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Guest post by
John Morgan.
John
runs R&D programmes at a Sydney startup company. He has a PhD in
physical chemistry, and research experience in chemical engineering in
the US and at CSIRO. He is a regular commenter on BNC.
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Introduction
Liquid hydrocarbons account for about
one third of fossil carbon dioxide emissions,
and while transition to electric vehicles is possible for some
passenger transport, it is simply not feasible to substitute for liquid
fuel in most long haul transport, aviation, or agricultural and
industrial prime movers. Synthesizing fuel from carbon dioxide extracted
from air is possible in principle but horrendously expensive. Yet, if
we are to achieve CO
2 levels of 350 ppm from our current 392 ppm, CO
2 removal from the biosphere appears necessary.
Two
papers published last year described a new approach to zero emissions
synfuel, looking at direct carbon dioxide extraction from seawater. The
new insight in these papers is that CO2 is very soluble in
seawater, where the concentration is about 140 times higher than in the
atmosphere. This could make seawater extraction a lot cheaper than
direct air capture.
The work was done by the
US Navy (
full text here), and by the
Palo Alto Research Center (PARC),who each developed membrane processes to extract CO
2 from seawater.
The
Navy’s interest is military – shipboard production of synthetic jet
fuel far from supply lines – but I figure we can beat this sword into a
ploughshare.
Rather than going after the CO2 directly with chemical scrubbers, they use electrochemical processes to split seawater into an acid and base stream, and the CO2
bubbles off from the acidified water. The two streams are recombined
and returned to the ocean. While these processes are novel, they are
very similar to a number of ion exchange processes, including
desalination, which are currently deployed at scale.
The
Navy costed the production of jet fuel at sea. But they neglected to
include the cost of energy for the carbon capture process. I used the
PARC research to estimate it and include it in the Navy costings. I
arrived at $1.78 per litre. I was also able to calculate the cost of
just the carbon capture part of the process at about $114 per tonne of
CO2.
But
if we don’t insist on running these processes on an expensive
ocean-going platform, the cost drops to $0.79 per litre for synfuel and
$37 /tCO
2. The costs are rough and there are a number of
caveats, but this is surprisingly low. To put it in context, the
American Physical Society recently
reviewed carbon capture from air, and “optimistically” costed it at about $600/tonne.
The
Navy costings are based on commercially available equipment whose
capital and operating costs are understood for all processes except the
membrane CO2 extraction. Analogous processes like
desalination are available for a cost baseline for membrane extraction.
The costing assumed power from Navy nuclear reactors. (They also costed
OTEC power - Ocean Thermal Energy Conversion – but this is not a
commercially available technology.)
I describe the CO2
capture and fuel synthesis processes below, and show how the costings
were derived. I also consider how the costs would change for civilian
nuclear electricity (Table 1). In brief, accepting the Navy’s
assumptions leads to plausible prices for synfuel and carbon capture,
but the amount of new power generation required makes very large volume
production unlikely.
CCS - Carbon capture from seawater
Concepts
for carbon capture from air have been developed, but never realized.
The basic idea is to pass air over alkaline scrubbers, such as amine or
carbonate solutions, extract the CO2, and recycle the scrubber solution. Because the concentration of CO2 in air is so low, a very large surface area is required, and the process is energy intensive and overall very expensive.
The American Physical Society prepared a
technology assessment on this approach in 2011. The results weren’t promising. A 1 Mt/yr CO
2 extractor comprised five 1 m x 1 m x 1 kilometre long air contactors, occupying about 1.5 km
2.
The cost, so far as it could be determined for an undeveloped
technology, and making optimistic assumptions, was about $600 per
tonne. Another 2011 study estimated costs based on current experience
with trace gas removal systems at about
$1000 per tonne.
![Graphic – cover of the APS report, with link]()
Graphic – cover of the APS report, with link
But CO2
is very soluble in water, and its concentration in the ocean is about
140 times higher than in air. So we are using the whole of the ocean
surface as an air contactor right now – for better or worse! The
extraction system is ‘built’, we just need to recover the CO2.
