Because biomass will naturally released CO2 when decomposing. Burning it won't add to the footprint.
On the other hand, when you are burning coal, you released CO2 that was previously sequestered.
On the other hand, when you are burning coal, you released CO2 that was previously sequestered.
Add that biomass usually releases CH4 (methane) which is a considered a much more potent greenhouse gas. Even simply burning the methane to CO2 -- ignoring energy recapture -- is an environmentally beneficial action, which is why landfills sometimes have flares.