The concept of co-firing coal with waste material is the most promising option to contribute to the thermal recycling of significant waste quantities. The utilization of this fuel mixture in the existing combustion systems may have serious effects on the fuel preparation, the combustion behaviour, the emissions and the ash composition. This paper describes the co-combustion performance at the 13.8 MWth steam boiler of PINDOS SA, in north-western Greece. Several trials with various waste wood species and Greek lignite were performed in the combustion facility, which comprises a moving grate system and a multi-fuel burner. The waste wood species were uncontaminated wood, MDF and power poles. Before the co-combustion trials, a theoretical estimation of the ash deposition tendency using several indices was accomplished. During the experimental measurements, O2 concentration in the flue gas and the emissions of CO, SO2 and NO were continuously monitored. The operation data of the boiler were recorded and ash samples from the combustion chamber and the multi-cyclone were collected. The results showed that co-combustion is technically feasible provided that agglomeration problems could be confronted. Additionally, MDF use in the fuel blend resulted in improved combustion efficiency. The burning of a MDF/uncontaminated waste wood/lignite blend for a long time period indicated that no additional maintenance costs of the mechanical equipment of the boiler or costly waste gas scrubbing are needed.