Direct air capture of CO2 is only done at great expense. ..Ditto for capturing CO2 from hot flue gases. Contrast this with:Indirect Air Capture which is remarkably cheap.
The better alternative
Carbon dioxide in the air is the source of carbon which plants use to form their tissues. ..So when a ferment of annual plantstuff produces CO2 as by·product, _as several ferments do_ its collection may be termed indirect air capture of CO2, since the C of it was in the air some weeks or months earlier. ..Should the ferment itself be profitable, by·product CO2 as a dense gas can be collected: _at minor expense.
Only a very foolish government puts hundreds of millions $$ into carbon capture projects. _ For the superior choice is Indirect Air Capture of CO2. So if CO2 is to be liquified and sent underground, let it be from a cheap source.. not a ridiculously expensive one. –As expected, Ottawa is being very foolish about this. Typical bad decision.
Ferments
Say “ferment” and perhaps making liquor from starchy grains first comes to mind — beer from barley, whisky from rye… Such ethanol production also gives a considerable amount of CO2 gas: _by weight nearly as much as the ethanol.
Fuel ethanol is made from various grains.. typically corn [maize] in the USA, though in Brasil using sugar cane juice. ..A huge number of hectares are devoted to growing sugar cane for this purpose. ..The gaseous CO2 by·product could be liquified and sent deep underground, which in North Dakota one fuel ethanol plant now is demonstrating.
The starch of grains is a polymer that yields glucose molecules when broken down, ..glucose being a simple six carbon sugar (C6). ..Sugar from sugar cane is comprised of two C6 sugars.. Cellulose is also a polymer of glucose molecules. ..So when starch or cellulose is fermented, it is as glucose the microbes use.
Other ferments are equally giving of CO2. ..Example is a ferment to butanoic acid from a cellulosic material. …Each glucose can yield either [2.ethanol + 2.CO2] _or_ [1.butanoic + 2.CO2 + 2.H2].
Wheat chaff or straw may be what is used. According to an old British patent [565.773], ..the ferment may be septic using a mix of wild microbes ..plus one butanoic acid bacteria. ..Particularly useful is a bacteria which converts lactic acid directly into butanoic acid*. ..Given time to develop, the endpoint will be mostly butanoic acid or its calcium salt. ..The ferment is shorter if much of the lignin in the material is first removed (though it need not be). …[*USPat 4138.498]
From the butanoic acid di·propyl ketone (PPK) can be had in one step. ..This clear liquid has a pleasant odour and is almost insoluble in water. For blending in gasoline it is way better than anhydrous ethanol, which tends to attract moisture. ..Plus, PPK has greater energy density. ..Since wheat chaff or similar material is far cheaper than starchy grain, the pathway: .chaff·to·PPK. should be more profitable than .grain·to·ethanol. _True also for maize cobs for their cellulose content. The starchy grain is not needed.
What more need be said?