The paper addresses the combined gasification of biomass and brown coal in an internal circulating fluidized bed (ICFB) for generating a valuable producer gas. A primary method for tar abatement and gas improving was adopted, consisting in a Ni-based catalyst in the bed together with sand. The hydrogen concentration in the producer gas was high (up to 35% in the best case, >20% typically) demonstrating the validity of the ICFB process for co-gasification of coal with biomass. Higher H-2 and CO contents and lower tar yields were obtained with greater coal loadings. The tar was reduced by up to three times when changing from inert to a partly catalytic bed, under steam gasification conditions. However, this good figure is contrasted by a rather fast decay in catalytic activity, and a rather low carbon conversion even in presence of the catalyst. A mathematical model based on thermodynamic computations predicts the trend and values of the measured gas concentrations at different steam/fuel ratios, providing useful indications for its optimal choice. The findings of the research reinforce the idea that dual bed gasification is suitable even when a relatively low reactivity fuel is used, such as the brown coal in this study. (C) 2011 Elsevier B.V. All rights reserved.