The effects on the combustion behaviour and the emitted pollutants from the co-firing of waste wood with coal are described in this paper. Wood residuals, such as uncontaminated wood, MDF and power poles, were used in conjuction with Greek lignite from the Ptolemais reserve, for the preparation of fuel blends. The fuel preparation process, including the mixing of the raw materials, was studied and tested. The co-combustion tests were performed in a 13.8 MWth steam boiler, which comprises a moving stoker furnace and a multi-fuel burner. During the co-combustion tests 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 was collected from the combustion chamber and the multi-cyclone particulate removal system. Ash samples were analysed for the unburnt fuel concentration in a LECO CHN-800 analyser. A long term co-combustion test followed with the burning of the optimum fuel blend, i.e. MDF / uncontaminated waste wood / lignite in the proportion of 60/20/20 %wt.
The experimental tests and the results from the demonstrative waste wood and lignite co-combustion proved the feasibility of this operating mode. The use of MDF as the base fuel in the blend resulted in improved combustion efficiency. No additional maintenance costs of the mechanical equipment of the boiler were needed and, thus, no technical modifications were necessary at the installation. Moreover, no substantial emissions (CO, SO2, NO) problems were recorded, indicating that no costly waste gas scrubbing will be required, in case of the permanent co-combustion application.
Keywords: co-combustion, waste wood, Greek lignite, moving stoker, emissions.