SINTERING OF ASH DURING FLUIDIZED-BED COMBUSTION

Agglomeration of bed material and fuel ash may sometimes cause problems during fluidized bed combustion. In this paper a laboratory test method has been applied on different coal ashes to predict how they behave in temperatures typical for circulating fluidized bed boilers. The method is also useful when the influence of the different bed compounds on the sintering is studied or when the effect of the surrounding gas phase is investigated. The method is based on compression strength measurements of sintered cylindrical pellets and has been used earlier when slagging and fouling tendencies of different coal ashes in pulverized coal fired boilers have been studied. The results showed clear differences in sintering tendencies between the five different coal ashes studied. Temperatures where the sintering was initiated could vary between 500 and 900-degrees-C, depending on the ash. The sintering tendency seemed also to correlate well with the experiences achieved from full-scale and pilot-scale operation. Addition of limestone decreased sintering of one coal ash when the amount exceeded the Ca/S ratio of 1.3. The decrease was even greater when an Al-Si-based clay mineral was used.