Operators of underground coal mines around the world are all too familiar with methane (CH4), an explosive gas found in coal seams as a by-product of coal formation. Coal seams often contain significant quantities of CH4 and when released to the atmosphere from gassy mines during coal extraction it constitutes a major source of greenhouse gas emissions. In fact, CH4 emissions from coal mining activities account for approximately 8% of total global human-related methane emissions.
Impact of methane
Methane is a potent greenhouse gas, over 20 times as powerful as CO2 at trapping heat in the atmosphere (on a mass basis over a 100 year timeframe). Since methane has a much shorter atmospheric lifetime than CO2 (about 12 years compared to about 200 years for CO2), reducing methane emissions can achieve significant climate benefits over the next 25 years. In addition, as the principal component of natural gas, methane can provide a clean, locally-available energy resource for coal companies and the surrounding region.
The source of emissions
Coal mine ventilation exhaust air contains very dilute concentrations of CH4, typically below 1%. Yet the flow rates are so large that ventilation air exhaust constitutes the largest single source of coal mine methane emissions to the atmosphere. In recent years, there have been exciting new developments in harnessing the dilute methane in coal mine ventilation exhaust systems. Each year, underground coal mines throughout the world emit more than 500 billion ft3 of CH4 from their ventilation systems. Mines that choose to mitigate these emissions can often obtain carbon credits as an additional revenue stream.
There are several technologies that can be used to destroy ventilation air methane (VAM), including catalytic and thermal oxidation, gas turbines, and a hybrid waste coal/VAM rotary kiln. Thermal oxidation using a flow reversal reactor has emerged as a tested and commercially available solution to VAM emission mitigation. If the technology is employed at active underground coal mines, it offers the potential to mitigate substantial quantities of global CH4 emissions. In addition, oxidisers have the ability to tap the excess heat to produce steam for electric power generation. Two existing projects employing oxidisers are the 6 MW WestVAMP electric power generation facility in Australia (MEGTEC VOCSIDIZER equipment) and the Jim Walter Resources pilot project in Alabama (Biothermica VAMOX technology).
Author: Jayne Somers
Jayne Somers, Ph.D., P.E., United States Environmental Protection Agency Climate Change Division. The US EPA Coalbed Methane Outreach Program promotes the cost-effective recovery and use of methane emitted from coal mining activities.
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