Capturing CO2 from ambient air - David Keith

In order to calculate CO2 absorbed, outlet CO2 concentration must be subtracted from background.
This posed some challenges because background CO2 sometimes varied dramatically, with spikes and
drift on a time scale of 10 seconds to many minutes. These variations were often similar in magnitude to
the absorption signal (4–50 ppm). Much of this appeared to be due to local variation from the ventilation
system and from experimenters breathing near the inlet. We reasoned that connecting the inlet to the
outside air would stabilize the CO2 background. In fact outside concentration was at least as variable as
inside concentration, perhaps due to signals from cars, plants, and drift from diurnal variation. The most
stable background was achieved in trials 8–12 when the inlet was placed in a corner of the room and all
experimenters stayed at the base of the tower, separated by about 3 m in height and 5 m in lateral distance
from the inlet. Repeated measurements of the same conditions over time and across trials were largely
consistent, so we were generally able to overcome background variation.
Variation of absorption by NaOH concentration was measured by running separate trials, changing
the working solution but holding other conditions constant. The main confound to this approach is that
different concentrations have different densities and viscocities, so the drop distribution was different
among the trials. The measured differences in CO2 absorption are thus due to both changes in the reaction
rate of CO2 hydrolysis and to differing drop size.
Variation of absorption by nozzle pressure was measured by changing the pressure (by adjustment of
a needle valve) within the same trial. Using manufacturer data for changes of drop size and flow rate with
pressure, the effect of drop size on absorption was inferred. The calculation is distorted by the use of 1.3
M NaOH solution instead of water, which is what the manufacturer data apply to. However, since the
solution is constant for the three pressures, the trend should be similar as for water.
B.3 Data Analysis
In the continuous mode trials, data were relatively easy to analyze when significant drift was not present.
Figure B.8 shows the CO2 concentration over a such a trial. The difference between the average background
concentration and average steady-state outlet concentration gives the rate of CO2 absorption. Some
subjective judgment is required to choose the bounds of each period to average over. Periods were chosen
to exclude transient regions between measuring states and regions which appear excessively noisy.
We have samples of the solution taken periodically through some of the trials, including the one shown
in Figure B.8, Trial 2. The CO 2−
3 concentrations in those samples were measured with a Shimadzu Total
Organic Carbon (TOC) analyzer after dilution and partial neutralization of the remaining NaOH. The
results are shown in Figure B.9. If the rate of CO2 uptake is taken as roughly constant, we would expect
carbonate (CO 2−
3 ) concentration in solution to increase linearly with operating time, assuming a constant
solution volume. But the solution volume changes due to evaporation (and entrainment and solution stuck
on surfaces, but we take these to be comparatively minor). Using the measured evaporation rate from
Trial 3 (which had similar conditions) to adjust the solution volume over time, we can make a prediction
of CO 2−
3 concentration based on the LiCOR measurements. This prediction is shown along with the TOC
measurements in Figure B.9.
For Trials 6–12, a more nuanced method of analysis was needed. The procedure in these trials involved
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