Biogas is a renewable fuel obtained from fermentation of animal and food wastes. Usage of biogas as a fuel in domestic and industrial applications has increased in recent times. The disadvantage of biogas is the presence of carbon dioxide, which varies in the range of 30% to 45% by volume. Stability of biogas flames strongly depend upon the amount of carbon-dioxide present in biogas, which varies with the source of biomass and reactor. Numerical simulations of biogas flames reveal flow, temperature and species fields, which are important to understand the flame characteristics. In this paper, a comprehensive study on the stability and flame characteristics of coflow biogas diffusion flames is reported. Numerical simulations are carried out using reactive flow module in open source software OpenFOAM, incorporated with variable thermophysical properties, Fick’s and Soret diffusion. A short reaction mechanism with 25 species and 121 reactions obtained after eliminating all hydrocarbon species higher than C2 and nitrogen containing species except N2 along with their associated reactions from the GRI 2.11 mechanism is employed to model the reactive flow. Effects of carbondioxide content in the biogas, temperatures of the fuel or coflowing air streams (preheated reactant) and hydrogen addition to fuel or air streams are analyzed. Entropy generation in these flames is also predicted. Results show that the flame temperature increases with the degree of preheat of reactants and the flames show better stability with the preheated air stream. Preheating the air contributes to increased flame stability and also to significant decrease in entropy generation. Hydrogen addition, contributing to the same power rating, is seen to be relatively more effective in increasing the flame stability when added to the fuel stream. Results in terms of flow, temperature, species and entropy fields, are used to describe the stability and flame characteristics.