Sorbent behaviour 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 determined and explained in terms of the occurrence of thermally induced fractures (TIFs). Data were obtained in a bench-scale fluidized bed reactor, a pilot-scale down-fired combustor and a 30 MW(e) circulating fluidized bed combustor (CFBC). Finer particle size fractions (100 × 400 mesh, 38-150 μm) had lower Ca/S molar ratios than coarser size fractions (> 100 mesh, > 150 μm) in the bed ash and recycle ash from the 30 MW(e) CFBC. Upon further sulfation of the ashes in a thermogravimetric analyser, the < 400 mesh (< 38 μm) fraction captured the most additional sulfur, indicating that these particles did not have sufficient residence time in the CFBC to be fully sulfated. For larger particles, the slow rate of SO₂ diffusion through the product layer limited the extent of sulfation. Hotstage scanning electron microscopy and microprobe 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 diffusion into the particle and, as a consequence, the sulfation behaviour of such sorbents was less dependent on particle size than was that for sorbents which did not develop TIFs.