[Atomic-S (OP)]

A way to clean up a coal-fired power plant may be to pipe its emissions into a large greenhouse, where vegetation would exchange the carbon dioxide for oxygen, resulting in clean emissions. How many acres of greenhouse would be required to clean the emissions of a 1 megawatt power plant, using what  type of vegetation?

[chris]

The answer will be a huge amount, but the problem is not just soaking up the CO2 in the short term. Fixing the carbon into crops locks it away only temporarily because, as when the plant matter eventually breaks down, the CO2 is released back into the atmosphere. Coal, on the other hand, is a stable "fossil" form of carbon storage. So you really need the plants to turn back into coal in order to redress the CO2 balance in the atmosphere, and that will take millions of years as well as millions of acres...

[Atomic-S (OP)]

Or, the process should be shortened by turning the plants into fuel directly, as wood pellets, which would reduce the need to mine additional coal.  Or, the plants could be turned into a fixed product that would hold the carbon over a long time, such as lumber.

[Atomic-S (OP)]

An  advantage of piping stack emissions directly into a greenhouse rather than into the open atmosphere is that with the higher concentrations of CO₂, plant growth would be speeded up, and the CO₂ would not be as available to trap solar heat while it was happening.