Methods for characterization of sorbents used in fluidized bed boilers

Abstract

Limestone addition to fluidized bed coal combustors permits the reduction of SO 2 levels emitted into the atmosphere. However, for its industrial use it is necessary to develop simple laboratory methods for the determination of parameters that, inserted in a model, permit the reliable prediction of the behaviour of the sorbent in the boiler. In this paper, the principal methods used for the characterization and determination of these parameters, thermogravimetric and batch fluidized bed, are analysed. Three limestones with different pore size distributions and hardness have been used for this analysis. The effect of the principal process variables affecting sulfur retention, i.e. calcination conditions, sorbent particle size, temperature and SO 2 concentration, has been analysed and compared using the two above mentioned methods. The very different sorbent behaviour, depending on particle size, requires the use of different particle size intervals for the characterization. Moreover, different effects of CO 2 and SO 2 concentration, over the sorbent sulfation behaviour in each method, have been found. The characterization parameters obtained depend on the method and operation conditions used. Therefore, different predictions of limestone requirements in the boiler are obtained. Depending on the type of pore size distribution in the limestones, thermogravimetric analysis cannot be used for sorbent characterization. Batch fluidized bed characterization must be made at an average CO 2 and SO 2 partial pressure, similar to those present during coal combustion.