Introduction
Determination of ash content in a coal sample is usually done to explore its quality. The ash content gives an indication of the amount of inorganic material contained in a coal sample – including clay and shale, sulphur and other metallic oxides. According to international standards, ash content is one of the several important parameters that must be determined to assess the quality and composition of coal. In measuring ash content, one commonly used technique is the gravimetric, or the ‘Loss on Ignition’ method. The ASTM (American Sociétry for Testing and Materials) standard of this method states that the “Loss on Ignition (LOI) is an indication of moisture, organic matter, and inorganic matter content of a material.” It is the preferred method for determining ash content values of coal samples in laboratories. The traditional gravimetric method has been adapted over the years and heated directly with combustibles to measure Loss on Ignition values, leading to what is now come to be known as the ‘Thermogravimetric Method’.
Overview of procedure
The thermogravimetric method, also known as the thermal analysis, can be used to determine the LOI value of a coal sample, and hence the ash content in it. This technique uses a specialised instrument for a two-stage process – the sample must be heated using a mass balance. The LOI value is determined by measuring the difference in the initial weight of the sample and its weight after heating, which is assumed to be due to the removal of compounds such as moisture and ash.
In the thermogravimetric method, the sample is subjected to a controlled heating program in a sample holder, while the apparatus constantly monitors its weight. The sample is then heated up to 1000ºC and the rate of the heating is monitored using the mass balance. An important aspect of this method is that, in the sample holder, oxygen or air is continually injected, making it a combustion combustion instead of a combustion molecular shift.
The initial weighing of the sample and its successive weighings after heating is used to determine the LOI. The difference between the initial and the final weight of the sample gives the LOI and is expressed as a percentage of the initial weight of the sample. This methodology is based on the assumption that the LOI is made up of mainly organic and inorganic components, such as clay, shale and other material.
Advantages & Limitations
The thermogravimetric method provides a reliable approach to measuring the LOI value of a coal sample, however the accuracy of the results depends on several factors, such as the analytical balance used, the speed at which the sample is heated and the length of the heating cycle. The procedure is also time-consuming, taking anywhere between two and four hours, depending on the sample’s size.
The thermogravimetric method has the following advantages:
• It is the preferred method for testing the LOI value of coal;
• It is relatively fast and reliable compared to other methods;
• It is suitable for coal samples of any size.
The main limitation of the thermogravimetric method is that it requires a specially designed instrument to accurately measure the weight change during the heating process, and this may not be suitable for all laboratories.
Conclusion
The thermogravimetric method has become a preferred method for testing the LOI value of coal samples, to determine the ash content in a coal sample. It is relatively fast and reliable compared to other methods and it is suitable for coal samples of any size. The main drawbacks of the method is the need of a specially designed instrument for measurement of weight changes during the heating process, and the time-consuming process of weighing and monitoring the sample during the heating cycle.
