Lowering the cost of carbon sequestration by ocean nourishment

The level of primary production in the ocean and the ocean's uptake of carbon dioxide appear limited by the supply of nutrients to the upper ocean. Ocean nourishment is the concept of providing the macro nutrients that would increase carbon dioxide sequestered in the ocean. Such a strategy would allow the burning of fossil fuel to supper? the economic improvement in many developing countries while keeping climate change rates within acceptable bounds. The cost of sequestering carbon by an ocean nourishment plant depends on the cost of feed stock for nitrogen fixation, together with the cost of servicing the capital needed for a chemical plant and a pipeline. For offshore natural gas feedstock there may be opportunities to reduce the feedstock and pipeline costs by fixing nitrogen on a floating platform at the edge of the continental shelf. Such a location is the desired release point for the nutrient and so pipeline costs can be reduced to zero. The floating nitrogen fixation plant will cost more than a shore based plant. The economic advantages are considered in this paper and found to be mostly in the flexibility to exploit low coast natural gas reserves. One technical advantage of the floating ocean nourishment plant is that the initial dilution of reactive nitrogen can be chosen over a wide range with little economic penalty. Sea water is available in copious amounts. The concentration of reactive nitrogen dissolved in sea water influences its density. There is an optimum distribution of nutrient throughout the euphotic zone to promote phytoplankton growth and minimise the losses of nutrient to the atmosphere. Adjusting the initial dilution allows depth control through buoyant diffusion and limits the maximum phytoplankton concentration achieved before exhaustion of the introduced nutrient. Issues such as exploiting presently uneconomic gas fields located near the edge of the continental shelf to reduce the cost of ocean nourishment and the relative ease of relocation of the floating ocean nourishment plants are explored.