One of the great advantages of using Biomass as your renewable heating source is the low CO2 emissions, which is significantly lower than traditional fossil fuels and even lower than ground or air source heat pumps.
Because burning biomass releases only the same amount of CO2 into the atmosphere that would be released if the biomass was to die naturally and left to rot, it is considered part of the natural carbon cycle. The CO2 released is absorbed as new trees grow and therefore the overall release of CO2 is said to be zero. There is some CO2 release related to manufacture and transport of biomass, but this is generally very low if sourced locally.
Burning fossil fuels releases large amounts of CO2 into the atmosphere. It would take millions of years for this CO2 to be turned back into coal, oil or gas and therefore this is not considered as renewable.
Using Ground Source or Air Source Heat Pumps releases more CO2 into the atmosphere than biomass as they require electricity to operate. Electricity which is typically made by burning fossil fuels.