(178d) Comparing Beccs and DAC for Climate Change Mitigation: The Water-Land-Energy Nexus

The Paris Agreement signaled global commitment to limit average global temperature rise to 2˚C and to pursue efforts to achieve 1.5˚C. The IPCC AR5 cites negative emission technologies, particularly bio-energy with carbon capture and storage (BECCS), as critical to achieving this¹. BECCS’ ability to deliver negative emissions has been questioned with many uncertainties around land-use change emissions. Furthermore, large-scale biomass use poses several biophysical and socioeconomic risks such as exacerbating scarce water resources, biodiversity loss, and large opportunity costs of land-use for biomass. Capturing carbon dioxide (CO₂) directly from the atmosphere, direct air capture (DAC), presents an alternative source of negative emissions. Established capture technologies can be used to extract it from the atmosphere, seemingly without the significant natural resource requirements of BECCS. However, the low concentration of CO₂ in air necessitates a high energy cost of capture.

In this work, a comparison of the strengths and weaknesses of each technology based on key performance metrics (land, water and energy requirements) has been done. A negative emissions target of 12 Gt_CO2/y was used; this is the central estimate of what is required from RCP2.6¹ and has been cited as the target beyond which BECCS is no longer economically feasible. A ‘farm-to-NET plant’ model of BECCS accounting extensively for the biomass supply chain and power plant specifications (coal co-firing proportions, capture rates) has been built to determine the overall GHG emission balance of the technology. Data from DAC processes proposed in the literature and existing pilot projects have been used to quantify the potential land, water and energy requirements of a large-scale system.

For BECCS, high variability in carbon balance outcomes was observed depending on the feedstock type and source, and assumptions on indirect land-use changes. Depending on decisions taken throughout the supply chain, a BECCS facility could take over 50 years to begin removing CO₂ from the atmosphere, or it could start removing CO₂ from day one². Stark contrasts in resource requirements to meet the mitigation target were found for BECCS and DAC. Depending on the conditions of its deployment, a BECCS plant operating at 100% load factor could need 363-2100 Mha of land and 4-16 Tm³/yr of water dedicated.

DAC’s needs were 3-5 orders of magnitude lower at 0.04-3.3 Mha and 180 km³/yr³, thus posing little to these resources. It is important to note that DAC’s water requirements are very sensitive to relative humidity and hence location of the plant³. Furthermore, the land area associated with the electricity used for DAC process was not accounted for, and could be significant when considering low carbon power, such as solar and wind.

BECCS’ status as an energy-positive mitigation option was found to be untrue in some scenarios. Its annual energy output in the case of US-sourced Switchgrass could be as high as 22 EJ/yr, however up to 37 EJ/yr could be required if Brazil-sourced Miscanthus is the choice biomass. DAC energy costs, however, are still significantly higher at 81-274 EJ/y, even when the lower-bound from literature is assumed.

Surplus resource availability, particularly land and water, are highly-dependent on geography. BECCS and DAC have been shown to require contrasting amounts of these. Therefore, the preferred technology must be discussed within the context of a specific location, and in the case of BECCS, biomass source if not locally-grown.

References

1. IPCC. Climate Change 2014: Mitigation of Climate Change. Working Group III Contribution to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (2014). doi:10.1017/CBO9781107415416

2. Fajardy, M. & Mac Dowell, N. Can BECCS deliver sustainable and resource efficient negative emissions? Energy Environ. Sci. 0–42 (2017). doi:10.1039/C7EE00465F

3. Socolow, R. et al. Direct Air Capture of CO 2 with Chemicals Panel on Public Affairs. Am. Phys. Soc. - Panel Public Aff. 100 (2011).