When you think about it, the real problem is the mismatch between how much CO2 is released and how much is absorbed. It took millions of years to absorb the CO2. It will take us 400 to "burn it all". If we had to burn coal within the limits of what the planet can absorb it wouldn't be a very effective power source. It could barely compete with trees.
Yes. The real real problem is energy. We are expending stored energy at a rate much higher than it was stored. Using plants to sequester carbon is a slow and long-winded way of extracting solar energy, although it has a number of advantages too - low-tech, self-sustaining, self-storing, convenient medium for immediate low-tech use i.e. burning.
To sustain our current per-capita energy consumption beyond our accumulated biomass stores, the limiting factor is land area. There's probably not enough space on Earth to devote to growing biomass purely for energy. Solar panels reach 10 times the generated power per unit area, easily, and we can do even better by exploiting energy accumulation that happens without technology, like wind.
