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Conversion of biomass - prediction and solution methods for ash agglomeration and related problems


The work performed within the project has lead to the following results. Chemical analysis of the fuel can give a first indication of whether there might be an agglomeration problem during thermal conversion. In general a high K-content means an increased risk for agglomeration. However, the K-content alone is not a good indicator. Also chlorine proved to be very important. From the methods used in the project, laboratory fluid bed agglomeration experiments seem to give the most reliable information about conditions and temperatures where agglomeration takes place. Contrary to methods like DTA, compression strength and ash melting temperatures, all processes that might be relevant for agglomeration actually can occur during fluid bed experiments: fuel-bed material interactions, volatilisation and condensation, shear forces, temperature homogeneity and accumulation. Standard tests have been developed where process temperature is gradually increasing until agglomeration. These tests have been applied in the project at three different laboratories: ECN (NL), ETC (S) and VTT (FIN). They have proved to be accurate and reproducible. It has been shown that the addition of kaolin, magnesite, dolomite and sewage sludge significantly reduce the risk of agglomeration. The agglomeration temperatures increased with at least 60 degrees Celsius in these cases. During combustion experiments, measured particle temperatures appeared to be up to 100 degrees Celsius higher than the bed temperature. This might have a large influence on agglomeration. Because gasification, contrary to combustion, is a process where peak temperatures are lower or even absent, one might expect that agglomeration during gasification will occur at higher temperatures than during combustion. This however cannot be concluded from the experiments, illustrating the importance of other factors like the design of fluid beds (the gas distribution, the type of nozzles, etc.). It can be concluded that not only the type of fuel and other chemical “input” is determining the agglomeration temperature, also other factors like gas distribution, size of bed material, type of nozzles, cyclone efficiency in CFB’s, etc. can have an important role. This means that results from lab-scale facilities can be interpreted in a relative way (comparing fuels and evaluate possible solution) but should always be used with great care when trying to draw conclusions for full-scale plants. Nevertheless, standardised lab-scale bubbling fluidised bed experiments, as developed and used in this project, seem to be the most reliable tools for the prediction of agglomeration.