It's not just the emission of carbon that is the cause of climate change. It's the release of carbon that was sequestered as fossil fuels for millennia, causing an imbalance, which is the problem.

Biomass is essentially solar-powered, short term sequestering of carbon*. The process is only temporary though, because vegetation left alone will release its carbon back into the atmosphere when plant matter decomposes. Burning biomass that would otherwise decompose or was produced specifically as a sequestered surplus is renewable: it does not create a carbon imbalance, and it does substitute burning fossil fuels which do create a carbon imbalance.

/* There are a lot of details to argue over. But one thing is for sure, burning fossil fuels is far worse than any other alternative.