(178d) Comparing Beccs and DAC for Climate Change Mitigation: The Water-Land-Energy Nexus
AIChE Annual Meeting
2017
2017 Annual Meeting
Sustainable Engineering Forum
The Food-Energy-Water Nexus
Monday, October 30, 2017 - 1:30pm to 1:50pm
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 GtCO2/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 âfarm-to-NET plantâ 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.
DACâs 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 DACâs 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.
BECCSâ 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. 0â42 (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).