On the Operation of CCS Within a Diverse Energy System

That CCS will be required to operate in a flexible and load following fashion in the diverse energy landscape of the 21st century is well recognised. However, what is less well understood is how these plants will be dispatched at the unit generator scale, and what effect this will have on the performance and behaviour of the plant at the individual unit operation level. To address this gap, we couple an investment and unit commitment energy system model with a detailed plant-level model of a super-critical coal-fired power station integrated with an amine-based post-combustion CO2 capture process. We provide insight into the likely role of coal and gas CCS plants in the UK’s energy system in the 2030s, 2040s and 2050s. We then evaluate the impact that this has on the performance of an individual coal CCS plant operating in this system, and chart its evolution throughout this period. Owing to the increased frequency and duration of part-load operation, asset utilisation and average efficiency suffer. This leads to a substantially increased LCOE. This reflects the growing inadequacy of this metric for evaluating CCS technology within a diverse energy landscape. Further, as a direct consequence of the dynamic operation, the interaction of the CCS plants with the downstream CO2 transport network is characterised by highly transient behaviour, including periods during which no CO2 is injected to the transport network, implying that the transport system must therefore be designed to incorporate this variability of supply

Can BECCS Deliver Sustainable and Resource-efficient Negative Emissions?

Negative emissions technologies (NETs) in general and Bioenergy with CO2 Capture and Storage (BECCS) in particular are commonly regarded as vital yet controversial to meeting our climate goals. In this contribution we present a whole-systems analysis of the BECCS value chain associated with the cultivation, harvesting, transport and conversion in dedicated biomass power stations in conjunction with CCS, of a range of biomass resources – both dedicated energy crops (miscanthus, switchgrass, short rotation coppice willow), and agricultural residues (wheat straw). We explicitly consider the implications of sourcing the biomass from different regions, climates and land types. The water, carbon and energy footprints of each value chain were calculated, and their impact on the overall system water, carbon and power efficiencies were evaluated. An extensive literature review was performed and a statistical analysis of the available data is presented. In order to describe the dynamic greenhouse gas balance of such as system, a yearly accounting of the emissions was performed over the lifetime of a BECCS facility, and the carbon breakeven time (Withers et al., 2015) and lifetime net CO2 removal from the atmosphere were determined. The effects of direct and indirect land use change were included (Searchinger et al., 2008; Fargione et al., 2008; Plevin et al., 2010), and were found to be a key determinant of the viability of a BECCS project. Overall we conclude that, depending on the conditions of its deployment, BECCS could lead to both carbon positive and negative results. The total quantity of CO2 removed from the atmosphere over the project lifetime and the carbon breakeven time were observed to be highly case specific. This has profound implications for the policy frameworks required to incentivise and regulate the widespread deployment of BECCS technology. The results of a sensitivity analysis on the model combined with the investigation of alternate supply chain scenarios elucidated key levers to improve the sustainability of BECCS: 1) measuring and limiting the impacts of direct and indirect land use change, 2) using carbon neutral power and organic fertilizer, 3) minimising biomass transport, and prioritising sea over road transport, 4) maximising the use of carbon negative fuels, and, 5) exploiting alternative biomass processing options, e.g., natural drying or torrefaction. A key conclusion is that, regardless of the biomass and region studied, the sustainability of BECCS relies heavily on intelligent management of the supply chain. References Fargione, J., Hill, J., Tilman, D., Polasky, S., & Hawthorne, P. (2008). Land Clearing and the Biofuel Carbon Debt. Science, 319(February), 1235–1237. Plevin, R. J., O’HARE, M., Jones, A. D., Torn, M. S., & Gibbs, H. K. (2010). The greenhouse gas emissions from indirect land use change are uncertain, but potentially much greater than previously estimated. Environmental Science & Technology., 44(21), 8015–8021. Searchinger, T., Heimlich, R., Houghton, R. A., Dong, F., Elobeid, A., Fabiosa, J., … Yu, T. (2008). Emissions from Land-Use Change. Science, 423(February), 1238–1241. Withers, M. R., Malina, R., & Barrett, S. R. H. (2015). Carbon, climate, and economic breakeven times for biofuel from woody biomass from managed forests. Ecological Economics, 112, 45–52

Can BECCS efficiently and sustainably remove CO2 from the atmosphere?

