Inhibiting agglomeration of bed particles in CFB burning high-alkali fuel: Experiment, mechanisms and criteria for recirculating bottom ash or selecting alternative bed materials

Abstract

Bed material agglomeration during circulating fluidised bed (CFB) combustion of high-alkali fuels not only affects stable fluidisation of the solid particles, but also requires extra expenditure to refresh or substitute the bed materials. This work therefore verified experimentally the role of three different bed materials in initiating or mitigating particle agglomeration, led to a proposal of criteria for selecting effective bed materials or recirculating bottom ash. Using XRF, SEM-EDX, XRD analysis and FactSage calculations, results show that the formation of low melting-point K/Na/Ca silicates up to 41.4% at the combustion temperature was responsible for agglomerate formation. However, this may be mitigated by using alternative bed materials rich in either aluminosilicate or alkali-resistant constituents, to form high melting-point akermanite, forsterite and merwinite meanwhile inhibit the slag-liquid phase formation. FactSage calculation also confirmed that a decrease in SiO2 and an increase in Al2O3, CaO and MgO would minimise liquid phase formation. Consequently, two indices including ((K2O + Na2O) × SiO2)1/2/Al2O3 and (K2O + Na2O)/(CaO + MgO), representing the likelihood of alkali silicates formation over the inhibiting effect of Al2O3, CaO and MgO, were proposed and also supported by the available literature data, which provides a reference for selecting alternative bed materials or recirculating bottom ash in CFB.