A 2. Cost estimates
These calculations enable us to make cost estimates for Direct Ah' Capture, assuming that the technology applied is similar in nature to the MEA-based post-combustion capture plants reported by Metz et al. (2005). The SRCCS costings, referring to a European or North American location, were adjusted and brought to a consistent 2013-basis by Rubin et al. (2015): some values for Pulverised Coal and Natural Gas combustion power plants are shown in Table Al. Note that these reported costs include compression of the CO, product, but exclude transport and storage.
The engineering duty of these capture plants is represented by the reversible work of separation W (MJ/tCO,), calculated from equation (11), shown in Table Al. Note that the spread in costs around the representative value is about +/- 25%. For process plant of a similar nature, where similar assumptions har e been made in calculating fixed and variable costs, we might expect the performance cost to scale with the duty, and indeed the cost/duty ratio is found to be close to 0.326 US$/MJ(rev work) for both coal and natural gas plants. The reversible work for DAC (Figure 4 in main text) is some 21 MJ/kmolCO,, or 477 MJ/tCO,. Using the scaling described, we might therefore expect the cost of carbon captured by DAC from the air to be about 155 US$/tCO„ exclusive of transport and storage.
Table Al. Scaling cost with reversible work. Range of cost of CO, is given as Low (Representative) High, indicating observed spread. Costs are adjusted SRCCS values, con'ectedto2013 (Rubin et al., 2015), and exclude transport and storage.
The estimate of 155 USS/tCO, for DAC can be compared with the representative cost of CO, avoided by applying FGC at power plants. At pulverised coal plants the cost of avoided CO, is 67 USS/tCO, so DAC is 88 USS/tCO, (131%) more expensive; at NGCC (gas) plants the cost of avoided CO, is 83 USS/ tCO, so DAC is 72 USS/tCO, (87%) more expensive.
The costs of transport and storage of carbon dioxide will vary with the location, and many other factors relating to specific projects (Metz et al., 2005). Arauge 1-19 USS/tCO, can be taken as consistent with other costs given here (Rubin et al., 2015). An indicative range of cost of DACCS is then 156-174 USS/tCO,.
The amount of carbon dioxide that the DAC process sends to storage can be estimated. We estimate the DAC plant to require heat energy of 14 x Wtev, which is 6678 MJ/tCO,. In addition there is 51 MJ/ kmol or 1159 MJ/tCO, required to din e the compression, thus, 7837 MJ(tli)/tCO, in total. If this energy is derived from natural gas with an emission factor of 0.0561 kg CO,/MJ, then another 440 kg of CO, are generated for every tonne captured from the ah. Assuming 90% of this extra CO, is captured by the DAC plant, the ratio of carbon stored to net carbon captured is (1 + 0.9*0.440)/(l-0.044), which is 1.46. This is similar to the ratio of carbon stored to carbon avoided for coal-fired power plants with post-combustion capture.
A large volume of air must be blown though the DAC contactor where CO, is absorbed, so it is very important that the pressure drop in this device be kept low, to minimise power consumption by the air blowers. The CE DAC process uses a novel structured packing arrangement with intermittent liquid flow (Keith et al., 2018), and its reported pressure drop is about 100 Pa. a very low value. Assuming this pressure drop and a fan efficiency of 70%, the work required is some 11.72 MJ(e)/kmolCO„ or 266 MJ(e)/tCO, when the recovery a is 74.5%. Deriving this work from heat at an efficiency of 40%, gives a requirement for the air blowers of 665 MW(th)/tCO,. This is a fraction 665/7837 of the total heat demand for the DAC process, or 8.5%. We note that in a study of post-combustion capture using MEA at a natural gas fired power plant, the flue gas booster fan required 16% of the energy required for cap true and compression (Smith et al., 2013). The DAC plant estimate is significantly less than this, but does rely on novel low-pressure drop absorption technology. Using a conventional packed contactor would increase the pressure drop, power requirement and cost of DAC.
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