The advance of methane recovery monitoring equipment technology is helping mining companies to improve safety and generate new revenue. Phil McClean, senior engineer at gas sensing specialist Trolex, explains the opportunities ahead.
Underground mines are the single largest source of coal mine methane (CMM) emissions in most countries, although CMM is also produced from surface mines and as a result of post mining activities such as coal processing, storage and transportation. It is a greenhouse gas that has a global warming potential (GWP) 21 times as great as CO2, thus its escape into the atmosphere is clearly to be avoided. To put the scale of methane emissions into perspective, it is estimated that by 2020 CMM emissions from the world’s coalmines will be some 40 billion cubic metres per annum, about a 30% increase from current levels and about 8% of the total methane emissions from human activity.
Removing methane gas from underground coal mines is a very necessary safety measure and it is only recently that there has been a realisation that methane is a valuable commodity which can be burnt to create electricity, either for localised use or to be sold on to aggregators. In fact methane can be a big revenue earner as demonstrated by UK Coal, which has pioneered the technology, and which in 2007 realised some £4.3 million profit from gas extracted from its mines, and generated enough electricity to power over 40,000 homes.
With the practice of methane drainage for power generation now on the increase, it has become vital to ensure that effective methane recovery monitoring is in place. Accurate continuous monitoring is crucial as changes in gas composition in a system can lead to engine down time and possibly even damage, reducing revenue and endangering both plant and personnel.
The shortcomings of current methane monitoring methods
Current methane monitoring systems have a number of limitations. They use either thermal conductivity or infra-red detectors. Thermal conductivity detectors measure the thermal conductivity of the whole gas sample, giving a collective reading for all the constituents of the gas, not just methane, and are therefore unreliable. Infra-red detection specifically looks for carbon-hydrogen bonds in hydrocarbon, which provides an ideal means of measuring the methane content. Unfortunately any other hydrocarbon components within the sample will also contribute to the response in an infra-red detector. The heavier the hydrocarbon, the disproportionately greater the effect on the detector response is. The complex mixture of hydrocarbons in CMM gas results in significantly higher responses than that due to the methane content alone, and the response can often exceed what would be expected for even 100% methane.
At many mines gas chromatography is used to analyse the CMM sample gas stream, periodically measuring individual amounts of each component gas. Because samples have to be collected from the gas stream and sent to a laboratory for analysis, and the results then have to be sent back to site, this clearly takes too much time and is far from a continuous measurement process.
New developments in methane monitoring
After much research and development at Trolex, a new methane monitoring system has now been introduced which uses infra-red detection coupled with a unique method to measure the methane content from the sample on a continuous basis. The way this works is that an initial gas analysis is used to “train” the system and subsequent analysis can be used to fine-tune the system. The contributions to the overall signal from the detector caused by methane and other hydrocarbon gases are evaluated and the methane content is derived from the overall signal and the individual contributions from the different gases. This technique provides a continuous online, high accuracy monitoring capability for methane drainage.
Known as Sentinel, Trolex’s highly accurate new system for methane recovery monitoring is a huge step forward in addressing methane recovery, either as a stand-alone gas-to-energy project, or as part of the Clean Development Mechanism (CDM) and the Joint Initiative (JI) flexible mechanisms agreed under the Kyoto Protocol. It is a fully integrated system with the advantages of maximising safety, meeting legal and regulatory requirements as well as generating new revenue streams, all in parallel.
The scope for new methane monitoring
So what are the wider opportunities which this advanced method of methane monitoring is now helping to facilitate? To give some examples, captured methane can be used for purposes such as natural gas pipeline injection, electric power generation, co-firing in boilers, district heating, mine heating, coal drying, vehicle fuel, and manufacturing uses such as feedstock for carbon black, methanol and dimethyl ether production. For the very low concentration methane in mine ventilation air, technological development has progressed to the point that this CMM source can be oxidised and the resulting thermal energy used to produce heat, electricity, and refrigeration.
The new methane monitoring technology is the result of decades of expertise in gas monitoring, and most systems are bespoke precision-engineered to meet the needs of each individual project in order to maximise performance. Long-term system integrity is also ensured by dedicated support for initial installation and ongoing service.
Even though the Sentinel system is new, the technology has already been installed in two coal mines in the UK and one in China and is being effectively used in a gold mine in South Africa. Most schemes are either approved or registered with the UNFCCC clean development mechanism (CDM) for gas to energy generation, and the system has been proven in dynamic testing conditions by independent authorities.
Now that there are significant advances in methane monitoring technology available, we will undoubtedly see many more projects around the world not only benefiting from increased site safety, but also protecting the environment and generating substantial additional revenue for their organisations at the same time.