Optimum sulfation temperature for sorbents used in fluidized bed coal combustion

An investigation was performed to identify the factors responsible for the existence of a characteristic optimum sulfation temperature for limestones and dolomites used as sorbents during the fluidized bed combustion of coal. The optimum sulfation temperature for a particular sorbent was found to be a function of the operating conditions, and ranged between 800 and 900°C. A reduction in the level of sulfur uptake by the sorbents at high temperatures was found to be a consequence of pore blockage by product formation. As the sulfation reaction rate increased with increasing temperature, the CaSO₄ produced accumulated at the external surface of the sorbent particles, blocking access to the porous interior. The parameters which most affected the optimum sulfation temperature were the physical structure (pore size and surface area) of the sorbent, and the operating temperature and residence time. For fixed operating conditions it was necessary to have a proper combination of surface area and porosity to ensure high utilization. For short particle residence times, the optimum temperature for high sulfur retention by the sorbent was several hundred degrees higher than when the particles were allowed to react for longer periods of time. This was attributed to kinetic rather than equilibrium constraints.