Optimisation of the Improved Ash Fusion Test

A new method of the measurement of Ash Fusion Temperatures has been developed using essentially the same equipment as is used for the measurement of ash fusibility under Standards such as AS1038.15-87 and ASTM D1857-87. However, unlike the standard method the new method produces quantitative results of the progressive change during melting of the ash and has a very much improved precision in determination of the temperatures at which these changes take place.  This project has defined the optimum conditions for the test. It was found that the ash pillar size should be kept as 2mm height and 2mm diameter. Increasing the size of the pillars casued undesirable expansion or swelling in the samples. Four ash pillars can be used but six pillars are desirable.

Video recording with image analysis was found to be a visable option for registering the tile separation but the data should require smoothing to give interpretable results. However, in the absence of video equipment, photographic intervals are quite adequate. Decreasing the photographic intervals from 20°C to 10°C and 5°C produced corresponding improvements in precision, approaching that of the video recorded data.

The results show that the method is very sensitive and the smaller the recording interval, the greater the precision in recording real detail and not noise. Therefore the choice of recording frequency depends on the amount of detail required. For a routine test basis, recording every 10°C is a recommended choice.

Heating rate was found not to affect the ash fusion behaviour in the ash furnace. Greater heating rates could be adopted in the new ash fusion test, but this must be fully investigated on a case by case basis, as the higher rates could damage some ash-fusion furnaces.

The new test offers a further capability of producing information on the changes occurring in the x-axis direction and ash volume. This should be relatable to the stickiness properties of the ash and might be useful in the evaluation of boiler operation conditions.

The aim of the project was to optimise the new improved ash fusion test developed by Coin and others (1994) for Australian coals.

The current standard ash fusion test [AS1038.15] suffers from subjectivity based on the operator's judgement of temperatures at which the ash sample reaches and passes through certain defined stages of fusion and flow, when heated at a specified rate in a controlled atmosphere. The repeatability [single operator and apparatus] of 30°C and reproducibility [different operator and apparatus] of 50-80°C reflect the subjectivity of the current test.

The change from the original form of the sample pellet through all the stages until complete melting, is a continous process. In cases of congruent melting, the entire transformation can occur over a very narrow temperature range.

The method proposed by Coin and others [1994, 1995] demonstrated that the new test method is more reliable and detailed than the current standard ash fusion test [AS1038.15]. This present project aims to define the optimum set-up conditions for the new method.

Objectives

The major objectives and benefits from this project include:

• providing a simple, inexpensive, objective, quantitative and highly repeatable ash fusion test, applicable world wide with no significant additional capital investment; 
• better measurement of the melting and viscosity of the ash than the current test; 
• technical data that can be used by Australian coal exporters for marketing and promoting their coals in overseas markets.

Conclusions

It has been shown in this project that the improved ash fusion test is far superior to the existing AS1038.15 ash fusion test. The information gained from the new ash fusion test is objective rather than subjective and contains much more information than the current AS1038.15 test. The following are the main conclusions from this study.

• The use of ash pillars > 3mm can cause expansion due to periodic gas release. 

• The number of pillars can be four but six is more desirable for mechanistic reasons and for reasons of increased sample size. 
• Video recording with image analysis is also a good option for registering the tile separation. However in the absence of video equipment, photographic intervals of 5°C and 10°C can give very similar results with adequate reproducible detail.  Increasing the frequency of recording to 5°C or 10°C does not significantly change the outcome. 
• Within the limits of heating rate examined in ths study, heating rate does not affect the ash fusion behaviour in the ash fusion furnace.  Greater heating rates could be adopted in the new ash fusion test but this should be fully investigated on a case basis as high rates could damage some furnaces. If a greater heating rate is chosen, it could significantly reduce the testing time. 
• Holding the temperature at a specific level does not affect the ash fusion outcome. This supports the theory that the ash fusion is a series of incongruent melting processes which happen when specific temperatures are reached. 
• The new test offers a further capability of producting information on the changes occurring in x-axis direction and ash volume. This may be related to the stickiness properties of the ash and may be used in the evaluation of boiler operation conditions.

Recommendations

• The ash pillar size should be kept as 2mm height and 2mm diameter. 

• Photographic recording at either every 5°C or 10°C is recommended as the standard option although continous video recording with image analysis also produces excellent reproducible results. 
• The extra data on the pillar volume opens up a new area of research, some of which will be investigated in a subsequent project. In this regard, the nature of the substrate on which the pillar is supported becomes important and hence it is recommended that some of the work in this study by repeated using substrate products may deposit