Agglomeration mechanisms in a laboratory bubbling fluidized bed due to addition of different phosphate compounds

Agricultural waste and biomass (agrofuels) are increasingly utilized in fluidized bed incineration for heat and power production. However, agrofuels are rich in phosphorus which has been shown to play a big role in agglomeration of fluidized beds. Phosphates in the fuel can react with alkali and form low melting compounds that may lead to agglomeration. The effect of phosphate compounds has been studied earlier by addition of different phosphates to non-agglomerated fluidized bed ash with subsequent heating in a furnace. Tests with different agrofuels rich in phosphorus have also been reported previously. However, no studies have been done with (semi)-continuous feeding at constant temperature with pure chemicals and quartz to separate and reveal the actual agglomeration reactions. In this work, six different phosphate compounds were tested: NH4H2PO4, KH2PO4, K3PO4, NaH2PO4, NaPO3 and CaHPO4. The tests were performed in an electrically heated bubbling laboratory fluidized bed reactor with a quartz sand bed at four temperatures: 750, 800, 850 and 900°C. Small batches of the phosphate (0.5 g) were fed onto the bed via a steel pipe with an air pulse once every 10 minutes until either the bed defluidized or nothing happened after a total of 10 g had been fed and the test was terminated. The defluidization was detected by a rapid decrease of the pressure drop over the bed. Furthermore, the upper and lower bed temperatures were also logged, as the temperature difference will increase when the bed starts to defluidize. The method has been proven previously in tests with potassium salts. After each test, samples of the bed material and agglomerates were taken out of the reactor, cast in epoxy, ground and polished to reveal the cross-sections to be studied by means of SEM/EDX. This paper presents the agglomeration mechanisms of a fluidized bed due to different phosphorus compounds and the influence of temperature; it also compares the results to tests with phosphorus-rich fuels.