Paywalled, so excerpting the key paragraphs of a discussion about capturing carbon from corn ethanol production for use as motor fuel:

If you just follow the carbon, it started in the atmosphere and ended up underground. In between, the corn sucked up carbon through photosynthesis; when it was processed into ethanol, about a third of that carbon went into the fuel, a third was left behind as dried grain, and the remainder was captured as it wafted out of the fermentation tank and stashed underground. “That is, in a broad sense, how that looks like carbon removal,” Daniel Sanchez, an assistant professor at the University of California, Berkeley who studies biomass carbon removal, told me.

But if you zoom out, the picture changes. For the carbon to get from the atmosphere to the ground, a few other things had to happen. The corn had to be grown, harvested, and transported in trucks to the plant. It had to be put through a mill, cooked, and then liquified using heat from a natural gas boiler. And this was all in service, first and foremost, of producing ethanol to be burned, ultimately, in a car engine. If you account for the CO2 emitted during these other steps, the process as a whole is putting more into the atmosphere than it’s taking out.