Pointers to Policy
The scale of carbon dioxide capture and storage required to meet climate targets is a daunting challenge both for policy formulation and technology development; however, it makes no thermodynamic sense to contemplate removing carbon dioxide from the atmosphere later this century whilst continuing massive uninhibited emissions now. Capture from flue gas can be done using about half of the energy that will be required for ah' capture later, and at much less cost. At present, there is little evidence that other methods of CDR will prove to be significantly cheaper and more convenient than CCS—after all, BECCS actually relies on CCS technology. Capture from flue gas should, therefore, be incentivised and applied as soon as possible, and not delayed so that future generations face the worse problem of capturing even more CO, from the ah, yet the IEA reports that industry is “woefully off-track’’ in installing CCS plant to meet even the very modest target of 500 Mt СО,/у by 2030 (IEA, 2019).
The problem is that virtually cost-free discharge of flue gas to the atmosphere, as available at present, makes it almost impossible for commercial operations to justify the investment and operating expense of CCS. Future generations will regret this lack of action. One remedy for this would be for governments to incentivise the storage of carbon dioxide and penalise the failure to store it. It would be possible perhaps to adapt the Extended Producer Responsibility (EPR) approach which has been used in Europe since the 1990s for the reduction and management of various waste streams and to promote more sustainable use of resources (EU, 2014). Under EPR. regulations might require organisations which extract and/or import fossil fuels to pay the costs of capture and permanent storage of a fraction of the emissions for which they are responsible. Initially, the Removal Fraction (that proportion of emissions that is stored) should be set at a small fraction of the total production, as there is currently insufficient capability to store all the carbon dioxide emitted. Over time, as storage capacity increases, so too should the Removal Fraction. Ultimately, the goal is to match the whole national, and thus global, production of carbon dioxide with an equal amount of storage. The price of fossil fuels would rise under this regime, at first modestly as the removal fraction is small, but ultimately to meet the frill cost of capture and storage. However, the scheme avoids the arbitrariness of permits and carbon tax, coupling the consumers’ extra price burden to the actual cost of pollution control (Allen et al., 2009).
Such an EPR policy would achieve several ends—(i) the costs of clean-up are paid by those parties that are responsible for producing the emissions, an application of the ethical principle of ‘the polluter pays’; (ii) it signals to emitting industries that in the long-term they will be responsible for the costs of clean-up. This will incentivise them to reduce emissions and also to find ways in which they can reduce the costs of capture and storage; (iii) it signals to those using developing techniques for capture and storage, working with flue gas or ah', that doing so is a worthwhile endeavour; (iv) it acts as a price-discovery mechanism for the avoidance of harms caused by carbon dioxide emissions. Eventually, this could create a rational pricing of carbon dioxide emissions—the cost of ‘cleaning up the mess’. Incentivising CCS in the UK context was the subject of a report by Oxburgh (2016).
The need for rapid movement on emissions reduction is underlined by Fuss et al. (2018) in their review of NETs. They looked at the characteristics, potential and limitations of BECCS, DACCS, Afforestation and forest management, EW, OF, Biocliar. and Soil carbon sequestration (schemes described in Table 1 above). They concluded that “from a risk management perspective, the uncertainties and risks around large-scale NETs deployment suggest a need for swiftly ratcheting up emissions reductions over the next decade in order to limit our dependence on NETs for keeping temperature rise below 2 °C”. A portfolio of several NETs would be necessary in order to meet the stated climate goal sustainably. They point out that there are many gaps in our knowledge concerning the claimed benefits, and that research on the side effects is basically non-existent. The technique DACCS emerged in this study as ... “a relatively promising long-term option beyond 2050. being limited in potential only by the economic (and energetic) feasibility of scale-up” (Fuss et al., 2018). Doubts about the economic and energetic feasibility of the scale-up of DACCS really need to be addressed in order to develop new technology and schemes, ready for potential deployment. There is huge scope for novel chemistry and process engineering, but all NETs schemes also raise questions relating to environment and society that need considering.
The expected scale and cost of CDR suggest that, later this century, it could become a growing and innovative trillion USS/y industry, managing carbon. The magnitude of the investment required to develop this capability justifies much more effort on research, development and design than is currently evident. We have to improve and demonstrate CDR schemes and understand the full range of impacts. The shortening timescale of climate change lends urgency to this programme.
