Sustainable development requires the research on new energy sources that will be safe and at the same time help to solve other societal problems. The paper is focusing on the analysing the heavy metals concentration of biomass and fuel mixtures of biomass and municipal solid waste. Analysis confirmed that the co-combustion of dirty fuels and biomass reduced the risk of releasing minerals and heavy metals from fuel into the natural environment. At a waste concentration of 20%, almost all heavy metals have passed from fuel to ash. Based on the experiment results and measurements, it can be stated that the co-combustion of biomass and municipal solid waste is a possible energy source that will significantly support sustainable energy development, but it is necessary to control the heavy metals concentrations during the whole process.