Table 1 Assumptions and parameters used for the upper and lower bounds in the process model.

From: Ambient weathering of magnesium oxide for CO2 removal from air

Lower boundUpper bound
 Calcination temperature [°C]6001200Literature values from 500 to 1200 °C45
 Calcination time [h]20.5Literature values from 0.5 to 4 h45
 Time between calcination loads [h]0.250.25 
 Heat of decarbonation [kJ mol−1]118118Literature value70
 Kiln efficiency [%]9090Assumed industry state-of-the-art
 Calcination efficiency [%]9090At 600 °C, the decomposition is completed within 2 h. At 800 °C, the decomposition is completed within 30 min45. Additional studies show decomposition efficiencies near 90% using 600 and 650 °C for 1.5 h71.
 CO2 uptake capacity [mol CO2 molMgO−1]11Assumed stoichiometric value consistent with magnesite formation
 MgO layer thickness [m]0.10.1 
 Particle size [µm]2020 
 Environmental losses [% cycle−1]510 
 Carbonation efficiency [%]9090 
 Stirring equipment [acres unit equipment−1]125125Literature value72
 Number of plots3,50410,512Determined to keep the calciner operating continuously at the given calcination conditions
Energy costs and emissions
 Natural gas [$ GJ−1]3.53.5Literature value7,20
 Natural gas [kgCO2 GJ−1]5959Literature value26
 Gasoline [$ gallon−1]2.602.60Average market price for 2019 from EIA73
 Gasoline [kgCO2 gallon−1]8.898.89Literature value from EIA74
 Grid electricity [$ GJ−1]16.716.7Literature value7,20
 Grid electricity [kgCO2  GJ−1]150150Literature value26
 Solar electricity [$ GJ−1]16.716.7US national average for utility-scale solar ($0.06 kWh−1)44
 Future solar electricity [$ GJ−1]88Projected value ($0.03 kWh−1)44
 Solar electricity [kgCO2  GJ−1]6.96.9Literature value7
 Raw material (mining) emissions [kgCO2 tMineral−1]1010Literature values from 2 to 12.126
Economic parameters
 Capacity factor [%]9090Consistent with Keith et al.20
 Plant economic lifetime [yr]2020Consistent with Keith et al.20
 Discount rate [%]411 
 Capital recovery factor [%]7.412.6Similar to 7.5 and 12.5% used in Keith et al.20