Characterising Coal Ash

The Thermo-Mechanical Analysis Technique (TMA)

Thermomechanical analysis (TMA) has distinct advantages over other characterisation techniques, such as the standard ash fusion temperature test (AFT), in that it is very reproducible.

The TMA measurement provides an indication of the extent of melting in an ash sample at a particular temperature. This technique allows the comparison of coal ash properties in terms of performance criteria such as the deposit formation potential. It provides information on the:

• fusibility and melting characteristics of a coal ash sample,
• the sintering behaviour of the coal ash,
• an indication of the deposit strength and,
• its influence on heat transfer.

This information can be used to determine:

• deposition potential for the coal,
• deposit character and effect on operation and,
• deposit remediation measures such as sootblowing frequency.

TMA and Coal Ash Melting and Fusibility

Studies have shown that the melting events observed during the heating of an ash sample can be reproduced using TMA with much greater accuracy (typically 10^oC) when compared to the AFT test (reported 50^oC for the spherical temperature ST, and 80^oC for the deformation temperature DT). A further advantage is that the TMA provides a flexible measuring technique in terms of selection of gas atmosphere and temperature range for assessment, thereby enabling the measurement to be tailored to suit the application.

TMA and the Sintering Temperature for Coal Ash

Sintering is an important process that defines ash behaviour in a number of processes, including:

Figure 1: Sintering temperatures determined using TMA

• deposit formation and strength development and,
• particulate removal systems e.g. bag filters.

The CCSD has developed a new method for determining sintering temperatures based on the TMA technique. The technique shows good agreement with standard sintering tests such as the compressive strength test, however, it is a more rapid test (~2 hours) with greater reproducibility. Typical measurement results for two different coal ashes are presented in Figure 1. The sintering temperature is denoted by a sharp rise in the shrinkage rate.

Figure 2: Understanding slagging potential from TMA measurements

Figure 2 presents the ramifications of differences in sintering temperatures for coal ashes on the operation of a boiler. Coal A has a lower sintering temperature than Coal B, however, the lower slope for coal A indicates that

small changes in temperature will not greatly increase the likelihood of slagging. For Coal B sintering will not start until a much higher temperature, however, a small increase in temperature above the sintering point will result in rapid slagging.

Figure 3: Heat transfer performance predicted from characteristic TMA temperatures

TMA and Heat Transfer

TMA measurements have been used to characterise real boiler operations. Data obtained from TMA measurements have been applied to the prediction of deposit character and the resulting effect on heat transfer. Figure 3 provides an example of this type of application. If an ash lies in the lower left region in this figure, the furnace design temperature has to be low to avoid slagging and the deposit growth will be sensitive to small changes in the furnace exit gas temperature, i.e. the deposit has to be removed via sootblowing to avoid slagging. If a sample lies in the top-right corner, the deposit needs to be removed by sootblowing to avoid reductions in heat flux and the coal will be less sensitive to changes in operating temperature with respect to slagging.

For more information contact:

Prof Terry Wall
Program Leader
University of Newcastle