Carbon dioxide removal from the oceans: Carbon dioxide emission and techno-economic analyses of producing renewable synthetic methane

The capture of carbon dioxide from ocean water and utilizing the captured carbon as a stock for carbon-neutral renewable methane production may reduce a considerable amount of carbon in the atmosphere. However, given the lack of comprehensive analyses to reveal the potential, the examination proceeded through techno-economic and carbon dioxide emission analyses with several possible options of carbon dioxide extraction, location, electricity sources, and transport media. Through the economic analysis, the unit methane supply costs range from 3.5 to 7.5 $ kgCH4- 1 according to the different options, and the majority of the total cost is found to be the electrolysis for generating hydrogen. The results of the emission analysis indicate cumulative emissions of the considered pathways which are highly negative values of around -80,000 tonCO2 y-1 due to the utilization of carbon by extraction. However, the utilization of synthesized methane in the natural gas power plant makes the final cumulative emission amount become highly positive values of more than 70,000 tonCO2 y-1. Here, the required capture rates in the power plant are suggested for the respective pathways to achieve carbon neutrality. A capture rate of 54.44 % is required for the pathway using column and OTEC, while 77.62 % is required for the pathway using BPMED and PV. Thus, given that it could be verified that the suggested pathways are competitive in the environmental aspect if the carbon capture in the utilization plant is possible, which can take advantage of the rate less than the common existing rate, a significant reduction in the cost, especially the levelized cost of electricity, will be crucial for the pathways to attaining competitiveness in the economic aspect as well.