Capturing CO2 from ambient air - David Keith

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
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