Understanding the Behavior of Calcium Compounds in Petroleum Coke Fluidized Bed Combustion (FBC) Ash

With growing understanding of the differences between solid residues from the fluidized bed combustion of petroleum coke and of coal, the significance of fuel-derived and sorbent-derived components of the ash has become clearer. It is well documented that hydration of the ashes is necessary prior to disposal or utilization or as a reactivation method. Initially, hydration of the lime was thought to involve water reacting only with $CaO$ to form $Ca(OH)2$ but when the free lime content of the ashes is looked at before and after hydration, it is apparent that the process is more complex. Detailed analyses have shown that the free lime can decrease and vary within the same ash in different particle size ranges. The complexity of the reactions is reflected in problems with the assessment of the free lime content of the materials and the effect of hydration on different particle size fractions of the ash. The free lime content of the ash is significantly lower than expected based on the elemental analysis. Bed ash from the circulating fluidized bed combustion boilers owned and operated by the Nelson Industrial Steam Company Ltd. (NISCO) was examined in detail to elucidate the fate of calcium in the ash during hydration, using a range of techniques. The objective of the study is to determine the amount of $CaO$ available for hydration/reactivation and to better understand interactions of $Ca$ and other mineral components of the ash. Analysis results indicate that in NISCO ashes up to about 6% of the analytical $CaO$ may be combined as acid soluble and insoluble OCCs (other calcium compounds). This implies up to about 10% less free lime than would be inferred from standard chemical analyses. About 1% of the missing $CaO$ can be present as acid insoluble $Ca$ and $Mg$ vanadates, with up to 2% bound in soluble OCCs. The remaining $3-4%$ is still not accounted for. It is clear that even very minor quantities of mineral matter, other than $CaO$ or $CaSO4$⁠, associated mainly with the coarser size fractions, are important. The amount of bound water in the hydrated ash, other than that combined in portlandite or brucite, can be as large as $3-5%$⁠. This cannot be ignored when sample mass change on hydration or heating is used as a measure of the extent of $CaO$ to $Ca(OH)2$ conversion.