Deep Ocean Water for Carbon Removal

Posted in Algae by Robert Tulip on the September 27th, 2017

Proposals to raise deep ocean water (DOW) to the surface as a climate mitigation technology have been criticised for producing warming. These problems may not arise if DOW is used for algae production with full recycling of nutrients.

Atmospheric consequences of disruption of the ocean thermocline, (Kwiatkowski et al 2015 Environ. Res. Lett. 10 034016) found that artificial vertical mixing of ocean water for local cooling, such as in the proposed ‘Lovelock Pipes’, would actually produce wider warming, reversing the intended benefit. Any proposed applications of large scale ocean pumping to mitigate climate change would need to address the problems modelled by this and related studies.

In considering use of ocean pumping for large scale algae production, recycling of deep ocean nutrients may be a key method to address such problems. A recent scientific paper, Phosphorus and nitrogen recycle following algal bio-crude production via continuous hydrothermal liquefaction, ( Edmundson, S., Algal Research (2017), http://dx.doi.org/10.1016/j.algal.2017.07.016), explains how hydrothermal liquefaction (HTL) to produce algae biocrude can separate and recycle the phosphorus and nitrogen in algae for ongoing reuse as fertilizer.

This finding could enable ocean based algae production to mitigate climate change by recycling oceanic phosphorus and nitrogen in combination with carbon dioxide mined from the air. Subject to modelling assessment, my hypothesis is that the climate benefit of efficient removal of carbon from the air in this way would outweigh any warming effects.

If nitrogen and phosphorus from deep ocean water are used to fertilize a contained algae pond at sea, and the algae is then converted to biocrude by HTL, then the finding that phosphorus and nitrogen in the algae can be separated from the biocrude enables continuous reuse of these nutrients. This changes the parameters for analysis of deep ocean water climate impact. Recycling of oceanic nutrients in algae farms presents a possible path to enable efficient mining of carbon from the air at scale.

If nutrients can be used in combination with atmospheric CO2 for ongoing repeat fertilization of the algae farm, this HTL nutrient separation process means DOW could potentially provide the nutrients required to grow algae on the scale needed to reverse global warming.

For algae factories at sea using HTL and recycling nutrients, my calculations of orders of magnitude are as follows.
· Ocean water below the thermocline has 3 micromoles of phosphate per litre, equal to 90 tonnes of phosphorus per cubic kilometre (per Sverdrup).
· The scale of carbon removal to reverse climate change requires removal of more than the ten gigatons of carbon in CO2 added to the air every year.
· To push back from the brink of possible climate tipping points, a reasonable goal is to remove twenty gigatons (petagrams) of carbon from the air every year (one gigaton of water has volume one cubic kilometre).
· Converting twenty gigatons of carbon from CO2 to hydrocarbons and other products for storage in stable useful form (plastic, soil, bricks, roads, etc) would require 172 million tonnes of phosphorus and 2.6 billion tonnes of nitrogen to grow algae at the Redfield Ratio (C:N:P=117:14:1).
· With complete retention of mined phosphorus and nitrogen via HTL, about two million cubic kilometres of water would need to be processed to obtain that amount of nutrient.

The attached diagram of a tidal pump may be one way to shift this volume of water. For the entire annual goal of 20gt of carbon, my estimate is that pumping arrays of 500,000 km2 located on continental shelves with twice daily tidal range 0.5 metres would take ten years to pump two million km3 of water to the surface, in order to mine the required amount of phosphorus and nitrogen from deep ocean water.

These nutrients would then be available for permanent recycling. This process would also deliver other useful dissolved minerals, and would continue indefinitely, enabling economic use of the vast dissolved mineral wealth of the seas. That scale of pumping operation is about 2% of the world continental shelf area as an eventual goal, and would depend on the availability of suitable locations, which in turn would depend on demonstrated environmental benefit. The algae farm area would be about six million km2, or 2% of the world ocean surface.

The use of hydrothermal liquefaction at such a scale would require innovative technology. HTL requires pressure equal to water pressure at two kilometres deep in the ocean, and temperature above 300°C, to make the algae cell wall break down to produce biocrude. The best way to subject algae slurry to such heat and pressure may be to pump it down to the deep ocean floor, and develop controlled automated sea floor systems for processing, as per the attached sketch. The feasibility of that method has not been assessed.

In summary, the demonstration that ‘Lovelock Pipes’ would have unforeseen warming effects does not mean raising DOW is unfeasible as a climate change response, and the ability to use HTL to recycle oceanic nutrients means that large scale ocean based algae production could be an effective method for carbon mining to deliver climate stability.

Robert Tulip

Leave a Reply