ACARP ACARP ACARP ACARP
Technical Market Support

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

Technical Market Support » Thermal Coal

Published: March 97Project Number: C5058

Get ReportAuthor: Harry Hurst | CSIRO Energy Technology

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.

Underground

Health and safety, productivity and environment initiatives.

Recently Completed Projects

C34019Longwall Bretby Cable Handling Monitoring With Fibre Optics

This project examined the potential of using fibre optic sensing tec...

C27049Mine Machine Radar Sensor Integration

The aim of this project was to develop an integrated radar sensor an...

C29007Innovative Coal Burst System To Investigate The Influence Of Confinement Loss And Pre-Conditioning On Coal Burst Mechanism

The challenges associated with designing and operating a safe and pr...

Underground

Open Cut

Safety, productivity and the right to operate are priorities for open cut mine research.

Recently Completed Projects

C33035High Water Recovery, Low Cost Desalination Using PV-Powered Membrane Capacitive Deionisation (Mcdi)

Capacitive deionization is a robust, energy efficient and cost effec...

C28035Topsoil Deficits In Site Rehabilitation Accelerated Transformation Of Spoils To Functional Soils

The incorporation of commercial biological amendments (compost, worm...

C34036Tyre Handler Testing Rig Stage 2: Lifting Trials

Tyre handling is a major source of risk in surface mining operations...

Open Cut

Coal Preparation

Maximising throughput and yield while minimising costs and emissions.

Coal Preparation

Technical Market Support

Market acceptance and emphasising the advantages of Australian coals.

Recently Completed Projects

C34060In-Situ Investigation Of Coke Structure Formation Under Stamp Charged Coking Conditions

Stamp charged cokemaking has emerged as an effective technique to im...

C34062Improving The Classification Of Microstructure Distribution In Coke CT Images Using Deep Learning And Lineal Path Calculations

This project builds on a number of earlier projects that have helped...

C29071Source Of Variability Of Reactivity Of Coke In The CSR Test

The Coke Strength after Reaction (CSR) test is used worldwide to det...

Technical Market Support

Mine Site Greenhouse Gas Mitigation

Mitigating greenhouse gas emissions from the production of coal.

Recently Completed Projects

C28076Selective Absorption Of Methane By Ionic Liquids (SAMIL)

This third and final stage of this project was the culmination of a ...

C29069Low-Cost Catalyst Materials For Effective VAM Catalytic Oxidation

Application of ventilation air methane (VAM) thermal oxidiser requir...

C23052Novel Stone Dust Looping Process For Ventilation Air Methane Abatement

This multi‐phase project is concerned with the mitigation of m...

Mine Site Greenhouse Gas Mitigation

Low Emission Coal Use

Step-change technologies aimed at reducing greenhouse gas emissions.

Recently Completed Projects

C17060BGasification Of Australian Coals

Four Australian coals were trialled in the Siemens 5 MWth pilot scale ga...

C17060AOxyfuel Technology For Carbon Capture And Storage Critical Clean Coal Technology - Interim Support

The status of oxy-fuel technology for first-generation plant is indicate...

C18007Review Of Underground Coal Gasification

This report consists of a broad review of underground coal gasification,...

Low Emission Coal Use

Mining And The Community

The relationship between mines and the local community.

Recently Completed Projects

C16027Assessing Housing And Labour Market Impacts Of Mining Developments In Bowen Basin Communities

The focus of this ACARP-funded project has been to identify a number...

C22029Understanding And Managing Cumulative Impacts Of Coal Mining And Other Land Uses In Regions With Diversified Economies

The coal industry operates in the context of competing land-uses that sh...

C23016Approval And Planning Assessment Of Black Coal Mines In NSW And Qld: A Review Of Economic Assessment Techniques

This reports on issues surrounding economic assessment and analysis ...

Mining And The Community

NERDDC

National Energy Research,Development & Demonstration Council (NERDDC) reports - pre 1992.

Recently Completed Projects

1609-C1609Self Heating of Spoil Piles from Open Cut Coal Mines

Self Heating of Spoil Piles from Open Cut Coal Mines

1301-C1301Stress Control Methods for Optimised Development...

Stress Control Methods for Optimised Development and Extraction Operations

0033-C1356Commissioned Report: Australian Thermal Coals...

Commissioned Report: Australian Thermal Coals - An Industry Handbook

NERDDC