DAC & CCS
While reducing GHG emissions is imperative, it is becoming clear that reductions alone are unlikely to be enough: it will also be necessary to remove GHG from the atmosphere to limit the global temperature increase to 1.5 °C [MKY, 2022].
Carbon capture technologies prevent the release of CO2 (carbon dioxide) to the atmosphere. In the most commonly used arrangement today, a chemical that can “grab” CO2 is placed in or near the stream of CO2 at a source [CRS, 2020].
CO2 can be captured from gases emitted by industrial processes or directly from the air (thru direct air capture - DAC - tech: CO2 is about 0.04% of the atmosphere). In many tech approaches, air is forced over a chemical that can “grab” CO2.
CO2 can then be used as a feedstock to an industrial process or permanently stored (sequestered) underground. The full process is called carbon capture, utilization, and storage (CCUS), or even CCS. Both techs, DAC & CCUS, are in early stages of development, with a few examples of operating projects worldwide. Of the two, CCUS is more mature. After capture, the process for DAC is similar to that used for CCUS and can use the same equipment for compression, transfer, and storage.
CO2 origin / production
The CO2 is predominantly sourced from industrial processes that produce high-purity CO2 as a by-product, such as ammonia production & biomass fermentation, or extracted from natural underground CO2 deposits, mainly for enhanced oil recovery (EOR) purposes [IEA, 2019].
Globally, some 230 Mt of carbon dioxide (CO2) are used every year (demand in 2015). The largest consumer of CO2 is the fertilizer industry, where over half is used in urea manufacturing, followed by the oil sector (for EOR), with about a third of CO2 consumption.
CO2 is also used as a refrigerant in chilling systems, in fire extinguishers (eliminating flames by smothering), for inflating life rafts and life jackets, foaming rubber & plastics, in greenhouses to promote the growth of plants, immobilizing animals before slaughter, and, the most obvious industry application, in carbonated beverages.
In theory, some CO2 use applications, such as fuels & chemicals, could grow to scales of multiple billions of tons of CO2 use per year [IEA, 2019]. It is important to say, however, that CO2 use does not necessarily reduce emissions, as quantifying climate benefits is a complex task, requiring a comprehensive life-cycle assessment and understanding of market dynamics. Figure 1 shows CO2 uses in 2015.
DAC plants currently operate at a small scale, but with plans to grow. Currently 18 DAC facilities are operating in Canada, Europe, and the United States. All but two of these facilities sell their CO2 for use, and the largest such plant (commissioned in Iceland in Sep/2021) is capturing 4,000 tCO2/year for storage. The world’s first large-scale DAC plant with 1 MtCO2/year capacity in the U.S. will not be operational until the mid-2020s [IEA, 2022].
DAC costs are dependent on i) the capture technology, ii) energy costs (price of heat & electricity), and iii) the plant configuration. Two technological approaches are currently being used to capture CO2 from the air: Solid DAC (S-DAC) is based on solid adsorbents operating at ambient to low pressure (i.e., under a vacuum) and medium temperature (80-120°C).
Liquid DAC (L-DAC) relies on an aqueous basic solution (such as KOH), which releases the captured CO2 thru a series of units operating at high temperature (btw 300 °C and 900 °C). Figure 2 compares S-DAC & L-DAC.
Costs are projected to fall
Future capture cost estimates for DAC are wide-ranging and uncertain, reflecting the early stage of technology development, but are estimated at btw USD 125 and USD 335 per ton of CO2 for a large-scale plant built today.
DAC deployment for carbon removal, which is an energy intensive process (and therefore expensive than capturing it from a point source), relies on the availability of low-carbon energy sources and CO2 storage. With deployment and innovation, capture costs could fall to under USD 100/tCO2. In locations with high renewable energy potential, like the Middle East, and using best available techs for electricity and heat generation, DAC costs could fall below USD 100/tCO2 by 2030.
Figure 3 shows a CCSU system, reducing CO2 emissions from the refinery sector in India.
Figure 1: CO2 use in 2015
Figure 2: S-DAC vs L-DAC
Figure 3: CCSU at the refinery sector in India