The goal of this study is to provide a more effective use of wheat straw for energy production by co-firing it with a gaseous fossil fuel – propane. The study includes experimental work and mathematical modelling of the processes developing at co-firing – the influence of the additional heat supply on the thermal decomposition of straw, on the formation, ignition and combustion of volatiles, on the heat output from the device and on the flue gas composition. Experimental results give evidence that the straw co-firing with propane enhances the thermal decomposition of wheat straw pellets providing so faster formation, ignition and more complete burnout of the volatiles. By increasing the propane supply into the device up to 0,6 l/min (≈ 0,9 kW), the volume fractions of H2 and CO at the gasifier outlet increase by about 36 % and 45 %, respectively. The improved combustion of the volatiles downstream the combustor results in correlating increase of the temperature in the flame reaction zone (from 1060 K up to 1200 K), hence, increasing the heat power of the device by about 44%. In addition, the wheat straw co-firing with propane improves the flue gas composition, decreasing the mass fraction of unburned CO in the products from 1800 ppm to 800 ppm, H2 from 250 ppm to 40 ppm and the mass fraction of the hazardous NOx emission from 400 ppm to 250 ppm. A mathematical model of the combustion dynamics at co-firing straw with propane has been developed using MATLAB, considering the variations of the additional heat energy supply from the propane flame flow.