The Application of Differential Thermal Analysis (DTA) to Coal Ash Characterisation and Ash Fusion T

This report presents the results of a scoping study which details the potential for application of Differential Thermal Analysis (DTA) combined with thermogravimetric analysis (TGA), for investigation of the ash fusion characteristics of thermal coals. DTA/TGA results have been recorded on selected power station coal and fly ashes, on selected coal ashes with low ash fusion characteristics, and on synthetic mixtures of the common materials found in Australian bituminous coal ashes.  While the work was necessarily limited in scope in this preliminary study, the DTA/TGA facility was successfully operated up to a temperature of 1600oC and reasonable operating procedures were established for the initial tests. DTA/TGA results obtained suggest that the best applications are for elucidation of melting, decomposition and recrystallization reactions which occur in coal ashes during ash fusion testing and in consolidation of slag deposits in conventional boilers. The technique shows some promise as a new ash fusion test but thermomechanical analysis (TMA) would be preferred in this regard.

A number of the samples analysed were common to other ACARP supported projects on alternate ash fusion testing by ACIRL and the CSIRO. In this regard, the present results show that the recrystallization of metakaolin, as revealed by an exothermic DTA peak with no mass loss at about 1000oC, is often associated with the first shrinkage effect recorded in the TMA curves of both ACIRL and CSIRO.

Decomposition of anhydrite (CaSO4) formed during ashing occurs in the range 1000-1200oC as revealed by a an endothermic DTA peak and an associated mass loss. It appears that this decomposition is often associated with the second shrinkage event recorded by both ACIRL and CSIRO.

The DTA/TGA technique is unique in providing the means to discriminate between endothermic reactions caused by melting or decomposition reactions during ash fusion testing and slag deposit formation. In addition, DTA has also identified exothermic reactions associated with crystallization of molten slags similar to those from slagging gasifiers.

DTA was also found to have potential as an alternate ash fusion test method, but probably correlated best with the softening, hemispherical and flow temperatures rather than the deformation temperature. While there is scope for improvement through redesign of the sample holders and increasing temperature limits to say 1650oC, it is difficult to see advantages over TMA. Thus, TMA would be preferred for ash fusion testing, but DTA/TGA could be of great assistance in elucidation of the processes which occur in such tests. In this latter regard, the combination with hot stage XRD and SIROQUANT appears particularly promising.

Project Objectives

To demonstrate the usefulness of high temperature DTA in explaining the ash fusion and slagging characteristics of thermal coals in the following areas:

• To complete a scoping DTA study on coal ash samples which have been examined in other ACARP sponsored ash fusion projects. 

• To correlate the DTA results with conventional and alternative ash fusion test results (Australian standard, ACIRL method and CSIRO thermomechanical analysis, TMA). 
• To define and recommend directions for future work, to develop and apply the DTA technique in thermal coal evaluation.

The DTA study of the laboratory and combustion fly ashes from coals used in power stations provided some additional information to the other techniques.

The power station laboratory ashes showed the presence of an exotherm at 1000oC due to transformation of meta-kaolin to form a silicon spinel phase. This often corresponds with an observed peak on the derivative curve from TMA measurements. The size of a DTA endotherm and TG mass loss over the range 950-1200oC could also be correlated with calcium sulfate found by the SIROQUANT technique and calculated from the SO3 content of the ashes.

The combustion ashes all gave exotherms around 700oC which could be attibuted to the oxidation of residual char from incomplete combustion, but very little signal above 800oC, consistent with the previous melting and reaction of the flyash at higher temperatures.

Recommendations

DTA/TGA provides an experimental technique that measures when phase changes and reactions occur by measurement of associated endothermic and exothermic heat. Thus, it supplies additional information to the other techniques for measurement of ash fusion characteristics.

Future work should concentrate on the the strengths of the DTA technique, and suggested areas are:

• The study of reactions of simple mixtures of the mineral components of coal ashes. The identification of these reactions in DTA/TGA measurements of coal ashes could then be confirmed by spiking to see whether the size of the signal increased. DTA/TGA, combined with hot stage X-ray diffraction (XRD) and SIROQUANT provide powerful tools for the elucidation of the mineral transformations occuring during the Australian Standard, the ACIRL, or the CSIRO thermomechanical (TMA), methods of ash fusion testing. DTA/TGA can identify the transformation of metakaolin (exothermic with no mass loss at about 1000oC), the decomposition reaction of anhydrite (endothermic with mass loss in the range 1000-1200oC) and melting reactions (endothermic with no mass loss) which occur during ash fusion testing. 

• The study of the melting and crystallization of coal ashes. This DTA work has shown that some samples show melting endotherms on the heating cycle and crystallization exotherms at a lower temperature on cooling. Crystallization temperatures may well provide a measure of the temperature of critical viscosity (Tcv), which sets a lower operating temperature for operation of a slagging gasifier. Expensive measurements of slag viscosity versus temperature curves currently provide the only method for determining Tcv. There is particular interest in evaluation of molten slag characteristics in relation to the use of Australian bituminous coals in entrained flow slagging gasifiers (ACARP Project C4058) because they form an important part of new advanced power generation technologies using coal based integrated gasification combined cycle (IGCC).   A DTA investigation of coal ashes and fluxed coal ashes with known Tcv values (from ACARP Project C4058) is recommended to determine the suitability of the technique for determination of Tcv. 
• Further development of DTA/TGA procedures for determination of melting temperatures. Endothermic melting effects were observed for the coal ashes with ash flow temperatures <1450oC and broadly correlated with the hemispherical and flow temperatures determined with the Australian standard method. However, equipment limitations on sample weight and operating temperature curtailed its applicability to many Australian coal ashes. It is considered that these limitations can be overcome by redesign of the sample holders and other factors. 
• This work suggests two future applications for DTA measurements. The first is a study of the reactions of simple mixtures of minerals found in coal in order to identify the reactions taking place during the AFT measurements. The second makes use of the capability to measure slag melting and recrystallisation temperatures, which may be used to determine the amount of supercooling of melts below the melting point. This may be closely related to the temperature of critical viscosity of slags, which cannot be predicted, and which is of great importance for the continuous operation of IGCC slagging gasifiers.