The Cost of Sucking

I’m talking about carbon dioxide scrubbers. This post will just be an extended quote from an excellent book, which is free online:

• David McKay, Sustainable Energy: Without the Hot Air.

It will help us begin to understand the economics. But some numbers may have changed since this was written! Also, the passage I’m quoting focuses on taking carbon dioxide out of the air. This not really what I’m researching now: I’m actually interested in removing carbon dioxide from the exhaust from coal-fired power plants, at least until we manage to eliminate these plants. But the two problems have enough similarities that it’s worth looking at the former.

Here is what McKay says:

The cost of sucking

Today, pumping carbon out of the ground is big bucks. In the future, perhaps pumping carbon into the ground is going to be big bucks. Assuming that inadequate action is taken now to halt global carbon pollution, perhaps a coalition of the willing will in a few decades pay to create a giant vacuum cleaner, and clean up everyone’s mess.

Before we go into details of how to capture carbon from thin air, let’s discuss the unavoidable energy cost of carbon capture. Whatever technologies we use, they have to respect the laws of physics, and unfortunately grabbing CO2 from thin air and concentrating it requires energy. The laws of physics say that the energy required must be at least 0.2 kWh per kg of CO2 (table 31.5). Given that real processes are typically 35% efficient at best, I’d be amazed if the energy cost of carbon capture is ever reduced below 0.55 kWh per kg.

Now, let’s assume that we wish to neutralize a typical European’s CO2 output of 11 tons per year, which is 30 kg per day per person. The energy required, assuming a cost of 0.55 kWh per kg of CO2, is 16.5 kWh per day per person. This is exactly the same as British electricity consumption. So powering the giant vacuum cleaner may require us to double our electricity production – or at least, to somehow obtain extra power equal to our current electricity production.

If the cost of running giant vacuum cleaners can be brought down, brilliant, let’s make them. But no amount of research and development can get round the laws of physics, which say that grabbing CO2 from thin air and concentrating it into liquid CO2 requires at least 0.2 kWh per kg of CO2.

Now, what’s the best way to suck CO2 from thin air? I’ll discuss four technologies for building the giant vacuum cleaner:

A. chemical pumps;
B. trees;
C. accelerated weathering of rocks;
D. ocean nourishment.

A. Chemical technologies for carbon capture

The chemical technologies typically deal with carbon dioxide in two steps.

  concentrate   compress  
0.03% CO2 Pure CO2 Liquid CO2

First, they concentrate CO2 from its low concentration in the atmosphere; then they compress it into a small volume ready for shoving somewhere (either down a hole in the ground or deep in the ocean). Each of these steps has an energy cost. The costs required by the laws of physics are shown in table 31.5.

In 2005, the best published methods for CO2 capture from thin air were quite inefficient: the energy cost was about 3.3 kWh per kg, with a financial cost of about $140 per ton of CO2. At this energy cost, capturing a European’s 30 kg per day would cost 100 kWh per day – almost the same as the European’s energy consumption of 125 kWh per day. Can better vacuum cleaners be designed?

Recently, Wallace Broecker, climate scientist, “perhaps the world’s foremost interpreter of the Earth’s operation as a biological, chemical, and physical system,” has been promoting an as yet unpublished technology developed by physicist Klaus Lackner for capturing CO2 from thin air. Broecker imagines that the world could carry on burning fossil fuels at much the same rate as it does now, and 60 million CO2-scrubbers (each the size of an up-ended shipping container) will vacuum up the CO2. What energy does Lackner’s process require? In June 2007 Lackner told me that his lab was achieving 1.3 kWh per kg, but since then they have developed a new process based on a resin that absorbs CO2 when dry and releases CO2 when moist. Lackner told me in June 2008 that, in a dry climate, the concentration cost has been reduced to about 0.18–0.37 kWh of low-grade heat per kg CO2. The compression cost is 0.11 kWh per kg. Thus Lackner’s total cost is 0.48 kWh or less per kg. For a European’s emissions of 30 kg CO2 per day, we are still talking about a cost of 14 kWh per day, of which 3.3 kWh per day would be electricity, and the rest heat.

