Biomass co-firing in coal-fired power plants has been widely accepted to reduce the environmental burden. In this study, food waste (FW) and sewage sludge (SS), which are the main types of municipal organic waste, were selected as solid refuse fuel (SRF). To compensate for the limitations of FW and SS, a mixture of FW and SS with varying ratios was processed using pyrolysis and desalination. The fuel properties such as the calorific value, chlorine content, alkali and alkaline earth metallic species (AAEMs) content, and heavy metal content were determined. The calorific values of all biochars were greater than 12.6 MJ/kg, which satisfies the national threshold of Bio-SRF in Korea. Chlorine and AAEMs contents exhibited clear trends for the FW ratio and pyrolysis temperature. Increasing concentrations of heavy metals were observed with increasing SS ratio and pyrolysis temperature. These results provide important insights into the practical application of municipal waste-based biochar in coal-fired plants, as well as the influence of mixing ratio and pyrolysis temperature.