Capturing CO2 from ambient air - David Keith

A summary of the assumptions about capital cost and energy requirements used to calculate total costs
is presented in Table 4.1. To reach the total cost per ton CO2 we sum the cost of each component per net
ton CO2 captured by the system. We assume that capital and energy requirements scale linearly with plant
capacity (a conservative assumption which ignores economies of scale) so only the unit cost matters. This
allows us to estimate total costs using source data for components of different capacities. To adjust the unit
costs to refer to net tons of CO2 captured, we introduce a CO2 multiplier, RC/Cnet , defined as the number
of tons of CO2 processed in the component per net ton captured in total system. In the base system, the
amine capture plant is assumed to be only 90% efficient, so 10% of the captured carbon and 10% of the
calciner fuel carbon is lost to the atmosphere during operation. Consequently, the contactor and caustic
recovery plant must process about 18% “extra” CO2 for each net ton captured (RC/Cnet = 1.18). The amine
plant processes fuel carbon in addition to atmospheric carbon, giving RC/Cnet of about 1.6. Since capture
in an oxyfuel system is nearly 100% efficient, the multiplier is 1 for contacting and caustic recovery. In
both systems, atmospheric and fuel CO2 must be compressed, bringing RC/Cnetfor compression to 1.6 in
the base system and 1.4 in the improved system. Immediately we can see that substantially more CO2
must be sequestered than is captured from the air. We can view (1 − RC/Cnet ) for compression as a sort
of “carbon penalty” of air capture. 60%, in the base case, is the extra carbon that must be burned to
capture and compress CO2 from air. It depends on the fuel used for thermal energy and the method of
electricity generation. If the calciner were fired with fuel oil, for instance, the penalty would be larger.
If the electricity were generated by fossil fuel combustion with CCS, the penalty including electricity
generation would be higher.
Even a carbon penalty above 100% does not invalidate air capture. Since the extra carbon is being
sequestered, there is still net capture. However there is significant added burden on the fossil fuel supply
with associated upstream and non-carbon impacts of fossil fuel use. Also, compared with point-source
sequestration costs, the cost of sequestration (after the compression step) will be higher per unit of CO2
captured, since the air capture system must also sequester carbon from the fuel. The cost of CO2 transport
and injection is not included in the figures given here, although it is expected to be comparatively small
(McCoy and Rubin, 2005; IPCC, 2005).
We calculate the unit cost of each component in analogy with Equation 3.18 for the contactor cost
where, again, Cap and O&M are the amortized capital and maintenance costs. We introduce ˙Etherm, the
rate of thermal energy use by a component. We then sum across the components adjusting the unit costs
by the carbon multiplier:
total cost
CO2 captured = Cap+O&M + pelec( ˙Eelec)+ ptherm( ˙Etherm)
∑
× RC/Cnet components
capacity
The same financial assumptions are made as for Section 3.4: 15% capital charge rate, 4% operation and
maintenance, and carbon-neutral electricity for 19 $/GJ.
The results of the cost estimation are given in Table 4.2. The total cost is 250 $/t-CO2 (900 $/t-C)
for the base case and 130 $/t-CO2 (500 $/t-C) for the improved system. In the base system, capital and
operating costs comprise half the total with energy cost as the other half. In the improved system energy
becomes slightly more important, making up 60% of the total.
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