Technical Market Support » Thermal Coal
The current ash fusibility temperature (AFT) test can give differences in estimates of up to 300°C for some coal ashes. This irreproducibility led to the testing of a range of methods for determining the fusibility temperatures of coal ashes. The use of shrinkage levels for an ash sample measured using thermo-mechanical analysis (TMA) is shown to be a valid method of characterising ash fusibility behaviour. Thermo-mechanical analysis is shown to be a reliable procedure, with a typical accuracy of ± 10oC for a particular shrinkage level.
Alternative ash fusibility temperatures based on the TMA test are proposed. These temperatures correspond to particular shrinkage levels (denoted as T(S%) for S of 25%, 50%, 75% and 90%). The new temperatures suggested as indicators are:
- Initial melting (T25%, ~25% melting),
- Intermediate melting (T50%, ~60% melting), completion of melting (T75%, ~80% melting),
- slag flow (T90%).
These new temperatures are shown to correlate with the observed extent of melting and calculated viscosity, which is an indication of particle stickiness in a furnace.
- to investigate alternative procedures, for characterising ash fusibility.
- establish the scientific understanding necessary to explain the results, including relationships with plant performance as well as with the standard AFT procedure.
- develop a correlation between ash shrinkage and ash stickiness, and therefore between shrinkage and ash deposition in furnaces
Various approaches to evaluating the ash deposition tendencies of a coal were reviewed:
- simultaneous electrical resistance-shrinkage measurement (HRL test)
- improved ash fusion test (ACIRL test)
- thermomechanical analysis (CSIRO test)
The effect of particle size, heating rate, ash chemistry and the sample configuration on the various ash fusion events observed during a measurement for each technique have also been discussed. The samples used in the study were far ranging, but were grouped into Australian export coals, overseas export coals and Australian power station feed coals. Base data on all coals and blends have been collected and collated. Combustion ashes from a selection of power stations, mineral mixtures and synthetic glasses supplemented the coal ashes studied. For the TMA work, the behaviour of specific ash classes was investigated and compared to AFT results. The classifications include:
- refractory ashes, further divided into low basic components and high basic components, where the basic components are K2O and Fe2O3
- ashes which contain Fe2O3 and CaO as the major fluxing components
- ashes with unusual chemistries eg very high CaO, or Na2O
- pure mineral mixtures and synthetic glasses
The study of melting was undertaken using ash pellets a technique successfully used in previous projects.
The data and observations show that the measured shrinkage reflects the extent of melting. An initial shrinkage event (ie minor peak temperature) in the TMA test is shown to relate to significant particle deformation. This event can account for up to 25% shrinkage, therefore the deformation temperature measured using the traditional AFT test does not necessarily represent the initial melting events.
Shrinkage at around the 50% level can be related to substantial melting. These events are related to the ash chemistry - mineralogy. The temperature of these peaks relate to the various eutectic temperatures from the known system SiO2-Al2O3-X where X = FeO, CaO, K2O.
In general, the major peak temperature for refractory ashes ranges from 1400-1600oC. However, there is a modification of the observed TMA behaviour with K2O content. The “K2O effect” is proportional to the absolute K2O content of the ash. Ashes with a high K2O content demonstrate major peaks up to 1200oC. Ashes with low K2O contents have weak peaks in the low temperature regions for deformation temperature, resulting in poor precision for deformation temperature measurement. The deformation temperature is found to represent the appearance of a substantial melt phase. The melt is the result of low melting point minerals containing K2O such as illite.
A major peak and substantial melting is observed in the temperature interval of 1100oC to 1200oC for ashes that contain high amounts of combined Fe2O3 and CaO (total basic components >10wt%). Ashes with a low SiO2/Al2O3 ratio and small amounts of Fe2O3 or CaO show a wide range of melt temperatures, whereas those with high SiO2/Al2O3 ratios or with high iron contents show a narrow range of melt temperatures. For these types of ashes shrinkage measurements adequately reflect the extent of melting.
The effect of particle size is found to be of secondary importance compared to ash chemistry when determining the rapid shrinkage events, with fine particles giving rapid shrinkage events only 10oC lower in temperature than coarser particles.
TMA shrinkage at a temperature greater than the major peak temperature is mainly associated with the dissolution of SiO2 into the existing melt phases.