Defluidization of a quartz bed – Laboratory experiments with potassium salts

Abstract

As the need for CO2 neutral and renewable fuels increase, the use of biomass in combustion for heat and power becomes more and more important. Biomass, however, has a tendency to cause problems for the boilers such as fouling and corrosion of super heaters as well as agglomeration of the bed material in the case of fluidized bed boilers. The problems arise because of the reactive alkali in biomass. Previous agglomeration studies have been done with laboratory scale fluidized beds but usually by burning biomass or by using biomass ash. This study focused on the separate alkali components in biomass to get a better understanding of the agglomeration mechanisms. Pure salts were used which in this case were potassium chloride, potassium carbonate and potassium sulfate. An electrically heated laboratory scale fluidized bed reactor was used for the defluidization experiments and quartz sand was used as bed material. The bed was heated to the set temperature and salt was added until defluidization occurred. The experiments were done in a temperature range of 750–900°C under both dry and wet conditions. The pressure drop and the bed temperatures were used to detect the onset of agglomeration and defluidization. The bed was then analyzed with SEM/EDX and ion chromatography to determine the bed composition. It was found that potassium chloride did not react with the quartz bed but in fact only acted as glue between the sand particles. Potassium carbonate on the other hand reacted with the bed particles forming potassium silicates. Potassium sulfate did not cause defluidization as was expected.