Achieving Supercritical Fluid CO2 Pressures Directly from Thermal Decomposition
of Solid Sodium Bicarbonate
Roger D. Aines,1 Joshuah K. Stolaroff,1 Megan M. Smith,1 and
William L. Bourcier1
1
Lawrence Livermore National Laboratory, Livermore, California, USA
Compressing CO2 to supercritical pipeline pressures is one of the major costs of carbon capture and
storage. We have now quantified through experimental and modelling studies that it is possible to
obtain CO2 pressures of 150 bar by thermal decomposition of solid sodium bicarbonate (Nahcolite,
NaHCO3). Precipitation of solid sodium bicarbonate has been observed previously when using
sodium carbonate solutions in carbon dioxide capture, but the benefits of separating the solid from
liquid before regenerating were not well understood. We examined the theoretical pressures
available because our encapsulated solvent process (Vericella et al.) is capable of separating
precipitated solid from most of the process water. Using an ion association model we predicted that
in the absence of water, Nahcolite reaches supercritical CO2 pressures over the solid at 125ºC (blue
line below). This encouraged us to pursue experimental determination with realistic amounts of
water (red line below). High pressures (150 bar) are shifted to higher temperatures (205ºC) when
there is some water present – this amount of water is that generated by the continuous
decomposition of Nahcolite during heating. The experimental determination was conducted in an
autoclave starting with dry, reagent Nahcolite. As it decomposes it generates water (L-V curve in
grey) and CO2. The composition of the CO32--HCO3- equilibria, and the solubility of Nahcolite,
shifts in the fluid with temperature. The re-precipitation reaction shows significant hysteresis upon
cooling. We have not fully examined the nature of this hysteresis.
The ability to reach supercritical pressures with thermal regeneration alone opens new approaches
to carbon capture. Small, modular reactors could be used to both capture and regenerate the fluid
CO2 in the same unit, permitting the size of a capture plant to be adjusted freely without the current
constraint imposed by the size of a large, fixed mechanical compressor. Although the design
requires high pressure regeneration, we believe that the advantages of modularity and capital
equipment savings will more than offset that cost. We are working on such a design using our
encapsulated carbonate system, and there are a variety of other carbonate processes in development
at other institutions that could benefit from this capability. We are experimentally examining the
behaviour of the temperature-pressure- CO32--HCO3- variations in order to facilitate design of
thermally-pressured CO2 capture systems.
Vericella, John J. et al., (2015) “Encapsulated Solvents for Carbon Dioxide Capture” Nature Communications 6, Article
number: 6124 doi:10.1038/ncomms7124 Published 05 February 2015
1
160 140 Calculated pressure (pure Nahcolite) H2O/NaHCO3 = 0 120 Pressure, bars 100 80 CO2 CriKcal Pressure 60 Experimentally Observed H2O/NaHCO3=3.2 40 20 L-­‐V water 0 25 45 65 85 105 125 145 165 185 205 Temp, C The observed pressure of CO2 determined in heating pure Nahcolite. The blue curve shows the calculated
CO2 pressure from an ion association model that assumed no liquid water. In practice, water is released at
the same rate as CO2 from the dissociation reaction 2NaHCO3 = Na2CO3 + H20 + CO2. This increases the
temperature at which high pressures are reached due to the gradual conversion of carbonate to bicarbonate,
and the ensuing dissolution in water. The red curve shows the experimentally observed CO2 pressure
beginning with pure Nahcolite. The CO2 pressure was obtained by subtracting the L-V curve pressure for
water from the observed total system pressure, which is the sum of CO2 and H2O gas pressure.
2