Three-dimensional compressible Direct Numerical Simulations have been used to investigate the localised forced ignition of statistically planar biogas/air mixing layers for different levels of turbulence intensity and biogas composition. The biogas is represented by a $$\hbox {CH}_4$$
CH
4
/$$\hbox {CO}_2$$
CO
2
mixture and a two-step mechanism capturing the variation of the unstrained laminar flame speed with equivalence ratio and $$\hbox {CO}_2$$
CO
2
dilution was used. The mixture composition was found to significantly affect the flame kernel development which was reflected in the diminished growth rate of the burned gas volume with increasing $$\hbox {CO}_2$$
CO
2
dilution. A successful ignition of $$\hbox {CH}_4$$
CH
4
/$$\hbox {CO}_2$$
CO
2
/air mixing layer gives rise to a tribrachial flame structure involving fuel-rich and lean premixed branches on either side of the diffusion flame stabilised on the stoichiometric mixture fraction iso-surface. The most probable edge flame speed decreases in time and converges to a value that is at most equal to its laminar theoretical limit, and can even locally become negative for large values of the dilution and/or turbulence intensity. The decomposition of the edge flame speed showed a negligible or negative contribution of the mixture fraction surface displacement speed, while the displacement speed of the fuel mass fraction surface appeared as the dominant contributor. Finally, the edge flame speed dependences to the fuel mass fraction and mixture fraction gradients, fuel mass fraction iso-surface curvature and tangential strain rate have been analysed and found, within the dilution values considered, qualitatively similar to those of undiluted mixtures regardless of the amount of $$\hbox {CO}_2$$
CO
2
, although quantitative differences were observed.