In this study, effects of straw co-firing with a renewable fuel – low heating value biogas – on the combustion characteristics, heat output from the device and composition of flue gas, are studied and analyzed with the aim to achieve more efficient use of straw for energy production. The study combines experimental research and mathematical modelling of the processes developing at co-firing of straw pellets with a low heating value biogas, which consists of 60 % methane and 40 % CO2. The complex research of the effects of straw co-firing on the gasification and combustion characteristics suggests that the increase of the biogas supply from 0 up to 1.35 l/min (additional heat input up to 0.4 kW) stabilizes the after-flame stage of the combustion of straw pellets, thus completing the burnout of volatiles, increasing the average value of the axial flow velocity, the flame temperature and the heat output from the device by about 25 – 30 %. The mathematical modelling of the straw co-firing with biogas is provided with account of variations of combustible volatiles by increasing the biogas supply into the experimental setup. A mathematical model of the combustion dynamics has been developed using MATLAB. A system of dimensionless parabolic type partial differential equations was used to describe the formation of the 2D compressible reacting swirling flow, the variations of the mass fraction of volatiles, flame species and flame temperature at thermo-chemical conversion of volatiles. Results of the numerical simulation were analyzed considering the processes developing downstream the combustor.