Modelling for the high-temperature sulphation of calcium-based sorbents with cylindrical and plate-like pore geometries

A distributed pore model has been developed to analyse the simultaneous calcination, sintering and sulphation processes suffered by the small CaCO3 or Ca(OH)(2) particles injected into the post-flame zone of pulverised coal boilers to reduce the SO2 emissions. The sorbent decomposition is considered to follow a shrinking core model with a changing pore size distribution in every shell and the sulphation takes place on the porous surface of CaO generated during sorbent calcination. A new submodel is used to define the sintering phenomena of the CaO-CaSO4 eutectic, which also modifies the sorbent pore structure during sulphation. Moreover, two different geometries, cylindrical and plate-like, have been used to define the pore structure of calcium oxide derived from different calcium-based sorbents. The plate-like pore geometry developed by some limestones and calcium hydroxides has demonstrated to have several advantages over the cylindrical one producing a higher sorbent reactivity and delaying pore plugging. The model has been used to determine the kinetic parameters of three sorbents (a natural limestone and two calcium hydroxides prepared from the former, one of them modified with calcium lignosulfonate) at furnace injection conditions, i.e., temperatures between 950 degrees C and 1150 degrees C and residence times lower than 1.5 s. Very similar product layer diffusion coefficients, D-s, were obtained for calcium carbonate and for calcium hydroxides, with activation energies ranging From 20 to 27 kcal mol(-1). The kinetic parameters have been obtained with a realistic thickness of product layer by using a volume ratio for product to reactant, Z(s), of 2.72 and an adequate pore geometry for each Ca-based sorbent. The use of a higher value for Z(s), 3.09, normally used in previous works on sorbent sulphation, produces higher predictions of D-s while it hardly affects the kinetic parameters obtained for sintering. Moreover, the use of an untrue cylindrical pore geometry to define the pore structure of calcium hydroxides can produce values of D-s up to 20 times higher than those obtained with the real plate-like pore geometry of these sorbents detected by nitrogen physisorption data. (C) 2000 Elsevier Science Ltd. All rights reserved.