The world energy demand growth is more and more supplied by renewable energy sources. In this scenario, biomass has a key role in both heat and power generation. Particularly, biomass combustion can be used in small size micro and distributed generation systems, or in smart grids together with wind and solar energy where a programmable energy source is necessary to keep the electric grid stable. In this paper, a small size 140 kWth biomass fixed bed boiler of the University of Pisa, located at the Biomass to Energy Inter-University Research Centre (CRIBE), was studied experimentally to characterize the biomass combustion process. Emissions of NOR, o(2), CO2 and CO, together with the temperature data, were measured in the flue gases. Moreover, the spatial distribution of volatile products from the fixed bed surface, such as H-2, CO, CO2, CH4, C2H6, C2H4, C2H2, together with temperature data, was studied in the early combustion stage. The parametric variation of the feed air flow and its effect on the emissions and performances was also investigated. The results indicate that the primary air mainly affects the volatiles distribution on the biomass combustion surface. Therefore, the CO emission was minimized at values of 0.03%vol with an air excess equal to 2 and a 0.06 secondary to primary air mass flow ratio.