Capturing CO2 from ambient air - David Keith

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Figure 3.14: Equilibrium speciation of carbonates added to a 5M NaOH Solution at 20C. Formation of the solid Natron is predicted at CO 2− 3 concentrations as low as 0.4 M, but not observed experimentally. Another solid, Nahcolite, is predicted at about 4M CO 2− 3 , which is out of the range tested in the prototype. 48
3.5.6 Solids formation – scaling and clogging At higher carbonate concentrations, solids formation may become a problem. To explore this issue, calculations with the chemical equilibria modeling software Chess were performed. The results are displayed in Figure 3.14. This analysis predicts that solids will form at carbonate concentrations larger than about 0.4 M in a solution with an initial NaOH concentration of 5 M. However, this was not observed in the prototype. Also, Apelblat and Manzurola (2003) report the solubility of sodium carbonate as more than 2 M at 20◦C. The formation of this species may be kinetically limited. Still, if very high NaOH concentrations are used to combat water loss and the solution is recirculated to collect a high concentration of CO 2− 3 , solid formation is likely. If solids are present, they can be managed, although this may add complexity and capital cost. The main concern is if solid particles larger than the minimum free passage of a nozzle get into the spray supply line they will clog the nozzle. This problem is typically solved with inlet screens, which we used in the prototype to keep the line clear of debris and foreign particles pulled in at the air inlet. The long term scaling (formation of a layer of solids adhered to a surface) of contactor walls, pipes, and equipment can probably be managed with a periodic water wash. Unlike calcium and magnesium compounds, the typical cause of scaling problems, sodium compounds are very soluble in water. Trace elements in air and process water may cause scaling not easily managed with a water wash, but this would be a similar problem as experienced in cooling towers and other industrial operations. 3.6 Conclusions from contactor analysis The prototype demonstrates four key features of a potential NaOH spray-based contactor: (1) off-the-shelf single-fluid spray nozzles can produce a spray which efficiently absorbs CO2 from ambient air (in terms of energy required for lifting the solution), (2) such nozzles can produce such a spray at pressures which are not prohibitive, (3) The pressure drop across a particle trap which controls entrainment of small drops from such a spray is not prohibitive (in terms of energy required for blowing air), and (4) materials compatibility and safety concerns in handling NaOH do not pose significant challenges to the design and operation of a contactor. However, substantial uncertainties remain about the cost of a full-scale contactor. In particular, scaling up the mass transfer process observed in the prototype to meet the needs of global carbon mitigation scheme is a complex engineering challenge. Cost estimates for a full scale contactor scaled up from prototype observations came out high and highly variable. Overall, this seems to suggest that the current approach to contactor design and cost estimation is inadequate. The costs are high but do not appear to run up against any absolute limits to improvements. On the contrary, the results suggest drastic improvements can be made with both modest and radical redesign. On the other hand, the results suggest serious pitfalls that may easily render spray-based contactors infeasible. No absolute conclusion can be drawn, but the way is pointed for further investigation. Specifically, the feasibility and cost of spray-based contactors could be established with (1) a more exhaustive investigation of basic designs including counter-current flows, dual-fluid nozzles, and multi-stage spraying, (2) a better understanding of capital cost for potential contactors and how they scale with various design parameters, and (3) a precise model of, or empirical 49
Page 1 and 2:
Capturing CO2 from ambient air: a f
Page 3 and 4:
For my family, friends, and everyon
Page 5 and 6:
etween the National Science Foundat
Page 7 and 8: Contents Acknowledgments iv Abstrac
Page 9 and 10: List of Figures 1.1 Global CO2 emis
Page 11 and 12: 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: Although, the cost of water loss ma
Page 61 and 62: Chapter 4 Cost of air capture Altho
Page 63 and 64: Capital Capacity Electric a Thermal
Page 65 and 66: Component Capital + O&M Energy cost
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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