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Appendix D: CO2 Flux Calculation
Pages 469-478

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From page 469...
... E = enhancement factor, which is only present when a chemical reaction takes place kl = mass-transfer to exhibit the the case of a solvent, this is of a CO separation process for an For simplicity, and coefficient. In unique requirements requiredstrictly the liquid-phase 2 extremely dilute system (air)
From page 470...
... l H = Henry's the case of a solid of atmΒ·cm3 coefficient. Inlaw constant in unitsonly present/mol a chemical reaction takes place E = enhancement factor, which issorbent or mineral, this could be an effective mass-transfer coefficient, when pCO = partial pressure of CO2 based2upon all of the dominating diffusion resistances present in the system CO2 Ruthven, 1984 for for an For simplicity, and to exhibit the unique requirements required of a (see separation process kl = mass-transfer coefficient.
From page 471...
... and Henry's law constant between 10,000-70 000 atm-cm3/mol, spanning that of ionic liquids to potassium carbonate. SOURCE: Wilcox et al., 2014.
From page 472...
... This approach, applying mass and energy balances to each individual step in the overall process, calculating energy requirements, and then assessing the costs of the necessary capital equipment, has been applied to various scenarios to arrive at overall costs for the separations process. In the analysis employed, an array of critical parameters was defined within a range of values deemed physically realistic, and from these parameters other key parameters are calculated (Figure D.2)
From page 473...
... In these calculations, the model envisaged that the steam is directly contacted with the sorbent, providing both a concentration and thermal driving force for desorption. In this approach, the sorbent quickly attains a pseudo-steady-state capacity of adsorbed water, becoming hydrated after steaming, with some water desorption occurring via evaporative cooling upon exposure to fans blowing air through the material in the next adsorption step.
From page 474...
... 0.2/1 Final desorption temperature (K) 340/373 Thermal Energy Outputs MJ/mole 0.08/0.85 GJ/tCO2 1.85/19.3 kW-hr/tCO2 514/5367 Electrical Energy Outputs MJ/mole 0.003/0.167 GJ/tCO2 0.08/3.79 kW-hr/tCO2 20/1055 Cost $/tCO2 14/1065 NOTE: Energy expenditure associated with upper- and lower bound-process configurations.
From page 475...
... The committee does not feel that the lower bound cost is practically achievable, though no physical bounds prevent the cost of direct air capture from dropping below $100/tCO2. RENEWABLE ROUTES WITH SOLVENT-BASED SYSTEMS Costing Air Capture from Renewables -- achieving 1Mt CO2 avoided per year Two routes to costing air capture using 100 percent renewables were considered: (1)
From page 476...
... This estimate demonstrates that an air capture process using solid sorbents and operating near ambient conditions, using low temperature thermal energy, can be surprisingly efficient. Despite the high dilution of the source gas, a process suitably designed TABLE D.2  Economic Costs Associated PV + Storage for Solvent-Based Air Capture CAPEX Cost ($M)
From page 477...
... Assuming direct electricity needs of 21-27 kJ/ molCO2. Assuming electric kiln and electric heater have efficiencies of 80%, requires an electricity demand to meet thermal energy demands of 485 643 kJ/molCO2.
From page 478...
... or (2) battery storage of renewable electrons for direct electric heating of an electric-fired kiln.


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