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 fore-
most 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physical system,” has been promoting an as yet unpublished technology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.

B. What about trees?

Trees are carbon-capturing systems; they suck CO2 out of thin air, and they
don’t violate any laws of physics. They are two-in-one machines: they are
carbon-capture facilities powered by built-in solar power stations. They
capture carbon using energy obtained from sunlight. The fossil fuels that
we burn were originally created by this process. So, the suggestion is, how
about trying to do the opposite of fossil fuel burning? How about creating

concentrate 0.13
compress 0.07
total 0.20
Table 31.5. The inescapable energy-cost of concentrating and compressing CO2 from thin air.