Biomass has a significantly lower heating value than most coal.  This is in part due to the generally higher moisture content and in part due to the high oxygen content.   One might be led to believe that the lower heating values lead to lower flame temperatures.  In the case of low heating values caused by high moisture contents, this is true.  However, low heating values caused by high oxygen contents are not associated with low flame temperatures.  As indicated here, dry biomass and dry coal have similar adiabatic flame temperatures even though their as-fired heating values differ by over 33%.

Biomass also has a higher volatile yield than coal.  The ASTM tests for volatile yields consistently under predict the actual yields during combustion, but in both cases biomass yields exceed those of coal by a substantial margin.