Speakeasy Science

Firedamp

The old mining term for explosive gases in coal mines is “firedamp”. It seems illogical – I mean, a damp fire? – until you realize that it comes from the German word “dampf” for vapors.

There are other “damps” in mining terminology – “afterdamp”, for instance, refers to the poisonous gas carbon monoxide, which tends to build up in mines after an explosion. But firedamp explicitly refers to a gas mixture rich in the flammable gas methane, which – as the recent disaster in West Virginia’s Upper Branch Mine reminds us – burns like a devil’s torch. Twenty-five miners were killed outright in the methane explosion at the mine and this week’s efforts to rescue four workers still unaccounted for were stymied when rising smoke signaled that fires were still burning nearly 1,000 feet underground.

It’s no real surprise that methane ignites and burns – we depend on that quality in fact. The gas is the primary component in the natural gas that we burn for energy purposes. And it’s no surprise either that methane is readily found in coal mines. Like coal, natural gas is a hydrocarbon material – the chemical formula is CH4 (one carbon atom and four of hydrogen)  – created when ancient organic material from long-dead plants and animals is subjected to underground heat and pressure. The gas methane is usually found in the same seams that contain coal itself, released when miners drill into those areas.

The trick is, then, avoiding igniting a highly flammable gas that easily builds up in the enclosed space of a mine. Mining companies tend to deal with this through a combination of methane detectors (dating back to the early 19th century Davy safety lamp) and ventilation systems that remove methane from underground tunnels and also blow fresh air into the system to dilute methane levels. As the operators of the Upper Branch Mine had been cited previously for problems with their ventilation system, questions have been raised as to whether that contributed to the recent disaster.

The amount of methane in air has to be very carefully monitored. The gas is only combustible when it builds up to levels between 5 and 15 percent in the surrounding air. Below that range, there’s not enough to ignite and above it, the mixture is too dense to be explosive (although it can be suffocating). Chemists say that the riskiest amount of methane in the air is when it reaches 9.5 percent saturation – achieving a perilously unstable balance with naturally explosive oxygen.  In that situation, it only takes one spark from a piece of machinery to ignite the gases into a literal fireball.

The basic formula for methane combustion is:

CH4 + 2O2 —> CO2 + 2H2O + heat

In other words, methane and oxygen combine and form carbon dioxide and water. That process releases a blast of heat energy. In a mine, the burning methane can ignite surrounding coal dust, known to be highly explosive, so that a fiery blast of methane can simply serve as the fuse for a much bigger explosion.

This is well known, obvious, straightforward chemistry. We’ve known it as long as we’ve known that firedamp was a problem. There’s no reason – at all – to keep repeating the experiment.