Bioenergy combined with carbon capture and storage, BECCS, could provide firm base load power while removing CO2 from the atmosphere. This unique feature makes it the predominant solution in the IPCC AR5 scenarios (IPCC, 2014) where it accounts for a substantial fraction of primary energy supply in half of the emissions pathways (Fuss et al., 2014). Such a demand for biomass would require large, integrated supply chains, whose embedded emissions could not only challenge the carbon negativity of BECCS – the underpinning concept of this option – but would also compete with other resources – such as water and land - already affected by global warming (UN Water, 2005). In this contribution, we present a whole-systems analysis of the biomass supply chain associated with a range of bioenergy materials – both energy dedicated crops (miscanthus, switchgrass, short rotation coppice willow), and agricultural residues (wheat straw) – supplied from different regions and land types, and converted in dedicated fired power stations in conjunction with post-combustion CCS technology. The water, carbon and energy footprints of each combination were calculated and their impact on the overall system net water intensity, power generation efficiency, and carbon intensity was measured. The model was evaluated with a range of values for each input parameter to capture the high variability and uncertainty in literature data. In order to describe the dynamic greenhouse gases (GHG) emissions of such as system, a yearly accounting of the emissions was carried out over a BECCS power plant lifetime, and the system carbon breakeven time was determined (Withers et al., 2015). Finally, a sensitivity analysis was carried out on the dynamic GHG emissions profile, and alternate scenarios involving organic chemicals, biofuels with and without CCS and carbon neutral electricity were investigated. Direct and indirect land use changes (Fargione et al., 2008; Plevin et al., 2010; Searchinger et al., 2008) effects were measured on both static and dynamic balances, and were found to be driving the results and uncertainty range. Overall we concluded that depending on conditions of its deployment, BECCS could lead to both carbon positive and negative balances. The most sustainable case study, miscanthus-based BECCS from Brazil, could lead to break-even times between 1 year if grown on marginal land, and 50 years on a forest land. Regulating and rewarding policies will have to integrate this local specificity in order to assure BECCS sustainable development. References Fargione, J., Hill, J., Tilman, D., Polasky, S., & Hawthorne, P. (2008). Land Clearing and the Biofuel Carbon Debt. Science, 319(February), 1235–1237. Fuss, S., Canadell, J. G., Peters, G. P., Tavoni, M., Andrew, R. M., Ciais, P., … Yamagata, Y. (2014). Betting on negative emissions. Nature Climate Change, 4(10), 850–853. IPCC. (2014). Climate Change 2014, Mitigation of Climate Change. Contribution of Working Group III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. Plevin, R. J., O’HARE, M., Jones, A. D., Torn, M. S., & Gibbs, H. K. (2010). The greenhouse gas emissions from indirect land use change are uncertain, but potentially much greater than previously estimated. Environmental Science & Technology., 44(21), 8015–8021. Searchinger, T., Heimlich, R., Houghton, R. A., Dong, F., Elobeid, A., Fabiosa, J., … Yu, T. (2008). Emissions from Land-Use Change. Science, 423(February), 1238–1241. UN Water. (2005). Coping with water scarcity: Challenge of the twenty-first century. Waterlines, 24(1), 28–29. https://doi.org/10.3362/0262-8104.2005.038 Withers, M. R., Malina, R., & Barrett, S. R. H. (2015). Carbon, climate, and economic breakeven times for biofuel from woody biomass from managed forests. Ecological Economics, 112, 45–52

What is the Value of CCS in the Future Energy System?