Al. Process calculations
The IPCC Special Report on Carbon Capture and Storage (SRCCS) collected data for processes removing carbon dioxide from flue gas in power plants, mostly using amine solvents like MEA (Metz et al., 2005). In a power plant, the energy used by the carbon capture unit is a fraction taken from the energy that would otherwise be used to generate electricity. The SRCCS reported this fraction to be in the range 24 to 40% of Lower Fleating Value (LED') in new pulverised coal (PC) plants and for new natural gas combined- cycle (NGCC) power plants it was between 11 and 22% LHV (Metz et al., 2005). Table 2 shows data for PC and NGCC plants at the limits of the effectiveness ranges, and also, for comparison purposes, for the Carbon Engineering (CE) DAC process (Keith et al., 2018). The reversible work is calculated from equation (11) for the parameters shown.
For both FGC and DAC plants, the heat used for the separation is calculated from IPCC’s Default Emission Factors using a net calorific basis, 0.0946 kg CO./MJ for bituminous coal, and 0.0561 kg CO,/MJ for natural gas (Eggleston et al., 2006—Table 2.2). However the SRCCS reported energy demand includes the energy for post-capture compression, with the delivery pressure, suitable for piping the product to underground storage, being in the range 8-14 MPa. Since we wish to compare the thermodynamic performance of the separation processes with each other we har e therefore subtracted the compression energy. We can then compare the heat requirement of the separation process, Q , with the reversible work w for the separation duty at the same temperature and pressure. The CE DAC process employs a 4-stage compressor with intercooling for the compression of almost pure carbon dioxide from atmospheric pressure (0.1 MPa) to 15 MPa. This includes a glycol dehydration system prior to the last stage, important for diying the gas before pipeline transport. The compression power consumption is
20.4 MJ(e)/kmol, which at a conversion efficiency of 40% requires 51 MJ(th)/kmol. We have subtracted this amount from the reported heat usage of the FGC units when calculating the net heat demand of the separation process, 0,ef shown in Table 2.
The CE DAC process absorbs carbon dioxide from ah into aqueous potassium hydroxide. This is followed by a cation switch in a separate reactor, where calcium carbonate is precipitated. The carbonate is then calcined to produce the CO,-enriched gas stream, and calcium oxide which is slaked to hydroxide and recycled to facilitate the cation switch. Data for this process included in Table 2 are those reported by Keith et al. (2018) based on process modelling, equipment testing and experience with a pilot plant capturing 1 t/day CO,. The calculated value of the ratio Qsep/wm for this process is 16.9.
Socolow et al. (2011) reported an analysis of DAC using aqueous sodium hydroxide followed by a cation switch and calcining of calcium carbonate. The process was similar in general outline to that described by Keith et al. (2018). The feed air composition was taken as 0.05% CO, and only 50% was captured (a = 0.5). The carbon dioxide produced, almost pure, was compressed to 10 MPa, requiring
0.42 MJ(e)/kg, which can be subtracted from the overall power usage 1.78 MJ(e)/kg. The total energy consumed by the separation process is then 1.36 MJ(e)/kg of electrical power and an additional 8.1 MJ(th)/ kg. Converting power to thermal energy at the usual efficiency (40%) gives a total requirement, Q , of 506 MJ(th)/kmol. The minimum reversible work for this separation is 19.45 MJ/kmol, so the estimated value of Q.ep/irm is 26. This value is significantly greater than that found for the CE DAC process (16.9) and is at the top end of the range of 0,e/firw for FGC processes in Table 2. For comparison purposes, Socolow et al. (2011) present study data from a United States’ National Energy Technology Laboratory (NETL) PC plant with flue gas carbon capnrre using aqueous MEA (data: yc = 12.8%,yco = 99%, a = 0.9, T0 = 40 °C, delivery pressure 10 MPa (Ramezan et al., 2007). Again, using a heat-to-power conversion efficiency of 40%. the total thermal energy requirement was 3.33 MJ/kg CO,; subtracting the compression energy yields a value Q =100 MJ/kmol and the ratio Q,ep/wm is 14.4 for the separation process. This is nicely within the range expected from Table 2.
At 40 °C the minimum work of compression of CO, from 0.1 to 15 MPa is 10.87 MJ/kmol, for a reversible isothermal process. From the simulation data for compressor power of Keith et al. (2018), we calculated that the heat required to generate this power was 51 MJ/kmol, giving a ratio for the compressor (О/= 4.7. This shows that compression is much more efficient than a separation process, which is why we prefer to analyse them separately as far as possible. In practice, some energy saving in the compression may be possible in an optimised compressor system (Jackson and Brodal, 2018).