Characterization of ashes and deposits from high-temperature coal-straw co-firing

Fly ashes, bottom ashes, and deposits collected on air-cooled probes at a P F-fired boiler cofired with coal and straw (0%, 10%, and 20% straw on an en ergy basis) have been studied with respect to chemical composition, mineral ogy, sintering, and melting. The varying straw share was found not to influ ence the overall chemical composition of the fly ashes, which were quite al ike on an oxide basis, whereas computer-controlled scanning electron micros copy data revealed a change in the species present, meaning that the more p otassium that was available for reaction (i.e., the higher the straw share burned), the higher was the fraction of alumino-silicates having reacted to form potassium-alumino-silicates. Comparing compositions of fly ashes and deposits, it was found that K-, Ca-, Fe-rich silicates were concentrated in deposits, probably as an effect of relatively low viscosities of these par ticles. Based on simultaneous thermal analysis, STA, all ashes examined sho wed melting in the temperature range from 1000 to 1390 degrees C, and despi te the mineralogical differences, no significant difference was found betwe en the melting behavior of the different fly ashes and bottom ashes. When c omparing results from the STA melting quantification method to results from the standard ash fusion test, moderate quantities of melt (1-36%) were fou nd at the initial deformation temperature, IDT. Comparing the IDT to the on set of melting as determined by the STA, it was found that the first meltin g occurred as much as 150 degrees C below the IDT. This stresses that the s tandard ash fusion method should be used with care when determining melting behavior and thereby ash deposition propensity. Sintering experiments reve aled that strength was built up in all ashes at temperatures below the firs t melt appearance. For the fly ash collected during coal combustion, high s trengths were built up in the absence of a liquid phase, whereas for the as hes produced during coal-straw co-combustion, only low strengths were obtai ned without melt present. On the basis of viscosity calculations it was fou nd that for all ashes the sintering onset was equivalent with an average vi scosity of (1-3) x 10(6) P.