Hurray for technical progress! But please don’t think that this is a small cost. We would require roughly a 20% increase in world energy production, just to run the vacuum cleaners.


Okay, this is me again: John Baez.

If you want to read about the other methods—trees, accelerated weathering of rocks, and ocean nourishment, go to McKay’s book. I’m not saying that they are less interesting! I am not trying, in this particular series of posts, to scan all technologies and find the best ones. I’m trying to study carbon dioxide scrubbers.

16 Responses to The Cost of Sucking

  1. Bob says:

    NIST is/was exploring the use of octagonal pores to filter CO2::
    Researchers Study Zeolite for Filtering Out Carbon Dioxide

  2. You might have a look at this:

    Powdered limestone is currently used in scrubber baghouses to remove 2.5 micron particulate matter from smoke, it could get the CO2 as well. The waste gypsum goes into the ash pit with the coal ash, so the only thing to worry about is lining the ash pit with a geomembrane, especially if the soil is permeable or semi-permeable. if there’s a good clay layer, then this problem is of less importance.

    See also

  3. ecoquant says:

    I just learned that Prof Wally Broecker, great scientist and oceanographer, died Monday at 87. Among his many, many other accomplishments, he wrote about and spoke about clear air capture of CO2, and emphasized the need to pursue something like that.

  4. domenico says:

    I am thinking that it is possible the carbon dioxide separation in ions CO+O with laser (exciting the molecular orbitals)
    there is an energy of 19.466 eV for molecule dissociation, so that if a single frequency laser dissociate the carbon dioxide in ions, and a constant electric field separate the ions in a gas flow, then a repeated laser injection on a gas flow could increase the carbon dioxide without high pressure.
    The only example that I found is
    so that there is a technology, but I did not evaluate the required energy, but I think that would be strange not to use it for air separation plants (if it were energetically favorable).

    • You’ve got to power your laser somehow – and you wind up with highly reactive radicals – a carbon monoxide radical and an oxygen radical. Chances are that they recombine to form CO2, unless they are immediately captured on some sort of medium or other.

      • domenico says:

        If there are ions in the dissociation CO+ and O-, then it is possible an electric drift in the chamber, and after a possible recombination with the right flows, with paths that connect carbon monoxide ions and oxygen radicals (it is possible the recovery of the light energy emitted in the recombination, like thermal or optical energy); it could be possible a increase of the concentration of the carbon dioxide step by step (it is not necessary a high concentration, but a good concentration of carbon dioxide), repeating the procedure; it could be possible the increase of the efficiency with reflective chambers, to reflect the laser photons.
        I read an article
        that contain many of the methods of separation, and I think that there are many explored alternatives, but strangely this was missing (perhaps because of the ineffectiveness).

  5. “Now, let’s assume that we wish to neutralize a typical European’s CO2 output of 11 tons per year, which is 30 kg per day per person. The energy required, assuming a cost of 0.55 kWh per kg of CO2, is 16.5 kWh per day per person. This is exactly the same as British electricity consumption.”
    “At this energy cost, capturing a European’s 30 kg per day would cost 100 kWh per day – almost the same as the European’s energy consumption of 125 kWh per day.”
    Something wrong there… Maybe it should read “almost 10 times the European energy consumption of 12.5kWh a day”?

    • John Baez says:

      Yes, something funny is going here: the British are not particularly abstemious compared to other Europeans when it comes to consuming energy!

      Wikipedia says the UK’s total power consumption per capita was about 4 kilowatts in 2013. I find “kilowatt-hours per day” to be an annoying unit of power, but since there are 24 hours per day it equals 24 kilowatts, right? So then the UK’s total power consumption is 24 × 4 = 96 kilowatts per day.

      That’s pretty close to McKay’s claimed “European’s energy consumption energy consumption of 125 kWh per day”.

  6. Seth R Trotz says:

    Have you seen this work out of Australia on carbon capture using a novel catalyst? Seems interesting:

  7. Jason Doege says:

    CO2 capture could conceivably take place anywhere on earth. There is a place where energy is incredibly abundant but impractical to use because it is so far away from population centers. I imagine that CO2 capture mechanisms could be put on large barges in the the oceans using wave and wind energy, far from land.

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