Ambitions to produce electricity at low, zero, or negative carbon emissions are shifting the priorities and appreciation for new types of power generating technologies. Maintaining the balance between security of energy supply, carbon reduction, and electricity system cost during the transition of the electricity system is challenging. Few technology valuation tools consider the presence and interdependency of these three aspects, and nor do they appreciate the difference between firm and intermittent power generation. In this contribution, we present the results of a thought experiment and mathematical model wherein we conduct a systems analyses on the effects of gas-fired power plants equipped with Carbon Capture and Storage (CCS) technology in comparison with onshore wind power plants as main decarbonisation technologies. We find that while wind capacity integration is in its early stages of deployment an economic decarbonisation strategy, it ultimately results in an infrastructurally inefficient system with a required ratio of installed capacity to peak demand of nearly 2.. Due to the intermittent nature of wind power generation, its deployment requires a significant amount of reserve capacity in the form of firm capacity. While the integration of CCS-equipped capacity increases total system cost significantly, this strategy is able to achieve truly low-carbon power generation at 0.04 tCO2/MWh. Via a simple example, this work elucidates how the changing system requirements necessitate a paradigm shift in the value perception of power generation technologies

The water-energy-carbon-land nexus: Optimising the BECCS supply chain

Negative Emissions Technologies (NETs) are necessary to meet the climate change targets identified at the 2015 Paris COP. In particular, Bioenergy with CO2 Capture and Storage (BECCS) is being put forth as a key mitigation option to decarbonise the atmosphere in most IPCC AR5 scenarios (IPCC, 2014). Incurring high emissions through biomass supply chain and competing with land and water, BECCS sustainability and carbon negativity has been shown to be greatly dependent on the conditions – feedstock production, processing, transport and conversion – of its deployment (Fajardy and Mac Dowell, 2017). In this contribution, we present a biomass supply chain optimization framework, implemented in the AIMMS software, based on a BECCS whole-systems model. This model calculates the carbon, energy, water and land intensity associated with biomass production, processing, transport and conversion in a 500 MW UK based BECCS power plant, for a range of biomass feedstock, either energy dedicated crops – miscanthus, switchgrass, willow or agricultural residues – wheat straw, importing regions and land types. This last parameter is particularly significant as it accounts for the impact of direct and indirect land use change and hence plays a decisive role in the determination of BECCS carbon break-even time. Given an UK annual CO2 removal target and constrained amount of arable land per importing region, the optimal combination of feedstocks, importing regions and land types is determined to minimize either water or land use. Regardless of the criteria which is being minimised, marginal land is found to be the optimal choice, as it is associated with very limited land use changes. Finally, the choice of the resource to be conserved drives the biomass and region selection. For example, when minimizing BECCS water use, wheat straw is prioritised over other feedstocks. However, when minimizing the land use, miscanthus is preferred because of its relatively high yield. Overall, we conclude that given an annual carbon removal target and constrained amount of arable lands, the resources aimed to be preserved – arable land or fresh water – will have profound implications on the potential feedstocks and supply chains for BECCS. References Fajardy, M., & Mac Dowell, N. (2017). Can BECCS deliver sustainable and resource efficient negative emissions? Energy Environ. Sci. https://doi.org/10.1039/C7EE00465F IPCC. (2014). Climate Change 2014, Mitigation of Climate Change. Contribution of Working Group III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change

Advanced thermodynamic and processing modelling integration for amine scrubbing in post-combustion CO~2~ capture

The reduction in CO~2~ emissions from anthropogenic sources has become a topic of widespread interest over the past number of years. As the power generation sector is by far the largest stationary-point-source of CO~2~, being responsible for approximately 35% of total global CO~2~ emissions^1^ this question has special relevance for this industry. As the inclusion of carbon capture facilities incurs a significant energy penalty on the efficiency coal-fired power-stations, there is a strong requirement for the improvement of these systems in terms of the minimisation of operation and maintenance costs, capital costs and the maximisation of efficiency and flexibility. This last issue has relevance for start-up times and ramp-rates. Post-combustion capture methods based on the chemisorption of CO~2~ in aqueous amine solutions are among the most mature and accepted technologies for CO2 capture from power plants^2^. However, amines are complex, associating solvents requiring a sophisticated thermodynamic model, capable of modelling the hydrogen bonding interactions that occur in these systems. One such model is provided by the statistical associating fluid theory (SAFT^3^). This is a molecular approach, specifically suited to hydrogen-bonding, chain-like fluids. In this contribution we use the SAFT approach for potentials of variable range (SAFT-VR^4^) to model the thermodynamics and phase equilibria of a number of amines including ammonia and monoethanolamine. The molecules are modelled as homonuclear chains of tangentially bonded square-well segments of variable range, and a number of short-ranged off-centre attractive square-well sites are used to mediate the anisotropic effects due to association in the fluids. We also determine values of the binary parameters for mixtures and then use these parameters to predict the phase equilibria of amine+water, amine+carbon dioxide as well as water+carbon dioxide mixtures. We then consider the phase equilibria of the ternary mixtures of amine+water+carbon dioxide and finally that of quaternary mixtures of amine+water+carbon dioxide+nitrogen. A good quantitative understanding of the phase behaviour of these quaternary mixtures is essential for accurate modelling of absorption processes for carbon dioxide capture. 

