The organizers of the Direct Air Capture Summit in Calgary, Canada, put together a thought-provoking and stimulating event. What struck me most was not the occasionally heated debate on cost but the discussions on how early niche applications could shape the long-term view of direct air capture. With regard to cost, those at the meeting agreed that the specific process studied by the American Physical Society comes out at six hundred dollars per ton or more. The debate is how much the price of air captured carbon dioxide could drop in the future and how to present the prospect of such a drop. Since at this point, nobody offers a commercial process under one hundred dollars, any claim by developers that they will break through this barrier is a forward-looking statement with all the risks and uncertainties this entails. Having said that, I challenge anyone to rule out, based on fundamental principles, a future air capture price of thirty dollars per ton of carbon dioxide.
Far more interesting was the debate on commercial applications. Rapidly growing demand for enhanced oil recovery (EOR) could give air capture not only an economic boost but also an environmental black eye, as it might result in the mobilization of large amounts of carbon that otherwise would stay safely underground. This is a valid concern, but it needs to be discussed in the context of oil production rather than air capture. Air capture might aid EOR; it certainly can be advanced by EOR; however, it is unlikely to drive EOR. Therefore, the desirability of air capture as an environmentally useful technology should not be judged by its immediate usefulness for EOR. Conversely, advocates of air capture technology should not claim environmental benefits for applications that are driven by economic considerations that have little to do with the environment.
Air capture has few competitors when it comes to removing carbon dioxide from the atmosphere and delivering it to a carbon storage facility. Therein lies its environmental importance. It is what motivated our work at the outset and still motivates it today. However, for EOR which simply demands pressurized carbon dioxide, capture from air is just one of many options. Air capture devices could be installed at the point of use, and in many implementations, collectors could be moved if demand patterns shift. This flexibility comes at the price of a higher production cost for carbon dioxide. Direct air capture for EOR will be limited to locations where low cost carbon dioxide is in short supply. Nonetheless, opportunities for air capture in EOR remain, because transport of liquid carbon dioxide by truck is expensive and low cost transport via pipeline requires large scales and long-term commitments. Direct air capture for commodity carbon dioxide is unlikely to capture more than a slice of the market. However, the potential market for EOR is so large that even a small slice would create the revenue stream necessary to establish direct air capture as a real technology with a demonstrated market price.
Setting environmental sensibilities aside, simply using coal to produce carbon dioxide at the point of use could provide carbon dioxide for EOR at a very competitive price and with a high degree of flexibility. Transporting coal is far cheaper than transporting carbon dioxide, and a ton of coal can produce about three tons of carbon dioxide when combusted with oxygen extracted at the spot from air. The combustion heat might even prove useful to enhanced oil recovery and offset some of the cost of oxygen separation. High temperature-mixed oxide membranes could lead to particularly cost effective implementations. In any such system, the costs would be dominated by the price of oxygen. Without doubt EOR needs carbon dioxide, but suffers from environmental shortcomings. However, as the above example indicates, these shortcomings cannot be blamed on air capture. EOR is not driven by the availability of direct air capture; it is driven by the high price of oil.
In the debate over EOR, energy security and climate change collide. Nevertheless, EOR stores carbon dioxide underground. If the oil industry wants to claim a carbon credit for this, it will have to argue this case itself. Air capture advocates should be extremely careful before claiming environmental benefits. It is true that the case for the carbon footprint of EOR will, in part, rest on the life cycle analysis of the carbon dioxide source. Such a life cycle analysis needs to be performed by people who have no stake in the technology. It is my expectation that the life cycle analysis of air capture will come out rather well. Air capture is intrinsically carbon negative if the produced carbon dioxide is indeed stored. In contrast, for carbon dioxide from a coal-fired power plant, one will still have to ask whether the power plant is running at any given moment because there was no other option for generating electricity or whether it was the need for carbon dioxide that tipped the balance in favor of running.
Affordable direct air capture could shape long-term climate mitigation options. Point source emissions may be the first to be addressed, but the climate change problem is not resolved until the carbon dioxide from the transportation sector is also dealt with. Either airplanes learn how to fly without liquid fuels or some form of air capture is required. Thus, development of the technology is an important public goal. Early markets for direct air capture could largely defray development costs. It would therefore be a shame if conflating air capture with a carbon dioxide consuming application like the environmentally questionable EOR would end up blocking future use of direct air capture for carbon management. To avoid such a fate, it is important that air capture developers are extremely careful when claiming carbon benefits for their early commercial applications.