Tuesday, December 22, 2009

To Bury CO2 or Recycle It

While not the most powerful of the greenhouse gases produced by humanity, CO2 is certainly the most prevalent, if you don't count water vapor. To a very large extent, addressing climate change depends on three main strategies for dealing with the excess CO2 our activities emit: avoiding its creation by switching to other energy sources, such as renewables or nuclear power; capturing and storing it in trees, other vegetation or underground; and recycling it into useful fuels and products. Most of the work to date on the third option has focused on biofuels, which employ photosynthesis to convert CO2 into vegetable oils or fermentable sugars. However, another strategy now attracting interest involves non-photosynthetic pathways for turning CO2 back into hydrocarbons. If practical, this approach has much to recommend it, though the laws of Thermodynamics suggest it will always require more energy than the resulting fuels can deliver when used. A recent conversation with the CEO and CTO of Carbon Sciences, Inc., a start-up pursuing CO2-to-fuel technology, shed some interesting light on the subject.

The magnitude of global emissions of CO2 makes managing them a daunting prospect. Carbon Capture and Sequestration (CCS), which creates an artificial carbon cycle, has garnered much political and financial support in the last year, though it is still in the development stage and faces significant hurdles. CO2-to-fuel conversion offers another interesting option, because it could either work in parallel to CCS to enhance the reduction of emissions from fossil-fuel power plants and other stationary sources, or in competition with sequestration as an outlet for the captured CO2 from such facilities. If the resulting synthetic fuel displaced a like quantity of petroleum, natural gas or coal, the effect on the atmosphere would be largely equivalent to CCS and likely better than conventional biofuels, which appear to result in substantial non-combustion releases of CO2 and other GHGs. Fuels produced from recycled CO2 could finesse many of the NUMBY concerns about CCS while beating corn ethanol and some biodiesel on overall "green-ness" and compatibility with existing fuel infrastructure and transportation fleets. So why aren't we already doing this?

The answer is simple. When we burn the carbon compounds found in fossil fuels, they produce CO2 and a specific quantity of energy that is unique for each molecule. Turning CO2 back into the original fuel compound requires the input of that same amount of energy--that's from the First Law of Thermodynamics--and in practice a bit more, thanks to the Second Law. Chemists have known for a long time that CO2 could be converted into fuel and chemicals, but outside the laboratory this wasn't regarded as useful, because it inherently consumed more energy than it could return. Biofuels get caught up in this same conundrum, though in their case much of the energy required is supplied by the sun, rather than from other fuels and energy inputs we must produce. So I was quite intrigued when I received an email inviting a conversation with the CEO of Carbon Sciences, the start-up I mentioned earlier, which claims to have solved this problem using "biocatalysts", nanotechnology, and a unique multi-step process.

The company's website includes animation showing how this would work, though from my perspective it omits the key factor: where does the energy come from to drive the process? Catalysts and enzymes can reduce the threshold for the reaction to take place and improve its speed--the reaction kinetics, in engineering terms--so that what would otherwise take nature years or millennia to produce can be accomplished in a commercially-practical interval. However, catalysts can't alter the basic energy requirement of the reaction. What is the source of that energy?

My discussion with Carbon Sciences' CEO Byron Elton and Chief Technology Officer Naveed Aslam, Ph.D. assuaged my immediate concern that this was yet another perpetual motion machine dressed up with technical jargon and fancy graphics. They struck me as pragmatic and realistic about the challenges they face, though with the customary optimism required for entrepreneurial risk-taking. Dr. Aslam clarified that their process for converting CO2 to methanol for later conversion into hydrocarbons or petrochemicals involves a hydrogen-and-energy carrier molecule that must be regenerated from a "sacrificial substrate." That substrate effectively provides the energy required for uplifting the CO2, which is at a very low energy state, and acts as the fuel source for the whole sequence. The value of the entire CO2-to-fuel process in energy, economic and emissions terms thus hinges on the characteristics, cost and supply potential of this energy-donating material.

The process developed by Carbon Sciences can apparently use a variety of substances for this purpose, which is fortunate. Initial laboratory tests apparently involved glucose, a commercially-available sugar, but the company is now using another, undisclosed feedstock because of their concerns that glucose supplies couldn't keep up with a large-scale CO2-to-fuel industry without affecting food prices. Dr. Aslam indicated that in the long run they would likely use a mineral-based compound that was widely available. Without knowing the specific chemical involved, it's impossible to assess the overall energy balance, lifecycle emissions, or usefulness of the process, but I at least came away with a sense that Mr. Elton and Dr. Aslam understand the constraints involved very well.

And while the global supply of CO2 certainly looks large enough, it has to be provided in the right form: highly concentrated and free of contaminants that could degrade their catalyst or retard the reaction rate. That is a very different requirement from biofuels that extract their CO2 from the air, and it would put CO2-to-fuel in direct competition with carbon sequestration and enhanced oil recovery, which also effectively produces incremental fuel from CO2. It's not obvious to me which technology will advance the fastest, offer the largest overall CO2 reduction, or the most attractive economics. Markets are usually the best way to sort that out, if given the right signals.

Nor are Carbon Sciences the only ones working on this problem. A team at Sandia Laboratory has been developing a "Sunshine to Petrol" system using CO2 and concentrated sunlight, while the new Advanced Research Projects Agency-Energy (ARPA-e) is looking into a variety of novel ways to convert CO2 into fuel without photosynthesis.

It's important to note that Carbon Sciences' conversion technology is still at an early phase of development--lab-scale, rather than demonstration-scale. "Milliliters per day" won't solve our energy or emissions problems, but if this can be scaled up to many thousands of barrels per day with a cheap and readily-available source of chemical energy and a suitable supply of CO2, it has the potential to deliver fuels that are 100% compatible with our current infrastructure and vehicle fleets. That's a big advantage, and it would certainly explain the interest that Carbon Sciences has apparently been getting from large energy firms. I was told that Carbon Sciences hopes to develop a commercially-attractive package by the third quarter of 2010 and are exploring a "strategic partnership" to take the process--and the company--to the next phase. They have also applied for DOE technology funding under the category of "Innovative Concepts for Beneficial Uses of CO2". I will be watching their progress with great interest.

Since I don't expect to post again until next week, I'd like to wish my readers a Merry Christmas and happy Boxing Day.

I also have a housekeeping matter to bring to your attention. Haloscan, the comment system I have used since 2004, is being discontinued. I must decide by Monday whether to switch to Haloscan's successor, Echo, use Blogger's comment feature, or find another comment platform. Although I will do my best to ensure the migration of the many thousands of comments you've left here, I can't guarantee it. If there are any you'd like to refer to again, I encourage you to copy them to another medium.

No comments: