Environmental issues raised by the use of fossil fuels lead to the search for alternatives that promote the reduction of emissions of greenhouse gases. CO2 has been identified as being the most important and urgent to control. Co-firing is a technique that allows the simultaneous combustion of different types of fuels, for example coal and biomass, combining the advantages of both. This study characterises the advantages of the system and the possibilities of using waste biomass as fuel in a coal-fired thermal power plant. For this, co-firing biomass waste, from forestry operations, with bituminous coal was simulated. Then reductions in CO2 emissions into the atmosphere from Sines Thermal Power Plant in Portugal were calculated, showing a reduction of more than 1,000,000 tons/year of CO2. Also it was verified that although environmentally advantageous, co-firing is still not economically viable due to the high cost of the residual biomass, combined with its low-energy density and high transportation costs.