BIOMASS BFB COMBUSTION: THE CUTRO (IT) AND AFFERDE (D) PROJECTS

摘要

Biofuels include a large variety of biomasses with different properties. However, for boiler process dimensioning, biofuels have similarities because their dry material heat value does not vary significantly. Hence, the fuel moisture is the most decisive factor for process dimensioning. The greatest difficulty in biofuels is to predict their ash behavior. A special tool, Fuel Tool, has been developed to combine easily available information on the chemical structure with more advanced laboratory analyses and pilot plant test results. By combining this information, a reliable basis for furnace modeling and boiler dimensioning can be achieved. This method was applied in two extremely different cases of bubbling fluidized bed, BFB, combustion. The Cutro greenfield plant burns various wood species and the possibility of alkali-induced sintering and corrosion was a decisive factor for boiler design. In Afferde, an existing coal-firing BFB was converted for demolished wood combustion. Fuel Tool showed that the southern Italian wood species in Cutro were prone for fouling and corrosion, while this tendency was less pronounced for demolished wood used in Afferde, Germany. To avoid ash sintering and fouling, the boiler dimensioning has to be revised. The flue gases should be cooled to low temperatures without harming the combustion result. In Cutro, the furnace volume is increased to absorb the energy in flue gases to evaporator walls. However, in the Afferde refurbishment projects this was not possible due to limitations in the existing furnace. Therefore, in Afferde flue gas recirculation is used for cooling hot gases. In both cases, reliable furnace modeling is important to optimize the location of the air and flue gas nozzles. In fuel handling and bottom ash systems modular design has been used in both plants. The incoming fuel is unloaded from trucks directly on a moving discharge bottom. The unloading station is composed of several moving bottom sections which can be built as modular design and their size can be increased flexibly. By adjusting the discharge speed of each section the primary mixing of different fuels can be done prior the fuel enters to sieving and conveying. The final mixing is achieved in boiler bin. From the bin fuel is fed directly into boiler to avoid time lags in the load control. The fuel is dropped above the fuel and the feeding chute is equipped with air jets that allow the fuel to spread evenly in the bed. Below the fuel feeding points bottom ash discharge hoppers are located. These hoppers remove coarse material from bed. The removal rate is controlled by water cooled screw that allow an adjustable and safe removal into closed containers. The possibility to recirculate the bed material is dependent on the enrichment of alkali compounds, metals or glass in the bed.