Capturing methane emissions from coal mines around the world could significantly reduce the amount of heat-trapping gases in the atmosphere, as well as lead to improvements in mine safety and local air quality, according to a report by the Clean Air Task Force (CATF).
Emissions from fossil fuel consumption account for the majority of global greenhouse gas emissions, but extraction of oil, natural gas and coal also emits greenhouse gases, particularly methane that frequently mingles within coal seams or oil and gas deposits.
Emissions generated by the extraction of fossil fuels accounts for nearly half as much near-term warming as is caused by burning of fossil fuels.
Typically, the impacts of greenhouse gas emissions are studied on 20- or 100-year horizons. Carbon dioxide, which makes up nearly 80 percent of total global greenhouse gas emissions, can linger in the atmosphere for centuries, trapping heat long after it was released from a factory, automobile tailpipe or dried-up piece of tropical peat moss.
Methane remains in the atmosphere for a much shorter time period but is 70 times as potent a heat-trapping substance as carbon dioxide. That has made mitigating methane emissions a short-term goal for many environmentalists and policymakers interested in addressing global warming.
Methane released from coal mines accounts for 8 percent of global methane emissions, according to CATF.
"[Methane emissions from coal mines] are important on many levels," said Jonathan Banks, senior climate policy adviser at CATF and lead author of the report. "I think, first, they are a significant concern because methane emissions are 70 percent more potent than other climate pollutants. That's a big deal. Even more so, this is not just about climate change but about mine safety and traditional air quality also."
Methane concentrations between 5 and 15 percent represent an "explosive range," Banks said, that contributes to the deaths of tens of thousands of miners each year due to underground explosions.
"[D]epending on where you are, coal mine methane can lead to local problems with ozone smog," Banks said. " So if the mine is in the middle of nowhere and there are not other sources of pollution, then the methane released simply becomes part of the global background. However, if a mine is in a region that has the requisite factors like other sources of methane or nitrogen oxide or other volatile organic compounds, then the coal-derived methane adds to that local impact."
Capturing a major energy source
The CATF report describes how existing technologies could capture methane gas during coal extraction and offer benefits beyond mine safety and environmental improvements. "When methane is released, it's a waste of energy. It's a product that has a value. It can be burnt quite cleanly," Banks said.
Depending on the quality of the gas, he said, mine companies could use captured methane to power equipment or generate electricity. Methane-powered burners might be used to dry coal or companies could trap the gas and sell it directly to pipelines.
"There's a whole lot of uses and a whole lot of reasons for why we should be capturing methane," Banks said.
Large-scale ventilation systems are used to move air in and out of underground mine shafts. The systems keep miners safe by reducing the risk of methane concentrations entering the "explosive range." Yet those systems release into the atmosphere low concentrations of methane that could otherwise be processed by existing technologies that oxidize the gas, suggests CATF.
Another method for reducing methane emissions, whether underground or on tracks of surface mining, lies in degasifying coal seams prior to extraction.
Small molecules of methane are produced during the same millions-of-years-long process that converts organic material to hard blocks of opaque coal. So when those lumps of fuel are pulled from deep below the surface in northern China or exposed by a blast of dynamite on an Appalachian mountaintop, invisible plumes of methane gas seep out, as well.