TROLEXPERT is a new information source for the coal and mining industry from Trolex Ltd, a leading manufacturer of safety monitoring systems and sensors, and Energy Global. Each month will provide answers to topical questions and challenges facing the industry. This month, TROLEXPERT asks: what is the importance of methane concentration when looking to extract and exploit methane technology projects?

Methane is a flammable gas when mixed with air at concentrations between 5-15% CH4 at atmospheric temperature and pressure. Methane combusts explosively when its combustion products are restricted by the local environment (i.e. confined areas such as mine workings, pipes, tanks, etc).

At elevated temperatures and pressures, the flammability range will widen considerably. This is particularly apparent above 100°C and 1BarG. When compressing coal mine methane, be aware that the flammability limits change.

CMM (Coal Mine Methane) or VAM (Ventilation Air Methane) mitigation or utilisation creates an ignition source. If a flammable mix is ignited by the utilisation equipment, the flame will travel down the connecting roadway, duct or pipe towards the underground mine workings. If the gas is outside the flammable range, there is no chance of a flame propagating towards the mine. CMM installations should have a minimum methane concentration trip-off setting of between 25% and 50% CH4, depending on the process safety risk assessment. 

VAM installations should have a maximum methane concentration trip-off setting associated with the legal requirements of the project location, with an absolute maximum of 2% CH4, depending on the findings of the process safety risk assessment.

Answer provided by Neil Butler, Technical Director at HEL-East Ltd.

Source: Presentation at the Methane Gas in Coal Mining Southern Africa, Extraction and Exploitation Strategies, Technologies and Practices Conference in June 2012.