SORBENT BEHAVIOR IN CIRCULATING FLUIDIZED-BED COMBUSTORS - RELEVANCE OF THERMALLY-INDUCED FRACTURES TO PARTICLE-SIZE DEPENDENCE

The particle size dependence of the performance of various limestones and dolostones in capturing SO2 in fluidized bed combustors was determ ined and explained in terms of the occurrence of thermally induced fra ctures (TIFs). Data were obtained in a bench-scale fluidized bed react or, a pilot-scale down-fired combustor and a 30 MW(e) circulating flui dized bed combustor (CFBC). Finer particle size fractions (100 x 400 m esh, 38-150 mu m) had lower Ca/S molar ratios than coarser size fracti ons (> 100 mesh, > 150 mu m) in the bed ash and recycle ash from the 3 0 MW(e) CFBC. Upon further sulfation of the ashes in a thermogravimetr ic analyser, the < 400 mesh (< 38 mu m) fraction captured the most add itional sulfur, indicating that these particles did not have sufficien t residence time in the CFBC to be fully sulfated. For larger particle s, the slow rate of SO2 diffusion through the product layer limited th e extent of sulfation. Hot-stage scanning electron microscopy and micr oprobe analysis of the sulfur distribution in the particles indicated that some sorbents developed thermally induced fractures, while others with comparable CaCO3 contents did not. The TIFs promoted SO2 diffusi on into the particle and, as a consequence, the sulfation behaviour of such sorbents was less dependent on particle size than was that for s orbents which did not develop TIFs. Copyright (C) 1996 Elsevier Scienc e Ltd.