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Ocean liming is probably economic (though that alone isn't enough)

Updated: Apr 10, 2023

There is a lot of interest in scaling up lime production to remove CO2 from the atmosphere. The idea is simple: Take calcium carbonate, calcine it to remove and capture CO2, and then introduce it to the ocean to sequester CO2 as it dissolves. The solution is simple and elegant and prone to a horror-show of geopolitical wrangling. But mostly, the problem is that it just isn't as economic as land-based CO2 capture. This blog post is a brief summary of the famous 2013 paper "Engineering challenges of ocean liming", updated slightly for modern assumptions.


The carbon capture capability of lime is simple to remember: About 1 ton of lime will capture 1 ton of carbon. Capital costs for lime production using a pre-calciner system with oxy-fuel firing, waste heat recovery, and CO2 capture are estimated at approximately US$290 million for a plant processing one million tonnes of hydrated lime per year. The paper assumed a typical plant life of 40 years, and a 4% annual discount rate, delivering hydrated lime at a cost of about US$12 per tonne. The discount rate will likely be higher, while inflation has raised these prices since - a better modern estimate for costs might be $20-30/ton, depending on how hard inflation continues to hit us. A major variable cost in operation is fuel, constituting about half of total running costs, so these actual costs can vary quite a bit in practice, but this is a good place to start.


Post-combustion CO2 capture costs are primarily associated with gas compression, with sequestration costs being the major uncertainty. The energy cost of distribution is estimated at MUS$2 per tonne of lime in today's dollars, while shipping costs are included as a 'hired' service at US$200,000 per day. The maximum distance between the processing plant and port has been limited to around 10 km to avoid costly land transport. It would require about 100 dedicated 300,000 ton carriers, each discharging about 1 ton per second, to sequester 1 Gt/y of carbon. This is not a small undertaking!


Using these methods, total capital and operational costs of the proposed method were calculated against net CO2 removal from the atmosphere. The cost for net sequestering one tonne of CO2 using oxy-fuel firing of limestone comes to about US$150 per tonne of CO2 today.


This is not nuts! But it is also not a slam-dunk. And the environmental consequences of modifying ocean chemistry are much more severe than the consequences of underground carbon injection, so the pushback against ocean liming will be severe. Still, the cost of liming is still less than the best estimates for the social cost of carbon, at roughly $190/ton, and less than the Inflation Reduction Act's DAC subsidy of $180/ton. The political and environmental risks around liming are probably too great to allow it to be done successfully, but it shouldn't be taken off the table quite yet.

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