Study suggests that decarbonizing US transport sector by converting waste CO2 to fuels would require economical air-capture of CO2

Although technologies for CO2 Capture to produce Transportation Fuels (CCTF)—referring to systems that use sunlight and captured CO2 to make transportation fuels—have the ability to improve domestic energy security, a significant long-term role for CCTF in decarbonizing the US transportation sector (assuming decarbonization of the power generation sector) is “difficult to see” absent the economical capture of CO2 from air, according to a paper by Dr. Tom Kreutz at the Princeton Environmental Institute, Princeton University.

Kreutz used two examples of CCTF systems in his analysis: biodiesel from microalgae and Sandia National Laboratory’s S2P process (an effort to utilize concentrated solar energy to convert waste CO2 into synthetic fuels, earlier post). His paper, Kreutz noted, is only a preliminary scoping study designed to sketch out the rough outlines of each system’s prospective performance and economics as related primarily to GHG emissions.

Kreutz presented the paper at the 10th International Conference on Greenhouse Gas Control Technologies (GHGT-10) earlier this fall in The Netherlands.

Kreutz used what he called a bifurcated climate regime—i.e., pre- and post- decarbonization of the electric power sector—to which he referred as pre-CCS and post-CCS, respectively (although decarbonization was not necessarily via CCS—carbon capture and storage).) In the near-term pre-CCS era, with a low cost of carbon, the economical solution for power providers is to vent the CO2 and pay the fees, passing on the costs to customers.

Over time, however, as the CO2 price increases, it eventually becomes more economical to either retrofit plants to capture and store most of their CO2 (e.g. ~90%) or to “repower” using lower carbon feedstocks or generation technologies (e.g. natural gas, nuclear energy, renewable energy, etc.)...In the pre-CCS regime, CCTF plants must be co-located with (but not necessarily owned by) a power plant; careful integration between the two plants will reduce costs (not studied in detail here).

In the post-CCS regime, fossil-based power plants either employ CCS or have been replaced by nuclear power and/or renewable generators; as a result, large point sources of vented fossil CO2 are relatively rare...In short, large supplies of CO2 are expected to be widely available only as supercritical CO2 in pipelines destined for geologic storage. The amount of pipeline CO2 available for CCTF will depend upon the economic competition (not studied here) between fossil plants with CCS (or CCTF) and non-carbon generators; for simplicity, we assume here that the former is competitive, and pipeline CO2 is plentiful.

—Kreutz (2010)

In CCTF, the source of CO2 determines the net carbon intensity of the fuel, Kreutz says. Direct capture of CO2 from air, or from an exhaust stream vented to the atmosphere, represents negative emissions. When that carbon is converted into a fuel, burned, and exhausted to the atmosphere, the overall cycle is roughly neutral.

However, in the post-CCS regime, if CCTF employs captured CO2 from a pipeline destined for geologic storage (i.e. displaces CCS), the GHG emissions from the combusted synfuel should be comparable to those of traditional petroleum-based fuels, i.e. minimal climate benefit. (Note that the climate benefit is independent of origin of the carbon, e.g. fossil fuels vs. biomass.) It is worth reiterating that, if CCTF employs CO2 captured directly from the air, the resulting fuels are roughly climate neutral. However, because the concentration of atmospheric CO2 is very low, direct air capture is widely believed to be quite costly, and is therefore not considered quantitatively in this study.

—Kreutz (2010)

Among the other conclusions of the study are:

  • In their most economical configuration (without CO2 buffer storage), the low carbon utilization (13-25%) of CCTF severely limits the fraction of US transportation fuels that can be supplied by CCTF, but the carbon utilization can be roughly tripled at modest cost.

  • CCTF most readily provides a significant climate benefit when coupled with large, point source emitters of CO2 that are actively harming the atmosphere, but these are expected to be “scarce resources” in the post-CCS era.

  • CCTF may have an important interim role to play for climate mitigation under a steadily increasing CO2 until the power sector becomes decarbonized, especially if widespread decarbonization is significantly delayed. (This raises the unusual possibility of the transportation sector becoming decarbonized before the power sector, Kreutz notes.) However, after decarbonization, CCTF has the potential to hinder climate mitigation efforts by providing a ready source of only mildly decarbonized domestic transportation fuels. CCTF will only employ direct CO2 capture from air when the CO2 emission price exceeds the cost of air capture.

At sufficiently high oil prices, CCTF will always displace CCS, but from a climate perspective, CCTF (without air capture) is clearly not a replacement for CCS. “Using the carbon twice” fails to meet the objective of deep GHG emission reductions across the entire energy economy; only one sector (either power or transportation)—but not both—can claim the benefit of carbon neutrality. Alternative CCR [CO2 capture and recycle – or reuse]schemes like CCBF [CO2 Capture to produce Boiler Fuel] where carbon is captured and recycled many times, can produce very low carbon energy, but unfortunately not convenient hydrocarbon transportation fuels whose inherently distributed GHG emissions can only be economically mitigated by systems—natural (e.g. biomass) or man-made—that reverse the process by re-capturing CO2 from the atmosphere.

—Kreutz (2010)


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