Slagging and Fouling in HELE plants: Assessment of Fusibility and Particle Size in Deposition

Key markets for Australian black coal have been moving to supercritical and ultra-supercritical plants, often referred to as HELE boilers, which operate at higher steam temperatures. Increased steam temperature means increased fire-side temperatures. Additionally, most countries are encouraging the inclusion of biomass combustion in their energy mix to decrease carbon emissions and attain a more sustainable energy source. Biomass generally contains alkalis and other fluxing components which often lower the temperatures where ash sinters and begins to melt. Both increasing boiler temperatures and increasing fluxing components in the ash are expected to increase deposition in both the radiant and convective sections of the boilers.

Deposition mechanisms are associated with certain particle sizes, meaning only a portion of the ash is involved in deposition. Laboratory ash represents all the ash produced in the combustion of the coal, and is morphologically very different to the ash produced in boilers. Drop tube furnace (DTF) ash, on the other hand, is morphologically very similar, being produced at similar heating rates and forming in a flowing gas stream, as in industrial boilers.

This project explores the suitability of drop tube derived fly ash for assessing ash fusibility characteristics and deposit potential.

Industrial deposits and feed coals were collected from two power stations, while a third power station provided coal, fly ash and economiser ash. The deposits were characterised by Scanning Electron Microscopy (SEM), thermomechanical analysis (TMA) and other standard techniques. TMA has previously been used to study ash deformation and involves the measurement of linear penetration into a softening ash sample during heating. The fusibility curves produced by the deposits on the TMA were compared to curves produced by laboratory ash made from the feed coals and ash produced from combustion of the coals in a drop tube furnace. The impact of particle size of the drop tube furnace ash was also investigated.

Comparison of commercial ash fusion temperatures and TMA traces show that Ash Fusion Temperatures (AFT) overestimate deformation significantly and do not provide any valuable insight into the behaviour of coal ash during deposition.

The expected changes in deposition behaviour associated with changes in steam temperatures in HELE boilers compared to sub-critical boilers is discussed. Deposition would be expected to increase due to the higher tube wall temperatures, the higher furnace temperatures, the higher fuel loading and the higher gas velocity. Addition of biomass material would increase this further.