Controlling Air Pollution from Straw Burning in China Calls for Efficient Recycling (Li
et al Environ. Sci. Technol., 2012, 46 (15), pp 7934–7936)
Steel requires a
billion tons of carbon at current production levels, and, I note that straw is only fractionally carbon.
It is difficult to imagine that a non-food crop of any kind could find enough land area without inducing starvation. Not only that, considerable energy is required to collect and process
grain never mind hemp or any other fast growing plant.
It is generally reported that photosynthesis on the entire planet captures about 100 billion tons of carbon dioxide per year, while the dangerous fossil fuel industry releases about 1/3 that quantity.
To the extent that carbon dioxide can be captured by photosynthesis, probably the most viable approach would be to use algae, which grows faster than land plants and takes up more carbon - this is an issue in eutrophication - but even this has an energy cost connected with dewatering. Since dewatering can be addressed by waste heat, or better yet, by supercritical water oxidation, this may be a lower hanging fruit that even straw, which will always be far more available than plants such as hemp grown for putative fuel, fiber or other purposes.
Again, from my perspective, thermochemical carbon dioxide splitting has a high energy to mass ratio when the heat source is nuclear, and is likely to have much lower environmental impact. To the extent carbon dioxide is obtained by reforming algae, this might be useful, but personally I think we will need direct carbon dioxide capture at this point, which is only remotely conceivable from the direct processing of seawater in supercritical settings. (This would have the benefit of rapid desalination as a side product.)
This is not to say that nuclear is risk free - clearly it isn't - but it is lower risk than all of its alternatives, which in a sensible world, as opposed to the one we live in, would be enough.
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