A bit of thermodynamics

In conjunction with the ADP meeting in Bonn last week, the UNFCCC held a Technical Expert Meeting (TEM) on Carbon Capture and Storage. It was really good to see this critical technology finally getting some airtime at the UNFCCC and even more importantly the attendance at the meeting by the Parties was good. There was plenty of interest, lots of good questions and a real desire to understand how CCS could be further advanced and more importantly deployed. On a historical note, the meeting was held in what was the German Bundestag between 1949 and 1999. The pigeon holes used by the MPs were still there and dutifully labelled with names such as Dr. A. Merkel.

The morning session covered the technology more broadly and focussed in on some major CCS projects either in operation or under development around the world. I had been invited to speak about the Shell Quest Project in Canada. The afternoon session had a focus on the “U” in a new acronym now entering the discussion, CCUS or Carbon Capture Use and Storage.

Carbon Capture and Use sounds like a great way forward; why not capture the CO2 and put it to good use? That way there will be an economic incentive to tackle emissions and the problem will be solved. This took me back to a dinner at COP19 in Warsaw last year where one participant suggested that CO2 be commoditized such that it would be used even more widely. Unfortunately, this is where thermodynamics gets in the way.

Carbon dioxide sits at the bottom of a deep thermodynamic well.

Thermodynamic well

It is a by-product from a very energetic chemical reaction, the oxidation of a hydrocarbon molecule (i.e. combustion). This releases a tremendous amount of energy (which is why fossil fuels have such value), but leaves us with CO2, which then doesn’t have the energy to react quickly with anything. As such, it is very stable and any chemical use for CO2 which converts it into something else requires a similar amount of energy to that produced when the CO2 was created in the first place. One of the key presentations at the CCS TEM was about the manufacture of polycarbonates (and other chemicals) from CO2. This was an excellent piece of work, but the tricky subject of process energy doesn’t jump out of the presentation.

There is also another tricky subject that needs to be opened up; for the most part, the use of CO2 in chemical processes has no impact on the atmospheric CO2 balance, unless of course the chemical is eventually sequestered. Otherwise the CO2 simply returns to the atmosphere when the chemical is used or the plastic degrades. Even if sequestration is the end point, chemical processes will never operate on the scale necessary to manage global CO2 emissions from energy use.

I did a bit more searching on these subjects and found an excellent paper at the University of Bath online publication store (originating from Imperial college), but beware it is long, detailed and very technical. However, an extract from the early part of the paper highlights the above points:

The development of methods to activate and use CO2 to prepare chemicals and materials is an attractive research goal. Carbon dioxide is abundant, renewable, of low toxicity and is emitted as a waste product from a myriad of industrial processes. A longstanding goal of synthetic chemistry has been to develop catalysts and processes which consume it, however, such reactions pose significant challenges. As the most highly oxidized state of carbon, CO2 is the lowest energy state of all carbon-containing binary neutral species: indeed, CO2 and water are the end-products of most energy releasing processes, including combustion and metabolic pathways. The table below illustrates the free energy of formation of carbon based molecules: the large energy required to reduce it is the most significant obstacle. This energy can either be directly input as physical energy or indirectly via the use of reactive chemical species as reagents; it is the latter strategy which powers the copolymerisation of epoxides and CO2. Free energy of Formation

Nature is successful in transforming approximately 200 billion tonnes/year of CO2 into carbohydrates via photosynthesis. Synthetic chemistry has been less successful, so far there are only a limited range of reactions which can transform CO2 to useful products, those that yield materials with high market volumes and/or economics are even scarcer. Successful reactions include the synthesis of urea (146 Mt/y, 2008), inorganic carbonates (45 Mt/y, 2008, mostly Na2CO3 via the Solvay process), methanol (6 Mt/y), salicylic acid (60 kt/y, 2003, via the Kolbe-Schmitt process), organic carbonates (100 kt/y, 2009; the subject of recent reviews) and polycarbonates (a few kt/y). Current production volumes for aliphatic polycarbonates produced from CO2 are small, however, the polycarbonates sector as a whole is large and growing. In Asia alone the sector is forecast to grow by 8-10%, resulting in the construction of new polycarbonate plants and opportunities for new technologies. Finally, it is important to note that CO2 consumption by chemical processes (approx. worldwide ~ 100 Mt/y) cannot impact global CO2 levels, nor are they a means to address climate change (UK CO2 emissions in 2008 from power stations exceeded 200 Mt/y). However, they could be a means to add value to a portion of the CO2 from carbon sequestration and storage (CSS) processes.

The last point is critical and it is why processes such as described above and the use of CO2 for enhanced oil recovery (EOR) are so important. All of these give real value to a modest amount of CO2. This is nowhere near the scale necessary to impact atmospheric concentrations, but enough to allow carbon dioxide capture plants to be built, which in turn allows capture technology to develop and become more cost competitive. This then makes CCS a more attractive option over the longer term and gives confidence that it is commercially viable at a certain carbon price.