1. Steeneveldt, R., Berger, B. & Torp, T.A., ChERD, 84(A9): 739-763, 2006
2. Rao, A.B.; Rubin, E.S., 2002. A Technical, Economic, and Environmental Assessment of Amine-Based CO2 Capture Technology for Power Plant Greenhouse Gas Control. Environ. Sci. Technol. 36, 4467-4475
3. Chapman, W.G., Gubbins, K.E., Jackson, G. & Radosz, M., Ind. Eng. Chem. Res., 1990. 29, 1709-1721
3. Gil-Villegas, A., Galindo, A., Whitehead, P. J., Mills, S. J. & Jackson, G., J. Chem. Phys. 106 (10), 8 March 199

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)

Power-to-transport: Using curtailed wind to run CCU processes

The Paris Agreement signaled global commitment to limit average global temperature rise to 2˚C and to make efforts to achieve 1.5˚C increase (UNFCCC, 2015). The IPCC AR5 cites carbon capture and storage (CCS) as a necessary technology to achieve this (IPCC, 2014). 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 (Styring, et al., 2011). 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). This work investigates two possible options for CCU integrated with DAC: Option A is the storage of curtailed wind as methanol which can be used in road vehicles to displace gasoline emissions (power-to-transport), and Option B is the use curtailed wind directly to run a DAC plant in order to capture decentralised carbon emissions and provide CO2 feedstock for CCU processes. A high-level analysis has been carried out to determine the gasoline substitution and emissions offset potential of both options, the overall conversion efficiency of the power-to-transport process, and the economics of each option. The UK was used as a case study and the methanol synthesis process described by (Rihko-Struckmann, et al., 2010) was used as the reference. Work by (Messiou, 2012) investigated the curtailment levels for the UK electricity system with increased wind generation; the ‘medium integration’ scenario assumed the UK had 5 times the current wind generation capacity. A corresponding curtailment level of 2.5% (of total electricity dispatched) was determined, this has been used as the base case in our analysis. The gasoline substitution potential of methanol produced via option A was ~0.12% (equivalent to ~0.05 MtCO2/y) with the overall power-to-transport efficiency being ~11%. Surplus energy (~64% of the curtailed electricity) was required to run the DAC plant and an associated air separation unit. The methanol production plant was found to be economically infeasible unless current methanol price increased by a factor of 1.8 to $988/t, the cost of hydrogen fell by a factor of 2.3 to$1811/t or a carbon price of $313/t was in effect. For option B, the emissions offset potential of the DAC process was ~0.18% for the same curtailment, capturing ~0.07 MtCO2/y. Therefore, the utilisation of curtailed electricity for direct capture of CO2 from the atmosphere results in greater avoided emissions than if it was stored as methanol. References IPCC, 2014. Climate Change 2014: Mitigation of Climate Change. Working Group III Contribution to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, s.l.: Cambridge University Press. Messiou, A., 2012. Investigating the role of power storage in accommodating the future wind integration into UK’s power system. London: Imperial College London. Rihko-Struckmann, L. K., Peschel, A., Hanke-Rauschenbach, R. & Sundmacher, K., 2010. Assessment of Methanol Synthesis Utilizing Exhaust CO2 for Chemical Storage of Electrical Energy. Industrial and Engineering Chemistry Research, Issue 49, pp. 11073-11078. Styring, P., Jansen, D., de Coninck, H. & Reith, H., 2011. Carbon Capture and Utilisation in the green economy: Using CO2 to manufacture fuel, chemicals and materials, s.l.: The Centre for Low Carbon Futures