Methane: curse or blessing?
by: Paul Kando
Methane (CH4) is a fossil fuel. Methane is an abundant energy-source continually produced by nature. Methane is a potent greenhouse gas. Methane is a hydrocarbon, the simplest alcane -- a class of organic chemicals that consist of a carbon chain with only carbon and hydrogen atoms and no double bonds. Natural gas contains about 75% methane. The rest is 15% ethane (C2H6), and a mixture other hydrocarbons, such as propane (C3H8) and butane (C4H10). Methane is lighter than air, colorless and odorless -- the smell of fuel gas comes from mercaptan, a sulfur-containing substance added to warn of a gas leak. The abundance and relatively clean burn of methane makes it attractive as a fuel, however, because under normal conditions of temperature and pressure it is a gas, it must be piped from source to user. In contrast, propane, butane, and common mixtures of the two are bottled as liquid petroleum (LP) gas.
"City-gas" works have been producing methane from coal for generations. In nature microbes known as methanogens produce it in a complex, multi-step anaerobic respiration process called methanogenesis or biomethanation. Methanogens are not bacteria, although they live together with anaerobic bacteria in the guts of animals, (especially cattle and other ruminants), termites, and also in landfills. Methane production is the final step in the decomposition of biomass -- a process by which nature recycles living matter and organic substances are broken down into simpler chemicals.
As a greenhouse gas, methane is 25 times as potent as carbon dioxide (CO2), i.e. over the upcoming century the effect on global temperature of a methane emission will be 25 times greater than that of a CO2 emission of similar mass. However -- because it remains in the atmosphere for 10 years or less, while the less potent CO2 remains longer than a century - the comparative short-range warming potential of methane is even greater.
Dinosaur flatulence probably warmed the Earth. Today 16% of the world's methane emissions come from cattle and, according to studies, cattle, chickens, and pigs together produce 37% of all human-induced methane. Taken together, the supply chain, production, byproducts, and transportation of meat and dairy products add up to more than half of global greenhouse gas emissions. Dietary changes, medical treatments and research to capture the methane for its energy content have all been proposed - so far with very limited success. Rice fields, too, generate large amounts of methane during plant growth, however direct emissions of methane by plants have not been documented.
There is a large amount of methane locked up in methane clathrates - ice-like combinations of methane and water in the Earth's crust and the ocean floor. In 2010, measured methane levels in the Arctic have been more than twice as high as at any time in 400,000 years prior to the industrial revolution. Arctic methane releases from permafrost and once-frozen lakes are a feedback effect - both a consequence and a contributor to global warming.
Methane burns readily, producing carbon dioxide and water vapor. Oxidizing in the atmosphere, it is a major source of stratospheric water vapor - another potent greenhouse gas. On the other hand, burning methane for energy on the ground, instead of releasing it directly into the atmosphere, reduces the greenhouse effect, since CO2 is a weaker greenhouse gas than CH4. Methane easily reacts with the hydroxyl(-OH) radical. Thus the more atmospheric methane, the lower the concentration of remaining -OH radicals. Therefore, as its concentration grows methane increases its own atmospheric life-span and cumulative greenhouse effect. The oxidation of methane can also produce ozone, yet another greenhouse gas.
Atmospheric methane concentration has increased by about 150% since 1750. It now accounts for a fifth of the total climate-impact of all long-lived greenhouse gases other than water vapor. Fossil methane (natural gas) is best left in the ground, since - even ignoring the untold damage caused by hydrofracking -- its use or release contribute to global climate change. On the other hand currently produced natural methane is a valuable source of energy. Allowed to waste into the atmosphere it damages the climate, but captured as fuel it can help preserve it. Recently, methane emitted from coal mines has been used to generate electricity. As biogas, methane is generated from such organic matter as manure, sewage sludge, municipal solid waste, and any other biodegradable organic feedstock. The process is straightforward and environmentally benign. Methane generators are widely used by farmers in China and India. A growing number of cities and towns use biomethane as transportation fuel or to generate electricity. Methane generation from sewage and municipal waste is a smart economic move: it captures a valuable fuel, avoids the cost of waste disposal and minimizes environmental degradation.
Methane may even have a key role in a 100% renewable energy-based economy. More on that next week.