Biomass co-firing with pulverized coal in conventional, large-scale-power plants is considered a low-cost strategy to reduce CO2 emissions and fossil-fuel dependence. This work analyzes the effects of biomass co-firing in the physical characteristics and oscillation patterns of the flame, by means of a digital imaging system installed in a 500 kW(th) semi-industrial scale swirl burner. The system is based on a monochrome CCD (Charged Couple Device) camera of high-speed frame rate (120 frames per second). Processing stage comprises the digital analysis in the spatial and spectral domain of recorded videos. Several co-firing flames with two biomass fuels (Cynara cardunculus and Populus sp.) were investigated, at substitution percentages ranging from 0% to 15%, energy basis. Spatial analysis of swirling flames highlights the effects of biomass addition in terms of luminous fluctuation rates and symmetry properties. With the joint analysis of flame derived parameters and CO emissions, flame flicker has been proved as a potential indicator of combustion performance. Additionally, a deeper investigation of luminous flame signal throughout its resulting histogram, have found new outcomes in the identification of instabilities of combustion process. Kurtosis and skewness parameters have shown an interesting response when CO emissions increase resulting in a promising option to develop monitoring and control systems based in non-intrusive measurements. (C) 2014 Elsevier B.V. All rights reserved.