Reusing any CO2 means that the produced fuel is zero-emissions and the car/drone/generator/whatever using it is a true zero-emissions product. Whether the CO2 came from flue gas or the ocean or a cement plan is irrelevant. If there is zero CO2 flue gas because somehow we have converted all these industrial processes to...(to what?) from fossil fuels, then it will be from a cement plant, and if that is zero too, it will be from the ocean. These "ifs" are interesting 2050 scenarios. In the meantime, zero-emissions liquid fuel solves a variety of problems like heavy-duty trucks, aviation, off-grid power, energy storage. Also it is equivalent to green hydrogen (hydrogen+co2=methanol) but without the 100x cost of a hydrogen delivery, storage, dispensing infra (which the developing world will not be able to afford for many decades). As for "where will the extra energy come from" there is way too much potential sun and wind energy. If you really want to solve the environment problem we really need mass production of renewable fuels.

Reusing any CO2 means that the produced fuel is zero-emissions and the car/drone/generator/whatever using it is a true zero-emissions product. Whether the CO2 came from flue gas or the ocean or a cement plan is irrelevant. If there is zero CO2 flue gas because somehow we have converted all these industrial processes to...(to what?) from fossil fuels, then it will be from a cement plant, and if that is zero too, it will be from the ocean. These "ifs" are interesting 2050 scenarios. In the meantime, zero-emissions liquid fuel solves a variety of problems like heavy-duty tracks, aviation, off-grid power, energy storage. Also it is equivalent to green hydrogen (hydrogen+co2=methanol) but without the 100x cost of a hydrogen delivery, storage, dispensing infra (which the developing world will not be able to afford for many decades). As for "where will the extra energy come from" there is way too much potential sun and wind energy. If you really want to solve the environment problem we really need mass production of renewable fuels.