Agglomeration in Fluidized Bed Combustion and Gasification─Effect of Biomass Type with Different Si/K Ratios and Char Reactivity

The agglomeration tendency in combustion and gasification of two batches of wheat straw (WS, named WS-1 and WS-2) and sunflower husk (SFH) was studied in a lab-scale fluidized bed reactor with sand as the bed material. Wheat straw and sunflower husk have significant differences in molar ratios of silicon to potassium (Si/K), which may result in different agglomeration mechanisms. Although the influence of the equivalence ratio (ER) and addition of steam shows a similar trend on the agglomeration tendency of the two types of biomass in gasification, the mechanisms do not appear the same: A reducing atmosphere and the presence of steam enhance the agglomeration of WS, probably caused by a decreased ash sintering temperature, while the increased agglomeration tendency under a reducing atmosphere and the presence of steam for SFH seem to be attributed to the enhanced effect on the interaction between SFH ash and bed material. Thermogravimetric analysis experiments show that SFH has the highest char reactivity, followed by WS-2 and then WS-1. An increase in char reactivity leads to a low char amount in the bed, thus increasing physical contact of sand and ash as well as K released from char. An increase in the ER during gasification of biomass causes a decrease in the inhibition effect of accumulated carbon.