Biogas issuing is a renewable and biodegradable clean energy source, that can be used to generate power and heat. The main component of both biogas and natural gas (NG) is methane diluted with inert gases like carbon dioxide (CO2). However, despite their advantages, it is not widely used because the composition can vary with the feedstock and therefore lead to uncertain combustion/engine performance and damage the hardware. This effect of composition variation (CO2 dilution) on the flame characteristics and pollutant emission is experimentally investigated in a partially premixed CH4 -air flame generated in a swirl and bluff body stabilized burner (IIST-GS2) placed inside a combustor. Optical and laser diagnostic methods are employed for this study. The non-premixed burner produced a converging-diverging flowfield at the burner exit and a lifted flame is produced at all test cases, with an upstream movement of the flame with decreasing global equivalence ratios (ϕg). Based on variations in ϕg, two flame stabilization modes – bluff body influenced and swirl stabilized – with a transition mode in-between is observed for the cases with (flame FB) and without dilution (flame FM). It is observed that the characteristics of the heat release zone are influenced by dilution, with the FB flames being longer and also less intense when compared to FM flames. Pollutant measurement at 30 mm downstream from the combustor exit highlighted the ultra-low NOx capability of the IIST-GS2 burner. CO2 dilution leads to a reduction in NOx emission due to both thermal and chemical effects. For ϕg ≥ 0.7 extreme low levels of CO and unburned hydrocarbons (UHC) are observed for both cases. For ϕg ≤ 0.6 the dramatic increase of both CO and UHC, may be due to the lower flame temperatures and shorter flame zone residence times, respectively.