Ash Behaviour in Coal-Fired Furnaces: Technology Review

This project conducted a state-of-the-art review of ash behaviour, in terms of the ability to predict slagging and fouling, either by comparison with other coals, or for a given coal fired in a given pf fired power station, and the ability to trouble-shoot slagging problems and to establish necessary adjustments to the coal supply or to the furnace operation to avoid problems. Recommendations for future research are made.

The capability to predict fouling and slagging has improved dramatically in recent years due to:

• the development of novel coal analysis and ash characterisation techniques, and
• the understanding of deposition mechanisms.

Together with mathematical predictive tools based on these mechanisms, traditional fouling and slagging indices are being improved, and can now be based on the new analysis techniques.

The recent application of very sophisticated microanalytical techniques such as the scanning electron microscope complete with microanalytical capabilities and the computer, has made it possible to characterise fuels and their impurities in greater depth. Models have been developed employing the analytical techniques along with engineering design parameters and configuration to reflect all aspects of ash deposition. Australia is at the forefront of recent developments. Their status is given in the following table, with the poor aspects highlighted:

* Note: Understanding of the mechanisms for strength development of deposits, however, are poor.

The areas of poor capability are those recommended as those of greatest research need.

The State-of-the art is summarised by the following points:

• In general, techniques for the characterisation of minerals in coal and the associated mechanisms for deposition on furnace walls are adequate. The mechanisms of the deposit related processes - growth, sintering, strength development and shedding - together with the tools to characterise deposits are inadequate.
• Indices are inherently poor prediction tools, although particular indices apply for the coals and conditions from which they are derived. Consolidation of the literature to better define these limits is necessary as, at present, indices are applied when they should not be.
• Predictors have been developed and are now in the long process of proving their applicability. As predictors are now based on the fundamental mechanisms, which are established through research, in the long term they must be expected to provide better predictions than indices.

It is known that three factors - the coal, the furnace design and the furnace operation - can cause operational problems. At present, coals associated with severe problems, or no problem at all, can readily be identified and rankings for coals within the uncertain band can be predicted. Traditional indices do not take the latter two of the three factors into account, are of high uncertainty, but there are many data sets to support their use. Recently developed "good" predictors do consider all three factors, but case histories to prove their applicability are few. The way forward is to progress the applicability of predictors through case histories. Realistically, progress will be slow, collaboration with the furnace manufacturing and power generation industry is essential and collaboration with the international community recommended to share costs of industrial testing.