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:

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.

Conclusion

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.