Maximizing the Mitigation Potential of Curtailed Wind: A Comparison Between Carbon Capture and Utilization, and Direct Air Capture Processes for the UK

Carbon capture and storage (CCS) with fossil fuel or biomass plants (BECCS) is considered a critical technology to meet mitigation targets set by the Paris Agreement1. However, several drawbacks including high upfront investment costs, significant energy penalty and long-term permanent storage challenges have limited the uptake of CCS on the required scale. Carbon capture and utilisation (CCU) provides an alternative route to recycle CO2 into chemical feedstock and/or synthetic transport fuels (e.g. methanol, DME) that can displace fossil-derived fuels. As the carbon is only transformed, CCU must be integrated with capture/storage to actually offset subsequent emissions from the vehicles consuming them. The mitigation of decentralised emissions poses significant challenges and necessitates the use of carbon dioxide removal technologies (CDR), one of which is direct capture of CO2 from the atmosphere (DAC). The last decade has seen increasing penetration of wind power in the UK electricity system to meet mitigation targets. Because of this, periods of surplus wind generation and low demand or limited/full storage capacity arise. Constraint payments then have to be made to wind farms to curtail generation. This work investigates two possible options to achieve mitigation with this curtailed electricity. In Process A, curtailed electricity is used to produce electrolytic hydrogen and operate methanol synthesis plants. It is then integrated with a direct air capture (DAC) plant to recapture and recycle emissions from the vehicles. Process B assumes curtailed electricity is used to run a DAC plant directly in order to capture decentralised carbon emissions and provide CO2 feedstock for CCU processes. The UK was used as a case study and the methanol synthesis process described by Rihko-Struckmann et al.2 was used as the reference. A range of energy requirements for DAC are cited in literature; the lower and upper bounds of 6.7 GJ/tCO2 and 12.6 GJ/tCO23, respectively, were used. This work has taken a base case curtailment level of 2.5% of the UK total electricity demand, which is equivalent to 390 GWh/y4. Both processes have been compared on the basis of mitigation potential, defined by the proportion of CO2 emissions from gasoline vehicles that are avoided, and mitigation costs per tonne of CO2 captured. Process A resulted in avoiding 0.12% of gasoline emissions (~0.05 MtCO2/y). Surplus energy (~64% of the curtailed electricity) was required to run the DAC plant and an associated air separation unit. The mitigation of potential of Process B was 0.10% or 0.18%, depending on energy requirement used. Therefore, the process that maximises mitigation potential depends on the DAC process considered; using the lower-bound energy requirement, surplus electricity for DAC only is preferable. Neither process is economically viable. CCU costs ($905/tCO2) were found to be double the DAC-only costs ($449/tCO2), mainly due to high H2 costs. It will remain financially-unattractive unless the methanol production becomes profitable. This is unlikely as it requires methanol price to almost double, a carbon price of $313/t to be in effect, or H2 price to reduce to a third of today’s price to$1800/t. 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. Rihko-Struckmann, L. K., Peschel, A., Hanke-Rauschenbach, R. & Sundmacher, K. Assessment of methanol synthesis utilizing exhaust CO2 for chemical storage of electrical energy. Ind. Eng. Chem. Res. 49, 11073–11078 (2010). 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). 4. Messiou, A. Centre for Environmental Policy Investigating the role of power storage in accommodating the future wind. (2012)

Copper manganese oxides as oxygen carriers for chemical looping air separation for near-zero emission power generation

In this thesis, copper manganese oxides were evaluated as oxygen carriers for chemical looping air separation. This process allows for the production of a O2+CO2 mixture, which can be used for oxy-fuel combustion of biomass for near-zero emission generation of power. Unsupported oxygen carriers were prepared via co-precipitation at low and at high super-saturation and via mechanical mixing. Furthermore, TiO2, ZrO2 and Al2O3-supported oxygen carriers were prepared. The oxygen carriers were characterised to study the effect of synthesis conditions on the material properties, and experiments in the TGA and fluidised bed were performed to assess their kinetic and long-term cycling performance. The unsupported oxygen carriers showed a highly stable cycling behaviour over a high number of redox-cycles. The O2 capacities and O2 equilibria were measured and the data reported. Cycled oxygen carriers were characterised to study the particle evolution for different redox-cycling modes (isothermal, non-isothermal, fuelled process with CH4) and at various conditions. The characterisation also elucidated the relevant redox-reactions and reaction pathways, as well as interactions between active phases and the supporting materials. During redox-cycling, the unsupported oxygen carriers were prone to swelling (an increase in the specific particle volume). It was demonstrated that the use of ZrO2 as a supporting material inhibits the swelling and result in a shrinking. Kinetic parameters were isolated from experiments in the fluidised bed, and the homogeneous reaction model was found to represent the O2 release and oxidation reaction best.Open Acces