monitoring System (RGMS) software was written from the ground up to undertake the necessary calculations and to establish a process for operators to follow, to arrive at meaningful results. The RGMS software is a fully functional PASCAL language (Borland Delphi 2.0), 32 bit WINDOWS program.

Because it is a separate program to CITECT, it has the ability to process data derived from any SCADA package - not just CITECT.

The system achieved shift by shift assessment of gas emission. It was built with the capability to undertake intra shift analysis, by inclusion of the belt weigher output. The belt weigher worked long enough at Tahmoor Colliery to establish that this aspect works. The system hardware and the CITECT software take full account of the belt weigher output. Additional programming of the RGMS data processing software is required to include belt weigher output. Other algorithms will need to be developed in this software to use belt weigher output for intra shift assessments.

The aim of "defining and documenting the process of using this data to back analyse gas drainage effectiveness" has

been achieved, but without physically undertaking an actual analysis.

The subsidiary aim to "assess the potential for using the technique to quantitatively define seam gassiness on a sub shift period basis" was not addressed due to inability to obtain belt weigher data.

The aim of "defining indices relating the gas emission response (rate of emission, peak emission rate, quantity, composition) to outburst proneness in terms significant to outbursting" has been reasonably achieved. The detailed reporting process as defined in the RGMS data processing software, guides the user toward arriving at a decision concerning the level of gassiness of the coal being mined and implications from a gas emission and outburst view point.

The system is put forward as an additional barrier to outburst preventation. Until more use is made of the technique, it is not seen as a replacement for existing barriers.

A small outburst at South Bulli Colliery triggered a real life need for this type of monitoring. Day by day updates on the levels of gassiness during mining were forwarded to the mine. Data files were accessed via MODEM, processed, and a report faxed back to the mine. This showed that the concept had application, even though the data processing at the time, was quite cumbersome.

A significant benefit beyond the scope of the project is the ability to use this data to more accurately model gas emissions on development for design of ventilation and as necessary, gas drainage and gas capture control measures.