An innovative 1000 kWth pilot plant based on a bubbling fluidized bed gasifier with internal recirculation was operated in experimental campaigns of biomass gasification. The evaluations were focused on the gasifier performances and quality of the product gas. To this aim, tests were carried out at atmospheric pressure using almond shells as a feedstock and three defined gasification mediums (i.e. steam/O2 mixture, 35wt.% and 50wt.% O2 enriched air); process temperature was in the range 820-880 °C. The result assessment allowed to evaluate the system flexibility to the gasifying agent and acquire data on the gas producible with this specific configuration. Based on the dry compositions, LHVs in the range 5.9-6.7 MJ/Nm3 dry, 6.3-8.4 MJ/Nm3 dry and 10.9-11.7 MJ/Nm3 dry were respectively calculated for the three product gases. Correspondingly, an increase in the cold gas efficiency from 0.5 up to 0.7 was also estimated. Concerning the contaminant loads, in the case of the tests related to steam/O2 biomass gasification, particle and tar contents were found in the range 6-10 g/Nm3 dry and 12-18 g/Nm3 dry, respectively, while H2S, HCl and NH3 were at concentrations below 100 ppms (v). © 2015 Elsevier B.V. All rights reserved.