Wednesday, June 25, 2008

CO2 sequestration -- size of the kill zone

Sometimes, the underground reservoirs that store natural gas explode. Drilling wells into them makes this more likely. When wells explode, the gas generally ignites, making a spectacular flame that can be seen for miles. Aside from the loss of valuable fuel and equipment damage, well explosions generally aren't too big a problem for people living nearby.

One less noteworthy effect of a well explosion is that the CO2 generated from the combustion of the methane is carried high up into the atmosphere by the heat of combustion, where it is mixed by high-altitude winds (routinely 100 MPH).

One plan for CO2 sequestration from coal-fired powerplants is to inject the CO2 into old, empty gas wells. Like the methane, the CO2 is in a supercritical state in the well -- not so much a liquid as a very dense high pressure gas.

The difference between CO2 and CH4 comes when the well explodes. CO2 does not start a fire. Instead, it expands, and cools, and the cold CO2 will flow with the wind, against the ground, eventually dissipating.

A 1 GW (electrical) coal-fired powerplant will burn 2.2 GW (thermal) of coal (because it's about 45% efficient). That's about 7000 metric tons every day. It will produce 4.7 cubic kilometers of carbon dioxide per year, at standard temperature and pressure. That CO2 is fatal to mammals at concentrations greater than 4%.

So, if a sequestration field explodes after 10 years of sequestering the output from a 1 GW coal plant, it will create an invisible blob of CO2 that will be at least 7 km across before it dissipates to the point of being nonlethal.

Think about this thing for a bit. CO2 inhalation is fatal within a couple of minutes, and I suspect it is disabling well before that. Who is going to detect this blob of gas before being overcome? You cannot see it. You cannot run from it. You cannot stay indoors to escape. You cannot start your car to drive away from it. As the wind wafts it across the scenery, it kills every animal in its path. It could go for 50 kilometers or more before wind shear mixes it with enough air to become safe.

Not in my back yard, if you please.


  1. I agree with you that CCS in gas fields has unproven safety and may be a serious concern.

    One solution might be to use offshore deposits.

    But still, there aren't enough gas fields for this to have a huge impact on CO2 emissions. So something else has to be figured out.

    Perhaps mineral sequestration makes more sense. Magnesium silicates (eg olivine) for example are fairly abundant. Mining it, crushing it into a fine powder and dispersing it over the ocean will sequester the CO2 in bicarbonate by reaction of the magnesium silicate with H2O and CO2. This also solves the ocean acidity issue by increasing alkalinity.

  2. It looks like worldwide CO2 emissions are on the scale of 10 gigatons/year. I'd expect you'd need scale 10 billion tons of olivine/year to zero this. If that source is a few hundred miles from the coast, you are talking about 10 billion gallons of diesel to move it, which is 1% of current world petroleum production. It would probably take more to mine and crush it, and to spread it over the water.

    Maybe you could find a place to mine olivine near a big river that you wouldn't mine driving alkaline.

    Or we could power the olivine mining, crushing, transport and distribution from nuclear plants.

  3. We're talking about a lot more than 10 GT/year if all emissions are counted. According to Wikipedia, it's more than 27 GT/year.

    Most olivine mines in use today are close to the shore. Electric rail could go a long way in reducing petroleum consumption. If we're in a pinch, we might even consider the use of steam locomotives powered by coal! But electrified rail is better even though it requires quite a bit of infrastructure.

    We don't really need a lot of energy for ships, spreading the crushed olivine out just along the shoreline would work very well. But considering the quantity, a bit further off might be necessary to spread it out. Hmm, I wonder if ships could be powered by wind, or is that too audacious an idea? ;)

  4. Check out the Lake Nyos incident--
    1700 people killed by a natural CO2 cloud.

    --Carl Feynman