Study on the critical amount of liquid for bed material agglomeration in a bubbling fluidized bed

Abstract

Agglomeration of bed materials at a high temperature is one of the most important and challenging problems for fluidized-bed biomass boilers for thermal/power generation. Inorganic alkali compounds derived from the biomass ash, mainly potassium (K) and sodium (Na) can be problematic as they form low-melting alkali compounds and may also react with the bed material (silica sand) forming low-melting alkali silicates. These low-melting alkali compounds, if surpassing a critical amount, could coat the sand particles to form agglomerates, eventually leading to partial or complete de-fluidization of the reactor. In the present study, the critical amount of liquid (molten ash in real biomass boiler operations) that would result in severe bed agglomeration and defluidization was studied a small pilot-scale cold bubbling fluidized bed (BFB) test rig filled with silica sand particles as bed materials equipped with non-invasive capacitance sensors and differential pressure transducers. In the cold BFB test rig, a solution of glycerol–water (30% v/v), employed to simulate molten ash in real biomass boiler operations, was injected to the bed at different quantities during the tests. It was found that in the present fluidization system the critical liquid amount causing bed agglomeration is likely 0.2wt% (in relation to the weight of bed material loaded) and 0.7wt% would cause severe channeling and de-fluidization conditions.