Problems in PFBC boiler (2): characterization of bed materials found in a commercial PFBC boiler at different load levels

Abstract

Bed materials sampled from a commercial pressurized fluidized bed combustion (PFBC) of coal boiler at different loads of 270 and 285 MW output levels (bed materials; namely BM270 and BM285, respectively) were characterized in term of their morphology, composition, and thermal properties, since the operation at a little higher output suffered the problem of poor fluidization, resulting in a shutdown due to the slagging at the bottom of the boiler. BM270 was uniform grains with diameter around 3–5 mm, consisting principally of CaCO 3 grains, which carried fine calcium aluminosilicate particles on their surface. In contrast, BM285 carried much larger grains among grains similar to BM270. The larger grains were classified into two groups, sintered egg (SE) and agglomerated grain (AG). The former grains were fused as well as adhered fine particles of calcium oxide and aluminosilicate, while the latter grains were adhered ones without extensive fusion. Higher temperature at a particular location occurring in 285 MW output operation may accelerate the calcination of CaCO 3 forming CaO. Yellow particles present dominantly in BM285 adhered each other to form AG. Low viscosity of melted calcium aluminosilicate formed the smooth surface of BM285 yellow particles that carried more numbers of very fine aluminosilicate. CaO particles successively adhered through SO 2 absorption to the half-solidified surface, forming porous surface of CaSO 4 found over AG. AG was degraded into fines particles when saved in the air atmosphere. CaO particles which bridged the BM285 yellow particles in AG may lose their binding force through their hydration into Ca(OH) 2 by absorbing humidity. Bed materials and ash particles are locally heated and softened to form SE through softening and adhesion of powder materials such as very fine calcium oxide, alumina, and silicate particles into a compact grain. Unique properties of very fine particles must be clarified. Unburnt char particles were found among the BM285, providing with another evidence of poor fluidization once larger grains are produced. A little more charge rate of feed at 285 MW output may critically increase the chance of poor heat dispersion at some locations, causing the agglomeration of bed materials and coal ash, which leads to the poorer fluidization first locally and then in the whole boiler.