Rheological Investigation of Coal Ash

Coal ash is a by-product of coal combustion in thermal power plants. This material not only settles as sediment to the bottom of the boiler and is blown out with the flue gas; but it also settles on the reactor walls, creating coal ash slag. This slag on the reactor wall can become problematic and therefore needs to be removed. Hence the rheology/flow behavior of the coal ash slag as a function of the temperature is crucial.

This investigation shows the rheological characterization of coal ash slag using a high temperature rheometer from Anton Paar. Thereby critical values at the onset of solidification/crystallization as a change in the viscosity-temperature-curve slope, the so-called T250 value and the viscosities at the most representative shear rate of 1 s-1 could be shown.


Coal ash is the by-product of coal combustion in thermal power plants. 5-15 %w/w of the coal ash is coarse “bottom ash”, which falls down through the airflow to the bottom of the boiler. It is mechanically removed. Opposed to that, the fine “fly ash”, which accounts for 85-95 %w/w of the coal ash, refers to the particles in the flue gas[1]. Fly ash is diminished when the boiler is operated at a flame temperature over the melting point of the coals ash. Residual particles of the fly ash settle on the reactor walls. Although most boilers have some degree of wall slag, excessive wall slag leads to several problems such as:

  1. Wall slag that flows to the bottom of the furnace and solidifies can seal the outlet for the bottom ash removal.
  2. The slag can act as insulator, impeding heat transfer. Thus, the furnace exit temperature is elevated, resulting in the deposit of molten ash in the convection gas regions.
  3. Slag built up around the burner regions interferes with the coal and air flow[2].

Knowledge about rheology, such as melting and flow properties of the coal ash slag can help to sustain stable operation of the boiler by draining the slag out of the system. The temperature-viscosity relationship is important to determine temperatures which are required in different parts of the system and which may be considered in the construction of the plant. The T250 value was introduced, describing the temperature at which the coal ash slag has 250 poise equaling 25 Pa∙s[3]. The background for this value is that substances with a viscosity of 25 Pa∙s should flow on flat surfaces.


1.) Yao, Z. T., Ji, X. S., Sarker, P. K., Tang, J. H., Ge, L. Q., Xia, M. S., & Xi, Y. Q. (2015). A comprehensive review on the applications of coal fly ash. Earth-Science Reviews, 141, 105-121.

2.) Hatt, R. (1990). Influence of Coal Quality and Boiler Operating Conditions on Slagging of Utility Boilers. Power Production in the 21th Century: Impact of Fuel Quality and Operations, 28.

3.) Laumb, J., Benson, S. A., Katrinak, K. A., Schwalbe, R., & McCollor, D. P. (1999). Predicting slag viscosity from coal ash composition. Microbeam Technologies Inc., Grand Forks, ND (US).

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