Friday, April 26, 2013

Engineering Carbon Out of Energy

  • Because of the slow progress in displacing fossil fuels with renewables, carbon capture and sequestration should receive much more attention as a game-changing technology.
  • The challenges that must be overcome for CCS to be deployed on a large scale remain significant.

Yesterday I ran across an excellent article in The Atlantic on the importance of carbon capture and sequestration (CCS).  In light of last week's warning from the International Energy Agency that efforts to reduce the carbon intensity of global energy have yielded minimal results over the last two decades, the authors' chosen title, "Learning to Live with Fossil Fuels", seems particularly apt. Although neither they nor the IEA are suggesting we abandon renewable energy, they do effectively question the conventional wisdom that climate change can only be addressed by abandoning coal, oil and natural gas within the next decade or two. 

I'm predisposed to their argument, because it aligns with my own view--the result of long and careful analysis--that the transition to a low-carbon economy is going to take a lot longer than optimists hope.  A speaker at yesterday's policy briefing on renewable energy from the Worldwatch Institute and REN21, marking the annual release of the latter group's always-useful Renewables Global Status Report, stated that long-term energy scenarios in which renewables don't significantly increase their market penetration are no longer credible, and that only scenarios including medium-to-high penetration rates by mid-century are credible today.  I had to wonder whether he had been looking at the same data as the IEA, even though he cited their "2DS" scenario in support of his view.  Sarewitz and Pielke, Jr. appear to take quite the opposite view in The Atlantic: We cannot ignore the potential of CCS, because it is not self-evident that renewables will sweep away carbon-based energy any time soon, for reasons of economics, politics, and "complex social arrangements." 

In their brief article, they do a good job of summing up the major options for capturing CO2, including some of the major challenges to be overcome, as well as how the captured CO2 might be used or disposed.  Underground storage, enhanced oil recovery, and conversion back into fuels are all technically feasible, despite significant obstacles of public acceptance, logistics, and cost. However, I believe they seriously underestimate the challenges of capturing CO2 from the air, instead of power plant smoke stacks. 

The desirability of doing so is clear; the atmosphere is everywhere, convenient to whatever use to which me might put the captured CO2, while power plants aren't always located near the oil fields, saline aquifers, or fuel markets that offer the best potential for storage or reuse. The problem is that, while 397 parts per million (ppm) of CO2 in the air is high enough to cause great concerns about global warming, it is still quite low in engineering terms.  Expressing it as a percentage it's 0.04%, or about 1/1000th the typical concentration of CO2 in flue gas.

Before writing this post I literally dusted off one of my old chemical engineering texts to look up the equations of mass transfer.  I was reminded that the flux, or flow, of molecules from one fluid into another--from air into the capture medium, for example--is proportional to the difference in their concentration in the two fluids.  What that means in practical terms is that extracting the same quantity of CO2 from the air as from flue gas will entail larger and more complex hardware, more energy, and probably a much higher cost per ton, barring a breakthrough that emulates green plants, which use chlorophyll, sunlight, water and nutrients to do this cheaply on a vast scale every second of the day during the growing season.

In any case, have a look at the article and give some thought to how CCS might, as the authors suggest, "transform the political debate" around mitigating climate change.


jtf said...

A simple exercise I did in college seems illustrative to relate.

Since at minimum to extract carbon dioxide from some gas we have to overcome the free energy of mixing, it stands to reason that the free energy of mixing per unit of carbon dioxide is a reasonable basis for the comparison of the difficulty in extracting from stack gas or from ambient air.

If we take as a simple approximation the free energy of mixing some carbon dioxide as being determined by some relationship of the partial pressure of carbon dioxide x and of the remaining air y - call it xy-ln(xy) - and assume ideality so that we have a partial pressure-mole correspondence, then using your numbers (in stack gas, x ~ 0.4 and in air x ~ 0.0004) then the energy cost per unit comes out roughly 5000 times higher for air than in stack gas.

Todd said...

I find this Calgary, Canada based company and their technology appropriate for this article.

Geoffrey Styles said...

The technology is interesting for several reasons, including the choice of a hydroxide/carbonate cycle for CO2 capture, release and recharge of the capture solution, as well as the choice of firing the natural gas driven regenerator with pure oxygen to keep N2 and nitrates out of the captured CO2. However, the economic estimates included in the company's FAQ ( validate the point made by JTF above and in my post concerning how much harder it is to extract CO2 from air than from flue gas.

They estimate a capital cost of "a significant fraction of $1 billion" for a plant sized to extract a million tons (metric or short?) per year of CO2, which is the emissions equivalent of a coal-fired power plant generating only about 150 MW. They also indicate a full cost of capture of "less than $250 per ton of CO2." Most estimates for power plant capture that I've seen were in the range of $40-100/ton.

So it certainly looks feasible, but will it prove economical? $250/ton looks very steep for enhanced oil recovery, let alone as the starting point of any CO2-to-fuel process. That's effectively $100 per barrel just for the carbon content of the feedstock.

Alden Hathaway said...

Renewable Energy such as solar and wind are already approaching the cost of conventional electricity on the margin. If utilities were forced to eliminate carbon from their existing fossil plants which for carbon capture is about 20% of the cost of wind and solar then they would do well to switch to wind and solar in larger numbers and to reduce the cost of generating electricity.

Geoffrey Styles said...

Unfortunately, neither wind nor solar is dispatchable. I.e., they do no generate either on demand or around the clock, as fossil plants, along with hydro and geothermal do. NREL just looked at that in the context of Concentrated Solar Power (which has at least the capability for storage and dispatchability) and found that it has a significant value when penetration of intermittent renewables is already high. See:

This premium for "capacity" explains why a utility or grid manager might prefer coal or gas with CCS over a similar increment of power generated from wind or solar, even if the marginal cost were equal. This is also why low-cost electricity storage for PV and wind is such a high R&D priority.