Capturing CO2 from ambient air - David Keith

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A summary of the assumptions about capital cost and energy requirements used to calculate total costs is presented in Table 4.1. To reach the total cost per ton CO2 we sum the cost of each component per net ton CO2 captured by the system. We assume that capital and energy requirements scale linearly with plant capacity (a conservative assumption which ignores economies of scale) so only the unit cost matters. This allows us to estimate total costs using source data for components of different capacities. To adjust the unit costs to refer to net tons of CO2 captured, we introduce a CO2 multiplier, RC/Cnet , defined as the number of tons of CO2 processed in the component per net ton captured in total system. In the base system, the amine capture plant is assumed to be only 90% efficient, so 10% of the captured carbon and 10% of the calciner fuel carbon is lost to the atmosphere during operation. Consequently, the contactor and caustic recovery plant must process about 18% “extra” CO2 for each net ton captured (RC/Cnet = 1.18). The amine plant processes fuel carbon in addition to atmospheric carbon, giving RC/Cnet of about 1.6. Since capture in an oxyfuel system is nearly 100% efficient, the multiplier is 1 for contacting and caustic recovery. In both systems, atmospheric and fuel CO2 must be compressed, bringing RC/Cnetfor compression to 1.6 in the base system and 1.4 in the improved system. Immediately we can see that substantially more CO2 must be sequestered than is captured from the air. We can view (1 − RC/Cnet ) for compression as a sort of “carbon penalty” of air capture. 60%, in the base case, is the extra carbon that must be burned to capture and compress CO2 from air. It depends on the fuel used for thermal energy and the method of electricity generation. If the calciner were fired with fuel oil, for instance, the penalty would be larger. If the electricity were generated by fossil fuel combustion with CCS, the penalty including electricity generation would be higher. Even a carbon penalty above 100% does not invalidate air capture. Since the extra carbon is being sequestered, there is still net capture. However there is significant added burden on the fossil fuel supply with associated upstream and non-carbon impacts of fossil fuel use. Also, compared with point-source sequestration costs, the cost of sequestration (after the compression step) will be higher per unit of CO2 captured, since the air capture system must also sequester carbon from the fuel. The cost of CO2 transport and injection is not included in the figures given here, although it is expected to be comparatively small (McCoy and Rubin, 2005; IPCC, 2005). We calculate the unit cost of each component in analogy with Equation 3.18 for the contactor cost where, again, Cap and O&M are the amortized capital and maintenance costs. We introduce ˙Etherm, the rate of thermal energy use by a component. We then sum across the components adjusting the unit costs by the carbon multiplier: total cost CO2 captured = Cap+O&M + pelec( ˙Eelec)+ ptherm( ˙Etherm) ∑ × RC/Cnet components capacity The same financial assumptions are made as for Section 3.4: 15% capital charge rate, 4% operation and maintenance, and carbon-neutral electricity for 19 $/GJ. The results of the cost estimation are given in Table 4.2. The total cost is 250 $/t-CO2 (900 $/t-C) for the base case and 130 $/t-CO2 (500 $/t-C) for the improved system. In the base system, capital and operating costs comprise half the total with energy cost as the other half. In the improved system energy becomes slightly more important, making up 60% of the total. 54
Component Capital + O&M Energy cost Unit cost Base System Contactor 60 30 90 Caustic recovery 50 80 120 Amine capture 20 4 20 CO2 compression 5 10 20 Total 250 Improved system Contactor 10 10 20 Caustic recovery 40 50 90 Oxygen separation 5 8 10 CO2 compression 4 10 20 Total 140 Table 4.2: Cost of example system and improved system by component. All costs in $/t-CO2. Base ΔE Base Δ$ Impr. ΔE Impr. Δ$ [GJ/t-CO2] [$/t-CO2] [GJ/t-CO2] [$/t-CO2] Switch to oxyfuel -4 -54 Packed tower with low capital costa -2 -62 +.06 -5 Fuel cost up to 8 $/GJ +24 +16 Fuel is stranded natural gas at 3 $/GJ -36 -24 Spray constant increased ×2 b -2 -34 -2.5 -5 Capital charge rate is 12% -19 -9 Economy of scale: cost×0.5 capital -61 -29 Table 4.3: Sensitivity of cost estimates to changes in assumptions. “ΔE” refers to change in energy requirement compared with results in Table 4.2 and “Δ$” refers to change in total cost. “Base” and “Impr.” refer to the base and improved systems in Table 4.2. a Using the packed tower energy requirement from Baciocchi et al. and assuming a per-ton capital cost half that of the optimized spray tower. b kspray becomes 6 × 10−3 m s , which is only a 50% increase in the improved system, where it was already increased. 55
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Capturing CO2 from ambient air: a f
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For my family, friends, and everyon
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etween the National Science Foundat
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Contents Acknowledgments iv Abstrac
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List of Figures 1.1 Global CO2 emis
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Chapter 1 Introduction The climate
Page 13 and 14: egeneration at a high enough concen
Page 15 and 16: 1.4 Routes to air capture 1.4.1 Org
Page 17 and 18: The NaOH approach is the primary su
Page 19 and 20: there is no compelling reason to ca
Page 21 and 22: 7Na2CO3(l)+5Na2Ti3O7(s) ⇋ 4Na8Ti5
Page 23 and 24: changing the process design to acco
Page 25 and 26: Component / source GJ/t-CO2 electri
Page 27 and 28: Chapter 3 Contactor In this chapter
Page 29 and 30: the CO2 capture rate in a spray sys
Page 31 and 32: CO 2 absorption per pass [mol/l] 10
Page 33 and 34: CO 2 absorption [mmol/pass] 16 14 1
Page 35 and 36: Figure 3.5: Water loss measured in
Page 37 and 38: 3.3.1 CO2 depletion in air With lon
Page 39 and 40: where vt,1 and vt,2 are the termina
Page 41 and 42: surface area [m 2 -surface / m 3 -r
Page 43 and 44: spray mass density [kg / ln(μm)] 1
Page 45 and 46: average surface area [m 2 -spray /
Page 47 and 48: As a bound on the effects of coales
Page 49 and 50: Draft type Height [m] Crosssectiona
Page 51 and 52: 2. Median assumptions, no coalescen
Page 53 and 54: capture system. We can also see tha
Page 55 and 56: Multistage spray One possible solut
Page 57 and 58: Although, the cost of water loss ma
Page 59 and 60: 3.5.6 Solids formation - scaling an
Page 61 and 62: Chapter 4 Cost of air capture Altho
Page 63: Capital Capacity Electric a Thermal
Page 67 and 68: Chapter 5 Discussion Overview In th
Page 69 and 70: 5.2 Lessons for assessing of future
Page 71 and 72: Bibliography Adams, P. J. and Seinf
Page 73 and 74: IPCC (2001). Climate Change 2001: M
Page 75 and 76: Tzivion, S., Feingold, G., and Levi
Page 77 and 78: F Liquid flow rate in the contactor
Page 79 and 80: Appendix B Experimental details and
Page 81 and 82: Figure B.2: Photograph of completed
Page 83 and 84: Figure B.4: Two cranes lift the rea
Page 85 and 86: fastened together and lined, the re
Page 87 and 88: The prototype has three particle fi
Page 89 and 90: Figure B.7: Sampling points of CO2
Page 91 and 92: In order to calculate CO2 absorbed,
Page 93 and 94: Absorbed CO 2 [mol/l] 0.3 0.25 0.2
Page 95: periodic switching of the spray on
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Capturing CO2 from ambient air - David Keith

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