In August of 1986, over 1700 villagers living as far away as 25 km from Cameroon's Lake Nyos died when a mysterious, invisible suffocating cloud enveloped them. They were victims of one of the two known limnic eruptions in recorded history. For years, the waters of Lake Nyos had absorbed carbon dioxide from underground sources, probably volcanic in origin. Because of temperature differences in the lake, the gas-saturated water remained at the bottom until something, possibly as insignificant as high winds, triggered a lake turnover. Once the eruption began, the lake began to boil like a soft-drink bottle you leave out in the sun and open by mistake. A giant cloud of carbon dioxide spilled out of the lake and smothered people and animals for miles around.
This is the same compound that, if numerous carbon sequestration projects now underway are successful, will be buried under tremendous pressure in dozens or hundreds of locations all over the world. The question is: will it stay there?
Ever since humans discovered fire, we have been adding to the amount of carbon dioxide in the atmosphere. Until the 1800s, the quantity of carbon dioxide humans put into the air was negligible compared to that contributed by natural causes such as forest fires and volcanic activity. The concern with rising levels of atmospheric carbon dioxide, of course, is that it tends to raise the Earth's temperature, other things being equal (which they never are). There is a general scientific consensus that (a) human activity has caused much if not most of the rise in carbon dioxide levels in the past two hundred years and (b) this will cause some increase in the Earth's average temperature, though how much and for how long is a matter of debate. Some theories even posit that a short temperature rise will trigger an instability that will wind us up in another Ice Age a few decades afterwards. Whatever actually happens, the political fact these days is that reducing one's carbon footprint has become a virtue, while emitting carbon, even for a good cause such as bringing the blessings of electric power to poor people, is a sin.
The business world has seen the politicians writing "cap-and-trade" on the wall, and so there is big private money to be made in developing systems that will capture the carbon dioxide generated when fossil fuels (most notably coal) are burned in power plants. A French firm called Alstom is currently building several pilot plants around the country, including one attached to a coal-fired power plant in New Haven, West Virginia.
The technology itself is rather clever. After cooling and washing the flue gas with cold water, they bubble it through a solution of ammonium bicarbonate (contains hydrogen) and ammonium carbonate (no hydrogen). Nearly all the carbon dioxide combines with the ammonium radicals. They pump the fizz-rich liquid under pressure to a heater where the carbon dioxide boils off and is compressed to send it underground. And there, in my opinion, the real trouble begins.
Never mind that the whole pile of machinery is doing something that engineers of an earlier era would have considered ludicrous: capturing the main gaseous combustion product and shooting it underground. The operation adds nothing to the efficiency of the plant, takes a fair amount of energy itself, and creates a long-term hazard compared to which nuclear waste is relatively harmless.
Look at it this way: would you rather live five miles from some well-shielded solid radioactive stuff whose emissions can't even be detected outside the plant boundaries, and which will just sit there and gradually cool off for the next few hundred years; or, would you like to live an equal distance from the wellhead of a giant underground reservoir which, if released, will suddenly spew out and make Lake Nyos look like a minor traffic accident? For my money, I'll take the nuclear stuff any day.
Presumably, geologists have been careful to select locations where the underground carbon dioxide is relatively safe and isolated. Okay, but our experience with large underground gas reservoirs of artificial origin is limited, to say the least. While natural gas has been stored underground for many years (often in depleted gas fields, not coincidentally), the two cases are significantly different. For one thing, natural gas storage is limited to transient market-related storage needs, and so the pressures and volumes required are relatively modest. By contrast, carbon sequestration will be "permanent"—the whole point is to send it down there and make it stay there indefinitely. If it escapes to the atmosphere we are back at Square One after spending billions of dollars for nothing, plus quite likely having numbers of dead citizens on our hands. The pressures and volumes eventually needed for carbon sequestration, if carried out on a large scale, will dwarf the current natural-gas underground storage facilities. While I am unaware of any major accidents that have happened with underground natural gas storage, there may have been some. Of course, carbon dioxide doesn't burn and natural gas does, but suffocation from a non-flammable gas makes you just as dead as if you had burned to death.
What makes a whole lot more sense from a technical point of view is to replace coal-fired power plants with nuclear plants as fast as we can. Nuclear energy generates zero carbon emissions, the nuclear waste problem is manageable even without the ill-fated Yucca Mountain disposal facility that the Obama Administration recently axed, and there are no particular concerns about running out of nuclear fuel any time soon. If we get low we can switch to the kind of reactor that makes more than it consumes.
That is the technical reality. But the political reality right now, which engineers as well as everyone else has to deal with, is that nuclear power is under the same emotion-laden mushroom cloud that has characterized it ever since nuclear weapons ended World War II, and has never freed itself from the almost superstitious fear that the word "nuclear" inspires in many people. Some of that fear has now been transferred to plain old carbon dioxide, a gas which each living human being emits every time we take a breath. When you end up being afraid of yourself, there's no place to hide.
It is still early in the carbon sequestration business, and there is time for the political winds to change before we all get burdened by carbon cap-and-trade taxes to pay for giant sequestration plants that send carbon dioxide into the ground, only to have some of it pop up one day in an unexpected place. Let's hope that cooler heads prevail and we reach a consensus that does sensible things about carbon emissions without burying a lot of unwelcome surprises for our descendants.
Sources: A good article originating at the Washington Post, describing the technology and politics of the carbon sequestration process pioneered by Alstom can be found at http://www.pittsburghlive.com/x/pittsburghtrib/business/s_638488.html. A description of the geology surrounding the Lake Nyos disaster can be found at http://www.cevl.msu.edu/~long/nyos.htm.