Evaluation of sorbents for capturing SO₂ in fluidized-bed combustion systems

Chemical composition, porosity, surface area, pore size distribution and particle size are the parameters which most affect the performance of Ca-based sorbents for in situ removal of SO₂ from the gaseous product from coal combustion in fluidized beds. There is reason to believe that an optimum range of CaCO₃ content exists since overall stone performance is determined by a balance between the calcium supplied and the calcium used. Porosity and pore size distribution are extremely important physical parameters for large particles (100-1,000 μm), whereas surface area is more crucial for small particles. The effect of particle size is pronounced, although in an operating fluidized-bed combustor (FBC) the minimum particle size is limited by the resulting residence time. That is, residence times of 2-3 seconds may not be sufficient to justify feeding a large amount of fines in an attempt to minimize Knudsen and/or product layer diffusion limitations. There is a need for a standardized, industrially-verified testing procedure for determining sorbent performance. In fact, once a proven methodology is developed and an index of performance established, it will be possible to identify the best sorbent for a particular coal-fired FBC.