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Improving gas sensing in underground mines

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World Coal,

Mario Moura, marketing director for Honeywell Life Safety in EMEA, explains how new innovations in oxygen sensing are improving the reliability of gas detectors in underground mines, while reducing costs.

Mining gas risks are numerous and ever-present. Methane is both explosive and toxic and a common risk in coal mines. Additional risks are created by explosion shockwaves that can cause coal dust, which is prone to spontaneous combustion. Hydrogen is also an explosive threat, whilst black damp – a mixture of carbon dioxide and nitrogen – formed from corrosion, removes oxygen, creating a risk of suffocation. Carbon monoxide is also prevalent and highly toxic, as is stink damp – hydrogen sulphide. This makes the use of robust gas detectors an essential aspect of mine safety.

In reality, gas detectors are only as good as the sensing elements they contain, so as much attention should be given to the sensor itself as the detector’s capability.

Sensor use in mines

Sensors have a hard remit in tough mining environments. Pellistors are used predominantly for detecting flammable gases, with infrared (IR) sensors being increasingly used as an alternative to measure hydrocarbons. Electrochemical sensors are widely used to measure oxygen and toxic gases, thanks to their fast speed of response and stability. UK coal mines were among the first to use pellistors for methane detection, followed by electrochemical sensors for oxygen, carbon monoxide and other toxics.

One of the constant challenges for electrochemical sensors in mines is the extreme temperature and humidity conditions they can encounter. It can cause premature sensor failure and, in the case of transients, result in nuisance alarms. Pressure shocks can also occur during mine shaft lift descents, resulting in nuisance alarms.

Cross-sensitivity is another challenge for measurement accuracy, particularly from hydrogen and methane gases on carbon monoxide sensors, requiring robust filters to prevent such interference. Hydrogen cross-sensitivity is a particular concern in South African Freestate mines. These combined environmental factors make nuisance alarms a prevalent and costly issue that must be minimised by the use of high quality, ultra-stable sensors.

Safety is of course the main concern in a mine; every second counts and can mean the difference between life and death for miners. Not only does the sensor have to be accurate and dependable it needs to achieve rapid response times.

Developments in oxygen sensing

Delivering improved safety, performance and reduced operating costs are the key requirements in developing new oxygen sensors for use in mines. With the 4OxLL, City has strived to create a new benchmark in long life oxygen sensors that delivers on all these requirements.

City Technology has built on its proven “oxygen pump” technology, which has been successfully used in the field for over four years. Using novel design tools, such as finite element modelling, has resulted in a unique internal design that ensures electrolyte and electrodes remain in contact in all environmental conditions – in particular this significantly reduces failure in extreme RH environments. Consequently the 4OxLL has the widest environmental operating range of any of the company’s sensors making it ideal for use in the hot and wet conditions found in many mines. This coupled with fast response and recovery times and reduced impact of environmental transients means the sensor reduces nuisance alarms.

Many gas detectors use oxygen sensors with a 2 year lifetime. By removing the consumable anode used in galvanic oxygen sensors the 4OxLL breaks through this lifetime barrier. With 4OxLL the sensor will last as long as the instrument so removing the need for regular and costly sensor replacement during the detector’s life.

To ensure the 4OxLL delivers we have undertaken extensive testing throughout development against a range of standards, including the AN/NZS 4641 mining standard.

Written by Mario Moura. Edited by

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