The effect of the natural gas to H-2 ratio on the flame structure and NO, formation in lean premixed porous combustion was determined by combining Comsol and Premix codes. The thermal pathway was of secondary importance while the NNH, N2O and prompt pathways were the most important routes involved in NO formation. H-2 enrichment resulted in the penetration of the combustion zone into the porous burner and in the decrease of the importance of the prompt pathway, which leads to the decrease of NO formation. The chemical effect of NO recirculation was also investigated. (c) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.