Superheater ash deposit ageing – Impact of melt fraction on morphology and chemistry

The initial melting behaviour of synthetic ash deposits, with a focus on how the amount of melt formed, affects the final deposit morphology, was studied. Binary and ternary reciprocal deposits were exposed to a temperature gradient with varying exposure time, steel temperature, and furnace temperature. Two different archetype deposit morphologies emerged. The final deposit morphology was identified to depend on the amount of melt formed during the experiment. A skeletal deposit morphology developed in systems with less than 30 wt-% melt. Short-term experiments proved that the formation of the skeletal structure is accelerated by the presence of melt, accumulating at contact points of distinct particles. If melt fractions exceeded 30 wt-%, a molten morphology developed, as the amount of melt present was sufficient to fill the pores within the deposit. For molten deposits of ternary reciprocal systems, enrichment of K in the formed melt and concurrent movement toward the steel was observed. The results confirmed a deposit ageing mechanism that has been proposed earlier, based on full-scale superheater deposit samples. The experiments showed that deposit melting and ageing behaviour are strongly connected, as the two identified morphologies differed in their ageing behaviour. The presented results improve the understanding of the initial melting behaviour of deposits and illustrate the connection between deposit morphology and deposit ageing. The results can be utilised as input data for deposition models and help in predicting the final deposit morphology and its associated ageing behaviour, aiding boiler operators and manufacturers dealing with deposit-related issues.