All of the convoluted biofuel hustles and many of the bio-sequestration hustles are just shell games that capture some co2 here and release it there. This fools regulators but not mother nature. For example consider this pataphysical device. [via Green Car Congress]

Biological carbon sequestration, in particular engineered photosynthesis systems, offers advantages as a viable near-to-intermediate term solution for reduced carbon emissions in the energy sector. Such systems could provide a viable option for “other-than-ocean” sequestration for smaller fossil generation units located in the midwest. Photosynthetic (or “natural” sequestration) systems produce usable byproducts (biomass). . .

An engineered photosynthesis system could be placed at the source of the emissions to allow measurement and verification of the system effects, rather than being far removed from the emissions source, as is the case with forest-based and ocean-based natural sinks. The byproducts of an engineered system, biomass, could be used as a fuel, fertilizer, feedstock, or source of hydrogen. And even though some carbon is released from biomass through decomposition, bioconversion is the fastest and safest method to add carbon to natural terrestrial sinks.

So they take carbon that had been sequestered as fossil fuel, coal for example, and burn it to produce electricity. But they run the exhaust gasses though a bioreactor where bacteria turns the gasses into more bacteria rather than blasting it into the sky.

Sounds good so far except bacteria carcasses aren't very durable, aren't really sequestration. It's more of a temporary capture. Gas in a green bottle. It can be carried away and used for fuel and such which releases the carbon into the air somewhere else. No carbon is actually sequestered. Emissions are not reduced, they are displaced. Whenever you hear the word "biomass" used by an energy hustler keep a hand on your wallet because he's trying to pick your pocket.

When people glibly talk about "forest-based and ocean-based natural sinks" they fail to grasp the process. It isn't tree leaves that sequester the carbon, it's the wood and roots. In the ocean it isn't cyanobacteria or other green squishy things that sequester carbon, it is the calcium carbonate exoskeletons and shells that fall to the deep ocean where they won't decompose, and so are stored for long periods.

With forests even the wood and roots are pretty temporary. They'll eventually rot and release their carbon back to the atmosphere too. Increasing the land used for forests can increase the amount of carbon held temporarily in wood. Though each tree will one day die and rot another in the forest will grow. It's the forest not the trees that sequesters carbon. How permanent is a forest? Well, we have seen them cleared, and burned, so it all depends.

Grasslands do a better job of sequestering carbon long term. See Special Sauce about mollisols, the deep, carbon rich soils of grassland ecosystems. Still, we can plow them up to grow grain crops and release that carbon too. With both forest and grasslands we have to manage them and care for them to keep the carbon sequestered.

Carbon that was sequestered for millions of years as coal or oil is not sequestered by temporary capture as "biomass" that is then used as "a fuel, fertilizer, feedstock, or source of hydrogen.". If this is all they mean when they say "clean coal" then it's just a ruse to fool regulators since the carbon still ends up in the air after a few more processes. It doesn't get put back in the ground for a few million years more, or even a few years more. It becomes part of the working set of the current, active carbon cycle of the biosphere. To solve our carbon problem we have to reduce the amount of carbon in the biosphere by putting it back into the geosphere.

We can buy some time by storing it in a durable form, such as in soil carbon or forests, but that's not a complete solution. And the forests and soil carbon we have released by burning, plowing, cutting and the whole agro-industrial civilization is still in the air too. All the trees and prairies we can grow will only get us back to even on that score, leaving the fossil carbon in the air still.

We can use bio-systems powered by the sun to capture carbon, but we have to stash it to accomplish anything. We can't use it again in ways that would result in carbon release and claim to have done anything for the atmosphere. Maybe they could inject the cyanobacteria into deep wells or something for long term storage, but we could do that with any "biomass". We don't, and won't, because it's valuable. The only way we'll keep our hands out of the cookie jar is if the carbon is captured and stored in a worthless form, or far away at the bottom of the sea where it's not worth going after.