The use of biomass is climate-neutral. During growth, trees and plants absorb CO2. This CO2 is released again during combustion. Over time, just as much wood will be harvested from forests as is planted. This means that the forest absorbs as much or more CO2 than is released during combustion in the power plant. The system provides the perfect balance and is therefore sustainable.
Biomass is also a renewable source: it will not run out, unlike fossil fuels. It is an unending cycle.