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Industrially produced quicklime from a coal-fired rotary kiln was analyzed and compared with laboratory-scale studies of ash-quicklime interaction using i) two coal ashes ii) a coal-olive pomace ash mixture iii) olive pomace ash, and iv) a case without any ash representing an electrically heated process. Multicomponent equilibrium calculations were performed in order to predict ash melting behavior.

The industrially produced quicklime showed intrusion of mainly Al and Si up to 800 μm into the quicklime in accordance with the expected ash composition of the coal fuel used.

Laboratory-scale studies were performed at 1,100°C and 1,350°C followed by SEM-EDX analyses of the ash-quicklime interface to determine changes in quicklime microstructure and depth of ash infiltration.

At 1,100°C, coal ashes appeared solid and no distinctive microstructure differences of quicklime was observed underneath the ash-quicklime interface. At 1,350°C, the ashes appeared molten resulting in densification of the quicklime microstructure.

Potassium-rich olive pomace ash resulted in a coarsening of the quicklime microstructure, most obvious at 1,350 °C and probably as a result of a carbonate melt intrusion. Coal-olive pomace ash mixture resulted in a less severe quicklime microstructure coarsening. Without ash, the quicklime showed enhanced densification at the higher temperature. The results show that the different elemental compositions in biomass ash and coal ash results in different melting behavior, impurities and quicklime microstructure. This is important knowledge in the transition to a more sustainable quicklime industry.

The effect on quicklime microstructure seen in this study is not necessarily translatable to industrial scale and more studies are needed, for example by introducing the complete fuels resulting in a flue gas atmosphere more comparable to industrial conditions.